Al2O3 Catalysts for

Article pubs.acs.org/IECR

Enhanced Investigation of CO Methanation over Ni/Al2O3 Catalysts for Synthetic Natural Gas Production Dacheng Hu,†,‡ Jiajian Gao,† Yuan Ping,† Lihua Jia,‡ Poernomo Gunawan,§ Ziyi Zhong,§ Guangwen Xu,† Fangna Gu,*,† and Fabing Su*,† †

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China 100190 ‡ College of Chemistry and Chemical Engineering, Qiqihaer University, Qiqihaer, Heilongjiang Province, China 161006 § Institute of Chemical Engineering and Sciences, A*star, 1 Pesek Road, Jurong Island, Singapore 627833 ABSTRACT: CO methanation reaction over the Ni/Al2O3 catalysts for synthetic natural gas production was systematically investigated by tuning a number of parameters, including using different commercial Al2O3 supports and varying NiO and MgO loading, calcination temperature, space velocity, H2/CO ratio, reaction pressure, and time, respectively. The catalytic performance was greatly influenced by the above-mentioned parameters. Briefly, a large surface area of the Al2O3 support, a moderate interaction between Ni and the support Al2O3, a proper Ni content (20 wt %), and a relatively low calcination temperature (400 °C) promoted the formation of small NiO particles and reducible β-type NiO species, which led to high catalytic activities and strong resistance to the carbon deposition, while addition of a small amount of MgO (2 wt %) could improve the catalyst stability by reducing the carbon deposition; other optimized conditions that enhanced the catalytic performance included high reaction pressure (3.0 MPa), high H2/CO ratio (≥3:1), low space velocity, and addition of quartz sand as the diluting agent in catalyst bed. The best catalyst combination was 20−40 wt % of NiO supported on a commercial Al2O3 (S4) with addition of 2−4 wt % of MgO, calcined at 400−500 °C and run at a reaction pressure of 3.0 MPa. On this catalyst, 100% of CO conversion could be achieved within a wide range of reaction temperature (300−550 °C), and the CH4 selectivity increased with increasing temperature and reached 96.5% at a relatively low temperature of 350 °C. These results will be very helpful to develop highly efficient Ni-based catalysts for the methanation reaction, to optimize the reaction process, and to better understand the above reaction.

1. INTRODUCTION As a versatile energy carrier, synthetic natural gas (SNG) attracts increasing attention due to the continuous rising of natural gas prices, increasing concern on depletion of natural gas, and greenhouse effect of the released CO2.1 The advantage of SNG includes its high combustion efficiency and the already existing gas distribution infrastructures, such as gas pipelines, together with the well-established downstream technologies. Generally, SNG is produced via gasification of coal2−6 and biomass7−10 that generates synthetic gas (syngas), followed with subsequent methanation process, in which CO methanation (reaction 1: CO + 3H2 = CH4 + H2O) is a key reaction that is highly exothermic (206.1 kJ/mol) and thermodynamically feasible.11,12 Thermodynamically, this reaction is favorable at low temperatures, but kinetically, it is favorable at high temperatures. Since the 1970s, efforts have been made to develop a number of methanation reactors for SNG production, including fixed bed and fluidized bed reactors.1 However, it is still a challenge to develop methanation catalysts that are highly efficient and extremely stable against sintering and carbon deposition. Many metals, such as Ni, Ru, Rh, Co, Fe, etc., can be used as methanation catalysts.13−19 However, some noble metals such as Ru and Rh are not economical for large-scale production of SNG due to their high cost, albeit demonstrating very high activity at low temperatures for the removal of trace CO in an H2-rich stream.20−23 Thus, Ni is often regarded as the most © 2012 American Chemical Society

practical choice because of its relatively high methanation activity and low price.24−26 Ni-based catalysts, e.g., Ni/Al2O3, have been explored for the methanation of carbon monoxide,6,21,24,27,28 but they suffer from rapid deactivation at high temperatures due to the sintering of Ni particles, facile carbon deposition, and severe sulfur poisoning.29−31 It was reported that Ni/Al2O3 catalysts prepared by a solution combustion32 or a single-step sol−gel method33 performed well in CO methanation with good thermal stability. Ma et al.30 reported that the Ni/Al2O3 catalysts with a coral reef morphology exhibited high activity and resistance to carbon deposition. Furthermore, MgO was found to be an effective promoter to improve resistance to carbon deposition and to minimize Ni particles sintering.34−36 Although there have been many studies on CO methanation over Ni-based catalysts,6,37−42 there are still plenty of room and opportunities for further improvement. Herein, we report the preparation and characterization of various Ni/Al2O3 catalysts supported on different commercial Al2O3 supports for CO methanation. Their catalytic properties are systematically investigated under wide reaction conditions. We aim not only to obtain and optimize efficient Ni catalysts Received: Revised: Accepted: Published: 4875

January 7, 2012 March 9, 2012 March 10, 2012 March 10, 2012 dx.doi.org/10.1021/ie300049f | Ind. Eng. Chem. Res. 2012, 51, 4875−4886

