Study of Noble Metal Catalysts by Extended X-ray Absorption Fine Structure Spectroscopy PDF Download
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Author: Yu Miao
Publisher:
ISBN:
Category :
Languages : en
Pages : 198
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Author: Yu Miao
Publisher:
ISBN:
Category :
Languages : en
Pages : 198
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Author: Yasuhiro Iwasawa
Publisher: World Scientific
ISBN: 9789810223236
Category : Science
Languages : en
Pages : 438
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X-ray absorption fine structure (XAFS) is a powerful technique in characterization of structures and electronic states of materials in many research fields including, e.g., catalysts, semiconductors, optical ingredients, magnetic materials, and surfaces. This characterization technique could be applied in a static or a dynamic state (in-situ condition). The XAFS can provide information that is not accessible by other techniques for characterization of materials, particularly catalysts and related surfaces. Furthermore, XAFS can provide a molecular-level approach to the study of reaction mechanisms for the understanding of catalysts and development of new catalysts. A number of synchrotron radiation facilities have been planned to be built in Asian countries in addition to the high-brilliant synchrotron radiation facilities under construction in the USA, Europe, and Japan. The applications of XAFS have now expanded to catalytic chemistry and engineering, surface science, organometallic chemistry, materials science, solid-state chemistry, geophysics, etc. This book caters to a wide range of researchers and students working in the domain or related topics.
Author: Lung-Shan Kau
Publisher:
ISBN:
Category :
Languages : en
Pages : 512
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Author: David Do
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Nanoparticles have received much attention due to their unique structure and physical and chemical properties compared to their bulk material counterparts, which allow them to be used in a variety of applications, such as imaging, medical applications, and catalysis. As such, close examination of the structure and electronic properties of various nanoparticles is of incredibly high interest. In this work, X-ray absorption spectroscopy (XAS), supported by other characterization techniques, was used to study the structure and bonding properties of a number of different nanoparticle catalysts. First, a series of W-doped TiO2 nanoparticles was examined, in order to determine the changes in atomic structure that allowed for the W-TiO2 4% doping to exhibit greater photothermalcatalytic activity than the other samples. Extended X-ray Absorption Fine Structure (EXAFS) fittings results showed that the dopant W atoms replaced Ti atoms within the lattice, creating Ti vacancies. As the W doping concentration increased to 4%, the Ti vacancies also increased, as did the number of dangling oxygen and oxygen vacancies, which could act as catalytic sites. Formation of WO3 outside of the lattice at 10% doping was also seen in the fitting results, showing why the 4% sample exhibited greater catalytic activity. Next, a series of manganese cobalt oxides supported on carbon nanotubes were studied, with XAS results allowing for a clearer picture of the phase transition from a cubic structure to a tetragonal structure as the amount of manganese in the samples increased and the amount of cobalt decreased. Mn1.5Co1.5O4, which contains an equal ratio of Co and Mn, was found to have a dual-phase structure, based on both EXAFS fitting results and linear combination X-ray absorption near-edge structure (LC XANES) fitting. This was unlike the other samples studied, which were either completely cubic phase or tetragonal phase structures, and further understanding of the dual-phase structure would potentially allow for better fine-tuning of the structure of the sample, and as such, its catalytic applications. Finally, a number of ruthenium nanoparticle samples, both monometallic and bimetallic, were synthesized and supported on three different zeolite supports (Y, beta, and mordenite) with varying concentrations of the ruthenium ion precursor to determine the effect of concentration and support on the local structure and bonding properties. It was found that bimetallic samples of Ru and Pd displayed strong indications of metal core formation, whereas Ru and Co bimetallic samples showed little metal core formation, as seen in the EXAFS fitting results for all samples. This work highlights the importance of the relationship between the structure of nanoparticles and their properties. X-ray absorption spectroscopy gives a deep understanding of the structure and bonding properties of representative sites of nanoparticles, and by using the information obtained from such studies, a clearer picture of the nanoparticle structure can be formed. These results could lead to further development into tailoring the structure of certain nanoparticles in order to optimize their performance in relevant applications.
