annex v. stsm reports (14) - uma reports.pdf20/07/2009 to 16/08/2009 cost-stsm-d36-4802 mazharul...

ANNEX V. STSM reports (14)

Realized Short Term Scientific Missions (STSM), COST Action D36; Jan 2009-Dec 2009

Period STSM Beneficiary Home Institution Host Budget (Euro)

1 WG 6 20/02/2009 to 08/03/2009 COST-STSM-D36-4271 Anna E Lewandowska Post doc M. Banares, ICP, CESIC, Madrid, SP M. Calatayud, UPMC, Paris, FR 1830

2WG 1 17/02/2009 to 17/03/2009 COST-STSM-D36-3961 Giorgio Volpi Post Doc C. Nervi Univ. Torino, IT L. Pospisil, Heyrovsky Inst. Prague, CZ 2300

3 WG 5 01/03/2009 to 30/04/2009 COST-STSM-D36-3903 Markus Vogelsang PhD student E.Santos, Ulm Univ.DE R. Dryfe, Univ. Mamchester, UK 2500

4 WG 3 15/04/2009 to 15/05/2009 COST-STSM-D36-4037 Ivan Bogev Ivanov Post Doc D. Andreeva,Inst. of Catal., BAS, Sofia,BG A.M.Venezia, Inst. of Nanostructured Mat. Palermo, IT 2300

5 WG 3 09/03/2009 to 21/03/2009 COST-STSM-D36-4089 Gèrôme Malet PhD student N.Kruse ULB, Brussels, BE A.M.Venezia, Inst. of Nanostructured Mat. Palermo, IT 1250

6 WG 6 14/04/2009 to 31/05/2009 COST-STSM-D36-4307 Ricardo Medina PhD student M. Banares, ICP, CESIC, Madrid, SP M. Ziolek, Univ. Poznan, PL 1500

7 WG 6 15/05/2009 to 15/07/2009 COST-STSM-D36-4354 Elisabeth Rojas Garcia PhD student M. Banares, ICP, CESIC, Madrid, SP M. Calatayud, UPMC, Paris, FR 2500

8 WG 6 08/06/2009 to 10/07/2009 COST-STSM-D36-4576 Daniela Plana PhD student R. Dryfe, Univ. Mamchester, UK M. Koper, Leiden University,Leiden,NL 2450

9 WG 6 20/07/2009 to 16/08/2009 COST-STSM-D36-4802 Mazharul M.Islam Post Doc M. Calatayud, UPMC, Paris, FR Paccino, Univ. Milano, IT ACTION D43 2400

10 WG 6 14/09/2009 to 23/09/2009 COST-STSM-D36-4798 Elisabeth Santos Post Doc Universität Ulm,Ulm, DE F.Tielens,UPMC, Paris, FR 110011 WG 6 14/09/2009 to 13/11/2009 COST-STSM-D36-4845 Ana Rita Almeida PhD student TUDelft,Delft Frederik Tielens,Université Pierre et Marie Curie Paris(FR), 250012 WG 6 15/10/2009 to 15/12/2009 COST-STSM-D36-5153 Noelia Beatriz Luque Post Doc Universität Ulm,Ulm(DE) Monica Calatayud,Université Pierre et Marie Curie,Paris(FR), 250013 WG 3 12/09/2009 to 16/12/2009 COST-STSM-D36-4772 Gabriella Di Carlo Post Doc CNR,Palermo(IT) Magali Boutonnet,KTH Stockholm (SE) 350014 WG 8 09/12/ 2009 to 23/12/ 2009 COST-STSM-D36-5478 Jordi Morros PhD student CSIC,Barcelona Maria Miguel,Faculdade de ciencias e tecnologia,Coimbra(PT), 1200

Total : 14 STSM at total cost: 29830

Magali Boutonnet STSM coordinator 2010-01-25

COST STSM D36-04450

Electrocatalytic activity of particles deposited at liquid/liquid interface

Scientific Report

Markus Vogelsang, University of Ulm STSM within the COST workgroup D36/005/06: Structure-Reactivity Relationships of Pt and Pd Nanoarrays Host: Prof. Robert Dryfe, The University of Manchester 01.03.2009 – 30.04.2009

COST STSM D36-04450 Manchester, UK, March/April 2009

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1. Introduction

Electrodeposition of metal particles e.g. Pt, Pd, Au, Ag and Cu or bimetallic particles

e.g. Pd-Pt, Au-Pd, Au-Pt at the polarised interface between two immiscible electrolyte

solutions (ITIES1) has become an important research field during the last years. The

ITIES formed between an aqueous solution containing mainly hydrophilic ions and an

organic solution containing mainly lipophilic ions represents an ideal background for

the metal nucleation because of the absence of a solid support2. Also the metal

deposition at ITIES is a novel method to prepare catalysts3 or photocatalysts4 in

comparison to traditional chemical or electrochemical techniques.

The deposition of metal and bimetallic particles at ITIES was studied in detail by

Dryfe et al.3, 5, 6 (Pd, Pt, Pd-Pt), Kontturi et al.2, 7, 8 (Pd) and Unwin et al.9 (Ag).

Catalysis by electrochemically prepared nanoparticles at the ITIES has been

investigated by Samec et al.10 and Kontturi et al.11 The deposited nanoparticles were

used to catalyse various reactions e.g. the dehalogenation of bromoacetophenone to

acetophenone at the water/1,2-dichloroethane (DCE) interface by Pd11, the reduction

of oxygen in aqueous solution by decamethylferrocene in DCE by Pt10 or the

photoreduction of tetracyanoquinodimethane by Pd4.

In this work the deposition of Pd nanoparticles at the water/DCE interface and the

usage of the synthesised nanoparticles as a catalyst for proton reduction have been

investigated.

2. Experimental section and results

All deposition experiments were realised in a four electrode electrochemical cell,

shown schematically in figure 1. The glass cell contained about 2 mL of each phase

and the diameter of the liquid/liquid interface was about 10 mm.


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Figure 1: Scheme of the electrochemical cell

In both phases platinum mesh electrodes were used as the counter electrodes. The

reference electrodes were silver/silver chloride electrodes freshly prepared by

oxidation of a silver wire in a potassium chloride solution. All potentials are indicated

in relation to these reference electrodes and all experiments were run with an

Autolab PGStat100 potentiostat (Eco Chemie B.V., Utrecht, The Nederlands).

The first deposition experiments of Pd nanoparticles at the water/DCE interface were

realised with the following electrochemical cell (1):

The aqueous phase comprised of ammonium tetrachloropalladate(II) (1mM) and

lithium chloride (100 mM) as aqueous electrolyte (both Sigma-Aldrich, Dorset, United

Kindom). The organic phase consisted of the electron donor decamethylferrocene

(DMFc, 10 mM, Sigma-Aldrich, Dorset, United Kingdom) and Bis(triphenyl-

phosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate (BTPPA TPFB,

20 mM) as the organic electrolyte. The BTPPA TPFB was synthesised by metathesis

of equimolar amounts of BTPPACl (Sigma-Aldrich, Dorset, United Kingdom) and

Ag (s) AgCl (s) 10 mM LiCl

1 mM BTPPACl

(aq)

AgCl (s) Ag (s) 100 mM LiCl

1 mM (NH4)2PdCl4

(aq)

10 mM DMFc

20 mM BTPPA TPFB

(DCE)


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LiTPFB . n Et2O (Boulder Scientific Company, Mead, USA)12. Pure water (resistance

> 18 MΩ.cm) was generated with a Millipore Milli-Q water purification system and

1,2-dichloroethane (DCE, HPLC grade) was obtained from Lancaster Synthesis

(Heysham, United Kindom).

The black cyclic voltammogram (CV) in figure 2 shows the electrochemical cell (1) in

the absence of the palladium salt. The straight baseline in the potential window

indicates that there is no ion transfer through the interface at an applied potential

between 0 and +0.6 V. At lower and higher potentials the electrolytes begin to pass

through the water/DCE interface. The red CV done in the electrochemical cell with

the ammonium tetrachloropalladate(II) shows the current produced from the

reduction of the Pd salt by the DMFc.

Figure 2: Cyclic voltammograms without (black) and with (red) ammonium tetrachloropalladate(II),

scan rate 100 mV/s

For the deposition of the Pd nanoparticles a constant potential of 0.55 V was applied

for 2 min. Then the aqueous solution was acidified with HCl to see if the deposited

Pd nanoparticles at the water/DCE interface would be able to catalyse the reduction

of protons to hydrogen. The obtained CV (blue line), which shows an irregular but

reproducible response, is shown in figure 3.


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Figure 3: CV after deposition of Pd and acidification of the aqueous phase (blue), scan rate: 100 mV/s

Because, without acidification, the same response appeared due to a gradual

alteration of the deposited Pd nanoparticles (> 2 hours) the response does not

depend on the acidification, but on the slow, spontaneous aggregation of the

nanoparticles which occurs more quickly in an acidic aqueous phase.

The obtained nanoparticles were characterised by high resolution TEM with the help

of the School of Chemical Engineering and Analytical Science at The University of

Manchester. The particles were directly collected from the interface with a carbon

coated Cu grid. The images (figure 4) show a particle size of about 5 nm which is

corresponding with previous reported particle sizes for Pd nanoparticles in the

literature5.

Figure 4: TEM images of the deposited Pd nanoparticles


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To reduce the effects of aggregation of the nanoparticles we wanted to stabilise the

Pd particles. In the literature there are many reported methods to stabilise

nanoparticles in solution, but we are not aware of any corresponding methods for

particles at liquid/liquid interfaces. The most common solution phase stabilising

agents are polymers13, 14 (e.g. PVP = polyvinylpolypyrrolidone), ligands15-17 (e.g.

citrate), surfactants18 (e.g. SDS = sodium dodecyl sulfate) or tetraalkylammonium

salts19-23.

The first attempt to stabilise the electrodeposited Pd particles at the water/DCE

interface was realised by adding sodium citrate to the aqueous phase.

The related CVs are shown in figure 5. The black CV originates from the cell in the

absence and the red one from the cell in the presence of the Pd salt. The green CV

shows the response after the deposition of the Pd particles by applying a constant

potential of 0.55 V for 2 min. The response is similar to the one without citrate in the

aqueous solution. However no aggregation was visible to the eye at the interface, nor

did the CV response become unstable by allowing the cell to stand for more than 2

hours. But after adding HCl to acidify the aqueous solution, there was a visible

aggregation of the Pd particles at the interface most likely caused by protonation

(and consequent removal) of the stabilising citrate ligands. The blue CV shows the

response after the aggregation.

Ag (s) AgCl (s) 10 mM LiCl

1 mM BTPPACl

(aq)

AgCl (s) Ag (s) 100 mM LiCl

1 mM (NH4)2PdCl4

2 mM Na3C6H5O7 . 2 H2O

(aq)

10 mM DMFc

20 mM BTPPA TPFB

(DCE)


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Figure 5: Deposition of Pd nanoparticles in the presence of citrate, scan rate: 100 mV/s

The TEM images (figure 6) show also a particle size of about 5 nm for the citrate

stabilised Pd nanoparticles.

Figure 6: TEM images of the citrate stabilised Pd nanoparticles

Due to the fact that the Pd nanoparticles deposited in the presence of citrate still

aggregate when acidifying the aqueous solution, stabilisation of the Pd with

tetraoctylammonium ions was attempted. Therefore the organic electrolyte was

changed to tetraoctylammonium tetrakis(penta-fluorophenyl)borate (TOA TPFB).

This electrolyte was synthesised by metathesis of TOACl (Sigma-Aldrich, Dorset,

United Kingdom) and LiTPFB . n Et2O (Boulder Scientific Company, Mead, USA)

according to a reported method24. Because the previously used

tetrachloropalladate(II) ion formed an insoluble salt with the tetraoctylammonium ion

the Pd salt had also to be changed to tetraamminepalladium(II) chloride.


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Figure 7 shows the related CVs. The black CV was measured in the cell without the

Pd salt and the red CV with the Pd salt. The green CV shows the response after the

deposition of the Pd particles by applying a constant potential of 0.55 V for 2 min. In

contrast to the previous measurements, there was no visible aggregation at the

water/DCE interface seen either by allowing the cell to stand for a longer time nor by

adding HCl to acidify the aqueous solution.

Figure 7: Deposition of Pd nanoparticles in the presence of tetraoctylammonium ions, scan rate:

100 mV/s

This indicates that the tetraoctylammonium ions stabilise the deposited Pd

nanoparticles at the water/DCE interface.

Again the TEM images (figure 8) show a particle size of the deposited

tetraoctylammonium stabilised Pd nanoparticles of about 5 nm. Also it can be seen

that aggregation of the Pd nanoparticles still occurs, but that the aggregates visible in

the TEM images have a diameter of at most 100 nm. In contrast the aggregates

resulted without the tetraoctylammonium stabilisation (figure 4) are much larger.

AgCl (s) 100 mM LiCl

1 mM Pd(NH3)4Cl2 . H2O

(aq)

10 mM DMFc

20 mM TOA TPBF20

(DCE)

10 mM LiCl

1 mM TOACl

(aq)

AgCl (s) Ag (s) Ag (s)


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Figure 8: TEM images of the tetraoctylammonium stabilised Pd nanoparticles

For additional characterisation of the deposited Pd nanoparticles XRD mesurements

(with help of the X-Ray Crystallography Facility, School of Chemistry) and XPS

measurements (with help of the School of Chemical Engineering and Analytical

Science) were carried out.

However the present XRD measurements showed no observable diffraction peak in

the expected region of the diffraction angle (2θ) of about 40 degrees. To estimate the

particle size by XRD a longer measurement should be done with the deposited Pd

nanoparticles to see if there is a broad peak in this region related to small particles

with a diameter in the lower nm range.

The XPS measurements were done by the School of Chemical Engineering and

Analytical Science at The University of Manchester with prepared Pd nanoparticles

after finishing the STSM. The results of these measurements should be transferred

within the next weeks.

After the deposition of stabilised Pd nanoparticles at the water/DCE interface we

wanted to investigate if these particles are able to catalyse the reduction of protons to

hydrogen in the acidified aqueous solution. Small visible gas bubbles at the platinum

counter electrode in the aqueous phase after a large amount of recorded CVs

indicated that hydrogen evolution occurred. Therefore a conventional three-electrode

configuration was incorporated into the aqueous phase of the electrochemical cell, to


10/12

oxidise the emerging hydrogen back to protons. However, applying this method the

problem occurred that a large current was obtained in the CVs of the three-electrode

setup only with an acidic aqueous solution of the used Pd salt in the potential region

where the hydrogen reduction was expected. Thus until the end of the STSM we

were unable to verify that the stabilised Pd particles at the water/DCE interface can

catalyse proton reduction.

3. Conclusion and Outlook

Stabilised Pd nanoparticles have been synthesised at the water/DCE interface by

electrochemical reduction of tetraamminepalladium(II) chloride in the presence of

tetraoctylammonium chloride. The obtained nanoparticles were characterised by

TEM, XRD and XPS resulting in a particle size of about 5 nm.

Investigation of the catalytic activity of the deposited nanoparticles for proton

reduction in the aqueous phase could not be completed until the end of the STSM

period. Further experiments to this topic are scheduled both in the group at the

University of Ulm and at The University of Manchester.

The group in Ulm will also try to investigate the electrochemical deposition of

nanoparticles at liquid/liquid interfaces with spectroscopic methods e.g. evanescent

wave cavity ring-down spectroscopy. This would be a new method to measure

simultaneously electrochemical and spectroscopic information at liquid/liquid

interfaces.


11/12

4. Literature

(1) Vanysek, P.; Ramirez, L. B.: Interface between two immiscible liquid electrolytes: a review. J. Chil. Chem. Soc. 2008, 53, 1455-1463.

(2) Johans, C.; Lahtinen, R.; Kontturi, K.; Schiffrin, D. J.: Nucleation at liquid-liquid interfaces: electrodeposition without electrodes. J. Electroanal. Chem. 2000, 488, 99-109.

(3) Platt, M.; Dryfe, R. A. W.; Roberts, E. P. L.: Controlled deposition of nanoparticles at the liquid-liquid interface. Chem. Commun. (Cambridge, U. K.) 2002, 2324-2325.

(4) Lahtinen, R. M.; Fermin, D. J.; Jensen, H.; Kontturi, K.; Girault, H. H.: Two-phase photocatalysis mediated by electrochemically generated Pd nanoparticles. Electrochem. Commun. 2000, 2, 230-234.

(5) Platt, M.; Dryfe, R. A. W.: Structural and electrochemical characterisation of Pt and Pd nanoparticles electrodeposited at the liquid/liquid interface: Part 2. Phys. Chem. Chem. Phys. 2005, 7, 1807-1814.

(6) Platt, M.; Dryfe, R. A. W.; Roberts, E. P. L.: Structural and electrochemical characterisation of Pt and Pd nanoparticles electrodeposited at the liquid/liquid interface. Electrochim. Acta 2004, 49, 3937-3945.

(7) Johans, C.; Kontturi, K.; Schiffrin, D. J.: Nucleation at liquid-liquid interfaces: galvanostatic study. J. Electroanal. Chem. 2002, 526, 29-35.

(8) Johans, C.; Liljeroth, P.; Kontturi, K.: Electrodeposition at polarisable liquid-liquid interfaces: The role of interfacial tension on nucleation kinetics. Phys. Chem. Chem. Phys. 2002, 4, 1067-1071.

(9) Guo, J.; Tokimoto, T.; Othman, R.; Unwin, P. R.: Formation of mesoscopic silver particles at micro- and nano-liquid/liquid interfaces. Electrochem. Commun. 2003, 5, 1005-1010.

(10) Trojanek, A.; Langmaier, J.; Samec, Z.: Electrocatalysis of the oxygen reduction at a polarized interface between two immiscible electrolyte solutions by electrochemically generated Pt particles. Electrochem. Commun. 2006, 8, 475-481.

(11) Lahtinen, R.; Johans, C.; Hakkarainen, S.; Coleman, D.; Kontturi, K.: Two-phase electrocatalysis by aqueous colloids. Electrochem. Commun. 2002, 4, 479-482.

(12) Fermin, D. J.; Dung Duong, H.; Ding, Z.; Brevet, P. F.; Girault, H. H.: Photoinduced electron transfer at liquid/liquid interfaces Part II. A study of the electron transfer and recombination dynamics by intensity modulated photocurrent spectroscopy (IMPS). Phys. Chem. Chem. Phys. 1999, 1, 1461-1467.

(13) Toshima, N.; Wang, Y.: Polymer-protected Cu/Pd bimetallic clusters. Adv. Mater. 1994, 6, 245-247.

(14) Henglein, A.: Physicochemical properties of small metal particles in solution: "microelectrode" reactions, chemisorption, composite metal particles, and the atom-to-metal transition. J. Phys. Chem. 1993, 97, 5457-5471.

(15) Schmid, G.: Large clusters and colloids. Metals in the embryonic state. Chem. Rev. 1992, 92, 1709-1727.

(16) Poulin, J. C.; Kagan, H. B.; Vargaftik, M. N.; Stolarov, I. P.; Moiseev, I. I.: Scanning tunneling microscopy observation of giant palladium-561 clusters. J. Mol. Catal. A: Chem. 1995, 95, 109-113.

(17) Amiens, C.; de Caro, D.; Chaudret, B.; Bradley, J. S.; Mazel, R.; Roucau, C.: Selective synthesis, characterization, and spectroscopic studies on a novel class of reduced platinum and palladium particles stabilized by carbonyl and phosphine ligands. J. Am. Chem. Soc. 1993, 115, 11638-11639.


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(18) Toshima, N.; Takahashi, T.: Colloidal dispersions of platinum and palladium clusters embedded in the micelles. Preparation and application to the catalysis for hydrogenation of olefins. Bull. Chem. Soc. Jpn. 1992, 65, 400-409.

(19) Boennemann, H.; Brijoux, W.; Brinkmann, R.; Dinjus, E.; Joussen, T.; Korall, B.: Production of colloidal transition metals in organic phase and their use as catalysts. Angew. Chem., Int. Ed. Engl. 1991, 30, 1312-1314.

(20) Boennemann, H.; Brijoux, W.; Brinkmann, R.; Fretzen, R.; Joussen, T.; Koeppler, R.; Korall, B.; Neiteler, P.; Richter, J.: Preparation, characterization, and application of fine metal particles and metal colloids using hydrotriorganoborates. J. Mol. Catal. 1994, 86, 129-177.

(21) Reetz, M. T.; Helbig, W.: Size-Selective Synthesis of Nanostructured Transition Metal Clusters. J. Am. Chem. Soc. 1994, 116, 7401-7402.

(22) Reetz, M. T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Breuer, N.; Vogel, R.: Visualization of surfactants on nanostructured palladium clusters by a combination of STM and high-resolution TEM. Science 1995, 267, 367-369.

(23) Reetz, M. T.; Quaiser, S. A.: A new method for the preparation of nanostructured metal clusters. Angew. Chem., Int. Ed. Engl. 1995, 34, 2240-2241.

(24) LeSuer, R. J.; Buttolph, C.; Geiger, W. E.: Comparison of the Conductivity Properties of the Tetrabutylammonium Salt of Tetrakis(pentafluorophenyl)borate Anion with Those of Traditional Supporting Electrolyte Anions in Nonaqueous Solvents. Anal. Chem. 2004, 76, 6395-6401.

COST PROJECT PRAGUE 17/02-17/03-2009

Giorgio Volpi 20/03/2009

COST REPORT Purpose of the visit: During the period of the COST STSM project, we intended to study the electrochemical properties of seven Rhenium complexes, synthesized in the Turin group, containing 1-pyridylimidazo[1,5-a]pyridine ligands and 2-dipyridil ketone. The electrochemical behaviour of these transition metal complexes is crucial for their possible application in luminescent and electrochemiluminescent devices. Here are reported the structures of the seven complexes studied:

N

Re

OC

OC

Cl

CON

R

N

ReGVn°

R= phenyl=GV6

R= methyl=GV16

R= 4-nitro phenyl=GV8

R= 4-dimethylamino phenyl=GV26

R=4-trif luoromethyl phenyl=GV34

R=4-tertbuthyl phenyl=GV38

NRe

OC

OC

Cl

CO

N

ReDPK

ODPK=Di-2-pyridyl ketone

Further task was to functionalize glassy carbon electrodes with different molecule, following the different method reported in literature1,2,3. For this purpose we wanted to employ four different aromatic ammines: 4-nitroaniline, 1,10-phenanthrolin-5-amine (NH2-pnt), 9-diazo4,5-diazafluorene (DPPZ) and 4-((4-((dipyridin-2-ylmethoxy)methyl) phenyl)-ethynyl)aniline (GV148). The first molecule is for comparison with the literature results1,2,3 while the other molecules available are possible ligands for transition metals (the corresponding complexes are

1 Allan Hjarbæk Holma, Karina Højrup Vase, Bjørn Winther-Jensen, Electrochimica Acta 53, 2007, 1680–1688.

2 Jianyun Liu, Long Cheng, Baifeng Liu, and Shaojun Dong, Langmuir, 16, 2000, 7471-7476.

3 Hideaki Tanaka, Akiko Aramata, Journal of Electroanalytical Chemistry 437, 1997, 29-35.

already known in the literature). The aim is to bond these four ligands on the electrode surface before complexation. GV148 is a new ligand synthesized from 2-dipyridyl ketone (that we have also used previously as ligand) in Turin group; the presence of the amino aromatic group gives the possibility to functionalize this ligand on the GC electrode.

