Comparative Study of Ni-based Mixed Oxide ... - ACS Publications

Energy & Fuels 2008, 22, 3575–3582

3575

Comparative Study of Ni-based Mixed Oxide Catalyst for 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 8, 2008. ReVised Manuscript ReceiVed July 22, 2008

Carbon dioxide reforming of methane (CDRM) to synthesis gas was studied over various Ni-based catalysts. It is shown that the mixed oxide supports CeO2-ZrO2, CeO2-Al2O3, and La2O3-Al2O3, prepared using surfactant, exhibit a high catalytic activity and stability for CDRM. Temperature program reduction (TPR) results demonstrate that the presence of CeO2, ZrO2, or La2O3 leads to the enhancement of the Ni reducibility compared to Al2O3, which is an important indicator of high activity and stability of these Ni catalysts for CDRM. Our thermodynamic calculations indicate that CeO2 could react with CH4 to produce synthesis gas, and then CO2 might reoxidize CeO2-x to its oxidation state. Furthermore, CeO2 might help in gasification of deposited carbon to inhibit the carbon formation and therefore improve catalyst stability. The presence of alumina tends not to affect the stability of the catalyst as well.

1. Introduction The dry-reforming process involves the conversion of CH4 and CO2, which are two of the cheapest and most abundant carbon-containing materials, into synthesis gas. This reaction also has very important environmental implications because both methane and carbon dioxide are greenhouse gases, which may be converted into valuable product such as hydrogen.1 Further, due to its large heat of reaction and reversibility, this process has potential thermo-chemical heat-pipe applications for the recovery, storage, and transmission of solar and other renewable energy sources. In addition, carbon dioxide reforming of methane (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 the WGS process is brought to about the same level as that of dry reforming, as was done in our earlier work,2,3 then the temperature mismatch between the two processes could be minimized. Although, catalysts based on noble metals have been found to be less sensitive to carbon deposition at low temperature.4 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. Several studies have been focused on the development of a promising support material for CDRM. The types of supports also become important when certain strong metal-support interactions might result in properties that * Corresponding author; fax: 1-306-337-2594; e-mail: Prashant.Kumar@ 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. World Intellectual Property Organization, PatentWO 2006/099716, September 28, 2006. (3) Kumar, P.; Idem, R. O. Energy Fuels 2007, 21, 522. (4) Lercher, J. A.; Hally, W.; Niessen, W.; Seshan, K. Stud. Surf. Sci. Catal. 1996, 101, 463.

make them interesting reforming catalysts. The formation of NiAl2O4 during pretreatment of the Ni/Al2O3 system and working with a 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.5,6 Also, the improved stability of the Ni/La2O3 catalyst was attributed to the isolation of nickel ensembles by LaOx species.7 Similarly, coke deposition also decreases when Ni is impregnated on supports that present a marked Lewis alkalinity/acidity and improved thermal stability, such as ZrO2 and La2O3,5-10 or on supports that have been modified for that purpose with alkaline metals such as Li, K, or Mg.5,11,12 The use of CeO2-ZrO2 as support material has been explored by several authors5,13-15 to utilize the inherent properties of this mixed oxide, such as oxygen storage capacity (OSC) and thermal stability. A few studies on CeO2-ZrO2 for CDRM at high temperatures (>800 °C) have been reported. For example, Montoya et al.16 studied a Ni/Ce-ZrO2 system; however, the problem of support sintering at 800 °C could not be completely avoided. Potdar et al.15 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 (5) Hu, Y. H.; Ruckenstein, E. AdV. Catal. 2004, 48, 297. (6) Verykios, X. E. Int. J. Hydrogen Energy 2003, 28, 1045. (7) Bartholomew, C. H. Catal. ReV. Sci.-Eng. 1982, 24, 67. (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) Bradford, M. C. J.; Vannice, M. A. Catal. ReV. Sci. Eng. 1999, 41, 1. (11) Pompeo, F.; Nichio, N. N.; Souza, M. M. V. M.; Cesar, D. C.; Ferretti, O. A.; Schmal, M. Appl. Catal., A 2007, 316, 175. (12) Kumar, P.; Sun, Yanping.; Idem, R. O Energy Fuels 2007, 21, 3113. (13) Stagg-Williams, S. M.; Noroha, F. B.; Fendley, G.; Resasco, D. E. J. Catal. 2000, 194, 240. (14) Akpan, E.; Sun, Y.; Kumar, P.; Ibrahim, H.; Aboudheir, A.; Idem, R. O. Chem. Eng. Sci. 2007, 62, 4012. (15) Potdar, H. S.; Roh, H.-S.; Jun, K. W.; Ji, M.; Liu, Z. W. Catal. Lett. 2002, 84, 95. (16) Montoya, J. A.; Pascual, E. R.; Gimon, C.; Angel, P; Del, A.; Monzon, A. Catal. Today, 2000, 63, 71.

