English

• 0.   Introduction

  With the rapid development of China's economy, natural gas is becoming an increasingly important clean fuel source domestically^[1]. As a high-quality secondary energy source, coke oven gas is rich in H₂ (54%-59%), CH₄ (23%-28%), CO (5%-8%), CO₂ (2%-4%), and another hydrocarbon C_nH_m (2%-4%)^[2], which can be synthesized into natural gas by methanation reaction of CO, CO₂, and H₂ under the action of nickel-based catalyst^[3-6]. This process has become an important supplement to the natural gas market gap in China. Trace sulfides in coke oven gas can cause permanent catalyst deactivation^[7-8]. Therefore, improving the sulfur resistance of catalysts for the coke oven gas methanation process is a key issue that needs to be addressed urgently.

  In order to relieve the sulfur poisoning of the catalyst, optimization can focus on two areas: firstly, enhancing the technology of fine desulfurization of coke oven gas with the accuracy of desulfurization from 0.14 mg·m^-3 to below 0.029 mg·m^-3; secondly, carrying out the development of sulfur-resistant methanation catalysts. Compared to desulfurization, sulfur-resistant catalyst development shortens the reaction process and lowers energy consumption. The traditional sulfur-resistant methanation catalysts are mainly Mo-based catalysts, their drawbacks are that Mo is extremely easy to sublimate and the one-way conversion of CO is significantly lower than that of Ni-based catalysts^[9], which leads to their poor economics in industrial applications. Therefore, the current practical research direction is to enhance the sulfur resistance of Ni-based catalysts.

  The addition of promoters to Ni-based catalysts is considered a plausible alternative to prevent physicochemical deactivation. It is well-reported that the presence of Mo can enhance the sulfur resistance of high-performance Ni-based catalyst^[10-17]. Fowler et al.^[18] discovered that Mo is not an effective catalyst for methanation, but the addition of Mo improves the capacity of catalysts to adsorb H₂S. Moreover, Ni-Mo catalysts show the same or slightly higher methanation activity and significantly improved sulfur resistance compared to nickel catalysts. H₂S desorption and in situ deactivation measurements confirmed the strong and irreversible nature of sulfur-metal interactions leading to relatively rapid and severe methanation catalyst poisoning. Wang et al.^[10] examined the performance of catalysts with various Ni-Mo ratios that utilized MCM-41 as support. Their findings indicate that the sulfur resistance stability of modified catalysts increases gradually with an increase in Mo content. Huo et al.^[12] found that the Ni species in the newly generated Ni₃Mo₃N phase of the Ni-Mo catalyst have strong CO dissociation adsorption ability and high CH₄ selectivity, which can alleviate the activity decrease brought by sulfur poisoning. Niu et al.^[13] identified the MoNi₄ alloy generated in Mo-Ni/γ-Al₂O₃ methanation catalysts as the main sulfur-resistant active phase, which is responsible for the improved sulfur resistance. Méndez-Mateos et al.^[14] showed that the introduction of Mo in Ni-based catalysts inhibits the active sites of nickel at higher concentrations, whereas the H₂S uptake capacity and the sulfur resistance of the catalyst were improved. Aksoylu et al.^[15-17] studied the interaction between active sites of Ni and MoO_x in Ni-Mo/Al₂O₃ catalyst leading to the electron transfer from MoO_x species to Ni particles. During the methanation process, the increase of the electronegativity of Ni atom surface to effectively hinder the adsorption of S on the Ni surface. According to Zhang et al.^[19], the introduction of Mo in Ni-based catalysts not only weakens the binding of sulfur to the surface metal center but also increases the barrier to sulfur diffusion. Zhi et al.^[20] proved that the strong interaction between Mo and O that weakens the C—O bond and promotes its breaking, and thereby leads to a highly productivity to CH₄. Besides, the strong S3p-Mo4d state weakens the hybridization of S3p-Ni3d, and thereby results in an enhanced S resistance of Ni-Mo/Al₂O₃ catalyst. Previous research has shown that incorporating Mo promoters into Ni-based methanation catalysts can enhance their performance in terms of activity, stability, and sulfur resistance. However, these studies have primarily focused on low nickel loading methanation catalysts prepared by co-impregnation and stepwise impregnation^[21-22], and the performance of the prepared catalysts is far from industrial high nickel loading methanation catalysts^[23]. Therefore, further research on the introduction of Mo promoters on high nickel loading catalyst applied to the coke oven gas methanation is necessary. In this study, MoO₃ was impregnated into high Ni-loading methanation catalysts prepared through double hydrolysis co-precipitation, the effect of MoO₃ on the low-temperature activity and sulfur resistance stability of Ni-based catalysts was investigated by adjusting the MoO₃ loading. Through the characterization results, the mechanism of the MoO₃ effect on Ni-based catalysts was also well discussed.

