The potential contribution of Ag + reaction with holes (i.e., MPEO) can be investigated by employing a biased WO 3 photoanode, since the positive potential bias can suppress the electron transfer to Ag + (retarding recombination with hole), but maximize the chance of the hole transfer to Ag +. The presence of Ag + (as AgNO 3) increases the hole lifetime and hence its reactivity 27, as its reduction potential ( E° = + 0.80 V SHE) 13 is positive enough to scavenge the photogenerated CB electrons 28. However, the water oxidation efficiency is limited by its sluggish kinetics that renders a partial oxidation of water to adsorbed peroxo species, eventually causing surface deactivation 26. 2.6–2.7 eV), high oxidation potential of photogenerated holes, and remarkable stability to photocorrosion in acidic conditions (pH < 4) 22, 23, 24, 25. WO 3 is a common n-type semiconductor that has been actively investigated for the photooxidation of water, owing to its visible light activity (bandgap ca. Here we show an example of successful mediated photoelectrocatalytic oxidation (MPEO) of water using a biased WO 3 mesoporous electrode in the presence of Ag +, under the conditions of a mild acidic nitrate solution, zero overpotential, and simulated solar light to photogenerate Ag 2+ that enhances water oxidation via a reversible homogeneous redox cycle of Ag 2+/Ag +. This limits its practical application, as it requires extreme acidic conditions on the one hand, and high applied potentials on the other. 1– 4) 7, 11:Īs the stability of the complex is paramount for the continuous operation of the above mechanism, rather high concentrations of nitric acid (1–10 M) have been used to prevent the depletion of Ag(II) 17, 21, which is normally generated using electrodes with large overpotentials for water oxidation (Pt, boron-doped diamond, etc.). Nitrate has been frequently used because of its high solubility, low viscosity, and stability 7 it generates Ag IINO 3 +, a dark brown complex 17, 20, 21 that can promote homogeneous water oxidation (Eqs. However, Ag(II) is so reactive that it needs to be stabilized using nitrate 7, 8, 9, 11, 14, 15, 16, perchlorate 14, 17, 18, sulfate 9, 16, or phosphate 19 as complexing ligands. Ag(II)/Ag(I) couple is the best candidate for the mediated electrocatalytic oxidation, as its redox potential is very positive in acidic media ( E° = + 1.98 V SHE Supplementary Table 1) 13. Recently, homogeneous Ag(I) complex ions of AgCl 2 − and AgCl 3 2− were further utilized for electrochemical oxidation of chloride to chlorine 10. It has been used for the degradation of a variety of recalcitrant organic and inorganic compounds 7, 8, 9, 10, 11, 12, as well as for water oxidation as a side process 7, 8, 9, 12. The mediated electrocatalytic oxidation is based on the electrochemical cycling of highly reactive redox shuttles (e.g., Ag 2+/Ag +, Co 3+/Co 2+, Ce 4+/Ce 3+, and Mn 3+/Mn 2+) that can easily oxidize many contaminants in a continuous cycle, yielding minimum byproducts 7, 8. We have noted a potential role of mediated electrocatalytic oxidation involving Ag(II)/Ag(I) redox couple in photocatalytic (PC) and photoelectrocatalytic (PEC) processes. Regarding the effects of Ag(I), some concerns on its role as a water oxidation promoter through the formation of Ag(II) species (i.e., acting as a hole scavenger) have been raised 6 yet, to date no direct photo(electro)catalytic evidence has been reported supporting the claims. For scavenging CB electrons, the former case commonly uses Ag(I) ions as an electron scavenger to promote oxidative reaction pathways 4, whereas the latter case applies a potential that is more positive than the onset potential to draw electrons out of the photoelectrode 5. The selective use of photogenerated charge carriers to induce either substrate oxidation with valence band (VB) holes or reduction with conduction band (CB) electrons on the surface of an irradiated photocatalyst can be generally achieved by suppressing their mutual recombination 1, 2, mainly through the addition of sacrificial scavengers of electrons or holes and the application of an external potential that depletes one of the carriers from the photocatalyst 3.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |