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Applied Catalysis A: General 390 (2010) 35–44

Contents lists available at ScienceDirect

Applied Catalysis A: General

journa l homepage: www.e lsev ier .com/ locate /apcata

ethanation of CO, CO2 and selective methanation of CO, in mixtures of CO andO2, over ruthenium carbon nanofibers catalysts

icente Jiméneza,∗, Paula Sáncheza, Paraskevi Panagiotopouloub, José Luís Valverdea, Amaya Romeroa

Facultad de Ciencias Químicas/Escuela Técnica Agrícola, Departamento de Ingeniería Química, Universidad de Castilla-La Mancha, 13071 Ciudad Real, SpainDepartment of Chemical Engineering, University of Patras, GR-26504 Greece

r t i c l e i n f o

rticle history:eceived 4 August 2010eceived in revised form 3 September 2010ccepted 21 September 2010vailable online 29 September 2010

eywords:O methanationO2 methanationelective methanation of COutheniumuel cell applicationsarbon nanofibers

a b s t r a c t

The catalytic performance of ruthenium catalysts supported on carbon nanofibers for the methanation ofCO, CO2 and their mixture has been investigated with respect to the nature of carbon nanofibers (orienta-tion of graphite planes): platelet, fishbone and ribbon. Experiments were conducted in the temperaturerange of 200–500 ◦C using feed compositions relevant to those of reformate gas streams, both in theabsence and in the presence of water. It has been found that, under conditions of solo-CO methanation,all the investigated catalysts are able to completely and selectively convert CO at temperatures around340 ◦C, with the conversion of CO being somewhat higher for the Ru/platelet sample. For hydrogenationof CO2 alone, catalytic performance is not affected by the nature of the carbon nanofibers used as support.In combined hydrogenation of CO/CO2 mixtures, catalytic performance for all the investigated catalysts ispoor since they promote the undesired reverse water–gas shift reaction. However, addition of 30% watervapour in the feed inhibits the reverse water–gas shift, thereby enhancing CO hydrogenation. Results of

kinetic measurements show that the turnover frequency of CO conversion becomes 2–3 times higherin the presence of steam over Ru/fishbone and Ru/platelet samples over the whole temperature rangeexamined, whereas in the case of Ru/ribbon catalyst temperatures higher than 250 ◦C are required inorder to achieve higher turnover frequency values. Carbon dioxide hydrogenation is not affected by thepresence of steam. For all experimental conditions investigated, selectivity toward methane increaseswith increasing temperature at the expense of higher hydrocarbons and is enhanced with the addition

s mix

of water vapour in the ga

. Introduction

One the major problems for the application of low temperatureolymer electrolyte membrane fuel cells (PEM FCs) as power sourceor electrically operated vehicles is the delivery of “nearly-CO-free”eed gas, which becomes problematic whenever H2 is generatedrom fuels. As the FC anodes can be poisoned even by traces impu-ities of CO, this gas has to be removed to a level below 50 ppm fort–Ru anode electrocatalyst [1–3] and below 10 ppm for Pt anodelectrocatalysts [4–6].

Theoretically, there are several methods to reduce carbononoxide to the levels acceptable for a fuel cell [7]. It is feasible to

eparate hydrogen by diffusion thorough a CO filtering membrane

ut the membrane is expensive and it usually requires a compressorwing to high pressure [8]. The most studied system for the removalf final traces of CO over the last years has been the selective oxida-ion of CO in a H2-rich atmosphere (PROX) since it has the ability to

∗ Corresponding author. Tel.: +34 926295300; fax: +34 926295318.E-mail address: vicente.jimenez@uclm.es (V. Jiménez).

926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2010.09.026

ture.© 2010 Elsevier B.V. All rights reserved.

remove CO down to 10 ppm [9–11]. The catalytic hydrogenation ofcarbon monoxide and carbon dioxide also produces a large varietyof products ranging form methane and methanol to higher molec-ular weight hydrocarbons and alcohols [12]. The methanation ofCO and CO2 as well as the related Fischer–Tropsch reaction havebeen extensively studied and reviewed. COx methanation may haveanother practical application as a means of CO removal from pro-cess gases for gas separation purposes and is also being discussedas an alternative to PROX in fuel processors for mobile fuel cellapplications [13]. However, in the last years, the onboard hydro-gen production has been a minor system for the application of fuelprocessing to FCV, except some particular cases. The methanationreaction (Eq. (1)) has been widely used as a gas-purification pro-cess. It is used on a large scale for the purification of hydrogen inammonia and hydrogen plants where CO is a catalyst poison [14].The interest for the reaction has grown significantly during the last

few years as a promise route to remove CO in the reformate streamdown to 50 ppm, poisoning limit for the use of polymer electrolytefuel cells [15,16].

CO + 3H2 ↔ CH4 + H2O, �H◦ = −206 kJ/mol (1)

3 alysis A

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C

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6 V. Jiménez et al. / Applied Cat

This approach has the advantage that no oxidizing and/or inertases are mixed with the reformate stream and that the methaneroduced, which is inert to the PEM fuel cell, can be utilized inesidential PEFC systems. However, depending on the operatingonditions and catalyst employed, reaction (1) may run in parallelith the undesired methanation of CO2, which consumes signifi-

ant quantities of valuable hydrogen (Eq. (2)), as well as with theeverse water–gas shift (RWGS) reaction (Eq. (3)), which shift CO2o CO [17].

O2 + 4H2 ↔ CH4 + 2H2O, �H◦ = −165 kJ/mol (2)

O2 + H2 ↔ CO + H2O, �H◦ = 41.1 kJ/mol (3)

Consequently, it is important to develop selective CO metha-ation catalysts characterized by high activity at sufficiently lowemperatures, able to retard both the CO2 and the RWGS reactions.

