Rate Law Analysis of Water Oxidation on a Hematite Surface

Abstract

Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.

Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO2 Photoanode: A Rate Law Analysis

“…36 Similar to this study, a third order reaction was also observed (above a surface hole density threshold). Ma et.…”

“…This novel approach was recently used to examine the reaction kinetics, and thus deduce the rate law with respect to holes, for the photocatalytic oxidation of water on α-Fe2O3 in alkali 36 and on BiVO4 in neutral pH. 37 Using PIAS, we investigate the water oxidation reaction on anatase TiO2 in acid, neutral and alkali pH.…”

“…36,37 A logarithmic plot of photocurrent against the steady-state surface hole density yields a gradient of (the reaction order) and intercept of (the rate constant). The absorption of holes measured by PIAS was converted into a surface hole density (h + .nm -2 ) using the hole extinction coefficient determined from TPC and TAS measurements (εh+ ~2000 M -1 .cm -1 , Figure S15).…”

Rate Law Analysis of Water Oxidation and Hole Scavenging on a BiVO4 Photoanode

“…The initial photocurrent decay most probably derives from a decrease in band bending due to hole accumulation, reducing the photoanode charge separation efficiency, as discussed previously. 11 When the excitation light is switched off, the OD signal decays to zero on the seconds timescale, with the slow timescale of this decay being assigned primarily to slow water oxidation by these surface holes, as discussed further below. In contrast, the current decreases to zero rapidly with no negative current transients, indicative of negligible losses from surface back electron/hole recombination under the strongly anodic bias conditions employed.…”

“…The reaction kinetics were obtained using the rate law analysis employed previously for hematite photoanodes. 11 These data are compared against those obtained using potassium ferrocyanide (K4[Fe(CN)6]) as a facile hole scavenger.…”