DFT study of the thermodynamic stability of Pd-Pt bulk oxide phases

The catalytic combustion activity of PdPt bimetallic catalysts is governed by oxide phases which form under oxygen-rich operation conditions. We investigate the thermodynamic stability of mixed Pd_xPt_1−xO_y phases with the crystal structures of PdO, PtO₂, and Pt₃O₄ within an ab initio thermodynamics framework based on density-functional theory calculations. Our results suggest a complex mixing−demixing behavior of the Pd−Pt−O system upon variations of temperature and oxygen partial pressure. At atmospheric pressure mixed oxides are predicted to be stable only at temperatures below 400−500 K, whereby the presence of Pd⁴⁺ ions is stabilized with increasing Pt amount in a Pd_xPt_1−xO₂ phase. At intermediate temperatures a mixture of phases becomes stable where PdO coexists with PtO₂ or Pt₃O₄. At high temperatures the oxides decompose directly to form PdPt alloys, except for Pd-rich systems, where PdO and metallic Pt coexist within a small temperature window.