Periodic adsorption-desorption processes

Due to the great interest in harmonically disturbed surfaces, such as capillary or longitudinal waves or oscillating bubbles and drops, models of periodic adsorption-desorption processes are... 

Abstract. The degradation mechanism of field emission cathodes based on carbon nanostructural materials was investigated in the presented work. Emission current instability came from the adsorption desorption processes on the cathode surface. During the long-time tests the periodical variation of the electron work function (caused by switching on and off of the cathode) was detected. The numerical model was proposed to explain the experimentally observed variations of electron work function. 

This brings up the question of how this scheme has to be modified when equilibrium is not attained. The answer is that the identity of thermod3mamical and mechanical measurement persists, but that the value obtained for y differs from y (eq.) in fact, often y (non-eq.) > y (eq.). Suppose a given interface is created very rapidly and then starts to relax with a time scale r. At any time t equilibrium state, characterized by the pertaining y (non-eq.). Even then, this non-equilibrium surface tension can be measured, provided the time-scale of our measurement is short in comparison with t. When different methods of measurement, either thermodynamical or mechanical, do not yield the same y this may either mean that there have been errors in the measurement or that they apply to different moments (or time intervals) of the relaxation period. The downward tendency of y(f) reflects the general trend of F(V,T,n) and G(p,T, n) to become minimal at equilibrium (sec. 1.2.12). When only relaxation of the interface takes place, y must decrease. However, when the bulk phases also relax slowly or when the relaxation is determined by adsorption-desorption processes, y may also increase. For instance, this would be observed if... 

Fig. 4. A continous sequence of 50 nm x 50 nm images revealing the effects of stepping the potential between -0.25 and -0.65 V vs. Cu/Cu in 10 mmol/1 HCl with tip = 0.050 V, /t = 9 nA and frame time of 22 s The black horizontal line in images (d), (i), (p) and (t) correspond to the instant vvhen the potential was stepped between the two values. A (V2 x /2)R45 chlorine adlattice is stable at -0.25 V whereas the adlattice is reductively disrupted at -0.65 V. For images (a) - (d) E =-0.25 V, (d) -(i) E—0.65 V, (i) - (p) =-0.25 V, (p) - (t) E—0.65 V and (t) - (x) E=-0.25 V. The rapid movement and development of step curvature in images (d) - (i) and (p) - (t) corresponds to the reductive disruption and desorption of the chloride adlattice. The images, which take 18 s to collect were obtained over a period of 8 min this reveals the remarkable effects of surface rearrangement associated with the adsorption/desorption process.

The kinetics of the adsorption/desorption process are complex and consist of an initial period where equilibrium is achieved essentially in a few seconds. 

Useful information about the adsorption kinetics, mobility of the adsorbed polynucleotide segments, and mechanism of electrode processes can be obtained by measurement of the frequency dependence of the impedance of the electrode double layer (EIS) [31, 88, 207-209]. If the adsorption/desorption process is slow with respect to the period of the a.c. potential used for the impedance measurement, the measured capacitance values decrease with increasing frequency (dispersion of the capacity). The frequency effect is most remarkable around the potentials of adsorption/desorption peaks. With more flexible ss polynucleotides, the frequency effect is larger than with the more rigid ds ones [210]. 

Recent concepts to achieve NOx purification from lean exhaust gas are emerging. In the NSR system, the material favors the reactive adsorption of NO to N02 which is stored as a nitrate complex in 02 rich atmosphere. The exhaust gas is then switched to a stoichiometric or HC rich environment in which the nitrate is thermodynamically unstable. The stored NOx is then released and catalytically reacts with excess HC species in the exhaust gas to form N2. In the SNR system, the NOx are temporarily stored on an adsorbent and periodically recirculated to the combustion chamber to be decomposed in the combustion process. In these processes the key point is the adsorption/desorption of N02 which has been... 

