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Published: February 04, 2011

r 2011 American Chemical Society 864 dx.doi.org/10.1021/ef101479y| Energy Fuels 2011, 25, 864877

ARTICLE

pubs.acs.org/EF

Influence of the Catalyst Preparation Method, Surfactant Amount, andSteam on CO2 Reforming of CH4 over 5Ni/Ce0.6Zr0.4O2 Catalysts

Thitinat Sukonket,, Ataullah Khan, Bappy Saha, Hussameldin Ibrahim, Supawan Tantayanon,

Prashant Kumar, and Raphael Idem*,

Process Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina,Saskatchewan S4S 0A2, CanadaGreen Chemistry Research Lab, Faculty of Science, Department of Chemistry, Chulalongkorn University, Bangkok 10330, ThailandHTC Purenergy Inc., #150-10 Research Drive, Regina, Saskatchewan S4S 7J7, Canada

ABSTRACT: A series of ceria-zirconia mixed oxide supports with nominal composition Ce0.6Zr0.4O2 were synthesized by two dif-ferent routes, namely, a surfactant-assisted route anda coprecipitation route. Among thesupports obtained by the surfactant-assistedroute, different surfactant/metal molar ratios (namely, 1.25, 0.8, and 0.5) were employed to study the influence of the surfactantamount on the catalyst performance. A nominal 5 wt % Ni was impregnated on the supports by a wet impregnation method. These

catalysts were evaluated for CO2 reforming of CH4 in both the presence and absence of steam. The textural, structural, andphysicochemical characteristics of the catalysts were thoroughly investigated with the help of various bulk and surface characteriza-tion techniques. The activity results indicate the superior nature of the catalysts obtained by the surfactant-assisted route over theone obtained by coprecipitation. Also, within the limits of the surfactant ratios used, the amount of surfactant employed during thecourse of support preparation seems to affect the activity, with catalysts prepared with the higher surfactant/metal molar ratioexhibiting better activity and enhanced stability. Structure-activity relationships (SARs) were formulated for some of the char-acteristics in order to explain the marked difference in activity between the catalysts obtained by the surfactant-assisted andcoprecipitation methods and between the catalysts prepared by the surfactant-assisted route but with different surfactant/metalmolar ratios. The SARs helped to identify that high oxygen storage capacity, high surface area, high reducibility, higher nickel surfacearea, better nickel dispersion, and higher surface nickel content are necessary for good performance in the CO 2 reforming of CH4.On the whole, catalysts obtained by the surfactant-assisted route exhibit a reasonably good performance in the CO2 reformingreaction but were prone to deactivation in the presence of steam. The inherent hydrophilic nature of the ceria-zirconia support isthe main cause for the apparent deactivation in the presence of steam.

1. INTRODUCTION

The CO2 reforming of CH4 to produce synthesis gas hasattracted attention from academia and industry in recent years

because this reaction involves gases that are intimately related tothe greenhouse effect and energy supply. This reaction directlyconverts two potent greenhouse gases (CH4 and CO2) to syn-thesis gas with a low H2/CO ratio (1), which satisfies therequirement of many important processes, such as Fischer-Tropsch synthesis and carbonyl production.1 However, CO2reforming uses a high C/H feedstock, which results in carbondeposition on the catalyst by CO disproportionation (2CO fCO2 C) and/or methane decomposition (CH4f 2H2 C)reactions. This problem has become the major issue in CO2reforming of methane.2 Thus, current intense research effortshave been focused in the development of catalysts that show highactivity and are also resistant to carbon formation and sintering.

With its relative availability, low cost, and activity comparable to anoble metal catalyst, nickel-based catalysts have shown goodpotential as a catalyst for the reforming of methane.3 Nickel has

been supported on various materials such as MgO,4,5 Al2O3,6,7

SiO2,8,9 (MgFe)2SiO4,

10CeO2,11ZrO2,

12 andCe1-xZrxO2.13-19

Currently, the Ce1-xZrxO2 solid solution is considered as apromising support13-19 for nickel-based catalysts. The addition

of ZrO2 to CeO2 has been found to improve the oxygen storagecapacity (OSC), redox property, thermal stability, and catalyticactivity.1,13,14,16,20,21Moreover, the zirconium content affects thestructure and redox properties of the ceria-zirconia mixedoxides. Usually, there are two phases (tetragonal and mono-clinic) in the CexZr1-xO2 samples with xe 0.5.

22However, onlya cubic fluorite phase is formed in the CexZr1-xO2 samples whenx is higher than 0.5.23 Generally, CexZr1-xO2 solid solutions

where 0.8 > x > 0.6 are preferredfor catalyticapplications. Severalmethods have been used to prepare CexZr1-xO2 solid solutionsfor catalytic applications. These include the high-temperaturefi

ring or high-energy milling of a mixture of the oxides,24,25

coprecipitation,1,20-22,26-28 sol-gel techniques,13-16 and a sur-factant-assisted templating route.29-33 Among these methods,the surfactant-assisted route, a modified coprecipitation assisted

with the surfactant, has attracted considerable interest because ofits effective soft template effect, reproducibility, and simplicity.29

This method can be used to prepare the solid solutions with highsurface area and thermal stability that are essential for high-temperature catalytic application.32,33 The Ce0.6Zr0.4O2 systems

Received: September 2, 2010Revised: January 5, 2011

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865 dx.doi.org/10.1021/ef101479y |Energy Fuels 2011, 25, 864877

Energy & Fuels ARTICLE

have already been identified by our group as a promising supportmaterial for nickel-based catalysts for the CO2 reforming pro-cess.32,33 It is reported that 5% Ni/Ce0.6Zr0.4O2 prepared by thesurfactant approach is a good catalyst for CO2 reforming of CH4

with stable activity when used for 100 h at 650 and 700 C, whileat 800 C, its activity remained stable when used for more than

230 h.32-34

However, nickel-based catalysts are readily deacti-vated by carbon deposition at low-temperature operating condi-tions. Much effort has been devoted toward coke suppression inthe case of nickel-based catalysts. Recently, theaddition of a smallamount of steam to the dry reformingfeedstock was attempted inorder to eliminate carbon formation.2,35-45 Hence, the simulta-neous steam and carbon dioxide reforming of methane, known asthe mixed reforming reaction, allows limited control of the H2/CO ratio while overcoming carbon deposition. In our previousstudy,32,33 a surfactant/metal molar ratio of 1.25 was used.

Although it yields an excellent catalyst, the waste materialgenerated during the course of support preparation was large.In the present study, we are also looking at the potential todevelop nickel-based catalysts supported on carriers prepared

using the surfactant-assisted route with optimal utilization of asurfactant, thereby reducing the generation of chemical wastesand, thus, the production costs.

