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Fem and volumetric studies of silver-nitrous oxide and silver-oxygen interactions
Authors:MMP Janssen  J Moolhuysen  WMH Sachtler
Institution:Koninklijke/Shell-Laboratorium, Amsterdam (Shell Research B.V.), Amsterdam, The Netherlands
Abstract:The non-dissociative and the dissociative adsorption of nitrous oxide and the adsorption of oxygen on silver have been studied by field-emission microscopy using whiskers and epitaxial layers on tungsten tips and volumetrically, with the aid of ultraclean thin films. At 77 K non-dissociative adsorption of nitrous oxide takes place, leading to a decrease in work function. At 273–473 K slow face-specific dissociative adsorption of nitrous oxide occurs, which causes an increase in work function and proceeds with an activation energy at low coverages of 29 ± 5 kJ mol?1. The adsorption of oxygen in this temperature range is more than 104 times faster and for low coverages work function-oxygen exposure plots yield an activation energy of 16 ± 3 kJ mol?1. The coverages reached above 1 Pa are constant and occur in the ratio 1:2:3.5 at 296, 373 and 473 K, the corresponding increases in work function being approximately 0.4, 0.6 and 0.8 eV. The oxygen adsorbed at low temperatures (≈ 273 K) is bound more loosely than that adsorbed at higher temperatures, which is shown by the partial desorption upon evacuation to low pressures (10?8 Pa) at 273 K and application of high electric fields (5 V/nm). The adsorbate formed in the presence of oxygen at 273 K can further be distinguished from the adsorbates formed in the presence of nitrous oxide at 273 K and oxygen at 473 K (both probably O=ads) by the higher reactivity towards hydrogen reduction and the easier thermal desorption, indicating that at 273 K molecular adsorption (O?2, ads) occurs.
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