Unprecedented large solvent (H₂O vs D₂O) isotope effect in semiconductors photooxidation

Abstract

Glucose in water (L 2 O, L = H or D) at pH = 7 (phosphate buffer) is oxidized in presence of Bi 2 WO 6 and light. An unusually large solvent isotope effects, k H 2 O =k D 2 O ¼ 7:8 and 6.8, have been measured using solar light and solar simulator, respectively. These large values come from the contribution of the equilibrium L 2 O ⇄ L + + OL − previous to the rate-limiting step (rls) and the kinetic one (proton transfer) involved at the transition state. The reaction is faster when [L + ] increases; therefore, L + species instead of OL − one participates in the photooxidation. The rls is the 1 e − reduction-adsorption of H + on the Bi 2 WO 6 surface (Bi 2 WO 6 /Bi 2 WO 6 -H, −0.6 V vs NHE). Subsequently, O 2 reduction to L 2 O, as driving force, occurs at the catalyst conduction band (CB). Linear sweep voltammetry when Bi 2 WO 6 is used as cathode shows two reduction processes: H + /H 2 and O 2 /O 2 .− . The last one-electron reduction occurs at −0.2 V vs NHE and the first one at −0.4 V vs NHE. Both show solvent isotope effect, although the i 0H 2 O =i 0D 2 O are 2.4 and 1.9, respectively, quite smaller than the value obtained in the photooxidation due to the influence of the bias on the transition state symmetry in the first case and through water O 2 reduction in the second. Cyclic voltammetry indicates that the reduced and adsorbed H (Bi 2 WO 6 -H 2 ) is oxidized to Bi 2 WO 6 -H at approximately 0.2 V vs NHE. Positive conduction band potential of Bi 2 WO 6 (+0.5 V vs NHE) establishes the difference with other semiconductors where oxidation (L 2 O/L + ,OL . )instead of reduction has been proposed as rls in photooxidation.

Detoxifying SARS-CoV-2 antiviral drugs from model and real wastewaters by industrial waste-derived multiphase photocatalysts

“…The respective onset potentials (vs. Ag-AgCl) of the WW6 and WU6 photoanodes are 0.41 V and 0.28 V in the presence of SO 3 2– and 0.50 V and 0.48 V in the absence of SO 3 2– , respectively. This is consistent with previous reports on WO 3 -based electrodes ( Ng et al, 2012 , Gomis-Berenguer et al, 2018 , Patil and Patil, 1996 , Costa et al, 2020 , Núñez et al, 2019 ).

Fig.

…”

Solid-State Schikorr Reaction from Ferrous Chloride to Magnetite with Hydrogen Evolution as the Kinetic Bottleneck

“…We observed a substantially large kinetic isotope effect (KIE) of k H / k D = ν H /ν D ∼ 2 even though the reaction was conducted at 500 °C (773 K). This value corresponds to k H / k D > 7 at 25 °C (298 K), and this is larger than previously reported KIE for the hydrogen evolution in photocatalysis and thermolysis . Although there is a possible diffusion limit, such a large KIE strongly suggests that the irreversible hydrogen evolution process involves the cleavage of H­(D)–O bonds or the formation of H–H/D–D bonds at the transition state rather than the structural rearrangement of iron oxides is the rate-determining step for the second step.…”

Unlocking the effect of Zn2+ on crystal structure, optical properties, and photocatalytic degradation of perfluoroalkyl substances (PFAS) of Bi2WO6

“…33 As one of the simplest members of the Aurivillius family (Bi 2 A n−1 B n O 3n+3 ), Bi 2 WO 6 has an orthorhombic layer structure constructed from alternating [Bi 2 O 2 ] n 2n+ and perovskite-like 34 In general, this layer structure not only favors the separation and transfer of photoexcited charge carriers due to the built-in electric field vertical to the layer direction but also reduces the surface trapping of photoexcited charge carriers. 35 The valence band edge is more positively positioned, 36 providing the sufficient potential to thermodynamically drive a hole-mediated oxidation reaction. By changing the synthesis parameters and doping, the morphology and exposed facets of Bi 2 WO 6 can be easily tuned, resulting in a large number of oxygen vacancies that act as electron scavengers and binding sites for adsorbates.…”