Theoretical study of the chemisorption and surface reaction of HCOOH on a ZnO(10(1)over-bar0) surface

The mechanisms of the molecular and dissociative adsorption of formic acid HCOOH on a ZnO(<10(1)over bar 0>) surface were investigated by means of the ab initio molecular orbital method using a Zn4O4 cluster embedded in an electrostatic field represented by 464 point charges at the crystal ZnO lattice positions. cis-Formic acid molecularly adsorbs on a ZnO(<10(1)over bar 0>) surface without activation energy. It dissociates into a formate anion HCOO- and a proton H+ with an activation energy of 11.7 kcal/mol. The formate anion is geometrically stable in the bridging structure of two Zn atoms and the unidentate structure of a Zn atom interacting with a surface OH species. The adsorption energy is about 80 kcal/mol for both structures. In contrast, the bidentate structure is about 24 kcal/mol less stable. Possible reaction pathways for the dissociation of the O-H bond of cis-formic acid were also examined. trans-Formic acid is dissociatively adsorbed on a ZnO(<10(1)over bar 0>) surface to form a formate anion and a surface OH species without activation energy. The initial formate anion generated from trans-formic acid continues to interact with the surface OH species. This interaction produces the stability of the unidentate structure. Interconversion between the unidentate and bridging structures was also examined. The formate anion can be easily tilted on the surface by a small perturbation, leading to increased interaction between the formate species and the surface.