Water vapour surface treatment

The latter authors found that a reversible water isothenn was obtained after the low-temperature (i.e. 40°C) evacuation of a carbon cloth, which had been activated by oxidative HNO, treatment. The molecular sieve character of this material was reduced by evacuation at 400°C and this also led to the appearance of hysteresis in the water vapour isothenn. Barton and Koresh (1983) conclude that such hysteresis is mainly due to the concentration of surface oxides which dictate the adsorption value at which the change from cluster adsorption to a continuous adsorbed phase takes place . The relationship between the adsorption of water and the surface concentration of chemisorbed oxygen was first established by Walker and Janov (1968). Bansal et al. (1978a,b) also investigated the influence of the surface oxygen on the adsorption of water they concluded that at p/p° < 0.5 the level of water uptake is determined by the concentration of surface oxygen-containing structures. 

A study of the hydrophilic sites on the surface of activated carbon fibres has been made recently by Kaneko et al. (1995) with the aid of X-ray photoelectron spectroscopy (XPS). In this work cellulose (CEL)- and polyacrylonitrile (PAN)-based activated carbon fibres were used and samples were either chemically treated with H202 or heated in H2 at 1000°C. As expected, surface oxidation by the H202 treatment increased the initial uptake of water, while the H2 reduction caused a marked decrease in the amount of water adsorbed at low p/p°. Measurement of the peak areas of the XPS spectra provided a means of determining the fractional surface coverage by the hydrophilic sites. In this way a linear relationship was found between the low-pressure adsorption of water vapour and the number of hydrophilic sites (mainly —COOH). 

The uniformity of the MgO(l 0 0) surface was demonstrated by the stepwise (Type VI) character of the isotherms of Rr, Xe and Ar (Coulomb etah, 1984), CH4 (Madih et ah, 1989) and C2H5 (Trabelsi and Coulomb, 1992). To obtain the required high degree of surface uniformity, it was necessary to exclude water vapour and control both the oxygen concentration and the conditions of thermal treatment (Coulomb and... 

Figure 1.26. influence of surface treatment on the shape of the isotherm open circles, adsorption closed circles, desorption. Adsorbent Cab-O-Sil. Fig. (a), adsorption of water vapour on the original sample, outgassed at 413 K fig. (b), the same, after outgassing at 1173 K fig. (c) adsorption of N2 at 77.5 K on a sample made hydrophobic by methylation. using chlorosilanes. (Redrawn from P.B. Barraclough, P.G. Hall, J. Chem. Soc., Faraday Trans. I 74 (1978) 1360.)... 

The water vapour isotherms for the treated MCM-41 sample, HX16-2T, are shown in Fig.6 together with the nitrogen isotherm obtained before water exposure. For a better comparison the data is expressed as equivalent liquid volumes by assuming the normal liquid densities. It is evident that this silicification treatment was very effective in making the surface extremely hydrophobic, as the uptake of water vapour is very low. Only when a 3 consecutive water isotherm was done some water molecules were able to penetrate the pore system, and even then, only at quite high values of relative pressure. Furthermore, the nitrogen isotherms shown... 

The adsorption of different materials is also very dependent on the nature of surface complexes. Thus, for example, some very early work showed that the amount of water vapour adsorbed by charcoal increased significantly by pre-treatment with oxygen. Dubinin et al have suggested that the oxygen provides active centres at which sorption of water proceeds in the form of isolated clusters through hydrogen bonds. 

An additional corroboration of the formation of Si-C bonds on silica surface is provided by the data on the hydrolytic stability of organosilicas produced as a result of the solid-phase hydrosilylation reaction. The treatment of these organosilicas with water vapours in the presence of pyridine [141] does not lead to any variation in the character of their IR spectra (Fig.10, spectra 1- 4). At the same time in the case of methoxysilica under similar conditions one can observe the destruction of Si-O-C bonds and formation of silanol groups (Fig. 10, spectra 5,6) which is evidenced for by the decrease in the intensity of the absorption bands in the region of 3000 - 2800 cm attributed to ether groups, and by the appearance of the band 3750 cm" A... 

To prove if an enrichment in phosphorus is the reason for this effect, different catalyst samples were exposed to wet feeds containing different levels of water vapour and, thereafter, the chemical composition of the catalyst surfaces was determined. Figure 2 shows the change in P V ratio as determined by XPS versus the duration of water vapour treatment. 

The beneficial action of surface treatments generally hes in the fact that they prolong the period of initiation of corrosion. Once corrosion has begun, only those treatments that effectively obstruct the penetration of water, both hquid and vapour, will reduce the corrosion rate. This effect may be significant, in particular, if corrosion is due to carbonation [27]. Chloride-induced corrosion processes attract moisture so strongly that in general, surface treatments cannot stop it [13]. 

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