Theoretical Study of the Electronic Structure and Reactions Involving Oxygen-Bridged Cu-Pairs in Zeolite Catalysts for Lean Nitric Oxide Abatement PDF Download
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Languages : en
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Author: Bryan Roger Goodman
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Category :
Languages : en
Pages : 258
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Author: Isabella Nova
Publisher: Springer Science & Business Media
ISBN: 1489980717
Category : Science
Languages : en
Pages : 715
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Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts presents a complete overview of the selective catalytic reduction of NOx by ammonia/urea. The book starts with an illustration of the technology in the framework of the current context (legislation, market, system configurations), covers the fundamental aspects of the SCR process (catalysts, chemistry, mechanism, kinetics) and analyzes its application to useful topics such as modeling of full scale monolith catalysts, control aspects, ammonia injections systems and integration with other devices for combined removal of pollutants.
Author: Eduardo J. Lamas
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Languages : en
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In the last few years the quest towards a hydrogen based economy has intensified interest for effective and less expensive catalysts for fuel cell applications. Due to its slow kinetics, alternative catalysts for the oxygen electroreduction reaction are actively researched. Platinum alloys with different transition metals (for example: Ni, Co and Fe) have shown improved activity over pure Pt. The design of a Pt-free catalysts is also highly desirable, and different alternatives including metalloporphyrins and Pd-based catalysts are being researched. Pd-based catalysts constitute an attractive alternative to Pt alloys in some fuel cell applications, not only because of lower costs but also because of the lower reactivity of Pt alloys towards methanol, which is important for improved methanol crossover tolerance on direct methanol fuel cells. In this work we apply density functional theory (DFT) to the study of four catalysts for oxygen electroreduction. The electronic structure of these surfaces is characterized together with their surface reconstruction properties and their interactions with oxygen electroreduction intermediates both in presence and absence of water. The energetics obtained for the intermediates is combined with entropy data from thermodynamic tables to generate free energy profiles for two representative reaction mechanisms where the cell potential is included as a variable. The study of the barriers in these profiles points to the elementary steps in the reaction mechanisms that are likely to be rate-determining. The highest barrier in the series pathway is located at the first proton and charge transfer on all four catalytic surfaces. This is in good agreement with observed rate laws for this reaction. The instability of hydrogen peroxide on all surfaces, especially compared with the relatively higher stability of other intermediates, strongly points at this intermediate as the most likely point where the oxygen bond is broken during oxygen reduction. This adds to the argument that this path might be active during oxygen electroreduction. A better understanding behind the reaction mechanism and reactivities on these representative surfaces will help to find systematic ways of improvement of currently used catalysts in the oxygen electroreduction reaction.
Author: Oliver Kröcher
Publisher: MDPI
ISBN: 3038973645
Category : Science
Languages : en
Pages : 281
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This book is a printed edition of the Special Issue "Selective Catalytic Reduction of NO
x" that was published in Catalysts
Author: Daniel Neal Briggs
Publisher:
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Category :
Languages : en
Pages : 194
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Abstract Structure and Reactivity of Zeolite- and Carbon-Supported Catalysts for the Oxidative Carbonylation of Alcohols by Daniel Neal Briggs Doctor of Philosophy in Chemical Engineering University of California, Berkeley Professor Alexis T. Bell, Chair The oxidative carbonylation of alcohols to produce dialkyl carbonates is a process that takes place commercially in a slurry of cuprous chloride in the appropriate alcohol. While this process is chemically efficient, it incurs costs in terms of energy (for product separation) and materials attrition (due to the corrosive nature of the chloride anion) that can be alleviated in a gas-phase process. Efforts to develop a supported copper catalyst for making dialkyl