Proceedings of MATSUS Spring 2024 Conference (MATSUS24)
Publication date: 18th December 2023
The Haber-Bosch industrial process was fundamental to enable the population growth over the 20th century. However, the fixation of atmospheric nitrogen caused a disturbance in global nitrogen cycle, leading to the accumulation of nitrogenous contaminants in water streams, especially nitrate, its most oxidized species. This nitrate leaching leads to serious environmental and human health consequences. Copper-based electrocatalysts for nitrate electrochemical reduction reaction (NO3RR) have received increasing attention in the last years due to their enhanced selectivity and activity toward ammonia. We investigate kinetically and spectroscopically how electrochemical cathodic conditions for nitrate conversion to ammonia modifies an electrodeposited Cu/Cu2O catalyst. Combining the kinetic evaluation of differently pre-reduced Cu/Cu2O composite compared with pure Cu alongside in situ Raman and X-ray absorption spectroscopies, we found that copper oxide reduces during NO3RR, leading to the formation of oxygen vacancies that boost the ammonia production at lower overpotentials (from −0.6 to −0.77 V vs. SHE), while copper itself is active at −1.1 V vs. SHE. We detected the formation of hydroxylamine through in situ Fourier transform infrared spectroscopy experiments and conclude that copper oxide’s previous reduction leads to the formation of hydroxylamine. Using neutral non-buffered electrolyte, we detected an increase in the catholyte’s pH from 5.8 to around 12 within 1 minute of electrolysis at the entire potential range we have evaluated (from −0.6 to −1.1 V vs. SHE). Considering that NO3RR to NH3 consumes 8 moles of H+ per mole of NH3, understanding the role of the electrolyte pH and identifying the proton source for this reaction is crucial to determine the best electrolyte conditions. We investigate the kinetics of Cu intrinsic activity for nitrate reduction at different pHs using phosphate buffer electrolytes. By employing rotating disk electrode, we can use the close control of mass transport to extract only kinetic current from nitrate NO3RR. We report a shift of reaction regime from pHs lower than 7 to pHs greater than 7. These findings would pave the way for both catalyst and electrolyte design for nitrate electrochemical reduction to ammonia.
Authors acknowledge support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) process numbers: G.F.C 2019/18847-6, 2022/01799-1; M. R. P. 2019/08244-2; R.N. 2023/02841-4, 2022/14169-6. M. W., T. M. and R.N. acknowledge Shell-CINE (Center for Innovation on New Energies; Division 1: Dense Energy Carriers (2017/11986-5). I. M. acknowledge Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) grant number 88887.339501/2019-00. N. S. acknowledges support from NSF Grant no. 2247194. This research used resources of the Brazilian Synchrotron Light Laboratory (LNLS) and Brazilian Nanotechnology National Laboratory (LNNano), both part of the Brazilian Center for Research in Energy and Materials (CNPEM), a private non-profit organization under the supervision of the Brazilian Ministry for Science, Technology, and Innovations (MCTI). The Carnaúba beamline staff are acknowledged for their assistance during the experiments.
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