Oxidation, reduction, and reactivity of supported Pd nanoparticles: Mechanism and microkinetics

We have studied the oxidation and reduction kinetics of Pd nanoparticles on Fe3O4 as well as the CO oxidation kinetics on partially oxidized Pd nanoparticles. The structural properties of the Pd/Fe3O4 model catalyst as well as its adsorption behavior have been studied in detail previously. Here we present the results of fully remote-controlled pulse sequence molecular beam (PSMB) experiments, using CO and O-2 molecular beams of variable intensity. It is found that at 500 K and above large quantities of oxygen are incorporated into a Pd interface oxide and, subsequently, into a Pd surface oxide. The Pd oxide coexists with metallic Pd over a broad range of conditions. We identify two reaction regimes as a function of oxygen coverage: fast CO oxidation on the O-precovered metallic Pd and slow CO oxidation involving reduction of the Pd oxide phases. The reaction orders for both reaction regimes are determined, showing a complex flux dependent behavior in the latter case. Experiments at 500 K reveal that there is a slow equilibrium between chemisorbed oxygen on the Pd metal and oxygen incorporated in the Pd oxide phases. The corresponding rate and equilibrium constants are determined, showing that at low to intermediate oxidation levels the equilibrium strongly favors the Pd oxide. As a consequence, there is a continuous depletion of chemisorbed oxygen on the metallic Pd metal at 500 K and above. An analysis of the CO oxidation kinetics on partially oxidized Pd particles in combination with microkinetic modeling suggests that the reduction of the Pd oxide is likely to proceed via two competing reaction channels: The first channel proceeds via decomposition of the oxide and release of oxygen onto the metal, followed by reaction with CO. The second channel is likely to involve a direct reaction with the oxide, possibly via a minority of active sites on the Pd oxide.