Bimetallic layer-by-layer films and their application in catalytic hydrogenation of olefin

Abstract

Since bimetallic catalysts exhibit enhanced activity and stability over their monometallic counterparts, bimetallic film catalysts (Pd/bpy/Pt/bpy)_n, were first prepared by alternating immersions of a substrate in a K₂PdCl₄ precursor (or K₂PtCl₄ precursor) and bpy (bpy = 4,4′-bipyridyl) solutions through a layer-by-layer (LbL) self-assembly method. The self-assembly of bimetallic films was characterized by UV-vis spectra. The absorbance increases with the number of bilayers and the film growth shows a good linear correlation between the optical absorption and the number of (Pd/bpy/Pt/bpy)_n. Pd and Pt contents of (Pd/bpy/Pt/bpy)₅ films were determined to be 2.0 × 10⁻⁷ and 8.4 × 10⁻⁸ mol using an inductively coupled plasma OES spectrometer (ICP-OES), respectively. The as-prepared (Pd/bpy/Pt/bpy)₅ film demonstrates a remarkable catalytic activity toward hydrogenation of olefin bearing different characteristics under mild conditions. The relationship between the catalytic activity and the number of bilayers was investigated. The catalytic activity of the (Pd/bpy/Pt/bpy)_n films increases with the number of bilayers below 4 bilayers for styrene hydrogenation and remains unchanged after 4 bilayers. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) illustrate that Pd and Pt ions of (Pd/bpy/Pt/bpy)_n films are in situ reduced into Pd and Pt nanoparticles (NPs) during the hydrogenation process, resulting in high catalytic activity. In addition, the control experiments show that the catalytic activity of our bimetallic catalysts is higher than that of the prereduced bimetallic catalyst and the physical mixtures of the Pd and Pt film catalysts. As compared to traditional heterogeneous catalysts, the film catalysts have superior advantages of easy separation and good recyclability, because they are easily removed from the reaction mixture without separation filtration.