Nickel Catalysts Supported on Barium Hexaaluminate for Enhanced

Jul 16, 2012 - by a coprecipitation method using aluminum nitrate, barium nitrate, and ammonium carbonate as the precursors. The Ni catalysts supporte...
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Nickel Catalysts Supported on Barium Hexaaluminate for Enhanced CO Methanation Jiajian Gao,†,‡ Chunmiao Jia,† Jing Li,† Fangna Gu,*,† Guangwen Xu,† Ziyi Zhong,§ and Fabing Su*,† †

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China 100190 ‡ Graduate University of Chinese Academy of Sciences, Beijing, China 100049 § Institute of Chemical Engineering and Sciences, A*star, 1 Pesek Road, Jurong Island, Singapore 627833 ABSTRACT: We report the preparation and characterization of Ni nanoparticles supported on barium hexaaluminate (BHA) as CO methanation catalysts for the production of synthetic natural gas (SNG). BHA with a high thermal stability was synthesized by a coprecipitation method using aluminum nitrate, barium nitrate, and ammonium carbonate as the precursors. The Ni catalysts supported on the BHA support (Ni/BHA) were prepared by an impregnation method. X-ray diffraction, nitrogen adsorption, transmission electron microscopy, thermogravimetric analysis, H2 temperature-programmed reduction, O2 temperature-programmed oxidation, NH3 temperature-programmed desorption, and X-ray photoelectron spectroscopy are used to characterize the samples. The CO methanation reaction was carried out at pressures of 0.1 and 3.0 MPa, weight hourly space velocities (WHSVs) of 30 000, 120 000, and 240 000 mL·g−1·h−1, with a H2/CO feed ratio of 3, and in the temperature range 300−600 °C. The results show that although the BHA support has a relatively low surface area, Ni/BHA catalysts displayed much higher activity than Al2O3-supported Ni catalysts (Ni/Al2O3) with a similar level of NiO loading even after high temperature hydrothermal treatment. Nearly 100% CO conversion and 90% CH4 yield were achieved over Ni/BHA (NiO, 10 wt %) at 400 °C, 3.0 MPa, and a WHSV of 30 000 mL·g−1·h−1. Long time testing indicates that, compared to Ni/Al2O3 catalyst, Ni/ BHA is more stable and is highly resistant to carbon deposition. The superior catalytic performance of the Ni/BHA catalyst is probably related to the relatively larger Ni particle size (20−40 nm), the high thermal stability of BHA support with nonacidic nature, and moderate Ni−BHA interaction. The work demonstrates BHA would be a promising alternative support for the efficient Ni catalysts to SNG production.

1. INTRODUCTION Natural gas is an efficient energy carrier because of its high calorific value and easiness for complete combustion with smoke- and slag-free properties.1 However, due to its poor reserves in some regions of the world, the production of synthetic natural gas (SNG) from coal and biomass thus becomes attractive.2,3 Coal and biomass can be gasified to produce synthetic gas (syngas, the mixture of H2 and CO),2 which can be subsequently converted to SNG via the CO methanation process (CO + 3H2 → CH4 + H2O, ΔH298 K = −206.1 kJ·mol−1). This reaction is strongly exothermic and thermodynamically feasible.4,5 Since Sabatier and Senderens discovered that some metals such as Ni, Ru, Rh, Pt, Fe, and Co could be used in the methanation reaction in 1902,6 many methanation catalysts have been developed. However, Ni is still the most favorable choice for industrial applications because of its relatively high activity and low cost. On the other hand, Al2O3,7,8 TiO2,9 SiO2,10,11 and SiC12 have been investigated as supports for Ni catalysts, but Al2O3 is still the most widely employed.1,10,12−15 It should be noted that most methanation catalysts used for removal of trace CO in the H2 stream16−19 are not suitable in SNG production. This is because the methanation for SNG production has a much higher concentration of CO20 (normally a H2/CO ratio of more than 312,15,21). As a result, the overall reaction becomes highly exothermic leading to the sintering of the active metal and/or support,12,22,23 as well as severe carbon deposition.24 Hwang © 2012 American Chemical Society