Accordingly, Ce0.6Zr0.4O2 supports were prepared by thesurfactant-assisted route with different surfactant/metal molarratios.The catalytic activity of the resulting catalysts when 5 wt %Ni was impregnated on these supports was examined for CO2reforming of CH4 in both the presence and absence of steamand compared with those of 5 wt % Ni impregnated on sup-ports prepared by the conventional coprecipitated method. Theresults of these comparisons are presented and discussed in thispaper.

2. EXPERIMENTAL SECTION

2.1. Catalyst Preparation. In order to prepare each Ce0.6Zr0.4O2mixed oxide support by the surfactant-assisted route, appropriate quan-tities of Ce(NO3)3 3 6H2O (99% purity, Aldrich) and ZrO(NO3)2 3H2O(99.9% purity, Alfa Aesar) precursor salts were dissolved in deionized(DI) water. Separately, a calculated amountof cetyltrimethylammonium

bromide (CTAB;g98% purity, Sigma) was dissolved in DI water at60 C. The metal nitrate solution was then added to the surfactantsolution to obtain a mixture solution. The molar ratios of [CTAB]/[Ce Zr] were 1.25, 0.8, and 0.5. Aqueous ammonia [28-30% (w/w);reagent grade, ACS-Pur] was gradually added to the aforementionedmixturesolution undervigorous stirringuntil precipitationwas complete(pH 11.8). The addition of ammonia induced precipitation of a

gelatinous yellow- brown colloidal slurry. The slurry was stirred for60 min in a glass reactor, subsequently transferred into Pyrex glass

bottles, sealed, and aged hydrothermally in autogenous pressureconditions for 5 days at 90 C. After this timeframe, the bottles werecooled and the resulting precipitate was filtered and washed repeatedly

with warm DI water. The resulting cakes were oven-dried at 120 Covernight and finally calcined at 650 C for 3 h in flowing air. Similarly,pristine CeO2 was prepared by the surfactant-assisted route followingthe exact procedure detailed above.

The Ce0.6Zr0.4O2 mixed oxide support was also prepared by theconventional coprecipitation method, wherein calculated amounts ofCe(NO3)3 3 6H2O and ZrO(NO3)2 3 2H2O salts were dissolved in DI

water and hydrolyzed with aqueous ammonia until precipitation was complete. The resulting precipitate was filtered, oven-dried at120 C overnight, and finally calcined at 650 C for 3 h in an airenvironment.

A nominal 5 wt % Ni was loadedover theabove-prepared supports bya standard wet impregnation method. In a typical impregnation, about14 g of the catalyst support was immersed in 128 mL of a 0.1 MNi(NO3)2 solution (99.999% purity, Aldrich). The mixture was sub-

jected to slow heating under constant stirring in a hot water bath so as to

remove the excess water; the dried powders thus obtained were calcinedat650 Cinairfor3h.PristineNiOinbulkformwaspreparedbyasolid-state preparation route, wherein a known amount of Ni(NO3)3 3 6H2Osalt (99.999% purity, Aldrich) was calcined at 650 C for 3 h in flowingair in order to yield bulk crystalline NiO.

For the sake of brevity, the supports and corresponding catalystsemployed in the present investigation are abbreviated as detailed inTable 1. For instance, CZ and NCZ correspond respectively toCe0.6Zr0.4O2 and 5 wt % Ni/Ce0.6Zr0.4O2 , while for the samplesobtained by the surfactant-assisted route, the numbers in parenthesescorrespond to the CTAB/[Ce Zr] (surfactant-to-metal) molar ratio.

2.2. Catalyst Characterization. Surface Area and Pore SizeDistribution Analysis. The Brunauer-Emmett-Teller (BET) surfacearea and pore size distribution analyses for all catalysts were obtained by

N2 physisorption at liquid-N2 temperature using a Micromeritics ASAP2010 apparatus. Prior to analysis, all of the samples were degassedovernight at 180 C under vacuum. The pore size distribution andaverage pore volume were analyzed using the desorption branch of theN2 isotherm. Each sample was analyzed by N2 physisorption at leasttwice in order to establish repeatability. The error in these measure-ments was e1%.

Metallic Surface Area and Metal Dispersion Measurements. Themetallic surface area and metal dispersion in the catalyst samples wereestimated by H2 chemisorption at 35 C using a Micromeritics ASAP2010C instrument. Prior to analysis, the catalyst samples were dried at120 C and then reduced in situ in flowing H2 gas (UHP grade) at700 C for 3 h (in order to mimic the reduced state formed during thecourse of a typical catalytic run) followed by evacuationat 700 Cfo r1h

Table 1. Textural Characterization

samplepreparation

routea acronym

CTAB/[Ce Zr]molar ratio

BET surfacearea (m2 g-1)

porevolume

(cm3 g-1)

averagepore

diameter ()

pore volume/BET surface

area (10-9m)

OSC[mol of O

(g of cat.)-1]

Nidispersion(%) DNi

Ni surfacearea [m2 (g of

cat.)-1] SNi

averagecrystallite sizeCeO2 (nm)

b

Ce0.6Zr0.4O2 CP CZ n/a 93 0.06 32 0.6 866 4.05% Ni/Ce0.6Zr0.4O2 WI NCZ n/a 59 0.06 51 1.0 4.2 1.4 3.8

Ce0.6Zr0.4O2 SA CZ(1.25) 1.25 201 0.3 41 1.5 1093 4.65% Ni/Ce0.6Zr0.4O2 WI NCZ(1.25) 1.25 184 0.2 41 1.1 7.4 2.5 4.1Ce0.6Zr0.4O2 SA CZ(0.8) 0.8 203 0.4 57 2.0 1031 4.35% Ni/Ce0.6Zr0.4O2 WI NCZ(0.8) 0.8 169 0.3 53 1.8 6.4 2.0 4.2Ce0.6Zr0.4O2 SA CZ(0.5) 0.5 232 0.4 51 1.7 937 4.25% Ni/Ce0.6Zr0.4O2 WI NCZ(0.5) 0.5 215 0.3 51 1.4 5.4 1.8 3.2

a CP = coprecipitation route; WI = wet impregnation route; SA = surfactant-assisted route. b Calculated using the Debye-Scherrer equation.

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before cooling down to 35 C. The metallic surface area (SNi) wascalculated with the help of the following expression:

SNi 13:58 10-20NM m

2 g of cat:-1

where NM is the number of hydrogen molecules adsorbed in themonolayer per gram of catalyst. The above expression was derived byconsidering the surface occupied per atom of nickel as 6.49 2 atom-1

(considering the density of nickel as 8.91 g cm-3 and a face-centered-cubic lattice) and the adsorption stoichiometry as two surface nickelatoms per hydrogen molecule. The nickel dispersion (D%) was thencalculated as the percentage of surface nickel atoms with respect to thetotal nickel atoms in the catalysts.46 The H2 chemisorption analysis wasrepeated for a few of the samples in order to check reproducibility. Theerror in these measurements was

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studied. A few samples were analyzed by TPR at least twice in order toestablish reproducibility. The error in the Tmax values was found to beless than (4 C.

