Description
Detection of active species in (photo)catalysts is a crucial step towards understanding and optimization of reaction mechanisms. X-ray absorption spectroscopy (XAS) is an extremely powerful tool to selectively probe local atomic and electronic structure around element of interest. Its sensitivity towards active species can be further enhanced by a modulation-excitation (ME) approach, using periodic stimuli, such as light. However, in liquid-phase reactions, diffusion limitations, product accumulation, and gradual pH shifts can obscure the direct photoinduced response of the catalyst. This makes the detection of light-induced structural changes elusive and limits the mechanistic understanding.
In this work, we overcome these limitations by synergistic combination of microfluidic setups with in situ and operando XAS. We have designed several microfluidic reactors for chemical synthesis [1], homogenous and heterogenous catalysis [2] and photocatalysis. The latter was used with the ME-XAS coupled with real-time UV-Vis analysis of the reaction products. The efficiency of the approach was demonstrated using photocatalytic degradation of Rhodamine B over Pt/TiO₂. Pt species were found to be present in Pt(0) state, not showing any XAS changes during catalysis in the conventional batch cell. To get deeper insights on the structural changes, a thin layer of the catalyst was deposited on a 1 mm channel of the microfluidic reactor with built-in directional UV source. The changes of both XAS and UV-vis data were monitored over periodic light ON/OFF experiment flowing RhB solution at 1, 20 and 60 mL/h. We demonstrate that high flow rates of the substrate through the microreactor are indispensable to resolve light modulated differences in Pt L₃-edge spectra – undetectable and low flow rates of the solution. Supported by ab initio modelling and comparing with the experimental data collected under gas phase conditions, the observed spectral modulations were successfully assigned to the formation of platinum hydrides.
Beyond advancing the mechanistic understanding of several chemical processes, this approach establishes a general platform for time-resolved operando XAS studies of metal speciation across diverse liquid-phase (photo)catalytic reactions.
[1] Bugaev et al. J. Phys. Chem. C. 2023, 127, 42, 20727–20733
[2] Moragues et al. Angew. Chem. Int. Ed. 2024, 63, e202401056