A novel Pd3O9@α-Al2O3 catalyst under a hydroxylated effect: high activity in the CO oxidation reaction

Abstract

Considering the importance of palladium-based and doped metal-oxide catalysts in CO oxidation, we design a new Pd₃O₉@α-Al₂O₃ catalyst and simulate its efficiency under a hydroxylated effect. The structure, electronic structure and oxidation activity of the hydroxylated Pd₃O₉@α-Al₂O₃(0001) surface are investigated by density functional theory. Under the O-rich growth conditions, Pd preferentially replaces Al. The lowest formation energy of the Pd-doped α-Al₂O₃(0001) surface is 0.21 eV under conditions wherein the coverage of the Pd-doped α-Al₂O₃ is 0.75 on a pre-hydroxylated surface and the water coverage is 0.25, which leads to formation of a Pd₃O₉ cluster embedded in the Al₂O₃(0001) surface. The reaction mechanisms of CO oxidization have been elucidated first by CO adsorption and migration, second by O_v formation with the first CO₂ release, then by the first foreign O₂ filling and CO co-adsorption, and finally by the second CO₂ desorption and restoration of the hydroxylated Pd₃O₉@α-Al₂O₃(0001) surface. The rate-determining step is the formation of the first CO₂ in the whole catalytic cycle. The results also indicate that the energy barrier for CO oxidization is obviously reduced compared to that of the undoped surface, which implies that the introduction of Pd can efficiently improve the oxidation reactivity of the α-Al₂O₃(0001) surface. Compared to the synthesized Ir₁/FeO_x (1.41 eV) and Pt₁/FeO_x (0.79 eV) catalysts, the reaction activation barrier of CO oxidation is lowered by 0.65 eV and 0.03 eV, respectively. Therefore, the Pd₃O₉@α-Al₂O₃ catalyst shows superior catalytic activity in CO oxidation. The present results enrich the understanding of the catalytic oxidation of CO by palladium-based catalysts and provide a clue for fabricating palladium-based catalysts with low cost and high activity.