Volume Editors' Introduction

In this volume of Comprehensive Inorganic Chemistry, we focus on solid catalysts of inorganic nature, that is, materials containing at least one metallic element. In Section 7.01, we give a detailed introduction of heterogeneous catalysis, including the definition of catalysis: a phenomenon in which a substance, the catalyst, accelerates chemical reactions without being consumed in the process. Catalysts are indispensable. In 2013, an estimated 85–90% of all chemical products are manufactured in catalytic processes. These products are transportation fuels, bulk and fine chemicals, or materials such as polymers. In particular, catalysts play a very important role in environmental applications, such as the automotive exhaust converter.\u3cbr/\u3e\u3cbr/\u3eIn heterogeneous catalysis, the catalyst is in a different phase than the reactants, contrary to homogeneous catalysis (Volume 6). It is a solid material, whereas the reactants are either in the gas or in the liquid phase.\u3cbr/\u3e\u3cbr/\u3eHeterogeneous catalysts are nanosized particles, often stabilized by a support.\u3cbr/\u3e\u3cbr/\u3eThey can be metals, metal oxides, zeolites, sulfides, carbon-based materials, and also metal carbides, nitrides, phosphides, chlorides. Chapter 7.01 introduces catalysis as a concept, and places it briefly into context. Chapters 7.02 and 7.03 present the molecular description that forms the basis for the potential energy considerations and the kinetic descriptions. Kruse and co-workers first describe elementary reaction steps such as adsorption, dissociation, reaction, and desorption on some well-defined metal surfaces, along with some of the typical spectroscopic and imaging methods used in surface science. A catalytic reaction mechanism is a sequence of elementary steps, and the reaction rate of the entire process is the resultant of those of the individual steps. In the chapter 7.03, Zaera discusses the kinetics of mechanisms on the idealized surfaces of model systems. These two other rather elementary chapters illustrate the power of surface science in unraveling catalytic mechanisms and in providing kinetic and thermodynamic data including activation barriers and heats of adsorption.\u3cbr/\u3e\u3cbr/\u3eWet chemistry metal nanoparticle preparations and the nature of the solid–liquid interface is dealt with in Chapter 7.04 and attention to seeing catalyst particles in all of their morphologic detail using imaging methods is the subject of Chapter 7.05. Supported oxides as catalysts with the emphasis on monolayer systems, their preparation, and industrial use are treated in Chapters 7.06 and 7.07. Heteropoly oxidic compounds and their use in catalysis are dealt with in Chapter 7.08. Solid acids, zeolites, and mesoporous materials are discussed in Chapters 7.09 (concepts), 7.10 (characterization an catalysis in zeolites) and 7.11 (micro- and mesoporous compounds). Sulfides, carbon polymorphs, and carbides and nitrides in catalysis are covered in Chapters 7.12 (hydrotreating reactions), 7.13 (coke and deactivation), and 7.14 (carbides as co-catalysts).\u3cbr/\u3e\u3cbr/\u3eComputational modeling in catalysis is important nowadays and four chapters deal with it in detail. Chapter 7.15 presents an introduction to the theoretical foundations, whereas Chapter 7.16 details DFT calculations on catalysts. Chapter 7.17 treats theory and practice of the Haber–Bosch process, and in Chapter 7.18, the computational modeling of reacting systems in presented.\u3cbr/\u3e\u3cbr/\u3eThe last part of this volume deals with catalysis in environmental and energy technology. In Chapter 7.19, the now famous three-way catalyst for cleaning automotive exhaust is presented, whereas an overview of the cobalt and iron catalysts used in Fischer-Tropsch synthesis is given in Chapter 7.20. Biomass as a renewable feedstock for energy and chemicals is treated in Chapter 7.21, whereas the last chapter (Chapter 7.22) of the volume is devoted to photocatalysis and water splitting using solar energy.\u3cbr/\u3e\u3cbr/\u3eThe editors believe that the valuable collection of chapters in this volume gives an excellent impression on the state of knowledge in heterogeneous catalysis. They are very grateful to the authors for writing these authoritative overviews

Elementary reactions of CO and H\u3csub\u3e2\u3c/sub\u3e on C-terminated χ-Fe \u3csub\u3e5\u3c/sub\u3eC\u3csub\u3e2\u3c/sub\u3e(0 0 1) surfaces

\u3cp\u3eCO and H\u3csub\u3e2\u3c/sub\u3e (co-)adsorption, direct and H-assisted CO activation, and surface carbon hydrogenation were investigated on C-terminated χ-Fe \u3csub\u3e5\u3c/sub\u3eC\u3csub\u3e2\u3c/sub\u3e(0 0 1) surfaces. Periodic DFT simulations at different surface carbon contents on the carbide surface showed that CO adsorbs preferably linearly on Fe top sites; CO and H\u3csub\u3e2\u3c/sub\u3e adsorptions being stable. The perfect carbide surface favors carbidic carbon hydrogenation (i.e. CH formation), whereas carbon-free surface favors direct CO dissociation and restoration of the carbide structure. In partially carbon-vacant intermediate situations, both direct and H-assisted CO activations are energetically feasible, the latter being the preferred path. Considering CH\u3csub\u3ex\u3c/sub\u3e and CH\u3csub\u3ex\u3c/sub\u3eO species as initiators for different product types can explain the catalytic behavior and selectivity patterns of iron carbide catalysts. The catalytically active surfaces are concluded to be dynamic, where carbon atoms of the carbide surface participate in the surface reactions, and CO dissociation on vacant sites leads to restoration of the carbide structure.\u3c/p\u3