N O

HN

NH2

4-((4-((dipyridin-2-ylmethoxy)methyl)phenyl)ethynyl)anilineGV148=

Description of the work carried out during the visit: During the first fortnight we studied all the Rhenium compounds; the electrochemical behaviour of these complexes was checked with cyclic voltammetry, polarography and in same cases with IR spectroelectrochemistry (ReGV6, ReGV8, ReDPK). In all cases, cyclic voltammetry measurement was repeated at different scan rate to check the reversibility of the electrochemical process and possible coupling to a chemical reaction. Redox potentials of each compound was referred to the ferrocene/ferrocenium redox couple and in some cases the measurement was repeated in different solvent (CH3CN, CH2Cl2 and EtOH). During the second fortnight we tried to functionalise glassy carbon electrodes with different amminoaromatics compounds. We tested different ways because in the literature there is not a uniform procedure for this kind of functionalization. We tried different solvents for every molecule (CH3CN, absolute ethanol, ethanol 96%, CH3CN 80% in water and CH2Cl2) and increasing concentrations of the substrate (from 0.5 to 10 mM). Functionalization have been carried out during simple scan rate or during time delay at fixed potential (from 10 second to 45 minute); to avoid interferences due to adsorption effects, before and after every measurement the electrode was washed with solvent (CH3CN, ethanol, CH2Cl2 or water) and sonicated (for 1 to 15 minute). We checked also the presence of adsorption effect by means of measurement at different scan rates and plotting the current vs. scan rate (or square root) to distinguish adsorption or diffusion controlled process. Description of the main results obtained: The results relative to the Rhenium complexes are summarized in the following table. We used CH3CN as solvent for every complexes with tetrabutylammonium hexafluorophosphate 0.1 M as electrolyte. The concentration of the complexes was about 1.0 mM as well as the concentration of the ferrocene used as reference. All data in the table are from measurements at a scan rate 200 mV/sec.

Compound V (vs FC) type Compound V (vs FC) type Compound V (vs FC) type

ReGV6 ReGV38 ReGV26 cathodic -2.175 irr cathodic -2.244 irr cathodic -2.216 irr

-2.43 irr -2.565 irr -2.927 irr -2.872 irr -3.008 irr anodic 0.542 rev

anodic 0.916 irr anodic 0.877 irr 0.819 rev 1.114 irr 1.07 irr 1.307 irr 1.47 irr 1.408 irr polarography -2.165

polarography -2.136 polarography -2.13 -2.541 -2.835 -2.496 -2.869 -3.151 -2.833

ReGV8 ReDPK ReGV34 cathodic -1.353 rev cathodic -1.289 rev cathodic -2.147 irr

-2.043 irr -1.903 rev -2.734 irr -2.269 irr -2.588 irr -2.926 irr -2.966 irr anodic 1.087 rev anodic 0.943 irr

anodic 0.959 irr 1.793 irr 1.167 irr 1.183 irr polarography -1.284 1.591 irr 1.565 irr -1.912 polarography -2.092

polarography -1.365 -2.616 -2.612 -1.988 -2.242 -2.885

ReGV16 4-nitroaniline V (vs

CoFC) DPPZ V (vs

CoFC)

cathodic -2.185 irr cathodic -0.632 rev cathodic -1.148 irr -2.893 irr anodic 2.258 irr -2.359 irr

anodic 0.926 irr 2.397 irr anodic 1.557 irr 1.074 irr

1.464 irr (NH2-pnt) V (vs

CoFC)

polarography -2.147 cathodic -1.268 irr -2.513 -1.58 irr -2.82 anodic 1.878 irr 2.187 irr

Following picture shows an example of CV of the ReGV26 complex (1.06 mM, CH3CN, Fc 0.84mM):

Three IR stretching frequencies of the carbonyl groups of the complexes ReDPK, ReGV8 and ReGV6 were monitored during electrochemical reduction by means of IR spectroelectrochemistry. The compounds exhibit three separated absorptions (one sharp absorption at high energy and two broader absorptions at lower energies) as expected for non-symmetrically substituted fac-Re(CO)3 moieties. For example, the CV of ReDPK is reported in the voltammogram below. The IR spectroelectrochemistry performed at a potential of about 0.05 V more negative than the first mono electronic cathodic peak (E°=1.289 V vs. Fc/Fc+) confirms a simple charge-transfer character of the process. The presence of an isosbestic point outlines the reversibility and the total conversion into the corresponding anion [ReDPK]–.

Similar results were obtained for the other compounds. With the aim to functionalize the GC electrode, we tested the electrochemical behaviour of the ligands 4-NO2-aniline and 1,10-phenanthroline-5-amine. An example of CV measurement (1° and 2° scan) after functionalization (at 1.7V for 20sec) of 4-NO2-aniline (1.5 mM, in CH3CN, electrolyte tetrabutylammonium hexafluorophosphate 0.1 M, scan rate 200mV/sec) is shown in the figure.

The amino compounds were always functionalized in the anodic region. After functionalization, it is possible to check the different signals derived from the molecules bonded on the electrode surface, in a clean solution. In difference from 4-aminobenzoic acid reported earlier2 the layer formed on the electrode was not stable after polarization to negative potentials and also not after sonication. Therefore we cannot yet unambiguously confirm, if a covalent bond to the electrode was formed or the observed effects was due to an adsorption only. As third molecule, we tried the 9-diazo-4,5-diazafluorene (DPPZ). This molecule could be able to bond GC electrode as mostly employed diazonium salts do, releasing N2 during reductive functionalization. The DPPZ compound gave promising results. The current profile during functionalization decreases in a similar way as reported in the literature1,2,3. A strong surface interaction is observed since the CV peak current decreased during functionalization does not increase after sonication. After functionalization it’is possible to see irreversible waves, in the cathodic region, stable and reproducible also after cleaning with solvents or sonification. This layer, formed on the electrode, changes the reference ferrocene wave profile already after the first functionalization cycle. We discover, with surprise, that for this molecule the functionalization happens only in anodic region, this suggests possible different mechanism from the diazonium salts reported in the literature. The next figure shows the CV of functionalization cycles of DPPZ (1.07mM, EtOH 96%, LiClO4 1M) performed with a delay of 10 second at 1.7V, in presence of ferrocene. It’s possible to see a significant decrease of the current (attributable to increasing of the covered surface) and a modification of ferrocene signal.

We noted that the functionalized layer was destroyed during scan in the cathodic region where in a clean solvent appeared a new peak (about -1.2V) attributable at the new layer on the surface. The modification of GC electrode was carried out also with a new ligand GV148 (see above), synthesized in Turin group, which involves amino aromatics group. This ligand bonds the GC surface in the anodic region (about 1.3V) with strong interaction that cannot be eliminated by sonication (15 minute in the same solvent used during the functionalization process). The layer on the electrode in clean solution shows an irreversible peak in the cathodic region. The next two figures illustrate the observed effects. When CV scanning in the solution of the ligand and ferrocene is performed up to cathodic region (first figure) the influence on the ferrocene wave is smaler, because the layer is partially destroyed at negative potentials. However, continued scanning only in anodic region (second figure) completely inhibits the ferrocene redox process.

Future collaboration with host institution: We hope to investigate the possibility to use the bonded ligands on the GC surface for complexation of Re and Ir precursors. Furthermore we’d like to investigate the presence of this complexes on the electrode surface by mean of electrochemistry measurement. Moreover in Turin group we have already synthesized six new Iridium complexes and three new Rhenium complexes that show interesting properties for luminescent and electrochemiluminescent devices. The study of the electrochemical properties of these compounds is essential for their possible application in the luminescent field.

Projected publications/articles resulting or to result from the STSM (if applicable):

A joint manuscript based on the electrochemical and spectroelectrochemical data obtained in Prague is in preparation.

Confirmation by the host institute of the successful execution of the mission:

This is to confirm that Mr. Giorgio VOLPI of University of Turin completed the STSM at the J. Heyrovsky Institute of Physical Chemistry, Acad. Sci. of the Czech Republic His mission was part of the COST project D36/001. The work program included an electrochemical characterization of new transition metal complexes designed as possible compounds for electroluminescent devices. After a thorough electrochemical study he attempted several procedures for modification of the graphite surface in such a way that luminescent compounds could be attached to the surface. Spectroelectrochemical in-situ techniques were used for evaluation of redox processes. Series of investigated compounds is a good starting point for finding correlation between the activity and the structure of new complexes. Participating institutes plan further work in the future. It is realistic that soon a joint communication will be completed.

Dr. Lubomir Pospisil, Coordinator for the host institute

Scientific report

of Dr. Ivan Ivanov Institute of Catalysis, BAS, Sofia

REFERENCE: Short Term Scientific Mission, COST D36

Host: Prof. Anna Maria Venezia, Institute of Nanostructured Materials, CNR, Palerm, Italy Project 06/0003/06 “Interfacial functionalization of (bi)-metallic nanoparticles to prepare highly active and selective catalysts: understanding synergy and/or promotion effect” Period: from 15/04/2009 to 15/05/2009 Place: 90146 Palermo (IT) Reference code: COST-STSM-D36-04455

I received a STSM grant from COST and I did experiments at the host Institute of Nanostructured Materials, CNR, Palermo, Italy. I was accepted at the laboratory of Prof. Anna Maria Venezia, coordinator of our common project 06/0003/06 “Interfacial functionalization of (bi)-metallic nanoparticles to prepare highly active and selective catalysts: understanding synergy and/or promotion effect”. We did all the experimental works with Chem. Eng. Giuseppe Pantaleo from the same laboratory using the apparatus for catalytic activity tests - the catalytic test was PROX process on gold based catalysts. The PROX process is very important in connection with hydrogen purification of CO traces for application in PEM fuel cells. Two series of gold catalysts based on ceria, doped by the different reducible metal oxides (CoOx, SnOx, Fe2O3, MnOx), have been prepared in the Institute of Catalysis, BAS, Sofia. The 1st route of the mixed oxide supports synthesis included co-precipitation technique (CP) and the 2nd one – mechano chemical activation (MA). During my stay at the Institute of Nanostructured Materials I studied the apparatus for PROX catalytic tests: the scheme for connecting the modules for measuring concentrations of N2O, NO, CO, H2O and CO2, and of O2 and CH4 as well.

The research work has included the following steps: First, it was studied the influence of the pretreatment of the

catalysts before catalytic test on the catalytic activity and selectivity of the samples. The pretreatment was carried out in the presence of oxygen at 150оС for 30 min, or in the presence of hydrogen at 150оС for 30 min. After each pretreatment, the PROX reaction in the temperature range from 20оС to 300оС was carried out. The obtained results have showed that the pretreatment with oxygen at 150оС, has a little influence on the catalytic activity in PROX process in comparison to the pretreatment with hydrogen at 150oC.

On the basis of these experiments and the preliminary obtained TPR data the optimal conditions for the pretreatment with oxygen was chosen.

All the catalysts have been tested applying the above described conditions of pretreatment in the PROX reaction. On the basis of registered CO, CO2 and oxygen, the selectivity of each catalyst has been calculated.

In Fig.1 - A, B are presented the PROX activity and selectivity of the samples doped by iron oxide, prepared by the both methods. The highest PROX activity was obtained for gold catalysts, prepared by MA – 99.8 % conversion of CO at the lowest temperature - 51оС and this conversion was above 99 % in all range to 150оС, the selectivity was at about 38 %.

A - Au/CeFeCP B - Au/CeFeMA

0

20

40

60

80

100

120

0 200 400

Con

v. %

, Sel

. % F

orm

. %

T [°C]

Conv. CO

Conv. O2

Selettività

0

20

40

60

80

100

120

0 100 200 300 400

Con

v. %

, Sel

. % F

orm

. %

T [°C]

Conv. CO

Conv. O2

Selettività

Fig.1. PROX catalytic activity and selectivity obtained for gold

catalysts doped with Fe2O3, prepared by CP – A and prepared by MA – B.

Another very interesting result - the highest PROX selectivity was obtained for gold catalysts doped with MnOx (Fig.2). For the sample, prepared by MA, very high value of selectivity was registered - 61.5 % at the temperature very close to the condition in PEM Fuel Cells - 75оС.

A - Au/CeMnCP B - Au/CeMnMA

0

20

40

60

80

100

120

0 200 400

Con

v. %

, Sel

. % F

orm

. %

T [°C]

Conv. CO

Conv. O2

Selettività

0

20

40

60

80

100

120

0 100 200 300 400

Con

v. %

, Sel

. % F

orm

. %

T [°C]

Conv. CO

Conv. O2

Selettività

Fig.2. PROX catalytic activity and selectivity obtained for gold

catalysts doped with MnOx, prepared by CP – A and prepared by MA – B.

In Fig.3 there are summarized the all studied catalysts at T=75оС.

AuCeCoCPcon

AuCeCoCPsel

AuCeCoMAsel

AuCeFeCPcon

AuCeFeMAcon

AuCeFeMAsel

AuCeMnCPcon

AuCeMnCPsel

AuCeMnMAcon

AuCeMnMAsel

AuCe_con

AuCe_sel

AuCeCoMAcon

AuCeSnCPcon

AuCeSnCPsel

AuCeSnMAcon

AuCeSnMAsel AuCeFeCPsel

0

20

40

60

80

100

120

Catalysts

CO

con

vers

ion,

sel

ectiv

ity %

Fig.3. PROX catalytic activity and selectivity at 75оС on gold catalysts prepared with different methods CP (co-precipitation) and MA (mechano chemical activation).

The best catalysts are tested also in the presence of water and CO2. On the contrary to literature data, we do not obtain improving of the activity and selectivity in the presence of water. The addition of CO2 additionally decreases the activity and selectivity. The obtained interesting results will be summarized in a common publication of the both teams.

- 1 -

Melaet Gérôme’s STSM Report:

About the effect of water and sulphur dioxide on palladium based catalysts in the frame of methane complete oxidation.

- 3 -

1. Formal Introduction

As the global warming is one of the first worldwide preoccupations, many countries all

over the world apply an environmental politics forcing most of the industries to lower their pollutant

emission. When politics think about pollution, its main concern is to work on the abatement of exhaust

gas pollution from cars. Therefore, all the automotive companies compete, creating less consuming

engines or developing solution for automotive exhaust emission such as the Three Way Catalyst

converter on gasoline engines or the particles filters on diesel engines.

Solutions such as methane propelled vehicles are a solid alternative to lower the emission

from cars. Nevertheless, the lean-burn conditions of these engines lead to the emission of small

quantities of methane. Unburned methane is quite a problem since it is more harmful to the

greenhouse effect than the carbon dioxide. In fact, its activity as a greenhouse gas is more significant -

about twenty times - than CO2. Therefore, the use of catalytic exhaust system is a good mean to follow

the ecologic standards increasing more drastically.

Nowadays scientists agree on the fact that palladium is maybe the best suited metal to be

used in exhaust system to burn methane slips coming form the engine. An important drawback comes

with the use of such metal as it is known to be quickly deactivated in presence of poisons such as

sulphur oxide and/or water.

2. Work plan

In a previous STSM at the ISMN in Palermo, we obtained modified catalysts using

titanium(oxide) to dope the SiO2 support. These catalysts were produced by the sol-gel method

varying the quantity of Ti loading from 5 to 20 wt%. Importance of this modification has been observed

on the methane oxidation performance and on the tolerance to SO2. The results of this collaboration

have been published in the journal Applied Catalysis B: Environmental(1) and therefore will not be

discussed here.

Regarding this new STSM, we plan to study the effect of water on our most active catalyst:

palladium supported on titanium 10% doped silica (Pd-Ti10Si). To study this influence, we suggested

to feed the catalyst with water in different conditions similarly to those used to study the SO2 poisoning

effect. The process is divided in eight different cycles, the first cycle is a reference cycle under reactive

conditions (0,3%CH4 + 2,4%O2 diluted in He). During the second run, 5% of water is co-fed to the

reactive flow in order to observe the deactivation of our catalyst in the presence of water. A

subsequent run, which can be associated to a regeneration cycle, is operated in lean burn conditions

to observe if the catalyst regains its activity. The third cycle consists of co-feeding both water and

- 4 -

sulphur dioxide together in the reactive flow in order to observe if the deactivation is more severe in

the presence of both these compounds. Again, a regeneration cycle is done in lean-burn condition,

and the methane conversion is measured. A fifth cycle will consist in an overnight treatment in which

the catalyst is fed with water and SO2 during 15 hours. Finally to understand the impact of H2O and

SO2 ageing, we will operate cycle seven and eight in lean burn condition aiming to observe if the

catalyst regains its activity.

3. Results and Discussion

The first step in this collaboration was to determine if the conversion observed was not

dependent on effects such as an inhomogeneous temperature distribution or transport limitation of

diffusing gaseous reactants and products

inside the catalytic bed. In order to see if

such effects can be avoided we ran two

cycles under lean-burn conditions,

increasing the temperature step wise

during the first one while decreasing the

temperature in the second cycle. As one

can observe in Figure 1, Cycle I is different

from Cycle II and this second one seems

more active as the T50% (temperature of

fifty percent conversion) is 34°C lower than

in the first cycle. While increasing the

temperature (Figure 1 Cycle III curve) in a

third run, we observed that this latest cycle

is identical to the second cycle, concluding that we now have a stable catalyst. Therefore, cycle III will

serve as a reference cycle in order to compare the effect of a wet feed in presence or absence of SO2.

This cycle III will be later referred as "Cycle I (Reference)".

These first tests lead us to the conclusion that to have a stable catalyst a first cycle under

lean-burn condition must be run as a mean of an activation cycle. At this point of time, we do not know

if this activation is chemically induced or if we face a temperature activation as we are reaching 600°C

in that first cycle where we only pre-treat the catalyst under O2 at 350°C for an half hour. Further

experiments should allow us to determine the predominant factor of this activation.

Finally, the lack of hysteresis between Cycle II and III suggests that there are no diffusion

effects, thus the conversion observed is supposedly due to the catalytic activity only.

Figure 1 : Conversion versus temperature under lean-burn condition (0,3%CH4 + 2,4%O2 in Helium). Cycle I increasing the temperature, Cycle II decreasing the temperature and Cycle III increasing the temperature

- 5 -

Figures 2 represent the conversion of methane against temperature in different conditions.

One can observe that a wet feed leads to

a partial deactivation of the catalyst

(Cycle II) but it will regain its activity while

running a regeneration cycle under lean-

burn conditions (Cycle III).

Furthermore, combining the

effect of water with that of sulphur in the

reaction mixture does not lead to a

higher deactivation than in the presence

of water only - see Figure 2, conversion

curve from Cycle II and IV are

overlapped.

The activation energies have been calculated from the data obtained, supposing a first order

reaction in methane and a pseudo-zero order reaction with respect to oxygen. These data have been

reported in the Table 1.

Condition Ea (kJ/mol) Ea(a)

(kJ/mol) Ln A R² T50%(°C) Cycle I (Reference) 70 17 0.9917 289 Cycle II – Wet feed 101 31 23 0.9975 327 Cycle III – Regeneration 74 4(b)

18 0.9915 289 Cycle IV – Wet feed + 10ppm SO2 101 31 22 0.9975 329 Cycle V – Regeneration 70 0 17 0.9954 298 Cycle VI – 1st cycle after overnight (350°C 10ppm SO2 + 5%vol. H2O) 168 99 33 0.9997 373 Cycle VII – 2nd cycle after overnight (350°C 10ppm SO2 + 5%vol. H2O) 77 7(b) 18 0.9915 288

Table 1. Activation energy (Ea) and pre-exponential factor A calculated (a) Ea obtain by subtracting the Ea of a given Cycle and the one from the

reference Cycle. (b)Value falling into the calculation error.

According to the results listed in Table 1, one can draw two foremost conclusions: firstly the

effect of the presence of both water and sulphur dioxide are not more severe than the presence of

water alone. This conclusion can be made because the Ea calculated either during Cycle II or Cycle IV

are very similar (101 kJ/mol). Furthermore, comparing the Ea of these precedent cycles (=31kJ/mol)

to the one obtained formerly in the presence of sulphur dioxide alone in the reactive flow (Ea =

35kJ/mol), we can conclude that the presence of water has neither a positive nor a negative effect on

the activity of our catalyst.

The second conclusion, less obvious but nevertheless important, is the high stability of our

catalyst. In fact, one can observe that the Ea calculated from Cycle I, III and VII is quite similar thus the

effect of poisons can be considered reversible. So as to recover the activity, a single cycle under lean-

burn conditions is sufficient. At this time, literature has never reported on regeneration during lean-

burn reaction conditions. Instead, either a reducing atmosphere or a high temperature treatment was

necessary.

Figure 2: Conversion versus temperature under different conditions.

- 6 -

4. Conclusion

The conclusion can be made that the mixed oxide catalyst is highly active and suffers neither

from the contact with water, nor from that with sulphur dioxide. To this end, it seems extremely stable.

As we demonstrate it during this collaboration, the adsorption of water or/and sulphur seems

reversible as the original activity can always be reproduced. Furthermore, the fact that the catalyst

recovers its activity, even after multiple cycles, shows that this catalyst is highly stable. Finally, one

should point out that the regeneration step is identical to a lean-burn cycle. This latter fact has its

importance since it was never before reported in the literature.

Future studies and collaborations should be done to understand why the use of combined

sulphatable and non-sulphatable supports helps prevent severe deactivation by water and SO2.

(1)

Applied Catalysis B: Environmental, In Press, Corrected Proof, Available online 8 November 2008 A.M. Venezia, G. Di Carlo, G. Pantaleo, L.F. Liotta, G. Melaet, N. Kruse, doi:10.1016/j.apcatb.2008.10.023

http://dx.doi.org/10.1016/j.apcatb.2008.10.023

1/9

REPORT

Short Term Scientific Mission, COST D36 Beneficiary: Dr Anna Elzbieta LEWANDOWSKA, Institute of Catalysis, CSIC

Host: Mònica CALATAYUD, Laboratoire de Chimie Théorique - UMR 7616 UPMC/CNRS Period: from 20/02/2009 to 08/03/2009

Place: Paris (FR)

Reference code: COST-STSM-D36-04271

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The objective of the COST-STSM-D36-04271 was to get acquaint with theoretical calculation

methodologies to describe catalytic systems of titania supported vanadia system in the

hydration/dehydration conditions and use in combination with experimental analysis of their

reactivity and structure. The work was focused on two goals:

• Training in theoretical modelling and apply it to study hydration process

• Comparing the modelling and experimental results

Methods and models All calculations are performed with the ab initio plane-wave approach implemented in the VASP code 1, 2. The Perdew-Burke-Ernzerhof (PBE) functional 3, 4 is used. The valence electrons (O:2s2 2p4, Ti:4s2 3d2, V 4s2 3d3, H: 1s1) are treated explicitly while their interactions with the ionic cores are described by the Projector Augmented-Wave method (PAW) 5, 6. A cut off equal to 400 eV is used for the plane-wave basis. A (3 x 3 x 1) k-point grid is used in the Brillouin-zone. A vacuum of at least 8 Å prevents interactions between successive slabs. The positions of all the atoms in the super cell are relaxed with the conjugate gradient method. Vibrational spectra have been calculated for selected surface species within the harmonic approximation. The vanadia unit, water molecules and first support layer are relaxed, the bottom support layers are kept fixed. The Hessian matrix is computed by the finite difference method followed by a diagonalization procedure. The eigenvalues of the resulting matrix lead to the frequency values. The assignment of the vibrational modes is done by inspection of the corresponding eigenvectors. The model used in the calculations has been successfully employed for describing reactivity of vanadia supported on titania catalysts 7, 8, 9. It contains a V2O5 unit deposed on an anatase (001) slab support as shown in Figure 1. The cell dimensions are 7.57 × 7.57 × 25 Å3. This model simulates dehydrated conditions and exhibits representative features of the vanadia/titania catalyst: presence of vanadyl V=O groups, interface V-O-Ti bonds, vanadium in its highest oxidation state (+V), vanadium coverage is 0.5 monolayers (3.5 V atoms nm-1).

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Figure 1. side view of the slab used in the calculations for dehydrated conditions (triple unit cell). The vanadia V2O5 units are deposed on an anatase (001) five TiO2 layers thick. OT: terminal oxygen (vanadyl group). Selected distances in Å. Hydration is modeled by the addition of water molecules to this slab. First, one water molecule is added to the cell in different geometries: dissociated, molecular, in interaction with vanadyl V=O, interface V-O-Ti, support Ti-O groups. A set of 10 geometries has been tested. Rather than a systematic study, we have focused on the analysis of trends and preference for certain spatial arrangements. For the most stable structures, a second water molecule is added and a set of possible geometries is studied. Again, the most stable structures are retained for the addition of a third water molecule. The most representative structures obtained are described below, and their stability is tested by molecular dynamics simulations at 273 K.

