A unified model for surface electrocatalysis based on observations with enzymes

Abstract

Despite being so large, many enzymes are not only excellent electrocatalysts - making possible chemical transformations under almost reversible conditions - but they also facilitate our understanding of electrocatalysis by allowing complex processes to be dissected systematically. The electrocatalytic voltammograms obtained for enzymes attached to an electrode expose fundamental aspects of electrocatalysis that can be addressed in ways that are not available to conventional molecular or surface electrocatalysts. The roles of individual components, each characterisable by diffraction or spectroscopy, can be tested and optimised by genetic engineering. Importantly, unlike small-molecule electrocatalysts (RMM < 1000) that are structurally well-defined but invariably altered by being attached to a surface, the enzyme is a giant, multi-component assembly in which the active site is buried and relatively insensitive to the presence of the electrode and solvent interface. A central assertion is that for a given driving force (electrode potential) a true catalyst has no influence on the direction of the reaction; consequently, 'catalytic bias', i.e. the common observation that an enzyme or indeed any electrocatalyst operates preferentially in one direction, must arise from secondary effects beyond the elementary catalytic cycle. This Perspective highlights and extends a general model for electrocatalysis by surface-confined enzymes, and explains how two secondary effects control the bias: (i) the electrode potential at which electrons enter or leave the catalytic cycle; (ii) potential-dependent interconversions between states of the catalyst differing in catalytic activity due to changes in the composition and arrangements of atoms. The model, which is easily applied to enzymes that have been studied recently, highlights important considerations for understanding and developing surface-confined electrocatalysts.

Carbon‐Nanotube‐Supported Bio‐Inspired Nickel Catalyst and Its Integration in Hybrid Hydrogen/Air Fuel Cells

“…Thestrong influence of the pH on this redox system is consistent with the behavior reported in solution. [31] Figure 2C shows the faradaic contribution of CV scans performed under H 2 at pH 0.3, 2, 4, and 6, obtained after background subtraction to remove the capacitive contribution of CNTs.Atheoretical model was developed by Armstrong et al for the study of reversible hydrogen catalysis for hydrogenases, [32,33] but this model has not yet been applied to molecular systems.W es uccessfully modeled the electrocatalytic behavior of 1-f-SWCNT using equations developed in ref. This pH dependence accounts for aP CET process involving a2 e À /2 H + mechanism.…”

Influence of haem environment on the catalytic properties of the tetrathionate reductase TsdA from Campylobacter jejuni

“…In contrast, the Cj TsdA N254G variant exhibited lower specific activity in the oxidative direction than the wild-type enzyme and proved more adapted to catalysing the tetrathionate-reducing direction in electrochemical experiments. These differences might be explicable by altered redox potentials of Haem 2, but more subtle changes in intramolecular electron transfer or reorganization energy might also apply [35,36]. …”

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“…[24][25][26] Figure 4A displays the simulated curve obtaineda fter fitting of the CV curve for MWCNT,A E-MWCNT and CN-MWCNTsu nder O 2 at pH 5. [24][25][26] Figure 4A displays the simulated curve obtaineda fter fitting of the CV curve for MWCNT,A E-MWCNT and CN-MWCNTsu nder O 2 at pH 5.…”