Study of CO molecules on Pd/Al2O3/NiAl(110) surface by atomic force microscopy and Kelvin probe force microscopy

Reactant adsorption sites of novel metal catalysts are difficult to characterize precisely, which is vital for understanding heterogeneous reactions and designing efficient catalytic systems. However, even at cryogenic temperatures, a complete atomic understanding of catalytic reaction sites remains elusive, such as the variation in reactant molecule adsorption sites on metal nanoclusters (NCs). Here, we studied CO adsorption on the Pd NC of an Al2O3/NiAl(110) surface with atomic resolution by noncontact atomic force microscopy and Kelvin probe force microscopy at room temperature. We found that CO molecules are preferentially adsorbed on the Pd NC (~2 nm) on line defects. We investigated the consecutive scanning topographic AFM images of CO molecules on the Pd/Al2O3/NiAl surface and found the most stable adsorption site of CO molecules on bridge site and the most unstable adsorbed site on step_110, which are supported by density functional theory (DFT) calculations. This result reveals that the electronic and geometric properties of Pd NCs and CO molecules are expected to provide insight into the mechanism of Pd-based heterogeneous catalysis. Graphical abstract