S. Padmaja

Bio: S. Padmaja is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Flash photolysis & Sodium metaborate. The author has an hindex of 2, co-authored 4 publications receiving 8 citations.

Redox and acidic properties of the borate radical B(OH)˙4. A flash photolysis study

Abstract: Flash photolysis of an aqueous solution of sodium metaborate and potassium peroxydisulphate at pH 115 gives rise to a new transient species with an absorption maxima at 590 nm (Iµ= 80 m2 mol–1) and 650 nm (Iµ= 90 m2 mol–1) The laser-Raman and 11B nmr spectra of metaborate solutions at pH 115 are consistent with the presence of the tetrahydroxyborate ion, B(OH)–4 The pKa and reduction potential of the radical B(OH)˙4, produced on flashing a mixture of B(OH)–4 and S2O2–8 are found to be 1075 ± 002 and + 14 V, respectively The rate constants for the oxidation of several amines and phenols by this one-electron oxidant have been determined Salt-effect studies are consistent with the reactive species being B(OH)3O˙–

Reactions of the borate radical with para-substituted benzoic acids (photo-kolbe reaction)

Abstract: Rate constants for the reactions of the borate radical B(OH) 3 O⨪ with various para -substituted benzoic acids were measured by flash kinetic spectrophotometry. The product analyses and kinetic studies are consistent with an electron transfer mechanism followed by decarboxylation. In the case of the p -aminobenzoate anion the —NH 2 and —COO − groups react simultaneously with the borate radical providing two parallel pathways with different rate constants.

Generation and reactions of the borate radical — A flash photolysis study

Abstract: The borate radical is produced by the flash photolysis of an aqueous alkaline solution of NaBO2 and K2S2O8. The absorption spectrum of the transient borate radical shows two maxima. The self decay of this radical is second order. Rate constants for the reactions of this radical with some organic substrates have been determined.

Redox and Acidic Properties of the Borate Radical B(OH)· 4. A Flash Photolysis Study.

Abstract: Flash photolysis of an aqueous solution of sodium metaborate and potassium peroxydisulphate at pH 11.5 gives rise to a new transient species with an absorption maxima at 590 nm (Iµ= 80 m2 mol–1) and 650 nm (Iµ= 90 m2 mol–1). The laser-Raman and 11B n.m.r spectra of metaborate solutions at pH 11.5 are consistent with the presence of the tetrahydroxyborate ion, B(OH)–4. The pKa and reduction potential of the radical B(OH)˙4, produced on flashing a mixture of B(OH)–4 and S2O2–8 are found to be 10.75 ± 0.02 and + 1.4 V, respectively. The rate constants for the oxidation of several amines and phenols by this one-electron oxidant have been determined. Salt-effect studies are consistent with the reactive species being B(OH)3O˙–.

Effects of Anions and pH on the Stability of ZnO Nanorods for Photoelectrochemical Water Splitting

Abstract: This work demonstrates the improved stability of zinc oxide nanorods (ZnO NRs) for the photoanode of solar water splitting under voltage biases by the addition of borate or carbonate ions in the aqueous electrolyte with suitable pH ranges. The ZnO NRs prepared by the hydrothermal method are highly active and stable at pH 10.5 in both borate and carbonate buffer solutions, where a photocurrent higher than 99% of the initial value has been preserved after 1 h polarization at 1.5 V (vs reversible hydrogen electrode) under AM 1.5G. The optimal pH ranges with a minimum morphological change of ZnO NRs for photoelectrochemical (PEC) water splitting in borate and carbonate buffer solutions are 9-13 and 10-12, respectively. The working pH range for PEC water splitting on ZnO NR photoanodes can be extended to 8.5-12.5 by the combination of borate and carbonate anions. The lifetime of ZnO NR photoanodes can be synergistically prolonged for over an order of magnitude when the electrolyte is the binary electrolyte consisting of borate and carbonate in comparison with these two anions used individually. On the basis of the experimental results, a possible mechanism for the protective behavior of ZnO in borate and carbonate solutions is proposed. These findings can be used to improve the lifetime of other high-performance ZnO-based catalysts and to understand the photocorrosive and protective behaviors of ZnO NRs in the borate and carbonate solutions.

Radiolytic Reactions of Monochloramine in Aqueous Solutions

Abstract: 30) L mol -1 cm -1 and 580 ) (56 ( 30) L mol -1 cm -1 . The ¥ NHCl radical undergoes self-decay and can react also with O2 to form a peroxyl radical. It is suggested that the peroxyl radical exists in equilibrium NHClO 2 ¥ / ¥ NHCl + O2 with an estimated equilibrium constant of (3 ( 2) 10 -3 mol L -1 . The reaction of chloramine with the carbonate radical is suggested to form a complex [CO3NH2Cl] ¥- with kf ) 2.5 10 5 L mol -1 s -1 and kr ) 4 10 2 s -1 , and this complex decomposes with k ) 7 10 2 s -1 to form ¥ NHCl.

Spectroscopic and Kinetic Properties of the Radical Zwitterion and Related Intermediates in the One-Electron Oxidation of p-Aminobenzoic Acid†

Abstract: The time-resolved resonance Raman spectra and acid−base properties of the transient prepared on the submicrosecond time scale by N3• oxidation of aqueous p-aminobenzoic acid (PABA) in near-neutral solutions identify it as the charge-neutral 4--O2C-aniline•+ radical. This zwitterion intermediate undergoes slow thermal dissociation (k ∼ 2.4 × 103 s-1) by intramolecular electron transfer and CO2 elimination. It reacts with common bases, such as OH-, N3-, and HPO42- ions, at the rate constants of 1.9 × 1010, 2.7 × 106, and 2.2 × 108 M-1 s-1, respectively, and converts into the nondissociative anilino radical form (pKa 6.7). In the •OH oxidation, formation of the zwitterion radical occurs via OH adducts (hydroxycyclohexadienyl radicals) of p-aminobenzoate anion, at a rate of 1.4 × 105 s-1 which is independent of the base concentration. In strongly acidic solutions the OH adduct of p-aminobenzoic acid reacts with H+, at a rate constant of 4.7 × 108 M-1 s-1, to form the 4-HO2C-aniline cation radical (pKa 1.1). T...

Borate-Mediated Hole Transfer from Irradiated Anatase TiO2 to Phenol in Aqueous Solution

Abstract: The development of a highly active TiO2 photocatalyst for energy and environmental use is a great challenge. In this work, we report that the addition of sodium borate to an aqueous suspension of anatase TiO2 at neutral pH can result in a significant enhancement in the rate of phenol degradation. Similar results were also observed from 2,4-dichlorophenol degradation, spin-trapped OH radical formation, H2O2 decomposition, and chromate reduction in the presence of phenol. This borate-induced rate increase for phenol degradation was determined not only by the amount of borate adsorption but also by the structure of borate species (pH effect). A (photo)electrochemical measurement with the TiO2 film revealed that upon addition of borate, the hole consumption by phenol and the electron consumption by O2 were accelerated and decelerated, respectively. Moreover, the flat band potential of TiO2 was negatively shifted by 81 mV. Since the hole oxidation of water to O2 remained unchanged, it is proposed that a borate...