Preparation of an Industrial Ni-Based Catalyst and Investigation on

An industrial cylinder Ni-based catalyst, LaNiOx/ZSM-5 (ϕ = 3 mm), was prepared by a sol−gel method, and its catalytic performance was investigated...
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Energy & Fuels 2009, 23, 607–612

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Preparation of an Industrial Ni-Based Catalyst and Investigation on CH4/CO2 Reforming to Syngas L. Tian,† X. H. Zhao,‡ B. S. Liu,*,† and W. D. Zhang† Department of Chemistry, and Department of Mathematics, School of Science, Tianjin UniVersity, Tianjin 300072, People’s Republic of China ReceiVed August 7, 2008. ReVised Manuscript ReceiVed NoVember 7, 2008

An industrial cylinder Ni-based catalyst, LaNiOx/ZSM-5 (φ ) 3 mm), was prepared by a sol-gel method, and its catalytic performance was investigated for CH4/CO2 reforming reaction not only in laboratory scale but also in pilot plant scale. The result revealed that the conversion of CH4 and CO2 was 97% and 94%, respectively, at 850 °C and gas hour space velocity ) 1.3 × 103 h-1 and remained a constant in the 100 h range, whereas carbon yield was ca. 5%. The properties of the catalyst were characterized by means of techniques such as X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area, high-resolution transmission electron microscopy (HRTEM), temperature-programmed reduction (TPR), and temperature-programmed oxidation (TPO) and compared to those of La2NiO4/MCM-41 and La2NiO4/γ-Al2O3 catalysts.

1. Introduction The reforming of methane with carbon dioxide to produce synthesis gas, CO and H2, has received significant attention in recent years. Reforming reaction utilizes two greenhouse gases, CH4 and CO2, and therefore is a promising reaction for the control of global warming.1 CH4/CO2 reforming (CO2 + CH4 f 2CO + 2H2, ∆H298 ) 247 kJ/mol) can get synthesis gas with a low H2/CO ratio (e1), which is more suitable for the production of formaldehyde, polycarbonate, or methanol.2,3 Because of the high endothermicity of the reaction, it is also considered as a means of storing and transporting solar and atomic energy to chemical energy.4-6 Another advantage of CH4/CO2 reforming is that it can be applied directly for the utilization of natural gas containing a large amount of CO2 without preseparation.7 Landfill gas commonly consists of 50% CH4 and 50% CO2, and power plants emit a large amount of * To whom correspondence should be addressed. Telephone: 86-2227892471. Fax: 86-22-87892946. E-mail: [email protected]. † Department of Chemistry. ‡ Department of Mathematics. (1) Hao, Z. G.; Zhu, Q. S.; Lei, Z.; Li, H. Z. CH4-CO2 reforming over Ni/Al2O3 aerogel catalysts in a fluidized bed reactor. Powder Technol. 2007, 179, 157. (2) Wender, I. Reactions of synthesis gas. Fuel Process. Technol. 1996, 48, 189. (3) Slagtern, A.; Schuurman, Y.; Leclercq, C.; Verykios, X. E.; Mirodatos, C. Specific features concerning the mechanism of methane reforming by carbon dioxide over Ni/La2O3 catalyst. J. Catal. 1997, 172, 118. (4) Cui, Y. H.; Zhang, H. D.; Xu, H. Y.; Li, W. Z. Kinetic study of the catalytic reforming of CH4 with CO2 to syngas over Ni/R-Al2O3 catalyst: The effect of temperature on the reforming mechanism. Appl. Catal., A 2007, 318, 79. (5) Chubb, T. A. Characteristics of CO2-CH4 reforming-methanation cycle relevant to the solchem thermochemical power system. Sol. Energy 1980, 24, 341. (6) Richardson, J. T.; Paripatysdar, S. A. Carbon dioxide reforming of methane with supported rhodium. Appl. Catal., A 1990, 61, 293. (7) Cheng, D. G.; Zhu, X. L.; Ben, Y. H.; He, F.; Cui, L.; Liu, C. J. Carbon dioxide reforming of methane over Ni/Al2O3 treated with glow discharge plasma. Catal. Today 2006, 115, 205.

