Size effects of PVP-Pd nanoparticles on the catalytic Suzuki reactions in aqueous solution

A series of poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized Pd nanoparticles with varying particle size are prepared by using the stepwise growth reaction. The effect of Pd particle size on the Suzuki reaction between phenylboronic acid and iodobenzene is investigated by the use of four Pd catalysts with mean particle sizes of 3.0, 3.9, 5.2, and 6.6 nm, respectively. The catalytic activity of the Pd nanoparticles expressed in terms of the initial turnover frequency (moles of the biphenyl product per mole of total surface Pd atoms per min) is found to be in the order of Pd (3.9 nm) > Pd (3.0 nm) approximate to Pd (5.2 nm) > Pd (6.6 nm), indicating that surface Pd atoms do not all have the same reactivity in this reaction. The general trend of increased catalytic activity with the decrease in the particle size suggests that the low-coordination number vertex and edge atoms on the particle surface are active sites for the Suzuki reaction. The lower catalytic activity for the smallest Pd nanoparticles might be due to stronger adsorption of the reaction intermediates on the particle surface, in which the strongly adsorbed species act as a poison to the reaction thereby decreasing the rate of the reaction.