First-principle analysis of the dissociative adsorption of formic acid on rutile TiO2(110)

The adsorption of formic acid (HCOOH) on the rutile TiO2(110) surface, where HCOOH dissociates into formate (HCOO) and hydrogen (H), has been analyzed with first-principles density functional calculations. A p(2 x 1) geometry can be observed for high exposure, while only local c(4 x 2) domains are detected for lower exposure. After desorption of some HCOOH at higher temperatures the p(2 x 1) geometry becomes unstable and the remaining molecules form a disordered structure. A direct and simultaneous observation of PI and HCOO is difficult, but the calculations show that coadsorption of H is necessary to stabilize the p(2 x 1) geometry. Some of the vibrational modes also show a slight dependence on the supply of H and might give experimentally accessible hints concerning the role of H. An interesting aspect concerns the surface diffusion of HCOO: while HCOO molecules behave rather immobile on the surface in the c(4 x 2) domains and in the disordered structure, they show a high mobility along the [001] direction in a p(3 x 1)-like environment. As it turns out, the reason for these findings can be found in the complex movement of HCOO molecules, which includes the coadsorbed HCOO-H compound as a whole. (C) 2000 Elsevier Science B.V. All rights reserved.