Adsorption of small organic molecules on anatase and rutile surfaces: a theoretical study

The adsorption of several small organic molecules on rutile (110) and (100) as well as on anatase (101) surfaces was investigated by Car-Parrinello molecular dynamics in aqueous solution and a new approach to the calculation of adsorption energies is proposed, taking into account the potential energy fluctuation of larger systems. Acetylene and ethylene insert into twin oxygen vacancies in the surface and form polarized covalent Ti-C bonds. In one case spontaneous coupling of two acetylene molecules to a C(4)H(3) molecule with a structure similar to trans-butadiene was observed. Neutral catechol and the singly charged anion were not reactive on any titanium dioxide surface, but the twofold-charged anion attained stable mono- and bidentated geometries on anatase. Methanol, ethanol, formaldehyde and acetaldehyde adsorbed with their functional groups. Very stable geometries provide a Ti-O bond and have adsorption energies of 60-200 kJ/mol. The adsorbates compete with water molecules for similar adsorption sites in point defects as well as on perfect surfaces.