Activation of Silver Powder for Ethylene Epoxidation at Vacuum and Atmospheric Pressures

Svoboda, G. D.; Gleaves, John

doi:10.1021/bk-1993-0523.ch014

Cited by 7 publications

(3 citation statements)

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“…"High-pressure" mode experiments have been utilized and compared at vacuum and atmospheric pressures. 72 Similar types of information can be obtained using long series of very large pulses. The time window for the investigated processes addressed, however, is shifted to longer durations compared to typical TAP experiments.…”

Section: Other Types Of Tap Operating Modementioning

confidence: 92%

Forty years of temporal analysis of products

Morgan

Maguire

Fushimi

et al. 2017

Catal. Sci. Technol.

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131

101

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A detailed understanding of reaction mechanisms and kinetics is required in order to develop and optimize catalysts and catalytic processes. While steady-state investigations are known to give a global view of the catalytic system, transient studies are invaluable since they can provide more comprehensive insight into elementary steps. For almost forty years temporal analysis of products (TAP) has been successfully utilized for transient studies of gas phase heterogeneous reactions, and there have been a number of advances in instrumentation and numerical modeling methods in that time. Since TAP is a complex methodology it is often viewed as a niche specialty. With the purpose to make TAP more relevant and approachable to a wider segment of the catalytic research community, part of the intention of this work is to highlight the significant contributions TAP has made to elucidating mechanistic and kinetic aspects of complex, multistep heterogeneous reactions. With this in mind, an outlook is also disclosed for the technique in terms of what is needed to revitalize the field and make it more applicable to the recent advances in catalyst characterization (e.g. operando modes).

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Section: Other Types Of Tap Operating Modementioning

confidence: 92%

Forty years of temporal analysis of products

Morgan

Maguire

Fushimi

et al. 2017

Catal. Sci. Technol.

Self Cite

131

101

View full text Add to dashboard Cite

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“…In this case the dissolved oxygen acts as a reservoir for chemisorbed oxygen, but a type with a reactivity different from that of the species already referred to as chemisorbed oxygen. Rigas et al [51] suggest that dissolved oxygen species modify the electronic properties of the surface and thus change the reactivity of adsorbed oxygen species for the ethylene epoxidation on silver powder. A mechanism for oxygen segregation is proposed by Lauterbach et al [40], who studied the reaction between subsurface oxygen and CO as well as H2 on Pt(100).…”

Section: Reaction Mechanismmentioning

confidence: 99%

An investigation on the reaction mechanism for the partial oxidation of methane to synthesis gas over platinum

1995

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. M. M. (1995). An investigation on the reaction mechanism for the partial oxidation of methane to synthesis gas over platinum. Catalysis Letters,, 291-304. DOI: 10.1007/BF00814232 General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. The partial oxidation of methane to synthesis gas has been investigated by admitting pulses of pure methane, pure oxygen and mixtures of methane and oxygen to platinum sponge at temperatures ranging from 973 to 1073 K. On reduced platinum the decomposition of methane results in the formation of surface carbon and hydrogen. No deposition of carbon occurs during the interaction of methane with a partly oxidised catalyst. Oxygen is present in three different forms under the conditions studied: platinum oxide, dissolved oxygen and chemisorbed oxygen species. Carbon monoxide and hydrogen are produced directly from methane via oxygen present as platinum oxide. Activation of methane involving dissolved oxygen provides a parallel route to carbon dioxide and water. Both platinum oxide and chemisorbed oxygen species are involved in the oxidation of carbon monoxide and hydrogen. In the presence of both methane and dioxygen at a stoiehiometric feed ratio the dominant pathways are the direct formation of CO and H2 followed by their consecutive oxidation. A Mars-van Krevelen redox cycle is postulated for the partial oxidation of methane: the oxidation of methane is accompanied by the reduction of platinum oxide, which is reoxidised by incorporation of dioxygen into the catalyst.

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“…A pretreatment of the catalyst is performed to obtain the specific oxidized surface required for an active and selective operation. 63,120,121 The oxygen partial pressure during this treatment greatly influences the amounts of the different oxygen species present on the surface, in turn influencing the activity and selectivity of the catalyst. 118 This led to the discovery that atomically adsorbed oxygen can also catalyze the selective reaction path when subsurface oxygen is present.…”

Section: Figure 13: Eley-rideal Reaction Mechanism Proposed Bymentioning

confidence: 99%

Supported Silver Catalysts for Ethylene Epoxidation

Reijen¹,

Elias²

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12or to increase the stability of the catalyst. [74][75][76] The activity of the catalyst is less important as high conversions decrease the ethylene oxide selectivity due to the secondary reaction (r 3 in Figure 1.2, the full oxidation of ethylene oxide).The presence of chlorine on the surface suppresses the full oxidation of ethylene, 74,77,78 as the presence of chlorine on the catalyst surface prevents the formation of certain surface species responsible for the unselective pathway, leading to an increase in the intrinsic selectivity. 75 The competitive adsorption of chlorine on the silver surface indiscriminately suppresses oxygen adsorption on the silver surface and therefore decreases the overall activity by suppressing both the selective (r 1 ) and unselective (r 2 ) pathways. 47,77,79,80 A secondary effect of chlorine promotion leads to a suppression of the secondary, unselective reaction (r 3 ); the decomposition of ethylene oxide to form carbon dioxide and water. 79,81 In a typical epoxidation reactor a feed containing oxygen, ethylene and ppm-levels of the chlorine moderator in an inert diluent gas is fed over the reactor at 200 -280 °C at 15 bar. 55,72 The reactor is designed to have good control over the temperature as to prevent hot-spots. 82 The oxygen content is important to maintain an oxide overlayer on the silver, which is required to obtain high selectivities. However, the oxygen content is kept below 10 % per volume to prevent the formation of explosive mixtures. 55,72,83 When the oxygen content is below this value, the ethylene concentration can be maintained at a relatively high value of 30 -40 vol% safely. 55,72,83 As selectivity decreases with conversion, the (ethylene) conversion is kept low to maintain a high yield. Commercially, a yield of 1 -3 vol% EO is viable. 55 Modern industrial catalysts obtain selectivities toward ethylene oxide of 90 % 77,84 compared to about 50-70 % for early epoxidation catalysts. 85,86 Besides (promoted) silver catalysts, also other catalytic systems are being investigated for this reaction. [87][88][89][90] High selectivities towards ethylene oxide are reported by Jankowiak and Barteau for CuAg bimetallic catalysts, 89,91 and later also for AgPd bimetallic catalysts. 87 Due to the bimetallic nature of these catalysts, the environment during reaction can greatly influence composition of the catalyst surface, as for example in the CuAg catalysts segregation of the metals occurs under reaction conditions. 92,93 Depending on the metal ratios and reaction conditions, alloying, but also promotion, additional metals can therefore be either beneficial or unfavorable for the catalytic performance of the silver catalyst. 88,92,93 As silver is capable of selectively oxidizing ethylene, the simplest olefin, it seems straightforward that this system is also an interesting candidate for the epoxidation of other olefins. However, silver appears to be unable to selectively oxidize higher olefins in gas phase, due to activation of the β-hydrogen (one of the hydrogen atoms b...

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Activation of Silver Powder for Ethylene Epoxidation at Vacuum and Atmospheric Pressures

Cited by 7 publications

References 0 publications

Forty years of temporal analysis of products

Forty years of temporal analysis of products

An investigation on the reaction mechanism for the partial oxidation of methane to synthesis gas over platinum

Supported Silver Catalysts for Ethylene Epoxidation

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