15 Scientific Article The Israel Chemist and Chemical Engineer Issue 8 · November 2021 · Kislev 5782 One might imagine the difficulty in applying an exact squarewave function of gas pulses, yet as stated above this approach to demodulation still holds valid due to Fourier’s theorem stating that any arbitrary function can be described by an arbitrary combination of sine and cosine functions. In essence, phase-sensitive detection is a Fourier transformation, with the addition of phase sensitivity to reduce noise. Some benefits of such analysis have already been proven by the study of basic systems such as the reversible oxidation of noble metals [30], where the sensitivity of such detection was greatly enhanced. In the case of the application of such spectroscopies to the complex and demanding systems that are relevant to catalysis, it may have some additional benefits. For example, after demodulation, for every phase angle one obtains one demodulated spectrum, and for every higher harmonic we obtain another demodulated spectrum. In principle one should be able to use the phase angle and higher harmonic as a descriptor for the kinetic behavior of the spectral signal under investigation, as the degree of attenuation is dependent on the phase offset with respect to the initial pulse onset and harmonicity. Via simulation of spectroscopic data, it was shown that transient species possessing fast kinetics are enhanced relative to those possessing slower kinetics in the fundamental harmonic [26]. As such, by examining higher harmonics one should – in principle – be able to distinguish species with faster kinetics from those with slower. 2.3 Perspective The past two decades have seen significant improvements in the application of spectroscopic techniques such as infrared and UV-Vis spectroscopy, or X-ray absorption spectroscopy to study catalysis at work, an approach that is termed operando spectroscopy. Nevertheless, with traditional application of such spectroscopic techniques it can be very difficult to determine the most relevant information (active sites, active reaction intermediates, active phases, and even dynamic changes) due to the high degree of complexity of the catalystreactant matrix e.g., in terms of pressure, temperature, and material complexity. Advances in the combination of spectroscopic techniques with increasingly higher time resolution, along with e.g., operando spectroscopic reactors, digital valve control and a posteriori data analysis, offer novel overall approaches to study catalytic reactions at work. In this article some examples have been given of achievable results. In our new research group at the Technion these, and several other approaches, will be developed and applied with the overall goal of obtaining novel fundamental insights into catalytic processes and driving the discovery time of new catalysts and materials down significantly. References 1. J. A. Dumesic, G. W. Huber, M. Boudart, in Handbook of Heterogeneous Catalysis, ed. G. Ertl, H. KnÖzinger, F. Schüth, J. Weitkamp, Wiley-VCH, Weinheim, 2nd edn., 2008, 1445-1462. 2. H. F. Rase, Handbook of Commercial Catalysts - Heterogeneous Catalysts, CRC Press, Boca Raton, 2000. 3. G. A. Somorjai and K. McCrea, Appl. Catal. A Gen., 2001, 222, 3–18. 4. C. Vogt, E. Groeneveld, G. Kamsma, M. Nachtegaal, L. Lu, C. J. Kiely, P. H. Berben, F. Meirer and B. M. Weckhuysen, Nat. Catal., 2018, 1, 127–134. 5. S. Bordiga, E. Groppo, G. Agostini, J. A. Van Bokhoven and C. Lamberti, Chem. Rev., 2013, 113, 1736–1850. 6. H. S. Taylor, Proc. R. Soc., 1925, 108, 105–111. 7. G. A. Somorjai, A. M. Contreras, M. Montano and R. M. Rioux, Proc. Natl. Acad. Sci., 2006, 103, 10577–10583. 8. J. Wolff, A. G. Papathanasiou, I. G. Kevrekidis, H. H. Rotermund and G. Ertl, Science, 2001, 294, 134–137. 9. G. Ertl, Angew. Chem. Int. Ed., 2008, 47, 3524–3535. 10. G. Kleinle, V. Penka, R. J. Behm, G. Ertl and W. Moritz, Phys. Rev. Lett., 1987, 58, 148–151. 11. G. A. Somorjai, Catal. Letters, 1992, 12, 17–34. Figure 3. Schematic example of possible results with modulated excitation type experimentation. FT-IR spectra before and after demodulation, and the separation of active and spectator species. Top half of figure is reproduced with permission from reference [31].

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