Non-linear correlations between formal potential and Hammett parameters of substituted iron phthalocyanines and catalytic activity for the electro-oxidation of hydrazine

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

“…This figure clearly shows that hydrazine oxidation currents were not observed on the bare electrode and modification with FeTPyPz results in the onset of oxidation currents at potentials near -0.55 V. Furthermore, a comparison of Figure 1A and Figure 2A indicated that the hydrazine oxidation reaction on FeTPyPz modified electrodes begins at potentials close to the formal potential of the Fe(I)TPyPz/Fe(II)TPyPz redox process, showing that the electrocatalytic activity of the complex can be associated to the metal center. 20,35,36 Based on the facts that the voltammetric profile of the hydrazine oxidation reaction on the OPG electrode modified with FeTPyPz is similar to those associated with irreversible processes and the dependence of the peak currents I P (A) with the square root of the electrode potential scan rate v 1/2 (V 1/2 s -1/2 ) is linear ( Figure 2B), the total number of electrons n can be determined using the equation: 39,40 ( 2) where C 0 is the reactant concentration in the bulk solution (C N 2 H 4 = 5.0 × 10 -3 mol L -1 ); D 0 the diffusion coefficient in solution (D N 2 H 4 = 1.4×10 -5 cm 2 s -1 ); 37 A the geometrical area of the electrode (A = 0.4 cm 2 ); n the total number of electrons involved in the oxidation of hydrazine, n 0 the number of electrons involved in the rate-determining and a the electron transfer coefficient, which can be determined by the linear dependence of peak potential E P with the logarithm of the potential scan rate v ( Figure 2C), according to: Vol. 19, No.…”

“…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.…”

“…The activity of this iron complex was compared with those of substituted and unsubstituted iron phthalocyanines reported in the literature. 35 …”

“…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.…”

“…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.…”