XAFS Techniques for Catalysts, Nanomaterials, and Surfaces

X-ray absorption fine structure (XAFS) is an element-specific, short-range, core-level X-ray absorption spectroscopy (XAS) with high sensitivity, which enables high-quality researches in a variety of scientific fields such as physics, chemistry, bioscience, materials science and engineering, energy science, environmental science, geoscience, metallurgy, mineralogy, etc. The history and progress of XAFS are summarized in this Chapter, showing principal achievements and landmarks for the theories, calculation methods, computer programs, methodologies and techniques, and applications to catalysts, nanoparticles and surfaces. Now we find new XAFS paradigms of frontier sciences and technologies for catalysts, nanoparticles and surfaces, including biomaterials, such as real time analysis of in situ chemical states, short-lived dynamics, 2D and 3D in situ/operando imaging of real spaces, spatially-resolved analysis of inhomogeneous materials, identification of diluted atoms/species, inelastic X-ray scattering analysis at same chemical sites with different bonding and valence states, in situ analysis under extreme conditions, etc.

Owing to its element specific and short-range nature, core-level X-ray absorption spectroscopy (XAS) is now widely used to elucidate the local structural, vibrational, and other physical properties of complex, aperiodic materials. In this Chapter we review the current theory of XAS including both extended (EXAFS) and near-edge (XANES) properties together with modern analysis methods.

A synchrotron radiation (SR) source is one of the best light sources for XAFS measurements, which need an intense energy-tunable X-ray source with a wide range. Two types of SR sources are available: a storage ring and a linear accelerator. Sects. 3.1–3.3 describe the storage-ring source and associated beamlines. Chapter 4 describes an X-ray free electron laser (XFEL) as an advanced linear-accelerator source.

The twenty-first century saw the arrival of an exciting new light source, X-ray free electron lasers (XFELs). XFELs generate ultra-brilliant, coherent, and femtosecond X-ray pulses. X-ray spectroscopy is one of the most promising applications using XFEL, allowing one to directly probe ultrafast changes in electronic states and geometric structures during real chemical reactions at angstrom and femtosecond resolution using X-ray absorption and/or emission measurements.

XAFS spectra are measured in several modes such as transmission, fluorescence, and electron yield. Most of XAFS spectra are taken by using synchrotron radiation as an X-ray source, and the system may be set up by beamline scientists. But highly precise measurement is required to obtain reliable experimental results since EXAFS oscillation is very small especially at high k region.

The design of appropriate spectroscopic cells for in situ and operando XAFS studies of heterogeneous catalysts has been a very active field during the past decades as the investigation of catalysts at work has become a powerful approach to improve the activity and selectivity of catalysts in a rational manner. This chapter reviews criteria for choosing the appropriate cell design and underlines its significance using several examples of in situ and operando cells for studying heterogeneous catalysts, sensors, and electrocatalysts and for deriving structure-performance relationships. This strongly contributes to a better understanding of the dynamics of functional materials and their knowledge-based improvement.

Time-resolved XAFS techniques are powerful tools for investigating the local structure and chemical state during physical and chemical reaction processes and have been used worldwide in synchrotron facilities. There are two major techniques: quick scan XAFS (QXAFS) and energy dispersive XAFS (DXAFS). In this section, the QXAFS techniques are described. The DXAFS techniques are described in Chap. 8.

This chapter presents an overview of energy dispersive X-ray absorption spectroscopy (EDXAS), as developed and used at synchrotron radiation sources. It mainly covers time resolved studies, with emphasis on technical aspects.

Changes of structure and chemical states of catalysts have been successfully observed with XAFS under reaction conditions. Nowadays, a XAFS spectrum can be obtained even within several milliseconds by using quick XAFS (QXAFS) and dispersive XAFS (DXAFS) techniques. However, much faster fundamental processes of catalysts should be also key phenomena to understanding catalytic reactions. For example, lifetimes of photocarriers, transfer processes and active sites of catalysis should govern the photocatalytic activity. In order to observe such fast events, the demanding time resolution of XAFS measurements is less than microseconds, which is much shorter than the time resolution of QXAFS and DXAFS techniques. A pump-probe XAFS technique is applicable if the processes can take place repeatedly. The pump-probe XAFS technique has been developed since 1980s [1]. Nowadays, the time range of phenomena observed by the pump-probe XAFS technique is from sub-picoseconds to several hundreds of microseconds.