Sample preparation

The catalysts were prepared by wet impregnation with an aqueous solution of ammonium

metavanadate (Sigma, 99.99%) of titanium (IV) oxide (Alfa Aesar, 100% anatase, SBET = 166

m2/g) 10. Oxalic acid (Panreac, 99.5%) was added to an aqueous solution of NH4VO3 to

facilitate dissolving the salt. Dissolution of vanadium precursor mixture was carried out by

stirring at 323 K for 50 min. The suspension was evaporated in rotatory evaporator at 338 K.

The obtained materials were dried at 383 K for 16 h and the samples were calcined at 673 K

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for 4 h in air (heating rate = 3 K min-1). The total vanadium content were calculated to

corresponds 1.2 and 3.5 V atoms/nm2 of TiO2 support. Catalysts are labeled as xV/TiO2,

where ‘‘x’’ indicates the number of atoms per nm2 of vanadia.

In situ Raman

Raman spectra were run with a single monochromator Renishaw System-1000 microscope

Raman equipped with a cooled CCD detector (-73 oC) and holographic super-Notch filter for

removing the elastic scattering. The powder samples were excited with the 488 nm excitation

line; spectral resolution was near 4 cm-1 and spectrum acquisition consisted of 5

accumulations of 33 s. The spectra were obtained under in situ conditions in a hot stage

(Linkam TS-1500). The samples were previously hydrated in humid synthetic air flow at

room temperature for 20 min. Subsequently catalyst was gradually dehydrated in a flow of dry

synthetic air. Dehydration process was carried out from 323 K to 673 K with a Raman

spectrum acquisition each 50 K. The rate of the heating was 10 K min-1. Vanadia catalysts

dehydration process were compared to pure titania catalyst support.

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Theoretical results Geometry and energetics Figure 2 displays selected hydrated structures obtained upon addition of one, two and three water molecules to the dehydrated model shown in Figure 1. Table 1 shows selected parameters (calculated V=O distance and interaction energy between water and catalyst).

1.946 1.7801.608

2.097

1.6431.6281.630 1.8441.828

A B

1 H2O

2.151

1.6541.622

1.8061.955

A B

A B2.177

1.6691.624

1.8021.649

1.622

1.6321.7331.7722 H2O

3 H2O

Figure 2. selected structures representing hydrated models. Distances in Å. Table 1 shows the relative energy. Molecular water molecules are stabilized.

The addition of one water molecule has a strong effect on the molecular structure of surface vanadia. It dissociates and forms V-OH and V-OH-Ti bonds, increasing the coordination of the vanadium center from four to five (structure A in Fig. 2). The interaction is exothermic, -1.28 eV. It is thus expected that even in dehydrated conditions some residual water is present. The hydroxyl groups formed are in close interaction with neighbouring vanadia units and

6/9

cause a stretch of the vanadyl groups, from 1.612 Å in the dehydrated model to 1.619 Å. Another stable structure is model B, 0.19 eV higher in energy than model A. In this model water is in molecular form, in interaction with two vanadia units and the support. Hydrogen bonds are created between the vanadyl groups and the water molecule and the former are elongated with respect to the bare dehydrated model. It is concluded from these results that the water molecule is in equilibrium between its dissociated and molecular forms, forming hydroxyl groups and causing a stretch of vanadyl V=O bonds. Further hydration leads to similar conclusion. Molecular water is found to coexist with dissociated form, and a network of hydrogen bonds is created. The vanadyl V=O group stretches. Monomeric vanadia units stabilize for three water molecules (structure A in Fig. 2). In a previous work carried out for gas-phase clusters, monomeric species are stabilized for four water molecules 11. In all dehydrated and hydrated models, vanadyl groups are always present and are sign of high stability. Table 1. relative energy ΔE for isomers, and interaction energy Eint (calculated as the difference between the hydrated model energy and the sum of bare plus water, given per water molecule) in eV. Average distance V=O (dV=O) in Å.

Water molecules

Model ΔE (eV) Eint dV=O

0 bare 0.00 0.00 1.612 1 A 0.00 -1.28 1.619 B 0.19 -1.09 1.636 2 A 0.00 -1.07 1.638 B 0.55 -0.79 1.632 3 A 0.00 -0.90 1.647 B 0.36 -0.78 1.636

Vibrational frequency analysis The vibrational frequency spectra for the hydrated models has been calculated in the harmonic approximation. The complete analysis is complex since the different groups are in close interaction and the corresponding modes are coupled. A fine analysis is however possible for the vanadyl V=O vibration which is a representative band present in IR and Raman spectra, clearly visible at values around 1040 cm-1. It has been shown above that hydration causes a stretch of the V=O bond due to the interaction of this group with hydroxyl or water molecules by hydrogen bonding. Thus, isolated V=O groups are calculated to vibrate between 1085 cm-1 and 1059 cm-1. However, the V=O bonds in interaction with water molecules (such as in

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model 1waterB or 2waterA) are shifted toward lower frequencies, between 1043 cm-1 and 1000 cm-1. Interaction of the vanadyl group with surface hydroxyl groups causes a larger shift and the corresponding bands are found at values between 1017 cm-1 and 972 cm-1. A correlation chart for the vanadyl vibrations is displayed in Figure 3.

95010001050110011501200

VO

OH

VO

OH H

VOdehydrated

coordinated to hydroxyl

cm-1

coordinatedto H2O

Figure 3. Correlation chart for the calculated vanadyl V=O vibrations in different environments: isolated (1085-1059 cm-1), in interaction with molecular water (1043-1000 cm-1) and in interaction with surface hydroxyl groups (1017-972 cm-1). Hydration causes a shift of the corresponding band to lower wavenumbers, in agreement with experimental results.

Note that the calculated values are overestimated with respect to experimental measurements. This is a typical feature of the method used (PBE functional) and the use of the harmonic approximation. The shift to lower frequencies is however clearly visible in experimental Raman spectra in a hydration/dehydration processes, in agreement with our calculated trends. Figure 4. exhibits Raman spectra recorded during dehydration process of 1.2 V/TiO2 catalyst. Coordination of a water presence clearly affects the Raman spectra of the vanadia catalyst. The vanadyl (V=O) - water interaction depends on the temperature. The V=O bonds of polyvanadate ions ~940 cm-1 exists at low temperature range 298 – 323 K (violet range at Figure 4) 12, 13. A higher water content favours a formation of a higher polymerized species. A catalyst surface is highly hydrated, since is saturated by OH groups and molecular water. The isolated vanadyl species gradually forms with temperature increasing. Two Raman bands at 1017 and 923 cm-1 appear at temperature 373 K (green range at Figure 4). They are assigned to the stretching mode of terminal V=O bond. First of them appears to be essentially to isolated VOx species and the former in polymerized species. The band location at 1017 - 1020 cm-1 indicates its interaction with OH groups on the surface. Raman spectra exhibit only

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a vanadyl V=O band ~1023 cm-1 above temperature 523 K (blue range at Figure 4). The isolated VOx species dominate on the titania surface at higher temperature. A removing of OH group causes a band shift to higher frequency. Figure 4. Raman spectra of 1.2V/TiO2 catalyst during dehydration experiment.

Conclusions Water interaction with vanadia/titania is exothermic, some coordination with water is expected, even at high temperature in essentially dry conditions. Both molecular and dissociated water would be in equilibrium. Hydroxyl V-OH and V-OH-Ti groups are stable. The structures are stabilized by hydrogen bonding. Vanadyl V=O groups elongate with respect to dehydrated models, and this elongation is found to increase with increasing coordination to water molecules. References 1. G. Kresse and J. Hafner, Phys. Rev. B, 1993, 47, 558. 2. G. Kresse and J. Hafner, Phys. Rev. B, 1994, 49, 14251. 3. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865.

1100 1000 900 800 700

Raman Shift, cm-1

905

1026

1026

1024

1023

1021

1020

911

928

1017

935

940

298K

323K

373K

423K

473K

523K

573K

623K

673K

9/9

4. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1997, 78, 1396. 5. P. E. Blöchl, Phys. Rev. B, 1994, 50, 17953. 6. G. Kresse and J. Hafner, J. Phys. Condens. Matter, 1994, 6, 8245. 7. M. Calatayud, B. Mguig and C. Minot, Surf. Sci., 2003, 526, 297. 8. M. Calatayud and C. Minot, J. Phys. Chem. B, 2004, 108, 15679. 9. M. Calatayud and C. Minot, J. Phys. Chem. C, 2007, 111, 6411. 10. A.E. Lewandowska, M. Calatayud, E. Lozano-Diz, C. Minot, M.A. Bañares, Catal.

Today 139 (2008) 209 11. M. Calatayud, B. Mguig and C. Minot, accepted Surf. Sci. Reports, 2004. 12. G.T. Went, L.-J. Leu, A.T. Bell, J. Catal. 134 (1992) 479 13. G. Deo, I.E. Wachs, J. Phys. Chem. 95, (1991) 5889 14. M.A. Vuurman, I.E. Wachs, A.M. Hirt, J. Phys. Chem. 95 (1991) 9928 15. N.-Y. Topsøe, M. Anstrom, J.A. Dumesic, Catal. Lett. 76 (2001) 11

10th

of June 2009

Mr. Ricardo López Medina

STSM Scientific Report

In recent years, the catalysts have the new developing direction due to the development

of nano technology. Because nano sized particles can have new characteristics including

high surface area, high binding energy and very high chemical activation, so nanoscale

particle catalyst was called the fourth generation.

Purpose of the visit. The objective of the present study is to improve the methanol

oxidation approach to quantify the number of surface active sites for supported oxide

catalysts by optimizing the experimental conditions. The objectives are to develop a

general method for identification of active surface metal oxide sites and for calculating

methanol oxidation TOF's in supported metal oxide catalysts.

Description of the work carried out during the visit. The reactivity/selectivity

properties of the nano catalysts are chemically probed with steady-state catalytic studies

of methanol oxidation reactions to determine optimum conditions for selective

oxidation of methanol. First the methanol adsorbs in a weakly held precursor state on

the surface, i.e. physisorbs. In such form it has a short lifetime, but high diffusivity. This

reaction produces methoxy species and a terminal OH group. The OH reacts with

another incoming methanol molecule producing a second methoxy species and water

which desorbs from the surface. This methoxy groups decompose to formaldehyde and

leaves the surface.

MoVNbO and MoVNbTeO supported catalysts corresponded to approximately

monolayer coverages, to improve oxygenate yield relative to the base case catalysts and

to provide a preliminary understanding of how these promoters and support actually

affect catalyst properties. Clearly, an investigation of a catalyst is an investigation of

many simultaneous processes in a complex system. The aim of the present work was to

study the gas-phase conversion of acetonylacetone over MoVNb(Te)O catalysts in order

to test Br nsted acid-base properties of the catalyst surfaces.

Description of the main results obtained. Oxidation of methanol on oxide surfaces is

very sensitive to the nature of active catalytic sites at low conversions. It reflects the

nature of the surface active sites since redox sites activate ODH (oxidative

dehydrogenation) process to yield formaldehyde, basic sites give rise to CO2 formation

and and Lewis and Brønsted acid sites lead to dehydration of methanol to yield

dimethyl ether.

The results of methanol oxidation reaction reveal that there are acid sites on the surface

of submonolayer coverages of MoVNb(Te)O catalysts (4 atoms/nm2) and the results

obtained showed that at monolayer and monolayer and half coverage (8 and 12

atoms/nm2

respectively) only redox sites are active on the surface of the catalysts.

Simultaneously, it was found that when the atomic ratio of Mo/V/Nb is 6/3/1, the

conversion was the best. This atomic ratio gives ~ 90% selectivity to redox products and

10 % to acidic products, without the formation of any products originating from surface

basic sites.

The selectivity-activity relationships of these catalysts as a function of temperature were

discussed in relation to the composition of the catalysts. These findings suggested that

several surface VOx sites located in the structures may be responsible for the selective

8-electron transformation of propane to acrylic acid reaction. The interaction of the

supports or other components with vanadium species can generally influence both the

dispersion and the structure of vanadium species, resulting in different chemical

reactivities. It has been reported that methanol could be converted into dimethyl ether,

formaldehyde, methyl formate, dimethoxymethane on acidic, oxidative, and acidic and

oxidative bifunctional surfaces.

For MoVNb(Te) supported oxide systems, methanol oxidation decreased with

increasing surface density, MoVNb(Te)O/nm2 of an oxide support.

The reaction of acetonylacetone, [1,4-diketone (2,5-hexanedione)], is known to undergo

both acid- and base-catalyzed intramolecular cyclizations leading to 2,5-dimethylfuran

(DMF) and 3-methyl-2-cyclopenten-1-one (MCP), respectively. Thus, the incorporation

of increasing amounts of atoms on surface of MoVNb(Te)O oxide causes a continuous

decrease in DMF selectivity. Moreover, with the catalysts with 12 atoms/nm2, (DMF)

was produced at greater than 60 % selectivity. In contrast on a catalyst even below

surface coverage for the supported MoVNb(Te)O (4 atoms/nm2), MCP was obtained

with selectivity approaching to 50 % or better.

The results presented above show that acetonylacetone (2,5-hexanodione) conversion is

a good test reaction to confirm the acid or base surface properties of typical solid acid or

base catalysts.

Future collaboration with host institution.

Projected publications/articles resulting or to result from the STSM. The result of

this work will be published in a Serial Publication Dealing with Topical Themes in

Catalysis and Related Subjects and presented in a “Structure-performance relationships

at the surface of functional materials” meeting in Benahavís, (Málaga, Spain) (COST

Chemistry D36 3rd Workshop and 5th Management Committee Meeting), 21st to 23rd

of October, 2009

Confirmation by the host institute of the successful execution of the mission.

This is the confirmation of the successful execution of the mission Mr. Ricardo López

Medina in my laboratory for 6 weeks (from April 14 to May 31, 2009) within COST

action D36. Mr. R. López Medina performed a great number of experiments and

obtained very interesting results presented below. These results will be published soon.

Prof. Maria Ziolek

A. Mickiewicz University

Poznan, Poland

1O2 4Mo5V4Nb1-air

2O2 8Mo5V4Nb1-air

3O2 12Mo5V4Nb1-air

4O2 4Mo6V3Nb1-air

5O2 8Mo6V3Nb1-air

6O2 12Mo6V3Nb1-air

10O2 4Mo8V1Nb1-air

11O2 8Mo8V1Nb1-air

12O2 12Mo8V1Nb1-air

1N2 4Mo5V4Nb1-inert

2N2 8Mo5V4Nb1-inert

3N2 12Mo5V4Nb1-inert

4N2 4Mo6V3Nb1-inert

5N2 8Mo6V3Nb1-inert





1TeO 4Mo5V4Nb0.5 Te0.5-air









1TeN 4Mo5V4Nb0.5 Te0.5-inert









1O2 2O2 3O2 4O2 5O2 6O2 10O2 11O2 12O20.0

0.5

1.0

1.5

2.0

2.5

3.0

ACIDIC

amount Mo

at/nm2

at/nm2

Catalysts

MC

P/D

MF

at/nm2

amount V

BASIC

acid-base

1N2 2N2 3N2 4N2 5N2 6N2 10N2 11N2 12N20.0

0.5

1.0

1.5

2.0

2.5

3.0

ACIDIC

acid-base

BASICat/nm2

amount Mo

amount V

MC

P/D

MF

Catalysts

at/nm2

at/nm2

1TeO 2TeO 3TeO 4TeO 5TeO 6TeO 10TeO 11TeO 12TeO0.0

0.5

1.0

1.5

2.0

2.5

3.0

ACIDIC

acid-base

BASIC

at/nm2

at/nm2

amount Mo

Catalysts

MC

P/D

MF

amount V

at/nm2

1TeN 2TeN 3TeN 4TeN 5TeN 6TeN 10TeN 11TeN 12TeN0.0

0.5

1.0

1.5

2.0

2.5

3.0

acid-base

ACIDIC

BASIC

at/nm2

amount Mo

at/nm2

at/nm2

Catalysts

MC

P/D

MF

amount V

at/nm2

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

% C

onve

rsio

n / S

elec

tivity

Time (min)

conversion CO2 HCOH CH3-O-CH3 HCOCH3 CH3O-CH2-OCH3

250 C

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity


0 50 100 150 200 250 3000

20

40

60

80

100

300 C

% C

onve

rsio

n/S

elec

tivity

Reaction Time [min]

% conversion HCOH CH3-O-CH3

HCOCH3

CO2

250 C

4Mo5V4Nb1 [1O2] 8Mo5V4Nb1 [2O2] 12Mo5V4Nb1 [3O2]

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity


0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity


0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity


4Mo6V3Nb1 [4O2] 8Mo6V3Nb1 [5O2] 12Mo6V3Nb1 [6O2]

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity


0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity conversion

CO2 HCOH CH3-O-CH3 HCOCH3 CH3O-CH2-OCH3

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity


4Mo5V4Nb1 inert [1N2] 8Mo5V4Nb1 inert [2N2] 12M05V4Nb1 inert [3N2]

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

% C

onve

rsio

n / S

elec

tivity

Time (min)


0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity


0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity


4Mo6V3Nb1 inert [4N2] 8Mo6V3Nb1 inert [5N2] 12Mo6V3Nb1 inert [6N2]

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100 conversion CO2 HCOH CH3-O-CH3 HCOCH3 CH3O-CH2-OCH3

% C

onve

rsio

n / S

elec

tivity

Time (min)0 20 40 60 80 100 120 140

0

10

20

30

40

50

60

70

80

90

100


% c

onve

rsio

n / S

elec

tivity

Time (min)0 20 40 60 80 100 120 140

0

10

20

30

40

50

60

70

80

90


% C

onve

rsio

n / S

elec

tivity

Time (min)

4Mo5V4Nb0.5Te0.5 [1TeO] 8Mo5V4Nb0.5Te0.5 [2TeO] 12Mo5V4Nb0.5Te0.5 [3TeO]

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100


% C

onve

rsio

n / S

elec

tivity

Time (min)

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90


% C

onve

rsio

n / S

elec

tivity

Time (min)0 20 40 60 80 100 120 140

0

10

20

30

40

50

60

70

80

90

100


% C

onve

rsio

n / S

elec

tivity

Time (min)

4Mo6V3Nb0.5Te0.5 [4TeO] 8Mo6V3Nb0.5Te0.5 [5TeO] 12Mo6V3Nb0.5Te0.5 [6TeO]

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90


Time (min)

% C

onve

rsio

n / S

elec

tivity

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity


0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

% C

onve

rsio

n / S

elec

tivity

Time (min)


4Mo5V4Nb0.5Te0.5 inert [1TeN] 8Mo5V4Nb0.5Te0.5 inert [2TeN] 12Mo5V4Nb0.5Te0.5 inert [3TeN]

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90


Time (min)

% C

onve

rsio

n / S

elec

tivity

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity conversion

CO2 HCOH CH3-O-CH3 HCOCH3 CH3O-CH2-OCH3

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

70

80

90

100

Time (min)

% C

onve

rsio

n / S

elec

tivity


4Mo6V3Nb0.5Te0.5 inert [4TeN] 8Mo6V3Nb0.5Te0.5 inert [5TeN] 12Mo6V3Nb0.5Te0.5 inert [6TeN]

Short Term Scientific Mission, COST D36

In-situ Spectroscopic Investigation of the Electrocatalytic

Oxidation of Dimethylamine Borane

Daniela Plana

Supervisor: Robert Dryfe, University of Manchester Host: Marc Koper, Leiden Institute of Chemistry, Leiden University

June 8 – July 10, 2009 Reference code: COST-STSM-D36-04910

2

In-situ Spectroscopic Investigation of the Electrocatalytic Oxidation of

Dimethylamine Borane

1. Purpose of the visit

The aim of the visit was to combine in-situ spectroscopy with

electrochemical techniques, as a way of studying the mechanism of dimethylamine

borane (DMAB) oxidation. Of specific interest was the understanding of the

adsorption processes involved in the mechanism. The use of spectroscopic

techniques, in addition to electrochemistry, would not only aid in the understanding

of the potential dependency of the adsorption, but also in the investigation of the

effects of factors such as surface structure and pH.

The work plan set before the visit included three main tasks, which are

summarised below:

Task ONE: Study DMAB oxidation on model gold electrode surfaces, in

highly alkaline aqueous media, through in-situ IR spectroscopy.

Task TWO: Continue the study described in Task ONE, as a function of

solution pH.

Task THREE: Study the adsorption process in DMAB oxidation using in-

situ Raman spectroscopy.

2. Description of the work carried out during the visit

A voltammetric study of DMAB (2.0 mM) oxidation at pH 13 (0.1 M KOH)

was carried out on model gold surfaces, including Au (111), Au (110), Au (100),

and a poly-oriented electrode. The working electrodes were flame-annealed and

quenched with de-ionized water before each voltammogram was recorded. A

conventional three-electrode configuration was used, with a gold counter electrode

3

and a RHE reference electrode. All solutions were degassed for at least 20

minutes with argon prior to experiments and an inert atmosphere was maintained

by circling argon above solution at all times. A potential range from -0.10 to 1.70 V

vs. RHE was studied at two different scan rates (10 and 50 mV/s). The work was

repeated at pH 11 (1.0 mM KOH), using 0.1 M KClO4 as a supporting electrolyte.

In-situ IR spectroscopy was employed, using the conditions described

above. A higher concentration of DMAB (50 mM) was needed, in order to have

better band definition; however, the electrochemical behaviour remained

unchanged. In this case, the working electrodes were pressed against an optical

window at the bottom of the cell. Polarized light (“p” and “s” was used, to probe

solution and surface-bound species. Spectra were recorded every 80 mV, in a

range from 4000 to 800 cm-1. Transmittance spectra of DMAB, borohydride and

boric acid were also recorded in alkaline media, as references for band

assignments.

Raman spectroscopy was attempted without success, as violent gas

evolution made it impossible to retrieve any useful data. Alternative experiments

were then performed; rotating disk electrodes were used to probe the mass

transfer effects. The three model gold surfaces were studied, at pH 11 and pH 13.

Transfer experiments were carried out at pH 11; these consisted in

oxidizing DMAB up to a certain potential, holding the potential, and then

transferring the electrode into a cell with no DMAB present. In this second cell, a

reduction scan was performed. In-situ IR spectroscopy was also employed to

study the reduction.

3. Main results obtained

DMAB voltammetry was studied at pH 13 on three model gold surfaces; the

results are presented in Figure 1. Although the main features are present in all

three cases, such as oxidizing currents in the forward scan, with no discernable

4

diffusion-limited peaks, a reactivation peak on the back scan and no reduction

currents present, there are some important differences with the surface structure.

j /

A c

m-2

0.00

1.00e-2

2.00e-2

3.00e-2

4.00e-2

5.00e-2

6.00e-2

E / V vs. RHE

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

j /

A c

m-2

0.00

1.00e-3

2.00e-3

3.00e-3

4.00e-3

Figure 1. Cyclic voltammograms of 2.0 mM DMAB at 50 mV/s, at pH 13, on three different gold surfaces: Au (111) - black, Au (110) – blue, and Au (100) - red. The bottom graph zooms in on the

top graph.

The potential at which DMAB oxidation starts changes as a function of

surface structure; it starts at less positive potentials on the Au (110) surface, then

the Au (111), and later on the Au (100). Two distinct processes can be observed

5

the Au (110) surface, where on the other two electrodes the processes are closer

together and harder to distinguish, especially on the Au (100). The reactivation

peak is much less prominent on the Au (110) than on the Au (111) and Au (110)

electrodes. The potential at which it appears seems to follow the same trend than

the onset of the oxidation.

Of particular interest was that although the first scan appears to have

plateaus, the second scan in each case presented lower currents and peaks more

easily associated with diffusion limited processes, particularly in the early stages of

oxidation. The two different stages of oxidation are more clearly recognised on all

three surfaces. The reactivation peak currents drop dramatically.

The marked differences in the electrochemical behaviour with the surface

structure and with consecutive scans seem to indicate that the interaction with the

surface has an important effect on the oxidation, possibly confirming the

suspected adsorption process involved.

