Reactivity of SO2 and NH3 on copper well-defined surfaces: an IRAS investigation

FT-IR spectroscopy has been used in the reflection mode on metal surfaces t o in situ investigate several systems involved in the current catalytic pro cesses as well as in environment-related problems. The reactivity Of SO2, S O2 + O-2, SO2 + H2O, SO2 + i-C4H8 and also NH3 with well controlled copper surfaces has been monitored by IRAS at room temperature in the presence of reactants (P = 10(-5)-10(-4) Torr). Various molecular surface complexes hav e been identified their geometry and binding mode have been deduced from th e position and the number of vibration bands. Both the reactivities Of SO2 and NH3 appear to be highly sensitive to the structure and to the level of oxidation of the surface. SO2 mostly dissociates on metallic coppers it does react with adsorbed oxyg en or lattice oxygen of an oxide layer and leads to sulphites and sulphates coordinated to the surface via oxygen. As an example, on Cu( 1 1 0), biden tate sulphates are predominant when oxygen is adsorbed and mobile on the su rface, conversely, monodentate sulphites are formed by interaction Of SO2 m olecules with lattice surface oxygen. A similar evolution was observed on C u(1 0 0). NH3 is adsorbed on Cu(1 1 0) and, in the presence of oxygen in th e gas phase, Cu-NH2, Cu-NH species are detected on the surface as intermedi ates of the selective oxidation of NH3 into N-2. In the presence of water, SO, leads to complex species; hydrogen-sulphates are formed by an electroph ilic attack of OH groups by the S atom Of SO2. Finally, the interaction Of SO2 and i-C4H8 with Cu(1 1 0) has revealed that these two species co-adsorb and react in the adsorbed layer. These results have been used to explain t he mechanism of poisoning of the reduction of NO on copper in the presence of isobutene and oxygen. The data, here presented, show how performing the IRAS technique is for a real-time in situ monitoring of the reactivity of p lanar surfaces. (C) 2001 Elsevier Science B.V. All rights reserved.