skip to main content

Title: The roles of chalcogenides in O 2 protection of H 2 ase active sites
At some point, all HER (Hydrogen Evolution Reaction) catalysts, important in sustainable H 2 O splitting technology, will encounter O 2 and O 2 -damage. The [NiFeSe]-H 2 ases and some of the [NiFeS]–H 2 ases, biocatalysts for reversible H 2 production from protons and electrons, are exemplars of oxygen tolerant HER catalysts in nature. In the hydrogenase active sites oxygen damage may be extensive (irreversible) as it is for the [FeFe]–H 2 ase or moderate (reversible) for the [NiFe]–H 2 ases. The affinity of oxygen for sulfur, in [NiFeS]–H 2 ase, and selenium, in [NiFeSe]–H 2 ase, yielding oxygenated chalcogens results in maintenance of the core NiFe unit, and myriad observable but inactive states, which can be reductively repaired. In contrast, the [FeFe]–H 2 ase active site has less possibilities for chalcogen-oxygen uptake and a greater chance for O 2 -attack on iron. Exposure to O 2 typically leads to irreversible damage. Despite the evidence of S/Se-oxygenation in the active sites of hydrogenases, there are limited reported synthetic models. This perspective will give an overview of the studies of O 2 reactions with the hydrogenases and biomimetics with focus on our recent studies that compare sulfur and selenium containing more » synthetic analogues of the [NiFe]–H 2 ase active sites. « less
Authors:
;
Award ID(s):
1665258
Publication Date:
NSF-PAR ID:
10199349
Journal Name:
Chemical Science
Volume:
11
Issue:
35
Page Range or eLocation-ID:
9366 to 9377
ISSN:
2041-6520
Sponsoring Org:
National Science Foundation
More Like this
  1. A biomimetic study for S/Se oxygenation in Ni(μ-EPh)(μ-SN 2 )Fe, (E = S or Se; SN 2 = Me-diazacycloheptane-CH 2 CH 2 S); Fe = (η 5 -C 5 H 5 )Fe II (CO) complexes related to the oxygen-damaged active sites of [NiFeS]/[NiFeSe]-H 2 ases is described. Mono- and di-oxygenates (major and minor species, respectively) of the chalcogens result from exposure of the heterobimetallics to O 2 ; one was isolated and structurally characterized to have Ni–O–Se Ph –Fe–S connectivity within a 5-membered ring. A compositionally analogous mono-oxy species was implicated by ν (CO) IR spectroscopy to be the corresponding Ni–O–S Ph –Fe–S complex; treatment with O-abstraction agents such as P( o -tolyl) 3 or PMe 3 remediated the O damage. Computational studies (DFT) found that the lowest energy isomers of mono-oxygen derivatives of Ni(μ-EPh)(μ-SN 2 )Fe complexes were those with O attachment to Ni rather than Fe, a result consonant with experimental findings, but at odds with oxygenates found in oxygen-damaged [NiFeS]/[NiFeSe]-H 2 ase structures. A computer-generated model based on substituting − SMe for the N-CH 2 CH 2 S − sulfur donor of the N 2 S suggested that constraint within the chelate hindered O-atom uptake at thatmore »sulfur site.« less
  2. Metal-ligand cooperativity is an essential feature of bioinorganic catalysis. The design principles of such cooperativity in metalloenzymes are underexplored, but are critical to understand for developing efficient catalysts designed with earth abundant metals for small molecule activation. The simple substrate requirements of reversible proton reduction by the [NiFe]-hydrogenases make them a model bioinorganic system. A highly conserved arginine residue (R355) directly above the exogenous ligand binding position of the [NiFe]-catalytic core is known to be essential for optimal function because mutation to a lysine results in lower catalytic rates. To expand on our studies of soluble hydrogenase-1 from Pyrococcus furiosus (Pf SH1), we investigated the role of R355 by site-directed-mutagenesis to a lysine (R355K) using infrared and electron paramagnetic resonance spectroscopic probes sensitive to active site redox and protonation events. It was found the mutation resulted in an altered ligand binding environment at the [NiFe] centre. A key observation was destabilization of the Nia3+-C state, which contains a bridging hydride. Instead the tautomeric Nia+-L states were observed. Overall, the results provided insight into complex metal-ligand cooperativity between the active site and protein scaffold that modulates the bridging hydride stability and the proton inventory, which should prove valuable to design principlesmore »for efficient bioinspired catalysts.« less
