Catalyst activation porous

The accumulation of matter on the surface of the catalyst restricts gas passage into the catalyst by a mechanism known as pore mouth plugging (see Fig. 10b). It takes only a smaU amount of material on the surface of the catalyst to restrict the free passage of gases into and out of the active porous... 

On the other hand, as already discussed in Chapter 11 in connection to the effect of metal-support interactions, it appears that a fully dispersed noble metal catalyst on porous YSZ is already at a NEMCA or electroche-mically-promoted state, i.e. it is covered by an effective double layer of promoting backspillover O2 ions. This can explain both the extreme catalytic activity ofZr02- and Ti02- supported commercial catalysts, as well as the difficulty so far to induce NEMCA on fully dispersed noble metal catalysts deposited on YSZ. 

The objective of this work is to determine the influence of the porous structure (size and shape) and acidity (number and strength of the acid sites) on isomerization selectivity during the conversion of ethylbenzene on bifunctional catalysts PLAI2O3/ 10 MR zeolite. The transformation of EB was carried out on intimate mixtures of Pt/Al203 (PtA) and 10 MR zeolites (ZSM-5, ZSM-22, Ferrierite, EU-1) catalysts and compared to Mordenite reference catalyst activity. 

Let us complicate Problem 5 assuming that the supported catalyst is porous. Allocate the partitions for this PS, and determine (a) a partial volume of the active component and (b) the true density of the active component. 

The catalyst activity depends not only on the chemical composition but also on the diffusion properties of the catalyst material and on the size and shape of the catalyst pellets because transport limitations through the gas boundary layer around the pellets and through the porous material reduce the overall reaction rate. The influence of gas film restrictions, which depends on the pellet size and gas velocity, is usually low in sulphuric acid converters. The effective diffusivity in the catalyst depends on the porosity, the pore size distribution, and the tortuosity of the pore system. It may be improved in the design of the carrier by e.g. increasing the porosity or the pore size, but usually such improvements will also lead to a reduction of mechanical strength. The effect of transport restrictions is normally expressed as an effectiveness factor q defined as the ratio between observed reaction rate for a catalyst pellet and the intrinsic reaction rate, i.e. the hypothetical reaction rate if bulk or surface conditions (temperature, pressure, concentrations) prevailed throughout the pellet [11], For particles with the same intrinsic reaction rate and the same pore system, the surface effectiveness factor only depends on an equivalent particle diameter given by... 

Catalysts are porous and highly adsorptive, and their performance is affected markedly by the method of preparation. Two catalysts that are chemically identical but have pores of different size and distribution may have different activity, selectivity, temperature coefficient of reaction rate, and response to poisons. The intrinsic chemistry and catalytic action of a surface may be independent of pore size, but small pores appear to produce different effects because of the manner and time in which hydrocarbon vapors are transported into and out of the interstices. 

This is the reason why, for example, the zero order formic acid dehydrogenation may easily be measured on bulk metal catalysts at 200-300°C. whereas the approximately first order ethanol dehydrogenation requires highly activated porous metals of large specific surface in order to become measurable under the same conditions. The same has been shown for the decomposition of formaldehyde, acetic acid, and hydrazine hydrate. In these cases, the fractional surface coverage is simply 1000 times lower than in the case of a zero order reaction. 

All types of catalytic reactors with the catalyst in a fixed bed have some common drawbacks, which are characteristic of stationary beds (Mukhlyonov et al., 1979). First, only comparatively large-grain catalysts, not less that 4 mm in diameter, can be used in a filtering bed, since smaller particles cause increased pressure drop. Second, the area of the inner surface of large particles is utilized poorly and this results in a decrease in the utilization (capacity) of the catalyst. Moreover, the particles of a stationary bed tend to sinter and cake, which results in an increased pressure drop, uneven distribution of the gas, and lower catalyst activity. Finally, porous catalyst pellets exhibit low heat conductivity and as a result the rate of heat transfer from the bed to the heat exchanger surface is very low. Intensive heat removal and a uniform temperature distribution over the cross-section of a stationary bed cannot, therefore, be achieved. The poor conditions of heat transfer within... 