Methanation of CO over different carried metal catalysts includ-ng Ni [18–21], Ru [18,22–24] and Rh [21,24–26], has been widelynvestigated for the production of CH4 from syngas [4,27–29] andecently also from a viewpoint of residual CO removal for PEMC applications [30–36]. Ruthenium catalysts dispersed on metalxide carriers have been found to exhibit high activity for the solo-ethanation of CO [18,19,37] or CO2 [20,21] as well as for the

o-methanation of CO/CO2 mixtures [19,25,33]. In a recent study25] the effects of the nature of the metallic phase on the per-ormance of Al2O3-supported Ru, Rh, Pt and Pd catalysts for the

ethanation of CO, CO2 and their mixtures both in the absence andn the presence of water in the feed has been studied. It has beenound that Ru and Rh are much more active hydrogenation cata-ysts as compared to Pt and Pd, which promote the undesired RWGSeaction. Furthermore, the nature of the support may play a crucialole in the mechanism of CO/CO2 hydrogenation reactions, sinceetal-support interactions can modify the catalytic properties of

he metallic phase [26,37,38]. Supports as carbon nanofibers coulde an alternative to the classical ones (alumina, silica, TiO2, etc)idely studied on the literature [25,16,38] due to their excellent

haracteristics as the high purity of the material, high mechani-al strength and the mesopore nature which result in low internalass-transfer resistances [39].In the present study, the catalytic performance of Ru cata-

yst on selective COx methanation using different types of carbonanofibers (platelet, fishbone and ribbon) has been investigated.he influence of water vapour in the feed was also studied.

. Experimental

.1. Support/catalyst preparation

CNFs synthesis was carried out by CVD method. The fixed-bedeactor, consisting on a quartz tube of 9 cm diameter and 100 cmength, was located in a horizontal electric furnace (JH Hornos) withn effective heating zone of 80 cm. Thermocouple type K was usedor monitoring the temperature of the bed. Hydrogen and ethaneow rates were controlled by mass controllers (Brooks Instruments,odel 5850). Supported catalyst was taken in a quartz boat, whichas kept inside the heating zone during the experiment. Carbonanofibers were grown at atmospheric pressure at different tem-eratures: 450 ◦C (platelet type), 600 ◦C (fishbone type) and 850 ◦Cribbon type). In each synthesis run 5 g of the prepared catalystNi/SiO2) was placed in the centre of the reactor and activated byeating (10 ◦C min−1) in a flow dry 20% (v/v) H2/He at the desired

eaction temperature. The reduced activated catalyst was thor-ughly flushed with dry He for 1 h before introducing the H2/C2H4atio desired feed. The growth time was 1 h and the space veloc-ty 25,000 h−1. Separation of CNFs and catalyst particles to recoverhe carbon material was carried out by repeated dissolution of the

: General 390 (2010) 35–44

support in hydrofluoric acid (70%) for 15 h under vigorous stirring,filtering and washing with deionised water [40]. Upon treatment ofthe catalyst–carbon mixture, the metal component of the catalystwas generally transferred into the solution.

Methanation catalysts were prepared employing the wetimpregnation method [25] by using Ru(NO)(NO3)3 (Alfa Products)as metal precursor salt and the different types of CNFs obtained. Theresulting slurry was heated at 1 ◦C min−1 up to 90 ◦C under continu-ous stirring and maintained at that temperature until nearly all thewater evaporated. The solid residue was dried at 110 ◦C for 24 h andthen reduced at 400 ◦C in H2 flow for 2 h. The metal loading of thecatalysts thus prepared was 0.5 wt.%.

2.2. Support/catalyst characterization

Surface area/porosity measurements were carried out using aMicromeritics ASAP 2010 sorptometer apparatus with N2 at 77 Kas the sorbate. The samples were outgassed at 453 K under vac-uum (6.6 × 10−9 bar) for 16 h prior to analysis. Specific surface areaswere determined by the multi point BET method, total pore vol-ume and sizes were evaluated using the standard BJH treatmentand micropore volume were evaluated using the t-plot equation.

The crystallinity of CNFs, the mean crystallite size and Ru specieswas determined using XRD analyses. These analyses were carriedout on a Philips X’Pert instrument using nickel filtered Cu K� radia-tion through a primary monochromator. The samples were scannedat a rate of 0.02◦ step−1 over the range 5◦ ≤ 2� ≤ 80◦ (scan time = 2 sstep−1). The metal particle size was determined after the catalystreduction in H2 flow to 400 ◦C for 2 h. Once the sample was cooled,it was passivated in a 1% (v/v) O2/He mixture to prevent bulk oxi-dation. The primary crystallite size of Ru (dRu) was calculated bymeans of Scherrer’s equation [41]:

dRu = 0.9�

B cos �(4)

where � is the X-ray wavelength corresponding to Cu-K� radiation(0.15406 nm), B is the broadening (in radians) of the ruthenium(1 0 1) reflection and � is the angle of diffraction corresponding tothe peak broadening.

The chemisorption measurements were carried out using adynamic pulse technique with an argon flow of 50 mL min−1 andpulses of H2 (99.9995% purity) using a Micromeritics AutoChem2950 HP apparatus according to the procedure described in ref.[40,42,43]. In order to calculate the metal dispersion, an adsorptionstoichiometry of metal/H = 1 was assumed [44]. Dispersion mea-surements with H2 pulses were carried out at 60 ◦C to avoid the spillover phenomenon [45]. Previously, the sample was pre-treated byheating at 15 ◦C min−1 in argon flow up to 250 ◦C and kept constantat this temperature for 20 min. Then, the sample was reduced insitu. Next, the hydrogen was removed by flowing argon for 30 min,being the temperature 10 ◦C higher than the reduction one. Finally,the sample was cooled to the experiment temperature in an argongas flow. The mean crystallite size of the dispersed metal was alsoestimated from hydrogen chemisorption data, assuming sphericalparticles, a H:Ru stoichiometry of 1:1 and an atomic surface area ofRu equal to 8.6 A2 [17], using the relation:

dM = 6�MSM

(5)

where dM is the mean crystallite diameter, �M is the density of Ru(12.3 g cm−3) and SM is the exposed surface area per gram of metal.