Raman Analysis. The Raman analyses were performed on a Re-nishaw inVia Raman microscope using a Ar laser (Spectra Physics)operating at 514.5 nm.The laser beam (10 mW at the laser) was focusedonto a pelletized sample using a Leica 20X NPLAN objective (NA =0.40). TheRaman spectrawere acquired using a 10 s detector acquisitiontime, and the spectra were accumulated to achieve sufficient signal-to-noise intensities. The spectra were baseline-corrected using the Re-nishaw Wire V3.1 software provided with the instrument. The wave-numbers obtained from the spectra are accurate to within 2 cm-1.

X-ray Photoelectron Spectroscopy (XPS) Measurements. The XPSmeasurements were performed on a Leybold MAX 200 X-ray photo-electron spectrometer using Al KR (1487 eV) radiation as the excitationsource. Prior to analysis, the samples were outgassed in a vacuum ovenovernight. Charging of the samples was corrected by setting the bindingenergy (BE) of the adventitious carbon (C 1s) at 285 eV.47XPS analysis

was performed at ambient temperature and at pressures typically on theorderof

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3. RESULTS AND DISCUSSION

3.1. Catalyst Characteristics. The textural characteristics ofthe supports and those of the catalysts prepared in this study aresummarized in Table 1. From Table 1, it is noted that thesupports and catalysts prepared by the surfactant-assisted routeexhibit larger specific surface area and pore volume compared tothe supports and catalysts prepared by the coprecipitation method.Similar results were reported by Terribile et al.,29 wherein thehigher surface areas were obtained as a result of interaction

between a hydrous mixed-metal hydroxide gel and cationicsurfactants under basic conditions. At pH g 11.0, the surfacehydroxyl protons [CeZr(O-H)4] are exchanged with thecetyltrimethylammonium cation [(C16H33)N

(CH3)3], result-ing in the incorporation of the surfactant cations into a hydrousceria-zirconia mixed hydroxide oxide gel. This incorporationdecreases the interfacial energy and eventually decreases thesurface tension of water that exists in the hydrous support pores.

As a result, the degree of shrinkage and pore collapse that wouldoccur in the hydrous support during drying and calcination isreduced, thereby imparting a high surface area to the sample. The

current results reveal that the supports obtained by the surfac-tant-assisted route possess very high surface areas (Table 1) andchanges in the surfactant/metal molar ratios do not seem to havea strong influence over the resultant surface area. Upon impreg-nation of the supports obtained by surfactant-assisted andcoprecipitation routes, with a nominal 5 wt % Ni, a loss in thesurface area can be seen, as shown in Table 1. This is a generalphenomenon observed in thecase of supported catalysts whenanactive component is impregnated over its surface and into itspores. The observed decrease is mainly due to penetration of thedispersed nickel oxide into the pores of the support, therebynarrowing its pore diameter and blocking some of the pores.32

Likewise, there can be instances wherein pore collapse could leadto increases in the pore volume and pore diameter, as could be

noted with the coprecipitated sample (Table 1.).The N2 isotherms of catalysts obtained from supports pre-pared by the surfactant-assisted route are shown in Figure 1A.The isothermsbelong to the type IV class and exhibit type H2hysteresis. The isotherms point to the existence of mesoporosity

with network effects in the analyzed catalysts. Additionally, thepore volume versus poresize distribution patterns of the catalystsobtained by the surfactant-assisted route are shown in Figure 1B.

Analysis of the observed trends in the pore volume distributionpatterns reveals that the size of the existing pores is not dispersedover a wide range, and it is, in fact, limited to a narrow range of2-10 nm. The average pore sizes reported in Table 1 are in goodagreement with those in Figure 1B. Furthermore, it is noted thatpores of >10nm diameter are nonexistent in the analyzed catalyst

samples.The calcination temperature (650 C) employed in the

current study is lower than the operating temperature of thecatalytic studies (700 and 800 C). In order to evaluate thethermal stability of the catalyst at reaction conditions (essentiallythe potential for sintering under reaction conditions), two samplesof the 650 C calcined NCZ(1.25) catalyst were recalcined, one at700 C and the other at 800 C for 3 h in flowing air. Therecalcined samples were analyzed by the N2 physisorptiontechnique.The results are presented in Table 2 and are compared

with the results obtained for the 650 C calcined sample. As isevident from Table 2, there is a loss in the surface area andporosity as the temperature increases. This change is considered

to be due to the loss of surface capping hydroxyl and oxygen ionsfrom the catalyst, thereby opening up the pores with a corre-sponding decrease in the surface area (see Table 2). The majoreffect of the temperature appears to be that the pore volume/surface area value increases with an increase in the temperature(Table 2). On the other hand, the N2 physisorption isotherms ofthe NCZ(1.25) catalyst samples calcined at different tempera-

tures (shown in Figure 1C) show that the isotherms for the 700and 800 C calcined samples are similar to the isotherm for the650 C calcined sample, and all three isotherms can be categor-ized into the type IV isotherm. The type IV isotherm is generallyassociated with mesoporous materials. Analysis of the pore

volume versus pore size distribution patterns (Figure 1D) ofthe above calcined samples revealsthata majority of the poresareconcentrated in the size range of 4-5 nm. Thus, it can beconcluded that, at higher temperature reaction conditions, thecatalyst sample retains its mesoporosity, and there is no sintering

because the latter would imply a loss of surface area due toagglomeration and the collapse of the pore structure of thematerial.

The OSC is the ability of the catalysts to undergo cyclic

reduction/oxidation under the given operating conditions. Thisunique feature in Ce0.6Zr0.4O2 solid solutions is due to theirinherent ability to easily and reversibly reduce to O2-deficientnonstoichiometric compounds with a nominal compositionCe0.6Zr0.4O2-x (where 0 < x < 0.178) when exposed to reducingatmospheres and subsequently regain their original stoichiome-try when exposed to oxidizing atmospheres. Interestingly,Ce0.6Zr0.4O2 solid solutions retain the same fluorite crystalstructure in both reduced and oxidized states, thus facilitating theredox process.48,49 The OSC measurements obtained for variousCe0.6Zr0.4O2 supports investigated in the current study areshown in Table 1. The Ce0.6Zr0.4O2 supports prepared by thecoprecipitation and surfactant-assisted routes with varying sur-factant/metal ion molar ratios exhibit variable OSC functionality

