The influence of nitrate concentration and acidity on the electrocatalytic reduction of nitrate on platinum

A study was performed to determine the influence of nitrate concentration and acidity on the reaction rate and selectivity of the electrocatalytic nitrate reduction on platinum. There are two different nitrate reduction mechanisms on platinum: a direct mechanism (0.4–0.1 V vs. SHE) and an indirect mechanism (0.9–0.5 V vs. SHE). In the direct mechanism the dependence of the reaction rate on the nitrate concentration changes with increasing nitrate concentration. Whereas at low concentrations (<0.1 M) the reaction order in nitrate is positive, at high concentrations (>0.1 M) the reaction order is negative. This suggests that at high concentrations the amount of free surface sites determines the reaction rate. These free surface sites are needed either for the adsorption of a second species necessary for the reaction (water or hydrogen) or for the dissociation of nitrate to nitrite. Both at low and high nitrate concentrations the direct reduction is mainly selective towards ammonia, although small amounts of N2O and N2 were observed using differential electrochemical mass spectrometry (DEMS) at potentials between 0.4 and 0.2 V at high concentrations of nitrate. This N2 and N2O formation seems to be related to the NOads coverage on the electrode. The indirect reduction mechanism is autocatalytic as is illustrated by its unusual stirring behavior. Large amounts of NO were observed using DEMS. This suggests that not nitrate but NO+ (⇆ HNO2) is involved in the actual electron transfer. NO+ is reduced to NO, which then reacts with HNO3 to reproduce NO+, resulting in an overall reaction of nitrate to nitrite. Both nitrite and a high acidity are needed for this mechanism to develop, but addition of nitrite is not necessary since nitrite is present in small amounts in HNO3 solutions of concentrations over 4 M. The autocatalytic reduction mechanism slows down and eventually terminates when NO starts to react to N2O, as was observed using DEMS.