Temperature-programmed oxidation (TGA/TPO) was used todetermine the CNFs crystallinity. Analyses were performed on10 mg samples using a Perkin-Elmer TGA7 termogravimetric ana-lyzer with a flow of 50 cm3 min−1 of 20% (v/v) O2/He mixture andwith a heating rate of 5 ◦C min−1 up to 1000 ◦C.

V. Jiménez et al. / Applied Catalysis A: General 390 (2010) 35–44 37

F SEM pC

masUaedo

s

TP

s

o

t

ig. 1. Representative images of the different type of carbon nanofibers (CNFs): (a)NFs and (d) TEM picture of ribbon CNFs.

Support morphology was examined by scanning electronicroscopy (SEM) using a JEOL JSM-6535F SEM unit with an

ccelerating voltage of 10 kV. The samples were deposited on atandard aluminium SEM holder and coated with gold (Balzersnion, Model MED 010). Transmission electron microscopy (TEM)nalysis employed was a Philips Tecnai 20T, operated at an accel-

ration voltage of 200 keV. Samples were prepared by ultrasonicispersion in acetone with a drop of the resultant suspension evap-rated onto a holey carbon supported grid.

The Ru metal loading was determined by atomic absorption (AA)pectrophotometry, using a SPECTRA 220FS analyzer. Samples (ca.

able 1hysicochemical properties of the CNFs and the reduced Ru catalysts.

CNFs support

Platelet Fishbone

BET surface area (m2/g) 286.0 202.0Micropore area (m2/g)a 57.0 (20%) 25.0 (13%)Mesopore area (m2/g) 229.0 177.0Micropore volume (cm3/g)b 1.194 (0.62) 0.546 (0.62)Mesopore volume (cm3/g)c 0.409 (8.7) 0.977 (10.1)Average pore size (nm) 6.9 9.3npgd 7.7 8.6Weight loss temperature range(◦C)e

380–585 (519) 430–618 (546)

NaOH added (mmol/g) 0.75 0.56Ru dispersion (%) – –dRu,TEM (nm)f – –dRu, H2 (nm)g – –

a In brackets: percentage of micropore area with respect to the total surface area.b Cumulative pore volumes obtained using Horvath–Kawazoe method. In brackets: mec Cumulative pore volumes obtained using BJH method. In brackets: mean mesopore sd Number of graphene planes in the crystallites Lc/d0 0 2 where Lc is the average crystal

tructure, and d0 0 2 is the average interlayer spacing.e In brackets: temperature at which the maximum of the oxidation temperature peak af Average diameters of Ru particles determined by counting around 200 particles on th

f particle of diameter di .g Average diameter of Ru particles measured using H2 chemisorption technique using

he exposed surface area per gram of metal [20].

icture of general CNFs, (b) TEM picture of platelet CNFs, (c) TEM picture of fishbone

0.5 g) were treated in 2 cm3 HCl, 3 cm3 HF and 2 cm3 H2O2 followedby microwave digestion (523 K).

2.3. Catalytic activity

Catalytic performance tests and kinetic measurements have

been carried out using an apparatus, consisting on a flow mea-suring and control system, the reactor and an on-line analysissystem [25]. The flow system is equipped with a set of mass-flowcontrollers (MKS) and a set of valves, which allows introductionof the gas mixture to the reactor or to a by-pass loop, through

Ru-based CNFs catalysts

Ribbon Platelet Fishbone Ribbon

68.0 183.6 175.9 93.92.0 (3%) 11.7 (6%) 20.6 (12%) 28.7 (31%)66.0 171.9 155.4 65.20.052 (0.64) 0.476 (0.67) 0.514 (0.66) 0.116 (0.67)0.246 (16.7) 0.403 (9.9) 0.378 (10.9) 0.232 (11.4)14.1 6.1 8.8 9.712.3 – – –545–715 (640) – – –

0.19 0.78 0.58 0.40– 67.7 63.5 58.5– 1.04 1.48 1.93– 1.41 1.50 1.63

an micropore size in nm.ize in nm.domain size along a direction perpendicular to the basal planes in a graphitic-type

ppears.e TEM images using the equation: dRu =

∑nid

3i/∑

nid2i

where ni is the number

the expression dM = 6/�MSM where, �M is the density of Ru (12.3 g cm−3) and SM is

38 V. Jiménez et al. / Applied Catalysis A: General 390 (2010) 35–44

1.00.80.60.40.20.0

0

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350 Fishbone CNFs Ru/Fishbone CNFs

N2

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(cm

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N 2 v

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Mic

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Ribbon CNFs

1.00.80.60.40.20.0

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180 Ribbon CNFs Ru/Ribbon CNFs

Relative pressure (P/P0 )

Relative pressure (P/P0 )

/meso

sst1flrmtdtml

Relative pressure (P/P0 )

Fig. 2. N2 adsorption–desorption isotherms and micropore

tainless steel tubing. When desired, water is introduced to theystem with the use of an HPLC pump (Marathon Scientific Sys-ems), vaporized in a stainless steel evaporator maintained at70 ◦C and mixed with the gas stream coming from the mass-ow controllers. The resulting gas mixture is then fed to theeactor through stainless steel tubing maintained at 150 ◦C byeans of heating tapes. The reactor consists of a 40 cm long quartz

ube (6 mm OD) with an expanded 1 cm long section in the mid-le (8 mm ID), in which the catalyst sample is placed. Reactionemperature is measured in the middle of the catalyst bed by

eans of a K-type thermocouple placed within a quartz capil-ary well, which runs through the cell. The reactor is placed in

Pore diameter (nm)

pore volume distribution of the catalysts and the supports.

an electric furnace, the temperature of which is controlled usinga second K-type thermocouple placed between the reactor andthe walls of the furnace. A pressure indicator is used to mea-sure the pressure drop in the catalyst bed. The analysis systemconsists on a gas chromatograph (Shimadzu) equipped with twopacked columns (Porapak-Q Carbosieve) and two detectors (TCD,FID) and operates with He as the carrier gas. The response fac-

tors of the detectors were determined with the use of gas streamsof known composition (Scott specialty gas mixtures). Reactiongases (He, 15% CO/He, CO2, H2) are supplied from high-pressuregas cylinders (Messer Griesheim GMBH) and are of ultra-highpurity.

