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Author: Publisher: ISBN: Category : Languages : en Pages : 119
Book Description
The kinetic, electronic and spectroscopic properties of two-dimensional oxide-supported catalysts were investigated in order to understand the role of charge transfer in catalysis. Pt/TiO2 nanodiodes were fabricated and used as catalysts for hydrogen oxidation. During the reaction, the current through the diode, as well as its I-V curve, were monitored, while gas chromatography was used to measure the reaction rate. The current and the turnover rate were found to have the same temperature dependence, indicating that hydrogen oxidation leads to the non-adiabatic excitation of electrons in Pt. A fraction of these electrons have enough energy to ballistically transport through Pt and overcome the Schottky barrier at the interface with TiO2. The yield for this phenomenon is on the order of 10-4 electrons per product molecule formed, similar to what has been observed for CO oxidation and for the adsorption of many different molecules. The same Pt/TiO2 system was used to compare currents in hydrogen oxidation and deuterium oxidation. The current through the diode under deuterium oxidation was found to be greater than under hydrogen oxidation by a factor of three. Weighted by the difference in turnover frequencies for the two isotopes, this would imply a chemicurrent yield 5 times greater for D2 compared to H2, contrary to what is expected given the higher mass of D2. Reversible changes in the rectification factor of the diode are observed when switching between D2 and H2. These changes are a likely cause for the differences in current between the two isotopes. In the nanodiode experiments, surface chemistry leads to charge flow, suggesting the possibility of creating charge flow to tune surface chemistry. This was done first by exposing a Pt/Si diode to visible light while using it as a catalyst for H2 oxidation. Absorption of the light in the Si, combined with the band bending at the interface, gives rise to a steady-state flow of hot holes to the surface. This leads to a decrease in turnover on the surface, an effect which is enhanced when a reverse bias is applied to the diode. Similar experiments were carried out for CO oxidation. On Pt/Si diodes, the reaction rate was found to increase when a forward bias was applied. When the diode was exposed to visible light and a reverse bias was applied, the rate was instead decreased. This implies that a flow of negative charges to the surface increases turnover, while positive charges decrease it. Charge flow in an oxide supported metal catalyst can be modified even without designing the catalyst as a solid state electronic device. This was done by doping stoichiometric and nonstoichiometric TiO2 films with F, and using the resulting oxides as supports for Pt films. In the case of stoichiometric TiO2, F was found to act as an n-type dopant, creating a population of filled electronic states just below the conduction band, and dramatically increasing the conductivity of the oxide film. The electrons in those states can transfer to surface O, activating it for reaction with CO, and leading to increased turnover for CO oxidation. This reinforces the hypothesis that CO oxidation is activated by a flow of negative charges to the surface. The same set of catalysts was used for methanol oxidation. The electronic properties of the TiO2 films again correlated with the turnover rates, but also with selectivity. With stoichiometric TiO2 as the support, F-doping caused an increase in selectivity toward the formation of partial oxidation products, formaldehyde and methyl formate, versus the total oxidation product, CO2. With non-stoichiometric TiO2, F-doping had the reverse effect. Ambient Pressure X-Ray Photoelectron Spectroscopy was used to investigate this F-doping effect in reaction conditions. In O2 alone, and...
