CO oxidation, although seemingly a simple chemical reaction, provides us with a panacea that reveals the richness and beauty of heterogeneous catalysis. The Fritz Haber Institute is a place where a multidisciplinary approach to study the course of such a heterogeneous reaction can be generated in house. Research at the institute is primarily curiosity driven, which is reflected in the five sections comprising this Review. We use an approach based on microscopic concepts to study the interaction of simple molecules with well-defined materials, such as clusters in the gas phase or solid surfaces. This approach often asks for the development of new methods, tools, and materials to prove them, and it is exactly this aspect, both, with respect to experiment and theory, that is a trade mark of our institute.

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CO oxidation as a prototypical reaction for heterogeneous processes

Although there are only a few known examples of supported single-atom catalysts, they are unique because they bridge the gap between homogeneous and heterogeneous catalysis. Here, we report the CO oxidation activity of monodisperse single Pt atoms supported on an inert substrate, θ-alumina (Al2O3), in the presence of stoichiometric oxygen. Since CO oxidation on single Pt atoms cannot occur via a conventional Langmuir–Hinshelwood scheme (L–H scheme) which requires at least one Pt–Pt bond, we carried out a first-principles density functional theoretical study of a proposed pathway which is a variation on the conventional L–H scheme and inspired by the organometallic chemistry of platinum. We find that a single supported Pt atom prefers to bond to O2 over CO. CO then bonds with the oxygenated Pt atom and forms a carbonate which dissociates to liberate CO2, leaving an oxygen atom on Pt. Subsequent reaction with another CO molecule regenerates the single-atom catalyst. The energetics of the proposed mechanism ...

CO Oxidation on Supported Single Pt Atoms: Experimental and ab Initio Density Functional Studies of CO Interaction with Pt Atom on θ-Al2O3(010) Surface

Many catalytic reactions exhibit oscillatory behaviour but these oscillations are not well understood for catalysts consisting of supported nanoparticles. The study of oscillatory CO oxidation catalysed by Pt nanoparticles now reveals that periodic changes in the CO oxidation are synchronous with a periodic refacetting of the Pt nanoparticles.

Visualization of oscillatory behaviour of Pt nanoparticles catalysing CO oxidation

Since the discovery of the role of oxidative etching in shape-controlled metal nanostructure synthesis in 2004, it has become a versatile tool to precisely manipulate the nucleation and growth of metal nanocrystals at the atomic level. Subsequent research has shown that oxidative etching can be used to reshape nanocrystals via atomic addition and subtraction. This research has attracted extensive attention from the community because of its promising practical applications and theoretical value, and as a result, tremendous efforts from numerous research groups have been made to expand and apply this method to their own research. In this review, we first outline the merits of oxidative etching for the controlled synthesis of metal nanocrystals. We then summarize recent progress in the use of oxidative etching to control the morphology of a nanostructure during and after its synthesis, and analyze its specific functions in controlling a variety of nanocrystal parameters. Applications enabled by oxidative etching are also briefly presented to show its practical impact. Finally, we discuss the challenges and opportunities for further development of oxidative etching in nanocrystals synthesis.

Oxidative etching for controlled synthesis of metal nanocrystals: atomic addition and subtraction