Industrial & Engineering Chemistry Research

Article

to obtain 20−40 mesh particles. Catalyst (0.5 g) diluted with 2.5 g of quartz sand was filled into a reactor with an 8 mm diameter (quartz tube at 0.1 MPa; stainless steel tube at 3.0 MPa) and reduced at 600 °C for 4 h in a continuous flow of pure H2 (30 mL/min). Finally, it was tested over a temperature range of 100−600 °C. The mixed reactant gas consisted of CO/H2/N2 with a molar ratio of 3:1:1, in which N2 was added as an internal standard gas for GC analysis. Gas hourly space velocity (GHSV, mL(gas)/g(catalyst)·h) was selected to be 30 000 except for specific clarification. The outlet gas stream was cooled using a cold water trap. To determine CO conversion and CH4 selectivity, the products were collected after half an hour of steady-state operation at each temperature and analyzed by Micro GC (3000A; Agilent Technologies). The amounts of H2, N2, CH4, and CO in the outlet gas were analyzed by a thermal conductivity detector (TCD), while CO2, C2H4, C2H6, C3H6, and C3H8 were analyzed by another TCD with a Plot Q column. For temperature-programmed methanation, the heating rate was set at 2.5 °C/min. The calculation formulas were described as follows:

suitable for industrial applications but also to get scientific understanding on the CO methanation reaction and process.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. Four commercial Al2O3 materials were used as catalyst supports, which are denoted as S1 (>94%, trileaf shape, Wenzhou Jingjing Alumina Co. Ltd., China), S2 (>84%, trileaf type, Wenzhou Jingjing Alumina Co. Ltd., China), S3 (>95%, trileaf pattern, Wenzhou Jingjing Alumina Co. Ltd., China), and S4 (>95%, spherical shape, Gongyi Huayu Alumina Co. Ltd., China). These supports were crushed to powders of about 20−40 mesh and subsequently calcined at 400 °C for 4 h prior to further use. All analytical grade chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used without further purification. The Ni catalysts were prepared by the conventional impregnation method, as described as follows: Ni(NO3)2·6H2O and Mg(NO3)2·6H2O were first dissolved in distilled water, followed by the addition of Al2O3 powder. The slurry was stirred at room temperature for 8 h and then dried at 120 °C for 4 h. The solid sample was afterward calcined at a given temperature for 4 h in air. The prepared catalysts were denoted as xNyM/Sz-t, where N, M, and S represent NiO, MgO, and Al2O3 support, respectively; x, y, z, and t are the content of NiO (x wt %) and MgO (y wt %), support number, and calcination temperature, respectively. Here, x = 5, 10, 20, and 40; y = 0, 2, 4, and 8; z = 1, 2, 3, and 4; t = 400, 500, and 600 °C. 2.2. Characterization. The porous structure of the samples was investigated using a Quantachrome surface area and pore size analyzer NOVA 3200e at 77 K. Prior to the measurement, the samples were degassed at 200 °C for 4 h under vacuum. The pore size distribution (PSD) was calculated with the Barett−Joyner−Halenda (BJH) method using the adsorption isotherm branch. The specific surface area was determined according to the Brunauer−Emmett−Teller (BET) method in the relative pressure (p/p0) range of 0.05−0.3. Powder X-ray diffraction (XRD) patterns were collected using a PANanalytical X’Pert PRO MPD with Cu Kα radiation of wavelength λ = 0.1541 nm at 40 kV and 40 mA. The crystallite size of the particles was calculated using Debye−Scherrer equation. The particles morphology was observed by field-emission scanning electron microscopy (SEM) (JEM-6100F, JEOL) and transmission electron microscopy (TEM) (JEM-2010F, JEOL). Temperature-programmed reduction with H2 (H2-TPR) was carried out on an automated chemisorption analyzer (chemBET pulsar TPR/TPD, Quantachrome). Prior to the measurement, 0.1 g of catalyst placed in a quartz U-tube was pretreated in an Ar stream at 300 °C for 0.5 h and then cooled to room temperature. H2-TPR was then conducted with a gas mixture of 10 vol % H2 in Ar at 30 mL/min. The temperature was raised to 1000 °C at a heating rate of 5 °C/min. The amount of H2 uptake was detected by a thermal conductivity detector (TCD). Thermogravimetric analysis (TGA) was conducted on a thermogravimetric analyzer (Seiko Instruments EXSTAR TG/ DTA 6300) in air with a flow rate of 100 mL/min and a heating rate of 5 °C/min. The carbon content in the catalysts was analyzed with CS-344 infrared carbon−sulfur analyzer (Leco, US). 2.3. Evaluation of Catalyst Performance. The evaluation of Ni catalysts for CO methanation was carried out in a continuous flow fixed-bed reactor. Prior to the measurement, the catalyst powder was pressed into tablets and further crushed

Gas hourly space velocity Vmixedgas,in GHSV (mL/g· h) = mcat CO conversion:

XCO(%) =

(1)

VCO,in − VCO,out × 100 VCO,in (2)

CH 4 selectivity:

SCH4(%) =

VCH4,out VCO,in − VCO,out

× 100 (3)

CH 4 yield: YCH4(%) =

H2 conversion:

XCO × SCH4 100

X H2(%) =

=

VCH4,out VCO,in

× 100 (4)

VH2,in − VH2,out × 100 VH2,in (5)

where GHSV is the gas hour space velocity (mL(gas)/ g(catalyst)·h), Vmixed gas,in (mL/min) is the volumetric flow rate of the mixed reactant gas, X is the conversion, S is the selectivity, Y is the yield, Vi,in (mL/min) and Vi,out (mL/min) are the inlet and outlet volumetric flow rate of species i (i = CO, CO2, H2, and CH4), respectively.