carbonates have been undertaken, using carbons or oxidic supports (including zeolites). However, the activity, selectivity and stability of the supported catalysts are not yet competitive with the slurry process. Little is understood regarding the nature of the active species or the mechanism by which carbonates and byproducts are formed. Catalyst properties that lead to favorable activity and selectivity have not been clearly outlined. To improve supported catalysts for this process, we have carried out detailed investigations of the structure and catalytic behavior of zeolite- and carbon-supported Cu catalysts for the synthesis of dimethyl or diethyl carbonates. The aim of the work on Cu+-exchanged zeolites was to establish the effects of zeolite structure/chemical composition on the activity and selectivity of Cu-exchanged Y (Si/Al = 2.5), ZSM-5 (Si/Al = 12), and Mordenite (Si/Al = 10) for the oxidative carbonylation of methanol to DMC. Catalysts were prepared by solid-state ion-exchange of the H-form of each zeolite with CuCl, and were then characterized by FTIR and X-ray absorption spectroscopy (XAS). The XANES portion of the XAS data showed that all of the copper is present as Cu+ cations, and analysis of the EXAFS portion of the data shows the Cu+ cations have a Cu-O coordination number of ̃2.1 on Cu-Y and ̃2.7 on Cu-ZSM-5 and Cu-MOR. Dimethyl carbonate (DMC) was observed as the primary product when a mixture of CH3OH/CO/O2 was passed over Cu-Y, whereas dimethoxy methane was the primary product over Cu-ZSM-5 and Cu-MOR. The higher activity and selectivity of Cu-Y for the oxidative carbonylation of CO is attributed to the weaker adsorption of CO on the Cu+ cations exchanged into Y zeolite. In situ infrared observations reveal that under reaction conditions, adsorbed CO is displaced by methoxide groups bound to the Cu+ cations. The kinetics of DMC synthesis suggests that the rate-limiting step in the formation of this product is the insertion of CO into Cu-OCH3 bonds. The yield of DMC is observed to decline with methanol conversion due very likely to the hydrolysis of DMC to methanol and carbon dioxide. Next, the investigation turned to the synthesis of diethyl carbonate (DEC) by oxidative carbonylation of ethanol, using catalysts prepared by the dispersion of CuCl2 and PdCl2 on amorphous carbon. Catalysts were characterized extensively by XRD, XAFS, SEM and TEM with the aim of establishing their composition and structure after preparation, pretreatment, and use. It was observed that after preparation and pretreatment in He at 423 K, copper is present almost exclusively as Cu(I), most likely in the form of [CuCl2]- anions, whereas palladium is present as large PdCl2 particles. Catalysts prepared exclusively with copper or palladium chloride are inactive for DEC synthesis, indicating that both components must be present together. Evidence from XANES and EXAFS suggests that the DEC synthesis may occur on [PdCl2-x][CuCl2]x species deposited on the surface of the PdCl2 particles. As-prepared catalysts exhibited an increase in DEC synthesis activity and selectivity with time on stream, but then reached a maximum activity and selectivity, followed by a slow decrease in DEC activity. The loss of DEC activity was accompanied by a loss in Cl from the catalyst and the appearance of paratacamite. Further work was undertaken on carbon-supported catalysts, building on insights regarding the active species, this time with activated carbon or carbon nanofibers as the support. The objectives of this last study were to establish the effects of carbon support structure and pretreatment on the dispersion of the catalytically active components and, in turn, on the activity and selectivity of the catalyst for DEC synthesis. At the same surface loading of CuCl2 and PdCl2, partially oxidized carbon nanofibers resulted in a higher dispersion of the active components and a higher DEC activity than could be achieved on activated carbon. Catalyst characterization revealed that nearly atomic dispersion of CuCl2 and PdCl2 could be achieved on the edges of the graphene sheets comprising the carbon nanofibers. Over oxidation of the edges or their removal by heat treatment of the nanofibers resulted in a loss of catalyst activity. The loss of catalyst activity with time on stream could be overcome by the addition of ppm levels of CCl4 to the feed. While catalysts prepared with CuCl2 alone were active, a five-fold increase in activity was realized by using a PdCl2/CuCl2 ratio of 1/20. It was proposed that the Pd2+ cations interact with [CuCl2]- anions to form Pd[CuCl2]2 complexes that are stabilized through dative bonds formed with oxygen groups present at the edges of the graphene sheets of the support. A mechanism for DEC synthesis was outlined and a role for the Pd2+ cations as part of this mechanism was proposed.
