Synergetic effect of gold in Pd/Au catalysis during HDS of model compounds

The simultaneous formation of colloidal dispersion of Pd and Au metal particles protected by the organic polymer, polyvinilpyrrolidone (PVP), in reducing alcohol solution, was used for the synthesis of new silica supported monometallic and bimetallic AuxPdy catalysts with different x/y ratios. The samples, with a 2 wt% total metal loading, after air calcinations at 673 K, to remove the PVP, were characterised by X-ray diffraction, X-ray photoelectron spectroscopy and FTIR of chemisorbed CO. Formation of alloyed AuxPdy particles of different metal composition with consequent modification of the electronic and geometric properties was ascertained. The catalytic activity of the samples as a function of the gold – palladium content was tested in the hydrodesulfurization of thiophene and dibenzothiophene. For the former reaction, performed at low hydrogen pressure, the promoting effect of gold on the palladium activity was confirmed. For the latter reaction, carried out under high hydrogen pressure, pure gold catalyst exhibited larger activity and different selectivity than pure palladium catalyst. In both reactions the maximum of activity was observed in correspondence of the bimetallic sample with 50/50 metal composition. The results are discussed in terms of electronic and ensemble size effects.