Selecting between two transition states by which water oxidation intermediates decay on an oxide surface

Abstract

Although catalytic mechanisms on electrode surfaces have been proposed for decades, the pathways by which the product’s chemical bonds evolve from the initial charge-trapping intermediates have not been resolved. Here, we discover a reactive intermediate population with states in the middle of a semiconductor’s bandgap that reveal the dynamics of two parallel transition state pathways for their decay. After phototriggering the water oxidation reaction from the n-SrTiO3 surface, the microsecond decay of the intermediates affirms transition state theory through two distinct time constants, the primary kinetic salt and H/D kinetic isotope effects, realistic activation barrier heights and transition state theory pre-factors. Furthermore, we show that the reaction conditions can be adjusted to allow selection between the two pathways, one characterized by a labile intermediate facing the electrolyte (the oxyl), and the other by a lattice oxygen (the bridge). In summary, we experimentally isolate an important activation barrier in multi-electron transfer water oxidation and, in doing so, identify competing mechanisms for O2 evolution at surfaces.

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Fig. 1: Spectra and kinetics of the oxyl and bridge radicals probed through subsurface vibration and the mid-gap electronic levels.
Fig. 2: The pH dependence and H/D KIE at the same ion density (0.2 M [Na+]).
Fig. 3: Ionic strength and temperature dependence at constant pH.
Fig. 4: Two competing reaction pathways.

Data availability

The representative data and all of the analysis from the extended dataset that support the findings of this paper are available in the paper and the Supplementary Information. The extended dataset that supports the findings in this paper is available from the corresponding author on reasonable request.

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Acknowledgements

The experimental work was supported by the Director, Office of Science, Office of Basic Energy Sciences, and by the Division of Chemical Sciences, Geosciences and Biosciences of the US Department of Energy at LBNL under contract no. DE-AC02–05CH11231. We thank H. Frei and J. Eaves for fruitful discussions.

Author information

T.C. and X.C. conceived the project and T.C. wrote the manuscript with input from all authors. X.C and D.J.A constructed the transient set-up, collected transient data and prepared samples. X.C and T.C. analysed the transient data.

Correspondence to Tanja Cuk.

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Supplementary Information

Supplementary Figs. 1–17, Tables 1–5, Notes 1–5 and references.

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Chen, X., Aschaffenburg, D.J. & Cuk, T. Selecting between two transition states by which water oxidation intermediates decay on an oxide surface. Nat Catal 2, 820–827 (2019). https://doi.org/10.1038/s41929-019-0332-5

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