Adsorption state of NO on Ir(111) surfaces under excess O2 coexisting condition

•The adsorption state of NO under O2 coexisting conditions on Ir(111) single-crystal surfaces was studied by NAP-XPS.•At room temperature, O2 causes a partial replacement of NO at hollow sites with atomic oxygen.•At elevated temperatures (200–250 °C), the adsorption of NO is significantly suppressed by high-density chemisorbed atomic oxygen. Iridium (Ir) exhibits a good efficiency for reduction of NO under excess O2 coexisting conditions at temperatures higher than 250 °C. On the other hand, the advantage is lost at lower temperatures below 250 °C under O2 existence, compared to any other platinum group metals. In this study, adsorption behavior of NO on Ir(111) single-crystal surfaces under excess O2 coexisting conditions at the temperatures from room temperature to 250 °C has been studied by in-situ X-ray Photoelectron Spectroscopy (XPS). It is revealed that the coexisting O2 induces formation of a high-density chemisorbed atomic oxygen (O) phase and the atomic O phase suppresses adsorption of NO at on-top sites significantly, which suggests that the suppression of NO adsorption causes a decrease in the ability of NO reduction at the lower temperatures. [Display omitted]