molybdenum sulfide: catalytic properties in nanoclustersleung.uwaterloo.ca/chem/750/talks/chris...

Molybdenum Sulfide: Catalytic Properties in Nanoclusters

Chris Choy

Department of Chemical Engineering

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2

IntroductionObjective:• Highlight aspects of MoS2 nanoclusters relevant to catalysis

Outline:• Brief Review of Catalysis• Applications• Structural and Electronic Properties of MoS2

• STM studies of model MoS2 nanoclusters: implications for HDS • Photocatalytic properties of MoS2




3

Brief Review of Catalysis by Transition Metals

• Catalyst – increases rate of reaction by lowering activation energy of pathway

• Stepwise Process1. Adsorption

• Coordination of reactant to vacant metal orbital or “dangling” bond

• η1 vs η4 modes2. Reaction3. Desorption

• Desirable Catalyst Characteristics– Cheap– Reasonable activity (conversion rate)– Physical and Chemical Durability

Wikipedia. 2007

Mo

S

Mo

Sη4 η1

Coordination/Adsorption Modes




4

Applications of MoS2

• Solid Lubricant

• Hydrodesulfurization of petroleum and transport fuels– Reduction of allowable diesel sulfur content from 50 ppmà 15 ppm

• Potential for oxidative destruction of environmental organic pollutants




5

Molecular Structure of Bulk MoS2

• Hexagonal Close Packed (HCP)– a = 3.1604 Å– c = 12.295 Å

• 2-D sheets consist of Mo sandwiched between S

• Van der Waals bonding between MoS2layers

1. Lide, D.R. ed. “CRC Handbook of Chemistry and Physics.”Crystallographic Data on Minerals. 2007. http://www.hbcpnetbase.com/

2. Chianelli et al. Cat. Rev. 2006, 48, 1-41.3. Wilson, J.A. and Yoffe, A.D. Adv. Phys. 1969, 18, 193-335.


http://www.hbcpnetbase.com/



6

Electronic Properties of MoS2

• Absorption spectrum changes with MoS2 size1

• Influence of 2-D structure

1. Wilcoxon, J.P. et al. J. App. Phys. 1997, 81, 7934-7944.

Bulk MoS2




7

Synthesis of MoS2 Nanocluster

Model Supported Systems1

– Mo evaporated in H2S (1*10-6 mbar) at 400 K onto Au (111)

– Annealed at 673 K to form clusters ~ 30 Å wide

Solution– Mo(IV) halide sulfided with H2S in inverse micelles– Nanocluster size controlled by micelle size

1. Helveg et al. Phys. Rev. Lett. 2000, 84, 951-954.

2. Wilcoxon, J.P. and Samara, G.A. Phys. Rev. B. 1995, 51, 7299-7303.




8

MoS2 in Catalysis: Hydrodesulfurization (HDS) of Transport Fuels

• Why HDS? – requirement for 15 ppm S in Diesel– Alberta bitumen (oil sands) contains 4-6 wt% S

• Why MoS2?– Relatively cheap– Good activity and conversion– Not poisoned by sulfur compounds (ie. H2S)

• Commercial Catalysts– Co promoted MoS2 supported on alumina– Typical conditions: 300 - 345 °C, 500 – 1000 psi of H2, 0.5-1.5 h1

Speight, J.G. Petroleum Refining Processes in Kirk-Othmer Encyclopedia of Chemical Technology. 2001. John Wiley and Sons.

SR R' O

SO

Combustion Atmospheric reactions+ O2 Acid Rain + Smog




9

HDS Mechanisms of Model Compounds

• Sulfur containing species in petroleum and bitumen are complex aromatic species

• Two significant pathways

• Direct Desulfurization (DDS)– Inhibited by H2S

• Hydrogenation with subsequent desulfurization (HYD)

S

S

a)

b)

+ 2H2

+ H2S

-H2SH2

HYD

DDS




10

Mechanistic Considerations in HDS: “Rim and Edge” Model

• Degrees of MoS2 “Layers” strongly influences selectivity1

• Basal plane is inactive• C-S bond rupture occurs on Edge sites with sulfur vacancies• Hydrogenation and C-S rupture occurs on Rim sites• H2S inhibits DDS pathway

1. Chianelli et al. Cat. Rev. 2006, 48, 1-41.




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Mechanistic Considerations in HDS: Metallic Edge States

• Method: STM/DFT• Contrary to previous opinions, fully sulfided cluster edges are reactive1

• Two edge states with metallic versus semi-conductor character1) Combination of px orbitals of S-S edge dimers2) 1st row Mo-Mo d-d bonds and p-d bond between 2nd row S and Mo

Fig. 2. (a) Atom-resolved STM image (Vt =−6.1 mV, It = 1.29 nA) of a triangular single-layer MoS2 nanocluster (67 × 69 Å2). White dots superimposednear the edge of the cluster indicate the registry of edge protrusions. Lines in the image refer to the line scans marked with triangles in Fig. 3. (b) Ball modelsin top and side view of the edge structure, the Mo edge with S dimers (S, bright; Mo, dark). (c) Ball model of a MoS2 triangle exposing this type of edge termination.

