| Energy consumption in a unbalanced manner causes the air pollution;whereas,natural gas,known as its advantage in environmental aspect,has been accepted and played an important role in energy policy formulation.Coal-based natural gas is able to not only achieve green and efficient coal usage,but also alleviate the contradiction between supply and demand of natural gas in China.In this process,methanation catalysts is the key point to make it as reality.Supported Ni catalysts have been widely applied in methanation process,due to the low price,has similar catalytic performance with the noble metal-based catalysts.However,supported Ni catalysts tend to carbon deposition,which probably cause the deactivation of Ni catalysts.Recently,metal organic framework?MOF?has been used to prepare porous materials with high specific surface area,which exhibits high performance after a certain treatment,such as high temperature calcination.In this regards,MOF containing aluminum was selected as one precursor to impregnate with Ni for the preparation of Ni/Al2O3.The metal-support interaction can be tuned to medium strong interaction,originating from the unique structure of MOF precursor.In this work,MIL-100?Al?was used as the precursor of the carrier,which was impregnated by 5 wt%nickel nitrate solution,and then calcined at 600?in air for 3 h to synthesize NiO/Al2O3-M catalyst.For comparison,NiO/Al2O3-C was prepared by the same method using conventional industrial alumina as a carrier.For the NiO/Al2O3-M,X-ray diffraction result showed that the catalyst was in amorphous form.N2 adsorption-desorption characterization showed that the catalyst has large surface area of 350 m2/g,and the pore size distribution was uniform.H2 temperature-programmed reduction results proved that 98%NiO existed in?form,indicating that the interaction between the NiO species and the carrier is moderate strength.The X-ray absorption near-edge structure and extended X-ray absorption fine structure patterns showed that coordination state and coordination environment of Ni on the NiO/Al2O3-M and NiO/Al2O3-C were the same.The O 1s X-ray photoelectron spectroscopy indicated there was a large amount of oxygen vacancies on the surface of NiO/Al2O3-M,which was three times higher that of those on NiO/Al2O3-C.In addition,after reduction Ni dispersion of Ni/Al2O3-M was 8.1%obtained by hydrogen chemical adsorption,and the Ni/Al2O3-C sample was only 4.9%.The catalytic performance of Ni/Al2O3-M and Ni/Al2O3-C were studied with CO methanation by using CO/H2?1:3?as feed gases.The results showed that CO conversion of Ni/Al2O3-M could reach 94%at 450?,0.10 MPa with a WHSV of 15000 mL·g-1cat·h-1;while CO conversion for Ni/Al2O3-C catalyst was only 20%under the same reaction conditions at 550?.According to the criteria of Mears and Weisz-Prater,the effect of internal and external diffusion on the CO methanation reaction were eliminated.Turn over frequency?TOF?stability of the two catalysts was studied.It was found that Ni/Al2O3-M catalyst has good stability.The TOF value was maintained between 0.06 s-11 and 0.07 s-11 for 100 h;while the TOF value of Ni/Al2O3-C decreased from 0.043 s-11 to 0.028 s-11 for first 20 h,and then slowly dropped to0.018 s-1.In addition,activation energy of the two catalysts was calculated by the Arrhenius formulation.For Ni/Al2O3-M,the activation energy was 52 kJ/mol,which is lower than that of NiO/Al2O3-C of 73 kJ/mol.Diffuse reflectance Fourier transform infrared?DRIFT?spectra was used to further explore the reaction pathway in CO methanation.The results showed that carbonate-hydroxyl species?COH?appeared on the Ni/Al2O3-C in the wavenumber range of 1300 cm-11600 cm-1,but for the Ni/Al2O3-M catalyst,except for COH species,the formyl?CHO?species also was found at 1265 cm-1and 1120 cm-11 in large portion.This is originated from the sufficient oxygen vacancy on Ni/Al2O3-M promoting the formation of low-energy formate intermediates,and lowering the activation energy for CO methanation. |
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