Piezo X-ray Absorption Spectroscopy for the Investigation of Solid-State Transformations in the Millisecond Range

Abstract

The piezo XAFS technique in combination with an in situ cell has been used for the investigation of fast solid-solid transformations with millisecond time resolution. The technique records X-ray absorption spectra (XAFS) in a continuous mode by making use of piezo tilt tables in the X-ray monochromator, which are actuated by an oscillatory high voltage. The application of this technique in the field of solid-state chemistry and catalysis is illustrated by investigations of the X-ray absorption near-edge structure (XANES) of the Cu K edge during the autocatalytic reduction of a Cu/ZnO methanol catalyst and the spontaneous decomposition of (NH 4 ) 2 Cr 2 O 7 to Cr 2 O 3 . The reduction of Cu(II) to Cu(0) occurs within a few seconds. The existence of an intermediate Cu(I) phase could be revealed by the analysis of the preedge position and the white line intensity, but the Cu(I) intermediate is very unstable. The spontaneous decomposition of (NH 4 ) 2 Cr 2 O 7 occurs within about 30 s and shows after a short acceleration period a rather constant reaction rate and a quite long decay period.

Reduction and re-oxidation of Cu/Al2O3catalysts investigated with quick-scanning XANES and EXAFS

“…methanol synthesis, steam reforming and for gas phase oxychlorination. QEXAFS studies on Cu/ZnO systems have been reported in detail previously and indicate that Cu(I) species are very short-living intermediates in many reactions [2,3]. In other cases Cu(I) is much more stable, especially if spinels can be formed [11,12].…”

“…Considering nano-sized particles it is even possible to measure surface structures in absorption mode under conditions where other surface sensitive methods cannot be applied since they require a vacuum and/or a flat surface. Thus the application of QEXAFS in catalysis is most interesting due to the possibility of studying the dynamic changes of a catalyst in situ under realistic conditions [1][2][3][4][5]. Since the setup of a QEXAFS experiment is equal to conventional EXAFS setups and thus nowadays rather simple, it provides an easy access to such experiments and has recently attracted more and more attention.…”

“…With the piezo-QEXAFS technique spectra were taken on catalysts with a time resolution of 17 ms/spectrum [3], but only the XANES region was monitored. More recently, full EXAFS data were taken with a cam-driven mechanical system with 1 Hz [8] and 2 Hz [4].…”

“…Interestingly the reduction behaviour of copper catalysts prepared with different impregnation times varies tremendously [13]. Thus, the dynamic behaviour of copper on alumina prepared with different impregnation times was studied here and compared to the results on Cu/ZnO in literature [2,3]. For this purpose, the Cu/Al 2 O 3 catalysts were reduced at 350°C and the gas atmosphere was periodically switched between oxidizing and reducing conditions using 21%O 2 /He and 5%H 2 /He, respectively.…”

Time-Resolved and Operando XAS Studies on Heterogeneous Catalysts — From the Gas Phase Towards Reactions in Supercritical Fluids

“…For round-shaped particles the reaction time τ can be calculated according to the formula: with the size of the particles R p . For an effective diffusion coefficient of 10 -6 -10 -8 m 2 /s, τ amounts to 20 ms to 2 s. This has important consequences for time resolved studies in heterogeneously catalyzed reactions: Both the QEXAFS and the DEXAFS technique allow monitoring of structural changes in the millisecond regime [6,7]. Considering the reduction of the CuO/ZnO catalyst and equation (1), we can conclude that a reaction faster than ≈ 1 ms may be hampered by internal diffusion or even external mass transport, even in the ideal case of a sieved catalyst.…”