  1.   Experimental

  1.1   Materials

  Nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, AR), sodium aluminate (NaAlO₂, AR), and ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O, AR) were purchased from Sinopharm Chemical Reagent Co., carbon dioxide (CO₂, 99.99%), carbon monoxide (CO, 99.999%), helium (He, 99.999%), argon (Ar, 99.999%), high purity oxygen (99.999%), and H₂/Ar (10%, V/V) were purchased from Shanxi Yihong Gas Industry Co. Ltd. Deionized water for the experiment was made in the laboratory. A commercial γ-Al₂O₃ powder purchased from BEIGE industry and trade Co. Ltd.

  1.2   Catalyst preparation

  According to our previous work^[24], the catalyst was prepared by double hydrolysis precipitation method (Ni²⁺+Al(OH)₄^-+OH^- → Ni(OH)₂↓+Al(OH)₃↓). Specific steps: the aqueous solutions of Ni(NO₃)₂·6H₂O and NaAlO₂ were dissolved into 100 mL of deionized water separately under vigorous stirring. Ni(NO₃)₂·6H₂O solution was added evenly dropwise to NaAlO₂ solution using a peristaltic pump at 60 ℃ and stirred evenly for 1 h. After aging at 60 ℃ for 12 h, the precipitate was filtered, washed, dried at 120 ℃ for 12 h, and calcined at 450 ℃ for 2 h with a heating rate of 5 ℃·min^-1. The obtained catalyst was denoted as NiO-Al₂O₃ with a NiO mass fraction of 40%.

  The modified catalysts were prepared by the incipient-wetness impregnation method. A certain amount of (NH₄)₈Mo₇O₂₄·4H₂O was dissolved in deionized water, and then the NiO-Al₂O₃ catalyst was added into (NH₄)₆Mo₇O₂₄·4H₂O solution. After this, the samples were dried and calcined under the same conditions. The prepared catalysts were denoted as xMoO₃/NiO-Al₂O₃, where x% represented the mass fraction of MoO₃ (x=5, 7.5, 10, 12.5, 15, 20). For comparison, a catalyst without MoO₃ was also prepared by the same method. The γ-Al₂O₃ support was immerged into an aqueous solution of (NH₄)₆Mo₇O₂₄·4H₂O, and then dried at 120 ℃ for 12 h and calcined at 450 ℃ for 2 h with a heating rating of 5 ℃·min^-1 (denoted as 15MoO₃/Al₂O₃).

  1.3   Catalyst characterization

  The specific surface area, pore volume, and pore size distribution of the catalysts were determined by a V-Sorb4800P adsorption apparatus of Beijing Gold Essence Technology Co. The samples were degassed under vacuum at 300 ℃ for 4 h before measurement, and the N₂ adsorption-desorption isotherm was tested at 77.4 K. The crystal structure of the catalyst was measured on a Rigaku MiniFlex 600 X-ray diffractometer equipped with Cu Kα radiation (λ=0.154 056 nm) with 2θ=10°-90° and a scanning speed of 10 (°)·min^-1. Hydrogen temperature-programmed reduction (H₂-TPR), H₂ pulse adsorption, and CO₂ pulse adsorption of the catalyst samples were measured on a FINSORB-3010 chemisorption instrument of Zhejiang Fantai Instruments Co. For H₂-TPR, 0.1 g of catalyst sample was placed in a U-shaped quartz sample tube and pre-treated with Ar atmosphere at 100 ℃ for 30 min and then cooled down to room temperature. The catalyst was heated to 900 ℃ at a rate of 10 ℃·min^-1 with a flow of 10% H₂/Ar. For H₂ pulse adsorption, 0.2 g of catalyst was pretreated at 100 ℃ for 30 min under an Ar atmosphere and then heated to 550 ℃ at a heating rate of 10 ℃·min^-1. Subsequently, the catalyst was reduced in a 10% H₂/Ar mixture at 550 ℃ for 60 min and then cooled to room temperature. Finally, pure H₂ was pulsed for 30 min to achieve saturated adsorption. The H₂ capacity and nickel surface area were calculated by the equation in the previous work^[25]. For CO₂ pulse adsorption, 0.2 g catalyst was pre-treated for 60 min at 200 ℃ under He atmosphere to remove water and other impurities from the sample surface. Then, the catalyst was cooled to room temperature, and pulsed with pure CO₂ until saturation. The sulfur content of the catalyst was measured by HCS-800 high-frequency infrared carbon and sulfur analyzer of Sichuan Sainty Instruments Co. Ltd. 0.5 g of iron particles, 50 mg of sample, and 1.5 g of tungsten particles were added into the treated crucible, and the crucible was placed on the high-temperature quartz crucible rack of the high-frequency infrared carbon and sulfur analyzer. The data display area would show the release curve of sulfur in the sample and the result of sulfur determination^[26-27]. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Fisher Scientific ESCALAB 250Xi X-ray photoelectron spectrometer, using a monochromatized Al Kα radiation with energy step of 0.05 eV under ultrahigh vacuum (8×l0^-10 Pa). Transmission electron microscopy (TEM) was taken on JEM-2100F at an acceleration voltage of 200 keV.

