Chemistry of a catalyst surface during catalysis is crucial for a fundamental understanding of mechanism of a catalytic reaction performed on the catalyst in the gas or liquid phase. Due to the pressure- or molecular density-dependent entropy contribution of gas or liquid phase of the reactants and the potential formation of a catalyst surface during catalysis different from that observed in an ex situ condition, the characterization of the surface of a catalyst under reaction conditions and during catalysis can be significant and even necessary for understanding the catalytic mechanism at a molecular level. Electron-based analytical techniques are challenging for studying catalyst nanoparticles in the gas or liquid phase although they are necessary techniques to employ. Instrumentation and further development of these electron-based techniques have now made in situ/operando studies of catalysts possible. New insights into the chemistry and structure of catalyst nanoparticles have been uncovered over the last decades. Herein, the origin of the differences between ex situ and in situ/operando studies of catalysts, and the technical challenges faced as well as the corresponding instrumentation and innovations utilized for characterizing catalysts under reaction conditions and during catalysis, are discussed. The restructuring of catalyst surfaces driven by the pressure of reactant(s) around a catalyst, restructuring in reactant(s) driven by reaction temperature and restructuring during catalysis are also reviewed herein. The remaining challenges and possible solutions are briefly discussed.

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Chemistry and structure of surface of a catalyst under a reaction condition is the crucial information for understanding catalytic mechanism since in many cases an authentic, active surface catalyzing a catalytic reaction is formed in a pretreatment or/and in a reaction between nominal catalyst and reactants. Ambient pressure X-ray photoelectron spectroscopy can be used to track surface of a catalyst under a reaction condition as the instrumentations in last decades have made characterization of catalyst surface in a gas phase at Torr pressure or higher possible. It can characterize surface chemistry of a catalyst including surface composition, surface phase and surface oxygen vacancies and other information under a reaction condition and track their evolutions when the reaction condition is changed to another.

The controllable mass transfer and reaction rate for phase transfer hydrogenation of acetophenone across a well‐defined boundary were investigated. The effect of solvent was found important and 1‐butanol exhibited the best performance among the five investigated homologous alcohol solvents, consistent with its higher solubility in water and greater dielectric constant. Initial reaction rates increased with increasing electric potential, consistent with enhanced mass transfer across the aqueous/organic boundary. At longer reaction times, deactivation was apparent. It correlated with increasing voltage and is ascribed to lower equilibrium concentration of reactive species at the interface. External control over reaction rate was demonstrated by switching the applied electric potential over the course of the reaction. Effects of external electric field on enantioselectivity were also explored with reversal field direction. The changes correlate with catalyst decomposition.