Iron-Only Hydrogenase Mimics. Thermodynamic Aspects of the Use of Electrochemistry to Evaluate Catalytic Efficiency for Hydrogen Generation

Abstract

Voltammetry is widely used for the evaluation of iron-only hydrogenase mimics and other potential catalysts for hydrogen generation using various dipolar aprotic solvents. Effective catalysts show enhanced current in the presence of a proton donor at the potential where the catalyst is reduced. To facilitate the comparison of catalytic efficiencies, this paper provides a simple means of calculating the standard potential for reduction of the acid, HA, according to the half reaction 2HA + 2e- <==> H2 + 2A-. This standard potential depends on the pKa of HA in the solvent being used. It is thermodynamically impossible for reduction of HA to occur at less negative potentials than the standard potential, and the most effective catalysts will operate at potentials as close as possible to the standard potential. In addition, direct reduction of HA at the electrode will compete with the catalyzed reduction, thus complicating evaluation of the rate of the catalyzed reaction. Glassy carbon electrodes, commonly used in such evaluations, show a quite large overpotential for direct reduction of HA so that the necessary corrections are small. However, catalysis at very negative potentials will be contaminated by significant direct reduction of HA at glassy carbon. It is demonstrated that direct reduction can be almost completely suppressed by using a mercury or amalgamated gold electrode, even at very negative potentials.

Singly versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen‐Evolution Reaction

“…These results indicated that acetic acid was not strong enough to protonate the Ni center of [ Ni‐P ] − , and catalytic H 2 evolution was initiated upon 2 e − reduction to [ Ni‐P ] 2− . Importantly, upon the addition of TFA (p K a =12.7 in acetonitrile),46 the first reduction wave exhibited pronounced catalytic activity (Figure 2 B). At low TFA concentrations, the catalytic current i c increased linearly with the acid concentration (see Figure S6), but reached an acid‐independent region with more than 108 m m of TFA (see Figure S4).…”

Electrocatalytic Hydrogen Production by an Aluminum(III) Complex: Ligand‐Based Proton and Electron Transfer

“…With excess lutH + ,the current density increased to j c /j p = 10; this result suggested that ac atalytic event had occurred ( Figure 2a). [15] Controlled potential electrolysis (CPE) experiments performed with 0.3 mm 1 and 20 equivalents of acid at À1.16 Vo ver 1.5 h ( Table 1a nd Figure S1 in the Supporting Information) indicated that H 2 was produced with 70 or 85 %F aradaic efficiency( FE) using lutH + or 4-dimethylaminopyridinium (DMAPH + ), respectively.Q uantification of H 2 was performed by using gas chromatography with at hermal conductivity detector (GC-TCD) on samples taken from the headspace of the CPE experiment. To [12] Ar = diisopropylphenyl.…”

“…Our previous reports on the [c] Thermodynamic potentialf or H + reduction calculated from pK a in THF (see the SupportingInformation for details). [15][16][17] [d] With respect to E cat/2 for reduction of the catalyst. [18] [e] With respect to applied potential of the CPE experiment: À1.16 Vfor 1 and À1.22 Vf or 2. .…”

Wasserspaltung mit Cobalt

“…Als letzte Klasse wären noch Organometallverbindungen mit Cyclopentadienyl- (30)(31)(32)(33) [63,65,66] oder Diphosphanliganden (34) [67,68] [72] Somit erhält man ein Maß für den Energieaufwand, der betrieben werden muss, damit die Reaktion mit einer signifikanten Geschwindigkeit ablaufen kann. [72][73][74] Der zweite charakteristische Parameter eines Katalysators ist seine Umsatzfrequenz ("turnover frequency", TOF). Weil der Strom an einer Elektrode mit der Zahl an Elektronen korreliert, die pro Sekunde mit den redoxaktiven Verbindungen in Lösung -in diesem Fall mit dem Katalysatorausgetauscht werden, besteht eine direkte Beziehung zwischen der Umsatzfrequenz und dem cyclovoltammetrisch gemessenen Katalysestrom.…”