To improve the activity of Au/TiO2 catalysts toward hydrogenation without decreasing chemoselectivity, a number of isolated and supported Au, Pt, and Au-Pt nanoparticles have been investigated by means of density functional theory (DFT) calculations. H-2 dissociation on Pt and Au Pt nanoparticles is considerably faster than that on Au, because H-2 adsorption on Pt atoms is dissociative and no activation barriers are involved. The high chemoselectivity of Au/TiO2 catalysts does not exist in Pt/TiO2 catalysts no matter how small the Pt particles are, but can be preserved in Au-Pt/TiO2 catalysts if the Au/Pt ratio is high enough as to keep Pt atoms isolated and not at the active metal support interface.