Construction and evolution of active palladium species on phase-regulated reducible TiO2 for methane combustion

Abstract

Active palladium species play a pronounced role in determining the catalytic performance of supported catalysts. Although extensive studies have been made, the variation of palladium species on the surface of reducible supports still needs to be clarified due to the complex metal–support interactions (MSI). Herein, investigations were performed on Pd/TiO₂ catalysts, which were tailored by regulating the phase structure of TiO₂. The thermal treatment of TiO₂ at 1000 °C gave rise to a “snow cookie”-like catalyst structure comprising amorphous surface palladium species and rutile TiO₂ supports. The as-prepared catalyst possessed abundant Pd²⁺ species, Ti³⁺ ions and adsorbed oxygen species, which meanwhile could facilitate the re-oxidation of Pd⁰ back to PdO, and thus completely catalyzed the combustion of CH₄ to CO₂ at as low as 370 °C. However, some Pd²⁺ would be oxidized into Pd⁴⁺ on the working catalyst, and then enter the TiO₂ lattice to form Pd_xTi_1−xO₂, thus exerting a detrimental effect on the oxygen exchange between bulk TiO₂ and surface Pd species. Consequently, the re-oxidation ability was weakened, accompanied by the decrease of active PdO species, resulting in the loss of activity during on-stream tests.