Catalyst-, Potential-, and pH-Dependent Product Selectivity of NO Electroreduction on Transition Metals Studied by...

The electrochemical NO reduction reaction (NORR) has been exploited as an alternative method to scrub NO from industrial and residential effluents as well as a route for ammonia synthesis. However, the product selectivity and electroactivity are strongly affected by numerous factors, and the detailed mechanism remains elusive. In this work, the NORR on Pd, Ru, Ir, Au, Ag, and Cu in both alkaline and acidic media was studied with coupled differential electrochemical mass spectrometry and attenuated total reflection-surface-enhanced infrared absorption spectroscopy. The potential-dependent product formation in both alkaline and acidic media and surface-adsorbed species could be simultaneously monitored, enabling us to correlate the product selectivity with adsorbed species and to elucidate the NORR mechanism. At potentials >0.4 V, N₂O was the dominant product on all studied metals, and the kinetics for N₂O formation were enhanced in alkaline media when compared to acidic media. Cu was the most selective and effective catalyst for the NORR to NH₃/NH₂OH. NH₃/NH₂OH were also the predominant products on Pt, Ru, and Ir in both alkaline and acidic media and on Ag and Au in alkaline media at potentials of hydrogen evolution/adsorption. The NORR on Pd selectively generated N₂ at 0.175 V in alkaline media but not in acidic media. These in situ experimental data provide new mechanistic insights into the NORR selectivity that could inform/guide the design of more effective and selective NORR catalysts and establish optimal operation conditions for the electrochemical synthesis of selected products.