8A series of bimetallic Pd-Ag/SiO 2 catalysts were prepared by galvanic displacement with 9 increasing loadings of Pd on Ag. The catalysts were characterized by atomic absorption 10 spectroscopy, Fourier-transform infrared spectroscopy of CO adsorption and X-ray photoelectron 11 spectroscopy. An actual Pd deposition beyond the theoretical limit for galvanic displacement 12 suggested that the large difference in surface free energy for Pd and Ag resulted in Pd diffusion 13 into the bulk of Ag particles, or Ag diffusion to the surface to provide fresh Ag atoms for further 14 galvanic displacement. Characterization results indicated that on this series of catalysts the Pd 15 atoms are distributed in very small ensembles or possibly even atomically on the Ag surface, and 16 there was a transfer of electrons from Pd to Ag at all Pd loadings. For comparison, the catalysts 17 were also evaluated for the selective hydrogenation of acetylene in excess ethylene at the 18 conditions used in our previous study of the reverse Ag-Pd/SiO 2 catalysts. The selectivities for 19 C 2 H 4 formation remained high and constant due to the geometric effects that Pd atom existed as 20 small ensembles. However, the electronic effects resulted in lower selectivities for C 2 H 4 21 formation than those from the catalysts with high coverage of Ag on Pd. 22 the base material. This method is different from ED, in that GD does not require chemical 1 reducing agents since the base metal itself already serves as the reducing agent. However, this 2 redox reaction is usually limited by the accessibility of the base metal, resulting in passivation of 3 the reaction when the entire surface of the base metal is covered. 14 4 Over the past decade, GD has been widely used for the development of metal 5 nanostructures with highly active surfaces for a variety of applications such as 6 electrocatalysts 13,16-22 , biomedicine 15 , functional coatings 23,24 , and deposition of metals on 7 semiconductors 14,25 . Xia et al. 15,26-28 have thoroughly studied the mechanism of GD for tuning 8 the properties of metal nanostructures through adjustment of composition, size, shape and 9 morphology. Complex hollow nanostructures of Pd-Ir, Au-Ag, Pd-Ag, and Pt-Ag have been 10 generated with potential for many applications. Yan et al. 13,20-22 also developed electrocatalysts 11 with Pd-Au, Pt-Cu, Pt-Pd nanotubes or nanowires prepared by GD for the oxygen reduction and 12 methanol oxidation reactions. Lee et al. 17-19 have investigated the performance of different forms 13 of hollow and porous Pd-Ag or Pt-Ag nanomaterials prepared by GD as electrocatalysts for the 14 oxygen reduction reaction. Maboudian and Carraro 14,23-25 have used GD to coat Au, Pt, Ag and 15 Cu onto Si in thin film or nanoparticle forms for surface-enhanced Raman spectroscopy, and for 16 improved interfacial behavior of semiconductors. 17 Although GD has been widely used in synthesizing electrocatalysts, it has had little 18 application for the development of catalysts used in hydrogenation reactions....
