Selectivity and efficiency of nitrite electroreduction catalyzed by a series of Keggin polyoxometalates

Abstract

In this paper we have studied the influence of the charge and redox potential for a series of polyoxometalates (POMs) belonging to the Keggin family on the electrochemical reduction of nitrite or nitrous acid (NO2– or HNO2). We have focused on the correlation of gaseous products and the redox properties of the POMs. Analysis by differential electrochemical mass spectrometry (DEMS) showed that the gaseous products (NO and N2O) are formed at pH 2 and 5 by reaction of HNO2 or NO2– with the reduced POM. Only NO is detected when the POM ([PW12O40]3- , [SiW12O40]4-, [BW12O40]5-) undergoes the first one-electron reduction wave. However, N2O is obtained along with NO as soon as the POM undergoes two-electron reduction. The potential of the first reduction wave of the POM is correlated with its charge. Consequently NO and/or N2O appear at lower overpotential for [PW12O40]3- and [SiW12O40]4- followed by [BW12O40]5- and [H2W12O40]6-. Microkinetic simulations of the experimental current–potential curves were used to estimate the rate constant kc of the reaction between the reduced POM and HNO2 or NO2– in solution and the turnover frequency (TOF). At low overpotentials, the values of TOF are decreasing in a trend following the redox potential of the POM: [PW12O40]3- > [SiW12O40]4- > [BW12O40]5- while at high overpotentials the TOFs are dominated by kc following the reverse order. At pH 5 kc values are about 70 times lower than at pH 2. This is attributed to the decrease in the proton concentration and to the electrostatic repulsion between the negatively charged POMn- and NO2–.