Electrocatalytic behaviour of an electrode coated with a nitrite-sensitive layer based upon an iron-substituted heteropolytungstate doped poly(N-methylpyrrole)

Abstract

An electrode modified by a poly(N-methylpyrrole) film doped with [(H ₂ O)Fe ^III PW ₁₁ O ₃₉ ] ^4- (between 4×10 ^-9 and 5×10 ^-8 mol cm ^-2 ) displays the electrochemical response of both partners. The catalytic properties of this film are illustrated by the multielectronic reduction of nitrite. The first step is the formation of an iron–nitrosyl complex [(NO)Fe ^II PW ₁₁ O ₃₉ ] ^5- , generated from the replacement of H ₂ O initially coordinated to the iron centre by an NO group, and of which the reduction leads to the catalytic conversion of nitrite into ammonium ions. The measured catalytic currents reach a maximum for an amount of catalyst deposited on the electrode around 1.5×10 ^-8 mol cm ^-2 and are linear with the nitrite concentration over the range 1×10 ^-4 –3×10 ^-2 M. Anions such as NO ₃ ^- , Cl ^- , SO ₄ ^2- , PO ₄ ^3- and CO ₃ ^2- are found not to interfere. From electrolysis experiments, the current efficiency for NH ₄ ⁺ is around 80% and the turnover is 100–120 (h of electrolysis) ^-1 . The kinetic analysis shows that the catalytic reaction is the rate-determining step when thin films are investigated (up to 1.3×10 ^-8 mol cm ^-2 of incorporated anion). For thicker films, the system progressively shifts towards the general kinetic situation where the three processes (the catalytic reaction, the electron and substrate diffusions in the film) can control the rate.