and co-workers prepared Ni−Al2O3 xerogel catalysts by a single-step sol−gel method and found that a Ni content of 40 wt % was optimal for the methanation reaction.25 Ma et al.1 prepared coral-reef-like Ni/Al2O3 catalysts by a coprecipitation method and found their activities for CO methanation substantially decreased after a 120 h reaction due to carbon deposition. Recently, a solution combustion method was also employed to prepare Ni/Al2O3 catalysts for syngas methanation and their activities decreased at 600 °C after a 50 h test because of Ni particle sintering.15 A recent report presented a complete methanation from syngas to SNG over Ni catalyst embedded in a microchannel reactor.21 At the temperature of 550 °C, pressure of 3.0 MPa, and a gas hourly space velocity of 71 000 h−1, the CO conversion and CH4 selectivity could remain above 98% and 92%, respectively. However, no more detailed information about the stability and carbon deposition testing on Ni catalysts is disclosed in this paper. In short, although Ni/ Al2O3 catalysts have been extensively explored, their thermal stability and resistance to carbon deposition still need to be improved. Considering the methanation process operated at elevated temperatures (≥400 °C), together with the presence of steam product, the control of the thermally induced sintering Received: Revised: Accepted: Published: 10345

March 1, 2012 July 1, 2012 July 16, 2012 July 16, 2012 dx.doi.org/10.1021/ie300566n | Ind. Eng. Chem. Res. 2012, 51, 10345−10353

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Kα radiation of Cu (λ = 1.5418 Å) at 40 kV and 40 mA. The crystal size of the sample was calculated using the Debye− Scherrer equation. Adsorption−desorption isotherms of the samples were measured using N2 at −196 °C with a Quantachrome surface area and pore size analyzer, NOVA 3200e. Prior to the measurements, the sample was degassed at 200 °C for 4 h under vacuum. The specific surface area was determined according to the Brunauer−Emmett−Teller (BET) method in the relative pressure range of 0.05−0.2. The microscopic features of the samples were observed by transmission electron microscopy (TEM; JEM-2010F, JEOL, Tokyo, Japan). To make the TEM observations on the H2reduced catalysts, the samples were first reduced in a fixed bed quartz tube reactor under H2 at 600 °C for 2 h. After the samples were cooled to room temperature under H2 flow, 1 vol % O2/Ar gas mixture was introduced for 30 min to passivate the samples. Thermogravimetric analysis (TGA) was conducted on a TG/DTA 6300 thermogravimetric analyzer (Seiko Instruments EXSTAR) in air with a flow rate of 200 mL·min−1 and a temperature ramp rate of 10 °C·min−1. The surface chemical composition of the samples was determined by X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB 250 spectrometer (Thermo Electron, U.K.), using a nonmonochromatized Al Kα X-ray source (1486 eV). The deposited carbon content was measured using a CS-344 infrared analyzer (Leco, USA). H2 temperature-programmed reduction (TPR), O2 temperature-programmed oxidation (TPO), and NH3 temperature-programmed desorption (TPD) were carried out on a Quantachrome automated chemisorption analyzer (chemBET pulsar TPR/TPD). A 0.1 g sample was loaded in a quartz U-tube and heated from room temperature to 300 °C at 10 °C·min−1 and maintained for 1 h in Ar flow. Then, the sample was cooled to room temperature and followed by heating to 1000 °C at 10 °C·min−1 under a binary gas (10.0 vol % H2/Ar) with a gas flow of 30 mL·min−1. After the TPR test, the reduced catalyst was cooled to room temperature in flowing Ar and then subsequently heated to 1000 °C under another binary gas (4.9 vol % O2/He) with a total gas flow of 30 mL·min−1. For NH3-TPD, a 0.3 g sample was loaded in a quartz U-tube and heated from room temperature to 500 °C with 10 °C·min−1 and maintained for 1 h under Ar flow. Then, the sample was cooled to 100 °C and saturated with ammonia (10.0 vol % NH3/He). After removing the physically adsorbed ammonia by flushing with Ar for 1 h, the sample was heated to 600 °C ramping at 10 °C·min−1 in argon flow (30 mL·min−1). The H2 or O2 consumption or NH 3 desorption was continuously detected as a function of increasing temperature using a thermal conductivity detector (TCD). 2.3. Catalytic Measurement. The CO methanation reaction was carried out in a fixed bed reactor equipped with a quartz tube at 0.1 MPa and a stainless steel tube at 3.0 MPa (both with a diameter of 8 mm), respectively. First, a 0.5 g catalyst sample (20−40 mesh) was homogenously mixed with 2.5 g of quartz sand (20−40 mesh) and loaded in the tube reactor. The catalyst was reduced at 600 °C in pure H2 (50 mL·min−1) for 2 h and cooled to the starting reaction temperature in pure H2. For the temperature-programmed methanation at a heating rate of 5 °C min−1, a gas mixture containing H2, CO, and N2 (N2 is used as an internal standard) was introduced into the reactor with a molar stoichiometric ratio of H2/CO/N2 = 3/1/1 and the total flow rate was set to 250 mL·min−1 (the weight hourly space velocity (WHSV) is 30 000 mL (gas)·g−1 (catalyst)·h−1). The test was conducted in

