Mechanistic Study of Carbon Monoxide Methanation over Pure and Rhodium- or Ruthenium-Doped Nickel Catalysts

Carbon monoxide (CO) methanation has been studied through periodic density functional theory calculations on flat and corrugated nickel surfaces. The effect of doping the catalyst was taken into account by impregnating the nickel surfaces with Rh or Ru atoms. It was found that the methanation of CO as well as the synthesis of methanol from CO and hydrogen (H2) evolve through the formyl (HCO) intermediate on all the surfaces considered. The formation of this intermediate is the most energy-consuming step on all surface models with the exception of the Rh- and Ru-doped Ni(110) surfaces. In the methanation reaction, the CO dissociation is assisted by hydrogen atoms and it is the rate-determining step. Also, surfaces displaying low-coordinated atoms are more reactive than flat surfaces for the dissociative reaction steps. The reaction route proposed for the formation of methanol from CO and H2 presents activation energy barrier maxima similar to those of CO methanation on pure nickel and Rh- or Ru-doped flat nickel surfaces. However, the CO methanation reaction is more likely than the methanol formation on the doped stepped nickel surfaces, which is in agreement with experimental results available in the literature. Thus, the different behavior found for these two reactions on the corrugated doped surfaces can then be used in the optimization of Ni-based catalysts favoring the formation of methane over methanol.