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Author: Erjia Guan
Publisher:
ISBN: 9780438630031
Category :
Languages : en
Pages :
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Book Description
Metals are dominant catalysts, being used in forms ranging from simple atomically dispersed (single-site) metal complexes to few-atom clusters to nanoparticles to bulk metals. Investigations of atomically dispersed metal complexes are drawing wide attention because their well-defined structures facilitate fundamental understanding of catalysis as well as offering new catalytic properties. In this work, we extend the field of atomically dispersed supported metal catalysts to dinuclear clusters to build a bridge between atomically dispersed metal complexes and few-atom clusters. Thus, the research extends the subject of atomically dispersed supported catalysts to supported metal pair-site catalysts, which have heretofore been little investigated because of their instability, lack of uniformity, and difficulty of precise synthesis. A separate, collaborative project reported on here includes characterization by in-situ X-ray absorption spectroscopy of the structures of single-site supported metals present as promoters in complex catalysts that contain metal nanoparticles for selective hydrogenation of nitroarenes. Iridium and rhodium pair-site catalysts supported on MgO were synthesized and characterized with infrared (IR) and X-ray absorption spectroscopies and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), supported by density functional theory (DFT) calculations done by collaborators. In-situ IR and X-ray absorption near edge structure (XANES) spectra were used to characterize the structural changes of the pair-sites under various treatment conditions, including ligand substitution reactions involving CO and hydrogen. Catalytic properties for ethylene hydrogenation and H-D exchange in the H2 + D2 reaction were tested and compared with those of single-site iridium and rhodium analogues as well as few-atom clusters of these metals supported on MgO. The pair-site catalysts on MgO activated by removal of ligands facilitate H2 dissociation much more rapidly than their single-site analogues and catalyze ethylene hydrogenation one to two orders of magnitude faster than their single-site analogues on MgO. The pair sites are active for ethylene hydrogenation even after being partially poisoned by CO, and, in contrast, the analogous single-site catalysts are fully poisoned. The results provide understanding of the roles of neighboring metal sites and the effects of ligands on pair sites catalysts, opening opportunities for synthesis of stable pairs of various metals on various supports. The benefits of such stable metal pair sites may extend to numerous reactions other than those investigated in this work. The single-site promoters investigated in this work are Sn cations on TiO2 supports that incorporate noble metal nanoparticle catalysts. These catalysts decidedly outperform the comparable unpromoted supported metals for hydrogenation of nitroarenes substituted with various reducible groups. X-ray absorption spectroscopy at the Sn K edge was used to characterize the structural changes in the single-site Sn in the catalysts as influenced by H2 and by nitrobenzene at 353 K and 1 atm. The changes in Sn–O coordination numbers and distances give evidence that the high activity and selectivity of these catalysts result from the creation of oxygen vacancies on the TiO2 surface associated with single-site Sn sites that lead to efficient, selective activation of the nitro group (in contrast to the other reducible group) coupled with reaction involving hydrogen atoms activated on the nearby metal nanoparticles.
Author: Erjia Guan
Publisher:
ISBN: 9780438630031
Category :
Languages : en
Pages :
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Book Description
Metals are dominant catalysts, being used in forms ranging from simple atomically dispersed (single-site) metal complexes to few-atom clusters to nanoparticles to bulk metals. Investigations of atomically dispersed metal complexes are drawing wide attention because their well-defined structures facilitate fundamental understanding of catalysis as well as offering new catalytic properties. In this work, we extend the field of atomically dispersed supported metal catalysts to dinuclear clusters to build a bridge between atomically dispersed metal complexes and few-atom clusters. Thus, the research extends the subject of atomically dispersed supported catalysts to supported metal pair-site catalysts, which have heretofore been little investigated because of their instability, lack of uniformity, and difficulty of precise synthesis. A separate, collaborative project reported on here includes characterization by in-situ X-ray absorption spectroscopy of the structures of single-site supported metals present as promoters in complex catalysts that contain metal nanoparticles for selective hydrogenation of nitroarenes. Iridium and rhodium pair-site catalysts supported on MgO were synthesized and characterized with infrared (IR) and X-ray absorption spectroscopies and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), supported by density functional theory (DFT) calculations done by collaborators. In-situ IR and X-ray absorption near edge structure (XANES) spectra were used to characterize the structural changes of the pair-sites under various treatment conditions, including ligand substitution reactions involving CO and hydrogen. Catalytic properties for ethylene hydrogenation and H-D exchange in the H2 + D2 reaction were tested and compared with those of single-site iridium and