CO2 at relatively high temperature.8 Therefore, from the environmental and industrial viewpoints, the study on the dry reforming has an important significance and a promising future. Although the CH4/CO2 reforming concept has environmental benefits and economic advantages, there are only a few commercial processes based on the CH4/CO2 reforming reaction.9-11 Group VIII metals12-15 such as Pt, Ru, Ir, Rh, and Co have extensively been studied as catalysts for CH4/CO2 reforming. In comparison to the precious metals, nickel is more economical to use as a catalyst.16 However, the major obstacle for commercialization of the CH4/CO2 reforming over Ni-based catalyst is the rapid deactivation of catalysts induced by carbon deposition.17 Recently, we18,19 reported La2NiO4/γ-Al2O3 and (8) Wang, S. B.; Lu, G. Q. Carbon dioxide reforming of methane to produce synthesis gas over metal-supported catalysts: state of the art. Energy Fuels 1996, 10, 896. (9) Ginsburg, J. M.; Pina, J.; El Solh, T.; de Lasa, H. L. Coke formation over a nickel catalyst under methane dry reforming conditions: Thermodynamic and kinetic models. Ind. Eng. Chem. Res. 2005, 44, 4846. (10) Teuner, St. C.; Neumann, P.; Vonlinde, F. The calcor standard and calcor economy processes. Oil Gas Eur. Mag. 2003, 3, 44. (11) Udengaard, N. R.; Bak Hansen, J. H.; Hanson, D. C.; Stal, J. A. Oil Gas J. 1992, 90, 62. (12) Bitter, J. H.; Hally, W.; Seshan, K.; Ommen, J. G.; Lercher, J. A. The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane. Catal. Today 1996, 29, 349. (13) Perera, J. S. H. Q.; Couves, J. W.; Sankar, G.; Thomas, J. M. The catalytic activity of Ru and Ir supported on Eu2O3 for the reaction, CO2 + CH4 S 2H2 + 2CO: a viable solar-thermal energy system. Catal. Lett. 1991, 11, 219. (14) Solymosi, F.; Knozinger, H. Infrared spectroscopic study of the adsorption and reactions of CO2 on K-modified Rh/SiO2. J. Catal. 1990, 122, 166. (15) Zhang, J. G.; Wang, H.; Dalai, A. K. Effects of metal content on activity and stability of Ni-Co bimetallic catalysts for CO2 reforming of CH4. Appl. Catal., A 2008, 339, 121. (16) Luo, J. Z.; Gao, L. Z.; Ng, C. F.; Au, C. T. Mechanistic studies of CO2/CH4 reforming over Ni-La2O3/5A. Catal. Lett. 1999, 62, 153. (17) Li, L.; Liu, B. S.; Leung, J. W. H.; Au, C. T.; Cheung, A. S. C. CO2/CH4 reforming over La2NiO4 and 10%NiO/CeO2-La2O3 catalysts under the condition of supersonic jet expansion via cavity ring-down spectroscopic analysis. Catal. Today 2008, 131, 533. (18) Liu, B. S.; Au, C. T. Carbon deposition and catalyst stability over La2NiO4/γ-Al2O3 during CO2 reforming of methane to syngas. Appl. Catal., A 2003, 244, 181.

10.1021/ef800647n CCC: $40.75  2009 American Chemical Society Published on Web 01/15/2009

