High energy surface x-ray diffraction applied to model catalyst surfaces at work

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.

Oxidation and Reduction of Ir(100) Studied by High-Energy Surface X-ray Diffraction

“…This implies that the IrO 2 islands grew larger in size, and their coverage increased. At the same time, the timescan revealed that the IrO 2 (110) diffraction peaks progressively smeared out in a radial direction, implying a gradual tilting of the IrO 2 (110) islands with respect to the sample surface normal, as we have described earlier and which is also elucidated in Supplementary Note 3 and in Figure S6 of the Supporting Information. By the end of the timescan, about 15 min into the set condition of 5 mbar of O 2 , the tilt angle comprised around ±18° with respect to the sample surface normal, suggesting a corresponding tilt of the bulk oxide islands with respect to the sample surface normal.…”

“…For the HESXRD data acquisition two distinct measurement modes were employed. ,, (1) Timescans during which, with a stationary sample, 2D images were acquired in a distinct reciprocal space plane of interest and with a high temporal resolution up to 5 Hz. (2) Rotational scans during which the sample was rotated around its surface normal while 2D images were acquired with an angular resolution of 0.1°, covering 100° of reciprocal space with a total acquisition time of 260 s. Schematic representations of these two data acquisition modes and of the successive data processing are shown in Figure b–g and will be elucidated in the following.…”

A combined rotating disk electrode–surface x-ray diffraction setup for surface structure characterization in electrocatalysis

“…This directly translates to the measurement of a particular CTR and the range in L over which it is recorded. 4 The total number of photons collected in a single detector image measured in this way may be too low to give statistically relevant data and therefore the measurement over the same angular range is repeated a number of times, in a similar way as pump-probe data are collected. 13 A complicating factor in the above mentioned scheme is a fluctuating rotator speed, which may cause ill-defined angular ranges over which the detector is exposed in case fixed delay and exposure times are used.…”

“…Surface x-ray diffraction (SXRD) has proven to be a powerful technique for structural characterization experiments and its applicability in electrochemistry has been boosted strongly by the development of high-energy (20-100 keV, HE-SXRD) beamlines and large 2D detectors. [2][3][4] As a result, the time needed for the measurement has decreased drastically, from hours to minutes, while also unwanted, photo(electro)chemical, side-reactions are damped. In practice, this means that the only movement necessary for crystal truncation rod (CTR) measurements, is an in-plane sample rotation.…”

Operando Reflectance Microscopy on Polycrystalline Surfaces in Thermal Catalysis, Electrocatalysis, and Corrosion

“…The technique must also be able to probe the sample surface through the surrounding gas and electrolyte. Techniques such as surface X-ray diffraction (SXRD) can be used both in gas phase and in electrochemistry but provide no spatial information about the surface. Others, such as low-energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM), can provide 2D spatial information about the chemical state of the sample surface but are limited to UHV or high vacuum (HV) due to the low mean free path of electrons. − …”