Clays and Clay Minerals; December 2005; v. 53; no. 6;
p. 622-630; DOI: 10.1346/CCMN.2005.0530608
© 2005 Clay Minerals Society
NICKEL CATALYSTS SUPPORTED ON MgO/SMECTITE-TYPE NANOCOMPOSITES FOR METHANE REFORMING
Alexander Moronta*,
,
Nobuhiro Iwasa,
Shin-Ichiro Fujita,
Masahide Shimokawabe and
Masahiko Arai
Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Kita 12, Nishi 8, Kita-ku, Sapporo 060-8628, Japan
* E-mail address of corresponding author: amoronta{at}luz.edu.ve
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Abstract
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MgO-clay nanocomposites were prepared from a synthetic smectite-type clay, TS, using three different non-ionic surfactants (Igepal CA-720, Brij 30 and Brij 56) and the resulting clay nanocomposites were impregnated with Ni for the methane reforming reaction with carbon dioxide to synthesis gas. A Ni/TS catalyst was also prepared for comparison. The prepared supports and catalysts were characterized by X-ray diffraction, X-ray fluorescence, thermogravimetric analysis and N2 adsorption/desorption isotherms. The thermal stability, pore structure and the surface area strongly influence the catalytic behavior of the catalysts. The methane conversions (at 700°C for 4 h) were 91, 95 and 97% for Ni/TSIGE, Ni/TSBR30 and Ni/TSBR56, respectively, indicating that the surface properties and the catalytic performance of the resulting solids slightly improved as the polyethylene oxide number of the surfactant increased. A reduced conversion (10%) and a rapid deactivation was observed in the Ni/TS catalyst, attributed to its Na content and low thermal stability, which led to sintering and coke deposition.
Key Words: Clay Nanocomposites • Dry Reforming of Methane • Ni Catalysts • Synthesis Gas
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INTRODUCTION
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The process of carbon dioxide reforming of methane to synthesis gas has received considerable attention in recent years because of the possibility of converting two of the cheapest carbon-containing materials into useful feedstock for further chemical processes (Erdohelyi et al., 1994; Zhang and Verykios, 1994). The renewed interest is also due to environmental considerations as this reaction consumes carbon dioxide and methane, both greenhouse gases. The factor leading to increased environmental interest in this process is the endothermic nature of the reaction that makes it possible to store solar energy as synthesis gas. However, due to the deactivation of the catalyst, no industrial technology has been established thus far.
Many catalysts have been tested, mainly the transition metals such as Rh, Pt, Ru and Ir and non-noble Ni (Edwards and Maitra, 1995; Wang and Lu, 1998; Prabhu et al., 1999). Due to its availability and price, Ni is seen as the most appropriate catalyst for the reforming process (Prabhu et al., 1999). Nevertheless, under the conditions used, this reaction leads to a fast deactivation of this catalyst, caused by the deposition of carbon species at active metal sites. Therefore, there is great industrial interest in the development of stabilized Ni catalyst. Several supports for the metal have been studied, indicating that MgO is better than Al2O3 for preventing coke formation (Choudhary and Rajput, 1996; Chen et al., 1997; Wang and Lu, 1998).
Clay minerals are interesting materials as catalyst supports due to their great abundance, low cost and particular properties. The incorporation of large inorganic cations onto the clay’s gallery leads to the formation of metal oxide pillars after calcination, which improve the available surface area and structural integrity; these materials have the added advantage that the pillars themselves may be catalytically active. In particular, montmorillonite and its pillared derivates have been used for hydrogenation and dehydrogenation (Andersen et al., 1993; Gonzalez and Moronta, 2004). Excellent activity for selective reduction of NO catalyzed by ion-exchanged pillared clays has been observed (Yang and Li, 1995).