  1.4   Catalyst evaluation

  0.5 g (40-60 mesh) catalyst pellets were filled in the reaction tube, which consisted of a quartz tube inside and a stainless steel tube outside. The catalyst was reduced by passing 20 mL·min^-1 reducing gas H₂ for 60 min at T=550 ℃ and p=0.1 MPa before the reaction. The catalytic experiments were performed at H₂/CO (3∶1, V/V), p=0.1 MPa, and WHSV (weight hourly space velocity)=20 000 mL·g^-1·h^-1. The low-temperature activity of the catalysts was evaluated at 200-360 ℃ at 20 ℃ intervals, and the sulfur resistance of the catalysts was tested at 550 ℃ by introducing 143 mg·m^-3 H₂S/H₂ mixture at the reactor inlet to simulate the composition of coke oven gas. The outlet gas was separated from gas and liquid, and then the gas entered into a Shanghai Haixin GC-950-SD gas chromatograph equipped with a Thermal conductivity detector (TCD) for composition and content analysis. The CO conversion (X_CO) and CH₄ yield (Y_CH4) were calculated by the following equation:

  $ X_{\mathrm{CO} }=\frac{N_{\text {in }} y_{\mathrm{CO, } \text { in }}-N_{\text {out }} y_{\mathrm{CO}, \text { out }}}{N_{\text {in }} y_{\mathrm{CO} \text {, in }}} \times 100 \% $

  (1)

  $ Y_{\mathrm{CH}_4}=\frac{N_{\text {out }} y_{\mathrm{CH}_4, \text { out }}}{N_{\text {in }} y_{\mathrm{CO, } \text { in }}-N_{\text {out }} y_{\mathrm{CO}, \text { out }}} \times 100 \% $

  (2)

  where N_in or N_out denotes the flow rate (mL·h^-1) at the inlet or outlet and y_in or y_out represents the molar fraction of CO or CH₄ at the inlet or outlet.

  2.   Results and discussion

  2.1   Low-temperature methanation activity

  Fig. 1 shows the results of low-temperature activity evaluation of the xMoO₃/NiO-Al₂O₃ catalysts. The CO conversion and CH₄ yield of the NiO-Al₂O₃ catalysts were above 98.9% and 97.2% at a low temperature of 220 ℃, respectively. As the MoO₃ loading increased, the CO conversion and CH₄ yield of the xMoO₃/NiO-Al₂O₃ catalysts gradually decreased. The CO conversions of the xMoO₃/NiO-Al₂O₃ catalysts reached above 99% only when the reaction temperature reached 260 ℃, and the CH₄ yield decreased to about 93%. The higher the MoO₃ loading, the faster the CH₄ yield decreased. When the MoO₃ loading was greater than 12.5%, the CH₄ yield decreased to about 90%. The reason for the decrease in activity is the formation of the MoO₃ crystalline phase by agglomeration of MoO₃ on the catalyst surface, which covers more nickel active sites. The conclusion of the above discussion is consistent with the results of Zhang et al.^[28], where the introduction of Mo promotes the dispersion of the active component Ni when the MoO₃ loading is lower than 3%, but when the MoO₃ loading is higher than 3%, the catalytic activity of the catalyst decreases with the increase of MoO₃ loading.

  Figure 1

  

  Figure 1.  Low-temperature activity evaluation of the catalysts: CO conversion (a) and CH₄ yield (b)

  Reaction conditions: V_H2/V_CO=3∶1, T=200-360 ℃, p=0.1 MPa, WHSV=20 000 mL·g^-1·h^-1

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  2.2   Catalysts sulfur resistance evaluation

  Fig. 2 shows the evaluation results of the sulfur resistance of the xMoO₃/NiO-Al₂O₃ catalysts. It can be seen that MoO₃ addition improved the sulfur resistance of the catalysts, and the sulfur resistance of the xMoO₃/NiO-Al₂O₃ catalysts increased and then decreased with the increase of MoO₃ loading. When the reaction time reached 4.5 h, the activity of NiO-Al₂O₃ and 20MoO₃/NiO-Al₂O₃ catalysts decreased significantly. The CO conversion and CH₄ yield of the 12.5MoO₃/NiO-Al₂O₃ catalyst can afford 81.1% and 63.4% respectively at a reaction time of 6.5 h, while the catalysts with MoO₃ loadings of 5%, 7.5%, 10%, 15%, and 20% were all deactivated. These results indicated that the optimum loading of MoO₃ was 12.5% for the NiO-Al₂O₃ catalyst with the best resistance to sulfur poisoning. Table 1 shows the comparison of the catalytic activity of 12.5MoO₃/NiO-Al₂O₃ and the related catalyst reported in the literature. It can be seen that the NiO-Al₂O₃ loaded with 12.5% MoO₃ had the best CO conversion and CH₄ selectivity. Therefore, the 12.5MoO₃/NiO-Al₂O₃ catalyst was selected as the optimal catalyst by combining both activity and sulfur resistance performance.

