The electrochemical ammonia oxidation reaction (AOR) is a crucial process for addressing environmental challenges, enabling energy conversion, and producing value-added chemicals. Selectively oxidizing ammonia into nitrogen (N2) or products like nitrite (NO2−) and nitrate (NO3−) offers applications in nitrogenous wastewater remediation and ammonia-based fuel cells. However, the mechanisms of AOR, especially on NiOOH catalysts, are poorly understood, particularly regarding the concurrent oxygen evolution reaction (OER) and the role of dissolved oxygen (O2) in influencing AOR activity and selectivity.
This study employs in situ analytical techniques to explore the interplay between AOR and OER on NiOOH surfaces. Differential Electrochemical Mass Spectrometry (DEMS), Raman
Figure 1. DEMS electrochemical cell during operation.
spectroelectrochemistry, UV–vis spectroelectrochemistry, and Attenuated Total Reflection Infrared Reflection (ATR-IR) spectroelectrochemistry were used to monitor reaction intermediates, product formation, and structural transformations on NiOOH in real time. These tools provide a detailed understanding of how OER and dissolved O2 affect AOR pathways and catalyst stability.
In this study, HPR-40 DEMS was used in monitoring gaseous intermediates and products formed during the AOR on NiOOH surfaces, enabling real-time detection of key species, such as N2, N2O, and NO, under varying electrochemical conditions. This capability provided critical insights into reaction pathways, product selectivity, and the role of dissolved O2 in modulating the process. By combining DEMS with ex situ ion analysis, it was revealed that AOR selectivity is highly dependent on the applied potential. N2 production begins at ~1.52 VRHE, coinciding with the onset of the OER. However, at higher potentials, selectivity shifts toward the formation of NOx and NO2/3− species due to the competition between the OER and the AOR-to-N2 pathway.
Combining ATR-IR with DEMS further confirmed the significance of dissolved O2 in shaping product distribution and improving catalyst performance. Dissolved O2 not only facilitates NOx formation by interacting with surface intermediates, such as *ONHx species, but also mitigates catalyst deactivation by regenerating active sites on the NiOOH surface. At higher potentials, O2 promotes the formation of additional *OOH groups on the catalyst, which are essential for generating nitrogen-oxygen compounds. These findings highlight the dual role of dissolved O2 and OER in enhancing AOR selectivity and stability.
Figure 2 (left). Hiden Analytical DEMS electrochemical cell.
A critical challenge identified in AOR research is the deactivation of NiOOH in O2-deficient conditions. Without sufficient dissolved O2, *NHx intermediates accumulate on the catalyst surface, blocking active sites and suppressing OER activity. This results in rapid catalyst deactivation, as observed during long-term electrolysis experiments. In contrast, O2-rich environments prevent surface poisoning by facilitating the regeneration of NiOOH active sites, allowing efficient AOR and OER to proceed simultaneously. Isotopic labeling experiments with 18O2 confirmed that dissolved O2 is directly involved in NOx formation, further emphasizing its role in modulating reaction outcomes. Electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) analysis provided kinetic insights into these reactions. In O2-deficient conditions, NH3 adsorption slows down OER, leading to increasing resistance and catalyst deactivation. In contrast, O2-rich environments reduce the resistance associated with NH3 adsorption and charge transfer, improving reaction kinetics and catalyst performance.
By combining DEMS with spectroelectrochemical techniques, the study highlights the value of correlative in situ analysis for understanding AOR mechanisms. This integrated approach provides insights into NiOOH as a cost-effective catalyst for AOR and highlights the importance of real-time monitoring in unravelling complex electrochemical processes.
Figure 3. (right) Staircase electrolysis of AOR on Ni OOH/Ni(OH)2 in Ar/O2-saturated electrolytes: (A) Electrochemical signals; (B-C) DEMS results (N2, O2, NO,
N2O) in Ar- and O2-saturated electrolytes (1.0 M KOH + 100 mM NH3).
Project Summary by: Weiran Zheng, Associate Professor of Chemistry, Guangdong Technion–Israel Institute of Technology, Shantou, China Technion–Israel Institute of Technology, Haifa, Israel.
Paper Reference: Chen, J. Chen, S., Gao, J., Huang, X., Stavrou, E., Vogt, C., Zheng,
W. (2024) ‘Correlative in situ analysis of the role of oxygen on ammonia electrooxidation selectivity on NiOOH surfaces’. Journal of Catalysis. Elsevier BV, 438, October, p. 115720. DOI: 10.1016/j.jcat.2024.115720.
Hiden Product: HPR-40 DEMS.
