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Contributors xiii
1 Introduction 1Franklin (Feng) Tao, William F. Schneider, and Prashant V. Kamat
2 Chemical Synthesis of Nanoscale Heterogeneous Catalysts 9Jianbo Wu and Hong Yang
2.1 Introduction 9
2.2 Brief Overview of Heterogeneous Catalysts 10
2.3 Chemical Synthetic Approaches 11
2.3.1 Colloidal Synthesis 11
2.3.2 Shape Control of Catalysts in Colloidal Synthesis 12
2.3.3 Control of Crystalline Phase of Intermetallic Nanostructures 14
2.3.4 Other Modes of Formation for Complex Nanostructures 17
2.4 Core-Shell Nanoparticles and Controls of Surface Compositions and Surface Atomic Arrangements 21
2.4.1 New Development on the Preparation of Colloidal Core-Shell Nanoparticles 21
2.4.2 Electrochemical Methods to Core-Shell Nanostructures 22
2.4.3 Control of Surface Composition via Surface Segregation 24
2.5 Summary 25
3 Physical Fabrication of Nanostructured Heterogeneous Catalysts 31Chunrong Yin, Eric C. Tyo, and Stefan Vajda
3.1 Introduction 31
3.2 Cluster Sources 34
3.2.1 T hermal Vaporization Source 34
3.2.2 Laser Ablation Source 36
3.2.3 Magnetron Cluster Source 37
3.2.4 Arc Cluster Ion Source 38
3.3 Mass Analyzers 39
3.3.1 Neutral Cluster Beams 40
3.3.2 Quadrupole Mass Analyzer 41
3.3.3 Lateral TOF Mass Filter 42
3.3.4 Magnetic Sector Mass Selector 43
3.3.5 Quadrupole Deflector (Bender) 44
3.4 Survey of Cluster Deposition Apparatuses in Catalysis Studies 44
3.4.1 Laser Ablation Source with a Quadrupole Mass Analyzer at Argonne National Lab 44
3.4.2 ACIS with a Quadrupole Deflector at the Universität Rostock 46
3.4.3 Magnetron Cluster Source with a Lateral TOF Mass Filter at the University of Birmingham 47
3.4.4 Laser Ablation Cluster Source with a Quadrupole Mass Selector at the Technische Universität München 48
3.4.5 Laser Ablation Cluster Source with a Quadrupole Mass Analyzer at the University of Utah 49
3.4.6 Laser Ablation Cluster Source with a Magnetic Sector Mass Selector at the University of California, Santa Barbara 49
3.4.7 Magnetron Cluster Source with a Quadrupole Mass Filter at the Toyota Technological Institute 51
3.4.8 PACIS with a Magnetic Sector Mass Selector at Universität Konstanz 52
3.4.9 Magnetron Cluster Source with a Magnetic Sector at Johns Hopkins University 53
3.4.10 Magnetron Cluster Source with a Magnetic Sector at HZB 53
3.4.11 Magnetron Sputtering Source with a Quadrupole Mass Filter at the Technical University of Denmark 54
3.4.12 CORDIS with a Quadrupole Mass Filter at the Lausanne Group 56
3.4.13 Electron Impact Source with a Quadrupole Mass Selector at the Universität Karlsruhe 56
3.4.14 CORDIS with a Quadrupole Mass Analyzer at the Universität Ulm 58
3.4.15 Magnetron Cluster Source with a Lateral TOF Mass Filter at the Universität Dortmund 59
3.4.16 Z-Spray Source with a Quadrupole Mass Filter for Gas-Phase Investigations at FELIX 60
3.4.17 Laser Ablation Source with an Ion Cyclotron Resonance Mass Spectrometer for Gas-Phase Investigations at the Technische Universität Berlin 61
4 Ex Situ Characterization 69Minghua Qiao, Songhai Xie, Yan Pei, and Kangnian Fan
4.1 Introduction 69
4.2 Ex Situ Characterization Techniques 70
4.2.1 X-Ray Absorption Spectroscopy 71
4.2.2 Electron Spectroscopy 72
4.2.3 Electron Microscopy 74
4.2.4 Scanning Probe Microscopy 75
4.2.5 Mössbauer Spectroscopy 76
4.3 Some Examples on Ex Situ Characterization of Nanocatalysts for Energy Applications 77
4.3.1 Illustrating Structural and Electronic Properties of Complex Nanocatalysts 77
4.3.2 Elucidating Structural Characteristics of Catalysts at the Nanometer or Atomic Level 81
