Adsorbents, weak

Finally, it should be pointed out that all the results on dynamical testing presented here are correct only if the tracer gas is not significantly adsorbed at the solid surface. If it is adsorbed weakly, so that a linear isotherm is appropriate, the equations should be modified by the following replacements ... 

Campbell reported that propylene adsorbs weakly on gold surfaces and adsorbs moderately on T1O2 (11 0) with a desorption activation energy of 11.3kcal/mol and that propylene adsorbs most strongly at the perimeter of gold islands on Ti02 (11 0) [68]. 

Oxygen gas is normally present in electroless solutions unless steps are taken to reduce its concentration by inert gas purging, the concentration of dissolved O2 usually approaches 10 4 mol dm 3 at room temperature. In contrast to deliberately added solution stabilizers, the O2 molecule tends to adsorb weakly on catalytically active surfaces, where it may or may not undergo dissociation [125], Since at many metallic surfaces it is capable of being reduced via either a 2-electron or a 4-electron reaction route, dissolved O2 gas affects the mixed potential of the surface being coated in the electroless solution. Shown here is the case of a 4-electron reduction reaction in basic solution ... 

For a solid-catalyzed reaction between two different molecules all the possibilities for the rate-determining step of the one-molecule reaction still exist. Thus, adsorption of either molecule can be rate determining a surface reaction involving only one of the molecules can be the rate-determining step and so forth. The two-molecule case introduces some new possibilities for the rate-determining step. In both the one-molecule and the two-molecule case the rate-determining step can be a surface bimolecular reaction. With the surface reaction in the two-molecule case, however, the reaction depends upon the relative abilities of the molecules to adsorb on the active sites. There are several possible cases. Both can adsorb weakly one can adsorb moderately well as the other adsorbs weakly they can both adsorb moderately well, competing effectively with each other for sites and one can adsorb very well as the other adsorbs weakly. Another problem arises when the two... 

Microbalances (stainless steel) can be made to handle pressures as high as 120 Mpa (120 atm) since gases that adsorb weakly or boil at very low pressures can still be used. 

Rh > Ir > Ni > Pd > Co > Ru > Fe A plot of the relation between the catalytic activity and the affinity of the metals for halide ion resulted in a volcano shape. The rate determining step of the reaction was discussed on the basis of this affinity and the reaction order with respect to methyl iodide. Methanol was first carbonylated to methyl acetate directly or via dimethyl ether, then carbonylated again to acetic anhydride and finally quickly hydrolyzed to acetic acid. Overall kinetics were explored to simulate variable product profiles based on the reaction network mentioned above. Carbon monoxide was adsorbed weakly and associatively on nickel-activated-carbon catalysts. Carbon monoxide was adsorbed on nickel-y-alumina or nickel-silica gel catalysts more strongly and, in part, dissociatively,... 

Benzene adsorbs weakly on Cu and strongly on Ni. It is interesting to study how the differences in adsorption strength are reflected in the electronic structure of the adsorbate-substrate complexes as determined based on the XAS and XES spectra for benzene on Cu(l 10) and Ni(100) shown in Figures 2.39 and 2.40, respectively [83,90]. 

The observed effects of structure on rate and on orientation, confirmed by the Brown selectivity relationship, show that there is no basic difference between heterogeneous catalytic alkylation of aromatic compounds and homogeneous electrophilic aromatic substitution, cf. nitration, sul-phonation etc. This agreement allows the formulation of the alkylation mechanism as an electrophilic attack by carbonium ion-like species formed on the surface from the alkene on Br0nsted acidic sites. The state of the aromatic compound attacked is not clear it may react directly from the gas phase (Rideal mechanism ) [348] or be adsorbed weakly on the surface [359]. 

Infrared spectral studies of pyridine adsorbed on alkali metal ion-exchanged faujasites have demonstrated the absence of Brpnsted acidity, as reported by Eberly (151), Ignat eva et al. (208), and Ward (156, 209-211). Pyridine is adsorbed weakly by coordination to the alkali metal ions (151, 156). Addition of small amounts of water does not result in formation of Br0nsted acid sites, indicating that the coordinate bound pyridine is not associated with Lewis acid sites in the zeolite framework (210). 

