Ammonia electrooxidation on dendritic Pt nanostructures in alkaline solutions investigated by in-situ FTIR spectroscopy and online electrochemical mass spectroscopy

Abstract

Dendritic Pt nanostructures were prepared through electrodeposition on a substrate Pt electrode by applying square-wave potential method. It has been found that the as-prepared dendritic Pt nanostructure exhibited enhanced IR absorption with an enhancement factor up to 10 folds for adsorbed CO species. Such an enhanced in-situ FTIR spectroscopy (FTIRS) has been applied in studies of reaction mechanism of ammonia electrooxidation. From in-situ FTIR spectra recorded during ammonia electrooxidation, two characteristic IR bands at 1430cm−1 and between 1227 and 1250cm−1 were observed at low potential region (E<−0.50V vs. SCE), and are assigned to adsorbed NH2,ad and N2H4,ad, respectively. The assignment of the band between 1227 and 1250cm−1 has been also confirmed through studies by using isotopic 15NH3−NaOH and NH3−NaOH – D2O systems. Furthermore, in spectra collected at high electrode potential region (E>0.10V vs. SCE), two IR bands at 2231cm−1 and 1236cm−1 were observed and ascribed respectively to N2O and NO2− species, which are the ultimate oxidation products detected under present investigation conditions. In addition to in-situ FTIR spectroscopy, online electrochemical mass spectroscopy (OEMS) was used to detect volatile products. The clear OEMS signals of m/e=30 and m/e=46 measured at potentials above −0.5V and −0.30V, respectively, indicate the production of N2 (IR inactive) and confirm the generation of N2O. Based on results of cyclic voltammetry, in-situ FTIRS and OEMS, the reaction mechanism is therefore elucidated with molecular details of intermediates and products involved in ammonia electrooxidation in alkaline solutions.