Temperature adsorption kinetics

Adsorption Kinetics. In zeoHte adsorption processes the adsorbates migrate into the zeoHte crystals. First, transport must occur between crystals contained in a compact or peUet, and second, diffusion must occur within the crystals. Diffusion coefficients are measured by various methods, including the measurement of adsorption rates and the deterniination of jump times as derived from nmr results. Factors affecting kinetics and diffusion include channel geometry and dimensions molecular size, shape, and polarity zeoHte cation distribution and charge temperature adsorbate concentration impurity molecules and crystal-surface defects. 

Zverev, S.M., Smirov, K.S., and Tsyganenko, A.A. (1988) IR spectroscopic study of low-temperature adsorption of molecular nitrogen on the surface of oxides. Kinet. Katal.,... 

In order to have localized adsorption with only physical interaction it is clear that either the interaction must be strong, or the kinetic energy of the adsorbed molecules must be small. As an example of the latter condition we have the work of Keesom and Schweers (24, 25) for low temperature adsorption of hydrogen and neon on glass. They assumed that the actual area of the glass was equal to the apparent area, and the results in Table IX were worked out for = 3 on that basis. The... 

Interaction between Ti02 concentration and temperature (b45), the adsorption kinetics, for example, depending on the temperature. 

At present we have evidence for the complexity of higher temperature adsorption/desorption phenomena while, in general, the kinetic characteristics observed for many catalytic reactions are perhaps deceptively simple. The estimations of the concentrations of the participating surface intermediates are, in contrast, experimentally very difficult. Mechanistic investigations of many heterogeneous catalytic processes yield insufficient information to allow clear distinctions to be drawn between alternative reaction modelsf 125). 

While exploring the kinetic consequences of variations in surface occupancy upon reaction rate, a further mechanistic explanation of compensation behavior, of particular relevance in the consideration of adsorption kinetics, became apparent. If the total quantity of gas adsorbed by a surface df varies with temperature and the rate of adsorption dd/dt is proportional to the... 

The study of adsorption kinetics of a surfactant on the mineral surface can help to clarify the adsorption mechanism in a number of cases. In the literature we found few communications of this kind though the adsorption kinetics has an important role in flotation. Somasundaran et al.133,134 found that the adsorption of Na dodecylsulfonate on alumina and of K oleate on hematite at pH 8.0 is relatively fast (the adsorption equilibrium is reached within a few minutes) as expected for physical adsorption of minerals with PDI H+ and OH". However, the system K oleate-hematite exhibits a markedly different type of kinetics at pH 4.8 where the equilibrium is not reached even after several hours of adsorption. Similarly, the effect of temperature on adsorption density varies. The adsorption density of K oleate at pH 8 and 25 °C is greater than at 75 °C whereas the opposite is true at pH 4.8. Evidently the adsorption of oleic acid on hematite involves a mechanism that is different from that of oleate or acid soaps. 

For laboratory-scale modification, distinction has to be made between static and dynamic adsorption procedures. In a static procedure, the substrate is contacted with a known volume of gas at a well-defined pressure. The modifying gas may be stationary or circulating in a closed loop. Modification in a static gas adsorption apparatus allows the careful control of all reaction parameters. Temperature and pressure can be controlled and easily measured. Adsorption kinetics may be determined by following the pressure as a function of the reaction time. Figure 8.13 displays a volumetric adsorption apparatus, in which mercury is used, as a means to change the internal volume and for pressure measurement. 

Desorption is always an activated process and may conveniently be studied by temperature-programmed techniques. Information is obtained in this way on the adsorption kinetics and the energetics of the gas/solid interactions. 

Fortunately, the effects of most mobile-phase characteristics such as the nature and concentration of organic solvent or ionic additives the temperature, the pH, or the bioactivity and the relative retentiveness of a particular polypeptide or protein can be ascertained very readily from very small-scale batch test tube pilot experiments. Similarly, the influence of some sorbent variables, such as the effect of ligand composition, particle sizes, or pore diameter distribution can be ascertained from small-scale batch experiments. However, it is clear that the isothermal binding behavior of many polypeptides or proteins in static batch systems can vary significantly from what is observed in dynamic systems as usually practiced in a packed or expanded bed in column chromatographic systems. This behavior is not only related to issues of different accessibility of the polypeptides or proteins to the stationary phase surface area and hence different loading capacities, but also involves the complex relationships between diffusion kinetics and adsorption kinetics in the overall mass transport phenomenon. Thus, the more subtle effects associated with the influence of feedstock loading concentration on the... 

