Identification of active sites for CO and CH4 oxidation over PdO/Ce1−xPdxO2−δ catalysts

A PdO/Ce1−xPdxO2−δ catalyst containing surface PdOx (x = 1–2) species and Ce1−xPdxO2−δ solid solution was prepared by a solution combustion method. It was found that the surface PdOx species could be removed by nitric acid treatment. Also, partial Pd4+ cations in the CeO2 lattice migrated to the surface to form surface PdOx species after high temperature calcination (500 °C). The catalyst was tested for catalytic CO and CH4 oxidation. For CO oxidation, the specific reaction rate of the surface PdOx species (673.4 μmolCO gPd−1 s−1) was 249 times as high as that of the Ce1−xPdxO2−δ solid solution (2.7 μmolCO gPd−1 s−1), due to the fact that the surface PdOx species provided CO chemisorption sites. While for CH4 oxidation, the specific reaction rate of the Ce1−xPdxO2−δ solid solution (7.5 μmolCH4 gPd−1 s−1) was higher than that of the PdO/Ce1−xPdxO2−δ catalyst (2.8 μmolCH4 gPd−1 s−1), due to the covered Ce1−xPdxO2−δ surface by surface PdOx species.

Figure optionsDownload as PowerPoint slideHighlights
► PdO/Ce1−xPdxO2−δ contains surface PdOx (x = 1–2) and Ce1−xPdxO2−δ solid solution.
► Surface PdOx are more active for CO oxidation than Pd4+ cations in Ce1−xPdxO2−δ.
► Pd4+ cations in Ce1−xPdxO2−δ are more active for CH4 oxidation than surface PdOx.
► A portion of Pd4+ in Ce1−xPdxO2−δ transform to surface PdOx at high temperature.