Controlled synthesis of PDDA stabilized Au-Pd bimetallic nanostructures and their activity in hydrogenation of acetylene

Pd hydrosol was synthesized by reducing PdCl2 solution with dissolved H-2 in the presence of PDDA [poly(diallyldimethylammonium chloride)] as ionic stabilizer. O-2 treatment resulted in the reappearance of ligand to metal charge transfer bands of PdCl42-. Repeated oxidation and hydrogen treatments caused aggregation/agglomeration of the Pd particles yielding big spherical aggregates of 30-50 nm but still keeping the size of the 5-6 nm primary particles. Aggregation was attributed to the reduction of PdCl42- anion having high local densities around Pd particles surrounded by PDDA polycation. Au-Pd bimetallic hydrosols were synthesized by reduction with 2-propanol and by pulse radiolysis technique in the presence of PDDA. Both reduction modes synthesize Au core-Pd shell structures. The mode of reduction was observed to affect the particle size and the thickness of the Au core and Pd shell. At the Au rich samples the differences in the optical spectra were attributed to different dispersions of the Au-Pd particles. Au-Pd bimetallic hydrosols were adsorbed on Aerosil 200 and the supported samples were tested in gas phase hydrogenation of acetylene.