Transient adsorption behavior

Characterization of the Adsorbed Layer of a Silver Catalyst in the Oxidation of Ethylene from Its Transient Adsorption Behavior... 

In order to analyze the adsorption behavior of carbon dioxide on silver it was necessary to understand the adsorption behavior of and its reactivity, because the adsorption of CO strongly relatedto the adsorbed oxygen species as will be described later. For this reason, the following transient experiments were purformed. 

Usually, for a potential-decay experiment, the system is at steady state just before the circuit is opened. Therefore the value of K(0) to be used to define the initial conditions for solution of the differential equations is the potential at which the system was held prior to the transient. The initial value of 6 is the corresponding steady-state value, obtained by inserting K(0) into Eq. (54), setting Eq. (54), equal to zero, and solving for 6. It is this 6 that is required for evaluation of the adsorption behavior of the electroactive intermediate. The required differential kinetic equations can be solved numerically for various mechanisms and forms of transients t) t) or V t) derived. 

The adsorption behavior of coumarin (2H-l-benzopyran-2-one) at the mercury-electrolyte interfaces was investigated by electrocapillary, capacitance, and transient experiments [162-164, 540-543]. 

Santerre JP, ten Hove P, VanderKamp NH, Brash JL. Effect of sulfonation of segmented polyurethanes on the transient adsorption of fibrinogen from plasma possible correlation with anticoagulant behavior. J Biomed Mater Res 1992 26(1 ) 39-57. 

In total, the differences between Ci adsorbate stripping and COad stripping, in both potentiodynamic measurements and potentiostatic transients, can be qualitatively explained by the lower COad coverage after Ci adsorption. A quantitative comparison with a lower coverage CO adlayer reveals, however, that these coverage effects are not sufficient and that contributions from other effects, most likely related to the structure of the CO adlayer, are important for the different oxidation behavior of the respective adsorbates as well. 

The same group has looked into the conversion of NO on palladium particles. The authors in that case started with a simple model involving only one type of reactive site, and used as many experimental parameters as possible [86], That proved sufficient to obtain qualitative agreement with the set of experiments on Pd/MgO discussed above [72], and with the conclusion that the rate-limiting step is NO decomposition at low temperatures and CO adsorption at high temperatures. Both the temperature and pressure dependences of the C02 production rate and the major features of the transient signals were correctly reproduced. In a more detailed simulation that included the contribution of different facets to the kinetics on Pd particles of different sizes, it was shown that the effects of CO and NO desorption are fundamental to the overall behavior... 

Propagation problems. These problems are concerned with predicting the subsequent behavior of a system from a knowledge of the initial state. For this reason they are often called the transient (time-varying) or unsteady-state phenomena. Chemical engineering examples include the transient state of chemical reactions (kinetics), the propagation of pressure waves in a fluid, transient behavior of an adsorption column, and the rate of approach to equilibrium of a packed distillation column. 

Jain, A.K., Hudgins, R.R. and Silveston, P.L., "Adsorption/ Desorption Models How Useful to Predict Catalyst Behavior under Transient Conditions", paper submitted to Seventh North American Meeting, The Catalysis Society, Boston, 1981. 

Although the purpose of this investigation was to study the initial transient behavior of heatless adsorption, the best removal of hydrogen sulfide occurred in the run with 3.01% H S and a y = 2.88, cycle time of 12 minutes and feed rate of 190 sCCM. Removal was down to 99.55% in three hours. For the 6.32% feed the best removal for the same time period was 98.49%. 

Heterogeneously catalyzed reactions are usually studied under steady-state conditions. There are some disadvantages to this method. Kinetic equations found in steady-state experiments may be inappropriate for a quantitative description of the dynamic reactor behavior with a characteristic time of the order of or lower than the chemical response time (l/kA for a first-order reaction). For rapid transient processes the relationship between the concentrations in the fluid and solid phases is different from those in the steady-state, due to the finite rate of the adsorption-desorption processes. A second disadvantage is that these experiments do not provide information on adsorption-desorption processes and on the formation of intermediates on the surface, which is needed for the validation of kinetic models. For complex reaction systems, where a large number of rival reaction models and potential model candidates exist, this give rise to difficulties in model discrimination. 

The results of Yamada et al, 111) experiment are shown in Fig. 14 there is desorption of at the switch. There must be two different reservoirs of CO on the surface one for the usual strongly adsorbed CO and the other for CO that can desorb, probably in a precursor state. A material balance shows that at ti, d ldt + d /dt = d /dt slopes b and c are not equal. This segregation of the two kinds of CO is a transient effect. If one stops the adsorption and performs a TPD, the C 0 and the C 0 should become indistinguishable in their behavior. This complicated matter has been considered by Yates and Goodman (113), Lombardo and Bell (114), and Tamaru and colleagues (110). Again, the explanation of transient and isotopic experiments requires a more detailed understanding of the processes involved than that needed merely to explain steady-state experiments. 

Among the unit operations, adsorption may be considered a prototype for all fluid-solid separation operations. When it is conducted under countercurrent conditions, the calculation methods required are entirely analogous to those for countercurrent absorption or extraction (H3). Often, however, it is most economical to conduct adsorption in a semi continuous arrangement, in which the solid phase is present as a fixed bed of granular particles. The fluid phase passes through the interstices of this bed at a constant flow rate and for an extended period of time. The concentration gradients in the fluid and solid phases display a transient or unsteady-state behavior, and their evolution depends upon the pertinent material balances, rates, and equilibria. 

Figure 3 illustrates the effect of the adsorption/desorption kinetics on the transient profile, in the absence of surface diffusion, where KJKd = 1, i.e., half the surface sites are occupied at equilibrium. The results are presented as i/i() versus r 1/2 in order to emphasize the short-time behavior. At very short times, i.e., the largest r 1/2, the UME response is identical for all values of Ka, since under these conditions, the diffusion field adjacent to the electrode is much smaller than the tip/sample separation and so does not sense the presence of the substrate (30,33). At times sufficient for the dif-... 

Thus, the most suitable route for obtaining information on the adsorption/desorption kinetics is from the short-time transient behavior. Under these conditions, surface diffusion effects are negligible and the short-time current response depends only on Ka, Kd, and A for a given tip/substrate separation. Provided that an independent measurement of A can be made, an absolute assignment of the interfacial kinetics is possible. Furthermore, analysis of the long-time current allows the importance, and magnitude, of surface diffusion to be determined. 

The analytical solution to Equation 2 for a range of boundary conditions is a model of pesticide fate that has been used under a variety of laboratory situations to study the basic principles of soil-water-pesticide interaction. It is in fact limited to such laboratory cases, as steady state water flow is an assumption used in deriving the equation. As a modeling approach it is useful in those research studies in which careful control of water and solute fluxes can be used to study degradation and adsorption. For example, Zhong et al. (11) present a study of aldicarb in which the adsorption and degradation of aldicarb, aldicarb sulfone and aldicarb-sulfoxide were simultaneously determined from laboratory soil column effluent data. The solution to a set of equations of the form of Equation 2 was used. A number of similar studies for other chemicals could be cited that have provided useful basic information on pesticide behavior in soil (4,12,13). Yet, these equations are not useful in the field unless re-formulated to describe transient water and solute fluxes rather than steady ones. Early models of pesticide fate based upon Equation 2 (14) were constrained by such assumptions, but were... 

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