Figure 2 shows IR spectra taken during DMAB oxidation on Au (111) at pH

13. Mayor features can be observed in the regions between 3600 - 3000 and 1700

– 1600 cm-1, which are related to O-H bonds and possibly also N-H bonds. Initially

they can be assigned to water molecules, due to the large changes in water that

occur at high pHs in the wide potential range studied. However, there are several

bands within each of the two regions, possibly indicating OH (or NH) groups not

belonging to water. They could be related to the dimethylamine or to hydroxides

joined to boron atoms. Some of these bands shift with potential, indicating

adsorbed species [1,2].

6

Figure 2. Consecutive spectra obtained every 80 mV during a cyclic voltammetry, at 10 mV/s, from -0.20 to 1.80 V vs. RHE. Au (111) electrode, 50 mM DMAB, pH 13. -0.20 V was used as reference.

Smaller bands are also present, and can be seen more easily in Figure 3.

Three bands are present between 2500 and 2200 cm-1. These bands go upwards,

indicating a species which is disappearing as the oxidation process occurs. They

can be assigned to the B-H bonds in the DMAB molecule [1,2]. A small peak close

to 1500 cm-1 seems to show a disappearing bond just before or at the oxidation

onset. It can be assigned to a breaking of the N-B bond, which would indicate that

the DMAB molecule does not fully dissociate when dissolved, as it has been

claimed before [3].

At higher potentials, three more peaks appear; in this case they indicate

species that are being formed. The first one appears around 1210 cm-1,

approximately at 0.2 V vs. RHE, while a second band appears 200 mV later at

1130 cm-1. The first can be can be associated to the stretching of B-O bonds, while

the second appears to be the bending of B-O-H bonds, these structures can be

formed as intermediates or oxidation products. Well into the second oxidation

stage, a third peak emerges around 1415 cm-1; this region can be assigned to B-O

bonds, in species such as boric acid or polyborate anions [1,2].

100015002000250030003500

Wavenumber cm-1

7

Figure 3. Expanded regions of the spectra depicted in Figure 2.

Interestingly, although boric acid has been in some cases named as one of

the likely oxidation products [3-5], some of the characteristic bands associated

with it are not present. Comparison with spectra obtained of an alkaline boric acid

solution seems to prove that under the conditions studied, boric acid is not formed.

More likely products appear to be BO2- or B4O7

2-, for example.

Some smaller signals appear in the region between 2200 and 2000 cm-1

which could be associated to the oxidation products of the dimethylamine, such as

N-O or C-O bonds [1,2].

Not many differences can be found between the three gold surfaces

studied, other than the potentials at which the bands appear; the number of bands

and their positions appear to be the same. However, when the pH was lowered to

11, quite a few differences were observed. The band around 1210 cm-1 does not

appear, while the signal at 1130 cm-1 is much more prominent than at pH 13. This

appears to indicate a preference for one of the oxidation products over another.

On the other hand, there are no bands in the region between 2200 and 200

cm-1, possibly indicating that if the amine is oxidised, its oxidation is not as

extensive, producing different species which do not include the N-O or C-O bonds

previously seen, or simply follows a different mechanism.

200021002200230024002500

Wavenumber cm-1

11001200130014001500160017001800

Wavenumber cm-1

8

I /

A

0.0

5.0e-5

1.0e-4

1.5e-4

2.0e-4

Col 1 vs Col 4

Col 13 vs Col 14

I /

A

-2.0e-5

0.0

2.0e-5

4.0e-5

6.0e-5

8.0e-5

1.0e-4

1.2e-4

E / V vs. RHE

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

I /

A

-2.0e-5

0.0

2.0e-5

4.0e-5

6.0e-5

8.0e-5

1.0e-4

1.2e-4

1.4e-4

Au111

Au100

Au110

Figure 4. Cyclic voltammograms of 2.0 mM DMAB at 50 mV/s, at pH 11, on three different gold surfaces. In each case the first scan is shown in black and the second in red.

9

At pH 11, there are two distinct processes in the forward scan which can be

clearly seen as limiting currents in Figure 4. Unlike at pH 13, the currents do not

drop to zero and there is no reactivation peak. If the potential is taken further

positive, oxygen evolution currents can be seen. Again the oxidation starts at

lower potentials on Au (110), however the second process starts earlier on the

other two surfaces.

The difference between the first and second scans is very marked on all

three surfaces. The current of the first oxidation process drops to different

degrees, becoming almost non-existent on Au (111). The second process

becomes markedly diffusional and a reduction current appears on the backscan.

This last was unexpected, as the oxidation of this type of reducing agents is

irreversible [6-8]. In order to study this new feature more thoroughly consecutive

scans were made, increasing the turning potential in each case. Figure 5 shows

that the reduction current does not appear until the second limiting current is

reached; indicating that what is reduced was first formed at those potentials. Once

the cathodic process starts, the current of the first oxidation disappears almost

completely and the shape of the second oxidation wave changes to a markedly

diffusional process. Whatever is produced during this process appears to poison

the electrode surface, thus inhibiting the initial oxidation.

10

E / V vs. RHE

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

I /

A

-4.0e-5

-2.0e-5

0.0

2.0e-5

4.0e-5

6.0e-5

8.0e-5

1.0e-4

1.2e-4

Figure 5. Consecutive cycles at 50 mV/s on a Au (111) electrode, with a higher turning potential in each scan. 2.0 mM DMAB, pH 11.

In view of the voltammetric and spectroscopic behaviour observed at pH 11,

transfer experiments were performed. Figure 6 shows consecutive

voltammograms recorded in a DMAB-containing solution (on the left), the potential

was then held during the second oxidation step, and the electrode was then

transferred to a solution without DMAB, where the voltammograms on the right

were recorded. The reduction of gold oxide is clearly visible, meaning that

whatever is adsorbed on the surface only covers a small part of it. The reduction

currents between 0.20 and -0.10 V are present, confirming the strong adsorbance

of the species formed during the second wave of oxidation. The reduction currents

decrease with consecutive scans, also confirming the adsorbed state of the

reacting species which then goes into solution as it is reduced. IR spectra that

needs further analysis was recorded during the reduction scan performed after the

transference to a blank cell, in order to identify the products of both the oxidation

and of the reduction processes.

11

Figure 6. First (black), second (red) and third (red) cycles for transfer experiments; oxidation (left), reduction after transferring into blank cell (right). Au (111) electrode, scan rate 50mV/s, 2.0 mM

DMAB, pH 11.

In order to further understand the effect of mass transport, a rotating disk

electrode configuration was used. This seemed an interesting study as it is

unusual to find limiting currents while not under forced convection in the system.

Figure 7 shows the resulting cyclic voltammograms at pH 11 on a Au (111)

electrode. The limiting currents scale up well with the rotation speed, however,

unlike the system with no convection, the second scan is identical to the first; no

current is lost and there are no reduction currents present. Whatever poisons the

electrode in static conditions appears to be removed by agitation or it is removed

from the surface before it can react.

E vs. RHE

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

I / A

-2.00e-5

0.00

2.00e-5

4.00e-5

6.00e-5

8.00e-5

1.00e-4

1.20e-4

1.40e-4

1.60e-4

E vs. RHE

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

I / A

-1.00e-5

-8.00e-6

-6.00e-6

-4.00e-6

-2.00e-6

0.00

2.00e-6

4.00e-6

12

E vs. RHE

-0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

I /

A

0.00

5.00e-5

1.00e-4

1.50e-4

2.00e-4

2.50e-4

Figure 7. Cyclic voltammograms recorded at a rotating Au (111) electrode, at 50 mV/s, of 2.0 mM DMAB at pH 11. Rotating speeds: 100 (black), 200 (red), 300 (blue), 400 (green), 500 (purple) and

600 (gray) RPM.

While a traditional Levich plot does not fit the data well, an adjustment with

Koutecký-Levich (Figure 8) seems to work well for both plateaus. This would

indicate that a kinetic factor is involved in the process. This behaviour was also

observed at pH 13.

13

ω-1/2

/ s1/2

0.05 0.10 0.15 0.20 0.25 0.30 0.35

I-1 /

A-1

0

10000

20000

30000

40000

50000

60000

70000

y = 223660x - 6224

R2 = 0.9949

y = 52128x - 2420.9

R2 = 0.9851

Figure 8. Koutecký-Levich plots of the data shown in Figure 7.

4. Future collaborations and projected publications

Although more in-depth analysis of the data collected, specially the IR

spectra, is needed before making a final decision, it is likely that one or two

manuscripts on the mechanism of DMAB oxidation will be prepared for publication.

It would be interesting to continue this study by using differential

electrochemical mass spectrometry (DEMS) to further understand the mechanism

of DMAB oxidation. The use of such a technique would allow the identification of

the gaseous products observed during oxidation. It would also improve the

understanding of the initial dissociation step.

14

5. References

[1] Socrates, G. Infrared Characteristic Group Frequencies. 3rd edition. John

Wiley and Sons Ltd. New York, 2001.

[2] White, R. G. Handbook of Industrial Infrared Analysis. Plenum Press. New

York, 1964.

[3] Burke, L.D. and Lee, B.H., Journal of Applied Electrochemistry, V22 (1), 48-

56, (1992).

[4] Patterson, J.C., et al., Applied Surface Science, 91, 124-128, (1995).

[5] Sverdlov, Y. and Shacham-Diamand, Y., Microelectronic Engineering, 70

(2), 512-518, (2003).

[6] Chatenet, M., et al., Electrochimica Acta, 51 (25), 5459-5467, (2006).

[7] Mirkin, M.V., Yang, H., and Bard, A.J., J. Electrochem. Soc., 139 (8), 2212-

2217, (1992).

[8] Denuault, G., Mirkin, M.V., and Bard, A.J., Journal of Electroanalytical

Chemistry, 308 (1), 27-38, (1991).

1

Gabriella Di Carlo’s STSM report

Cobalt-based catalysts on SBA-15

for Fischer-Tropsch synthesis

2

Introduction

The aim of the present STSM is the development of cobalt-based catalysts on ordered mesoporous

supports for Fischer-Tropsch synthesis (FTS).

The increasing demands for high-quality and environmentally friendly transportation fuels lead to a

renewed interest in diesel production by Fischer-Tropsch synthesis (FTS), which is nowadays one of

the most promising ways for the conversion of coal, biomass and natural gas to clean fuels and

chemicals via syngas (H2 + CO). The FT technology offers significant environmental and efficiency

benefits over crude oil, as the products are mainly composed by linear paraffins having high cetane

numbers and free of sulfur and aromatics pollutants [1].

Cobalt-based catalysts are widely used in FTS if diesel is the desired product because of their high

activity, high selectivity to long chain paraffins (C5+) and low water–gas shift activity [2-3].

In the present work, cobalt nanoparticles supported over SBA-15 based materials were prepared by

using both wetness impregnation and microemulsion method. The attention was focused on the effect

of the preparation procedure as well as on the role of morphological and chemical properties of the

support on the selectivity of the reaction.

The deposition of cobalt nanoparticles over ordered mesoporous materials is considered an attractive

way to improve the metal dispersion and catalytic performances [4]. In particular, among

mesostructured silica the SBA-15 has attracted much attention for its notable hydrothermal stability,

thick pore wall and adjustable pore size (from 5 to 30 nm). These characteristics make it suitable for

use as support for FTS catalysts.

The effect of dopant agent insertion (as Ti or Al) into the framework of silica support on the catalytic

performance was investigated. It is expected that the addition of modifiers can affect both the

dispersion and the reducibility of supported cobalt species and consequently it can play a key role on

the catalytic activity and selectivity of the catalyst in FT synthesis.

The promoter effect of noble metals (as Au or Ru) acting as structural or electronic modifiers was also

investigated.

During the STSM, the attention was also focused on the use of microemulsion method for the

synthesis of cobalt nanoparticles with a tailored and narrowed particle size distribution. The

microemulsion procedure is considered very attractive for metal particle size control by tuning the

composition of the microemulsion. In particular, water-in-oil (W/O) or oil-in-water (O/W) system were

compared and the role of the nature of surfactant, cobalt precursor or precipitating agent on the

structural and morphological properties of cobalt nanoparticles was investigated. In order to obtain the

cobalt catalyst, the nanoparticles confined into the microemulsion were subsequently deposited over

silica support, such as commercial silica and mesoporous SBA-15.

3

Therefore, the relationship between the synthetic procedure and the morphological and structural

properties of supported cobalt nanoparticles was studied.

Synthesis of mesostructured SBA-15 based supports

Ordered mesoporous silicas, such as HMS [5] and SBA-15 [6], have recently attracted many

attentions as supports for metal and metal oxide nanoparticles. The confinement of supported species

into a mesoporous host support having a high specific surface area and a stable well-ordered pore

structure could limit the growth of the supported nanoparticles, enhancing the resistance to sintering

as well as the catalytic performance. In particular, our recent studies have shown a remarkable effect

of the mesoporous HMS and SBA-15, limiting the agglomeration of supported PdO nanoparticles and

enhancing the catalytic activity [7-8].

In the present project, the attention was focused on mesoporous SBA-15 having a well-ordered

hexagonal pore structure (Fig. 1). The development of SBA-15 based materials doped with titanium

and aluminium was also investigated.

HMS SBA-15

1-dodecylamine P123

A.-H. Lu, F. Schüth / C. R.

Chimie 8 (2005) 609–620

Fig. 1. A scheme of the pore structure in SBA-15.

Mesoporous SBA-15 was prepared according to a published procedure [6] using triblock copolymer

Pluronic P123 as templating agent and tetraethoxysilane (TEOS) as precursor. Ti-SBA-15 and Al-

SBA-15 samples were synthesized with similar procedures by adding an appropriate amount of

titanium or aluminium precursor (i.e. titanium(IV) isopropoxide or aluminium sec-butoxide respectively)

during the synthesis of the mesoporous silica, the so-called direct synthesis. The obtained materials

were labelled as TixSBA-15 and AlxSBA-15, where x correspond to the titania and alumina weight

percent (5-10 wt% as TiO2 or Al2O3).

The morphological and structural properties of the prepared materials were investigated by BET

surface area measurements, X-ray diffraction, small-angle X-ray scattering (SAXS) and high resolution

TEM analysis.

The use of a templating agent during the synthesis of doped-SBA-15 leads to materials with high

surface area, narrow pore size distribution and a well-ordered 2D hexagonal structure.

4

The ordered pore structure, arising from the use of a structure-directing agent, was detected by SAXS

measurements. According to the SAXS profiles (Fig. 2), both Ti-SBA-15 and Al-SBA-15 composites

retained the original ordered hexagonal structure with a long range order.

0.5 1.0 1.5 2.0 2.5

SBA-15 Ti5SBA Ti10SBA Al5SBA Al10SBA

Inte

nsity

(a.u

.)

2 theta (°)

Fig. 2. SAXS patterns of SBA-15 based materials.

Morphological and textural properties of the SBA-15 based supports are compiled in table 1. The

incorporation of titania or alumina into the silica framework by direct synthesis is responsible for a

slightly increase of surface area, average pore diameter and pore volume with respect to pure silica.

Table 1. BET surface area (S), average pore diameter (dp) and pore volume (Vp) of the supports.

Sample Sa (m2/g) dpb (nm) Vp

c (cm3/g)

SBA-15 749 7.2 0.79

Ti10SBA 902 7.7 1.05

Al10SBA 830 9.1 1.12 a the BET values are calculated using the range 0.05 – 0.2 p/p0. b the mean pore diameter is determined using the BJH model. c the pore volume is determined considering the range p/p0 from 0.1 until to 0.98.

High resolution TEM images of mesoporous SBA-15 were recorded in order to investigate the long

range order of the pore structure. The picture, which is reported in Fig. 3, allows detecting the

presence of a highly ordered porosity.

5

Fig. 3. High resolution TEM image of mesoporous SBA-15.

Synthesis of cobalt catalysts by wetness impregnation

Cobalt nanoparticles were supported over SBA-15, Ti(x)SBA-15 and Al(x)SBA-15 (with x = 5-10 wt%

as TiO2 or Al2O3) by wetness impregnation, which is a widely used procedure for the production of

commercial catalysts.

In particular, cobalt-based catalysts, with a loading equal to 12 wt% as Co, were prepared by wetness

impregnation with an aqueous solution of Co(NO3)2·6H2O. The samples were dried at 120 °C for 5 h

and calcined in air at 400 °C for 4 h using a ramp rate of 1 °C/min.

Ru and Au promoted cobalt catalysts were prepared using a similar procedure. The addition of the

noble metal as promoter were performed by adding an appropriate amount of Ru(NO)(NO3)3 or

HAuCl4 to the aqueous cobalt solution in order to yield a cobalt loading equal to 12 wt% and a molar

ratio Ru/Co or Au/Co equal to 0.024 (corresponding to 0.50 wt% of Ru and 0.97 wt% of Au

respectively). The samples were dried at 120 °C for 5 h and calcined in air at 400 °C for 4 h using a

ramp rate of 1 °C/min.

The catalysts were characterised by BET surface area measurements, X-ray diffraction, small-angle

X-ray scattering (SAXS), temperature programmed reduction (H2-TPR) and hydrogen chemisorption.

Cobalt deposition leads to a slight decrease of surface area, average pore size and pore volume with

respect to the values observed for the support. For example, in CoSBA the surface area decreases

from 749 to 623 m2/g, the pore size from 7.2 to 5.5 nm and the pore volume from 0.79 to 0.66 cm3/g.

These findings suggest that cobalt deposition inside the pore structure of the support occurs.

6

In order to evaluate how the insertion of Ti or Al into the SBA-15 framework affects the reducibility and

the dispersion of supported cobalt nanoparticles, the cobalt catalysts were characterized by H2-TPR

and XRD measurements and the results are shown in Figure 4 and 5 respectively.

Both titania and alumina addition to mesoporous SBA-15 leads to a shift of hydrogen consumption

towards higher temperature. The peak around 300 °C, which is present in CoSBA, is shifted at higher

temperature in CoTi10SBA and CoAl10SBA (Fig. 4 a).

The effect of Ti loading on the TPR profiles was also investigated and the curves reported in Fig. 4 b

show that the shift of the peaks at high temperature is proportional to the amount of titanium.

The different reducibility of cobalt species can be explained by taking into account the metal-support

interaction, which is usually stronger over both alumina and titania.

0 100 200 300 400 500 600 700 800 900

CoSBA CoTi10SBA CoAl10SBA

TCD

sig

nal (

a.u.

)

Temperature (°C)

(a)

0 100 200 300 400 500 600 700 800 900

CoSBA CoTi5SBA CoTi10SBA

TCD

sig

nal (

a.u.

)

Temperature (°C)

(b)

Fig. 4. TPR profiles of Co catalysts. The effect of Ti or Al addition (a) and of Ti loading (b).

XRD patterns of cobalt catalysts are displayed in Fig. 5 and the presence of Co3O4 crystallites was

detected. Cobalt particle size was calculated from the full width at half maximum (FWHM) of the most

intense reflection peak using the Scherrer equation and the values are listed in Table 2. A weak and

broad peak at 25.3 ° 2 theta, which can be attributed to the presence of titania anatase, was also

detected in the sample containing a 10 wt% of TiO2.

7

20 25 30 35 40 45 50

CoSBA CoAl10SBA CoTi10SBA

Inte

nsity

(a.u

.)

2 theta (°)

Fig. 5. XRD patterns of Co catalysts.

Table 2. Average diameter of Co3O4 and Au crystallites (d) in supported cobalt catalysts.

Sample d (nm)

CoSBA 13

CoAl10SBA 11

CoTi10SBA 14

CoRuSBA 12

CoAuSBA 12 (Co3O4), 38 (Au)

The addition of Ru or Au to CoSBA does not have any effect on the cobalt dispersion. The average

diameter of Co3O4 crystallites is almost unchanged as results from the values in Tab. 2.

On the contrary, the presence of the noble metal, Ru or Au, leads to enhanced reducibility of cobalt

with respect to pure CoSBA (Fig. 6). In particular, the Ru catalyst shows higher hydrogen consumption

at low temperature with an intense peak around 200 °C and the reduction is completed below 600 °C.

Concerning the Au catalyst, the hydrogen consumption at high temperature is shifted below 500 °C,

whereas at low temperature the reducibility is comparable to the unpromoted catalyst. As reported in

Tab. 2, Au particle size is around 38 nm (Fig. 7). Therefore, the lack of highly dispersed gold

nanoparticles could be the main reason for the not enhanced reducibility at low temperature in the Au

catalyst.

8

0 100 200 300 400 500 600 700 800 900

CoSBA CoRuSBA CoAuSBA

TCD

sig

nal (

a.u.

)Temperature (°C)

Fig. 6. TPR profiles of Co, CoRu and CoAu nanoparticles over SBA-15.

25 30 35 40 45 50


Inte

nsity

(a.u

.)

2 theta (°)

Fig. 7. XRD patterns of Co, CoRu and CoAu nanoparticles over SBA-15.

SAXS measurements were recorded for Co, CoRu and CoAu catalysts over SBA-15. The profiles

reported in Fig. 8 show that all the samples retain a long range order.

0.5 1.0 1.5 2.0 2.5


Inte

nsity

(a.u

.)

2 theta (°)

Fig. 8. SAXS patterns of Co, CoRu and CoAu nanoparticles over SBA-15.

9

High resolution TEM images of CoSBA are displayed in Figs. 9-11. In Fig. 10 it is possible to discern

the crystalline structure and in Fig. 11 the presence of the lines indicate the ordered structures of the

pores.

Fig. 9. TEM images of CoSBA.


I


10

Synthesis of cobalt catalysts by microemulsion method

In the present project, the attention was focused on the development of synthetic procedures which

allows preparing nanostructured cobalt-based materials with a well-defined and controlled structure [9-

11].

Different microemulsion procedures were compared. In particular, the role of the microemulsion

composition and the nature of the precipitation agent on the morphological and structural properties of

cobalt nanoparticles were investigated.

Cobalt nanoparticles were prepared by using different nonionic surfactant, such as Synperonic® 10/6,

Pluronic® P123 and a mixture of SPAN® 20-TWEEN® 80. The role of the surfactant on the shape and

size of the micelles and consequently on the properties of cobalt nanoparticles was investigated.

Cobalt microemulsions prepared with Synperonic® 10/6, Pluronic® P123 and a mixture of SPAN® 20-

TWEEN® 80 were labelled as CoME1, CoME2 and CoME3 respectively. A oil-in-water (O/W)

procedure was used for CoME1 and CoME3, whereas water was the only solvent for the synthesis of

CoME2. For all these procedures NaBH4 was used as precipitating agent.

The attention was focused on the system with Synperonic® 10/6 (CoME1). To go into more depth, oil-

in-water (O/W) and water-in-oil (W/O) microemulsions were prepared using Synperonic® 10/6 as

surfactant and the cobalt microemulsions were labelled as CoME1 and CoME4.

The O/W microemulsion with Synperonic® 10/6 was also selected in order to investigate the influence

of the precipitating agent.

The use of NaBH4 for cobalt reduction under inert atmosphere (N2) was compared with the use of

NaOH and (COOH)2 for cobalt precipitation as hydroxide and oxalate respectively.

A scheme with nature of the surfactant, microemulsion composition and precipitating agent is reported

in Tab. 3.

Table 3. Nature of the surfactant, composition and precipitation agent in cobalt microemulsions.

Sample’s label Surfactant Composition Precipitating agent

CoME1 Synperonic® 10/6 O/W NaBH4

CoME2 Pluronic® P123 In water NaBH4

CoME3 SPAN® 20-TWEEN® 80 O/W NaBH4

CoME4 Synperonic® 10/6 W/O NaBH4

CoME5 Synperonic® 10/6 O/W NaOH

CoME6 Synperonic® 10/6 O/W (COOH)2

11

The microemulsion procedure allows the synthesis of cobalt nanoparticles with tailored morphological

and structural properties. Then, the microemulsion was destabilized by adding an appropriate solvent

(like acetone or water) and the so-obtained cobalt nanoparticles were deposited over a silica support.

The catalyst was recovered by centrifugation and it was washed with an ethanol/acetone mixture with

volume ratio 1/1 in order to remove the surfactant and the precursors.

All the samples were dried at 120 °C for 5 h and calcined in air at 400 °C for 4 h using a ramp rate of 1

°C/min in order to get the final catalyst.

A commercial silica (Merck) was used as support in order to compare the different microemulsion

procedures. The cobalt catalysts over commercial silica were labelled as CoMExS with x included

between 1 and 5 depending on the microemulsion procedure (Tab. 3).