Chapter 1. Introduction Abstract 1.1. The early years 1.2. The postwar revival References Chapter 2. Basic Interactions Abstract 2.1 Fundamental theories of electron emission 2.2 Photoemission 2.2.1 General considerations 2.2.2 The free electron gas approximation 2.2.3 Band structure UV photoemission 2.2.4 Spin effects in UV photoemission 2.2.5 Surface plasmon photoemission 2.2.6 X-ray photoemission 2.2.6.1 Photoelectron emission 2.2.6.2 Secondary electron emission 2.3 Electron reflection 2.3.1 General considerations 2.3.2 Elastic scattering 2.3.3 Inelastic scattering 2.3.4 Surface effects 2.3.5 VLEED, LEETS, TCS 2.3.6 Quantum well effects 2.3.7 Other aspects References Chapter 3. Instrumentation Abstract 3.1 Instruments: from simple to complex3.1.1 PEEM 3.1.2 LEEM 3.1.3 Aberration-corrected instruments 3.1.4 Spectroscopic imaging instruments 3.1.5 Spin-resolved imaging instruments 3.2 Components 3.2.1 Objective lens and other axial-symmetric lenses 3.2.2 Magnetic deflectors 3.2.3 Electron mirrors 3.2.4 Aberration correctors 3.2.5 Energy filters 3.2.6 Wien filters 3.2.7 Photon sources 3.2.8 Electron sources 3.2.9 Other components (image detectors vacuum system including airlock and specimen preparation chamber, electronics) References Chapter 4. Theory of image formation Abstract 4.1 Introduct ion 4.2 Wave propagation: The contrast transfer function 4.2.1 Low energy electron microscopy. The wave amplitude| 4.2.2 The image intensity 4.2.3 Mirror electron microscopy 4.2.4 Emission electron microscopy 4.3 Through-focus series image improvement 4.4 Information transfer in the image acquisition system 4.5 Summary and outlook References Chapter 5. Applications in surface science Abstract 5.1 Surface microstructure 5.1.1 Metals 5.1.2 Semiconductors 5.1.2.1 Si(111) 5.1.2.2 Si(100) 5.1.2.3 Other Si surfaces 5.1.3 Other inorganic semiconductor surfaces 5.1.4 Other inorganic compound surfaces 5.2 Adsorption 5.2.1 Adsorption on metals 5.2.1.1 Nonmetallic adsorbates 5.2.1.2 Coadsorption and reaction: catalysis 5.2.1.3 Metallic adsorbates 5.2.2 Adsorption on Semiconductors 5.2.2.1 Metallic adsorbates 5.2.2.2 Nonmetallic adsorbates 5.3 Film growth and structure 5.3.1 Films on semiconductors 5.3.1.1 Metal films 5.3.1.2 Ge on Si 5.3.1.3 Other films on semiconductors 5.3.1.4 Nanostructures and droplets on semiconductors5.3.2 Films on metals 5.3.2.1 Metal films 5.3.2.2 Inorganic compound films 5.3.3 Organic films References Chapter 6. Applications in other fields Abstract 6.1 Graphene 6.1.1 Introduction 6.1.2 Graphene on SiC 6.1.2.1 Growth and microstructure 6.1.2.2 Intercalation 6.1.3 Graphene on metals 6.1.3.1 Introduction 6.1.3.2 Growth and microstructure 6.1.3.3 Intercalation 6.2 Plasmons 6.2.1 Introduction 6.2.2 Linear structures 6.2.3 Nanostructures 6.2.4 Complex wave fields 6.2.5 Limitations 6.3 Technological applications 6.3.1 General materials applications 6.3.2 Electronics 6.4 Biology 6.5 A multimethod case study References Chapter 7. Magnetic imaging Abstract7.1 Introduction 7.2 Ferromagnetic films 7.2.1 Single layers 7.2.2 Quantum well effects 7.2.3 Bilayers 7.2.4 Trilayers 7.2.5 Multilayers 7.2.6 Compound layers 7.3 Bulk magnetic materials 7.3.1 Ferromagnetic materials 7.3.2 Antiferromagnetic materials 7.4 Ferromagnetic-antiferromagnetic interfaces 7.5 Nanostructures 7.5.1 Introduction 7.5.2 Static domain structure 7.5.3 Field and current influence 7.5.4 Nanodots and nanostructure arrays 7.6. Ferroelectrics / Multiferroics 7.6.1 Ferroelectries 7.6.2 Multiferroics Chapter 8. Other surface imaging methods with electrons Abstract 8.1 Scanning Low Energy Electron Microscopy 8.2 Scanning Low Energy Electron Diffraction Microscopy 8.3 Reflection Electron Microscopy 8.4 Secondary and Auger Electron Microscopy 8.5 Scanning Electron Microscopy with Spin Analysis 8.6 Scanning Photoelectron Emission Microscopy 8.7 Concluding remarks

https://www.springer.com/cda/content/document/productFlyer/productFlyer-WEST_978-1-4939-0934-6.pdf?SGWID=0-0-1297-176716717-bookseller

