DFT study of difference caused by catalyst supports in Pt and Pd catalysis of oxygen reduction reaction

Based on an experimental phenomenon that catalytic activity of Pt and Pd for oxygen reduction reaction (ORR) changes with catalyst supports from C to TiO₂, density function theory (DFT) was used to elucidate the cause behind the difference in catalysis caused by catalyst supports. First, factors closely associated with the first electron transfer of the ORR were assessed in the light of quantum chemistry. Then intermediate (atomic oxygen, O) adsorption strength on the catalyst surface was calculated. The results show that, in terms of minimum energy difference, the best orbital symmetry match, and the maximum orbital overlap, TiO₂ does bring about a very positive effect on catalysts Pd/TiO₂ for the first electron transfer of the ORR. Especially, TiO₂ remarkably expands the space size of Pd/TiO₂ HOMO orbital and improves orbital overlap of Pd/TiO₂ HOMO and O₂ LUMO. The analysis of deformation density and partial density of state shows that the strong interaction between Pt and Ti leads to a strong adsorption of intermediate O on Pt/TiO₂, but the strong interaction between Pd and surface O causes positive net charge of Pd and a weak adsorption of intermediate O on Pd/TiO₂. Thus, the ORR can proceed more smoothly on Pd/TiO₂ than Pt/TiO₂ in every respect of maximum orbital overlap and rate delay by intermediate O. The research also discloses that several factors lead to less activity of TiO₂-supported Pt and Pd catalysts than the C-supported ones for the ORR. These factors include the poor dispersion of Pt and Pd particles on TiO₂, poor electric conduction of TiO₂ carrier itself, and bigger energy difference between HOMO of TiO₂-carried metallic catalysts and LUMO of O₂ molecule due to electrons deeply embedded in the semiconductor TiO₂ carrier. Supported by the National Natural Science Foundation of China (Grant No. 20676156), the Chinese Ministry of Education (Grant No. 307021), the National 863 Program (Grant Nos. 2006AA11A141 and 2007AA05Z124), and the Chongqing Sci &Tech Key Project (Grant No. CSTC2007AB6012)