Preliminary results, using a Ca-montmorillonite (STx-1, Texas, USA) modified by different activation procedures (acid activation, Al2O3-pillared, La-exchanged and combinations of these treatments) and then impregnated with Ni loading of 5 wt.%, did not produce good results for methane reforming (conversions of <25% achieved) due to the weak thermal stability found, which gave low mesoporosity as well as sintering. Hwang et al.(2001) reported that the pore structure of pillared clay supports affects the catalytic activity in CO2 reforming of methane; in this regard, mesoporous materials are good catalyst supports for this reaction.
The aim of this work was to prepare materials of large surface area and mesoporosity, based on Ni catalysts supported on MgO/smectite-type nanocomposites for methane reforming. The textural characteristics and the catalytic performance of synthesized MgO clay nanocomposites in the presence of non-ionic surfactants of different polyethylene oxide number were studied.
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MATERIALS AND METHODS
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Support preparation
A synthetic smectite-type clay, TS-1 (referred to hereafter as TS, trioctahedral Mg clay) obtained from CO-OP Chemical Co. Inc., was used as received. This clay was selected because of its large surface area (SBET = 488 m2g–1); naturally occurring clays usually have small surface areas.
The clay composites were prepared using a similar procedure to that described by Zhu and Lu (2001). In this case, 2.5 g of TS clay were dispersed in 70 mL of de-ionized water. The clay suspension was stirred for 1 h. 5 g of surfactant of different polyethylene oxide number (PEO, Table 1
) (Igepal CA-720, Brij 30 and Brij 56), obtained from Aldrich Chemical Company, were added to the clay suspension. Stirring was maintained for 2 h to allow sufficient mixing. To this mixture, 2.5 mL of a 2.5 M magnesium acetate tetrahydrate (99.0%, Wako) solution was added drop-wise with continuous stirring for 2 h. The suspension was transferred to an autoclave and kept at 100°C for 2 days. A white precipitate was recovered from the mixture by centrifugation and washed three times with de-ionized water. The solid collected was dried at 110°C overnight, calcined in air at 500°C for 2 days and then stored in sample vials. A support prepared in this manner is identified, for example, as TSIGE, which means that the starting TS clay was treated with the Igepal surfactant and Mg solution; the other two clay nanocomposite supports using Brij 30 and Brij 56 are identified as TSBR30 and TSBR56, respectively.ss
Catalyst preparation
5 wt.% of Ni was loaded onto the nanocomposite supports by a wet impregnation method, using acetone solutions of nickel acetylacetonate complex (98%, Merck). For comparison purposes, Ni-impregnated TS was also prepared. The suspensions were stirred and heated at 60°C to evaporate the solvent and dried in an oven at 110°C overnight. The Ni-loaded supports were then calcined in air at 500°C for 4 h. These Ni-loaded samples are referred to hereafter as fresh catalysts.
The chemical compositions of the catalysts prepared, determined by X-ray fluorescence (XRF), are given in Table 2. The starting TS clay is composed largely of 67.7% Si, 28.0% Mg and 4.3% Na. After Mg2+ incorporation, it is confirmed that the Na content is displaced by Mg2+ cations to form a MgO-clay nanocomposite after calcination. In all cases, the Ni content was ~5 wt.%, as expected.
Characterization techniques
The surface area and porosity of the supports and the prepared catalysts were measured by nitrogen adsorption isotherms at 77 K, conducted on a NOVA 1000 Quantachrome Instrument. Samples were outgassed at 120°C for 2 h before the measurements. The surface area (SBET) was calculated using the BET method. The total pore volume (Vt) was estimated from the amount of adsorption at a relative pressure close to unity. The micropore surface area (Smicro) was determined by the t-plot method. The mesopore surface area (Smeso) was calculated by subtracting the Smicro value from that for SBET. The average pore diameter (Dp) was calculated from the pore volume, assuming a cylindrical pore geometry using the equation Dp = 4Vliq/SBET, where Vliq is volume of liquid adsorbate contained in the pores.
