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.