**2. Photon-in photon-out X-ray spectroscopy: principles and techniques**

*scientists must use these tools to achieve a fundamental understanding of catalytic processes occurring in multiscale, multiphase environments.* However, this is applicable for any kind of catalytic

At its most fundamental level, catalysis is directly related to the electronic structure of the valence shells because they control chemical bond formation/rupture [2]. Several spectros‐ copies can provide information on valence shells but only X-rays do it directly. Hard X-rays are the epitome of spectroscopic probes because of their great penetration depth, and element specificity, which enables studies of catalytic states under working conditions [3]. X-ray photon-in photon-out core level spectroscopy is a powerful tool to understand catalytic reactions because it enables us to map the entire electronic structure of the catalyst under

In this book chapter, we summarize our latest efforts in the use of synchrotron-based highresolution X-ray spectroscopy to study a plethora of academic and industrial relevant catalytic systems. We will start by covering in a succinct manner the developments in the field that allow for the understanding of catalysis in situ/operando and time-resolved, keeping the highenergy resolution and element specificity. This section includes an overview on techniques, spectrometers, and experimental setups. After that we will describe the studies we carried out to understand catalysis and materials. Our work spans over three fundamental aspects of

Vast majority of the studies were carried out on a dispersive von Hamos-type spectrometer [4]; however, when a higher peak-to-background signal level was necessary the experiments were carried out with a Johann-type spectrometer [5]. The studies were carried out both in homo‐ geneous and heterogeneous catalytic systems. The latest includes also studies on photocatal‐

The future of this spectroscopy concerns its application at the X-ray free electron lasers (XFELs). The advent of XFELs enabled scientist to achieve the goal of producing a movie of a catalytic transformation. XFEL light sources are in its beginning but their effect on the field of timeresolved X-ray science will be deep and comprehensive because reactions can be followed not only in situ but also in real time, this of course if the current limitations in respect to selective triggering and sample stability are overcome. Quoting Sá & Szlachetko*: in a cinematographic analogy, we have the camera (von Hamos Spectrometer), the film (HEROS), the set (catalytic reactor), the script (catalytic reaction) and the actors (catalyst and reagent molecules); what is missing is the director to shout 'action' and direct the scenes (pulse shaping), and that the actors do not fall ill (sample refreshment)* [6]. The insights gathered from the high-resolution X-ray spectroscopy provide deeper understanding of the systems and reactions, which can be used to improve or develop

**c.** Study of catalytic reactions under real working conditions (*in situ*).

system.

catalysis:

ysis.

catalytic relevant conditions.

4 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**a.** Materials characterization

novel catalysts with enhanced properties.

**b.** Molecular adsorption

Unique properties of X-rays provide wide range of applications to study the catalytic materials, not only in steady-state environment but also more importantly at in situ and material's operating conditions. Ability to probe metal-site of catalytic material enclosed in gas or liquid environment is possible thanks to penetrating properties of X-rays. In the hard X-ray regime of few keV range, the attenuation length of X-rays is in the order of few thousands of micro‐ metres for low Z materials (like carbon) to few micrometres in lead. Therefore, X-ray spectro‐ scopy provides the depth of probe extensively larger as compared, for example, to electronbased techniques. The second important aspect of X-ray application is the ability to probe both, lowest unoccupied and highest occupied electronic states of an atom. By tuning the X-ray energy close to ionization threshold, the interaction of incidence photon with core-electron leads to electron excitation into unoccupied level above Fermi energy. The fine-tuning of incidence X-ray energy, nowadays easily possible at any synchrotron source, allows thus for precise scanning through the available excitation levels, and as a consequence provides electronic picture of above Fermi electronic states. The photon–electron interaction leading to the creation of the excited intermediate atomic state, decays radiatively to the final state by electron transition from higher electronic level into the created core-hole. This decay channel is accompanied by emission of an X-ray and may be used as probe of occupied electronic states. Therefore, the combination of X-ray absorption process and following X-ray emission event allow for obtaining the complete electronic picture, and hence chemical configuration, of absorbing atom.

In the following, we will discuss the latest developments and trends in X-ray spectroscopy as applied to study catalytic systems, ranging from steady-state measurements to real-timeresolved studies. The combination of X-ray absorption (XAS) and X-ray emission spectros‐ copies (XES) allowing for resonant X-ray emission spectroscopy (RXES) studies will be presented along with examples of technical developments allowing to extend RXES method‐ ology into the time domain. Finally, the high-energy resolution off-resonant spectroscopy (HEROS) will be described in detail. The advantages and disadvantages of HEROS approach in in situ measurements will be illustrated further on with several practical examples.

## **2.1. Dispersive-type X-ray emission spectrometers**

The high-energy resolution resonant and off-resonant X-ray emission spectroscopy relies on measurements of incidence and emission X-ray energies and intensities. In order to obtain meaningful X-ray emission data, the experimental resolution should be of the order of corehole lifetime of probed atomic species. Typically, the core broadening is in the order of subeV up to few eV for K- and L-shells in the X-ray energy range of few keV [7]. The high-energy resolution of incidence X-rays is provided commonly at any synchrotron source by use of double-crystal monochromators. For X-ray detection, dedicated X-ray spectrometers are being developed, with different spectrometer geometries and arrangements depending on particular needs and goals of experiments. In the present section we focus solely on dispersive-type spectrometer because of its particular parameters and operating characteristics.

characteristics.

