The adsorption of NO on Ru(001) and its co-adsorption with oxygen studied by vibrational spectroscopy

Abstract The adsorption of NO on Ru(001), and its co-adsorption with oxygen, has been studied by LEED, TPD, EELS and IRAS (with particular emphasis on the vibrational spectroscopies) over a wide range of temperatures. It has been well established (ref.1,2) that the adsorption of NO on Ru(001) at room temperature is initially dissociative with molecular adsorption taking place only after a dissociative layer is formed. It therefore seemed appropriate to study the effect of oxygen co-adsorption on the adsorption of NO under well defined conditions. The dissociation layer is characterized by a (2x2) LEED structure and is found to influence the subsequent molecular adsorption of NO in exactly the same way as a saturated, pre-chemisorbed layer of oxygen. One effect of oxygen co-adsorption is to suppress the ν 1 intensity in the desorption spectra (ref.1) (the ν 2 peak remains essentially unchanged), while simultaneously producing a new TPD peak of NO ( p 1 (O)) with lower binding energy. In the corresponding EELS spectra these two species ( ν 1 and ν (O)) are clearly distinguished. Our inability to observe anything other than the absorption band at ∼1800cm −1 in the IRAS experiments led us to repeat the EELS experiments (ref.2) together with TPD to elucidate more clearly the nature of the ν 1 molecular species (ref.1). The results indicate that the adsorption of NO at low temperature (∼95K) initially produces a species of NO with an N-O stretch frequency of 1400cm −1 . This is the only species observed up to an exposure of 0.5 Langmuirs, and on warming to room temperature it dissociates completely to produce the (2x2) pattern described above. Pre-adsorption of oxygen prevents the formation of this species of NO. Only subsequent to the saturation of this species during adsorption at low temperature do two bands at ∼ 1490cm −1 and ∼ 1810cm −1 , associated with the ν 1 and ν 2 molecular species (ref.1), appear. We suggest that the low temperature, low coverage species is lying down, and the molecular species ν 1 and ν 2 are both adsorbed on “on top” sites but corresponding to the bent and linear forms of the NO molecule, respectively.