Study On Reaction Mechanism Of Dry Reforming Of Methane On Ni Catalysts

Dry reforming of methane is a reaction of CH4 and CO2 to produce a synthesis gas (CO and H2), which can be used as feed gas for chemical processes such as Fischer-Tropsch synthesis and oxo synthesis, thus playing an important role in reducing CO2 emissions. Further, it is also an effective way to develop the technology of methane dry reforming for high-value value utilization of CH4 and CO2. Ni-based catalysts are found to be the best choice to realize methane dry reforming, but it is liable to transport and accumulate in high-temperature reactions and cause serious carbon deposits, making it fail to achieve its industrial application till present. Therefore, the design of high-activity, anti-coke Ni-based catalysts for methane dry reforming is still the key in the field research. In this paper, the mechanism of action behind Ni-based catalysts in methane dry reforming is explored by using the quantum chemical calculations, with the purpose of laying a theoretical foundation for the design of catalysts which exhibit higher activity and stronger resistance to carbon deposition. To begin with, the reaction network for methane dry reforming is calculated, finding that alkoxy species may be rich in the reaction system, and discovering alkoxy species in a steady state in the system by adopting Diffuse Reflectance Infrared Fourier Transform Spectroscopy. On this basis, the activation energy and enthalpy change of the reaction elementary step are analyzed, and the critical rate-determining step and optimal path of the reaction system has been acquired. Secondly, by making an analysis of the charge population, orbital energy and symmetry, among others, of the main reaction steps, it is elaborated that the electron transfer of CH4 dehydrogenation and CO2 dissociation from the perspective of state-specific reactions. The results show that for CH4 dehydrogenation, the activation energies for dehydrogenation are the least, mainly because there are not only electronic supplies of Ni-based catalysts, but also σC-H bonding orbital to feed back to Ni, directly undermining the C-H bond. Moreover, the most likely dissociation path for CO2 is the hydrogen assisted dissociation path, and the reaction path activation energies for the intermediate formyl (HCOO·) are smaller. Subsequently, based on the optimal reaction path calculated, the catalyst model is expanded to the cluster size for simulation study. The results show that the rate-determining step of the methane dry reforming reaction system grounded on a Ni-based catalyst model of cluster size is consistent with the results obtained by the monatomic catalyst model which was used previously. Last but not least, the grand- canonical Monte Carlo method is employed to systematically study the competitive adsorption of CO2 and CH4 on the Ni surface, to observe the impact of operating parameters (the molar ratio of reactants, operating temperature, etc.) on the activity and selection of catalysts for dry reforming of methane. The calculation finds that n(CO2)/n(CH4)>1 and higher reaction temperatures are conducive to the reactions, which comply with the thermodynamic calculations and experiment findings reported in the literature.