It is interesting to compare the morphology of cobalt nanoparticles in microemulsion (Fig. 12 a) with

the one supported over silica (Fig. 12 b). TEM images of cobalt nanoparticles in O/W microemulsion

prepared with Synperonic 10/6 (CoME1) and the corresponding supported catalyst (CoME1S) are

shown in Fig. 12. The cobalt nanoparticles in microemulsion appear highly dispersed and with uniform

particle size distribution. Even after deposition over silica the cobalt nanoparticles retain high

dispersion and average diameter lower than 3 nm as they are not detectable by X-ray diffraction (Fig.

13).

The reduction of cobalt with NaBH4 using both O/W and W/O procedures leads to final catalysts with

highly dispersed cobalt nanoparticles (CoME1S and CoME4S respectively), which are not detectable

by X-ray diffraction measurements (Fig. 13). The precipitation of cobalt with NaOH (CoME5S)

significantly affects the morphological and structural properties of the nanoparticles. Co3O4 crystallites

were detected by X-ray diffraction (Fig. 13) with an increase of cobalt particle size up to 8 nm. The

precipitation with oxalic acid (CoME6S) leads to a further increase of the average diameter up to 20

nm (Fig. 13).

(a) (b)

Fig.12: TEM images of CoME1 (a) and CoME1S (b).

12

10 15 20 25 30 35 40 45 50

CoME1 CoME2 CoME3 CoME4 CoME5 CoME6 Co3O4 reference

Inte

nsity

(a.u

.)

2 theta (°)

Fig.13: XRD patterns of cobalt catalysts over commercial silica prepared by microemulsion methods.

Cobalt nanoparticles supported over commercial silica were characterized by high resolution TEM

analysis and the images are displayed in Fig. 14.

CoME1S shows the presence of highly dispersed cobalt nanoparticles with a uniform particle size

distribution (Fig. 14 a), whereas CoME3S and CoME4S present some particle aggregation.

The morphology was totally different in CoME5S, in which the formation of cobalt nanorods was

observed (Fig. 14 d).

(a) (b)

(c) (d)

Fig.14: TEM images of CoME1S (a), CoME3S (b), Co ME4S (c) and CoME5S (d).

13

Selected microemulsion procedures were used for the deposition of cobalt nanoparticles over SBA-15.

In particular, cobalt nanoparticles, which were prepared according to CoME4, CoME5 and CoME6

procedures, were deposited over mesoporous SBA-15. The catalysts, characterized by XRD

measurements, show textural properties which are similar to the ones over commercial silica. TEM

analyses are in progress.

Catalytic results in Fischer-Tropsch synthesis

The relationship between the morphological and chemical properties of cobalt catalysts and their

catalytic performance in the Fischer-Tropsch synthesis was investigated.

The catalytic experiments were performed in a tubular fixed bed reactor using 0.7 g of sample. The

catalysts (pellet size 53-90 μm) were reduced in high purity H2 at 350 ºC for 16 h using a ramp rate of

1 °C/min. The experiments were carried out at 210 ºC and 20 bar with an inlet H2/CO ratio of 2.1 at

different gas hourly space velocities (GHSVs). The effect of water addition to the reactant mixture was

also evaluated.

In order to compare the performance of the catalysts in Fischer-Tropsch synthesis, the same

procedure was used for all the experiments.

It consists of the following steps:

- I period: at constant GHSV (equal to …………)

- II period: at constant conversion (equal to 40 %)

- III period: by adding 20 % of water vapor (same GHSV of period II)

- IV period: by adding 33 % of water vapor (same GHSV of period II)

- V period: without water vapor (same condition of period II)

14

R1

-10.0 %

0.0 %

10.0 %

20.0 %

30.0 %

40.0 %

50.0 %

60.0 %

0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00

TOS [h]

CO

co

nv

ers

ion

R1

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0.00 20.00 40.00 60.00 80.00 100.00 120.00

TOS [h]

CH

4 s

ele

cti

vit

y

65.00

70.00

75.00

80.00

85.00

90.00

95.00

100.00

C5

+ s

ele

cti

vit

y

R1-CH4 R1-C5+

Fig.15: CoSBA

15

R1

-10.0 %

0.0 %

10.0 %

20.0 %

30.0 %

40.0 %

50.0 %

60.0 %

0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00

TOS [h]

CO

co

nv

ers

ion

R1

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0.00 20.00 40.00 60.00 80.00 100.00 120.00

TOS [h]

CH

4 s

ele

cti

vit

y

65.00

70.00

75.00

80.00

85.00

90.00

95.00

100.00

C5

+ s

ele

cti

vit

y

R1-CH4 R1-C5+

Fig.16: CoTi10SBA

16

R1

-10.0 %

0.0 %

10.0 %

20.0 %

30.0 %

40.0 %

50.0 %

60.0 %

0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00

TOS [h]

CO

co

nv

ers

ion

R1

CO2-free selectivities

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0.00 20.00 40.00 60.00 80.00 100.00 120.00

TOS [h]

CH

4 s

ele

cti

vit

y

65.00

70.00

75.00

80.00

85.00

90.00

95.00

100.00

C5

+ s

ele

cti

vit

y

R1-CH4 R1-C5+

Fig.17: CoAl10SBA

17

R1

-10.0 %

0.0 %

10.0 %

20.0 %

30.0 %

40.0 %

50.0 %

60.0 %

0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00

TOS [h]

CO

co

nv

ers

ion

R1

CO2-free Selectivities

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0.00 20.00 40.00 60.00 80.00 100.00 120.00

TOS [h]

CH

4 s

ele

cti

vit

y

65.00

70.00

75.00

80.00

85.00

90.00

95.00

100.00

C5

+ s

ele

cti

vit

y

R1-CH4 R1-C5+

Fig.18: CoRuSBA

Conclusions

18

References

[1] G.P. Van der Laan, A.A.C.M. Beenackers, Catal. Rev.-Sci. Eng., 41 (1999) 255.

[2] E. Iglesia, Appl. Catal. A, 161 (1997) 59.

[3] A.Y. Khodakov, W. Chu and P. Fongarland, Chemical Reviews, 107 (2007) 1692.

[4] A.Y. Khodakov, A. Griboval-Constant, R. Bechara, V.L. Zholobenko, J. Catal., 206 (2002) 230.

[5] P.T. Tanev, T.J. Pinnavaia, Science, 267 (1995) 865.

[6] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science, 279

(1998) 548.

[7] A.M. Venezia, G. Di Carlo, L. F. Liotta, G. Pantaleo, Europacat IX, Salamanca (Spain), P13-35

(2009).

[8] A.M. Venezia, R. Murania, G. Pantaleo, G. Deganello, J. Catal., 251 (2007) 94.

[9] M. Sanchez-Dominguez, M. Boutonnet, C. Solans, J. Nanopart. Res., 11 (2009) 1823.

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[11] N. Bao, L. Shen, X. Lu, K. Yanagisawa, X. Feng, Chemical Physics Letters, 377 (2003) 119.

SHORT-TERM SCIENTIFIC MISSION COST Action Number: D36 Beneficiary's Name and Institution: Dr Elizabeth Santos, Universität Ulm Host's Name and Institution: Dr. Monica Calatayud, Dr. Frederik Tielens Université Pierre et Marie Curie Period: from 14/09/2009 to 23/09/2009 Place: Paris (FR) Reference code: COST-STSM-D36-05009

Scientific report

Investigation of the adsorption mechanism of alkyl thiols on Au(111) surfaces, using theoretical ab initio tools

The aim of this Short Term Scientific Mission was to discuss preliminary results on the adsorption process of thiol molecules on gold surfaces. Experimental results show that thiols can chemisorb on gold, by loosing the H atom from the thiol forming strong Au-S chemical bonds. Whereas the final adsorption geometry starts to be understood, one has no clear view on the adsorption process it self. Some theoretical studies have discussed simple reaction mechanisms of adsorption using ab initio calculation technique, however, without taking into account the collaborative effect of two thiol molecules in the adsorption process releasing molecular hydrogen. Our collaboration makes it possible to combine, two state of the art of theoretical techniques in order to study this particular reaction, which is at the basis of the formation of the technologically important self assembled monolayers (SAMs). Computational Details and investigated systems during the visit The geometry optimization and minimization of the total energy were performed using the VASP code. In the periodic Density Functional Theory framework used, the Kohn-Sham equations have been solved by means of the Perdew-Burke-Ernzerhof (PBE) functional. The electron-ion interaction was described by the Projector Augmented-Wave method (PAW). The Au(111) surface has been modelled using a slab containing 5 layers. The two bottom layers were fixed to the bulk positions. The cell parameters were obtained after optimization of the bulk at the same level of calculation as the thiol SAM. The atom positions of the thiol together with the upper three layers were relaxed without geometrical constraints (optimized at 0K). The optimization was performed at a 4x4x1 k-point mesh for the Brillouin-zone integration with an energy cutoff of 500 eV. After the geometry optimization, different geometries have been built. In the first case,

the S-H distance has been elongated from the equilibrium distance (1.35Å) until 2.29 Å. In this case only the S-H distance was constrained in the relaxation. In the second case, the radical thiol (without the hydrogen atom) has been approached to the surface. In the third case, the thiol molecule has been approached to the distance of the surface where the equilibrium position of the chemisorbed radical has been found, and all the coordinates of the thiol were allowed to relax. Results – Geometrical configurations Firstly, the physical adsorption of a propane thiol molecule on Au (111) has been analysed. The initial geometry has been build started from the one obtained in our former study on undecanethiol SAMs, which agrees with the results found in other studies. It has been found that the thiols adsorb on different possible sites simultaneously forming unit cells containing up to four chains thiol chains. Our group evidenced on the ab initio level backed up with IR and XPS experiments that at least two adsorption sites are present in the same unit cell. Nevertheless, the energy differences between the different adsorption configurations are very small, and one can approximate the SAMs of thiols on the Au(111) surface being adsorbed on the same type of site, namely a bridge-like site somewhere between the hollow hpc of fcc and the bridge site. In summary, since we will concentrate on the S-H bond cleavage, our model consist of the simple (√3x√3)R30º unit cell containing one thiol chain. The following structure has been found to be the more stable configuration after relaxation when the thiol molecule “physiadsorbes” on gold: It is noticeable that the equilibrium position reached by the sulphur atom of the thiol is practically on top of the Au atom designed in the figure as “13”. However, this equilibrium position (d=3.173 Å) is much far away from the surface in order to form a chemisorbed species by interacting with gold. A similar situation happens with the hydrogen atom. It lies on almost a bridge position between the Au atoms “13” and “15”, but the distance to the surface (> 3.3 Å) is to large in order to interact with the metal to form an adsorbed species. Nevertheless, the gold atoms of the upper layer are softly displaced of their equilibrium sites in comparison to the slab in the absence of the thiol.

Gold atom Distance to the Sulphur atom

Distance to the Hydrogen atom

Au(13) 3.173 Å 3.353 Å Au(14) 4.356 Å 5.265 Å Au(15) 4.470 Å 3.759 Å

15 13 14

13 13

13 13

14

14

14

14

15 15

15 15

In the case where a thiol radical is approached to the gold surface, the equilibrium configuration obtained after relaxation is the following: For this condition, a bridge-like site between the Au “13” and “15” almost shifted to the hollow hpc of fcc sites is the more suitable. The distances to the surface are now much shorter and thus a strong interaction with gold takes place. The shift in the equilibrium position of the gold atoms of the two upper layers is also much stronger than in the case described above. In order to investigate the dissociation of the thiol molecule into the radical thiol and hydrogen atom, we started with the equilibrated physiadsorbed molecule (a) and changed succesively the distance between the hydrogen and the sulphur atoms, fixing their x and y coordinates and relaxing all other coordinates. Up to a distance of about 2 Å the structure did not change significatively. When the starting separation distance between the sulphur and hydrogen atoms is higher, the bond is broken (d) and finally the hydrogen atom adsorbs on a bridge position on gold between the atoms “13” and “15” (f). The thiol approached to the surface pointing the sulphur atom to an almost ontop position on the atom “13” at a distance where the chemisorption can take place. However, if after that all coordinates x, y and z are allowed to relax, the thiol displaced the hydrogen from the surface and chemiadsorbs in the shifted hollow site (e).

Gold atom Distance to the Sulphur atom

Au(13) 2.454 Å Au(14) 3.288 Å Au(15) 2.606 Å

13 14 15

13 13

13 13

14

14

14

14

15 15

15 15

Energetics

Single point calculations have been performed for the dissociation of the thiol molecule approaching to the surface with the vertical orientation. A contour plot of the energy is shown in the figure. The minimum at the right bottom corner corresponds to the equilibrium state of the physisorbed molecule. The minimum at the left top corner corresponds to the equilibrium state of the chemisorbed radical.

S—H : 1.346 Å S—H : 1.752 Å S—H : 2.106 Å

S—H : 2.292 Å S—H : 2.204 Å S—H : 2.761Å

(a) (b) (c)

(d) (e) (f)

Chem Ads

Phys Ads

Electronic Properties The electronic structure has also been investigated. The density of states (DOS) projected on the different atoms of the system has been analysed and they reveal the formation of a chemical bond between the thiol and the gold atoms when the molecule approaches near the surface. The contribution of the different orbitals to the bond can be

inferred. Although we are still analysing these data, some important conclusions can be drawn. The interaction of the different components of the d-band of gold with the sp orbitals of the sulphur and the s orbital of the hydrogen changes for the different geometrical structures. It is well known that a set of three different d orbitals can be distinguished for the surface of the slab in the absence of any adsorbate:

the components projected on the plane of the surface (dxy and dx2-y2) at 45° of the surface (dxz and dyz ) and perpendicular to it (dz2). When an adsorbate is present at the interface, the symmetry is broken and all the components are different. This effect is clearly observed for the chemisorbed species and specially for the nearest gold atom (Au “13”). The effect of the interaction with the d band on the sp orbitals of the sulphur atom in comparison between the physisorbed and chemisorbed species can be observed in the following figure:

-8 -6 -4 -2 00,0

0,2

0,4

0,6

0,8

E - EF / eV

DO

S / e

V -1

dxy dxz dz2 dyz dx2y2

d-bands Au Slab

-8 -6 -4 -2 00,0

0,2

0,4

0,6

0,8

E - EF / eV

DO

S / e

V -1


d-bands Au-13 Phys Ads

-8 -6 -4 -2 00,0

0,2

0,4

0,6

0,8

E - EF / eV

DO

S / e

V -1



-8 -6 -4 -2 00,0

0,2

0,4

0,6

0,8

E - EF / eV

DO

S / e

V -1



-8 -6 -4 -2 00,0

0,2

0,4

0,6

E - EF / eV

DO

S / e

V -1


d-bands Au-13 ChemAds

-8 -6 -4 -2 00,0

0,2

0,4

0,6

E - EF / eV

DO

S / e

V -1



-8 -6 -4 -2 00,0

0,2

0,4

0,6

E - EF / eV

DO

S / e

V -1



The broadening is stronger for the chemisorbed species since according to the Anderson – Newns model the coupling is in this case more important. We have also calculated the components for the different S—H distances:

-10 -8 -6 -4 -2 0 2 40,0

0,2

0,4

E - EF / eV

DO

S / e

v-1

PhysAds

dTot s pTot

-10 -8 -6 -4 -2 0 2 40,0

0,2

0,4

DO

S / e

v-1

E - EF / eV

ChemAds

pTot dTot s

-6 -4 -2 0 2 4 60 ,0

0 ,2

0 ,4

A u -S 3 .1 1

E - E F / e V

DO

S / e

v-1

p y E q u ilP h y s A d s

1 .3 5 1 .4 9 1 .6 5 1 .7 9 1 .9 4

D if fe r e n t S -H

- 6 - 4 - 2 0 2 4 60 , 0

0 , 2

0 , 4

A u - S 3 . 1 1

E - E F / e V

DO

S / e

v-1

E q u i l 1 . 3 5 1 . 4 9 1 . 6 5 1 . 7 9 1 . 9 4

p x D i f f e r e n t S - H

- 6 - 4 - 2 0 2 4 60 ,0

0 ,2

0 ,4

A u - S 3 .1 1

E - E F / e V

DO

S / e

v-1

p z E q u i l 1 . 3 5 1 . 4 9 1 . 6 5 1 . 7 9 1 . 9 4

D i f f e r e n t S - H

bonding

anti-bonding

bonding

bonding

anti-bonding

anti-bonding

In the figure above are also shown the bonding and anti-bonding components related to the S—H bond. Because of the orientation of the adsorbate, the oribitals py does not contribute to this bond. When the distance between S and H increases the bonding components shift to the Fermi level and finally, at distances larger than 2 Å they cross it and become occupied producing the dissociation of the molecule:

The same effect is observed for the s orbital of the hydrogen atom:

Conclusions and Perspectives We have investigated the adsorption of propanethiol on Au(111) through a mechanism where the thiol molecule approaches to the surface with a “vertical” orientation. The geometric structure, the energetic and the electronic properties of this system have been analysed. These results will be shown at the next COST Workshop in Málaga, Spain, and a manuscript for a publication is in preparation. We have also received an invitation to give a conference about this topic on quantum chemical simulations in the Symposium 9 at the next Meeting of the International Society of Electrochemistry in Nice. Next, we plan to investigate other alternative mechanisms such as the approach of the thiol molecule parallel to the surface and in the presence of defects (gold adatoms and vacancies).

- 6 - 4 - 2 0 2 4 60 ,0

0 ,2

0 ,4 E q u i l 1 . 3 5 2 .1 3 2 .2 9

A u - S 3 .1 1

E - E F / e V

DO

S / e

v-1

p yD i f f e r e n t S - H

- 6 - 4 - 2 0 2 4 60 , 0

0 , 2

0 , 4 E q u i l 1 . 3 5 2 . 1 3 2 . 2 9

A u - S 3 . 1 1

E - E F / e V

DO

S / e

v-1

p xD i f f e r e n t S - H

-6 -4 -2 0 2 4 60 ,0

0 ,2

0 ,4

A u -S 3 .1 1

E - E F / e V

DO

S / e

v-1

p z E q u il 1 .3 5 2 .1 3 2 .2 9

D if fe r e n t S -H

-8 -6 -4 -2 0 2 4 60,0

0,1

0,2

0,3

E - EF / eV

DO

S / e

V-1

Equil 1.35 1.49 1.65 1.79 1.94

Diffrent S-H s H

-8 -6 -4 -2 0 2 4 60,0

0,1

0,2

0,3 Equil 1.35 2.13 2.29

Diffrent S-H s H

Mechanisms of hydrogen diffusion on the anatase TiO2(101)

surface: Investigation from the first principles

Introduction

Titanium dioxide, TiO2, is one of the most intensively studied materials due to its wide

range of technological and industrial applications. Unlike most other materials, TiO2 is

reactive with water as well as exhibits an outstanding resistance to corrosion and photo-

corrosion in aqueous environments. Due to these unique properties, TiO2 has a wide range of

potential environmentally friendly applications, such as, for photocatalytic water purification

[1,2,3], for solar hydrogen production [4,5,6,7], as photocatalysts for self cleaning building

materials [1] and for purification of air [1].

There are essentially four polymorphs of TiO2 including anatase (I41/amd), rutile

(P42/mnm), brookite (Pbca), and a high pressure phase. While rutile is thermodynamically the

most stable bulk phase, most nanomaterials are of anatase form [8]. Moreover, below a

particle size of ~ 14 nm, the anatase phase appears to be more stabilized than the rutile phase

which is attributed to the lower surface energy of anatase [9,10]. Anatase has the same

tetragonal symmetry as rutile. Ttitanium atoms are located at (0,1/4,3/8) and (0,3/4,5/8), and

oxygen atoms are located at (0,1/4,z), (0,1/4,1- z), (0,1/4,1/4- z) and (0,1/4,z-1/4); where

z=0.16675 is the internal coordinate. The anatase structure contains four titanium and eight

oxygen atoms in the unit cell with a slightly distorted TiO6 octahedron as shown in Figure 1.

The experimental investigations on single crystalline anatase are just starting.

Meaningful surface science investigations necessitate single-crystalline samples. While rutile

crystals are readily available, sufficiently large and pure anatase crystals are more difficult to

obtain. As anatase is a metastable phase, it transforms into rutile at relatively low

temperatures [8], with the transition temperature dependent on impurities, crystal size and

- 2 -

sample history. In spite of a rapid interest in anatase TiO2, the conclusive study on the anatase

surfaces are still lacking.

The hydrogen interaction with the TiO2 surfaces is less intensively studied so far. It has

been shown that molecular hydrogen does not interact strongly with rutile TiO2(110) surface

[11,12], while atomic hydrogen readily sticks to the surface oxygen atoms [13,14].

Investigation regarding the reducibility of TiO2 surfaces is also limited. Recent theoretical

investigations predict that rutile TiO2 surfaces can be reduced by hydroxylation of the surface

oxygen atoms via adsorption of hydrogen [15,16].

Our present study is motivated by a very recent combined experimental and DFT study

on the hydrogen diffusion on rutile (110) surface [17]. There it was observed that hydrogen

atoms diffuse into the bulk rather than desorbs from the surface. In the present study, the

diffusion mechanisms on the anatase surfaces have been investigated theoretically by using

quantum chemical approaches from the first principles. The adsorption of hydrogen has been

investigated on the various possible positions at the surface and subsurface of anatase

TiO2(101) surface. The investigation has been performed by using periodic DFT and DFT+U

approaches as described in the next section.

2. Computational Methods

All the calculations were performed with the density functional theory (DFT) based

on the generalized gradient approximation (GGA) of Perdew and Wang [18]. This method

was used as implemented in the plane-wave program VASP [19-21]. The projector-

augmented wave (PAW) potentials [22,23] were used for the core electron representation. For

comparison, hydrogen adsorption and diffusion on the TiO2 surfaces were investigated by

using the DFT+U approach, where the value of U was 4. The transition state search for the

diffusion and migration processes was conducted with the nudged-elastic-band (NEB) [24]

method as implemented in VASP.

- 3 -

3. Results and Discussion

3.1 Optimization of bulk anatase TiO2

Full optimization of the lattice parameters and fractional coordinates was performed for bulk

anatase TiO2. The converged values of the optimized lattice parameters (a and c) and Ti-O

bond distances are presented in Table 1 together with corresponding experimental values.

The experimental lattice parameters obtained at 15 K are a=3.782 Å and c=9.502 Å and the

two apical bond lengths (1.979 Å) are greater than the other four basal bond lengths (1.932 Å)

[25].

The calculated lattice parameters a and c for bulk anatase are overestimated by only

1.32 % and 1.70 % respectively with respect to the experimental value. Similarly, the

calculated bond lengths agree well with the experimental values.

3.2 Hydrogen adsorption on anatase TiO2(101)

Based on the optimized structure of bulk TiO2, slab of (101) surface was modeled with

four Ti2O4 layers. A vacuum layer of 15 Å was employed to prevent spurious interactions

between the repeated slabs. The surface contains fivefold- and sixfold-coordinated Ti atoms

and both two-fold and three-fold-coordinated oxygen atoms as shown in Figure 2(a). The

exposed oxygen and titanium atoms of the bottom layer are saturated by the dissociation of

H2O as depicted in Figure 2(b). The hydroxylated bottom Ti2O4 layer was kept fixed while

the three top layers are relaxed during the optimization.

The adsorption of hydrogen was investigated by the attachment of atomic hydrogen

bonded to different surface sites in a (1×1) unit cell. The surface sites considered are the two-

fold coordinated O2C, three-fold coordinated O3C-1 and O3C-2, and subsurface OSub-1 and OSub-2.

The existence of two different O3C and OSub oxygen atoms are related to different O – T bond

distances as explained in Table 2.

- 4 -

The binding energy Eb for the hydrogen bonded to different surface sites are calculated

with the following equation,

Eb = (EHydoxylated-Slab + EH) – EH/Hydoxylated-Slab (1)

where EHydoxylated-Slab and EH/Hydoxylated-Slab are the energy of bottom layer hydroxylated

slab without and with hydrogen adsorbed on the surface respectively, and EH is the energy of

hydrogen. The calculated Eb for the anatase (101) surfaces are compiled in Table 3 along with

those obtained in rutile (110) surface [17]. Our DFT results reveal that single H atoms adsorb

preferentially on top of the bridging two-fold coordinated oxygen atom (O2C) similar to that in

the rutile (110) surface. The next preferential adsorption site would be OSub-2, followed by the

three-fold coordinated oxygen site O3C-2. For the pairs of H atoms, we find that adsorption on

O2C and OSub-2 is the most stable among the all considered co-adsorption sites.

