Nickel-Based Ceria, Zirconia, and CeriaZirconia Catalytic Systems for

Energy & Fuels 2007, 21, 3113–3123

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Nickel-Based Ceria, Zirconia, and Ceria–Zirconia Catalytic Systems for Low-Temperature Carbon Dioxide Reforming of Methane Prashant Kumar,†,‡ Yanping Sun,†,§ and Raphael O. Idem*,† Hydrogen Production Research Group, Process Systems Engineering, Faculty of Engineering, UniVersity of Regina, Regina, SK, Canada S4S 0A2, and HTC Pure Energy, #001, 2305 Victoria AVenue, Regina, SK, Canada S4P 0S7 ReceiVed May 11, 2007. ReVised Manuscript ReceiVed July 18, 2007

Carbon dioxide reforming of methane (CDRM) was studied over a variety of ZrO2-, ceria-doped ZrO2-, and CeO2–ZrO2-supported Ni catalysts. Different techniques were used to prepare supports material having different physicochemical properties, and a correlation was established to show the importance of a robust support material. Various characterization of the catalyst further established that the coking behavior of the catalyst depends on the support preparation techniques. Compared to zirconia and ceria-doped zirconia, the use of ceria–zirconia (CexZr1–xO2) solid solution as a support prepared by using a surfactant was found to be the most stable for low-temperature CDRM. It seems the inhibition of reactions leading to carbon deposition is prominent in systems having ZrO2. Temperature-programmed oxidation (TPO) experiments indicated excellent resistance toward carbon formation for Ni supported on CexZr1–xO2 compared with other catalysts studied. H2-TPR (temperature-programmed reduction) analyses also showed that the stability of CexZr1–xO2 solid solution is a function of its enhanced reducibility at lower temperatures as compared to either pure ceria or ceria-doped ZrO2. Based on all the catalysts studied, 5% Ni Ce0.6Zr0.4O2 was found to be the best catalyst as activity was stable for up to 100 h at 650 and 700 °C, while at 800 °C the catalyst activity remained stable for more than 200 h.

1. Introduction In recent years, the catalytic carbon dioxide reforming of methane (CDRM) process (CH4 + CO2 T 2CO + 2H2, ∆H298 ) 247.3 kJ/mol) to produce synthesis gas has become an attractive but challenging subject for the chemical utilization of carbon-containing “greenhouse” gases (CH4 and CO2).1 The CDRM can be carried out in a fixed-bed reactor followed by purification steps. Also, if it becomes possible to work with membranes at the dry reforming temperature, it could offer the possibility of overcoming the thermodynamic limitations of this endothermic reaction, thus allowing for methane conversion at lower temperatures. In addition, CDRM in conjunction with the water-gas shift (WGS) reaction may be applied to produce additional hydrogen. Moreover, if the dry reforming temperature is comparatively low and WGS process is brought to about the same level as that of dry reforming as was done in our earlier work,2,3 the temperature mismatch between the two processes could be minimized. Thus, there is a lot of merit in pursuing the idea of developing a catalytic process involving a lowtemperature CDRM coupled with a high-temperature WGS reaction. The dry reforming reaction has been studied over numerous supported metals catalysts, especially Ni-based catalysts. How* Corresponding author: Fax 1-306-585-4855; e-mail Raphael.Idem@ uregina.ca. † University of Regina. ‡ HTC Pure Energy. § Current address: CSIRO Energy Centre, Newcastle NSW 2300, Australia. (1) Wang, S.; Lu, G. Q. M. Energy Fuels 1996, 10, 896. (2) Idem, R. O.; Kumar, P.; Sun, Y. WO 2006/099716, September 28, 2006. (3) Kumar, P.; Idem, R. O. Energy Fuels 2007, 21, 522.

ever, the major issue encountered in the application of CDRM process is the rapid deactivation of the nickel-based catalysts. Most of the authors1–5 agree that coke formation is induced by either CH4 decomposition (eq 1) or CO disproportionation (the Boudouard reaction, eq 2). However, carbon formation by CH4 decomposition is a structure-sensitive reaction.4,5 Specifically, the Ni (100) and Ni (110) surfaces are more active in the decomposition of CH4 to carbon than the Ni (111) surface. CH4 f C + 2H2 2CO f C + CO2

∆H0298 ) 75 kJ/mol

(1)

∆H0298 ) -172 kJ/mol

(2)

The CO disproportionation is an exothermic reaction and generally favored at temperatures below 973 K.5 The form of carbon on metal surfaces generated during the reaction depends on the reaction conditions. Amorphous and filamentous carbons predominate in the lower temperature range of 673–873 K,6 and a graphitic structure predominates at 973 K or higher temperatures.7 The diffusion and segregation of carbon are also dependent on metal surface structure. For example, the carbon on Ni (110) can diffuse more readily into the bulk than that on Ni (100), and consequently, carbon formation is more difficult on Ni (111).4 Furthermore, the carbon adsorbed on smaller metal particles diffuses with greater difficulty than that on the larger particles.5 This means that structure sensitivity of carbon formation provides the possibility for inhibition of the carbon deposition by modification of the catalyst surface structure. For (4) 41, 1. (5) (6) (7)

Bradford, M. C. J.; Vannice, M. A. Catal. ReV.—Sci. Eng. 1999, Hu, Y. H.; Ruckenstein, E. AdV. Catal. 2004, 48, 297. Verykios, X. E. Int. J. Hydrogen Energy 2003, 28, 1045. Bartholomew, C. H. Catal. ReV.— Sci. Eng. 1982, 24, 67.

10.1021/ef7002409 CCC: $37.00  2007 American Chemical Society Published on Web 09/11/2007

