Modulated Cr(III) oxidation in KOH solutions at a gold electrode: Competition between disproportionation and stepwise electron transfer

The electrochemical oxidation of aqueous Cr(III) was examined using cyclic voltammetry with a polycrystalline Au electrode in KOH solutions of varying pH and Cr(III) concentration. The mechanism and kinetics for the oxidation of Cr(III) is a quasi-reversible diffusion-controlled reaction and is largely dependent on the solution pH. The reaction mechanism is initiated by an irreversible electrochemical electron transfer to form Cr(IV) which is the rate-determining step (RDS). Following the RDS, subsequent oxidation of Cr to its hexavalent state occurs by the disproportionation of Cr(IV) at low KOH concentrations and electron transfer at high KOH concentrations due to the involvement of OH(-) in the disproportionation reaction. As the solution pH increases, the Cr(III) oxidation peak potential shifts negatively owing to the involvement of OH(-) in the RDS. The competitive adsorption of OH(-) and CrO(2)(-) on the electrode surface also plays an important role in the oxidation behavior. (C) 2011 Elsevier Ltd. All rights reserved

Indirect Electrochemical Cr(III) Oxidation in KOH Solutions at an Au Electrode: The Role of Oxygen Reduction Reaction

The indirect electro-oxidation of Cr(III) by in situ generated superoxide at a gold electrode has been investigated in KOH solutions using cyclic voltammetry and UV-vis spectroscopy. It is observed that the indirect Cr(III) oxidation behavior is substantially affected by the media pH and there is a pH-modulated oxygen reduction reaction (ORR) process to generate reactive oxygen species which promotes Cr(III) oxidation. The ORR in KOH solutions is attributed to a quasi-reversible diffusion-controlled reaction. In dilute KOH solution (0.2 M), 4e reduction occurs and no reactive oxygen species are generated for the indirect Cr(III) oxidation. Moreover, Cr(III) oxidation is inhibited due to competition for the electrode active sites. As the alkaline concentration increases (3.0 M), the protonation of superoxide is greatly suppressed, and thus, le ORR to generate superoxide is observed. This change in mechanism facilitates the indirect Cr(III) oxidation through the superoxide as a mediator to oxidize Cr(III) to Cr(IV), which is the rate-determining step of Cr(III) oxidation to Cr(VI)

Intensified decomposition of vanadium slag via aeration in concentrated NaOH solution

Abstract A new metallurgical process via aeration for the decomposition of vanadium slag in concentrated NaOH solution was proposed. The improvement of oxygen mass transfer coefficient when using aeration at different NaOH concentration was studied and the effects of critical reaction parameters on vanadium extraction were systematically investigated. The optimal condition was determined to be: alkali concentration of 60%, reaction temperature of 130 °C, alkali-to-ore mass ratio of 6:1, stirring speed of 500 rpm. The yield of vanadium could reach to 97.41% after reacting for 6 h under this reaction condition. The reaction temperature in this new method is 50–270 °C lower than the current liquid oxidation methods reported in the literatures, and the medium alkaline concentration declined from 85% to 60%, exhibiting significant advantages in energy consumption as well as reactor design. Kinetics study indicated that the extraction of vanadium was governed by internal diffusion, and the apparent activation energy was calculated to be 17.57 kJ/mol

Tunable nano-interfaces between MnOx and layered double hydroxides boost oxygen evolving electrocatalysis

The development of low overpotential, non-precious metal oxide electrocatalysts is important for sustainable water oxidation using renewable energy. Here we report the fabrication of nano-interfaces between MnOx nanoscale islands and NiFe layered double hydroxide (LDH) nanosheets, which were chosen as baseline electrocatalysts for OER activity tuning. The MnOx nano-islands were grown on the surfaces of NiFe-LDH nanosheets by atomic layer deposition (ALD). Morphological and structural characterization indicated that the MnOx formed flat nanoscale islands which uniformly covered the surfaces of NiFe-LDH nanosheets, giving rise to a large density of threedimensional nano-interfaces at the NiFe-LDH/MnOx/electrolyte multi-phase boundaries. We showed by X-ray spectroscopic characterization that these nano-interfaces induced electronic interactions between NiFe-LDH nanosheets and MnOx nano-islands. Through such modifications, the Fermi level of the original NiFe-LDHwas lowered by donating electrons to the MnOx nano-islands, dramatically boosting the OER performance of these electron-deficient NiFe-LDH catalysts. Using only 10 cycles of ALD MnOx, the MnOx/NiFe-LDH nanocomposites exhibited remarkable and enhanced electrocatalytic activity with an overpotential of 174 mV at 10 mA cm(-2). This work demonstrates a promising pathway for tuning transition metal electrocatalysts via a generic ALD surface modification technique

Hydrogen evolution activity tuning via twodimensional electron accumulation at buried interfaces

Developing efficient earth-abundant transition metal-based electrocatalysts for the hydrogen evolution reaction (HER) is crucial for hydrogen production at scale. This paper reports that the buried electrocatalytic interfaces between Ni-Fe sulfide (NiFeS) nanosheets and TiO2 conformal coatings (about 5 nm) achieved remarkable HER activity improvement, lowering the HER overpotential from 170 mV to 107 mV at 50 mA cm 2 in a base. Non-HER active, permeable TiO2 coatings grown by atomic layer deposition (ALD) achieved continuous fine-tuning of the electronic properties at the buried TiO2/ NiFeS interfaces, as a novel strategy and the main factor for electron accumulation at the interface. Core-level and valence band X-ray photoelectron spectroscopy (XPS) was used to investigate the TiO2 electronicstructure tuning effect on the charge-transfer energetics during the HER. Their alkaline HER mechanism was elucidated by supplementing characterizations of membrane permeation, Tafel slope, and synchrotron X-ray absorption spectroscopy, which verified that the buried TiO2/ NiFeS interfaces are electrocatalytically active. This study offers a general strategy for improving the charge-transfer kinetics of an electrocatalytic system by confining catalysis at a permeable solid-solid interface. The broad applicability of permeable and tunable coatings potentially accelerates the optimization of earthabundant catalysts to achieve high performance under operationally relevant conditions