Author: Publisher: ISBN: Category : Languages : en Pages : 119
Book Description
The kinetic, electronic and spectroscopic properties of two-dimensional oxide-supported catalysts were investigated in order to understand the role of charge transfer in catalysis. Pt/TiO2 nanodiodes were fabricated and used as catalysts for hydrogen oxidation. During the reaction, the current through the diode, as well as its I-V curve, were monitored, while gas chromatography was used to measure the reaction rate. The current and the turnover rate were found to have the same temperature dependence, indicating that hydrogen oxidation leads to the non-adiabatic excitation of electrons in Pt. A fraction of these electrons have enough energy to ballistically transport through Pt and overcome the Schottky barrier at the interface with TiO2. The yield for this phenomenon is on the order of 10-4 electrons per product molecule formed, similar to what has been observed for CO oxidation and for the adsorption of many different molecules. The same Pt/TiO2 system was used to compare currents in hydrogen oxidation and deuterium oxidation. The current through the diode under deuterium oxidation was found to be greater than under hydrogen oxidation by a factor of three. Weighted by the difference in turnover frequencies for the two isotopes, this would imply a chemicurrent yield 5 times greater for D2 compared to H2, contrary to what is expected given the higher mass of D2. Reversible changes in the rectification factor of the diode are observed when switching between D2 and H2. These changes are a likely cause for the differences in current between the two isotopes. In the nanodiode experiments, surface chemistry leads to charge flow, suggesting the possibility of creating charge flow to tune surface chemistry. This was done first by exposing a Pt/Si diode to visible light while using it as a catalyst for H2 oxidation. Absorption of the light in the Si, combined with the band bending at the interface, gives rise to a steady-state flow of hot holes to the surface. This leads to a decrease in turnover on the surface, an effect which is enhanced when a reverse bias is applied to the diode. Similar experiments were carried out for CO oxidation. On Pt/Si diodes, the reaction rate was found to increase when a forward bias was applied. When the diode was exposed to visible light and a reverse bias was applied, the rate was instead decreased. This implies that a flow of negative charges to the surface increases turnover, while positive charges decrease it. Charge flow in an oxide supported metal catalyst can be modified even without designing the catalyst as a solid state electronic device. This was done by doping stoichiometric and nonstoichiometric TiO2 films with F, and using the resulting oxides as supports for Pt films. In the case of stoichiometric TiO2, F was found to act as an n-type dopant, creating a population of filled electronic states just below the conduction band, and dramatically increasing the conductivity of the oxide film. The electrons in those states can transfer to surface O, activating it for reaction with CO, and leading to increased turnover for CO oxidation. This reinforces the hypothesis that CO oxidation is activated by a flow of negative charges to the surface. The same set of catalysts was used for methanol oxidation. The electronic properties of the TiO2 films again correlated with the turnover rates, but also with selectivity. With stoichiometric TiO2 as the support, F-doping caused an increase in selectivity toward the formation of partial oxidation products, formaldehyde and methyl formate, versus the total oxidation product, CO2. With non-stoichiometric TiO2, F-doping had the reverse effect. Ambient Pressure X-Ray Photoelectron Spectroscopy was used to investigate this F-doping effect in reaction conditions. In O2 alone, and...
Author: Antoine Hervier Publisher: ISBN: Category : Languages : en Pages : 119
Book Description
The kinetic, electronic and spectroscopic properties of two-dimensional oxide-supported catalysts were investigated in order to understand the role of charge transfer in catalysis. Pt/TiO2 nanodiodes were fabricated and used as catalysts for hydrogen oxidation. During the reaction, the current through the diode, as well as its I-V curve, were monitored, while gas chromatography was used to measure the reaction rate. The current and the turnover rate were found to have the same temperature dependence, indicating that hydrogen oxidation leads to the non-adiabatic excitation of electrons in Pt. A fraction of these electrons have enough energy to ballistically transport through Pt and overcome the Schottky barrier at the interface with TiO2. The yield for this phenomenon is on the order of 10−4 electrons per product molecule formed, similar to what has been observed for CO oxidation and for the adsorption of many different molecules. The same Pt/TiO2 system was used to compare currents in hydrogen oxidation and deuterium oxidation. The current through the diode under deuterium oxidation was found to be greater than under hydrogen oxidation by a factor of three. Weighted by the difference in turnover frequencies for the two isotopes, this