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

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.