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this reason, a number of studies4–7 have been focused on the development of a promising support material for CDRM. The catalysts based on noble metals have been found to be less sensitive to carbon deposition. However, considering the high cost and limited availability of noble metals, it is more practical from the industrial standpoint to develop Ni-based catalysts with high performance and high resistance to carbon deposition. The development of robust nickel-based catalysts, which are resistant to carbon deposition and which will exhibit stable operation at low temperature for an extended period of time, is urgently needed. Several studies5–15 have been carried out in order to improve the coke resistance of Ni-based catalysts. The formation of NiAl2O4 during pretreatment for the Ni/Al2O3 system and working with solid solution of NiO–MgO were found to be useful, at least, in terms of stability, for more than 100 h at relatively higher temperatures.5,8 Also, the improved stability on Ni/La2O3 catalyst was attributed to the isolation of nickel ensembles by LaOx species.9 Similarly, coke deposition also decreases when Ni is impregnated on supports that presents a marked Lewis alkalinity/acidity and improved thermal stability such as ZrO2 and La2O35–10 or on supports that have been modified for that purpose with alkaline metals such as Li or K or Mg.5,11,12 Similarly, strong metal–support interaction between Pt and Zr is shown to be a good recipe for a reforming catalyst. In this regard, Pt/ ZrO2 catalyst was shown to have a strong metal–support interaction, resulting in a better catalyst.13 A similar observation was reported14 on the performance of CDRM at 750 °C for 30 h without significant deactivation on 13.2% Ni/ZrO2 catalyst. Another report concluded15 that Ni/ZrO2 catalytic stability depends greatly on the preparation method and the support precursor. The catalyst prepared by impregnation of ultrafine Zr(OH)4 particles with nickel nitrate showed high and stable activity for CDRM. Ceria (CeO2) was also studied as an effective promoter and/or support for Ni catalysts. CeO2 containing materials are interesting due to its redox and high oxygen mobility. The addition of ZrO2 to CeO2 also leads to improvements in oxygen storage capacity (OSC), thermal stability, and improved metal dispersion.5,16–18 Furthermore, it has been observed that surface reduction of ceria is greatly enhanced when ZrO2 is incorporated into CeO2 or when the two form a solid solution.16 A few studies on CeO2–ZrO2 for CDRM at high temperatures (>800 °C) have been reported. For example, Montoya et al.19 studied the Ni/ Ce–ZrO2 system; however, the problem of support sintering (8) Djaidja, A.; Libs, S.; Kiennemann, A.; Barama, A. Catal. Today 2006, 113, 194. (9) Tsipouriari, V. A.; Verykios, X. E. Catal. Today 2001, 64, 83. (10) Pompeo, F.; Nichio, N. N.; Souza, M. M. V. M.; Cesar, D. C.; Ferretti, O. A.; Schmal, M. Appl. Catal. A 2007, 316, 175. (11) Nandini, A.; Pant, K. K.; Dhingra, S. C. Appl. Catal. A 2006, 308, 119. (12) Chang, J.-S.; Park, S.-E.; Yoo, J. W.; Park, J.-N. J. Catal. 2000, 195, 1. (13) Lercher, J. A.; Hally, W.; Niessen, W.; Seshan, K. Stud. Surf. Sci. Catal. 1996, 101, 463. (14) Li, X.; Chang, J.-S.; Park, S.-E. React. Kinet. Catal. Lett. 1999, 67, 375. (15) Wei, J.-M.; Xu, B.-Q.; Li, J.-L.; Cheng, Z.-X.; Zhu, Q.-M. Appl. Catal. A 2000, 196, L167. (16) Trovarelli, A. Catalysis by Ceria and Related Materials; Imperial College Press: Universita di Udine, 2002; p 2. (17) Kaspar, J.; Fornasiero, P.; Graziani, M. Catal. Today 1999, 50, 285. (18) Pengpanich, S.; Meeyoo, V.; Rirksomboon, T.; Bunyakiat, K. Appl. Catal. A 2002, 234, 221. (19) Montoya, J. A.; Pascual, E. R.; Gimon, C.; Angel, P. D.; Monzon, A. Catal. Today 2000, 63, 71.

Kumar et al.

at 800 °C could not be avoided completely. A similar observation was reported by Stagg-Williams et al.20 for 1 wt % Pt/Ce–Zr–O catalysts at 800 °C and a CH4:CO2 ratio of 2:1 that increasing the loading of Ce in the Ce–Zr–O support from 5 to 18 wt % resulted in a slight decrease in the initial activity of the catalyst and that the amount of carbon deposits on 1 wt % Pt /Ce–Zr–O catalysts was not less than 1 wt % Pt /ZrO2 catalyst. Potdar et al.21,22 have recently examined the CDRM reaction over coprecipitated Ni–Ce–ZrO2 catalysts at 800 °C. The authors observed that the 15% Ni (w/w) catalyst exhibited a high and stable activity (about 90% CH4 and CO2 conversions) for almost 100 h at a certain space velocity. Another report23 on Ni/Ce–ZrO2/θ-Al2O3 catalyst showed that this catalyst gave a CH4 conversion of more than 97% at 800 °C. The activity of this catalyst could be maintained for 40 h. These authors considered that the high stability of the catalyst stemmed from the beneficial precoating effect of Ce–ZrO2, resulting in the existence of stable NiOx species, a strong interaction between Ni and the support, and an abundance of mobile oxygen species in CeO2 itself. A very interesting study by the same group was also reported where the activity was found to be stable for more than 250 h at 790 °C in the presence of feed having a feed of CO2/CH4 ratio of more than one.24 In the present work, we have tried to evaluate how the coke formation can be inhibited by controlling the size of the ensembles of the nickel on the surface, since the ensembles necessary for carbon formation are larger than those needed for methane reforming.4,5,15 An example is the use of surfactantmediated support materials with just enough nanocomposite characteristics that might lead to a strong metal–support interaction, resulting into smaller metal ensembles.25 Based on this, a report on both steam reforming and dry reforming of methane has been published, albeit at relatively high temperatures.26,27 Similarly, we have attempted to show how the preparation approach of both ZrO2 and CeO2–ZrO2 supports materials can affect the stability for low-temperature CDRM. A variety of techniques such as hydrogel and alcogel were used to prepare ZrO2 support for this purpose. Similarly, a ceriadoped zirconia was used to prepared low ceria containing ZrO2, and finally a cationic surfactant assisted method was used to prepare CexZr1–xO2 solid solution with reduced pore volume. The use of surfactant is intended to lead to the reduction of particle size of the support material to such an extent that it becomes comparable to the metal particles resulting in a nanocomposite of supported metal catalyst. A correlation on catalytic activity and stability was established with respect to support choice, nickel loading, and preparation method of the catalysts for low-temperature CDRM. The focus would be to establish a catalytic system that shows the required stability at (20) Stagg-Williams, S. M.; Noroha, F. B.; Fendley, G.; Resasco, D. E. J. Catal. 2000, 194, 240. (21) Potdar, H. S.; Roh, H.-S.; Jun, K. W.; Ji, M.; Liu, Z. W. Catal. Lett. 2002, 84, 95. (22) Roh, H.-S.; Potdar, H. S.; Jun, K. W.; Kim, J. W.; Oh, Y. O. Appl. Catal. A 2004, 276, 231. (23) Roh, H.-S.; Jun, K.-W.; Baek, S.-C.; Park, S.-E. Catal. Lett. 2002, 81, 147. (24) Liu, Z.W.; Roh, H.-S.; Jun, K.-W. J. Ind. Eng. Chem. 2003, 9 (6), 753. (25) Laosiripojana, N.; Assabumrungrat, S. Appl. Catal. A 2005, 290, 200. (26) Laosiripojana, N.; Assabumrungrat, S. Appl. Catal. B 2005, 60, 107. (27) Laosiripojana, N.; Sutthisripok; Assabumrungrat, W. S. Chem. Eng. J. 2007, 127, 31.

