Despite the general inertness of gold, finely dispersed gold nanoparticles on suitable oxide supports can demonstrate remarkable catalytic activity for the epoxidation of propene or the oxidation of CO, for example. [1][2][3][4][5][6][7] Gold-based catalysts have potential applications in automotive emission control, because unlike platinum or palladium catalysts, they remain active at low temperatures (room temperature).[8] While various support materials, particle synthesis routes, and deposition techniques have been investigated over the years, [9,10] the mechanisms responsible for the catalytic activity are still under debate, because of the complexity of the particle-support interactions and the reaction pathways.Research to date has shown that the particle size, type of support material, and particle-support contact structure play major roles. [6,11,12] In contrast to supported gold catalyst systems, unsupported systems, such as gold powder, have not yet drawn much attention, although remarkably high catalytic activity for CO oxidation has been attained with such systems. [13] Moreover, unsupported gold catalysts allow the relevant catalytic mechanisms to be more easily understood and also make new applications accessible. Herein, we demonstrate that high catalytic activity is not necessarily linked to the presence of finely dispersed particles. Nanoporous gold with a spongelike morphology, formed through the selective leaching of silver from a gold-silver alloy, [14][15][16] has an unexpectedly high catalytic activity for CO oxidation at ambient pressures and temperatures down to À20 8C. Sintering can hamper the catalytic applications of gold particles; in contrast, nanoporous gold has good thermal stability, and its morphology can be easily reproduced.The spongelike morphology of the nanoporous gold used herein consists of interconnecting ligaments with diameters[*] Dr.

The unique properties of gold especially in low temperature CO oxidation have been ascribed to a combination of various effects. In particular, particle sizes below a few nanometers and specific particle−support interactions have been shown to play important roles. In contrast, recent reports revealed that monolithic nanoporous gold (npAu) prepared by leaching a less noble metal, such as Ag, out of the corresponding alloy can also exhibit a remarkably high catalytic activity for CO oxidation, even though no support is present. Therefore, it was claimed to be a pure and unsupported gold catalyst. We investigated npAu with respect to its morphology, surface composition, and catalytic properties. In particular, we studied the reaction kinetics for low temperature CO oxidation in detail, taking the mass transport limitation due to the porous structure of the material into account. Our results reveal that Ag, even if removed almost completely from the bulk, segregates to the surface, resulting in surface concentrations of up to 10 atom %. Our data suggest that this Ag plays a significant role in activating of molecular oxygen. Therefore, npAu should be considered a bimetallic catalyst rather than a pure Au catalyst.

Nanostructured materials are governed by their surface chemical properties. This is strikingly reflected by np-Au. This material can be generated by corrosion of bulk Ag-Au alloys. Based on a self-organisation process, a 3 dimensional sponge like gold structure evolves with ligaments in the range of only a few tens of nanometers. Due to its continuous porosity, the material can be penetrated by gases which then adsorb and interact with the surface. In this perspective we will review potential applications of np-Au resulting from this effect, namely heterogeneous gas phase catalysis, surface chemistry driven actuation, and adsorbate controlled stability of the nanostructure. We will summarize the current knowledge about the low temperature oxidation of CO as well as the highly selective oxidation of methanol. Furthermore, we will address the question how surface chemistry can influence the material properties itself. In particular, we will deal with (a) the actuation of np-Au by the reversible oxidation of its surface using ozone and (b) the adsorbate controlled coarsening of ligaments, using annealing experiments under ozone or inert gas atmosphere.

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.

Colloidal solutions of metal nanoparticles are currently among the most studied substrates for sensors based on surface-enhanced Raman scattering (SERS). However, such substrates often suffer from not being cost-effective, reusable, or stable. Here, we develop nanoporous Au as a highly active, tunable, stable, biocompatible, and reusable SERS substrate. Nanoporous Au is prepared by a facile process of free corrosion of AgAu alloys followed by annealing. Results show that nanofoams with average pore widths of ∼250nm exhibit the largest SERS signal for 632.8nm excitation. This is attributed to the electromagnetic SERS enhancement mechanism with additional field localization within pores.

The evolution of the grain structure, internal strain, and the lattice misorientations of nanoporous gold during dealloying of bulk (3D) Ag-Au alloy samples was studied by various in situ and ex situ X-ray diffraction techniques including powder and Laue diffraction. The experiments reveal that the dealloying process preserves the original crystallographic structure but leads to a small spread in orientations within individual grains. Initially, most grains develop in-plane tensile stresses, which are partly released during further dealloying. Simultaneously, the feature size of the developing nanoporous structure increases with increasing dealloying time. Finally, microdiffraction experiments on dealloyed micron-sized nanoporous pillars reveal significant surface damage introduced by focused ion beam milling.

Nanoporous gold (np-Au) represents a novel nanostructured bulk material with very interesting perspectives in heterogeneous catalysis. Its monolithic porous structure and the absence of a support or other stabilizing agents opens up unprecedented possibilities to tune structure and surface chemistry in order to adapt the material to specific catalytic applications. We investigated three of these tuning options in more detail: change of the porosity by annealing, increase of activity by the deposition of oxides and change of activity and selectivity by bimetallic effects. As an example for the latter case, the effect of Ag impurities will be discussed. The presence and concentration of Ag can be correlated to the availability of active oxygen. While for the oxidation of CO the activity of the catalyst can be significantly enhanced when increasing the content of Ag, we show for the oxidation of methanol that the selectivity is shifted from partial to total oxidation. In a second set of experiments, two different metal-oxides were deposited on np-Au, praseodymia and titania. In both cases, the surface chemistry changed significantly. The activity of the catalyst for oxidation of CO was increased by up to one order of magnitude after modification. Finally, we used adsorbate controlled coarsening to tune the structure of np-Au. In this way, even gradients in the pore- and ligament size could be induced, taking advantage of mass transport phenomena.

The growth of titanium oxide nanoparticles on reconstructed Au(111) surfaces was investigated by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Ti was deposited by physical vapor deposition at 300 K. Regular arrays of titanium nanoparticles form by preferential nucleation of Ti at the elbow sites of the herringbone reconstruction. Titanium oxide clusters were synthesized by subsequent exposure to O 2 at 300 K. Two-and three-dimensional titanium oxide nanocrystallites form during annealing in the temperature range from 600 to 900 K. At the same time, the Au(111) surface assumes a serrated, <110> oriented step-edge morphology, suggesting step-edge pinning by titanium oxide nanoparticles. The oxidation state of these titanium oxide nanoparticles varies with annealing temperature. Specifically, annealing to 900 K results in the formation of stoichiometric TiO 2 nanocrystals as judged by the observed XPS binding energies. Nano-dispersed TiO 2 on Au(111) is an ideal system to test the various models explaining the enhanced catalytic reactivity of supported Au nanoparticles. IntroductionMetal oxides are of considerable interest because of their technological importance, e.g. in the field of heterogeneous catalysis, where they are used as catalyst supports for a wide variety of metals. The interaction of the adsorbed metal particles with the oxide support is complex and gives rise to interesting phenomena such as the strong metal support interaction (SMSI). This term describes the change in catalytic activity (favoring hydrogenation) observed when group VIII B metals supported on reducible oxides (TiO 2 , TaO 5 , CeO 2 , NbO) are annealed in a reducing atmosphere [1]. Transition metal oxides are also used to support other oxides in the socalled monolayer catalysts. For example, V 2 O 5 /TiO 2 systems are used for the partial oxidation of methanol, and have been extensively studied in single crystal as well as in powder form [2][3][4][5].