In Situ Oxidation Study of Pt Nanoparticles on MgO(001)

Noble metal nanoparticles supported by oxide carriers are widely employed in heterogeneous catalysis. To improve catalyst efficiency, it is important to understand oxidation processes on the atomic scale. Here, we studied oxygen-induced shape changes of Pt nanoparticles on MgO(001) by means of in situ surface X-ray diffraction and X-ray reflectivity measurements. The X-ray results on the particle morphology were complemented by transmission electron microscopy studies. The samples were prepared by means of physical vapor deposition and differed in average particle size and lateral particle size distribution. The oxygen-induced particle shape changes were found to be independent of particle size and were characterized by the emergence of higher indexed facets. We find the bulk oxide formation to be kinetically hindered and particle size-dependent. The formed bulk oxide structures were concluded to be (110)-oriented Pt3O4 and a modified structure of (0001)-oriented α-PtO2. CO exposure of the oxidized particles did not lead to a shape change reversibility.