Surface basicity characterization

Table 3.12 Molecular probes applied for surface basicity characterization.

Molecular Probes Applied for Surface Basicity Characterization... 

In the present work low temperature adsoi ption of fluoroform and CO, were used to characterize surface basicity of silica, both pure and exposed to bases. It was found that adsorption of deuterated ammonia results in appearance of a new CH stretching vibration band of adsorbed CHF, with the position typical of strong basic sites, absent on the surface of pure silica. Low-frequency shift of mode of adsorbed CO, supports the conclusion about such basicity induced by the presence of H-bonded bases. 

Lavalley, J.C. (1991) Use of probe molecules for the characterization of the surface basicity of divided metal oxides, Trends Phys. Chem., 2, 305. 

For basicity measurements, the number of acidic probes able to cover a wide range of strength is rather small [166]. The most common acidic probe molecules used are CO2 (p/fa = 6.37) and SO2 (p/fa = 1.89). Carboxylic acids such as acetic acid can also be used but dimmers can be formed, particularly at high coverage. Pyrrole may also be used, particularly at low adsorption temperature, but has sometimes shown some amphoteric character [103]. Hexafluoroisopropanol has also been used to characterize the surface basicity of some solids [145]. 

The Br0nsted basicity of a surface is related to its deprotonation ability, which can be probed by investigating the dissociative adsorption of protic molecules (Bailly et al., 2005a Chizallet et al., 2006). The 0Lc2 0Lc H transformation thus induced can be followed by PL, which is one of the few techniques able to simultaneously characterize oxide ions and their protonated forms. The same kind of equilibrium is also involved when a hydroxylated surface is undergoing thermal pretreatment (Section 2.1). PL is thus an interesting tool to evaluate the surface basic properties of alkaline earth oxides. 

In contrast to acidity characterization with basic probes, the use of acidic molecules to probe surface basicity is far less satisfactory. In fact, all acidic (or electrophilic) molecules (Table 3.12) also contain accessible nucleophilic (basic) atoms. It seems impossible to find a molecule that actually only interacts specifically with basic sites. On the other hand, metal oxides that display significant surface basicity... 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

Characterization data evidenced that the prepared NiMo04 is stoichiometric and that Cs is deposited only on the catalyst surface (atomic ratio Cs/Mo = 0.03) not affecting the molybdate structure. However, Cs doping causes a decrease of the catalyst surface area Sbet (NiMo04) = 44.1 m /g and Sbet (3% Cs-NiMo04) = 28.7 mVg. Moreover, the promoted sample exhibits a higher surface basicity, electrical conductivity and also a larger resistance to reduction [4,5,12]. 

Finally, a more precise method for the characterization of the acidity and the basicity of the active carbon sites is needed. TPD of weak acids and bases may contribute. Basidty is probably more important, since we have shown that i) global surface basicity is more affected by nitric acid treatment than surface acidity, and ii) the amount of the different surface acidic sites is not directly related to the catalytic effects. 

SmP04, Cs2HP04 was also characterized by the MBOH test. Results reported in Table 5 show that the presence of the cesium salts exalt the surface basicity. The addition of the cesium salts induces a totally basic like behaviour of this catalyst. The observed difference in activity should be interpreted with some caution due to the high basic activity of cesium oxide, the presence of which, even in small quantities, can not be excluded. 

For basicity measurements, the number of acidic probes able to cover a wide range of strength is rather small [41]. Moreover, a difficulty stems from the fact that some acidic probe molecules may interact simultaneously wifri cations (such as Na ). The ideal probe molecule should be specific to basic sites and should not be amphoteric. It should not interact with unwanted types of basic sites or give rise to chemical reactions [41]. For instance CO2 (pKa = 6.37) is a suitable probe to determine and characterize, simultaneously, the surface basicity as well as the Lewis acidity of acidic metal systems. It can form caibonate-like species on the former sites, whereas it can be molecularly coordinated in a linear form at the latter sites [42]. Moreover, the energetic features of the adsorption of CO2 on various molecular sieves, over a large domain of temperature and pressure, can provide interesting information on the nature of the adsorbate-adsorbent interactions [43]. Similar problems may arise when using SO2 as an acidic probe, despite the fact that SO2 (pKa = 1.89) is more acidic than CO2 and, thus, more likely to probe the total basicity of the surface. 

The surface acidity and basicity of iron oxide (a-Fe203) catalysts, pure and surface-doped with variable amoimts of sulfate groups, have been characterized by microcalorimetry of CO adsorption at room temperature. An appreciable decrease of Q with surface coverage was detected [108]. The surface basicity of iron oxide, tested by adsorption of CO2 and monitored by FTIR, which revealed the formation of carbonate-like surface species, is gradually decreased by sulfates, but not suppressed. The vacuum reducibility of iron oxide, which can be spectroscopically evidenced both by a colour change (UV-VIS spectra) and by the formation upon CO adsorption of surface carbonyl-like species with a... 

Based on the results of the current study, the following conclusions have been drawn. Processes for the selective surface phosphonylation of polyethylene and nylon-12 and the subsequent conversion to other biologically relevant moieties have b n developed and basic characterization of prevailing functionalities was achieved. Immobilization of calcium ions on a modified polyethylene surface is feasible and has been achieved by incubation of tiie modified surface in a saturated calcium phosphate solution. An exposure to fibroblasts in a typical culture medium to a phosphonate bearing surface decreased cell attachment but elicited no discernible toxicity. 

Lara and Schreiber assumed that Eqs. 16 and 17 describe the acidic and the basic character of the surface, respectively. The donor number, DNs, is a measure of the electron-donor (basicity) ability of the molecular probe , while the acceptor number, ANs, is a measure of the electron-acceptor (acidity) ability of molecular probe Based on the values of ANg and DNs, surfaces are characterized as follows ... 

Characterization studies determined that the transition aluminas also present significant surface basicity. In fact, CO2 adsorption sites on 3/-AI2O3 is quite strong although the density of sites for strong adsorption (AH- 50-180 kJ/mol) is... 

---