Surface Microscopy with Low Energy Electrons

Electron microscopy and modelling are used to study CO oxidation on oxide-supported Pd. The perimeter of the metal/oxide interface is shown to affect CO tolerance of the entire particle, demonstrating a long-range effect over micrometre length scales.

The role of metal/oxide interfaces for long-range metal particle activation during CO oxidation.

Shedding light on light-off: Photoemission electron microscopy, DFT, and microkinetic modeling were used to examine the local kinetics in the CO oxidation on individual grains of a polycrystalline sample. It is demonstrated that catalytic ignition ("light-off") occurs easier on Pd(hkl) domains than on corresponding Pt(hkl) domains. The isothermal determination of kinetic transitions, commonly used in surface science, is fully consistent with the isobaric reactivity monitoring applied in technical catalysis. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

http://publications.lib.chalmers.se/records/fulltext/165764/local_165764.pdf

Local Catalytic Ignition during CO Oxidation on Low‐Index Pt and Pd Surfaces: A Combined PEEM, MS, and DFT Study

Mapping the local reaction kinetics by PEEM: CO oxidation on individual (100)-type grains of Pt foil

The surfaces of noble metal single crystals such as Pt(111) have served as successful model systems for heterogeneous catalysts. To account for the fact that supported Pt nanoparticles of technological catalysts typically exhibit different low Millerindex facets to the reactive gases, single-crystal studies were most frequently carried out for Pt(111), Pt(110) and Pt(100) terminations. Catalytic reaction properties were, for example, determined by temperature-programmed methods or molecular beam studies (both relying on mass spectroscopic product analysis) or, in case of atmospheric pressure conditions, by gas chromatography. In order to evaluate and compare the catalytic properties of the different crystallographic orientations, a relatively large parameter space of reactant gas pressure and reaction temperature must be investigated. Since this represents a significant body of work, there is hardly any catalytic system with a complete set of reported reactivity data. We present an alternative approach that allowed us to determine catalytic properties of different crystallographic terminations in a more efficient way, at least for specific surface reactions. Photoemission electron microscopy (PEEM) was employed to study the CO oxidation reaction on polycrystalline Pt foil, consisting of micrometer-sized domains of (100), (110) and (111) terminations. PEEM imaging was performed in situ, that is, during the reaction of CO and oxygen to give CO2, and the analysis of the local photoemission intensity of selected domains on the Pt foil enabled us to obtain locally resolved kinetic information for (100), (110) and (111) surfaces. It is important to note that at a given time the reactant gas pressure and temperature were identical for all terminations, allowing a direct simultaneous comparison of their catalytic properties. The kinetic behavior of the (100), (110) and (111) surfaces is described by “kinetic phase diagrams” (also known as bifurcation diagrams; for details see the Supporting Information), representing the domain-specific catalytic properties. It is shown that the (111)-, (100)and (110)-oriented grains behaved almost identical to the corresponding single crystals and that the superposition of the weighted contributions of the individual domains (as measured by PEEM) reproduced the global kinetics of CO oxidation on Pt foil (as measured by MS). Under the applied experimental conditions, the (100), (110) and (111) domains behaved independently to a large extent, indicating that diffusion and gas-phase coupling between the different facets were not sufficient to synchronize their kinetic transitions. The microscopic observation of reaction fronts that were confined within the domain boundaries corroborated the insufficient coupling. Implications of the current results on CO oxidation on supported Pt nanoparticles of technological catalysts are also discussed. Kinetic phase transitions on the Pt(111) single-crystal surface have been previously observed by PEEM (by averaging the whole image intensity, since the homogeneity of such a surface did not ask for spatially-resolved analysis) and PEEM images of CO oxidation on Pt foil have been reported (but without analysis of phase transitions). However, the present contribution shows for the first time how local kinetic phase diagrams of a catalytic reaction can be obtained for individual differently oriented domains of a polycrystalline material. The oxidation of CO on platinum surfaces is a seemingly simple reaction, but it exhibits several complex phenomena such as hysteresis (bistability), oscillations, dissipative structures and chaotic behaviour. Herein, we have concentrated on bistability and deliberately did not examine the parameter space (higher temperature and pressures) of oscillations which were extensively studied before. To illustrate the kinetic behavior of Pt foil, Figure 1 (right inset) shows the global (overall) reaction kinetics measured by mass spectroscopy. At a reaction temperature of 417 K, the Pt foil was exposed to a constant partial pressure of oxygen (1.3 10 5 mbar) while the CO partial pressure (pCO) was cycled and the CO2 production rate (RCO2 ) was monitored by mass spectroscopy. Upon exposing the oxygen-covered Pt surface to increasing CO pressure, RCO2 increased (high activity) until a kinetic transition point at pressure tA was reached. When pCO exceeded tA, the Pt surface switched from oxygen-covered to CO-covered, marked by the loss of catalytic activity due to CO self-poisoning (oxygen cannot adsorb on the CO-covered surface). When pCO was decreased, the catalyst remained in the poisoned (low reactivity) steady state until a second transition pressure tB was approached. Accordingly, at pCO tB<pCO<pCO , the catalyst can adopt one of two steady states, depending on the prehistory. The resulting hysteresis is characteristic for a bistable reaction behavior that originates from the asymmetric inhibition of dissociative oxygen adsorption by CO:oxygen needs two neighboring adsorption sites per molecule for dissociation and can not adsorb on a densely packed CO-covered [a] Prof. Dr. Y. Suchorski, C. Spiel, D. Vogel, Dr. W. Drachsel, Prof. Dr. G. Rupprechter Institute of Materials Chemistry Vienna University of Technology Veterin rplatz 1, 1210 Vienna (Austria) Fax: (+43)1-58801-16599 E-mail : grupp@imc.tuwien.ac.at [b] D. Vogel, Prof. Dr. R. Schlcgl Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg, 4–6, 14195 Berlin (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201000599.