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Table 1. Physicochemical Properties (Surface Area, Pore Volume, and Pore Size) of the Materials Studied support

BET (m2/g)

PV (cm3/g)

PS (nm)

crystallite sizea (nm)

ZrO2 (commercial) ZrO2 (HY) ZrO2 (AL) ZrO2 (Reflux) (4% CeO2)–ZrO2 (AL) (50% CeO2)–ZrO2 (AL) (60% CeO2)–ZrO2 (AL) (85% CeO2)–ZrO2 (AL) CeO2 (CTAB) Ce0.40Zr0.60O2 (CTAB) Ce0.50Zr0.50O2 (CTAB) Ce0.60Zr0.40O2 (CTAB) Ce0.68Zr0.32O2 (CTAB) Ce0.78Zr0.22O2 (CTAB) Ce0.85Zr0.15O2 (CTAB) Ce0.92Zr0.02O2 (CTAB)

2.4 38.0 56.3 75.9 60.3 36.9 99.0 110.4 164.0 216.5 206.8 149.5 189.6 154.0 159.8 139.0

0.018 0.10 0.23 9.0 0.24 0.07 0.17 0.31 0.60 0.23 0.23 0.23 0.24 0.24 0.28 0.24

25.9 7.4 12.6 0.20 12.2 4.1 4.8 8.7 11.8 3.4 3.5 4.5 3.9 4.7 5.3 5.1

134 54 25.9 16.4 20.2 42.8 38.4 34.6 7.4 5.9 7.7 7.5 7.3 7.8 8.4 8.8

a

Crystallite size measured by XRD.

low temperature and also compares our results with the ones that are available in the literature. 2. Experimental Section 2.1. Catalyst Preparation. 2.1.1. ZrO2 Supported Nickel Catalysts. Five different types of Ni/ZrO2 catalysts were prepared by using different techniques as follows: (a) Ni/ ZrO2 (Comm): The catalyst Ni/ ZrO2 (Comm) was prepared by wet impregnation of commercial zirconia (Aldrich, 99.9%) calcined in flowing air at 800 °C for 6 h, using an aqueous solution of nickel nitrate as a precursor in the desired concentration. The resulting solution was stirred at room temperature followed by water removal and drying at 110 °C overnight. The samples were then calcined in flowing air at 650 °C for 5 h. (b) Ni/ZrO2 (CP): The catalyst Ni/ZrO2 (CP) was synthesized by the coprecipitation technique. A predetermined amount of Ni(NO3)2 (Aldrich, 99.9%) and ZrOCl2 (Aldrich, 99.9%) was used to make an aqueous solution, and the solution was slowly mixed into a 2.5 wt % ammonia–water under vigorous stirring to make a coprecipitate of Ni(OH)2–Zr(OH)4. This coprecipitate was kept under stirring overnight followed by washing until it was free of chloride ions. The above precursor was dried in an oven at 110 °C overnight and then calcined in flowing air at 650 °C for 5 h.

(c) Ni/ZrO2 (HY), Ni/ZrO2 (AL), and Ni/ZrO2 (Reflux): Three samples of nickel containing zirconia as a support were prepared by wet impregnation of an aqueous solution of nickel nitrate onto ZrO2 obtained by different preparation techniques. They are denoted as Ni/ZrO2 (HY), Ni/ZrO2 (AL), and Ni/ZrO2 (Reflux). After being dried at 110 °C overnight, they were then calcined in flowing air at 650 °C for 5 h. ZrO2 (HY) was prepared by addition of a certain volume of ZrOCl2 solution to 2.5 wt % aqueous ammonia under vigorous stirring with careful control of pH ) 10. The precipitated Zr(OH)4 hydrogel was kept under stirring for another 2 h and then left to digest overnight at room temperature. The resulting gel was filtered and washed with deionized water until it was free of chloride ions. The “wet cake” was further divided into two. The first half was dried in an oven at 110 °C overnight and then calcined in flowing air at 650 °C for 5 h to obtain ZrO2 (HY). In order to obtain ZrO2 (AL), the other half was washed and filtered by ethanol several times followed by drying in flowing nitrogen at 270 °C overnight and then calcination at 650 °C for 5 h. The preparation of ZrO2 (Reflux) involves the following steps: A certain volume of zirconium n-propoxide (70% in propanol) was dissolved in anhydrous ethanol to obtain a 0.1 M solution. Deionized water was added dropwise to the zirconium n-propoxide solution to generate a white precipitate. The precipitate was aged at room temperature for 24 h, followed by removal of the solvent by rotary evaporation. The obtained wet solid was suspended in a certain amount of deionized water with the pH adjusted to 9 with 28% ammonia. The suspension was refluxed at 100 °C for 96 h. Then, the sample was filtered and washed free of ammonia with deionized water followed by drying at 100 °C overnight and then calcining in flowing air at 650 °C for 5 h. 2.1.2. CeO2–ZrO2, CeO2, and CexZr1–xO2 Supported NickelBased Catalysts. A ceria-doped (4 mol %) ZrO2 was prepared by using the coprecipitation technique. A predetermined amount of CeCl3 · 7H2O (Aldrich) and ZrOCl2 · 8.33H2O (Aldrich) was used to make an aqueous solution, and the solution was added to a 2.5 wt % ammonia–water to make a coprecipitate of Ce(OH)4–Zr(OH)4. The coprecipitate was then washed by ethanol several times to convert Ce(OH)4–Zr(OH)4 hydrogel into Ce(OH)4–Zr(OH)4 alcogel. The resulting Ce(OH)4–Zr(OH)4 alcogel was dried in flowing nitrogen at 270 °C overnight and then calcined in flowing air at 650 °C for 5 h to obtain a series of materials where the ceria doping was varied from 4 to 85 mol % CeO2–ZrO2(AL). CeO2 and a series of CexZr1–xO2 supports were prepared by using a cationic surfactant with the hydrous mixed oxide produced by

Table 2. Physicochemical Properties of the Catalysts Studied catalysts

BET (m2/g)

PV (cm3/g)

PS (nm)

NiOa (nm)

Nic dispersion (%)

Nic surface area (m2/g)

13% Ni/ZrO2 (Reflux) 13% Ni/ZrO2 (HY) 13% Ni/ZrO2 (CP) 13% Ni/ZrO2 (AL) 20% Ni/ZrO2 (AL) 7% Ni/ZrO2 (AL) 5% Ni/ZrO2 (AL) 5Ni/(4% CeO2)–ZrO2 (AL) 5Ni/(50% CeO2)–ZrO2 (AL) 5Ni/(60% CeO2)–ZrO2 (AL) 5Ni/(85% CeO2)–ZrO2 (AL) 5Ni/CeO2 (CTAB) 5Ni/Ce0.40Zr0.60O2 (CTAB) 5Ni/Ce0.50Zr0.50O2 (CTAB) 5Ni/Ce0.60Zr0.40O2 (CTAB) 5Ni/Ce0.68Zr0.32O2 (CTAB) 5Ni/Ce0.78Zr0.22O2 (CTAB) 5Ni/Ce0.85Zr0.15O2 (CTAB) 5Ni/Ce0.92Zr0.02O2 (CTAB)

60.4 28.7 27.9 33.0 27.9 36.0 37.6 44.7 8.90 36.5 68.9 158 168.2 154.3 115.8 145.3 109.8 129.8 86.9

0.18 0.13 0.10 0.17 0.17 0.19 0.19 0.21 0.03 0.14 0.24 0.50 0.19 0.19 0.19 0.19 0.2 0.22 0.17

10.1 14.4 7.5 16.6 19.9 16.7 17.3 14.7 7.77 11.8 10.8 11.1 3.5 3.8 5.0 4.1 5.5 5.4 5.6

124 190 210 160 185 152 138 84.4 12.5 18.2 21.6 10.7 4.2 NAb NAb NAb NAb NAb NAb

0.98 1.86 1.95 0.9 1.53 1.35 2.09 2.23 3.6 4.30 2.62 2.47 1.99 1.96

3.53 12.40 12.95 5.96 10.16 8.99 13.91 14.87 23.78 28.64 17.45 16.46 13.24 13.05

a Nickel oxide crystallite size measured by XRD after calcinations. b Not available due to very broad peak and weak XRD peaks. c By using the H 2 chemisorption technique.


