ELECTROCATALYTIC BEHAVIOR OF AN ELECTRODE COATED WITH A NITRITE-SENSITIVE LAYER BASED UPON AN IRON-SUBSTITUTED HETEROPOLYTUNGSTATE DOPED POLY(N-METHYLPYRROLE)

An electrode modified by a poly(N-methylpyrrole) him doped with [(H2O) (FePW11O39)-P-III](4-) (between 4 x 10(-9) and 5 x 10(-8) mol cm(-2)) displays the electrochemical response of both partners. The catalytic properties of this film are illustrated by the multielectronic reducti on of nitrite. The first step is the formation of an iron-nitrosyl com plex [(NO)(FePW11O39)-P-II](5-), generated from the replacement of H2O initially coordinated to the iron centre by an NO group, and of which the reduction leads to the catalytic conversion of nitrite into ammon ium ions. The measured catalytic currents reach a maximum for an amoun t of catalyst deposited on the electrode around 1.5 x 10(-8) mol cm(-2 ) and are linear with the nitrite concentration over the range 1 x 10( -4)-3 x 10(-2) M. Anions such as NO3-, Cl-, SO42-, PO43- and CO32- are found not to interfere. From electrolysis experiments, the current ef ficiency for NH4+ is around 80% and the turnover is 100-120 (h of elec trolysis)(-1). The kinetic analysis shows that the catalytic reaction is the rate-determining step when thin films are investigated (up to 1 .3 x 10(-8) mol cm(-2) of incorporated anion). For thicker films, the system progressively shifts towards the general kinetic situation wher e the three processes (the catalytic reaction, the electron and substr ate diffusions in the film) can control the rate.