(Table 1), which implies that they differ in their redox proper-ties. Supports with higher OSC values will possess a highly facileCe4 S Ce3 redox couple compared to the ones with lowerOSC values. The OSC of the support obtained by the surfactant-assisted route is higher than the OSC of supports obtained by thecoprecipitation route. Within the family of supports obtained bythe surfactant-assisted route, it is observed that the OSC increases

with increasing surfactant/metal molar ratios (Table 1). Recentliterature reports suggest that the OSC of the ceria-zirconiasolid solution is independent of its specific surface area.48,49

The XRD patterns of the supports and catalysts investigated inthe current study are shown in Figure 2 along with the XRDprofiles of pristine ceria and nickel oxide samples. The XRD

Table 2. Effect of the Calcination Temperature on theTextural Characteristics of NCZ(1.25)

calcination

temperature

(C)

duration

(h)

BET surface

area

(m2 g-1)

pore

volume

(cm3 g-1)

average

pore

diameter

()

pore volume/

BET surface

area

(10-9 m)

5% Ni/Ce0.6Zr0.4-NCZ(1.25)

650 3 184 0.25 41 1.1

700a 3 153 0.22 47 1.4

800a 3 102 0.15 51 1.5a Using the 650 C calcined NCZ(1.25) sample.

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profile of pristine ceria is characteristic of the presence of a cubicfluorite-type crystal structure. The diffraction patterns of thesupports and catalysts exhibit diffraction patterns exactly thesame as that of pristine ceria, thus confirming the existence of asingle-phase cubic fluorite-type structure. A distinct shift towardhigher 2 values could be noted in the case of Ce0.6Zr0.4O2 and5Ni/Ce0.6Zr0.4O2 samples (2 29.0), when compared withpure CeO2 (2 = 28.65). The shift in the peak positions can beattributed to the structural distortion caused by the substitutionof smaller Zr4 ions (0.84 ) in place of larger Ce4 ions(0.97 ) in the cubic fluorite lattice.47 The XRD profile of

pristine NiO was used to identify the existence or nonexistence ofcrystalline NiO in the nickel-impregnated catalyst samples. 5%Ni/Ce0.6Zr0.4O2 prepared by the coprecipitation method showedthe presence of crystalline NiO, while the 5% Ni/Ce0.6Zr0.4O2catalysts obtained by the surfactant-assisted route are free of anycrystalline NiO. The presence of crystalline NiO in the catalystsample indicates that the impregnated NiO species are nothomogeneously dispersed over the support surface, resulting inthe formation of bulk crystalline structures. The occurrence ofcrystalline bulk NiO is not favorable for the dry reformingprocess because it is known to catalyze coke-forming reactionssuch as CO disproportionation and CH4 decomposition.

32

The crystallite size (DXRD) of Ce0.6Zr0.4O2 in each sample wascalculated by using XRD data of the most prominent lines 111,

200, 220, and 311 and is given in Table 1. In the case of theCe/Zr supports obtained by the coprecipitation route, theaverage crystallite size was 4 nm, which is smaller than theaverage crystallite size observed in the Ce/Zr supports obtained

by the surfactant-assisted route. Among the Ce/Zr supportsprepared by the surfactant-assisted route, the average crystallitesize decreased slightly with a decrease in the CTAB/[Ce Zr]ratio. The crystallite size of CeO2 in pristine ceria wasfound to be7.2 nm. The formation of a solid solution between cerium andzirconium retards the crystallite growth, thus paving the way forthe formation of thermodynamically metastable phase(s), as wasobserved in the previous study.47 The crystallite size of Ce0.6Z-r0.4O2 in the case of nickel-impregnated catalyst samples varied in

the range of 3.2-4.2 nm, with NCZ(0.5) exhibiting the smallestcrystallite size (3.2 nm).

A H2 chemisorption technique was employed to estimate themetallic surface area and metal dispersion of the active compo-nent (nickel); the observed findings are given in Table 1. It isimportant to once again emphasize that all of the catalystformulations were prepared by a standard wet impregnationmethod and were loaded with the same amount of nickel, i.e.,5 wt %. During a chemisorption experiment, the sample is dried,reduced in hydrogen, evacuated, then cooled to the analysistemperature (35 C), and finally evacuated before performing

actual measurements. In a volumetric H2 chemisorption mea-surement, known amounts of hydrogen are dosed and subse-quently adsorbed at different partial pressures, resulting in achemisorption isotherm. This isotherm measurement is repeatedafter applying an evacuation step at the analysis temperature toremove weakly adsorbed species (back-sorption or a dual-isotherm method). The difference between the two isothermsrepresents the chemically bonded reactive gas and is used tocalculate the active metal surface area. This information iscombined with information on metal loading to calculate themetal dispersion. The relative measurement of chemically boundhydrogen was used to distinguish the four catalyst formulationsinvestigated in the current study. The dispersion of nickel in thecase of a coprecipitated catalyst (CZ-CP) was found to be lower

than that observed in any of the surfactant-assisted catalysts and,furthermore, the nickel dispersion decreased with a decrease inthe surfactant/metal molar ratio. The nickel surface area mea-surements also exhibit similar trends. From the above findings, itcan be inferred that a support prepared with a higher surfactant/metal molar ratio is able to support nickel in a highly dispersedstate and can also control the size of the nickel crystallites,thereby improving the nickel surface area and nickel dispersion as

well as the surface nickel content. With the coprecipitatedcatalyst, agglomeration of nickel crystallites was confirmed fromthe XRD studies (Figure 2). These findings prove that the H2chemisorption measurements are in good agreement with the

XRD results.

Figure 2. XRD patterns of Ce0.6Zr0.4O2 supports and 5% Ni/Ce0.6Zr0.4O2 catalysts.
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The Raman spectroscopic characterization was performed in

order to get valuable information on both M-O (M = Ce, Zr)bond arrangement and lattice defects. In the present study, only afew representative samples, viz., CZ(0.5) and NCZ(0.5) corre-spondingto the lowest surfactant/metal ratio used and CZ(1.25)and NCZ(1.25) corresponding to the highest surfactant/metalratio used, were characterized by Raman spectroscopy and areshown in Figure 3. Raman spectroscopy of all of the samples ischaracterized by an intense sharp Raman feature at 470 cm-1 andtwo broad features at 620 and 285 cm-1. The band at 470 cm-1

corresponds to the triply degenerate F2gmode and can be viewedas a symmetric breathing mode of the oxygen atoms aroundcerium ions.16 The weak band observed near 600 cm-1 corre-sponds to a nondegenerate longitudinal optical mode of CeO2,

which normally should not be observed by Raman spectroscopy.

