Oxidation with Catalytic Nanoparticles

The continued development of new single-source molecular precursors should lead to increasingly complex mixed-element oxides with novel properties. Continued work with grafting methods will provide access to novel surface structures that may prove useful for catalytic apphcations. Use of molecular precursors for the generation of metal nanoparticles supported on various oxide supports is another area that shows promise. We expect that the thermolytic molecular precursor methods outlined here will contribute significantly to the development of new generations of advanced materials with tailored properties, and that it will continue to provide access to catalytic materials with improved performance. 

We showed that the application of PEG/CO2 biphasic catalysis is also possible in aerobic oxidations of alcohols [15]. With regard to environmental aspects it is important to develop sustainable catalytic technologies for oxidations with molecular oxygen in fine chemicals synthesis, as conventional reactions often generate large amoimts of heavy metal and solvent waste. In the biphasic system, palladium nanoparticles can be used as catalysts for oxidation reactions because the PEG phase both stabilises the catalyst particles and enables product extraction with SCCO2. 

Bulk Au is a noble metal. Goodman and co-workers,1301 however, found that Au nanocrystals supported on a titania surface show a marked size-effect in their catalytic ability for CO oxidation reaction, with Au nanoparticles in the range of... 

FIGURE 13 Graded oxide nanoparticle. Mo02 was oxidized with air at 723 K to give a core-shell structure of molybdenum dioxide and possibly molybdenum trioxide that was identified by its different electron energy loss spectrum. No structural description of the highly disordered and catalytically relevant outer oxide shell could be determined, either with XRD or with TEM, (or even with EXAFS spectroscopy) as the signals are dominated by the core structure. 

K. Mallick, M. S. Scurrell, CO oxidation over gold nanoparticles supported on TiOi and TiOi-ZnO Catalytic activity effects due to surface modification of TiOi with ZnO, Appl. Catal. A 253 (2003) 527. 

Apparent advantage of the prefabricated nanoparticles hes in the opportunity to use well-developed procedures to control particle size and particle size distribution and to characterize the particles by all possible means. Prefabricated particles can be introduced in the mesoporous materials in two ways. One avenue is direct incorporation of particles in sol-gel mixture as was recently reported in [60]. Here mesostructured nanoparticle-sihca monoliths have been synthesized by dispersing prefabricated Au or zeohte (sihcate) nanoparticles in sol-gel precursors containing SiCl4 and a Pluronic triblock copolymer template. To improve the compatibility of the Au nanoparticles with the poly(ethylene oxide) block, the nanoparticle surface was modified with a SiOj layer. Therefore, this technique requires good compatibihty of nanoparticles with the components of the sol-gel reaction, as it influences the distribution of nanoparticles through the material. Another hmitation of this method is encapsulation of particles within mesoporous material, but not necessary location in the pores. As was discussed above, this limitation is especially crucial for catalytic applications. 

The catalytic properties of mesoporous materials with embedded nanoparticles are mainly determined by the type of the inclusion (particle). All catalytic reactions, which are normally known for the particular metals or alloys, can be carried out with mesoporous soHds containing nanoparticles. The important advantage of mesoporous oxides is their stability at high temperatures. Due to this feature, mesoporous oxides with nanoparticles can be successfully used as catalysts in such reactions where nanoparticles embedded in polymeric systems cannot be employed. Another probable advantage of mesoporous catalysts is an appropriate use of pores as nanoreactors of certain size. This can be applicable to large molecules or to cyclization reaction where pore size and shape will influence the reactive path [90]. However, for mesoporous solids with nanoparticles such applications are not reported so far. 

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