  Figure 2

  

  Figure 2.  Evaluation of sulfur resistance of the catalysts: CO conversion (a) and CH₄ yield (b)

  Reaction condition: V_H2/V_CO=3∶1, T=550 ℃, p=0.1 MPa, WHSV=20 000 mL·g^-1·h^-1

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  Table 1

  

  Table 1.  Summary of catalytic performance of CO methanation catalysts

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  Catalyst	Reaction atmosphere	p/MPa	T/℃	WHSV/(mL·g-1·h-1)	XCO/%	SCH4/%	Ref.
  12.5MoO3/NiO-Al2O3	H2/CO (3∶1, V/V)	0.1	260	20 000	99.8	91.9	This work
  0.5MoO310NiO/KIT-6	H2/CO/N2 (3∶1∶1, V/V)	0.1	350	15 000	99.8	85.5	[21]
  15NiO1MoO3/MCM-41	H2/CO/N2 (3∶1∶1, V/V)	0.1	340	60 000	100.0	87.3	[29]
  15NiO-1MoO3/OMA	H2/CO/N2 (3∶1∶1, V/V)	0.1	375	60 000	97.9	93.8	[30]
  1MoO3-10NiO/SBA15	H2/CO (3∶1, V/V)	0.1	350	15 000	100.0	94.1	[31]

  2.3   N₂ adsorption-desorption analysis

  As shown in Table 2, the specific surface areas and pore volumes of the catalysts decreased progressively with the increase of MoO₃ loading, whereas the average pore size showed an increasing trend. This trend was attributed to the dispersion of MoO₃ in the form of particles on the surface of the NiO-Al₂O₃ catalyst, leading to the partial blockage of some pore channels^[32-33]. When the MoO₃ loading was below 12.5%, the decline in specific surface area was minimal. However, the specific surface area of the catalyst declined rapidly as the MoO₃ loading exceeded this value. The significant decrease in the specific surface area of the catalyst was caused by the accumulation of excess MoO₃ aggregates on the catalyst surface, which deepens the blockage of the pore structure.

  Table 2

  

  Table 2.  Textural properties of the xMoO₃/NiO-Al₂O₃ catalysts

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  Catalyst	SBETa/(m2·g-1)	VBJHb/(cm3·g-1)	Daverageb/nm	Scatc/(m2·gcat)	DNic/%	VCO3d/(mL·g-1)	Se/%
  NiO-Al2O3	236	0.35	5.8	36.86	4.08	1.89	0.48
  5MoO3/NiO-Al2O3	194	0.33	6.2	7.18	0.68	0.10	0.58
  7.5MoO3/NiO-Al2O3	193	0.31	6.5	5.80	0.50	0.067	0.68
  10MoO3/NiO-Al2O3	188	0.30	7.7	4.50	0.42	0.044	0.69
  12.5MoO3/NiO-Al2O3	187	0.29	7.9	4.24	0.40	3.06×10-4	0.71
  15MoO3/NiO-Al2O3	162	0.28	8.5	2.04	0.19	2.02×10-4	0.64
  20MoO3/NiO-Al2O3	133	0.26	8.7	1.51	0.14	1.81×10-4	0.65
  a BET (Brunauer-Emmett-Teller) specific surface area derived from the BET equation; b Average pore volume obtained from BJH (Barrette-Joyner-Halenda) desorption; c Active nickel surface areas (Scat), nickel metal dispersion (DNi) determined by hydrogen pulse chemisorption of the fresh catalyst; d Volume of CO2 adsorbed per gram determined by carbon dioxide chemisorption; e Sulfur content of catalyst surface after sulfur resistance evaluation.

  2.4   XRD analysis

  To analyze the surface crystalline phase structure of the xMoO₃/NiO-Al₂O₃ catalysts, the catalyst samples were tested for XRD. In Fig. 3, the peaks at 2θ=37.6°, 43.3°, 62.9°, 75.5°, and 79.5° for each sample are characteristic diffraction peaks of NiO (PDF No.89-7130), the peaks at 2θ=19.4°, 37.6°, 45.8°, and 67.1° are attributed to γ-Al₂O₃ (PDF No.10-0173), the peaks at 2θ=23.4°, 25.7°, 29.4°, 34.7°, and 48.0° are MoO₃ characteristic diffraction peaks (PDF No.05-0528), and the peaks at 2θ=26.6° and 32.4° are NiMoO₄ characteristic diffraction peaks (PDF No.12-0348) formed by the interaction of NiO and MoO₃^[34]. Due to the interaction between MoO₃ and NiO to generate the NiMoO₄ phase and MoO₃ covering the NiO on the catalyst surface, the distinctive diffraction peak intensity of NiO gradually decreased as MoO₃ loading increased^[35]. When the loading of MoO₃ was lower than 12.5%, the typical diffraction peaks of MoO₃ and NiMoO₄ were very weak in the catalyst. The distinctive diffraction peaks of MoO₃ and NiMoO₄ may be readily seen when the loading of MoO₃ was more than 12.5%, and their strength steadily rose as the MoO₃ loading increased.

