Raman spectroscopy is the technique of choice to obtain the structure of atomically and nanodispersed metal oxides of transition elements [1] supported, e.g. on Al₂O₃, SiO₂ and TiO₂ that are active for a number of catalytic industrial processes. Nevertheless, when used under reaction conditions, it becomes difficult to isolate the contribution of that fraction of M=O and M-OH species that are effectively involved in adsorption and reaction from that of the overall phase constituted by the transition metal oxide. We have applied the modulated excitation (ME) approach [2] to increase our sensitivity to such species in the case of typical catalysts used for the selective catalytic reduction of NO_x by NH₃, e.g. V₂O₅-TiO₂ and V₂O₅-WO₃-TiO₂. ME consists in subjecting the sample to repeated pulses, e.g. alternate reducing and oxidizing conditions, while acquiring spectroscopy data with sufficient time resolution. Finally, the spectroscopic data are processed by phase sensitive detection (PSD) to eliminate the contribution of species not responding to the perturbation of the gas phase applied to the sample.

Here, we show that in simple ME experiments where water is repeatedly added to an O₂ feed on V₂O₅-TiO₂ V=O and V-O-V signals are greatly enhanced. These signals represent only a small fraction of the overall time-resolved signals suggesting that only a fraction of the available V is perturbed and can be detected with high sensitivity upon PSD: the fwhm of the V=O signal at 1024 cm^-1 was of only 14 cm^-1 after PSD. Additionally, we show by the same approach that NH₃ adsorption can be monitored on V₂O₅-TiO₂ and V₂O₅-WO₃-TiO₂ when pulsing NH₃ in various feeds also by the perturbation of the TiO₂ signal at 700-300 cm^-1. This region is otherwise omitted when experiments are performed under steady state conditions because of the strong intensity and the subtle changes. Comparison between V₂O₅-TiO₂ and V₂O₅-WO₃-TiO₂ clearly demonstrated that PSD enables a careful differentiation of the contribution of the responsive V=O (1028 cm^-1) and W=O (1012 cm^-1) species within the broad signal of unresponsive species.

The sensitivity obtained by the ME approach potentially defines new frontiers for the application of Raman spectroscopy to the characterization of dispersed transition metal oxide catalysts. Moreover, it calls for combination with detailed theoretical simulation in order to understand precisely the origin of signal shifts.

[1] Israel E. Wachs, Charles A. Roberts, Chem. Soc. Rev., 2010, 39, 5002-5017
[2] Davide Ferri, Mark A. Newton, Marco Di Michiel, Gian Luca Chiarello, Songhak Yoon, Ye Lu, Jerôme Andrieux, Angew. Chemie Int. Ed., 2014, 53, 8890-8894; Philipp Müller, Ive Hermans, Ind. Eng. Chem. Res., 2017, 56, 1123-1136