Operando NAP-XPS Observation and Kinetics Analysis of NO Reduction over Rh(111) Surface: Characterization of Active Surface and Reactive Species

NO reduction by CO on Rh(111) was investigated by near-ambient pressure X-ray photoelectron spectroscopy, mass spectrometry, and kinetic analysis. Under exposure to NO + CO mixed gases and with heating the surface from room temperature to 450 °C, NO dissociation and NO reduction reaction start simultaneously independent of gas pressure ratio of NO/CO, which indicates that NO dissociation triggers this reaction. From kinetic analyses based on observed adsorbate coverages under reaction conditions, the following two points are suggested: (i) NO_hollow is a reactive species for N₂ and N₂O formation via N + NO reaction. (ii) At low temperatures, the N + NO reaction is dominant for N₂ production, whereas above around 400 °C, the N + N reaction becomes dominant, which leads to an increase in N₂ selectivity at the higher temperatures. Compared with the NO + CO reaction on Ir(111) surfaces, which exhibits a high N₂ selectivity, the adsorption site of reactive NO and the availability of vacant surface sites could be key factors for the lower N₂ selectivity for Rh(111).