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 presented. This approach serves to support and extend information gleaned by infrared spectroscopy on reaction mechanisms. The theoretical method uses Green's matrix to evaluate the electronic potential of a supermolecule embedded in a periodic lattice substrate. The supermolecule includes 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 account by direct perturbation of Green's matrix. The final matrix gives the electronic density matrix after Fourier transformation. This approach has been programmed both at semi-empirical and ab initio levels by one of the authors. The semi-empirical GEOMOS package with an MNDO hamiltonian generalized to include d orbitals is used for the present application to CO2 insertion in hydroxyl or methoxy groups on Al2O3 and TiO2. The mechanistic information obtained by in situ FT-IR is confirmed: the first reaction step is an electron donor-acceptor interaction between the oxygen of type I hydroxyl groups and the CO2 carbon of the adsorbed species.