Improving the sensitivity of QEXAFS using modulation excitation spectroscopy

Stötzel, Jan; Lützenkirchen‐Hecht, Dirk; Frahm, R.; Grunwaldt, Jan‐Dierk

doi:10.1088/1742-6596/430/1/012126

Cited by 8 publications

(10 citation statements)

References 9 publications

(6 reference statements)

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“…Averaging the data collected over several modulation periods from this point onwards provides already an improvement in the signal-to-noise ratio. 13,14 The PSD algorithm 12 is then applied to the set of time-resolved data after data acquisition in order to further enhance the sensitivity of the analytical method. The data are treated using the following equation to obtain a new set of phase-resolved (demodulated) data:…”

Section: Introductionmentioning

confidence: 99%

Increasing the Sensitivity to Short-Lived Species in a Modulated Excitation Experiment

et al. 2017

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The combination of spectroscopic and diffraction methods to study chemical transformations is fundamental for the understanding of reaction mechanisms. The identification of short-lived species, likely active species, is often hindered by the contribution of spectator species not directly involved in the reaction. The present study considers two different approaches to obtain increased sensitivity to transient species for experiments obeying the modulated excitation paradigm and exploiting phase sensitive detection (PSD). First, the variation of the frequency of the external stimulation (ω) during the experiment is considered. We demonstrate using the Fourier analysis that the increase of ω, i.e., the decrease of the modulation period T, enhances the sensitivity to short-lived species. The second alternative is the use of a single stimulation frequency (ω) during the measurement and the variation of the demodulation frequency (nω) during data analysis. The absolute intensity of the phase-resolved signals is reduced by increasing n. However, species with slow kinetics are more attenuated than species with fast kinetics. Thus, transient species possessing fast kinetics are enhanced relative to other components and can be, in principle, discerned with improved sensitivity in the phase-resolved data obtained with n > 1. Experimental results in the field of heterogeneous catalysis are provided that support our findings.

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Section: Introductionmentioning

confidence: 99%

Increasing the Sensitivity to Short-Lived Species in a Modulated Excitation Experiment

et al. 2017

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“…Using a combination of time resolved DRIFTS and impedance measurements, conductivity changes in ZSM-5-based catalysts during the selective catalytic reduction of nitrogen oxides were successfully assigned to a certain adsorbed NH 3 species, by comparing the time constants of the electrical response and the different adsorbed NH 3 species [ 39 ]. Other approaches using time-resolved methods are Modulation Excitation Spectroscopy (MES) [ 70 , 72 , 73 , 74 ], Steady State Isotopic Transient Kinetic Analysis (SSITKA) [ 68 , 75 ] and the evaluation of time-resolved data by Multivariate Curve Resolution (MCR) [ 76 , 77 ]. These rather sophisticated approaches are applied by various authors to study catalysts, but today there is just a small number of articles dedicated to the investigation of gas sensing materials [ 28 , 39 , 73 , 78 ].…”

Section: Identifying Active and Inactive Speciesmentioning

confidence: 99%

Trends and Advances in the Characterization of Gas Sensing Materials Based on Semiconducting Oxides

Degler

2018

Sensors

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The understanding of the fundamental properties and processes of chemoresistive gas sensors based on semiconducting metal oxides is driven by the available characterization techniques and sophisticated approaches used to identify structure-function-relationships. This article summarizes trends and advances in the characterization of gas sensing materials based on semiconducting metal oxides, giving a unique overview of the state of the art methodology used in this field. The focus is set on spectroscopic techniques, but the presented concepts apply to other characterization methods, such as electronic, imaging or diffraction-based techniques. The presented concepts are relevant for academic research as well as for improving R&D approaches in industry.

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“…Recently, Sto ¨tzel et al have raised some doubts about the advantage of coupling ME with EXAFS followed by PSD data treatment. 17 Thus, a more careful description of the possibilities at hand for application of PSD to XAS datasets is required even on a qualitative base. This has also been stimulated by the theoretical accounts of ME-XRD.…”

Section: Introductionmentioning

confidence: 99%

Modulated excitation extended X-ray absorption fine structure spectroscopy

Chiarello

Ferri

2015

Phys. Chem. Chem. Phys.

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The sensitivity of extended X-ray absorption fine structure spectroscopy (EXAFS) for minute structural changes can be enhanced by combination with the modulated excitation approach and making use of phase sensitive analysis. A modulated EXAFS experiment of a reversible periodic Pd to PdO partial oxidation has been simulated in order to understand the effect of the phase sensitive analysis on the shape and meaning of the resulting phase-resolved EXAFS spectra. In particular, the simulation comprises either a synchronous or a delayed sinusoidal variation of the EXAFS parameters, i.e. coordination number (N), interatomic distance (R) and Debye-Waller factor (σ(2)), of first Pd-Pd, first Pd-O, and second Pd-(O)-Pd coordination shells. The effect of these variations on the resulting phase-resolved Fourier transform EXAFS spectra is discussed. The results of the simulation are validated by an in situ EXAFS experiment at the Pd K-edge over 1.6 wt% Pd/Al2O3 undergoing reversible partial oxidation in a H2vs. O2 modulation at 573 K. It is shown that phase sensitive detection (PSD) is able to separate the minor contribution at ca. 2.8 Å corresponding to the growth of the Pd-(O)-Pd shell that is otherwise hidden under the static signal of the Pd-Pd shell of reduced Pd particles. The fitting of the phase-resolved EXAFS spectra suggests that the fast H2 to O2 switch leads the partial oxidation of the Pd surface with the formation of a PdO shell covering a metallic Pd core. Therefore, the dynamics of the full system can be described with greater detail than in conventional EXAFS. The intention of this work is to provide the tools and therefore a solid guidance to qualitatively and quantitatively understand the nature of the shape of phase-resolved FT-EXAFS spectra that may prove helpful in the analysis of a wide range of functional materials.

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Improving the sensitivity of QEXAFS using modulation excitation spectroscopy

Cited by 8 publications

References 9 publications

Increasing the Sensitivity to Short-Lived Species in a Modulated Excitation Experiment

Increasing the Sensitivity to Short-Lived Species in a Modulated Excitation Experiment

Trends and Advances in the Characterization of Gas Sensing Materials Based on Semiconducting Oxides

Modulated excitation extended X-ray absorption fine structure spectroscopy

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