The selective reduction of NO with NH3 catalyzed by isolated VOx species grafted onto TiO2 (anatase) is studied by means of periodic density functional calculations. NH3 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 VOx species, which provide V5+ Lewis acid sites and V-OH sites. Neither Ti-OH nor submonolayer V-OH groups act as Bronsted acids toward NH3. Reaction pathways where both Lewis-bonded and H-bonded NH3 complexes yield a NH2NO intermediate are found. In the former case, a (rate-determining) deprotonation step of NH3 is required, whereas, in the latter, NH2NO is formed directly through a concerted mechanism. This suggests that many channels may contribute to the NO reduction process.