Role of pH and the transition metal for the electrocatalytic reduction of nitrite with transition metal substituted Dawson type heteropolyanions .2.

The parent Dawson and lacunary anion were both seen previously to have a catalytic effect (redox mediation) upon the reduction of nitrite at a pH equal to 2.00. However, at a pH value of 4.50 no electrocatalytic effect was observed for low concentrations of nitrite. It was shown that at the latter pH value the presence of certain transition metals substituted into the lacunary anion is necessary to observe the electrocatalytic reduction (chemical catalysis) of low concentrations of nitrite. It was found that the Fe-III and Cu-II substituted Dawson heteropolyanions Fe(III)HPA and Cu(II)HPA respectively were the most efficient at pH 4.50 for the electroreduction of nitrite. In addition, despite the lack of direct electroactivity of the Ni-II site in Ni(II)HPA, the Ni-II site is demonstrated by the observation of a catalytic effect for nitrite reduction at pH 4.50. (C) 1997 Elsevier Science S.A.