Adsorption of Electropositive Atoms with an Odd Number of Electrons (H, K, Au) on a Perfect TiO2(110) Surface

The adsorption of electropositive atoms with an odd number of electrons (H, K, Au) on a perfect TiO2(110) rutile surface is investigated using periodic DFT calculations (VASP code). The rutile surface, a closed shell system when uncovered, becomes an open-shell system when covered by one of these adsorbates. One possibility for adsorption is to transfer electrons from the adsorbate A. to the rutile-surface, (the rutile is reduced): A.A. -> A(+) + e(-). The cationic A(+) moiety will then adsorb on basic 02, surface sites forming OA groups. This is the best adsorption mode for H, K and Au. Another solution is to couple two adsorbates A to give A(+) and A(-) : 2A. -> A(+) and A(-) and subsequent adsorption on basic and acidic surface sites respectively regenerating the closed-shell system. This is the best adsorption mode for the coadsorption of Au with H or K. Then, Au- adsorbs on the surface acidic site, Ti+4, while W (or K+) does on the surface basic sites, O-2. Our results confirm that K coadsorption should have a significant effect on the binding of Au particles on TiO2 surfaces and hence on the growth. Adsorption of atomic hydrogen at room temperature leads to the formation of ordered H, however upon annealing the sample to above 600 K, surprisingly, no H-2 (or H2O) molecules were found to desorb from the surface. An explanation for this unexpected behavior is provided by ab-initio calculations: the activation energy for H-atoms migrating into the bulk, 1.11 eV, is significantly smaller than that for recombinative desorption of H-2 (or H2O).