Chemical process of hydrogen and formic acid on a Pd-deposited Cu(111) surface studied by high-resolution X-ray photoelectron spectroscopy and density functional theory calculations

Formic acid (HCOOH) is one of the essential molecules for CO₂ utilization including methanol synthesis and hydrogen carriers. In this study, we have investigated the chemical processes of hydrogen and HCOOH on a dilute-alloy Pd-Cu(111) surface using high-resolution X-ray photoelectron spectroscopy (HR-XPS) and density functional theory (DFT) calculations. The present Pd-Cu(111) surface was prepared at 500 K, and the observed core-level shifts of Pd 3d_5/2 indicate that Pd atoms were located at the surface and subsurface sites: 335.3 eV at the surface and 335.6 eV at subsurface sites, respectively. The coverage of surface Pd atoms was estimated to be 0.05 ML, indicating that the present Pd-Cu(111) surface acted as a single atom alloy catalyst. The observed C 1s and O 1s XPS spectra indicate that the surface chemistry of HCOOH on the present Pd-Cu(111) surface is almost equivalent to a bare Cu(111) surface; HCOOH is dissociated into monodentate formate and atomic hydrogen at 150-160 K, followed by conversion to bidentate formate species at 300 K, and finally it is decomposed and desorbed as CO₂ + ½H₂ at ∼450 K. The conversion ratio of adsorbed HCOOH to bidentate formate species on Pd-Cu(111) was 12%, almost the same as that on Cu(111). That monodentate formate species and atomic hydrogen aggregate around the Pd atom is supported by the observed core-level shift of Pd 3d_5/2 and systematic DFT calculations. The present DFT calculations also show that formate species are preferably adsorbed on the Cu site; thus, the Pd site is unoccupied by formate species at this stage. This implies that the present single atom alloy catalyst Pd-Cu(111) has an advantage during CO₂ hydrogenation, where the Pd site can act as the H₂ dissociation site without poisoning by formate intermediate species.