Infrared spectroscopic, thermal desorption and X-ray photoelectron spectroscopic studies of NO, NO + CO and NO + O2 adsorbed on palladium surfaces
Abstract
The adsorbed state of NO on polycrystalline Pd metal surfaces, its variation with heat treatment and the effect of CO and O2 have been studied by infrared spectroscopy (silica-supported), thermal desorption (powder) and X-ray photoelectron spectroscopy (powder, foil). The i.r. spectra of NO adsorbed at 298 K showed that there exist mainly bent or bridged NO (1660–1650, 1580–1570 cm–1) for θNO < 0.5 and linear NO (1750–1730 cm–1) for θNO > 0.6. Upon heat treatment, these i.r. bands changed in different ways. X.p.s. showed a broad N 1s peak of molecular NO (398.7 eV) at 298 K and another peak due to NO2-like surface species (404.0–404.2 eV) at 453 K. For θNO < 0.6, the t.d. spectra exhibited a main N2 peak at ca. 490 K, whereas for θNO > 0.6 additional peaks of NO appeared at 373–473 K. It is proposed that in N2 formation the dissociation of NO is rate-determining. Changes in the adsorbed state of NO upon heat treatment are explained by the interaction of NO with O atoms remaining on the surface after part of the NO is decomposed. The effect of preadsorbed oxygen supports this view. Preadsorbed CO works as a scavenger for this O atom and accelerates the formation of N2.