A NEW METHOD OF CALCULATING INTERACTIONS BETWEEN ADSORBATES AND METAL-OXIDE SURFACES - APPLICATION TO THE STUDY OF CO2 INSERTION IN HYDROXYL OR METHOXY GROUPS ON AL2O3 AND TIO2

A new method of investigating adsorbate-substrate interactions is pres ented. This approach serves to support and extend information gleaned by infrared spectroscopy on reaction mechanisms. The theoretical metho d uses Green's matrix to evaluate the electronic potential of a superm olecule embedded in a periodic lattice substrate. The supermolecule in cludes the adsorbate and sufficient atoms of the substrate active site to cater for specific interactions. The combined system is allowed to relax, subject to the interaction potential. This is taken into accou nt by direct perturbation of Green's matrix. The final matrix gives th e electronic density matrix after Fourier transformation. This approac h has been programmed both at semi-empirical and ab initio levels by o ne of the authors. The semi-empirical GEOMOS package with an MNDO hami ltonian generalized to include d orbitals is used for the present appl ication to CO2 insertion in hydroxyl or methoxy groups on Al2O3 and Ti O2. The mechanistic information obtained by in situ FT-IR is confirmed : the first reaction step is an electron donor-acceptor interaction be tween the oxygen of type I hydroxyl groups and the CO2 carbon of the a dsorbed species.