The functions of solid materials are highly related to their structures, and the visualization of heterogeneous structures of solid materials is one of the interesting targets for XAFS analysis. Conventional XAFS measurement is performed by micron-size X-ray beams, and it can provide average structural information of a target sample in a beam spot. In the case of a powder sample, the information of local coordination is averaged for all powders with heterogeneous morphology, composition, surface structures, etc. in a beam spot of X-rays, and the local coordination of inhomogeneous assemblies cannot be separated by the conventional XAFS measurement.

Three-dimensional imaging of a solid sample is attractive for high-throughput and non-destructive characterization methods, and computed tomography (CT) is well developed and widely used for several analytical methods such as electron microscopy. X-ray computed tomography (XCT) can obtain three-dimensional structural data of a solid sample, in particular morphology and elemental information.

In this section we focus on the use of transmission X-ray microscopy (TXM) to measure the XAS spectra. In the last decade a range of soft X-ray and hard X-ray TXM microscopes have been developed, allowing the measurement of XAS spectra with 10–100 nm resolution. In the hard X-ray range the TXM experiments pose the same restrictions on in situ experiments as bulk XAS experiments, allowing experiments with capillaries to study catalysts under working conditions. In the soft X-ray range, dedicated transmission nanoreactors are needed. Considering catalysts the main result the in situ TXM experiments are the determination of nanometer range variations of catalysts under working conditions. An important property of X-rays is their short wavelength below 1 nm. This allows direct imaging of catalysts in scanning mode or full field mode. In contrast, visible light with an energy of 1 eV has a diffraction limited resolution of approximately 500 nm and VUV light with an energy of 10 eV has a diffraction limit of ~50 nm.

The continuous improvement of X-ray sources, optics, and detectors since the start of the twenty first century has paved the way for unprecedented studies of functional materials at work. Studying these typically highly complex materials while they are working requires a sophisticated combination of analysis techniques that can provide chemical information about the ongoing processes at multiple time- and/or length scales. This makes X-rays an excellent probe for such studies, as they are (usually) non-destructive, can be used for relatively fast processes (0.01–2 s), and can operate in harsh environments, for example under high pressure or temperature. In order to link the (spatio-)temporal chemical information obtained by X-ray absorption spectroscopy (XAS) to the task performed by the functional material, it is necessary to collect additional complementary information about the running process (performance). It is this simultaneous measurement together with a combined data analysis that defines “operando” studies.

The EXAFS is usually measured in a transmission mode. For dilute system signal is too weak and hindered by the large background. In that case one may use a fluorescence mode. The sensitivity of fluorescence mode depends on the S/B ratio. In the low concentration system, energy-resolved detector can be used. In the ultra dilute crystal monochromator can effectively distinguish the fluorescence and the background. We can now have a signal of the dilute system with less than 1 ppb.

“Reflection XAFS” or “ReflEXAFS” is a particular collection mode of X-ray Absorption Spectroscopy (XAS) data with the probe beam impinging on the sample in total external reflection conditions. The main advantage of total reflection XAS is that the extinction length of the probe beam is greatly reduced respect to the normal incidence. This makes ReflEXAFS a surface-sensitive technique in the range of a few nm with the considerable advantage that it does not need Ultra High Vacuum conditions like electron-based techniques. Then, this method can be applied to a variety of interfaces (gas-solid, liquid-solid, liquid-liquid and solid-solid) realizing ‘in operando’ conditions for the materials under study. In this contribution the basic theoretical aspects of this technique are reviewed and then a list of remarkable experimental apparata is presented. Successively, some examples of experiments using Total Reflection XAS are described and in the final section some perspectives of development of this technique are given.

