Electrocatalytic reduction of HNO2 by a silicomolybdate polyanion:: A differential electrochemical mass spectrometry study

Electrocatalytic activities towards the reduction of important analytes such as nitrite, bromate, chlorate and hydrogen peroxide have been demonstrated for numerous heteropolyanions (HPAs). In this paper, the behavior of a Keggintype HPA, SiMo12O404-, on the one hand in solution and on the other hand immobilized in a conducting polymer, polypyrrole (PPy), in the presence of nitrous acid (HNO2) is examined. It is shown that the PPy films codoped with SiMO12O404+ and HSOT ions could be used as sensitive part of an amperometric sensor even if the response is limited by HNO2 diffusion into the PPy film. Differential electrochemical mass spectrometry (DEMS) was used to determine the gas formed during nitrous acid reduction by H2SiMo12O404-. Under an applied cathodic potential, when both HNO2 and SiMo12O404- are present in the medium, NO gas is produced whereas, when SiMo12O404- ions are immobilized in a PPy film, N2O evolves and the NO concentration, notably due to the disproportionation of HNO2, decreases.