Therefore, its presence may be caused by lattice defects, whichlead to relaxation in the selection rules. In particular, this bandhas been linked to oxygen vacancies in the CeZrO2 lattice. Thesubstitution of Zr4 ions in place of Ce4 ions causes displace-ment of the oxygen atoms from their idealfluorite lattice positions,resulting in the appearance of a weak band at 285 cm-1.47 Therelative intensities of the band at 600 and 470 cm-1 (I600/I470)

were calculated and compared in order to get a comparativeestimate of the oxygen vacancy (V) concentration in theselected samples. In the case of CZ(1.25) and CZ(0.5), the ratioI600/I470 is found to be 0.25 and 0.2, respectively, while in thecase of NCZ(1.25) andNCZ(0.5), it is found to be 0.625 and 0.4,respectively. From the I600/I470 values, it is clear that the Vconcentration is higher in the CZ(1.25) and NCZ(1.25) samples

compared to the CZ(0.5) and NCZ(0.5) samples. On the basis ofthese results, it is established that the V concentration increases

with an increase in the surfactant/metal molar ratio. Further-more, the OSC measurements obtained by TGA also follow asimilar trend; i.e., the OSC increases with and increase in thesurfactant/metal molar ratio, showing that the Raman spectro-copy measurements are in agreement with the TGA measure-ments.

The TPR profiles of the support and catalyst powders ob-tained by surfactant-assisted and coprecipitation routes areshown in Figure 4, along with the TPR profiles of pristineCeO2 and NiO. The TPR profile of pristine NiO shows a sharpfeature at440 C, which occurs due to the reduction of NiO to

Ni. In the case of pristine CeO2, the one at the lower temperature(Tmax 600 C) was ascribed to the reduction of the surfaceoxygen species, and the other two broad peaks at highertemperatures (Tmax 780 and 950 C) were due to thereduction of bulk oxygen species.50The coordinately unsaturatedsurface-capping oxygen ions can be easily removed in the low-temperature region. However, bulk oxygen needs to be trans-

ported to the surface before their reduction. Consequently, thebulk reduction takes place at a higher temperature compared tothe surface reduction. The bulk reduction begins only after thecomplete reduction of the surface-capping oxygen ions.50 Pris-tine ZrO2 does not show any sign of reduction below 1000 C

because of its refractory nature.50 Ceria-zirconia solid solutionsexhibit two distinct reduction zones; the former at lower tem-peratures pertains to the surface shell reduction, while the latterat higher temperatures pertains to the bulk reduction. In the caseof the NCZ series of catalysts, a reduction peak at about440-490 C is associated with the reduction of NiO to Ni andthe other peaks at the higher temperature are associated with thesurface and bulk reduction of Ce4 to Ce3 species. A compar-ison between the catalysts revealed that the catalysts whose

supports were obtained by the surfactant method exhibit higherreducibility (i.e., lower Tmax) than the one obtained by copreci-pitation. A comparison among the reduction profiles of catalystsprepared from supports with variable CTAB/[Ce Zr] molarratios reveal that, in the case of NCZ(0.5), the reduction of NiOoccurs at higher temperatures compared to that in the case ofNCZ(1.25). The TPR studies revealed thatthe preparation routestrongly affects the reducibility of the NiO species in a givencatalyst. Forinstance, the reduction of NiO to Ni occurs at a Tmaxof 490 C in the case of the coprecipitation route and at a Tmaxof430-470 C in the case of the surfactant-assisted route. Withinthe family of catalysts obtained by the surfactant-assisted route, a

variation in the surfactant/metal molar ratio alters the reducibility of the surface NiO species. Among them, the order of

reducibility is as follows: NCZ(1.25) > NCZ(0.8) > NCZ(0.5).Reducibility is inversely proportional to the transition tempera-ture (Tmax) of NiO to Ni species. A lower Tmax implies higherreducibility. Among the various catalysts tested in the currentstudy, the ones obtained by the surfactant-assisted route exhibithighly complex reduction profiles compared to the one obtained

by the coprecipitation route. The complexity of the TPR profilesis a measure of the interaction between theactivecomponent andsupport.

XPS analysis was performed in order to understand the natureof interactions between the various surface species and to getinformation on their oxidation state and relative surface compo-sition. Accordingly, the measured electron binding energies (eV)of the O 1s, Zr 3d, Ce 3d, and Ni 2p photoelectron peaks and

the corresponding surface atomic composition are presented inTable 3. In the present study, only a few representative samples,

viz., CZ(0.5) and NCZ(0.5) corresponding to the smallestsurfactant/metal ratio used and CZ(1.25) and NCZ(1.25)corresponding to the highest surfactant/metal ratio used, werecharacterized by XPS. The corresponding O1s, Ce 3d, and Zr 3dpeaks of the chosen samples are shown in Figure 5. It is apparentfrom Figure 5a that the Ce 3d spectra are complex and made upof many individual overlapping peaks. The assignment of the

various peaks into two sets of spin-orbital multiplets was done byfollowing the notation of Burroughs et al.51 In brief, the featuresv, v00, v00 0 and u, u00, u00 0 correspond to cerium in a 4 oxidationstate, while the features v0, v

0 and u0, u0 correspond to cerium in a

Figure 3. Raman spectra of Ce0.6Zr0.4O2 supports and 5% Ni/Ce0.6Z-r0.4O2 catalysts.

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3 oxidation state. A closer look at Figure 5a reveals that thesamples investigated in the current study exhibit similar types ofCe 3d XPS features. The three main features of 3d5/2 at ca. 883.5

(v), 890.0 (v00), and 899.5 (v00 0) and three prominent features of3d3/2 atca. 902.5 (u),912.8(u

00),and917.5(u00 0) were observed,indicating the presence of cerium in a 4 oxidation state. Thepresence of Ce3 was established from the appearance of v0

and u0 features in the Ce 3d spectra at 887.5 and 908.5,respectively. Thus, from the above finding, it can be concludedthat both the support and catalyst samples have surface ceriumspecies in both 4 and 3 oxidation states, with the cerium(IV)species being predominant. Figure 5b shows the O 1s photo-electron peaks of selected samples. The O 1s peaks shown inFigure 4b are, in turn, composed of two peaks due to thenonequivalence of the surface oxygen chemical environments.The main feature at 530.6-531 eV corresponds to the surfacelattice oxygen of a CeZrO2 solid solution, where both cerium and

zirconium are present in their highest 4 oxidation state.According to the literature, the oxygen ions in pure CeO2 andZrO2 exhibit intense peaks at 528.6-530.1 and 530.0 eV,respectively.47 The broad feature (indicated by the blue circle)in Figure 5b correspondsto the oxide ions bonded to cerium(III)species,which are formed as a result of oxygen vacancyformation(which helps to maintain the charge neutrality), and as a result,an additional O 1s peak at the higher BE end (2.0-2.2 eV) isformed.52 Figure 5c shows the Zr 3d photoelectron peaks ofselected samples investigated in the current study. The corre-sponding BE values are shown in Table 3. As can be noted fromTable 3, the BE values are in the range of 182.7-183.0 eV, whichagrees well with earlier literature reports and corresponds to

zirconium in a 4 oxidation state.53 Galtayries et al.,54 whostudied ceria-zirconia solid solutions of varying composition,reported a BE of Zr 3d5/2 of 181.7-181.8 eV. The Ni 2p3/2 peak

(850-870 eV), not shown here, was used for chemical stateidentification and quantification purposes. In the case of NCZ-(1.25)and NCZ(0.5), the Ni 2p3/2 peak BE valueswere recordedat 856.9 and 856.8 eV, respectively. The above observed BE

values agree well with those reported for NiO and Ni(NO3)2,where Ni is present in its most stable oxidation state, i.e.,nickel(II).55On the whole, the XPSstudies revealed the presenceof nickel and zirconium in their highest oxidation states, i.e.,nickel(II) and zirconium(IV), respectively, while cerium wasfound to exist in both 4 and 3 oxidation states.