would imply a chemicurrent yield 5 times greater for D2 compared to H2, contrary to what is expected given the higher mass of D2. Reversible changes in the rectification factor of the diode are observed when switching between D2 and H2. These changes are a likely cause for the differences in current between the two isotopes. In the nanodiode experiments, surface chemistry leads to charge flow, suggesting the possibility of creating charge flow to tune surface chemistry. This was done first by exposing a Pt/Si diode to visible light while using it as a catalyst for H2 oxidation. Absorption of the light in the Si, combined with the band bending at the interface, gives rise to a steady-state flow of hot holes to the surface. This leads to a decrease in turnover on the surface, an effect which is enhanced when a reverse bias is applied to the diode. Similar experiments were carried out for CO oxidation. On Pt/Si diodes, the reaction rate was found to increase when a forward bias was applied. When the diode was exposed to visible light and a reverse bias was applied, the rate was instead decreased. This implies that a flow of negative charges to the surface increases turnover, while positive charges decrease it. Charge flow in an oxide supported metal catalyst can be modified even without designing the catalyst as a solid state electronic device. This was done by doping stoichiometric and nonstoichiometric TiO2 films with F, and using the resulting oxides as supports for Pt films. In the case of stoichiometric TiO2, F was found to act as an n-type dopant, creating a population of filled electronic states just below the conduction band, and dramatically increasing the conductivity of the oxide film. The electrons in those states can transfer to surface O, activating it for reaction with CO, and leading to increased turnover for CO oxidation. This reinforces the hypothesis that CO oxidation is activated by a flow of negative charges to the surface. The same set of catalysts was used for methanol oxidation. The electronic properties of the TiO2 films again correlated with the turnover rates, but also with selectivity. With stoichiometric TiO2 as the support, F-doping caused an increase in selectivity toward the formation of partial oxidation products, formaldehyde and methyl formate, versus the total oxidation product, CO2. With non-stoichiometric TiO2, F-doping had the reverse effect. Ambient Pressure X-Ray Photoelectron Spectroscopy was used to investigate this F-doping effect in reaction conditions. In O2 alone, and in CO oxidation conditions, the O1s spectrum showed a high binding energy peak that correlated in intensity with the activity of the different films: for stoichiometric films, the peak decreased in intensity with F-doping, while for nonstoichiometric films, the opposite was observed. No such changes were visible in the C1s spectrum, confirming the role of O activation in the reaction. This thesis adds to the body of knowledge on the importance of charge transfer at the metal-oxide interface in shaping the reactivity of heterogeneous catalysts, and provides examples of how this can be the basis for new methods to tune reactivity.
Author: John B. Moffat Publisher: Springer Science & Business Media ISBN: 9401098824 Category : Technology & Engineering Languages : en Pages : 548
Book Description
Catalysis involves just about every field of scientific study. This means that a multidisciplinary approach is needed in catalytic studies. Catalysis involves breaking and forming new bonds and this requires an under standing of either adsorption by bonding to an extended structures or bonding in a coordination sphere. Any understanding of catalytic action must necessarily involve an understanding of this bonding. Even 200 years ago scientists were aware that a properly treated mate rial, such as charcoal, could adsorb an enormous quantity of gas. In 1812, de Sassasure (English translation, Annal Philosphy, 6, 241 (1815» pro posed that the ability of a material to increase the rate of chemical reac tion was due to adsorption of the material in the fine structure of the solid so that the concentrations of the reactants were significantly increased, and this increase in concentration led to an increase in reaction rate. During the 1800s, little advance was made in the understanding of adsorp tion.
Author: Publisher: ISBN: Category : Languages : en Pages : 127
Book Description
Kinetic, electronic, and spectroscopic characterization of model Pt–support systems are used to demonstrate the relationship between charge transfer and catalytic activity and selectivity. The results show that charge flow controls the activity and selectivity of supported metal catalysts. This dissertation builds on extensive existing knowledge of metal–support interactions in heterogeneous catalysis. The results show the prominent role of charge transfer at catalytic interfaces to determine catalytic activity and selectivity. Further, this research demonstrates the possibility of selectively driving catalytic chemistry by controlling charge flow and presents solid-state devices and doped supports as novel methods for obtaining electronic control over catalytic reaction kinetics.