rhodium analogues as well as few-atom clusters of these metals supported on MgO. The pair-site catalysts on MgO activated by removal of ligands facilitate H2 dissociation much more rapidly than their single-site analogues and catalyze ethylene hydrogenation one to two orders of magnitude faster than their single-site analogues on MgO. The pair sites are active for ethylene hydrogenation even after being partially poisoned by CO, and, in contrast, the analogous single-site catalysts are fully poisoned. The results provide understanding of the roles of neighboring metal sites and the effects of ligands on pair sites catalysts, opening opportunities for synthesis of stable pairs of various metals on various supports. The benefits of such stable metal pair sites may extend to numerous reactions other than those investigated in this work. The single-site promoters investigated in this work are Sn cations on TiO2 supports that incorporate noble metal nanoparticle catalysts. These catalysts decidedly outperform the comparable unpromoted supported metals for hydrogenation of nitroarenes substituted with various reducible groups. X-ray absorption spectroscopy at the Sn K edge was used to characterize the structural changes in the single-site Sn in the catalysts as influenced by H2 and by nitrobenzene at 353 K and 1 atm. The changes in Sn–O coordination numbers and distances give evidence that the high activity and selectivity of these catalysts result from the creation of oxygen vacancies on the TiO2 surface associated with single-site Sn sites that lead to efficient, selective activation of the nitro group (in contrast to the other reducible group) coupled with reaction involving hydrogen atoms activated on the nearby metal nanoparticles.
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Languages : en
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The objectives of our work are: (i) to create solid catalysts with active sites that can function in a cooperative manner to enhance reactivity and selectivity, and (ii) to prepare solid catalysts that can perform multiple reactions in a network that in some cases would not be possible in solution due to the incompatibilities of the various catalytic entities (for example an acid and a base). We carried out extensive reactions to test the nature of the cooperative effect caused by thiol/sulfonic acid interactions. The acid/thiol combination provided an example where the two organic groups should be positioned as close to one another as possible. We also studied a system where this is not possible (acid-base). We investigated simultaneously incorporating acid and base groups into the same material. For the case of acid and bases, there is an optimal separation distance (too close allows for neutralization while too far eliminates any cooperative behavior).
Author: Dicle Yardimci
Publisher:
ISBN: 9781303155192
Category :
Languages : en
Pages :
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Solid catalysts incorporating transition metals are important in industry, providing cost- effective syntheses, ease of separation from products, and control of selectivity. The metal is often expensive and thus often constitutes only about one percent of the catalyst mass, being highly dispersed on a high-area support. Dispersed metals in industrial catalysts are usually highly nonuniform in structure and challenging to characterize, and consequently relationships between structure and catalyst performance are typically less than fully understood. Our approach to the investigation of supported metal catalysts involves the synthesis of uniform catalytic sites that have essentially molecular character. Supported molecular catalysts can be characterized spectroscopically to provide fundamental understanding of the catalyst structure under reactive atmospheres, and thereby determination of structural changes of working catalysts that can be correlated with the catalytic activity and selectivity. The sample characterization techniques used in this work included infrared (IR), extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES) spectroscopies, as well as gas chromatography (GC) and mass spectrometry (MS) to characterize reaction products. The catalysts were prepared from the organometallic precursor Rh(C2H4)2(C5H7O2) and the supports MgO and zeolite HY. These catalysts initially incorporated site-isolated, mononuclear rhodium complexes on the supports. The complexes on MgO were treated in H2 at elevated temperatures to form the smallest supported rhodium clusters--rhodium dimers. These catalysts are essentially molecular in character and allowed tailoring of the rhodium nuclearity, the ligands bonded to the rhodium, and the rhodium-support interface. The catalysts incorporated mononuclear Rh(C2H4)2 and Rh(CO)2 complexes; dimeric rhodium clusters with ethyl ligands, and dimeric rhodium clusters with CO ligands. These were tested for the hydrogenation of ethylene. Rhodium in various forms is highly active for catalytic hydrogenation of olefins. However, rhodium has been little investigated for diene hydrogenation, because, like other noble metals in the form of supported clusters or particles, it is unselective. We postulated that new catalytic chemistry of rhodium could emerge if the catalytic species were essentially molecular so that they could be tuned by the choice of the rhodium nuclearity and ligands. Thus, we investigated the influence of the following catalyst design variables on the activity and selectivity of supported rhodium for 1,3-butadiene hydrogenation: (a) the metal nuclearity, ranging from one to several; (b) the electron-donor properties of the support (MgO vs. zeolite Y); and (c) other ligands on the rhodium, including reactive hydrocarbons (ethylene or ethyl) and CO. The data show that extremely small MgO-supported rhodium clusters that are partially carbonylated are highly active and selective for the hydrogenation of 1,3-butadiene to give n-butenes. The support, the rhodium nuclearity, and the ligands on rhodium are crucial to the catalyst selectivity, transforming a metal that is typically regarded as unselective for 1,3-butadiene hydrogenation into one that is highly selective even at high conversions. Transition metals in complexes and clusters tend to aggregate to form of more stable, bulk particles under reactive atmospheres, causing catalyst deactivation. We investigated the initial steps of the aggregation of supported metal species that were highly dispersed on MgO and zeolite HY, synthesizing samples that incorporated supported rhodium complexes bonded to ligands with different reactivities (including the support), and then spectroscopically investigated the formation of extremely small rhodium clusters in the presence of H2. The stability of the rhodium complexes and the stoichiometry of the surface-mediated transformations are regulated by the support and the other ligands bonded to the rhodium, being prompted at a lower temperature with zeolite HY than the better electron-donor MgO when the rhodium complexes incorporate ethylene ligands, but occurring more facilely on the MgO than on the zeolite when the ligands are CO. The preparation of highly uniform rhodium dimers is possible. We infer that results such as those presented here may be useful in guiding the design of stable, highly dispersed supported metal catalysts by choice of the support and other ligands on the metal.
Author: Hermenegildo Garcia
Publisher: John Wiley & Sons
ISBN: 3527350896
Category :
Languages : en
Pages : 501
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Book Description
Understanding the synthesis and applications of porous solid catalysts Heterogeneous catalysis is a catalytic process in which catalysts and reactants exist in different phases. Heterogeneous catalysis with solid catalysts proceeds through the absorption of substrates and reagents which are liquid or gas, and this is largely dependent on the accessible surface area of the solid which can generate active reaction sites. The synthesis of porous solids is an increasingly productive approach to generating solid catalysts with larger accessible surface area, allowing more efficient catalysis. Catalysis in Confined Frameworks: Synthesis, Characterization, and Applications provides a comprehensive overview of synthesis and use of porous solids as heterogeneous catalysts. It provides detailed analysis of pore engineering, a thorough characterization of the advantages and disadvantages of porous solids as heterogeneous catalysts, and an extensive discussion of applications. The result is a foundational introduction to a cutting-edge field. Catalysis in Confined Frameworks: Synthesis, Characterization, and Applications readers will also find: An editorial team comprised of international experts with extensive experience Detailed discussion of catalyst classes including zeolites, mesoporous aluminosilicates, and more A special focus on size selective catalysis Catalysis in Confined Frameworks: Synthesis, Characterization, and Applications is an essential reference for catalytic chemists, organic chemists, materials scientists, physical chemists, and any researchers or industry professionals working with heterogeneous catalysis.
Author: Claudia Martinez Macias
Publisher:
ISBN: 9781339065441
Category :
Languages : en
Pages :
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Essentially molecular supported catalysts were synthesized by using organometallic complexes as precursors, such as Rh(CO)2(acac), Rh(C2H4)2(acac), Ir(CO)2(acac), and Ir(C2H4)2(acac) (where acac is acetylacetonate) and HY zeolite as a support. A goal was to obtain highly uniform solid catalysts with well-defined structures. Characterization by X-ray absorption (XAS) and infrared (IR) spectroscopies confirmed the anchoring of the metal to the support with a high degree of uniformity. IR and 29Si and 27Al nuclear magnetic resonance (NMR) spectra characterize the presence of amorphous regions in the zeolite, and scanning transmission electron microscopy (STEM) identifies these amorphous regions, where iridium is more susceptible to aggregation than in the crystalline regions. Treatment of Ir(CO)2/HY zeolite with C2H4 and H2 at room temperature led to a family of species which includes Ir(CO)2, Ir(CO)(C2H4), Ir(CO)(C2H4)2, Ir(CO)(C2H5) and, tentatively, Ir(CO)(H). The identification of the species is based on XAS and IR spectra (including spectra of samples made with isotopically labeled ligands, 13CO and D2O) and density functional theory (DFT) calculations. The catalytic performance of isostructural rhodium and iridium species incorporating CO as a ligand was measured for the ethylene conversion; the CO not only acts as an inhibitor but it also as a probe molecule providing information about the electronic properties of the metal and of the species present during reaction. When isostructural rhodium and iridium diethylene species are bonded near each other on HY zeolite, the iridium complexes alter the selectivity of rhodium by spilling over hydrogen that hinders the interaction between ethylene and the acidic sites of the zeolite that act in concert with the rhodium, causing it to favor ethylene hydrogenation over dimerization. All these results show how structurally simple solid catalysts can be used to facilitate fundamental understanding of catalysts and their performance.