The surface atomic compositions of the selected supports andcatalysts, as determined by XPS, are presented in Table 3. Thetrends observed in Table 3 provide valuable information on thenature of the surface-active sites, which is vital for explainingtheir

resultant catalytic activities. From Table 3, it is evident that thesurface atomic composition varied with a variation of thesurfactant/metal molar ratio. The surface of NCZ(1.25) wascomposed of 31.0% surface cerium and 14% surface nickelspecies, while the surface of NCZ(0.5) had 32.8% surface ceriumand 6.3% surface nickel species. The marked superior perfor-manceof NCZ(1.25) over that of NCZ(0.5) for a CO2 reformingreaction can therefore be partly attributed to their surface atomiccomposition. From the current observation, it is apparent that ahigher surface nickel content (active component) and/or a lowersurface cerium content (redox component), either individuallyor combined, is/are necessary to obtain a superior catalystformulation. The bulk enrichment of cerium helps to promote

Table 3. XPS Characterization

binding energy (eV) surface atomic composition

support/catalyst CTAB/[Ce Zr] molar ratio acronym O 1s Zr 3d Ce 3d Ni 2p Zr % Ce % Ni %

Ce0.6Zr0.4O2 1.25 CZ(1.25) 530.6 182.7 883.5 59.2 40.8

Ce0.6Zr0.4O2 0.5 CZ(0.5) 530.8 182.9 883.8 62.2 37.8

5% Ni/Ce0.6Zr0.4O2 1.25 NCZ(1.25) 530.8 182.8 883.7 856.9 55.0 31.0 14.0

5% Ni/Ce0.6Zr0.4O2 0.5 NCZ(0.5) 531.0 183 884.0 856.8 60.9 32.8 6.3

Figure 4. TPR patterns of Ce0.6Zr0.4O2 supports and 5% Ni/Ce0.6Zr0.4O2 catalysts.

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the oxygen transport/buffer capacity of the catalyst.48 From the

current results, it seems that there is a correlation between theamount of surfactant employed and the degree of nickel disper-sion. With an increase in the amount of surfactant employed, it ispossible to improve the dispersion of nickel over the supportsurface. The above findings on the surface atomic composition ofnickel agree well with the H2 chemisorption measurements,presented earlier in Table 1.

3.2. Catalyst Performance Evaluation. 3.2.1. Effect of thePreparation Method and CTAB/[Ce Zr] Molar Ratio on theCatalyst Performance for CO2 Reforming of CH4. All of thecatalytic tests were performed under identical operating condi-tions. The performance of the different catalyst formulations wasmeasured in terms of the activity and stability. The tests wereperformed at 800 C using feed composed of 1:1:0.5 (vol %)

CH4/CO2/N2 at a flow rate of 100 sscm; the results obtainedthereof are presented in Figure 6. The direct measurement of thetextural and thermal stability of any given catalyst is its catalyticactivity and stability. From Figure 6, it is found that the catalystsobtained by the surfactant-assisted route exhibit high initialactivity with reasonable stability for the given time on stream(TOS) of operation. The initial activity of the coprecipitatedcatalyst was similar to that of the catalysts obtained by thesurfactant-assisted route. However, the activity of the coprecipi-tated catalyst started to deteriorate from 1 h TOS, losing 12%activity (% CH4 conversion) in the first 8 h. A comparison of theactivity data of catalysts prepared with different surfactant/metalmolar ratios indicates that the amount of surfactant employedplays a significant role in the resultant activity and stability, and

only NCZ(1.25) and NCZ(0.8) catalysts deliver a stable perfor-mance. All of the catalysts prepared in the current study exhibitalmost similar initial catalytic activity in terms of CH4 conversion(Figure 6; % CH4 conversion, 1 h data). However, beyond that,their relative catalytic activities vary significantly. The reason

behind the identical initial activity can be explained in terms ofnickel loading (active component) and its rate of sintering andagglomeration. Because all of the Ce0.6Zr0.4O2 supports preparedin the current work were impregnated with a nominal amount of5 wt % Ni under exactly identical conditions, all of the freshcatalysts would exhibit almost identical initial responses to theCH4/CO2 feed. However, with time, the one obtained by thecoprecipitation route seems to deactivate faster because of coke

deposition, thereby leading to a decline in the activity. On the

other hand, the catalysts obtained by the surfactant-assistedroute, followed by considerable (5 days) hydrothermal aging,maintain a steady active site density over the surface of thecatalyst. This reasoning is supported by the XRD studies(Figure 2) performed on the fresh catalyst samples, which revealthe existence of crystalline NiO in the sample obtained by thecoprecipitation route and the nonexistence of any crystallineNiO in the case of catalysts obtained by the surfactant-assistedroute. The crystalline NiO (large ensembles) is known tocatalyze coke formation reactions, while the nickel present in ahighly dispersed state is found to be beneficial for the dryreforming of methane.56 The overall results show that a highdispersion of NiO leads to better stability, as noted in Figure 6.On the whole, from the current study, it can be inferred that the

use of a high surfactant/metal molar ratio led to better dispersionof the NiO species, as noted from the H2 chemisorption studies;thus, a stable catalyst formulation [NCZ(1.25)] for the dryreforming of methane is formed.

Figure 6 also shows activityvariations with TOS in terms of theTOF of CH4 observed for the investigated catalysts. In thepresent study, it is noted that the catalyst formulation[NCZ(1.25)] that exhibits the highest CH4 conversion has themost surface nickel (Table 1) but possesses the lowest TOF formethane. From the above results, it can be hypothesized that notall of the surface nickel sites present in the NCZ(1.25) formula-tion participate in the dry reforming process. According to theliterature, both the surface-active metal sites and lattice oxygendefects (oxygen vacancies) have to be present in the right

proportion for the nickel site to participate effectively in thedry reforming reaction.57Numerical analysis of the surface nickelsites and oxygen vacancies was performed for all of the catalystformulations used in this study. It can be considered that (i) theatomic cross section of each nickel atom is about 6.49 2 (1 2 =10-20 m2), (ii) the density of nickel metal is 8.91 g cm-3, and(iii) nickel can be assumed to be present in the face-centered-cubic lattice. According to the spherical model for the metallicparticles, the NCZ(1.25) catalyst having a nickel surface area of2.45 m2 (g of cat.)-1will have 3.77 1019 nickel atoms pergram of catalyst. With similar calculations, the NCZ(0.8), NCZ-(0.5), and NCZ(CP)catalysts will have 3.06 1019, 2.74 1019,and 2.14 1019 nickel atoms per gram of catalyst, respectively.

