Catalysis by unsupported skeletal gold catalysts.

Catalysis is one of the key technologies for the 21st century for achieving the required sustainability of chemical processes. Critical improvements are based on the development of new catalysts and catalytic concepts. In this context, gold holds great promise because it is more active and selective than other precious metal catalysts at low temperatures. However, gold becomes only chemically and catalytically active when it is nanostructured. Since the 1970s and 1980s, the first type of gold catalysts that chemists studied were small nanoparticles on oxidic supports. With the later onset of nanotechnology, a variety of nanostructured materials not requiring a support or organic stabilizers became available within about the last 10 years. Among these are gold nanofoams generated by combustion of gold compounds, nanotube membranes prepared by electroless deposition of gold inside a template, and corrosion-derived nanoporous gold. Even though these materials are macroscopic in their geometric dimensions (e.g., disks, cubes, and membranes with dimensions of millimeters), they are comprised of gold nanostructures, for example, in the form of ligaments as small as 15 nm in diameter (nanoporous gold, npAu). The nanostructure brings about a high surface to volume ratio and a large fraction of low coordinated surface atoms. In this Account, we discuss how unsupported materials are active catalysts for aerobic oxidation reaction in gas phase (oxidation of CO and primary alcohols), as well as liquid phase oxidation and reduction reactions. It turns out that the bonding and activation of molecular oxygen for gas phase oxidations strongly profits from trace amounts of an ad-metal residue such as silver. It is noteworthy that these catalysts still exhibit the special gold type chemistry, characterized by activity at very low temperatures and high selectivity for partial oxidations. For example, we can oxidize CO over these unsupported catalysts (npAu, nanotubes, and powder) at temperatures well below water's freezing point (-30 °C) and with turnover frequencies up to 0.5 s(-1) (at 30 °C). Yet, we can anticipate the surface chemistry of these unsupported and extended gold surfaces based on model experiments under UHV conditions. We have demonstrated this for the selective oxidation of primary alcohols at low temperatures employing npAu catalysts. Chemists have paid growing interest to oxidation and reduction reactions in liquid phase catalysis, most suitable for synthetic organic chemistry. Early work on the aerobic oxidation of d-glucose in 2008 using Raney type npAu already showed the potential of this type of catalyst for liquid phase reactions. Since then, researchers have investigated further oxidation reactions (silanes to silanols) and reduction reactions of alkynes, as well as C-C coupling reactions ([4 + 2] benzannulation) and azo compound decomposition, with likely several more reactions to be reported in the next years. The advantage of this unsupported skeletal type of catalyst is its recyclability and retrievability without leaching of gold into the reaction medium, owing to its monolithic structure. Even though these materials contain nanoscopic structures, they are macroscopic in their geometric dimensions and pose no threat to the environment or health as discussed for other nanomaterials.