Chemisorption of NO on polycrystalline Pd surface studied by TDS, AES, UPS and XPS

Abstract

The adsorbed state of NO on polycrystalline Pd surfaces has been studied by thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES), ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). UPS results showed that molecular adsorption was characterized by additional maxima at -3.5, -8.8 and -14.2 eV at room temperature below the Fermi level which were derived from the highest occupied molecular orbitals of NO. These peaks almost disappeared by heating to about 500 K, and another additional maximum appeared at -6 eV above 400 K which was derived from atomic orbitals of N and O, and persisted up to 700 K. XPS and TDS results indicated the presence of two states in NO adsorption on the Pd surface at room temperature which were characterized by adsorption in bridge sites and linear single on-top sites, and the NO adsorption states depended on the amount of CO in the residual gases. Small amounts of N 2O and CO 2 were observed to desorb during heating of adsorbed NO.