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Figure 1. Schematic of the reaction setup: packed bed tubular reactor (PBTR).

coprecipitation under basic conditions. A starting composition had CeO2–ZrO2 (mol %) of 100/0, 92/8, 85/15, 78/22, and 60/40. In a standard experiment, the materials were prepared by adding an aqueous solution of the appropriate cetyltrimethylammonium bromide (CTAB, 0.1 M, Aldrich) to an aqueous solution containing the stoichometric quantities of CeCl3 · 7H2O (Aldrich) and ZrOCl2 · 8.33H2O (Aldrich) (CTAB/[Ce] + [Zr] ) 1). The mixture was stirred for 40 min, and then aqueous ammonia (28–30%) was added dropwise under vigorous stirring until the pH reached 11.5. The mixture was stirred for 2 h in a glass reactor, then sealed, and placed in an oven at 90 °C for 5 days. After that, the mixture was filtered and washed with hot water until it was free of chlorine and bromine. The light-yellow powder was dried at 90 °C for 1 day and then calcined at 650 °C for 5 h. Further, nickel was loaded on these supports by wet impregnation of aqueous solution of Ni(NO3)2 · 6H2O to obtain Ni/(4% CeO2)–ZrO2, Ni/CeO2, and Ni/ CexZr1–xO2 catalysts. The samples were then calcined in flowing air at 650 °C for 5 h before using in the reactor. All the materials obtained were free of any measurable silica impurity as determined by ICP analyses. 2.2. Catalyst Characterization. All the catalysts were characterized using different analytical techniques. Micromeritics ASAP 2010 was used for BET surface area (using nitrogen) and chemisorption (using H2) measurements. For chemisorption analyses the samples (0.55 g) were first reduced in situ in flowing hydrogen at 923 K for 3 h. After reduction, the samples were evacuated at 923 K for 1 h before cooling to 308 K. The method of double isotherm was used to determine the irreversibly bound chemisorbed hydrogen, determined by extrapolating the linear part of the isotherm to zero pressure, which should correspond to hydrogen adsorbed on the metal surface. The results obtained are presented in Tables 1 and 2. Further, the temperatureprogrammed reduction (TPR, Chembet 3000, Quantachrome) was used to evaluate the reducibility pattern of the catalyst, while X-ray diffraction (XRD, Bruker, AXS) was used for diffraction pattern and crystallite size from X-ray broadening by using the well-known Scherrer equation.28 Elemental composition was determined by ICP-MS (Varian). Carbon deposition on the catalysts reacted for 3 or 8 h time-on-stream (TOS) was measured by temperature-programmed oxidation (TPO) on a TG-DSC1100 (Setaram) thermoanalyzer in air with a flow rate of 40 mL/ min. The temperature was raised from ambient temperature to 780 °C at 5 K/min. The amount of carbon deposited was determined from the weight loss during TPO measurement. (28) Chen, C. L.; Weng, H. S. Appl. Catal. B 2005, 55, 115.

Figure 2. Equilibrium conversion of methane for CDRM as a function of temperature (total pressure Ptot ) 1 atm and CH4/CO2/ N2 ) 2/2/1).

2.3. Experimental Setup and Product Analyses (CDRM). In order to investigate the CDRM, an experimental reactor system was constructed as shown in Figure 1. An Inconel packed bed tubular reactor (i.d. ) 6.3 mm) housed in a furnace with a single heating zone was used for catalyst performance evaluation. The gas flows were metered and regulated by an Aalborg digital flow controller (GFC171S). The catalyst bed temperature was measured by means of a sliding thermocouple dipped inside the catalyst bed. The diluent used in the catalyst bed was quartz sand (Aldrich) having a particle size range of 0.2–0.3 mm (i.e., passing through ASTM mesh no. 50 and holding on mesh no. 70) similar to the catalyst particle size range. Pure quartz was also used in the preheating zone. In order to approach plug flow conditions and minimize backmixing and channeling, certain operating criteria as prescribed by Froment and Bischoff27 were used. Accordingly, the ratio of catalyst bed length to catalyst particle size (L/Dp) was 100, and the ratio of the inside diameter of the reactor to particle size (D/Dp) was 21. Prior to each experimental run for catalyst evaluation, the catalyst was activated by in-situ reduction at 973 K with 5% H2 in N2 (Praxair, Regina, Canada). The feed and product gases were analyzed with an online gas chromatograph (HP-6890, Agilent Technologies) equipped with a TCD using Haysep Q column and Molsieve (29) Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design, 2nd ed.; Wiley: New York, 1990.



Figure 3. Comparative plot for different ZrO2 samples on (a) CH4 conversion and (b) H2 selectivity over 13% Ni/ZrO2 catalysts for CDRM at 700 °C with a feed (CH4/CO2 )1:1) rate of 3.76 × 104 mL/(h g-cat).

13X columns (Alltech Associates) for complete separation of the gaseous components. The conversions of CH4 and CO2, and selectivity of H2 are defined as follows: conversion (CH4) % )

(CH4)in – (CH4)out × 100 (CH4)in

(3)

conversion (CO2) % )

(CO2)in – (CO2)out × 100 (CO2)in

(4)

selectivity of (CO2) % )

(H2)in 100 × (CH4)in – (CH4)in 2

(5)

3. Results 3.1. Thermodynamics and Equilibrium Conversion. A thermodynamic equilibrium analysis was performed to determine the limit for CH4 conversion at various temperatures. The main reactions that generally take place during CDRM process are as follows. CH4 + CO2 T 2CO + 2H2

(6)

H2 + CO2 T H2O + CO

(7)

CO2 + 4H2 T CH4 + H2O

(8)

CO + 3H2 T CH4 + H2O

(9)

CH4 + H2O T CO + 3H2

(10)