The system also proved to be useful for the analysis of CO adsorbed on noble metal particles in different zeolites [23], Series of samples could be analyzed in relatively short periods of time, and it could be estimated, that the parallel setup could be used for samples with a platinum content as low as 0.1 % at an acceptable signal-to-noise ratio. One can expect that with the rapid scan capabilities implemented by Lauterbach and coworkers even kinetics of fast adsorption and desorption processes could be monitored in a parallel fashion. 

Instead of UHV conditions, a wet process was applied by Kunitake and coworkers in order to realize an equilibrium polymerization on the surface [136]. In this case, iodine-modified Au(111) substrates (I/Au(lll)) were dipped into aqueous solutions of tetraamine 64 and dialdehyde monomers 65-67 (Figure 28.28b). A dynamic adsorption-desorption equilibrium and a high lateral mobility of the adsorbed monomers on I/Au(lll) allowed for their surface polycondensation based on Schiff-base formation. The polycondensation was found to be sensitive to the solution conditions (such as pH), as well as to the choice of substrates. When the products were observed directly using STM, fragments with a periodic order were seen to have formed on the surface, although the lateral sizes still required some improvement (Figure 28.29). 

In our modeling, we used a square N x N lattice (N = 400-1600) with periodical boundary conditions. The states of square cells were set according to the rales determined by the detailed mechanism of the reaction (e.g., in the case of Pd(l 1 0) each lattice cell can exist in one of five states , COads, Oads, [ Osub], [COads Osub])- The time was measured in terms of the so-called Monte Carlo steps (MC step) consisting of x trials to choose and realize the main elementary processes. For an MC step, each cell was called once in the average. The probability of each step for the processes of adsorption, desorption, and reaction was determined by the ratio of the rate constant of a given step to the sum of the rate constants of all steps. 

Most previous theoretical studies of dynamic operating reactors used kinetic expressions obtained under steady-state operation. These models do not account for the detailed dynamics of the adsorption and desorption rate processes, and they may lead to erroneous predictions in periodic operation of the reactor. Thus, simulations of periodic processes may require use of kinetic models that are much more detailed than those used for predicting steady-state operation. These dynamic models also need to account for the rate of adsorption, desorption, and adsorption capacity of the catalyst. As mentioned above, the hot-temperature zone in a cooled RFR may exhibit complex dynamic features under... 

Industrial adsorption processes normally are cyclic processes in which adsorption and desorption steps of the sorbent material alterate periodically. Often the desorption or regeneration step is cmcial and essentially determine the period and the energetic efficiency of the cycle [1.2, 1.14-1.16]. An important quantity to characterize the desorption process is the (molar) enthalpy (AH ) needed to desorb the leading component either of product or waste - of a gas mixture from the sorbent. In Table 1.2 some examples of desorption processes and their industrial applications together with typical values of the molar desorption enthalpy are given. Summarizing it can be stated that in reversible physidesorption processes molar enthalpies of about (10-50) kJ/mol are needed whereas in irreversible chemisorption processes (70-200) kJ/mol are necessary for desorption. ... 

Dielectric measurements also can be performed in capacitors placed inside adsorption columns. Here they provide a means to detect adsorption processes on site , i. e. to observe changes in the electric capacitance of the capacitor due to gas adsorption or desorption processes on the sorbent material in the capacitor. In this way not only breakthrough curves but also periodic loading and unloading curves of sorptive gases in laboratory or industrial adsorption reactors can be observed. 

Figure 12. Combined pressure (p) and dielectric (er) measurements of a periodic ad- and desorption process of nitrogen (N2) on molecular sieve MSNalBX (UOP) at 293 K taken inside an industrial sized adsorption column (PSA).

It is obvious from works devoted to the investigation of adsorption-desorption dynamics of chains on attractive surfaces that chains attached to the surface, even by strong specific bonding, are able to detach completely due to thei-mally activated processes [62-65]. Thus, the chain once attached to the surface can leave the surface aftei- a period correlated to the time characteristic of cooperative multiple adsorption-desorption steps. For instance, Grauick and co-workers [62,64] have shown that the time needed for complete desorption of PS chains from the oxygen-plasma treated silicon surface in cyclohexane solution may reach the value 1(1 s and higher (in dependence on molecular-weight). 

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