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Photoinduced Catalysis of Redox Reactions. Turnover Numbers, Turnover Frequency, and Limiting Processes: Kinetic Analysis and Application to Light-Driven Hydrogen Production
journal contribution
posted on 2022-05-11, 12:06 authored by Cyrille Costentin, Fakourou Camara, Jérôme Fortage, Marie-Noëlle CollombThe
energy of light is likely to be used to drive thermodynamically
unfavorable redox reactions with the goal of storing energy in chemical
bonds, for example, via hydrogen production. To this end, molecular
systems involving at least four components (substrate, photosensitizer,
sacrificial donor, and catalyst) are designed as a step toward building
photoelectrochemical devices. The efficiency of such photoinduced
catalysis of redox reactions is often reported as turnover numbers
over time, leading to maximal turnover numbers and initial turnover
frequencies. How these figures of merit are related to the properties
of the system (light absorption, excited state quenching, catalytic
rate constants, back electron transfers, etc.) is however lacking,
thus making reliable comparison of systems difficult. Herein, we propose
a general analytical kinetic framework for the analysis of photoinduced
catalytic processes. In particular, we show that even for ideal systems,
the turnover number does not increase linearly with time due to the
increasing unproductive cycles over time via back electron transfers.
We then incorporate limiting processes corresponding to the photosensitizer
or catalyst degradation in the kinetic analysis, and we provide analytical
expressions for the maximal turnover numbers in such situations. Finally,
the kinetic model is used to successfully rationalize experimental
data corresponding to light-driven hydrogen production in water using
ascorbate as a sacrificial donor, a cobalt tetrazamacrocyclic complex
as a catalyst, and two different photosensitizers, the classical [Ru(bpy)3]2+ and a robust triazatriangulenium organic dye
TATA+.
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water using ascorbatevia hydrogen productiontwo different photosensitizersprovide analytical expressionsleast four componentsincreasing unproductive cyclesexcited state quenchingdriven hydrogen productioncobalt tetrazamacrocyclic complexcatalytic rate constantsback electron transfers2 + supinitial turnover frequenciesmolecular systems involvingphotoinduced catalytic processesmaximal turnover numbersturnover numbersphotoinduced catalysislimiting processesturnover numberturnover frequencysystems difficultideal systemssacrificial donorredox reactionsoften reportedkinetic modelincrease linearlyhowever lackingetc .)chemical bonds