First principles studies of vanadia - Titania monolayer catalysts: Mechanisms of NO selective reduction

The selective reduction of NO with NH ₃ catalyzed by isolated VO, species grafted onto TiO ₂ (anatase) is studied by means of periodic density functional calculations. NH ₃ is adsorbed molecularly by the bare support both as a Lewis-bonded complex at (101) 5-fold coordinated Ti sites, and as a H-bonded complex at (001) Ti-OH sites. Analogous interactions are predicted for stable submonolayer VO _x species, which provide V ⁵⁺ Lewis acid sites and V-OH sites. Neither Ti-OH nor submonolayer V-OH groups act as Brønsted acids toward NH ₃. Reaction pathways where both Lewis-bonded and H-bonded NH ₃ complexes yield a NH ₂NO intermediate are found. In the former case, a (rate-determining) deprotonation step of NH ₃ is required, whereas, in the latter, NH ₂NO is formed directly through a concerted mechanism. This suggests mat many channels may contribute to the NO reduction process.