10.1021/ef800326q CCC: $40.75  2008 American Chemical Society Published on Web 09/10/2008

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conversions) for almost 35 h. However, beyond this time, there was an apparent decrease in activity as compared to the initial activity of the catalyst. Another report17 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. Our work2,3,12,14 on cationic surfactant resulted in a CexZr1-xO2 solid solution with reduced pore-volume followed by nickel impregnation. We have shown that by utilizing the nanocomposite characteristics of the support a smaller nickel ensemble can be created that could results into a stable catalyst. The 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 nanocomposites of supported metal catalyst. In this work we have synthesized and evaluated a few other mixed oxides, such as La2O3-Al2O3 and CeO2-Al2O3, and have compared their catalytic activity with CeO2-ZrO2. Becasue the γ-Al2O3- or La2O3-Al2O3-based catalysts are more costeffective, they are interesting from the commercial application point of view. Moreover, we have found that the major problem associated with CeO2-ZrO2 catalyst has been its poor mechanical strength and attrition loss after a prolong run in a fixedbed reactor. The use of γ-Al2O3 that would provide the required mechanical strength and does not interfere with the catalyst property as such could be an option. However, acidic sites on γ-Al2O3 either crack or polymerize hydrocarbon and leave residual carbon on the surface, thereby deactivating the catalytic activity. Also, at elevated temperatures of the CDRM process, the transition of γ-Al2O3 to a relatively denser R-Al2O3 crystalline phase reduces the surface area and, hence, the catalytic activity. In addition, the presence of lanthanum in stabilizing the alumina is remarkable. Therefore, it was important for us to evaluate the catalytic system having an alumina or a lanthanum-alumina system and compare the stability with the CeO2-ZrO2. 2. Experimental Section 2.1. Preparation of Catalysts. CeO2-ZrO2 with 60 atomic % of Ce balanced by Zr, CeO2-Al2O3 with 60 atomic % of Ce balanced by Al, and La2O3-Al2O3 with 60 atomic % of La balanced by Al supports were prepared by a surfactant-assisted method as previously described elsewhere.11 The materials used for synthesis were either nitrate or chloride salts (Aldrich) of lanthanum, cerium, zirconium, and aluminum. The supports were prepared by adding an aqueous solution of the appropriate cetyltrimethylammonium bromide, C16 (Aldrich) to an aqueous solution containing the desired amount of metal ions (all metal concentrations ) 0.1 M). Aqueous ammonia (28∼30%) was added to the mixed solution dropwise under vigorous stirring until the pH reached 11.5. The mixture was stirred for 2 h and then aged at 90 °C for 5 days. After that, the resulting mixture was filtered and washed with hot, distilled water until it was free of chlorine. Then, the light-yellow residue was dried at 90 °C for 1 day, followed by calcination at 650 °C in a flow air for 5 h. About 5 wt % of Ni was loaded on to the support by a wet impregnation method. A required amount of the support was mixed with an aqueous solution of Ni(NO3)2 · 6H2O in a roundbottom flask. The resulting slurry was stirred at room temperature overnight, and then the excess water was evaporated at 70 °C using (17) Roh, H.-S.; Jun, K.-W.; Baek, S.-C.; Park, S.-E. Catal. Lett. 2002, 81, 147.

Kumar et al. a rotary vacuum evaporator. The residue was dried in an oven at 110 °C overnight and then calcined with continuous aeration at 650 °C for 5 h. 2.2. Catalyst Characterization. All the catalysts were characterized using different analytical techniques. A Micromeritics ASAP 2010 was used for Brunauer-Emmett-Teller (BET) surface area and chemisorption (using H2) measurements whereas thermogravimetric analysis/differential scanning calorimetry (TGA/DSC, TGDSC-1100, Setaram), temperature-programmed reduction (TPR, Chembet 3000, Quantachrome), and X-ray diffraction (XRD, Bruker, AXS) were used to evaluate the crystal structure of catalysts. Elemental composition was determined by inductively coupled plasma mass spectrometry (ICP-MS, Varian). The crystal size of samples was measured from X-ray broadening by using the wellknown Scherrer equation,

d)

