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• 

  With the deepening of natural gas shortage^[1],many efforts were took to find a clean and efficient way to synthetic natural gas (SNG)^{[2, 3]}. According to the research results,the technology of SNG production from coal not only realizes the clean utilization of coal,but also supplements the natural gas resources^[4-7]. Methanation reaction is the key step in the process of SNG. Compared with Ni-based catalyst,the Mo-based catalysts have good resistance to sulfur performance^[8-11]. Moreover,since the good performance in water-gas shift (WGS) reaction^[11],Mo-based catalysts could also maintain great catalytic activity at low H₂/CO ratios in methanation reaction^[12]:

  As an effective support of catalyst,CeO₂ was used for many common chemical reactions due to its good oxidation-reduction ability,such as methane reforming with CO₂^[13] and WGS reaction^[14],etc. It is well known that ceria is usually expressed as Ce⁴⁺/Ce³⁺,resulting that it could facilitate the storage and release of oxygen in catalysts^[15]. Zhuang et al^[16] indicated that CeO₂ could decrease the carbon formation on the catalyst surface during the methanation. In order to enhance the catalyst surface area,Wang et al^[17] prepared CeO₂-Al₂O₃ composite supports by different methods and suggested that the weak interaction of CeO₂-Al₂O₃,the large specific surface area of catalysts and small crystal size of CeO₂ was benefit to high activity toward CO methanation.

  With the development of modern instrumental analysis technology,the role of chelating agent were extensively and in-depth researched^{[18, 19]}. Typically,the use of citric acid (CA) in Mo-based catalyst preparations was found to be very effective in increasing catalytic performance towards hydrogenation reaction^[20-22]. Bergwerff et al^[23] reported that the edge dispersion of Mo could be improved throughout the preparation process because the agglomeration of Mo was restrained by adding citric acid,resulting in the increasing activity of HDS reaction. Rinaldi et al^[24] showed that the addition of citric acid could weaken the interactions between Mo and supports due to the formation of Mo-CA surface complexes. Wang et al^[25] synthesized highly HDS active catalysts by adding citric acid to precursor metal salt solutions. On the one hand,the MoO₃/CeO₂-Al₂O₃ catalysts possess higher surface area and lower average pore size than the corresponding samples prepared without citric acid because of the thermal decomposition in the roasting process. On the other hand,CA could prevent the agglomeration of metal effectively.

  Obviously,citric acid is a promising chelating agent to prepare highly active hydrogenation catalysts. The present work was an attempt to research the modification of citric acid on the CeO₂-Al₂O₃ composite support,as well as the interaction between citric acid and CeO₂-Al₂O₃ composite support. For this purpose,the MoO₃/CeO₂-Al₂O₃-CA catalysts were prepared by impregnation method and the catalysts were characterized by BET,XRD,H₂-TPR and XPS. Meanwhile,the prepared catalysts were applied to sulfur-resistant methanation in order to explore their structure-activity relationship.

  1   Experimental

  1.1   Composite support preparation

  25% CeO₂-Al₂O₃ composite support and a series of 25% CeO₂-Al₂O₃-CA composite supports were prepared by an impregnation technique. For the preparation of 25% CeO₂-Al₂O₃ composite support,the γ-Al₂O₃ was added into a solution of a known amount of the cerium nitrate,and then the mixed solution was stirred at room temperature. The natural dried mixture was further dried at 120℃ for 4h. Then,it was calcined at 600℃ for 4h in air with a heat rate of 5℃/min.

  In the procedure of 25% CeO₂-Al₂O₃-CA composite support preparation,the cerium nitrate was added into appropriate volume of solution which had dissolved exact amount citric acid in it. Then,the powdery γ-Al₂O₃ was added in the mixed solution of cerium nitrate and citric acid. The extra procedures were the same as 25% CeO₂-Al₂O₃ composite support as described above.

  The composite supports were denoted as: CeAl,CeAl-0.5CA,CeAl-1CA and CeAl-3CA. The meaning of the number was the molar ratio of citric acid and cerium nitrate.