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La2NiO4/ZSM-5 catalysts and observed that these catalysts presented high activity and stability at 700 °C in the laboratory scale. In addition, MCM-4120 had a large specific surface area and uniform mesoporous structure; its utilization as a support can efficiently enhance the dispersion of nickel particles. Despite all this, the preparation of industrial Ni-based catalyst in the large scale has still not been reported. In order to meet the requirement of commercialization, the objective of this work is to fabricate a catalyst suitable for use in industry. Hence, we reported an industrial cylindrical LaNiOx/ZSM-5 catalyst for CH4/CO2 reforming at 850 °C by the optimization of different catalysts; the catalytic performance investigated in the laboratory and pilot plant scale revealed that the catalyst could be used in industry. 2. Experimental Section 2.1. Support and Catalyst Preparation. A highly ordered hexagonal siliceous MCM-41 was synthesized as described in a previous method20 using tetraethoxysilane (TEOS) as the silica source and hexadecyltrimethyl ammonium bromide (CTAB) as a template. A composition of 1TEOS/0.12CTAB/8NH4OH/114H2O was used to form a gel. The gel was transferred into a Teflon-lined autoclave for hydrothermal synthesis at 110 °C for 52 h. After filtration, washing, and drying, it was calcined at 550 °C in air for 6 h to remove template. The 9% La2NiO4/MCM-41 catalyst was prepared from Ni(Ac)2 · 6H2O, La(NO3)3 · 6H2O, and the aforementioned MCM-41 by means of a sol-gel technique. Nickel acetate (1.0752 g) and lanthanum nitrate (3.5275 g) were dissolved in deionized water, and then citric acid was added to the solution. The molar amount of citric acid was 1.5 times that of the total metal ions. The solution was then heated to 50 °C with constant stirring, and MCM-41 (1 g) was added to the solution. After the removal of the water by evaporation, a translucent green gel was formed. Next, the cogel obtained was aged and dried in a beaker at room temperature (RT) for 3 days. Subsequently, it was calcined at 500 °C for 5 h. The industrial La2NiO4/ZSM-5 catalyst was prepared by the mixture of a La2NiO4 precursor sol with cylindrical ZSM-5 (φ ) 3 mm) support which molded with 10% aluminum sol. The catalyst was then calcined at 500 °C for 5 h; the obtained catalyst will be referred to as “8.2% LaNiOx/ZSM-5” hereafter. In addition, powder La2NiO4/ZSM-5 catalyst was prepared as described in the literature.19 2.2. Characterization of Catalyst. The properties of the catalysts were characterized using Brunauer-Emmett-Teller (BET) surface area, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and temperature-programmed reduction and oxidation (TPR and TPO). BET measurements were conducted using N2 as the adsorbate at 77 K on a homemade apparatus. The powder XRD patterns of MCM-41 and the catalysts were recorded on a BDX3300 diffractometer with a Cu KR radiation source () 1.54056 Å) at a voltage of 30 kV and a current of 20 mA. The average size of reduced Ni particles was estimated by the Scherer equation,21 D ) 0.89 λ/β(θ)2 cos θ, where θ stands for the diffraction angle of the Ni(111) peak, β(θ) is the peak width at half-height, and D is the crystallite size. The fine structure of

Figure 1. Nitrogen adsorption isotherms of different samples: (a) MCM41, (b) La2NiO4/MCM-41, (c) ZSM-5, (d) used, and (e) fresh industrial cylinder-shaped LaNiOx/ZSM-5.

MCM-41 was observed by HRTEM on a Tecnai G2 F20 electron microscope equipped with a field emission gun.22 TPR was carried out according to the following procedure, as a rule of thumb.23 The sample (ca. 35 mg) was heated from RT to 150 °C at a rate of 10 °C/min in a flow of N2 (50 mL/min) and kept at this temperature for another 30 min. The sample was then cooled to RT, and the carries gas was switched to an 8% H2/N2 mixture (62 mL/min). A linear rise in temperature from RT to 900 °C at a rate of 10 °C/min was adopted. H2O produced by reduction was removed before TC detection with a 13X sieve at ambient temperature. The amount of hydrogen consumption was measured with a thermal conductivity detector (TCD) and calibrated via reduction of CuO (99.95%) sample. TPO of used catalyst was carried out according to the following procedure. About 20 mg of the catalyst used in a reaction was placed in a quartz microreactor that was placed in a furnace coupled to a temperature controller. The catalyst was first heated from RT to 120 °C at a rate of 10 °C/min in a flow of He (40 mL/min) and kept at this temperature for 30 min before cooling to RT. This pretreatment step was conducted at a relatively low temperature with the purpose of removing physisorbed water, while not thermally modifying the nature of the deposition carbon. Subsequently, the catalyst was heated to 850 °C under the flow of 7% O2/He at a rate of 10 °C/min. The signal of effluent was detected with a TCD. 2.3. CH4/CO2 Reforming Reaction. CH4/CO2 reforming was carried out at 850 °C over a fluidized bed reactor. The catalyst was placed between two quartz-wool plugs in the reactor. The temperature was measured with a K-type thermocouple placed at the center of the catalyst bed. Before the CH4/CO2 reforming, the catalyst was reduced in a flow of H2 (40 mL/min) at 850 °C for 1 h (industrial catalyst was reduced for 3 h). The gas effluent was analyzed by means of on-line gas chromatography (102G) with a TCD. A TDX-01 column was used for the separation of H2, CO, CH4, and CO2. The conversion of CO2 and CH4, the selectivity of H2 and CO, as well as carbon yields were calculated according to the report in the literature.19

3. Results and Discussion (19) Zhang, W. D.; Liu, B. S.; Zhu, C.; Tian, Y. L. Preparation of La2NiO4/ZSM-5 catalyst and catalytic performance in CO2/CH4 reforming to syngas. Appl. Catal., A 2005, 292, 138. (20) Lou, L. L.; Yu, K.; Ding, F.; Peng, X. J.; Dong, M. M.; Zhang, C.; Liu, S. X. Covalently anchored chiral Mn(III) salen-containing ionic species on mesoporous materials as effective catalysts for asymmetric epoxidation of unfunctionalized olefins. J. Catal. 2007, 249, 102. (21) Wang, L.; Yu, Y.; Chen, P. C.; Zhang, D. W.; Chen, C. H. Electrospinning synthesis of C/Fe3O4 composite nanofibers and their application for high performance lithium-ion batteries. J. Power Sources 2008, 183, 717.