0.089λ B(2θ)cos θ

(1)

where B(2θ) is the width of the XRD pattern line at half-peakheight (rad), λ is the wavelength of the X-ray, θ is the angle between the incident and diffracted beams (°), and d is the crystal size of the power sample (nm). NIST standard LaB6 (SRM 660) was used to determine the instrument broadening. With this standard, all broadening that was considered due to instrumental parameters were established. 2.3. Experimental Setup and Product Analyses (CDRM). To investigate the CDRM, an experimental reactor system was constructed as shown in Figure 1. An Inconel packed bed tubular reactor (PBTR, id ) 6.3 mm) housed in a furnace with a single heating zone was used for catalyst performance evaluation. The gas flows (100 mL/min) were metered and regulated by an Aalborg digital flow controller (GFC171S). A premixed CO2/CH4/N2 with a composition of 2:2:1 was used as a feed. The catalyst bed temperature was measured by means of a sliding thermocouple dipped inside the catalyst bed. The diluents (quartz sand, Aldrich) used in the catalyst bed had a particle size of 0.3 mm, similar to the catalyst. Both catalyst (0.15 g) and diluents (17.6 g) were mixed together before loading into the reactor. Pure quartz (5.5 g) was also used in the preheating zone. To approach plug flow conditions and to minimize back mixing and channelling, 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 13X columns (Alltech Associates) for complete separation of the gaseous components. The conversions of CH4 and CO2 and the selectivity of H2 are defined as follows:

Conversion (CH4) % )

(CH4)in-(CH4)out × 100 (CH4)in

(2)

Conversion (CO2) % )

(CO2)in-(CO2)out × 100 (CO2)in

(3)

Selectivity of H2 (%) )

(H2)out 100 × (CH4)in-(CH4)out 2

(4)

3. Results and Discussion 3.1. Feasibility of Oxidation/Reduction Reactions between Gases and Metal-Support. CeO2-based materials have high oxygen storage capacity and oxygen mobility. These characteristics are related to their rapid reduction/oxidation capability by releasing and uptaking oxygen owing to the

Ni-based Mixed Oxide Catalyst for CDRM

Energy & Fuels, Vol. 22, No. 6, 2008 3577

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

Figure 2. Variation of standard Gibbs free energy with temperature for CeO2-based Ni catalyst in CDRM processes.

reversible reaction of CeO2 ) CeO2-x + (x/2)O2 (0 e x e 0.5).18 The following main reactions between feed and support are possible during the CDRM process. 2CeO2 + CH4 T Ce2O3 + CO + 2H2

(5)

Ce2O3 + CO2 T 2CeO2 + CO

(6)

In addition to these reactions, carbon would also be formed as a result of the decomposition of CH4 and CO disproportionation. As a lattice oxygen provider, CeO2 may oxidize the solid carbon in the following reaction. 2CeO2 + C(s) T Ce2O3 + CO

(7)

In presence of CeO2, Ni oxidation may also take place according to the following reaction. 2CeO2 + Ni T Ce2O3 + NiO

(8)

To study the possibility of the above reactions, standard Gibbs-free energy changes (∆G°) have been calculated as a

function of temperature, as shown in Figure 2, where straight lines are obtained on the basis of ∆G° ) ∆H° - T∆S°. Under standard conditions, the reaction of CH4 with CeO2 is thermodynamically feasible only above 780 °C, whereas reaction 6 is favored below 1000 °C. The estimated values of temperatures at which reactions 5 and 6 occur differ from the reported values.19 According to Otsuka’s calculations,19 reaction 5 is favored above 600 °C, and 6 is favored below 700 °C. The discrepancy may be due to difference in databases used for the calculation. Because gasification of solid carbon is thermodynamically favored above 900 °C, the presence of CeO2 could inhibit carbon formation during the CDRM process. In addition, it is unlikely that CeO2 oxidizes Ni to NiO, which has less (18) Trovarelli, A.; Catalysis by Ceria and Related Materials; Imperial College Press: Universita di Udine, 2002; 2. (19) Otsuka, K.; Wang, Y.; Sunada, E.; Yamanaka, I. J. Catal. 1998, 175, 152.


Kumar et al.

