Henry-type isotherms

At room temperature only a small amount of hydrogen is stored in porous materials and the adsorption isotherm typically does not show any saturation described by a plateau. Instead a linear increase in the hydrogen uptake is observed up to relatively high pressure. Therefore, at room temperature the adsorption is better described by a linear Henry type isotherm [Ij. 

Figure 8. Henry type isotherms for from O2/N2 mixtures.

Oxygen adsorption isotherms, in the experimental region, is Henry type isotherm (Figure 8). Table IV gives Henry constants. 

Hydrogen storage by physisorption is typically applied between 77K and room temperature. Since the temperature is considerably higher than the critical temperature of H2 no multilayer adsorption takes place and type I or Henry-type isotherms... 

One obvious deficiency of the D-A equation is that it does not approach the Henry type equation at lower concentrations. According to chromatographic measurement using helium gas as a carrier (Chihara, Suzuki and Kawazoe, 1978), the adsorption equilibrium coefficient of the Henry type equation, which was assumed to hold at an extreme of C=0, could be determined for MSC SA. The results are shown in Fig. 3.14. In this experiment, since helium gas exists in large excess compared with adsorbable tracer gas, the coadsorption effect of helium may not have been negligible, making it possible to assume the Henry type isotherm for the tracer gases. 

The most simple type, A, is that of a linear increase. It is described by the Henry adsorption isotherm equation ... 

Figure 30 shows the CTL glow curves for the catalyst pre-adsorbed ethanol or acetone vapor. The catalyst is heated at a rate of 0.5 °C/s in synthetic air. The CTL intensity increases at high temperatures, and the total amount of CTL intensity L depends on the adsorption time At and the gas concentration C during adsorption. We can measure the gas at a very low concentration by measuring the value of L because L is proportional to the product of At and C in a region of low AtC where a Henry-type adsorption isotherm holds. 

Figure 34 shows the results for alcohol (methanol, ethanol, 1-propanol and 1-butanol), ketone (acetone and diacetyl), terpene (pinene and linalool), aldehyde (n-nonyl aldehyde) and ester (acetic acid n-amyl ester and n-butyric acid ethyl ester) of various concentrations. Because of the linear characteristics of the CTL-based sensor, the plots are located in a similar region for a certain type of gas of various concentrations where the Henry-type adsorption isotherm holds. Thus, we can identify these gases with various concentrations by simple data-processing. 

On the other hand, kinetics of reactions occiuring on a solid surface, that is, catalysis or photocatalysis, must be significantly different. There may be two representative extreme cases. One is so-called a diffusion controlled process, in which siuface reactions and the following detachment process occur very rapidly to give a negligible surface concentration of adsorbed molecules, and the overall rate coincides with the rate of adsorption of substrate molecules. In this case, the overall rate is proportional to concentration of the substrate in a solution or gas phase (bulk), that is, first-order kinetics is observed IS). The other extreme case is so-called surface-reaction limited, in which surface adsorption is kept in equilibrium during the reaction amd the overall rate coincides with the rate of reaction occurring on the surface, that is, reaction of e and h+ with surface-adsorbed substrate (l9). Under these conditions, the overall rate is not proportional to concentration of the substrate in the bulk unless the adsorption isotherm obeys a Henry-type equation, in which the amount of adsorption is proportional to concentration in the bulk (20). In the former case, the rate... 

The heat of adsorption of Ar was also measured for acidity evaluation. In the case of Ar-TPD, an effect of the probe molecule diffusion in micropores is observed with some samples, such as zeoUtes, at high temperature-programmed rates. The adsorption method is not influenced by diffusion of the adsorbed molecule because the Ar isotherm is measured at static equilibrium. It is also advantageous that the usual BET apparatus can be used to obtain the adsorption isotherm. In addition, the adsorption behavior of Ar is of the Henry type at temperatures around room temperature. 

Experimental results by means of the volumetric method using a conventional BET system were observed to be Henry type in the temperature range 233-313 K and the pressure range P= 5-30 kPa, achieving the condition of small coverage. An example adsorption isotherm, for H-mordenite, is shown in Figure 17.3. 

When r = 1, 9 is very small and the adsorption increases linearly with the increase in 0 (Henry s type isotherm). On the other hand, when r = 10 the isotherm deviates from such a straight line and approaches a Langmuirian type with a near-plateau value at high 0 values. However, when r > 20, high affinity isotherms are obtained. In this case, the adsorption rises very steeply at low polymer concentrations and thereafter it reaches a pseudo-plateau region. The adsorption isotherms for chains with r = 100 and above are typical of those obtained experimentally for most polymers that are not too polydisperse (i.e. showing a steep rise followed by a nearly horizontal pseudo-plateau (which only increases by few percent per decade increase in 0. ... 

The above equation is called the Langmuir isotherm. When the amount adsorbed, g, is far smaller compared with the adsorption capacity of the adsorbent, go, Eq. (3-3) is reduced to the Henry type equation ... 

As shown in Figs. 9.3 and 9.5, adsorption isotherm can be given as a function of concentration of eluant in the carrier stream. In the case of chromatographic separation, for the sake of simplicity, an adsorption isotherm relation often is written as a Henry type equation. 