V. Jiménez et al. / Applied Catalysis A: General 390 (2010) 35–44 39

30

Ru (101)

Ru (002)

Ru (100)

2θ

b

50403020100

Ru (101)

Ru (002)

Ru (100)

ca

Ru (101)

Ru (002)

Ru (100)Inte

nsity

(a.

u.)

es of C

trCpHcrp(fsphe

S

wnsmce

wmvp

3

3

fcbctwbwt((

210050403020100

2θ

Fig. 3. XRD pattern for Ru catalysts supported on different typ

The catalytic performance of the prepared samples for the selec-ive methanation of CO has been investigated in the temperatureange of 200–500 ◦C using a feed stream consisting of 1% CO, 15%O2 and 50% H2 (balance He). When water was added in the feed,art of the balance gas (He) was replaced by water vapour (30%2O). The mass of catalyst used in these experiments was typi-ally 150 mg (particle size: 0.18 < d < 0.25 mm) and the total flowate was 200 cm3 min−1. Prior to each experiment the catalyst sam-le was reduced in situ at 300 ◦C for 1 h under hydrogen flow60 cm3 min−1), purged with He and then conditioned at 170 ◦Cor 1 h with the reaction mixture. Conversions of reactants andelectivities toward products were then measured at that tem-erature using the analysis system described above. Selectivity toydrogenation product “i”(Si) was calculated using the followingxpression:

i = Ci,out/vi

˙iCi,out/vi(6)

here Ci,out is the outlet concentration of the product i and vi is theumber of carbon atoms of product “i”. It should be noted that theystem was left at each temperature for about 1 h under isother-al conditions in order to achieve steady state conditions. In all

ases, data points are overages of at least three measurements. Allxperiments were performed at near atmospheric pressure.

Results obtained, along with measurements of metal dispersion,ere used to determine the turnover frequencies (TOFs) of carbononoxide and carbon dioxide, defined as moles of CO or CO2 con-

erted per surface Ru atom per second. Details of the methods androcedures employed can be found elsewhere [25].

. Results and discussion

.1. Support characterization

In the representative TEM images showed in Fig. 1 are clearly dif-erentiated the different structures of the CNFs used in this study asatalyst supports. Carbon nanofibers (CNFs) are an allotrope of car-on that has crystalline structure (Fig. 1a). Different types of CNFsan be obtained depending on their synthesis temperature. Plateletype structures (hexagonal planes perpendicular to the fiber axis)ere produced at a synthesis temperature of 450 ◦C (Fig. 1b); fish-

one type structures (graphene layers terminate on the surfaceith a determinate inclination angle) were predominant at a syn-

hesis temperatures of around 600 ◦C (Fig. 1c), whereas, ribbonhexagonal planes staked parallel to the fiber axis) type structuresFig. 1d) started to be produced at high temperatures (i.e. 850 ◦C).

504002θ

NFs: (a) platelet CNFs, (b) fishbone CNFs and (c) ribbon CNFs.

The graphitic character of the different supports was eval-uated by XRD, where the npg values (Table 1) suggested thatribbon CNFs were the most graphic carbon nanofibers. Thus, Lc val-ues increased in the following order: platelet < fishbone < ribbon,and d0 0 2 values in the following one: ribbon < fishbone < platelet.Higher values of Lc may reduce the adsorptive sites on CNFs sur-face which will be verified by N2 adsortion [46]. Carbon structuralorder was also measured by means of TPO analyses. All CNFs exhib-ited only a single oxidation peak from the DTA curve (not shown),indicating high product purity [40]. Table 1 presents the maxi-mum weight loss temperature showing that an increase in theCNFs synthesis temperature lead to more crystalline structures(ribbon > fishbone > platelet).

Nitrogen adsorption/desorption isotherms and pore size dis-tribution are presented in Fig. 2. The isotherms can be clearlyassigned to the type IV IUPAC classification, characteristic of meso-porous materials, where the mean size (see Table 1) falls within theaccepted mesoporous (2–50 nm) range. Also, a well defined hys-teresis (H3 type according to IUPAC classification) was observedindicating the existence of a substantial volume of mesoporeswhere irreversible capillary condensation occurred [47]. Texturalparameters data are shown in Table 1. It is observed that for increas-ing CNFs synthesis temperature, the specific BET surface area andthe total pore volume of the resulting carbon material decreased,in agreement with the results obtained by others authors [48].Their corresponding pore size distribution curves are also showedin Fig. 2. Average mesopore size increased in the following order:platelet < fishbone < ribbon, being the principal mesopore size inthe range 15–30 nm, accompanied with a secondary mesoporesize of 3–5 nm. Larger mesopores have been attributed to inter-stices between interlaces filaments whereas smaller ones havebeen associated with the CNFs surface roughness. Obtained resultswere indicative of the higher amount of adsorption sites present inplatelet CNFs [46]. Thus, platelet CNFs had the largest BET surfacearea and the smallest average pore diameter because of the pres-ence of a higher amount of 3–5 nm pores. Nevertheless, fishboneand ribbon CNFs had larger average pore diameters than plateletones because of much of their pores resulting mainly from aggre-gation.