Table 1. Physicochemical Properties of Supports and Corresponding Ni Catalysts

support and catalyst 60 (atom) % CeO2-ZrO2 60 (atom) % CeO2sAl2O3 60 (atom) % La2O3-Al2O3 5 wt % Ni/60 (atom) % CeO2-ZrO2 5 wt % Ni/60 (atom) % CeO2sAl2O3 5 wt %/60 (atom.) % La2O3sAl2O3

BET surface areas (m2/g)

pore diameter (nm)

pore volume (cm3/g)

178.9 107.9 46.9 136.1

3.87 4.41 6.96 4.22

0.23 0.17 0.1 0.19

83.1

4.85

0.14

33.9

6.03

0.07

catalyst activity than Ni species,20 whereas reduced Ce2O3 possibly reduces NiO to Ni, maintaining Ni activity for CDRM. The role of CeO2 as an oxidant for the conversion of CH4 to H2 and CO in the absence of O2 has been investigated by many researchers.21,22 Otsuka et al.19 reported that the gas-solid reaction between CeO2 and CH4 produced synthesis gas with a H2/CO ratio around 2, whereas CeO2 is reduced to CeO2-x at 700 °C under anaerobic conditions. This reduced phase of Ce could be reoxidized back to CeO2 by CO2. The findings are consistent with our thermodynamic predictions. The addition of ZrO2 to CeO2 can improve the reactivity of the lattice to promote the conversion of CH4 to syngas at 500 °C,22 which is a much lower temperature than predicted from the above thermodynamic calculation. On the basis of the thermodynamic analysis, CeO2-based catalyst support can actively participate in CDRM at certain temperatures and pressures. That is, CeO2 reacts with CH4 to produce synthesis gas, whereas CO2 present in CH4 may reoxidize Ce2O3 back to its original oxidation state. Also, CeO2 could react with deposited carbon on the surface Ni catalysts and prevent coking. Finally, reduced CeO2-x could also spontaneously reduce NiO to Ni and prevent oxidation of active Ni metal on the catalyst surface. This is why the activity of the catalyst is maintained, as reported in one of our publications.2,3,12 3.2. Physicochemical Properties. The BET surface area, pore volume, and pore size distribution of the catalyst support and corresponding Ni catalysts employed in this study are summarized in Table 1. As shown in Table 1, the BET surface area of CeO2-ZrO2 is 178.9 m2/g, which is the largest surface area among these catalyst supports, followed by 107.9 m2/g for CeO2sAl2O3 and 46.9 m2/g for La2O3sAl2O3. It is evident that the addition of Al2O3 to CeO2 and also to La2O3 leads to a significant decrease in the support surface area and pore volume as compared to CeO2-ZrO2. A drastic loss of surface area is either due to smaller species of CeO2 or La2O3, which block the pores of Al2O3,23 or due to the stabilizing effect of La2O3 or CeO2 on Al2O3,24 which prevents the transformation of γ-Al2O3 to R-Al2O3. Loading of Ni metal onto various supports results in a decrease in surface areas to different degrees, so these Ni catalysts have a lower surface area than their pure supports. The results are similar to Natesakhawat and Ozkan’s finding.25 3.3. X-Ray Diffraction (XRD). To understand the effect of composition on the phases and crystallinity of these materials, (20) Roh, H.-S.; Potdar, H. S.; Jun, K. W.; Kim, J. W.; Oh, Y. O. Appl. Catal. A, 2004, 276, 231. (21) Otsuka, K.; Wang, Y.; Nakamura, M. Appl. Catal., A 1999, 183, 317. (22) Pantu, P. K. K.; Gavalas, G. R. Appl. Catal., A 2000, 193, 203. (23) Damyanova, S.; Bueno, J. M. C. Appl. Catal., A 2003, 253, 135. (24) Beguim, B.; Garbowski, E.; Primet, M Appl. Catal. A, 1991, 75, 119. (25) Natesakhawat, S.; Oktar, O.; Ozkan, U. S. J. Mol. Catal. A 2005, 241, 133.

