Tracking the Structural and Electronic Configurations of a Cobalt Proton Reduction Catalyst in Water

Abstract

Time resolved X-ray absorption spectroscopy (X-TAS) has been used to study the light induced hydrogen evolution reaction catalyzed by a highly stable tetradentate macrocyclic cobalt complex with the formula [LCo III Cl 2 ] + (L = macrocyclic ligand), [Ru(bpy) 3 ] 2+ photosensitizer and an equimolar mixture of sodium ascorbate/ascorbic acid electron donor in pure water. XANES and EXAFS analysis of a binary mixture of the octahedral Co(III) pre-catalyst 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 timescales. 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 Xray 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 unequivocally 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). 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.

Atomic‐Level Insight into Optimizing the Hydrogen Evolution Pathway over a Co1‐N4 Single‐Site Photocatalyst

“…However, the photocarriers separation are not very efficient in pristine C 3 N 4 . Here, the anchoring of isolated Co atoms and phosphidation would be quite beneficial for electron–hole separation . To confirm this point, steady‐state fluorescence (SSF) and the corresponding time‐resolved fluorescence (TRF) spectra were recorded (Figure c,d).…”

Alternative difference analysis scheme combining R-space EXAFS fit with global optimization XANES fit for X-ray transient absorption spectroscopy

“…Our framework does the fit directly to the difference spectrum instead to the ground state and the excitation state spectrum separately. Moreover, the pre-process of spectrum in EXAFS such as the background subtraction is no need in difference fit as they are same in ground state and (Moonshiram et al, 2016). It would fail when the analogue sample is hard to synthesize or is unstable in the experiment, which occurs in most cases.…”

“…Zhang obtained 0.01 Å high resolution structure change of an excited state of osmium complex (Zhang et al, 2014). Moonshiram revealed intermediate molecule structure of Co(II) and Co(I) photocatalyst in a real H2 production (Moonshiram et al, 2016). Wen studied BiFeO3 nanofilm and found anisotropic unit cell change of in-plane contraction and out-plane extension (Wen et al, 2015).…”

Structure of the CoI Intermediate of a Cobalt Pentapyridyl Catalyst for Hydrogen Evolution Revealed by Time‐Resolved X‐ray Spectroscopy

“…[36][37][38][39][40] The microsecond range is most relevant to studying intermediates of catalysts in multicomponent photocatalytic systems. [41][42][43][44][45][46] Thus, the time-resolved XANES enables the investigation of the structure of the cobalt photocatalytic intermediate. Time-resolved XASm easurements were performed by using ap ump-sequential-probess etup at the Su-perXASb eamline of the Swiss Light Source.…”