In comparison to other spectrometer solutions employed at synchrotron beamlines [5, 8] that use two-dimensional focusing, the dispersive-type spectrometer employs only one-dimen‐ sional focusing of X-rays. Dispersive-spectrometers are characterized by lower detection efficiency in comparison to, for example, spectrometers working in Johann geometry. On the other hand, the dispersive-spectrometer geometry allows for measurements of X-ray emission spectra in dispersive-mode, which enables detection of X-ray emission in a wide range of energies (few tens to few hundreds of eV) without any scanning elements during measure‐ ment. For the experiments requesting short acquisition times, the dispersive-spectrometer geometry may be thus regarded as optimal solution. geometry. On the other hand, the dispersive‐spectrometer geometry allows for measurements of X‐ray emission spectra in dispersive‐mode, which enables detection of X‐ ray emission in a wide range of energies (few tens to few hundreds of eV) without any scanning elements during measurement. For the experiments requesting short acquisition times, the dispersive‐spectrometer geometry may be thus regarded as optimal solution.

are being developed, with different spectrometer geometries and arrangements depending

on particular needs and goals of experiments. In the present section we focus solely on

dispersive‐type spectrometer because of its particular parameters and operating

In comparison to other spectrometer solutions employed at synchrotron beamlines [5, 8] that

use two‐dimensional focusing, the dispersive‐type spectrometer employs only one‐

dimensional focusing of X‐rays. Dispersive‐spectrometers are characterized by lower

**Figure 1:** (Left) Schematic representation of von Hamos spectrometer geometry (from [4]). **Figure 1.** (Left) Schematic representation of von Hamos spectrometer geometry (Reprinted with permission from [4]. Copyright (2012), American Institute of Physics). (Right) Schematic view of the geometrical setup used in Johansson geometry (Reprinted with permission from [9]. Copyright (2012), American Institute of Physics)

(Right) Schematic view of the geometrical setup used in Johansson geometry (from [9]). There are two common geometries allowing for spectrometer setup in dispersive mode: Johannson‐type [10] and von Hamos‐type [11]. Schematic representation of von Hamos geometry is shown in Figure 1 (left) [4]. In such arrangement, the X‐ray fluorescence from the sample is dispersed on cylindrically bent crystal. The dispersion axis and therefore energy range covered by the setup is limited by the length of the crystal/detector along There are two common geometries allowing for spectrometer setup in dispersive mode: Johannson-type [10] and von Hamos-type [11]. Schematic representation of von Hamos geometry is shown in Figure 1 (left) [4]. In such arrangement, the X-ray fluorescence from the sample is dispersed on cylindrically bent crystal. The dispersion axis and therefore energy range covered by the setup is limited by the length of the crystal/detector along dispersion axis. One-dimensional bending of the crystal aims at increasing the efficiency of the setup, as compared to flat crystal geometry, by focusing the diffracted X-rays onto the detector plane. The von Hamos setup provides good energy resolution being often below 1eV at relatively large Bragg angles. The Bragg angle domain is changed by linear displacement of the crystal and detector along dispersion axis, where the detector distance from the sample is always twice that of the crystal. Because of linear motions of the crystal/detector, the von Hamos spectrom‐ eter allows for flexible arrangements around the sample environment. Moreover, because of application of short curvature radiuses without loss on energy resolution, the spectrometer requires relatively small space for operation. Finally, the spectrometer geometry can be easily extended into multicrystal operation allowing for enhanced spectrometer efficiency or measurements of multiple X-ray emission lines [12].

5 In Johannson geometry, schematically shown in Figure 1 (right), the X-ray fluorescence from the sample is diffracted by cylindrically bent crystal; however, unlike in von Hamos geometry, the crystal curvature is positioned in dispersive axis. For Johannson geometry, the Bragg angle planes are not parallel to the crystal surface and therefore the energy broadening at lower Bragg angles is eliminated. Thanks to this, a wider Bragg angle range may be applied at highenergy resolution of the spectrometer. As a consequence, only few diffraction crystals are necessary to operate the spectrometer over broad energy range. By operating the Johannson spectrometer in the off-Rowland geometry the X-ray emission spectrum over certain band‐ width is measured, which is given by the detector length along dispersion axis. The drawback of off-Rowland setup is that only small part of the crystal contributes to the X-ray diffraction at one energy channel; however, this efficiency loss is somehow compensated by recording entire X-ray emission spectrum without scanning components.

Both spectrometers, von Hamos and Johannson types, yield absolute energy resolution significantly below the lifetime of characteristic emission lines, being crucial for detailed analysis of spectral features. The provided dispersive type of detection can be exploited to record time-resolved off-resonant, resonant and nonresonant X-ray emission studies. Because of ability of performing quick acquisitions of X-ray emission spectra, the spectrometers may be applied for in-situ spectroscopic studies of dynamic systems.