  1.2   Catalyst preparation

  All catalysts were prepared by impregnating the desired support which was moulded sized to 180 mesh with aqueous solutions of ammonium heptamolybdate. The mixtures were continuously stirred at room temperature and dried at 120℃ for 4h in air. Then the samples were calcined at 600℃ for 4h with a heat rate of 5℃ /min. After calcinations,the catalysts were underwent pressurization molding for 30 min by 30 MPa with the pressurization pressing machine. Finally,the catalyst after molding was sized to 20-40 mesh. For all samples,the amount of MoO₃ was about 15% of supports. The catalysts were denoted as: Mo/CeAl,Mo/CeAl-0.5CA,Mo/CeAl-1CA and Mo/CeAl-3CA,respectively.

  1.3   Catalyst characterization

  1.3.5   Catalytic activity evaluation

  The catalysts for sulfur-resistant methanation activity measurements were carried out in a continuous fixed-bed reactor continuous fixed bed reactor with a gas chromatograph (Agilent 7890A) equipped with two thermal conductivity detectors and one hydrogen flame ionization detector. The temperature was controlled and detected by K-type thermocouples in the different position of the catalyst bed. For Mo-based catalyst,the sulfuration procedure was essential as pretreatment. The catalyst (3mL) was sulfurized under gas mixture about 3.0% H₂S/H₂ at 400℃ for 4h.

  Following pretreatment,the evaluation of sulfur-resistant methanation activity was performed under the pressure of 3MPa and at 550℃ for about 24h. The volume space velocity of feed gas (H₂/CO volume ratio of 1,20% N₂ and 1.2% H₂S) for the reaction should be hold 5000h^-1. The calculation method of reaction products including CO conversion and CH₄ selectivity was shown as folows:

  where x_CO and s_CH4 refer to the CO conversion and CH₄ selectivity,respectively.

  1.3.2   X-ray diffraction analysis

  X-ray diffraction profiles were detected with D/MAX 2500 V/PC X-ray Diffractometer oprated at 40kV and 100mA using Ni-filtered Cu Kα radiation source (λ=0.154056nm). The range of the scanning was from 5° to 75° with the rate of 8(°)/min. The phase identification of the acquired profiles should refer to the Joint Committee on Powder Diffraction Standards (JCPDS).

  1.3.1   N₂ physisorption analysis

  Textural properties of prepared catalysts were acquired by N₂ physisorption analysis with a Tristar-3000 apparatus of Micromeritics from the United States. As a pretreatment of the N₂ physisorption analysis,the fresh samples should be degassed at 300℃ for 4h in vacuum.

  1.3.3   H₂-temperature-programmed reduction analysis

  Temperature-programmed reduction was performed on a 2910 Automatic chemical adsorption instrument made by Micromeritics from the United States. In the prior treatment,the samples were purged with 99.999% Ar at 200℃ for 40min. Then,the samples were heated from 60 to 1000℃ at 10℃/min,while continuously flowing 10% H₂/Ar at 30mL/min.

  1.3.4   X-ray photoelectron spectroscopy analysis

  X-ray photoelectron spectroscopy was analyzed by using Perkin Elmer PHI-1600 XPS spectrometer with Mg Kα X-ray radiation. The binding energy of C 1s at 284.6 eV was a reference to the binding energies.

  2   Results

  2.1   N₂ physisorption

  The N₂ physisorption results for CeAl support and Mo-CeAl catalyst samples with/without the addition of citric acid are shown in Table 1. For CeAl support samples,the specific surface area increases by the addition of citric acid,while the average pore diameter decreases. For CO₂ and water vapor produced from the burning of citric acid can block the agglomeration of CeO₂ and Al₂O₃,leading to smaller support particles. Therefore,the specific surface area and average pore diameter of support are affected due to the change of CeO₂ and Al₂O₃ particle size. It is suggested that the textural properties of CeAl support are greatly modified by adding citric acid. The supported catalysts show relatively smaller specific surface area than supports due to the deposition of MoO₃ into the prepared composite carriers.

