Surface Orientation Dependent Dissociation of Formic Acid and Adsorption States of Formate on Copper

The surface orientation dependence of the dissociation of formic acid and the adsorption structures of formate on single-crystalline Cu surfaces at 300 K were investigated by infrared reflection absorption spectroscopy (IRAS) combined with density functional theory (DFT) calculations. A monotonic increase in peak intensity of the formate OCO symmetric stretching (νs(OCO)) mode was observed on Cu(111), indicating the dissociation of formic acid exposed on Cu(111) at 300 K and a uniform adsorption state of formate at terrace sites. In contrast, multiple νs(OCO) peaks of formate at step sites were detected on Cu(997) at a much lower formic acid exposure compared with Cu(111). After the saturation of the step sites, a slow reaction at the terrace sites was observed, which shows the site-specific reactivity of formic acid on Cu(997). In the case of Cu(110), the reaction rate of the formic acid to produce formate was significantly higher than other crystal faces, leading to densely packed formate adsorption. Novel periodic superstructures of formate on Cu(110) were detected by low-energy electron diffraction. Much broader νs(OCO) bands were observed on Cu(100), indicating various adsorption states compared with the other faces. DFT calculations show that the peak position of the νs(OCO) mode is affected mainly by the interaction between adsorbed formate molecules. Shorter intermolecular distances at higher formate coverages lead to a blueshift of the formate νs(OCO) mode, which is induced by the repulsive electrostatic intermolecular interaction. The experimental and theoretical results indicate that the adsorption structure of formate depends significantly on the surface orientation and coverage.