Henry s law corresponds physically to the situation in which the adsorbed phase is so dilute that there is neither competition for surface sites nor any significant interaction between adsorbed molecules. At higher concentrations both of these effects become important and the form of the isotherm becomes more complex. The isotherms have been classified into five different types (9) (Eig. 4). Isotherms for a microporous adsorbent are generally of type I the more complex forms are associated with multilayer adsorption and capillary condensation. 

At different types of adsorption isotherms plotted for adsorption of donor particles on oxides (see section 1.5) expressions (1.112) - (1.115) provide the rise in and decrease in with the growth of partial pressure of gas P, the functions themselves being different. Thus, in case of applicability of the Henry isotherm at small P we have the function oi - exp const-P becoming a power function <7s P with the rise in P which is often observed in experiments [154, 155, 169]. 

In other words, the model predicts a limiting form of a linear (Henry s Law) type of isotherm as the polymer concentration tends to zero. 

For propane, n-pentane and n-hexane the differential heats of adsorption over FER dropped more rapidly, right after 1 molecule was adsorbed per Bronsted acid site. Similar results were obtained with TON. In contrast, with MOR and FAU the drop in the differential heats of adsorption for n-alkanes occurred at lower coverages, indicating that only a certain fraction of the Bronsted acid sites were accessible to the adsorbing alkane probe molecules. With MFI the drop did not occur until 2 molecules of n-alkane were adsorbed per Bronsted acid site, suggesting perhaps a higher stoichiometry of about two n-alkanes per Bronsted acid site. In the cases of i-butane and i-pentane the drop occurred around one alkane per Bronsted acid site. Finally, n-butane adsorption isotherms measured over TON framework type catalysts having three different A1 contents (Si/Al2 = 90, 104, 128) showed Henry coefficients to increase with increase in the A1 content [5], Based... 

The CB-MC method has been used to simulate the adsorption isotherms of various alkanes in silicalite (170, 171). Using potential parameters that were fitted to obtain good agreement with experimental Henry s law coefficients, Smit and Maesen (170,171) have simulated the adsorption isotherms of straight-chain alkanes in silicalite. Good agreement was obtained for ethane and propane in comparison with the different type-I curves measured experimentally. The overall agreement with experimental isotherms was found to be satisfactory with hexane and heptane, and a kink is seen... 

The results of experimental studies of the sorption and diffusion of light hydrocarbons and some other simple nonpolar molecules in type-A zeolites are summarized and compared with reported data for similar molecules in H-chabazite. Henry s law constants and equilibrium isotherms for both zeolites are interpreted in terms of a simple theoretical model. Zeolitic diffusivitiesy measured over small differential concentration steps, show a pronounced increase with sorbate concentration. This effect can be accounted for by the nonlinearity of the isotherms and the intrinsic mobilities are essentially independent of concentration. Activation energies for diffusion, calculated from the temperature dependence of the intrinsic mobilitieSy show a clear correlation with critical diameter. For the simpler moleculeSy transition state theory gives a quantitative prediction of the experimental diffusivity. 

For the sorption of hydrocarbons in type-A zeolites at ordinary temperatures, the region of linearity of the isotherm is limited to very low pressures, and Henry constants are usually obtained by extrapolation from measurements outside the linear region. 

Thus, in terms of a, the sorption isotherm for osmotically ideal solutions is of Type III in Brunauer s classification (29) and reduces to Henry s law for (Mi/M2)a< 1. 

Xenon Adsorption Isotherms and 129Xe NMR Measurements. Figure 1 displays the room temperature (22 °C) xenon adsorption isotherms of the coadsorbed xenon for the three different zeolite samples loaded with various amounts of benzene. A consistent decrease of adsorption with increasing 6 was found for each benzene/zeolite system. By comparing the slope at low xenon pressures, i.e. in the Henry s Law region, we obtained for the adsorption strength NaX(1.23) > NaY(2.49) > NaY(2.70). Moreover, the saturation benzene concentration in faujasite-type zeolites with different Si/Al ratios follows the relation NaX(1.23) < NaY(2.49) < NaY(2.70). 

The majority of physisorption isotherms may be grouped into the six types shown in Figure 2. In most cases, at sufficiently low surface coverage the isotherm reduces to a linear form (i.e. na oc p), with is often referred to as the Henry s Law region (On heterogeneous surfaces this linear region may fall below the lowest experimentally measurable pressure). 

The extent of adsorption can have a profound effect on the rate of the surface reaction. Equilibrium isotherms of many kinds have been reported for adsorption from solution and have been classified by Giles et al. [24-27], The shapes of these adsorption curves often furnish qualitative information on the nature of the solute-surface interactions. Several of the types of isotherm observed in dilute solution are represented reasonably well by three simple and popular isotherm equations, those of Henry, Langmuir, and Freundlich. Their shapes are illustrated in Fig. 1. Each of these isotherms relates the surface concentrations cads (mol m"2) to the bulk equilibrium concentration c of the solute species in question. When few surface sites are occupied, Henry s law adsorption... 

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