Author: Colin Forest Dickens
Publisher:
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Category :
Languages : en
Pages :
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The efficient storage of intermittent, renewable energy in the form of chemical bonds is critical to successfully transition away from fossil-derived fuels and chemicals. The oxygen evolution reaction (OER) plays a central role in many of these storage technologies by splitting water molecules to supply reactive protons and electrons. The activity of OER electrocatalysts directly influences the efficiency of these technologies by reducing the overpotential required to achieve reasonable production rates. Density functional theory (DFT) is a useful tool for investigating potential OER catalyst active sites at the atomic scale in an effort to either explain experimental observation or suggest new experiments to perform. Such theoretical studies are popular for a variety of electrochemical and thermochemical reactions, but OER catalysts are particularly challenging to model because of the highly oxidizing conditions at which they operate, leading to corrosion via oxidation and dissolution of the catalyst surface. In the first part of this thesis, we examine two state-of-the-art OER catalysts, SrIrO3 and RuO2, that are known experimentally to dissolve under reaction conditions and use DFT to explore possible active sites that might form. In the case of SrIrO3 we consider various Sr-deficient surface structures, while for RuO2 we consider defect motifs such as Ru-vacancies, steps, and kinks, and in both cases we identify sites with higher theoretical activities than the ideal, defect-free surfaces. These studies are computationally expensive because they require individually probing the activity of possible active sites by calculating the stability of OER intermediates OH*, O*, and OOH* at each site. Towards circumventing these calculations, we identify an electronic structural descriptor, namely the average 2p-state energy of adsorbed atomic oxygen, that correlates strongly with the theoretical OER activity and allows for screening multiple active sites at once with a single DFT calculation. In the second part of this thesis, we attempt to move beyond the conventional thermodynamic analysis of theoretical OER activity with microkinetic modeling, which allows for a more direct comparison to experimental results. This involves explicitly modeling the aqueous-solid electrochemical interface and computing kinetic barrier heights for reactions that involve charge transfer across the interface. We find that the intrinsic barrier height for one elementary step in particular, OOH* formation, is significantly higher than the others for rutile (110) surfaces and directly accounts for the non-negligible OER overpotential observed experimentally. The resultant microkinetic model, which assumes OOH* formation to be the sole rate determining step, is analyzed in the context of experimental observations including Tafel behavior and is used to construct an OER volcano consisting solely of experimental data.
Author: Stephen M. Ouellette
Publisher:
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Category : Copper catalysts
Languages : en
Pages : 32
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Author: Kelsey Ann Stoerzinger
Publisher:
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Category :
Languages : en
Pages : 181
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The intermittent nature of renewable energy sources requires a clean, scalable means of converting and storing energy. Water electrolysis can sustainably achieve this goal by storing energy in the bonds of oxygen and hydrogen molecules. The efficiency of this storage-conversion process is largely determined by the kinetic overpotential required for the oxygen evolution and reduction reactions (OER and ORR), respectively. This thesis focuses on transition metal oxides as alternative oxygen catalysts to costly and scarce noble metals. In order to develop descriptors to improve catalytic activity, thus reducing material cost for commercial technologies, this work studies fundamental processes that occur on model catalyst systems. Electrochemical studies of epitaxial oxide thin films establish the intrinsic activity of oxide catalysts in a way that cannot be realized with polydisperse nanoparticle systems. This thesis has isolated the activity of the catalyst on a true surface-area basis, enabling an accurate comparison of catalyst chemistries, and also revealed how different terminations and structures affect the kinetics. These studies of epitaxial thin films are among the first to probe phenomena that are not straightforward to isolate in nanoparticles, such as the role of oxide band structure, interfacial charge transfer (the "ligand" effect), strain, and crystallographic orientation. In addition, these well-defined surfaces allow spectroscopic examinations of their chemical speciation in an aqueous environment by using ambient pressure X-ray photoelectron spectroscopy. By quantifying the formation of hydroxyl groups, we compare the relative affinity of different surfaces for this key reaction intermediate in oxygen electrocatalysis. The strength of interaction with hydroxyls correlates inversely with activity, illustrating detrimental effects of strong water interactions at the catalyst surface. This fundamental insight brings molecular understanding to the wetting of oxide surfaces, as well as the role of hydrogen bonding in catalysis. Furthermore, understanding of the mechanisms of oxygen electrocatalysis guides the rational design of high-surface-area oxide catalysts for technical application.
Author: Jiří Čejka
Publisher: Royal Society of Chemistry
ISBN: 1782627847
Category : Science
Languages : en
Pages : 547
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Accessible references for researchers and industrialists in this exciting field, covering both developments and applications of catalysis.