4.3.3 Pinpointing the Nature of the Active Sites on Nanocatalysts 85
4.4 Conclusions 88
5 Applications of Soft X-Ray Absorption Spectroscopy for In Situ Studies of Catalysts at Nanoscale 93Xingyi Deng, Xiaoli Gu, and Franklin (Feng) Tao
5.1 Introduction 93
5.2 In Situ SXAS under Reaction Conditions 96
5.3 Examples of In Situ SXAS Studies under Reaction Conditions Using Reaction Cells 99
5.3.1 Atmospheric Corrosion of Metal Films 99
5.3.2 Cobalt Nanoparticles under Reaction Conditions 101
5.3.3 Electrochemical Corrosion of Cu in Aqueous NaHCO3 Solution 108
5.4 Summary 112
6 First-Principles Approaches to Understanding Heterogeneous Catalysis 115Dorrell C. McCalman and William F. Schneider
6.1 Introduction 115
6.2 Computational Models 116
6.2.1 Electronic Structure Methods 116
6.2.2 System Models 117
6.3 NOx Reduction 118
6.4 Adsorption at Metal Surfaces 119
6.4.1 Neutral Adsorbates 119
6.4.2 Charged Adsorbates 122
6.5 Elementary Surface Reactions Between Adsorbates 125
6.5.1 Reaction Thermodynamics 125
6.5.2 Reaction Kinetics 129
6.6 Coverage Effects on Reaction and Activation Energies at Metal Surfaces 131
6.7 Summary 135
7 Computational Screening for Improved Heterogeneous Catalysts and Electrocatalysts 139Jeffrey Greeley
7.1 Introduction 139
7.2 T rends-Based Studies in Computational Catalysis 140
7.2.1 Early Groundwork for Computational Catalyst Screening 140
7.2.2 Volcano Plots and Rate Theory Models 141
7.2.3 Scaling Relations, BEP Relations, and Descriptor Determination 144
7.3 Computational Screening of Heterogeneous Catalysts and Electrocatalysts 148
7.3.1 Computational Catalyst Screening Strategies 149
7.4 Challenges and New Frontiers in Computational Catalyst Screening 153
7.5 Conclusions 155
8 Catalytic Kinetics and Dynamics 161Rafael C. Catapan, Matthew A. Christiansen, Amir A. M. Oliveira, and Dionisios G. Vlachos
8.1 Introduction 161
8.2 Basics of Catalyst Functionality, Mechanisms, and Elementary Reactions on Surfaces 163
8.3 T ransition State Theory, Collision Theory, and Rate Constants 166
8.4 Density Functional Theory Calculations 168
8.4.1 Calculation of Energetics and Coverage Effects 169
8.4.2 Calculation of Vibrational Frequencies 172
8.5 T hermodynamic Consistency of the DFT-Predicted Energetics 172
8.6 State Properties from Statistical Thermodynamics 176
8.6.1 Strongly Bound Adsorbates 177
8.6.2 Weakly Bound Adsorbates 177
8.7 Semiempirical Methods for Predicting Thermodynamic Properties and Kinetic Parameters 178
8.7.1 Linear Scaling Relationships 178
8.7.2 Heat Capacity and Surface Entropy Estimation 179
8.7.3 Brønsted-Evans-Polanyi Relationships 180
8.8 Analysis Tools for Microkinetic Modeling 181
8.8.1 Rates in Microkinetic Modeling 181
8.8.2 Reaction Path Analysis and Partial Equilibrium Analysis 181
8.8.3 Rate-Determining Steps, Most Important Surface Intermediates, and Most Abundant Surface Intermediates 184
8.8.4 Calculation of the Overall Reaction Order and Apparent Activation Energy 186
8.9 Concluding Remarks 187
9 Catalysts for Biofuels 191Gregory T. Neumann, Danielle Garcia, and Jason C. Hicks
9.1 Introduction 191
9.2 Lignocellulosic Biomass 192
9.2.1 Cellulose 192
9.2.2 Hemicellulose 194
9.2.3 Lignin 195
9.3 Carbohydrate Upgrading 195
9.3.1 Zeolitic Upgrading of Cellulosic Feedstocks 196
9.3.2 Levulinic Acid Upgrading 199
9.3.3 GVL Upgrading 201
9.3.4 Aqueous-Phase Processing 202
9.4 Lignin Conversion 205
9.4.1 Zeolite Upgrading of Lignin Feedstocks 206
9.4.2 Catalysts for Hydrodeoxygenation of Lignin 208
9.4.3 Selective Unsupported Catalyst for Lignin Depolymerization 211
9.5 Continued Efforts for the Development of Robust Catalysts 212
