THEORETICAL-STUDY ON THE CHEMISORPTION AND THE SURFACE-REACTION OF HCOOH ON A MGO(001) SURFACE

The routes and the mechanisms of the molecular and dissociative adsorptions of a formic acid HCOOH and the succeeding migration of a proton on MgO(001) surface are investigated by means of the ab-initio molecular orbital method using the Mg8O8 cluster embedded in the electrostatic field due to the 284 point charges on the crystal lattice positions. Both cis- and trans-formic acids are molecularly adsorbed on an MgO(001) surface with no energy barrier, the trams form being more stable than the cis form. They are dissociated into a formate anion HCOO- and a proton H+ sat on two Mg and O atoms, respectively. We examine the favorable pathway for the dissociation of the O-H bond. The most stable structure of the formate anion is the bridging type on two Mg atoms and the heat of adsorption is 36 kcal/mol. The formate anion adsorbed on the surface activates the surface O atom next to it, because the electrons in the pi molecular orbital of the formate anion repel the electrons in the p orbital of this surface O atom, raising it up to the HOMO of the system. The proton adsorbed on the surface O atom hardly migrates because of the high energy barrier of 83-94 kcal/mol, but the existence of the adsorbed formate ion helps this migration by reducing the energy barrier to 44 kcal/mol, through the formation of the molecularly adsorbed formic acid intermediate. The formate anion is shown to be easily tilted on the surface by a small perturbation, leading to an increased interaction between the formate species and the surface.