The species taken into consideration for the thermodynamic equilibrium calculations were CH4, CO2, CO, and H2. The equilibrium conversions of CH4 are calculated under our experimental conditions (Ptot ) 1 atm; CH4/CO2/N2 ratio ) 2/2/ 1) in a broad temperature range from 673 to 1173 K for CO2–CH4 reforming. This has been plotted in Figure 2 as a function of temperature. 3.2. CDRM over Ni/ZrO2 Catalysts. The catalytic performances of various ZrO2 supported 13 wt % Ni catalysts were evaluated in a fixed bed reactor at 700 °C at atmospheric

pressure and gas hourly space velocity (GHSV) of 37600 mL/hg(cat). All the catalyst samples were found to be catalytically active for both CH4 and CO2 conversions (∼70–75%) for the first hour of reaction time-on-stream (TOS) after which deactivation took over (Figure 3a). However, a remarkable difference appeared when the activity of the catalysts was compared for longer TOS. The catalysts with 13 wt % Ni/ZrO2 (CP), 13 wt % Ni/ZrO2 (Comm), and 13 wt % Ni/Ni/ZrO2 (HY) deactivated rapidly. On the other hand, 13 wt % Ni/ZrO2 (Reflux) and 13 wt % Ni/Ni/ZrO2 (AL) catalysts were relatively stable, and H2 selectivity (about 90%) was relatively high during this longer TOS as can be seen in Figure 3b. These results indicate that the catalytic activity and stability of Ni/ZrO2 catalyst depend largely on the support preparation. In order to derive a better nickel dispersion and determine its impact on stability, the Ni loading were varied from 5 to 25 wt % of Ni/ZrO2 (AL). The results in terms of turnover number (TON) for CH4 conversion, defined as the number of moles of CH4 converted per gram of nickel per gram of catalyst, are presented in Figure 4. It can be seen that the 5 wt % Ni/ZrO2 (AL) catalyst has the highest catalytic activity in comparison to other amount of nickel loading. Similar results were also obtained for CO2 turnover numbers. This indicates that a nickel loading of 5 wt % on ZrO2 (AL) is sufficient for a high initial activity, and the catalyst gives the turnover number compared to catalysts of other Ni loadings tested. The comparison confirms that the preparative control of the support precursor is important for the development of a highly stable Ni/ZrO2 catalyst. This performance, to a certain extent, can also be related to the textural properties such as high surface area of the materials and an improved Ni dispersion of 5 wt % on ZrO2 (AL) compared to 7 wt % on ZrO2 (AL) as presented in Table 2. Based on these, the zirconia containing materials with high surface area and controlled porosity coupled with


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Figure 4. Impact of various Ni loading on catalyst activity measured as turnover number (TON) for methane conversion (mol CH4/(g-Ni g-cat.)) vs time on stream (TOS) over Ni/ZrO2 (AL) catalysts for CDRM at 700 °C with a feed (CH4/CO2 ) 1:1) rate of 3.76 × 104 mL/(h g-cat.).

nanoparticles and better nickel dispersion can possibly be developed as a potential catalyst. 3.3. CDRM over Ni/ZrO2 Catalysts Doped with Ceria. In order to improve the long-term stability of Ni/ZrO2 (AL), the catalyst was doped with ceria. The incorporation of ceria into the system brings about certain properties such as improved oxygen storage capacities and redox characteristics. Moreover, it also changes the crystal structure from being monoclinic to tetragonal. The catalyst activity carried out at 700 °C shows some improvement in stability compared to Ni loaded on ZrO2 (AL). However, the stability was still not sustained as the conversions of both CH4 and CO2 declined monotonically with time on stream (Figure 5, a and b, respectively). On the other hand, it was observed that the addition of ceria into Ni/ZrO2 (AL) catalyst clearly improved the stability and also improved the H2 selectivity (Figure 5c) significantly. Therefore, the possibility of either using ceria or a combination of a ceria–zirconia support system with relatively high surface area could be a good option. 3.4. CDRM over Ni/CeO2 Catalysts. The results on pure ceria as a support for nickel are presented in Figure 5a,b. It can be seen in the figure that conversions of CH4 and CO2 still declined with time on stream. The other downside of using pure ceria as a support was the reduced H2 selectivity (Figure 5c) compared to both Ni/ZrO2 (AL) and ceria-doped Ni/ZrO2 (AL). This suggests that a reverse water-gas shift reaction (CO2 + H2 f CO + H2O) is apparently taking place over Ni/CeO2 catalyst along with CDRM, thus reducing the hydrogen selectivity. This was also reflected in the high ratio of CO/H2 in the products over Ni/CeO2 catalyst. This is consistent with the results reported in the literature,12–22 confirming that the CO/ H2 product ratio is a function of the water-gas shift equilibrium. In addition, although Ni/CeO2 catalyst has a larger surface area in comparison with ceria-doped Ni/ZrO2 (AL) catalysts, the catalyst does not exhibit the desired stability pattern. Trovarelli et al.16 also claimed that CeO2-supported nickel catalysts do not always contribute to improving the activity and the stability for CDRM because of a strong interaction between the metal and the support. A similar report by Laosiripojana et al.25 also indicated that such type of ceria can be used for CDRM at a relatively high temperature (900 °C) without any deactivation. 3.5. CDRM over Ni/CexZr1–xO2 Catalysts. Further, the Ce0.6Zr0.4O2 solid solution prepared by surfactant-assisted method (CTAB) was used as a support for the nickel catalysts. The XRD patterns of the support after calcination at 650 °C for 5 h are shown in Figure 6a,b. The patterns indicate the presence of a true mixed-oxide phase with cubic fluorite structure and show reflections corresponding to (111), (200),

Figure 5. Comparative plots over 5% Ni catalysts for (a) CH4 conversion, (b) CO2 conversion, and (c) H2 selectivity over different supports for CDRM at 700 °C with a feed (CH4/CO2 ) 1:1) rate of 3.76 × 104 mL/(h g-cat).

(220), (311), (222), and (400) planes. There is no indication of the presence of other phases such as ZrO2 or CeO2. This also is indicative of the fact that Ce and Zr ions are homogeneously mixed. When x changes from 0.5 to 0.92 in CexZr1–xO2, the XRD pattern has a little shift in the reflections for all peaks toward higher angles as shown in Figure 6a due to the insertion of Zr4+ ions in the lattice of CeO2, which is also manifested in the crystallite size (Table 1) of CexZr1–xO2 in a subtle way. It should also be noticed that the diffraction peaks shift to higher degrees with increasing amounts of ZrO2 (Figure 6a), which is attributed to the shrinkage of the lattice due to the replacement of Ce4+ (0.097 nm) with a smaller cation Zr4+ radius (0.086 nm). A similar observation can be seen in the TPR profile (Figure 7) of the 5% Ni/CexZr1–xO2 (x ) 0.5–0.68) where the variation of x does not apparently affect the positions of the two peaks with x in the range 0.5–0.68. This indicates that 5% Ni/CexZr1–xO2 (x ) 0.5–0.68) catalysts have more or less same reducibility. The first sharp peak (at 440 °C) is attributed to the reduction of Ni species, and the second one is a broad peak at about 662 °C ascribed to the reduction of highly dispersed CeO2 in CexZr1–xO2 solid solution. The similar reducibility of 5% Ni/ CexZr1–xO2 (x ) 0.5–0.68) is also likely to be associated with the equivalent oxygen storage capacity (OSC) of CexZr1–xO2 (x ) 0.5–0.68) and having the same cubic fluorite structure.



