Titania, adsorption isotherms

Adsorption isotherms for AMP-QS onto kaolin, titania and calcium carbonate are shown in Figure 4. Viscosity profiles obtained when AMP is used to disperse titania and kaolin are shown in Figures 5-7. 

Figure 4 Adsorption isotherms for AMP-95 onto kaolin, titania and calcium carbonate...

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . 

Parameters of the porous structure of titania samples (pores volume Vs, specific surface area Ssp) were calculated using BET theory [34] from the adsorption isotherms of methanol. The average pore diameter (Dp) values were estimated from the differential curves of pore size distribution. 

The textural nature of the titania and TiCex were characterised by nitrogen ad/desorption isotherms. The specific surface areas are presented in Table la. None of the materials were found to be microporous from t-plot analyses of the adsorption isotherms. From the desorption curves, the mesopore size distribution was calculated using the BJH method. All of the samples had bimodal mesopore size distributions. The volumes of the narrow and wide mesopores, presented in Table lb), were calculated from the minima between the two distributions. These results indicated, that as the amount of ceria incorporation rose, the bimodal mesopore size distribution became narrower. For the titania sample the mesopores were centred in diameters of approximately 14 and 17 nm. With ceria incorporation both of these diameters were reduced until at the highest ceria content they were approximately 9 and 15 nm. 

Figure 10. Adsorption isotherm for aqueous silica on titania at pH 9.5.

Figure 14. Adsorption isotherms of water vapor (at 25 °C) on titania (O) and 0.6 wt % DS-coated titania (%). The out gassing temperature was 150 °C. (Reproduced with permission from reference 42. Copyright 1986.)...

The surface hydration-hydroxylation structure of titania, proved previously mainly by IR studies using dry titania powders, also seems to hold when these powders are dispersed in water. An interesting approach, therefore, is to probe directly the uptake of water from the gas phase by DS-coated rutile surfaces (42). Water adsorption isotherms are presented in Figure 14. The dual nature of titania surface sites, a property not seen... 

Fig. 3. Niteogen adsorption isotherms (A) and the corresponding BJH pore size distribution curves (B) of the mesoporous titania as-prepared (MTi02 80), modified by ceria mesoporous titania support as-prepared (CeMTi 80) and calcined at 400 °C (CeMTi 400), and gold-based catalysts calcined at 400 °C with different gold content (2 Au/CeMTi 400 and 5 Au/CeMTi 400).

The surface hydration-hydroxylation structure of titania, proved previously mainly by IR studies using dry titania powders, also seems to hold when these powders are dispersed in water. An interesting approach, therefore, is to probe directly the uptake of water from the gas phase by DS-coated rutile surfaces [42]. Water adsorption isotherms are presented in Figure 52.14. The dual nature of titania surface sites, a property not seen with other common oxides such as silica and alumina, leads to an unusual type of water adsorption isotherm for titania. The isotherm shows two distinct knees (Figure 52.14) connected by a region where adsorption increases linearly with the partial vapor pressure of water. The explanation for this adsorption behavior is rather complex [42] and beyond the scope of this chapter. This behavior is believed to be due to the presence of hydrated surface cation sites. 

FIGURE 2.33 DTF IPSDs with respect to (a) the pore volume and (b) the specific surface area calculated on the basis of the water adsorption isotherms for nanooxides. (Adapted from Powder TechnoL, 195, Gun ko, V.M., Zarko, V.L, Turov, V.V. et al.. Morphological and structural features of individual and composite nanooxides with alumina, silica, and titania in powders and aqueous suspensions, 245-258, 2009g, Copyright 2009, with permission from Elsevier.)... 

Fig.l shows the pore size distributions of y-alumina, titania (anatase) and tetragonal zirconia (after calcination at 450°C for 3 hours). The pore structure data of these three adsorbent samples are compared in Table 2. The pore structure data were obtained from nitrogen adsorption isotherms measured by Micromeritics ASAP-2000 adsorption porosimeter. As shown in Fig.l, the pore size distributions of these materials are rather narrow, with an average pore diameter of about 3 nm. Such narrow size distribution and nanoscale average pore size are determined by the primary crystallite particles. The particles of the... 

Figure 8. Adsorption isotherms of phoi horus on titania and alumina constmcted from the integrated TPD ectra resulting from e q)osure to calcium pho hate solution, as in Fig. 7.

