Dry (CO2) reforming of methane over Pt catalysts studied by DFT and kinetic modeling

摘要

Dry reforming of methane (DRM) is a well-studied reaction that is of both scientific and industrial importance. In order to design catalysts that minimize the deactivation and improve the selectivity and activity for a high H-2/CO yield, it is necessary to understand the elementary reaction steps involved in activation and conversion of CO2 and CH4. In our present work, a microkinetic model based on density functional theory (DFT) calculations is applied to explore the reaction mechanism for methane dry reforming on Pt catalysts. The adsorption energies of the reactants, intermediates and products, and the activation barriers for the elementary reactions involved in the DRM process are calculated over the Pt(1 1 1) surface. In the process of CH4 direct dissociation, the kinetic results show that CH dissociative adsorption on Pt(1 1 1) surface is the rate-determining step. CH appears to be the most abundant species on the Pt(1 1 1) surface, suggesting that carbon deposition is not easy to form in CH4 dehydrogenation on Pt(1 1 1) surface. In the process of CO2 activation, three possible reaction pathways are considered to contribute to the CO2 decomposition: (I) CO2* + * -> CO* + O*; (II) CO2* + H* -> COOH* + * -> CO* + OH*; (III) CO2* + H* -> mono-HCOO*+* bi-HCOO* + * [CO2* + H* -> bi-HCOO* + *] -> CHO* + O*. Path I requires process to overcome the activation barrier of 1.809 eV and the forward reaction is calculated to be strongly endothermic by 1.430 eV. In addition, the kinetic results also indicate this process is not easy to proceed on Pt(1 1 1) surface. While the CO2 activation by H adsorbed over the catalyst surface to form COOH intermediate (Path II) is much easier to be carried out with the lower activation barrier of 0.746 eV. The C-O bond scission is the rate-determining step along this pathway and the process needs to overcome the activation barrier of 1.522 eV. Path III reveals the CO2 activation through H adsorbed over the catalyst surface to form HCOO intermediate firstly. This reaction requires a quite high activation barrier and is a strongly endothermic process leading to a very low forward rate constant. In conclusion, Path II is the dominant reaction pathway in CO2 activation. Additionally, there are two pathways of CH oxidation by O: (A) CH* + O* -> CHO* + * -> CO* + H*; (B) CH* + O* -> COH* + * -> CO* + H*. Both the activation barriers and kinetic results demonstrate that Path A is the prior reaction pathway. Furthermore, in the two pathways of CH oxidation by OH: (C) CH* + OH* -> CHOH* + * -> CHO* + H*; (D) CH* + OH* -> CHOH* + * -> COH* + H*. Path C is easier to proceed. In conclusion, the main reaction pathway in CH oxidation according to the mechanism: CH* + OH* -> CHOH* + * -> CHO* + H* -> CO* + 2H*. These results could provide some useful information for the operation of DRM over Pt catalysts, and are helpful to understand the mechanisms of DRM from the atomic scale. (C) 2016 Elsevier B.V. All rights reserved.