Desorption of Molecules

The opposite of adsorption, desorption, represents the end of the catalytic cycle. It is also the basis of temperature-programmed desorption (TPD), an important method of studying the heats of adsorption and reactions on a surface, so that the activation 

The molecule reaches the transition state and from there it desorbs into the gas phase. To evaluate the rate constant we use the same procedure as above Write down the partition functions for the participating species, equalize the chemical potentials, and find an expression for the number of molecules in the transition state. Since it is much more practical to do this in terms of coverages we immediately obtain  

This is not strictly correct, since the vibrational terms are often extracted out of hv and included in the activation energy, which then deviates slightly from AE or as given by Eq. (188). 

Note that if the ratio of partition functions in Eq. (189) leaves a nonvanishing temperature dependence, it generates the additional terms ksTin a and factors e in hv. Consequently, the prefactor hv strongly depends on the nature of the transition state. 

Often the adsorbed species are bound rather strongly and can be considered immobile at the bottom of a vibrational well. The transition state may, however, have several possibilities, being, for example, a precursor that is highly mobile in two 

If k l j2 is the probability that a collision between adatoms will result in molecular desorption, we have 

We suppose that the temperature dependence of Kx 2 can be represented by an Arrhenius-type expression, i.e. 

In the manner of Section 2.1.3, we now again consider limiting cases. (i) Case RT Em. From eqns. (10), and (16), we obtain 

Highly exothermic adsorption of molecular hydrogen, oxygen and nitrogen into the atomic state on clean metals is virtually unactivated [Fig. 2(a) and (d)],so we can write for such systems E12 = — AU21. When— AU2, is less than Em, molecular adsorption is activated and, in such cases, we suppose that Ed is zero and then E 2 = Em [see Fig. 2(b) and (c)]. 

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By applying the machinery of statistical thermodynamics we have derived expressions for the adsorption, reaction, and desorption of molecules on and from a surface. The rate constants can in each case be described as a ratio between partition functions of the transition state and the reactants. Below, we summarize the most important results for elementary surface reactions. In principle, all the important constants involved (prefactors and activation energies) can be calculated from the partitions functions. These are, however, not easily obtainable and, where possible, experimentally determined values are used. 

Upon selective absorption of analyte molecules from the ambient environment, the zeolite thin film increases its refractive index. Correspondingly, release of adsorbed molecules from the zeolite pore results in the decrease of its refractive index. The absorption/desorption of molecules depends on the molecule concentration in the environment to be monitored. Therefore, monitoring of the refractive index change induced phase shift in the interference spectrum can detect the presence and amount of the target analyte existing in the environment. 

Dreisewerd, K. Schiirenberg, M. Karas, M. Hillenkamp, F. Influence of the Laser Intensity and Spot Size on the Desorption of Molecules and Ions in MALDI With a Uniform Beam Profile. Int. J. Mass Spectrom. Ion Proc. 1995, 747, 127-148. 

This classification based on water splitting is important to understanding the redox potential of a given semiconductor. Although this classification is simple, it is convenient in selecting a semiconductor that is appropriate for a desired reaction. For a more detailed reactor design, factors such as the lifetimes of carriers energy levels of surface states adsorption and desorption of molecules on the surface kinetic nature of the surface and electron kinetics must be considered (Serpone and Pelizzetti, 1989). 

Sundqvist, B., Ed. "Ion Induced Desorption of Molecules From Bioorganic Solids" Nucl. Instrum. Methods 1982, 198,... 

Finally, one may use charging or polarization of surfaces, induced by external electric fields, to control the adsorption and desorption of molecules and the state of these adsorbed molecules, in order to control their chemical reactivity. This is an upcoming field that has not yet been explored to its fullest potential. It involves aspects of nanotechnology and nanoscience, like the fabrication of structures of several nanometers and stimuli generated by scanning tunneling microscopic probes. The outcome of the research in this field is generally of a fundamental nature. The topic of electronic control of reactions at surfaces will be discussed in the last section of this chapter. 