Figure 6. (a) XRD patterns of different ceria–zirconia support systems: red, Ce0.4Zr0.6O2; black, Ce0.5Zr0.5O2; blue, Ce0.6Zr0.4O2; brown, Ce0.9Zr0.1O2. (b) Representative example of XRD patterns for (a) Ce0.6Zr0.4O2 (CTAB) and (b) 5% Ni/Ce0.6Zr0.4O2 (CTAB).

Compared to the XRD profiles of only support material, the nickel-loaded system (5 wt %) shows some evidence of nickel [111] species (Figure 6b), and the Ni dispersion appears to be at its best, as can be seen in Table 2. However, the XRD results revealed that increased loading of nickel from 5 to 15 wt % results into agglomeration of nickel particle. On the other hand, the presence of nickel greatly improves the reducibility patterns of the cubic phase of CexZr1–xO2. This can be seen in the TPR profile of Ni/CexZr1–xO2 (x ) 0.5–0.92) system in Figure 8 in which the effect of Ni loading (5–20%) of the reducibility of

NiO and CexZr1–xO2 are presented. A comparison of the TPR profile of Ce0.6Zr0.4O2 with that of 5% Ni/Ce0.6Zr0.4O2 shows that the presence of 5% Ni species shifts the CeO2 reduction peak to a lower temperature from 662 to 652 °C. It indicates that CeO2 can easily be reduced in the presence of 5% Ni species. However, with an increase in Ni loading from 5% to 15% and to 20%, the NiO reduction peak shifts to higher temperatures from 440 °C to 500 and 520 °C, respectively. The CeO2 reduction peak also shifts to higher temperatures from 652 °C to 667 and 679 °C, respectively. The results suggest


Figure 7. TPR-H2 profiles of the catalysts: (a) 5% Ni/Ce0.5Zr0.5O2 (CTAB), (b) 5% Ni/Ce0.6Zr0.4O2 (CTAB), and (c) 5% Ni/Ce0.68Zr0.32O2 (CTAB).

Figure 8. Comparative TPR-H2 profiles of catalysts: (a) Ce0.6Zr0.4O2, (b) 5% Ni/Ce0.6Zr0.4O2, (c) 15% Ni/Ce0.6Zr0.4O2, and (d) 20%Ni/ Ce0.6Zr0.4O2.

Figure 9. Effect of change in Ce:Zr ratios in CexZr1–xO2 (CTAB) support on (a) CH4 conversions and (b) H2 selectivity over 5% Ni/CexZr1–xO2 catalysts for CDRM at 700 °C.

that the 5% Ni/Ce0.6Zr0.4O2 catalyst possesses the highest reducibility of NiO and CeO2. Furthermore, the degree of NiO interacting with the Ce0.6Zr0.4O2 support results into smaller particle size of the nickel as nickel loading is reduced, as expected. The catalytic performance of various CexZr1–xO2 supported Ni catalysts were further evaluated in a packed bed tubular reactor at 700 °C at atmospheric pressure and GHSV of 37600 mL/(h g). The activities of 5% Ni loading on CexZr1–xO2 (x ) 0.5–0.92) for CDRM are shown in Figure 9a where catalytic activity of various Ni/CexZr1–xO2 (CTAB) catalysts are pre-

Kumar et al.

Figure 10. Profile of CH4 conversion over 5% Ni/Ce0.6Zr0.4O2 catalysts for CDRM at three different temperatures.

sented. It can be seen that the ratio of Ce:Zr in CexZr1–xO2 (CTAB) support affects CH4 conversions as well as the deactivation characteristics of Ni/CexZr1–xO2 (CTAB) catalysts during the CO2 reforming of methane. When x is in the range 0.50–0.92, CH4 conversions are very stable and do not show any decrease with time on stream. Outside this range, CH4 conversions decline with time on stream. CH4 conversion of CexZr1–xO2 (CTAB) supported Ni catalysts was of the order Ce0.6Zr0.4O2 (CTAB) ∼ Ce0.85Zr0.15O2 (CTAB) > Ce0.5Zr0.5O2 (CTAB) ∼ Ce0.68Zr0.32O2 (CTAB) ∼ Ce0.78Zr0.22O2 (CTAB) ∼ Ce0.92Zr0.08O2 (CTAB) > Ce0.4Zr0.6O2 (CTAB). Similar results were also obtained for CO2 conversions. The selectivity toward H2 also follows a similar trend as seen in Figure 9b over 8 h TOS. These results are entirely different from the pure CeO2 or ZrO2 or doped zirconia-based nickel catalysts (Figures 3–5), which were found to be highly active at the beginning of the reaction, and then their activities decreased in varying degrees due to carbon deposition. In contrast, the CexZr1–xO2 supported Ni-based catalysts exhibited remarkably high activity and stability during the 8 h of reaction. Also, H2 yield and H2 selectivity (around 100%) were very stable. The increased nickel loading from 5% to 15% did not apparently enhance the catalyst activity as concluded from the CH4 and CO2 conversion turnover numbers. The 5% Ni/Ce0.6Zr0.4O2 was found to be the best among all the catalyst tested and were selected for further longterm activity test. This catalyst also showed remarkable stability even at lower temperature (600–700 °C) as seen in Figure 10. The catalyst was found to remain active for more than 8 h even at such low temperatures, indicating the possibility of using this catalyst at temperatures lower than those reported in the literature. 3.6. Effect of Support Preparation Method (CTAB vs Alcogel). Another evidence that the surfactant-assisted method (CTAB) preparation of support could lead to a better catalyst can be appreciated when the stability profile of methane conversion is compared in Figure 11. It can be seen that the 5% Ni/CexZr1–xO2 (CTAB) catalysts have a high and stable activity in the broad range of x ) 0.5–0.85 while 5% Ni/CexZr1–xO2 (alcogel) catalysts exhibit a high and stable activity only at x ) 0.6. The high stability and high catalytic activity of 5% Ni/CexZr1–xO2 (CTAB) catalysts could be attributed to their higher surface areas with the resultant high dispersion of the Ni species on the support coupled with better thermal stability. These results show that the surfactant-assisted method (CTAB) is more effective to prepare highly active and stable Ni/CexZr1–xO2 catalysts than the sol–gel method (alcogel). A similar conclusion can also be derived based on TPR profile of both 5% Ni/Ce0.6Zr0.4O2 alcogel (AL) and 5% Ni/Ce0.6Zr0.4O2 (CTAB). It can be seen in Figure 12 that the preparation method of Ce0.6Zr0.4O2 support affects the reducibility of both nickel



Figure 11. Comparative plot for alcogel and CTAB preparation techniques for the CexZr1–xO2 support on CH4 conversions over 5% Ni/CexZr1–xO2 catalysts.

