Catalytic nanoparticles

The technique had to be capable of quickly S5m-thesizing catalytic nanoparticles - with diameters less than 5 nm - on a variety of support materials regardless of the material s lEP. 

L. Sun and R. M. Crooks, Dendrimer-mediated immobilization of catalytic nanoparticles on flat, solid supports, Langmuir 18, 8231-8236 (2002). 

Dai JH, Bruening ML (2002) Catalytic nanoparticles formed by reduction of metal ions in multilayered polyelectrolyte films. Nano Lett 2 497-501... 

Catalysts were some of the first nanostructured materials applied in industry, and many of the most important catalysts used today are nanomaterials. These are usually dispersed on the surfaces of supports (carriers), which are often nearly inert platforms for the catalytically active structures. These structures include metal complexes as well as clusters, particles, or layers of metal, metal oxide, or metal sulfide. The solid supports usually incorporate nanopores and a large number of catalytic nanoparticles per unit volume on a high-area internal surface (typically hundreds of square meters per cubic centimeter). A benefit of the high dispersion of a catalyst is that it is used effectively, because a large part of it is at a surface and accessible to reactants. There are other potential benefits of high dispersion as well— nanostructured catalysts have properties different from those of the bulk material, possibly including unique catalytic activities and selectivities. 

Lu Y, Spyra P, Mei Y, Ballauff M, Pich A (2007) Composite hydrogels robust carriers for catalytic nanoparticles. Macromol Chem Phys 208 254-261... 

These facts obviously raise the question of what constitutes the best computational model of a small catalytic particle. As catalysis is often a local phenomenon, a cluster model of the reactive or chemisorption site may give quite a reasonable description of what happens at the real surface [1,3,30]. However, the cluster should still be large enough to eliminate cluster edge effects, and even then one must bear in mind that the cluster sizes employed in many computational studies are still much smaller than real catalytic particles (say 10-50 versus 50-1000 atoms, respectively). Hence, a slab model of a stepped surface may provide a much more realistic model of the active site of a catalytic nanoparticle. Hammer [31,32] has carried out quite extensive DFT-GGA slab calculations of N2 and NO dissociation at stepped Ru and Pd surfaces, showing how the dissociation energy is significantly lower at the low-coordination step sites compared to terrace sites. The special reactivity of step sites for the dissociation of NO and N2 has been demonstrated in several recent surface-science studies [33,34]. Also, the preferential adsorption of CO on step sites has been demonstrated in UHV [35], under electrochemical conditions [36], as well as by means of DFT-GGA slab calculations [37]. 

FIGURE 22 Cross-section of a microchannel (200 x 100 pm) coated with catalytic nanoparticles (photo) [159]. (Adapted with permission from Reuse.)... 

The method described above has been used to prepare a variety of supported catalytic nanoparticles. In all the studies presented below the argon pressure and target-substrate distance was the same. One feature of the nanoparticles prepared in these studies is that they are preferentially formed on the outer surface of the support material, rather than within the pores of the supports or interior of the powder agglomerates. Whereas techniques such as... 

During fuel cell operation the membranes are stressed mainly by mechanical interference. Differences in the local gas and water distribution would lead to different processes in chemical reactions, shrinkage or expansion. The mechanical stress can induce changes in the distribution of the catalytic nanoparticles by e.g. agglomeration at fissures. 

The catalytic nanoparticles possess unique catalytic properties due to their large surface area and considerable number of surface atoms leading to an increased amount of active sites [1-3]. The catalytic properties of nanoparticles depend on the nanoparticle size, nanoparticle size distribution, and nanoparticle environment [4]. Moreover, the surface of nanoparticles plays an important role in catalysis, being responsible for their selectivity and activity. As was demonstrated in the last decade, the formation of nanoparticles in a nanostructured polymeric environment allows enhanced control over nanoparticle characteristics, yet the stabilizing polymer (its functionality) is of great importance, determining the state of the nanoparticle surface [5-8]. 

Heterogenization of catalytic nanoparticles stabilized by block copolymers can be carried out by their incorporahon into porous membranes based on poly(acrylic acid) crosslinked with a difunctional epoxide [20-22]. Membranes with defined porosities and amounts of palladium were studied in the selective hydrogenahon of propyne to propene as a model reaction. The porosity of the polymer membrane, the content of catalyst, and the residence time of the reaction mixture were found to influence the conversion and selectivity. The main advantage of these membranes compared to other heterogeneous catalysts is simple adjustment to reaction conditions and fadhtated mass transfer. 

Another most popular nanostructured system for the stabihzation of catalytic nanoparticles is dendrimers. These catalysts are prepared via sorphon of metal ions into dendrimers (normally using commercially available poly(amido-amines). 

We studied the behavior of catalytic nanoparticles formed in a nanostructured polymeric environment in the hydrogenation of long chain acetylene alcohols and the direct oxidation of a monosaccharide (L-sorbose). These reactions were chosen because of their industrial relevance and also because of the special importance of high selectivity, which can be achieved using a polymeric environment in mild... 

It is well established that low molecular weight modifiers such as quinoline, pyridine, etc. [51] increase the selectivity of the hydrogenation of acetylene alcohols, but often the modifiers leach and selectivity deteriorates. In the case of pyridine units of the P4VP block, the modification is fairly permanent. The stability of modification, which governs the stability of catalytic properties and high selectivity, is one of the important advantages of catalytic nanoparticles stabilized in the polymeric media [47]. 

Por the computation we have used the integral method using cubic spline and the combined gradient method of Levenberg-Marquardt [57, 58]. The kinetic models chosen describe well the hydrogenation kinetics. In the formulas presented in Table 3.1 k is the kinetic parameter of the reaction and Q takes into account the coordination (adsorption) of the product (LN) and substrate (DHL) with the catalyst (the ratio of the adsorption-desoprtion equilibrium constants for LN and DHL). Parameters of the Arrhenius equation, apparent activation energy kj mol , and frequency factor k, have been determined from the data on activities at different temperatures. The frequency factor is derived from the ordinate intercept of the Arrhenius dependence and provides a measure of the number of collisions or active centers on the surface of catalytic nanoparticles. 

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