Product controlling desorption

The initial reactant product conversion rate should increase at higher temperature because kinetic rate constants for elementary steps, particularly the desorption of gas D, increase at higher temperature. In summary, there is no total pressure dependence of the initial reactant product conversion rate when (1) A -h B C -h D, (2) single-site adsorption is appropriate for each component, and (3) desorption of one of the products controls the Hougen-Watson kinetic rate law. 

If the surface reaction is reversible and desorption of product controls, the overall rate still depends on the fraction of surface covered by the product, but this may be much less than unity. If the surface reaction is at equilibrium and reactants A and B are also at adsorption-desorption equilibrium but product C is not, the value of 6c is obtained from modified equations for 0 and 6 ... 

Clearly, the dynamics of radical transfer (radical absorption or entry and desorption or exit) has a great influence on the kinetics of emulsion polymerization. Thus, a better understanding of these processes is crucial for both process and product control. 

Case III Desorption of product controlling This is the step where Sb decomposes to give gaseous product B and regenerates Sy. Both the adsorption of A and the surface reaction converting Sa to Sb are fast ... 

For the desorption of product-controlling case. Case III, k3SoK) CA- )... 

Pulse radiolysis results (74) have led other workers to conclude that adsorbed OH radicals (surface trapped holes) are the principal oxidants, whereas free hydroxyl radicals probably play a minor role, if any. Because the OH radical reacts with HO2 at a diffusion controlled rate, the reverse reaction, that is desorption of OH to the solution, seems highly unlikely. The surface trapped hole, as defined by equation 18, accounts for most of the observations which had previously led to the suggestion of OH radical oxidation. The formation of H2O2 and the observations of hydroxylated intermediate products could all occur via... 

Heterogeneous catalytic studies should also be concerned with the significance of adsorption and desorption rates and equilibria of the reactants, intermediates and products. Yang and Hougen (1950) tabulated the expressions for surface catalyzed reactions controlled by various steps. 

The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

The reaction of Si02 with SiC [1229] approximately obeyed the zero-order rate equation with E = 548—405 kJ mole 1 between 1543 and 1703 K. The proposed mechanism involved volatilized SiO and CO and the rate-limiting step was identified as product desorption from the SiC surface. The interaction of U02 + SiC above 1650 K [1230] obeyed the contracting area rate equation [eqn. (7), n = 2] with E = 525 and 350 kJ mole 1 for the evolution of CO and SiO, respectively. Kinetic control is identified as gas phase diffusion from the reaction site but E values were largely determined by equilibrium thermodynamics rather than by diffusion coefficients. 

Since the recombination step (c) does not principally differ from a recombination of two H or D atoms to the respective hcmonuclear imole-cule there is no reason to assume a special activation barrier for a H and a D atom to recombine to the HD molecule. Therefore the rate of the HD production is solely determined by the rates of adsorption of H and D, respectively (as long as the reaction is adsorption-controlled, i.e., at hi enou tenperatures), or by the rate of desorption of HD (provided the reaction is desorpticai-oontrolled, i.e., at low temperatures). If wie deal with the first case only we may w/rite ... 

The observation of negative apparent activation energy can most simply be interpreted in terms of the competition between the adsorption and desorption of methylacetylene on the surface. This qualitative explanation is illustrated in Figure 3, where the steady-state production of trimethylbenzene is compared with the TPD trace of methylacetylene. The fall off in steady state cyclotrimerization rate matches the tail of the desorption spectrum and illustrates the role of reactant desorption at higher temperatiu-es controlling the availability of alkyne monomers and thus the overall cyclotrimerization rate in this temperatime/pressure regime. 

In general, a preparation of mixed monolayer can be realized by either a kinetic control or a thermodynamic control (Figure 1, left). Kinetic control is based on a suggestion that for an initial deposition step the desorption rate is ignorable in comparison with the adsorption rate. In this case, the concentration ratio of the adsorbed species A and B on the surface corresponds to the ratio of products of their adsorption rate constant ( a or b) and concentration (Ca or Cb) A aCa/A bC b. The validity of the initial assumption on low desorption rate means that the total surface coverage obtained under kinetic control is essentially lower than 100%. This non-complete coverage does not disturb most of optical applications of the... 

Which reaction pathway dominates is dependent on the O s) concentration and the relative rates of carbonate formation and desorption of the gaseous C4, C6 and C7 gaseous products some control of these is possible38 by varying the propene-to-oxygen ratio and also the oxidant, such as substituting N20 for 02. 

Kinetic Term The designation kinetic term is something of a misnomer in that it contains both rate constants and adsorption equilibrium constants. For thfe cases where surface reaction controls the overall conversion rate it is the product of the surface reaction rate constant for the forward reaction and the adsorption equilibrium constants for the reactant surface species participating in the reaction. When adsorption or desorption of a reactant or product species is the rate limiting step, it will involve other factors. 

Now consider the case where the controlling step is the rate of desorption of a product species R for a reversible surface reaction of the form... 

Toluene alkylation with isopropyl alcohol was chosen as the test reaction as we can follow in a detail the effect of zeolite structural parameters on the toluene conversion, selectivity to cymenes, selectivity to para-cymene, and isopropyl/n-propyl ratio. It should be stressed that toluene/isopropyl alcohol molar ratio used in the feed was 9.6, which indicates the theoretical toluene conversion around 10.4 %. As you can see from Fig. 2 conversion of toluene over SSZ-33 after 15 min of T-O-S is 21 %, which is almost two times higher than the theoretical toluene conversion for alkylation reaction. The value of toluene conversion over SSZ-33 is influenced by a high rate of toluene disproportionation. About 50 % of toluene converted is transformed into benzene and xylenes. Toluene conversion over zeolites Beta and SSZ-35 is around 12 %, which is due to a much smaller contribution of toluene disproportionation to the overall toluene conversion. A slight increase in toluene conversion over ZSM-5 zeolite is connected with the fact that desorption and transport of products in toluene alkylation with isopropyl alcohol is the rate controlling step of this reaction [9]... 

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