Figure 12. Comparative temperature-programmed reduction H2 (TPRH2) profiles of catalysts: (a) 5% Ni/Ce0.6Zr0.4O2 (CTAB) and (b) 5% Ni/Ce0.6Zr0.4O2 (AL).

and ceria. The catalyst 5% Ni/Ce0.6Zr0.4O2 (AL) synthesized by an alcogel method shows a reduction peak of NiO centered at 440 °C, which is also seen in the case of 5% Ni/Ce0.6Zr0.4O2 (CTAB) synthesized by a surfactant-assisted method. However, the main peak corresponding to the reduction of CeO2 in Ce0.6Zr0.4O2 in 5% Ni/Ce0.6Zr0.4O2 (AL) shifts to a higher temperature (747 °C) as compared to 664 °C in 5% Ni/ Ce0.6Zr0.4O2 (CTAB). This shows that Ce0.6Zr0.4O2 which is prepared by a surfactant-assisted method is more easily reducible compared to the one prepared by alcogel method. This difference could be related to the morphological and physicochemical properties of the two different types of catalysts. The surfactantassisted method leads to a higher specific surface area, smaller pore size, and higher dispersion of Ni species. The higher reducibility of Ce0.6Zr0.4O2 enables the support to make good use of its oxygen storage capacity and participate in the redox function of the catalyst, thus increasing its stability during the dry reforming of methane. 3.7. Long-Term Stability Test. Further, the catalyst 5% Ni/ Ce0.6Zr0.4O2 (CTAB) was tested under two experimental conditions for long-term stability test. One condition was to perform the CDRM reaction over the 5% Ni/Ce0.6Zr0.4O2 (CTAB) catalyst at a reduction temperature of 710 °C and a reaction temperature of 700 °C. The other condition was to use a reduction temperature of 650 °C and a reaction temperature of 650 °C. The CO2 reforming reaction data are presented in Figure 13. These show that 5% Ni/Ce0.6Zr0.4O2 (CTAB) catalyst has a CH4 conversion of more than 68% up to 70 h at 700 °C and a CH4 conversion of more than 53% up to 80 h at 650 °C without any deactivation. To our knowledge, it is a rare case that 5% Ni catalyst shows such a high activity and stability at both 700 and 650 °C. Another long-term stability test was carried out at 800 °C using a reduction temperature of 710 °C. This is shown in Figure 14 where the methane conversion is very stable up to 240 h. This is a remarkable result in the sense that such activities are not reported on the catalyst without incorporating any

Figure 13. Long-term stability test of 5% Ni/Ce0.6Zr0.4O2 (CTAB) catalyst for CDRM at (a) 650 °C and (b) 700 °C with a feed (CH4: CO2:N2 ) 2:2:1) rate of 4.70 × 104 mL/(h g-cat.) with reduction temperature of 650 and 700 °C, respectively.

Figure 14. Long-term stability test of 5% Ni/Ce0.6Zr0.4O2 (CTAB) catalyst at 800 °C, feed (CH4:CO2:N2 ) 2:2:1) rate of 5.04 × 104 mL/ (h g-cat.), and reduction temperature of 710 °C.

alkalinity associated with the support material. Supports with alkaline character (such as Mg, Ca, or K) tend to enhance the relative abundance of adsorbed CO2 on the catalyst surface, which prevent CH4 decomposition as well as CO disproportionantion reaction, and as a result, the carbon deposition is decreased. However, the ability of the catalyst to remain stable under such condition implies that the mechanism and therefore the kinetics of the reaction are different on this type of catalysts.30 The conversion and selectivity remained unchanged during the entire reaction time employed, indicating that the catalyst had a high stability. The role of ceria–zirconia as a support material therefore imparts stability and reduced coking characteristics, which can be attributed to the redox properties of (Ce–Zr)O2, which can react directly with carbon containing species to generate CO and (Ce–Zr)Ox, followed by the reoxidation of (Ce–Zr)Ox by CO2 back to (Ce–Zr)O2. 3.8. Carbon Propensity. The amount of carbon deposited on the used catalysts was measured by TPO experiments on (30) Akpan, E.; Sun, Y.; Kumar, P.; Ibrahim, H.; Aboudheir, A.; Idem, R. O. Chem. Eng. Sci. 2007, 62, 4012.


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Table 3. Coking Propensity of Nickel Catalysts coke (g/g-cat.) used catalysts for CDRM

total weight loss

weight loss based on heat of oxidation

5 wt % Ni/Ce0.60Zr0.40O2a 10 wt % Ni/Ce0.60Zr0.40O2 15 wt % Ni/Ce0.60Zr0.40O2a 5 wt % Ni/Ce0.78Zr0.22O2a 5 wt % Ni/Ce0.85Zr0.15O2a 5 wt % Ni/Ce0.92Zr0.02O2a 5 wt % Ni/CeO2b 5 wt % Ni/ZrO2b

0.025 0.044 0.042 0.022 0.019 0.019 0.020 0.019

0.011 0.022 0.022 0.011 0.011 0.014 0.012 0.011

a After 8 h CDR at 700 °C with a feed (CH /CO ) 1:1) rate of 3.76 4 2 × 104 mL/(h g-cat.). b After 3 h CDR at 700 °C with a feed (CH4/CO2 ) 1:1) rate of 3.76 × 104 mL/(h g-cat.).

TGA to adjudge the coke propensity of the catalyst studied. The weight losses observed are presented in Table 3, which shows that carbon depositions accumulated on the 5Ni/ CexZr1–xO2 (x ) 0.6, 0.78, 0.85, 0.92) catalysts all had the same amount of about 1.1% within the first 8 h TOS. A similar amount of coking was found to be deposited on 5% Ni/CeO2 and 5% Ni/ZrO2 catalysts only within the first 3 h TOS. This suggests the idea of using a mixed oxide system of CexZr1–xO2 is better than using a single oxide system. Table 3 also shows that increasing Ni loading from 5 to 15 wt % leads to an increase in carbon deposition over these catalysts. It can be seen that not only are there small amounts of carbon deposition present on Ni/CexZr1–xO2 (x ) 0.6, 0.78, 0.85), but also, these carbons are amorphous and filamentous in nature,4,30 which may not lead to any loss of activity for these catalysts. In contrast, 5% Ni/CeO2 and 5% Ni/ZrO2 catalysts accumulated a similar amount of carbon within 3 h TOS, and apparently, their catalytic activities decreased during the period. This implies that the type of carbon formed also depends on the type of support used. It is also likely that the extremely stable catalytic performance of Ni/CexZr1–xO2 (x ) 0.6, 0.78, 0.85) is probably due to an efficient periodic cycle of carbon deposition and elimination on the nickel surface measured in terms of H2/CO ratio, as can be seen in Figure 15. Using CexZr1–xO2 as a support provides this platform and results in a stable catalyst. This could be in terms of oxygen atoms from activated CO2 being able to migrate to the metal surface to oxidize the carbon atoms or CHx intermediates from CH4 decomposition. The rates of oxygen atoms transfer from CO2 activation and then migration to the metal surface are limited. This becomes evident by combining these rates with the results obtained for the effect of space velocity on carbon deposition at different temperatures. In that result, we observed that when the space velocity was larger than a certain value (based on the temperature), carbon deposition started setting in. These space velocity values represent the limit, in terms of rate, of utilizing the redox/oxygen transfer properties

Figure 15. Periodic cycling of carbon deposition and elimination represented by plotting H2/CO ratio vs time on stream (TOS).