of support is critical for maintaining the catalyst activity. Therefore, it requires a stable support which interferes with the growth of Ni crystals.23 It has been known that hexaaluminate materials possess a unique layer structure with alternative stacked spinel blocks separated by mirror planes. These materials, which can be easily synthesized by a sol−gel method,26 reverse microemulsion,27 and coprecipitation methods,28 are thermally stable29 and have been widely used as composite ceramics,30 thermal barrier coatings,31 catalysts and/or catalyst supports in high temperature reactions such as N 2 O decomposition, 29,32 CH 4 combustion,7,33 CO2 reforming of CH4,8 and catalytic steam reforming of CH4.34 It is thus expected that hexaaluminatesupported Ni catalysts could be promising CO methanation catalysts. Herein, we report the investigation of Ni catalysts supported on barium hexaaluminate (Ni/BHA) for CO methanation to the production of SNG. The results show that, compared with Ni catalysts supported on commercial Al2O3, the Ni/BHA catalysts exhibit much higher catalytic activity and thermal stability, as well as stronger resistance to carbon deposition, showing a promising alternative support for efficient Ni catalysts for SNG production.

2. EXPERIMENTAL SECTION 2.1. Preparation of Supports and Ni Catalysts. All the chemicals of analytical grade including aluminum nitrate nonahydrate (Al(NO3)3·9H2O), barium nitrate (Ba(NO3)2), ammonium carbonate ((NH4)2CO3), and nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O) were purchased from Sinopharm Chemical Reagent Co. Ltd., China, and used without further treatment. Barium hexaaluminate (BHA) support was prepared by the coprecipitation method.28 Typically, 0.180 mol of Al(NO3)3·9H2O and 0.015 mol of Ba(NO3)2 were dissolved in 200 mL of deionized water at 60 °C to get a BHA precursor solution. Likewise, 0.855 mol of (NH4)2CO3 was dissolved in 100 mL of deionized water and then heated to 60 °C. Subsequently, the BHA precursor solution was added to the (NH4)2CO3 solution, and the mixture was vigorously stirred for 4 h to obtain a gel, which was further filtrated under vacuum and washed with deionized water. The filtered solid was dried at 110 °C overnight and crushed to form the powder. After calcination at 1200 °C for 6 h with a heating rate of 5 °C·min−1 in air, the BHA support was obtained. The Ni/BHA catalysts with different NiO loadings were prepared by the wet impregnation method. A 0.078 g sample of Ni(NO3)2·6H2O was dissolved in 10 mL of distilled water and then 1.0 g of BHA was added. The mixture was stirred at room temperature for 12 h. Then, the mixture was evaporated at 80 °C under stirring to obtain solid samples and dried at 110 °C overnight. After calcination at 400 °C for 2 h in air, the NiO/ BHA with a NiO loading of 2 wt % was obtained and denoted “2Ni/BHA”. Similarly, other NiO/BHA catalysts with NiO loadings of 5 and 10 wt % were prepared by using 0.195 and 0.389 g of Ni(NO3)2·6H2O and were named “5Ni/BHA” and “10Ni/BHA”, respectively. For a comparison, commercial porous γ-Al2O3 (purity >95%, GongYiHuaYu Alumina Co. Ltd., China) calcined at 400 °C in air for 4 h prior was used as the support, and two catalysts of 5Ni/Al2O3 and 10Ni/Al2O3 with NiO loadings of 5 and 10 wt % respectively were prepared by the same method. 2.2. Characterization. X-ray diffraction (XRD) patterns were recorded on a PANalytical X’Pert PRO MPD using the 10346

