Nanoporous gold: a new gold catalyst with tunable properties

Abstract

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.

Nanoporous Gold Electrodes and Their Applications in Analytical Chemistry

“…In the catalytic oxidation of CO, the presence of small amounts of silver has been shown to play an important role [95,96,112]. In contrast, for methanol oxidation, increases in Ag do not improve activity [113]. Ag has also been shown to contribute to the SERS enhancement observed at nanoporous gold [114] and has also been shown to influence the electrochemically measured surface area [64,83].…”

“…There have been several approaches to deal with mass transport limitations. Increasing the pore sizes via controlled coarsening can help [113]. Early on, Ding and Erlebacher proposed the use of a two-step dealloying process to generate a multimodal pore network [119].…”

“…Restricted diffusion and pore blocking can be very relevant in nanoporous with very small pore sizes and hence large surface areas. Mass transport restrictions can be very important in high surface area materials with narrow pore size distributions [113]. There have been several approaches to deal with mass transport limitations.…”

Catalysis by gold: New insights into the support effect

“…For example, depositing TiO 2 or PrO 2 onto the surface of NPG could greatly enhance the reactivity for CO oxidation [101], which could be regarded as inversely RMO supported gold catalyst. The strong synergy between gold and the RMO further confirmed that the active sites for CO oxidation were located at the interfaces.…”

Nanoporous Gold by Alloy Corrosion: Method-Structure-Property Relationships

“…for CO oxidation) is under discussion. [22][23][24][25] As another instance, reports on the mechanical strength and stiffness of nominally comparable structures disagree by up to one order of magnitude. 26,27 Therefore it is significant to note that a number of different dealloying protocols are applied within these studies.…”