3. RESULTS AND DISCUSSION 3.1. Pore Structure of the Al2O3 Supports and Ni Catalysts. Figure 1a shows the N2 adsorption−desorption isotherms of different Al2O3 supports. S1, S2, and S3 are found to exhibit similar N2 adsorption/desorption profiles with a hysteresis loop at p/p0 = 0.6−0.9, which can be tentatively ascribed to either the mesoporosity and the capillary condensation at high pressure43 or the interparticle space between oxide particles.44 In contrast, S4 exhibits a H3 along with H4 hysteresis, possibly attributed to the slit-like pore.45 Furthermore, the inflection point occurs at lower relative pressure (p/p0 = 0.4), compared to that of other supports (p/p0 4876

dx.doi.org/10.1021/ie300049f | Ind. Eng. Chem. Res. 2012, 51, 4875−4886


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Figure 2. XRD patterns (a) and H2-TPR curves (b) of supports and Ni catalysts.

Figure 1. N2 adsorption isotherms (a) and PSD curves (b) of all Al2O3 supports. (For clarity, the isotherm of S1, S2, and S3 was vertically shifted for 350, 209, and 350 cm3/g, respectively.)

Ni catalysts. It is found that the S1, S3, and S4 supports are γtype Al2O347 while S2 is a composite of Ti−Al support, as indicated by the diffraction peaks at around 25.6°, 48.4°, and 55°, which are assigned to (101), (200), and (211) planes of anatase TiO2 (JCPDS 02-0406). For all of the prepared catalysts, a group of diffraction peaks at 37.4°, 43.3°, and 63.0° are observed, which correspond respectively to (111), (200), and (220) diffractions of NiO.48 In addition, the peak at approximately 45.1° can be assigned to the (400) diffraction of NiAl2O4 (JCPDS 10-0339). The NiO crystal size on different Al2O3 supports is estimated by the Debye−Scherrer equation and presented in Table 1. It is observed that the crystallite size of the supported NiO particles is in the range of 4.9−10.7 nm, depending on the supports, among which 20N2M/S4-500 catalyst exhibits the smallest NiO crystallite size (4.9 nm). This

> 0.6), implying the existence of mesopores with smaller sizes, which is confirmed by the pore size distribution in Figure 1b. Table 1 summarizes the specific surface areas, pore volumes, and pore sizes of the various Al2O3 supports. As seen in Table 1, the surface area and pore volume decrease in the order S4 > S3 > S1 > S2. An average pore diameter of 5.6 nm is measured for S4, much smaller than that of other supports (9.4−16.8 nm). The isotherms and PSD curves of Ni catalysts are not shown here and their surface areas, pore volumes, and pore sizes are listed in Table 1. It is observed that the specific surface area and pore volume gradually decrease with the increase of NiO loadings and calcination temperatures.46 3.2. Effect of Al2O3 Supports. Figure 2a shows the XRD patterns of different Al2O3 supports and the Al2O3-supported Table 1. Physical Parameters of Supports and Ni Catalysts

300−400 °C samples S1 S2 S3 S4 20N2M/S1-500 20N2M/S2-500 20N2M/S3-500 20N2M/S4-500 5N2M/S4-500 10N2M/S4-500 40N2M/S4-500 20N0M/S4-500 20N4M/S4-500 20N8M/S4-500 20N2M/S4-400 20N2M/S4-600

SBETa

2

(m /g)

160 146 259 305 108 109 206 196 252 231 150 213 196 175 229 180

b

3

c

d

V (cm /g)

D (nm)

crystal size (nm)

MAXCOconversion(%)

MAXCH4selectivity(%)

0.53 0.49 0.62 0.48 0.28 0.28 0.39 0.31 0.42 0.38 0.25 0.29 0.29 0.29 0.32 0.32

16.8 16.7 9.4 5.6 12.1 9.4 6.5 5.0 6.5 5.6 5.6 4.9 4.9 6.5 4.3 6.5

/ / / / 8.4 9.5 10.7 4.9 / / 16.7 4.8 5.7 11.7 3.7 4.2

/ / / / 100 97.4 3.7 100 82.3 93.8 99.3 100 100 60.1 99.3 97.9

/ / / / 83.7 76.7 44.9 82.6 89 79 89.3 82.1 81 73.3 81.4 78.7

a

Surface area, derived from BET equation. bPore volume, obtained from the volume of nitrogen adsorbed at the relative pressure of 0.97. cPore size, derived from BJH method using adsorption branch. dCrystal size of NiO, derived from XRD by Debye−Scherrer equation. 4877



