Desorption, isotherm

Figure 5.3. Oxygen thermal desorption spectra after electrochemical O2 supply to Pt/YSZ at 673 K (I = +12 pA for 1800 s) followed by isothermal desorption at the same temperature at various times as indicated on each curve.4,7 Reprinted from ref. 7 with permission from Academic Press.

In summary, it must be concluded that amides as a category do not seem to have a substantial advantage over LiAlH system doped with catalytic precursors. There is no evidence that the amides could reversibly desorb/absorb at temperatures lower than 200°C. At the temperatures higher than 200°C amides and related materials barely desorb 5 Their kinetics of isothermal desorption at the atmospheric pressure... 

The techniques described in the foregoing do not easily provide information about the chemisorption bond strength. The Clausius-Clapeyron equation is not applicable in the range of irreversible adsorption. Only by measurements of the desorption rates during the thermal desorption processes at two slightly different temperatures can the activation energy of desorption be estimated. This method has been used by Kubokawa (55). Desorption rates can be calculated from the evolution curves obtained during isothermal desorption as shown, for example, by Czanderna (54). 

Ethylene molecules are known to physisorb at low crystal temperature. The binding energy in this state was estimated to be 0.25 eV from isothermal desorption experiments on Ag(l 0 0) [84]. Near edge X-rays absorption fine structure showed that the admolecules occupy the fourfold hollows on Ag(l 0 0) with the axis parallel to the surface [85, 86]. The sticking probability into the physisorption well is inhibited for rotationally excited gas-phase molecules [84]. 

The last Issue to be dealt with Is the apparent irreversibility of the adsorption. One quite often encounters the opinion, especially In the older literature, that polymer adsorption would be an Irreversible phenomenon. These ideas are based on the hysteresis found In the adsorption isotherms desorption Isotherms (obtained by dilution with solvent) do not coincide with adsorption Isotherms (obtained by adding more polymer at given amount of solvent). Qualitatively, this was already discussed in sec. 5.3d. An experimental example Is given in fig. 5.31, for the adsorption of a polydisperse rubber from heptane on two types of carbon black (differing In specific surface area) ). The desorption Isotherms are found to He considerably above the adsorption Isotherms, the extent of desorption being very small. 

Secondary drying is generally carried out under higher vacuum when the product has reached an above-zero temperature (or its electric impedance has reached the upper limit). Indeed, it has been shown by different authors that isothermal desorption was faster and allowed lower final residual moistures when the pressure lay in the level of 10 mbar, but that higher vacuums (10 mbar) did not drastically improve the operation. 

Figure 10.9 shows the isothermal desorption characteristics of pure C02 from a column (packed with BPL carbon or 5A zeolite) by purge with pure H2 at 30 °C and 1.0 atm.30 The figure plots the fraction of C02 desorbed from a column that was initially equilibrated with pure C02 at 1.0 atm and 30 °C as a function of the specific amount of H2 leaving the column. 

Figure 10.13 compares the isothermal desorption characteristics of pure (a) C2H6 and (b) C3H8 by H2 purge from the BPL carbon and the silica gel samples at 1.0 atm and 30 °C.31 These data were also generated in the same fashion as those for the components of the SMROG. They show that desorption of C3H8 from the carbon by... 

Figure 8 shows the H/D exchange kinetics between 50 nm thick D2O layer and the surrounding H20 polycrystalline ice at -5 C. As shown in the figure, only half of the initial D2O layer is converted into HDO on the time scale of our FTDS experiment. Analysis of the isothermal desorption spectra of HDO along with the observed reaction kinetics for D2O layers of various thickness show that the H/D exchange is controlled by... 

Minimum Purge Requirement 0 = AqlAY kg purge/kg bed Figure 7.10 Type III isotherm desorption [10],... 

This chapter describes the results of the acidity characterization of a selected silica surface with VT-DRIFT spectroscopy. Examples of the capabilities of the method are demonstrated by the qualitative determination of the adsorption and thermal desorption characteristics of pyridine on amorphous, porous silica gel. Procedures for the determination of isothermal desorption rate constants and activation energy of desorption are presented and discussed as a means of assessing acid site strength. 

Fig. 6. Sorption and desorption of imazethapyr on (a) Sassfras sandy loam and (b) Webster clay loam (45) 0 = sorption isotherm, = desorption isotherm at 0.1728 J mol/L, and V — desorption isotherm at 3.4561 J mol/L.

Using isothermal desorption of CO from Ni lll Ibach et al. [342] found that v = 1017s-1, and eqn. (86) indicated a value of 6x 1016 s-1 at low coverages, in good agreement with the experiment. 

As pointed out above, the desorption order markedly effects the shape of the desorption curve and the behaviour of the peak temperature with variation of initial coverage. Zero-order kinetics are shown by an increase in peak temperature with coverage and zero-order surface processes have now been observed for many systems [281—286]. Schwartz et al. [287] performed isothermal desorption measurements on the H2/Ti system and determined an order of 1.5, explaining this finding in terms of surface compound formation, with a stoichiometry of TiHl s. A very good example of the confusion which can reign in this field is exhibited by the... 

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