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Overall Catalytic Properties

Another characteristic feature of enzymatic catalysis was demonstrated for the Diels-Alderase ribozymes, namely enantioselective bond formation. While the uncatalysed [Pg.388]


The properties of the zeolite play a significant role in the overall performance of the catalyst. Understanding these properties increases our ability to predict catalyst response to changes in unit operation. From its inception in the catalyst plant, the zeolite must retain its catalytic properties under the hostile conditions of the FCC operation. The reaclor/regenerator environment can cause significant changes in chemical and structural composition of the zeolite. In the regenerator, for instance, the zeolite is subjected to thermal and hydrothermal treatments. In the reactor, it is exposed to feedstock contaminants such as vanadium and sodium. [Pg.88]

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. [Pg.410]

Ill-defined carbon materials that provide a distinct nanostructure, such as spherical particles in the case of soot and carbon black, or hexagonally ordered cylindrical pores in the case of ordered mesoporous carbons, are not discussed here. Surface chemical, thus catalytic properties of these material are closer to carbon black or activated carbon [13], which is frequently reviewed [2-4]. Here, the higher degree of sp3 hybridization often results in a higher reactivity, however, at lower selectivity, as compared to nanocarbons exposing large basal plane fractions of the overall surface. [Pg.396]

The reasons for the effect of pH on the catalytic properties of enzymes are numerous and will not be discussed here. For most enzymes, however, there is a pH at which they are optimally effective changing the pH to lower (more acidic) levels or to higher (more basic) levels will decrease the overall rate at which the associated chemical reaction occurs. In the region of the optimum pH, the reaction rate vs. pH response surface can usually be approximated reasonably well by a second-order, parabolic relationship. [Pg.199]

Synergistic Promotion Effects. As was already mentioned promoter elements are not considered themselves to be catalytically active, but it is fair to say that this is not always the case. This promoter activity may indirectly affect the behaviour of the catalytic active element since it will alter, e.g., the local feed composition or may, due to its catalytic properties, influence the overall reaction product distribution. The following effects, illustrated in Figure 5, are expected to occur in a promoted Co F-T catalyst. [Pg.25]

The active biochemical constituents of cells are a particular group of proteins which have catalytic properties. These catalytic proteins, or enzymes, are in some ways similar to inorganic catalysts but are distinctive in other, quite important respects. Enzymes are very powerful catalysts, capable of enhancing the overall rates of reactions much more markedly they are much more specific than the average inorganic catalyst. [Pg.252]

In summary, the preparation of bimetallic catalysts by surface redox reaction using a reductant preadsorbed on the parent monometallic catalyst has been studied in detail. Unfortunately, the method is intricate and time consuming, especially if several successive operations are required. Furthermore, when the modifier has a standard electrochemical potential higher than that of the parent metal (AUCI4 deposited on Pt°), the overall reaction is a complex one involving a reduction by adsorbed reductant but also direct oxidation of the metallic parent catalyst. The relative rate of the two parallel reactions determines the catalytic properties of the resulting bimetallic catalyst. [Pg.223]

We distinguish integral and differential characteristics of catalytic properties. One of the integral characteristics is the extent of reaction Generally, any chemical reaction, whether overall or an elementary step, can be represented by a stoichiometric equation ... [Pg.541]

However, this assumption is not necessarily justified. Even for a well-faceted nanoparticle there are a number of nonequivalent adsorption sites. For example, in addition to the low-index facets, the palladium nanoparticle exhibits edges and interface sites as well as defects (steps, kinks) that are not present on a Pd(l 1 1) or Pd(lOO) surface. The overall catalytic performance will depend on the contributions of the various sites, and the activities of these sites may differ strongly from each other. Of course, one can argue that stepped/kinked high-index single-crystal surfaces (Fig. 2) would be better models (64,65), but this approach still does not mimic the complex situation on a metal nanoparticle. For example, the diffusion-coupled interplay of molecules adsorbed on different facets of a nanoparticle (66) or the size-dependent electronic structure of a metal nanoparticle cannot be represented by a single crystal with dimensions of centimeters (67). It is also shown below that some properties are merely determined by the finite size or volume of nanoparticles (68). Consequently, the properties of a metal nanoparticle are not simply a superposition of the properties of its individual surface facets. [Pg.139]

MPO is a covalently linked dimer which is ellipsoidal in shape with overall dimensions of 110 x 60 x 50 A. The dimer can be cleaved by reduction of a disulfide bond into two identical halves. Each half of the dimer termed hemi-MPO has the same optical and catalytic properties of the dimer. Hemi-MPO consists of two polypeptides of466 and 108 amino acid residues, and a heme prosthetic group covalently bound to the large polypeptide. Like CcP and LIP, MPO is largely a helical bundle protein with very little -sheet stmcture. The bulk of the large polypeptide folds into five separate domains and one... [Pg.1949]

The copolymerization theory presented in Chapter is of limited applicability to processes involving heterogeneous Ziegler-Natta catalysis. The simple copolymer model assumes the existence of only one active site for propagation, whereas the supported catalysts described above have reaction sites that vary in activity and stereoregulating ability. In addition, the catalytic properties of the active sites may vary with polymerization time. The simple copolymer model can be used with caution, however, by employing average or overall reactivity ratios to compare different catalysts and monomers. [Pg.339]


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Catalytic properties

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