Catalysts diffusivity

Vibrational Spectroscopy. Infrared absorption spectra may be obtained using convention IR or FTIR instrumentation the catalyst may be present as a compressed disk, allowing transmission spectroscopy. If the surface area is high, there can be enough chemisorbed species for their spectra to be recorded. This approach is widely used to follow actual catalyzed reactions see, for example. Refs. 26 (metal oxide catalysts) and 27 (zeolitic catalysts). Diffuse reflectance infrared reflection spectroscopy (DRIFT S) may be used on films [e.g.. Ref. 28—Si02 films on Mo(llO)]. Laser Raman spectroscopy (e.g.. Refs. 29, 30) and infrared emission spectroscopy may give greater detail [31]. 

For a catalytic reaction of a reactant from a single fluid phase (either gas or liquid) to take place on a solid catalyst, diffusion processes also play a role, so in the complete process the following steps can be distinguished ... 

In emulsion polymerization, a solution of monomer in one solvent forms droplets, suspended in a second, immiscible solvent. We often employ surfactants to stabilize the droplets through the formation of micelles containing pure monomer or a monomer in solution. Micelles assemble when amphiphilic surfactant molecules (containing both a hydrophobic and hydrophilic end) organize at a phase boundary so that their hydrophilic portion interacts with the hydrophilic component of the emulsion, while their hydrophobic part interacts with the hydrophobic portion of the emulsion. Figure 2.14 illustrates a micellized emulsion structure. To start the polymerization reaction, a phase-specific initiator or catalyst diffuses into the core of the droplets, starting the polymerization. 

In situ dynamic surface structural changes of catalyst particles in response to variations in gas environments were examined by ETEM by Gai et al. (78,97). In studies of copper catalysts on alumina, which are of interest for the water gas shift reaction, bulk diffusion of metal particles through the support in oxygen atmospheres was shown (78). The discovery of this new catalyst diffusion process required a radical revision of the understanding of regeneration processes in catalysis. 

The irradiation of films prepared from 1% triphenylsulfonium salts in poly(4-t-butoxycarbonyloxystyrene) with lithographically useful doses of 254 nm light generates acid which is less than 0.1% of the t-BOC groups. The efficiency of the photochemistry is several times less than the efficiency of acid generation from triphenylsulfonium salts in solution. The catalytic chain is about 1000 for the t-BOC deprotection step at 100°C. This implies that catalyst diffusion during postbake is on the order of 50A... 

Keywords reesterification, kinetics, catalyst, diffusion, scaling, mean-field approximation. 

D Catalyst diffusivity a Bead radius Cpc Specific heat... 

Parameters in the model are listed in Table I. The flow, structural, and boundary conditions are known quantities. The frequency factor and activation energy for coke burning were the values determined by Weisz and Goodwin (1966) from the experiments discussed earlier, and the catalyst diffusivity D was measured directly in the laboratory. The value of a was determined from direct observations of the CO/CO2 ratio in each zone of the operating kiln. The remaining parameters are known quantities. Thus, there are no adjustable parameters available to tune the fitting of predicted values to observed data, for the fraction of coke remaining and for the vertical temperature versus distance from the top of the kiln. 

The morphology of the carbon on the surface can assume several forms a two-dimensional film or so-called whisker carbon, which is formed when the carbon dissolves in the supported metal catalyst, diffuses through the metal, and forms a growing filament that lifts the metal from the catalyst surface. Whisker carbon is typically associated with Ni-based catalysts because carbon is soluble in Ni at reforming conditions. Whisker carbon tends to form at higher temperatures, low steam to hydrocarbon ratios and higher aromatic content of the feeds. This type of carbon formation may be minimized by the use of precious metals as catalysts, because these metals do not dissolve carbon. On a nickel surface, the whisker mechanism can be controlled by sulfur passivation. 

Three fundamental processes can limit overall reaction rates in heterogeneous catalysis mass transfer of the reactants from the bulk liquid phase(s) to the surface of the solid catalyst, diffusion of the reactants from the catalyst surface to the active site, and the intrinsic reaction at the active site 61,62). Each of these processes depends on one or more experimental parameters, as shown in Fig. 1. 

Table 4 contains a collection of diffusion coefficients determined experimentally for a variety of adsorbate systems. It shows that the values may vary considerably, which is of course due to the specific bonding of the adsorbate to the surface under consideration. Surface diffusion plays a vital role in surface chemical reactions because it is one factor that determines the rates of the reactions. Those reactions with diffusion as the rate-determining step are called diffusion-limited reactions. The above-mentioned photoelectron emission microscope is an interesting tool to effectively study diffusion processes under reaction conditions [158], In the world of real catalysts, diffusion may be vital because the porous structure of the catalyst particle may impose stringent conditions on molecular diffusivities, which in turn leads to massive consequences for reaction yields. 