High-energy resolution X-ray absorption refers to the measurement of XANES spectra with the use of a detector signal that is measured coherently with the XANES measurement. The detector signal can be fluorescence, which is known under the name High-Energy Resolution Fluorescence Detected (HERFD) XANES. Similarly one can define High-Energy Resolution Auger Detection (HERAD) XANES.

In analogy to the connections between Raman scattering and absorption in the optical regime, X-ray Raman scattering (XRS) is an alternative to X-ray absorption spectroscopy, particularly for sub-keV excitations. XRS is the nonresonant inelastic X-ray scattering (NIXS, or NRIXS) from core or semi-core electrons and has a similar energy-dependence as X-ray absorption, albeit with a much smaller overall cross section. However, as a high-energy photon-in/photon-out approach, XRS has found widespread use for samples at extreme conditions, such as high pressure experiments in diamond anvil cells, as well as samples incompatible with vacuum conditions, like many liquids. Compared to absorption spectroscopy, XRS has key technical advantages beyond bulk sensitivity, including lack of self-absorption and the ability to access dipole-forbidden final states in certain cases. It also shares many of the same components as hard X-ray absorption/emission spectroscopy and has found a niche as a complementary technique available at multiple synchrotron facilities worldwide [1–9].

MD-XAFS (Molecular Dynamics X-ray Adsorption Fine Structure) makes the connection between simulation techniques that generate an ensemble of molecular configurations and the direct signal observed from X-ray measurement. Due to the fact that the signal is most sensitive to the structure nearest to a photoelectron source, an understanding of XAFS signal is an exquisite tool for decoding the nearest neighbor coordination and correlated structure of the solvent molecules surrounding the photo-electron source. The XAFS signal can be constructed from an ensemble of scattering paths. Often the signal cannot be decomposed into a few dominant paths with characteristics that can be fitted. MD-XAFS takes advantage of the direct correspondence between the ensemble of molecular configurations and the ensemble of scattering paths, taking into account the complex correlation between them resulting in the observed signal. Due to the fact that significant phenomena are controlled by solvent response and fluctuations, such as diffusion and speciation of species, the establishment of the connection between molecular simulation and experiment has established utility in materials and catalysis systems.Below we will expand on a variety of approaches that enhance the interpretation of MD-XAFS analysis. We will give examples of its utility in a variety of systems, including 1) the solvation of transition metal ions in water 2) an example of an analysis of a reactive chemistry corresponding to homogeneous catalysis 3) the distribution of reaction centers in a heterogeneous catalysis system as well as 4) fundamental analysis of acid/base equilibrium as a function of concentration.

Metal catalysts in nanometer size range are under worldwide investigations due to their fascinating electronic and atomic strucutures which play essential roles in tuning catalytic properties of metal catalysts. Owing to intrinsically high disorder, asymmetric bond distributions, heterogeneity in particle sizes and compositions, as well as strong coupling between the structural properties and environment, nanosized metal catalysts present a number of challenging problems in EXAFS analysis for determining the size, structure, shape, support orientation of nanocatalysts in real time and in reaction conditions. In this chapter we review methods of EXAFS analysis developed in the last two decades for structural characterization of mono- and bi-metallic nanocatalysts.

Oxide materials find widespread application in industry, for example as semiconductor, solar cell, catalyst, and sensor. New materials and applications continue to be reported, which is a main driver for technological development and slowly makes our society more sustainable. The function of these materials strongly depends on their structure, which is why their structural characterization receives much attention.

