Elucidation of the atomic-scale processes of dissociative adsorption and spillover of hydrogen on the single atom alloy catalyst Pd/Cu(111).

Hydrogen spillover is a crucial process in the selective hydrogenation reactions on Pd/Cu single atom alloy catalysts. In this study, we report the atomic-scale perspective of these processes on the single atom alloy catalyst Pd/Cu(111) based on the experimental and theoretical results, including infrared reflection absorption spectroscopy (IRAS), temperature programmed desorption (TPD), high-resolution X-ray photoelectron spectroscopy (HR-XPS), and density functional theory (DFT) calculations for core-level excitation. The hydrogen spillover onto Cu(111) was successfully observed in real time using time-resolved IRAS measurements at 80 K. The chemical shifts of Pd 3d indicate that H is dissociated and adsorbed at the Pd site. In addition, a "two-step" chemical shift of the Pd 3d binding energy was observed, indicating two types of hydrogen adsorption states at the Pd site. The proposed mechanism of the hydrogen dissociation and spillover processes is as follows: (i) a hydrogen molecule is dissociated at a Pd site, and the hydrogen atoms are adsorbed on the Pd site; (ii) the number of hydrogen atoms on the Pd site increases up to three; and (iii) the hydrogen atoms will spill over onto the Cu surface.