XRD analysis was conducted. The diffractograms are shown in Figures 3-5 for CeO2-ZrO2, CeO2sAl2O3, and La2O3sAl2O3 supports, respectively. The patterns in Figure 3 indicate the presence of a true mixed oxide phase with cubic fluorite structure. Its reflections correspond to (111), (200), (220), (311), (222), and (400) planes, and no segregation phases such as CeO2 or ZrO2 have been detected. This single cubic phase CeO2-ZrO2 has the maximum degree of structural defects and oxygen storage capacity as reported in one of our earlier publications.12 Compared to the XRD profiles of only support material, the nickel-loaded system (5 wt %) shows some evidence of nickel [111] species and seems to have not affected the cubic phase structure of CeO2-ZrO2 support. The X-ray profile of the CeO2sAl2O3 support is shown in Figure 4a. As compared to the X-ray profile of CeO2 reported18 in the literature, it shows that CeO2 with cubic fluorite structure is the only detectable phase, and other crystalline phases such as of Al2O3 or CeAlO3 are not present, indicating that Al2O3 is present in its amorphous form in this sample. This implies that there probably is no strong chemical interaction between CeO2 and Al2O3 in CeO2sAl2O3, even after calcinations at 650 °C for 5 h in flowing air. There is some evidence of a nickel oxide peak (Figure 4b), although the structural robustness is maintained. The X-ray profile of the La2O3-Al2O3 (Figure 5a) support appears to have only a measurable amount of hexagonal La2O3 formed in this binary oxide, which means Al2O3 exists in an amorphous state. The nickel loading, on the other hand, on this support results in a very good dispersion without having any nickel oxide peak in the diffraction pattern. Our results are consistent with those reported by Toops et al.26 3.4. Temperature Programmed Reduction (TPR). The H2TPR has been extensively used to characterize the Ni reducibility of the reforming Ni catalysts. The TPR-H2 profiles for the mixed oxides-based Ni catalysts are presented in Figure 6. For comparison, the TPR profile of pure NiO is also included in this figure. It can be seen that the reduction peak of unsupported NiO appears at 490 °C, which is a little higher than the published results.27 This may be due to the difference in the experimental setup and operating conditions. The CeO2-ZrO2 support has two broad peaks at 650 and 960 °C. The peak at 650 °C results from the surface reduction of Ce4+ in mixed CeO2-ZrO2 support, and the peak at 960 °C is related to the bulk reduction of CeO2 occurring at higher temperature.28 According to the reduction mechanism of CeO2 proposed by Pias et al.,29 the outermost layers of smaller crystalline CeO2 are first reduced at lower temperature due to a lower enthalpy of reduction, and then bulk reduction occurred at higher temperatures. The TPR profile of 5 wt % Ni/CeO2-ZrO2 catalyst possesses three peaks: the first one is a peak at 450 °C, and the other two broad peaks are at 650 and 960 °C. The first peak is attributed to the reduction of NiO to Ni0 and is comparable to the 490 °C peak in the pure NiO TPR profile. It is obvious that the reduction peak of NiO is shifted to a lower temperature when it is deposited on the high surface area CeO2-ZrO2 support. It is likely that the better-dispersed Ni and an appropriate interaction between metal and support increase the Ni reducibility. There is an electronic interaction between CeO2 and Ni30 because Ce is rich in d-electrons and Ni has unfilled d orbits. The unfilled (26) Toops, T. J.; Walters, A. B.; Vannice, M. A. Appl. Catal., A 2002, 233, 125. (27) Mori, H.; Wen, C.; Otomo, J.; Eguchi, K.; Takahashi, H. Appl. Catal., A 2003, 245, 79. (28) Tuller, H. L.; Nowich, A. S. J. Electrochem. Soc. 1979, 126, 209. (29) Pias, A.; Trovarelli, A.; Dolcetti, G. Appl. Catal., B 2000, 28, L77L81.



Figure 3. A representative example of XRD patterns for (a) Ce0.6Zr0.4O2 (CTAB) and (b) 5 % Ni/Ce0.6Zr0.4O2 (CTAB).

Figure 4. A representative example of XRD patterns for (a) 60 (atom) % CeO2-Al2O3 and (b) 5 % Ni/60 (atom) % CeO2-Al2O3.

d orbits of Ni atoms can accept d-electrons from Ce, resulting in increase in d-electron density of Ni atom. The peaks at 650 and 960 °C correspond to the reduction of surface and bulk CeO2, respectively, in CeO2-ZrO2 support.

The TPR profile of 5 wt % Ni/CeO2-Al2O3 catalyst (Figure 6) has four peaks at 415, 490, 780, and 890 °C. The first peak at 415 °C corresponds to the reduction of NiO. It should be noticed that the reduction of NiO in the catalyst is now possible


Kumar et al.

Figure 5. A representative example of XRD patterns for (a) 60 (atom) % La2O3-Al2O3 and (b) 5 % Ni/60 (atom) % La2O3-Al2O3.

Figure 6. The TPR-H2 profiles of the materials: (a) Ce0.6Zr0.4O2 (support) (b) 5 % Ni/Ce0.6Zr0.4O2 (catalyst), (c) 5 % Ni/CeO2-Al2O3 (catalyst), (d) 5 % Ni/La2O3-Al2O3 (catalyst), and (e) pure NiO.