  Table 1.  Textural properties of CeAl supports with/without the addition of citric acid and their corresponding catalysts

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  Support BET surface area
  A/(m²·g^-1) Pore size d/nm CeO₂ diameter
  D/nm Catalyst BET surface area
  A/(m²·g^-1) Pore size d/nm CeAl 97 13.6 7.7 Mo/CeAl 76 13.0 CeAl-1CA 117 9.6 7.5 Mo/CeAl-1CA 89 10.2

  Table 1.  Textural properties of CeAl supports with/without the addition of citric acid and their corresponding catalysts

  2.2   XRD analysis

  The XRD spectra of both CeAl-1CA support and CeAl support are shown in Figure 1. The diffraction peaks characteristic of CeO₂ are observed at 28.5°,33.3°,47.5° and 56.4°^[26]. Only one diffraction peak for Al₂O₃ is observed at 66.6°^[27]. The crystal forms of them are almost the same. And the crystal size of (111) CeO₂ plane (28.5°) is calculated by using the Scherrer equation:

  Figure 1.  XRD patterns of support samples a: CeAl; b: CeAl-1CA

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  In the equation,λ is stand for the wavelength (Cu Kα₁,0.154nm),θ represents the diffraction angle,k represents a constant (0.89) and β is the half-width of the diffraction peak.

  The results of crystal size for CeO₂ are listed in the Table 1. It is shown that the crystal size of CeO₂ in CeAl-1CA support is 7.5nm,which is slightly smaller than CeAl sample. Because the difference of CeO₂ crystal size for CeAl and CeAl-1CA are very small,the addition of citric acid has little impact on CeO₂ crystal size.

  2.3   H₂-TPR analysis

  H₂-TPR profiles of Mo/CeAl catalyst and Mo/CeAl-1CA catalyst are shown in Figure 2.

  Figure 2.  H₂-TPR patterns of catalyst samples a: Mo/CeAl; b: Mo/CeAl-1CA

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  In both of two catalysts,the peaks derived from H₂ consumption are detected at 400-600℃ and 850-1000℃. The H₂-TPR peaks of the low temperature region are attributed to the reduction of octahedrally coordinated Mo⁶⁺ to Mo⁴⁺ on catalysts surface and the reduction of small crystalline MoO₃ to MoO₂^{[28, 29]}. The high temperature region contains the reduction of tetrahedrally coordinated Mo⁶⁺ to Mo⁴⁺ as well as the further reduction of the agglomerate MoO₃ species to crystalline MoO₂^[30]. It can be seen that the reducibility and interaction among different components of catalysts are changed by addition of citric acid.

  The addition of citric acid in catalysts could reduce the reduction temperature of Mo⁶⁺ in low-temperature region. And the reduction temperature shifts lower due to the decrease of interaction between octahedrally coordinated Mo⁶⁺ species and supports.

  2.4   XPS analysis

  XPS analysis was carried out to characterize the Mo/CeAl-CA catalysts with addition of different citric acid amounts. As shown in Figure 3,the ratio of Ce species to Al species on the surface of Mo/CeAl-CA catalysts is steadily increased with increasing citric acid amounts. Especially,when the value of n(CA)/n(Ce) is below 1,the Ce/Al atomic ratio increases linearly with citric acid content.

  Figure 3.  Elemental distribution of Ce in different catalysts

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  When the value of n(CA)/n(Ce) reaches up 1,the increasing rate of Ce/Al atomic ratio decreases gradually because the other part of the citric acid is calcinated to CO₂ and water vapor in the high-temperature calcination procedure.

  2.5   Catalytic activity evaluation

  The methanation activities of a series of Mo/CeAl-CA catalysts were tested and the results are illustrated in Figure 4.

