First-principles based modeling of alloying and promotion in the catalytic selective hydrogenation of acetylene

The ethylene polymerization process requires acetylene traces in the ethylene feedstock be lowered from 1--2% by volume to less than 5 parts per million by volume. The previous supported Pd-only catalysts that were used to hydrogenate acetylene to ethylene were limited in their selectivity and activity owing to ethylene over-hydrogenation and carbonaceous deposit formation. These limitations led to several modifications in the catalyst formulation as well as control strategies to improve performance. Two modifications that have resulted in significant improvements in ethylene selectivity and catalyst activity are the alloying of Pd with Ag and the selective poisoning of the active surface by CO addition. While a qualitative understanding of the role of Ag and CO in the hydrogenation of olefins and acetylenic intermediates is well-established, their explicit effects on the elementary steps and fundamental mechanism are unknown. The aim of this dissertation is therefore to provide an atomic level understanding of the effects of Ag and CO on the selective hydrogenation of acetylene over Pd catalysts using Density Functional Theory (DFT) quantum mechanical methods.;The adsorption energies, reaction energies and activation barriers for different elementary reactions that occur during the hydrogenation of acetylene were calculated over model Pd(111) and Pd-Ag surfaces. The results show that the presence of Ag lowers the adsorption energies for all of the intermediates studied. While both geometric and electronic effects are responsible for this lowering of adsorption energies, the geometric effects that result from Ag addition are found to be dominant. The changes in the binding/adsorption energies subsequently impact the overall reaction energies and activation barriers for both bond-making and bond-breaking elementary steps. The weaker binding energies enhance bond-forming reactions such as hydrogenation. In addition, the overall reaction energies for the unselective bond C-H and C-C bond breaking reactions become more endothermic and will therefore be inhibited by increases in the Ag surface composition.;In the case of the effect of co-adsorbed CO on acetylene hydrogenation, it is found that CO exhibits weak attractive interactions on the different C1 and C2 co-adsorbates at 11% CO. At the higher coverages (25% and 33% CO) however, CO-CO interactions become much more repulsive thus significantly lowering their adsorption energies. CO repulsive interactions also affect the adsorption (or desorption) of ethylene and acetylene. Further, at the coverages of up to 33% CO, atomic surface hydrogen is found to be insensitive to these interactions. At higher coverage (greater than 50% hydrogen) however, kinetic Monte-Carlo studies show that small amounts of CO can lower the surface hydrogen adsorption energies significantly.;The DFT calculated interaction energies between CO and the different C1 and C2 intermediates are also compared with different model predicted interaction energies (Bond Order Conservation model and Bond Order Conservation + Van der Waal model). These models under-predict the interaction energies (compared to the DFT calculated values) and therefore need to be corrected. Some simple correction schemes are therefore investigated and recommendations for further improvements are suggested.