Although the general trend of the nature of hydrogen adsorption and calculated Eb on

the anatase (101) and rutile (110) surfaces are similar, the absolute values of Eb for the anatase

surface are always smaller than those for the rutile surface at the DFT level. These findings

give an indication that hydrogen binding is stronger on the rutile surface than on the anatase

surface.

However, the most interesting part is to find out the fate of the electron(s) due to the

adsorption of hydrogen(s). Because of the well-known self-interaction error, the semi-local

DFT functionals are unable to properly describe the localization of these electrons. The self-

interaction error causes an excessive repulsion between electrons in localized states, often

resulting in a spurious and unphysical delocalization [26]. Therefore, in order to check the

discrepancies in the calculated absolute values of Eb between the anatase and rutile surfaces,

we have employed the DFT+U approach. Here we observe that the binding energy for the

adsorption of hydrogen on both anatase and rutile surface are nearly same (Table 3). It gives

- 5 -

an indication that unpaired electron(s) might be localized on the 3d orbitals of neighboring Ti

atoms of the adsorption sites. A detail investigation of the electronic properties of the

hydrogen adsorbed surfaces is underway.

3.3 Diffusion of hydrogen on anatase TiO2(101) surface

The diffusion of hydrogen on the anatase (101) surface was investigated for the

surface with two adsorbed hydrogen atoms such as O2CH+O3C-1H and O2CH+O3C-2H. Our

investigation (Table 3) reveals that subsurface OSubH is stable species, indicating the

possibility of migration of hydrogen from the surface to the subsurface region. In the present

study, we have considered several migration paths of H atom from the O2C, O3C-1 and O3C-2

sites. In each case, hydrogen can migrate through either a small cavity or a large cavity.

Therefore, there will be six possible H migration pathways from the surface to subsurface

regions. For the sake of brevity, we have explained only the migration pathways of hydrogen

from the O3C-2 site to the subsurface sites in Figure 3. Figure 3(a) shows that H passes

through the small cavity and Figure 3(b) shows that H passes through the large cavity. For all

the migration pathways, hydrogen passes through a barrier which we call as activation barrier

Eact.

The calculations of the Eact are underway. Some of the already finished activation

barriers (Table 4) showed that Eact is not symmetric in the anatase surface as the initial and

final oxygen sites are not equivalent. We define Eact-1 as the energy difference between the

transition state and initial position of the migrating hydrogen and Eact-2 as the energy

difference between the transition state and final position of the migrating hydrogen.

4. Work on the progress

- 6 -

The electronic properties investigation of the hydrogen adsorbed anatase surface and

calculation of activation barriers for hydrogen migration are on the progress with both DFT

and DFT +U approaches.

(1) A. Fujishima, K. Hashimoto, T. Watanabe, TiO2 Photocatalysis. Fundamentals and

Applications; BKC, Inc.: Tokyo, 1999; pp 14-176

(2) A. L. Linsebigler, G. Lu, Jr. J. T. Yates, Chem. Rev. 1995, 95, 735

(3) J. Ryu, W. Choi, Environ. Sci. Technol. 2008, 42, 294.

(4) A. Fujishima, K. Honda, Nature 1972, 238, 37.

(5) J. Nowotny, T. Bak, M. K. Nowotny, L. R. Sheppard, Int. J. Hydrogen Energy 2007, 32,

2609.

(6) J. Nowotny, T. Bak, M. K. Nowotny, L. R. Sheppard, J. Phys. Chem C 2008, 112, 5275

(7) J. Tang, J. R. Durrant, D. R. Klug, J. Am. Chem. Soc. 2008, 130, 13885.

(8) M. R. Ranade, A. Navrotsky, H. Z. Zhang et al., Proc. Natl. Acad. Sci. U. S. A. 2002, 99,

6476.

(9) M. Lazzeri, A. Vittadini, A. Selloni, Phys. Rev. B 2001, 63, 155409.

(10) J. M. C. Amores, V. S. Escribano, G. Busca, J. Mater. Chem. 1995, 5 1245.

(11) V. E. Henrich, R. L. Kurtz, Phys. Rev. B 1981, 23, 6280.

(12) M. Kunat, U. Burghaus, Ch. Wöll, Phys. Chem. Chem. Phys. 2004, 6, 4203.

(13) S. Suzuki, K. I. Fukui, H. Onishi, Y. Iwasawa, Phys. Rev. Lett. 2000, 84, 2156.

(14) T. Fujino, M. Katayama, K. Inudzuka, T. Okuno, K. Oura, Appl. Phys. Lett. 2001, 79,

2716.

(15) P. M. Kowalski, B. Meyer, D. Marx, Phys. Rev. B. 2009, 79, 115410

(16) J. Leconte, A. Markovits, M. K. Skalli, C. Minot, A. Belmajdoub, Surf. Sci. 2002, 497,

194.

(17) X. –L. Yin, M. Calatayud, H. Qiu, Y.Wang, A. Brikner, C. Minot, Ch. Wöll,

ChemPhysChem 2008, 9, 253.

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(18) J.P. Perdew, Y Wang, Phys. Rev. B 45 (1992) 13244.

(19) G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558.



(22) G. Kresse, J. Joubert, Phys. Rev. B 59 (1999) 1758.

(23) P. E. Blöchl, Phys. Rev. B 50 (1994) 17953.

(24) H. Jónsson, G. Mills, K. W. Jacobsen, in Classical and Quantum Dynamics in

Condensed Phase Simulations, edited by B. J. Berne, G. Ciccotti, and D. F. Coker (World

Scientific, Singapore, 1998), p. 385.

(25) J. K. Burdett, T. Hughbanks, G. J. Miller, J. W. Richardson, J. V. Smith, J. Am. Chem.

Soc. 1987, 109, 3639.

(26) A. J. Cohen, P. Mori-Sanchez, W. Yang, Science 2008, 321, 792.

- 8 -

Tables

Table 1 Calculated lattice parameters a, c and bond distances (Å) for bulk anatase TiO2 and

comparison with experimental values.

Properties Calculated Experimental

a 3.83 3.78

c 9.66 9.50

Ti-O (Apical) 2.00 1.98

Ti-O (Basal) 1.96 1.93

Table 2 Calculated O – Ti and O – H distances (Å) for different surface and subsurface

oxygen atoms.

Bonds O2C O3C-1 O3C-2 OSub-1 OSub-2

Before

Opt

After

Opt

Before

Opt

After

Opt

Before

Opt

After

Opt

Before

Opt

After

Opt

Before

Opt

After

Opt

O – Ti1 2.05 1.99 2.05 2.05 1.99 2.01 1.86 1.92 1.97 2.27

O – Ti2 2.16 2.25 2.08 2.05 1.97 2.23 1.93 2.58 1.99 2.11

O – Ti3 2.68 2.53 1.99 2.01 2.13 2.25 1.98 2.18

O – H 1.11 0.98 1.11 0.99 1.11 0.99 1.11 0.99 1.11 0.99

- 9 -

Table 3 Comparison of calculated Eb (eV) for the hydrogen adsorption on the anatase (101)

and rutile (110) surfaces.

Types Anatase (101) Rutile (110)

DFT DFT+U

DFT (a)

DFT+U

1 H: O2C 2.31 2.35 2.52 2.50

1 H: OSub-1 1.37

1 H: OSub-2 2.04 1.90 2.34 1.87

1 H: O3C-1 1.33

1 H: O3C-2 1.65 1.85 1.76 1.83

2 H: O2C+OSub-1 0.89

2 H: O2C+OSub-2 1.53 1.82 1.88

2 H: O2C+O3C-1 1.02

2 H: O2C+O3C-2 1.15 1.56 1.54

2 H: O3C-2+OSub-1 1.35

2 H: O3C-2+OSub-2 1.60 (a)

Reference [17]

Table 4 Calculated activation barriers Eact (eV) for the hydrogen migration on the anatase

(101) surface.

Migrating H Cavity Type DFT DFT+U

Eact-1 Eact-2 Eact-1 Eact-2

O2C-H Small Cavity

Large Cavity 2.25 1.20

O3C-1-H Small Cavity

Large Cavity

O3C-2-H Small Cavity

Large Cavity 1.91 2.05

- 10 -

Figure captions

Figure 1. Tetragonal crystal structure of anatase TiO2 bulk. A 2×2×1 supercell is shown. The red and

blue spheres represent the oxygen and titanium atoms, respectively.

Figure 2. 2×2 supercells of the slab of anatase TiO2(101) surface (a) Bare surface showing various

surface and subsurface oxygen and titanium atoms, (b) Dissociation of H2O at the bottom of the slab.

Figure 3. Migration of hydrogen atoms from the O3C-2 site to the OSub site. (a) Migration through the

small cavity, (b) Migration through the large cavity. In all cases, the O2C surface sites are always

hydroxylated.

- 11 -

Figure 1

Ti

O

- 12 -

Figure 2

a b

O2C

Ti2O4

O3C-1

O3C-2 Ti5C

Ti6C

OSub-1

OSub-2

OH

H

- 13 -

Figure 3

a

b

Initial step

Small Cavity

Large Cavity

Three intermediate steps

Final step

O3C-2

O2C T.S

Initial step

Small Cavity

Large Cavity

Three intermediate steps Final step

O3C-2

O2C T.S

STSM Scientific Report

DFT methods for modelling rutile VSbO4 surfaces and their interaction with gas

phase molecules.

MAY, 15th 2009 to JULY, 15

th 2009

ELIZABETH ROJAS GARCIA

HOST: Université Pierre et Marie Curie

Laboratoire de Chimie Théorique - UMR 7616 UPMC/CNRS

Site Ivry, case courrier 137

4, place Jussieu, 75252 Paris cedex 05

The aim of this STSM was to combine experimental results obtained during ethane and

propane ammoxidation with the theoretical study in order to characterize the surface

reactivity in such system. The calculations were carried out for the adsorption of

reactant molecules (ethane, propane, ammonia and oxygen) approaching the surface

along a reaction pathway parallel or perpendicular to the VSbO4 surface in order to

determine the active sites of the rutile active phase. Such studies provided a detailed

picture of the changes in the electronic structure and VSbO4 surface sites orbitals. In

addition, the study of the adsorption of possible intermediates as propylene, ethylene

and oxygenates, will provide a detailed study about the mechanism pathway during both

ethane and propane ammoxidation reaction.

The investigation of the surface of solids and identification of the mechanism of

heterogeneous catalytic reactions in solid catalysts is difficult due to the presence of a

non uniform chemical composition and to the local structure of the surface sites, with

some of them inactive or catalyzing chemical reactions by different pathways and

according to different mechanisms. This problem is especially critical in mixed-metal

oxides due to (i) the presence of multiple sites, (ii) a surface composition often different

from that of the bulk, (iii) different crystallographic facets and habits, and (iv) a

multicomponent composition. Further specific effort directed towards obtaining a better

understanding of the nature of surface sites in mixed-metal oxides and identification of

their catalytic role with reference to the possible pathways of transformation of reactants

is thus necessary[1].

A basic problem in such studies is understanding the gas-solid interaction (i.e., whether

the gas phase reactants simply chemisorbed at the surface sites as the initial step in the

mechanism of their transformation or whether the chemisorption of reactants also

changes the intrinsic surface reactivity characteristics of the solid). Due to the surface

heterogeneity of oxide catalysts, in the second case it may be expected that the

pathways of surface transformation of reactants are a function of the chemisorption of

the reactants; that is, the reaction mechanism depends on the nature and amount of

adspecies and the reactants induce a self-modification of the surface characteristics.

Acrylonitrile (ACN) is produced nowadays by the ammoxidation of propene on

catalysts made of promoted Fe-Bi-Mo-O (SOHIO process) or promoted Fe-Sb-O (Nitto

process); nevertheless, in recent years some companies have decided to invest in the

research of propane ammoxidation, and one of the more interesting catalytic systems for

the direct ammoxidation of this paraffin is Sb/V/O [2-4]. Catalyst design and/or

optimisation often require a good understanding of the reaction network, and of the

surface reactivity and catalytic chemistry of the catalyst. The lower selectivity and

productivity to acrylonitrile when using propane rather than propene as a feed result

from the different chemisorption properties of these two hydrocarbons and to the

difficulty to activate selectively alkane C–H bonds [5]. The presence of a reactive

function (a π-bond which can lead to an allylic hydrogen) in propene makes possible a

much stronger and specific interaction of the olefin with the surface active sites and thus

favours its faster activation. No specific functionality exists in propane. This

dissimilarity in reactive sites for these two molecules leads to different populations of

ad-species (reactants, intermediates, and spectators) during the catalytic reaction. In the

selective ammoxidation of propene to acrylonitrile, a single dominant reaction pathway

exists whereas for propane ammoxidation, multiple pathways can lead to the products.

A description of the reaction network for the ammoxidation of propane using classical

unsupported mixed metal oxides, e.g., VSb oxides (redox type), has been proposed [6].

The activation of propane occurs initially on Lewis acidic sites and leads to the

formation of propene. Oxygen insertion yields acrolein or an alcoholate intermediate

which is susceptible to ammonia attack, giving eventually acrylonitrile. Ammonia plays

not only an indirect role in the change of the surface pathways of reaction, but also a

direct role as reactant. However, different ammonia adspecies participate in the various

steps of the reaction network of propane ammoxidation. In the absence of ammonia

these ``oxidation products'' are converted to carbon oxide.

Previous studies of our team have shown that VSbO4 is the active phase during

acrylonitrile formation, thus, the rutile-VSbO4 have been selected for the theoretical

study during present STSM [7]. Thus, in this host, we have used density functional

theory using the Perdew−Wang exchange and correlation funtionals PW91 functional

for the prediction of adsorption energies of ammonia, propane, ethylene molecule in the

(110)- VSbO4 cluster. Density functional theory using the Perdew−Wang exchange and

correlation functionals PW91 functional can provide useful information about the

electronic structure of transition metal oxides as well as about the interactions between

the adsorbed hydrocarbon molecule and the catalyst surface. The application of these

methods to the study of toluene–VSbO4 interactions, has allowed us to establish

systematic tendencies, which satisfactory reproduce experimental results.

RESULTS

1. Active sites and adsorption model

In our theoretical calculation the VSbO4 oxide has been modelled using a trirutile

tetragonal super cell (Fig. 1), which contains the most probably metal-oxygen

combinations as reported by Hansen et. al [8]. The lattice parameters obtained are: a=b=

4,674 Å and c´´= 9,373 Å (c´´=3c, c=3,1243 Å).

Fig. 1. Superstructure of VSbO4 with lattice parameters

a= 4,674 Å and c´´= 9,373 Å.

The rutile structure is formed by infinite chains of metal–oxygen octahedra with shared

edges and corners. Each metal center binds to six oxygen atoms (O) while each oxygen

binds to three metal atoms (M). The metal–metal distances in the resulting structure of

coordination 6 : 3 are always relatively long and there is no effective O...O or M...M

interactions. Additionally, open channels parallel to z axis are formed in the crystal. The

(110)- VSbO4 cluster used in our calculations has 72 atoms and a charge of -61. This

electron excess has been assigned to keep the oxygen anions in the oxidation state O2-

.The plane (110) is chosen because it appears to be one of the most stable crystal face of

oxides of rutile and results from breaking the smallest number of M-O bonds. This

surface exhibits an oxygen extra plane and contains the most probable Sb–V

combinations reported by Hansen et al [8].

We have been modeled four alternative arrangement of Sb and V cations, two of which

are similar to the reported ones for the adsorption of toluene on V-Sb oxides [9]. The

hypothetical structures, referred to as SA, SB, SC and SD, exhibit two V –cations

separated by one Sb cation, two neighboring Sb-cations separated by one V ion, only V-

cations and Sb-cations, respectively( Fig. 2).

Fig. 2. Modeled arrangement for (110)-VSbO4 face

The nature of the adsorption process of ammonia, propene, ethene, ammonia + propene,

ammonia + ethane on the (110)-VSbO4 face has been studied performing the calculation

of its adsorption energy. The interaction of these molecules with (110)-VSbO4 face is of

considerable interest for ethane and propane (amm)oxidation reaction. The propane

(amm)oxidation reaction on metal oxides involves complex mechanisms resulting in a

(110)-VSbO4 face

SbSb

V

Sb

VV

OO

SbVV

Sb

VSb

OO SB

VVV

Sb

SbSb

OO SC

SbSb

Sb

V

VV

OO SD

SA

Sb V O

great variety of products under different reaction conditions. The knowledge of these

mechanisms requires not only the characterization of the solid surface properties, but

also information on the transition complex formed. The electronic structure of the

surface is responsible for the interaction and binding with the adsorbate; thus, it is

valuable to determine the role of the different active sites. Also, the modifications in the

electronic structure of the hydrocarbon molecule influence the reactivity of its different

bonds, determining the type of product.

2. Ammonia adsorption on the VSbO4 (110) cluster surface

Ammonia is a reactant in two main classes of reactions of industrial interest: (i) the

ammoxidation of hydrocarbons (alkenes, alkanes, alkylaromatics) to the corresponding

organic nitriles and (ii) the reduction of NO by NH3, in the presence of O2. An ammonia

molecule can be retained on the surface of oxides in several ways: 1) Hydrogen-bonding

via one of its hydrogen atoms to a surface oxygen atom (or to the oxygen of a surface

hydroxyl group), 2) Hydrogen-bonding via its nitrogen atom to the hydrogen of a

surface hydroxyl group (Brønsted acid site) to form an ammonium ion, 3) Coordination

to an electron-deficient metal atom (Lewis acid site), 4) As NH, (or NH) species after

dissociative chemisorption (i.e., after abstraction of one or more hydrogen atoms by

surface oxygen sites to (from) OH species).

In our study of the ammonia adsorption in the VSbO4 oxide we considered the

interaction of ammonia and ammonia dissociation (as NH2 , NH, and N species) on the

(110)-VSbO4 face for the different SA, SB, SC and SD arrangement models. In the SA,

the Sb and V ions sites are explored; in SB, V and Sb ions; in SC, V ions; and in SD, the

Sb ions. However, the formation of OH species after of the abstraction of one or more

hydrogen atoms of ammonia dissociation as NH2, NH and N were localized in the

oxygen extra plane that contains the most probable Sb–V combinations as shows in the

Fig. 3.

During the NH3 adsorption, the energetically most favoured sites on the SA structure,

which exhibits two V– cations separated by one Sb cation, the interaction over a V atom

(i.e A.v site in Fig. 3) is more favourable and the system reached a total energy value

of -32.8 kcal/mol (Fig. 4 and Fig. 5). Ammonia adsorption on V(III) is very exothermic,

which is expected considering the electronically and coordinatively unsaturated V site.

There are several possible pathways for activation of NH3, and the most favourable

pathway is one where both hydrogen atoms of NH3 are transferred to the oxygen extra

plane that contains the most probable Sb–V combinations formed OH species. The

highest barrier for that process is 28.4 kcal/mol. These results suggest that once the

reaction is initiated and V (III) sites start appearing in higher ratios, ammonia will be

activated more rapidly (Fig. 5).

Fig. 3. Activation of NH3 molecule in the SA structure.

The physisorbed states may be tentatively considered as precursors to chemisorptions

and can indicate the presumably initial steps of a catalytic reaction.

A.sb2.o2h.o3h

A.v

A.v.01

A.v.02

A.sb1.o3A.sb1.o2

A.v.03

A.sb2.o2A.sb2.o1

A.v.o2h.03h

A.sb1

A.sb2.o3A.sb1.o1

A,sb1.o2h.o3h

A.sb2 B.v2B.v1

B.v1.o1h

B.v1.o2h

B.v2.o1hB.v1.o3h

B.v2.o2h.o3h

B.v1.o2h.o3h

B.v2.o3hB.v2.o2h

B.sb

B.sb.o1h

B.sb.o2h

B.sb.o2h.o3h

C.v

C.v.h+nh2

C.v.h2+nh

D.sb

D.sb.h+nh2

D.sb.h2+nh

-538,0000

-537,5000

-537,0000

-536,5000

-536,0000

-535,5000

-535,0000

-534,5000

-534,0000

Ad

sorp

tio

n e

ner

gy

(eV

)

SA SB SC SD NH3

Fig. 4. Total energy of the NH3 adsorption in (110)-VSbO4 face.

Different N-species are discussed in the literature to be active in the ammoxidation

reaction on different catalysts. For Fe-Sb oxides Sb-NH-Sb sites have been proposed for

acrylonitrile formation from propene [10], which is generally agreed to be the primary

intermediate in propane ammoxidation. In contrast to this hypothesis, NH, groups are

suggested to be active in the N insertion on Ga-Sb oxides [11]. NH4+, NH3ads, NH2

- and

imido groups (V = NH) are discussed for (VO)2P2O7 [12].

Sb

V

V

O

O

O

V

S b

Sb

O

O

Sb

V

V

N

H

H

H

O

O

O

O V O V O

O

O

Sb

V

V

O

O

O

V

S b

Sb

O

O

Sb

V

V

N

H

H

H

O

O

O

O V O V O

O

O

Sb

V

V

O

O

O

V

S b

Sb

O

O

Sb

V

V

N

H H

H

O

O

O

O V O V O

O

O

S b

V

V

O

O

O

V

S b

Sb

O

O

Sb

V

V

N

H

H

H

O

O

O

O V O V O

O

O

S b

V

V

O

O

O

V

Sb

Sb

O

O

S b

V

V

O

O

O

O V O V O

O

O

0.0

+ NH3

- 32.8

- 19.3

-4,4

65.4

Fig. 5. Potential energy surface for activation of NH3 on V (III) sites, ∆Eads (kcal/mol).

3. Propene adsorption on the VSbO4 (110) clusters surface

Propene is the primary intermediate in the conversion of propane to acrylonitrile [6].

The hydrocarbon approaches to the oxide following three main adsorption routes, with

the methyl group parallel or perpendicular to the catalyst surface and C=C bond centre

in the catalytic sites ( i.e Sb, V ). The different active sites explored in the sequences are

shown in the fig. 2.

3.1 Propene parallel and perpendicular adsorption on the VSbO4 (110) surface

The parallel propene adsorption exhibits the lowest energy for the Sb atoms in the SA

structure. However, sites with V atoms have the same probability for propene molecule

adsorption with very small energy difference. Is important to underline that both

propene and ammonia may adsorb in sites with V atoms in the SA structure; thus, both

molecules compete for V sites (Table 1).

Table 1.Total energy of propene molecule Adsorption in all structures.

Catalytic sites Total Energy

(eV) Catalytic sites Total Energy

(eV) A.sb1.c3h.c3h6paral -566,54372 A.sb1.c3h.c3h6perp -566,24891 A.sb2.c3h.c3h6paral -566,47569 A.sb2.c3h.c3h6perp -566,71460 A.v.c3h.c3h6paral -566,33790 A.v1.c3h6perp -566,39183 B.sb.c3h.c3h6paral -565,84893 B.sb.c3h.c3h6perp -565,69706 B.v1.c3h.c3h6paral -565,82640 B.v1.c3h.c3h6perp -565,62461 B.v2.c3h.c2h6paral -565,75033 B.v2.c3h.c2h6perp -565,68574 C.v.c3h.c3h6paral -564,60955 C.v.c3h.c3h6perp -564,53545 D.sb.c3h.c3h6paral -566,09915 D.sb.c3h.c3h6perp -565,76973

4. Adsorption of ethylene on the VSbO4 (110) surface.

Parallel and perpendicular ethylene interactions with the cluster surface has been made

approaching ethylene to Sb and V sites; for parallel adsorption, the C=C bond centre

binds to the specific sites (see Fig. 6). From the adsorption energy (see Fig. 7) it could

be said that perpendicular adsorption of ethylene on Sb cation has the lowest energy and

it is thus energetically more probable that other sites. However, parallel adsorption of

ethylene with C=C bond centre on V site is energetically possible too.

