Modelling of the adsorption of formic acid and formaldehyde over rutile TiO2(110) and TiO2(011) clusters

Abstract

While several computational works have addressed the energetics of adsorption over TiO2(1 1 0) surface no work has, so far, addressed the adsorption energy of any molecule over TiO2(0 1 1) surface. Yet, the (0 1 1) surface, also known as phase I of reconstructed TiO2(0 0 1), is extremely active towards simple oxygenates as shown for over a decade. In this work, the adsorption of formaldehyde and formic acid has been studied using the molecular orbital semi-empirical method PM3 over a rutile Ti8O29H26 cluster representing TiO2(0 1 1) surface. A study over a rutile Ti11O42H40 cluster representing TiO2(1 1 0) surface has been made for comparison. The adsorption energy of formaldehyde was found smaller than that of formic acid over both surfaces. However, while the adsorption energy of formaldehyde was comparable over both TiO2 surfaces, that of formic acid was dramatically larger over TiO2(0 1 1) surface (the one containing all Ti4+ cations five-fold-coordinated to oxygen anions, Ti5c 4+) than that overTiO2(1 1 0) surface (the one containing alternating rows ofTi5c 4+ andTi6c 4+). This adsorption energy was computed equal to 1.6 and 1.1 eV, respectively. Repulsive interaction between bridging oxygen anions, in the case of the (1 1 0) surface is most likely the reason for the relatively smaller energy of adsorption.