Understanding the kinetics of carbon-hydrogen reaction: Insights from reaction mechanisms on zigzag edges for homogeneous and heterogeneous formation of methane

Although mechanisms for the methane formation in the carbon-hydrogen reaction (C + 2H₂ → CH₄) have been proposed from molecular modeling, there is still no compelling evidence about their kinetic feasibility. In this work, we provide evidence about mechanisms for the methane formation in the carbon-hydrogen reaction from a kinetic point of view. Kinetic parameters for all elementary steps were explored within the framework of the density functional theory and the conventional transition state theory from 298 K to 1500 K (at 0.1 MPa). Variation with time of concentration and reaction rates for the species involved in the proposed mechanisms were calculated numerically by solving the systems of ordinary differential equations accounting for the behavior of each species concentration. The proposed mechanisms allow to explain the effect that variation of i) partial pressure of H₂, ii) graphitic degree of carbonaceous materials and, iii) H radical concentration, has on the overall methane formation rate. Moreover, the calculated profiles of reaction rates reproduced qualitatively well the widely experimentally observed sharp decrease of the overall methane formation rate. This decrease is rationalized via the calculated concentration profiles of the species involved in the proposed mechanisms.