Electrosynthesis of nitrate by FeS 2-TiO 2 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, FeS 2 -TiO 2 heterostructures are welldispersed 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 exhibit the excellent NOR electroactivity. The highest NO 3 - yield by the obtained FeS 2 - TiO 2 @2-MeIm/PPy/GO reaches to 137.04 mu g h -1 mg -1 act. 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 NO 3 - yield and FE present a trend of rising and then declining in the cyclic test of FeS 2 -TiO 2 @2-MeIm/PPy/GO. The irreversible conversion of the electrocatalysts can be confirmed through detailed characterization towards the crystal structures and chemical states of FeS 2 -TiO 2 @2-MeIm/PPy/GO after long term NOR test. The electrochemical step of NOR mainly occurs at FeS 2 , converting the inert N 2 to active *NO intermediates. SO 4 2- 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 FeS 2 phase, FeS 2 -TiO 2 as the catalytic center is finally irreversibly converted to Fe and S ions doped TiO 2 , resulting in the sharp drop of both the NO 3 - yield and FE at the sixth cycle. This work will provide valuable guidance for the designing and preparing NOR electrocatalysts.