3.2. Catalyst characterization

The physicochemical characteristics of the synthesized Ru cat-alysts are summarized in Table 1.

XRD patterns for the activated catalysts are presented in Fig. 3.Peaks at 2� ≈ 26◦ correspond to the (0 0 2) graphite plane of carbon

4 alysis A: General 390 (2010) 35–44

ntarrwoam

RwspetaCp(wRmgCsaatdgsswaitstra

bostotsw

3

uhccri1cwhps

0 V. Jiménez et al. / Applied Cat

anofibers (JCPDS-ICDD Card No. 41-1487), where the implica-ion in terms of support graphitic character has been discussedbove. Reflections appearing at 38.2◦, 42.3◦ and 44.1◦ correspond,espectively, to the (1 0 0), (0 0 2) and (1 0 1) planes of metallicuthenium (JCPDS-ICDD Card No. 06-0663), which are consistentith an exclusive hexagonal geometry and establish the presence

f Ru0 [49,50]. These results indicate that after activation of cat-lysts with H2 flow previous to the methanation reaction all theetallic phase was in the Ru0 form.BET surface area and micropore/mesopore volume values of

u platelet and fishbone CNFs (Table 1) decreased as comparedith the parent CNFs (before metal incorporation). This effect is

hown in Fig. 2. However, ribbon CNFs surface area and micro-ore volume significantly increase after metal incorporation. Toxplain these results is necessary to have into account the posi-ion of the Ru particles on the different supports. Thus, the surfacerea decrease that took place with the platelet and fishbone typeNFs has been ascribed to the occupation of the adsorptive sitesresent on the CNF surface by highly dispersed Ru nanoparticlesnote that the highest Ru dispersion values (Table 1) were obtainedith the platelet and fishbone type CNFs). A high dispersion ofu nanoparticles on the CNFs is normally associated to a strongetal-support interaction between the Ru nanoparticles and the

raphitic edges of the CNFs, as has been previously observed for aNF-supported Pd catalyst [51]. On the other hand, the microporeurface area increase that was observed in ribbon CNFs could bescribed to the presence of additional oxygen containing groupsnd defects created on the CNF surface during the Ru deposi-ion treatment. These groups would lead to new adsorption sitesuring N2 physisorption [52]. The presence of oxygen-containingroups, which could significantly influence the adsorptive and sub-equent catalytic behaviour [53], can be confirmed by means oftandard acid–base titrations. In such titrations the acid groupsould be neutralized by NaOH. The quantities of acidic sites, which

re presented as millimoles of NaOH needed to neutralize the acid-ty groups of pKi ≤ 7, are shown in Table 1. Results showed thathe total acidic groups determined by titrations decreased in theequence: platelet > fishbone > ribbon. It is interesting to note thathe presence of oxygen-containing groups, after incorporation ofuthenium, was the same in platelet and fishbone CNFs, but it waslmost two times higher on the surface of ribbon CNFs.

Finally, the average diameter of ruthenium particles measuredy H2-chemisorption and TEM are shown in Table 1. It can bebserved that the results obtained by both techniques were quiteimilar. Ru particle size varied with the type of CNFs support. Par-icle size in platelet CNFs was the lowest if compared with thatf fishbone and ribbon CNFs. As expected, the larger the Ru par-icles were, the lower metal dispersion was observed. In Fig. 4 ishown the representative TEM images of different types of CNFsith Ru.

.3. CO and CO2 hydrogenation

The effect of the type of CNFs (platelet, fishbone and ribbon)sed as support on catalytic performance for the solo-CO and CO2ydrogenation was investigated over Ru catalysts of the same metalontent (0.5 wt.%). Results obtained are summarized in Fig. 5, whereonversions of CO (XCO) and CO2 (XCO2 ) are plotted as functions ofeaction temperature. It is observed that conversion of CO (Fig. 5a)ncreases with increasing temperature above ca. 240 ◦C and reaches00% at 340 ◦C for all catalysts investigated. It should be noted that

atalytic performance is somewhat higher for Ru/platelet sample,ith XCO taking values of ca. 92% at 320 ◦C, i.e. more than two timesigher than that measured for Ru/ribbon and Ru/fishbone sam-les. This small difference may be attributed to the higher specificurface area of platelet CNFs.

Fig. 4. Representative TEM image illustrating dispersion of Ru particles in the CNFs:(a) platelet CNFs, (b) fishbone CNFs and (c) ribbon CNFs.

V. Jiménez et al. / Applied Catalysis A

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20

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100aX

CO

(%

)

Temperature (ºC)

CNFs Platelet

Ribbon

Fishbone

500450400350300250200

0

20

40

60

80

100b

XC

O2 (

%)

Temperature (ºC)

CNFs Platelet

Ribbon

Fishbone

Fig. 5. (a) Conversions of solo-CO (XCO) and (b) CO2 (XCO2 ) as function of reactiontemperature obtained over Ru (0.5 wt.%) supported on the indicated CNFs. Experi-mental conditions: mass of catalyst: 150 mg; particle diameter: 0.18 < dp < 0.25 mm;feed composition: (a) 1% CO, 50% H2 (balance He) and (b) 15% CO2, 50% H2 (balanceHe); total flow rate: 200 cm3/min.

4804203603002400

20

40

60

80

100

a

Sel

ectiv

ity (

%)

CO/H2 CO

2/H

2

, CH4

, C2H

4

, C2H

6

300240

Tempera

CO

Fig. 6. Effect of reaction temperature on the selectivity to the indicated reaction products onation over Ru (0.5 wt.%) supported on (a) platelet, (b) fishbone and (c) ribbon CNFs. Experifeed composition: CO/H2: 1% CO, 50% H2 (balance He) and CO2/H2: 15% CO2, 50% H2 (bala

: General 390 (2010) 35–44 41

Regarding the CO2 hydrogenation, it is initiated at about 300 ◦Cand conversion reaches 50% at 500 ◦C (Fig. 5b). As it can be seencatalytic performance is not affected by the nature of the CNFs usedas support.

