Preparation Of Slag-based Geopolymer Microsphere Catalyst And Its Application In The CO₂ Methanation

CO₂ methanation can not only solve the environmental problems caused by the greenhouse effect,but also alleviate the current situation of natural gas supply shortage in China.Ni-based catalysts have the advantages of easy availability of raw materials and low prices,and exhibit excellent catalytic performance in the CO₂ methanation reaction,which has attracted extensive attention of researchers.The slag-based geopolymers have the synergistic catalysis of alkaline earth metals and excellent metal adsorption capacity,and are ideal catalyst supports.In this paper,a series of CO₂ methanation catalysts were produced by using slag and chemically synthesized slag as the main raw materials,used KOH as the alkali activator and prepared microsphere supports by suspension dispersion solidification method,and then loaded the microsphere supports with active metal Ni,and studied the catalytic activity of the catalysts.N₂ adsorption-desorption,XRD,SEM,TEM,H₂-TPR,CO₂-TPD and other analytical methods were used to characterize the catalyst and investigate the feasibility of slag-based geopolymer microspheres as CO₂ methanation supports.The research content of the paper are as follows:（1）A series of catalysts with different Ni loadings were prepared by impregnation method using slag-based geopolymer microspheres as the supports.It was found that the catalytic activity of the support was best at 15 wt.%Ni loading,and the CO₂ conversion was 72.8%and the CH₄selectivity was about99%at 400℃/0.1 MPa and a mass hourly space velocity of 12000 m L·g^-1·h^-1.The results of XRD,TEM,and activity tests showed that excessive Ni loading affected the Ni dispersion and caused the agglomeration phenomenon of Ni particles to increase.This agglomeration phenomenon will reduce the number of active sites on the catalysts,resulting in a decrease in the catalytic activity of the catalysts.（2）The chemically synthesized slag-based geopolymer microspheres were loaded with 15 wt.%Ni by impregnation method to produce a chemically synthesized slag-based geopolymer catalyst with higher catalytic activity.Compared with the slag-based geopolymer catalyst with the same loading amount,The chemically synthesized slag-based geopolymer catalyst could achieve CO₂conversion of 80.2%and the CH₄selectivity of 99.2%at400℃/0.1 MPa and a mass hourly space velocity of 12000 m L·g^-1·h^-1.The chemically synthesized slag-based geopolymer catalyst still showed excellent anti-sintering performance in the stability test lasting 100 h.BET,XRD and TEM analysis showed that the chemically synthesized slag-based geopolymer microsphere supports had a larger specific surface area than the slag-based geopolymer microsphere supports,which made the particles of active metal Ni on the catalyst smaller.Moreover,the chemically synthesized slag-based geopolymer catalyst possesses more basic sites,which were favorable for the adsorption and desorption of CO₂.The comparative analysis of the two catalysts showed that the substances that played a positive role in catalysis in the slag-based geopolymer catalysts were CaO,SiO₂,Al₂O₃,and MgO.In addition,the reaction pathway of the slag-based geopolymer catalyst was obtained by in situ DRIFTS analysis for direct hydrogenation of formate to methane.（3）When exploring the effect of different MgO content in the slag-based geopolymer microsphere supports on the catalyst activity,it was found that when the MgO content in the slag-based geopolymer was 3.11%,the Ni-based catalyst had the best activity,at 400℃/0.1 MPa and a mass hourly space velocity of 12000 mL·g^-1·h^-1,which achieved the CO₂conversion of 83.6%and the CH₄selectivity of 99.3%,and the catalyst was still stable in the stability test lasting 100 h.Combined with XRD,SEM,TEM and activity test analysis,it was found that the presence of a small amount of calcite on the supports can improve the dispersion of active metal Ni and reduce the particle size of Ni.In addition,the results of CO₂-TPD analysis showed that the presence of a small amount of MgO in the supports increased the number of moderately basic sites on the catalysts,while the presence of a large amount of MgO in the supports decreased the number of moderately basic sites on the catalysts.