Mechanisms Involving Adsorption

The CV response involving only adsorbed species is characterized by symmetric cathodic and anodic waveshapes. The relative positions of the two 

Note that rather than the aquare root dependence found for the diffusion-controlled response, here we have the peak current proportional to the scan rate. A log-log plot of peak current versus scan rate will give a slope = 0.5 for diffusion and 1 for adsorption. This is the primary quantitative method by which simple adsorption peaks are distinguished from diffusion peaks. To avoid misleading results, however, care must be taken to subtract capacitive and faradaic background currents. 

Adsorption peaks can occur along with diffusion peaks. The extent of overlap is determined by the relative strength of adsorption of the oxidized and reduced species. For strong interactions, adsorption pre- or postwaves can be clearly 

Quantitative treatments of adsorption require a knowledge of the adsorption Isotherm, which may be potential dependent  

It is reasonable to suggest that hemin is involved in a reductive decomposition of artemisinin, in analogy with known action of hemin on inorganic peroxide. To test this idea, Zhang et al. examined the cyclic voltammetric responses of hemin and artemisinin separately and in the same solution. The results supported the earlier models for the action of artemisinin and showed that electron transfer is a viable mechanism for the interaction of hemin and artemisinin. 

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This reaction is actually slightly endothermic (A// = 88 kJ/mol), but the large net increase in entropy and the nonequilibrium nature of most CVD processes lead to significant tungsten deposition. As with the Ge example, the deposition mechanism involves adsorption steps and surface reactions. At low pressures and under conditions of excess hydrogen gas, the deposition rate follows the general form ... 

In the interpretation of the kinetics, it was concluded that a mechanism involving adsorption equilibrium between methylcyclohexane in the gas phase and methylcyclohexane molecules adsorbed on platinum sites was not very likely. If Eq. (1) were interpreted on such a basis, then b would be an adsorption equilibrium constant. From the temperature dependence of b, one would calculate a heat of adsorption of 30 kcal./mole, which seems too high for adsorption of methylcyclohexane molecules as such. Furthermore, the small inhibition by aromatics casts doubt on such a picture, since the extent of adsorption of aromatics would be expected to be considerably greater than that of methylcyclohexane molecules at equilibrium, in view of the unsaturated nature of aromatics. 

The mechanism of steam reforming of higher hydrocarbons discussed in Section 2.2.2.3 is also valid for the steam reforming of liquid hydrocarbons as well. As noted, the mechanism involves adsorption of the hydrocarbon on the catalyst surface followed by the formation of Ci species by the successive a-cleavage of the C-C bonds. The steps involved in the steam reforming of higher hydrocarbons are also given in Equations 2.29-2.37.120... 

Most workers favour a 2-site mechanism, involving adsorption and disruption of the hydrocarbon molecule on the metal to produce single-carbon species such... 

Furthermore, this separation problem, which is theoretically simple, is also highly relevant to the pharmaceutical industry. An example is the separation of mixtures of N-benzoyl-D- and L-alanine on immobilized BSA (see Figures 11.20). We have explained in Chapters 3 and 4 (i) that a competitive bi-Langmuir isotherm can be employed to account for the competitive behavior of these components (Figure 4.25c) and (ii) that, because the chiral selective retention mechanism involves adsorption of the enantiomers in the hydrophobic cavity of BSA, the column saturation capacity of the chiral selective mechanism is the same for the two enantiomers. This competitive bi-Langmuir isotherm model is simply derived from the parameters obtained from single-component isotherm measurements. 

A general model of a multistep mechanism involving adsorption and diffusion was recently given by Harrington. 

Ammonia Synthesis. - Ammonia (NH3) synthesis is another well-known structure-sensitive reaction. The two most widely accepted mechanisms involve adsorption of nitrogen as the rate limiting step. A main question has been whether nitrogen adsorption is dissociative [37,38] or associative [38]. For NH3... 

Hetero-coagulation This mechanism involves adsorption of oppositely charged particles, e.g., complexes of resin acids and aluminum sulfate, on the surfaces of fibers and filler particles. Hetero-coagulation is sensitive to soluble anionic wood polymers and electrolytes, with which cationic sizing particles, preferentially interact. 

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 presence of oxygen the SCR activity mechanism involved adsorption of NO at the metal sites followed by oxidation of adsorbed NO to NO2. The adsorbed NO2 was then reduced to nitrogen gas and water by ammonia and the products were desorbed. 

Some mechanisms involving adsorption steps (see Section 3.L5.5) do not obey Eq. (3.146). The reader can easily test other mechanisms this way. 

The reaction is done with a metal catalyst. Catalysts based upon iron or cobalt have been used commercially for hydrocarbon synthesis [5], The mechanism involves adsorption of hydrogen and carbon monoxide on the metal surface [6]. The Fischer-Tropsch process enables natural gas to be converted to liquid synthetic fuel. First, the natural gas is oxidized to syn gas which is then converted by the Fischer-Tropsch process to the liquid hydrocarbon mixture, which is useful as fuel. 

Step 1 of the dual mechanism involves adsorption of a paraffin molecule to a metal site, followed by reversible dehydrogenation to form an olefin. In Step 2, the olefin migrates to an acid site, where it reacts with a proton to form a carbenium ion. The carbenium ion can rearrange into a more-stable carbenium ion (Step 3), which explains why products from hydrocrackers are relatively rich in iso-paraffins. In Step 4, P-scission of the carbenium ion produces an olefin and a smaller carbenium ion. The olefin can undergo further cracking on an acid site, or it can react with hydrogen at a metal site... 

An emulsifier is a single chemical or mixture of components having a capacity for promoting emulsion formation and a short-term stahUity hy interfadal action. A stahiUzer is a single chemical component, or a mixture of components, that can offer long-term stability of an emulsion, possibly by a mechanism involving adsorption, but not necessarily so. 

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