(716c) Preparation, Characterization and Evaluation of Group IB-Pd Bimetallic Catalysts Prepared by Electroless Deposition
Preparation, Characterization and Evaluation of Group IB-Pd Bimetallic Catalysts Prepared by Electroless Deposition
Yunya Zhang, Jayakiran Rebelli, Christopher Williams, and John R. Monnier
Department of Chemical Engineering, University of South Carolina, Columbia SC 29208, USA
Research has shown that bimetallic catalysts often enhance activity, selectivity and stability relative to their corresponding monometallic catalysts because of interactions between these two metals. Co-impregnation and successive impregnation are two widely used methods in industry to synthesize bimetallic catalysts. However, these two methods commonly produce both monometallic and bimetallic catalyst particles. This complex mixture of monometallic and bimetallic particles (of variable composition) results in poor control of the final catalyst composition and makes it very difficult to characterize these catalysts and correlate catalyst performance with bimetallic catalyst composition.
Electroless deposition (ED) is an alternative, industrially-feasible method to produce a wide range of truly bimetallic catalysts. This methodology uses a controlled chemical reaction to deposit a second metal onto a pre-existing (primary) metallic surface from a solution containing a reducible metal salt and a reducing agent. Because the reducing agent is catalytically activated by the pre-existing metallic surface, the second metal is deposited only on the primary metal. Since the ED process is kinetically-controlled, the final composition of a particular bimetallic catalyst can be controlled to give rather precise combinations of the two metallic components.
We have used ED to prepare an extensive series of Au-, Ag-, and Cu-Pd/SiO2 bimetallic catalysts having controlled, incremental surface coverages of each group IB metal. Electroless deposition baths were developed using the bis-cyano metal (1+) salts of each Group IB metal as precursors and hydrazine (for Au and Ag) or dimethylamine borane (for Cu) as the reducing agents.
The Group IB bimetallic catalysts have been characterized by atomic absorption spectroscopy, selective H2 chemisorption, and Fourier transform infrared spectroscopy (FTIR) of CO. Because Cu, Ag, and Au do not chemisorb H2 or CO at room temperature, chemisorption and FTIR are specific for the exposed Pd component of the bimetallic catalysts. The analytical results demonstrate that electroless deposition is an effective method to prepare bimetallic catalysts with controlled coverages of the IB metal on the Pd surface. These bimetallic catalysts have been evaluated for selective hydrogenation of acetylene in the presence of ethylene, an important industrial process. The kinetic results indicate superior performances for high coverages of the IB metal on the Group IB-Pd bimetallic catalysts. Explanations will be given for the enhanced performances of the compositions.