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Table 2. TPR Quantitative Data of the Catalysts Tm (°C)

fraction of total area (%)

catalyst

α

β1

β2

γ

α

β1

β2

γ

20N2M/S1-500 20N2M/S2-500 20N2M/S3-500 20N2M/S4-500 5N2M/S4-500 10N2M/S4-500 40N2M/S4-500 20N0M/S4-500 20N4M/S4-500 20N8M/S4-500 20N2M/S4-400 20N2M/S4-600

474 470 501 − − − 471 491 500 − 462 526

595 544 565 565 606 592 557 581 593 613 558 628

713 713 647 702 686 720 708 720 712 723 694 720

810 − 805 806 784 802 825 779 802 830 809 810

1.9 17.4 74.9 − − − 23.0 9.2 5.5 1.0 0.7

72.4 78.9 13.0 57.7 31.7 50.0 63.7 73.0 33.8 24.3 84.6 26.2

22.3 3.7 10.9 40.0 48.4 46.2 12.3 13.8 58.9 71.0 12.8 64.3

3.4 − 1.2 2.3 19.9 3.8 1.0 4.0 1.8 4.7 1.6 8.8

suggests high dispersion of NiO nanoparticles on the S4 support, possibly because of its large surface area. Figure 2b shows the H2-TPR curves of supported Ni catalysts on different Al2O3 supports. It is observed that H2 consumption commences at about 400 °C for all the Ni catalysts. The reducible NiO species are usually classified to four types: α, β1, β2, and γ.49 The peaks located in the low temperature region (410−500 °C) are assigned to α-type NiO species, which is attributed to free nickel oxides species possessing a weak interaction with alumina support.46,50 The mild-temperature peaks (580−740 °C) represent β-type NiO species, which has a stronger interaction with alumina than the α-type NiO,32,51 and can be further classified into β1-type and β2-type. The former is located at 580−637 °C, attributed to the more reducible NiO in Ni-rich mixed oxide phase, while the latter is at 697−740 °C, ascribed to the less reducible one in Al-rich phase. The hightemperature peaks (790−840 °C) are assigned to γ-type NiO species, which is stable nickel aluminate phase with the spinel structure.32,46,50,51 The TPR profile of 20N2M/S1-500 is similar to that of 20N2M/S4-500, both of which show a broad and superimposed H2 consumption peak, which can be deconvoluted using Gaussian-type functions (e.g., 20N2M/S4-500 showed in Figure2b). The TPR quantitative results are listed in Table 2. It is revealed that β1- and β2-type NiO are the dominant species in 20N2M/S1-500 and 20N2M/S4-500. However, a weaker peak also appears at about 870 °C, indicating the formation of NiAl2O4 spinel structure, as observed in their XRD patterns (Figure 2a). This is because Ni species in the lattice of nickel aluminate are normally reduced at approximately 800 °C.30 On the other hand, β1-type NiO is the major species in 20N2M/S2-500 and α-type NiO is in 20N2M/S3-500. These results may hence imply the stronger interaction of NiO particles with S1 and S4 supports compared with S2 and S3, consistent with the obtained XRD patterns (Figure 2a).30 Figure 3 shows the catalytic properties of CO methanation on different Ni/Al2O3 catalysts at 0.1 MPa. 20N2M/S1-500 and 20N2M/S4-500 catalysts show a comparable CO conversion (Figure 3a), CH4 selectivity (Figure 3b), and yield (above 80%) (Figure 3c), which are much higher than those of 20N2M/S2500 and 20N2M/S3-500 catalysts. With the increase of GHSV from 30 000 to 60 000, CO conversion remains the same for the former samples (Figure 3d), but they exhibit slightly lower CH4 selectivity (Figure 3e) and yield (Figure 3f) compared to the latter at the same methanation temperature. The difference in their catalytic properties may be related to the nature of

Figure 3. CO methanation properties on different Ni/Al2O3 catalysts with different GHSVs at 0.1 MPa: (a, d) CO conversion, (b, e) CH4 selectivity, and (c, f) CH4 yield.

supports, such as large surface area that is resulting in the small particle size and high dispersion of active phase, as well as strong metal−support interaction that leads to the formation of inactive spinel phases in the extreme case.39,52−54 20N2M/S4500 catalyst perform the best catalytic properties possibly because of its high surface area (Table 1), small NiO particle size (Figure 2a), and relatively strong NiO-Al2O3 interaction (Figure 2b). Therefore, S4 support was selected for further study. 3.3. Effect of NiO Loading. Figure 4a shows XRD patterns of xN2M/S4-500 catalysts with different NiO contents (where, 4878



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Figure 4. XRD patterns (a) and H2-TPR curves (b) of Ni catalysts with different NiO loadings.

