Potential- and Time-Dependent Operando X-Ray Absorption Study of Cu₂O Microcrystals Transformations during Nitrate...

Area of Science:

• Electrochemistry
• Materials Science
• Catalysis

Background:

• The Haber-Bosch process for ammonia synthesis is energy-intensive and relies on fossil fuels.
• Electrochemical nitrate reduction reaction (NO3RR) presents a sustainable, carbon-neutral pathway for ammonia production.
• Copper-based catalysts are promising for NO3RR, but understanding their surface evolution is crucial.

Purpose of the Study:

• To investigate the chemical states and structural evolution of copper(I) oxide (Cu2O) microcrystals during electrochemical nitrate reduction.
• To elucidate the role of nitrate adsorption in passivating the copper surface and influencing reaction pathways.
• To determine the optimal conditions for selective ammonia synthesis via NO3RR.

Main Methods:

• Time-resolved operando X-ray absorption spectroscopy (XAS) was employed to monitor catalyst changes in situ.
• Electrochemical measurements were performed across a potential range of +0.6 to -0.7 V versus reversible hydrogen electrode (VRHE).
• Copper(I) oxide microcrystals deposited on carbon fiber (Cu2O/C) were used as the catalyst.

Main Results:

• Nitrate adsorption passivates the Cu2O surface, retarding its reduction to metallic copper (Cu(0)) at lower potentials.
• Ammonia formation initiates at -0.3 VRHE and its Faradaic efficiency (FE) increases with more negative potentials.
• Fully reduced Cu particles achieved an 89.7% ammonia FE at -0.7 VRHE, demonstrating high activity and selectivity.

Conclusions:

• Surface passivation by nitrate adsorption is key to promoting selective electron transfer for ammonia synthesis.
• The transformation of Cu2O to Cu(0) is essential for efficient NO3RR, with metallic copper particles facilitating intermediate hydrogenation.
• Optimized copper catalysts can achieve high ammonia yields, positioning NO3RR as a viable sustainable alternative.