Abstract
X-ray absorption spectroscopy and X-ray diffraction are suitable probes of the chemical state of a catalyst under working conditions but are limited to bulk information. Here we show in two case studies related to hydrothermal aging and chemical modification of model automotive catalysts that enhanced detailed information of structural changes can be obtained when the two methods are combined with a concentration modulated excitation (cME) approach and phase sensitive detection (PSD). The catalysts are subject to a modulation experiment consisting of the periodic variation of the gas feed composition to the catalyst and the time-resolved data are additionally treated by PSD. In the case of a 2 wt% Rh/Al2O3 catalyst, a very small fraction (ca. 2%) of Rh remaining exposed at the alumina surface after hydrothermal aging at 1273 K can be detected by PSD. This Rh is sensitive to the red-ox oscillations of the experiment and is likely responsible for the observed catalytic activity and selectivity during NO reduction by CO. In the case of a 1.6 wt% Pd/Al2O3-Ce(1-x)Zr(x)O2 catalyst, preliminary results of cME-XRD demonstrate that access to the kinetics of the whole material at work can be obtained. Both the red-ox processes involving the oxygen storage support and the Pd component can be followed with great precision. PSD enables the differentiation between Pd deposited on Al2O3 or on Ce(1-x)Zr(x)O2. Modification of the catalyst by phosphorous clearly induces loss of the structural dynamics required for oxygen storage capacity that is provided by the Ce(4+)/Ce(3+) pair. The two case studies demonstrate that detailed kinetics of subtle changes can be uncovered by the combination of in situ X-ray absorption and high energy diffraction methods with PSD.
Highlights
X-ray absorption spectroscopy and X-ray diffraction are suitable probes of the chemical state of a catalyst under working conditions but are limited to bulk information
Al2O3 is sensitive to high temperature conditions that are encountered during engine operation and the associated well-known phase transformations promote the loss of precious metal surface area by particle sintering
Whereas chemisorption data indicate very low dispersion for the catalyst aged at 1273 K, suggesting the presence of large Rh particles (Table 1), scanning transmission electron micrographs (STEM) confirm the disappearance of the Rh nano-particles observed in the fresh catalyst (Fig. 1), only EDX analysis demonstrates the presence of Rh
Summary
X-ray absorption spectroscopy and X-ray diffraction are suitable probes of the chemical state of a catalyst under working conditions but are limited to bulk information. This observation demonstrates that during the CO–NO modulation at 573 K the fresh catalyst undergoes periodic partial re-oxidation of Rh compared to the initial reduced state.[15] In marked contrast, Rh/Al2O3 aged at 1273 K exhibits the least intensity variation, and above 23.27 keV the spectral oscillations are strongly damped.
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