Author: Keishla R. Rivera-Dones
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Catalytically driven processes account for over ninety percent of industrial chemical manufacturing today. Developments in manufacturing processes are largely driven by continued improvements in catalytic materials, which aim to increase production volumes while minimizing costs along with safety and environmental hazards. In order to achieve these goals, however, a rational approach in catalyst design must be pursued that aims to understand and build upon the fundamental structural, electronic, and chemical properties governing catalytic performance. To that purpose, the work presented in this dissertation makes use of kinetic experiments, theoretical models, and advanced characterization techniques to generate a fundamental understanding of noble metal surfaces employed in a variety of catalytic reaction systems. In Chapter 2, we discuss the use of N2 physisorption, CO chemisorption, and NH3 temperature programed desorption to evaluate the effect of support acidity on the reactivity profiles of various zeolite-supported Pt and Pt-Sn catalysts for the non-oxidative coupling of methane to ethylene and aromatics. Reactivity studies for Pt-Sn/H-ZSM-5 catalysts at 973 K showed that, while all catalysts produced ethylene as the primary product, increasing support acidity led to an increase in naphthalene selectivity at the expense of benzene selectivity. Volcano-shaped profiles observed for the generation of aromatic products suggest that the formation of a reactive hydrocarbon pool on acidic support surfaces could be responsible for the oligomerization of ethylene. Notably, the Pt-Sn/H-ZSM-5 (SiO2:Al2O3 = 50) catalyst was found to be comparable to the state-of-the-art Mo/H-ZSM-5 catalysts in terms of carbon product generation and resistance to coke formation. In Chapter 3, x-ray absorption spectroscopy (XAS) was used to highlight the effect of local electronic and structural environments in specially synthesized metallic catalysts. The local coordination and nearest-neighbor distance of Pd species were evaluated to understand metal dispersion and the effect of catalyst support on the extent of bimetallic particle formation in Pd, AgPd, CuPd, and AuPd catalysts synthesized by controlled surface reactions (CSR) for a variety of amination, hydrodechlorination, and hydrogenation reactions. Near-edge structure analyses were also used on these Pd catalysts, as well as on a set of Mo-containing multi-metallic catalysts prepared by atomic layer deposition (ALD) for synthesis gas conversion, to understand catalyst reducibility along with potential support and hydrogen spillover effects on the extent of metal reduction. Chapter 4 evaluates the effects of catalyst support and pretreatment conditions on the hydrogenation of acetone over SiO2-, Al2O3-, and ZSM-5-supported platinum catalysts. Pt/ZSM-5 catalysts were found to have specific conversion rates and turnover frequencies that were 2 - 3 orders of magnitude higher than those observed over Pt/SiO2 and Pt/Al2O3 catalysts, regardless of zeolite acidity or pretreatment conditions. For Pt/ZSM-5 catalysts, the higher activity was achieved by increasing calcination and decreasing reduction temperatures, likely due to the effects of these treatments on the morphology of the platinum particles. CO-FTIR measurements showed a shift to higher frequencies of the Pt-CO band in Pt/ZSM-5 catalysts compared to Pt/SiO2, which alluded to the interactions between Pt and the porous zeolite structure as a source of the activity enhancements observed. Chapter 5 introduces the use of transient kinetics studies and theoretical modeling to explore the importance of surface coverage effects in the hydrogenation of acetone over platinum. Transient models based on steady-state microkinetics using static and dynamic inclusion of surface coverage via the Langmuir and Bragg-Williams approximations, respectively, predicted notable differences in the decay profiles of the most abundant reactive intermediate (MARI) from the catalytic surface. Experimental studies using steady-state isotopic transient kinetic analysis (SSITKA) methods served to validate the theoretical predictions for transients induced by complete acetone removal from or its substitution in the reactant feed and provided tangible evidence for the importance of surface coverage effects in understanding the reactivity of platinum catalysts for acetone hydrogenation. Lastly, Chapter 6 addresses possible future research directions in the field of transient kinetics studies.
Author: Jeroen A. van Bokhoven
Publisher: John Wiley & Sons
ISBN: 1118844238
Category : Science
Languages : en
Pages : 940
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X-Ray Absorption and X-ray Emission Spectroscopy: Theory and Applications During the last two decades, remarkable and often spectacular progress has been made in the methodological and instrumental aspects of x-ray absorption and emission spectroscopy. This progress includes considerable technological improvements in the design and production of detectors especially with the development and expansion of large-scale synchrotron reactors All this has resulted in improved analytical performance and new applications, as well as in the perspective of a dramatic enhancement in the potential of x-ray based analysis techniques for the near future. This comprehensive two-volume treatise features articles that explain the phenomena and describe examples of X-ray absorption and emission applications in several fields, including chemistry, biochemistry, catalysis, amorphous and liquid systems, synchrotron radiation, and surface phenomena. Contributors explain the underlying theory, how to set up X-ray absorption experiments, and how to analyze the details of the resulting spectra. X-Ray Absorption and X-ray Emission Spectroscopy: Theory and Applications: Combines the theory, instrumentation and applications of x-ray absorption and emission spectroscopies which offer unique diagnostics to study almost any object in the Universe. Is the go-to reference book in the subject for all researchers across multi-disciplines since intense beams from modern sources have revolutionized x-ray science in recent years Is relevant to students, postdocurates and researchers working on x-rays and related synchrotron sources and applications in materials, physics, medicine, environment/geology, and biomedical materials
Author: Jacinto Sa
Publisher: CRC Press
ISBN: 1466592990
Category : Science
Languages : en
Pages : 242