10 Development of New Gold Catalysts for Removing CO from H2 217Zhen Ma, Franklin (Feng) Tao, and Xiaoli Gu
10.1 Introduction 217
10.2 General Description of Catalyst Development 218
10.3 Development of WGS catalysts 220
10.3.1 Initially Developed Catalysts 220
10.3.2 Fe2O3-Based Gold Catalysts 221
10.3.3 CeO2-Based Gold Catalysts 221
10.3.4 TiO2- or ZrO2-Based Gold Catalysts 223
10.3.5 Mixed-Oxide Supports with 1:1 Composition 223
10.3.6 Bimetallic Catalysts 224
10.4 Development of New Gold Catalysts for PROX 225
10.4.1 General Considerations 225
10.4.2 CeO2-Based Gold Catalysts 226
10.4.3 TiO2-Based Gold Catalysts 227
10.4.4 Al2O3-Based Gold Catalysts 228
10.4.5 Mixed Oxide Supports with 1:1 Composition 228
10.4.6 Other Oxide-Based Gold Catalysts 229
10.4.7 Supported Bimetallic catalysts 229
10.5 Perspectives 229
11 Photocatalysis in Generation of Hydrogen from Water 239Kazuhiro Takanabe and Kazunari Domen
11.1 Solar Energy Conversion 239
11.1.1 Solar Energy Conversion Technology for Producing Fuels and Chemicals 239
11.1.2 Solar Spectrum and STH Efficiency 242
11.2 Semiconductor Particles: Optical and Electronic Nature 244
11.2.1 Reaction Sequence and Principles of Overall Water Splitting and Reaction Step Timescales 244
11.2.2 Number of Photons Striking a Single Particle 245
11.2.3 Absorption Depth of Light Incident on Powder Photocatalyst 247
11.2.4 Degree of Band Bending in Semiconductor Powder 248
11.2.5 Band Gap and Flat-Band Potential of Semiconductor 250
11.3 Photocatalyst Materials for Overall Water Splitting: UV to Visible Light Response 251
11.3.1 UV Photocatalysts: Oxides 251
11.3.2 Visible-Light Photocatalysts: Band Engineering of Semiconductor Materials Containing Transition Metals 253
11.3.3 Visible-Light Photocatalysts: Organic Semiconductors as Water-Splitting Photocatalysts 255
11.3.4 Z-Scheme Approach: Two-Photon Process 257
11.3.5 Defects and Recombination in Semiconductor Bulk 257
11.4 Cocatalysts for Photocatalytic Overall Water Splitting 259
11.4.1 Metal Nanoparticles as Hydrogen Evolution Cocatalysts: Novel Core/Shell Structure 259
11.4.2 Reaction Rate Expression on Active Catalytic Centers for Redox Reaction in Solution 261
11.4.3 Measurement of Potentials at Semiconductor and Metal Particles Under Irradiation 264
11.4.4 Metal Oxides as Oxygen Evolution Cocatalyst 266
11.5 Concluding Remarks 268
12 Photocatalysis in Conversion of Greenhouse Gases 271Kentaro Teramura and Tsunehiro Tanaka
12.1 Introduction 271
12.2 Outline of Photocatalytic Conversion of CO2 273
12.3 Reaction Mechanism for the Photocatalytic Conversion of CO2 276
12.3.1 Adsorption of CO2 and H2 276
12.3.2 Assignment of Adsorbed Species by FT-IR Spectroscopy 279
12.3.3 Observation of Photoactive Species by Photoluminescence (PL) and Electron Paramagnetic Resonance (EPR) Spectroscopies 281
12.4 Summary 283
13 Electrocatalyst Design in Proton Exchange Membrane Fuel Cells for Automotive Application 285Anusorn Kongkanand, Wenbin Gu, and Frederick T. Wagner
13.1 Introduction 285
13.2 Advanced Electrocatalysts 288
13.2.1 Pt-Alloy and Dealloyed Catalysts 288
13.2.2 Pt Monolayer Catalysts 290
13.2.3 Continuous-Layer Catalysts 293
13.2.4 Controlled Crystal Face Catalysts 296
13.2.5 Hollow Pt Catalysts 298
13.3 Electrode Designs 299
13.3.1 Dispersed-Catalyst Electrodes 299
13.3.2 NSTF Electrodes 302
13.4 Concluding Remarks 307
Index 315
Franklin (Feng) Tao1, William F. Schneider2,3, and Prashant V. Kamat2,3,4
1 Department of Chemical and Petroleum Engineering and Department of Chemistry, University of Kansas, Lawrence KS USA