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the temperature range 200−600 °C with an interval of 50 °C. For the test at a high WHSV of 120 000 mL·g−1·h−1, 0.1 g of catalyst mixed with 2.5 g of quartz sand and a total flow rate of 200 mL·min−1 were used. Lifetime testing of 10Ni/BHA and 10Ni/Al2O3 at a high sand dilution ratio (60 times) was carried out under the following conditions: 0.05 g of catalyst (20−40 mesh) diluted with 3.0 g of quartz sand (20−40 mesh) and 200 mL·min−1 reaction gas mixture (H2/CO/N2 = 3/1/1). The reaction was carried out at 450 °C, 0.1 MPa, and WHSV of 240 000 mL·g−1·h−1. Hydrothermal treatment for catalyst aging was carried out in a fixed bed quartz tube reactor at 800 °C and 0.1 MPa for 5 h with 90 vol % H2O/H2 before testing. The outlet gas stream from the reactor was cooled using a cold trap. Inlet and outlet gases were analyzed on line with a Micro GC (3000A; Agilent Technologies). The concentrations of H2, N2, CH4, and CO contents in gas products were analyzed by a thermal conductivity detector (TCD) with a molecular sieve column after 1 h of steady-state operation at each temperature. The concentrations of CO2, C2H4, C2H6, C3H6, and C3H8 were analyzed by another TCD with a Plot Q column. Stability testing at 0.1 and 3.0 MPa was carried out using fresh catalysts. After reduction at 600 °C in pure H2 (50 mL·min−1) for 2 h, the catalyst was cooled to 400 °C and the H2 flow was changed to the reaction mixture gas to perform the stability test. The products are CO, CO2, CH4, H2O, H2, and a negligible amount of C2H6 (99%) over 2Ni/BHA and 5Ni/ BHA catalysts at 350 °C. With the increase of NiO loading for the 10Ni/BHA catalyst, the maximum peak of CO conversion (>99%) shifts to lower temperature (250 °C) and is maintained in the range 250−400 °C. These results are in agreement with the early reports.38 In contrast, for Ni/Al2O3 catalysts, the maximum CO conversion is only 93.5 and 21.7% obtained over 10Ni/Al2O3 and 5Ni/Al2O3 at 400−450 °C, respectively. However, a similar CH4 selectivity in the measured temperature range can be found in Figure 7b for all catalysts although 5Ni/ Al2O3 shows a little higher while 10Ni/Al2O3 shows slightly lower. Figure 7c shows the CH4 yield curves of all catalysts, which are obtained by integrating data of Figure 7a with those of Figure 7b. The highest CH4 yield of more than 80% can be obtained on 10Ni/BHA at 250−400 °C. After 400 °C, 2Ni/ BHA and 5Ni/BHA show yields comparable to that of 10Ni/ BHA. In contrast, the highest CH4 yield is only 68.9% for 10Ni/Al2O3 and 18.1% for 5Ni/Al2O3 catalysts. It is clearly seen that, in the whole temperature range, the CH4 yield on Ni/BHA is much higher than that on Ni/Al2O3 catalysts, demonstrating that Ni/BHA catalysts are much more active, even with low Ni loadings. Considering the CO conversion approaches the thermodynamic equilibrium data calculated on Ni/BHA catalysts as shown in Figure 7a, we carried out the catalytic measurement at a high WHSV (120 000 mL·g−1·h−1) and the results are shown in Figure 7d−f. Similarly, the CO conversion (Figure 7d) and CH4 yield (Figure 7f) on Ni/BHA catalysts are still much larger than on Ni/Al2O3 although the CH4 selectivity (Figure 7e) has no big change. In industry, high pressure (2.9−3.4 MPa) is often applied for SNG production because CO methanation is a volume-reduced reaction.40 Our thermodynamic analysis has shown that that high pressure is beneficial to CH4 production.4 Figure 7g−i

Figure 5. (a) H2-TPR curves of the supports and Ni catalysts and (b) O2-TPO curves of the supports and Ni catalysts after H2-TPR.