The bulk chemical composition was measured by XRF (using a JEOL JSX-3230Z Element Analyzer instrument). Powder X-ray diffraction (XRD) patterns of the MgO-clay nanocomposites were recorded on a JEOL JDX-8020 diffractometer, operating at 30 kV and 30 mA and at a scan speed of 1.2° min–1 from 2 to 80°2
, using CuK
radiation. The crystalline phase was identified using the tabulated powder diffraction files of the International Center of Diffraction Data (ICDD) d spacing files.
Thermogravimetry (TG) analysis for the desorption/decomposition of the surfactant from the dried and calcined clay composites were carried out on a thermogravimetric analyzer (TG-DTA 2000S Material Analysis Characterization) by passing nitrogen at a flow rate of 30 mL min–1, using a heating rate of 20°C min–1. Thermogravimetry in oxidizing conditions was carried out to examine the carbonaceous species formed on the used catalysts (after reforming reaction and further storage in sample vials), passing air at a flow rate of 80 mL min–1 and a heating rate of 15°C min–1.
Dry methane reforming
Carbon dioxide reforming of methane over Ni catalysts was carried out at 700°C and at atmospheric pressure with a tubular fixed-bed quartz reactor. 100 mg of catalyst were placed in the reactor and reduced in situ at 500°C for 3 h in hydrogen flow. The mixture of reactants of methane and carbon dioxide with a molar ratio of 1:1 (diluted to 60% in N2) was fed into the reactor at a flow rate of 100 mL min–1 (GHSV = 60,000 mL g–1 h–1). The analysis of the reactants and products was performed on two on-line gas chromatographs (GCs) using helium as carrier gas. One was a Shimadzu GC-4C, equipped with a flame ionization detector and a Porapak T (80–100 mesh) stainless column. The other was a Hitachi 063 with a thermal conductivity detector, equipped with a (molecular sieve 5A) stainless column (60–80 mesh).
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RESULTS AND DISCUSSION
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Characterization of supports and catalysts
The TG and DTA curves for the starting TS clay as well as those prepared using the three surfactants (dried at 110°C) are shown in Figure 1. The DTA curves for TS clay exhibited two large and small peaks at 35–280°C and 400–640°C (weight loss of ~10.7% and 4.8%, respectively), attributed to loss of sorbed water and dehydroxylation of the structure (Breen and Moronta, 1999). For the clay nanocomposites, a reduction in the weight loss percentage (~3.2%) of sorbed water is clearly observed as a consequence of the hydrophobic nature of the surface, caused by the presence of the organic molecules. In the region 200–420°C, three peaks are noticed and correspond to the desorption/breakdown of the surfactant and the weight loss increases with the number of ethylene groups in the surfactant molecule (Table 3). The weight loss in the dehydroxylation region for TSIGE and TSBR30 increased due to the evolution of CO2 arising from a combination of layer dehydration and oxidation of carbonaceous deposits not desorbed at 420°C (Table 3) (Breen and Moronta, 2000), but for TSBR56 the weight loss is less than that of the original clay. The incorporation of surfactant on the clay surface increased almost threefold the total weight-loss percentage of the starting TS.
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Figure 1. TGA and DTA curves for (a) starting TS and dried Mg-surfactant-exchanged TS (b) TSIGE, (c) TSBR30, and (d) TSBR56.
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Figure 2 shows the weight loss and the derivative curves of the clay nanocomposites after calcination at 500°C for 2 days and further storage in samples vials, the peaks in the 200–420°C region disappear completely and only those attributed to loss of sorbed water (produced by rehydration) and dehydroxylation combined with CO2 evolution are observed. In all three cases, the weight-loss percentage of these species was very small (Table 4). The combined dehydroxylation/CO2 desorption band disappears after Ni incorporation with further calcination and reduction at 500°C, as will be discussed later.
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Figure 2. TGA and DTA curves for calcined MgO-clay nanocomposites: (a) TSIGE, (b) TSBR30, and (c) TSBR56.
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Table 4. Weight loss percentage at different temperatures for the supports previously calcined at 500°C for 2 days.
