Preparation Of Ni/bentonite Catalyst Ang Its Application In The CO₂ Methanation

As a green fuel,methane is the main component of natural gas,and its demand has been increasing in recent years.The C02 methanation reaction is an efficient method of C02 conversion,which can not only reduce the C02 emissions but also provide fuel energy.With the advantages of low cost,easy availability of raw materials,Ni-based catalysts show good catalytic activity in catalytic hydrogenation processes,and have attracted a lot of attention from researchers.Bentonite is an advantageous resource in Guangxi province,and the bentonite used a carrier for catalysts would help to broaden its application in the field of catalysis.In the present work,modified bentonite was used as a carrier,and was used to prepare Ni/modified bentonite catalyst by an impregnation combustion method for CO2 methanation,and the catalytic activity of the catalyst was also studied.The catalysts were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),H2 temperature-programmed reduction(H2-TPR),X-ray photoelectron spectroscopy(XPS),N2-adsorption desorption,and H2-chemisorption.The effects of the preparation methods and additives on the structure and properties of Ni/bentonite catalysts were investigated.The main research contents of the work were showed in following.(1)Using urea as fuel and four modified bentonite as carriers,the Ni/bentonite catalysts were prepared by an impregnation combustion method and used for C02 methanation reaction,and the catalytic activity on Ni/bentonite was compared with the traditional impregnation method.The effects of the Ni/bentonite catalyst preparation methods on the microstructure of catalyst,catalytic activity and stability were also investigated.The results showed that the grain size of the active component Ni was reduced,the degree of dispersion was increased,and the specific surface area of Ni/bentonite catalyst prepared by the impregnation combustion method was increased when compared with the traditional impregnation method,which led to the higher catalytic activity and stability of the Ni/bentonite.Under the conditions of atmospheric pressure,300 ℃,Y(H2):V(CO2)= 4,and feed gas space velocity of 3600.mL gcat-1·h-1,the C02 conversion on the Ni/bentonite catalyst prepared by the impregnation combustion method was 84.7%,which was 10%higher than that of the catalyst prepared by impregnation method,and the CH4 selectivity was higher than 99%.After a 110-hour reaction,the C02 conversion of the Ni/bentonite catalyst prepared by the impregnation combustion method decreased by only 2.8%,while the CH4 selectivity remained unchanged.At the same time,the C02 conversion of the Ni/bentonite catalyst prepared by the impregnation method decreased by 6.3%.(2)To further enhance the low-temperature catalytic activity and stability of Ni/bentonite catalyst,Ni-Mn/bentonite catalyst was prepared by introducing Mn into Ni/bentonite catalyst.The effect of Mn addition on the catalytic performance and structure of Ni/bentonite catalyst was investigated.The results indicated that the addition of Mn increased the dispersion of the active component Ni and decreased the grain size of the active component Ni,increased the specific surface area and pore volume of the Ni/bentonite catalyst,and decreased the average pore size,which suppressed the aggregation of Ni particles growed during the C02 methanation process.At the same time,the doping of Mn increased the amount of oxygen vacancies on the Ni/bentonite catalyst surface,which promoted the activation of C02 in the methanation reaction,increasing the low-temperature activity and stability of the Ni/bentonite catalyst.Under the reaction condition of atmospheric pressure,270 ℃,V(H2):V(CO2)= 4,and feed gas space velocity of 3600 mL·gcat-1-h-1,,the C02 conversion on the Ni-Mn/bentonite catalyst with 2 wt%Mn was 85.2%,the selectivity of CH4 was 99.8%.In the other hand,when Mn was not added the C02 conversion reached 84.7%only the reaction temperature raised to 300 ℃.During a 150-hour stability test,the C02 conversion of Ni-2%Mn/bentonite catalyst decreased by 2.2%,while the C02 conversion of the Ni/bentonite catalyst decreased by 6.4%.