Van der Waals density functional study of formic acid adsorption and decomposition on Cu(111).

We present a density functional theory study on the adsorption and decomposition mechanisms of monomeric formic acid (HCOOH) on a Cu(111) surface. We used Perdew-Burke-Ernzerhof (PBE) functional, PBE with dispersion correction (PBE-D2), and van der Waals density functionals (vdW-DFs). We found that the adsorption energy of HCOOH by using the PBE functional is smaller than the experimental value, while the PBE-D2 and vdW-DFs give better agreement with experimental results. The activation energies of decomposition calculated by using PBE-D2 and vdW-DFs are lower compared with desorption energies, seemingly in contradiction with experimental findings at room temperature, in which no decomposition of HCOOH on Cu(111) is observed when the surface is exposed to the gas phase HCOOH. We performed the reaction rate analysis based on the first-principles calculations for desorption and decomposition processes to clarify this contradiction. We found that the desorption of monomeric HCOOH is faster than that of its decomposition rate at room temperature because of a much larger pre-exponential factor. Thus, no decomposition of monomeric HCOOH should take place at room temperature. Our analysis revealed the competition between desorption and decomposition processes of HCOOH.