  Figure 3

  

  Figure 3.  XRD patterns of the catalysts

  (a) NiO-Al₂O₃, (b) 5MoO₃/NiO-Al₂O₃, (c) 7.5MoO₃/NiO-Al₂O₃, (d) 10MoO₃/NiO-Al₂O₃, (e) 12.5MoO₃/NiO-Al₂O₃, (f) 15MoO₃/NiO-Al₂O₃, (g) 20MoO₃/NiO-Al₂O₃

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  2.5   H₂-TPR

  Fig. 4 shows the H₂-TPR measurements of the xMoO₃/NiO-Al₂O₃ catalysts. It can be seen that the 15MoO₃/Al₂O₃ catalyst shows two hydrogen consumption peaks, the low-temperature reduction peak at 450 ℃ is attributed to the reduction of Mo⁶⁺ → Mo⁴⁺ species of octahedral structure^[34], and the high-temperature peak above 700 ℃ belongs to the reduction of Mo⁴⁺ → Mo⁰ species of tetrahedral structure^[36]. The reduced NiO species can be divided into two types: β₁-NiO type and β₂-NiO type in the temperature region of 400-700 ℃. The interaction with support of β₁-NiO was weaker than β₂-NiO, so, β₁-NiO is usually identified as the main active site for methanation reaction^[37]. After adding the promoter MoO₃, the reduction peaks of both β₁-NiO and β₂-NiO of the xMoO₃/NiO-Al₂O₃ catalysts was shifted to high temperature. This phenomenon implied that the interaction between the active Ni component and the support is strengthened by the presence of the Mo element. It is worth noting that the reduction temperature of MoO₃ is lower than that of NiO. As a result, when MoO₃ undergoes reduction, the heat released during the process can pre-reduce NiO on the surface. This pre-reduction phenomenon will lead to the forward shift of the reduction peak of NiO. The decrease in both β₁-NiO and β₂-NiO peak area and the increase in Mo⁶⁺ → Mo⁴⁺ and Mo⁴ → Mo⁰ can be observed in Table 3 with the increase of MoO₃ content of xMoO₃/NiO-Al₂O₃ catalyst, which indicates the introduction of Mo covering the NiO sites. The lower content NiO species is unfavorable for the low-temperature reactivity in methanation reaction.

  Figure 4

  

  Figure 4.  H₂-TPR curves of the catalysts

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  Table 3

  

  Table 3.  Gaussian fitting analysis of H₂-TPR profiles of the fresh catalysts with different MoO₃ contents

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  Catalyst	Relative content/%
  	MoO2	β1-NiO	β2-NiO	Mo
  15MoO3/Al2O3	100	—	—	—
  NiO-Al2O3	—	57.49	42.51	—
  5MoO3/NiO-Al2O3	4.91	55.46	39.26	1.25
  7.5MoO3/NiO-Al2O3	6.60	52.41	39.27	1.70
  10MoO3/NiO-Al2O3	8.25	50.05	39.04	2.66
  12.5MoO3/NiO-Al2O3	23.30	49.96	22.36	4.36
  15MoO3/NiO-Al2O3	28.11	32.76	33.37	5.74
  20MoO3/NiO-Al2O3	34.40	30.64	28.58	6.38

  2.6   H₂ and CO₂ pulse chemisorption

  Table 2 shows the Ni dispersion and the nickel active specific surface area of the xMoO₃/NiO-Al₂O₃ catalysts. The active surface area is correlated to the number of surface active sites, which can directly affect the catalytic activity^[38]. As can be seen, with the increase of MoO₃ loading, Ni dispersion and active specific surface area in the catalyst gradually decreased. The results of H₂ pulse adsorption are consistent with the results of low-temperature activity evaluation of the catalysts in Fig. 1, which proves that the active specific surface area is the key influencing factor of the low-temperature activity of the catalysts. CO₂ could only chemisorb on the basic oxide Al₂O₃ and NiO surfaces, so CO₂ adsorption was used to determine the surface MoO₃ dispersion. Table 2 shows that there was more uncovered Al₂O₃ and NiO on the surface of the NiO-Al₂O₃ catalyst. After adding the promoter MoO₃, the CO₂ adsorption decreased gradually with the increase of MoO₃ loading, and when the MoO₃ loading was lower than 12.5%, blank Al₂O₃ and NiO surfaces still exist on the catalyst surface. The CO₂ adsorption capacity of 12MoO₃/NiO-Al₂O₃ catalyst started to appear significantly decline, which is due to the uniform diffusion of the MoO₃ over the whole catalyst surface. Combined with N₂ adsorption and XRD proved that the catalyst was close to the monolayer dispersion threshold when the MoO₃ content was 12.5%.