In the presence of catalysts, heterogeneous catalytic cracking occms on the surface interface of the melted polymer and solid catalysts. The main steps of reactions are as follows diffusion on the surface of catalyst, adsorption on the catalyst, chemical reaction, desorption from the catalyst, diffusion to the liquid phase. The reaction rate of catalytic reactions is always determined by the slowest elementary reaction. The dominant rate controller elementary reactions are the linking of the polymer to the active site of catalyst. But the selectivity of catalysts on raw materials and products might be important. The selectivity is affected by molecular size and shape of raw materials, intermediates and products [36]. 

Ocone, R., and Astarita, G., Lumping nonlinear kinetics in porous catalysts Diffusion-reaction lumping strategy. AIChEJ. 39,288 (1993). 

Diffusion-limited removal of products from catalyst pellets leads to enhanced readsorption and chain initiation by reactive a-olefins. These secondary reactions reverse chain termination steps that form these olefins and lead to heavier products, higher chain growth probabilities, and more paraffinic products. Diffusion-enhanced readsorption of a-olefins accounts for the non-Flory carbon number distributions frequently observed during FT synthesis on Co and Ru catalysts. Diffusion-limited reactant (H2/CO) arrival leads instead to lower selectivity to higher hydrocarbons. Consequently, intermediate levels of transport restrictions lead to highest selectiv-ities to C5+ products. A structural parameter containing the pellet diameter, the average pore size, and the density of metal sites within pellets, determines the severity of transport restrictions and the FT synthesis selectivity on supported Ru and Co catalysts. 

R. Mann and G. Thomson, Deactivation of a supported zeolite catalyst Diffusion, reaction and coke deposition in a parallel bundle, Chem. Eng. Sci. 42 555 (1987). 

Diffusion in porous catalyst affecting experimental activation energy. In case of a porous catalyst diffusion effects existing therein will also reduce the measured activation energy, depending on the temperature dependence of in... 

In 1979, Frechet and Willson put forward a very productive idea of a chemical amplification that was used in the development of a new generation of photoresists.They decided to use a photoresist comprising of a photochemical acid generator (PAG) and a polymer that was able to switch from hydrophobic to hydrophilic in the course of acid catalyzed hydrolysis. The PAG reacts with light to produce an acid catalyst. During a subsequent postexposure bake, the catalyst diffuses and reacts with the polymer component, causing many reaction events in the polymer and recovers the acid catalyst. The acid molecules catalyze the deprotection reaction and provide a prerequisite for chemical amplification. The number of the reaction events initiated by single quantum absorption has been estimated to be of order of 100. ... 

Coronene adsorbs on catalyst sites present on both the alumina support and on the active NiMo sulfide phase 3) It has been found that adsorption decreases with coke content to a very Jow value at high coke levels (4), Therefore, it appears that coronene adsorption on the coke is nil. On this basis, it is assummed that the loss in adsorption with increasing coke is proportional to the loss in pore surface area due to coverage by coke. The results of coronene adsorption measurements on the VGO-coked catalysts show an initial drop for the 2% C sample, but little change thereafter for higher coked catalysts (Fig. 1A) This implies that the coke is concentrated near the mouth of the pores, On the other hand, catalyst diffusivity measurements show a continual and sig-... 

In order to avoid carbon deposition, steam usually needs to be intentionally co-fed with the hydrocarbon vapor introduced into the reactor. Following initial fuel steam reforming, hydrogen, and to a lesser extent CO, are adsorbed on the high surface area fuel-side catalyst and react with oxygen anions to form H2O and CO2. The exothermic oxidation reactions produce heat which drives endothermic steam reforming reactions in a packed bed of catalyst adjacent to the membrane. Steam formed over the oxidation catalysts diffuses to the packed bed of steam reforming catalysts and reacts with hydrocarbon fuel by the reaction ... 

The first, and simplest, approach is to treat the reaction pseudo-homogeneously, whereby catalyst diffusion (including diffusion to the catalyst surface) and reaction are lumped into an overall reaction term. For heterogeneous reactions that are modeled in this way the liquid phase material balance is as given above and ej is given by the total amount of catalyst present on the stage under consideration. In this case, one only needs to specify catalyst mass and activity. 

In recent studies about new formulations of the high-temperature catalysts, diffuse reflectance infrared Fourier transform spectroscopy was used to elucidate the reaction mechanism. The results showed that the catalyst surface can provide oxygen to CO even in the absence of water. When the surface comes in contact with water, the surface of the catalyst recovers oxygen. The presence of intermediate species on the catalyst surface was not detected. This suggested that the WGSR over Fe based catalysts proceeds by redox mechanism. 

How far can the catalyst diffuse or ring walk along the polymer backbone before either oxidative addition to a carbon-halogen bond or dissociation of the catalyst ... 

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