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

Abstract

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.