Cation adsorption, temperature effects

The molecular sieve behaviour of zeolites can be controlled by a hydrolytic process at elevated temperatures. Water vapour in contact with zeolite crystals at elevated temperatures results in a variation of the zeolitic adsorption characteristics. The amount of water vapour, the pretreatment temperature and the pretreatment time, can control the effect pore size of zeolites. It appears, that a steam treatment causes a cation migration and a cation hydrolysis of the exchangeable cations. However, the effect of steam on the adsorption behaviour of zeolites is influenced by the nature of the initial exchangeable cations. 

Dove (1995) further summarizes evidence suggesting that adsorption of both Al " " and Fe3+ onto quartz surfaces inhibits reactivity of that phase. Inhibition of feldspar dissolution also occurs when Al is present in solution (Chou and Wollast, 1985 Nesbitt et al, 1991 Chen and Brantley, 1997). For example, Nesbitt et al. (1991) argued that adsorption of Al " " retarded the rate of dissolution of labradorite more than other cations. Furthermore, the effect of aqueous Al " " on dissolution of albite may increase with increasing temperature due to the enhanced adsorption of cations with temperature (Machesky, 1989 Chen and Brantley, 1997 note however that Oelkers (2001b) disputes this trend). In contrast, the addition of aqueous aluminum was not observed to affect the rate of forsterite dissolution at pH 3 and 65 °C (Chen and Brantley, 2000). It may be that aqueous aluminum becomes incorporated into surfaces and affects dissolution wherever the connectedness of surface silicon atoms is >0. Brantley and Stillings (1996, 1997) and Chen and Brantley (1997) suggest that Equation (51) can be used to model aluminum inhibition on feldspars. Sverdrup (1990) has reviewed the effects of aqueous Al on many minerals and incorporated these effects into rate equations. 

Temperature the results compiled in Tables 4.1-4.6 were obtained at different temperatures, and in some studies the temperature was not controlled. The results reported in Table 3.11 and Fig. 3.104 indicate that the PZC of oxides and related materials shifts to low pH when the temperature increases (with a few exceptions). Most surfaces carry more negative charge at elevated temperature (at given pH), and this creates favorable conditions for adsorption of cations and unfavorable conditions for adsorption of anions. Therefore elevated temperature would enhance uptake of cations, and low temperature would enhance uptake of anions at constant pH, if the electrostatic interaction was the only factor. On the other hand, the rate of chemical reactions and diffusion is enhanced at elevated temperatures. Thus, the kinetic and electrostatic effect on cation adsorption add up and the uptake increases with temperature. With anions these effects act in opposite directions the uptake increases with temperature when the kinetic factor prevails the uptake decreases with temperature when the electrostatic factor prevails, finally the both effects can completely cancel out. 

The quantification of temperature effects on surface charging has been discussed in Section 3,IV, and many problems and limitations considered in that section concern also specific adsorption. Interpretation of temperature effects on specific adsorption is even more complex, e.g. due to the discussed above complicated solution chemistry, which is also temperature dependent. While literature data relevant to speciation of solutions involving hydrolyzable cations and/or weak acids at one temperature (usually 20°C or 25°C) are readily available [22], the information on temperature effects on stability constants of water soluble complexes is rather incomplete. 

PZC of, 139-148, 167, 173, 177 temperature effect on, 286 sedimentation in nonaqueous media, 291 specific adsorption of cations on,... 

Kosmulski [137] found that the temperature dependence of cation adsorption is dependent on (1) their charge (higher charge causes a stronger effect of temperature) and (2) on total concentration of the metal. 

The effect of the reaction temperature on the rates of the Cu+ introduction and Na replacement alone, i.e., after removal of the temperature effect on the adsorption equilibrium, can be readily recognized. As expected, the rates increase with increasing temperature, whereas the final steady state of SSIE was independent of the temperature. For the temperature of 453 K this was confirmed by extending the reaction time to 25 h. Similar results to those obtained for CuCl/Na-Y were obtained for the system CuCl/Na-MOR [289,290] and for zeolites containing K+,Rb+ or Cs as (out-going) cations [292]. It was tentatively assumed that the exchange kinetics were diffusion-controlled. 

The use of supports such as Ti02, where the effect of a strong metal-support interaction (SMSI) was observed at high reduchon temperature, is one of the recommended routes. It is proposed that TiO having coordinatively unsaturated Ti cations that could interact with the electron pair donor site of the C=0 bond, facilitates adsorption of the unsaturated aldehyde in a favorable way to produce UOL [74, 75]. As for the metaUic phase, both theoretical and experimental studies indicate that larger particles improve the selectivity to UOL. In effect, it has been... 

The double-layer effect in the electrode kinetics of the amalgam formation reactions was discussed [67]. The dependences on the potential of two reduction (EE) mechanisms of divalent cations at mercury electrode, and ion transfer-adsorption (lA) were compared. It was suggested that a study of temperature dependence of the course of these reactions would be helpful to differentiate these two mechanisms. 

However, if the valences of the exchanging cations are equal, the selectivity coefficient or pseudo-equilibrium constant is not affected by concentration. As already mentioned, one isotherm corresponds to a specific temperature in the case of adsorption or ion exchange of equal valence ions, whereas additionally, the same normality is required for the existence of only one isotherm in the case of ion exchange of different valence ions, due to the concentration-valence effect (Helfferich, 1962). The determination of the true equilibrium constant should be based on the thermodynamic activities (activity coefficients) of the species rather than concentrations. It is clear that the difficulties in the determination of activity coefficients also complicate the determination of the true equilibrium constant (Culfaz and Yagiz, 2004). 

Iron molybdates, well known as selective methanol oxidation catalysts, are also active for the propene oxidation, but not particularly selective with respect to acrolein. Acetone is the chief product at low temperature (200°C), whereas carbon oxides, besides some acrolein, predominate at higher temperatures [182,257], Firsova et al. [112,113] report that adsorption of propene on iron molybdate (Fe/Me = 1/2) at 80—120°C causes cation reduction (Fe3+ -> Fe2+) as revealed by 7-resonance spectroscopy. Treatment with oxygen at 400°C could not effect reoxidation (in contrast to similarly reduced tin molybdate). The authors assume that this phenomenon is related to the low selectivity of iron molybdate. 

These observations suggest that the heterogeneous effect in the S02-modenite system represents an extreme case, so much so that chemisorp-tive bonds may be stipulated (probably between the S02 and the cations). These effects would, of course, involve energy emission and show up in the calorimetric measurements. However, the specificity of the adsorption would tend to show a relatively temperature-insensitive isotherm in the low-pressure region, thus rendering the isosteric techniques of obtaining heats of adsorption/chemisorption ineffective. 

Materials obtained from the LDH show a reduction in SSA with the temperature as reported in the literature. This reduction can be attributed to the crystallisation of the material [15,17]. However, post-treatment with mineral acid was an increase in the SSA for all temperatures. It is possible to attribute this increase to two combined effects, which can both increase the porosity of the materials, as well as yield more active adsorption sites (i) the elimination of ZnO and (ii) the elimination Zn(II) cations occupying octahedral sites in the spinel oxide structure. Even though the SSA had varied sensibly, the average pore size (APS) remained fairly constant with temperature. Acid treatment increases the APS value for all temperature tested, although the effect was very small (Figure 5b). Comparison of the materials obtained by the different synthesis methods showed that spinel oxides obtained from the LDH presented greater SSA values than those obtained by other methods, principally after the posttreatment with mineral acid. On other hand, the treatment with acid had little influence on the textural properties of the spinel oxides obtained by the other methods. 

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