Figure 5. XPS spectra of Ce0.6Zr0.4O2 supports and 5% Ni/Ce0.6Zr0.4O2 catalysts.

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From the OSC measurements, the estimated numbers of oxygenvacancies in the NCZ(1.25), NCZ(0.8), NCZ(0.5), and NCZ-(CP) catalyst formulations are 6.58 1020, 6.21 1020,5.64 1020 , and 5.21 1020 oxygen vacancies per gramcatalyst, respectively. By following the measurements of surface-active nickel sites and oxygen vacancy defects, it is clear that,

among all of the catalyst formulations, NCZ(1.25) possesses thelargest number of both surface-active nickel sites and oxygenvacancies. However, it is observed from the TOF results that it isthe ratio of the total number of oxygen vacancies per gram ofcatalyst to the total number of surface nickel atoms per gramof catalyst that is related to the observed initial TOF of CH4(1 h data). As this ratio increases, the initial TOF of CH 4 alsoincreases, as shown in Figure 7. Thus, these observations confirmthat both the surface-active metal sites and lattice oxygen defects(oxygen vacancies) have to be present in the right proportion foreach nickel site to participate effectively in the dry reformingreaction.57

3.2.2. Effect of the Addition of Steam on the Performance ofCatalysts for CO2 Reforming of CH4. Li et al.

45 reported that the

addition of steam to the dry reforming feed gas (mixed refor-ming) can significantly minimize catalyst deactivation caused bycokedeposition on the catalyst surface. The steam/methaneratioplays an important role in the overall steam-assisted dry reform-ing process. Generally, higher steam/methane ratios yield higherconversions, but excess steam over that required by the reactionstoichiometry is energetically unfavorable and also dilutes thefeed gas. On the contrary, the use of lower steam/methane ratiosleads to carbon deposition problems.37 In order to find out

whether the addition of steam is beneficial to the overall activity,two catalyst formulations, namely, NCZ(1.25) and NCZ(0.5),

were screened under identical operating conditions with H2O/CH4 = 1, CH4/CO2 = 1, T= 800 C, flow = 100 sscm, and feedratio H2O/CH4/CO2/N2 = 1/1/1/0.5 vol %. The results

obtained are shown in Figure 8. The addition of a stoichiometricamount of steam to the CO2 reforming feed gas mixture resultedin a decrease in CH4 conversion and led to catalyst deactivation.Steam addition increases the surface coverage by oxygen and,hence, surface nickel atoms are partly oxidized. Stabilization ofnickel cations by the support decreases the number of accessibleactive nickel atoms. Thisexplains the decline of activity.The CO2reforming reaction was predominant under the experimentalconditions employed in the current study, while the steamreforming reaction was negligible. The above observation isconfirmed by the obtained H2/CO ratios, which were1. Thus,the obtained results indicate a poor performance of theabove-tested catalysts for the CO2 reforming of CH4 reaction

in the presence of steam. The above results can be explainedfrom the perspective of hydrophilicity. According to Gonzalez-

Velascos findings,58 hydrophilicity is introduced into the ceriasystem upon the addition of zirconium, which increases with anincrease in the zirconium content. The inherently hydrophilicceria-zirconia-based catalysts strongly adsorb water, thereby

blocking the catalyst active sites (inhibition of active sites by water) and causing catalyst deactivation. The higher ioniccharacter of the Zr-O bond compared to that of the Ce-O

bond promotes the hydrophilic character of the Ce0.6Zr0.4O2surface, making surface rehydroxylation an easy process in thepresence of steam. It is important to mention that all 5% Ni/Ce0.6Zr0.4O2 catalysts prepared by the surfactant-assisted routeperform well in a CO2 reforming reaction but are prone todeactivation in the presence of steam. Further studies are neededto understand the complete surface chemistry and the aspects

responsible for the observed deactivation in the presence ofsteam.

3.2.3. Effect of the Temperature on the Catalytic Activity forCO2 Reforming of CH4. From the above tests, it is clear that theaddition of steam to the dry reforming reaction is not beneficial

withthe5%Ni/Ce0.6Zr0.4O2 catalysts obtained by thesurfactant-assisted route. In order to achieve one of the set goals of thecurrent study, the dry reforming reaction was performed at loweroperating temperature, i.e., 700 C, over catalysts obtained by thesurfactant-assisted route. The results thus obtained are comparedin Figure 9. The figure reveals a lower catalytic activity at loweroperating temperature because of the endothermic nature of thereaction. On the whole, the performances of both NCZ(1.25)

Figure 6. Performanceevaluation of 5% Ni/Ce0.6Zr0.4O2 catalysts for a typicalCO2 (dry) reforming reaction [feed composition (vol %) CH4/CO2/N2= 40/40/20; temperature = 800 C; feed flow rate = 100 sccm].

Figure 7. Plot showingthe relationship between theinitialTOF of CH4and the ratio of oxygen vacancies/surface nickel atoms.
http://pubs.acs.org/action/showImage?doi=10.1021/ef101479y&iName=master.img-006.jpg&w=186&h=151http://pubs.acs.org/action/showImage?doi=10.1021/ef101479y&iName=master.img-005.jpg&w=396&h=118

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and NCZ(0.5) catalysts are found to be stable even at 700 Coperating temperature. On the basis of these results, it appearsthat it is possible to employ 5% Ni/Ce0.6Zr0.4O2 catalystsobtained by the surfactant-assisted route even at an operatingtemperature as low as 700 C for the dry reforming reaction.

3.3. SARs. The performance evaluation results (Figure 6)show a marked difference in terms of activity and stability betweenthe catalysts where the supports are prepared through thesurfactant-assisted method and the other one is prepared by

coprecipitation. It also shows a marked difference in the activityand stability between the catalysts prepared by the surfactant-assisted method but using different CTAB/(Ce Zr) ratios. It isa well-known fact in the literature that the synthesis andpreparation conditions affect some properties of ceria-basedmaterials such as the formed phase, particle size, surface area,and OSC, which, in turn, influence the resultant catalyticactivities. The enhancement of the surface area of ceria-zirconia-based solid solutions prepared by the surfactant-assistedroute is related to the surfactant effect that reduces the surfacetension inside the pores through a decrease in the capillary stressduring drying and calcination processes. Better thermal stabilityof the above-obtained materials is related to the structural

arrangement and morphology of the inorganic-organic compo-sites, which are produced by an exchange between the deproto-nated hydroxyl group of the oxides and the alkylammoniumcations. These features contribute to the enhanced texturalstability of the catalysts obtained by the surfactant-assisted routein comparison with the one prepared by the conventionalprecipitation method.

