Characterization by Synchrotron-Radiation X-Ray Photoelectron Spectroscopy of NO Adsorption on Rh

Rhodium nanoparticles were deposited by the evaporation method on a silicon wafer. The process used He gas in a Rh evaporation chamber that was connected to the load-lock chamber of BL6N1 at Aichi Synchrotron Radiation Center (Aichi SR). NO gas was introduced into the evaporation chamber, and the deposited Rh nanoparticles were exposed to NO with and without an ambient atmosphere to study the effect of O-2 on the electronic properties of the Rh nanoparticles and the behavior of NO molecules using synchrotron-radiation X-ray photoelectron spectroscopy (SR-XPS). The diameter of the Rh nanoparticles was estimated by transmission electron microscopy to be 2.7 +/- 0.6 nm. The XPS analysis indicates that the deposited Rh nanoparticles before NO exposure were in the metallic state. After exposure to NO, a part of the outermost surface of the Rh nanoparticles changed into Rh oxide, and NO molecules, atomic N and O species, and NO2 were detected. On the other hand, after exposure to NO in ambient air, the surface of the Rh nanoparticles was preferentially oxidized. In addition, a few NO molecules were adsorbed on the surface of Rh, and a small amount of the adsorbed NO that reacted with the surface oxygen turned into NO2.