In situ and Operando Spectroscopies in Photocatalysis: Powerful Techniques for a Better Understanding of the Performance and the Reaction Mechanism

Abstract

In photocatalysis, a set of elemental steps are involved together at different time scales to govern the overall efficiency of the process. These steps are divided as follow: 1) photon absorption and excitation (in femtoseconds), 2) charge separation (femto-to picoseconds), 3) charge carrier diffusion/transport (nano-to microseconds), 4 & 5) reactant activation/conversion and mass transfer (micro-to milliseconds). The identification and quantification of these steps, using the appropriate tool/technique, can provide the guidelines to emphasize the most influential key parameter that improve the overall efficiency and to develop the "photocatalyst by design" concept. In this review, the identification/quantification of reactant activation/conversion and mass transfer (steps 4&5) is discussed in details using the in-situ/operando techniques, especially the infrared (IR), Raman and XAS spectroscopy. The use of these techniques in photocatalysis was highlighted by the most recent and conclusive case studies which allow a better characterization of the active site and revealing the reaction pathways in order to establish a structureperformance relationship. In each case study, the reaction conditions and the reactor design for photocatalysis (pressure, temperature, concentration, etc.) were thoroughly discussed. In the last part, some examples in the use of time-resolved techniques (time-resolved FTIR, photoluminescence and transient absorption) are also presented as an author's guideline to study the elemental steps in photocatalysis at shorter time scale (ps, ns and µs).

Green Thermo-Photo Catalytic Production of Syngas Using Pd/Nb–TiO2 Catalysts

“…The response of the samples is characteristic of the dominant anatase phase, presenting the so-called green and red de-excitation peaks, roughly below and above ca. 500–525 nm. , PL changes through the series of catalysts correspond to a decrease of the overall intensity together with a shift of the maximum of the signal to higher wavelengths as a function of the noble metal content of the powder. A relevant point concerns the strong effect on the intensity of the signal exerted by the Pd noble metal.…”

Identification and Understanding of Active Sites of Non‐Noble Iron‐Nitrogen‐Carbon Catalysts for Oxygen Reduction Electrocatalysis

“…Raman and infrared (IR) spectroscopy techniques, can be used to distinguish structural information of molecules and further identify the key reaction intermediates and dynamic process on active sites. [140,141] For example, Wan et al [142] clearly revealed the degradation products and the corresponding structural evolution pathways of FePc molecule by in situ Raman spectroscopy. The degradation mechanism between molecular FePc and practical Fe-N-C catalysts was highly similar, and free radical attack was the main cause for the instability of FePc in acidic media.…”

“…Raman and infrared (IR) spectroscopy techniques, can be used to distinguish structural information of molecules and further identify the key reaction intermediates and dynamic process on active sites. [ 140,141 ] For example, Wan et al. [ 142 ] clearly revealed the degradation products and the corresponding structural evolution pathways of FePc molecule by in situ Raman spectroscopy.…”

Operando FTIR study of the photocatalytic reduction of CO2 in the presence of water vapor over Pt/TiO2: on the role of surface residual C-species

“…In efforts at better depicting the photocatalyst at work, in situ or operando measurements are in some cases feasible based on spectroscopic methods such as infrared spectroscopy (which allows CO bond rupture and the formation of CH and CC bonds to be followed). 2,3 Despite numerous studies on photoreduction of CO 2 , answers to many key questions remain unclear, even for the most commonly studied TiO 2 -based photocatalytic systems. Herein, we investigated the surface chemistry evolution and reaction mechanisms occurring on TiO 2 and associated Pt/TiO 2 (2 wt% Pt) in the photoreduction of CO 2 using real-time operando Fourier transform infrared (FTIR) spectroscopy.…”