CO oxidation on gold tips :in situ imaging and local chemical probing by means of field ion microscopy and atom probe techniques

Gold has been known as being almost completely inactive as a catalyst. However, it was found recently that when dispersed as nanometre-size gold particles on metal-oxide supports it exhibits a surprisingly high activity in the CO oxidation at low temperatures. This has prompted a debate on the 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. In this context, the CO oxidation over Au crystal surfaces was studied under in-situ conditions at pressures below 10-1 Pa in the absence of a support material using video-FIM (Field Ion Microscopy). Chemical waves were observed to move across the catalyst surface. Local chemical probing (~300 surface sites) by short field pulses allowed to monitor the chemical surface composition during wave propagation. Partial positive charges at step sites due to the application of imaging fields of several V/nm cause the sticking probability of oxygen molecules to increase by many orders of magnitude. This ensures the presence of significant oxygen surface coverages to perform either titration or co-adsorption experiments with CO gas. In both cases chemical fronts were observed and associated with the formation of CO2. The mass spectra change substantially while waves move across the probe-hole: they contain only oxygen ions first and become dominated by Au carbonyls (AuCOn+ and Au(CO)2n+) later on. The results suggest CO oxidation over pure Au to involve step sites. In the present study, the electric field helps activating the reactants while in gold-based catalysts this activation is most likely provided in the interface between the Au particles and the metal-oxide support.