Nanoporous metals have many technologically promising applications, but their tendency to coarsen limits their long-term stability and excludes high temperature applications. Here, we demonstrate that atomic layer deposition (ALD) can be used to stabilize and functionalize nanoporous metals. Specifically, we studied the effect of nanometer-thick alumina and titania ALD films on thermal stability, mechanical properties, and catalytic activity of nanoporous gold (np-Au). Our results demonstrate that even only 1 nm thick oxide films can stabilize the nanoscale morphology of np-Au up to 1000°C, while simultaneously making the material stronger and stiffer. The catalytic activity of np-Au can be drastically increased by TiO2 ALD coatings. Our results open the door to high-temperature sensor, actuator, and catalysis applications and functionalized electrodes for energy storage and harvesting applications.

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.

Porous bulk materials are of great interest in catalysis because they can be employed in heterogeneous gas and liquid phase catalysis, electrocatalysis, and in electrocatalytic sensing. Nanoporous gold gained considerable attraction in this context because it is the prime example of a corrosion-derived nanoporous bulk metal. The material was shown to be a very active and selective Au type catalyst for a variety of oxidation reactions. By leveraging the functionalization of the surface of the material with various additives, its catalytic applications can be extended and tuned. In this review, we will summarize recent developments in using nanoporous gold as the platform for the development of high performance catalytic materials by adding metals, metal oxides, and molecular functionalities as building blocks.

Inspired by model studies under ultrahigh vacuum (UHV) conditions, inverse monolithic gold/ceria catalysts are prepared using thermal decomposition of a cerium nitrate precursor on a nanoporous gold (npAu) substrate. Cerium oxide deposits throughout the porous gold material (pores and ligaments 30−40 nm) are formed. npAu disks and coatings were prepared with loadings of about 3 to 10 atom % of ceria. The composite material was tested for the water−gas shift (WGS) reaction (H 2 O + CO → H 2 + CO 2 ) in a continuous flow reactor at ambient pressure conditions. Formation of CO 2 was observed at temperatures as low as 135°C with excellent stability and reproducibility up to temperatures of 535°C. The considerably increased thermal stability of the material can be linked to the presence of metal oxide deposits on the nanosized gold ligaments. The loss of activity after about 15 h of catalytic conversion with heating to 535°C was only about 10%. Photoemission spectroscopy indicates a defect (Ce 3+ ) concentration of about 70% on the surface of the cerium oxide deposits, prior to and after WGS reaction. Raman spectroscopic characterization of the material revealed that the bulk of the oxide is reoxidized during reaction. ■ INTRODUCTIONDuring the last decades increasing demand for a novel type of water−gas shift (WGS) catalyst in the context of mobile and green energy harvesting such as in fuel cells surfaced. 1,2 For either low-temperature fuel cells (polymer electrolyte membrane fuel cells, PEMFC) or high-temperature fuel cells in mobile applications such as in cars a novel type of WGS catalyst is required. These catalysts need to be highly active at low temperatures, shifting CO almost quantitatively to hydrogen (<10 ppm for PEMFCs). They need to be safe, nonpyrophoric and oxidation resistant upon exposure to air (excluding, thus, traditional WGS catalysts), highly durable and long-lasting and have to withstand fast deactivation and shut down procedures. Last but not least they have to be readily applicable into small scale reactor designs, ideally as coatings or monolithic catalyst beds. The focus of this latest surge in research regarding WGS catalysts are precious metal based catalysts. 1,3Among possible candidates, gold is of particular interest as it is cheaper than, e.g., platinum and also a very selective catalyst for the oxidation of CO in the presence of H 2 , making it an ideal catalyst material for procession of hydrogen gas for fuel cells. 4 Recently, Au-CeO 2 nanomaterials have been reported to be very efficient catalysts for the WGS reaction. 2 The interplay between the oxide and the Au provides reactivity toward the dissociation of water. Similarly, as reported for oxidation reactions using molecular oxygen, the synergy between the two partners is critical for the catalytic activity. 5,6 Oxygen vacancies existing in the oxide nanoparticles are suggested to play a key role for the dissociation of water. 7 A blend of catalysts consisting of metal nanoparticles dispersed on an oxide support has been report...

Nanoporous gold (npAu) supports were prepared as disks and powders by corrosion of Au-Ag alloys. The npAu materials have pore sizes in the range of 40 nm as shown by scanning electron microscopy (SEM). The surface was modified by a self-assembled monolayer (SAM) with an azidohexylthioate and then functionalized by a zinc (II) phthalocyanine (ZnPc) derivative using “click chemistry”. By atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectrometry (ICP-MS) the content of zinc was determined and the amount of immobilized ZnPc on npAu was calculated. Energy-dispersive X-ray (EDX) spectroscopy gave information about the spatial distribution of the ZnPc throughout the whole porous structure. NpAu and ZnPc are both absorbing light in the visible region, therefore, the heterogeneous hybrid systems were studied as photocatalysts for photooxidations using molecular oxygen. By irradiation of the hybrid system, singlet oxygen is formed, which was quantified using the photooxidation of 1,3-diphenylisobenzofuran (DPBF) as a selective singlet oxygen quencher. The illuminated surface area of the npAu-ZnPc hybrid system and the coverage of the ZnPc were optimized. The synergistic effect between the plasmon resonance of npAu and the photosensitizer ZnPc was shown by selective irradiation and excitation of only the phthalocyanine, the plasmon resonance of the npAu support and both absorption bands simultaneously, resulting in an enhanced photooxidation activity by nearly an order of magnitude.