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The surface area and pore size of the prepared clay nanocomposite supports and the starting TS clay are listed in Table 5. The TS clay has a surface area of 488 m2 g–1 and a mesopore area of 373 m2 g–1. The incorporation of MgO, via surfactant, increases the surface area (by 37–48%), mesoporosity (by 57–67%) and total pore volume (by 77–148%) of the nanocomposites over that of the starting TS clay support; the extent of this increment depends slightly on the polyethylene oxide number of the surfactant added during the preparation process. Hwang et al.(2001) and Zhu and Lu (2001) reported similar results using an alumina-intercalated Laponite, and pointed out that the size of the pore structure was directly related to the amount of the surfactant added to the clay suspension.
Figure 3 shows the nitrogen adsorption/desorption isotherms of TS and its calcined derivate nanocomposites. It is clearly seen that the adsorption-desorption for TS is less than for the clay nanocomposites calcined at 500°C for 2 days. The removal of the surfactant contributes to a significant increase in the porosity of the MgO-clay nanocomposites, and thus the adsorption. The TS clay exhibits a shape of type IV isotherm with a hysteresis of type H4 while TSIGE, TSBR30 and TSBR56 also show an isotherm of type IV with much higher adsorption volume from mesopores.
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Figure 3. Nitrogen adsorption and desorption isotherms of the starting TS clay and calcined TSIGE, TSBR30 and TSBR56 MgO-clay nanocomposites.
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The d001 spacing from the XRD patterns can be used to examine the structure of MgO-clay nanocomposites. Figure 4 shows the XRD patterns of TS, TSIGE dried at 110°C, TSIGE calcined at 500°C for 48 h and fresh Ni/TSIGE catalyst; TSBR30 and TSBR56 samples showed similar patterns. The starting TS clay presents a scarcely noticeable d spacing of 15.8 Å and other well-defined reflection peaks which correspond to the planes 003, 104, 211 and 033 of MgO-SiO2 species. The incorporation of MgO-surfactant in the dried clay form does not cause significant structural changes due to the adsorption of MgO on the external surface of TS, and a clear d spacing is again scarcely observed. However, it has been reported that the incorporation of Igepal and Brij 56 into the interlamellar region (via ion exchange) of a bentonite, from Gonzalez, Texas, produced d spacings of 18 and 19 Å, respectively (Deng et al., 2003). There is no noticeable d001 signal in the calcined MgO-clay form. The absence of this signal is an indication that MgO/smectite-type nanocomposites were obtained. Similar observations were made by Zhu and Lu (2001) preparing Al2O3 clay nanocomposites. The 003, 104 and 211 peaks of Ni/TSIGE seem to shift to the higher-angle side after the impregnation of Ni, probably due to interactions of NiO with the support; additionally, a slight increment in the intensity in the reflection planes of 104 and 211 was detected, due perhaps to the combination with the planes 611 and 650 of MgNiSi2O6. Furthermore, the absence of NiO reflectance peaks in the XRD pattern is due to the large surface area of TS and its derivate nanocomposites, resulting in good dispersion of Ni in the pore structure. This observation is in line with the results reported by Wang et al.(1998).
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Figure 4. XRD patterns for (a) starting TS, (b) TSIGE dried at 110°C, (c) TSIGE calcined at 500°C for 2 days and (d) fresh Ni/TSIGE catalyst.
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The textural characteristics of Ni-supported catalysts prepared before and after methane reforming reaction are given in Table 6. The surface area of the clay supports decreased considerably after Ni impregnation (18–33% for all supports); however, the catalysts exhibit similar patterns of porous structure as the supports in Table 5
. All fresh Ni clay nanocomposites have greater surface area and mesopore area than that of Ni/TS catalyst. It is therefore anticipated that the pore surface of the MgO-nanocomposite supports will favor the adsorption of CO2 and thus high methane conversions are expected. Hwang et al.(2001) reported that the CO2 adsorption on Al2O3-clay nanocomposites is directly related to the surface area, and it was greater on highly mesoporous solids. After the reaction at 700°C, the area of the catalysts decreased because the treatment at high temperatures significantly affects the pore structure of the clay nanocomposites. The most dramatic surface area reduction is observed for the Ni/TS catalyst (49%) while in the other three Ni catalysts, surface-area reductions of 28% for Ni/TSIGE and Ni/TSBR30, and of 18% for Ni/TSBR56 were determined. On the other hand, Table 6 shows that the pore diameter increases after the methane reforming. This is due to the blocking of micropores by coke formation and sintering for used Ni/TS (as discussed below), leaving open pores of larger diameter and an increase in the pore distribution should be expected.