In addition to methane, hydrogenation of CO also results inthe production of higher hydrocarbons. Results obtained over theRu/CNFs catalysts are shown in Fig. 6 (solid symbols), where selec-tivities toward hydrogenated products (SCxHy ) are plotted as afunction of reaction temperature. It is observed that, in the caseof Ru/platelet (Fig. 6a) and Ru/fishbone (Fig. 6b) catalysts, SCH4increases from 85% to 100% with increasing temperature from240 ◦C to 400 ◦C. The main by-products formed below 400 ◦C areC2H6 and C2H4. At temperatures above 400 ◦C, where conversionof CO is complete (Fig. 5a), formation of higher hydrocarbons is sup-pressed and the only hydrogenation product formed is CH4 (Fig. 6aand b). Ruthenium catalyst supported on ribbon CNF was foundto be more selective toward CH4 (Fig. 6c) than Ru supported onplatelet and fishbone CNFs, with SCH4 being typically higher than96% in the whole temperature range, while the only by-productformed is C2H4. The above findings are in good agreement withresults reported previously over noble metal catalysts supportedon Al2O3 [25].

Qualitatively, similar results were obtained for the CO2 hydro-genation reaction. As it can be seen in Fig. 6 (open symbols)selectivity toward methane takes more or less the same values(for each sample investigated) with that obtained for CO hydro-genation, with Ru/ribbon catalyst exhibiting the highest SCH4 . Ithas been proposed over noble metal catalysts supported on metaloxides [17,25,26,56–63] that under conditions of CO2 hydrogena-tion appreciable amounts of CO2 are converted to CO via the reverseWGS reaction. It is believed that carbon monoxide acts as an inter-mediate, which is further hydrogenated to methane [59]. However,this is not the case for the results of the present study since noCO was detected under CO2 hydrogenation conditions. In pre-vious investigations [17,25,64] it was found that the occurrenceand the onset of the RWGS reaction under conditions of solo-CO2methanation and methanation of CO/CO2 mixtures depends sig-nificantly on the nature of metal and the support. For example, itwas found that, for solo-CO2 methanation reaction, selectivity toCO, at the expense of CH4, at a given temperature decreases in the

order of Pt > Pd � Ru > Rh, with Rh catalyst producing CO only attemperatures higher than 400–450 ◦C [25]. Moreover, it has beenreported that production of CO via the RWGS depends strongly onthe nature of the support over Ru [17] and Rh [64] catalysts. Regard-ing the results of the present study (Fig. 5b), it is possible that small

480420360300240

c

CO/H2 CO

2/H

2

, CH4

, C2H

4

480420360

b

ture (ºC)

/H2 CO

2/H

2

, CH4

, C2H

4

C2H

6

btained under conditions of solo-CO (solid symbols) and CO2 (open symbols) metha-mental conditions: mass of catalyst: 150 mg; particle diameter: 0.18 < dp < 0.25 mm;nce He); total flow rate: 200 cm3/min.

42 V. Jiménez et al. / Applied Catalysis A

400350300250200

-200-100

0

20

40

60

80

100CO CO2----- ------

, platelet, ribbon, fishbone

XC

O, X

CO

2 (%

)

Temperature (ºC)

Fig. 7. Conversions of CO and CO2 as functions of reaction temperature obtainedoCc1

amep

3

aoati2dbcRAtncCXwtt

ppa

TR32

ver Ru (0.5 wt.%)/CNFs catalysts for the selective methanation of CO. Solid symbols:O conversion; open symbols: CO2 conversion. Experimental conditions: mass ofatalyst: 150 mg; particle diameter: 0.18 < dp < 0.25 mm; feed composition: 1% CO,5% CO2, 50% H2 (balance He); total flow rate: 200 cm3/min.

mounts of CO may also produced under conditions of solo-CO2ethanation, which are not detectable with the analysis system

mployed, or significantly higher temperatures are required for theroduction of measurable amounts of CO.

.4. Selective methanation of CO

The effect of the type of CNFs (platelet, fishbone and ribbon) useds support on catalytic performance for the selective methanationf CO in CO/CO2 mixtures was investigated over Ru (0.5 wt.%) cat-lysts and results obtained are summarized in Fig. 7. It is observedhat XCO over the 0.5% Ru/ribbon catalyst increases with increas-ng temperature above 200 ◦C and goes through a maximum of ca.5% at 250 ◦C. Further increase of temperature results in a drasticecrease of XCO, which takes “negative” values above 275 ◦C. Thisehaviour can be explained by considering that, under the presentonditions, CO hydrogenation (Eq. (1)) runs in parallel with theWGS reaction (Eq. (3)), which becomes important above ca. 275 ◦C.t temperatures higher than 275 ◦C the rate of CO production via

he RWGS is higher than the rate of CO consumption via the metha-ation reaction, thereby resulting in the observed net increase of COoncentration at the effluent of the reactor. Regarding conversion ofO2, it is practically zero at temperatures up to ca. 250 ◦C, i.e., untilCO reaches its maximum value, and then progressively increasesith increasing temperature (Fig. 7). This behaviour reflects the fact

hat CO interacts more strongly with the catalyst surface, compared

o CO2, and is in good agreement with previous studies [17,25].

The Ru catalysts supported on platelet and fishbone CNFs areractically inactive for the selective methanation of CO under theresent experimental conditions, reaching a maximum of XCO of 7%t 250 ◦C (Fig. 7). This is attributed to the RWGS, which is operable at

able 2epresentative results of kinetic measurements obtained over the examined catalysts fo0% H2O in the feed. Mass of catalyst: 150 mg; particle diameter: 0.18 < dp < 0.25 mm; feed00 cm3/min.

