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.

Synthesis of stable AuAg bimetallic nanoparticles encapsulated by diblock copolymer micelles

We present a facile method for the preparation of bimetallic AuAg nanoparticles (NPs) with controlled size and composition rendering them ideally suitable for optical and catalytic applications. In analogy to methods for the generation of monometallic Au and Ag NPs, AuAg NPs were prepared inside polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block-copolymer micelles formed in toluene, by loading the P4VP cores of the micelles first with AgNO(3) and then with HAuCl(4). In contrast to the reverse sequence of loading, homogenously bimetallic AuAg particle arrays were achieved after reduction carried out in solution with hydrazine monohydrate as the reducing agent. TEM reveals that stable and spherical NPs can be prepared well separated from one another and with a narrow size distribution with diameters of ∼3 nm. The bimetallic NP composition was confirmed by energy-dispersive X-ray spectroscopy (EDX) of single NPs. The atomic ratio of Ag and Au contained in single particles is in good agreement with the relative concentrations of both metals used in the synthesis which was confirmed by atomic absorption spectroscopy. The atomic ratio Au : Ag was systematically varied between 3 : 1 and 1 : 3. For all ratios UV-vis spectra showed a single plasmon band. Its wavelength varied from 430 for Au : Ag = 1 : 3 to 515 nm for Au : Ag = 3 : 1, showing a linear dependence on the relative amount of gold within the range of plasmon wavelengths from monometallic gold (538 nm) to silver (415 nm).

Supported colloidal nanoparticles in heterogeneous gas phase catalysis: on the way to tailored catalysts

Using colloidally synthesized nanoparticles for the preparation of supported catalysts offers several advantages (e.g. precise control of particle size and morphology) when compared to traditional preparation techniques. Although such nanoparticles have already been very successfully used for catalytic applications in the liquid phase, applications in heterogeneous gas phase catalysis are still scarce. One aspect, usually considered as a problem, is organic stabilizers typically employed during the nanoparticle synthesis since they or their decomposition products are supposed to block catalytically active sites on the nanoparticle surface. Thus, in many studies so far, the removal of the organic ligands prior to use in gas phase catalysis has been proposed. In this perspective article, however, we will discuss a number of benefits such ligand shells may have for heterogeneous gas phase catalysis, including the protection against chemical modification, prevention of sintering and tuning of SMSI effects.

Colloidal synthesis and structural control of PtSn bimetallic nanoparticles

PtSn bimetallic nanoparticles with different particle sizes (1-9 nm), metal compositions (Sn content of 10-80 mol %), and organic capping agents (e.g., amine, thiol, carboxylic acid and polymer) were synthesized by colloidal chemistry methods. Transmission electron microscopy (TEM) measurements show that, depending on the particle size, the as-prepared bimetallic nanocrystals have quasi-spherical or faceted shapes. Energy-dispersive X-ray (EDX) analyses indicate that for all samples the signals of both Pt and Sn can be detected from single nanoparticles, confirming that the products are actually bimetallic but not only a physical mixture of pure Pt and Sn metal nanoparticles. X-ray diffraction (XRD) measurements were also conducted on the bimetallic particle systems. When compared with the diffraction patterns of monometallic Pt nanoparticles, the bimetallic samples show distinct shifts of the Bragg reflections to lower degrees, which gives clear proof of the alloying of Pt with Sn. However, a quantitative analysis of the lattice parameter shifts indicates that only part of the Sn atoms are incorporated into the alloy nanocrystals. This is consistent with X-ray photoelectron spectroscopy (XPS) measurements that reveal the segregation of Sn at the surfaces of the nanocrystals. Moreover, short PtSn bimetallic nanowires were synthesized by a seed-mediated growth method with amine-capped bimetallic particles as precursors. The resulting nanowires have an average width of 2.3 nm and lengths ranging from 5 to 20 nm.

ALD functionalized nanoporous gold: thermal stability, mechanical properties, and catalytic activity

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 1,000°C, while simultaneously making the material stronger and stiffer. The catalytic activity of np-Au can be drastically increased by TiO(2) ALD coatings. Our results open the door to high-temperature sensor, actuator, and catalysis applications and functionalized electrodes for energy storage and harvesting applications.

Colloidally prepared Pt nanowires versus impregnated Pt nanoparticles: comparison of adsorption and reaction properties

Ligand-capped Pt nanowires, prepared by colloidal synthesis and deposited on a high surface area γ-Al(2)O(3) support, were subjected to surface characterization by electron microscopy and FTIR spectroscopy using CO as a probe molecule. The structural, adsorption, and catalytic reaction properties of the colloidal Pt nanowires were compared to those of conventional, impregnated Pt nanoparticles on the same Al(2)O(3) support. In situ FTIR spectroscopy indicated ligand effects on the CO resonance frequency, irreversible CO-induced surface roughening upon CO adsorption, and a higher resistance of colloidal catalysts toward oxidation (both in oxygen and during CO oxidation), suggesting that the organic ligands might protect the Pt surface. Elevated temperature induced a transformation of Pt nanowires to faceted Pt nanoparticles. The colloidal catalyst was active for hydrodechlorination of trichloroethylene (TCE), but no ligand effect on selectivity was obtained.