Coupled Differential Electrochemical Mass Spectrometry and Surface-Enhanced Infrared Absorption Spectroscopic Studies Unravel the Mechanism of Nitric Oxide Electroreduction on Platinum

Abstract

The nitric oxide electroreduction reaction (NORR) has received considerable attention due to its importance in electrochemical denitrification of nitrogen oxides in groundwater and industrial waste gases and electrochemical ammonia synthesis. However, the detailed mechanism and the factors that affect product selectivity are far less understood. Employing coupled differential electrochemical mass spectrometry (DEMS) and attenuated total reflection–surface-enhanced infrared absorption (ATR–SEIRA) spectroscopy, adsorbed species and volatile solution products, during the adsorption of NO and NORR on Pt in both alkaline and acidic media, have been simultaneously studied, enabling us to correlate the potential-dependent product selectivity with the surface ad-species. NO_ad,M, NO_ad,B, NO_ad,L, and NO_2,ad were identified using SEIRA spectroscopy as surface ad-species, with their potential-dependent intensities having a strong correlation with the product selectivity. N₂O is the only reduction product at potentials beyond the hydrogen region and is attributed to the reduction of weakly adsorbed NO. In contrast, the formation of NH₃ and NH₂OH occurs only in the hydrogen region and is ascribed to the reaction between strongly adsorbed NO and adsorbed H. N₂ is a minor product, and is formed through further reduction of N₂O by adsorbed H. The formation of N₂ is significantly suppressed in acidic media due to the fast kinetics of NO reduction to NH₃/NH₂OH, and thus lowering of NO coverage in the hydrogen region. To achieve the selective reduction of NO to NH₃/NH₂OH, the potential should remain at 0.1–0.2 V (vs RHE) in both acidic and alkaline media while a slow NO supply, and acidic media are preferred over alkaline media due to the faster kinetics. These new spectroscopic results and insights about the NORR could advance the design of more effective NORR catalysts and help develop optimal conditions for selective ammonia synthesis.