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Book Description
Photon-in-photon-out core level spectroscopy is an emerging approach to characterize the electronic structure of catalysts and enzymes, and it is either installed or planned for intense synchrotron beam lines and X-ray free electron lasers. This type of spectroscopy requires high-energy resolution spectroscopy not only for the incoming X-ray beam b
Author: Erjia Guan
Publisher:
ISBN: 9780438630031
Category :
Languages : en
Pages :
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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: Adam Scott Hoffman
Publisher:
ISBN: 9781369795424
Category :
Languages : en
Pages :
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Supported metal catalysts are widely applied in technology, and a class of such catalysts, supported single-site metal catalysts, is drawing increasing attention because of prospects for increased application and the opportunities for fundamental understanding afforded by such catalysts made to have nearly uniform structures. Elucidation of structure-catalytic activity relationships of supported metals is complicated by the heterogeneity of the metal bonding sites on almost all support surfaces, especially those that are amorphous or incorporate multiple phases. The research summarized in this dissertation was carried out with the goal of fundamental understanding of the structures, reactivities, and catalytic properties of supported single-site catalysts with relatively well-defined structures. The catalyst samples consisted of Ir(C2H4) 2 or Ir(CO)2 supported on MgO or HY zeolite. The catalysts were synthesized by chemisorbing Ir(C2H4)2(acac) or Ir(CO)2(acac) (acac is acetylacetonate) on the above-mentioned supports that had been treated at elevated temperatures to remove water, or, in the case of MgO, to generate unique surface sites for bonding of the metal. The samples were characterized by infrared spectroscopy, X-ray absorption spectroscopy (including X-ray absorption near edge structure and extended X-ray absorption fine structure (EXAFS) analysis), scanning transmission electron microscopy, X-ray diffraction, N2 adsorption, and thermogravimetric analysis. The samples were tested as catalysts for the ethylene hydrogenation reaction. The research yielded four main results. First, a spectroscopic cell was developed for characterization of supported ultra-low loading metal catalyst, second, infrared spectroscopy characterizing metal carbonyls elucidated the uniformity of the metal-support interaction, third, high-energy-resolution X-ray absorption spectroscopy was used to identify ligands not detectable with infrared, and finally a limiting case of the metal-support interaction is identified and characterized. A specific goal was to generate unique surface sites on MgO as a support for samples with iridium loadings as low as 0.01 wt%, to determine sites for preferential bonding of iridium to the support. For characterization of samples with low iridium loadings using X-ray absorption spectroscopy, a novel experimental X-ray absorption spectroscopy cell was designed and fabricated; it allows for investigation of samples in transmission or fluorescence detection modes at temperatures up to 300°C under flow or vacuum conditions. Infrared characterization of the [nu][subscript CO] bands of supported Ir(CO)2 showed that narrower full bands corresponded to higher degrees of uniformity of the supported metal species, with the most uniform being those on zeolites, as a consequence of high degree of crystallinity and uniformity of bonding sites. The results also include new synthesis techniques, such as those involving low synthesis temperatures, high Si:Al ration in the zeolites, and high degrees of support crystallinity resulting from treatments that lead to high degrees of bonding site uniformity. High-energy-resolution fluorescence detection X-ray absorption spectroscopy was used to detect the dynamic exchange of C2H4 and CO ligands on the single-site Ir under conditions for which the sensitivity of conventional infrared spectroscopy was insufficient. The improved resolution in the XANES region also identified specific, assignable, features to C2H4 and CO ligands making it a valuable technique for catalyst characterization. EXAFS analysis of samples consisting of Ir(C2H4)2 supported on MgO demonstrated a change in coordination with decreasing loading and a limiting case of tripodal iridium on the MgO surface. The tripodal species have low initial activity for ethylene hydrogenation relative to the corresponding bipodal species, but they are characterized by greater thermal stability and can be used stably as catalysts at temperatures as high as at least to 300°C in reductive atmospheres. The results show that various surface sites can be involved in the bonding metals to MgO and that these sites exhibit distinct properties.
Author: Zizwe Atiba Chase
Publisher:
ISBN:
Category : Biomass energy
Languages : en
Pages : 226
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Book Description
The conversion of biomass into energy carriers in the condensed phase involves reactions in the presence of water mainly as a solvent. These chemical transformation of lignin-derived compounds to hydrocarbon fuels are performed at elevated temperature (473 K) and pressure (up to 50 bar H2) and therefore the state and stability of supported metal catalysts during reaction in the presence of water is not clear. Using in situ extended X-ray absorption fine structure spectroscopy, to better define the chemical state under dynamic variations of the environment, the noble metal Pd, the base metal Ni, and a bimetallic mixture of Ni and Fe were investigated under hydrothermal conditions. For Pd on various supports (C, Al2O 3, SiO2) water had no effect allowing its active phase (PdH x) to perform phenol hydrogenation. In the presence of water, Ni was found to be stable supported on HZSM-5 and sulfonated carbon while it transformed to Ni(II) phyllosilicate when supported on SiO2. This chemical state change lowers the hydrogenation activity of Ni on this support. Lastly, alloying Ni with Fe inhibits this undesired chemical state change for Ni(0) supported on SiO2 yet it also leads to a lowering of the hydrogenation activity that is speculated to be the result of a decreased amount of surface Ni available for reaction. Sum Frequency Generation was also used in a fundamental study to investigate diverse conformation of different interfaces to give a general understanding of surface interactions.
