Thermal Activation of Nanoporous Gold for Carbon Monoxide Oxidation

Heterogeneous catalysts based on gold have attracted considerable attention over the last 30 years. In spite of its chemical inertness, the research on gold catalysis has proven that in oxidation reactions, for instance, high activities and, in particular, selectivities can be achieved. While mostly Au nanoparticles supported on suitable oxides have been focused on in the literature, more recently, nanoporous gold (npAu) was found to be an alternative that shows similarly good catalytic performance, even though the characteristic structural length (diameter of ligaments vs particle diameter) is an order of magnitude larger. To date, however, no recipe has been reported to reliably activate npAu for total oxidations, such as CO oxidation. Here, we present such an activation procedure that allows obtaining high conversion levels on the time scale of an hour, independently of the prehistory of the samples. It consists of one or several steps where the catalyst is subjected to a thermal treatment in the reaction mixture (CO + O2) at 300 °C for 1 min only. Previously, it was assumed that such a high-temperature treatment would result in coarsening of the nanoporous structure and thus in an intolerable loss of surface area. According to our results, however, short annealing steps hardly alter the ligament size but effectively remove residuals from the preparation and/or modify the surface chemistry so that a catalytically active state is obtained quickly. Reproducibly, rates can be achieved, which are in good agreement with values previously reported in the literature and are stable for at least several days on stream. Our results open the way for a practical use of npAu as well as for further studies on the mechanistic origin of its catalytic activity.