According to the different exchange current densities, i0, for hydrogen oxidation and hydrogen evolution on Ni and Pt, the catalytic activity of platinum is by a factor of several hundred to a thousand higher than that of nickel. Therefore, if the utilization of Raney-nickel particles below 10 jum size approaches 100%, it is clear that Pt-activated porous soot particles must be by a factor of from 10 to 30 smaller than Raney-nickel particles to achieve full utilization, that is, vanishing fuel starvation of the catalyst. This happens to be the case with soot agglomerates that are by their very nature of correct size (dv < 0.1 /im) (150, 151). 

After an iPP particle reached the FBR, co-polymerization of ethylene-propylene starts preferrably inside the porous PP matrix. Depending on the individual residence time, the particle will be filled with a certain amount of ethylene-propylene rubber, EPR, that improves the impact properties of the HIPP. It is important to keep the sticky EPR inside the preformed iPP matrix to avoid particle agglomeration that could lead to wall sheeting and termination of the reactor operation. Ideally a "two phase" structure, see Fig.5.4-3, is produced. Finally, a "super-high impact" PP results that contains up to 70% EPR. How much EPR is formed per particle depends on three factors catalyst activity in the FBR, individual particle porosity, and individual particle residence time in the FBR. All particle properties are therefore influenced by the residence time distribution, and finally, a mix of particles with different relative amounts of EPR is produced - a so called "chemical distribution" see, for example, [6]. 

While catalytic HDM results in a desirable, nearly metal-free product, the catalyst in the reactor is laden with metal sulfide deposits that eventually result in deactivation. Loss of catalyst activity is attributed to both the physical obstruction of the catalyst pellets pores by deposits and to the chemical contamination of the active catalytic sites by deposits. The radial metal deposit distribution in catalyst pellets is easily observed and understood in terms of the classic theory of diffusion and reaction in porous media. Application of the theory for the design and development of HDM and HDS catalysts has proved useful. Novel concepts and approaches to upgrading metal-laden heavy residua will require more information. However, detailed examination of the chemical and physical structure of the metal deposits is not possible because of current analytical limitations for microscopically complex and heterogeneous materials. Similarly, experimental methods that reveal the complexities of the fine structure of porous materials and theoretical methods to describe them are not yet... 

Molecular-level design of catalytic ensemble structures on surfaces in a controllable manner, based on new chemical concepts and strategies regarding composition or structure, provides a promising opportunity for the development of novel and efficient catalysts active for selective oxidation. Novel strategies and concepts for the creation of active ensemble structures on flat and porous surfaces may emerge from self-assembly and in situ transformation of precursors immobilized on the surfaces, with the aid of in situ characterization by sophisticated physical techniques [1-6]. 

The main reaction i.e, benzene hydrogenation occuring inside porous Ni-catalyst pellets is accompanied by poisoning reaction in which the thiophene presented in the feed stream reacts irtevcrsibly with the catalytic active sites. An analysis was made assuming isothermal behaviour [7], the same effective diffusivity for reactant and poison and that the steady-state continuity equation represents a good approximation at all times [8,9]. Under these conditions the mass balances for benzene, thiophene and catalyst activity are... 

Unlike the RDE technique, which is quite popular for characterizing catalyst activities, the gas diffusion electrode (GDE) technique is not commonly used by fuel cell researchers in an electrochemical half-cell configuration. The fabrication of a house-made GDE is similar to the preparation of a membrane electrode assembly (MEA). In this fabrication, Nation membrane disks are first hot-washed successively in nitric acid, sulphuric acid, hydrogen peroxide, and ultra-pure water. The membranes are then coated with a very thin active layer and hot-pressed onto the gas diffusion layer (GDL) to obtain a Nation membrane assembly. The GDL (e.g., Toray paper) is very thin and porous, and thus the associated diffusion limitation is small enough to be ignored, which makes it possible to study the specific kinetic behaviour of the active layer [6],... 

