(539a) Directed Deposition Synthesis of Supported Bimetallic Overlayer Catalysts

The study and use of bimetallic catalysts has been commonplace for many years. Although these catalysts have proven very useful, they are typically a simple alloy of the two metals, which permits very little control over the catalyst surface where catalytic activity takes place. More recently, first principles quantum dynamical computational studies and single crystal surface studies have indicated that bimetallic pseudomorphic overlayer systems can have surface properties much different than, for example, is found with either metal alone or a uniform alloy of the two metals. These studies have indicated that a Pd_ML/Re system possesses weaker bond strengths for CO, H₂, and C₂H₄, which are expected to have a significant impact on the hydrogenation activity and poison-resistance of a practical supported catalyst. Other studies have indicated that a Pt_ML/Ni system may provide benefits for aqueous phase reforming of carbohydrates. Since computational and single crystal work has previously demonstrated the flexibility and benefits of these systems, the directed synthesis of real world supported Pd_ML/Re and Pt_ML/Ni catalysts have been investigated.

The synthesis method for these catalysts utilized previously synthesized and reduced alumina-supported catalysts as the base. Re/Al₂O₃ and Ni/Al₂O₃ base catalysts were synthesized by incipient wetness impregnation. The directed deposition technique preferentially deposits the overlayer metal onto the base metal particles and not the support. This is accomplished through the use of surface inhibitors to prevent the growth of overlayer particles on the support and direct the deposition onto the pre-existing base metal particles. The overlayer metal deposition is enhanced by the reaction onto the base metal surface where adsorbed hydrogen is available to react with the acetylacetone ligands in the overlayer precursor. Overlayer catalysts were compared against the base catalysts and separately synthesized Pd/Al₂O₃ and Pt/Al₂O₃ catalysts using H₂ chemisorption.

The chemisorption experiments were performed using a Micromeritics ASAP 2020. Isotherms were generated at temperatures from 35°C to 400°C and pressures from 1 mtorr to 800 torr for each catalyst. Reactivity studies were performed for the Re@Pd/Al₂O₃ catalysts using the ethylene hydrogenation reaction in a plug flow reactor at atmospheric pressure and temperatures from -10°C to 60°C, with total gas flow rates between 100-1000 mL/min. Analysis of the reactor effluent was performed by GC/MS. Transmission electron microscopy and X-ray energy-dispersive spectrometry studies were also used to characterize the catalysts.

Hydrogen chemisorption studies show that the adsorption characteristics of the standalone Re/Al₂O₃ and Pd/Al₂O₃ catalysts were markedly different from each other in regard to isotherm shape and the temperature dependence of H₂ adsorption, as well as calculated values for the isoteric heat of H₂ adsorption. Isotherms from Re@Pd/Al₂O₃ catalysts were found to be similar to Re/Al₂O₃ catalysts for H₂ chemisorption in general, but with significantly lower calculated heats of H₂ adsorption (30kJ/mol on Re@Pd/Al₂O₃ vs. 42kJ/mol on Re/Al₂O₃ and 69kJ/mol on Pd/Al₂O₃). Isoteric heat of adsorption values are also comparable to computationally predicted values. The decrease in H₂ heat of adsorption for the overlayer catalyst also agrees well with computational prediction and single crystal studies. Isotherms from a Re@Pd/Al₂O₃ catalyst synthesized without inhibitors to prevent formation of isolated Pd particles contain features of both Pd/Al₂O₃ and Re/Al₂O₃ catalysts. These results indicate that the surface treatment used to inhibit Pd particle growth is successful and that Pd is preferentially deposited on the Re particles.

Results from ethylene hydrogenation experiments indicate that, as expected, Pd is highly active in this reaction. In contrast, the base metal Re is about four orders of magnitude lower in activity. As the surface coverage of Pd on Re increases, the activity also increases and surpasses that of pure Pd. Changes in the apparent activation energy and reaction orders will also be used to compare the different catalysts.

The directed deposition technique has produced supported bimetallic catalysts having properties consistent with computational and single crystal predictions of pseudomorphic overlayers. Catalysts synthesized without directed deposition inhibitors indicated the formation of new Pd particles on the support instead of on the existing base metal particles. Reactivity studies for ethylene hydrogenation show these overlayer catalysts are at least as active as pure Pd catalysts.