Hydrogen Generation from Weak Acids:  Electrochemical and Computational Studies of a Diiron Hydrogenase Mimic

Abstract

Extended investigation of electrocatalytic generation of dihydrogen using [(mu-1,2-benzenedithiolato)][Fe(CO)3]2 has revealed that weak acids, such as acetic acid, can be used. The catalytic reduction producing dihydrogen occurs at approximately -2 V for several carboxylic acids and phenols resulting in overpotentials of only -0.44 to -0.71 V depending on the weak acid used. This unusual catalytic reduction occurs at a potential at which the starting material, in the absence of a proton source, does not show a reduction peak. The mechanism for this process and structures for the intermediates have been discerned by electrochemical and computational analysis. These studies reveal that the catalyst is the monoanion of the starting material and an ECEC mechanism occurs.

Kinetic and thermodynamic aspects of the electrocatalysis of acid reduction in organic solvent using molecular diiron-dithiolate compounds

“…Diiron-dithiolate compounds of the type [Fe 2 (µ-SRS)(CO) 6-x L x ] (R = organic group, L = electron-donor ligand, x ≤ 4) have been shown to electrocatalyze the reduction of acid in organic solvents [19][20][21] and, very recently, in aqueous micellar solutions [22,23](for examples of photo-driven H 2 -production by diiron-dithiolate compounds see [24,25]). The electrocatalytic pathway entails successive electron and proton transfers in a sequence that depends on the nature of the terminal ligand L and the strength of the acid used as proton source [16,26] We [27][28][29][30][31][32], and others [33][34][35][36][37] [27,28].…”

“…The two-electron one-proton intermediate 1H − must hence be further reduced to produce H 2 at an appreciable rate, but with a large thermodynamic cost of about 0.8 V [33]. In an attempt to obtain diiron-dithiolate electrocatalysts achieving balanced basicity and reduction potential, we focused on the mono-substituted derivative [Fe 2 (µ-bdt)(CO) 5 (P(OMe) 3 )] (2, Scheme 1) [29].…”

Bilayer Vesicles as a Noncovalent Immobilization Platform of Electrocatalysts for Energy Conversion in Neutral Aqueous Media

“…The results suggested that immobilized 1 on CB via DDAB liposomes underwent a quasi-reversible electrochemical process. CVs collected in Na 2 SO 4(aq) solution showed a redox 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 behavior similar to that in CH 3 CN solution [14] (Figure S7), and thus the reduction wave in aqueous solution was assigned to a two-electron reduction process.…”

“…The large deviation of a from 0.5 implies the force constants for the reactant and the product are unequal (according to the asymmetric Marcus theory), [16] which is in agreement with substantial structural changes upon reduction of 1 to 1 2À . [14] The apparent heterogeneous electron transfer rate constant (k s ) was calculated to be 1.33 AE 0.15 s À1 (see Supporting Information). The k s of 1@DDAB/CB was comparable with values of a range of electrodes modified with electroactive species in literature (Table S2).…”

Homogeneous ligand-centered hydrogen evolution and hydrogen oxidation : exploiting redox non-innocence to drive catalysis.

“…At acid concentrations above 0.4 M, the current is acidindependent indicating the solution is acid saturated, and that CV response is no longer limited by diffusion (Figure 12;inset). 121,[163][164][165] Under these pseudo-first order conditions, the turnover frequency (TOF), which is also the observed rate constant, is 30 ± 4 s -1 .…”