Melanie T. Schaal

Preparation of True Bimetallic Catalysts via
Electroless Deposition Methodologies

MELANIE SCHAAL

YARITZA LOPEZ

            Catalysts are materials that increase the rate of a given reaction without being consumed in the reaction.  These materials are often composed of a precious metal maintained on an inexpensive support such as alumina or silica.  Our research has focused on modifying these catalysts by adding a second, closely associated metal to form a bimetallic catalyst.  Such catalysts are often used to enhance the formation of a specific desired product.  However, the typical preparation methods are problematic since it is virtually impossible to ensure formation of only bimetallic particles; rather, catalysts containing particles of each metallic component are usually formed.  As such, an alternative and more precise method of preparation is essential. 

            For the past year we have worked to determine whether a preparative method known as electroless deposition (ED) can be modified to reliably produce true bimetallic catalysts.  In electroless deposition, metallic components are deposited by a controlled chemical reaction that is catalyzed by the pre-existing metal (catalysis) or the metal which is itself being deposited (auto-catalysis).  In principle, the secondary metal is deposited only on the primary metal or on the pre-existing secondary metal, and not on the catalyst support.  Thus, the problems associated with other preparation methods should not be observed using ED methodologies.

            In the electroless deposition process, a monometallic catalyst (Pt/SiO₂, in our study) is added to a “bath” that, in our case, is composed of silver nitrate (AgNO₃), formaldehyde (HCHO), which is a reducing agent, and a pH modifier (NaOH).  The required concentrations of these materials that allows for sustained bath stability with controllable silver deposition rates have been determined.  The specificity of the ED bath was tested using plain silica (SiO₂) support and no silver deposition was seen.  The same bath was then tested with Pt/SiO₂ (primary metal catalyst) and a steady rate of silver deposition was observed, proving that ED must be catalyzed by the presence of a pre-existing catalytic metal.  The effects of platinum catalyst concentration, silver salt concentration, and formaldehyde reducing agent concentration on the rate of silver deposition have been determined.  Overall, the concentrations of the reducing agent (formaldehyde) and the monometallic metal catalyst were found to have greater effects on the rate of silver deposition (0.7 and 0.9 reaction order, respectively) than the concentration of the silver salt (0.2 reaction order).  Thus, the rate of Ag deposition on Pt surfaces is nearly first order in Pt surface site and formaldehyde concentrations and close to zero order in silver salt concentration.  The effects of temperature and pH will be next examined to complete a full kinetic expression for the electroless deposition of Ag onto Pt/SiO₂. 

            Once the method of preparation was fully established, several catalysts containing varying amounts of silver were prepared.  The resulting catalysts have been characterized by atomic adsorption (AA) to measure the amount of silver deposited on the Pt/SiO₂ catalysts and by selective H₂ chemisorption to determine the influence of silver deposition on the concentration of catalytically-active, surface Pt sites.  In addition, CO adsorption on Pt has been studied via Fourier Transform-Infrared Spectroscopy (FT-IR) to determine the actual sites of the Pt surface where silver deposition has occurred.  Based on the current IR results it appears that the silver is selectively deposited on the Pt(111) crystalline planes.  Additionally, the peak for CO adsorption changes with increasing silver loadings, suggesting that higher Ag loadings dilutes the Pt surface into smaller clusters, or ensembles of Pt atoms on the surface of the bimetallic catalyst.  This geometric effect may also be enhanced by electronic effects of Ag on the Pt surface sites (alterations of the electronic properties of platinum atoms by adjacent silver atoms).  These effects will be studied in the next several months by coupling isotopic carbon monoxide  (C¹³O¹⁶)  adsorption with FT-IR analysis.  Also, further characterization of the catalysts by high resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS) are planned to better characterize the surfaces of these bimetallic catalysts. 

            Kinetic studies have been performed with these Ag-Pt/SiO₂ bimetallic catalysts using as a probe reaction the selective hydrogenation of 3,4-epoxy-1-butene (EpB), a multifunctional olefin that has been commercially used as an intermediate for production of specialty and pharmaceutical chemicals.  Background reactions with SiO₂ and Ag/SiO₂  have shown that both Ag and SiO₂ are inactive for EpB hydrogenation. Thus, the base Pt/silica catalyst would be expected to have the highest activity for EpB hydrogenation, since addition of Ag to the Pt surface should lower the concentration of surface Pt sites. However, we find that the addition of small amounts of silver via ED to Pt/SiO₂ markedly  increase (3x) the overall activity for EpB hydrogenation compared to that for the Pt/SiO₂ catalyst.  Addition of larger amounts of silver results in a sharp decrease in the EpB conversion, which again is expected since silver is inactive for this reaction.  The enhancement in activity with low silver weight loadings is also supportive of the presence of electronic or ensemble effects.  Additionally, it is interesting to note that silver addition does result in a modification of product selectivities.  For small amounts of silver addition, the most selective product is 3,4-epoxybutane (BO) formed by the hydrogenation of the C=C bond in EpB.  However, with the addition of larger amounts of silver, selectivity to BO decreases and the selectivity to products formed by epoxy ring opening increases.

            For comparison, a series of catalysts were prepared using traditional methods of catalyst preparation (incipient wetness) and contrasted to the results obtained for catalysts prepared by ED.  The traditional catalysts demonstrated the same general trends as the catalysts prepared by ED; however, the trends were much broader and not as pronounced.  These results indicate that for conventional methods of bimetallic catalyst preparation, the Ag is deposited on both the silica support and the platinum, while for ED methodologies, the Ag is selectively placed on the platinum surface.

            In summary, we have shown that from (1), the kinetic study of the electroless deposition of Ag onto Pt surfaces, (2), the FT-IR characterization of these bimetallic catalysts, and (3), the evaluation of both ED-derived and conventionally-prepared Ag-Pt bimetallic catalysts that electroless deposition methods are a viable and highly selective means for the preparation of potentially novel catalyst compositions.