In SituEXAFS Characterization of Nanoparticulate Catalysts

Abstract

X-ray absorption fine structure (XAFS) spectroscopy probes the structure and electronic properties of metal centers. Because it can be applied to noncrystalline materials, it is a key technique for probing nanoparticulate materials, such as colloidal and heterogeneous metal catalysts. The high brilliance of modern synchrotron radiation x-ray sources facilitates in situ studies, which provide direct structure–function relationships with both spatial and time resolution; this is especially effective when applied in combination with complementary techniques such as x-ray diffraction, mass spectrometry, and optical or vibrational spectroscopies. Tracking the particle formation of platinum-group metal catalysts, their behavior under reaction conditions, and the distribution of sites within a catalyst bed shows that this approach is essential for understanding the chemistry of these nanoparticles. Rather than behave as monolithic entities, nanoparticulate catalysts undergo rapid structural transformations induced by the gas environment and reaction conditions, and their lifetimes as catalysts depend on the reversibility of these changes.

Insights in the mechanism of selective olefin oligomerisation catalysis using stopped-flow freeze-quench techniques: A Mo K-edge QEXAFS study

“…14,15,16 In the stopped flow system the evolution of the reaction mixture can be monitored in time with EDE or Quick-XAS (QEXAFS), allowing full EXAFS spectra to be obtained in the millisecond to second time-scale. 17 The problem with this set-up is that the energy dispersive XAFS data acquisition (but also the QEXAFS if the high time-resolution wants to be achieved) is performed in transmission geometry and therefore a relatively high concentration of the species under investigation is required. At the same time, the complete reaction mixture, i.e.…”

Catalysis seen in action

“…Time-resolved XAS can be performed using two different acquisition methods, i.e. using a fast moving monochromator scanning through the energy range (QEXAFS) or an energy dispersive mode (EDXAS) [5]. The first can be used for either transmission or fluorescence detection, while the latter is limited to transmission, which requires higher concentrations of reactants, but enables faster time resolution.…”

Active Site Elucidation in Heterogeneous Catalysis via In Situ X-Ray Spectroscopies

“…When operated in its conventional photons in-/photons-out mode, XAS is also a bulk-averaging method, although some surface sensitivity is possible in total electron yield mode. [56][57][58] There are many detailed articles and reviews on the application of XAS to heterogeneous catalysis [59][60][61][62][63] and photochemistry, [64][65][66] hence only illustrative examples on the breadth of systems/materials, and wealth of information on reaction mechanisms, available by time-resolved XAS approaches are provided.…”