Operando observation of NO reduction by CO on Ir(111) surface using NAP-XPS and mass spectrometry: Dominant reaction pathway to N₂ formation under near realistic conditions

The nitric oxide (NO) reduction by carbon monoxide (CO) on Ir(111) surfaces under near ambient pressure conditions was studied by a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and mass spectrometry (MS), particularly paying attention to the dominant reaction pathway to formation of molecular nitrogen (N₂). Under a relatively low CO pressure condition (50 mTorr NO + 10 mTorr CO), two reaction pathways to form N₂ are clearly observed at different ignition temperatures (280 and 400 °C) and attributed to a reaction of NO adsorbed at atop site (NO_atop) with atomic nitrogen (N_ad) and associative desorption of N_ad, respectively. Since the adsorption of NO_atop is inhibited by CO adsorbed at atop site (CO_atop), the ignition of the NO_atop + N_ad reaction strongly depends on the coverage of CO_atop; the ignition temperature shifts to higher temperature as increasing CO pressure. In contrast, for the N_ad + N_ad reaction the ignition temperature keeps almost constant (∼400 °C). The online MS results indicate that the latter reaction is the dominant pathway to N₂ formation and the former one less contributes to N₂ formation with accompanying a small amount of nitrous oxide (N₂O). No evidence for contribution of the isocyanate (NCO) species as an intermediate was observed in the operando NAP-XP spectra. (Graph Presented).