Water desorb

At high temperatures (> 170 K), the water desorbs and so the autocatalytic reaction cannot be sustained and is an explanation for why the H2 + 02 reaction slows, the formation of OH species now being solely dependent on the H(a) + O(a) reaction, which is the slowest step in the above scheme. That the water + oxygen reaction was fast and facile was evident from the spectroscopic studies at both nickel and zinc surfaces, when the oxygen surface coverage was low and involving isolated oxygen adatoms. 

The amount of water desorbed can be calculated by the pressure rise after the valve between the chamber and condenser has been closed, divided by the time of closure and the chamber volume (see Section 1.2.3). 

During secondary drying, a small RM should be reached, since Tg of the dry product increases with decreasing water content. Tg- of amorphous, freeze dried sucrose increases from 16 °C with 8.5 % RM to 63 °C or 64 °C between 1.0 and 0.7 % RM. It should be taken into account, that RM cannot be taken only at the end of drying, but a possible increase during storage by water desorbed from the stopper has to be considered (see Section 1.3.2 and Pikal et al. [3.7]). 

During MD the Tke can be closely controlled and the change from MD to SD documented and, if required, automatically executed. During SD the pressure control can be switched of, the pressure will drop and the progress of SD can be followed by DR measurements. DRs give the amount of water desorbed/h in % of solids. By the integration over time (Eq. (20)) it can be decided, when e. g. 5 % desorbable water are reached and which result can be expected in another 48 h of drying. 

Figure 4. Pt(lll) HREELS spectra for a.) 1 L H 0 at 90 K (does not change upon annealing) 0.25 L CO followed By 1 L fLO b.) at 90 K, c.) after flash to 148 K (nothing desorbed), d.) after flash to 323 K (water desorbed). (Reproduced with permission from Ref. 5. Copyright 1988 Elsevier.)...

Electrochemical cells, particularly those with aqueous electrolytes, operate well above the freezing temperature of water. Since water desorbs from Pt below its freezing point ca. 180 K), there is a very low temperature ceiling for which reactions may take place. While the low temperature of water desorption precludes UHV study of many, if not most, electrocatalytic reactions, the surface reaction... 

In the reaction of 1 — 2, two hydrogen atoms have to be removed. Oxygen from the copper oxide is assumed to react with the hydrogen atoms and water desorbs. As the oxygen is removed gradually from the catalyst, the production of the unsaturated diketone 2 decreases and the selectivity to the corresponding saturated diketone 3 increases. This scenario is supported by the... 

Water desorbed from E glass fiber, molecules nm ... 

HC1 desorbs at approximately 65°C, water desorbs at approximately 120°C and benzene desorbs from 150-220°C. A typical DTA curve is shown in Figure 4. These results suggest that it should be possible to regenerate at low temperature (<200°C) and so help minimize any of the zeolite-catalyzed reactions of benzene. Goto et al. ( 7) report a regeneration scheme of three hours at 500°C for their study of the benzene-water system. However, they used a small-port (4A) zeolite which may have excluded the benzene from adsorption. 

Chamber pressure pch, left ordinate 2, desorption rate, DR = water desorbed in % of solids/h, right ordinate 3, desorbable water,... 

Tzolova-Miiller et al. (unpublished data) followed the dehydration of VOx/SBA-15, shown in Figure 14. The spectra provide a good example illustrating the usefulness of simultaneously acquired NIR data. The band at 1902 nm is typical for adsorbed water, and it can be seen by comparing the UV-vis and the NIR range that the vanadium environment changes the moment water desorbs (i.e., as the respective band vanishes). 

The PcFe-H20 bond is stable under vacuum up to 340 K. At higher temperatures, the adsorbed water desorbs quantitatively. The mean surface area of an adsorption site for water has been estimated to 0.83 nm2 from the lattice parameters of PcFe 8 This value is in fact close to the geometric area of a single PcFe molecule. 

An alternative way to determine acidity was to outgas the samples at increasing temperatures and to follow the weight losses by thermogravimetric analysis (TGA). Physisorbed water desorbs initially followed by water release from acid OH groups, according to the equations... 

TPD spectra obtained for the desorption of H2O from Ti02 and ZnO after the adsorption of a monolayer of H2O at 298 K showed that the amounts of water desorbed from Ti02 and ZnO in the 273-673 K temperature range are almost the same. These results suggest that differences in the photocatalytic reaction rates do not correlate with the amounts of adsorbed H2O in the steady state. 

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