Catalyst (0.5% Ru) Reaction

CO/CO2/H2

SCH4 at 250 ◦C (%) TOFCO at 250 ◦C (s−1) TOFCO2 at 330 ◦C

Platelet 93.2 0.017 0.56Fishbone 93.1 0.023 0.39Ribbon 91.8 0.080 0.26

: General 390 (2010) 35–44

temperatures as low as 275 ◦C, as evidenced by the “negative” val-ues of XCO. As a result, the CO content is higher at the effluent of thereactor, compared to that in the feed, and continuously increaseswith increasing temperature.

Results of kinetic measurements (Table 2) obtained under con-ditions of combined hydrogenation of CO/CO2 mixtures indicatedthat the specific reaction rate of CO conversion depends on thenature of CNF. In particular turnover frequency (TOF) of CO conver-sion at 250 ◦C is about four times higher when Ru is supported onribbon CNF (0.080 s−1) compared to platelet (0.017 s−1) and fish-bone CNFs (0.023 s−1). However, TOF of CO2 conversion does notdepend significantly on the nature of CNFs supports, exhibitingsimilar values at a given temperature (Table 2). Similar values ofTOF of CO and CO2 conversion have been previously reported overRu/Al2O3 catalysts under the same experimental conditions [25].

The observed increase of TOF of CO conversion when Ru is dis-persed on ribbon CNF may be due to the larger Ru crystallite sizeof this sample compared to the other catalysts examined (Table 1),which is well known to enhance catalytic activity for the selectivemethanation of CO [17,33,54,55]. For example, Panagiotopoulou etal. [17] reported, for Ru/TiO2 catalysts, that specific activity (TOF)increases by more than one order of magnitude with increasingRu crystallite size from 2.1 to 4.5 nm. It should be noted however,that the higher catalytic activity of Ru/ribbon catalyst can be alsoattributed to interactions between the Ru crystallites and the CFNsupport, which may be vary depending on the nature of the support.Clearly, a final conclusion on the role of the CNF support on the cat-alytic performance of Ru catalysts for the title reaction requires adetailed investigation of the reaction mechanism, which is beyondthe scope of the present study.

Selectivity to CH4 at a given temperature does not dependappreciably on the nature of the CNF support. Typical resultsobtained at 250 ◦C are presented in Table 2. In all cases, SCH4increases from 90 to 95% with increasing temperature from 225 ◦Cto 275 ◦C (not shown for clarity). It should be noted that at tem-peratures higher than 275 ◦C the estimation of selectivities towardhydrogenated products is doubtful, due to the production of highamounts of CO (i.e. “negative” values of XCO) at low reaction tem-peratures.

3.5. Effect of water vapour on catalytic performance

Normally, the reformate gases obtained from fuel processorscontain considerable amounts of steam, which may affect COmethanation characteristics. In order to investigate this issue,water vapour was added to the feed (30%) and results obtainedover Ru catalysts supported on CNFs are summarized in Fig. 8.The corresponding conversion curves obtained in the absence ofwater are also shown for comparison. It is observed that in all

cases the conversion of CO at a given temperature increases signif-icantly with the addition of water in the reaction mixture (Fig. 8).This is accompanied by an increase of XCO maximum from 7% (at250 ◦C) to 40% and 47% (at 300 ◦C) for Ru/platelet (Fig. 8a) andRu/fishbone (Fig. 8b), respectively, and from 25% (at 250 ◦C) to 53%

r the hydrogenation of CO and CO2 mixture, in the absence and in the presence ofcomposition: 1% CO, 15% CO2, 50% H2, 0% or 30% H2O (balance He); total flow rate:

CO/CO2/H2/H2O

(s−1) SCH4 at 250 ◦C (%) TOFCO at 250 ◦C (s−1) TOFCO2 at 330 ◦C (s−1)

100 0.058 0.51100 0.043 0.38100 0.070 0.50

V. Jiménez et al. / Applied Catalysis A

400350300250200-200-100

0

20

40

60

80

100a

0.5% Ru/platelet

XC

O, X

CO

2 (%

)

Temperature (ºC)

H2O Content (%)

0 30

, XCO, XCO2

400350300250200

-200

-100

0

20

40

60

80

100b

0.5% Ru/fishbone

XC

O, X

CO

2 (%

)

Temperature (ºC)

H2O Content (%)

0 30, XCO, XCO2

400350300250200

-200

-100

0

20

40

60

80

100c

0.5% Ru/ribbon

XC

O, X

CO

2 (

%)

Temperature (ºC)

H2O Content (%)

0 30

, XCO, XCO2

Fig. 8. Effect of addition of water vapour in the feed (30%) on the catalytic per-formance of Ru (0.5 wt.%) catalysts supported on (a) platelet, (b) fishbone and(s0H

(tbinlfs

c) ribbon for co-methanation of CO/CO2 mixture. Solid symbols: CO conver-ion; open symbols: CO2 conversion. Mass of catalyst: 150 mg; particle diameter:.18 < dp < 0.25 mm; feed composition: 1% CO, 15% CO2, 50% H2 0–30% H2O (balancee).

at 300 ◦C) for Ru/ribbon (Fig. 8c) CNF. Regarding CO2 hydrogena-ion, it is observed that this reaction remains practically unaffectedy increasing water content from 0 to 30% (Fig. 8) for all samples

nvestigated. Different results have been previously reported overoble metal catalysts supported on Al2O3 [25] and Ru/TiO2 cata-

ysts [17], where it was found that addition of water vapour in theeed does not affect CO hydrogenation but shifts the CO2 conver-ion curve toward higher temperatures. Similarly, Batista et al. [58]

: General 390 (2010) 35–44 43

reported that the CO2 methanation does not occur appreciably inthe presence of water in the feed.

