Determination of the Transition Between Supported Pdo and Pd Under Various Time-Temperature Trajectories

Palladium plays an important role as oxidation catalyst in automotive three-way catalyst applications and in catalysed combustion processes. It is still an open question in the literature whether Pd or PdO represent the active sites for oxidation reactions. It is also important to better understand how the reactive environments, i.e. reducing versus oxidizing conditions, affect the state of palladium and its degree of dispersion. Here, a systematic study is carried out to determine the time-temperature trajectories for the transition of supported PdO into metallic Pd and vice versa. In addition, the influence of the support and the Pd particle size on the transition are discussed with special emphasis on its relationship to the activity in various oxidation reactions. The catalysts are extensively characterized by hydrogen and carbon monoxide chemisorption, temperature programmed reduction and oxidation, ex-situ and in-situ XRD, and XPS. As probe reactions, the oxidation of CO, methane, and propylene are used.