Nanoscaled metal

The geometry of the nanoscaled metals has an effect on the fluorescence enhancement. Theoretically, when the metal is introduced to the nanostructure, the total radiative decay rate will be written as T + rm, where Tm corresponds to the radiative decay rate close to the metal surface. So, (1) and (2) should be modified and the quantum yield and lifetime are represented as ... 

It is well established that ultrasmall metal clusters on supports have catalytic properties distinct from those properties of large bulk-like particles, as illustrated by the selective oxidation of propylene to propylene oxide by gold, alkene and arene hydrogenation catalysis,and CO oxidation. In these examples, the catalytic properties improve as the clusters become smaller. On the other hand, a reduction in size of the metal cluster can lead to less desirable catalytic properties as seen for ammonia synthesis on iron. Various explanations have been offered to account for the unique properties of nanoscaled metal catalysts, however, much remains to be understood. Clearly, experimental and theoretical studies will be required to develop an in-depth under-... 

Precious metal catalysts supported on nanoscaled metal fluorides 151... 

Nanoscaled Metal (Hydroxide) Fluoride-Catalyzed Fine Chemical Synthesis (All-rac)-(x-tocopherol (vitamin E) synthesis 155... 

The classical sol-gel synthesis certainly is one of the most powerful synthesis routes in terms of the wide variety of synthesis approaches and techitical applications. For a long time, especially the aqueous (hydrolytic) sol-gel synthesis route, mainly forced by the development of sitica, has been the main focus of thousands of chemists and materials scientists worldwide. Thus, it is no wonder that many nanoscaled metal oxide-based catalysts have been intensively investigated and reviewed for many different catalytic reactions. 

However, due to recent developments of new synthesis approaches toward nanoscaled metal fluorides, the interest in metal fluoride-based catalysis has received a significant boost over the past few years. 

Thus, it is the intention of this chapter (1) to provide a short introduction to the new non-aqueous fluorolytic sol-gel approach that gives for the first time direct access to novel nanoscaled metal fluorides, (2) to present the most essential catalysis-related properties, and (3) to give a comprehensive overview about the new catalytic applications that have been explored so far. 

Although several slightly differing synthesis routes toward nanoscaled metal fluorides have been explored very recently (reviewed by, e.g., Fedorov et al. [4]), they are not sol-gel based but bear several drawbacks. However, besides and in fact earlier than the fluorolytic, another indirect sol-gel route - the TFA route - was developed that will be briefly mentioned as well. 

Although most of the nanoscaled metal fluorides are amorphous or at least highly distorted, a few single crystals of intermediately formed aluminum alkoxide fluorides were isolated and structurally determined, thus giving deeper insights about the species formed in the course of reaction progress (Figure 6.3). 

Another exciting example for the superior catalytic power of nanoscaled metal fluorides is the catalytic cleavage of C-F bonds, which became possible for the first time without any precious metal and surprisingly under room temperature conditions. There is evidence that silylium-like species are created at the surface of ACF and HS-AIF3, as was evidenced by H MAS NMR spectroscopy as shown in Figure 6.1 la [65]. 

Lewis acidic nanoscaled metal fluorides were successfully used as cataljdically active support for platinum and palladium. These catalysts were found to be very efficient in n-pentane hydroisomerization reactions [71]. In situ preparation is an efficient approach toward very small precious metal nanoparticles (2 nm) finely distributed over the fluoride catalyst that exhibit very good activity, stability, and selectivity toward isomerization (up to nearly 1(X)%) at 350°C. The exciting requisite for this superior catalytic behavior has been shown to be the combination of very strong Lewis acidity of the HS-AIF3 with C-H and/or C-F bond activation... 

Nanoscaled Metal (Hydroxide) Fluoride—Catalyzed Fine Chemical Synthesis... 

However, these nanoscaled metal fluorides and hydroxide fluorides not only represent new, catalytically active classes of compounds with very high surface areas but are also excellent candidates to be used as supports for active components such as precious (e.g., Au, Pd, Pt) or nonprecious (e.g., Sn, Nb) metal deposition by incipient wetness impregnation or by in situ incorporation during the fluorolytic sol-gel synthesis. 

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