Desorption of water

GP 11] ]R 5] The addition of up to 7 vol.-% of water has no detectable impact on the oxygen reaction rate [121]. This is a hint that desorption of water generated is not the rate-determining step (Figure 3.51). 

Water was also produced in the reduction and was adsorbed on the Ba sites to form Ba(OH)2. As already reported, this eventually accounted for the observed delay in the outlet H20 concentration, which slightly increased with temperature. The traces of H20 and H2 continued to change after the N2 outlet concentration has diminished to zero, in particular at low temperature. This could be ascribed both to desorption of water previously stored as Ba(OH)2, and to the reduction of poorly reactive catalyst oxygen species. 

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

The stoppers for vials contain a certain amount of water, which depends on the composition of the stoppers. De Grazio and Flynn [1.86] showed, that the selection of the polymer, the additives for the vulcanization, and the filler influence the adsorption and desorption of water. However even the best possible mixture increases the RM in 215 mg sucrose from 1.95 % to 2.65 % during 3 months storage time at room temperature. Other stopper mixtures show an increase up to 1.7 %. Pikal and Shah [1.87] demonstrated, that the desorption of water from the stopper and the absorption of water by the product depends, in the equilibrium state, on the mass and water content of the stopper and the water content and sorption behavior of the dry product. 

Further confirmation was provided by Darlow and Ross (175) who studied the desorption of water from saturated surfaces at 100-190°. The rate of desorption was proportional to the square root of the amount adsorbed. The authors concluded from this result that there was one water molecule adsorbed for every two silanol groups. 

Simulating Temperature Programmed Desorption of Water on Flydrated... 

Joubert, J. Fleurat-Lessard, P. Delbecq, F. Sautet, P. Simulating Temperature Programmed Desorption of Water on Hydrated y-Alumina from First-Principles Calculations. J. Phys. Chem. B 2006,110, 7392-7395. 

Fig. 2.19 ( ) Time-of-flight spectrum in photon stimulated field desorption of water using synchrotron radiation, obtained by Jaenike et al.m...

Sorption of water vapour to or from a food depends on the vapour pressure exerted by the water in the food. If this vapour pressure is lower than that of the atmosphere, absorption occurs until vapour pressure equilibrium is reached. Conversely, desorption of water vapour results if the vapour pressure exerted by water in the food is greater than that of the atmosphere. Adsorption is regarded as sorption of water at a physical interface between a solid and its environment. Absorption is regarded as a process in... 

With respect to the mechanism, the adsorption measurements of Matsuura and Schuit [207—209] are of interest. The assumption of A-and B-sites is reported in some detail for the propene oxidation in Sect. 2.2.2(d)(i). As for the oxidation of propene, the abstraction of (both) H-atoms is assumed to occur on the molybdenum layers by initially forming HOb groups, followed by H-transfer from Ob to Oa and desorption of water (H2Oa). 

Berlin, E., Kliman, P. G., Anderson, B. A. and Pallansch, M. J. 1971. Calorimetric measurement of the heat of desorption of water vapor from amorphous and crystalline lactose. Therm. Acta 2, 143-152. 

Drying. The conditioned frozen material to be dried is placed in a vacuum chamber, where sublimation and desorption of water occur. As soon as the chamber has been evacuated and the optimum vacuum has been reached (0.5 to 0.05 millimeters of mercury), heating is applied so that the ice sublimes. For large-scale production of food products, the combination of conduction and radiation that results from circulating a hot fluid through coils or plates has proved quite satisfactory for heating. 

Table III. Lowest Loading at which Appreciable Desorption of Water Occurs...

Correlating positively with the hydrophobicity of the solvent, different fractions of inactivated active centers were measured in different solvents with solid-state NMR spectroscopy (13C-cross-polarization/magic angle spinning (MAS) NMR) (Burke, 1992). Just as with tritiated water (see above), immediate desorption of water molecules from the protein surface was observed after addition to the organic solvent. 

Figure 7 shows the CTL response and partial pressure of the desorbed gases during heating (1 °C/s) from the y-AkOs catalyst which pre-adsorbed ethanol vapor at room temperature. In Fig. 7a, the mass spectrometer measurements show the desorptions of water (m/z 18), the physisorbed ethanol (m/z 46), diethylether (m/z 59), and ethylene (m/z 25) for the catalyst heated in Ar, but the CTL emission is not observed. In Fig. 7b, the desorptions of ethanol (80 °C), diethylether (230 °C) and ethylene (250 °C) are observed above the same temperature as in Ar, in the course of heating in a mixed gas of 21% O2 and 79% Ar. The desorption of ethylene in the atmosphere containing oxygen, however, begins to decrease at a lower temperature (320 °C) in Fig. 7b than the peak temperature (340 °C) of ethylene desorption in Fig. 7a. Desorption peaks of CO2 (m/z 44) and water appear at 340 °C, and the CTL peak is observed at the same temperature in Fig. 7c.

After adsorption of H2 in this way an enhancement of the tendency to chemisorb 02 should be expected which, indeed, is found to be the case. After desorption of water, one may expect chemisorbed oxygen to fill up... 

---