x = 5, 10, 20, 40). It is found that there is no obvious NiO diffraction peaks observed for low NiO loading (5 and 10 wt %), indicating the formation of extremely small NiO nanoparticles of free NiO species on the catalyst or crystalline nickel oxides.46 When NiO loading is increased to 20 and 40 wt %, diffraction peaks at 37.2°, 43.3°, and 63.2° appear, and their intensity increases with NiO loading, indicating the formation of larger NiO nanoparticles (4.9 and 16.7 nm, respectively). As shown in Table 1, the surface area decreases from 252 to 150 m2/g with the increase of NiO content from 5 to 40 wt %. Figure 4b shows the H2-TPR profiles of these catalysts, and they exhibit broad and overlapped H2 consumption peaks. The quantitative results of the TPR profiles are given in Table 2. With the increase of NiO loading, the reduction peak shifts to lower temperature with increasing α- and β1-type NiO species. It therefore suggests that low NiO content encourages the formation of strong NiO-Al2O3 interaction.32,49,55 The midtemperature band representing β-type NiO species has a stronger interaction with the support than α-type NiO, and it is often attributed to the NiO precursor in a fixed form, i.e., Ni− Al mixed oxide.49 Zhang et al. also reported that β1-type NiO species with Ni-rich mixed oxide phase are more reducible than β2-type NiO species with Al-rich mixed oxide phase.49 Therefore, NiO loading of 20 wt % is able to produce high density of reducible NiO on the surface with intermediate interaction with the support due to its high content of β1-type NiO species which eventually generate relatively small-sized and highly active Ni0 particles after activation. Figure 5 shows CO methanation performance of all xN2M/ S4-500 catalysts (x = 5, 10, 20, 40) at 0.1 MPa. Both 20N2M/ S4-500 and 40N2M/S4-500 catalysts show a comparable CO conversion (Figure 5a), CH4 selectivity (Figure 5b), and yield (Figure 5c), which are much higher than those of 5N2M/S4500 and 10N2M/S4-500. As suggested by the TPR results, low NiO content exhibits stronger NiO−Al2O3 interaction that leads to a small fraction of reducible NiO at 600 °C and thus poor performance for 5N2M/S4-500 and 10N2M/S4-500. In contrast, higher NiO content increases the fraction of reducible

Figure 5. CO methanation properties on Ni catalysts with different NiO loadings: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

NiO that results in higher activity for 20N2M/S4-500. However, further NiO loading (40N2M/S4-500) can give rise to high proportion of α-type NiO, which is easily segregated and forms large NiO particles (16.7 nm in Table1), slightly contributing to the activity. In addition, the GHSV used here may not be high enough to distinguish the difference in activity between 20N2M/S4-500 and 40N2M/S4-500, and further study is needed. Herein, 20 wt % NiO loading was selected for the following investigation. 3.4. Effect of MgO Loading. Figure 6a shows XRD patterns of the Ni catalysts with different MgO loadings. As the MgO content increases, initial NiO diffraction peaks at 37.4°, 43.5°, and 63.2° shift to 37.1°, 43.1°, 62.6°, as observed for 20N8M/S4-500, indicating the formation of a solid solution of NiO and MgO (MgNiO2, JCPDS 24-0712). Additionally, the Al2O3 peak at 45.7o (JCPDS 01−1303) shifts to 45.1°, suggesting the formation of stoichiometric and nonstoichiometric NiMg(Al)O spinel phase in fixed form.56,57 These results imply that the interaction among NiO, neighboring cations, and the support could be enhanced by the introduction of MgO,56 which also can be observed by their H2-TPR results as presented in Figure 6b. It is observed that the β-type NiO is the main phase in all cases, and increasing MgO content leads to the increase of β2-type at the expense of α- and β1-type, thus decreasing the reducibility of Ni particles due to the formation of NiMg(Al)O mixed oxide at the NiO-Al2O3 interface and a 4879



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(Figure 7b) are in average smaller than those in 20N8M/S4-500 (Figure 7e), consistent with the XRD results (Figure 6a). Figure 8 shows CO methanation properties of the Ni catalysts with different MgO loadings. 20N0M/S4-500,

Figure 6. XRD patterns (a) and H2-TPR curves (b) of Ni catalysts with different MgO loadings.

higher interaction between nickel atoms and the neighboring cations,56 consistent with the report by Romero et al.57 Therefore, further addition of MgO does not cause a large change in NiO particle size but the formation of MgNiO2 that substantially alters the reducibility of NiO, especially at high MgO content. TEM images of 20N2M/S4-500 and 20N8M/S4-500 catalysts are presented in Figure 7, and they show that NiO particles

Figure 8. CO methanation properties on catalysts with different Mg loadings: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

