Surface science studies of olefin oxidation on the silver surface

The constant urge of making industrial chemical processes economically viable and highly selective to desired products triggers much catalysis research. A broad goal of catalysis research is to design and develop new or improved catalysts for variety of processes. Direct oxidation (oxidation using O 2 To date, ethylene oxide (EO) and epoxybutene (EpB) are the only two epoxides that have been successfully manufactured at industrial levels by direct heterogeneous oxidation. Surface science studies of the mechanisms of these epoxidation processes have demonstrated the formation of reaction intermediates, or air) represents a major class of chemical processes utilizing heterogeneous catalysis. Ethylene epoxidation has 40-50% share in the total value of large volume chemicals produced by direct heterogeneous oxidation. Epoxidation remains one of the most important chemical reactions, but little is understood in terms of reaction mechanism. Most of the catalyst design and improvement in this area has been carried out by empirical approaches at industrial scale. Epoxidation also represents one of most active areas of surface science research, mainly due to the utility of surface science techniques to develop a molecular understanding of epoxidation processes. Understanding reaction mechanisms at a molecular level helps in creating a rational basis for design of more efficient and more selective epoxidation catalysts. The aim of the work included in this thesis is to develop molecular understanding of non-selective pathways in ethylene epoxidation and to isolate intermediates in the formation of epoxybutene on the Ag(111) surface.;To date, ethylene oxide (EO) and epoxybutene (EpB) are the only two epoxides that have been successfully manufactured at industrial levels by direct heterogeneous oxidation. Surface science studies of the mechanisms of these epoxidation processes have demonstrated the formation of reaction intermediates, oxametallacycles, in epoxidation reaction sequences. In the case of ethylene oxide, epoxidation proceeds via formation of an oxametallacycle, produced by reaction of ethylene and atomic oxygen on the silver catalyst surface. This oxametallacycle then reacts via one transition state to form EO and another transition state to form acetaldehyde and combustion products. The difference in the activation barriers for these two branching reactions is very small and hence selectivity to EO depends on these two competing pathways. Initial experiments in the current study concentrate on UHV studies of 'path 2', the non-selective pathway in ethylene epoxidation. In surface science experiments examining the reactions of acetaldehyde on an oxygen precovered Ag(111) surface, formation of acetate species was observed. At acetaldehyde exposures sufficient to completely consume oxygen to form acetates, the acetate species decompose to reaction products, predominantly CO2, at ∼550 K. Hence, the acetate species are found to be stable from 300 K to 500 K, within the temperature range at which ethylene oxide production is typically carried out in industry. At lower acetaldehyde coverages producing an adsorbed layer containing both acetates and oxygen atoms, the acetate species decompose near room temperature to form only combustion products, CO2 and H 2O, providing a combustion route accessible at the temperatures employed in ethylene epoxidation. These experiments are the first evidence of formation of acetate species on the Ag(111) surface and serve to map out the pathway for complete combustion in ethylene oxidation on silver catalysts.;In experiments with 1-epoxy-3-butene on Ag(111) surface, at a lower dosing temperature of 190 K, molecular desorption of EpB is observed. At a higher dosing temperature of 300 K, a high temperature state was observed desorbing at around 510 K, suggesting formation of an intermediate. The product analysis of the desorption spectra indicated formation of EpB, 2,5-dihydrofuran (2,5-DHF) and crotonaldehyde. These observations support the conclusions from steady state reactor experiments, where crotonaldehyde and 2,5-DHF were reported in the final products. In the reactor studies, the rate determining step of butadiene epoxidation was reported to be the desorption of the final EpB product and its isomers, which is also supported by formation of stable intermediate upto 510 K which reacted to form products like 2,5-DHF and crotonaldehyde.;For studying promoter effects in UHV, a cesium deposition source capable of producing controlled and very minute Cs loadings was developed. In studying Cs deposition on Ag(111), it has been observed that Cs stays on the Ag(111) surface up to 700 K. This observation is certainly helpful to carry out further research of promoter effects on the reactivity of the intermediate in the EpB formation, as the intermediate in the EpB formation is stable on the surface up to 510 K. Surface science experiments of EpB on the Cs-promoted Ag(111) surface can potentially explain, at the molecular level, the increase in selectivity observed in butadiene oxidation on the CsCl promoted silver catalyst in steady state reactor experiments. The ultimate impact of this research would be demonstration of successful approaches for designing more efficient and more selective epoxidation catalysts.