The photoexcitation of plasmonic nanoparticles has been shown to drive multistep, multicarrier transformations, such as the conversion of CO2 into hydrocarbons. But for such plasmon-driven chemistry to be precisely understood and modeled, the critical photoinitiation step in the reaction cascade must be identified. We meet this goal by measuring H/D and C-12/C-13 kinetic isotope effects (KIEs) in plasmonic photosynthesis. In particular, we found that the substitution of H2O with D2O slows hydrocarbon production by a factor of 5-8. This primary H/D KIE leads to the inference that holedriven scission of the O-H bond in H2O is a critical, limiting step in plasmonic photosynthesis. This study advances mechanistic understanding of light-driven chemical reactions on plasmonic nanoparticles.
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