20N2M/S4-500, and 20N4M/S4-500 catalysts exhibit a comparable CO conversion (Figure 8a), CH4 selectivity (Figure 8b), and yield (Figure 8c), which are much higher than those of 20N8M/S4-500. This is because high MgO content leads to the formation of MgNiO2 (Figure 6a), which lowers the reducibility of NiO at 600 °C (Figure 6b). These results are consistent with previous reports that adding a small amount of MgO into NiO/Al2O3 could improve the activity and stability.34,35 3.5. Effect of Calcination Temperature. Figure 9a shows TG curve of 20N2M/S4 in air before calcination. It is found that the decomposition of Ni(NO3)2 and Mg(NO3)2 in 20N2M/S4 is completed at 400 °C. Figure 9b shows XRD patterns of 20N2M/S4-t catalysts at different calcination temperatures (t = 400, 500, and 600 °C). It can be seen that a diffraction peak at 45.1° is observed when the calcination temperature was increased from 400 to 600 °C, suggesting the formation of NiAl2O4 spinel structure at 500 °C.57 It was reported that NiO could interact with Al2O3 at relatively low temperatures (400−450 °C) to form NiAl2O4 spinel oxide, and the amount of aluminate formed increases with the increase of

Figure 7. TEM images of 20N2M/S4-500 (a, b) and 20N8M/S4-500 (c, d, and e).

(Figure 7a) are more uniformly dispersed in 20N2M/S4-500 than in 20N8M/S4-500 (Figure 7c). Figure 7b shows a high resolution image of 20N2M/S4-500 catalyst with NiO lattice spacing of about 0.209 nm, in a good agreement with (200) plane of NiO (JCPDS 73-1523). Figure 7d,e shows the high resolution images of 20N8M/S4-500 with lattice spacing of 0.241 and 0.209 nm, which are assigned to (111) plane of MgNiO2 and (200) plane of NiO, respectively. In addition, it is also noted that NiO particles in 20N2M/S4-500 catalyst 4880



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Figure 9. TG curve of 20N2M/S4 in air before calcination (a), XRD patterns (b), and H2-TPR curves (c) of Ni catalysts calcined at different temperatures.

Figure 10. CO methanation properties on Ni catalysts calcined at different temperatures: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

the calcination temperature and time.58,59 The NiO particles size is observed to increase from 3.7 to 4.9 nm as the calcination temperature was increased from 400 to 500 °C, respectively. Inversely, the surface area of catalysts decreases with the calcination temperature. At 600 °C, the NiO particles size decreases to 4.2 nm, which could be due to the formation of NiAl2O4. Figure 9c displays the H2-TPR curves of these catalysts, and it is found that the reduction peak shifts to higher temperature with the increasing calcination temperature. The TPR quantitative results in Table 2 reveals the increasing fraction of β2-type NiO along with the diminishing α-and β1type NiO when the calcination temperature is raised, suggesting the stronger interaction between Ni atoms and the neighboring cations at the high calcination temperature.56,57 Therefore, it is crucial to choose an appropriate calcination temperature, at which the NiO particles interact with the support with an optimum strength. After calcination, they can be reduced at midtemperature to form highly dispersed and active Ni0 particles. Figure 10 shows the catalytic performance of these catalysts at 0.1 MPa. It is observed that catalysts obtained at different calcination temperatures show a comparable CO conversion at the methanation temperature above 350 °C (Figure 10a). However, at 300 °C, 20N2M/S4-600 exhibits much lower CO conversion (around 10%) compared to that of 20N2M/S4-400 and 20N2M/S4-500 (near 100%). The latter catalysts also exhibit slightly higher CH4 selectivity in Figure 10b, resulting in

higher CH4 yield in Figure 10c and better catalytic activity especially in the methanation temperature range of 300−400 °C. This may be due to the formation of less reducible β2-type NiO species (Figure 9b). The obtained results therefore indicate that low calcination temperature and moderate methanation temperature would favor the production of SNG. 3.6. Effect of Reaction Condition. Using the optimum catalyst (20N2M/S4-400), different reaction parameters, such as GHSV, H2/CO ratio, and the reaction pressure are examined. Figure 11 shows the catalytic property of 20N2M/ S4-400 catalyst under different GHSVs at 0.1 MPa. It is observed that the CO conversion (Figure 11a) is independent of GHSVs, but CH4 selectivity (Figure 11b) and yield (Figure 11c) decrease gradually as the GHSV increases, for example, 92% of CH4 yield at 400 °C under 10 000 to 84% under 60 000. These results reveal that low GHSV would be beneficial for the formation of CH4. However, it is not favorable for industrial application considering the need for large production capacity. In addition, the carbon deposition was investigated by sulfur− carbon analysis and it is found to be 0.56, 0.53, and 0.56 wt % at GHSV of 10 000, 30 000, and 60 000, respectively, implying that space velocity has no significant effect in the short term testing (5 h). Therefore, a GHSV of 30 000 was selected for the subsequent investigation. 4881