Chemical design of artificial enzyme catalysts with high activity, selectivity, and molecular recognition has been a long-term challenge in catalytic materials research. Metal complexes with well-defined coordination structures around metal centers may resemble active sites/ensembles of metal enzymes in a sense, where organic and/or inorganic ligands coordinated to a metal center significantly promote and regulate not only reactivity of the metal site electronically but also reaction space around the metal center geometrically. The metal center may constitute a single metal atom or a multimetals cluster. As the result, high activity and sharp selectivity under mild reaction conditions can be achieved with metal-complex catalysts, which may be difficult to obtain on metal particles and metal single crystal surfaces. However, homogeneous metal complexes in solutions tend to gather and decompose during catalytic cycles, resulting in loss of the catalytic properties. Hence, the transformation of homogeneous catalysts to a new class of heterogeneous catalysts with active structures and compositions in a molecular level has been accomplished by supporting metal complexes on oxide surfaces [1–10]. The supported metal complexes may be further transformed chemically at the surfaces to provide unique structures and compositions that are different from their homogeneous analogues and often remarkable catalytic properties. Molecular-level characterization of catalyst surfaces thus fabricated is the key issue for evidence of the design of new catalysts, which also leads to further development of new catalytic materials [1–10]. Unfortunately, most of these supported catalysts have no long-range ordered structures and distributed at surfaces or on pore walls of porous supports, which makes molecular-level characterization of their structures and electronic states very difficult. XAFS can provide the molecular-level structural and electronic information on the active metal sites and ensembles of designed catalysts under catalytic reaction conditions [11]. However, it is to be noted that molecular-level characterization of conventional supported metal catalysts, which are generally heterogeneous and complicated, is difficult particularly under catalytic reaction conditions.

The XAFS spectroscopy has been applied to studies on local geometric and electronic structures of chemical species at surfaces. In this chapter, the contribution of the XAFS spectroscopy to surface science studies are described particularly paying attention to elucidation of the static structures of adsorbates and in-situ observation of surface dynamic processes.

The advent of Li ion batteries (LIBs) changed the world of secondary batteries. With their excellent performance, the LIBs are widely used as electric energy storages for mobile phones, laptop computer, etc. New types of LIBs have been developed one after another and now it is a worldwide challenge to develop a large-scale battery with high capacity for hybrid and/or full electric vehicles [1]. Battery is, in principle, composed of cathode, separator, anode, and electrolyte, as shown in Fig. 23.1. It has two phases, charge and discharge, in which Li ions move from cathode to anode, and vice versa, respectively. It has several important key factors: high capacity, high cyclability, high charging rate, low cost, ease of preparation, and safety. To realize a battery with such high performances, a number of difficult subjects should be overcome. Since the battery is a very complicated target, many analytical techniques have been applied to elucidate the structures, electrochemical reactions of batteries, such as XRD (X-ray diffraction), neutron diffraction, Raman scattering, NMR, etc., as well as theoretical simulations. Especially, X-ray Absorption Fine Structure (XAFS) is a powerful tool and numerous studies with XAFS have been reported for secondary batteries. Due to the limit of page, however, this review is focused on the XAFS methodologies and how they are applied for characterization of the secondary batteries.

The XAFS spectroscopy has been applied to studies on local geometric and electronic structures of chemical species at surfaces. In this chapter, the contribution of the XAFS spectroscopy to surface science studies are described particularly paying attention to elucidation of the static structures of adsorbates and in-situ observation of surface dynamic processes.

Knowledge on structural properties is essential to understand the working principle of gas sensing devices based on Semiconducting Metal OXides (SMOX). For sensors, like for many other functional materials, nano-sized structures as well as the nature of surfaces and interfaces define the materials’ properties. As will be shown in this chapter, X-ray absorption spectroscopy (XAS) in terms of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) is ideal to explore model sensors with high dopant loading and exposed to elevated gas concentrations as well as to investigate real sensor devices. This implies studies on porous oxide layers with low dopant loading, at elevated temperatures (100–400 °C) and realistic reaction conditions such as low analyte gas concentrations (ppm) in oxygen (20–21 vol%) and interfering components such as water (10–60 % relative humidity). This chapter focuses mainly on the potential and limitations of XAFS and further related novel photon-in/photon-out techniques in the field of chemical gas sensors. After a brief introduction to the current state of the art and proposed mechanisms of gas sensing special focus is laid on the main advantages and some of the smart approaches presented in the literature when XAS is applied to the characterization of gas sensors. Both experimental aspects and selected examples are described.

In this chapter, we introduce MOFs as the new class of crystalline porous materials of remarkable potentialities. We underline the flexibility in the realization of different MOFs and the fact that they are ideal materials for performing X-ray absorption experiments at the metal K or L3 edges in transmission mode. A selection of relevant results appearing in the 2010-2016 period follows, highlighting the relevant role of both EXAFS and XANES in determining the structural and electronic configuration of metal centers inside MOFs.

