Investigation of “Current Doubling” Mechanism of Organic Compounds by the Rotating Ring Disk Electrode Technique

Abstract

The photo‐oxidation mechanisms of methanol, formaldehyde, and formic acid, which are potentially “current‐doubling agents,” were investigated using the RRDE technique. The disk electrode was normaln‐TiO2 and was illuminated by a xenon lamp while the ring electrode was Pt. Hydrogen ion, generated at the disk during the oxidation of water or the organic solute, was observed by its reduction near −1V vs.. SCE at the ring. Formic acid was also reduced to hydrogen at the ring electrode in this potential region. O2 , produced by H2O2 decomposition while in transit from the disk, was reduced at −0.4V vs. SCE at the ring. The experimental results are consistent with initial production of hydroxyl radicals by photogenerated holes, followed by oxidation of the organic solute. One pathway involves injection of an electron into the conduction band resulting in increased photocurrents. Although true current‐doubling (100% increase) was not observed, photocurrent increases of ∼50% were observed.

Boosting the H2 Production Efficiency via Photocatalytic Organic Reforming: The Role of Additional Hole Scavenging System

“…We think that a higher and steady formation of CO 2 radical anions occurred in such a higher concentration of oxalic acid, increasing the current doubling-like effect and, hence, the electrons available for the reduction of protons. A similar behavior has been reported in the literature through 1.6 times enhancement in the photogenerated current of TiO 2 electrode due to a 5-times increase in the concentration of different organic substrates [36].…”

“…Such a deficiency of the current doubling has been previously reported in the case of methanol and has shown to be related to the pH of the solution, the amount of methanol, and the photon flux [35]. Moreover, in another study, the photo-oxidation of different organic substrates using TiO 2 electrodes has never shown a maximum current doubling, rather an excess of ~50% in the photocurrent has been recorded [36]. Other reasons for the deficiency of H 2 are the adsorption of hydrogen on the surface of Pt NPs, retarding the evolution of molecular hydrogen, and/or the solubility of H 2 gas in the aqueous solution.…”

Photocatalytic and photoelectrochemical oxidation mechanisms of methanol on TiO2 in aqueous solution

“…The latter process is referred to as "photocurrent-doubling" and has been observed in many related systems including the photoanodic oxidation of alcohols on TiO 2 electrodes [31][32][33]. However, it was previously noted that the amount of the current doubling for methanol never reaches the same extent as the hole current and that the extent of the photocurrent doubling depends on many factors such as the degree of TiO 2 crystal doping, the methanol concentration, the acidity of the solvent and the light intensity [31,34,35]. For example, low values of the TiO 2 doping level and the solvent acidity were found to lead to a lower extent of the photocurrent doubling [34].…”

Continuous flow gas phase photoreforming of methanol at elevated reaction temperatures sensitised by Pt/TiO2

“…Consistent with the flux-matching mechanism, it is worth noting that the photo-oxidation of methanol (at a TiO2 photoanode), and concomitant dark reduction of water to hydrogen, is a very well-studied process. 29 In the flux-matching mechanism, after the photogeneration of an electron-hole pair (5) on the TiO2 photocatalyst particles , i.e.…”