Active sites relationship with activity

The multi-functionality of metal oxides1,13 is one of the key aspects which allow realizing selectively on metal oxide catalysts complex multi-step transformations, such as w-butane or n-pentane selective oxidation.14,15 This multi-functionality of metal oxides is also the key aspect to implement a new sustainable industrial chemical production.16 The challenge to realize complex multi-step reactions over solid catalysts and ideally achieve 100% selectivity requires an understanding of the surface micro-kinetic and the relationship with the multi-functionality of the catalytic surface.17 However, the control of the catalyst multi-functionality requires the ability also to control their nano-architecture, e.g. the spatial arrangement of the active sites around the first centre of chemisorption of the incoming molecule.1... 

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

The effects of pumiliotoxin B on sodium channels appear to be due to interaction with a subdomain of the site at which batrachotoxin acts scorpion toxins and brevetoxin can potentiate the effects of pumiliotoxin B and of congeneric pumiliotoxins and allopumiliotoxins (102,112). Certain pumiliotoxin congeners appear to block sodium channel activation and may act as antagonists or reverse agonists (106). Structure-activity relationships with respect to stimulation of sodium flux and phosphoinositide breakdown have been studied (106). The nature of the side chain is critical to activity. For example, whereas pumiliotoxin B is one of the most potent of these alkaloids, its 15,16-erythro isomer has much lower activity (106,112). 

Pai et al. (1983) measured hole mobilities of a series of bis(diethylamino)-substituted triphenylmethane derivatives doped into a PC and poly(styrene) (PS). The mobilities varied by four orders of magnitude, while the field dependencies varied from linear to quadratic. In all materials, the field dependencies decreased with increasing temperature. The temperature dependencies were described by an Arrhenius relationship with activation energies that decrease with increasing field. Pai et al. described the transport process as a field-driven chain of oxidation-reduction reactions in which the rate of electron transfer is controlled by the molecular substituents of the hopping sites. 

The deactivation of the catalysts coked by BIT shows a much different relationship with respect to activation energy (Fig. 1). The experimental data are displaced to higher activation energies from those of the AN-coked catalysts. Since the distribution parameter of the fresh catalyst must be the same for the coked catalysts, is it obvious that deactivation by BIT cannot be by SSD. Hence, the BIT-coked catalysts are considered to deactivate by site preference deactivation (SPD). A value of the site preference parameter, g, was selected and a plot generated by varying p. Another g was selected and the process repeated until the best fit to the experiment BIT data of Fig. 1 was obtained. This occurred for a value of g of 2.7, and the fit is shown by the curve on the right-hand side of Fig. 2. 

The active sites of enzymes are three-dimensional structures that are formed as a result of the overall tertiary structure of the protein. This results from the amino acids and co-factors in the active site of an enzyme being spatially structured in an exact, three-dimensional relationship with respect to one another and the structure of the substrate molecule. 

Of course, there is no direct relationship between the H storage and the catalytic performance. And this is true whatever the reaction (oxidation or hydrogenation), mainly because (i) the H species storage involves bulk and surface phenomena whereas the activity is related to catalytic sites located at the surface of the solid, and (ii) activity and specially selectivity for a given reaction depend on specific kind of sites. It is highly probable that the sites interacting with the alkane are much more specific (structurally for example) than... 

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