2 Department of Chemical and Biochemical Engineering, University of Notre Dame, Notre Dame IN, USA
3 Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN, USA
4 Radiation Laboratory, University of Notre Dame, Notre Dame, IN, USA
Heterogeneous catalytic transformations are responsible for improving the quality of our everyday life. Whether it is the ready availability of food and clothing, clean fuel for our cars, or new devices for energy conversion and storage, it is the catalytic process that makes possible the lifestyle that we all enjoy today. These capabilities are the result of more than a century of research, development, and application of heterogeneous catalytic materials and processes. Our society now faces grand challenges in energy sustainability. Heterogeneous catalysis again is at the forefront of new processes to harvest energy and convert it. Emergent areas of need and opportunity include but are not limited to conversion of nonedible biomass and natural gas to fuel molecules through thermal catalysis, the harvesting of solar energy to generate solar fuels through photocatalysis, and the conversion of chemical fuels such as hydrogen or methanol to electricity through electrocatalysis. These catalytic processes occur at solid-gas, solid-liquid, or even three-phase boundaries, as at an electrode-electrolyte-gas interface, and the efficiency of these energy harvesting and conversion processes is largely determined by catalytic performance at these interfaces. Because many of these desired energy-related conversions and harvestings are new and in many cases yet to be discovered, a summary of fundamental insights and understanding of those processes is critical to progress.
A catalytic event is envisioned to occur at a catalytic "site" [1, 2]. A catalytic site consists of one or many atoms arranged into a particular configuration that provides an ideal electronic structure and geometric environment for facilitating the event. A commercial catalyst is typically heterogeneous from macroscopic to microscopic length scales. It can consist of catalytic particles of different shapes and sizes of dimensions from less than 1 to more than 100?nm, most often supported on other particles or materials that provide structural integrity and access to the active sites. In an industrial catalyst, each particle can have a different composition, and this composition can vary from the bulk to the surface. Further, these compositions can be a strong function of the reactive environment. This diversity in structure and composition makes fundamental interrogation of catalytic events on a commercial catalyst at the level of a catalytic site quite challenging. To gain fundamental understanding of catalytic reactions at this microscopic level, a practical strategy is to employ model catalysts. These models can range from materials of a composition simpler than commercial materials all the way to catalytically active single crystals with well-defined surface structures. Extensive experiments in the last four decades on model systems have revealed precious insights into the chemistry and physics of heterogeneous catalysis. However, there are some limitations of these models. For instance, a single-crystal model catalyst presents a limited interfacial surface area, which can make the detection of reaction products over such a model catalyst challenging. Nanocatalysts of well-defined size, shape, and composition provide a further step forward in terms of a closer representation of practical catalysts and better access to questions about the impact of structural and compositional factors on their catalytic performances.
