Abstract
Catalysts are materials that accelerate the rate of a desired chemical reaction. As such, they constitute an integral part in many applications ranging from the production of fine chemicals in chemical industry to exhaust gas treatment in vehicles. Accordingly, it is of utmost economic interest to improve catalyst efficiency and performance, which requires an understanding of the interplay between the catalyst structure, the gas phase and the catalytic activity under realistic reaction conditions at ambient pressures and elevated temperatures. In recent years efforts have been made to increasingly develop techniques that allow for investigating model catalyst samples under conditions closer to those of real technical catalysts. One of these techniques is high energy surface x-ray diffraction (HESXRD), which uses x-rays with photon energies typically in the range of 70–80 keV. HESXRD allows a fast data collection of three dimensional reciprocal space for the structure determination of model catalyst samples under operando conditions and has since been used for the investigation of an increasing number of different model catalysts. In this article we will review general considerations of HESXRD including its working principle for different model catalyst samples and the experimental equipment required. An overview over HESXRD investigations performed in recent years will be given, and the advantages of HESXRD with respect to its application to different model catalyst samples will be presented. Moreover, the combination of HESXRD with other operando techniques such as in situ mass spectrometry, planar laser-induced fluorescence and surface optical reflectance will be discussed. The article will close with an outlook on future perspectives and applications of HESXRD.
Highlights
The term ‘catalysis’ was coined in 1836 by the Swedish chemist Jöns Jakob Berzelius in his annual progress report in chemistry, which he wrote after having reviewed previous works on homogeneous and heterogeneous systems [1]
The obtained data suggested a gradual depletion of Rh from the metallic core of the Rh-containing nanoparticles during oxidation, resulting in the formation of Rh oxide shells. These prevented the Pd inside the particle core from oxidation, even at the higher oxygen pressures of the second oxidation step. This experiment revealed that the combination of high energy surface x-ray diffraction (HESXRD) and a 2D detector for the investigation of epitaxial nanoparticle systems allows for the tracking of the structure of particular nanoparticle facets, where thanks to the combinatorial approach different particle characteristics can be probed under the same condition
In a recent publication we showed that HESXRD allowed for an in situ tracking of the alloy composition- and shapedependent vertical sintering of Pt–Rh alloy nanoparticles supported on Al2O3(0001) under realistic reaction conditions during CO oxidation [73]
Summary
The term ‘catalysis’ was coined in 1836 by the Swedish chemist Jöns Jakob Berzelius in his annual progress report in chemistry, which he wrote after having reviewed previous works on homogeneous and heterogeneous systems [1]. With the advent of high-pressure compatible in situ and operando techniques in the last three decades such as high pressure scanning tunnelling microscopy (HP-STM) [17], high pressure atomic force microscopy (HP-AFM) [17], x-ray absorption spectroscopy (XAS) [18, 19], high pressure x-ray photoelectron spectroscopy (HP-XPS) [20,21,22], polarizationmodulation infrared reflectance absorption spectroscopy (PMIRRAS) [23,24,25], summed frequency generation (SFG) [26], environmental transmission electron microscopy (E-TEM) [27], grazing incidence small angle x-ray scattering (GISAXS) [28] and surface x-ray diffraction (SXRD) [29], and with the investigation of more complicated sample systems, such as vicinal surfaces and epitaxial nanoparticle systems, the gaps are being progressively bridged One of these novel techniques is high energy surface x-ray diffraction (HESXRD) [30, 31], which will be discussed in this review. Due to the continued improvement of synchrotron source performance, insertion device characteristics and detector technology, the use of synchrotron-based high photon energy x-rays (E 40 keV) has in recent years progressively increased [32]. Subsection 2.3 will illustrate how HESXRD experiments for catalysis studies can be realized, giving an overview of typical high energy x-ray beamline set-ups and sample environments for in situ and operando investigations of model catalyst samples
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