°C), and γ-type (strong interaction, at about 750−840 °C).37 The TPR curves of the catalysts are fitted using Gaussian-type functions, and the quantitative results are listed in Table 2. Table 2. TPR Quantitative Data of the Catalysts Tm (°C)

fraction of total area (%)

catalyst

α

β

γ

α

β

γ

2Ni/BHA 5Ni/BHA 10Ni/BHA 5Ni/Al2O3 10Ni/Al2O3

418 449 447 422 403

552 545 539 635 598

− − − 770 760

42.6 85.2 92.1 18.9 15.9

57.4 14.8 7.9 58.2 62.1

− − − 22.9 22.0

Generally, the α-type NiO fraction for BHA-supported catalyst increases with the NiO loading. 2Ni/BHA is dominant with the β-type of NiO (57.4% in Table 2). In particular, almost all NiO belongs to the α-type for 5Ni/BHA (85.2% in Table 2) and 10Ni/BHA (92.1% in Table 2). No obvious γ-type NiO is found in all the BHA catalysts. In contrast, for the Ni/Al2O3 catalysts, β-type NiO becomes dominant for both 5Ni/Al2O3 (58.2% in Table 2) and 10Ni/Al2O3 (62.1% in Table 2), and the γ-type is also found (22.9 and 22.0% in Table 2), implying the presence of a relatively strong interaction of NiO with Al2O3,38 which could lead to the difficulty in reduction. From the O2-TPO curves of the supports and Ni catalysts obtained after H2-TPR in Figure 5b, no peaks can be seen for both BHA and Al2O3 supports. For the reduced 10Ni/BHA and 5Ni/ BHA, the oxidation mainly occurs in the range 300−500 °C. However, reduced 2Ni/BHA shows two weak oxidation peaks at 300 and 700 °C, suggesting that part of the reduced Ni particles have a relatively small size and a strong metal−support interaction at 700 °C. Obviously, the peaks of reduced Ni/ Al2O3 catalysts are much wider within the range 400−700 °C, indicating the formation of the different Ni−Al2O3 interactions. These results indicate the Ni−support interaction for Ni/BHA catalysts is relatively weaker than that of Ni/Al2O3. Figure 6 shows the Ni 2p3/2 and O 1s XPS spectra of the reduced 10Ni/Al2O3 and 10Ni/BHA catalysts. Figure 6a exhibits the same Ni peak position for both Ni catalysts except that the intensity in Ni/BHA is stronger than that in Ni/Al2O3, suggesting the presence of a higher Ni density on the surface of BHA. The peak of Ni 2p3/2 at 856.4 eV suggests that both 10349

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Figure 7. Catalytic properties of the different Ni catalysts: (a, d, and g) CO conversion, (b, e, and h) CH4 selectivity, and (c, f, and i) CH4 yield.

and 400 °C in Figure 8a, and at 3.0 MPa and 400 °C in Figure 8c, is Ni/BHA really deactivated? Figure 8e and 8f shows the lifetime testing results of 10Ni/BHA and 10Ni/Al2O3 with a higher sand/catalyst dilution ratio (60 times) at 0.1 MPa and a higher WHSV of 240 000 mL·g−1·h−1. It can be seen that CO conversion remains constant for 10Ni/BHA and its catalytic performance is also much better than that of 10Ni/Al2O3, although a slight decline in CH4 selectivity and CH4 yield are observed within 100 h. Thus, further improvement of Ni/BHA catalyst is indeed demanded. Figure 9 shows the XRD patterns of the fresh and used catalysts Ni/BHA after a 200 h and 10Ni/AlO3 after a 12 h durability test at 0.1 MPa. There is no obvious change seen for both 5Ni/BHA and 10Ni/BHA catalysts before and after the reaction (Figure 9a). However, for the used 10Ni/Al2O3 (Figure 9b), the intensity of the Ni diffraction peaks increased, implying that Ni particles on 10Ni/Al2O3 are partially sintered or aggregated after 12 h of reaction. These results indicate that Ni/BHA catalysts remain stable during long-term methanation reaction. It is reported that carbon deposition often occurs on the Ni catalysts.24 Figure 10 shows the TG and DTA curves of the used catalysts after durability testing at 0.1 MPa (for 5Ni/BHA and 10Ni/BHA, 200 h test; for 10Ni/Al2O3, 12 h test). This TG analysis was conducted in air. In Figure 10a, the weight of the used Ni/BHA catalysts first increased at 200−400 °C because of the oxidation of Ni to NiO, and then decreased

