Activity gas phase

Catalyst Activation Gas phase activation of supported DENs was examined using in-situ FTIR spectroscopy and FTIR spectroscopy of adsorbed CO. For in-situ dendrimer decomposition studies, the spectra were collected under a gas flow composed of 20% 02/He or 20% H2/He. The supported DEN sample was pressed into a self-supporting wafer, loaded into a controlled atmosphere IR cell, and collected as the sample background. The temperature was raised stepwise and spectra were collected at each temperature until little or no change was observed. After oxidation, the sample was reduced in 20% H2/He flow with various time/temperature combinations. The sample was then flushed with He for lhr at the reduction temperature. After cooling under He flow, a background spectrum was collected at room temperature. A 5% CO/He mixture was flowed over the sample for 15 minutes, followed by pure He. IR spectra of CO adsorbed on the catalyst surface were collected after the gas phase CO had been purged from the cell. 

Figure 4.26 shows a flow reactor of diameter D in which the downstream portion of the walls is catalytic. Assume that there is no gas-phase chemistry and that there is a single chemically active gas-phase species that is dilute in an inert carrier gas. For example, consider carbon-monoxide carried in air. Assume further a highly efficient catalyst that completely destroys any CO at the surface in other words, the gas-phase mass fraction of CO at the surface is zero. Upstream of the catalytic section, the CO is completely mixed with the carrier (i.e., a flat profile). The CO2 that desorbs from the catalyst is so dilute in the air that its behavior can be neglected. Thus the gas-phase and mass-transfer problem can be treated as a binary mixture of CO and air. The overall objective of this analysis is to... 

If a catalytic cycle should be maintained, oxygen diffusion out to the surface must be complemented by an inward diffusions of surface-activated oxygen resulting from accumulation of reduced metal centers required to activate gas-phase oxygen. Not all studies mentioned here ensured in their experiments that the conditions of lattice oxygen catalysis were such as to fulfill the conditions of cyclic reversibility [34, 51, 82,131,132] as opposed to stoichiometric and irreversible reduction [133] caused by a structural phase transition. As long as complex MMO oxides are being used and the extent of reduction is kept to levels where no bulk transformation can be detected this condition can be verified [20,99,118,121,134,135], The kinetics of re-oxidation of partly reduced oxide catalysts was found to be rapid [77, 78, 80, 82] and always faster than its reduction. 

An accurate accounting of mass-transfer is especially important in heterogeneous systems, and, first of all, in catalytic oxidation. In this case the simultaneous presence of active surfaces and an active gas phase and occurrence of different processes involving different species in these two reaction zones makes the system puzzling. Some additional reasoning and examples illustrating the complexity of such systems and possible solutions will be discussed in Sections III.E and III.F. However, we must confess that a thorough elucidation of this extremely important and complicated problem is far beyond the scope of this paper. 

Variations in morphology and adsorption properties of PTFE coatings along with the effect the substrate pretreatment have been studied at the initial stages of coating formation. PTFE coatings have been deposited from the active gas phase obtained by the electron-beam dispersion of the original powder of polymer in vacuum [4],... 

One of the advantages of lasers for activating gas phase reactions is the feasibility to control the dimension of the process zone by laser focus. In this way very small regions may be achieved where nucleation and particle growth happen. The same advantage is applicable to processes where the laser is used for evaporating coarse ceramic material in order to get nano-scale ceramic powders by re-condensation via homogeneous nucleation [232]. This technique has also been used to prepare SiC nanopowder with yields up to 100 g h [233]. However, with this approach, traces of free silicon were also detectable. 

Growth mechanisms link together activated gas phase, surface processes, and the resulting crystal. 

It should be noted that the possibility of the formation of an active gas phase during combustion of the Mo-B and Ta-C systems was pointed out earlier (22,23). However, the gas release in these systems was considered as a secondary process that has nothing in common with the fundamentals of chemical interaction between the reactants" (24). 

Catalytic gas-phase reactions play an important role in many bulk chemical processes, such as in the production of methanol, ammonia, sulfuric acid, and nitric acid. In most processes, the effective area of the catalyst is critically important. Since these reactions take place at surfaces through processes of adsorption and desorption, any alteration of surface area naturally causes a change in the rate of reaction. Industrial catalysts are usually supported on porous materials, since this results in a much larger active area per unit of reactor volume. 

Mention was made in Section XVIII-2E of programmed desorption this technique gives specific information about both the adsorption and the desorption of specific molecular states, at least when applied to single-crystal surfaces. The kinetic theory involved is essentially that used in Section XVI-3A. It will be recalled that the adsorption rate was there taken to be simply the rate at which molecules from the gas phase would strike a site area times the fraction of unoccupied sites. If the adsorption is activated, the fraction of molecules hitting and sticking that can proceed to a chemisorbed state is given by exp(-E /RT). The adsorption rate constant of Eq. XVII-13 becomes... 

The Nemst equation above for the dependence of the equilibrium potential of redox electrodes on the activity of solution species is also valid for uncharged species in the gas phase that take part in electron exchange reactions at the electrode-electrolyte interface. For the specific equilibrium process involved in the reduction of chlorine ... 

An important example for the application of general first-order kinetics in gas-phase reactions is the master equation treatment of the fall-off range of themial unimolecular reactions to describe non-equilibrium effects in the weak collision limit when activation and deactivation cross sections (equation (A3.4.125)) are to be retained in detail [ ]. 

Cluster research is a very interdisciplinary activity. Teclmiques and concepts from several other fields have been applied to clusters, such as atomic and condensed matter physics, chemistry, materials science, surface science and even nuclear physics. Wlrile the dividing line between clusters and nanoparticles is by no means well defined, typically, nanoparticles refer to species which are passivated and made in bulk fonn. In contrast, clusters refer to unstable species which are made and studied in the gas phase. Research into the latter is discussed in the current chapter. 

Unsaturated nitriles are formed by the reaction of ethylene or propylene with Pd(CN)2[252]. The synthesis of unsaturated nitriles by a gas-phase reaction of alkenes. HCN, and oxygen was carried out by use of a Pd catalyst supported on active carbon. Acrylonitrile is formed from ethylene. Methacrylonitrile and crotononitrile are obtained from propylene[253]. Vinyl chloride is obtained in a high yield from ethylene and PdCl2 using highly polar solvents such as DMF. The reaction can be made catalytic by the use of chloranil[254]. 

Drying. The single most common gas phase appHcation for TSA is drying. The natural gas, chemical, and cryogenics industries all use zeoHtes, siHca gel, and activated alurnina to dry streams. Adsorbents ate even found in mufflers. 

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