Cu model catalyst dynamics and CO oxidation kinetics studied by simultaneous in situ UV-Vis and mass spectroscopy

\u3cp\u3eThe oxidation state of Cu nanoparticles during CO oxidation in CO + O\u3csub\u3e2\u3c/sub\u3e gas mixtures was sensitively monitored via localized surface plasmon resonances. A microreactor, equipped with in situ UV-vis and mass spectrometry, was developed and used for the measurements. Cu nanoparticles of ∼30 nm average diameter were supported on optically transparent, planar quartz wafers. The aim of the study is 2-fold: (i) to demonstrate the performance and usefulness of the setup and (ii) to use the combined strength of model catalysts and in situ measurements to investigate the correlation between the catalyst oxidation state and its reactivity. Metallic Cu is significantly more active than both Cu(I) and Cu(II) oxides. The metallic Cu phase is only maintained under conditions where close to full oxygen conversion is achieved. This implies that kinetic measurements, aimed at determining the apparent activation energy for metallic Cu under realistic steady-state conditions, are difficult or impossible to perform.\u3c/p\u3

Understanding FTS selectivity:the crucial role of surface hydrogen

\u3cp\u3eMonomeric forms of carbon play a central role in the synthesis of long chain hydrocarbons via the Fischer-Tropsch synthesis (FTS). We explored the chemistry of C\u3csub\u3e1\u3c/sub\u3eH\u3csub\u3exad\u3c/sub\u3e species on the close-packed surface of cobalt. Our findings on this simple model catalyst highlight the important role of surface hydrogen and vacant sites for product selectivity. We furthermore find that CO\u3csub\u3ead\u3c/sub\u3e affects hydrogen in multiple ways. It limits the adsorption capacity for H\u3csub\u3ead\u3c/sub\u3e, lowers its adsorption energy and inhibits dissociative H\u3csub\u3e2\u3c/sub\u3e adsorption. We discuss how these findings, extrapolated to pressures and temperatures used in applied FTS, can provide insights into the correlation between partial pressure of reactants and product selectivity. By combining the C\u3csub\u3e1\u3c/sub\u3eH\u3csub\u3ex\u3c/sub\u3e stability differences found in the present work with literature reports of the reactivity of C\u3csub\u3e1\u3c/sub\u3eH\u3csub\u3ex\u3c/sub\u3e species measured by steady state isotope transient kinetic analysis, we aim to shed light on the nature of the atomic carbon reservoir found in these studies.\u3c/p\u3

In situ Moessbauer spectroscopy of carbon-supported iron catalysts at cryogenic temperatures and in external magnetic fields

A highly dispersed C-supported Fe catalyst was studied with in situ Moessbauer spectroscopy at temps. down to 5 K and with external magnetic fields. Measurements of spectra in the presence of large magnetic fields considerably improves the information obtained from Moessbauer spectra. [on SciFinder (R)

CO as a promoting spectator species of C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e conversions relevant for Fischer-Tropsch chain growth on cobalt:evidence from temperature-programmed reaction and reflection absorption infrared spectroscopy

\u3cp\u3eCobalt-catalyzed low temperature Fischer-Tropsch synthesis is a prime example of an industrially relevant reaction in which C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e intermediates involved in chain growth react in the presence of a large quantity of CO\u3csub\u3ead\u3c/sub\u3e. In this study, we use a Co(0001) single-crystal model catalyst to investigate how CO, adsorbed alongside C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e adsorbates affects their reactivity. Temperature-programmed reaction spectroscopy was used to determine the hydrogen content of the C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e intermediates formed at different temperatures, and infrared absorption spectroscopy was used to obtain more specific information on the chemical identity of the various reaction intermediates formed. Ethene, propene, and but-1-ene precursors decompose below 200 K. The 1-alkyne adsorbate is identified as a major product, and some alkylidyne species form as well when the initial alkene coverage is high. The surface hydrogen atoms produced in the low temperature decomposition step start leaving the surface >300 K. When an alkyne/H\u3csub\u3ead\u3c/sub\u3e-covered surface is heated in the presence of CO, the alkyne adsorbates are hydrogenated to the corresponding alkylidyne at temperatures <250 K. This finding shows that C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e surface species react differently in the presence of CO\u3csub\u3ead\u3c/sub\u3e, a notion of general importance for catalytic reactions where both CO and C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e species are present. In the context of Fischer-Tropsch synthesis, the observed CO-induced reaction is of specific importance for the alkylidyne chain growth mechanism. In this reaction, scheme hydrocarbon chains grow via coupling of CH\u3csub\u3ead\u3c/sub\u3e with a (C\u3csub\u3en\u3c/sub\u3e) alkylidyne adsorbate to produce the (C\u3csub\u3en+1\u3c/sub\u3e) alkyne. A subsequent hydrogenation of the alkyne product to the corresponding alkylidyne is required for further growth. The present work shows that this specific reaction is promoted by the presence of CO. This suggests that the influence of CO spectators on the stability of C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e surface intermediates is beneficial for efficient chain growth.\u3c/p\u3