Why Bimetallic Catalysts?
Information from our American colleagues
The field of heterogeneous catalysis has recently turned its attention to the study of
bimetallic catalysts because they offer the potential of increased activities and selectivities combined with enhanced stability as compared to their monometallic counterparts. Several families of bimetallic catalysts have been commercialised for use in industrial environmental treatment, chemical synthesis and petroleum refining processes.
Currently, commercial scale methods for the production of bimetallic catalysts –typically involving successive impregnation or co-impregnation of two metal salts suffer from an inability to carefully control the placement of the two different metals on to the substrate. As a result, a complex mixture of unwanted monometallic particles and bimetallic particles of varying compositions is obtained with no reproducibility from batch to batch. The catalytic performance of such mixtures is, not surprisingly, difficult to control.
Several alternative methods have been developed for the synthesis of bimetallic catalysts, but each of these approaches (ion exchange using a zeolite framework, polyvinyl alcohol or poly vinylpyrrolidone suspension/stabilization of bimetallic colloids, nanoparticles or clusters; and heteropolyatomic, organometallic complexes with a ligand -stabilised bimetallic core) also suffers to some extent from an inability to guarantee formation of the specific, desired bimetallic species as well as a limited combination of catalytically activated metals.
Galvanic Displacement and Electroless Deposition, however, rely on redox chemical reactions to control the placement of a secondary metal onto a monometallic primary catalyst. Galvanic Displacement is limited to the deposition of noble metal salts with a higher reduction potential onto less noble (or base metals with lower reduction potentials.
Electroless Deposition is more versatile, since all metals that can be deposited using electro-deposition can be used in this process as well.
Any bimetallic composition can be prepared as long as the overall redox reaction of the reducible metal salt by the reducing agent is thermodynamically favourable.
In addition,Electroless Deposition does not require high temperature calcinations or reduction treatments and thus avoids the potential for restructuring effects. Furthermore, Electroless Deposition has been extensively used at commercial scale for the production of continuous film coatings with applications in electronics and corrosion protection.
Electroless Deposition has been demonstrated as a feasible method for the preparation of bimetallic catalysts with more targeted placement of even small amounts of the secondary metal. Bimetallic catalysts that have been prepared using Electroless Deposition have been shown to have more intimate contact between the metals than catalysts prepared using traditional wet methods. Most importantly, the secondary metal species in these new bimetallic compounds is preferentially deposited onto certain sites of the primary metal, leaving other active sites available for catalysis