versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.50 n.1 Concepción mar. 2005
J. Chil. Chem. Soc., 50, N 1 (2005)
"Electrocatalysis of NO2- on poly-M-tetraaminophenylporphyrines (M = Ni, Cu) modified electrodes assisted by visible light".
FRANCISCO ARMIJO, MARÍA J. AGUIRRE*
Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile, Casilla 40, correo 33, Santiago, Chile.
Porphyrins and phthalocyanines of transition metals are good catalysts for many redox reactions [1-7]. In order to increase their catalytic activity it is often necessary to modify the macrocycle with donor or acceptor substituents making the species more reductive or oxidant depending on the analyte. However, this process is a one-way modification, because the molecule enhances or diminishes the electronic density of the metal by the substituent effect. .
On the other hand, an excited species is simultaneously more reductive and more oxidant than the same species in the ground state, as can be seen in Scheme 1.
When an electron is in the LUMO, it is easy to remove it from the molecule, and the species becomes more reductive. On the other hand, the HOMO is converted in a SOMO and is capable of receiving an electron in a lower energy site, becoming more oxidant [9-11]. For that reason, we tested the light effect on two modified electrodes obtained by the electropolymerization of ortho-tetraaminophenylporphyrin of Ni or Cu, (poly-M-o-TAPP) in an HCl medium  on a transparent conducting glass electrode (home made SnO2:F) [13, 14]. Both electrodes are capable of electrocatalyzing the reduction of nitrite into nitrous oxide and nitrate into nitrite and nitrous oxide in perchloric acid and sodium perchlorate [13, 14]. In spite of the short lifetime of the metals (Cu and Ni), the irradiation of continuous visible light with a simple 100-watt tungsten lamp through the modified conducting glass electrode shows an enhancement of the current as shown in Figures 1 and 2. The diagram of the experimental method is shown in Scheme 2.
In Figure 1, the response of the Poly-Cu-o-TAPP modified electrodes in HClO4 and NaClO4 is shown. The electrode shows an enhancement in current in both media. In HClO4, the current increases in ca. 20 mA, when the system is illuminated. In NaClO4, where the electrode is less active, the increase in the current is ca. 5 mA. However, this increase is practically constant (ca. 13%) compared with the maximum current observed at -600 mV vs. Ag|AgCl. Then, in this case, the effect of the light is the same in both electrolytes. On the other hand, the Poly-Ni-o-TAPP-modified electrode (Figure 2) shows a different behavior. Only in HClO4 we can find an effect. In this case, the enhancement at -600 mV is ca. 110 mA, indicating an increase of ca. 35%. In the case of NaClO4, there is no effect found, either with or without light. The reason for those differences is not known at present and is under research. The voltammetric responses of poly-Cu-o-TAPP in HClO4 and NaClO4 are different as shown in Figure 1. In our previous papers [13, 14] we compared the kinetics of polymerization of poly-Cu-o-TAPP and poly-Ni-o-TAPP and its morphology (by AFM). In the case of the poly-Cu-o-TAPP, the kinetics of polymerization is slower than that of the other complex, and its morphology corresponds to a compact and thin film. Probably, when the polymer is protonated, electrostatic repulsion causes its layers to be more separated, generating more available active sites for the reaction. In the case of the Ni-o-TAPP, where the kinetics of polymerization is faster and the morphology corresponds to a more porous electrode, this effect is smaller. Data from Table 1 were obtained by electrochemical impedance spectroscopy . In this Table it is possible to see a noticeable change in the concentration of redox sites when the pH of the media is acid or neutral. As mentioned above, this effect is more pronounced in the case of poly-Cu-o-TAPP.
On the other hand, in order to discard the effect of the heat in the response of the illuminated electrodes, the same experiment was performed with an electrolyte heated at the same temperature produced by the lamp, but in the absence of light, and no effect was observed. The products of the reaction under illumination are the same as those obtained in darkness. At this time we are working with M-porphyrins (M = Ni, Cu and Zn) in the presence of a sensitizer using two techniques, photoelectrochemistry and flash photolysis, in order to elucidate the "true" effect of the light in the enhancement of the electrocatalytic activity and the mechanisms involved.
The polymeric modified electrodes of ortho-M-tetraaminoporphyrins (M= Cu, Ni) show an increase in current toward the reduction of nitrite when the electrode is exposed to light. The short lifetimes of the porphyrins used in this work show a small effect which can be measured, however. In the case of poly-Ni-o-TAPP, light has no effect in NaClO4, but the reason is not yet understood.
The authors acknowledge support under Fondecyt project 1010695, and interesting discussion by Professor Guillermo Ferraudi. F.A. acknowledges "Beca de Ayuda de Tesis Doctoral" Conicyt, 2002.
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