  3. Electrochemical synthesis of hydrogen peroxide (H 2 O 2 ) in acidic solution can enable the electro-Fenton process for decentralized environmental remediation, but robust and inexpensive electrocatalysts for the selective two-electron oxygen reduction reaction (2e − ORR) are lacking. Here, we present a joint computational/experimental study that shows both structural polymorphs of earth-abundant cobalt diselenide (orthorhombic o -CoSe 2 and cubic c -CoSe 2 ) are stable against surface oxidation and catalyst leaching due to the weak O* binding to Se sites, are highly active and selective for the 2e − ORR, and deliver higher kinetic current densities for H 2 O 2 production than the state-of-the-art noble metal or single-atom catalysts in acidic solution. o -CoSe 2 nanowires directly grown on carbon paper electrodes allow for the steady bulk electrosynthesis of H 2 O 2 in 0.05 M H 2 SO 4 with a practically useful accumulated concentration of 547 ppm, the highest among the reported 2e − ORR catalysts in acidic solution. Such efficient and stable H 2 O 2 electrogeneration further enables the effective electro-Fenton process for model organic pollutant degradation.
  4. Platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) with atomically dispersed FeN 4 sites have emerged as a potential replacement for low-PGM catalysts in acidic polymer electrolyte fuel cells (PEFCs). In this work, we carefully tuned the doped Fe content in zeolitic imidazolate framework (ZIF)-8 precursors and achieved complete atomic dispersion of FeN 4 sites, the sole Fe species in the catalyst based on Mößbauer spectroscopy data. The Fe–N–C catalyst with the highest density of active sites achieved respectable ORR activity in rotating disk electrode (RDE) testing with a half-wave potential ( E 1/2 ) of 0.88 ± 0.01 V vs. the reversible hydrogen electrode (RHE) in 0.5 M H 2 SO 4 electrolyte. The activity degradation was found to be more significant when holding the potential at 0.85 V relative to standard potential cycling (0.6–1.0 V) in O 2 saturated acid electrolyte. The post-mortem electron microscopy analysis provides insights into possible catalyst degradation mechanisms associated with Fe–N coordination cleavage and carbon corrosion. High ORR activity was confirmed in fuel cell testing, which also divulged the promising performance of the catalysts at practical PEFC voltages. We conclude that the key factor behind the high ORR activity ofmore »the Fe–N–C catalyst is the optimum Fe content in the ZIF-8 precursor. While too little Fe in the precursors results in an insufficient density of FeN 4 sites, too much Fe leads to the formation of clusters and an ensuing significant loss in catalytic activity due to the loss of atomically dispersed Fe to inactive clusters or even nanoparticles. Advanced electron microscopy was used to obtain insights into the clustering of Fe atoms as a function of the doped Fe content. The Fe content in the precursor also affects other key catalyst properties such as the particle size, porosity, nitrogen-doping level, and carbon microstructure. Thanks to using model catalysts exclusively containing FeN 4 sites, it was possible to directly correlate the ORR activity with the density of FeN 4 species in the catalyst.« less
  5. Electrochemical conversion of CO 2 into value-added chemicals continues to draw interest in renewable energy applications. Although many metal catalysts are active in the CO 2 reduction reaction (CO 2 RR), their reactivity and selectivity are nonetheless hindered by the competing hydrogen evolution reaction (HER). The competition of the HER and CO 2 RR stems from the energy scaling relationship between their reaction intermediates. Herein, we predict that bimetallic monolayer electrocatalysts (BMEs) – a monolayer of transition metals on top of extended metal substrates – could produce dual-functional active sites that circumvent the scaling relationship between the adsorption energies of HER and CO 2 RR intermediates. The antibonding interaction between the adsorbed H and the metal substrate is revealed to be responsible for circumventing the scaling relationship. Based on extensive density functional theory (DFT) calculations, we identify 11 BMEs which are highly active and selective toward the formation of formic acid with a much suppressed HER. The H–substrate antibonding interaction also leads to superior CO 2 RR performance on monolayer-coated penta-twinned nanowires.