Nanoporous gold: a new gold catalyst with tunable properties.

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.