  Figure 4.  CO methanation activity of Mo-based catalyst samples (a): CO conversion; (b): CH₄ selectivity

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  As the curves displayed in Figure 4(a),the methanation activity improves by adding citric acid and the conversion of CO is increased by 8.3%-11.1%. As shown in Figure 4(b),there is little difference for the selectivity to the main product on all of catalyst samples and the selectivity of CH₄ on them is about 95% during the activity evaluation. Although the addition of citric acid could improve the conversion of CO obviously,the selectivity of CH₄ has no significant change.

  3   Discussions

  3.1   Effect of citric acid addition on the CeAl support and Mo/CeAl catalyst preparation

  The N₂ physisorption results clearly suggest that the specific surface area of CeAl supports increases with adding citric acid. It is indicated that citric acid could be effectively introduced into the pore of catalysts to modify the textural properties of catalysts.

  Moreover,XPS analysis indicates that the amounts of Ce species on catalyst surface increase obviously with increasing citric acid content. It is reported^[31] that MoO₃ loaded on CeO₂ supports is easier to reduce and sulfide than MoO₃ loaded on Al₂O₃ supports. In other words,the interactions between MoO₃ and CeO₂ are weaker than those between MoO₃ and Al₂O₃. Accordingly,combined with XPS and H₂-TPR results,the reduction of MoO₃ could be easier by adding citric acid. Additionally,as demonstrated in Figure 1,the crystal form of CeAl support cannot be changed by adding citric acid.

  3.2   Influence of the additive amount of citric acid on catalytic activity of Mo/CeAl catalyst

  According to XPS,the results of element content analysis demonstrate that the reaction between Ce(NO₃)₃ and citric acid can be occurred in the preparation process of CeAl support. The Ce species can be brought from inside of catalyst to outside by reacting with citric acid.

  As the results presented in Figure 4,the conversion of CO on Mo/CeAl increases with the increase of amount of citric acid gradually. It is reported^[32] that a weak interaction between the active phase and support could increase catalytic performance. According to XPS and H₂-TPR results,the presence of CeO₂ can decrease the interaction between MoO₃ and Al₂O₃. The catalytic behavior of Mo/CeAl catalyst is enhanced with increasing amount of Ce species on the surface of catalyst. Accordingly,the increase of the additive amount of citric acid is beneficial to increasing CO conversion.

  4   Conclusions

  A series of Mo/CeAl catalysts with adding different amount of citric acid were prepared by impregnation method and tested toward sulfur-resistant methanation of syngas. Among these tested catalysts,the experimental results showed that the additive amount of citric acid could affect the catalytic performance remarkably and the conversion of CO was improved gradually with the increase of additive amount. Based on the BET data,it was found that addition of citric acid to CeAl support could increase the specific surface area and decrease the average pore diameter which could to be favor to the high dispersion of Mo particles on CeAl support. The H₂-TPR and XPS results demonstrated that the amount of Ce species on the surface of the catalysts could increase with the increased addition of citric acid and further led to the weaker interaction between Mo species and composite support. Complementally,the XRD results illustrated that the crystal forms of Ce species were not changed by adding citric acid.

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  Figure 1  XRD patterns of support samples a: CeAl; b: CeAl-1CA

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  Figure 2  H₂-TPR patterns of catalyst samples a: Mo/CeAl; b: Mo/CeAl-1CA

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  Figure 3  Elemental distribution of Ce in different catalysts

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  Figure 4  CO methanation activity of Mo-based catalyst samples (a): CO conversion; (b): CH₄ selectivity

  ■: CA/Ce=0; ●: CA/Ce=0.5; ▲: CA/Ce=1; : CA/Ce=3

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  Table 1.  Textural properties of CeAl supports with/without the addition of citric acid and their corresponding catalysts

  Support BET surface area
  A/(m²·g^-1) Pore size d/nm CeO₂ diameter
  D/nm Catalyst BET surface area
  A/(m²·g^-1) Pore size d/nm CeAl 97 13.6 7.7 Mo/CeAl 76 13.0 CeAl-1CA 117 9.6 7.5 Mo/CeAl-1CA 89 10.2

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