The science of nanocatalysis is enabled by the ability to synthesize nanoparticles of well-controlled shape, size, and composition. To explore how structural factors impact catalytic performance, we need to decouple these interacting structural and compositional factors of an industrial catalyst through chemical synthesis [3-12]. For example, to explore potential surface-structure-dependent catalytic activity or selectivity we could keep the size and composition of nanoparticles of a catalyst the same but vary shape of a catalyst. The surface structure of a catalyst with a different shape can be varied through chemical synthesis [13-17]. Chapter 2 reviews the control of nanocatalyst structural parameters through chemical synthesis by which shape, composition, and nanostructure can be controlled. In this chapter, the fundamental mechanisms of growth of metal nanoparticles are introduced. Controlled syntheses of intermetallic nanocatalysts, nanostructured catalyst particles, and core-shell nanoparticles are reviewed.
Colloidal synthesis offers an elegant approach to manipulate the structure of a crystallographic surface, size, and composition of nanocatalysts. The surface of a 2?×?2?nm nanoparticle or larger likely presents multiple combinations of catalyst atoms packed with different distances and relative orientations. The occurrence of nonhomogeneous catalytic sites on larger surfaces may decrease catalytic selectivity by opening undesired reaction pathways. An alternative strategy is to synthesize a catalyst anchoring singly dispersed metal atoms on a given substrate [18-21]. Charge transfer between singly dispersed metal atoms and their nonmetallic substrates can tune the adsorption energy of reactant molecules [21] and thus potentially vary the activation barrier of a catalytic reaction. For example, the formation of a singly dispersed Pt atom bonded to oxygen atoms of FeOx substrate (Pt1-On-Fem, n is the number of Pt-O bonds) has been shown to exhibit high CO oxidation activity [18]. However, achieving singly dispersed catalytic sites on a substrate through chemical synthesis is quite challenging. Alternatively, subnanometer metal clusters with a specific number of atoms [22, 23] can be prepared through physical methods including thermal vaporization, laser ablation, and magnetron and arc cluster ion deposition techniques. These physical methods can produce clusters with a specific number of atoms on a given substrate. The ability to vary the number of atoms of a cluster offers the opportunity to study site-specific catalyses. Chapter 3 summarizes these physical approaches to the preparation of size-specific catalysts (Mn, n?=?1-20), including a discussion of methods and cluster sources.
Catalyst characterization is the primary window through which to obtain insights into structure and mechanism. Characterization of a catalytic site demands methods with fidelity at the nanoscale or smaller. It is particularly challenging to achieve this level of detail in the presence of a real reaction mixture at actual catalytic temperatures, and thus the first tier of analysis is often carried out ex situ, or outside of this environment. Spectroscopic and microscopic analysis carried out ex situ under ultrahigh vacuum (UHV) allows surface structures and processes to be studied in exquisite detail and are the foundations of much of our understanding of surface catalytic processes. The reaction environment can and often does have a significant modifying influence on surface properties and reactivity, and thus increasingly analytical methods have been developed to be applied in situ, or "in place" [9, 24-31]. There is some debate in the catalysis and surface science communities regarding the precise meanings of in situ and the related term operando. We draw no particular line between them here, recognizing instead that analysis under any set of conditions can provide useful insights into catalytic behavior.
Both surface and bulk properties are relevant to catalytic reactivity. Although heterogeneous reactions by definition occur at the interface between a catalyst and reactant/product phase, the process of catalysis actually includes activation of an as-synthesized catalyst, catalytic reaction, and adverse processes leading to the deactivation of a working catalyst. Activation may involve chemical transformations of both the catalyst surface and bulk. For example, the iron oxide Fe2O3 is chemically transformed into the active iron carbide during activation for the Fischer-Tropsch synthesis (FTS) from CO and H2 [32, 33]. There are numerous other examples of reduction of a metal oxide to an active metal or oxidation of a metal to an active oxide, carbide, sulfide, or similar. Characterization of chemistry and structure of the surface and bulk of a catalyst nanoparticle using representative techniques are presented in Chapter 4.
The surface energy of a material is sensitive to the environment it is exposed to, including the type, temperature, and pressure of any reactants. As a result, a catalyst may "adapt" to its environment by exposing different surface structures [34, 35]. To capture this relationship between environment, structure, and activity, it is necessary to characterize a catalyst as it undergoes reaction, in situ [9, 24-31, 33, 34]. In situ X-ray absorption spectroscopy, ambient pressure X-ray photoelectron spectroscopy, environmental electron microcopy, and high-pressure scanning...
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