Gold as a catalytic material has recently attracted much interest in the catalysis community (see, e.g. refs. [1, 2] and references therein). Although inert as a bulk material, it can be catalytically very active in the form of nanometer-sized particles on suitable supports as first pointed out by Haruta et al. [3] While a lot of questions regarding the underlying mechanisms are still under debate, it is evident that low-coordinated Au atoms play a decisive role in the catalytic cycle of reactions such as the low-temperature oxidation of CO or the selective oxidation of olefins. [3][4][5][6] Surface science studies and theoretical calculations strongly support this conclusion: At low-coordinated Au atoms, CO binds more strongly than on regular terrace sites, [7][8][9][10] oxygen is more easily dissociated, [11] and new size dependent reaction channels exist on nanoparticles. [12] Consequently, the stability of low-coordinated Au atoms is of crucial importance for any technical application of Au catalysts. Unfortunately, Au particles have a strong tendency to coalesce so that severe problems result from sintering of the particles to large inactive aggregates. [13][14][15] In order to elucidate the fundamental question of how lowcoordinated Au atoms can be stabilized we perform a UHV model study starting with a well-ordered flat AuA C H T U N G T R E N N U N G (111) singlecrystal surface. In a number of studies it has been demonstrated that a high concentration of low-coordinated sites can be generated in a controllable fashion by ion bombardment. Sputtering of gold surfaces by argon ions, for example, induces a variety of surface morphologies [16] which range from monatomic vacancy islands [17,18] to a rough pit-and-mound structure, [19] depending on the experimental parameters. In comparison, experimental results on surface morphologies generated by sputtering with oxygen ions are scarce, although a recent study indicates that oxygen stabilizes low-coordinated Au sites generated during oxygen ion bombardment. [20] Here, we use the adsorption of oxygen during oxygen ion bombardment to study oxidation reactions as well as the influence of the adsorbed oxygen on the surface morphology.
The stabilizing effect of oxygen is deduced from Figure 1 comparing scanning tunneling microscopy (STM) images of Arand oxygen-ion-bombarded AuA C H T U N G T R E N N U N G (111) surfaces. Both surfaces exhibit a similar pit-and-mound morphology, however the char-