The catalyst activity is determined by the coke extraction rate relative to the coke formation rate. The coke extraction rate is determined by the solubility of the coke compounds in the reaction mixture and the diffusivity of the extracted compounds through the porous catalyst. The coke formation on the catalyst occurs from the hexenes, and more significantly, from hexene oligomers formed in the fluid phase catalyzed by traces of peroxide impurities [3]. A detailed mathematical model is presented here to interpret the results presented in our earlier paper [2] and to develop a better understanding of the underlying physicochemical processes. 

Shown in Table 9.7 are some examples of incorporating catalysts into porous ceramic membranes. Both metal and oxide catalysts have been introduced to a variety of ceramic membranes (e.g., alumina, silica, Vycor glass and titania) to make them catalytically active. The impregnation/heat U eatment procedures do not appear to show a consistent cause-and-effeci relationship with the resulting membrane permeability. For example, no noticeable change is observed when platinum is impregnated into porous Vycor glass... 

When the catalyst is immobilized within the pores of an inert membrane (Figure 25.13b), the catalytic and separation functions are engineered in a very compact fashion. In classical reactors, the reaction conversion is often limited by the diffusion of reactants into the pores of the catalyst or catalyst carrier pellets. If the catalyst is inside the pores of the membrane, the combination of the open pore path and transmembrane pressure provides easier access for the reactants to the catalyst. Two contactor configurations—forced-flow mode or opposing reactant mode—can be used with these catalytic membranes, which do not necessarily need to be permselective. It is estimated that a membrane catalyst could be 10 times more active than in the form of pellets, provided that the membrane thickness and porous texture, as well as the quantity and location of the catalyst in the membrane, are adapted to the kinetics of the reaction. For biphasic applications (gas/catalyst), the porous texture of the membrane must favor gas-wall (catalyst) interactions to ensure a maximum contact of the reactant with the catalyst surface. In the case of catalytic consecutive-parallel reaction systems, such as the selective oxidation of hydrocarbons, the gas-gas molecular interactions must be limited because they are nonselective and lead to a total oxidation of reactants and products. For these reasons, small-pore mesoporous or microporous... 

Since catalyzed reactions take place on the surface of the catalytically active material, the most efficient catalysts are those in which a high percentage of the active species is exposed to the reaction medium. Some metal catalysts, such as those discussed in Chapter 12, are composed of very finely divided metal particles that have the high ratio of surface to bulk atoms needed for good catalyst activity. These fine particles, however, sinter on heating. As discussed in Chapter 9, the most common way of minimizing metal catalyst sintering is to distribute the active component over a porous, thermostable, support. 

Cr- FILM from the different cationic forms of this clay [4], These materials exhibited dehydration and dehydrogenation activity in the decomposition of 2-propanol and ethanol. Cr-PILM catalysts activated by HjS/Hj mixture displayed also an interestingly high activity for thiophene hydrodesulfurization (HDS). Now, the Cr-PILM s have been prepared form different clays with the objective to obtain new porous materials with potential catalytic applicability. Their physicochemical properties and the catalytic behaviour for thiophene HDS have been investigated. 

The sharp decrease in activity of the catalysts is difficult to explain by the small change in the porous structure. The main reason for the decrease in the catalysts activity should be chemical transformations in the catalyst, rather then a change in the physical structure. 

A reference case for the C02-selective WGS membrane reactor was chosen with the C02/H2 selectivity of 40, the C02 permeability of 4000 Barrer, the inlet sweep-to-feed molar flow rate ratio of 1, the membrane thickness of 5jum, 52,500 hollow libers (a length of 61 cm, an inner diameter of 0.1 cm, and a porous support with a porosity of 50% and a thickness of 30jLon), both inlet feed and sweep temperatures of 140 °C, and the feed and sweep pressures of 3 and latm, respectively. With respect to this case, the effects of C02/H2 selectivity, C02 permeability, sweep-to-feed ratio, inlet feed temperature, inlet sweep temperature, and catalyst activity on the reactor behavior were then investigated. 

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