Scanning Tunneling Microscopy Study of Formate Species Synthesized from CO2 Hydrogenation and Prepared by Adsorption of Formic Acid over Cu(111)

Abstract

The adsorption structure of formate species on Cu(111), prepared by two different methods, was studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM), that is, the synthesis by the hydrogenation of CO2 at atmospheric pressure and the adsorption of formic acid on an oxygen-precovered Cu(111). Linear chains of formate molecules were imaged by the hydrogenation of CO2 by STM at low formate coverage with the distance to the nearest neighbor of the formate species estimated to be 5.0 ± 0.2 Å, twice that of the nearest Cu−Cu neighbor. The adsorption phase of formate thus grows linearly at the initial stage by an anisotropic attractive interaction between the formate species. The ordered structure of the formate species changed in the order of p(2 × 4), c(2 × 8), (7 × 7), p(2 × 3), (5 × 5), and c(2 × 4) with increasing formate coverage, indicating that various ordered structures appeared corresponding to the small change in the formate coverage. All the formate structures, except for (7 × 7) and (5 × 5), consisted of the same chain as observed at low coverage, with the distance between the formate chains decreasing as the formate coverage increased. A repulsive interaction thus acts between the formate chains, resulting in a lowering of the adsorption energy of the formate species as reported in the literature. On the other hand, (4 × 4) and (3 × 7/2) structures were observed upon exposing the Cu(111) surface to formic acid at 2 and 15 L, respectively, showing no chain structure of the formate species. At low formate coverage, no molecular image was observed, indicating that the formate species diffuses more rapidly than the scanning of the STM tip. Thus, the adsorption structure of formate on Cu(111) was found to be different depending on the preparation method. It is suggested that the difference in the adsorption structure strongly influences the rate constant of the previously reported formate decomposition.