Photoelectron spectroscopic study of CO and NO adsorption on Pd(100) surface under ambient pressure conditions

Abstract We report molecular CO and NO adsorption structures on Pd(100) single crystal surfaces under sub-Torr pressure conditions at room temperature. A combination of ambient pressure X-ray photoelectron spectroscopy and density functional theory based calculations indicates that CO molecules adsorb at 2-fold bridge sites with (2√2 × √2)R45°, (3√2 × √2)R45° and (4√2 × √2)R45° structures under the CO gas pressures at room temperature. Furthermore a higher-density adsorption structure is reversibly formed at 0.5 Torr CO, where all the CO molecules keep sitting on the bridge sites. In case of NO adsorption, at 10 − 7  Torr, a (2√2 × √2)R45° structure with occupying bridge sites is formed, which is the same as the case of CO. However, subsequent gas introduction up to 0.5 Torr causes additional site occupation of on-top sites. Furthermore NO molecules partially dissociate into atomic N and O species but they disappear from the surface due to further reaction with NO under ambient conditions.