  2.7   Sulfur content measurement

  The data of sulfur absorption of the xMoO₃/NiO-Al₂O₃ catalysts after the evaluation of sulfur resistance performance are given in Table 2. The 12.5MoO₃/NiO-Al₂O₃ catalyst showed the best performance among the tested catalysts with sulfur adsorption (mass fraction) of 0.71%, and the result is consistent with the evaluation results of sulfur resistance of the xMoO₃/NiO-Al₂O₃ catalyst in Fig. 2. The sulfur absorption capacity of the 12.5MoO₃/NiO-Al₂O₃ catalyst was 1.48 times higher than that of the NiO-Al₂O₃ catalyst. MoO₃ was close to the monolayer dispersion threshold on the NiO-Al₂O₃ catalyst surface at the MoO₃ loading of 12.5%, and the distribution of specific surface area on the catalyst surface was the largest, which could increase the electronegativity of Ni atom surface to effectively hinder the adsorption of S on the Ni surface, thus prolonging the sulfur poisoning resistance of catalyst.

  2.8   XPS analysis

  To further investigate the chemical state of the deactivated xMoO₃/NiO-Al₂O₃ catalyst surface elements, the XPS characterization was carried out. For simplicity, the deconvolution of Mo3d and Ni2p XPS spectra for only catalysts with different loadings (0%, 5%, 7.5%, 10%, 12.5%, 15%, 20%) was reported (Fig. 5). The deconvolution of Mo3d XPS spectra has been well documented^[39-42]. Mo could exist as MoO₃ (Mo⁶⁺), where the doublet was located at (233.0±0.1) eV and (236.0±0.1) eV. The existence of MoO₂ (Mo⁴⁺) was elucidated by two contributions located at (229.5±0.1) eV and (232.1±0.1) eV. In addition, at (226.0±0.1) eV, a band appeared corresponding to the S2s contribution, which is attributed to the S^2- species^[43]. The content of S^2- reflects the content of MoS₂ on the catalyst surface. From Fig. 5A and Table 4, it can be seen that the Mo⁶⁺ content gradually decreased with the increase of MoO₃ content. The trend of S^2- content achieved a peak when the MoO₃ loading was 12.5%, this indicated that the most MoS₂ was generated at this point. Under the same reaction conditions, the sulfur content on the catalyst surface was proportional to the reaction time. The catalyst surface just reached the monolayer loading limit when the MoO₃ content was 12.5%, and the surface MoO₃ minimized the contact opportunity between the active component and sulfide, thus delaying the deactivation time of the catalyst. Fig. 5B shows the Ni2p_3/2 XPS spectra. It can be seen that there were mainly three peaks with different forms of Ni element, which are Ni⁰ (852.1±0.1) eV, NiO (855.5±0.1) eV, and NiO satellite peak (861.3±0.1) eV. Table 4 shows each peak Gaussian fitting results of xMoO₃/NiO-Al₂O₃ catalysts. It can be obtained that the content of Ni⁰ gradually increased with the increase of MoO₃ content, which indicates that the increase of MoO₃ content is beneficial to the reduction performance of NiO. This result agrees with the H₂-TPR conclusion. However, with increasing MoO₃ loading, the reducible NiO content of the catalyst decreased. This explains the easy reduction but low activity of the xMoO₃/NiO-Al₂O₃ catalyst.

  Figure 5

  

  Figure 5.  Mo3d (A) and Ni2p_3/2 (B) XPS spectra of the spent catalysts

  (a) NiO-Al₂O₃, (b) 5MoO₃/NiO-Al₂O₃, (c) 7.5MoO₃/NiO-Al₂O₃, (d) 10MoO₃/NiO-Al₂O₃, (e) 12.5MoO₃/NiO-Al₂O₃, (f) 15MoO₃/NiO-Al₂O₃, and (g) 20MoO₃/NiO-Al₂O₃

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  Table 4

  

  Table 4.  Mo3d and Ni2p_3/2 XPS parameters of the spent catalysts

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  x	Mo6+			Mo4+			S2-
  	Binding energy/eV	Atomic fraction/%		Binding energy/eV	Atomic fraction/%		Binding energy/eV	Atomic fraction/%
  0	—	—		—	—		—	—
  5	233.0	52.4		229.6	33.5		226.1	14.1
  7.5	233.0	51.4		229.6	32.9		226.0	15.7
  10	233.1	44.9		229.7	31.5		226.0	23.6
  12.5	233.1	43.3		229.7	30.1		226.0	26.6
  15	233.1	43.2		229.6	43.4		226.0	13.4
  20	233.1	34.6		229.5	54.4		226.0	11.0
  x	Ni0			NiO			Satellite
  	Binding energy/eV	Atomic fraction/%		Binding energy/eV	Atomic fraction/%		Binding energy/eV	Atomic fraction/%
  0	852.3	11.2		855.6	64.7		861.3	24.2
  5	852.0	6.0		855.5	66.7		861.5	27.4
  7.5	851.9	6.9		855.6	66.1		861.4	26.9
  10	852.0	7.5		855.6	65.0		861.6	27.5
  12.5	852.1	9.8		855.6	63.5		861.5	26.7
  15	851.9	12.0		855.4	62.3		861.3	25.6
  20	852.0	12.8		855.4	60.9		861.3	26.2