3.1. BET and HRTEM Analysis. As shown in Figure 1, the N2 adsorption isotherm of MCM-41 (Figure 1a) exhibited a (22) Liu, B. S.; Chang, R. Z.; Jiang, L.; Liu, W.; Au, C. T. Preparation and high performance of La2O3-V2O5/MCM-41 catalysts for ethylbenzene dehydrogenation in the presence of CO2. J. Phys. Chem. C 2008, 112, 15490. (23) Malet, P.; Caballero, A. The selection of experimental conditions in temperature-programmed reduction experiments. J. Chem. Soc., Faraday Trans. 1 1988, 84 (7), 2369.

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Figure 2. HRTEM image of MCM-41 taken with the electron beam perpendicular to the pore direction. Inset is the SAED pattern of MCM41.

typical character of mesopore.24 The sudden N2 adsorption within the narrow P/P0 range (3.2-4.5) reflected the capillary condensation of the N2 molecule in a uniform mesopore, similar to the observation on Al-MCM-41.25 The HRTEM image of the MCM-41 support was taken with the pore channels viewed perpendicularly (Figure 2) to the pore axis. One can see that the width of the inner channel (between two dark strips) is in the 3.6 ( 0.2 nm range, which is consistent with the result reported earlier.22 The selected area electron diffraction (SAED) pattern (inset of Figure 2) exhibits two bright spots on the diffraction circle, an indicator of the formation of single-channel structure. According to a report in the literature,22 there must be a bright spot on the center of the diffraction circle. Therefore, the distance between two diffraction spots, which mathematically corresponded to a Fourier transform, is 3.87 nm, which is close to a d 100 spacing of 3.8 nm deduced on the basis of XRD analysis. For the La2NiO4/MCM-41 (Figure 1b) sample, there was significant reduction in N2 adsorption capacity in the same P/P0 range compared to that of pure MCM-41. The specific surface areas of MCM-41 and La2NiO4/MCM-41 were 574 and 157 m2/g, respectively. Such a drastic drop in surface area might be caused by the formation of La2NiO4 species in the channel of MCM-41 and/or the partial collapse of MCM-41 structure during the preparation of catalyst, similar to our observation in LaVOx/MCM-4122 or LaVOx/SBA-15 catalysts.26 On the contrary, the N2 adsorption isotherms of the ZSM-5 sample and industrial cylinder-shaped LaNiOx/ZSM-5 exhibited a typical character of micropores, strongly depending on the properties of support ZSM-5, as shown in Figure 1c. Their specific surface areas were 285.7 and 129 m2/g (Figure 1e), respectively. After CH4/CO2 reforming reaction of 100 h, we observed that the adsorption capability and specific surface area (148 m2/g) of (24) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Reporting physisorption data for gas solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem. 1985, 57, 603. (25) Liu, B. S.; Xu, D. F.; Chu, J. X.; Liu, W.; Au, C. T. Deep desulfurization by the adsorption process of fluidized catalytic cracking (FCC) diesel over mesoporous Al-MCM-41 materials. Energy Fuels 2007, 21, 250. (26) Liu, B. S.; Rui, G.; Chang, R. Z.; Au, C. T. Dehydrogenation of ethylbenzene to styrene over LaVOx/SBA-15 catalysts in the presence of carbon dioxide. Appl. Catal., A 2008, 335, 88.