shows the catalytic performance of CO methanation at 3.0 MPa and a WHSV of 120 000 mL·g−1·h−1. It can be seen that CO conversion (Figure 7g), CH4 selectivity (Figure 7h), and CH4 yield (Figure 7i) on 5Ni/BHA and 10Ni/BHA are also much higher than those on 10Ni/Al2O3, especially below 500 °C, further demonstrating the superior property of Ni/BHA catalysts. In particular, a much higher CH4 yield of 87.5% can be achieved for the 10Ni/BHA catalyst (Figure 7i). Compared to the catalytic results at 0.1 MPa (Figure 7d−f), we can see that the high pressure is undoubtedly favorable for CO conversion and CH4 production. 3.3. Lifetime Test. Figure 8 shows the lifetime test results of 5Ni/BHA, 10Ni/BHA, and 10Ni/Al2O3 catalysts at different conditions. Both 5Ni/BHA and 10Ni/BHA catalysts in Figure 8a show excellent catalytic performance for CO methanation at 0.1 MPa during 200 h of testing without an obvious decline in CO conversion (∼99%), CH4 selectivity (∼86%), and CH4 yield (∼85%). In contrast, for the 10Ni/Al2O3 catalyst in Figure 8b, both the CO conversion and CH4 yield rapidly decline from 96% to only 20% after 10 h of reaction. Parts c and d of Figure 8 show the catalytic performances of 10Ni/BHA and 10Ni/ Al2O3 at 3.0 MPa and WHSV of 30 000 mL·g−1·h−1, respectively. 10Ni/BHA still reveals a relatively high CO conversion (∼98%), CH4 selectivity (∼92%), and CH4 yield (∼90%). However, the catalytic activity of 10Ni/Al2 O 3 decreases remarkably during the 5 h test. Although we did not observe the deactivation of Ni/BHA catalysts at 0.1 MPa 10350

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Figure 10. (a) TG and (b) DTA curves of fresh and used Ni catalysts under air.

344 infrared analyzer: 0.97 wt % for 5Ni/BHA, 1.12 wt % for 10Ni/BHA, and 3.08 wt % for 10Ni/Al2O3. Based on the latter carbon analysis result, the average carbon deposition rate is calculated to be 4.79 × 10−3 wt %·h−1 for 5Ni/BHA, 5.53 × 10−3 wt %·h−1 for 10Ni/BHA, and 2.56 × 10−1 wt %·h−1 for 10Ni/Al2O3. This means that the carbon deposition rate on 10Ni/Al2O3 is about 50 times higher than on 10Ni/BHA catalysts, suggesting the superior resistance to carbon deposition for Ni/BHA catalysts. Moreover, the N2 isotherms of three used catalysts show that, compared with the fresh catalysts, there is not much change in the surface area for both 5Ni/BHA (23.4 vs 22.7 m2·g−1) and 10Ni/BHA (23.8 vs 24.5 m2·g−1), but there is a large change for 10Ni/Al2O3 (220 vs 180 m2·g−1) possibly due to pore blocking by a large amount of deposited carbon on Al2O3 and the collapse of the pore structure of the Al2O3 support. Considering the presence of water steam as one of the products in methanation and that the steam is often added to the reactant gas mixture to control hot spots in industry, the hydrothermal stability of the Ni catalysts is thus examined. Figure 11 shows the catalytic properties of 10Ni/BHA and 10Ni/Al2O3 before and after hydrothermal treatment (labeled “HT”). It can be seen that 10Ni/BHA nearly maintained catalytic activity while 10Ni/Al2O3 decreased substantially, implying that the former is more stable than the latter. The phase transformation of the Al2O3 support (from γ-type to partial δ-type) and aggregation of small Ni particles (size from about 5 nm to about 25 nm) for 10Ni/Al2O3 observed from the XRD patterns (not shown here), together with the collapse of the pore structure of Al2O3 (surface area from 220 to 113 m2·g−1) which may bury the Ni particles and make them unavailable for reaction, lead to the severe deactivation of 10Ni/Al2O3. However, no obvious change for 10Ni/BHA is observed for Ni particle size from XRD patterns (24 nm), TEM images (25−40 nm), and the surface area (23.4 m2·g−1 vs 21.4 m2·g−1) before and after hydrothermal treatment, further suggesting the superior stability of the 10Ni/BHA catalyst. As reported, the CO methanation is a structure-sensitive reaction, and the catalytic performance of catalysts is significantly affected by metal crystallite size and the nature of the metal oxide support.42 The carbon deposition, which leads to the deactivation of Ni catalysts in methanation catalytic processes,41 is mainly affected by the nature of the active component and the support, the dispersion of active metal particles, and the reaction conditions.43 Takenaka et al. reported that Ni metal particles with relatively large diameters (about 20−100 nm) were more active for the CO methanation reaction.44 Aksoylu et al. found that methanation activity was

