title: Tracking the Structural and Electronic Configurations of a Cobalt Proton Reduction Catalyst in Water.
abstract: X-ray transient absorption spectroscopy (X-TAS) has been used to study the light-induced hydrogen evolution reaction catalyzed by a tetradentate macrocyclic cobalt complex with the formula [LCo(III)Cl2](+) (L = macrocyclic ligand), [Ru(bpy)3](2+) photosensitizer, and an equimolar mixture of sodium ascorbate/ascorbic acid electron donor in pure water. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analysis of a binary mixture of the octahedral Co(III) precatalyst and [Ru(bpy)3](2+) after illumination revealed in situ formation of a Co(II) intermediate with significantly distorted geometry and electron-transfer kinetics of 51 ns. On the other hand, X-TAS experiments of the complete photocatalytic system in the presence of the electron donor showed the formation of a square planar Co(I) intermediate species within a few nanoseconds, followed by its decay in the microsecond time scale. The Co(I) structural assignment is supported by calculations based on density functional theory (DFT). At longer reaction times, we observe the formation of the initial Co(III) species concomitant to the decay of Co(I), thus closing the catalytic cycle. The experimental X-ray absorption spectra of the molecular species formed along the catalytic cycle are modeled using a combination of molecular orbital DFT calculations (DFT-MO) and finite difference method (FDM). These findings allowed us to assign the full mechanistic pathway, followed by the catalyst as well as to determine the rate-limiting step of the process, which consists in the protonation of the Co(I) species. This study provides a complete kinetics scheme for the hydrogen evolution reaction by a cobalt catalyst, revealing unique information for the development of better catalysts for the reductive side of hydrogen fuel cells.
authors.forename: Dooshaye
authors.initials: D
authors.lastname: Moonshiram
authors.affiliation: Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology , Avinguda Països Catalans 16, 43007 Tarragona, Spain.
authors.forename: Carolina
authors.initials: C
authors.lastname: Gimbert-Suriñach
authors.affiliation: International Research Center "Smart Materials", Southern Federal University , 344090 Rostov-on-Don, Russia.
authors.forename: Alexander
authors.initials: A
authors.lastname: Guda
authors.forename: Antonio
authors.initials: A
authors.lastname: Picon
authors.forename: C Stefan
authors.initials: CS
authors.lastname: Lehmann
authors.forename: Xiaoyi
authors.initials: X
authors.lastname: Zhang
authors.forename: Gilles
authors.initials: G
authors.lastname: Doumy
authors.forename: Anne Marie
authors.initials: AM
authors.lastname: March
authors.affiliation: Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology , Avinguda Països Catalans 16, 43007 Tarragona, Spain.
authors.forename: Jordi
authors.initials: J
authors.lastname: Benet-Buchholz
authors.affiliation: International Research Center "Smart Materials", Southern Federal University , 344090 Rostov-on-Don, Russia.
authors.forename: Alexander
authors.initials: A
authors.lastname: Soldatov
authors.affiliation: Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology , Avinguda Països Catalans 16, 43007 Tarragona, Spain.|Departament de Química, Universitat Autònoma de Barcelona , 08193 Cerdanyola del Vallès, Barcelona, Spain.
authors.forename: Antoni
authors.initials: A
authors.lastname: Llobet
authors.forename: Stephen H
authors.initials: SH
authors.lastname: Southworth