Catalytic activity
Due to the endothermic nature of the CO2 methane reforming, the increment of the reaction temperature favors total conversion (Wang et al., 1998; Hwang et al., 2001). In this study, the catalytic activity of all the catalysts prepared was measured at 700°C. Figures 5 and 6 show that the methane and carbon dioxide conversions for all Ni-clay nanocomposite catalysts have similar values, which indicates that neither the nature nor the PEO number of the surfactant used had a significant effect on the overall catalytic activity. The CH4 and CO2 conversions in equilibrium at 700°C are very high for these catalysts (~95% and 85%, respectively). The CO2 conversion was greater than that of CH4 for Ni/TS. Nevertheless, for the Ni-MgO-clay nanocomposites, the methane conversions were always greater than those of CO2; this might indicate that the reverse water gas shift reaction did not occur in the reaction process for these samples. Another possibility might perhaps be a different reaction mechanism in which CO2 molecules are still adsorbed on the surface or to CO2 forming carbon deposits. The latter possibility is discarded, for reasons to be shown below by thermogravimetry in oxidizing conditions of used catalysts.
The methane conversions for Ni-MgO-clay nanocomposites are even greater than those reported, at the same temperature, for Al2O3-intercalated Laponite and Ni/(La)Al-PILC bentonite catalysts, prepared by different types of surfactant (methane conversions of 60% and 75%, respectively) (Wang et al., 1998; Hwang et al., 2001); a greater conversion, similar to those of our catalysts was achieved by the former of these two catalysts at a higher temperature of 850°C. Conversions are also competitive to those found using Ni supported on SiO2 and Al2O3 (Cheng et al., 1996; Tomiyama et al., 2003).
These observations clearly indicate that the formation of a continuous network of pores that mimic the size and shape of the template (MgO templates) in the starting clay has a profound effect upon the catalytic activity of methane reforming due to its high thermal stability, large surface area, as well as the generation of mesoporosity. This consideration is supported by the small conversion and very fast deactivation detected in the Ni/TS catalyst, which has a high concentration of Na, contains no MgO in the interlayer via surfactant incorporation and a smaller surface area and mesoporosity. These data demonstrate that there is a combined relationship between the surface area, thermal stability and pore structure and the catalytic activity for these materials. The absence of Ni peaks in the XRD patterns of the clay composites (as discussed above) indicates a good dispersion of metal particles on the surface favoring catalytic transformation.
The excellent conversions obtained using the synthetic Ni-MgO clay nanocomposites are also comparable to those reported using a Ni/MgO catalyst. The catalytic performance of the Ni/MgO catalyst has been attributed to the strongly basic properties, high thermal stability and resistance to carbon deposition of MgO (Arena et al., 1991; Ruckenstein and Hu, 1995; Hu and Ruckenstein, 1997).
The reduction in methane conversion in the Ni/TS catalyst is probably related to the amount of Na in the catalyst structure or perhaps to sintering as well as coke formation caused by the reduction in surface and mesopore area. Figure 7 shows the XRD patterns of Ni/TS and Ni/TSIGE catalysts after 4 h of reaction. For the used Ni/TS catalyst, prominent Ni peaks at 44.40 and 51.74°2 indicate the occurrence of a significant amount of metal sintering. In contrast, for the used Ni/TSIGE catalyst, the Ni peaks are barely detectable suggesting that only minor metal sintering occurred.
