Mechanistic Insights into Selective Acetaldehyde Formation from Ethanol Oxidation on Hematite Photoanodes by Operando Spectroelectrochemistry

Abstract

This study employed operando spectroelectrochemical l and photoelectrochemical methods to investigate the charge carrier dynamics of photogenerated holes in hematite for ethanol oxidation and its possible over‐oxidation. Ethanol oxidation was found to form acetaldehyde with around 100 % initial selectivity and faradaic efficiency. The overoxidation of acetaldehyde was suppressed by being unable to kinetically compete with ethanol oxidation in terms of turnover frequency by a factor of ten. Temperature‐dependent rate law analyses were applied to determine the activation energies of these two oxidations. For the ethanol oxidation, the activation energy was 195 meV, compared to 398 meV for acetaldehyde oxidation. These results were correlated with the valence band potential to elucidate the advantage of using hematite for safer and sustainable value‐added aldehyde synthesis compared to the industrial method. The dynamics of ethanol oxidation also addressed the challenges in broad‐spectrum deep oxidation of organic compounds in water purification using metal oxides.

Dynamic charge collecting mechanisms of cobalt phosphate on hematite photoanodes studied by photoinduced absorption spectroscopy

“…2d). 12,29,30 These results suggest that the charging sites of bare Fe 2 O 3 and CoPi/Fe 2 O 3 photoanodes at high potential (1.4 V RHE ) may be mainly on the Fe 2 O 3 surface. Moreover, the similar transient absorption results of bare Fe 2 O 3 and CoPi/Fe 2 O 3 photoanodes further confirm the assignment of same charging sites (Fig.…”

Charge carrier dynamics of surface back electron/hole recombination in BiVO4 and TiO2 photoanodes

“…Previous consideration has been made to the quasi-Fermi level of photogenerated holes altered by the light intensities, resulting from the quantity of surface accumulated holes with different energetics for metal oxide photoanodes. However, in our study of BiVO 4 and TiO 2 , the nearly independent activation energy strongly suggests the insignicant impact of the quasi-Fermi level upon the kinetics of photogenerated holes, particularly in water oxidation, 4,26,29 alcohol oxidation 30,31 and the BER reported herein.…”

ZnSe and CdS Co‐Sensitized TiO2Nanorod Arrays as Photoanodes for Bias‐Free Solar Hydrogen Evolution

“…The flat band potential, charge carrier density and space charge layer thickness of the prepared photoelectrodes were determined using (Figure 7) and the Mott‐Schottky Equation [38] $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {{1}\over{{C}^{2}}}={{2}\over{\varepsilon{} {\varepsilon{} }_{0}{A}^{2}q{N}_{d}}}({E}_{app}-{E}_{fb}-{{{k}_{B}T}\over{q}})\hfill\cr}}$$$ …”

“…The flat band potential, charge carrier density and space charge layer thickness of the prepared photoelectrodes were determined using (Figure 7) and the Mott-Schottky Equation (1). [38] 1…”