Figure 8. Lifetime tests under different reaction conditions: (a) 5Ni/ BHA and 10Ni/BHA, (c and e) 10Ni/BHA, and (b, d, and f) 10Ni/ Al2O3.

Figure 9. XRD patterns of fresh and used catalysts: (a) 5Ni/BHA and 10Ni/BHA, and (b) 10Ni/Al2O3.

when temperature reached 500 °C and maintained constant above 600 °C. Compared with the fresh catalysts, the weight loss of 10Ni/BHA and 5Ni/BHA was 1.5 and 0.9 wt %, respectively. In DTA curves of Figure 10b, the used 10Ni/BHA exhibits two exothermic peaks at 330 and 600 °C which are attributed to the oxidation of Ni and deposited graphitic carbon,41 respectively. It is known that deposited carbon on Ni catalysts has different morphologies and structures with different oxidation temperatures.24 For the 10Ni/Al2O3 catalyst, the carbon amount is around 3.3 wt % only after the 12 h test. These TG results are consistent with the analysis using a CS10351

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Figure 11. Catalytic properties of 10Ni/BHA and 10Ni/Al2O3 before and after hydrothermal treatment: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

Program of China (No. 2010BAC66B01), and Knowledge Innovation Program of the CAS (No. KGCX2-YW-396). F.G. is grateful for the support of K. C. Wang Postdoctoral Fellowships of the CAS and China Postdoctoral Science Foundation (Nos. 20100480026 and 201104151).

dependent on the Ni content, and high catalytic activity was obtained at high Ni loadings due to the formation of large Ni particles.45 On the other hand, carbon deposition occurs quickly on small Ni particles, especially more readily on their step planes.41 Moreover, carbon is easily formed in the developed pores of the Al2O3 support,46 resulting in the blocking of the pores, as well as prevention of gas diffusion. In our work, compared to high surface Al2O3, the use of BHA as a thermally stable support with undeveloped pore structure results in relatively large Ni particles and moderate Ni−support interaction, and thus NiO particles are much easier to completely reduce to produce more active sites for CO methanation and less deposited carbon. Last but not least, the much lower acidity of BHA than that of Al2O3 would cause much less carbon deposition.47,48