Figure 3. XRD patterns of (A) La2NiO4/MCM-41 and (B) industrial cylindrical LaNiOx/ZSM-5 catalyst: (a) support, (b) fresh, (c) reduced, and (d) after CH4/CO2 reforming reaction.

the catalyst slightly increased (Figure 1d). This phenomenon was attributed to carbon deposition and/or formation of nanotubes, which had been confirmed in previous literature.17,27,28 The BET surface areas of La2NiO4/MCM-41 (157 m2/g), powder La2NiO4/ZSM-5 (119.7 m2/g),19 and La2NiO4/γ-Al2O3 (26.4 m2/g)18 declined in the following order of supports: MCM41 > ZSM-5 > γ-Al2O3.18 This meant that large specific surface areas of supports were favorable for enhancement of the nickel dispersion and could improve the activity of the catalyst. 3.2. XRD Characterization. The XRD patterns of the support, La2NiO4/MCM-41, and industrial cylindrical LaNiOx/ ZSM-5 catalyst are shown in Figure 3. For fresh La2NiO4/MCM41 catalyst (Figure 3A, curve b), typical broad diffraction peaks at 2θ ) 29.7°, 31.4°, and 43.3°19 were observed, originating from La2O3 (2θ ) 29.7°), La2NiO4 (31.4°), and NiO (43.3°) distributed uniformly. After H2 reduction or reforming reaction, the diffraction peaks attributable to La2NiO4 (2θ ) 31.4°) and NiO (2θ ) 43.3°) (Figure 3A, curves c and d) almost disappeared, whereas those belonging to metallic Ni0 phase (2θ ) 44.5° and 52.2°) and H6.8LaNi529 (2θ ) 39.6°, Figure 3A, curves c and d) appeared and the peaks belonging to La2O3 (2θ (27) Liu, B. S.; Li, L.; Au, C. T.; Cheung, A. S. C. Investigation on reverse water-gas shift over La2NiO4 catalyst by cw-cavity enhanced absorption spectroscopy during CH4/CO2 reforming. Catal. Lett. 2006, 108 (1-2), 37. (28) Zhang, W. D.; Liu, B. S.; Tian, Y. L. CO2 reforming of methane over Ni/Sm2O3-CaO catalyst prepared by a sol-gel technique. Catal. Commun. 2007, 8, 661–667. (29) Calvert, L. National Research Council of Canada, Ottawa, Canada, ICDD Grant-in-Aid, 1980, PDF no. 330601.

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Tian et al. Table 1. Characteristics of the Supported La2NiO4 Catalysts

sample 9% La2NiO4/ MCM-41 9.0% La2NiO4/ ZSM-5 8.2% LaNiOx/ ZSM-5

Ni density (Ni0 per nm2)a

size of Ni particle (nm)b

H2 uptake (mmol)c

157

4.6

9.0

7.4

119.7d

6.1

13.5

5.15

129

5.1

14.2

2.43

SBET (m2/g)

a Ni density (Ni0/per nm2) ) (6.02 × 1023)(10-18)(w(Ni)/58.7)(S BET). Calculated by the Scherer equation. c H2 consumption in mmol H2/g of catalyst was estimated from the TPR peak area calibrated by the CuO sample. d Data from ref 19. b

Figure 4. TPR profiles of (a) powder La2NiO4/ZSM-5, (b) La2NiO4/ MCM-41, and (c) industrial cylinder-shaped LaNiOx/ZSM-5 catalyst.

