ReaxFF Reactive Molecular Dynamics Study on Methanation Reaction from Syngas

Abstract

The side reactions The side reactions in the syngas methanation process can lead to carbon deposition and deactivation of Ni-based catalysts. The existing ReaxFF reaction force field parameters are not suitable for the present study system; thus, new force field parameters of Ni-C/H/O need to be developed for the syngas methanation process. The DFT method was used to calculate the adsorption energies and transition state energy barriers for elementary reactions, which were used as the training set of the force field. The transition state was searched by DFT and reaction force field methods, respectively, to narrow the gap between the two energy barriers under similar structures. The comparison of the results of DFT and ReaxFF indicates that the developed force field suits the system. Based on the developed force field, ReaxFF was used to study the transformation process of CHx (x = 0–3) species on the Ni surface. The results show that at high temperatures and low H coverage, CHx species are easily dissociated and eventually become C species on the Ni(111) surface. Increasing H coverage can significantly increase the hydrogenation of CHx species to methane and inhibit the dissociation of CHx species. The methanation activity on the Ni(211) surface is higher, but the carbon deposition phenomenon is more serious. The reaction rate equations of each species were given, and the kinetic parameters of the elementary reactions were solved. Finally, the best reaction conditions with a higher yield of CH4 and less carbon deposition were found.