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(1) Ma, S.; Tan, Y.; Han, Y. Methanation of syngas over coral reeflike Ni/Al2O3 catalysts. J. Nat. Gas Chem. 2011, 20 (4), 435−440. (2) Kopyscinski, J.; Schildhauer, T. J.; Biollaz, S. M. A. Production of synthetic natural gas (SNG) from coal and dry biomass―A technology review from 1950 to 2009. Fuel 2010, 89 (8), 1763−1783. (3) Seemann, M. C.; Schildhauer, T. J.; Biollaz, S. M. A. Fluidized Bed Methanation of Wood-Derived Producer Gas for the Production of Synthetic Natural Gas. Ind. Eng. Chem. Res. 2010, 49 (15), 7034− 7038. (4) Gao, J.; Wang, Y.; Ping, Y.; Hu, D.; Xu, G.; Gu, F.; Su, F. A thermodynamic analysis of methanation reactions of carbon oxides for the production of synthetic natural gas. RSC Adv. 2012, 2 (6), 2358− 2368. (5) Kopyscinski, J.; Schildhauer, T. J.; Biollaz, S. M. A. Fluidized-Bed Methanation: Interaction between Kinetics and Mass Transfer. Ind. Eng. Chem. Res. 2011, 50 (5), 2781−2790. (6) Sabatier, P.; Senderens, J. B. New methane synthesis. Acad. Sci. 1902, 134, 514−516. (7) Zheng, J. D.; Ren, X. G.; Song, Y. J.; Ge, X. T. Catalytic combustion of methane over iron- and manganese-substituted lanthanum hexaaluminates. React. Kinet. Catal. Lett. 2009, 97 (1), 109−114. (8) Kundu, A.; Shul, Y. G.; Kim, D. H. Methanol Reforming Processes. In Advances in Fuel Cells; Elsevier Science: New York, 2007; Vol. 1, Chapter 7, pp 419−472. (9) Panagiotopoulou, P.; Kondarides, D. I.; Verykios, X. E. Mechanistic Study of the Selective Methanation of CO over Ru/ TiO2 Catalyst: Identification of Active Surface Species and Reaction Pathways. J. Phys. Chem. C 2011, 115 (4), 1220−1230. (10) Wang, Y. Z.; Wu, R. F.; Zhao, Y. X. Effect of ZrO2 promoter on structure and catalytic activity of the Ni/SiO2 catalyst for CO methanation in hydrogen-rich gases. Catal. Today 2010, 158 (3−4), 470−474. (11) Park, J.-N.; McFarland, E. W. A highly dispersed Pd−Mg/SiO2 catalyst active for methanation of CO2. J. Catal. 2009, 266 (1), 92−97. (12) Yu, Y.; Jin, G.-Q.; Wang, Y.-Y.; Guo, X.-Y. Synthetic natural gas from CO hydrogenation over silicon carbide supported nickel catalysts. Fuel Process. Technol. 2011, 92 (12), 2293−2298. (13) Urasaki, K.; Tanpo, Y.; Takahiro, T.; Christopher, J.; Kikuchi, R.; Kojima, T.; Satokawa, S. Selective Methanation of CO in Reformate Gas over Ni/TiO2 Catalyst. Chem. Lett. 2010, 39 (9), 972−973. (14) Mok, Y. S.; Kang, H. C.; Koh, D. J.; Shin, D. N.; Baik, J. H. Nonthermal Plasma-enhanced Catalytic Methanation of CO over Ru/ TiO2/Al2O3. J. Korean Phys. Soc. 2010, 57 (3), 451−457.

4. CONCLUSIONS In summary, we report a systematic investigation of the Ni/ BHA catalysts synthesized by the impregnation method for the production of SNG via CO methanation reaction. Compared to the Ni/Al2O3 catalysts, the Ni/BHA catalysts with the same Ni loading are much more active at different reaction conditions. Almost 100% CO conversion and 90% CH4 selectivity can be achieved over Ni/BHA (NiO loading, 10 wt %) at 400 °C, 3.0 MPa, and a WHSV of 30 000 mL·g−1·h−1. Lifetime tests at 0.1 and 3.0 MPa as well as after hydrothermal treatment of catalysts demonstrate that the Ni/BHA catalysts are highly stable, while the Ni/Al2O3 catalysts deactivate rapidly due to much more severe carbon deposition and sintering of Ni particles. Probably a nonacidic support with relatively low surface area, large Ni particle size (20−40 nm), and a moderate metal−support interaction are the necessary parameters for Ni catalyst that is highly active for the CO methanation reaction. Our work shows that BHA is a promising support candidate for methanation catalysts.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.G.); [email protected] (F.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the Hundred Talents Program of the Chinese Academy of Sciences (CAS), State Key Laboratory of Multiphase Complex Systems of China (No. MPCS-2009-C-01), National Key Technology R&D 10352

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