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Figure 12. CO methanation properties on 20Ni2M/S4-400 catalyst with different H2/CO ratios: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

temperature, which could be due to the occurrence of water gas shift reaction (reaction 4: CO + H2O = CO2 + H2) and methane cracking reaction (reaction 5: CH4 = C + 2H2). Consequently, high carbon deposition (25.2 wt %) was observed at H2/CO ratio of 1:1, compared to that of 3:1 (0.57 wt %) and 4:1 (0.66 wt %). It was reported that increasing H2/CO ratio could lower the rate of carbon deposition40 and deactivation of the catalyst.63 Thus, to obtain a relative high yield of CH4 and high conversion of H2, the optimum H2/CO ratio should be more than 3:1. Figure 13 shows the effect of pressure on CO methanation reaction. At 3.0 MPa, the CO conversion can reach 100% at 300−550 °C (Figure 13a), while the CH4 selectivity (Figure 13b) and yield (Figure 13c) are more than 95%. As expected, high pressure is beneficial to the catalytic performance since CO methanation is an volume reducing reaction. 3.7. Life Time Test. Catalyst lifetime is a key consideration in the economical production of SNG from coal-derived gases.64 As we know, deposited carbon is often an intermediate product during the methanation reaction that leads to catalyst deactivation.27,64−69 Moreover, sintering of Ni particles at high methanation temperature would also deteriorate the activity of Ni catalysts. Figure 14 exhibits the catalytic performance of 20N2M/S4-400 and 20N0M/S4-400 at 400 °C and 0.1 MPa within 196 h. In Figure 14a, without mixing with quartz sand in reactor, the CO conversion declines much quickly for 20N0M/ S4-400 than that for 20N2M/S4-400, especially after 100 h. Although the CH4 selectivity (73%) for both catalysts remains relatively unchanged even after 196 h (Figure 14b), 20N2M/ S4-400 produces higher CH4 yield than 20N0M/S4-400 after 100 h. These results further demonstrate the important role of MgO added with a proper amount. On the contrary, 20N2M/ S4-400 catalyst mixed with quartz sand as the dilution media (0.5 g catalyst: 2.5 g quartz sand) results in significantly higher CO conversion (∼97%), CH4 selectivity (∼87%), and yield (∼85%). Moreover, they can be maintained constant for 196 h. More importantly, compared to 20N2M/S4-400 without mixing

Figure 11. CO methanation properties on 20Ni2M/S4-400 catalyst under different GHSV: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

Since syngas derived from coal or biomass has a variable ratio of H2/CO,60 it is necessary to know the effect of this ratio on the methanation process. Tøttrup61 studied the influence of small H2/CO ratio (0.05) on carbon deposition, but it is far from the actual methanation conditions. Figure 12 shows the effect of H2/CO molar ratio on the catalytic activity at 0.1 MPa. It is found that CO conversion (Figure 12a), CH4 selectivity (Figure 12b), and yield (Figure 12c) increase with the H2/CO ratio increase. To obtain a relatively high CH4 yield, H2/CO ratio should not be lower than 3:1, as used in TREMPTM technology of Haldor Topsoe.62 It is worth noting that CH4 selectivity reaches only to about 90% although CO is completely converted at 3:1 of the H2/CO ratio, which is due to the formation of byproduct, such as deposited carbon and CO2 via side Boudouard reaction (reaction 2: 2CO = C + CO2) and carbon monoxide reduction (reaction 3: CO + H2 = C + H2O). Interestingly, the CO conversion and selectivity of CH4 can reach almost 100% at H2/CO ratio of 4:1 in the methanation temperature range of 300−400 °C, which indicates that rich H2 environment not only promotes the CO methanation but also suppresses the occurrence of side reactions. Figure 12d shows the H2 conversion at different H2/ CO ratios. It can be seen that small H2/CO ratio leads to high H2 conversion. In addition, there is always unreacted H2 detected in the outlet gas, especially at the high reaction 4882



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Figure 14. CO methanation properties on 20N2M/S4-400 and 20N0M/S4-400 changing with time: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

Figure 13. CO methanation properties on 20N2M/S4-400 catalyst under different pressures: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.

with quartz sand to mixing with quartz sand, a large gap (∼15− 20%) in CO conversion (Figure 14a), CH4 selectivity (Figure 14b), and yield (Figure 14c) are obviously observed. Without mixing with quartz sand, the temperature within the catalyst bed is normally much higher than the controlled temperature (400 °C), e.g., 500 °C, which worsened the catalyst performance (see Figure 8). This is because the CO methanation is highly exothermic, and the presence of quartz sand helps dissipate the hot spot in the catalyst bed, which normally leads to the more carbon deposition and sintering of Ni particles.29,30,70 Figure 15a shows XRD patterns of both reduced 20N2M/S4400 and 20N0M/S4-400 catalysts before and after a lifetime test with and without mixing with quartz sand. For the freshly reduced 20N2M/S4-400 and 20N0M/S4-400 catalysts, obvious diffraction peaks for Ni species were observed, indicating that NiO is reduced to Ni particles. However, after a 196 h reaction, the intensity of Ni diffraction peaks decrease, and the new peaks at about 26.3° and 50.8° assigned to (002) and (102) planes of carbon (JCPDS 02-0456), respectively, appear in the XRD pattern of 20N0M/S4-400 without quartz sand, whereas they were absent for 20N2M/S4-400 with and without mixing with quartz sand after the same period of reaction, implying that MgO plays an important role in inhibiting the formation of carbon deposition. The amount of deposited carbon can be

Figure 15. (a) XRD patterns (a) and TG curves (b) of fresh and used catalysts. 4883



