Supported nanoparticles, from metal

The reaction dynamics studies on supported nanoparticles are relatively not numerous. However, two reactions have been studied in detail CO oxidation and NO reduction by CO. From these studies it is clear that some differences exist between extended metal surfaces and supported particles. At least... 

Figure 4.4.1 Schematic representation of the model systems discussed within the chapter (A) nanoparticle growth influenced by dopants in the support, (B) nanoparticle deposition from solution, (C) strong metal support interaction, and (D) photochemistry at supported nanoparticles as a function of size.

Nonlinear optical infrared-visible sum frequency generation (IR-vis SFG) is a versatile surface-specific vibrational spectroscopy that meets the requirements mentioned above. SFG provides vibrational spectra of molecules adsorbed on a surface, while the molecules in the gas phase do not produce a signal. Consequently, SFG can be operated in a pressure range from UFIV to ambient conditions and still detects only the adsorbed species. A direct comparison of adsorbate structures under UFIV and elevated pressure is therefore feasible. Furthermore, SFG can be applied to molecules adsorbed on single crystals, thin films, metal foils, and supported nanoparticles (46,116-121) and is thus a promising tool to extend surface science experiments to more realistic conditions. 

Figure 3 TEM picture of a typical supported metal catalyst, silver nanoparticles (dark spots) deposited on a high surface area alumina support. (Reprinted from Ref 15. 1998, with permission from Elsevier)...

From catalysis it is well-known that the metal-substrate interaction influences the reactivity of supported nanoparticles. For instance, for noble metal particles on oxidic supports, the hydrogenation and hydrogenolysis activity is much greater if the support has a higher acidity (high concentration of acidic —OH groups at the surface) than for neutral or alkaline oxidic supports. The influence of the presence of a support on the catalytic activity of metal nanoparticles has been ascribed to [70, 75-79] ... 

In addition to the benefits of MEF from metal nanostructures deposited onto solid supports that are very useful in surface bioassays, MEF can also be observed from individual nanostructures in bioassays carried out in solution. In this regard, fluorophores and metal nanostructures can be assembled in core-shell architecture and can be used as fluorescent nanoparticles as indicators in biological plications such as imaging of cellular activity or single-molecule sensing. 

Prockl et al. [60] measured the concentration of leached Pd species from palladium nanoparticles supported on a metal oxide via atomic absorption spectroscopy as a function of time in solution. The data indicated that the largest concentration of Pd species in solution (Pd " and/or Pd(0)) occurred during the reaction (Fig. 18.6). As the reaction neared 100% conversion, the soluble Pd concentration returned to the original value, presumably due to readsorption onto the metal oxide substrate. The process was concluded by the authors to have clearly involved heterogeneous reactions [60]. This data supports a catalytic mechanism that is heterogeneous in nature, where the reaction occurs at the interface and causes the dissolution of surface atoms into solution. This explanation is supported by the report of Prockl et al. that individual reactants did not initiate nanoparticle dissolution but that dissolution was observed during the reaction over the 25-50 min time interval when the conversion was the highest (Fig. 18.6). 

With the advent of synthetic methods to produce more advanced model systems (cluster- or nanoparticle-based systems either in the gas phase or on planar surfaces), we come to the modern age of surface chemistry and heterogeneous catalysis. Castleman and coworkers demonstrate the large influence that charge, size, and composition of metal oxide clusters generated in the gas phase can have on the mechanism of a catalytic reaction. Rupprechter (Chap. 15) reports on the stmctural and catalytic properties of planar noble metal nanocrystals on thin oxide support films in vacuum and under high-pressure conditions. The theme of model systems of nanoparticles supported on planar metal oxide substrates is continued with a chapter on the formation of planar catalyst based on size-selected cluster deposition methods. In a second contribution from Rupprecther (Chap. 17), the complexities of surface chemistry and heterogeneous catalysis on metal oxide films and nanostructures, where the extension of the bulk structure to the surface often does not occur and the surface chemistry is often dominated by surface defects, are discussed. 

PEM fuel cells use a solid proton-conducting polymer as the electrolyte at 50-125 °C. The cathode catalysts are based on Pt alone, but because of the required tolerance to CO a combination of Pt and Ru is preferred for the anode [8]. For low-temperature (80 °C) polymer membrane fuel cells (PEMFC) colloidal Pt/Ru catalysts are currently under broad investigation. These have also been proposed for use in the direct methanol fuel cells (DMFC) or in PEMFC, which are fed with CO-contaminated hydrogen produced in on-board methanol reformers. The ultimate dispersion state of the metals is essential for CO-tolerant PEMFC, and truly alloyed Pt/Ru colloid particles of less than 2-nm size seem to fulfill these requirements [4a,b,d,8a,c,66j. Alternatively, bimetallic Pt/Ru PEM catalysts have been developed for the same purpose, where nonalloyed Pt nanoparticles <2nm and Ru particles <1 nm are dispersed on the carbon support [8c]. From the results it can be concluded that a Pt/Ru interface is essential for the CO tolerance of the catalyst regardless of whether the precious metals are alloyed. For the manufacture of DMFC catalysts, in... 

Figure 43 Schematic of an electrochemically promoted metal catalyst film supported on an 0 conductor (top) and schematic of cylindrical or, more generally, fixed cross-section nanoparticles deposited on an 0 conducting support (bottom). (From Ref. 138.)...

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