Substrate adsorption

In the case of 2-octanol and cyclohexanol, differences in stereochemistry might be due to a lower hydrogen availability. In the case of cyclohexanol, its high viscosity, modifying the substrate adsorption step on the surface, may also play a role. 

Figure 4.17 Substrate adsorption synthesis of ((S)-3,4-methylenedioxyphenyl)-2-propanol from 3,4-methylenedioxyphenylacetone applying a batch process followed by a filtration step and resin extraction...

Substrate inhibition physical binding of substrate adsorption of substrate on resin... 

In addition to the matching of the structures of the surfaces of the mineral to be nucleated and the substrate, adsorption or chemical bonding of nucleus constituents to the surface of the substrate can be expected to enhance the nucleation. Surface... 

Inhibitors form a film on the surface that blocks the dissolution of the substrate. Adsorption of an alkyl-thiol to the surface of the CU3AU alloy resulted in an inaeased surface tension of the gold film this is observed as an increase in the value of <., which depresses dissolution of copper. This behavior resembles inhibition of copper corrosion on a pure copper surface where benzotriazole increases the potential to start significant copper dissolution this was demonstrated by Cruickshank et using in situ AFM. However, when the critical potential for benzotriazole film... 

Surface type Topmost substrate Adsorption Adsorption Adsorption Equivalent Method References Comments... 

For many photoelectrochemical events, Langmuirian kinetics govern substrate adsorption. That is, the observed photochemically induced reaction rate is found to be proportional to the concentration (or pressure) of the reactant, which in turn can be related to the surface coverage 9 by the Langmuir expression, Eq. (10),... 

The solute—substrate interactions are responsible for the separation of low molecular weight solutes in conventional TLC. Solutes are partitioned between the mobile phase and substrate. Adsorption takes place at the head of the spot and desorption at the tail. For solutes with varying affinities for the substrate, relatively more or less time is spent in the absorbed state resulting in different migration distances. 

If one could use heterogeneous catalysts such as dispersed metals to promote this type of reaction, product separation would be facilitated and the more efficient flow processes could be used instead of the commonly employed batch mode. There are, however, a number of problems which must be overcome before such systems can be used practically. In the first place it must be shown that dispersed metals can promote these reactions and, secondly, a more detailed knowledge must be acquired of substrate adsorption on the catalyst and the interaction of the adsorbed species to give the product. 

Zhao J,WuT,Wu K, Oikawa K, Hidaka H, Serpone N. Photoassisted degradation of dye pollutants. 3. Degradation of the cationic dye rhodamine B in aqueous anionic sur-factant/Ti02 dispersions under visible light irradiation evidence for the need of substrate adsorption onTi02 particles. Environ Sci Technol 1998 32 2394-400. 

Still controversial [64] is the function of the deposit. It could either act as a dynamic cover for nonselective active sites, which is compressed laterally upon substrate adsorption and reexpanded after desorption of the product. The alternative function is to serve as a weakly interacting surface for the substrate adsorption [65]. 

Ideally one would like to determine the effective surface area of a catalyst using as adsorbate the reactant species in question. This has indeed proved possible in a few cases where the adsorbent area is large. One such favourable situation occurred in the racemisation of (+ )589- [Co(en)3]3+ catalysed by a carbon black [35] where fast initial adsorption of the substrate was followed by its slow isomerisation, as shown in Fig. 2. The difference between the initial reading and the intercept of the first-order line was a measure of the extent of the substrate adsorption. 

In principle one should even be able to determine the extent of substrate adsorption during the course of the reaction itself, provided an appreciable fraction of the reactant is adsorbed. If the experimenter knows the initial reactant concentration, a, and can measure both its bulk concentration, c, at a given time as well as the concentration, x, of a non-adsorbed product, then he can calculate the number of moles of reactant absorbed from the equation... 

Bulnes F., Ramirez-Pastor A.J. and Zgrablich G., Scaling Behavior in Adsorption on Bivariate Surfaces and the Determination of Energetic Topography. J. Chem. Phys. 115 (2001) 1513 Power Laws in Adsorption and the Characterization of Heterogeneous Substrates. Adsorption Sci. and Tech. 19 (2001) 229 Scaling Laws in Adsorption on Bivariate Surfaces Pf s. Rev. E, 65 (2002) 31603. 

All these photocorrosion processes are, of course, undesirable and it is obvious that their relative importance depends strongly on the presence of surface states which may facilitate recombination or redox reactions with adsorbed substrates. It is well known from ESR [69, 70, 94] and emission spectra [94] that most of these metal sulfide powders contain surface states. They are introduced during preparation of the powder as a result of lattice defects [72, 96], trapped holes [94], surface impurities [97] and metallization [38], and during the actual catalytic reaction as a consequence of irradiation and substrate adsorption. The stabilizing effect of plati-nization is exemplified by Figure 6 for the ZnS-catalyzed reduction of water in the presence of sodium formate [98]. Note that platinum does not accelerate the reaction but doubles the time of constant catalytic activity from 1 to 2 days. Similarly, the apparent product quantum yield of the 2,5-DHF dehydrodimerization is not increased but slightly decreases when platinizes ZnS is the photocatalyst [97]. 

To unravel the detailed mechanism, substrate adsorption, quenching, inhibition and kinetic studies were conducted for the ZnS-catalyzed photodehydrodimeriza-tion of 2,5-DHF [107, 148]. A plot of the amount of 2,5-DHF adsorbed ( eq) against the residual concentration in solution (cgq) exhibits saturation plateaus at eq(max) of 2.8 X 10 and 65 X 10 mol g . The first plateau is due to the formation of a mixed solvent-solute surface monolayer and the second corresponds to multilayer adsorption. Assuming that the formation of the monolayer can be described by competitive adsorption between water and 2,5-DHF, the data can be analyzed according to Hiemenz (Eq. 30) [149] ... 

Substrate adsorption studies as conducted with ZnS in aqueous solution were performed also with CdS, CdS--Si02 and SiOi in methanol, the solvent employed in... 

It is difficult to predict a priori the direction of the reaction, since its actual course may be affected by many parameters. Among these are the nature of the catalyst, the geometry of the substrate adsorption on the surface, the density of the adsorbed H atoms etc. One of the reactions mentioned above (equation 136) which was performed in MeOH can serve as an example of the effect of the conditions on product distribution. A change in catalyst and solvent inflicted a drastic change in the ratio of the products (equation 140). 

C) and stirred (1000 rpm) while samples were taken at stipulated times. Substrate adsorption was followed by GC. 

Ng = moles of adsorption sites per gram of substrate (adsorption capacity)... 

As shown in Scheme 1, the rate constants k, and k., in each pathway refer to the substrate adsorption and desorption steps on the modified sites, while the two kj s are constants for the irreversible hydrogenation steps for each pathway. Expressions for the intermediate species 0 on unmodified sites and 0 Epj o, and 0 EP.inod modified sites are given below. Adsorption-desorption equilibrium on unmodified sites is assumed, with Kgp and K referring to the... 

The use of the steady-state approximation instead of assuming substrate adsorption-desorption equilibrium a key feature of Halpem s studies (9) of pressure effects on enantioselectivity, and its implications for enantioselectivity in the current system will be discussed later in the text. 

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