Eletroconversão eficaz de nitrato em amônia usando um catalisador emaranhado de Co3O4 com nanofitas de grafeno

Abstract

There has been significant interest among scientists in the electrochemical reduction of nitrate (NO3⁻) to ammonium (NH4⁺) due to the useful application of NH4⁺ in fertilizers and nitrogen-based fuels. To carry out such a complex reduction reaction, which involves 8 electrons and 8 protons, it is necessary to develop high-performance and stable electrocatalysts that favor the formation of reaction intermediates selective for ammonia production. In the present study, a simple hydrothermal method was employed, involving the mixture of CoCl₂·6H₂O, urea, and graphene nanoribbons (GNR), to synthesize the following electrocatalysts: Co3O4(Cowt.%75), Co3O4(Cowt.%38)GNR, Co3O4(Cowt.%55)GNR, Co3O4(Cowt.%53)GNR, and GNR(Cowt. %0). The Co3O4(Cowt.%55)GNR catalyst exhibited the best performance, capable of converting NO3⁻ to NH4⁺ with a yield rate of 42.11 mg h⁻¹ mg⁻¹ catalyst, a Faradaic efficiency (FE) of 98.7%, a nitrate conversion efficiency of 14.71%, and an NH₄⁺ selectivity of 100%, using only 37.5 µg cm⁻2 of catalyst (20.6 µg cm⁻2 of Co applied), confirmed by loadings ranging from 19 to 150 µg cm⁻². The highly satisfactory results obtained for the proposed catalyst were favored by high values of electrochemically active surface area (ECSA) and low values of Rct, along with the presence of multiple planes in Co3O4 and the occurrence of a "complex (Co3(Co(CN)6)2(H2O)12)1.333" (entanglement involving the elements Co, C, O, and N) on the catalyst surface, as well as the effective migration of NO₃⁻ from the cathodic branch to the anodic one, which was confirmed in the experiment conducted using a H-cell separated by a Nafion 117 membrane. In-situ FTIR and Raman spectroscopy results helped identify the adsorbed intermediates, such as NO3⁻, NO2⁻, NO, and NH2OH, and the NH4⁺ product, which are consistent with the electroreduction mechanism. Density Functional Theory (DFT) calculations helped confirm that the Co3O4(Cowt.55%)GNR catalyst exhibited the best performance in terms of NO3⁻ electroreduction compared to Co3O4(Cowt.75%), considering the intermediates identified by in-situ FTIR and Raman spectroscopy results and the rate-determining step (RDS) observed for the transition from *NO to *NHO (0.43 eV).