Figure 9 shows several TPD spectra of mass 31 (P) at different exposure times to calcium phosphate solution (55). Phosphorus desorption from alumina riiows broad features from 450-700 K, the residual salt peak at 980 K, and a high tenq>erature feature, vAach occurs between 1400 and 1560 K. Variation in the tenq>erature associated with the high tenq)erature desorption feature may be a result of variations in heating rate or thermocouple placement. Unlike the titania spectra, the large feature at 1200 K is absent. In addition, at least one new feature can be observed at approximately 450-700 K Figure 9b shows the corre onding calcium and potassium desorption features. The ratio of calcium and potassium ions to pho horus is conq)arable, but for longer exposures, this ratio decreases dramatically. For a 30 hr. e q>osure, the calcium and potassium to phosphorus ratios drop to less than 1% of the solution concentration. Integration of several phosphorus desorption ectra provide the data for an adsorption isotherm shown in Figure 8. Onset of rapid uptake of pho hate is observed between 20-25 hr.

The rate of uptake of calcium and potassium pho hate on the alumina and titania sur ces can be directly conq>ared from their re ective adsorption isotherms. The onset of rapid uptake is between 200-250 min. for the titania surftice and between 1500 and 2000 min. for the alumina surfrce. This is conq)arable to in-situ studies wiiere the onset of rapid growth is 160 min. for titania powders and 1800 min. for ahimina powders (Canq)bell, A. A., Battelle, Pacific Northwest Laboratories, personal communication, 1996). This correlation between in-situ and ex-situ measurements fiuther validates our technique for making kinetic measurements on thin metal oxide films that have been e q)osed to solution and returned to vacuum. 

Such an application was introduced when adsorption isotherms of sol-gel films were recorded by ellipsometry (Martin, 1990). The refractive index of silica-titania films was... 

This method allows independent control over both the macropores, which are determined by the emulsion droplets in the first step of the process, and the mesopores, by the supramolecular self-assembly in the second step. Therefore, the two templating systems do not interfere vhth each other, and consequently the meso-and macro-structures can be optimized separately [21]. Inorganic oxides, such as silica, titania and zirconia, were obtained by this two-step process. The bimodal distribution of pores was characterized by highly ordered mesopores (5-10 nm) and by interconnected macropores (0.1 to 5 p,m). These materials possess high pore volumes (17 cm g ) and specific surface areas, as determined by the BET method applied to the nitrogen adsorption isotherm, of between 250 and 750 The... 

Influence of temperature on the Ni loading on titania Anatase A. A. was selected for this study due to its higher SSA (table 1). In figure 1 it is shown a representative adsorption isotherm (surface Ni concentration vs initial Ni concentration) at two temperatures of the impregnation suspension. 

It can be observed that at a constant pH, the plateau of the isotherm increases with T. This plateau corresponds to the maximum uptake of Ni achieved through interfacial deposition. The same trend with T was found in all adsorption isotherms. Thus, a considerable increase of well dispersed Ni species on titania surface was obtained by regulating the temperature of the impregnation suspension. 

Fig. 4.15 a,-plots for the adsorption of nitrogen on a sample of microporous titania, before and after nonane treatment. Curve (A), before nonane pre-adsorption curves (B), (C), (D) after nonane pre-adsorption, followed by outgassing at (B) 250° (C) 150°C (D) 25°C. The a,-plots were based on standard nitrogen isotherms having the same c-values as the isotherms of... 

A detailed study of the physical and chemical adsorption of water on three xerogels, ferric oxide, alumina and titania, as well as on silica (cf. p. 272) has been carried out by Morimoto and his co-workers. Each sample was outgassed at 600°C for 4 hours, the water isotherm determined at or near 20°C, and a repeat isotherm measured after an outgassing at 30 C. The procedure was repeated on the same sample after it had been evacuated at a... 

Volumetric measurements at room temperature showed that by far the greater part of the adsorption of carbon monoxide on Au/TiC>2 occurred on the support it followed the Langmuir equation and most of it was removable by pumping.23,83 About one-third of the titania surface was able to retain it, but there was little uptake on Au/SiC>2. Use of the double-isotherm method with Au/MgO showed that adsorption onto the metal was complete at about 1 atm, but on various samples the coverage never rose above 18%. On model Au/MgO(100) the maximum coverage attained using a pulsed molecular beam at room temperature was < 10%.54... 

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