Surface Orientation\nDependent Dissociation of Formic\nAcid and Adsorption States of Formate on Copper

The surface orientation dependence of the dissociation\nof formic\nacid and the adsorption structures of formate on single-crystalline\nCu surfaces at 300 K were investigated by infrared reflection absorption\nspectroscopy (IRAS) combined with density functional theory (DFT)\ncalculations. A monotonic increase in peak intensity of the formate\nOCO symmetric stretching (ν<sub>s</sub>(OCO)) mode was observed\non Cu(111), indicating the dissociation of formic acid exposed on\nCu(111) at 300 K and a uniform adsorption state of formate at terrace\nsites. In contrast, multiple ν<sub>s</sub>(OCO) peaks of formate\nat step sites were detected on Cu(997) at a much lower formic acid\nexposure compared with Cu(111). After the saturation of the step sites,\na slow reaction at the terrace sites was observed, which shows the\nsite-specific reactivity of formic acid on Cu(997). In the case of\nCu(110), the reaction rate of the formic acid to produce formate was\nsignificantly higher than other crystal faces, leading to densely\npacked formate adsorption. Novel periodic superstructures of formate\non Cu(110) were detected by low-energy electron diffraction. Much\nbroader ν<sub>s</sub>(OCO) bands were observed on Cu(100), indicating\nvarious adsorption states compared with the other faces. DFT calculations\nshow that the peak position of the ν<sub>s</sub>(OCO) mode is\naffected mainly by the interaction between adsorbed formate molecules.\nShorter intermolecular distances at higher formate coverages lead\nto a blueshift of the formate ν<sub>s</sub>(OCO) mode, which\nis induced by the repulsive electrostatic intermolecular interaction.\nThe experimental and theoretical results indicate that the adsorption\nstructure of formate depends significantly on the surface orientation\nand coverage.