Mapping the Ultrafast Mechanistic Pathways of Co Photocatalysts in Pure Water through Time-Resolved X-ray Spectroscopy

Abstract

Nanosecond time-resolved X-ray (tr-XAS) and optical transient absorption spectroscopy (OTA) are applied to study 3 multimolecular photocatalytic systems with [Ru(bpy)₃ ]²⁺ photoabsorber, ascorbic acid electron donor and Co catalysts with methylene (1), hydroxomethylene (2) and methyl (3) amine substituents in pure water. OTA and tr-XAS of 1 and 2 show that the favored catalytic pathway involves reductive quenching of the excited photosensitizer and electron transfer to the catalyst to form a Co^II square pyramidal intermediate with a bonded aqua molecule followed by a Co^I square planar derivative that decays within ≈8 μs. By contrast, a Co^I square pyramidal intermediate with a longer decay lifetime of ≈35 μs is formed from an analogous Co^II geometry for 3 in H₂ O. These results highlight the protonation of Co^I to form the elusive hydride species to be the rate limiting step and show that the catalytic rate can be enhanced through hydrogen containing pendant amines that act as H-H bond formation proton relays.

Keywords: Co hydrogen evolution catalysts; Optical transient absorption spectroscopy; TD-DFT calculations; Time-resolved X-ray absorption spectroscopy.