) 30-31°, 46.2°, 55.4°) intensified remarkably, indicating that La2NiO4 and NiO had been completely reduced and decomposed by hydrogen to form Ni0, H6.8LaNi5,29 and a new La2O3 cluster. Similarly, in the case of fresh industrial cylindrical LaNiOx/ ZSM-5, the active component was highly dispersed in the channel and/or on the surface of ZSM-5; no significant diffraction peak was observed (Figure 3B, curve b). After reduction and CH4/CO2 reforming reaction of 100 h, the diffraction peaks of La2O3 and elemental Ni0 appeared. However, there was not a significant difference of the diffraction peaks between samples reduced (Figure 3B, curve c) and reformed for 100 h (Figure 3B, curve d), which indicated that the catalyst was stable during the reforming reaction. In addition, the appearance of the Al2.427O3.6430 (2θ ) 38.47°) small diffraction peak in the industrial cylindrical LaNiOx/ZSM-5 catalyst (Figure 3B, curves c and d) was due to the addition of Al sol additives during the mold of the ZSM-5 support. 3.3. TPR Characterization. The TPR profiles of powder La2NiO4/ZSM-5, La2NiO4/MCM-41, and industrial cylindershaped LaNiOx/ZSM-5 catalyst are shown in Figure 4. In the case of La2NiO4/ZSM-5 (Figure 4a), the peak at ca. 340 °C could be assigned to the reduction of surface NiO and the one at 552 °C to the reduction of La2NiO4. The TPR profiles of pure NiO and La2NiO4 samples have verified the aforementioned argument.31 For La2NiO4/MCM-41 (Figure 4b), the shoulder peaks at 338 °C and the peak at 394 °C could be assigned to the reduction of surface and bulk NiO, as well as the H2 consumption reacted with nonstoichiometric oxygen. A large amount of nonstoichiometric oxygen in La2NiO4+δ had been observed by O2 TPD,31 and the hydrogen consumption estimated by TPR peak area of La2NiO4/MCM-41 sample was significantly higher than the stoichiometric value (Table 1) of NiO reduction. In addition, the reduction temperature of bulk NiO (394 °C, Figure 4b) in La2NiO4/MCM-41 was higher than that of NiO in La2NiO4/ZSM-5 (340 °C). This demonstrated that the interaction between NiO and MCM-41 was stronger than that between NiO and ZSM-5, i.e., MCM-41 exhibited a typical mesoporous structure; the Ni2+ could easily enter the mesoporous channel of MCM-41 and interacted strongly with surface H-OSi groups which made it difficult to be reduced. The peak at ca. 569 °C is related to the reduction of La2NiO4 in the (30) Zhou, R. S.; Snyder, R. L. Structures and transformation mechanisms of the eta, gamma and theta transition aluminas. Acta Crystallogr., Sect. B 1991, 47, 617. (31) Liu, B. S.; Au, C. T. Sol-gel generated La2NiO4 for CH4/CO2 reforming. Catal. Lett. 2003, 85 (3,4), 165.

channel of MCM-41 due to strong interaction of La2NiO4 with supports. We18,31 studied the reducibility of La2NiO4/γ-Al2O3 and La2NiO4 catalysts and observed similarly a large peak at about 400 °C and two small ones at around 350 and 550 °C. It indicated that the peak at around 350 °C was related to the reduction of surface NiO and the one at 400 °C to the reduction of bulk NiO. The one at around 550 °C was attributed to the reduction of La2NiO4. Here, we found that the amount of H2 consumption estimated by the integration of the TPR peak area was remarkably higher than that of NiO reduction (Table 1). This meant that, except for the reaction of H2 with NiO, the formation of surfacial HO-Si (Al) groups in MCM-41 (or ZSM5) and the H6.8LaNi5 phase, as well as the reaction of H2 with nonstoichiometric oxygen, would consume a large amount of hydrogen. Furthermore, the formation of HO-Si groups had been confirmed via Fourier transform infrared (FT-IR) spectra during hydrogenation of ethylbenzene to styrene over 10La15V/ MCM-41,22 and the formation H6.8LaNi5 species was observed by XRD characterization (Figure 3A). The TPR profile of industrial cylindrical LaNiOx/ZSM-5 catalyst is shown in Figure 4c. There were two main peaks at about 350 and 370 °C. We assigned the peak at 350 °C to the reduction of surface NiO and the peak at ca. 370 °C to reduction of bulk NiO combined with ZSM-5 support. It revealed that the amount of surface NiO was more abundant for industrial LaNiOx/ZSM-5 catalyst. This meant that La-Ni active species formed in the channel or on the surface of ZSM-5 existed mainly in the La2O3-NiO mixture during preparation of the catalyst due to the strong interaction of ZSM-5 with metal ions, such as Ni2+ and La3+. The TPR profiles of the sample (Figure 4c) also revealed that H2 consumption between 500 and 650 °C was relatively low. 3.4. Investigation on Catalytic Performance. The conversion of CH4 and CO2 over La2NiO4/MCM-41, powder La2NiO4/ ZSM-5, and La2NiO4/γ-Al2O3 is shown in Figure 5. The results indicated that conversion of CH4 and CO2 (Figure 5a-c) over all catalysts was higher than 90% at the initial reaction stage (850 °C and gas hour space velocity (GHSV) ) 2.4 × 104 mL · h-1 · gcat-1). For both La2NiO4/MCM-41 and powder La2NiO4/ ZSM-5 catalysts, the conversion of CH4 and CO2 was higher than 91% and almost a constant value during the first 10 h of reaction. One can see that conversion of CH4 and CO2 (95% and 93%) over La2NiO4/MCM-41 was a slightly higher than that over La2NiO4/ZSM-5 (Figure 5b, 91.5% and 90.5%) because the specific surface area of La2NiO4/MCM-41 (157 m2/ g) was slightly larger than that of powder La2NiO4/ZSM-5 (119.7 m2/g)19 and the size of nickel particles on the supported MCM-41 was significantly smaller (Table 1). It revealed that the La2O3-doped Ni particles being dispersed on support with high specific surface area was favorable for the enhancement of CH4 and CO2 conversion. However, for the La2NiO4/γ-Al2O3

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Figure 6. Conversion of CH4 and CO2, selectivity of CO and H2, as well as yield of coke observed over industrial LaNiOx/ZSM-5 catalyst during CH4/CO2 reforming at 850 °C as a function of time on stream. GHSV ) 1.3 × 103 h-1, CH4/CO2 ) 1.