Figure 8 shows the TG and DTA curves in oxidizing conditions of used Ni-clay catalysts. All catalysts show a peak near 90°C ascribed to the adsorption of water after reaction and further storage. The Ni/TS catalyst presents a reduction band in the region 430–640°C attributed to CO2 evolution from carbonaceous deposits left on the surface after reaction (weight loss of 1.7%). Additionally, this catalyst presents a weight gain at 450°C due to oxidation of big metallic Ni particles to NiO, not oxidized at room temperature. In the other three used Ni-clay nanocomposite catalysts, the appearance of this band is less pronounced and there is no weight gain detection at 500°C, indicating the formation of less carbon deposits (~0.4% for all clay-nanocomposite catalysts), limited sintering and complete oxidation of small Ni particles at room temperature. From Figure 8, it is worth pointing out that further calcination and reduction treatments of all Ni-impregnated clay nanocomposites reduce the carbon content remaining over Mg-surfactant clay supports after calcination at 500°C for 2 days.
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Figure 8. TGA and DTA curves of the clay catalysts in oxidizing conditions after reaction for 4 h at 700°C. (a) Ni/TS, (b) Ni/TSIGE, (c) Ni/TSBR30, and (d) Ni/TSBR56.
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In our preliminary results, it was found that the catalytic activity for the dry methane reforming was very limited and rapidly deactivated for acid, La-exchanged, Al2O3-pillared ST-1 clay as well as samples obtained combining these three treatments. The poor catalytic behavior was attributed to a weak thermal stability of the starting material the structure of which collapsed after calcination at 500°C for 4 h, as well as a significant reduction of the surface area and mesoporosity which led to sintering of Ni after reaction. In this work, however, better catalysts for methane reforming have been synthesized with large mesoporosity and high thermal stability.
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CONCLUSIONS
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The incorporation of magnesium oxide via surfactant stabilization to produce clay nanocomposites results in changes in the structural properties of the starting mesoporous TS clay, improving the catalytic performance of the methane reforming reaction with CO2. The synthesized supports are potential Ni catalysts for this reaction and the efficiency obtained compares very well with that reported for conventional Ni-impregnated Al2O3, MgO, SiO2 and Al2O3-pillared clays at the same reaction conditions. The excellent catalytic behavior was related to the thermal stability and improved surface and mesopore areas achieved, allowing better Ni dispersions and resistance to deactivation by coke formation. The selection of the starting material, as well as the activation treatment, play a key role in obtaining good catalytic supports for dry methane reforming.
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ACKNOWLEDGMENTS
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A.M. is indebted to The Matsumae International Foundation (Japan) for financial support to complete the research fellowship at Hokkaido University. The sabbatical year provided by La Universidad del Zulia (Venezuela) is also acknowledged.
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Footnotes
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Present address: Instituto de Superficies y Catálisis, Facultad de Ingeniería, Universidad del Zulia, PO Box 15251, Maracaibo 4003a, Venezuela 
Ms. 1001; A.E. James E. Amonette
(Received 13 January 2005;
revised 7 July 2005)
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REFERENCES
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|---|
Andersen, J.A., Rodrigo, M.T., Daza, L. and Mendioroz, S. (1993) Influence of the support in the selectivity of nickel/clay catalysts for vegetable oil hydrogenation. Langmuir, 9, 2485–2490.[CrossRef]
Arena, F., Parmaliana, A., Mondello, N., Frusteri, F. and Giordano, N. (1991) Influence of the calcination treatment on the surface chemical properties of nickel/magnesia catalyst: a carbon dioxide-temperature programmed desorption approach. Langmuir, 7, 1555–1557.[CrossRef]
Breen, C. and Moronta, A. (1999) Influence of layer charge on the catalytic activity of mildly acid-activated tetramethyl-ammonium-exchanged bentonites. Journal of Physical Chemistry B, 103, 5675–5680.
Breen, C. and Moronta, A. (2000) Characterization and catalytic activity of aluminum- and aluminum/tetramethyl-ammonium-exchanged bentonites. Journal of Physical Chemistry B, 104, 2702–2708.
