Reactivity of radiolytically- and photochemically-generated tertiary amine radicals towards a CO2 reduction catalyst
Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Brønsted acid/base, and sacrificial electron donor (SED). Tertiary amines, in particular triethylamine (TEA) and triethanolamine (TEOA), are ubiquitously deployed in photocatalysis applications as SEDs, and capable of reductively quenching the PS's excited state. Upon oxidation, TEA and TEOA form TEA•+ and TEOA•+ radical cations, respectively, which decay by proton transfer to generate redox non-innocent transient radicals, TEA• and TEOA•, respectively, with redox potentials that allow them to participate in an additional electron transfer step, thus resulting in net one-photon / two-electron donation. However, the properties of the TEA• and TEOA• radicals are not well understood, including their reducing powers and kinetics of electron transfer to catalysts. Herein, we have used both pulse radiolysis and laser flash photolysis to generate TEA• and TEOA• radicals in CH3CN, and combined with UV/Vis transient absorption and time-resolved mid-infrared (TRIR) spectroscopies, we have probed the kinetics of reduction of the well-established CO2 reduction photocatalyst, fac-ReCl(bpy)(CO)3 (bpy = 2,2'-bipyridine), by these radicals (kTEA• = (4.4 ± 0.3) × 109 M-1 s-1 and kTEOA• = (9.3 ± 0.6) × 107 M-1 s-1). The ~50× smaller rate constant for TEOA• indicates that in contrast to a previous assumption, TEA• is a more potent reductant than TEOA• (by ~0.2 V, as estimated using the Marcus cross relation). This knowledge will aid the design of photocatalytic systems involving SEDs. We also show that TEA can be a useful radiolytic solvent radical scavenger for pulse radiolysis experiments in CH3CN, effectively converting unwanted oxidizing radicals into useful reducing equivalents in the form of TEA• radicals.
- Research Organization:
- Brookhaven National Laboratory (BNL), Upton, NY (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- Grant/Contract Number:
- SC0012704
- OSTI ID:
- 2222380
- Report Number(s):
- BNL-225008-2023-JAAM
- Journal Information:
- The Journal of Chemical Physics, Vol. 159, Issue 24; ISSN 0021--9606
- Country of Publication:
- United States
- Language:
- English
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