We have used the characterization results in conjunction withthe activity results in an attempt to establish SARs for each

catalyst system. Figure 10 represents the various SAR plotsgenerated in this study. Certain parameters, namely, (1) OSC,(2) reducibility, and (3) nickel surface area, were used toestablish the SARs. The OSC measurements were obtained fromTGA under cyclic reductive/oxidative excursions, as describedearlier (Table 1). The reducibility values were obtained from theTPR measurements (Figure 4); more specifically, the Tmaxvaluespertaining to the reduction of NiO to Ni were used for calcula-tion of the reducibility (where reducibility = 1/Tmax 100). Thenickel surface area measurements were obtained from H2chemisorption experiments (Table 1). In the current work, theactivity is defined in terms of % CH4 conversion observed at 8 hof TOS.

Figure 8. Screening of 5% Ni/Ce0.6Zr0.4O2 catalysts for the CO2 reforming reaction in the presence of steam [feed composition (vol %) H2O/CH4/CO2/N2 = 40/40/40/20; temperature = 800 C; feed flow rate = 140 sccm].

Figure 9. Influence of the temperature on the catalytic activity of 5% Ni/Ce0.6Zr0.4O2 catalyst for CO2 reforming of CH4 [feed composition (vol %)CH4/CO2/N2 = 40/40/20; feed flow rate = 100 sccm; temperatures = 700 and 800 C].

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Figure 10A represents the correlation plot of activity withOSC. As is evident from Figure 10A, an increase in OSC leads toan improvement in the resultant catalytic activity. As mentioned

earlier, the OSC of a catalyst is its abilityto transition between thereduced and oxidized states rapidly. The magnitude of the OSCdenotes the ease of occurrence of the Ce4 S Ce3 redoxcouple, which is known to promote CO2 reforming processesand also assist in the decoking of the catalyst surface. Therelationship between the activity and catalyst reducibility ispresented in Figure 10B. The higher the reducibility, the loweris its reduction temperature (Tmax). The descending order ofreducibility observed for the various catalysts tested in thecurrent study is as follows: NCZ(1.25) > NCZ(0.8) > NCZ(0.5)> NCZ(CP). The trends show a somewhat monotonic increasein the relationship between the reducibility and activity, whichmeans that highly reducible catalysts exhibit enhanced stability

and high activity. The correlation between the catalyst stabilityand nickel surface area is presented in Figure 10C. It is notedfrom this figure that the catalyst activity improves with anincrease in the nickel surface area. The catalyst that possesses agreater nickel surface area and a greater proportion of surfacenickel compared to bulk nickel will exhibit longer life for theCO2 reforming reaction. Out of the four catalyst formulationstested in the current work, only NCZ(1.25) and NCZ(0.8)obtained by the surfactant-assisted route exhibit steady andhighest catalytic activity, while the other two, namely, NCZ-(0.5) and NCZ, are not stable and their catalytic activity decays

with TOS. On the basis of the current findings, it can beinferred that the surfactant-assisted route yields better catalyst

formulations compared to the conventional coprecipitationroute. Also, among the catalysts obtained by the surfactant-assisted route, the amount of surfactant used during the courseof the support preparation plays a pivotal role in imparting thedesired characteristics to the resultant catalyst formulation.From the current study, it can be inferred that, in order toobtain a better catalyst formulation, the optimum molar ratioof CTAB/[Ce Zr] should be g0.8.

Before this discussion is concluded, it is very important tomention here that, although there were many concrete correla-tions between the observed catalytic activity/stability and textur-al/physicochemical characteristics, it would be best to considerthat the right combination of all relevant characteristics isresponsible for the resultant overall activity and that the percen-

tagecontribution fromeach characteristic to the overallactivity isnot necessarily the same. On the basis of our studies, it was notedthat the absence of any desired characteristic in a given catalystformulation leads to a poor performance or even deactivation.From the present investigation, it is quite apparent that highOSC, high surface area, high reducibility, high nickel surface area,high nickel dispersion, and high surface nickel content lead toexcellent catalytic activity for CO2 reforming of CH4. Among the

various catalyst formulations investigated in the current study,the catalyst NCZ(1.25) has all of the desirable characteristics andsubsequently exhibits superior catalytic activity along with en-hanced stability, as is established by a long-term stability run for aduration of 150 h, as is shown in Figure 11.32 Thus, there is an

Figure 10. Structure-activity correlation plots for CO2 reforming ofCH4: (A) activity vs OSC; (B) activity vs reducibility; (C) activity vsnickel surface area.

Figure 11. Long-term TOS stability studies on the 5% Ni/Ce0.6Z-r0.4O2-NCZ(1.25) catalyst [feed composition (vol %) CH4/CO2/N2 =40/40/20; temperature = 800 C; feed flow rate = 100 sccm].

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immense potential in the NCZ(1.25) catalyst for plausible futurecommercialization.

4. CONCLUSIONS

1 The results showed a marked difference in the activity andstability between the catalysts where the supports wereprepared by surfactant-assisted route and the other one was

prepared by coprecipitation. It also showed a markeddifference in the activity and stability between the catalystsprepared by the surfactant-assisted route but using differentCTAB/(Ce Zr) molar ratios. Also, the use of steam toassist the dry reforming process produced differences interms of the activity and stability.

2 The surfactant-assisted route yields better catalysts than theconventional coprecipitation route.

3 Within the range of CTAB/[Ce Zr] molar ratios em-ployed, the amount of surfactant used during the course ofthe support preparation affects the stability, with catalystsprepared by a higher surfactant/metal molar ratio exhibiting

better stability.4 From structure-activity correlation (SAR) plots, it is noted

that the higher the OSC, surface area, nickel surface area,nickel dispersion, reducibility, and surface nickel content,the higher is the CH4 conversion and stability for the CO2reforming of CH4 reaction.

5 The initial TOF of CH4 increases as the ratio of the totalnumber of oxygen vacancies per gram of catalyst to the totalnumber of surface nickel atoms per gram of catalystincreases.

6 The catalysts obtained by the surfactant-assisted route fareextremely well in a CO2 reforming process but are prone todeactivation in the presence of steam. The inherent hydro-philic nature of the ceria-zirconia support offered reducedsensitivity to water inhibition of active sites, leading to

catalyst deactivation.

AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

ACKNOWLEDGMENT

The financial support provided by the Natural Sciences andEngineering Research Council of Canada is gratefully acknowl-edged.

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