XAFS is an excellent technique to capture the homogeneous catalyst structure under operando conditions – the actual operating catalyst under higher-temperatures and often under high-pressure conditions. Thus operando XAFS on homogeneous systems yields insights into the catalyst function in a way that neither ex situ nor in situ techniques can. Direct observation of the metal center using XAFS techniques goes far beyond the limitations of NMR (limited to a few active nuclei), or IR spectroscopy (limited to specific metal-ligand vibrational modes). When possible, the combination of XAFS with NMR, IR, UV-Vis, theoretical approaches such as advanced electronic structure calculation (e.g., DFT), or careful kinetics studies brings together powerful complementary methods. XAFS probes the metal center during the catalytic process, yielding oxidation states and local structure about the metal center. We showcase a few representative examples from the literature and additionally highlight some of our recent work. These examples cover themes in important topical areas including green chemistry, hydrogen storage, and classical hydrogenation chemistry

Many important redox-active metalloenzymes consist of 3d transition metals in their active sites. They catalyze multi-electron reactions in aqueous solution, under ambient temperature and pressure. To understand the functionality of the catalytic sites of these enzymes, X-ray spectroscopy methods provide important information of the electronic structure of the metal of interest and its local structural information, as well as their changes during the catalysis. Studying model compounds, that mimic enzyme catalytic site structures or their functional mimics, is as critical as studying enzymes themselves, in order to interpret X-ray spectroscopic data. In this chapter, various X-ray spectroscopic methods that are useful for structural and electronic structural studies of enzymes and model compounds are described.

Chemoselectivity is a cornerstone of catalysis, permitting the targeted modification of specific functional groups within complex starting materials [1–5]. This ability to activate and transform only certain chemical functionalities without the use of protecting groups, and attendant improvements in atom efficiency (and waste minimisation), also underpins catalysis’ green credentials [6, 7].

Gasoline engine vehicle emits several pollutants of carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) in the exhaust gases. Three-way catalysts reduce those toxic gases into harmless compounds, and are composed of several components, including noble metals such as platinum, rhodium, and palladium, alumina support, and ceria-based oxides as an oxygen storage component. X-ray absorption fine structure is suitable for analysis of heterogeneous catalysts including the three-way catalysts. In this section three applications are introduced. 1. EXAFS analysis of the local structure of ceria-zirconia oxide. 2. XAFS analysis of interactions between supported platinum and support oxides. 3. Operando XAFS analysis of catalyst.

Various interesting processes are taking place at solid–liquid interfaces, i.e., electrode–electrolyte interfaces, and therefore the electronic and geometric structures play crucial roles in those interfacial processes. X-ray absorption fine structure (XAFS) allows us to clarify not only the interfacial static structures but also the dynamic structural changes in situ and in real time, because x-rays can penetrate through the liquid phase without significant loss in intensity. In this chapter, applications of in situ XAFS to various electrode–electrolyte interfaces are briefly overviewed from the viewpoint of fundamental electrochemistry and several examples of in situ XAFS studies on electrocatalytic reactions are presented.

The surface structure of supported metal clusters can be finely controlled by using a well-defined metal complex and surface modifications, as described in the other chapter. The surface structures are stabilized by the interaction between the metal and support oxide. Investigation of a metal species on a single-crystal oxide surface will provide further knowledge regarding the structure of the metal species on oxide surfaces and the metal–oxide interaction. However, the investigation of a metal species on a single-crystal oxide surface is not easy because of the small amount of metal species and high penetration ability of the X-ray. As described in the other chapter, polarization-dependent total reflection fluorescence XAFS (PTRF-XAFS) will provide three-dimensional structural information of a metal species highly dispersed on flat surfaces. In this chapter, we discuss how to obtain an atomically dispersed metal species by controlling the oxide surface property and structure (mainly a TiO (110) surface) and how to determine the three-dimensional structure on an atomic level using PTRF-XAFS. The structure mainly depends on the strengths of the metal–metal and metal–oxide interactions. What is the metal–oxide interaction?