Local Reaction Kinetics by Imaging: CO Oxidation on Polycrystalline Platinum

The role of artificially created defects and steps in the local reaction kinetics of CO oxidation on the individual domains of a polycrystalline Pd foil was studied by photoemission electron microscopy (PEEM), mass spectroscopy (MS), and scanning tunneling microscopy (STM). The defects and steps were created by STM-controlled Ar+ sputtering and the novel PEEM-based approach allowed the simultaneous determination of local kinetic phase transitions on differently oriented μm-sized grains of a polycrystalline sample. The independent (single-crystal-like) reaction behavior of the individual Pd(hkl) domains in the 10–5 mbar pressure range changes upon Ar+ sputtering to a correlated reaction behavior, and the reaction fronts propagate unhindered across the grain boundaries. The defect-rich surface shows also a significantly higher CO tolerance as reflected by the shift of both the global (MS-measured) and the local (PEEM-measured) kinetic diagrams toward higher CO pressure.

https://pubs.acs.org/doi/pdf/10.1021/jp312510d

Diana Vogel

Papers

The role of metal/oxide interfaces for long-range metal particle activation during CO oxidation.

Local Catalytic Ignition during CO Oxidation on Low‐Index Pt and Pd Surfaces: A Combined PEEM, MS, and DFT Study

Mapping the local reaction kinetics by PEEM: CO oxidation on individual (100)-type grains of Pt foil

Local Reaction Kinetics by Imaging: CO Oxidation on Polycrystalline Platinum

The Role of Defects in the Local Reaction Kinetics of CO Oxidation on Low-Index Pd Surfaces.