Effect on catalytic activity

The turnover frequency (TOP) based on surface-exposed atoms significantly increases with a decrease in the diameter of the gold particle from 5 nm [66]. This feature is unique to gold, because other noble metals usually show TOFs that decrease or remain the same with a decrease in the diameter [7]. The decrease in particle size gives rise to an increase in corner or edge and perimeter of NPs and change in electronic structure however, the origin of size effects on catalytic activity for CO oxidation is not clear. 

FIGURE 1 2-2 Schematic diagram of the phosphorylation sites on each of the four 60kDa subunits of tyrosine hydroxylase (TOHase). Serine residues at the N-terminus of each of the four subunits of TOHase can be phosphorylated by at least five protein kinases. (J), Calcium/calmodulin-dependent protein kinase II (CaM KII) phosphorylates serine residue 19 and to a lesser extent serine 40. (2), cAMP-dependent protein kinase (PKA) phosphorylates serine residue 40. (3), Calcium/phosphatidylserine-activated protein kinase (PKC) phosphorylates serine 40. (4), Extracellular receptor-activated protein kinase (ERK) phosphorylates serine 31. (5), A cdc-like protein kinase phosphorylates serine 8. Phosphorylation on either serine 19 or 40 increases the activity of TOHase. Serine 19 phosphorylation requires the presence of an activator protein , also known as 14-3-3 protein, for the expression of increased activity. Phosphorylation of serines 8 and 31 has little effect on catalytic activity. The model shown includes the activation of ERK by an ERK kinase. The ERK kinase is activated by phosphorylation by PKC. (With permission from reference [72].)... 

Doskeland AP, Martinez A, Knappskog PM, Flatmark T. 1996. Phosphorylayion of recombnant human phenylalanine hydroxylase effect on catalytic activity, substrate activation and protection against non-specific deavage of the fusion protein by restriction protease. Biochem J 313 (Pt 2) 409-414. 

For adequate reaction rates, a high concentration of iodide anion is necessary. The cation portion of the salt appears to have little or no effect on catalytic activity or reaction selectivity. Inorganic iodides (such as potassium iodide) are the obvious first choice based on availability and cost. Unfortunately these catalysts have very poor solubility in the reaction mixture without added solubilizers or polar, aprotic solvents. These solubilizers (e.g., crown ethers) and solvents are not compatible with the desired catalyst recovery system using an alkane solvent. Quaternary onium iodides however combine the best properties of solubility and reactivity. 

Kaplan has proposed that ion pairing between rhodium complex anions and the positively charged counterions has an adverse effect on catalytic activity for ethylene glycol formation (96, 109, 110). The following scheme ... 

Fig. 22. Effect on catalytic activity to methanol (O) and ethanol (A) of adding HI to a Ru3(CO,2 catalyst in tri-n-propylphosphine oxide solvent (193). Reaction conditions 75 ml solvent, 6 mmol Ru, 850 atm, H2/CO = I, 230 C.

Despite numerous screening studies, the literature contains little evidence that homogeneous catalyst systems based on metals other than Co, Rh, or Ru have significant activity for catalytic CO reduction. As seen for the known active catalytic systems, however, properties of solvents and additives or promoters can have enormous effects on catalytic activities. Solvents and additives can serve many roles in these catalytic systems. One important function of promoters in the Rh and Ru systems appears to be that of stabilizing metal oxidation states involved in catalytic chemistry. Other... 

A wide variety of solid surfaces is used as catalysts in an even wider assortment of industrial processes (see, for example, Richardson 1989 and Somorjai 1994) we limit our discussion to metal catalysts. While these represent only a fraction of all catalytic systems, they do include a number of industrially important examples. Table 9.6 lists some metals used as commercial catalysts and indicates briefly the types of reactions for which they are employed. In this section we emphasize the effect on catalytic activity of the chemical and crystallographic properties of metal surfaces. 

Since the work of Inoue a wide variety of metal complexes have been shown to be highly active in CO2 reduction. These are usually hydrides or halides with phosphines as neutral ligands, and Rh and Ru proved to be the most active metals.9,100-104 Variation of the ligand has a considerable effect on catalytic activity. 