As NO dissociation produces two atoms from one molecule, the reaction can only proceed when the surface contains empty sites adjacent to the adsorbed NO molecule. In addition, the reactivity of the molecule is affected by lateral interactions with neighboring species on the surface. Figure 4.10 clearly illustrates all of these phenomena [38]. The experiment starts at low temperature (175 K) with a certain amount (expressed in fraction of a monolayer, ML) of NO on the Rh(100) surface. During temperature programming, the SIMS intensities of characteristic ions of adsorbed species are followed, along with the desorption of molecules into the gas phase, as in temperature-programmed desorption (TPD) or temperature-programmed reaction spectroscopy (TPRS) (see Chapter 2). 

When we come to consider experimental results, we will find (in Sect. 3.3) that the interaction of oxygen with tungsten or rhenium at high temperatures produces atoms or oxides, but not molecules. The reason for this has been given by Ehrlich [4]. The effect is associated with the case — AU21>D(X2), when the desorption of molecules requires more activation than the desorption of adatoms [see Fig. 2(d)]. The relative rate of desorption of molecules compared with atoms is obtained by combining eqns. (1) and (14), viz. 

The principle of microscopic reversibility enables us to say that, at equilibrium, the rates of adsorption and desorption of molecules are equal, and independently that the rates of adsorption and desorption of atoms are also equal. In the first case, we will not attempt to apply the discussion of Sect. 2.1, but will simply make use of the molecular and atomic sticking coefficients (s2 and s, respectively). This procedure is useful when s2 and Sj are constants, as in the case when the coverage is low and molecular and atomic adsorption are unactivated. The resulting kinetic description contains only the sticking coefficients as adjustable parameters. 

Hi) The transition region. As the movement of adatoms is frozen by lowering temperature and the coverage driven up by the flux of gaseous atoms to the surface, 7 falls. If r were zero, then the only recombination which could occur would be via the residual desorption of molecules and 6 would be very close to unity. Since in our model the desorption of molecules depends on 62, recombination with kz = 0 would be insensitive to Zx and the order of recombination with respect to atoms... 

In the case of very strong chemisorption [see Fig. 2(d)], the desorption of molecules is even more activated than the desorption of adatoms. At temperatures not high enough for appreciable desorption of adatoms, the very large value of — AU2 t ensures values of 6 close to unity. The virtual absence of molecular desorption rules out the Langmuir— Hinshelwood process, so that recombination is restricted to the Rideal— Eley mechanism, which is now activated, causing recombination to be... 

Second-order rate coefficient for the desorption of molecules by recombination of adatoms [see eqn. (14)]. constant of proportionality defined by eqn. (89) to give the rate of change of pressure in an atomisation system, rate coefficient for the production of atoms per unit are of surface at temperature T and gas pressure P2. rate coefficient for wall trapping of thermally excited molecules per unit area of filament, masses of atom (X) and molecule (X2), respectively, rate coefficient for the Rideal recombination mechanism [see equation (60)]. 

The zeolite crystal is modeled here as a finite, two-dimensional rectangular grid of intersecting channels. The adsorption and the desorption of molecules take place at border sites only according to the characteristics of zeolites, and the diffusion of the sorbed molecules in the channels is modeled as a random walk process. The reaction occurB in sorbed phase. The simulation technique was described elsewhere [2, 3], and the simulation results are calculated as the follows ... 

Immediately after immersion in a solution of MPA, the monolayer packing density on the metal surface is low and the trans conformation predominates. Over time, as more molecules adsorb to the metal surface, the gauche conformation becomes more common. With silver, the metal surface has a tendency to oxidize in aqueous environments which results in desorption of molecules from the monolayer over time thus the molecular orientation tends toward the trans configuration (Fig. 4.6). 

For unimolecular desorption of molecules that are mobile on the surface, the preexponential factor is approximately equal to = kT/h, and its magnitude is of the order of 10 s. If the adsorbed molecules are immobilized on the surface prior to desorption, the preexponential factor may be in the range of 10 to 10 s (23). If we consider mobile adsorption, for example, the rate of desorption per molecule adsorbed is... 

Adsorption and desorption of molecules in the gas (or liquid) phase (steps 7 and 7 ) lead to adsorption/desorption equilibria. 

Comparison of Calculated Values of si [Eq. (49)] and the Experimentally Determined Rate Constant k for the Desorption of Molecules from Tungsten... 

Dreisewerd, K.L. et al., Influence of the laser intensity and spot size on the desorption of molecules and ions in matrix-assisted laser desorption/ionization with a uniform beam profile, Ini. J. Mass Spectrom., 141, 127, 1995. 

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