Kinetics of desorption

Even if the peak behavior fits well for a given apparent desorption order, the real kinetic situation may be a different one. As a rate controlling step in a second-order desorption, random recombination of two particles is assumed most frequently. However, should the desorption proceed via a nonrandom recombination of neighboring particle pairs into an ordered structure, the resulting apparent first-order desorption kinetics is claimed to be possible (36). The term pseudo-first-order kinetics is used in this instance. Vice versa, second-order kinetics of desorption can appear for a nondissociative adsorption, if the existence of a dimer complex is necessary before the actual desorption step can take place (99). A possibility of switching between the apparent second-order and first-order kinetics by changing the surface coverage has also been claimed (60, 99, 100). 

The first-order and second-order kinetics of desorption are by far the most common and practically considered cases. Less than first-order desorption kinetics indicates multilayer adsorption or transport limited desorption (101). An actual significance of the third-order kinetics in desorption has been found recently by Goymour and King (102, 103). 

The kinetics of desorption control bioaccumulation of historical (e.g., aged) contamination (e.g., PAHs in benthic animals [225]) and historically contaminated soils are less toxic and/or lead to lower body burdens than equivalent amounts of spiked soils [226,227]. 

One can notice that in (1.13) the Fe " in the (FeH ) complex is reduced to metallic Fe, the process that requires a major rearrangement of atoms. Therefore it is not surprising that the kinetics of desorption was poor while being controlled by diffusion of the metal atoms. 

This behavior can be explained if we resort to the imaginary desorption PCT cnrve at 275°C in Fig. 1.33. The eqnilibrinm plateau pressure at 275°C is higher than 0.1 MPa at which desorption is carried ont and at this temperature MgH can desorb at atmospheric pressnre. However, the kinetics of desorption will depend on the driving force (as shown in Fig. 1.26). Since a larger mass of hydride will produce... 

While the number of parameters may be large, only a few of the parameters are usually significant. The significant parameters are easily determined by calculation of the sensitivity of the calculated rate at typical conditions to a small variation in the value of each parameter. For the NH3 synthesis, for instance, the rate of dissociation of N2 and the binding energy for N are the most significant parameters and the kinetics of desorption of N2 in a TPD experiment is rather closely related to the kinetics of NH3-synthesis. 

If the slightly soluble, surface-active compound is spread as a mono-layer, the film is unstable and desorbs from the surface. Analysis of the kinetics of desorption follows the treatment given by Ter Minassian-Saraga (29). Immediately upon spreading the monolayer, film molecules... 

An FI mass spectrum typically contains peaks due to ions formed in chemi- and physisorbed layers on the emitter. The expressions given in Sect. 2 would be unlikely to be applicable to these surface processes. The numbers of these ions detected will depend upon, among many other factors, the kinetics of desorption from the surface. 

The kinetics of desorption of water from a maize starch is given by the equation398 ... 

Elimination of the adsorbed molecules (O, Fig. 20) begins when a, becomes equal to a. During the interval required for a, to decrease from a( to a g, the kinetics of desorption is first-order with respect to a, i.e. 

Fig. 21. A typical example of many time-studies of desorption from liquid saturated poly(Sty-co-DVB) particles enmeshed in PTFE microfibers. The Inset records the number, at> of residual sorbed molecules [in this case CHC13 in (Sty)98(DVB)2] per phenyl group of polymer over the first 10 min of desorption at 23 °C. The main plot records the logarithm of at over the first 70 min of the time-study. The breakpoints in the kinetics of desorption at aj, otg and ag mark sequentially the compositions at incipient elimination of molecules immobilized by adsorption, incipient transition from the rubbery state of the system to the glassy state, and finally completion of this transition as discussed in the text...

These results (Eqs. 25-27) show that the residual composition at, at each breakpoint in the kinetics of desorption, which marks some form of physical change in the system, is proportional to as (Eq. 20), i.e. the number of adsorbed molecules per accessible phenyl group of (Sty)j X(DVB)X at liquid-saturation, or in true solution when x is zero. Moreover the residual compositions (ag , ag 2 etc.) in the glassy state, which mark the sequential depletions of populations present when a, became ag, are also related linearly [149] to as as expressed by ... 

Fig. 13. Kinetics of desorption (a) and adsorption (b) of 2-butyne on silicalite-I recorded by the pressure response following the rapid square-wave volume expansion (a) and compression (b). Experimental conditions 1.5 Torr of 2-butyne at 323 K o, without adsorbent , ZSM-5 present in the system (14).

A systematic study of the kinetics of desorption of lauric acid away from interfaces has recently been completed (Saraga, 31,32). If S represents the total area of the air-water interface, from which desorption is... 

Coalescence being the secondary process, the number of distinct droplets decreases leading to a stage of irreversibility and finally complete demulsification takes place. Coalescence rate very likely depends primarily on the film-film repulsion, film drainage and on the degree of kinetics of desorption. Kinetically, coalescence is a unimolecular process and the probability of merging of two droplets in an aggregate is assumed not to affect the stability at other point of contact (32). 

The ethylene temperature-programmed reaction spectrum of an oxygen-covered Cs/Ag/a-Al203 catalyst produced a peak for the co-evolution of EO and CO2 at 373 K (100 °C) with a selectivity to EO of 44%. The Cs had had no effect on the kinetics of desorption of oxygen from Ag(lll) nor on the amount of oxygen desorbing from that surface, nor on its selectivity in its oxidising ethylene to EO (Eig. 7.10). 

At the simplistic level the kinetics of desorption can be described by the following standard representation... 

The area of a TPD peak provides a good measure of the surface coverage. In cases where there are multiple species desorbed, the ratios of the TPD peaks provide the stoichiometry. (3) The shapes of the peaks, and how they change with surface coverage, provide detailed information on the kinetics of desorption. For example, the shapes of TPD curves differ for zeroth-, first- or second-order processes. 

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