Promoters textural

Promoters. Many industrial catalysts contain promoters, commonly chemical promoters. A chemical promoter is used in a small amount and influences the surface chemistry. Alkali metals are often used as chemical promoters, for example, in ammonia synthesis catalysts, ethylene oxide catalysts, and Fischer-Tropsch catalysts (55). They may be used in as Httie as parts per million quantities. The mechanisms of their action are usually not well understood. In contrast, seldom-used textural promoters, also called stmctural promoters, are used in massive amounts and affect the physical properties of the catalyst. These are used in ammonia synthesis catalysts. 

The industrial catalysts for ammonia synthesis consist of far more than the catalyticaHy active iron (74). There are textural promoters, alumina and calcium oxide, that minimise sintering of the iron and a chemical promoter, potassium (about 1 wt % of the catalyst), and possibly present as K2O the potassium is beheved to be present on the iron surface and to donate electrons to the iron, increasing its activity for the dissociative adsorption of N2. The primary iron particles are about 30 nm in size, and the surface area is about 15 m /g. These catalysts last for years. 

Ermakova and co-workers manipulated the Ni particle size to achieve large CF yields from methane decomposition. The Ni-based catalysts employed for the process were synthesized by impregnation of nickel oxide with a solution of the precursor of a textural promoter (silica, alumina, titanium dioxide, zirconium oxide and magnesia). The optimum particle size (10 0 nm) was obtained by varying the calcination temperature of NiO. The 90% Ni-10% silica catalyst was found to be the most effective catalyst with a total CF yield of 375 gcp/gcat- XRD studies by the same group on high loaded Ni-silica... 

From these results, it is concluded that, in a fully reduced catalyst, FeAl204 is not present furthermore, the aluminum inside the iron particle is present as a phase that does not contain iron (e.g., A1203), and this phase must be clustered as inclusions 3 nm in size. These inclusions may well account for the strain observed by Hosemann et al. From the Mossbauer effect investigation then, the process schematically shown in Fig. 17 was suggested for the reduction of a singly promoted iron synthetic ammonia catalyst. Finally, these inclusions and their associated strain fields provide another mechanism for textural promoting (131). 

A promoter which works by reducing the tendency for sintering and loss of area may be called a textural promoter (see 1.7.3). 

Industrial catalysts for oxidation reactions rarely incorporate only a single bulk phase. A number of promoter elements are usually added, which can act purely as textural promoters or can enhance the activity and selectivity of the bulk catalyst. The role of promoters on vanadium phosphate catalysts has been addressed mainly in the patent literature, and Hutchings (163) has provided an extensive review of these patents. 

To further optimize the performance of metal catalysts, promoters are often added. Textural promoters work by separating metal particles from one another to minimize sintering, whereas electronic and structural promoters change the electronic or crystal structure of the active metal. For instance, a small amount of potassium acetate is added to... 

The active agent is the component(s) that causes the main catalytic action. Without it, the catalyst will have no effect. A promoter is a substance added into the catalyst to improve either of the activity, selectivity or stability so as to prolong the life of catalyst. The promoter is often added in a small amount and by itself has little activity. There are various types of promoters, depending on how they improve the catalyst. Textural promoters are inert substance which inhibit sintering of the active catalyst by being present in the form of very fine particles, usually they have a smaller particle size than that of the active species, are well dispersed, do not react with or form a solid solution with the active catalyst and have a... 

Effect Emulsifier, whitener, gives body and texture, promotes resistance to feathering,... 

Textural promoters inhibit the growth of catalyst particles to form larger, less active structures during the reaction. Thus they prevent loss of active surface by sintering and increase the thermal stability of the catalyst. 

H. Topsoe, J. A. Dumesic, M. Boudart, Alumina as a textural promoter of iron synthetic ammonia catalysts, J. Catal. 28 (1973) 477-488. 

We have summarized in Figure 9.9 all our results relative to a surface whose texture promotes the trapping of air. We further assume that the surface is sufficiently simple for the previous discussion to be valid (crenellated profile). In this case, equation (9.9) describes the hydrophobic regime, whereas the hydrophilic regime obeys equations (9.3) and (9.6), depending on the value of the contact angle. 

Mossbauer spectroscopy is a technique which is particularly sensitive to the local chemical environment. If the chemical environment is modified locally by the presence of heteroatoms such as dispersed alumina arising from the addition of textural promoters, this would affect many iron atoms within the sample and the method should detect differences in the spectral parameters, i.e., linewidths of the iron metal in the catalyst would change. If the iron contained larger inclusions of hercynite, the spectra should exhibit a quadrupole split component, with a typical ferrous ion shift characteristic of the ternary aluminum iron oxide. Three groups have studied these problems and their results are reviewed critically in Ref. 14. 

The catalysts often used in commercial processes consist in hematite (a-Fe203) and promoters such as potassium oxide and chromium oxide, among others [92]. It is well-known [92-97] that potassium acts as electronic promoter increasing the iron activity and also contributes to decrease the coke formation. On the other hand, chromium acts as textural promoter increasing the specific surface area of hematite [92, 93]. These catalysts are cheap and very active and selective, but have some drawbacks such as the low specific smface area and the instability of the active oxidation state of iron hematite (a-Fe203) is preferred for... 

Catalyst constituents, which have little or no catalytic activity by themselves, but which increase the catalytic activity for the catalyst are referred to as promoters. Promoters which increase the catalytic activity primarily by increasing the active area of the sample are referred to as structural or textural promoters. Promoters which increase the activity of the catalyst primarily by increasing the reaction rate per area are referred to as chemical or electronic promoters. Constituents, which decrease the activity of the catalyst when present in small amounts are referred to as poisons. 

Fig. 2.18. The Schlieren and homeotropic textures promoted by the use of very clean surfaces. Crossed polarizers. Original magnification x 200. (Ref. 95.)...

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