Nanostructured materials catalytic properties

Due to their high surface-to-volume ratio and size-dependent electronic properties nanostructured materials like NPs are good as catalysts. NPs of different sizes and structures can show significantly different catalytic activities and thus provides an opportunity to understand the structure-function relationship. NPs prepared usually in ensembles of NPs immobilized on an electrode. Thus the electrocatalytic property result of the average properties of the ensemble. Optimization of the catalyst requires increasing the number of sites available for the reaction to occur, shape and size effect of NP and composition of particles (in case of mixed metal... 

K. S. Suslick, M. Fang, T. Hyeon, and A. A. Cichowlas, Sonochemical synthesis and catalytic properties of nanostructured molybdenum carbide, in Molecularily Designed Nanostructered Materials, K. E. Gonsalves., ed., M.R.S., Pittsburgh (1994). 

The introduction of new synthetic techniques has led to the discoveries of many new electronic materials with improved properties [20-22]. However, similar progress has not been forthcoming in the area of heterogeneous catalysis, despite the accumulation of considerable information regarding structure-reactivity correlations for such catalysts [14-19]. The synthetic challenge in this area stems from the complex and metastable nature of the most desirable catalytic structures. Thus, in order to minimize phase separation and destruction of the most efficient catalytic centers, low-temperature methods and complicated synthetic procedures are often required [1-4]. Similar challenges are faced in many other aspects of materials research and, in general, more practical synthetic methods are required to achieve controlled, facile assembly of complex nanostructured materials [5-11]. 

Ill-defined carbon materials that provide a distinct nanostructure, such as spherical particles in the case of soot and carbon black, or hexagonally ordered cylindrical pores in the case of ordered mesoporous carbons, are not discussed here. Surface chemical, thus catalytic properties of these material are closer to carbon black or activated carbon [13], which is frequently reviewed [2-4]. Here, the higher degree of sp3 hybridization often results in a higher reactivity, however, at lower selectivity, as compared to nanocarbons exposing large basal plane fractions of the overall surface. 

The activity of elemental carbon as a metal-free catalyst is well established for a couple of reactions, however, most literature still deals with the support properties of this material. The discovery of nanostructured carbons in most cases led to an increased performance for the abovementioned reasons, thus these systems attracted remarkable research interest within the last years. The most prominent reaction is the oxidative dehydrogenation (ODH) of ethylbenzene and other hydrocarbons in the gas phase, which will be introduced in a separate chapter. The conversion of alcohols as well as the catalytic properties of graphene oxide for liquid phase selective oxidations will also be discussed in more detail. The third section reviews individually reported catalytic effects of nanocarbons in organic reactions, as well as selected inorganic reactions. 

Both the discovery of new synthesis processes for nanostructured materials and the demonstration of the highly reactive properties of these materials have increased rapidly within recent years. The new synthesis processes have made available nanostructured materials in a wide variety of compositions of metal oxides and metals supported on metal oxides, which have led to recognition of their exceptional chemical, physical, and electronic properties. The objective of this review is to provide recent results on synthesis of nanostructured materials using the novel processes that were developed in these laboratories recently and to contrast them to other important, new methods. Because some of the most important applications of nanostructured materials are as catalysts for chemical processing, several key reports on enhanced catalytic reactivity of nanostructured grains will be discussed along with the pertinent theory responsible for controlling both activity and selectivity of these new catalysts. 

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. 

The chapters in this volume present detailed insights into the synthesis-structure-properties relationships of nanostructured materials. In particular, the catalytic and photocatalytic properties of nanoclusters and nanostructured materials with ultrahigh surface-to-volume ratio are demonstrated. The gas absorption characteristics and surface reactivity of nanoporous and nanocrystalline materials are shown for various separation and reaction processes. In addition, the structural manipulation, quantum confinement effects, transport properties, and modeling of nanocrystals and nanowires are described. The biological functionality and bioactivity of nanostructured ceramic implants are also discussed. 

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