at 415 °C, which is lower than reduction at 490 °C in pure NiO and at 600 °C in Ni/Al2O3.1 This means that the NiO species dispersed on CeO2-Al2O3 are easily reducible due to the presence of CeO2. The results are consistent with those published by Natesakhawat et al.,25 who observed that the addition of a Ce element results in a greater reduction of Ni species in the Ni-Ce/Al2O3 catalysts. Wang and Lu et al.1 have also reported that the addition of CeO2 in Ni/Al2O3 catalyst can enhance the dispersion of much finer particles of Ni metal. The peak at 490 °C corresponds to the reduction of surface CeO2 in 5 wt % Ni/CeO2-Al2O3 catalyst. In comparison to the reduction surface of CeO2 at 650 °C in 5 wt % Ni/CeO2-ZrO2 catalyst, there is significant lowering of surface CeO2 reduction temperature in 5 wt % Ni/CeO2-Al2O3 catalyst. The possible reasons could be (a) improved dispersion of CeO2 particles due to incorporation of Al2O3 into CeO2 and retardation of sinter-

ing31 and (b) a stronger interaction between CeO2 and Ni, which could be due to the overlapping of the NiO and CeO2 reduction peaks. This effect of promotion of CeO2 reduction by Ni is also described in the literature.18 However, this influence is not obvious in 5 wt % Ni/CeO2-ZrO2 catalysts. This implies that the interaction between Ni and support in 5 wt % Ni/ CeO2-Al2O3 catalyst is stronger than that in 5 wt % Ni/ CeO2-ZrO2 catalyst. The reason for the difference is not quite clear, and the possible explanation may be an enhanced interaction between Ni and CeO2 due to a wider dispersion of Ni and CeO2 in Al2O3. The peak at 780 °C is attributed to the reduction of bulk CeO2. The highest temperature peak at 890 °C usually corresponds to the CeAlO3 formation that occurs by occupation of Al3+ cations sites by Ce3+ cations on the surface of the support after the complete reduction of surface CeO2.29 Damyanova et al.32 have characterized the CeO2-Al2O3 mixed oxides using different techniques and have found that the CeAlO3 phase appears only at a higher reduction temperature, due to a diffusion limitation of Ce to form solid solution with Al2O3. In the case of 5 wt % Ni/La2O3-Al2O3 catalyst, there are four peaks at 455, 580, 850, and 985 °C. The first peak corresponds to the reduction of NiO to Ni0 at a lower temperature (455 °C) as compared to that for pure NiO as well as for Ni/ Al2O3, as described earlier. This proves that the addition of La2O3 to Al2O3 can further decrease the reduction temperature of NiO. Ross et al.33 have also reported that the addition of La2O3 to Ni-Al catalyst improves the dispersion (30) Yang, Y. L.; Li, W. Z.; Xu, H. Y. Kinet. Catal. Lett. 2002, 77, 155. (31) Fernandez-Garcia, M.; Martiner-Arias, A.; IglesiasJuez, A. J. Catal. 2000, 194, 385. (32) Damyanova, S.; Perez, C. A.; Schmal, M.; Bueno, J. M. C. Appl. Catal., A 2002, 234, 271. (33) Lansink-Rotgerink, H. G. J.; Paalman, R. P. A. M.; Van Ommen, J. G.; Ross, J. R. H. Appl. Catal. A, 1988, 45, 257.


Figure 7. CH4 conversion with time on a stream over 5 wt % Ni catalysts on mixed-oxide supports for CDRM at 700 °C.

Figure 8. H2 yield with time on a stream over 5 wt % Ni catalysts on mixed oxide supports for CDRM at 700 °C.

of Ni species in Ni/Al2O3 (also revealed through XRD, Figure 5b) catalyst and promotes the reducibility of NiO. Wang and Lu1 have also found that the enhancement of the reducibility is due to the ability of La2O3 to disperse Ni metallic crystallites on Al2O3 support. It is well-known that La2O3 lattice has surface and bulk oxygen vacancies,34 the density of which is related to La2O3 concentration in La2O3-Al2O3 binary oxide. Our XRD results indicate that La2O3 exists only as a hexagonal phase in La2O3-Al2O3 support without formation of LaAlO3 after calcinations at 650 °C. Hence, the peak at 580 °C may correspond to the reduction of surface La2O3 in the vicinity of Ni,35 whereas the peaks at 985 and 850 °C are probably due to the reduction of bulk La2O3 or LaAlO3,34 which are likely to be formed at higher temperatures. The reduction of La2O3-Al2O3 and CeO2-Al2O3 (not reported) supports were found to be very similar due to being iso-structural oxides with almost identical lattice parameters. 3.5. The Catalytic Activity for CDRM and Its Relationship with Catalyst Support. The catalytic activity of different mixed-oxides-based Ni catalysts for CDRM at 700 °C is shown in Figures 7-9. It can be seen that 5 wt % Ni on CeO2-ZrO2 catalyst exhibits about 63% CH4 conversion, 57% H2 yield, and 92% H2 selectivity at 700 °C. This is a high catalytic activity and selectivity at the relatively low temperature. The catalyst activity remains stable and no deactivation was detected during the course of reaction. Other catalysts such as 5 wt % Ni/CeO2-Al2O3 and 5 wt % Ni/La2O3-Al2O3 also shows similar catalytic activities and stabilities compared to 5 wt % Ni on CeO2-ZrO2. The high activity and stability of Ni/ (34) Huang, S. J.; Walters, A. B.; Vannice, M. A. Appl. Catal. B, 2000, 26, 101. (35) Figoli, N. S.; Largentiere, P. C.; Arcoya, A.; Seoane, X. L. J. Catal. 1995, 155, 95.


