First-principles Study On The Mechanism Of Pd-Cu-Cl_x/Al₂O₃Catalyst For Low-temperature CO Oxidation

As a typical represent of the composite catalysts, the supported Wacker-type catalyst (PdCl2-CuCl2/support) has been used in kinds of chemical reactions. The Pd-Cu-Clx/γ-Al2O3catalyst prepared by a NH3coordination-impregnation method exhibits an excellent activity for low-temperature CO oxidation. However, the atom-level theoretical research for its catalytic low-temperature CO oxidation has not been reported. In this paper, we systematically studied the reaction mechanism of low-temperature CO oxidation over the Pd-Cu-Clx/γ-Al2O3catalyst by using a first-principles density functional theory (DFT) and periodic boundary model, including the adhering of PdCl2, CuCl2components on γ-Al2O3(100), the active sites of CO and O2, the catalytic cycle of palladium and copper species. The adsorption energies and geometries for various intermediates and the reactants were computed, the transition state and the activation barrier for every step were also calculated. On base of the results calculated by DFT, the catalytic reaction mechanism of Pd-Cu-Clx/γ-Al2O3for CO oxidation was proposed. The obtained results are mainly as follows.(1) The adhering sites of palladium species on γ-Al2O3(100) facet is O3c-Al5c-O3c bridge sites, and copper species preferentially adhering on Alsc-O3c bridge sites;(2) The active sites for adsorbing CO is Pd+(PdCl and OH-Pd), and CuCl is the active sites for O2adsorption, which are consistent with the in situ infrared diffuse reflectance spectra of CO adsorbed on this catalyst;(3) Palladium catalytic cycle:OH-Pd+CO→OH-Pd-CO, OH-Pd-CO→cis-Pd-COOH, cis-PdCOOH+OH-→Pd0+CO2+H2O; copper catalytic cycle:CuClOH+PdCO→OH-Pd-CO+CuCl, CuCl+O2→CuClO2, CuClO2+H+-+CuClOOH→CuClO+OH-, CuClO+H+→CuClOH. The reaction barrier of OH-Pd-CO→cis-Pd-COOH is0.52eV, which is rate-controlling step in the reaction process;(4) Water plays a very important role in the catalytic cycle, and the H proton resulted from dissociation of H2O on Al3+sites promotes a series change of PdCl2and CuCl2to form OH-Pd and CuClOH active species and the copper sites for actively adsorbing oxygen, which supplies active oxygen species to the catalytic oxidation of CO.