Surface structure and reaction property of CuCl_2-PdCl_2 bimetallic catalyst in methanol oxycarbonylation: A DFT approach

摘要

Surface structure of CuCl_2-PdCl_2 bimetallic catalyst (Wacker-type catalyst) was built employing density functional theory (DFT) calculations, and the reaction mechanism of methanol oxycarbonylation over the CuCl_2-PdCl_2 surfaces was also investigated. On the CuCl_2-PdCl_2 surface, the active site for methanol oxidation was confirmed as Cu-Cl-Cu (Pd). Comparing with pure CuCl_2 surface, the introduction of Pd atom causes the electron repopulation on the surface and lowers the energy barrier for methanol oxidation, but the number of the active site decreases with the increasing of Pd doping volume. Agreed with previous experimental results, the Pd site is most favorable for the CO insertion, indicated by the lowest activation barrier for the formation of COOCH_3 on Pd atom. The lowest energy barrier for the formation of DMC appears when COOCH_3 species adsorbed on Pd atom and methoxyl adsorbed on Cu atoms, which is 0.42 eV. Finally, the reconstruction of the unsaturated surface is a spontaneous and exothermic process. Comparing with other surfaces, the rate-limiting step, methanol oxidation, on CuCl_2-PdCl_2 surface with Pd/Cu = 1:17 has the lowest energy barrier, which is agreed with the experimental observation that CuCl_2-PdCl_2 catalyst with Pd/Cu = 1:20 has the favorable activity. The adsorbed methoxyl will further lower the activation barrier of methanol oxidation, which is agreed with experimental observation that the Wacker-type catalysts have an induction period in the methanol oxidative carbonylation system.