Formation of N 2 O and (NO) 2 During NO Adsorption on Au 3D Crystals
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Catalysis Letters Vol. 98, Nos. 2–3, November 2004 (Ó 2004)
Formation of N2 O and (NO)2 during NO adsorption on Au 3D crystals T-D. Chau*, T. Visart de Bocarme´, and N. Kruse Chemical Physics of Materials, Universite´ Libre de Bruxelles, Campus Plaine, CP 243, B-1050 Brussels
Received 20 July 2004; accepted 27 July 2004
The adsorption of NO on Au 3D hemispherical crystals (‘‘field emitter tips’’) has been studied by means of pulsed field desorption mass spectrometry (PFDMS) under dynamic gas flow conditions and at 300 K. Local chemical probing of 200 Au sites in the stepped surface region between the central (111) pole and the peripheral (001) plane leads to the detection of NOþ , N2 Oþ and (NO)þ 2 species. Obviously, molecular NO adsorption on stepped Au surfaces can lead to dimerization. Nitrous oxide formation probably occurs via the dimer, (NO)2 . KEY WORDS: chemical probing; gold nano-crystals; nitric oxide; nitrous oxide.
1. Introduction Nitric oxide reduction on supported noble metal catalysts frequently leads to unwanted amounts of nitrous oxide (N2 O). Various formation mechanisms have been proposed but no general agreement seems to have been reached as yet. The relevant literature published until 1998 has been reviewed by Nieuwenhuys [1] from the viewpoint of a surface science approach to catalysis. More recently, on the basis of Steady state isotopic transient kinetic analysis (SSITKA) of the NO/O2 /H2 reaction over Pt/SiO2 catalysts [2] a mechanistic network of NO conversion into N2 O and N2 has been proposed. The scheme includes dimeric (NO)2 as an intermediate of N2 O formation. Although such dimers have not been explicitly identified by SSITKA, the authors emphasize that the route to N2 O formation, which is fast, must be through a final step involving a coupled intermediate weakly bound to the catalyst surface and in reversible equilibrium with the gas phase. Subsequent calculations using density functional theory (DFT) have indeed shown [3] that the activation barrier for the NOad + NOad reaction is considerably lower than the one for the Nad + NOad reaction. Studies of the NO reduction over Au based catalysts are scarce. Ueda and Haruta [4] have reported on N2 O formation at 323 K during the NO + CO reaction. Mechanistic aspects in this work have not been addressed. No report on N2 O formation has been made by Deckers et al. [5] during a study of the NO + H2 reaction. The authors mention that NO adsorption on Au/Al2 O3 was not detectable by infrared spectroscopy. We wish here to present first results of an ‘‘atomprobe’’ PFDMS study (Pulsed Field Desorption Mass Spectrometry) on the low-pressure adsorption of NO on pure Au at 300 K. We demonstrate the ultimate *To whom correspondence should be addressed.
sensitivity of the instrument in detecting species like nitrous oxide and dimeric (NO)2 at coverages far below the monolayer limit. 2. Experimental The experiments were performed in a home-built ‘‘atom-probe’’ device similar to the one described earlier [6]. Standard field ion micrographs were taken
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