Heterogeneous Catalysis at Nanoscale for Energy Applications

Franklin (Feng) Tao, William F. Schneider, Prashant V. Kamat, Franklin Tao

• Publisher: Wiley
• Year: 2015
• Print ISBN: 9780470952603
• eBook ISBN: 9781118843529
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Energy

This book presents both the fundamentals concepts and latest achievements of a field that is growing in importance since it represents a possible solution for global energy problems.  It focuses on an atomic-level understanding of heterogeneous catalysis involved in important energy conversion processes. It presents a concise picture for the entire area of heterogeneous catalysis with vision at the atomic- and nano- scales, from synthesis, ex-situ and in-situ characterization, catalytic activity and selectivity, to mechanistic understanding based on experimental exploration and theoretical simulation.

The book:

• Addresses heterogeneous catalysis, one of the crucial technologies employed within the chemical and energy industries
• Presents the recent advances in the synthesis and characterization of nanocatalysts as well as a mechanistic understanding of catalysis at atomic level for important processes of energy conversion
• Provides a foundation for the potential design of revolutionarily new technical catalysts and thus the further development of efficient technologies for the global energy economy
• Includes both theoretical studies and experimental exploration
• Is useful as both a textbook for graduate and undergraduate students and a reference book for scientists and engineers in chemistry, materials science, and chemical engineering

1
Introduction

Franklin (Feng) Tao¹, William F. Schneider^2,3, and Prashant V. Kamat^2,3,4

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 FeO_x substrate (Pt₁-O_n-Fe_m, 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 (M_n, 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 tunneling microscopy have enabled direct probing of the catalytic surface and its activity under reaction conditions. Chapter 5 provides a brief review of X-ray absorption spectroscopy, one of the more widely used in situ characterization techniques. It reviews the design of in situ reaction chambers for X-ray absorption spectroscopy and their application to catalytic energy conversion processes.

The recent rapid advances in heterogeneous catalysis science owes as much to the development of theoretical tools able to reliably model reactions at heterogeneous surfaces ab initio (or from “first principles”) as it does to the revolutions in synthesis and characterization described earlier. In fact, it is somewhat ironic that fast and cheap computing power made possible by the shrinking of microelectronics to the nanoscale has enabled computational models of catalytic reactivity at the same scale! Semiempirical bond-order conservation methods were the first to be widely applied to heterogeneous catalysis [36], but today density functional theory (DFT) models [37, 38] dominate the field. Fundamentally, DFT provides a mapping from the geometric arrangement of a set of atoms to the distribution of elect...

Table of contents

• Cover
• Title page
• Copyright page
• Contributors
• 1 Introduction
• 2 Chemical Synthesis of Nanoscale Heterogeneous Catalysts
• 3 Physical Fabrication of Nanostructured Heterogeneous Catalysts
• 4 Ex Situ Characterization
• 5 Applications of Soft X-Ray Absorption Spectroscopy for In Situ Studies of Catalysts at Nanoscale
• 6 First-Principles Approaches to Understanding Heterogeneous Catalysis
• 7 Computational Screening for Improved Heterogeneous Catalysts and Electrocatalysts
• 8 Catalytic Kinetics and Dynamics
• 9 Catalysts for Biofuels
• 10 Development of New Gold Catalysts for Removing CO from H2
• 11 Photocatalysis in Generation of Hydrogen from Water
• 12 Photocatalysis in Conversion of Greenhouse Gases
• 13 Electrocatalyst Design in Proton Exchange Membrane Fuel Cells for Automotive Application
• Index
• End User License Agreement