Size effects of PtcoreAgshell and AgcorePtshell nanoparticles on their catalytic activity in aqueous solutions

Platinum colloids in an aqueous solution catalyze the reduction of silver ions by hydrogen to form Pt-Ag core-shell bimetallic nanoparticles. In the presence of the silver nanoparticles, Pt^II ions are reduced by hydrogen to form Ag_corePt_shell nanoparticles. The effect of the structure and composition of the nanoparticles on the ability of platinum to catalyze the one-electron reduction of methyl viologen by hydrogen in an aqueous solution was studied. For the Pt_coreAg_shell nanoparticles, an induction period preceding the start of the reaction was found. The thicker the silver shell on platinum, the longer the lag time of the reaction, which is probably due to a decrease in the rate of hydrogen transfer to the platinum core. For the Ag_corePt_shell nanoparticles, the size effect was revealed: at the shell thickness less than 1 nm (～4 atomic layers of platinum), platinum loses the ability to catalyze the reaction. The mechanism of the catalytic process is discussed.