Dynamic chemical processes on ZnO surfaces tuned by physisorption under ambient conditions

Abstract

The catalytic properties of non-reducible metal oxides have intrigued continuous interest in the past decades. Often time, catalytic studies of bulk non-reducible oxides focused on their high-temperature applications owing to their weak interaction with small molecules. Hereby, combining ambient-pressure scanning tunneling microscopy (AP-STM), AP X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations, we studied the activation of CO and CO2 on ZnO, a typical non-reducible oxide and major catalytic material in the conversion of C1 molecules. By visualizing the chemical processes on ZnO surfaces at the atomic scale under AP conditions, we showed that new adsorbate structures induced by the enhanced physisorption and the concerted interaction of physisorbed molecules could facilitate the activation of CO and CO2 on ZnO. The reactivity of ZnO towards CO could be observed under AP conditions, where an ordered (2 × 1)–CO structure was observed on ZnO(101¯0). Meanwhile, chemisorption of CO2 on ZnO(101¯0) under AP conditions was also enhanced by physisorbed CO2, which minimizes the repulsion between surface dipoles and causes a (3 × 1)–CO2 structure. Our study has brought molecular insight into the fundamental chemistry and catalytic properties of ZnO surfaces under realistic reaction conditions.