Figure 9. H2 selectivity with time on a stream over 5 wt % Ni catalysts on mixed oxide supports for CDRM at 700 °C.

CeO2-ZrO2 and Ni/CeO2-Al2O3 catalysts in CDRM are attributed to high redox-ability of cubic CeO2-based materials and high reducibility of Ni. The XRD measurements indicate that the CeO2-ZrO2 and CeO2 in CeO2-Al2O3 have a cubic fluorite structure, which plays an important role in the properties of Ni on the catalyst surface where CH4 decomposition and carbon formation take place. It is reported18 that the CeO2-ZrO2 or CeO2 with a homogeneous cubic phase have a higher oxygen storage capacity and a better Ni dispersion than other phases. These characteristics reduce carbon formation and improve the catalyst activity. The presence of ZrO2 in ceria-based solid solution also helps largely to facilitate good thermal stability and excellent redox behavior. These two features are generally connected due to enhanced reducibility at lower temperature as well as an increase in oxygen mobility within the bulk with ZrO2.12 Also it has been reported36 that the fast transfer of O* from CeO2-ZrO2 or CeO2 cubic phase to active metal surface can improve the catalyst activity. TPR results confirm that the reducibility of NiO is enhanced in both Ni/CeO2-ZrO2 and Ni/ CeO2-Al2O3, whereas the reducibility of surface CeO2 in the latter is improved compared to the former. Swaan et al.37 have reported that the catalytic activity of CeO2-based Ni catalysts is strongly influenced by the state of metallic Ni phases, such as reduced and dispersed phases, and the redox-ability of CeO2. Roh et al.20 have investigated CDRM over coprecipitated Ni-Ce-ZrO2 at 800 °C. The catalyst exhibits a high catalytic activity (CH4 conversion >97%) and stability without significant loss of activity over a reaction period of 100 h. They have indicated that the higher activity and stability of the catalyst is due to a higher oxygen transfer rate from the cubic Ce-ZrO2 support, an easier reduction of Ce-ZrO2, and a better Ni dispersion on the catalyst surface. It is well-known18 that the addition of Al2O3 to CeO2, having the same effect as the addition of ZrO2 to CeO2, leads to the increase in oxygen storage capacity of CeO2 and to an improvement in the thermal stability of CeO2. The presence of CeO2 in Ni/Al2O3 catalysts can suppress the formation of inactive NiAl2O4 phase and stabilize Al2O3.29 Wang and Lu et al.1 have studied the effect of CeO2 on the catalytic activity of Ni/γ-Al2O3 for CDRM at the temperature range from 500 to 800 °C. They have demonstrated that the addition of CeO2 to γ-Al2O3 not only improves the catalyst activity and stability but also suppresses carbon formation because CeO2 can increase the Ni dispersion and react with deposited carbon. (36) Feio, L. S. F.; Hori, C. E.; Damyanova, S.; Noronha, F. B.; Cassinelli, W. H.; Marques, C. M. P.; Bueno, J. M. C. Appl. Catal., A 2007, 316, 107. (37) Swaan, H. M.; Kroll, V. C. H.; Martin, G. A.; Mirodatos, C. Catal. Today 1994, 21, 571.