The chapter by C. J. Swan and D. L. Trimm, which also emphasizes the effect on catalytic activity of the precise form of a metal complex, shows too that, depending on the metal with which it is associated, the same ligand can act either as a catalyst or inhibitor. The model reaction studied was the liquid-phase oxidation of ethanethiol in alkaline solution, catalyzed by various metal complexes. The rate-determining step appears to be the transfer of electrons from the thiyl anion to the metal cation, and it is shown that some kind of coordination between the metal and the thiol must occur as a prerequisite to the electron transfer reaction (8, 9). In systems where thiyl entities replace the original ligands, quantitative yields of disulfide are obtained. Where no such displacement occurs, however, the oxidation rates vary widely for different metal complexes, and the reaction results in the production not only of disulfide but also of overoxidation and hydrolysis products of the disulfide. 

The effect on catalytic activity of varying the 7r-bonding ligands for a series of chloro complexes was also studied. The cocatalyst was (CH3)3A12C13j and the solvent was PhCl. The results are displayed in Table II along with vn-o for the complexes. 

Complexes of Other Metals. Having studied in detail catalysts derived from molybdenum nitrosyl complexes, it was interesting to investigate the effect on catalytic activity of substituting other transition metals in both nitrosyl derivatives and in other related complexes. 

No particular significance is attached to the fact that the molar amounts of glyme and diglyme in these two catalysts are almost the same. Extensive vacuum drying of the cobalt catalyst, which reduced the water level from 4.29 to less than 1.0 mole per mole of the zinc salt (and, presumably, reduced the glyme to a similar or larger extent), gave no appreciable effect on catalytic activity. This result emphasizes the nonstoichiometric nature of the catalytic forms of the hexacyanometalate salt complexes. 

The experimental results described in this review support the concept that, in certain reactions of the redox type, the interaction between catalysts and supports and its effect on catalytic activity are determined by the electronic properties of metals and semiconductors, taking into account the electronic effects in the boundary layer. In particular, it has been shown that electronic effects on the activity of the catalysts, as expressed by changes of activation energies, are much larger for inverse mixed catalysts (semiconductors supported and/or promoted by metals) than for the more conventional and widely used normal mixed catalysts (metals promoted by semiconductors). The effects are in the order of a few electron volts with inverse systems as opposed to a few tenths of an electron volt with normal systems. This difference is readily understandable in terms of the different magnitude of, and impacts on electron concentrations in metals versus semiconductors. 

As will be discussed later, the particular form of contaminant distribution within the porous layer, as observed by the microprobe, does in some cases correlate with the contaminant effect on catalytic activity. 

Recently the effect of steaming on the presence of framework and extra-framework aluminium and its effect on catalytic activity has been the subject of a considerable research effort. 

The conclusion is that particle size effects on catalytic activity or selectivity due to variations in the inherent properties of small metal particles (geometric or electronic) are unlikely to be important for particles larger than about 1.5-2.0 nm. If size effects are observed for larger particles it is necessary to consider the nature and origin of such effects. 

The amount of water contained in the reactants is usually not controlled, although it may have a drastic influence reports of its effects on catalytic activity are not consistent (Table 6.2) and general features axe hard to discern. Careful studies44,45 have shown that the effect that water has on catalytic activity depends on the support and on its concentration (Figure 6.1). Activation energies shown by Au/TiC>2 and AU/AI2O3... 

Although the inclusion of sulfate in the Fe203 catalyst has a negative effect on catalytic activity in this study, we have shown elsewhere that sulfation of goethite (a-FeOOH) precursors may be a potentially simple route to the preparation of functionalised nanoporous haematite [18,19],... 

Scheme 3). Subtle structural differences can have a pronounced effect on catalytic activity and selectivity. 

In the presence of MAO or AlMe3 the 14-electron species is stabilized by reversible adduct formation, giving [Cp2Zr(//,-Me)2AlMe2]+.101 Very weakly basic anions such as B(QF5)4 are required to stabilize the cationic metallocene species anions have a strong effect on catalytic activity.105... 

Climent, L, Sj berg, B.-M., and Huang, C. Y., 1992, Site-directed mutagenesis and deletion of the carboxyl terminus of Escherichia coli ribonucleotide reductase protein R2. Effects on catalytic activity and subunit interaction. Biochemistry 31 4801fi4807. 

---