Author: Lawrence Robert Baker Publisher: ISBN: Category : Languages : en Pages : 252
Book Description
Kinetic, electronic, and spectroscopic characterization of model Pt-support systems are used to demonstrate the relationship between charge transfer and catalytic activity and selectivity. The results show that charge flow controls the activity and selectivity of supported metal catalysts. Using a Pt/n-Si Schottky junction, it is possible to externally control the electronic properties at the Pt/Si interface by applying bias or by exciting charge carriers with visible light. It is found that this device can control the rate of CO oxidation on Pt. Results show that negative charge on the Pt increases the reaction rate while positive charge on the Pt decreases the reaction rate. This is the first time that a solid-state device has been used to externally control the rate of a chemical reaction as determined by directly measuring the product yield. Similar experiments were performed for the H2 oxidation reaction and analogous results were obtained. Similarly, electronic control of catalyst performance can be achieved by substrate doping. It is shown that F doping can tune the electronic structure of TiO2. When F is incorporated into TiO2, the electrical conductivity is dramatically enhanced because F acts as an n-type dopant. It is shown that this highly n-type TiO2 acts as an electronically active support for Pt catalysts, increasing the rate of CO oxidation by electronically activating surface O. It is further shown that for a multipath reaction, the electronic structure of TiO2 also controls the reaction selectivity. The electronic activity of highly n-type TiO2 serving as a Pt support for methanol oxidation selectively enhances the production of formaldehyde relative to CO2. Finally, sum frequency generation (SFG) vibrational spectroscopy is used to probe the molecular nature of strong metal-support interactions (SMSI). This is the first time that SFG has been used to probe the highly selective oxide-metal interface during catalytic reaction, and the results demonstrate that charge transfer from TiO2 on a Pt/TiO2 catalyst controls the product distribution of furfuraldehyde hydrogenation by an acid-base mechanism. SFG spectra reveal that a furfuryl-oxy intermediate forms on TiO2 as a result of a charge transfer interaction. This furfuryl-oxy intermediate is a highly active and selective precursor to furfuryl alcohol, and spectral analysis shows that the Pt/TiO2 interface is required primarily for H spillover. Density functional calculations predict that O-vacancies on the TiO2 surface activate the formation of the furfuryl-oxy intermediate via an electron transfer to furfuraldehyde, drawing a strong analogy between SMSI and acid-base catalysis. This dissertation builds on extensive existing knowledge of metal-support interactions in heterogeneous catalysis. The results show the prominent role of charge transfer at catalytic interfaces to determine catalytic activity and selectivity. Further, this research demonstrates the possibility of selectively driving catalytic chemistry by controlling charge flow and presents solid-state devices and doped supports as novel methods for obtaining electronic control over catalytic reaction kinetics.
Author: Jens K. Nørskov Publisher: John Wiley & Sons ISBN: 1118888952 Category : Technology & Engineering Languages : en Pages : 228
Book Description
This book is based on a graduate course and suitable as a primer for any newcomer to the field, this book is a detailed introduction to the experimental and computational methods that are used to study how solid surfaces act as catalysts. Features include: First comprehensive description of modern theory of heterogeneous catalysis Basis for understanding and designing experiments in the field Allows reader to understand catalyst design principles Introduction to important elements of energy transformation technology Test driven at Stanford University over several semesters
Author: James J Spivey Publisher: Royal Society of Chemistry ISBN: 1839163127 Category : Science Languages : en Pages : 494
Book Description
This volume looks at modern approaches to catalysis and reviews the extensive literature. Chapters highlight application of 2D materials in biomass conversion catalysis, plasmonic photocatalysis, catalytic demonstration of mesoporosity in the hierarchical zeolite and the effect of surface phase oxides on supported metals and catalysis. Looking to the future a chapter on ab initio machine learning for accelerating catalytic materials discovery is included. Appealing broadly to researchers in academia and industry, these illustrative chapters bridge the gap from academic studies in the laboratory to practical applications in industry not only for catalysis field but also for environmental protection. Other chapters with an industrial perspective include heterogeneous and homogeneous catalytic routes for vinyl acetate synthesis, catalysis for production of jet fuel from renewable sources by HDO/HDC and microwave-assisted catalysis for fuel conversion. Chemical reactions in ball mills is also explored. The book will be of great benefit to any researcher wanting a succinct reference on developments in this area now and looking to the future.
