Operando Observation of NO Reduction by CO on Ir(111) Surface Using NAP-XPS and Mass Spectrometry: Dominant Reaction Pathway to N-2 Formation under Near Realistic Conditions

Abstract

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 (N-2). Under a relatively low CO pressure condition (50 mTorr NO + 10 mTorr CO), two reaction pathways to form N-2 are clearly observed at different ignition temperatures (280 and 400 degrees C) and attributed to a reaction of NO adsorbed at atop site (NOatop) with atomic nitrogen (N-ad) and associative desorption of N-ad, respectively. Since the adsorption of NOatop is inhibited by CO adsorbed at atop site (COatop), the ignition of the NOatop + N-ad reaction strongly depends on the coverage of COatop; the ignition temperature shifts to higher temperature as increasing CO pressure. In contrast, for the Nad + Nad reaction the ignition temperature keeps almost constant (similar to 400 degrees C). The online MS results indicate that the latter reaction is the dominant pathway to N-2 formation and the, former one less contributes to N-2 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.