Transient adsorption process

The Manifestation of Electrostatic Retardation in Transient Adsorption processes... 

When discussing the cyclic voltammograms, the question arises if the peaks in the voltammograms with alkali and alkaline earth oxides are due to transient adsorption processes or to continuous electrochemical processes. A second question is What are the products if continuous processes prevail Answers might come from recording the stationary polarisation curves and using continuous gas analyses by GC [3]. 

The foregoing is an equilibrium analysis, yet some transient effects are probably important to film resilience. Rayleigh [182] noted that surface freshly formed by some insult to the film would have a greater than equilibrium surface tension (note Fig. 11-15). A recent analysis [222] of the effect of surface elasticity on foam stability relates the nonequilibrium surfactant surface coverage to the foam retention time or time for a bubble to pass through a wet foam. The adsorption process is important in a new means of obtaining a foam by supplying vapor phase surfactants [223]. 

Structural changes on surfaces can often be treated as first-order phase transitions rather than as adsorption process. Nucleation and growth of the new phase are reflected in current transients as well as dynamic STM studies. Nucleation-and-growth leads to so-called rising transients whereas mere adsorption usually results in a monotonously falling transient. In Fig. 10 are shown the current responses to potential steps across all four current peaks in the cyclic voltammogram of Fig. 8a [44], With the exception of peak A, all structural transitions yield rising current transients sug-... 

From the slope of the straight line, the effective mixing cell volume was calculated to be 30.1 cm, with a 50% relaxation time of about 0.08 s. Similar mixing characteristics were observed following a step decrease (i.e., CO + N ), giving an effective mixing cell volume of 31.8 cm and a 50% relaxation time of 0.09 s. Since these response times of the reactor are not much faster than the time scale of the adsorption process (a halfscale relaxation time of about 0.2 s), the transients of the reactor cell were included in our analysis. For our simulations, the mixing cell volume was taken to be 31 cm. ... 

Chronopotentiometry, galvanostatic transients, 1411 as analytical technique, 1411 activation overpotential, 1411 Clavilier, and single crystals, 1095 Cluster formation energy of, 1304 and Frumkin isotherm, 1197 Cobalt-nickel plating, 1375 Cold combustion, definition, 1041 Cole-Cole plot, impedance, 1129, 1135 Colloidal particles, 880, 882 and differential capacity, 880 Complex impedance, 1135 Computer simulation, 1160 of adsorption processes, 965 and overall reaction, 1259 and rate determining step, 1260... 

If the processes just described are assumed to characterize the transfer of mass and energy in a fixed-bed adsorber, the conservation principles may be applied to them to describe the temperature and concentration as a function of time and position. Presenting the equations for a fixed-bed geometry has the advantage of including also equations, as special cases, for transient adsorption in single particles or groups of particles in batch systems. 

By the use of various transient methods, electrochemistry has found extensive new applications for the study of chemical reactions and adsorption phenomena. Thus a combination of thermodynamic and kinetic measurements can be utilized to characterize the chemistry of heterogeneous electron-transfer reactions. Furthermore, heterogeneous adsorption processes (liquid-solid) have been the subject of intense investigations. The mechanisms of metal ion com-plexation reactions also have been ascertained through the use of various electrochemical impulse techniques. 

The model fits adequately the experimental data allowing the extraction of the adsorption parameters and the diffusion coefficients for the transport inside the micropores. Table 3 reports these parameters for both samples. Similar diffusion parameters were found for both samples. Despite the difference in volume of the micropores as calculated by the HK method (Table 1), a big difference between Sbet and Sdr for each sample is found indicating problems of accessibility for nitrogen molecules at the low temperature at which the adsorption process is carried out (77 K). The micropore volume according to the DR method gives similar values for both samples (Table 1) The values of the diffusion parruneters found by modelling of the transient responses for both samples are very close. 

The sorption of ethane from dilute mixtures with helium by 4A sieve crystal powder and pellets made without binder has been studied with a microbalance in a flow system at temperatures between 25° and 117°C. Results show clearly that intracrystalline diffusion is the rate-controlling process and that it is represented well by a Pick s law diffusion model. Transient adsorption and desorption are characterized by the same effective diffusivity with an activation energy of 5660 cal/gram mole. 

In the A model. R- remains adsorbed until it suffers r. In a D model, transient adsorption is possible, although it has not been included thus lar in calculations. Provided that the only processes of transiently adsorbed R- are r ami desorption, product distributions will be independent of whether or not there is adsorption. 

The kinetics of the adsorption process taking place at the surface of a growing drop or bubble is important for the interpretation of data from drop volume or maximum bubble pressure experiments. The same problem has to be solved in any other experiment based on growing drops or bubbles, such as bubble and drop pressure measurements with continuous, harmonic or transient area changes (for example Passerone et al. 1991, Liggieri et al. 1991, Horozov et al. 1993, Miller at al. 1993, MacLeod Radke 1993, Ravera et al. 1993, Nagarajan Wasan 1993). 

Equation 6 together with Eqs. 10 and 11 describe a process of onedimensional diffusion, initiated by a change in the surrounding atmosphere so that the corresponding equilibrium concentration varies from Co to Coo-Equation 10 requires that immediately after the pressure step, the concentration at the boundary (namely for y = 0) assumes the new equilibrium value. This means that the existence of additional transport resistances at the surface of the system is excluded. The second term in Eq. 11 indicates that the process has to proceed as in a semi-infinite medium. This means in particular that the transient adsorption or desorption profiles originating from different crystal faces must not yet have met each other. 

Fig. 13 Current transients i(t) for Au (111), miscut < 0.5°, in 0.05 M H2SO4 obtained after a singie potentiai step from 1 = 0.75 V (region II) to various final potentials in region iii. The experimentai traces are given as individual data points, the solid lines represent theoretical curves calculated with the parameters of the numerical fit to a model combining (a) an adsorption process (Eq. 7) and (b) one-step nucleation according to an exponential law with surface diffusion-controlled growth (Eq. 34), (reprinted from Ref. [299]. Copyright 1997 by VCH Verlagsgesellschaft mbH Weinheim).

The real advantage of this approach is attained in the case of platinum electrodes where the time scale of the perturbation is selected to achieve the separation of double-layer and charge-transfer adsorption processes, by taking advantage of the fact that the double-layer response is usually much faster than charge-transfer adsorption phenomena. If the laser-pulsed experiments are performed under experimental conditions so that charge-transfer processes remain essentially frozen during the measurement time-scale, then the laser-induced potential transient reflects the temperature coefficient of the potential drop across the double-layer. 

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