Author: J.A. Moulijn Publisher: Elsevier ISBN: 0080886981 Category : Technology & Engineering Languages : en Pages : 485
Book Description
Catalysis is a multidisciplinary activity which is reflected in this book. The editors have chosen a novel combination of basic disciplines - homogeneous catalysis by metal complexes is treated jointly with heterogeneous catalysis with metallic and non-metallic solids. The main theme of the book is the molecular approach to industrial catalysis. In the introductory section Chapter 1 presents a brief survey of the history of industrial heterogeneous and homogeneous catalysis. Subsequently, a selection of current industrial catalytic processes is described (Chapter 2). A broad spectrum of important catalytic applications is presented, including the basic chemistry, some engineering aspects, feedstock sources and product utilisation. In Chapter 3, kinetic principles are treated. The section on fundamental catalysis begins with a description of the bonding in complexes and to surfaces (Chapter 4). The elementary steps on complexes and surfaces are described. The chapter on heterogeneous catalysis (5) deals with the mechanistic aspects of three groups of important reactions: syn-gas conversion, hydrogenation, and oxidation. The main principles of metal and metal oxide catalysis are presented. Likewise, the chapter on homogeneous catalysis (6) concentrates on three reactions representing examples from three areas: carbonylation, polymerization, and asymmetric catalysis. Identification by in situ techniques has been included. Many constraints to the industrial use of a catalyst have a macroscopic origin. In applied catalysis it is shown how catalytic reaction engineering deals with such macroscopic considerations in heterogeneous as well as homogeneous catalysis (Chapter 7). The transport and kinetic phenomena in both model reactors and industrial reactors are outlined. The section on catalyst preparation (Chapters 8 and 9) is concerned with the preparation of catalyst supports, zeolites, and supported catalysts, with an emphasis on general principles and mechanistic aspects. For the supported catalysts the relation between the preparative method and the surface chemistry of the support is highlighted. The molecular approach is maintained throughout. The first chapter (10) in the section on catalyst characterization summarizes the most common spectroscopic techniques used for the characterisation of heterogeneous catalysts such as XPS, Auger, EXAFS, etc. Temperature programmed techniques, which have found widespread application in heterogeneous catalysis both in catalyst characterization and simulation of pretreatment procedures, are discussed in Chapter 11. A discussion of texture measurement, theory and application, concludes this section (12). The final chapter (13) gives an outline of current trends in catalysis. Two points of view are adopted: the first one focusses on developments in process engineering. Most often these have their origin in demands by society for better processes. The second point of view draws attention to the autonomous developments in catalysis, which is becoming one of the frontier sciences of physics and chemistry. In this book emphasis is on those reactions catalyzed by heterogeneous and homogeneous catalysts of industrial relevance. The integrative treatment of the subject matter involves many disciplines, consequently, the writing of the book has been a multi-author task. The editors have carefully planned and harmonized the contents of the chapters.
Author: Rutger A. van Santen Publisher: John Wiley & Sons ISBN: 3527608346 Category : Science Languages : en Pages : 488
Book Description
An integrated approach to the molecular theory of reaction mechanism in heterogeneous catalysis, largely based on the knowledge among the growing theoretical catalysis community over the past half century, and covering all major catalytic systems. The authors develop a general conceptual framework, including in-depth comparisons with enzyme catalysis, biomineralisation, organometallic and coordination chemistry. A chapter dedicated to molecular electrocatalysis addresses the molecular description of reactions at the liquid-solid interphase, while studies range from a quantum-chemical treatment of individual molecular states to dynamic Monte-Carlo simulations, including the full flexibility of the many-particle systems. Complexity in catalysis is explained in chapters on self-organization and self-assembly of catalysts, and other sections are devoted to evolutionary, combinatorial techniques as well as artificial chemistry.
Author: R.M. Lambert Publisher: Springer Science & Business Media ISBN: 9401589119 Category : Science Languages : en Pages : 534
Book Description
Heterogeneous catalysis provides the backbone of the world's chemical and oil industries. The innate complexity of practical catalytic systems suggests that useful progress should be achievable by investigating key aspects of catalysis by experimental studies on idealised model systems. Thin films and supported clusters are two promising types of model system that can be used for this purpose, since they mimic important aspects of the properties of practical dispersed catalysts. Similarly, appropriate theoretical studies of chemisorption and surface reaction clusters or extended slab systems can provide valuable information on the factors that underlie bonding and catalytic activity. This volume describes such experimental and theoretical approaches to the surface chemistry and catalytic behaviour of metals, metal oxides and metal/metal oxide systems. An introduction to the principles and main themes of heterogeneous catalysis is followed by detailed accounts of the application of modern experimental and theoretical techniques to fundamental problems. The application of advanced experimental methods is complemented by a full description of theoretical procedures, including Hartree-Fock, density functional and similar techniques. The relative merits of the various approaches are considered and directions for future progress are indicated.
Author: 
Author: 
Author: Antoine Hervier
Author: John B. Moffat
Author: 
Author: Lawrence Robert Baker
Author: Jens K. Nørskov
Author: James J Spivey
Author: J.A. Moulijn
Author: Rutger A. van Santen
Author: R.M. Lambert