Bimetallic catalysts may be prepared using many different methodologies. Two of the most widely used industrial procedures are successive impregnation of different metal salts, in which the two metal salts are successively deposited and reduced on the support, and coimpregnation, where the two metal salts are simultaneously deposited and reduced on the support. These preparative methods provide little control over metal placement, since monometallic particles from each of the two metal salts as well as bimetallic particles of varying compositions are produced. This complex mixture of monometallic and bimetallic particles results in poor control of the final catalyst performance and makes any correlations between catalyst performance, catalyst characterization, and catalyst composition virtually impossible or fortuitous, at the very best. An alternative method is Electroless Deposition (ED) wherein a metal salt is deposited from solution by a controlled chemical reaction that is catalyzed by the pre-existing metal (catalysis) or by the metal which is being deposited (autocatalysis).

Two extremes of bimetallic catalysts prepared by ED can be realized. Bimetallic catalysts having low and partial coverages (< 1 monolayer of the ED metal based on the surface of the pre-existing metal) can result in bimetallic surfaces with new properties due to existence of ensemble effects, electronic effects, or bifunctional effects. Bimetallic catalysts with high coverages (> 1 layer) of an autocatalytic metal may exist as core-shell, pseudo-monometallic catalysts where the primary metal acts as a seed site for the ED metal. This methodology offers the potential to control size and distribution of the deposited metal by having kinetic control of electroless deposition. Further, if the shell layer is thin enough, electronic communication between the shell layer and seed component may provide a new catalyst with unique properties.

In this talk, we focus on bimetallic catalysts containing partial coverages of the ED metal on the base metal composition. Results are presented for a series of Ag-Pt/SiO₂ catalysts prepared by the deposition of Ag⁺ (using formaldehyde as the reducing agent) onto a base Pt/SiO₂ catalyst. The kinetics for the electroless deposition reaction have been determined and the results of catalyst characterization by selective chemisorption and FT-IR of adsorbed CO are presented. The characterization data indicate that Ag is preferentially located on the (111) facets, rather than the more reactive corner and edge atoms of the base Pt catalyst. The Ag-Pt/SiO₂ catalysts have been evaluated for the selective hydrogenation of 3,4-epoxy-1-butene (EpB), a multi-functional olefin; interestingly, the catalyst containing an optimum, fractional coverage of Ag on Pt exhibits much higher specific activity for EpB conversion as well as much higher selectivity to epoxybutane formation than the base Pt/SiO₂ catalyst. The mode of EpB adsorption is modified by the presence of Ag, which subsequently alters the reaction kinetics for the hydrogenation of EpB.