(514d) Correlation of H2 Heat of Adsorption and Ethylene Hydrogenation for Re@Pd Overlayer Catalysts

Bimetallic catalysts have long been an important area of catalysis research. Certain combinations of metals are known to improve catalyst activity, selectivity, or catalyst lifetime. Several mechanisms have been postulated to explain the catalytic properties observed using bimetallic catalysts such as: structure effects, bifunctional routes, and electronic modification of surface properties. Bimetallic overlayer catalysts consisting of a monatomic layer deposited on a different bulk metal have received widespread attention recently using first principles computation and single crystal techniques. Compared to their component metals, these overlayer systems have been found to display different surface properties. This is likely explained by electronic interactions which lead to a shift in the d-band center of the overlayer metal when it is deposited on a suitable base metal. However, there is still a need to develop broad-based methods for effective deposition of the overlayer catalysts directly on high surface area supports that can exhibit properties consistent with predictions. Based on previous computational studies, we have examined the Re@Pd (base@overlayer) system for the hydrogenation of ethylene.

Re/Al2O3 base catalysts were synthesized by standard incipient wetness impregnation techniques. This catalyst was then calcined and reduced to produce metal particles on the support. The directed deposition technique was used to preferentially deposit the overlayer metal on the base metal and not the support. First the alumina surface is hydroxylated using He saturated water. The catalyst is then exposed to H2 to deposit hydrogen atoms on the base metal particles. Finally, acetylacetone is introduced to block surface adsorption of Pd acetylacetonate which preferentially reacts with the hydrogen on the metal particle surface and deposits the Pd overlayer. Samples were made using the above procedure, the above procedure repeated three times, and the above procedure without any inhibitors. Pd/Al2O3 and Re/Al2O3 reference catalysts were also synthesized for comparison.

Hydrogen chemisorption experiments were performed using a Micromeritics ASAP 2020. Isotherms were generated from 35 to 400°C and pressures from 1 mtorr to 800 torr. Reactivity studies for the overlayer and reference catalysts were performed in a plug flow reactor at atmospheric pressure and temperatures from -10 to 60°C, with total gas flow rates between 100 and 1000 ml/min. Reactor effluent was monitored by GCMS.

Hydrogen adsorption characteristics for the Pd/Al2O3 and Re/Al2O3 reference catalysts were distinct in both isotherm shape and temperature dependence of H2 adsorption. Isotherms for the Re@Pd/Al2O3 catalysts were very similar to the Re/Al2O3 catalysts for H2 chemisorption, and there was no indication of isolated Pd metal particle formation. Measurement of adsorption isotherms at multiple temperatures also allowed for determination of isosteric heat of hydrogen adsorption. Heats for Pd/Al2O3 were 67 kJ/mol and similar to single crystal and computational prediction. For Re/Al2O3, heats were about 40 kJ/mol. All overlayer catalysts had lower hydrogen heats of adsorption. Heat of adsorption decreased from the no inhibitors catalyst to the standard synthesis to the triple deposition catalyst with a maximum heat of adsorption of 20 kJ/mol. This decrease in heat of adsorption is consistent with that predicted by computational studies and is similar to the 100% coverage prediction of 2 kJ/mol.

As expected, Pd is highly active for ethylene hydrogenation compared to Re. The Pd/Al2O3 reference catalyst was slightly more active than the Re@Pd/Al2O3 catalysts with activity decreasing from no inhibitors, to standard synthesis, to triple deposition synthesis. Reaction orders for all catalysts were first order in hydrogen. All Pd catalysts demonstrated slightly negative reaction orders in ethylene with Re being zero order. Apparent activation energies for Pd catalysts were very similar at 75 kJ/mol with Re slightly higher at 103 kJ/mol. The measured activation energy for the Pd catalysts is very similar to the forward activation barrier (78 kJ/mol) for ethylene hydrogenation to surface ethyl as predicted by literature first principles computation.

Finally, the hydrogen heat of adsorption for the Pd/Al2O3 and Re@Pd/Al2O3 catalysts can be correlated with activity for the ethylene hydrogenation reaction. This correlation demonstrates that as maximum hydrogen heat of adsorption decreases, the turnover frequency for ethylene hydrogenation decreases in a linear manner. This observation is consistent with the suggestion that ethylene hydrogenation on a Pd surface is affected by the availability of adsorbed hydrogen at these conditions. This also suggests that modification of the catalyst to make a slightly stronger hydrogen bond may increase catalytic activity. Since other works in the literature have demonstrated a linear relationship between the computed center of the d-band and hydrogen heat of adsorption, and we have demonstrated a linear relationship between hydrogen heat of adsorption and activity, it is now possible to correlate activity with the calculated center of the d-band for a high surface area supported metal catalyst containing a bimetallic overlayer structure. Thus, use of bimetallic overlayer catalysts synthesized using the directed deposition technique offer a powerful tool to precisely control catalytic activity.