~ 0.4 Å above basal plane

1. Lauritsen, J.V. et al. J. of Cat. 2004, 224, 94-106.

2. Byskov, L.S. et al. J. of Cat. 1999, 187, 109-122; Raybaud, P. et al. J. of Cat. 2000, 189, 129.




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Mechanistic Considerations in HDS: Adsorption of Hydrogen

Vertical distance (tunneling current) versus Line scan in perpendicular direction from edge □ – Clean SurfaceΔ – After exposure to atomic H

• Surprise! Adsorbed atomic hydrogen occurs as surface S-H and not Mo-H hydrides

• Explains why HYD sites not poisoned by H2S1. Lauritsen, J.V. et al. J. of Cat. 2004, 224, 94-106.




13

Thiophene Desulfurization on Single MoS2Nanoclusters

Conditions: Exposed to atomic H at 673 K; thiophene exposure at 500 K, followed by quenching to room temperature.

1. Lauritsen, J.V. et al. J. of Cat. 2004, 224, 94-106.

2. Helveg, S. Phys. Rev. Lett. 2000, 84, 951-954.

1. Dissociation of H2 and formation of reactive S-H bonds

2. π-adsorption of thiophene to brim site, hydrogenation and C-S cleavage

3. Movement of thiolate intermediates to sulfur vacancies

4. Cleavage of 2nd C-S bond

S*H H

HHH

SH H

HH

SH H

HH

HHS*H H

HHH H H

HHHH

+ 3Hads + Hads + 2Hads- H2S




14

Variations in Edge Structure with MoS2 Cluster Size

• Two competing tendencies govern edge reconstruction1

– Minimize edge free energy– Sulfur excess Ns/NMo < 3.5 favoured; optimum ~ 3

• n < 6, bright brim disappears and lattice of well-defined bright spots appear

• n > 6, 100% S coverage on nanocluster edge

• Maximum HDS activity has been shown to be associated with cluster sizes around the transition region

Jacoby, M. C&EN. 2007, 85, 13.

1. Lauritsen, J.V. et al. Nature Nanotech. 2007, 2, 53-58.




15

Considerations between Model and Commercial HDS Catalysts

• Industrial conditions operate at much higher pressure and temperature

• Supported active clusters exist as single or low-order layers on support1

• Typically promoted with Co forming Co-Mo-S

• Differences in physical properties– Crystallinity– Support Effects

1. Chianelli et al. Cat. Rev. 2006, 48, 1-41.

2. Lauritsen, J.V. J. of Cat. 2001, 197, 1-5.

Figure 16: Destacking effect under the hydrodesulfurization process: A) sulfided precursor,B) hydrogen activated catalyst, C) stabilized single layer catalyst.1




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Summary

• 2-dimensional properties responsible for unique catalytic properties of “rim and edge” states

• Hydrogenation occurs on metallic brim sites through reactive S-H bonds

• Further understanding into mechanistic insights

• Caution: Model studies often do not represent actual conditions in Industrial Reactors, ie. UHV vs. 1500 psi.




17

MoS2 Nanoclusters in Photocatalysis




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MoS2 Nanoclusters in Photocatalysis

• Absorption edge of MoS2 nanoclusters lie in the visible wavelength region

• High resistance to photo-oxidation– CB and VB derived from Mo 4d orbitals

• Redox potential of MoS2 capable of oxidizing water to form ●OH radical

1. Thurston, T.R. and Wilcoxon, J.P. J. Phys. Chem B. 1999, 103, 11-17.




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Photo-oxidation of Phenol by MoS2Nanoclusters

2MS2 (h+) + 2H2O + O2 à H2O2 + 2H+

H2O2à 2OH•

OH• + organic compounds à CO2+ H2O

• Oxidation of Phenol occurs only with 4.5 nm nanoclusters – any relation to brim or edge sites?

• Potential Problems:– Aggregation of nanoclusters in aqueous

solution

1. Thurston, T.R. and Wilcoxon, J.P. J. Phys. Chem B. 1999, 103, 11-17.

2. Ho, W et al. Langmuir. 2004, 20, 5865-5869.




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Conclusions

• 2-Dimensional structure gives MoS2 unique structural and electronic properties

• Edge reconstruction of single-layer nanoclusters of MoS2 is important for achieving active HDS sites

• Metallic edge states and reactive S-H bonds characterized in model HDS studies

• Quantum Confinement Effects shift MoS2 absorption edge visible wavelength region – Photo-oxidation of organic compounds

• Any relation to brim edge state and oxidation by photocatalysis?




molybdenum sulfide: catalytic properties in nanoclustersleung.uwaterloo.ca/chem/750/talks/chris...

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