Figure 5. Conversion of CH4 (A) and CO2 (B) observed over (a) La2NiO4/MCM-41, (b) powder La2NiO4/ZSM-5, (c) La2NiO4/γ-Al2O3 with GHSV ) 2.4 × 104 mL · h-1 · gcat-1, and (d) powder La2NiO4/ ZSM-5 with GHSV ) 3.6 × 104 mL · h-1 · gcat-1. CH4/CO2 ) 1; reaction temperature, 850 °C.

catalyst, the initial conversion of CH4 and CO2 (Figure 5c) was about 91%, but it gradually declined with the increase of time on stream (ca. 85% after 10 h) due to the formation of a large amount of carbon deposition on the surface of the catalyst. It had been verified by TPO profiles of used La2NiO4/γ-Al2O3 catalyst (Figure 7), which will be discussed in following section 3.5. Furthermore, as shown in Figure 5, curves b and d, the conversion of CH4 and CO2 over powder La2NiO4/ZSM-5 gradually decreased with the enhancement of GHSV under the condition of the same particle size. At low GHSV, the reactants, CH4 and CO2 could sufficiently contact with the catalyst to form syngas. Therefore, the conversion of CH4 and CO2 at GHSV ) 2.4 × 104 mL/g · h was high and kept constant in the range of 10 h. By contrast, the conversion of CH4 and CO2 was low at GHSV ) 3.6 × 104 mL/g · h (Figure 5d) and declined with increase of time on stream; it suggested that CH4/CO2 reforming reaction was not favorable under the high-GHSV condition. Considering the performances of the aforementioned catalysts and the cost and thermal stability of supports, La2NiO4/ZSM-5 was chosen as the most suitable catalyst for the preparation of industrial catalyst and was used for CH4/CO2 reforming in pilot plant scale.32 The catalytic activity of industrial LaNiOx/ZSM-5 catalyst (φ ) 3 mm) was investigated at a relatively low GHSV in order

to obtain a high yield of synthesis gas. As shown in Figure 6, the conversion of CH4 and CO2 was 97% and 94%, respectively, at the beginning of the reaction and almost remained a constant in the range of 100 h. However, it could be seen that the conversion of CH4 (97%) was higher than that of CO2 (94%) at 850 °C due to rapid thermal decomposition of methane. We18,19 investigated the catalytic performance of La2NiO4/γAl2O3 and powder La2NiO4/ZSM-5 at 700 °C and confirmed that the CO2 conversion was a little higher than that of CH4 due to the contribution of reversed water gas shift (RWGS) reaction (CO2 + H2 f H2O + CO, ∆H298 ) 41 kJ/mol, ∆G° ) -8545 + 7.84T). In the meantime, Luo et al.33 also verified that the highest temperature for the occurrence of RWGS reaction was 820 °C and CH4 decomposition (CH4 f C + 2H2, ∆H298 ) 75 kJ/mol, ∆G° ) 21 960 - 26.45T) increased with the rise in temperature, which agree on the aforementioned observation. In addition, the selectivity of CO and H2 was ca. 94% and 84%, and yield of carbon was about 5% in the range of 100 h. The outcome indicated that the industrial cylinder-

(32) Liu, B. S.; Wang, Q.; Lai, C. B.; Tang, D. C.; Zhang, W. D.; Liao, A. M.; Tian, L. An industrial catalyst for natural gas reforming of CO2 to synthesis gas. China Patent 200710172625.6, 2007.

(33) Luo, J. Z.; Yu, Z. L.; Ng, C. F.; Au, C. T. CO2/CH4 reforming over Ni-La2O3/5A: An investigation on carbon deposition and reaction steps. J. Catal. 2000, 194, 198.

Figure 7. TPO profiles of (a) La2NiO4/γ-Al2O3 (10 h), (b) powder La2NiO4/ZSM-5 (10 h), (c) La2NiO4/MCM-41 (10 h), and (d) industrial cylindrical LaNiOx/ZSM-5 catalyst (100 h); the data in parentheses is time on steam.

