Fixed nanomaterials

There are naturally occurring nanoparticles in soot and volcanic ash, and nanoparticles have been used since the Middle Ages to color stained glass. When used in products, nanomaterials can be either free or fixed. Free nanomaterials are in solution or suspension, and are in products such as sunscreens and fuels. Fixed nanomaterials are in composites such as resins... 

This work was produced as part of the activities of fixe ARC Centre for Functional Nanomaterials fiind by the Australian Res rch Council under the ARC Centres for Excellence Program. 

In initial experiments carbon nanomaterial-supported catalysts showed acceptable activities and comparatively high selectivities toward higher hydrocarbons. Nevertheless, the applicability of these new materials in large-scale fixed bed reactors is limited due to their powdery appearance. Concerning this challenge research has already started, and hopefully carbon nanomaterial pellets will... 

For nanomaterials this is especially true if the exposure scenarios used in the test system are not representative of those likely to be found in the field [91, 92]. For example, the degree of toxicity observed in aquatic invertebrates exposed to multi-walled nanotubes (MWNTs) in water and sediment was influenced by the functional groups on the MWNTs and their preparation for dispersal into the test systems [93]. As noted, even the concept of what constitutes nanomaterials is not fixed [87], so these emerging materials will likely require a rethinking of how their toxicity is assessed and the hazards and risks they might pose to ecosystems [90]. For more information on nanomaterials, including application of life-cycle concepts to their design, see Chapter 8. 

More recently, the upsurge of nanosized materials has encouraged researchers to devote efforts to the formation of several composites in which carbon nanostructures [184, 185] and metal NPs [173,186] are mixed with zeolite. Similar composites combine the electrocatalytic properties of the nanomaterials with the selectivity imparted by the inorganic structure or, more simply, with the enormous enlargement of the surface due to zeolite material, also suitable to fix nano-objects. In this case, the high conductivity of the nanostructured component overcomes the problem of the insulating nature of the zeolite component. 

On the other hand, the geometry of individual single-crystal NWs promotes a detailed analysis of-the gas-surface interactions, because there are no necks and boundaries. The decrease in the number of parameters which control the sensor response of ID structures should facilitate a better understanding of the nature of the observed effects. In addition, ID metal oxide nanomaterials have excellent crystallinity and clear faceting with a fixed set of planes. It is expected that these nanomaterials will have less concentration of point defects and specific adsorption and catalytic properties, conditioned by a particular combination of crystallographic planes. In other words, semiconducting ID metal... 

In addition, there are general safety considerations of nanomaterials, including nanofibres, because the potential toxicological effects are not well understood. There is a concern that free nanomaterials, which are not fixed within a structure, could be inhaled, ingested, or enter the body via the skin and then cause damage to cells. A report by the European Commission suggested that there is only a limited amount of scientific evidence to suggest that nanomaterials present a risk for human health [54]. 

Accordingly, nanomaterials cannot be treated as small copies of macroscopic materials, but, instead, their properties depend critically on the arrangement of the atoms. Thus, for nanomaterials one can no longer apply the above-mentioned scaling laws, i.e., the systems of interest have sizes far from the thermodynamic limit and one has left the scaling regime so that every single atom counts. Therefore, theoretical studies of the properties of nanostructures have to be based on electronic-structure methods, like those described in, e.g., ref. 1. In most cases one imposes the Born-Oppenheimer approximation, i.e., for a system with M nuclei and N electrons one fixes the structure, R = assumes that... 

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