Kinetics and Mechanisms of Proton Reduction

Background

For all the above-mentioned sections of artificial photosynthesis, we study with various analytical methods and multiple collaborations the kinetics of H2 formation and the underlying mechanisms. Beyond “it works”, progress in catalyst development and in all other chemically active components is only possible based on an in depth understanding of kinetic and thermodynamic data as well as structural feature involved in the cycles. Ultra-fast time-resolved spectroscopy is a major tool but electrochemical data are equally important. We include stopped-flow techniques where appropriate but also ATR-IR spectroscopy. A number of our cycles could be quantified and supported to alter catalysts or photosensitizers for improved H2 formation. We could for instance clearly show that in catalysis with ascorbic acid, its oxidized form shortcuts the process by electron back-transfer. Thus, the TONs previously obtained did not mirror the limiting factors of the catalyst but rather the situation in which the forward reaction was counter-balanced by the back-reaction. This allowed us for instance to mutate ascorbic acid / dehydroascorbate into an electron relay rather than an SED. Important to note that all studies are done by every student in a highly interdisciplinary fashion, involving syntheses of new catalysts, system development and mechanistic understanding. We therefore do not have projects in electron relays separately from e.g. WRC synthesis.

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Figure 1. FT-IR spectra of the CO bands in [Re(py)(bypy)(CO)3]+ after excitation with a 400 nm laser pulse 

  Figure 2. Transient kinetic trace of reductive quenching of excited [ReBr(bpy)(CO)3], the slow increase at t>200 ns is indicative for 2nd electron transfer from singly oxidized TEAO

Current projects

All our catalytic systems are studied with the methods mentioned before in order to reveal quantitatively the elementary steps involved in H2 formation. To identify species and long-lived intermediates, we are also applying Co-57 as a tracer on very low concentration levels. This information is needed for feedback into the synthetic and kinetic part of the overall project.

Coworkers

Johannes Windisch, Matthias Mosberger, Stephan Schnidrig, Nico Weder, (PhD students) Dr. Benjamin Probst, Margherita Orazietti, Prof. Peter Hamm,

Selected publications

  • Photocatalytic H2 Production from Water with Rhenium and Cobalt Complexes. Probst, B., Guttentag, M., Rodenberg, A., Hamm, P. and Alberto, R. (2011): Inorg. Chem. 50, 3404-3412.
  • Mechanism of Photocatalytic Hydrogen Generation by a Polypyridyl-Based Cobalt Catalyst in Aqueous Solution.Rodenberg, A., Orazietti, M., Probst, B., Bachmann, C., Alberto, R., Baldridge, K. K. and Hamm, P. (2015): Inorg. Chem. 54, 646-657.
  • A highly stable polypyridyl-based cobalt catalyst for homo- and heterogeneous photocatalytic water reduction.Guttentag, M., Rodenberg, A., Bachmann, C., Senn, A., Hamm, P. and Alberto, R. (2013): Dalton Trans. 42, 334-337.