Advanced vapor phase processes

The recorded history of short fiber technology starts over two thousand years ago with asbestos fibers and reaches into the future with silicon nano-whiskers and carbon nanotubes. Asbestos is derived from the solid phase, but today, the most important short inorganic fibers are derived from the vapor phase. 

The evolution of modern vapor phase processes starts with metal catalyzed chemical vapor deposition and ends with laser vaporization (see Table I). Most vapor phase processes require metal particle catalysts some proceed without the addition of metal particles. The growth temperatures range from 100 to 4000°C. The length of silicon nanowires is 10 nm [74] that of carbon nanotubes is 300 jm [76] but they can be potentially endless [81]. 

Metal catalyzed chemical vapor deposition has become the most versatile and therefore most important whisker growth process. This process and other vapor phase processes facilitate the formation of uniform and reproducible products for demanding applications, where they offer new premium electrical, magnetic, dielectric and near theoretical mechanical properties [1-2]. 

Three major breakthroughs in process technology have recently been made. These processes facilitate the growth from a liquid phase. They inciude the formation of (1) InP, InAs and GaAs whiskers [18] from other organic soivents by a soiution-iiquid-soiid phase transformation, (2) short carbon fibers from liquid pitch melts by centrifuging [19], and (3) silver nanowires by a novel self-assembly process [71]. Micro- and nanopillars (or micro-and nanocolumns) are a new class of short inorganic fibers. They are 

In summary however, vapor phase processes offer a more effective route to advanced inorganic fibers than solid or liquid phase processes because they facilitate greater control over diameter, length, aspect ratio, and properties of the resulting whiskers, microfibers, and nanotube structures. 

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Results of these investigations demonstrate that changes of the reactor surface can be an effective method for directing chemical reactions. Thus, developing a theory of how heterogeneous factors influence liquid-phase chain reactions is one of the important lines of advancement in this area. Only a few years ago it was thought, almost a priori, that there are practically no heterogeneous factors in liquid-phase oxidation and that liquid-phase processes differ from vapor-phase processes in this respect. 

T0834 Ultrox International/U.S. Filter, Ultrox Advanced Oxidation Process T0855 Vapor-Phase Biofiltration—General... 

Another opportunity for advancement in ethylbenzene synthesis is in the development of liquid phase processes that can handle low cost feedstocks, including dilute ethylene such as ethane/ethylene mixtures. The use of dilute ethylene has become increasingly attractive since it has the potential to debottleneck ethylene crackers. Currently higher temperature, vapor phase technologies can tolerate contaminants that enter with the dilute ethylene feed from FCC units. However, these same contaminants can accelerate catalyst aging in lower temperature, liquid phase operations because they are more strongly adsorbed at the lower temperatures. Acid catalysts that tolerate elevated levels of contaminants would facilitate the development of dilute ethylene-based processes. These same catalysts could be useful in applications where lower cost or lower quality benzene feeds are all that are available. 

Novel Catalysts. - One other promising option exists that might, in the long run, lead to a viable alternative to the above process concepts. Recent advances in biomimetic catalysis have resulted in the development of molecular catalysts that are selective in liquid-phase oxidation of C,-Cj aliphatic hydrocarbons under mild conditions. The catalysts have now been placed on suitable support materials for vapor-phase oxidation of methane to methanol. Significant laboratory research is still required on the properties and synthesis of these special molecular catalysts before process conditions, products, and yields can be defined, even on a laboratory scale. 

In this chapter we extend our treatment of mechanisms for metal-catalyzed reactions in the vapor phase to heterogeneous catalytic reactions carried out in aqueous media and electrocatalytic reactions. More specifically, we discuss what is known about the wa-ter/metal interface, its reactivity, and the influence of the aqueous phase on elementary surface processes including adsorption, reaction, diffusion and desorption and sofution-phase kinetic processes. We advance these ideas into the discussion of the mechanisms... 

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