Interaction of CO2 with small rutile crystallites-an EHMO study

Several possible adsorption sites and adsorption geometries of CO2 on small rutile fragments were studied by Extended Hückel Molecular Orbital (EHMO) calculations. The parameters for the rutile part were optimised to reproduce the experimental rutile bulk structure and were tested in several small clusters up to [(TiO2)31(OH)32]32−•6H2O, a 175 atoms cluster. It was found that the average experimental bond legth can be reproduced with good accuracy. However the slight distortion of the TiO6 octahedra is calculated with the wrong sign (four long and two short Ti−O bonds). The agreement for the angle αO-Ti-O is less satisfactory. The study shows that CO2 can adsorb on fivefold coordinated surface titanium sites as well as surface oxygen sites. This means that CO2 can act as either Lewis base or acid. In the case of binding as a Lewis base, CO2 can adsorb linearly forming a single Ti−OCO bond, or interact with two neighboring Ti4+ sites. A chelating structure forming two Ti−O bonds was found to be weakly stable at the most. When CO2 behaves as a Lewis acid, a carbonate-like structure is formed by interaction with either terminal oxygen ions or bridging oxygen centers.