Electrocatalytic reduction of nitrate ions in neutral medium at coinage metal-modified platinum electrodes.

Nitrate is a water-soluble toxic pollutant that needs to be excluded from the environment. For this purpose, several electrochemical studies have been conducted but most of them focused on the nitrate reduction reaction (NRR) in alkaline and acidic media while insignificant research is available in neutral media with Pt electrode. In this work, we explored the effect of three coinage metals (Cu, Ag, and Au) on Pt electrode for the electrochemical reduction of nitrate in neutral solution. Among the three electrodes, Pt-Cu exhibited the best catalytic activity toward NRR, whereas Pt-Au electrode did not show any reactivity. An activity order of Pt-Cu > Pt-Ag > Pt-Au was observed pertaining to NRR. The Pt-Ag electrode produces nitrite ions by reducing nitrate ions ([Formula: see text]. Meanwhile, at Pt-Cu electrode, nitrate reduction yields ammonia via both direct ([Formula: see text] and indirect ([Formula: see text] reaction pathways depending on the potential. The cathodic transfer coefficients were estimated to be ca. 0.40 and ca. 0.52, while the standard rate constants for nitrate reduction were calculated as ca. 2.544 × 10-2 cm.s-1 and ca. 1.453 × 10-2 cm.s-1 for Pt-Cu and Pt-Ag electrodes, respectively. Importantly, Pt-Cu and Pt-Ag electrodes execute NRR in the neutral medium between their respective Hydrogen-Evolution Reaction (HER) and Open-Circuit Potential (OCP), implying that on these electrodes, HER and NRR do not compete and the latter is a corrosion-free process.

Heterogeneous Kinetics of Thiourea Electro-Catalytic Oxidation Reactions on Palladium Surface in Aqueous Medium.

The electrochemical behaviors of thiourea (TU) oxidation have been studied at Palladium (Pd) electrode in the acidic medium by recording cyclic voltammograms (CVs). The influence of pH was investigated in the pH range of 1.0 to 9.0. Facilitated adsorption of TU on electrode surface results in enhanced catalytic response in acidic medium and maximum electro-catalytic response was found at pH∼3.0. Chronoamperometric (CA) experiment determined this oxidation as 1e- transfer process and the variation of TU concentration reveals a 1st order kinetics. In the CV responses, the large value of peak separation (▵Ep >380 mV) calculated by the variation of scan rate indicates that oxidation of TU is an irreversible process. With the aid of convolution potential sweep voltammetry (CPSV), the standard rate constant (k°) for the reaction was found to be 7.1×10-4 cm/s and the formal potential constant (E°' ) was evaluated to be ∼0.37 V vs Ag/AgCl (sat. KCl). The value of transfer coefficient (α) was found to vary from 0.74 to 0.40 with applied potential (E). From the potential dependent variation of transfer coefficient (α) and activation energy (▵G≠ ), it was concluded that the overall electrochemical oxidation of TU follows a stepwise mechanism at lower potential (<0.40) V and a concerted one at relatively higher potential (>0.40) V. The FTIR analysis of the product after oxidation of TU molecules confirmed the appearance of a new sharp band near 530 cm-1 due to the formation of S-S bonds suggesting formation of formamidine disulfide (FD) ions.