Catalysis of polymer-protected Ni/Pd bimetallic nano-clusters for hydrogenation of nitrobenzene derivatives

Poly(N-vinyl-2-pyrrolidone)-protected Ni/Pd bimetallic colloidal nanoparticles, prepared by the polyol reduction method, have been proved to have a nanometer-sized alloy structure with both metals at zerovalent state by our previous study of TEM, XRD, EXAFS, and XPS analyses. Here, dispersions of these bimetallic nanoclusters with different composition ratios are extensively examined as catalysts for the hydrogenation of various nitrobenzene derivatives: i.e., p-nitrotoluene, p-nitroanisole, 1-nitronaphthalene, p-nitrobenzonitrile, and methyl p-nitrobenzoate, at 30 degrees C under an atmospheric pressure of hydrogen. These bimetallic nanoclusters exhibit excellent catalytic properties for the reduction of a nitro group to an amino group with high selectivity. The catalytic activity strongly depends on the metal composition of the particles. The maximum catalytic activity can be observed at a certain intermediate composition ratio, being 3-4 times greater than that of a monometallic colloidal Pd catalyst. A bimetallic nanocluster with the mole ratio of Ni : Pd = 1/4 was the most active catalyst for the hydrogenation of para-substituted nitrobenzenes. An approximately linear relationship exists between the hydrogenation rate of the substrate with an electron-donating or electron-withdrawing group and the corresponding Hammett constant of the substituent, as well as between the hydrogenation rate and the LUMO energy level of the substrate.