Atomic force microscopy has been up to now only scarcely used by the plasma processing community. Results mainly concern low-resolution measurements, that is modification of the surface roughness induced by the plasma [43,44], Micro masking effects have been observed when processing Si with a SF6 plasma beam at low temperature (Fig. 11) and correlated to the multi-layer adsorption of plasma species as observed by XPS [45], Further development of vacuum techniques should allow high resolution surface probe microscopy measurements on plasma-treated samples, and possibly lead to complementary information on adsorption kinetics, surface density of states. 

An adsorption kinetic model was developed to evaluate the adsorption rates of five pure gases (Nj, O2, Ar, CO, and CH4) on a Takeda-3A CMS over a wide range of pressures up to ISatm. The kinetic characteristics of adsorption on the CMS were studied by using the adsorption equilibrium of five pure gases measured at three different temperatures and their physical properties. Since the diffiisional time constants of all the components showed much stronger dependence of pressure than those expected by the traditional Darken relation, a structural diffusion model was applied to predict the strong pressure dependence. The proposed model successfully predicted the dif ional time constant up to high pressure on the CMS. 

The adsorption of methylene blue by coir pith carbon was carried out by varying the parameters such as agitation time, dye concentration, adsorbent dose, pH and temperature. Equilibrium adsorption data obeyed Langmuir isotherm. Adsorption kinetics followed a second order rate kinetic model. The adsorption capacity was found to be 5.87 mg dye per g of the adsorbent. There was no significant change in the per cent removal with pH. The pH effect and desorption studies suggest that chemisorption might be the major mode of the adsorption process. 

Our data show that large temperature changes can occur during measurements of adsorption kinetics. When the temperature maximum occurs early in the process, no pronounced effect on the rate curve is observed, and the unwary experimenter may conclude that his data were obtained isothermally. 

It has been concluded that, for most cases, catalysis over zeolites occurs within the intracrystalline voids. Strong supporting evidence for this was provided by Weisz (71), who compared the rates of dehydration of 2-butanol over Linde lOX and 5A zeolites at relatively high temperatures and low conversion. The rate constant per unit volume of 5A was 1/lOO-l/lOOOth that for lOX, a magnitude consistent with the ratio of available surface areas for the external area of 1-5/x-sized 5A crystals and for lOX, where the internal surface area was available to the alcohol. The strong driving force for occlusion within the intracrystalline zeolite voids is exemplified by the rapid adsorption kinetics and rectangular adsorption isotherms observed for molecules whose dimensions are not close to those of the entry pores. 

The aim of this work was to evaluate the separation ability of CMS by means of their pore size distribution obtained from immersion calorimetry measurements. In this way, adsorption kinetics of different gases (N2, O2. CO2, CH4, n-butane and i-butane) on the CMS prepared have been carried out at room temperature and 760 Torr initial pressure. The most illustrative results are reported in Figure 5. 

The support plays an important effect in the adsorption kinetics of CO on supported clusters. Indeed CO physisorbed on the support is captured by surface diffusion on the periphery of the metal clusters where it becomes chemisorbed. The role of a precursor state played by CO adsorbed on the support is a rather general phenomenon. It has been observed first on Pd/mica [173] then on Pd/alumina [174,175], on Pd/MgO [176], on Pd/silica [177], and on Rh/alumina [178]. This effect has been theoretically modeled assuming the clusters are distributed on a regular lattice [179] and more recently on a random distribution of clusters [180]. The basic features of this phenomenon are the following. One can define around each cluster a capture zone of width Xg, where is the mean diffusion length of a CO molecule on the support. Each molecule physisorbed in the capture zone will be chemisorbed (via surface diffusion) on the metal cluster. When the temperature decreases, Xg increases, then the capture zone increases to the point where the capture zones overlap. Thus the adsorption rate increases when temperature decreases before the overlap of the capture zones that occurs earlier when the density of clusters increases. Another interesting feature is that the adsorption flux increases when cluster size decreases. It is worth mentioning that this effect (often called reverse spillover) can increase the adsorption rate by a factor of 10. We later see the consequences for catalytic reactions. 

Fig. 2. Constant volume apparatus for the study of adsorption kinetics, equipped for high temperature studies and high speed pressure measurements. From ref. 82.

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