Gas-phase condensation

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. 

Carbonaceous compounds can also form in the absence of a catalyst by free-radical, gas-phase condensation reactions. The formation of this pyrolytic carbon is known in steam-reforming reactors where it can be controlled to some extent by minimizing the free volume within the reactor chamber. This type of carbon does not form readily with methane but can be severe with larger hydrocarbons. The compounds formed by free-radical reactions tend to be quite different from the graphitic carbon formed by metal catalysts. For example, Lee et al. showed that the compounds formed by passing pure, undi-... 

Besides oxidative coupling of methane and double bond isomerization reactions (242), a limited number of organic transformations have been carried out with alkali-doped alkaline earth metal oxides, including the gas-phase condensation of acetone on MgO promoted with alkali (Li, Na, K, or Cs) or alkaline earth (Ca, Sr, or Ba) (14,120). The basic properties of the samples were characterized by chemisorption of CO2 (Table VI). 

Gas phase condensation dc- and rf-magnetron sputtering, 11 deposition of palladium on silicon carbide support, 12... 

J.A. Horsley et al. performed a study of gas phase, condensed and monolayer benzene on Pt( 111) (4). In order to identify correctly the four features observed in the spectrum of the solid, they looked at the polarization dependence of the monolayer spectrum and performed a multiple scattering Xa MO calculation. The first two features, maximized at glancing incidence, were assigned to the first and second highest it MO s (e2u and b2g). The second two features were most intense for normal incidence and were assigned to the first and second-plus-third highest a MO s (e2u and e2g + a2g). 

In the past, acrolein was produced by the gas phase condensation of acetaldehyde with formaldehyde on sodium silicate, until it was supplanted by the catalytic oxidation of propylene. Early catalysts based on cuprous oxide were only sufficiently selective at low conversions of propylene. The real breakthrough came with the discovery made by Sohio of bismuth molybdate catalysts, developed into formulations specifically optimized for the manufacture of acrylonitrile, acrolein, and methacrolein. 

The laser ablation or laser-heated gas phase condensation process could produce NPs less than 50 nm with broad-size distributions. For example. Filers and Tissue reported that the EU2O3 NPs could be made by a laser-heated gas phase condensation process. The particles are polydis-perse in a range of 2-30 nm (Filers and Tissue, 1995). 

Metal granules also have been found in cokes formed or deposited on iron, cobalt, and nickel foils in experiments using methane, propane, propylene, and butadiene (7-10). Platelet-type coke, whose properties match those of graphite also was produced in some cases. Lahaye et al. (11) investigated the steam cracking of cyclohexane, toluene, and n-hexane over quartz, electrode graphite, and refractory steel. They report that heavy hydrocarbon species form in the gas phase, condense into liquid droplets which then strike the solid surface, and finally react on the solid surfaces to produce carbonaceous products. The liquid droplets wet and spread out on certain surfaces better than on others. 

This is a new technology developed by Lucite (formerly Ineous, formerly ICI).[8] It involves a two-step process a liquid phase methoxy-carbonylation followed by a gas phase condensation. 

The term e/(ee — 1), which appears in equations 1 and 2, was first developed to account for the sensible heat transferred by the diffusing vapor (1). The quantity 8 represents the group M4-C 4 / hg, the ratio of total transported energy to convective heat transfer. Thus it may be thought of as the fractional influence of mass transfer on the heat-transfer process. The last term of equation 3 is the latent heat contributed to the gas phase by the fog formation. The vapor loss from the gas phase through both surface and gas-phase condensation can be related to the partial pressure of the condensing vapor by using Dalton s law and a differential material balance. 

Depending upon the vapour pressure-temperature relationship of the condensing material, its freezing point and effective concentrations in the gas phase, condensation may occur directly to the solid rather than the liquid phase. In this case the particle size of the powder produced will depend upon the crystal growth rate from the vapour since coalescence will not be effective, and the particles will tend to have regular crystallographic facets. MgO condensed from an arc, for example, consists of small cubic crystals... 

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). 

---