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

Journal Article · Tue Aug 09 00:00:00 EDT 2016 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/jacs.6b05680· OSTI ID:1390809

Moonshiram, Dooshaye; Gimbert-Suriñach, Carolina ^[1]; Guda, Alexander ^[2]; Picon, Antonio; Lehmann, C. Stefan; Zhang, Xiaoyi; Doumy, Gilles; March, Anne Marie; Benet-Buchholz, Jordi ^[1]; Soldatov, Alexander ^[2]; Llobet, Antoni ^[3]; Southworth, Stephen H.

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1. Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology, Avinguda Països Catalans 16, 43007 Tarragona, Spain
2. International Research Center “Smart Materials”, Southern Federal University, 344090 Rostov-on-Don, Russia
3. 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

Time resolved X-ray absorption spectroscopy (X-TAS) has been used to study the light induced hydrogen evolution reaction catalyzed by a highly stable cobalt complex, [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 square pyramidal Co(II) intermediate, with 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 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 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.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• DOE Contract Number: AC02-06CH11357
• OSTI ID: 1390809
• Journal Information: Journal of the American Chemical Society, Vol. 138, Issue 33; ISSN 0002-7863
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English