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Unprecedented large solvent (H 2 O vs D 2 O) isotope effect in semiconductors photooxidation
Author(s) -
Núñez Oswaldo,
Madriz Lorean,
Carvajal David,
Tatá José,
Vargas Ronald
Publication year - 2019
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3952
Subject(s) - chemistry , kinetic isotope effect , cyclic voltammetry , electron transfer , solvent , adsorption , proton , linear sweep voltammetry , rate determining step , analytical chemistry (journal) , electrochemistry , inorganic chemistry , catalysis , photochemistry , electrode , deuterium , biochemistry , physics , quantum mechanics , organic chemistry , chromatography
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 = 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 thei 0H 2 O / i 0D 2 Oare 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.