Electrocatalytic oxidation of hydrazine in alkaline media promoted by iron tetrapyridinoporphyrazine adsorbed on graphite surface

Article J. Braz. Chem. Soc., Vol. 19, No. 4, 720-726, 2008. Printed in Brazil - ©2008 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00 Electrocatalytic Oxidation of Hydrazine in Alkaline Media Promoted by Iron Tetrapyridinoporphyrazine Adsorbed on Graphite Surface Luiza M. F. Dantas,a Alaécio P. dos Reis,a Sônia Maria C. N. Tanaka,a José H. Zagal,b Yo-Ying Chenb and Auro A. Tanaka*a Departamento de Química, Centro de Ciências Exatas e Tecnologia, Universidade Federal do Maranhão, 65085-580 São Luís-MA, Brazil a Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile b A oxidação eletrocatalítica de hidrazina foi estudada sobre um eletrodo de grafite pirolítico ordinário modificado com tetrapiridinoporfirazina de ferro (FeTPyPz) com as técnicas de voltametria cíclica e de eletrodo de disco rotatório. Análise dos voltamogramas registrados a diferentes velocidades de varredura do potencial e das curvas de polarização para diferentes velocidades de rotação do eletrodo mostraram que a reação de eletroxidação de hidrazina sobre FeTPyPz processa-se de acordo com um mecanismo envolvendo 4 elétrons e com a formação de N2 como principal produto. Os parâmetros cinéticos sugerem que a segunda etapa de transferência de carga é a etapa determinante da velocidade da reação. A atividade eletrocatalítica do complexo FeTPyPz depende do potencial formal do processo redox Fe(II)/Fe(I), que apresentou bom ajuste num gráfico do tipo vulcano formado por diferentes ftalocianinas de ferro, indicando que este potencial formal é um bom indicador da reatividade destes complexos. The electrocatalytic oxidation of hydrazine was studied using an ordinary pyrolytic graphite electrode modified with iron tetrapyridinoporphyrazine complex (FeTPyPz), employing cyclic voltammetry and rotating disk electrode techniques. Analyses of the voltammograms recorded at different potential scan rates and the polarization curves at different electrode rotation rates showed that the reaction of electrooxidation of hydrazine on FeTPyPz occurs via 4-electrons with the formation of N2 as main product. The kinetic parameters suggest that the second electron transfer step is rate controlling. The activity of FeTPyPz depends on its Fe(II)/Fe(I) formal potential and fits well in a volcano plot that includes several iron phthalocyanines, indicating that such formal potential is a good reactivity index for these complexes. Keywords: hydrazine oxidation, modified graphite electrode, iron tetrapyridinoporphyrazine, volcano plot Introduction The study of chemically modified electrodes has attracted considerable interest in the last decades as researchers attempt to exert more control over the chemical nature of the electrode surface. Molecules of known reactivity are then incorporated or confined on the electrode surface, acting as mediators for electron transfer reactions. Applications include electrocatalysis, electroanalysis, sensors and biosensors, as well as in electrochemical detection systems used in flow-injection *e-mail: analysis or high performance liquid chromatography.1-30 One applicability of these electrodes refers to the oxidation and detection of hydrazine, an important chemical compound used in jet and rocket fuels and in the production of agricultural and textile chemicals, drugs, explosives, photographic developers, blowing agents used in the manufacture of foam rubber, and in the prevention of rusting in boilers and nuclear reactors.31 Furthermore, the detection of hydrazine and its derivatives is very important in pharmacology due to the recognition as carcinogenic and hepatotoxic substances.31,32 In order to reduce the typically large overpotentials and improve the kinetics of the direct oxidation of hydrazine Vol. 19, No. 4, 2008 Dantas et al. at most conventional electrodes, carbon, graphite and gold electrodes modified with metallophthalocyanines and metalloporphyrins have attracted special attention.1,20,30,33 In addition, the effect of substituents on the ligand on the catalytic activity of metallophthalocyanines for the oxidation of hydrazine has been investigated for cobalt and iron derivatives.34,35 It has been found that as the electronwithdrawing power of the substituents increases, activity also increases, but only up to some point. After this, the activity decreases. So when activity, measured as current at constant potential is plotted versus the M(II)/M(I) formal potential of the complex, or versus the sum of the Hammett parameters of the substituents on the ligand, a volcano plot is obtained. This implies that the M(II)/M(I) formal potential of the complex needs to be located in a rather narrow potential window to achieve maximum activity for the reaction.34,35 In this work, an ordinary pyrolytic graphite (OPG) electrode was modified with iron tetrapyridinoporphyrazine (FeTPyPz) and its activity was investigated for the oxidation of hydrazine in alkaline media. The activity of this iron complex was compared with those of substituted and unsubstituted iron phthalocyanines reported in the literature.35 Experimental All solutions were prepared with distilled water purified in a Milli-Q Academic system from Millipore S.A. All reagents were Merck® analytical grade and used without further purification. Iron tetrapyridinoporphyrazine (FeTPyPz) was synthesized and purified as previously reported.36 Prior to any electrochemical measurements, the electrolytic solution was saturated with White Martins 4.6 analytical grade argon gas. Experiments were carried out in 0.1 mol L-1 NaOH in water. The experiments were performed in a conventional electrochemical cell with three compartments. A disk of ordinary pyrolytic graphite (OPG) from Union Carbide, geometrical area of ca. 0.4 cm2, mounted in Teflon, and a saturated calomel electrode (SCE) served as working and reference electrodes, respectively. All potentials, unless specified, are referred to this reference electrode. A platinum foil (geometrical area of ca. 2 cm2) was used as auxiliary electrode. The OPG working electrode was first polished with 2000 grit emery paper, rinsed with purified water, sonicated in water for 2 min and rinsed with purified water several times. After drying at room temperature, the OPG surface was immersed in an air-saturated solution containing 2.0×10-4 mol L-1 FeTPyPz in 1.0 mol L-1 H2SO4 solution 721 and the electrode was rotated at 100 rpm for different adsorption times. Finally, the electrode was thoroughly rinsed with purified water, in order to remove any excess of the complex, and introduced into the electrochemical cell containing the electrolyte solution. The electrochemical measurements were performed with a BAS CV-50W potentiostat from Bioanalytical System and the electrode rotation rate was controlled by an AFASR rotator from Pine Instruments Co. Results and Discussion Characterization of the FeTPy (...truncated)