Scheme 1. Scheme of Reforming of Methane on Ni/CexZr1–xO2 Catalyst 2,4,5,30

of the catalyst to burn off the carbon that would potentially be deposited on the catalyst. 4. Discussion The high thermal stability of Ni/CexZr1–xO2 (x ) 0.5–0.92) catalysts in CDRM is attributed to the high oxygen mobility and defect structure in the CexZr1–xO2 by the formation of solid solutions. The ability of the cubic Ni/CexZr1–xO2 (x ) 0.5–0.92) solid solution is related to their rapid reduction/oxidation capability by releasing and uptaking oxygen owing to the reversible reaction of CeO2 ) CeO2–x + (x/2)O2 (0 e x e 0.5).15,16 The introduction of ZrO2 into CeO2 strongly decreases the reduction temperature of ceria through structural modifications of the fluorite type lattice due to the substitution of Ce4+ (0.97 Å) with Zr4+ (0.84 Å). In turn, this decreases the cell volume to lower the activation energy for oxygen ion diffusion, and this effectively inhibits the sintering of CeO2. Furthermore, an optimum OSC was observed for CexZr1–xO2 oxides with x between 0.6 and 0.82,15,21,31 and x in Ni/CexZr1–xO2 (x ) 0.5–0.85) catalysts falls in the range of 0.6–0.8. When x is equal to 0.92, apparently outside the range, the activity and stability of 5 wt % Ni/Ce0.92Zr0.02O2 catalyst were found to be lower than 5 wt % Ni/CexZr1–xO2 (x ) 0.5–0.85) catalysts (Figure 10a). On the other hand, reduced CexZr1–xO2 generates an anion vacant site after the removal of O2- ions from its lattice according to eq 11: 4Ce4+ + O2- f 4Ce4+ + 2e- ⁄ 0 + 0.5O2 f 2Ce4+ + 2Ce3+ + 0 + 0.5O2 (11) where 0 represents an empty position (anion-vacant site) originating from the removal of O2- from the lattice. These defect sites can promote CO2 dissociation in CDRM.25,30 The catalytic behavior of Ni/CexZr1–xO2 (x ) 0.5–0.85) can be further explained in Scheme 1. The top layer is constituted of relatively free metal particles; an intermediate layer where metal is strongly interacting with CexZr1–xO2 is then sandwiched between the top layer and the support. The decomposition of CH4 or CO disproportion reaction occurs on the Ni particles, resulting in the formation of H2, carbon, and CO2. Simultaneously, the reaction of carbon with oxygen occurs to produce CO. The required oxygen is available from the cubic CexZr1–xO2 (x ) 0.5–0.85) supports with high oxygen mobility near the Ni particles or from the decomposition of CO2.20,30,32 In this dual mechanism, the role of support is equally important in the dissociative adsorption of CO2. The cubic CexZr1–xO2 (x ) 0.5–0.85) supports have the ability to provide mobile oxygen species from the support to Ni particles and help in transferring (31) Letichevsky, S.; Tellez, C. A.; de Avillez, R. R.; Da Silva, M. P.; Fraga, M. A.; Appel, L. G. Appl. Catal. B 2005, 58, 203. (32) Rostrup-Nielsen, J. R. Catal. ReV.—Sci. Eng. 2004, 46, 247.



oxygen to coked metal.33 In this way, the removal of carbon is accelerated, and the activity of catalyst during the CDRM is maintained (Scheme 1).

presence of oxygen because it often results in the overoxidation of methane.36

In this context it would be prudent to mention that the oxygen from the ceria or CexZr1–xO2 (x ) 0.5–0.85) supports could be effectively utilized only if the calcinations temperature of the supports are relatively lower, between 800 and 1100 K,34 and the amount of oxygen that desorbs at the lower temperatures correlates with the catalytic activity. On the basis of these, it can be assumed that the presence of CO2 may influence the oxidation state of CexZr1–xO2 under operating conditions. In summary, the following reaction mechanism can be written for the CDRM as explained above.

5. Conclusions It is shown that the carbon deposition that generally takes place during the CDRM process can be controlled by manipulating the nickel ensembles on the catalyst surface. This is achieved by using a surfactant (CTAB)-mediated support preparation that results in materials with just enough nanocomposite characteristics leading to a strong metal–support interaction and smaller nickel ensemble. The stability of 5% Ni/ Ce0.6Zr0.4O2 (CTAB) catalyst was found to be close to 100 h at 923 and 973 K while at 1073 K the catalyst remained active for more than 200 h. The ZrO2 support obtained by using alcogel performed better compared to the hydrogel system in the beginning of the reaction. The 4% ceria doping of ZrO2 was however not as good as ZrO2 (alcogel). Temperature-programmed oxidation (TPO) experiments indicated excellent resistance toward carbon formation for Ni supported on CexZr1–xO2 as compared with other catalysts studied. H2-TPR (temperature-programmed reduction) profiles analysis also showed that the stability of CexZr1–xO2 solid solution is connected to the enhanced reducibility at lower temperature compared to either pure ceria or ceria-doped ZrO2. The mechanism assumes that (i) CH4 adsorbs and dissociates on Ni/ Ce0.6Zr0.4O2 support, (ii) the solid carbon formed further reacts with the lattice oxygen (Ox) on Ce0.6Zr0.4O2 surface, (iii) CO2 oxidizes the reduced site (Ox–1), (iv) two adsorbed hydrogen atoms react to produce H2, and (v) H2O could finally be formed as a byproduct.

CH4 + 2S T CHx + (4 - x)H(s)

(12)

(4 - x)H(s) T (4 - x ⁄ 2)H2 + S

(13)

CHx + 2(Ce - Zr)O2 T CO + x ⁄ 2H2 + (Ce - Zr)2O3 + S (14) CO2 + (Ce - Zr)2O3 T CO + 2(Ce - Zr)O2

(15)

where S represents an adsorption site on the metal. Similar conclusions were also reported30,35 that the use of an oxygen storage component of a catalyst as an oxidant, in the absence of gas-phase oxygen, is an innovative promising approach to circumvent the direct partial oxidation of methane to synthesis gas. The latter approach is not easy to achieve due to the (33) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287. (34) Bunluesin, T.; Gorte, R. J.; Graham, G. W. Appl. Catal. B 1997, 14, 105. (35) Otsuka, K.; Yang, Y.; Sunada, E.; Yamanaka, I. J. Catal. 1998, 175, 152.

Acknowledgment. The authors thank HTC Purenergy, Canada, for their financial support of this work. EF7002409 (36) Ramirez-Cabrera, E.; Atkinson, A.; Chadwik, D. Appl. Catal. B 2004, 47, 127.

Nickel-Based Ceria, Zirconia, and CeriaZirconia Catalytic Systems for

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