Dynamic behavior analysis of active species by modulation excitation spectroscopies: Application in heterogeneous catalysis – MESCAT

Heterogeneous catalytic steps are a synergetic interactions of reactives pecies with continuously evolving surface actives sites to promote the desirable reaction from complex manifolds of pathways. To achieve such a high level of selectivity, heterogeneous catalysts have to be tailored in a very precise way requiring an integrated strategy including the monitoring of reaction actives species and actives surface sites. However, the spectroscopic identification of surface active sites is problematic since they are only present in very small quantities. This is especially true for catalysts in the form of bulk materials for which the recorded spectroscopic signal corresponds to a global signal containing the active surface species, and the bulk species that are spectators. For supported catalysts, segregation of metal precursors at the surface of the support in active and non-active phases is observed making the identification of the active centers very difficult. Intermediates species have a very short lifetime and are in negligible amounts compared to reactants and final products. Hence, the recorded signal will often be saturated with the signal of reactants and products and it is very difficult to obtain a pure spectroscopic fingerprint of intermediate reaction species.

Despite the recent technical advances in in situ characterization, no available technique can by itself probe active species selectively. One approach to overcome these limitations is to apply modulation excitation (ME) combined with proper spectroscopic methods. The modulation excitation spectroscopy consists in stimulating the sample with oscillatory perturbation, (e.g. rapid looping from different atmospheres) and recording spectra all along these oscillations. This periodic perturbation of the system will influence only the concentration profile of the active species which will oscillate at the frequency of the excitation but with a phase delay. The concentration profile of species not responding to the periodic excitation (i.e. spectator species) will remain constant making possible their removal from the signal by a mathematical post-treatment known as PSD (phase sensitive detection). In this project X-ray Absorption (XAS) and Infrared (IR) spectroscopy will be applied under modulation excitation conditions. Identification of intermediate reaction species reached through ME-IR correlated with the structural evolution around catalyst actives site centers probed by ME-XAS will yield a clearer picture on the behavior of catalyst during the catalytic process.

ME-PSD is a major achievement to monitor selectively species directly involved in a reaction. However, its use is not widespread yet because the state of the art of MES signal processing is still in its early stages. Indeed, the PSD converts the original spectrum (a function of energy and time) into a demodulated signal (a function of energy and phase angle). The key question is how to process such a demodulated spectra to obtain accurate information on active species. There still a need to go beyond a qualitative interpretation to obtain a quantitative analysis of the phase domain signal: (1) determination of the number of species present during the perturbation, (2) isolation of species and their sequence of apparition and (3) back-transformation to the time domain to isolate pure spectra. We will pioneer a new strategy addressing the ME-PSD signal treatment. This strategy will combine a new adopted chemometric tool to ME-PSD signal treatment, homemade algorithm allowing a back transformation in time domain to isolate pure regular spectra, DFT calculation and spectra modelling. Besides the methodological approach, the questions addressed in this project are also about the catalytic behaviors of two major class of heterogeneous catalysis: partial oxidation catalysts to synthetize green platform molecules (Fe2(MoO4)3) and gasoline exhaust treatment catalysts (Alumina-supported Pt).