612 Energy & Fuels, Vol. 23, 2009

shaped LaNiOx/ZSM-5 catalyst (φ ) 3 mm) was excellent for CH4/CO2 reforming with good catalytic activity and long-time stability. 3.5. Carbon Deposition and Catalyst Stability. The carbon deposition was the main reason for catalyst deactivation during CH4/CO2 reforming, which originated from the Boudouard reaction (2CO f C + CO2, ∆H298 ) -172 kJ/mol, ∆G° ) -39 810 + 40.87T) and methane decomposition (CH4 f C + 2H2). The Boudouard reaction was exothermic and decreased with the rise in reaction temperature, whereas the decomposition of CH4 was endothermic and the equilibrium constant increased with increasing temperature.33 By thermodynamics equilibrium calculation of both reactions, Wang and Lu8 came to the conclusion that the highest temperature for the occurrence of the Boudouard reaction was 700 °C. Therefore, the carbon deposition generated during CH4/CO2 reforming at 850 °C mainly originated from CH4 decomposition and the carbon species was more reactive than that generated from the CO Boudouard reaction; it could easily be removed in the CO2 atmosphere. During CH4/CO2 reforming at low GHSV, the CO2 could sufficiently contact with deposited carbon species to form gaseous CO. So under the condition of reforming in high temperature (850 °C), for the industrial molded catalyst, we did not observe a large amount of carbon deposition even after CH4/ CO2 reforming of 100 h. Figure 7a-c shows TPO profiles of catalysts used after CH4/ CO2 reforming of ca. 10 h. There were three distinguishable exothermic peaks, which demonstrated that three kinds of carbon species were formed on the active sites of nickel or supports. A small peak at 300-350 °C can be assigned to the oxidation of active carbon, such as CHx (x ) 1, 2, 3). In our previous TPO-MS (mass spectrometer) study of used La2NiO4/γ-Al2O3 catalyst during CH4/CO2 reforming, there was a large amount of water in the oxidation atmosphere of coke except for COx species;18 this verified the existence of hydrogenated carbon species (CHx) on the surface of used catalyst. Additionally, a big peak at 582 °C for La2NiO4/γ-Al2O3 (Figure 7a), 513 °C for powder La2NiO4/ZSM-5 (Figure 7b), and 596 °C for La2NiO4/MCM-41 (Figure 7c) revealed a type of heavy carbon deposition, which could be assigned to amorphous carbon (such

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as encapsulated carbon).18,33 The small peaks above 650 °C observed over used catalysts were attributed to the formation of inert carbon/graphitic carbon as well as the formation of nanotubes. TPO profile analysis demonstrated that a large amount of carbon species was formed on the surface of La2NiO4/γ-Al2O3 due to the acidity of γ-Al2O3.18 With the industrial cylindrical LaNiOx/ZSM-5 catalyst used in CH4/CO2 reforming of a long time (100 h) (Figure 7d), we did not observe peaks below 650 °C. This meant that with the increase of time on stream, only partial active carbon could be changed into more stable inert carbon, such as graphite carbon and nanotubes. 4. Conclusion XRD patterns of fresh Ni-based catalysts with different supports revealed that La2O3-doped Ni active species were highly dispersed on support and the specific surface area of the catalyst depends upon the properties of supports. The TPR profiles of industrial catalyst revealed that Ni active species existed mainly in the Ni0 sites of La2O3 segregation on the support ZSM-5. The conversion of CH4 and CO2 observed over industrial LaNiOx/ZSM-5 catalyst at 850 °C and GHSV ) 1.3 × 103 h-1 was 97% and 94%, respectively, significantly higher than the results reported previously, and carbon yield remained ca. 5% in the 100 h range. The results investigated not only in laboratory scale but also in pilot plant scale indicated that cylindrical LaNiOx/ZSM-5 catalyst (φ ) 3 mm, length ) 10 to ∼15 mm) had high activity under the condition of low GHSV and could meet the requirement of the use in industry. Therefore, it is economically feasible, especially for the utilization of natural gas in remote areas due to transporting cost and the advantages of clean energy sources. Acknowledgment. We gratefully acknowledge the financial support of the Enterprise Collaboration Foundation for Shanghai Coking & Chemical Corporation and the Analytical Center of Tianjin University for XRD and HRTEM characterization of samples. EF800647N