Moisture, surface oxide-support

The vanadium oxide species is formed on the surface of the oxide support during the preparation of supported vanadium oxide catalysts. This is evident by the consumption of surface hydroxyls (OH) [5] and the structural transformation of the supported metal oxide phase that takes place during hydration-dehydration studies and chemisorption of reactant gas molecules [6]. Recently, a number of studies have shown that the structure of the surface vanadium oxide species depends on the specific conditions that they are observed under. For example, under ambient conditions the surface of the oxide supports possesses a thin layer of moisture which provides an aqueous environment of a certain pH at point of zero charge (pH at pzc) for the surface vanadium oxide species and controls the structure of the vanadium oxide phase [7]. Under reaction conditions (300-500 C), moisture desorbs from the surface of the oxide support and the vanadium oxide species is forced to directly interact with the oxide support which results in a different structure [8]. These structural... 

Under dehydrated conditions, the adsorbed moisture is removed and the in situ Raman spectra of the surface metal oxides differ markedly showing that the structures of the dehydrated species are very different from those of their hydrated counterparts (see references above in Section 6.2.2). These changes are not surprising since the influence of the net zero surface charge of the oxide support can only be exerted in an aqueous medium. For the dehydrated surface metal oxides, however, essentially the same molecular structures are seen on all the oxide supports for each supported metal oxide. ... 

AOO-4 is a small steam leak followed by cooldown using the SCS. Thermal and structural load conditions are essentially the same as AOO-l(A). Moisture ingress conditions result in <0.02 ram (0.001 in.) maximum local surface oxidation of 2020 graphite core support structural components. Based on 0.4 events per reactor year mean frequency, total surface oxidation of 0.3 mm (0.01 in.) thickness is predicted during the reactor lifetime. This cumulative oxidation is within the design corrosion allowance of 2 mm (0.08 in.) thickness on graphite core support structural components. 

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. 

As the supported metal oxide catalyst temperature is increased, the thin aqueous film evaporates and desorbs, 100 to 200" C, from oxide surfaces to yield dehydrated surfaces. If sufficient moisture is present in the environment at the elevated temperatures, however, it is still possible to maintain an extensively hydrated surface up to 230°C [35]. At higher temperatures, the desorption rate of the adsorbed moisture from oxide surfaces is very fast and the surfaces are essentially dehydrated (<5% of the surface contains adsorbed moisture at steady-state when moisture is present) [35],... 

In contrast to acidic surface metal oxides with cation oxidation states of +5 to +7, which are anchored to the support by surface hydroxyl groups, basic surface metal oxides with cation oxidation states of +1 to +3 are anchored at surface Lewis acid sites (Bredow et al., 1998 Cortez et al., 2003 Diebold, 2003 Jehng and Wachs, 1992 Vuurman et al., 1996). Raman spectra demonstrated that supported basic metal oxides are, in contrast to acidic supported metal oxides, insensitive to moisture. The Raman spectra of basic surface metal oxide species do not show the bands at about 1000 cm 1 that would indicate terminal M = 0 bonds. The spectra typically exhibit Raman bands in the wave number region of 500-700 cm-1, characteristic of M—O bonds (Chan and Wachs, 1987 Tian et al., 2006 Vuurman et al., 1996) similar behavior was observed for TiO, ZrOx, Pt02, and other oxide surface species with cations in the +4 oxidation state. 

The response of the supported nickel oxide on alumina to an in situ experiment is very different from that presented earlier for the supported tungsten oxide, as shown in Figure 13. The Raman band of the supported nickel oxide on alumina is not affected by the removal of the moisture present on the sample surface (compare Figures 13(a) and 13(d)) and suggests that water molecules do not coordinate to the supported nickel oxide. 

Figure 14 Schematic of surface nickel oxide in the Y-AI203 support Illustrating its inaccessibility to surface moisture.

Polymer supported Palladium(II) complex catalyst was synthesised using chloromethylated styrene-divinyl benzene copolymer as a support by sequential attachment of glycine and a metal salt solution. It was characterized using various methods such as IR, UV-Vis, ESR, DTA-TGA, SEM and surface area measurement. Swelling studies, moisture content and bulk density have also been investigated. The catalytic activity of the catalyst was tested for the oxidation of toluene. The effect on it of various parameters has been seen. The recycling efficiency has also been studied. 

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