Dry reforming of methane on Ni(1 1 1) surface with different Mo doping ratio: DFT-assisted microkinetic study

Abstract

Ni-based catalysts are one of the most promising Dry Reforming reaction (DRM) catalysts due to their cheapness and high catalytic activity, but the deactivation caused by severe carbon deposition is a common problem in Ni-based catalysts, and one solution is to dope with other elements. In this paper, density functional theory (DFT) combined with microkinetic simulation was used to study the effect of the surface doping concentration of Mo on the catalytic activity and carbon deposition resistance of Ni catalyst. In the surface doping model, the Mo surface doping concentration is set to 0, 1/9, 2/9, 3/9, 4/9 ML, namely Ni(111), NiMo1(111), NiMo2(111), NiMo3(111) and NiMo4(111). The electronic structure analysis shows that the doping of Mo improves the electron donating ability of the Ni catalyst and therefore generally increases the adsorption energy of the intermediate species in the DRM reaction. Compared with Ni(111), on NiMox(111)(x = 1–3), the activation energy of CH4 dissociation, the activation energy of CO2 dissociation, and the activation energy of C-O bond formation are generally reduced, while the activation energy of CH* dissociation and CHxO(x = 1–2) dehydrogenation increases. It was shown that the CH-O oxidation route is favorable for DRM and the Mo doped Ni catalyst showed lower free energy span. These DFT results clearly show the doping Mo not only improve the DRM catalytic activity but also enhance the carbon resistance ability. In the microkinetic model, the reaction sites on the catalyst surface were distinguished into sites affected by Mo (#) and sites not affected by Mo (*), and the influence of O coverage was considered on the activation energy of CO2 dissociation at the reaction sites affected by Mo. At 1073.15 K, the order of the catalytic activity of these catalysts for DRM reaction is NiMo2(111) > NiMo3(111) > NiMo1(111) > Ni(111), and NiMo2(111) shows the best catalytic activity for DRM. The microkinetic simulation results showed that without considering that adsorbed O may lead to the formation of NiMoO4 and thus affect the catalytic activity, the Mo-doped Ni catalyst, especially NiMo2(111), exhibits much higher catalytic activity than Ni(111) under high temperature conditions, and Ni(111) is mainly occupied by C* whereas NiMox(111)(x = 1–3) is basically not affected by C species when the simulation reaches a steady state. The strong electron donating ability of Mo is considered to be the basis for increasing the catalytic performance of Ni-based catalysts, which means that our conclusion can be more widely promoted, i.e. the catalytic activity and carbon deposition resistance of Ni-based catalysts can be improved by doping trace metals with strong electron donating ability.