Egg-shell structure

This is clearly not what we find experimentally Either abrasion or a non-uniform distribution of platinum could be responsible for this discrepancy. Since the observed rates are determined during initial reaction times (1-5 min), abrasion can not be the main reason for the independence of TOF on particle size. Hence, these experiments suggest that the catalyst used has indeed an "egg-shell" structure. 

We have identified reaction conditions where intrinsic kinetics can be obtained for the very fast enantioselective hydrogenation of ethyl pyruvate using a commercially available Pt/Al203 powder catalyst, modified with dihydrocinchonidine. We conclude that this is in pan due to i) the egg-shell structure of the catalyst, ii) the high turbulence achieved in the reactor and iii) the density and/or the viscosity of the solvent used. In solvents like ethyl pyruvate, liquid-solid transpon problems can arise. 

Similar to normal catalyst synthesis whereby ion exchange methods can result in egg-shell structures, in the preparation of monolith catalysts the majority of the metal can be deposited at the entrance of the monolith. Egg-shell structures can be attractive for catalyst particles, but for monoliths, analogous uneven distributions of the active phase are a disaster. Fortunately, extensive literature is available describing ion-exchange procedures for conventional catalysts that yield homogeneous metal distributions. This literature can be used as a guide for preparing satisfactory monolithic catalysts. 

It is possible to shape catalyst bodies, in which the catalytically active substances are not distributed over the complete bulk, but rather located in concentric areas. Many fluidized-bed catalysts, for instance, are spheres in which the active phase represents the core, and the shell is a porous, protective layer to prevent attrition. Bodies exhibiting the active phase in the core are denoted as egg-yolk catalysts. For fixed-bed applications, so-called egg-shell structures are more convenient, in which the catalytic material is located at the external surface, whereas the core is nonreactive. 

For Pt75Ni25 the optimal structure is an egg-shell of Pt covering a Ni-rich core. 

Two ways to reduce the diffusion length in TBRs are 1) use of smaller catalyst particles, or 2) use of an egg-shell catalyst. The first remedy, however, will increase pressure drop until it becomes unacceptable, and the second reduces the catalyst load in the reaction zone, making the loads of the TBR and the MR comparable. For instance, the volumetric catalyst load for a bed of 1 mm spherical particles with a 0.1 mm thick layer of active material is 0.27. The corresponding load for a monolithic catalyst made from a commercial cordierite structure (square cells, 400 cpsi, wall thickness 0.15 mm), also with a 0.1 mm thick layer of active material, is 0.25. 

While CaC03 crystals (calcite and aragonite) predominantly appear in egg shells and in biomineralisates from invertebrates, calcium phosphates are predominantly involved in processes which play an inportant role in medicine. They will be described here in detail the knowledge of their structures are most relevant for the understanding of the cellular and molecular processes in bones and teeth. 

The biocrystalline layer of avian egg shells is approximately 270-300 jum thick (chicken). It can be divided into morphological units as follows eisospherites, organic matrix core, zone of tabular aggregates, zone fish bone patterns, external zone, cuticle, and organic matrix. Details of the morphological structure have been reviewed recently28, 374). 

Amplification of DNA of chromosomes. During formation of oocytes parts of the DNA are "amplified" by repeated replication. This provides a way for the ovum to accumulate ribosomal RNA and various proteins in large amounts. Similarly, genes for two abundant proteins of the egg shell or chorion of insects are amplified. Bidirectional replication initiated at discrete positions yields an "onion skin" structure containing many copies of an 90-kb sequence containing the two genes. The polyploidy observed in some highly specialized cells such as the Purkinje cells... 

The brittle covering of avian eggs, chiefly calcium carbonate, lime, etc. The formation of proper shell structures in certain species of birds is said to be adversely affected by DDT and similar insecticidal contaminants of their food. 

Meldrum, 2002), the elaborately shaped spicules in ascidians, and finely the sculpted coccolith shells in coccolithophores (Mann, 2001). For silica structure, the diatoms produce the most exquisite siliceous cell wall (Sumper and Brunner, 2006). Also the calcium phosphate in bone shows a highly regulated organization and arrangement. Interestingly, avians have two main biomineralization systems, one produces calcium carbonate for egg shell and the other produces calcium phosphate for bone (Bauerlein, 2000 Mann, 2001). 

The structure, ultrastructure and formation of the hymenolepidid egg has been reviewed in detail by Ubelaker (888). Its general morphology is shown in Fig. 7.14). Although there are only the usual three basic embryonic membranes (p. 179) in the developing egg - shell/capsule, outer envelope, and inner envelope - the fully formed egg often appears to be more complex due to further differentiation of these layers. The following structures can be recognised (Fig. 7.11). 

In principle, deposition of an active phase (metal and/or oxide) on a monolithic catalyst support can be carried out in a manner similar to that used to prepare a t) ical catalyst. However, the large dimension of a monolith can easily enhance problems of nonhomogeneous deposition. For example, if in the preparation of conventional catalyst particles the active phase would be deposited at the external surface of the support, the result would be an egg-shell-t) e catalyst, which for many processes can be advantageous. However, if this pattern of deposition were applied to a monolithic support, it could result in a monolith with only the outer charmels of the structure having a significant catalytic activity, resulting in a dramatically poor catalytic reactor. The critical steps in the s)mthesis process are the deposition and drying steps, which are discussed separately below. Calcination, reduction, etc. for monolith catalysts are not different from those used to manufacture t) ical catalysts, and these steps are therefore not discussed here. 

Occurs only in combination, as limestone, marble, chalk (GaCO,) gypsum, selenite, alabaster (CaSO,), aud many other minerals. In bones, egg-shells, oyster-shells, etc., as Ca,(PO,), and CaCO and in many vegetable structures... 

The present research report deals with the preparation of egg-shell catalysts with controlled oxidic structure on metallic substrates like aluminum, titanium and magnesium. The influence of the substrate, electrolyte composition, and treatment time on the manufacture of egg-shell catalysts with tailor-made properties for the effective oxydehydrogenation of cyclohexane to cyclohexene will be presented. The selectivity control by the pore length is discussed. 

The examination of the experimental results presented in Fig. 4 (b) shows an enhancement of cyclohexene selectivity by using of the catalyst C6 with shorter pores. This is an evidence for the fact that short pores suppress the successive reactions of cyclohexene. The potential of the egg-shell catalysts with tailor-made pore structure can be further improved by their assembling in microchannel reactors. The good thermal conductivity of the metallic substrate... 

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