CO oxidation on Au tips :in-situ imaging and local chemical probing

Nanometre-size gold particles on metal-oxide supports have recently been shown to be surprisingly active in the CO oxidation at low temperatures. This has prompted a debate onthe use of such catalysts to reduce the cold-start emission of motor vehicles. An atomic-level understanding of the catalytic mechanisms in operation is currently not available. In particular, a clear distinction between metal-support interactions and metal-only effects is missing. We have therefore undertaken a video-FIM study to image the CO oxidation under truly in-situ conditions on the surface of small Au tips. To elucidate the local chemical composition while imaging, atom-probe studies have been performed using PFDMS (Pulsed Field Desorption Mass Spectrometry). The interaction of pure CO with Au field emitter tips at 300 K (monitoring each time a few up to several hundred atomic sites) causes Au carbonyl singly and doubly charged cations, AuCOn+ and Au(CO)2n+ to appear in PFDMS spectra. The formation of these species is promoted by applying a steady electric field of several V/nm. Partially positive charges in step sites seem to play an important role in this promotion. In a similar manner, oxygen molecules can be activated in step sites to overcome the barrier for dissociation. We have found that the sticking probability of O2 is higher by many orders of magnitude in the presence of an electric field than in its absence. Video-FIM reveals cluster formation during the extended interaction of oxygen gas with the Au surface. After adsorption of CO gas at 300 K titration of surface oxygen starts and extends nearly isotropically (like a “grass-fire”) from areas close to the top (111) pole toward the outskirts of the tip surface. Moreover, chemical waves have been observed to move across the surface while dosing the Au tip simultaneously with mixtures of O2 and CO. Local chemical probing with PFDMS has demonstrated the transition from an oxygen-covered to a COcovered surface (dominated by Au-carbonyls) while a wave is moving across the monitored area. The results demonstrate the usefulness of combined video-FIM and atom-probe studies to elucidate catalytic reaction mechanisms under model conditions. In the present study, the electric field helps activating the reactants while in Au based catalysts this activation is most likely provided in the interface between the Au particles and the metal-oxide support.