Nanoparticles dispersion, difficulty

Ni(CO)4 is the sole binary carbonyl complex of the elements of group 10 that is stable (Table 8.1). However, very few studies in which Ni(CO)4 is used in the preparation of catalysts have been reported [43]. This is probably due to the difficulty of manipulation of Ni(CO)4 and its very high toxicity. However, surface Ni(CO)4 species have been identified after the interaction of CO with highly dispersed supported nickel catalysts prepared by other routes [44]. Recent interest in the use of Ni(CO)4 has focused on the controlled production of nickel nanoparticles for specific purposes, such as in automotive converters [45]. The use of nickel tetracarbonyl as an agent for the nucleation process in the growth of single-wall carbon nanotubes has also been reported [46]. 

Particles produced in the gas phase must be trapped in condensed media, such as on solid substrates or in liquids, in order to accumulate, stock, and handle them. The surface of newly formed metallic fine particles is very active and is impossible to keep clean in an ambient condition, including gold. The surface must be stabilized by virtue of appropriate surface stabilizers or passivated with controlled surface chemical reaction or protected by inert materials. Low-temperature technique is also applied to depress surface activity. Many nanoparticles are stabilized in a solid matrix such as an inert gas at cryogenic temperature. At the laboratory scale, there are many reports on physical properties of nanometer-sized metallic particles measured at low temperature. However, we have difficulty in handling particles if they are in a solid matrix or on a solid substrate, especially at cryogenic temperature. On the other hand, a dispersion system in fluids is good for handling, characterization, and advanced treatment of particles if the particles are stabilized. 

Another immobilization technique proposed is nanoentrapment into NPs. In this method, a water-in-oil microemulsion system is used for the fabrication of NPs and for the dispersion of enzyme. This procedure leads to the creation of discrete NPs through polymerization in the water phase or on the interface, in which the enzyme is dispersed [195, 196], One of the challenges of this approach is the difficulty in controlling the size of reverse micelles, as well as the number of enzyme molecules within each reverse micelle, which will directly affect the final properties of enzyme-entrapped nanoparticles [6],... 

As shown later, in the case of water and various organic polar carriers, the preparation process [6, 9, 10, 11, 12, 13, 14] is quite different from that developed for nonpolar organic carriers. Owing to specific difficulties encountered in stable dispersion of magnetic nanoparticles in the strongly polar water carrier, most of the preparation procedures were used for low-magnetization (100-200 G) samples. The preparation of alcohol-based magnetic fluids was first reported in Ref [7] and implies a stabilization mechanism, different from that applied in the case of water carrier. 

So far, CMK-5 carbons have been much less explored than CMK-3. One possible reason is the difficulty of finding suitable precursors that permit a strict control of the pore wall thickness to obtain stable carbon replicas. There are reports indicating the possibility of preparing CMK-5 with controlled pore wall thickness by using different methods of furfuryl alcohol polymerization and ferrocene as carbon precursors. Also, there are reports on the incorporation of highly dispersed nanoparticles of platinum " and cobalt into CMK-5 to enhance its electrochemical and... 

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