Laosiripojana et al.38 have also found that doping of CeO2 as an additive promoter on Ni/Al2O3 can significantly improve the reforming reactivity and carbon resistance. To explain the role of CeO2 in the catalysts for CDRM, the reaction mechanism of CDRM over ceria-based Ni catalysts has been considered. It is reported that the decomposition of CH4 and dissociation of CO2 occur via two independent paths;10 CH4 is mainly activated on the metallic surface, whereas CO2 is hydrogenated on the alumina surface or at the metal-support interface to give CO. Our previous work on CDRM12,14 has demonstrated that the first H-abstraction step in the CH4 decomposition is the rate-controlling step, and the resulting C* can be accumulated on the metal surface up to the relative rates of the CH4 decomposition and of carbon removing by the reaction C* + O* S CO. Hence, the catalyst activity depends upon the activation of CH4 on metal sites, which can be partially blocked by carbon. From the thermodynamic analysis described earlier, it can be concluded that the reduction of CeO2 by CH4 and carbon are thermodynamically favorable at the reforming temperature range 650-950 °C. Also, the oxidation of surfacereduced CeOx by CO2 is likely to be at thermodynamic equilibrium in the range of reforming temperatures considered, therefore it is likely that the oxygen transfers from CO2 to the metal surface via interface metal-CeO2. Kim et al.39 have investigated catalyst properties of Ni-Ce mixed oxide for CDRM using XRD, XPS, pulse experiments, and regeneration tests. They have found that the catalytic activity and resistance to coking rely on the reactivity of lattice oxygen in Ni-catalyst support. The ceria lattice oxygen participates in the reforming reaction followed by the oxidation from CO2 dissociation. Therefore, it is reasonable to believe that the CeO2 actively participates in the CDRM via the above reactions, resulting in an increase in activity and stability of Ni/CeO2-ZrO2 and Ni/ CeO2-Al2O3 catalysts. The high activity and thermal stability of Ni/La2O3-Al2O3 catalysts in CDRM stems from the high Ni reducibility and the ability of La2O3 to disperse Ni species and react with deposited carbon. TPR results in Figure 6 verify the easy reduction of NiO in Ni/CeO2-Al2O3, and XRD results in Figure 5 show that the La2O3 appears as a pure hexagonal phase in La2O3-Al2O3. Martinez et al.40 have studied the influence of the addition of La2O3 to Al2O3 support on the activity of Ni/Al2O3 catalyst in CDRM using a fixed bed reactor (38) Laosiripojana, N.; Sutthisripok, P.; Assabumrungrat, W. S. Chem. Eng. J., 2007, 127, 31. (39) Kim, D. K.; Stowe, K.; Muller, F.; Maier, W. F. J. Catal. 2007, 247, 101. (40) Martinez, R.; Romero, E.; Guimon, C.; Bilbao, R Appl. Catal. A 2004, 274, 139.

Kumar et al.

at 700 °C. Their results show that the presence of La slightly increases the catalyst activity but significantly enhances the catalyst stability due to a substantial decrease in coke formation during the CDRM. Another report by Verykios et al.6 also investigated the Ni/La2O3 catalyst in CDRM and was characterized using FTIR and XPS techniques. They have concluded that the good catalytic performance of the catalyst results from the ability of La2O3 to form the oxy-carbonate species of La2O2CO3, which directly reacts with deposited carbon and then restores the activity of the catalyst. Matsui et al.41 have studied the reaction mechanism of the CO2 and CH4 reaction over Ru/Al2O3 and Ru/La2O3. They also confirm that La2O3 can rapidly form La2O2CO3 with CO2 whereas Al2O3 adsorbed very little CO2. Furthermore, the addition of La to γ-Al2O3 catalyst supports not only favors a decrease in carbon formation but also stabilizes the crystalline structure and crystallite size of Al2O3 against sintering;10 consequently, the presence of Al2O3 stabilizes La2O3. 4. Conclusions In this work, various Ni catalysts were investigated in CDRM on a variety of mixed oxide supports (Ce, La, Zr, and Al) prepared by a surfactant-assisted method. The synthesis method results in the formation of a homogeneous CeO2-ZrO2 solid oxide with a cubic phase after calcination at 650 °C. However, in case of CeO2-Al2O3 and La2O3-Al2O3, CeO2 appears in a cubic phase in the former, La2O3 appears in a hexagonal phase in the latter, and Al2O3 exists in an amorphous state in both mixed oxides. All the catalysts exhibit a high catalytic performance and stability for CDRM and were found to be comparable to CeO2-ZrO2. The TPR results have shown that the presence of Ce or La leads to the increase in Ni reducibility in mixed oxides-based Ni catalysts. Active participation of lanthanide in the CDRM and high Ni reducibility are crucial for the high catalytic activity and stability. The presence of alumina is found to be not detrimental, provided it is incorporated into the mixed oxide during the preparation of the support itself. The presence of alumina will provide the necessary mechanical strength to the catalyst for our scale-up endeavor. Acknowledgment. The financial support provided by HTC Purenergy, Regina, SK, Canada and facility of the International Test Centre (ITC) for CO2 capture, University of Regina, SK, Canada is greatly acknowledged. EF800326Q (41) Matsui, N.; Anzai, K.; Akamatsu, N.; Nakagawa, K.; Ikenaga, N.; Suzuki, T. Appl. Catal. A 1999, 179, 247.

Comparative Study of Ni-based Mixed Oxide ... - ACS Publications

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