  2.9   TEM analysis

  The TEM images and particle size distributions of the fresh catalysts are shown in Fig. 6. The NiO dispersion on the surface of the fresh NiO-Al₂O₃ catalyst was uniform, and its average particle size was around 5.5 nm. When loaded with 12.5% MoO₃, the average particle size increased to 6.7 nm, which indicates that the introduction of MoO₃ covers the NiO, thus causing an increase in particle size. MoO₃ exhibited a uniform dispersion of the 12.5MoO₃/NiO-Al₂O₃ catalyst without any discernible aggregation. When the MoO₃ loading increased to 20%, the average particle size on the catalyst surface grew to 7.3 nm, and significant MoO₃ aggregation was also observed on the catalyst surface. Excess MoO₃ on the one hand covered the active site causing a decrease in activity, on the other hand, MoO₃ would self-deposit, thus reducing the utilization of MoO₃. Therefore, the highest availability of surface MoO₃ was achieved when the loading of surface MoO₃ was 12.5%.

  Figure 6

  

  Figure 6.  TEM images and particle size distributions of (a, d, g) NiO-Al₂O₃, (b, e, h) 12.5MoO₃/NiO-Al₂O₃, and (c, f, i) 20MoO₃/NiO-Al₂O₃

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  In this paper, a series of the xMoO₃/NiO-Al₂O₃ catalysts were prepared by double hydrolysis co-precipitation combined with impregnation, and the performance of the catalysts was evaluated by a micro-reaction fixed-bed device and characterized using N₂ adsorption, XRD, H₂-TPR, H₂ pulse chemisorption, CO₂ pulse chemisorption, XPS analysis, carbon, and sulfur meter analysis, and TEM analysis and the following conclusions were drawn:

  (1) The NiO-Al₂O₃ catalyst exhibited the best low-temperature activity, with CO conversion and CH₄ yield reaching 98.9% and 97.2% at 220 ℃. With the increase of MoO₃ loading, the low-temperature activity of the catalyst gradually decreased due to the introduction of MoO₃ covering the NiO sites on the catalyst surface, thus reducing the active specific surface area of the catalyst. The CO conversion and CH₄ yield of the 12.5MoO₃/NiO-Al₂O₃ catalyst reaching 99.8% and 91.9% separately at 260 ℃, were superior to other catalysts in the literature and can meet the catalyst activity requirements for methanation reactions.

  (2) The NiO-Al₂O₃ catalyst exhibited the worst sulfur tolerance performance with a lifetime of only 4.5 h under a sulfur-containing atmosphere. With the increase of MoO₃ loading, the sulfur tolerance performance of xMoO₃/NiO-Al₂O₃ catalysts was optimized. The reason for the improvement of sulfur resistance after the introduction of MoO₃ is that the reduced MoO₂ competes with Ni⁰ for the adsorption of H₂S, thus reducing the contact opportunities between S and the active component Ni⁰. The best sulfur tolerance was achieved when the MoO₃ loading reached 12.5% and maintained the reaction lifetime for 7 h in the presence of 143 mg·m^-3 H₂S/H₂. The MoO₃ content on the surface of the 12.5MoO₃/NiO-Al₂O₃ catalyst reached the monolayer dispersion limit. MoO₃ was highly dispersed on the NiO surface, the competitive adsorption of H₂S reached the best and the protection of Ni⁰ was brought to the strongest. The sulfur tolerance performance started to decrease when the MoO₃ loading was above 12.5%, which is attributed to the reduction of MoO₃ dispersion caused by excessive MoO₃ deposition on the catalyst surface.

  
  
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• 

  Figure 1  Low-temperature activity evaluation of the catalysts: CO conversion (a) and CH₄ yield (b)

  Reaction conditions: V_H2/V_CO=3∶1, T=200-360 ℃, p=0.1 MPa, WHSV=20 000 mL·g^-1·h^-1

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  Figure 2  Evaluation of sulfur resistance of the catalysts: CO conversion (a) and CH₄ yield (b)

  Reaction condition: V_H2/V_CO=3∶1, T=550 ℃, p=0.1 MPa, WHSV=20 000 mL·g^-1·h^-1

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  Figure 3  XRD patterns of the catalysts

  (a) NiO-Al₂O₃, (b) 5MoO₃/NiO-Al₂O₃, (c) 7.5MoO₃/NiO-Al₂O₃, (d) 10MoO₃/NiO-Al₂O₃, (e) 12.5MoO₃/NiO-Al₂O₃, (f) 15MoO₃/NiO-Al₂O₃, (g) 20MoO₃/NiO-Al₂O₃

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  Figure 4  H₂-TPR curves of the catalysts

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  Figure 5  Mo3d (A) and Ni2p_3/2 (B) XPS spectra of the spent catalysts

  (a) NiO-Al₂O₃, (b) 5MoO₃/NiO-Al₂O₃, (c) 7.5MoO₃/NiO-Al₂O₃, (d) 10MoO₃/NiO-Al₂O₃, (e) 12.5MoO₃/NiO-Al₂O₃, (f) 15MoO₃/NiO-Al₂O₃, and (g) 20MoO₃/NiO-Al₂O₃