Weakly adsorbed Strongly adsorbed Weakly adsorbed Strongly adsorbed... 

Experiments have been conducted to simulate leaching of phthalates from blood bags by allowing human plasma to extract added phthalates from coated Celite (Albro and Corbett 1978). It was found that more than 80% of the DEHP was associated with lipoproteins, in the order LDL > VLDL > HDL > chilomicrons. The remaining DEHP was adsorbed weakly and nonspecifically to other proteins including albumin. MEHP was in equilibrium between free in solution and adsorbed to albumin, no MEHP was bound to lipoproteins. Rock et al. (1986) reported that in human plasma, the lipase that hydrolyzes DEHP copurified with the albumin, and once in the plasma, the MEHP bound to albumin. An earlier study by Jaeger and Rubin (1972) reported that in human blood stored in PVC bags, the bulk of DEHP was associated with lipoproteins, but a substantial amount was in a fraction likely to represent DEHP soluble in plasma water as well as bound to plasma proteins and cell membranes. 

Oxygen is adsorbed weakly onto the surface of the coal. 

Selective formation of cw-monochlorocyclopropanes in the absence of base or in the presence of monoamines has been explained by preferential homolytic cleavage of a less hindered C-Cl linkage to form the more stable surface intermediate Ic rather than the less stable It under the circumstances that the substrate can be strongly adsorbed. On the other hand, in the presence of an alkali hydroxide or a diamine, which may be adsorbed strongly to the catalyst, it is probable that the substrate is adsorbed weakly and formation of the more stable fran.s-monochlorocyclopropanes increases with an increased contribution of a nucleophilic reaction (Scheme 13.17).211... 

An excellent illustration of the LHHW theory is catalytic cracking of n-alkanes over ZSM-5 [8]. For this reaction, the observed activation energy decreases from 140 to -50 ( ) kj/mol when the carbon number increases from 3 to 20. The decrease appeared to linearly depend on the carbon number as shown in Fig. 3.11. This dependence can be interpreted from a kinetic analysis that showed that the hydrocarbons (A) are adsorbed weakly under the experimental conditions. The initial rate expression for a rate-determining surface reaction applies (3.30), which in the limiting case of weak adsorption of A reduces to Eqn. (3.52). The activation energy is then represented by equation (3.53). 

Phase system Adsorbent Weak solvent Strong solvent... 

In practice, the adsorption from solution behavior can often be predicted qualitatively in advance, in terms of the polar/non-polar nature of the solid and of the solution components. A non-polar adsorbent will tend to adsorb non-polar adsorbates strongly and polar adsorbates weakly, and vice versa. In addition, non-polar solutes will tend to be... 

At low concentrations, ionic surfactants adsorb chiefly as counterions in response to surface charge. Anionic species adsorb strongly when pH < pH zc where surface charge is positive, and weakly, if at all, when pH>pH 2. Cations adsorb weakly when pH pH zc range, electrokinetic behavior is identical to that of simple monovalent indifferent... 

Table II shows the results of butenes hydrogenation reactions. An interesting feature of the power function model for the hydrogenation reaction is the pressure dependencies obtained. As shown in Table II some experimental results indicated consistently that the hydrogenation rate is proportional to the 0.5 power of hydrogen and butenes partial pressure. This may imply, as discussed by Bond (I), that hydrogen is dissociated upon adsorption and that butenes are adsorbed weakly on a portion of active sites owing to the steric hindrance of adsorbing butenes molecules.

Haruta and coworkers [1] have suggested that different reaction mechanisms may be operative at different temperatures for CO oxidation. Extrapolating their suggestion, it is possible that different mechanisms occur on different active sites. The discussion below will focus only on the mechanism applicable to reactions near room temperature. Several mechanisms have been proposed in the literature. They can be classified into two categories those that occur entirely on the Au particles and those that involve the support. Based on FTIR and TAP reactor studies, it is generally agreed that CO is adsorbed weakly and reversibly on Au particles [42,43,44]. However, there are many proposals for the subsequent steps of reaction. 

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