Reduction and re-oxidation of Cu/Al<sub>2</sub>O<sub>3</sub>catalysts investigated with quick-scanning XANES and EXAFS

Stötzel, Jan; Lützenkirchen‐Hecht, Dirk; Frahm, R.; Kimmerle, Bertram; Baiker, Alfons; Nachtegaal, Maarten; Beier, Matthias; Grunwaldt, Jan‐Dierk

doi:10.1088/1742-6596/190/1/012153

Cited by 15 publications

(12 citation statements)

References 17 publications

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“…The majority of the literature dealing with time-resolved works using XANES is 'limited' to statements about 'oxidized' or 'reduced' states, thus with qualitative structural insights. Doing the same kind of timeresolved experiments with EXAFS is a much more difficult issue [20,41], yet it is EXAFS that contains the more readily comprehendible structural data.…”

Section: Discussionmentioning

confidence: 99%

“…Some XAS and XRD experiments of the type proposed by the MES approach have been reported [17][18][19], however, without application of the PSD analysis. A first step towards PSD was provided by Stötzel et al who averaged the XAS spectra of Pt-Rh/Al 2 O 3 over three modulation periods to demonstrate the improvement in the signal-tonoise ratio [20]. In a provocative way, it can be claimed that MES in combination with PSD can impart to XAS the sensitivity to surface and active species it needs for the analysis of heterogeneous catalysts under working conditions, within the terms indicated in the following.…”

Section: Introductionmentioning

confidence: 99%

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Modulation Excitation X-Ray Absorption Spectroscopy to Probe Surface Species on Heterogeneous Catalysts

2011

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The advantages and open questions of the combination of modulation excitation spectroscopy and phase sensitive detection (PSD) with X-ray absorption spectroscopy (XAS) for the analysis of heterogeneous catalysts at work are reviewed. The characteristic spectral signatures of two different edges (Pd K and Pt L 3 ) are described in relation to the red-ox chemistry of Pd/Al 2 O 3 and Pt/Al 2 O 3 with respect to NO reduction by CO and CO oxidation, respectively. Both examples demonstrate that PSD makes XAS sensitive to potentially active species for the catalytic reaction.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Modulation Excitation X-Ray Absorption Spectroscopy to Probe Surface Species on Heterogeneous Catalysts

2011

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“…The quality of noise suppression is similar to the one achieved with the FS/PSD approach, but it is still possible to localize the major changes in the spectra at 90 s. Considering these results low pass filtering seems to be the preferred choice and it cannot be confirmed that FS/PSD is an outstanding tool to for example increase surface sensitivity in catalytic experiments. As a second example a Cu/Al 2 O 3 catalyst at 322 °C was investigated during periodic reduction and reoxidation induced by switching between 21%O 2 /He and 5%H 2 /He gas streams every 60 s (see also [7]). A combination of XANES linear combination analysis (LCA) and EXAFS fitting was applied to the low pass filtered 1200 spectra of one cycle.…”

Section: Resultsmentioning

confidence: 99%

Improving the sensitivity of QEXAFS using modulation excitation spectroscopy

Stötzel

Lützenkirchen‐Hecht

Frahm

et al. 2013

J. Phys.: Conf. Ser.

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Abstract. Modulation excitation spectroscopy (MES) is a promising tool to analyze timeresolved EXAFS data during reproducible catalytic processes and was applied in two differentexperiments to evaluate the benefits compared to conventional analysis strategies. Thereby, phase sensitive detection (PSD) was applied directly to the x-ray absorption spectra as well as to the Fourier transformed extracted EXAFS. In both cases the results were discussed with respect to the achievable level of noise reduction as well as the possibilities to better understand the underlying mechanisms of the reactions, more precisely the dynamics and possible intermediate species. The results clearly demonstrate serious limitations for the application of MES in time-resolved EXAFS studies as soon as more sophisticated steps than averaging over several modulation cycles are performed IntroductionIn catalysis and materials science often only small but rapid structural changes occur under varying reaction conditions. In this case modulation excitation spectroscopy (MES) has received much attention which makes use of monitoring the structural changes by periodically changing the conditions [1,2]. In case of reversible structural changes the alternating part of the signal, which is the frequency response of the active species to the applied excitation, can be separated from the often dominant non-varying part. Furthermore, by considering only frequencies that can be assigned to the excitation, it is possible to efficiently suppress random noise. For catalytic applications, the technique has been widely used in infrared spectroscopy [2]. In addition, it has been found useful when studying materials by Raman spectroscopy and XRD [3].In a first step we straight forwardly improved the signal-to-noise ratio by averaging over all cycles [4] which is possible due to the exactly known cycle duration T and the fact that T is an integer multiple of the measurement time for one spectrum. With this approach the original time resolution of the experiment is maintained while photon statistics are significantly improved. Afterwards, the more advanced phase sensitive detection (PSD) approach can be applied, where the spectra of the cycle are studied with the modulation frequencies only and the measured response is resolved dependent on the phase shift to the applied modulation [5]. An alternative way to increase sensitivity is low pass filtering followed by conventional XAS data analysis. Presently, it is strongly discussed which of these approaches is more useful or how they complement each other. Hence, we present here a comparison of the two approaches by analysing QEXAFS data in two model reactions [6].

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“…4, where the oxidation/reduction behaviour of Cu/Al 2 O 3 was studied by fitting the measured data with Cu, Cu 2 O and CuO reference spectra. In this experiment the gas stream applied to the catalyst was switched between 5%-H 2 /He and 21%-O 2 /He every 60 s, while QEXAFS spectra were acquired with a crystal oscillation frequency of 10 Hz resulting in 20 full EXAFS spectra each second [see Stö tzel et al (2009) for experimental details]. This example demonstrates how valuable the LCA approach for studying fast processes by resolving the sample composition is.…”

Section: Linear Combination Analysis (Lca)mentioning

confidence: 99%

T-REX: new software for advanced QEXAFS data analysis

Stötzel

Lützenkirchen‐Hecht

Grunwaldt

et al. 2012

J Synchrotron Radiat

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New approaches to analyze the data generated by modern time-resolved X-ray absorption spectroscopy instrumentation are presented as part of a new analysis software to handle files containing typically a few thousand EXAFS spectra. Various filter techniques to remove high-frequency noise and run-away values are discussed as well as advanced analysis tools like linear combination fitting, EXAFS fitting, principal component analysis and phase-sensitive detection. These techniques were implemented in a user-friendly graphical user interface to analyse huge data files where it is not possible to treat each spectrum separately. New ideas to exploit existent tools more efficiently for time-resolved EXAFS data analysis are discussed theoretically as well as applied to real measurements, especially in situ catalytic experiments and surface-sensitive reflection-mode X-ray absorption studies of thin film growth.

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

Cited by 15 publications

References 17 publications

Modulation Excitation X-Ray Absorption Spectroscopy to Probe Surface Species on Heterogeneous Catalysts

Modulation Excitation X-Ray Absorption Spectroscopy to Probe Surface Species on Heterogeneous Catalysts

Improving the sensitivity of QEXAFS using modulation excitation spectroscopy

T-REX: new software for advanced QEXAFS data analysis

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

Cited by 15 publications

References 17 publications

Modulation Excitation X-Ray Absorption Spectroscopy to Probe Surface Species on Heterogeneous Catalysts

Modulation Excitation X-Ray Absorption Spectroscopy to Probe Surface Species on Heterogeneous Catalysts

Improving the sensitivity of QEXAFS using modulation excitation spectroscopy

T-REX: new software for advanced QEXAFS data analysis

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Product

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