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 θ_NO < 0.5 and linear NO (1750–1730 cm^–1) for θ_NO > 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 θ_NO < 0.6, the t.d. spectra exhibited a main N₂ peak at ca. 490 K, whereas for θ_NO > 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.