Oxygen-Adsorption-Induced Charge State Change of a Pd Nanocluster on the Al₂O₃/NiAl(110) Surface

Abstract

Following the chemical state evolution of a catalyst in the catalytic cycle is crucial for the identification of the catalyst’s active phase and reaction mechanism. However, it is difficult to ascertain the oxidation state of a metal catalyst following oxygen exposure. Here, we present a time-scale study of the charge state of Pd nanoclusters on a model catalyst system, Pd/Al2O3/NiAl­(110), during an exposure of molecular oxygen by noncontact atomic force microscopy and Kelvin probe force microscopy (KPFM). We speculate that the Pd nanocluster can be oxidized by O2 at room temperature, leading to the formation of metal-complex Pd x O y nanoclusters. Pd x O y shows a positive charge on the alumina surface in KPFM images, and it is the major sintering species in contrast with the stable Pd nanocluster. In addition, Pd x O y contains weak binding oxygen that can be removed after annealing to a higher temperature. Following the evolution from individual well-dispersed metal nanoclusters to the oxidized state enables the identification of the key processes that underlie gas-induced charge transition.