  下载: 全尺寸图片 幻灯片
  

  Figure 6  TEM images and particle size distributions of (a, d, g) NiO-Al₂O₃, (b, e, h) 12.5MoO₃/NiO-Al₂O₃, and (c, f, i) 20MoO₃/NiO-Al₂O₃

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  Table 1.  Summary of catalytic performance of CO methanation catalysts

  Catalyst	Reaction atmosphere	p/MPa	T/℃	WHSV/(mL·g-1·h-1)	XCO/%	SCH4/%	Ref.
  12.5MoO3/NiO-Al2O3	H2/CO (3∶1, V/V)	0.1	260	20 000	99.8	91.9	This work
  0.5MoO310NiO/KIT-6	H2/CO/N2 (3∶1∶1, V/V)	0.1	350	15 000	99.8	85.5	[21]
  15NiO1MoO3/MCM-41	H2/CO/N2 (3∶1∶1, V/V)	0.1	340	60 000	100.0	87.3	[29]
  15NiO-1MoO3/OMA	H2/CO/N2 (3∶1∶1, V/V)	0.1	375	60 000	97.9	93.8	[30]
  1MoO3-10NiO/SBA15	H2/CO (3∶1, V/V)	0.1	350	15 000	100.0	94.1	[31]

  下载: 导出CSV

  Table 2.  Textural properties of the xMoO₃/NiO-Al₂O₃ catalysts

  Catalyst	SBETa/(m2·g-1)	VBJHb/(cm3·g-1)	Daverageb/nm	Scatc/(m2·gcat)	DNic/%	VCO3d/(mL·g-1)	Se/%
  NiO-Al2O3	236	0.35	5.8	36.86	4.08	1.89	0.48
  5MoO3/NiO-Al2O3	194	0.33	6.2	7.18	0.68	0.10	0.58
  7.5MoO3/NiO-Al2O3	193	0.31	6.5	5.80	0.50	0.067	0.68
  10MoO3/NiO-Al2O3	188	0.30	7.7	4.50	0.42	0.044	0.69
  12.5MoO3/NiO-Al2O3	187	0.29	7.9	4.24	0.40	3.06×10-4	0.71
  15MoO3/NiO-Al2O3	162	0.28	8.5	2.04	0.19	2.02×10-4	0.64
  20MoO3/NiO-Al2O3	133	0.26	8.7	1.51	0.14	1.81×10-4	0.65
  a BET (Brunauer-Emmett-Teller) specific surface area derived from the BET equation; b Average pore volume obtained from BJH (Barrette-Joyner-Halenda) desorption; c Active nickel surface areas (Scat), nickel metal dispersion (DNi) determined by hydrogen pulse chemisorption of the fresh catalyst; d Volume of CO2 adsorbed per gram determined by carbon dioxide chemisorption; e Sulfur content of catalyst surface after sulfur resistance evaluation.

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  Table 3.  Gaussian fitting analysis of H₂-TPR profiles of the fresh catalysts with different MoO₃ contents

  Catalyst	Relative content/%
  	MoO2	β1-NiO	β2-NiO	Mo
  15MoO3/Al2O3	100	—	—	—
  NiO-Al2O3	—	57.49	42.51	—
  5MoO3/NiO-Al2O3	4.91	55.46	39.26	1.25
  7.5MoO3/NiO-Al2O3	6.60	52.41	39.27	1.70
  10MoO3/NiO-Al2O3	8.25	50.05	39.04	2.66
  12.5MoO3/NiO-Al2O3	23.30	49.96	22.36	4.36
  15MoO3/NiO-Al2O3	28.11	32.76	33.37	5.74
  20MoO3/NiO-Al2O3	34.40	30.64	28.58	6.38

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  Table 4.  Mo3d and Ni2p_3/2 XPS parameters of the spent catalysts

  x	Mo6+			Mo4+			S2-
  	Binding energy/eV	Atomic fraction/%		Binding energy/eV	Atomic fraction/%		Binding energy/eV	Atomic fraction/%
  0	—	—		—	—		—	—
  5	233.0	52.4		229.6	33.5		226.1	14.1
  7.5	233.0	51.4		229.6	32.9		226.0	15.7
  10	233.1	44.9		229.7	31.5		226.0	23.6
  12.5	233.1	43.3		229.7	30.1		226.0	26.6
  15	233.1	43.2		229.6	43.4		226.0	13.4
  20	233.1	34.6		229.5	54.4		226.0	11.0
  x	Ni0			NiO			Satellite
  	Binding energy/eV	Atomic fraction/%		Binding energy/eV	Atomic fraction/%		Binding energy/eV	Atomic fraction/%
  0	852.3	11.2		855.6	64.7		861.3	24.2
  5	852.0	6.0		855.5	66.7		861.5	27.4
  7.5	851.9	6.9		855.6	66.1		861.4	26.9
  10	852.0	7.5		855.6	65.0		861.6	27.5
  12.5	852.1	9.8		855.6	63.5		861.5	26.7
  15	851.9	12.0		855.4	62.3		861.3	25.6
  20	852.0	12.8		855.4	60.9		861.3	26.2

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•