Gas Phase Condensation Synthesis

O. Friedrichs, L. Kolodziejczyk, J.C. S4nchez-L6pez, C. Lopez-Cartes, A. Fernandez, Synthesis of nanocrystaUine MgH powder by gas-phase condensation and in situ hydrida-tion TEM, XPS and XRD study, J. Alloys Compd. 434-435 (2007) 721-724. 

The well-known nonequilibrium process of plasma chemical synthesis has found practical application for a large number of compounds and compositions. However, in recent years more attractive and well-developed processes of synthesis become gas phase condensation under quasi-equilibrium conditions of moderate heat and mass transfer. This process becomes preferable over latter plasma chemical synthesis due to its ability to control the thermal regime and more flexibility in regard to dispersion and purity of the synthesized product. Uniformity of particle size and chemical composition (powder purity) are essential for fabrication of nanocomposites or dense nanocrystalline materials with improved physical, chemical and mechanical properties. This is because the particle size distribution determines the stability of grains during consolidation of a polycrystal while the concentration of impurities affects properties of grain boundaries and entire material (Table 5.1). 

Nanophase materials in which the average grain size, phases or crystallites are in the nanometer regime have recently been the focus of intense research effort (1-3). This interest has developed due to their superior properties compared to conventional materials which have particle sizes on the order of a micron (4-5). Nanostructured materials have traditionally been prepared by a variety of techniques which include physical methods such as gas-phase condensation, metal evaporation, spray pyrolysis, laser ablation and plasma synthesis (6-12). Chemical methods to synthesize such materials have frequently been used due to the better control of the stoichiometry in the end-product, the molecular level mixing of the constituent phases and the feasibility of low cost bulk production of these materials. Various chemical... 

The maximum pressure that should be tolerated in a metal atom reactor is a point of controversy among various workers in this field. High pressures favor reaction in the gas phase with respect to those in the matrix. Where different products are obtained from the gas and condensed phases, the former products begin to appear at pressures of 10 4 torr. The molybdenum atom syntheses described in this volume are best carried out under 10 4 torr and with apparatus described in synthesis number 16. Skell and co-workers consider this apparatus necessary and appropriate for all work. 

In the gas phase, the co-condensation of chromium vapors with azines and subsequent cooling (77 K) led to the first 7r-complexes of pyridine (44, in the presence of PF3) [75AG(E)273] and (45) (76JA1044). The gas-phase synthesis was utilized to prepare the parent bis(pyridine) sandwich species (46) (88CB1983). An attempt to prepare the -complex of pyridine was... 

In natural processes, metal ions are often in high oxidation states (2 or 3), whereas in chemical systems the metals are in low oxidation states (0 or 1). This fact inverts the role of the metal center, such that it acts as a one-electron sink in a natural system, but as a nucleophile in an artificial ones (see other chapters of this book and the review by Aresta et al. [109]). Nevertheless, important biochemical processes such as the reversible enzymatic hydration of C02, or the formation of metal carbamates, may serve as natural models for many synthetic purposes. Starting from the properties of carbonic anhydrase (a zinc metalloenzyme that performs the activation of C02), Schenk et al. proposed a review [110] of perspectives to build biomimetic chemical catalysts by means of high-level DFT or ah initio calculations for both the gas phase and in the condensed state. The fixation of C02 by Zn(II) complexes to undergo the hydration of C02 (Figure 4.17) the use of Cr, Co, or Zn complexes as catalysts for the coordination-insertion reaction of C02 with epoxides and the theoretical aspects of carbamate synthesis, especially for the formation of Mg2+ and Li+ carbamates, are discussed in the review of Schenk... 

Since the concept of superelectrophilic activation was proposed 30 years ago, there have been many varied superelectrophiles reported both in experimental and theoretical studies. Superelectrophiles can be involved in both gas and condensed phase reactions, ranging from interstellar space down to the active sites of certain enzymes. Moreover, synthetic conversions involving superelectrophiles are increasingly used in the synthesis of valuable products. Superelectrophilic activation has also been useful in the development of a number of new catalytic processes. It is our belief that superelectrophilic chemistry will continue to play an increasing role in both synthetic and mechanistic chemistry. 

What effect do shocks have on the gas phase synthesis of complex interstellar molecules This question has been investigated at least for hydrocarbons through six carbon atoms in complexity by Mitchell (1983, 1984). He has found that if a shock passes through a dense cloud where much of the carbon is already in the form of carbon monoxide, complex hydrocarbons are not formed in high abundance. However, if a shock passes through a diffuse cloud, of density approximately 103 cm-3, where much of the cosmic abundance of carbon is in the form of C+ and to a lesser extent C, a different scenario is present. As the shock cools, the C+ and C, which remain in appreciable abundance for up to 10s yrs after the shock passage, react via many of the reactions discussed above as well as others to produce a rich hydrocarbon chemistry. The net effect is that large abundances of hydrocarbons build up as the cloud cools and eventually reaches a gas density of 3 x 104 cm-3. Do these results bear any relation to the results obtained from ambient gas phase models In both types of calculations, hydrocarbon chemistry appears to require the presence of C+ and/or C both to synthesize one-carbon hydrocarbons such as methane and then, via insertion reactions, to produce more complex hydrocarbon species. Condensation reactions do not appear to be sufficient. 

One major reason for the great interest in the processes of thin metal-containing films is that reactions on the surface of small metal clusters can be studied. Indeed, prior to the development of thin-film chemistry, reactions of similar particles were studied only in the gas phase at rather high temperatures. Under these conditions, most of the primary products are unstable and decompose in the course of further reaction, which is non-selective. As a result, the information obtained on the routes and mechanisms of reactions of disperse metals appears to be scarce, while the use of such reactions in synthesis is inexpedient. Conversely, low-temperature reactions in the films of co-condensates are very promising from the standpoint of determining the detailed reaction mechanism, as well as for synthesis of previously unknown complexes and organometallic compounds. It is important that atoms of only a few metals react with organic compounds immediately at the instant of their contact on the cooled substrate. Rather often, atoms and/or small (molecular) clusters are first stabilized in the film, and then their transformations are observed. 

Synthesis of metallic nanoparticles proceeds in many ways they can be divided into physical and chemical. Physical methods include inert gas condensation, arc discharge, ion sputtering, and laser ablation. The main idea behind these methods is condensation of solid particles from the gas phase, the substrate for nanoparticle generation being pure metals (or their mixtures/alloys in the case of complex particle composition). Chemical methods, in turn, include various methods utilizing... 

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