Electrocataltyic oxidation of hydrazine at polymeric iron-tetraaminophthalocyanine modified electrodes

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

“…Similar assignments have been also made for a glassy carbon electrode modified with iron tetraaminophthalocyanine and several substituted iron phthalocyanines adsorbed on OPG. 21,35,37 In order to establish an immersion time for modification of the OPG surface with FeTPyPz, the charge associated with the voltammetric peak 2 was plotted versus the immersion time (t/min) of the electrode surface in the FeTPyPz solution. Figure 1B shows that immersion time higher than 5 min is sufficient to reach the maximum charge value associated with FeTPyPz adsorbed species.…”

“…These curves were analyzed according to the characteristics of a first-order irreversible reaction controlled by diffusion, where the current I (A) is given by the Koutecky-Levich equation: 21,38 ( 5) where I K (A) and I D (A) represents the kinetic and diffusion controlled currents, respectively, with (6) where D 0 is the diffusion coefficient (D N 2 H 4 = 1.4×10 -5 cm 2 s -1 ), 37 C 0 the reactant concentration in the bulk solution (C N 2 H 4 = 5.0×10 -3 mol L -1 ), n the solution kinematic viscosity (n=1.02×10 -2 cm 2 s -1 , 37,41,43 A the electrode geometrical area (A OPG = 0.4 cm 2 ), n the number of electrons transferred/reactant molecule, f is the electrode rotation rate in radians s -1 and the other terms have their usual meanings. According to equations 5 and 6, a plot of I -1 vs. f -1/2 (Koutecky-Levich plot) should be linear with an angular coefficient of 1/B, as shown in Figure 4B for a fixed potencial of -0.25 V. Parallel lines were also observed for other potentials, which indicates a first order kinetics with respect to hydrazine in solution and a constant value of n over the potential range investigated.…”

“…These observations indicated that the redox process is reversible and involves only adsorbed species. 27,37 Based on these information, the amount of adsorbed complex Γ (mol cm -2 ) on the electrode surface could be calculated from the following equation: 36,38 ( 1) where I P (A) is the peak current; v (V s -1 ) the potential scan rate; A (cm 2 ) the electrode area; n the number of electrons involved in the redox process and the other symbols have their usual meanings. Thus, Γ was estimated as approximately 2.0×10 -10 mol cm -2 , which is close to the values 1.74×10 -10 mol cm -2 previously reported.…”

“…A similar reaction order has been also reported in studies involving the oxidation of hydrazine on glassy carbon electrodes modified with chlorogenic acid, iron tetraaminophthalocyanine and iron tetrasulfonated phthalocyanine. 37,41,42 Multiple cycling of the FeTPyPz/OPG surface in 0.1 mol L -1 NaOH solutions in this regime provides a stable surface whose current remains constant. However, the electrocatalytic activity of FeTPyPz for hydrazine oxidation was observed to decrease significantly at solutions with pH lower than 12 and unfortunately it was not possible to obtain useful electrocatalytic data due to low currents and lack of well defined limiting currents in the rotating disk experiments.…”

“…A Tafel plot was obtained from the rotating disk polarization curve recorded at 1600 rpm ( Figure 4C) and the calculated slope of aproximately 45 mV/decade suggested the second electron transfer as the rate determining step for the hydrazine oxidation reaction on the OPG electrode modified with FeTPyPz. 20,37,42,44,45 Based on the voltammetry and rotating disk electrode data, a possible mechanism that can be proposed for the hydrazine oxidation reaction in alkaline media catalyzed by FeTPyPz adsorbed on the OPG surface involves the following steps: This mechanism is compatible with the facts that: i) the reaction is mediated by the Fe(II)TPyPz/Fe(I)TPyPz redox process, ii) the reaction is first-order with respect to hydrazine iii) a second electron transfer step is the rate determining step (rds) and iv) molecular nitrogen is the reaction product.It is interesting to note that the peak potentials of the cyclic voltammograms for the hydrazine oxidation reaction on FeTPyPz shift to more positive potentials with the increase in the potential scan rate ( Figure 2C), which suggests kinetic limitations. In addition, the peak currents increase linearly with the square root of the scan rate ( Figure 2B), which indicates that, at sufficient overpotentials, the reaction is controlled by diffusion.…”