Electrosynthesis of nitrate by FeS2-TiO2 heterogeneous nanoparticles supported on 2-methylimidazolium functionalized polypyrrole/graphene oxide

Electrochemical nitrogen oxidation reaction (NOR) acts as a clean and sustainable approach for promisingly replacing the conventional industrial method for producing nitrate. Designing and preparing the effective electrocatalysts for NOR has aroused great research interests. Herein, FeS2-TiO2 heterostructures are well-dispersed on the surface of 2-methylimidazolium functionalized polypyrrole/graphene oxide (2-MeIm/PPy/GO) by the in situ growth due to the ion-exchange and the coordination between the 2-MeIm groups and the metal precursors, which exhibits the excellent NOR electroactivity. The highest NO3– yield by the obtained FeS2-TiO2@2-MeIm/PPy/GO reaches to 137.04 μg h−1 mg-1act. with the maximum Faradic efficiency (FE) of 4.38 % at 2.04 V (vs. RHE), better than most reported electrocatalysts. The good selectivity for producing nitrate is also achieved, but the stability of both the NO3– yield and FE present a trend of rising and then declining in the cyclic test of FeS2-TiO2@2-MeIm/PPy/GO. The irreversible conversion of the electrocatalysts can be confirmed through detailed characterization towards the crystal structures and chemical states of FeS2-TiO2@2-MeIm/PPy/GO after long term NOR test. The electrochemical step of NOR mainly occurs at FeS2, converting the inert N2 to active *NO intermediates. SO42- generated at high NOR potential gradually increase by the oxidation of S element, which can cause the promotion of the NOR performance before the fourth cycle. But along with the disappearance of FeS2 phase, FeS2-TiO2 as the catalytic center is finally irreversibly converted to Fe and S ions doped TiO2, resulting in the sharp drop of both the NO3– yield and FE at the sixth cycle. This work will provide valuable guidance for the designing and preparing NOR electrocatalysts.