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 Huckel Molecular Orbit al (EHMO) calculations. The parameters for the rutile part were optimi sed to reproduce the experimental rutile bulk structure and were teste d in several small clusters up to [(TiO2)(31)(OH)(32)](32-). 6H(2)O, a 175 atoms cluster. It was found that the average experimental bond le gth can be reproduced with good accuracy. However the slight distortio n of the TiO6 octahedra is calculated with the wrong sign (four long a nd two short Ti-O bonds). The agreement for the angle alpha(O-Ti-O) is less satisfactory. The study shows that CO2 can adsorb on fivefold co ordinated 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 b inding as a Lewis base, CO2 can adsorb linearly forming a single Ti-OC O bond, or interact with two neighboring Ti4+ sites. A chelating struc ture 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 forme d by interaction with either terminal oxygen ions or bridging oxygen c enters.