Transport mechanisms during drying supported

FIGURE 16.4 (a) Transport mechanisms during drying of supported catalysts (b) schematic of metal adsorption on an oxide support. 

According to Hagymassy, Brunauer and Mikhail [36], the thickness of an adsorbed water layer at the saturation pressure of water vapor is about 6 monolayers. When the thickness of a water layer is 0.3 nm, the fraction of the pore volume of the support taken up by an adsorbed film of water can be calculated. Knijff [35] established that the fraction of the pore volume filled with 6 monolayers of water agrees well with the moisture content, at which the drying rate displays the first, slight drop below the level of the constant rate period. The rate of evaporation that is maintained during that period indicates that transport of the water film proceeds more slowly when the thickness of the water layer decreases below about 6 mono-layers. However, the drying experiments clearly indicate that liquid water can be transported not only by capillary pressure, but also as a film adsorbed on the surface of oxidic supports. A prerequisite for this mechanism of transport may be that the surface of the support is hydrophilic. 

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