Regarding the mechanism of CO/CO2 hydrogenation reactions, ithas been proposed that CO methanation proceeds via dissociationof carbon monoxide to C and O atoms, followed by their hydro-genation into CH4 and H2O [17,54,55,65,66]. Results of Fig. 8 showthat this reaction is enhanced appreciably by the presence of water.This indicates that methanation of CO is favoured, compared to thereverse WGS, under these conditions. Concerning CO2 methana-tion, it is believed to involve conversion of CO2 toward CO via theRWGS (Eq. (3)), followed by CO hydrogenation to methane [17].If this is the case, then increase of water concentration in the feedshould shift Eq. (3) to the left, thereby resulting in an increase of COconversion in agreement with results of Fig. 8. This indicates thatthe addition of steam in the reaction mixture retards the RWGSwhich is responsible for the “negative” values of XCO (Figs. 7 and 8).

The addition of water vapour in the gas mixture also affectsselectivity toward hydrogenation products. Results show (Table 2)that for all catalysts examined selectivity toward methane isimproved in the presence of water. For example, SCH4 at 250 ◦Cfor Ru/ribbon catalyst increases from 91.8 to 100% with increas-ing the concentration of water in the feed from 0 to 30% (Table 2).It has been reported that SCH4 always increases with increasingXCO in a manner which does not depend on whether this is dueto an increase of reaction temperature [17,25], to the effect ofmetal crystallite size [17], to the effect of the oxide support [17]or to variations of space velocity [17,25]. Furthermore, Inui et al.[67] demonstrated that the formation of higher hydrocarbons hasa strong retarding effect on CO methanation, which is releasedwith increase of CO conversion. It may then be suggested thatthe observed increase of SCH4 with the addition of water vapour(Table 2) is related to the increase of XCO (Fig. 8).

Results of kinetic measurements (Table 2) showed that, forall catalysts examined, TOF of CO2 conversion remains practicallyunaffected by the presence of water, whereas TOF of CO conversionincreases with increasing water content from 0 to 30%. In partic-ular, TOF of CO over Ru/fishbone catalyst increases by a factor of2 (at 250 ◦C) with the addition of 30% water in the feed. In thecase of Ru/ribbon catalyst TOFs of CO are practically the same inthe absence and in the presence of steam at temperatures lowerthan 250 ◦C, but becomes higher in the latter case above 250 ◦C (notshown for clarity). The effect is more pronounced for the Ru/plateletcatalyst, for which the specific activity for the CO hydrogenationreaction becomes more than three three times higher (at 250 ◦C)with the addition of water (Table 2).

Results of the present study show that the 0.5%Ru/CNFs catalystsinvestigated exhibit similar catalytic activity with that obtained inprevious studies over the well known Ru/Al2O3 catalyst conductedunder the same experimental conditions [25]. However, it has beenreported that catalytic performance for the title reaction could befurther improved provided that catalyst characteristics are opti-mized and operating conditions are properly selected [17,25]. Forexample, it has been found that catalytic activity of Ru/TiO2 andRu/Al2O3 catalysts is greatly enhanced with increase of Ru load-ing or crystallite size [17,33]. Furthermore, results presented inFigs. 5–8 were obtained with a relatively high space velocity (SV)of ca. 48,800 h−1, compared to that usually used (5000 h−1) in fuelcell applications. It was found over 0.5% Ru/Al2O3 catalysts thatdecreasing SV from 48,800 h−1 to 12,200 h−1 resulted in a shift ofthe XCO curves toward lower temperatures by ca. 80 ◦C, whereasthe XCO2 curve was not affected significantly [17]. Thus, catalytic

performance of the investigated 0.5% Ru/CNFs catalysts could befurther improved by decreasing SV and/or increasing Ru loading(i.e. crystallite size), leading to completely methanation of CO andmaking them suitable for use in the selective methanation of COfor fuel cell applications.

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aotowvRapppwRtugffaipacRa

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4 V. Jiménez et al. / Applied Cat

. Conclusions

Results of the present study show that catalytic performancend selectivity to reaction products over Ru catalysts supportedn different types of CNFs depend strongly on the experimen-al conditions used (e.g. reaction temperature, solo-methanationf CO or CO2, co-methanation of CO/CO2 mixtures, addition ofater vapour etc.). Under solo-CO methanation conditions, CO con-

ersion reaches 100% at 340 ◦C over all catalysts examined, withu/platelet sample being somewhat more active than Ru/fishbonend Ru/ribbon. Under solo-CO2 methanation conditions, catalyticerformance is not affected by the nature of the CNFs used as sup-ort. In co-methanation of CO/CO2, hydrogenation of CO is theredominant reaction until a certain temperature is reached, abovehich conversion of CO starts to decrease due to the onset of theWGS reaction and hydrogen starts to be unselectively consumedo methanate CO2. All catalysts examined tend to enhance thendesired RWGS reaction, exhibiting low activity for CO hydro-enation reaction. However, addition of 30% water vapour in theeed enhances methanation of CO, inhibiting the RWGS reactionor all catalysts investigated, whereas CO2 hydrogenation is notffected by the presence of steam. For all experimental conditionsnvestigated selectivity toward CH4 increases with increasing tem-erature at the expense of higher hydrocarbons and increases bydding water in the reaction mixture. Among the various Ru/CNFsatalysts investigated, optimal results were obtained over theu/ribbon catalyst, which exhibits maximum CO conversion of 53%t 300 ◦C under realistic feed composition.

cknowledgement

The authors gratefully acknowledge financial support from Con-ejería de Ciencia y Tecnología de la Junta de Comunidades deastilla-La Mancha (Projects PBI-05-038 and PCI 08-0020-1239).

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