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analysis shows that the amount of carbon deposited on the 20N0M/S4-400 catalyst surface is 9.2 wt %, much more than that on the 20N2M/S4-400 catalyst (2.3 wt %), which is in a good agreement with the TG results (Figure 15b). To a great extent, the deactivation of methanation catalyst can be due to the formation of carbon deposition which can cause: (1) fouling of the metal surface, (2) blockage of catalysts pores and voids, and/or (3) detachment of active Ni particles from the support.67 This work confirms that the former two factors are the main reasons for deactivation of the Ni catalysts. In fact, both phenomena of metal sintering and carbon deposition often occur simultaneously.29 To conclude, the introduction of MgO can improve the Ni/Al2O3 catalysts by strengthening its stability and suppressing the formation of carbon deposits. Additionally, adding quartz sand into the reactor also significantly promotes catalytic activity because it can dilute the catalytic sites and facilitates heat dissipation.71

approximately determined by TG analysis, as presented in Figure 15b. The carbon content in 20N0M/S4-400 catalyst without quartz sand was estimated to be 9.4 wt %, while it was 2.4 wt % for 20N2M/S4-400 catalyst without quartz sand and only 1.5 wt % for 20N2M/S4-400 catalyst mixed with quartz sand. The Ni crystallite size in 20N2M/S4-400 mixed with quartz sand was almost unchanged (5.8 and 5.6 nm), while that of 20N2M/S4-400 without quartz sand increased slightly from 5.8 to 6.3 nm after a 196 h reaction, suggesting the important role of quartz sand in inhibiting Ni particles sintering due to the quick heat removal in the catalyst bed and thus increasing the catalyst stability. After the reaction, the surface area of 20N2M/ S4-400 changed from 163 to 123 m2/g, while that of 20N0M/ S4-400 increased from 154 to 176 m2/g due to the carbon deposition (weight increased 9.2 wt %). Thus, these results further demonstrate that addition of MgO in catalysts and quartz sand in reactor plays an important role in inhibiting the formation of carbon deposition. The morphology of 20N2M/S4-400 and 20N0M/S4-400 catalysts after a lifetime test with and without mixing with quartz sand is presented in Figure 16. It is observed that there is

4. CONCLUSIONS In summary, we have comprehensively investigated the CO methanation on a series of Ni/Al2O3 catalysts synthesized by the impregnation method for the production of SNG. It is found that the physicochemical properties of NiO species in the catalyst are strongly dependent on the nature of the support, the loading of NiO, the calcination temperature, and the existence of MgO additive. In general, a large surface area of Al2O3 support and a relatively strong interaction between NiO and Al2O3 promote the formation of small NiO particles and more reducible β-type NiO species, which result in high catalytic activity, strong resistance to the carbon deposition, and good thermal stability. Particularly, a moderate NiO loading (20 wt %) and a relatively low calcination temperature (400 °C) can produce a high density of reducible NiO on the S4 Al2O3 support. In this catalyst, after the activation, relatively small-sized and highly active Ni0 particles are generated. Moreover, the addition of MgO (2 wt %) is proved to be an efficient measure to enhance the resistance to the carbon deposition, thus significantly increasing the stability of the Ni catalyst. Other conditions proved to enhance CO conversion and CH4 selectivity include running the reaction at a relatively low GHSV of 30 000 mL/g·h, at a high pressure of 3.0 MPa, and a molar ratio of H2/CO more than 3:1. In addition, dilution of catalyst with quartz sand is favorable for CO methanation. The lifetime test shows that the synthesized 20 wt % NiO-2 wt % MgO/S4 catalyst calcined at 400 °C is highly active, thermally stable, and resistant to carbon deposition due to its moderate interaction between NiO and Al2O3 support and high density of reducible NiO. This work would therefore be important for developing highly efficient Ni catalyst and methanation process for SNG production and provides scientific understanding to the CO methanation reaction.

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Figure 16. SEM images of the used catalysts: (a) 20N2M/S4-400 mixed with quartz sand, (b) 20N2M/S4-400 without quartz sand, and (c) 20N0M/S4-400 without quartz sand.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-82544850. Fax: +86-10-82544851. E-mail: [email protected] (F.G.); [email protected] (F.S.). Notes

no visible deposited carbon on the surface of the 20N2M/S4400 catalyst mixed with quartz sand (Figure 16a), while rod-like carbon particles are observed on the catalyst without quartz sand (Figure 16b). Similarly, a large number of deposited carbon fibers are found on 20N0M/S4-400 catalyst without quartz sand (Figure 16c). Quantitatively, carbon−sulfur

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Hundred Talents Program of the Chinese Academy of 4884



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Sciences (CAS), National Key Technology R&D Program of China (No. 2010BAC66B01), Knowledge Innovation Program of the CAS (No. KGCX2-YW-396), State Key Laboratory of Multiphase Complex Systems of China (No. MPCS-2009-C01), K.C. Wang Postdoctoral Fellowships of the CAS, and China Postdoctoral Science Foundation (No. 20100480026 and 201104151).

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