Minimal Proton Channel Enables H₂ Oxidation and Production with a Water-Soluble Nickel-Based Catalyst

Abstract

Hydrogenase enzymes use first-row transition metals to interconvert H2 with protons and electrons, reactions that are important for the storage and recovery of energy from intermittent sources such as solar, hydroelectric, and wind. Here we present Ni(P(Cy)2N(Gly)2)2, a water-soluble molecular electrocatalyst with the amino acid glycine built into the diphosphine ligand framework. Proton transfer between the outer coordination sphere carboxylates and the second coordination sphere pendant amines is rapid, as observed by cyclic voltammetry and FTIR spectroscopy, indicating that the carboxylate groups may participate in proton transfer during catalysis. This complex oxidizes H2 (1-33 s(-1)) at low overpotentials (150-365 mV) over a range of pH values (0.1-9.0) and produces H2 under identical solution conditions (>2400 s(-1) at pH 0.5). Enzymes employ proton channels for the controlled movement of protons over long distances-the results presented here demonstrate the effects of a simple two-component proton channel in a synthetic molecular electrocatalyst.

Amino acid modified Ni catalyst exhibits reversible H 2 oxidation/production over a broad pH range at elevated temperatures

“…The carboxyl group of the amino acid plays an important role in the catalytic mechanism. As discussed previously (17,18), the introduction of a carboxylic acid in the outer coordination sphere results in rapid proton transfer, a critical component in eliminating the overpotentials required for reversibility. The endo positioned carboxyl group may also play a role in stabilizing the dihydrogen intermediate, than CyArgOMe for H 2 oxidation and 8 times faster for H 2 production, coupled with the observation that H 2 addition is rate limiting, indicates that the OH group of the carboxyl substituent facilitates H-H bond cleavage as shown in structure 3.…”

“…1) (17,18). Both of these complexes are active, water-soluble H 2 oxidation catalysts with TOFs up to 200 s −1 at 1 atm H 2 and 298 K, operating at low overpotentials (less than 180 mV) (17,18). Unexpectedly, catalysis was fastest under acidic conditions (from pH = 0 to pH = 1) (17,18).…”

“…Both complexes also demonstrated impressive TOFs for H 2 oxidation as a function of pressure, up to 144,000 s −1 at 133 atm H 2 , 298 K, and pH = 0.5, albeit at reduced energy efficiency (overpotential = 460 mV) (18). The carboxyl functional group in both complexes is thought to play a key role by coupling proton and electron transfer, facilitated by the ability of the COOH group to deprotonate the pendant amine (17,18). The CyArg derivative operates at similar overpotentials but with TOFs ∼6 times faster than those of the CyGly catalyst.…”

“…amino acid is a member of the P 2 N 2 ring ( Fig. 1) (17,18). Both of these complexes are active, water-soluble H 2 oxidation catalysts with TOFs up to 200 s −1 at 1 atm H 2 and 298 K, operating at low overpotentials (less than 180 mV) (17,18).…”

“…Both of these complexes are active, water-soluble H 2 oxidation catalysts with TOFs up to 200 s −1 at 1 atm H 2 and 298 K, operating at low overpotentials (less than 180 mV) (17,18). Unexpectedly, catalysis was fastest under acidic conditions (from pH = 0 to pH = 1) (17,18). Both complexes also demonstrated impressive TOFs for H 2 oxidation as a function of pressure, up to 144,000 s −1 at 133 atm H 2 , 298 K, and pH = 0.5, albeit at reduced energy efficiency (overpotential = 460 mV) (18).…”