Chen, Y.G., Tomishige, K. and Fujimoto, K. (1997) Formation and characteristic properties of carbonaceous species on nickel-magnesia solid solution catalysts during CH4-CO2 reforming reaction. Applied Catalysis A, 161, L11–L17.[CrossRef]
Cheng, Z., Wu, Q., Li, J. and Zhu, Q. (1996) Effects of promoters and preparation procedures on reforming of methane with carbon dioxide over Ni/Al2O3 catalyst. Catalysis Today, 30, 147–155.
Choudhary, V.R. and Rajput, A.M. (1996) Simultaneous carbon dioxide and stream reforming of methane to syngas over NiO-CaO catalyst. Industrial and Engineering Chemistry Research, 35, 3934–3939.[CrossRef]
Deng, Y., Dixon, J.B. and White, G.N. (2003) Intercalation and surface modification of smectite by two non-ionic surfactants. Clays and Clay Minerals, 51, 150–161.[Abstract/Free Full Text][CrossRef][ISI][GeoRef]
Edwards, J.H. and Maitra, A.M. (1995) The chemistry of methane reforming with carbon dioxide and its current and potential applications. Fuel Processing Technology, 42, 269–289.[CrossRef]
Erdohelyi, A., Cserenyi, J., Papp, E. and Solymosi, F. (1994) Catalytic reaction of methane with carbon dioxide over supported palladium. Applied Catalysis A, 108, 205–219.[CrossRef]
Gonzalez, E. and Moronta, A. (2004) The dehydrogenation of ethylbenzene to styrene catalyzed by a natural and an Al-pillared clays impregnated with cobalt compounds: a comparative study. Applied Catalysis A, 258, 99–105.[CrossRef]
Hu, Y.H. and Ruckenstein, E. (1997) The characterization of a highly effective NiO/MgO solid solution catalyst in the CO2 reforming of CH4. Catalysis Letters, 43, 71–77.[CrossRef]
Hwang, K.S., Zhu, H.Y. and Lu, G.Q. (2001) New nickel catalysts on highly porous alumina intercalated laponite for methane reforming with CO2. Catalysis Today, 68, 183–190.[CrossRef]
Prabhu, A.K., Radhakrishnan, S. and Oyama, S.T. (1999) Supported nickel catalysts for carbon dioxide reforming of methane in plug flow and membrane reactors. Applied Catalysis A, 183, 241–252.[CrossRef]
Ruckenstein, E. and Hu, Y.H. (1995) Carbon dioxide reforming of methane over nickel/alkaline earth metal oxide catalysts. Applied Catalysis A, 133, 149–161.[CrossRef]
Tomiyama, S., Takahashi, R., Sato, S., Sodesawa, T. and Yoshida, S. (2003) Preparation of Ni/SiO2 catalyst with high thermal stability for CO2-reforming of CH4. Applied Catalysis A, 241, 349–361.[CrossRef]
Wang, S. and Lu, G.Q.M. (1998) CO2 reforming of methane on nickel catalysts: effect of the support phase and preparation technique. Applied Catalysis B, 16, 269–277.[CrossRef]
Wang, S., Zhu, H.Y. and Lu, G.Q.M. (1998) Preparation, characterization, and catalytic properties of clay-based nickel catalysts for methane reforming. Journal of Colloid and Interface Science, 204, 128–134.[Medline]
Yang, R.T. and Li, W.B. (1995) Ion-exchanged pillared clays: a new class of catalysts for selective catalytic reduction of NO by hydrocarbons and by ammonia. Journal of Catalysis, 155, 414–417.[CrossRef]
Zhang, Z.L. and Verykios, X.E. (1994) Carbon dioxide reforming of methane to synthesis gas over supported Ni catalysts. Catalysis Today, 21, 589–595.[CrossRef]
Zhu, H.Y. and Lu, G.Q. (2001) Engineering the structure of nanoporous clays with micelles of alkyl polyether surfactants. Langmuir 17, 588–594.[CrossRef]
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Abstract
INTRODUCTION
