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The Adsorption of NO on Ru(001) and its CO-Adsorption with Oxygen Studied by Vibrational Spectroscopy
Journal of Electron Spectroscopy and Related Phenomena, 29 (1983) 261 ElsevierScientific Publishing Company, Amsterdam - Printed in The Netherlands
261
THE ADSORPTION OF NO ON Ru(OOl) AND ITS CO-ADSORPTION WITH OXYGEN STUDIED BY VIBRATIONAL SPECTROSCOPY W. Stenzel, H. Conrad, B. E. Hayden, K. Kretschmar andA. M. Bradshaw Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 1000 Berlin 33, West Germany
ABSTRACT The adsorption of NO on Ru(OOl), 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.l,2) that the adsorption of NO on Ru(OOl) 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 (2 x 2) 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 1/1 intensity in the desorption spectra (ref'.I) (the 1/2 peak remains essentially unchanged), while simultaneously producing a new TPD peak of NO (1/1 (0» with lower binding energy. In the corresponding EELS spectra these two species ("t and "t (0» are clearly distinguished. Our inability to observe anything other than the absorption band at ~1800cm-l in the IRAS experiments led us to repeat the EELS experiments (ref.2) together with TPD to elucidate more clearly the nature of the I/t molecular species (ref.l). 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-l and ~1810cm-l, associated with the 1/1 and 1/2 molecular species (ref.l), appear. We suggest that the low temperature, low coverage species is lying down, and the molecular species "I and "2 are both adsorbed on "on top" sites but corresponding to the bent and linear forms of the NO molecule, respectively. REFERENCES 1. 2.
P. Feulner, S. Kulkarni, E. Umbach and D. Menzel, Surf. Sci., 99 (1980) 489, and references therein. P. A. Thiel and W. H. Weinberg in A. T. Bell and M. L. Hair (Eds.) Vibrational Spectroscopies for Adsorbed Species, ACS Symposium Series, 137 (1980) 191, and references therein.
0368.2048/83/0000-0000/$03.00 © 1983 Elsevier Scientific Publishing Company
261
THE ADSORPTION OF NO ON Ru(OOl) AND ITS CO-ADSORPTION WITH OXYGEN STUDIED BY VIBRATIONAL SPECTROSCOPY W. Stenzel, H. Conrad, B. E. Hayden, K. Kretschmar andA. M. Bradshaw Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 1000 Berlin 33, West Germany
ABSTRACT The adsorption of NO on Ru(OOl), 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.l,2) that the adsorption of NO on Ru(OOl) 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 (2 x 2) 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 1/1 intensity in the desorption spectra (ref'.I) (the 1/2 peak remains essentially unchanged), while simultaneously producing a new TPD peak of NO (1/1 (0» with lower binding energy. In the corresponding EELS spectra these two species ("t and "t (0» are clearly distinguished. Our inability to observe anything other than the absorption band at ~1800cm-l in the IRAS experiments led us to repeat the EELS experiments (ref.2) together with TPD to elucidate more clearly the nature of the I/t molecular species (ref.l). 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-l and ~1810cm-l, associated with the 1/1 and 1/2 molecular species (ref.l), appear. We suggest that the low temperature, low coverage species is lying down, and the molecular species "I and "2 are both adsorbed on "on top" sites but corresponding to the bent and linear forms of the NO molecule, respectively. REFERENCES 1. 2.
P. Feulner, S. Kulkarni, E. Umbach and D. Menzel, Surf. Sci., 99 (1980) 489, and references therein. P. A. Thiel and W. H. Weinberg in A. T. Bell and M. L. Hair (Eds.) Vibrational Spectroscopies for Adsorbed Species, ACS Symposium Series, 137 (1980) 191, and references therein.
0368.2048/83/0000-0000/$03.00 © 1983 Elsevier Scientific Publishing Company