Leonardo M. Da Silva

Electrochemical ozone production: influence of the supporting electrolyte on kinetics and current efficiency

The nature of the electrolyte strongly influences the electrode kinetics of the oxygen evolution reaction (OER) and electrochemical ozone production (EOP) mainly by affecting the degree of coverage by the intermediates of both processes. The anomalous behaviour of the Tafel coefficient, b, as a function of temperature was attributed to surface adsorption of the electrolyte species, and the competition between them, as well as gas bubble adherence. Comparison of the current efficiencies of the EOP, ΦEOP, determined for different temperatures and supporting electrolyte compositions, showed the presence of fluorinated anions increases ΦEOP. The influence of the anion nature on ΦEOP, when analysed in the light of the proposed electrode mechanism, reveals introduction into the electrolyte of anions having a high electronegativity changes the double layer structure resulting in an increase of surface concentration of the active centres leading to EOP. The inhibition of the OER in the high overpotential domain during EOP provoked by fluoro-anion adsorption is supported by the activation energy data. In situ surface characterisation before and after EOP investigation revealed that even under drastic conditions (high current density, low interfacial pH) β-PbO2 can be considered an inert electrode material.

Electrochemistry and green chemical processes: electrochemical ozone production

-based electrodes) also are presented. Different chemical processes of interest to the solution of environmental problems involving ozone are discussed.

Simultaneous determination of paracetamol and ibuprofen in pharmaceutical samples by differential pulse voltammetry using a boron-doped diamond electrode

O presente trabalho apresenta uma metodologia simples, rapida e de baixo custo para determinacao simultânea de paracetamol (PC) e ibuprofeno (IB) em formulacoes farmaceuticas por voltametria de pulso diferencial usando eletrodo de diamante dopado com boro (BDD). Os analitos apresentaram picos de oxidacao definidos em 0,85 V para o PC e 1,72 V para o IB (vs. Ag/AgCl) sobre o eletrodo de trabalho de BDD em meio de uma solucao de H2SO4 0,1 mol L-1 com 10% (v/v) de etanol. As curvas de calibracao mostraram uma resposta linear para determinacao simultânea dos analitos entre 20 a 400 µmol L-1 (r2 = 0,999) e os limites de deteccao obtidos pelas regressoes foram de 7,1 µmol L-1 e 3,8 µmol L-1 para PC e IB, respectivamente. Os estudos de adicao e recuperacao nas amostras ficaram proximos de 100% e os resultados foram validados por metodos cromatograficos.

Influence of lead dioxide electrodes morphology on kinetics and current efficiency of oxygen-ozone evolution reactions

Influence of electrode morphology on electrochemical properties of lead dioxide electrodes (β-PbO2) for oxygen-ozone evolution reactions in acid medium was investigated using scanning electronic microscopy (SEM), cyclic voltammetry (CV), polarization curves (PC), and determination of the current efficiency (Φ). Experimental findings revealed that application of high electrodeposition current densities furnishes more rough β-PbO2 films. Surface characteristics were verified by SEM images and the analysis of interfacial pseudo-capacitances and morphology factor (φ). Kinetic study of the overall electrode process (O2 + O3) based on the analysis of the Tafel slope revealed that the electrode morphology and electrolyte composition considerably affect the electrode kinetics. In most cases, the existence of two Tafel slopes distributed in the low and high overpotential domains was observed. Abnormal Tafel slopes (b ≠ 120 mV) obtained for the primary water discharge step during water electrolysis were interpreted considering the apparent charge transfer coefficient (αapa). Optimum conditions for the ozone production were obtained for the less rough β-PbO2 electrode immersed in a sulfuric acid solution (1.0 mol dm−3) containing KPF6 (30 × 10−3 mol dm−3), where the current efficiency of 15 mass % for the ozone production was obtained.

Chlorine evolution reaction at Ti/(RuO2+Co3O4) electrodes

A systematic investigation of the mechanistic and electrocatalytic properties of the chlorine evolution reaction, ClER, on mixed rutile/spinel oxides of nominal composition Ti/[RuO2(x)+Co3O4(1-x)] was done. Electrodes were prepared using the thermal decomposition procedure (Tcalc. = 470oC; tcalc. = 1h), with x varied between 0 and 1, changed in 0.1 steps. Kinetics were studied recording quasi-stationary current-potential curves and reaction order determined with respect to H+ and Cl-. A constant Tafel slope of about 33 mV was found for the 0 - 80 mol% RuO2 concentration interval, increasing to 40 mV for higher RuO2 contents. Experimental data support the mechanism originally proposed by Erenburg et al. for the ClER. The edge effect exerts an influence on the electronic transfer coefficient values. Analysis of the electrocatalytic activity reveals an increase in the apparent electrocatalytic activity for the mixed oxides.

Influência das condições de resfriamento sobre as propriedades superficiais e eletroquímicas de anodos dimensionalmente estáveis

Surface and electrochemical properties of the dimensionally stable anode Ti/(Ru0.3Ti0.7)O2 were studied as a function of the annealing temperature using different conditions in order to perform the cooling process of the oxide films (conventional thermal shock and the slow cooling processes). It was found that surface and electrochemical properties for the oxygen evolution reaction are both affected through the cooling process, being the electrode prepared at 400 oC using the slow cooling process the less susceptible to wear. The Tafel slope obtained in the high overpotential domain was analysed in light of the apparent charge transfer coefficient.

Decolorization, Degradation and Toxicity of Dye Solutions Containing Orange Cassafix ® CA-2R after UV/H2O2 Oxidation under Laminar Flow Conditions

Abstract Dye solutions containing Orange Cassafix® CA-2R were treated using the UV/H2O2 advanced oxidation process under laminar flow conditions characterized by Reynolds numbers in the range of 322-295 using an annular photochemical reactor. The initial dye concentration [dye]0 and the initial hydrogen peroxide concentration [H2O2]0 were shown to affect the percentage of decolorization (P.D.) as a function of the irradiation time (t). A maximum P.D. of 90% was achieved at t > 120 min, [H2O2]0 = 800 ppm and [dye]0 = 300 ppm. The total organic carbon (TOC) depends on [H2O2]0 and [dye]0, while a maximum TOC reduction of 45% was achieved when t > 100 min, [H2O2]0 = 800 ppm and [dye]0 = 200 ppm. The toxicity of the treated samples was changed from 38 to 79%, depending on the [H2O2]0/[dye]0 ratio. A minimum electric energy consumption of 0.383 kW h m-3 order-1 was obtained at the [dye]0/[H2O2]0 ratio of 0.25.

Revisiting ideal gases and proposal of a simple experiment for determining atmospheric pressure in the laboratory

Relevant historical aspects concerning ideal gases were briefly reviewed to provide a condensed resource for the undergraduate student of chemistry. The importance of the barometer and the concept of atmospheric pressure were reviewed and discussed. A combination of Boyle’s law with the well-known equation for determining the pressure produced by a column of stationary fluid was used to obtain a graphical method for determining atmospheric pressure under isothermal conditions in the laboratory. Important aspects related to the study of ideal gases were reviewed in light of the pressure-volume data that can be obtained by physical chemistry students using a simple apparatus composed of a commercial hypodermic syringe connected to a mercury manometer. Relevant concepts associated with the measurement of a physical quantity, the importance of linear regression, as well as the use of a graphical method to test the validity of a theory were reviewed from a pedagogical perspective through the experimental study of Boyle’s law during regular experimental physical chemistry classes.