Molecular catalysts offer a potentially high activity and selectivity for solar fuels production. Rational design of catalysts requires mechanistic understanding beyond overall performance measurements and benchmarking. By time-resolved laser spectroscopy reaction intermediates can be identified and their rates of interconversion can be determined. We have used a laser flash-quench method with time-resolved UV/VIS and mid-IR spectroscopy to induce rapid (~100 ns) reduction and protonation steps of [FeFe]-hydrogenase mimics. In this way we can control single-electron reduction on a short time scale, and capture reactive intermediates that are not easily accessible by e.g. electrochemical methods. From the transient mid-IR spectra and comparison with calculated spectra, we could determine the structure of several intermediates of a photocatalytic proton reduction cycle. The results also show that the photocatalytic mechanism can be different from electrocatalytic mechanisms for the same catalyst, which illustrates the need for mechanistic understanding and underscores the problem of meaningful benchmarking of catalyst performance. The combination of ultra-fast UV-VIS and mid-IR spectroscopy is useful also to follow reaction steps of molecular dyes and catalysts immobilized on semi-conductor surfaces. We have been able to follow light-induced hole- and electron injection into the semi-conductor, ultrafast electron hopping from dye to catalysts on a NiO surface, as well as several steps in the photoreduction cycle of CO2-reducing catalysts.

Capturing intermediates of molecular solar fuels catalysts by femto- and nanosecond mid-IR spectroscopy

252nd National Meeting (2016)

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