Oxidation of Ammonium on Platinum in Acidic Solutions

Abstract

Oxidation of ammonium on polycrystalline Pt has been studied in perchloric and sulfuric acid solutions at room temperature. Ammonium was electrochemically active, and N2N2 and NO were detected using differential electrochemical mass spectroscopy. Cyclic voltammetry and electrochemical quartz crystal microbalance data showed that ammonium affects the formation and reduction of Pt oxides, probably due to formation of very stable adsorbed nitrogen or nitrogen-oxygen species stable down to the HadsHads region. An oxidation peak occurred at 0.8 VRHEVRHE in positively going scans, decreasing strongly with increasing sweep rate. A corresponding reduction shoulder at about 0.66 VRHEVRHE was seen, probably caused by formation and reduction of NHx,adsNHx,ads or NadsNads species. Formation of Pt oxides was shifted to slightly higher potentials in the presence of ammonium, and the reduction charge of the Pt oxide reduction peak at 0.80 VRHEVRHE was independent of sweep rate, indicating that the amount of Pt oxides formed was limited by other adsorbates. In particular, the most strongly adsorbed hydrogen was shifted to lower potentials by adsorbed species formed at high potentials as well as adsorption of bisulfate mutually stabilized by ammonia. The voltammetric response in the hydrogen desorption region was not affected, showing that all adsorbed species were desorbed at low potentials in the negatively going scan.