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

Poly(N-vinyl-2-pyrrolidone)-protected Ni/Pd bimetallic colloidal nanopartic les, prepared by the polyol reduction method, have been proved to have a na nometer-sized alloy structure with both metals at zerovalent state by our p revious study of TEM, XRD, EXAFS, and XPS analyses. Here, dispersions of th ese bimetallic nanoclusters with different composition ratios are extensive ly examined as catalysts for the hydrogenation of various nitrobenzene deri vatives: i.e., p-nitrotoluene, p-nitroanisole, 1-nitronaphthalene, p-nitrob enzonitrile, and methyl p-nitrobenzoate, at 30 degrees C under an atmospher ic pressure of hydrogen. These bimetallic nanoclusters exhibit excellent ca talytic properties for the reduction of a nitro group to an amino group wit h high selectivity. The catalytic activity strongly depends on the metal co mposition of the particles. The maximum catalytic activity can be observed at a certain intermediate composition ratio, being 3-4 times greater than t hat 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 hydrog enation of para-substituted nitrobenzenes. An approximately linear relation ship exists between the hydrogenation rate of the substrate with an electro n-donating or electron-withdrawing group and the corresponding Hammett cons tant of the substituent, as well as between the hydrogenation rate and the LUMO energy level of the substrate.