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Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
The selective oxidation of ethylene to ethylene epoxide (C2H4 + 1/2 O2 .-->. C2H4O) over Ag is the simplest example of kinetically-controlled, selective heterogeneous catalysis. We have studied the steady-state kinetics and selectivity of this reaction for the first time on a clean, well-characterized Ag(110) surface by using a special apparatus which allows rapid (approx. 20 s) transfer between a high-pressure catalytic microreactor and an ultrahigh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The effects of temperature and reactant pressures upon the rate and selectivity are virtually identical on Ag(110) and supported, high-surface-area Ag catalysts. The absolute specific rate (per Ag surface atom) is however some 100-fold higher for Ag(110) than for high-surface-area catalysts. This is related to the well-known structural sensitivity of this reaction. It is postulated that a small percentage of (110) planes (or (110)-like sites) are responsible for most of the catalytic activity of high-surface-area catalysts. The high activity of the (110) plane is attributed to its high sticking probability for dissociative oxygen adsorption, since the rate of ethylene epoxidation is shown in a related work to be proportional to the coverage of atomically adsorbed oxygen at constant temperature and ethylene pressure.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
The selective oxidation of ethylene to ethylene epoxide (C2H4 + 1/2 O2 .-->. C2H4O) over Ag is the simplest example of kinetically-controlled, selective heterogeneous catalysis. We have studied the steady-state kinetics and selectivity of this reaction for the first time on a clean, well-characterized Ag(110) surface by using a special apparatus which allows rapid (approx. 20 s) transfer between a high-pressure catalytic microreactor and an ultrahigh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The effects of temperature and reactant pressures upon the rate and selectivity are virtually identical on Ag(110) and supported, high-surface-area Ag catalysts. The absolute specific rate (per Ag surface atom) is however some 100-fold higher for Ag(110) than for high-surface-area catalysts. This is related to the well-known structural sensitivity of this reaction. It is postulated that a small percentage of (110) planes (or (110)-like sites) are responsible for most of the catalytic activity of high-surface-area catalysts. The high activity of the (110) plane is attributed to its high sticking probability for dissociative oxygen adsorption, since the rate of ethylene epoxidation is shown in a related work to be proportional to the coverage of atomically adsorbed oxygen at constant temperature and ethylene pressure.
Author: Ashay Dileep Javadekar Publisher: ISBN: 9781109393040 Category : Alkenes Languages : en Pages :
Book Description
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 CO 2, 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, CO 2 and H 2 O, 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.
Author: Adriano Zecchina Publisher: John Wiley & Sons ISBN: 1119181305 Category : Science Languages : en Pages : 352
Book Description
This book gradually brings the reader, through illustrations of the most crucial discoveries, into the modern world of chemical catalysis. Readers and experts will better understand the enormous influence that catalysis has given to the development of modern societies. • Highlights the field's onset up to its modern days, covering the life and achievements of luminaries of the catalytic era • Appeals to general audience in interpretation and analysis, but preserves the precision and clarity of a scientific approach • Fills the gap in publications that cover the history of specific catalytic processes
Author: Robert Bakhtchadjian Publisher: CRC Press ISBN: 1000084841 Category : Science Languages : en Pages : 302
Book Description
This book is devoted to the problems of oxidation chemical reactions and addresses bimodal reaction sequences. Chemical reactions of oxidation, occurring under certain conditions and in multicomponent systems are complex processes. The process of the oxidation essentially changes in the presence and contact of the solid substances with reactants. The role of solid substances and the appearance of this phenomenon in oxidation reaction are discussed. The reader will understand the "driving forces" of this phenomenon and apply it in practice. Written for chemists, physicists, biologists and engineers working in the domain of oxidation reactions. Key Selling Features: Covers the historical background, modern state of the art, and perspectives in investigations of the coupling between heterogeneous and homogeneous reactions Discusses the feasible pathways of the coupling of heterogeneous and homogeneous reactions in oxidation in man-made and natural chemical systems Addresses the abundance, peculiarities and mechanisms of the bimodal reaction sequences in oxidation with dioxygen in recent decades Discusses the existence of the bimodal reaction sequences in chemical systems investigations in atmospheric chemistry and heterogeneous photocatalysis Presented in a simple concise style, accessible for both specialists and non-specialists
Author: Gabriele Centi Publisher: Springer Science & Business Media ISBN: 1461541751 Category : Science Languages : en Pages : 519
Book Description
Selective Oxidation by Heterogeneous Catalysis covers one of the major areas of industrial petrochemical production, outlining open questions and new opportunities. It gives keys for the interpretation and analysis of data and design of new catalysts and reactions, and provides guidelines for future research. A distinctive feature of this book is the use of concept by example. Rather than reporting an overview of the literature results, the authors have selected some representative examples, the in-depth analysis of which makes it possible to clarify the fundamental, but new concepts necessary for a better understanding of the new opportunities in this field and the design of new catalysts or catalytic reactions. Attention is given not only to the catalyst itself, but also to the use of the catalyst inside the process, thus evidencing the relationship between catalyst design and engineering aspects of the process. This book provides suggestions for new innovative directions of research and indications on how to reconsider the field of selective oxidation from different perspectives, outlining that is not a mature field of research, but that new important breakthroughs can be derived from fundamental and applied research. Suggestions are offered on how to use less conventional approaches in terms of both catalyst design and analysis of the data.
Author: Costas G. Vayenas Publisher: Springer Science & Business Media ISBN: 0306475510 Category : Science Languages : en Pages : 597
Book Description
I knew nothing of the work of C. G. Vayenas on NEMCA until the early nineties. Then I learned from a paper of his idea (gas interface reactions could be catalyzed electrochemically), which seemed quite marvelous; but I did not understand how it worked. Consequently, I decided to correspond with Professor Vayenas in Patras, Greece, to reach a better understanding of this concept. I think that my early papers (1946, 1947, and 1957), on the relationship between the work function of metal surfaces and electron transfer reactions thereat to particles in solution, held me in good stead to be receptive to what Vayenas told me. As the electrode potential changes, so of course, does the work function at the interface, and gas metal reactions there involve adsorbed particles which have bonding to the surface. Whether electron transfer is complete in such a case, or whether the effect is on the desorption of radicals, the work function determines the strength of their bonding, and if one varies the work function by varying the electrode potential, one can vary the reaction rate at the interface. I got the idea. After that, it has been smooth sailing. Dr. Vayenas wrote a seminal article in Modern Aspects of Electrochemistry, Number 29, and brought the field into the public eye. It has since grown and its usefulness in chemical catalytic reactions has been demonstrated and verified worldwide.
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Author: Ashay Dileep Javadekar
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Author: Adriano Zecchina
Author: Robert Bakhtchadjian
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Author: Gabriele Centi
Author: Costas G. Vayenas