A Study Of The Size Effect Of Nickel-based Catalysts For Dry Reforming Of Methane

Dry reforming of methane(DRM)has attracted much attention as it can convert two major greenhouse gases CH4 and CO2 into syngas(H2/CO?1)with high industrial value,which can be used for subsequent production of chemical products.Nickel-based catalysts with high activity and low cost are the most common industrial catalysts for DRM reactions.However,the nickel-based catalysts are easy to sinter during the high-temperature reaction process,which leads to the deactivation of the catalysts by carbon deposition.Studies have shown that the amount of coke deposited on the catalyst is closely related to the size of Ni particles,and the formation of coke is due to the difference in activity of catalysts of different sizes.Therefore,we explored the size effect of Ni-catalyzed DRM reaction using combined theoretical and experimental methods.The research content and main conclusions are as follows:(1)Based on the density functional theory(DFT),in the CASTEP module of the Materials Studio calculation software,the models of metallic Ni clusters with atomic numbers ranging from 4 to 21 were constructed,and the optimal configuration was obtained for Ni clusters of different sizes.Then we discussed the adsorption of CH4 and CO2 on the surface,and analyzed the structure(bond parameters,etc.),coordination number of Ni atoms and adsorption energy respectively.Among Ni clusters,the adsorption energy of low-coordination(3-5 coordination)Ni for CH4 and CO2 has a good correlation with the cluster size.As the size increases,the adsorption energy(absolute value)decreases in order.The adsorption capacity of high coordination(6-8 coordination)Ni sites on two molecules no longer depends on its size.As the coordination number increases,the adsorption of CH4 and CO2 gradually becomes difficult.The adsorption energy(absolute value)of CO2 on the same Ni cluster is much greater than the adsorption energy of CH4,indicating that in the reaction system,CO2 molecules occupy the active sites on the Ni catalyst surface preferentially under the same conditions.(2)Based on the density functional theory(DFT),we construct the Ni periodic models in the CASTEP module of the Materials Studio calculation software,and discuss the adsorption of CH4 and CO2 on the surface of different coordinated Ni periodic models.In Ni periodic models with different coordination numbers,the adsorption energy of CH4 and CO2 has a good correlation with the atomic coordination number,that is,as the coordination number increases,the adsorption energy(absolute value)decreases sequentially.At the same time,the adsorption of CH4 and CO2 gradually changes difficult.(3)Ni-SiO2 catalysts with different Ni particle sizes were prepared by the combustion decomposition method,and their catalytic performances of dry reforming of methane was studied.The Ni particle size of the Ni-SiO2 catalyst greatly affected the catalytic performance of the catalyst and the degree of coke deposition in the reaction.The smaller the size of Ni particles,the better the catalytic activity and stability,and the less the coke deposition.