Nitrate reduction on tin-modified rhodium, ruthenium, and iridium electrodes

The reduction of nitrate ion was conducted on Sn-modified rhodium, ruthenium, and iridium electrodes in 0.1 M HClO4. These electrodes showed very high electrocatalytic activities for this reaction, although they were not as high as those of the previously reported Sn-modified palladium and platinum electrodes. The number of cleaved N–O bonds per reduced nitrate ion was calculated from the product distribution to be 2.2–2.5, independent of the noble metal including Pt and Pd. These results show that Sn-modified noble metal electrodes (Rh, Ru, Ir, Pd, and Pt) are good catalysts for N–O bond cleavage. On the other hand, the product strongly depends on the electrode. The main product formed at −0.2 V vs. Ag/AgCl on Sn/Rh was N2O (59%), while the remaining products were hydrogenated species such as NH 3 + OH (22%) and NH 4 + (19%). On Sn/Ru, only hydrogenated products were formed; NH 3 + OH : 46%, NH 4 + : 41%. Sn/Ir showed a product distribution similar to that on Sn/Pt; N2 (17%), NO 2 - (17%), N2O (11%), and NH 3 + OH (56%). The potential of the electrodes for the selective N2 formation, calculated on the basis of the number of cleaved N–O bonds, and formed N–N and N–H bonds per reduced nitrate ion, decreased in the order of Sn/Pd > Sn/Rh > Sn/Pt > Sn/Ir ≫ Sn/Ru, although the highest N2 production was obtained on Sn/Pt.