Diffusion desorption

NjO pulses, contributed to formation/regeneration of Rhodium-containing sites having activity for dissociation of each incoming NjO pulse to Nj plus Oj at 623 K. Possibilities for such formation/regeneration of active sites include the diffusion/desorption of blocking species away from Rhodium-containing sites, which are further considered below in respect of (NiOj) " and Oj. 

Diffusion desorption from surface ion exchange Particles... 

With this information in mind, we can construct a model for the deposition rate. In the simplest case, the rate of flux of reactants to the surface (step 2) is equal to the rate at which the reactants are consumed at steady state (step 5). All other processes (decomposition, adsorption, surface diffusion, desorption, and transport away from the substrate) are assumed to be rapid. It is generally assumed that most CVD reactions are heterogeneous and first order with respect to the major reactant species, such that a general rate expression of the form of Eq. (3.2) would reduce to... 

To demonstrate this, in Section 9.2.2 we have studied a stochastic model for an extended ZGB-model including diffusion, desorption and energetic interactions as additional steps. We have used different values of the diffusion and the desorption rates and different values for the energetic parameters. In the case of repulsive interactions the system s behaviour is strongly influenced by. Eaa for large values of Yqo and by for small values of kco-The former parameter leads to a smooth phase transition at yi and the latter to a sharp transition at 2/1 The sharpness and the location of the phase transitions depend also on the diffusion and desorption rate of the A particles. The A-diffusion leads to an increase of the value of 2/2 due to the higher reactivity of the A particles. At lower values of Yco the system behaviour is nearly not influenced by the diffusion. The A-desorption increases the values of the critical points and smoothes the phase transition at 2/2- This effect becomes very important if Ca is large. 

For heterogeneous systems, the set of reactions includes adsorption, dissociation, surface diffusion, desorption, and other processes. In this case, the rates of processes can differ by many orders of magnitude. In accordance with Eq. (30), the time step is determined by the fastest process in the system. This condition strongly restricts the maximum real time in the simulation and prevents modeling of rare processes. One of the ways of overcoming this problem can be to exclude all fast processes from the table of reactions and use equilibrium distributions for these processes. For example,... 

Figure 9 Adsorption process of NO on Pd particles supported on MgO(l 00). (a) Global adsorption probability as a function of surface temperature and for various particle sizes (from Ref. [89]). (b) Schematic representation of die elementary processes in die molecular adsorption of NO on supported Pd particles (1) quasi-elastic redection on die bare support, (2) physisorption-diffusion-desorption from the bare support, (3) direct chemisorption on die Pd particles, (4) NO chemisorption on the Pd particles via a precursor physisorbed state on die bare support. Xs is die mean diffusion length of die NO molecules on the support and p is die width of die collection zone around die Pd particles.

Lombardo and Bell (1991) reviewed stochastic models of the description of rate processes on the catalyst surface, such as adsorption, diffusion, desorption, and surface reaction, which make it possible to account for surface structure of crystallites, spatial inhomogeneities, and local fluctuations of concentrations. Comparison of dynamic MC and mean-field (effective) description of the problem of diffusion and reaction in zeolites has been made by Coppens et al. (1999). Gracia and Wolf (2004) present results of recent MC simulations of CO oxidation on Pt-supported catalysts. 

S. J. Lombardo and A. T. Bell, A Review of Theoretical Models of Adsorption, Diffusion, Desorption, and Reaction of Gases on Metal Surfaces., Surf. Sci. Rep., 13 (1991) 1. 

Some ceramic dense materials, such as Si02 [59], are also capable of being selectively permeated by hydrogen according to a solution-diffusion-desorption mechanism. The only difference with Pd-alloy membranes lies in the fact that hydrogen diffuses across silica membranes in a molecular form. 

The adsorption-surface diffusion-desorption mechanism of transport through the SSF membrane can simultaneously provide high separation selectivity between H2 and the impurities of the PSA waste gas and high flux for the impurities even when the gas pressure in the high-pressure side of the membrane is low to moderate (3-5 atm). 

SJ. Lombardo, A.T. Bell, A review of theoretical models for adsorption, diffusion, desorption and reaction of gases on metals, Surface Science Reports, 13 (1991) 1. 

After a multicomponent aqueous solution has been freeze-concentrated to the limit and the ice has been sublimed, any residual unfrozen water must be removed from the remaining solid solution by diffusion, desorption and evaporation (transfer to the condenser). This process is termed secondary drying . For an amorphous preparation, the amount of unfrozen water remaining after the removal of ice may be typically 20-30% w/w, but much higher values, even >50% w/w, have been found in some formulations. Attempts are on record to measure the amount of water that remains in the freeze-concentrated solution phase at 7, mainly by differential scanning calorimetry but such... 

Sorption Diffusion Desorption Fig. 3.1 Pervaporation, solution-diffusion mechanism. 

Integration of a H2 PSA process with an adsorbent membrane can meet this goal [23, 24]. A nano-porous carbon adsorbent membrane called Selective Surface Flow (SSF) membrane which selectively permeates CO2, CO and CH4 from their mixtures with H2 by an adsorption- surface diffusion-desorption transport mechanism may be employed for this purpose. The SSF membrane can produce an enriched H2 gas stream from a H2 PSA waste gas, which can then be recycled as feed to the PSA process for increasing the over-all H2 recovery. The membrane is prepared by controlled carbonization of poly-vinyledene chloride supported on a macro-porous alumina tube. The membrane pore diameters are between 6 -7 A, and its thickness is - 1-2 pm [25]. 

Surface-selective flow membranes made of nanoporous carbon, which is a variation of molecular sieving membranes, were developed by Rao et al. (1992) and Rao and Sircar (1993). The membrane can be produced by coating poly(vinylidene chloride) on the inside of a macroporous alumina tube followed by carbonization to form a thin membrane layer. The mechanism of separation is by adsorption-surface-diffusion-desorption. Certain gas components in the feed are selectively adsorbed, permeated through the membrane by surface diffusion, and desorbed at the low-pressure side of the membrane. This type of membrane was used to separate H2 from a mixture of H2 and CO2 (Sircar and Rao, 2000), and their main advantage is that the product hydrogen is at the high-pressure side eliminating the need for recompression. The membrane, however, is not industrially viable because of its low overall separation selectivity. In addition, since the separation mechanism involves physical adsorption, operation at low temperatures is required. 

Initial excitation of the molecule Phonon excitations Electron-hole pair excitation Nonadiabatic processes Diffusion Desorption... 

Oxygen, diffusion, desorption, pr eatment, thermo-oxidative degradation, gentle processing, injection molding... 

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