A detailed understanding of the mode of operation of heterogeneous catalysts largely benefits from the use of spectroscopic methods enabling access to the catalyst’s structural dynamics. Operando Raman spectroscopy has been shown to be a valuable tool to characterize heterogeneous catalysts under working conditions allowing the simultaneous measurement of the catalyst (defect) structure as well as adsorbed species. In this contribution, we will highlight the potential of multi-wavelength and time-resolved operando Raman spectroscopy combined with UV-Vis spectroscopy to obtain new insight into the structural dynamics of catalysts. For illustration recent results on supported vanadia and Au/ceria catalysts in the context of oxidation reactions will be discussed. As a first example, operando multi-wavelength and time-resolved Raman spectroscopy is applied to the oxidative dehydrogenation (ODH) of ethanol over silica-supported vanadia catalysts. Using multiple excitation wavelengths in the Vis and UV, the resonance Raman effect is exploited in a targeted manner offering the possibility of separating signals from differently structured vanadia components as well as from vanadia and support features. By applying this approach under reaction conditions, large oligomeric structures are shown to decompose into smaller vanadia units such as short-chain oligomeric and monomeric vanadia species, which are proposed as the catalytically active species. Secondly, Au/ceria catalysts are discussed, which are great interest in the context of low-temperature CO oxidation and water-gas shift. Here focus is put on the ceria support dynamics and its relevance for Au/ceria catalysts during room temperature CO oxidation. In particular, combined operando Raman / UV-Vis spectroscopy is employed to monitor the surface and subsurface defect dynamics of ceria quantitatively and in real time. The results show a dependence of catalytic activity on the ceria reduction state. This is highly relevant as pre-reduced ceria increases the activity during CO oxidation initially by 100%, thus highlighting the importance of support properties such as oxygen defects for Au/ceria catalysts. Our results underline that detailed operando spectroscopic analysis using optical methods offers a powerful approach to unravel the mode of operation of catalysts.

Shining light on catalysts at work: Multi-wavelength and time-resolved operando Raman spectroscopy

256th National Meeting (2018)

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