Fig. 6. Ethylene approaches to the VSbO4 (1 1 0) surface and the catalytic sites on the SA slabs. A) parallel adsorption of ethylene on Sb site, B) perpendicular adsorption of ethylene on Sb site, C) parallel

adsorption of ethylene with C=C bond centre on V site.

A.o2hc2h4.paral

A.o2hc2h4.perp

D.de.sb.paralC.de.v.paral

C.v.perp

C.v.c1.paral

C.v.c2.paral

B.v2.perp

B.de.v1.paral

B.de.v2.paral

B.de.sb.paral

B.v1.perp

B.v1.c1.paral

B.v1.c2.paralB.v2.paral

B.sb.perp

B.sb.C1.paral

A.de.sb1.paral

A.sb2.perp

A.v.perp

A.sb1.perpA.de.v.paral

A.de.sb2.paral

A.sb1.paralA.sb2.paral

A.v.C1.paral

A.v.c2.paral

D.sb.c2.paral

D.sb.c1.paral

D.sb.perp

-550,0000

-549,5000

-549,0000

-548,5000

-548,0000

-547,5000

-547,0000

-546,5000

-546,0000

-545,5000

-545,0000 SA SB SC SDC2H4

Fig. 7. Total energy of the ethylene adsorption in (110)-VSbO4 face.

Conclusion and perspectives

During the present STSM, a number of results concerning the adsorption of single

molecules related to the ammoxidation reaction were obtained. So far there are few

theoretical studies regarding this system, all of them using cluster models. Our

contribution is to employ periodic DFT slab models together with experiments to

provide fundamental information on the ammoxidation process over VSbO4 rutile. The

results obtained are of high quality and will lead to publication in specialized journals as

well as presentation in international meetings.

Reference

[1] Y. Iwasawa, Studies in Surface Science and Catalysis, 101, 1996, 21.

[2] Guerrero-Pérez, M.O., Fierro, J.L.G., Vicente, M.A, and Bañares, M.A., J. Catal.,

206, 339 (2002).

[3] Guerrero-Pérez, M.O., Fierro, J.L.G., and Bañares, M.A Catal. Today, 78, 387

(2003).

[4] Guerrero-Pérez, M.O., and Bañares, M.A., Chem. Matter.,19, 6621 (2007).

[5] D. Fraenkel, Appl. Catal. 67 (1990).

[6] G. Centi and S. Perathoner, CHEMTECH 28 (1998) 13.

[7] M.O. Guerrero-Pérez, M.A. Bañares, Chem. Commun. 12 (2002) 1292

[8] Hansen, S., Ståhl, K., Nilsson, R., and Andersson, A., J. Solid State Chem. 102, 340

(1993).

[9] Irigoyen B., Juan A., Larrondo S., Amadeo N., Surface Science, 523 (2003) 252.

[10] J.D. Burrington, C.T. Kartisek and R.K. Grasselli, J. Catal., 87 (1984) 363.

[11] V.D. Sokolovskii, A.A. Davydov and O.Y. Ovsitser, Catal. Rev.-Sci. Eng., 37(3)

(1995) 425.

[12] G. Centi and S. Perathoner, J. Catal., 142 (1993) 84..

1

Scientific report

Short Term Scientific Mission, COST D36

Molecular structure-performance relationships at the surface of functional materials

Beneficiary: Ana Rita Almeida, TUDelft, The Netherlands

Host: Frederik Tielens, Université Pierre et Marie Curie, Paris, France

Period: from 14/09/2009 to 13/11/2009

Reference code: COST-STSM-D36-05159

Study of cyclohexanone adsorption on TiO2 by DFT calculations

Purpose of the visit

The aim of the Short Term Scientific Mission was to study the adsorption of cyclohexanone

on different planes of hydrated anatase TiO2, by Density Functional Theory calculations.

Calculated adsorption energies and vibrational frequencies were compared with results

obtained experimentally by in situ ATR-FTIR. The nature of cyclohexanone adsorption on

TiO2, the influence of adsorbed H2O on the surface and the effect of TiO2 surface planes,

were analyzed. The obtained insight will aid the understanding of which exposed crystal plane

influences the interaction of products with the TiO2 catalyst. This will aid in the improvement

of the cyclohexanone desorption limitation occurring in the selective photo-catalytic oxidation

of cyclohexane.

2

Computational methods

Geometry optimization and molecular dynamics calculations were performed using the ab

initio plane-wave pseudopotential approach as implemented in VASP (Viena ab initio

simulation package). Periodically repeated calculations with the GGA density functional by

Perdew and Wang PW91 were performed. The valence electrons were treated explicitly and

their interactions with the ionic cores are described by the projector augmented-wave method

(PAW), which allows a low energy cut off equal to 400 eV for the plane-wave basis.

Geometry optimization calculations were performed at 0 K and at 3x2x1 k-point mesh.

Molecular Dynamics were performed at 300 K, 1x1x1 k-points mesh with an energy cutoff of

250 eV, to scan preferred geometries. For both optimization and molecular dynamics

calculations, half of the slab was frozen and only the upper TiO2 layers were able to relax

during the calculation. Vibrational spectra were calculated, with 1x1x1 k-points mesh, for a

limited number of atoms within the harmonic approximation. Only the adsorbed

cyclohexanone molecule and its neighboring/interacting OH groups on the surface were

allowed to vibrate, while the rest of the slab was kept “frozen”. The eigenvalues of the

resulting matrix lead to the frequency values. The assignment of the vibrational modes was

done by inspection of the corresponding eigenvectors in the Molekel software. Dipole

moments were also calculated and used to predict the intensities of each vibrational band.

Anatase (100), (101) and (001) planes were optimized with different extents of adsorbed H2O.

The distribution of water layers over (100), (101) and (001) TiO2 planes was based on

previous work by Arrouvel et al.1

In Table 1 the characteristics of the surface planes, with variable hydration are shown.

Table 1. Dimensions of (100), (101) and (001) unit cells and applied hydration levels.

Planes

Unit cell dimensions Hydration level

(H2O molecules/nm2) a (Å) b (Å) c (Å) A (Å 2)

(100) 7.6 9.5 40 72.5 0 6.2 9.3 18.5

(101) 7.6 10.9 40 83.2 0 4.8 9.6 16.8

(001) 7.4 11.6 40 85.1 0 2.3 7.0 16.4

Cyclohexanone physisorption and chemisorption on different TiO2 planes with variable

hydration levels, was induced with the ModelView software. Physisorbed cyclohexanone is

3

induced by one or more H-bonds between the oxygen atom of cyclohexanone and an

hydrogen atom of a surface active site or adsorbed H2O molecule. Cyclohexanone was

chemisorbed on TiO2 inducing a chemical bond between a free Titanium atom and the oxygen

atom of cyclohexanone.

Results

• H2O adsorption on TiO2

(100) plane

Water chemisorbs on the (100) surface plane at coverage up to 6.2 molecules/nm². Some

water molecules dissociate on free Ti sites and on bridging Ti-O-Ti sites of the bare (100)

surface, with the formation of Ti-OH sites and bridging Ti-OH-Ti sites, respectively.

Undissociated water molecules chemisorb on free Ti sites and form Ti-H2O sites. Free Ti-O-

Ti sites are also present at this hydration level, as can be seen in Figure 1.

Increasing the water coverage to 9.3 molecules/nm² leads to water physisorption on the

bridging Ti-O-Ti sites. The preferred energetic step is through the formation of a protonated

adsorbed water molecule. This protonation is induced by the abstraction of an H+ from a

bridging Ti-OH-Ti site. Furthermore, Ti-OH sites abstract an H+ from Ti-H2O sites, enabling

the formation of long H-bonding network. The strongest adsorption enthalpies of H2O (-73.8

kJ/mol) are obtained under these conditions, providing the most stabilized TiO2 (100) surface

(represented in Figure 1, structure A). A top view of this configuration is shown in Figure 2A,

where two kinds of H-bond chains can be observed: 1) formed by the Ti-O-Ti sites, H3O+ and

adsorbed H2O; and 2) formed by Ti-H2O and Ti-OH sites. At the same hydration level (9.3

molecules/nm²) and in the absence of these proton exchange on the surface, the adsorption

energy of H2O is lower (-61.7 kJ/mol). At these conditions the H-bond network is not favored

as can be seen in Figure 2B.

Increasing the surface hydration to 18.5 molecules/nm², induces the formation of a layered

H2O network, characterized by a lower affinity of water to the surface, of 61.0 kJ/mol.

4

Figure 1. Physisorption energy (kJ/mol) of H2O on a (100) surface as a function of H2O

coverage (molecules/nm2).

Figure 2. TiO2 (100) conformation with 9.3 H2O molecules per nm2: A) after proton exchange

and formation of H-bond networks; B) when proton exchange is absent.

(101) plane

Figure 3 shows the geometries of H2O adsorption on (101) TiO2 at different hydration levels

and its corresponding adsorption energies. Water chemisorbs without dissociation on the

(101) plane up to a surface coverage of 4.8 molecules/nm². For this coverage, an adsorption

-100

-80

-60

-40

-20

00 5 10 15 20

H2O/nm2E

ads

(kJ/

mol

)

Phys. H3O+ Phys. H2O

Ti-H2O Ti-OH Ti-H2O Ti-OH A B

A B

5

enthalpy of -65.7 kJ/mol was determined with the formation of only Ti-H2O, leaving bridging

Ti-O-Ti oxygen sites still free on the surface.

A surface hydration increase to 9.6 H2O molecules per nm2 is characterized by a similar

adsorption enthalpy, but the extra water molecules are now physisorbed on the Ti-O-Ti sites,

which are hydrogen bonded to neighboring Ti-H2O sites. A decrease in adsorption enthalpy (-

57.1 kJ/mol) is observed at higher hydration levels, where a multilayer of adsorbed water is

formed.



(001) plane

The (001) surface plane in its bare state (OH free) contains Ti-O-Ti sites with the oxygen

atom at a higher plane than the Ti atoms. Upon hydration one of the Ti-O-Ti bonds breaks and

two Ti-OH sites are created. Of these two new OH sites, one contains an oxygen atom

originally from the TiO2 lattice structure. Under low surface hydration (2.3 molecules/nm²),

the created Ti-OH sites are H-bonded, and free Ti-O-Ti sites are still present1,2, as can be seen

in Figure 4. On this TiO2 plane the energy for dissociative water chemisorption is much

higher than in other planes (-184.7 kJ/mol). At a surface hydration level of 7.0 molecules/nm²

undissotiative H2O physisorption occurs, with H2O molecules H-bridging on the Ti-O-Ti

sites, and a reduction of the H2O adsorption strength to -97.4 kJ/mol. The formation of H2O

-80

-60

-40

-20

00 5 10 15 20

H2O/nm2

Ead

s (k

J/m

ol)

6

multi layers on the surface, at coverages of 16.4 H2O molecules per nm2 is characterized by

an adsorption energy of -72.6 kJ/mol.



From the calculations on H2O adsorption on different TiO2 planes it can be concluded that the

(001) surface is the most hydrophilic plane, with chemisorption energies three times larger

than for the (100) and (101) planes. The (101) plane, which is the most stable and therefore

the less reactive one, does not dissociate H2O upon chemisorption. The (100) shows more

variety in active sites and under intermediate hydration levels extensive networks of H-

bonded sites and adsorbed H2O molecules can be formed.

• Cyclohexanone adsorption on TiO2

(100) plane

The calculated energies for cyclohexanone physisorption and chemisorption on the (100) TiO2

plane are summarized in Table 2, for different extents of water adsorption. The calculated

H2O adsorption energies (Figure 1) are also shown for comparison.

-200

-150

-100

-50

00 5 10 15 20

H2O/nm2

Ead

s (k

J/m

ol)

7

Table 2. Energies of cyclohexanone physisorption and chemisorption on the (100) TiO2 plane

at different levels of surface hydration, and the corresponding H2O adsorption energies.

H2O/nm2 ECnone phys (kJ/mol) ECnone chem (kJ/mol) EH2O ads (kJ/mol)

0 --- -45.6 ---

6.2 -48.8 -15.2 -68.5

9.3 (A) -121.4 --- -73.8

9.3 (B) -89.4 -51.4 -61.7

18.5 -34.0 > 0 -61.0

Cyclohexanone physorption energies are represented in Figure 5, with an image of the

corresponding adsorption conformation. On the (100) plane, adsorption of cyclohexanone is

enhanced at an intermediate hydration level of 9.3 H2O molecules per nm2. Especially for

configuration A, the formation of a water H-bond network between basic Ti-OH active sites

and physisorbed H2O molecules is established (as shown in Figure 2A), leading to an increase

in the adsorption strength of cyclohexanone on the Ti-OH site. In the absence of these H-bond

networks (configuration B), cyclohexanone adsorption occurs mainly by H-bonding with a Ti-

H2O site and an adsorbed H2O molecule, and is less strong than in configuration B. At lower

hydration levels (6.2 H2O/nm2), the adsorption of cyclohexanone on a Ti-OH site is

considerably reduced. At higher hydration levels (18.5 H2O/nm2), cyclohexanone is not able

to adsorb directly on a TiO2 active site, being pushed away by the H2O layers which show

higher affinity to the surface.

8

Figure 5. Physisorption energy (kJ/mol) of Cyclohexanone on a (100) TiO2 surface as a

function of H2O coverage (molecules/nm2).

Calculations on cyclohexanone chemisorption on bare (100) planes show that the energy

involved in the Ti-O bond is low, in the order of -50 kJ/mol, as can be seen in Figure 6. The

occurrence of cyclohexanone chemisorption is negligible at low hydration levels (6.2

molecules/nm2). The length of the Ti-O bond formed upon cyclohexanone chemisorption,

increases with surface hydration and at levels above 9.3 H2O molecules/nm2, cyclohexanone

chemisorption is not favored anymore.

Figure 6. Chemisorption energy (kJ/mol) of Cyclohexanone on a (100) TiO2 surface as a


-200

-150

-100

-50

00 5 10 15 20

H2O/nm²E

ads

(kJ/

mol

)

-80

-60

-40

-20

00 2 4 6 8 10

H2O/nm²

Ead

s (k

J/m

ol)

A

B

9

Comparing the calculated energies in Table 2, it can be observed that in the (100) plane,

physisorption will be preferred over chemisorption at all H2O coverages. Furthermore, water

adsorbs generally more strongly on (100) TiO2 except for intermediate hydration levels (9.3

H2O molecules per nm2), where cyclohexanone adsorption is able to displace H2O molecules

on the surface.

Frequencies

The vibration frequencies of cyclohexanone in the (100) unit cell, were determined and based

on the carbonyl frequency, a scaling factor was estimated:

C Oexperimental 1720Shift factor 0.990C Ocalculated 1738=

= = ==

This factor was applied on all calculated frequencies for the (100) surface. However, this

factor is only accurate for carbonyl vibrations, and in order to discuss other types of

vibrations, specific scaling factors should be estimated3. Furthermore this factor depends on

the size of the unit cell (Table 1), so a new factor will be calculated for each TiO2 plane.

The most favored configurations of cyclohexanone adsorption on (100) TiO2 plane, at each

hydration level are represented in Table 3 by a grey shading. In Figure 7, the calculated

frequencies of theses configurations are compared with a cyclohexanone adsorption spectrum,

obtained experimentally by ATR-FTIR.

10

Figure 7. Comparison between an adsorbed cyclohexanone reference spectrum, measured by

ATR-FTIR, and the spectra of cyclohexanone physisorbed on a (100) TiO2 at different

hydration levels, as determined by DFT calculations.

The experimental spectrum contains three main peaks in the carbonyl range, at 1720, 1694,

1681 cm-1, related to desorbed cyclohexanone and to two different adsorption states of

cyclohexanone, respectively. Other adsorption geometries can be expected as some small

shoulder peaks can be observed below the 1681 cm-1 band. The negative contribution at 1630

cm-1 is related to the displacement of H2O from the surface, during cyclohexanone adsorption.

The peaks observed below 1600 cm-1 can relate to a particularly strong adsorption with

redshifts above 100 cm-1. The spectra calculated by DFT, show carbonyl vibrations in the

region of the experimental peak observed at 1681 cm-1 and the small shoulders observed

below. At a hydration level of 9.3 H2O molecules/nm2, two configurations were proposed,

but configuration A, of higher adsorption energy doesn’t fit the experimental spectrum as well

as configuration B. So it is possible that theoretically the (100) surface is able to exchange

protons in the surface and create long H-bond networks that stabilize cyclohexanone

adsorption, but the experimentally it does not occur.

In general, the stronger the cyclohexanone adsorption, the larger is the redshift observed in

the carbonyl peak, as can be observed in Figure 8. This trend is not followed by configuration

B at 9.3 H2O molecules per nm2, so it is possible that the redshift of the carbonyl peak is

1500155016001650170017501800

Cyclohexanone reference

Phys. (100) 6.2 H2O/nm2

Phys. (100) 9.3 H2O/nm2 A

Phys. (100) 9.3 H2O/nm2 B

Phys. (100) 18.5 H2O/nm2

C=O str. H2O bend.

Ti-H2O bend.

1681

1694

1720

11

related not only to the adsorption strength but also to the nature of the adsorption site and to

the extent of H-bond network on the surface.

The calculated water bending vibrations appear at very low wavenumbers, when compared

with experimental results. Even without applying the scaling factor, the H2O bending

vibrations occurs 30-60cm-1 redshifted to the usual range observed. This is probably due to

the interaction of surface H2O molecules with adsorbed cyclohexanone or H2O layers (at the

highest hydration level).

The calculated infrared vibrations do not fully represent the experimentally obtained IR

adsorption spectrum. Though two main contributions were calculated for the carbonyl region

(1683 cm-1 for the yellow spectum and around 1672 cm-1 for the blue spectra), they are

redshifted when compared to the experimental reference. Furthermore, the calculated

frequencies don’t fit the two peaks below 1600 cm-1 in the experimental spectrum. Since this

is a typical region for adsorbed carboxylic acid vibration, these two bands could relate to such

a group as a result of cyclohexanone oxidation on a strongly adsorbed configuration. This

calculation was however not contemplated in this study.

Figure 8. Wavenumbers of carbonyl vibrations as a function of cyclohexanone adsorption

strength on TiO2 (100) plane, with different hydration levels.

-150

-100

-50

0164016501660167016801690

wavenumber (cm-1)

Eads (kJ/m

ol)

12

(101) plane



adsorption energies of H2O (Figure 3), are also shown for comparison.



H2O/nm2 ECnone phys (kJ/mol) ECnone chem (kJ/mol) EH2Oads (kJ/mol)

0 --- -45.4 ---

4.8 -23.5 -48.9 -65.7

9.6 -37.0 -31.2 -67.1

16.8 -62.1 > 0 -57.1

Cyclohexanone adsorption on a (101) active site at 4.8 H2O molecules per nm2, is relatively

weak. At a higher hydration level (9.6 H2O/nm2), the presence of neighboring adsorbed H2O

molecules, allow for further stabilization of cyclohexanone adsorption by H-bonding (Figure

9). Direct adsorption of cyclohexanone on the a TiO2 active site at 16.8 H2O molecules/nm2 is

not favored, with the cyclohexanone molecules being pushed away to top of the water layer,

where its adsorption is most stable. As in the (100) plane, the adsorption enthalpy is

considerably increased when the cyclohexanone is H-bonded to a network of adsorbed H2O.

In Figure 9, the increasing stabilization of cyclohexanone H-bonding to adsorbed water

molecules can be observed.

13

Figure 9. Physisorption energy (kJ/mol) of Cyclohexanone on a (101) TiO2 surface as a


In the (101) plane cyclohexanone chemisorption competes with physisorption as the energies

for both interactions are in the same range. In particular for low hydration levels (4.8 H2O

molecules/nm2), cyclohexanone chemisorption in the TiO2 (101) surface will be energetically

preferred.

In general, cyclohexanone adsorption on (101) TiO2 active sites is weaker than water

adsorption, so cyclohexanone is not able to displace H2O molecules from the (101) surface.

-80

-60

-40

-20

00 5 10 15 20

H2O/nm²Ea

ds (k

J/m

ol)

14

Figure 10. Chemisorption energy (kJ/mol) of Cyclohexanone on a (101) TiO2 surface as a


Frequencies




= = ==

The most stable configurations of cyclohexanone adsorption on (101) TiO2, for each

hydration level are represented by a grey shading in Table 4. In Figure 11, the calculated

spectra of these configurations are shown. The calculated carbonyl vibrations occur in the

same region of the 1681 cm-1 peak and shoulders, of the experimental reference.

Cyclohexanone physisorption at 9.6 H2O molecules/nm2 hydration show the best relation to

the experimental spectrum. The bending vibrations of H2O occur once again redshifted,

probably due to H-bonding interactions with other adsorbents.

Similarly to the (100) plane, the calculated infrared vibrations do not fully represent the

experimentally obtained IR adsorption spectrum.

-60

-50

-40

-30

-20

-10

00 2 4 6 8 10

H2O/nm2E

ads

(kJ/

mol

)

15

Figure 11. Comparison between an adsorbed cyclohexanone reference spectrum, measured by

ATR-FTIR, and the spectra of cyclohexanone physisorbed/chemisorbed on (101) TiO2 at

different hydration levels, as determined by DFT calculations.

(001) plane



adsorption energies of H2O (Figure 4), are also shown for comparison.



H2O/nm2 ECnone phys (kJ/mol) ECnone chem (kJ/mol) EH2O ads (kJ/mol)

0 --- --- ---

2.3 -19.6 -78.8 -184.7

7.0 -12.4 --- -97.4

16.4 --- --- -72.6

1500155016001650170017501800


Chem. (101) 4.8 H2O/nm2

Phys. (101) 9.6H2O/nm2

Phys. (101) 16.8H2O/nm2

C=O str.

Ti-H2O and ads. H2O bend

1681

16

Physisorption of cyclohexanone on the (001) TiO2 surface is very weak, below -20 kJ/mol at

low hydration levels. At 2.3 H2O molecules per nm2 a chemisorbed configuration is preferred

over the physisorbed one. In Figure 12, the energies and their correspondent configurations

are shown.

Figure 12. Physisorption and Chemisorption energies (kJ/mol) of Cyclohexanone on a (001)

TiO2 surface as a function of H2O coverage (molecules/nm2).

Frequencies




= = ==

The calculated and corrected spectra, show carbonyl vibrations in the range of the 1694 and

1681 cm-1 experimental bands. The 1694 cm-1 is therefore related to a rather weak adsorption

of cyclohexanone. The calculated adsorbed H2O band is in the same region of the negative

vibration of H2O in the experimental spectrum.

The calculated infrared vibrations don’t fully represent the experimentally obtained IR

adsorption spectrum, but represents better than the (100) and (001) plane the existence of two

main adsorption configurations.

-100

-80

-60

-40

-20

00 2 4 6 8

H2O/nm2

Ead

s (k

J/m

ol)

17

Figure 15. Comparison between an adsorbed cyclohexanone reference, measured by ATR-

FTIR, and the spectra of cyclohexanone physisorbed and chemisorbed on (001) TiO2 at

different hydration levels, as determined by DFT calculations.

Conclusions

The (100) plane shows the highest affinity towards cyclohexanone adsorption, which occurs

mainly through physisorption. This interaction is greatly increased when long H2O networks

are formed on the surface, especially with the formation of protonated H2O species on the

surface. However this strongly adsorbed cyclohexanone configuration does not agree with the

experimental adsorption results. At the same hydration level, an adsorption configuration of

slightly lower stability showed higher agreement with the experimental results. Furthermore,

in the (100) plane, displacement of water molecules from the surface by cyclohexanone

adsorption was found to be possible. From this, the (100) plane seems to represent well the

reactivity of TiO2 material used experimentally. However, this plane does not fully explain

the experimental spectrum of adsorbed cyclohexanone, contributing only to the strongest

adsorption bands. A mixture of different planes in the experimentally used TiO2 material may

be considered.

1500155016001650170017501800


Chem. (001) 2.3H2O/nm2

Phys. (001) 7.0H2O/nm2

ads. H2O bend

1681

1694

C=O str.

