(356f) Directed Synthesis And Characterization Of A Supported Bimetallic Overlayer Catalyst

The study and use of bimetallic catalysts has been commonplace for many years. Although these catalysts have proven very useful, they are typically nothing more than simple alloys of the two metals, which permits very little control over the composition of 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 may have a significant impact on the activity and poison-resistance of a practical supported catalyst. Since computational and single crystal work has previously demonstrated the flexibility and benefits of these systems, the directed synthesis of a real world supported Pd_ML/Re catalyst and its catalytic properties for ethylene hydrogenation have been investigated.

The synthesis method for this catalyst utilized a previously synthesized and reduced alumina-supported rhenium catalyst as the base. In order to preferentially deposit palladium onto the rhenium particles and not the support, a hydroxylation surface treatment was used to inhibit the growth of palladium particles on the support. The palladium is directly deposited onto the rhenium surface where adsorbed hydrogen is available to react with the Pd acetylacetone ligands. The Pd/Re catalyst was compared against the Re/Alumina base catalyst and a separately synthesized Pd/Alumina catalyst using H₂ chemisorption and reactivity studies of the ethylene hydrogenation reaction. The chemisorption experiments were performed using a Micromeritics ASAP 2020. Isotherms were generated at temperatures from 35^oC to 400^oC and pressures from 1 mtorr to 800 torr for each catalyst. Reactivity experiments were performed in a plug flow reactor at atmospheric pressure and temperatures from 30^oC to 100^oC, 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 will also be used to characterize the catalysts.

Hydrogen chemisorption studies show that the adsorption characteristics of the standalone Re/Alumina and Pd/Alumina catalysts are markedly different from each other, with the Pd/Re catalyst being nearly identical to the Re/Alumina catalyst in regard to isotherm shape and the temperature dependence of H₂ adsorption, as well as calculated values for the isoteric heat of H₂ adsorption. Isoteric heat of adsorption values are also comparable to computationally predicted values. These results indicate that the surface treatment used to inhibit Pd particle growth was successful and that the Pd was preferentially deposited on the Re particles. Results from ethylene hydrogenation experiments indicate that Pd is highly reactive in this reaction, while Re shows very little activity. The Pd/Re catalyst, rather than performing similar to rhenium as it did in the chemisorption studies, has an activity much closer to that of palladium. Activation energies and reaction orders for each of the catalysts will be compared. Taken together, these results indicate that a catalyst containing Pd/Re particles and few or no pure Pd particles has been synthesized.