Author: Wey Yang Teoh
Publisher: John Wiley & Sons
ISBN: 352781356X
Category : Technology & Engineering
Languages : en
Pages : 768
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Book Description
Presents state-of-the-art knowledge of heterogeneous catalysts including new applications in energy and environmental fields This book focuses on emerging techniques in heterogeneous catalysis, from new methodology for catalysts design and synthesis, surface studies and operando spectroscopies, ab initio techniques, to critical catalytic systems as relevant to energy and the environment. It provides the vision of addressing the foreseeable knowledge gap unfilled by classical knowledge in the field. Heterogeneous Catalysts: Advanced Design, Characterization and Applications begins with an overview on the evolution in catalysts synthesis and introduces readers to facets engineering on catalysts; electrochemical synthesis of nanostructured catalytic thin films; and bandgap engineering of semiconductor photocatalysts. Next, it examines how we are gaining a more precise understanding of catalytic events and materials under working conditions. It covers bridging pressure gap in surface catalytic studies; tomography in catalysts design; and resolving catalyst performance at nanoscale via fluorescence microscopy. Quantum approaches to predicting molecular reactions on catalytic surfaces follows that, along with chapters on Density Functional Theory in heterogeneous catalysis; first principles simulation of electrified interfaces in electrochemistry; and high-throughput computational design of novel catalytic materials. The book also discusses embracing the energy and environmental challenges of the 21st century through heterogeneous catalysis and much more. Presents recent developments in heterogeneous catalysis with emphasis on new fundamentals and emerging techniques Offers a comprehensive look at the important aspects of heterogeneous catalysis Provides an applications-oriented, bottoms-up approach to a high-interest subject that plays a vital role in industry and is widely applied in areas related to energy and environment Heterogeneous Catalysts: Advanced Design, Characterization and Applications is an important book for catalytic chemists, materials scientists, surface chemists, physical chemists, inorganic chemists, chemical engineers, and other professionals working in the chemical industry.
Author: Umit S. Ozkan
Publisher: John Wiley & Sons
ISBN: 352762533X
Category : Science
Languages : en
Pages : 340
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Book Description
This long-awaited reference source is the first book to focus on this important and hot topic. As such, it provides examples from a wide array of fields where catalyst design has been based on new insights and understanding, presenting such modern and important topics as self-assembly, nature-inspired catalysis, nano-scale architecture of surfaces and theoretical methods. With its inclusion of all the useful and powerful tools for the rational design of catalysts, this is a true "must have" book for every researcher in the field.
Author: Ming Bao
Publisher: Springer Nature
ISBN: 981974573X
Category :
Languages : en
Pages : 228
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Book Description
Author: Noritaka Mizuno
Publisher: John Wiley & Sons
ISBN: 3527627553
Category : Science
Languages : en
Pages : 356
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Book Description
Filling a gap in the current literature, this comprehensive reference presents all important catalyst classes, including metal oxides, polyoxometalates, and zeolites. Readers will find here everything they need to know -- from structure design to characterization, and from immobilization to industrial processes. A true must-have for anyone working in this key technology.
Author: Piet W.N.M. van Leeuwen
Publisher: Springer Nature
ISBN: 3030458237
Category : Science
Languages : en
Pages : 460
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Book Description
This book provides an overview of the latest developments in the field of nanoparticle catalysis. It not only discusses established topics in detail, but also explores several emerging topics. Catalysis with nanoparticles is expanding exponentially and is attracting significant interest due to the many exciting findings being reported. Mastering the synthesis, characterization, stabilization and use of these catalysts offers numerous possibilities that far exceed those of classic heterogeneous and homogeneous catalysis.