18

Perspectives

This short term scientific mission yielded further knowledge on the cyclohexanone adsorption

on hydrated TiO2 and this knowledge will be used for a publication, which gathers theoretical

work performed in the Universitet Pierre et Marie Currie and experimental work performed in

TUDelft.

References

(1) Arrouvel, C.; Digne, M.; Breysse, M.; Toulhoat, H.; Raybaud, P. Journal of Catalysis 2004, 222, 152-166.

(2) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gratzel, M. Physical Review Letters 1998, 81, 2954-2957.

(3) Islam, M. M.; Costa, D.; Calatayud, M.; Tielens, F. Journal of Physical Chemistry C 2009, 113, 10740-10746.

COST D36/WG06/0006/06

Reference of this STSM Request: COST-ONLINE_STSM-D36-5153

Period: from 15/10/2009 to 15/12/2009

Applicant: Dr. Noelia Luque, Institüt für Theoretische Chemie, Unverstät Ulm, Germany

Host: Dr. Monica Calatayud - Dr. F. Tielens, Université Pierre et Marie Curie, Laboratoire de

Chimie Théorique, Paris, France

Scientific report

Investigation of the adsorption mechanism of alkyl thiols on Au(111)

surfaces, using theoretical ab initio tools.

The aim of Short Term Scientific Mission is to continue [1,5] the investigation of the

adsorption process of thiol molecules on gold surfaces.

Immersion of a gold surface in thiolate solution leads to a spontaneous adsorption of thiols

giving as result a self-assembled monolayer (SAM). These types of layers have attracted

much attention because they may constitute ideal platforms for further binding or reactivity

[6]. In the framework of elaborating adjusted surface functionalities for biocompatibility,

biosensors or molecular electronics, special effort has been made to form two-component

monolayers, one of the aims being to avoid steric hindrance of functional tail groups or

disorder. Controlling the dispersion of the SAM domains makes it possible to control the

dispersion of e.g. biological systems to be attached on the SAM.

Concerning the initial step in the SAM formation still some questions remain unanswered

[7,22]. After decades of arguing about the precise site of adsorption of the thiol chains on the

gold surface one start to have a picture of the sorption process. Nevertheless, since

chemisorption is generally accepted above physisorption of the thiol molecule, the reaction

mechanism is not known completely. Some theoretical studies have shed some light on the

problem. In the present work we report some advances in the study of adsorption mechanism

of 1-propylthiol on Au(111).

Methodology

The calculations were carried out within the framework of Density Functional Theory (DFT)

with a plane-wave basis set, encoded in the Vienna Ab initio Simulation Package (VASP). In

the periodic DFT framework used, the Kohn-Sham equations were solved by means of the

Perdew-Burke-Ernzerhof (PBE) functional. The electron-ion interactions were described by

the Projector Augmented-Wave method (PAW). The optimizations were performed at a

convenient k-point mesh set for the Brillouin-zone integration with an energy cutoff of 400

eV. The different k-point mesh sets were customized for the several sized gold slabs. Table 1

shows the different set used.

The gold surface, Au(111), was modeled using different size slab. Each slab contains 5 gold

layers of which the two bottom ones were fixed to the bulk positions. The cell parameters

were obtained after optimization of the gold structure in the bulk. We used 15 Angstroms of

vacuum between the slabs in the z direction. Two types of cells were used, see figure 1. The

first type was based on the cell 3 3 R 30 and its derivatives, as double size in one

direction or in both, x and y direction. The second type of cells was straight on the first next

neighbor distance in the face centered cubic cell on gold bulk. The main key of different slab

cells was to emulate several coverage degree in the Au(111) surface.

The calculations were perform not only on perfect Au(111) surface but also on different

surface defects. There were two types of defects, vacancies and adatoms on the Au(111)

surface.

The atom positions of the adsorbed molecule (1-propylthiol or 1-propylthiolate) together with

the upper three layers were relaxed without geometrical constraints (optimized at 0K).

Size cell

3 3 R 30 2 3 3 R 30 2 4 2 5 2 3 2 3 R 30

k-points

mesh

7 7 1 6 3 1 3 6 1 3 5 1 3 3 1

Coverage

( )

1 0.5 0.38 0.3 0.25

Table 1. Different k-point mesh used for the different sized cell used in the calculations. The coverage, ,

is obtained by 1 thiol every 3 Au atoms.

Figure 1 Different sized cell used in the calculations. (a) 3 3 R 30 ,(b) 2 3 3 R 30 , (c)

2 3 2 3 R 30 , (d) 2x4, (e) 2x5.

The adsorption or binding energy, Eb, was calculated by:

Eb = Eb - EAu – Em (1)

Where Et, EAu and Em are the total energy of the system, the slab, and the isolated molecule,

respectively.

Although there are several kinds of propose in the initial step of SAMs formations, there is no

experimental result that proves in which reaction step the thiol loses its hydrogen to adsorb as

a thiolate on the surface. In several papers it is postulated that the thiol molecules lose its H in

a striped structure at low coverage. In this sense, our calculation in the lying down state were

performed and compared with the standing up state in order to make it clear whether the thiol

or the thiolate molecule is the most stable structure on the surface. Very resent results show

that surface defects could play important roles in the adsorption structures and resulting

domains, therefore in this work were performed calculations at perfect surface and two kinds

of surface defects.

Results and Discussion

In order to investigate the adsorption mechanism of thiols on Au(111), a good understanding

of the initial and final state is a critical point, therefore it is crucial in the goals of our studies

to analyze in detail and before to start with reaction mechanism itself different thiols and

thiolates coverage in its minimal energy structures.

We have calculated adsorption energies for 54 systems. Each type of cells represents different

coverage, and at each coverage there are three different kind of system, the first is a perfect

surface, and then comes two different surface defects, the vacancy and adatom on the surface.

It is worth mention that several stable or meta-stable structures could exist depending on the

tilt angle of the thiol chain with the surface. Our study was limited to the two extreme cases

and they are going to be mentioned as standing up (SU) and lying down (LD) systems. At low

coverage the molecule could be in both state but the lying down state become unfavorable

while the coverage increase due to the repulsion between the chains.

When the slab is a free defect surface, we will refer to that system as flat surface systems. The

other systems will be mention, depending on the kind of defect, as vacancy or adatom

systems.

It was found that the thiol and thiolate present different conformation in the same kind of

surface. In the case of the flat surface, the thiol prefers a lying down structure at low coverage

(see figure 2), while the thiolate is more stable in a standing up structure independent of the

coverage as it is shown in Figure 3. A similar behavior is observed in the case of a vacancy in

the surface as it is shown in the Figure 4 and 5. For a surface with an adatom, in the case of

the thiol, it present similar stabilities, that means that at low coverage the lying down state is

energetically favorable, as it is shown in Figure 6, on the other side, the thiolate, it presents a

cross in the stabilities even at low coverage (around 0.4), see figure 7.

It is clear in our results that there are several local minima in the potential energy surface of

the adsorption, that means that it could be possible to find several meta-stable state at

different distance S-Au or even at different tilt angle of the molecule chain.

And it is clear that for a better conformational determination in the range of low coverage it is

necessary to perform the calculations with a better description of the Van der Waals forces,

the last version of VASP 5.2, providing new functionalities such as the use of hybrid

functionals, may be used to improve the quantum description of the models.

1-propylthiol on flat surface

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0 0.2 0.4 0.6 0.8 1 1.2

Coverage

EbStanding Up

Lying Down

Figure 2 Adsorption or binding energy for 1-propylthiol on a flat surface (defect free surface) as

function of the thiol coverage.

1-propylthiolate on flat surface

-2.5

-2

-1.5

-1

-0.5

0

0 0.2 0.4 0.6 0.8 1 1.2

Coverage

EbStanding Up

Lying Down

Figure 3 Adsorption or binding energy for 1-propylthiolate on a flat surface (defect free surface) as

function of the thiol coverage.

1-propylthiol on vacancy surface

-0.5

-0.45

-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0 0.2 0.4 0.6 0.8 1 1.2

Coverage

EbStanding Up

Lying Down

Figure 4 Adsorption or binding energy for 1-propylthiol, on a surface with an a vacancy, as function of

the thiol coverage.

1-propylthiolate on vacancy surface

-3

-2.5

-2

-1.5

-1

-0.5

0

0 0.2 0.4 0.6 0.8 1 1.2

Converage

EbStanding Up

Ling Down

Figure 5 Adsorption or binding energy for 1-propylthiolate, on a surface with an a vacancy, as function

of the thiol coverage.

1-propylthiol on adatom surface

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0 0.2 0.4 0.6 0.8 1 1.2

Coverage

EbStanding Up

Lying down

Figure 6 Adsorption or binding energy for 1-propylthiol, on a surface with an adatom, as function of the

thiol coverage.

1-propylthiolate on adatom surface

-2.5

-2

-1.5

-1

-0.5

0

0 0.2 0.4 0.6 0.8 1 1.2

Coverage

EbStanding Up

Lying Down

Figure 7 Adsorption or binding energy for 1-propylthiolate, on a surface with an adatom, as function of

the thiol coverage.

Conclusions and Perspectives

We have investigated the different conformational state in the adsorption of 1-propanethiol

and 1-propanothiolate on Au(111). The geometric structure and the energetic of several

systems were analyzed and the electronic properties will be analysed. These are the

preliminary steps in the reaction mechanism of the SAMs formation and will be used for to

continue the studies on the reaction mechanism path in the SAMs formation. Next, we plan to

keep on investigate the concerted mechanism for the adsorption of two thiols per unit cell with

different geometries. The reaction path will be optimized and the transition states will be

obtained through the Elastic Nudged Band Method (NEB).

The results obtained in the framework of this project, are the extension of a longstanding

collaboration between both groups interested in surface phenomena. In particular, the focus of

the investigation by means of computational chemistry tools of the adsorption/reaction processes

on surfaces observed in heterogeneous catalysis and electrochemistry is the aim of this

cooperation.

References

[1] Tielens F., Humblot V., Pradier C.-M., Int. J. Quantum Chem. 108 (2008) 1792.

[2] Tielens F., Costa D., Humblot V., Pradier C.-M., J. Phys. Chem. C 112 (2008) 182.

[3] Doneux T., Tielens F., Geerlings P., Buess-Herman C., J. Phys. Chem. A 110 (2006) 11346.

[4] Tielens F., Humblot V., Pradier C.-M., Surf. Sci. 602 (2008) 1032.

[5] Tielens F., Humblot V., Pradier C-M., Calatayud M., Illas F., Langmuir 25 (2009) 9980.

[6] Love J.C., Estroff L.A., Kriebel J.K., Nuzzo R.G. and Whitesides G.M., Chem. Rev. 105 (2005) 1103.

[7] Schreiber F., Prog. Surf. Sci. 65 (2000) 151, Schreiber F., J. Phys. Condens. Matter 16 (2004) R881.

[8] Castner D.G. and Ratner B.D., Frontiers in Surface and Interface Science ed C.B. Duke and E.W. Plummer

(Amsterdam: North-Holland) 2002, p 28.

[9] Ulman A., An Introduction to Ultrathin Organic Films: from Langmuir–Blodgett to Self-Assembly (San

Diego, CA: Academic) 1991, Ulman A., Chem. Rev. 96 (1996) 1533.

[10] McGuiness C.L., Shaporenko A., Mars C. K., Uppili S., Zharnikov M. and Allara D. L., J. Am. Chem. Soc.

128 (2006) 5231.

[11] Pavlovic E., Quist A. P., Gelius U. and Oscarsson S., J. Colloid Interface Sci. 254 (2002) 200. Pavlovic E.,

Oscarsson S. and Quist A. P., Nano Lett. 3 (2003) 779.

[12] Whitesides G. M. and Grzybowski B., Science 295 (2002) 2418. Gates B. D., Xu Q., Stewart M., Ryan D.,

Grant Wilson C. and Whitesides G. M., Chem. Rev. 105 (2005) 1171.

[13] Alivisatos A. P., Barbara P. F., Castleman A. W., Chang J., Dixon D. A., Klein M. L., McLendon G. L.,

Miller J. S., Ratner M. A., Rossky P. J., Stupp S. I. and Thompson M. E., Adv. Mater. 10 (1998) 1297.

[14] Aviram A. and Ratner M., Molecular Electronics: Science and Technology (New York: NY Acad. Sci.) (ed)

1998.

[15] Kurth D. G., Lehmann P. and Schütte M., Proc. Natl Acad. Sci. USA 97 (2000) 5704.

[16] Pflaum J., Bracco G., Schreiber F., Colorado R. Jr., Shmakova O. E., Lee T. R., Scoles G. and Kahn A.,

Surf. Sci. 498 (2002) 89.

[17] Bain C. D., Troughton E. B., Tao Y-T., Evall J., Whitesides G. M. and Nuzzo R. G., J. Am. Chem. Soc. 111

(1989) 321.

[18] Cunningham A., Introduction to Bioanalytical Sensors 1998 (New York: Wiley–Interscience)

[19] Allara D. L., Dunbar T. D., Weiss P. S., Bumm L. A., Cygan M. T., Tour J. M., Reinerth W. A., Yao Y.,

Kozaki M. and Jones L., Ann. New York Acad. Sci. 852 (1998) 349.

[20] Briones C. and Martin-Gago J. A., Curr. Nanosci. 2 (2006) 257.

[21] Yu M., Woodruff D. P., Bovet N., Satterley C.J.. Lovelock K., Jones R.G. and Dhanak V., J. Phys. Chem. B

110 (2006) 2164.

[22] Vericat C.; Vela M. E.; Benitez G. A.; Matin Gago J. A.; Torrelles X.; Salvarezza R. C., J. Phys.: Condens.

Matter 18 (2006) R867.

STSM 2009 (Ref: COST-STSM-D36-05462) Scientific Report Author: Jordi Morros Camps

Title: Association between hydrophobically modified inulin (HMI) and

didodecyldimethylammonium bromide (DDAB).

Summary

We have studied the rheological behavior of the modified and unmodified inulin

polymer aqueous solutions, and the association activity between hydrophobically

modified inulin (HMI) and the vesicle-forming surfactant (DDAB). The interesting

results obtained during this research have increased our motivation to state new

applications of this kind of green polymers.

Introduction

Polymeric surfactants or surface active polymers, have gained in popularity during the

last two decades thanks to its characteristic features differ from low molecular weight

surfactants. They are now used commercially and represent an important class of

chemicals involved in many colloidal dispersed systems as stabilizers for oil in water

(emulsions), solid in liquid (dispersions) or rheological control.

Interaction between surface active hydrophobically modified polymers and surfactants

has been intensively investigated, with the main focus on surfactant micelles as the

surfactant aggregate in interaction. The main types of phase behaviour, driving forces

and structural/rheological effects at stake are now fairly well understood. Polymer–

vesicle systems, on the other hand, have received comparatively less attention from a

physico-chemical perspective.

Neutral hydrophobic β-hydroxyalkyl ethers of inulin with surfactant activity have been

prepared by our group under friendly environmental conditions. Since hydrophobic

modification (long chain etherification of different length) of inulin (biopolymer) could

be done in water media, a biological interest has emerged to know the capability of

these HMI ether derivatives to interact with surfactant vesicles. In this work, we will

cover different aspects of the associative behaviour between vesicles of DDAB and

these synthesized HMI polymers. We will address the effect of the type of hydrophobic

inulin modification (octyl, dodecyl or tetradecyl chains) and DDAB vesicles. The

results have been compared to those of the commercial analogue Inutec® SP1.

Experimental Part

Materials

The purified inulin was supplied by ORAFTI Bio Based Chemicals (Tienen, Belgium)

at present BENEO-BBC, with a main degree of polymerization of about 25 whose

commercial name is INUTEC®N25. It was dried at 70oC during 24 hours before use.

Inutec® SP1 supplied by BENEO-BBC, was used as reference of HMI. The cationic

surfactant DDAB was used to form vesicles. The other chemicals were from Aldrich

(St.Quentin Fallavier, France) and were used as received. MiliQ water was used for all

the experiments.

Preparation of Neutral hydrophobic β-hydroxyalkyl ethers of inulin

β-hydroxyoctyl ether of inulin (EC8), β-hydroxydodecyl ether of inulin (EC12) and β-

hydroxytetradecyl ether of inulin (EC14) as HMI ether derivatives were prepared in our

laboratory as follows: Hydrophobic inulin derivatives were prepared by reacting inulin

(INUTEC®N25) with 1,2-octylepoxide, 1,2-dodecylepoxide or 1,2-tetradecylepoxide

using with KOH as a basic catalyst in water-surfactants systems.

Rheology of each polymer

Polymers, unmodified (N25) and modified inulins (SP1, EC8, EC12 and EC14),

samples were analyzed in aqueous solution varying its concentration. Viscometric

measurements (shear = 0) were carried out using a Rheologica rheometer (cone), see

Figure 1. The temperature was regulated by a circulating bath. To solubilize completely

the polymer, the resulting suspension of mix the polymer in water was heated up to 85

ºC for 10 min and then quenched with an ice bath. The found viscosity values were

reanalyzed 72 hours after, in order to evaluate the kinetic effects associated to the self-

aggregation of inulin in water.

Gelation of DDAB by HMI

The mixtures of polymer and surfactant (DDAB) were prepared mixing a semi-dilute

solution of each one. The viscosity of the homogeneous mixtures of polymer with

several concentrations of DDAB was measured using a Rheologica rheometer (cone and

plate), see Figure 1, at 35 ºC. The same procedure was employed with DDAB solutions

without polymer to compare the results. Moreover, some representative samples were

observed by fluorescence microscopy with Nile Red (fluorescent hydrophobic probe) .

Figure 1 Rheologica rheometer, model: Stressteck®.

Results

Both, inulin and its derivatives (SP1, EC8, EC12, EC14) show interesting solubility

properties. At low concentrations (around 1%) these polymers are easy to solubilize in

water, only with gentle stirring. At higher concentrations, the solubilization requires

pre-heating during 10 min at 80ºC and 65ºC for unmodified and modified inulin,

respectively (Figure 2A and 2B). All solutions of dissolved polymers show phase

separation after 1 month. However, the higher the polymer concentration is, the earlier

the phase separation. While at 20% polymer concentration the phase separation is after

1 hour (Figure 2C), concentrations less than 5% remain soluble during more than one

week.

Figure 2 Pictures A and B shows the aspect of modified and unmodified polymers in aqueous solutions

after solubilization. Picture C are unmodified inulin 1 hour after solubilization. In picture D, samples of

unmodified inulin several hours after solubilization and centrifugation.

MODIFIED INULIN UNMODIFIED INULIN

UNMODIFIED INULIN (+1 hour)

A

DC

B

POLYMER IN SOLUTION

0,00E+00

5,00E-01

1,00E+00

1,50E+00

2,00E+00

2,50E+00

0,0001 0,001 0,01 0,1 1 10 100POLYMER CONCENTRATION (w.t %)

VISC

OSI

TY (P

a·s)

N25 (inmediately) SP1 (inmediately)

Figure 3 The plot displays the Newtonian viscosity of the unmodified and the hydrophobically modified

inulin in aqueous solution.

Figure 3 displays the viscosity results obtained for the INUTEC® SP1 (commercial

hydrophobically modified inulin) and INUTEC® N25 (unmodified inulin) in water

solution. Both polymers have practically the same behavior when its concentration in

aqueous solution is increased, although the modified polymer has a little bit more

viscosity because of the hydrophobic interactions between its hydrophobic chains. From

0,001 % to 10 % the viscosity is near the water viscosity, this is called diluted region.

At 10 % the solutions becomes more viscous. At this point, the distance between chains

is shorter being higher the interaction between each other, this is called the semi-diluted

region.

POLYMER CONCENTRATION = 1% at 35 oC

1,00E‐03

1,00E‐02

1,00E‐01

1,00E+00

1,00E+01

1,00E+02

1,00E+03

1,00E+04

1,00E+05

1,00E+06

1,00E+07

1,00E+08

1 10 100 1000

DDAB CONCENTRATION (mM)

VISCO

SITY

(Pa∙s)

Without polymer

[N25]=1%

[EC12]=1%

[SP1]=1%

Figure 4 The plot shows the Newtonian viscosities for the polymer-DDAB mixtures as a function of

DDAB concentration.

Figure 4 shows the viscosity results for the polymer-DDAB mixtures. The polymer

concentration is kept constant at Cp= 2 mM. In addition, the viscosities of the surfactant

without polymer at different concentrations are represented. The behavior of the

unmodified polymer with DDAB is the same that the behavior observed by the

surfactant alone. Thus, it can be assumed that there is not interaction between

unmodified inulin and the surfactant. Although, the profile defined by the surfactant has

to be investigated more extensively in further works.

A very different behavior is found for the hydrophobically modified inulin (commercial,

INUTEC® SP1) or the synthesized analog (EC12). When DDAB concentration is

below 25 mM, the viscosity do not increase, but between 25 mM and 100 mM the

viscosity remains unaltered while, at the same concentrations the surfactant, has

viscosities six or seven orders of magnitude higher. At 200 mM of DDAB, the viscosity

increase sharply at values even higher than incase of DDAB alone, approximately 10 or

100 times. This viscosity values are not well understood yet. The same behavior was

observed for other synthesized analogs with different chain length (EC8, EC12, EC14).

0,00E+00

5,00E+04

1,00E+05

1,50E+05

2,00E+05

2,50E+05

3,00E+05

UNMODIFIED EC8 EC12 EC14 SP1 DDAB

HYDROPHOBIC MODIFICATION

VISCO

SITY

/ Pa∙s

Figure 5 The plot presents the estimated Newtonian viscosity of different HMI induced DDAB-gel

system. DDAB without polymer is also represented.

Figure 5 represents the viscosity at 200 mM of DDAB with 2 mM of polymer

concentration. All modified polymers interact positively with the surfactant, although

little differences between them can be observed. The storage modulus of the gel forming

mixtures has to be measured. Moreover, SAXS analyses are in progress in our lab.

Waiting for the results, we expect to get enough data to determine the structural features

of this gel.

The photographic data obtained with the fluorescent hydrophobic probe (Figure 6) was

enough to do a qualitative analysis of the DDAB, EC12-DDAB and N25-DDAB

mixtures. The results indicate that there’s no appreciable difference between DDAB and

unmodified inulin-DDAB in all concentrations, where vesicles (1 to 5μm) are identified,

although in case of EC12-DDAB the observation is very different.

Figure 6 Pictures obtained by fluorescence microscopy shows the presence of vesicles between 1 and 5

μm in DDAB solutions and N25-DDAB mixtures. In case of EC12-DDAB mixtures, only few vesicles

can be observed.

25 mM DDAB 25 mM DDAB + 2 mM N25

25 mM DDAB + 2 mM EC12

126 mM DDAB

126 mM DDAB + 2 mM EC12

126 mM DDAB + 2 mM N25

200 mM DDAB

200 mM DDAB + 2 mM EC12200 mM DDAB + 2 mM N25

5 μm 5 μm 5 μm

5 μm 5 μm 5 μm

5 μm 5 μm 5 μm

5 μm 5 μm

5 μm

EC12-DDAB system has much less vesicles than in case of N25-DDAB or DDAB. This

evidence increases our motivation again to do SAXS analyses in order to understand

this change in the surfactant self-organization induced by the hydrophobically modified

inulin.

Conclusions

This short term scientific mission in Coimbra University has been an excellent and

profitable stage form the human and scientific point of view. After that we can conclude

that Inulin and its hydrophobic derivatives have similar solubility behaviors, both

products can be solubilized after a pre-heating, but they becomes unsoluble after a time

that depends on the polymer concentration. Only the presence of two different products

in equilibrium, the hydrated (insoluble) and the dehydrated (soluble) forms of inulin

would explain the solubility behavior of inulin and its hydrophobic derivatives.

On the other hand, the gelation properties of HMI have been evaluated by the rheology

HMI-DDAB mixtures. DDAB forms vesicles spontaneously when it’s dissolved in

water, but its self-organization behavior is modified by HMI. Unexpectedly, after the

observation under the microscope (fluorescent hydrophobic probe), the induced gel is

not formed by vesicles and polymers, thus SAXS analyses are in progress to determine

the structural features of this kind of gels.

annex v. stsm reports (14) - uma reports.pdf20/07/2009 to 16/08/2009 cost-stsm-d36-4802 mazharul...

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