Since Au turned out to be an active catalyst for CO oxidation at low temperatures, CO adsorption on various Au surfaces has been in the scope of numerous surface science studies. Interestingly, supported particles as well as stepped and rough single-crystal surfaces exhibit very similar adsorption behavior. To elucidate the origin of these similarities, we have performed temperature-programmed desorption and infrared absorption spectroscopy for a whole range of Au surfaces from nanoparticles grown on HOPG to Au(111) surfaces roughened by argon ion bombardment. In line with previous results, we have observed two desorption states at ∼130-145 and ∼170-185 K, respectively, and one infrared peak at around 2120 cm -1 in all cases. In addition to the experiments, we have carried out theoretical studies of CO adsorption on Au(332). The calculations show that CO desorption states above 100 K may be located at step-edges but not on terrace sites. Reducing the coordination of Au atoms further leads to successively higher binding energies with an unchanged anharmonic frequency. Therefore, we conclude that both desorption peaks belong to CO on low-coordinated Au atoms at steps and kinks. For the sputtered Au(111) surface, scanning tunneling microscopy reveals a rough pit-and-mound morphology with a large number of such sites. In annealing experiments we observe that the loss of these sites coincides with the loss of CO adsorption capacity, corroborating our conclusions.

Understanding the role of surface chemistry in the stability of nanostructured noble-metal materials is important for many technological applications but experimentally difficult to access and thus little understood. To develop a fundamental understanding of the effect of surface chemistry on both the formation and stabilization of self-organized gold nanostructures, we performed a series of controlled-environment annealing experiments on nanoporous gold (np-Au) and ion-bombarded Au(111) single-crystal surfaces. The annealing experiments on np-Au in ambient ozone were carried out to study the effect of adsorbed oxygen under dynamic conditions, whereas the ion-bombarded Au single-crystal surfaces were used as a model system to obtain atomic-scale information. Our results show that adsorbed oxygen stabilizes nanoscale gold structures at low temperatures whereas oxygen-induced mobilization of Au surface atoms seems to accelerate the coarsening under dynamic equilibrium conditions at higher temperatures.

Low‐coordinated gold atoms play a decisive role in the catalytic cycle of low‐temperature CO oxidation or the selective oxidation of olefins. A comparative study of argon and oxygen ion‐bombarded Au(111) surfaces (see microstructure images) reveals a threefold role of adsorbed oxygen: structure formation, stabilization of low‐coordinated gold atoms, and transfer during oxidation reactions.

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Gold with a nanoporous sponge-like morphology, generated by leaching of AuAg alloys is presented as a new unsupported material system for catalytic applications. The role of residual silver for catalytic activity towards CO oxidation in the temperature range from −20 to 50°C has been investigated by comparison with Au and Au/Ag zeolite catalysts. As revealed by a systematic variation of the silver content in the zeolite catalysts, bimetallic systems exhibit a significantly higher activity than pure gold, probably due to activation/dissociation of molecular oxygen by silver. By STEM tomography we can unambiguously prove that at least some of the particles form inside the zeolite lattice

Cobalt and bimetallic Co−Pd systems are well-known Fischer−Tropsch catalysts. Compared to Co, the bimetallic systems exhibit an increased activity toward CO hydrogenation and methane conversion, attributed to resistance against oxidation. To study the oxidation behavior, model catalysts have been generated by depositing either Co or first Co and subsequently Pd onto a thin epitaxial alumina film grown on NiAl(110). Pure Co particles and bimetallic particles with a Co core and a Pd shell have been studied before and after exposure to oxygen and after thermal treatments, using X-ray photoelectron spectroscopy (XPS), temperature-programmed CO desorption (TPD), ferromagnetic resonance (FMR), and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. Large doses of O2 (1000 langmuirs; 1 langmuir = 10-6 Torr·s) at 300 K lead to complete oxidation of Co particles. Upon annealing to temperatures above 530 K, XPS indicates that the cobalt oxide is mostly reduced by transfer of oxygen to the alumina support, resulting in its thickening. TPD, however, indicates the existence of persistent surface oxygen species, reducing the CO adsorption energy on the particles. Exposures to small doses of O2 (30−50 langmuirs) were also studied by a careful comparison of XPS, TPD, and FMR data. In this case, XPS indicates Co in a metallic state, whereas TPD and FMR indicate oxidic behavior. We conclude that small amounts of non-stoichiometric subsurface oxygen or subsurface and surface oxygen are present which are not detectable in the Co 2p XPS signal but have a pronounced effect on the surface chemistry and the magnetism, i.e., on certain bulk properties. In the case of bimetallic Co/Pd particles, an incomplete Pd shell on the Co particles even promotes oxygen uptake, while only a complete Pd shell inhibits oxidation.

Ion-beam-sputtering (IBS) single-layer and multilayer coating designs for UV applications were examined after the deposition process as well as after a defined postdeposition treatment. High internal compressive film stress as well as moderate absorption losses in the UV spectral range were measured at the as-deposited thin films. Due to a controlled postdeposition treatment process, the absorption losses and the high compressive stress can be reduced significantly. We show that the remaining thin-film stress of SiO2 and HfO2 multilayer designs can be specifically manipulated by the parameters of the postdeposition treatment. Even zero and tensile stress can be achieved for complex multilayer coatings.

Bimetallic nanoparticles often turn out to be superior to the corresponding monometallic systems with respect to their catalytic properties. To study such effects for the methanol decomposition reaction, model catalysts were prepared by physical vapor deposition of Pd and Co under ultrahigh-vacuum (UHV) conditions. Monometallic Pd and Co particles as well as CoPd core-shell particles were generated on an epitaxial alumina film grown on NiAl(110). The interaction with methanol is examined by temperature-programmed desorption of methanol and carbon monoxide and by X-ray photoelectron spectroscopy. The decomposition of methanol proceeds in two reaction pathways independent of the particle composition: complete dehydrogenation towards carbon monoxide and hydrogen, and C--O bond scission yielding carbon deposits. Pd is the most active material studied here. The relative importance of the two channels varies for the different particle systems: on Pd dehydrogenation is preferred, whereas the C--O bond cleavage is more pronounced on Co. The bimetallic clusters show a moderate performance for both pathways. Carbon deposition poisons the model catalysts by blocking the adsorption sites for methoxide, which is the first intermediate product during methanol decomposition. In particular on Co, large amounts of carbon deposits can also be caused by dissociation of the final product of the dehydrogenation pathway, carbon monoxide. A comparison with the results of methanol decomposition on Co, Pd, and CoPd catalysts in continuous-flow reactors demonstrates that the findings of the present UHV study are relevant for catalytic performance under high-pressure conditions.