Hydrogen adsorption on the tetragonal ZrO2(101) surface: a theoretical study of an important catalytic reactant

Abstract

In order to understand the fundamental properties of the activation of zirconia by hydrogen in relation to the dehydrogenation of hexane, we have performed first-principles calculations using the density functional formalism (PW91 functional) and a plane wave basis set to describe the valence electronic wavefunctions. The interaction of hydrogen atoms and molecules with the thermodynamically most stable, stoichiometric (101) surface has been examined in detail. Three main stages of the hydrogen–ZrO₂(101) interaction can be found: a weak interaction corresponding to molecular adsorption of H₂ on the top of one surface Zr atom (E_ad = −7.1 kJ mol⁻¹), dissociative adsorption, which leaves H atoms on top of one Zr and one O atom (−17.8 kJ mol⁻¹), and, finally, a repulsive interaction (+81.0 kJ mol⁻¹) as precursor of water formation when hydrogen atoms are located above oxygen positions. Desorption of water (+179.9 kJ mol⁻¹) forms a defect at the surface and creates therefore a zirconia suboxide. To separate these effects, atomic hydrogen adsorption has also been considered. Changes in the geometry and charge are discussed as well as the band structures of the adsorbates relative to the vacuum energy. The results are discussed and compared with the available experimental data.