Interaction of NO and O2 with Pd(111) surfaces. II |
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Authors: | H. Conrad G. Ertl J. Küppers E.E. Latta |
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Affiliation: | Institut für Physikalische Chemie der Universität München, D-8000 München, West-Germany |
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Abstract: | At least three different types of oxygen atoms may be present in the surface region of Pd(111) which may be distinguished by their thermal, chemical, structural and electronic properties. Exposure to O2 at low temperatures causes the formation of 2 × 2 and structures from chemisorbed oxygen, the latter being probably stabilized by small amounts of Hab or COab on the surface. The initial sticking coefficient was estimated to be about s0 ≈ 0.3, the adsorption energy ~55 kcal/mole. The photoelectron spectrum exhibits an additional maximum at 5 eV below EF. During thermal desorption dissolution of oxygen in the bulk strongly competes; on the other hand absorbed oxygen may diffuse to the surface giving rise to high temperature peaks in the flash desorption spectra. High temperature (~1000 K) treatment of the sample with O2 causes the formation of a more tightly bound surface species also characterized by a 2 × 2 LEED pattern which is chemically rather stable and which is considered to be a transition state to PdO. The latter compound is only formed by interaction with NO at about 1000 K via the reaction which offers a rather high “virtual” oxygen pressure. This reaction leads to drastic changes of the photoelectron spectrum and is also identified within the LEED pattern. |
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