Adsorption mechanisms alcohols

Alcohols are hydroxylated alkyl-compounds (R-OH) which are neutral in reaction due to their unionizable (OH) group (e.g., methanol, ethanol, isopropanol, and w-butanol). The hydroxyl of alcohols can displace water molecules in the primary hydration shell of cations adsorbed onto soil-solid and sediment-solid clay particles. The water molecule displacement depends mainly on the polarizing power of the cation. The other adsorption mechanisms of alcohol hydroxyl groups are through hydrogen bonding and cation-dipole interactions [19,65],... 

Maximum effectiveness in foam inhibition is exhibited by the alcohols in the middle of the homologous series (C7 - C9). In the whole range of their concentrations the foam inhibition occurs under heterogeneous conditions and hence, is a consequence of the joint action of the homogenous and heterogeneous adsorption mechanisms of foam breakdown. 

Mechanism The Mn02 oxidation reaction is believed to proceed by adsorption of alcohol on Mn02 followed by coordination of both alcohol and Mn02- Electron transfer then produces the radical, and Mn(IV) is reduced to Mn(III). Another electron transfer is followed by the release of adsorbed carbonyl product with the loss of water from Mn(OH)2 (Scheme 7.4). An ionic mechanism is also proposed for this oxidation which involves the formation of manganate ester. ... 

Mechanism The mechanism involves the adsorption of alcohol on silver carbonate followed by formation of the protonated carbonyl. The carbonate ion acts as a hydrogen ion acceptor, generating carbonic acid, which decomposes to CO2 and H2O (Scheme 7.11). 

We are left therefore with a situation that none of the mechanisms proposed can accurately describe the results obtained. In developing a new mechanism that can accommodate the results it is important to use as much of previous mechanisms as possible. As we have shown above, much of what has been proposed is in agreement with the results. However there has been much greater understanding of molecular adsorption/desorption since the publication of the Schwegler-Adkins mechanism. The adsorption of alcohols on Ni(l 11) [12] and our own exchange results show that the 0-H bond is the easiest to break in this system. Therefore the initial adsorption under reaction temperatures will result in the formation of an ethoxy species on the surfaee. Similarly the adsorption of... 

Calculations show that the model of a non-equilibrium surface layer is an alternative to kinetic-controlled adsorption models. On the basis of the purely diffusion-controlled adsorption mechanism the proper consideration of a non-equilibrium diffusion layer leads to a satisfactory agreement between theory and experimental data for various studied systems, systematically demonstrated for the short-chain alcohols [132], The non-equilibrium model is applicable in the concentration range from 10 to 10 mol/cm at different values of the Langmuir constant at- For l < 10 mol/cm a consideration of non-equilibrium layer effects is not necessary. For ai > 10 mol/cm and large surfactant concentration the Ay values calculated from the proposed theory do not compensate the discrepancy to the experimental data so that other mechanisms have to be taken into account. An empirical formula also proposed in [132] for the estimation of the non-equilibrium surface layer thickness leads to a better agreement with experimental data, however this expression restricts the validity of the non-equilibrium surface layer model as alternative to non-diffusional adsorption kinetics. 

Electrocatalysis in DAFC anodes is complex because the reaction mechanism involves adsorption of alcohol and several elementary reaction steps including the CO oxidation. Figure 8.2 shows a possible network of reaction pathways by which the electrochemical oxidation of methanol occurs. Since more than 50 years detailed catalysis studies have attempted to analyze possible reaction pathways to find the main pathway of methanol oxidation [11, 12] (see next Section). Most studies conclude that the reaction can proceed according to multiple mechanisms and that the most significant reactions are the adsorption of the alcohol and the oxidation of CO. 

In the case of NPLC, the stationary phase is polar (silica, alumina, or polar-bonded phase) and the mobile phase is nonpolar (hexane, heptane, etc.). Adsorption mechanism operates and involves adsorption of an analyte by its polar groups on the polar active sites of the stationary phase. Adsorption occurs by polar interactions and formation of hydrogen bonds. The nonpolar mobile phase cannot interact with the adsorbent-active sites and thus its adsorption practically does not occur. However, the mobile phase favors the analyte desorption interacting by dispersion forces with its nonpolar groups that are not involved in the adsorption. This is illustrated in Figure 2 for analysis of an alcohol on silica. This case will be discussed again below. Thus, analyte molecules adsorb and desorb constantly, being... 

Figure 2 Detailed adsorption mechanism of retention of an alcohol ROH in NPLC. A, adsorption by hydrogen bonds D, desorption by dispersion forces.

The difference in adsorption mechanism of multiprotic acids from aqueous solutions on the one hand and from alcoholic solutions on the other is that in aqueous solutimi, pre-existing ions are adsorbed, and the conductance is depressed. In alcoholic solution, adsorption of neutral molecules produces ions in solution, e.g.. 

The Mechanism of Dehydration of Alcohols over Alumina Catalysts Herman Pines and Joost Manassen Complex Adsorption in Hydrogen Exchange on Group VIII Transition Metal Catalysts... 

Obtaining of oxide film throu sedimentation or by electrophoretic depositing from water, alcohol, or benzene solution also makes it possible to produce a fairly thin film with sufficient sensitivity to adsorption of above particles. However, poor mechanical features of the films obtained are their major drawbacks. 

It is also observed in Fig. 5.3 that Pd(II) ions are partly adsorbed on AI2O3 before ultrasonic irradiation the concentration of Pd(II) just before irradiation becomes ca. 0.8 mM, although 1 mM Pd(II) was added in the sample solution. From a preliminary adsorption experiment, the rate of Pd(II) adsorption on A1203 was found to be slow compared with those of Pd(II) reduction in the presence of alcohols. Therefore, it is suggested that the sonochemical reduction of Pd(II) in the presence of alcohols mainly proceeds in the bulk solution. The mechanism of the Pd/Al203 formation is also described in the section of sonochemical synthesis of supported metal nanoparticles. 

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