Probing Surface Basicity

The number of acidic probes able to cover a wide range of strength is rather small. The ideal probe molecule should be specific to basic sites and should not be amphoteric. For example CO2 (pffi = 6.37) is commonly chosen to characterize the basicity of solids, but it may be either adsorbed on cations or physisorbed, or may react with hydroxyls and oxide ions to form carbonated species. 

CO2 is a poor donor but a good electron acceptor. Owing to its acidic character, it is frequently used to probe the basic properties of solid surfaces. IR evidence concerning the formation of carbonate-like species of different configurations has been reported for metal oxides [31], which accounts for the heterogeneity of the surface revealed by micro-calorimetric measurements. The possibility that CO2 could behave as a base and interact with Lewis acid sites should also be considered. However, these sites would have to be very strong Lewis acid sites and this particular adsorption mode of the CO2 molecule should be very weak and can usually be neglected [32]. 

The same 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. 

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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... 

Concluding this section, two interesting variants of the STM should be addressed. The spin-polarized STM (SPSTM), which works with a ferromagnetic tip, can be used to probe surface magnetism with high resolution [5.47, 5.48]. Other modifications of the STM involve electromagnetic radiation, whereby two basic concepts can... 

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

Lavalley, J.C. (1996) Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules, Catal. Today, 27, 377. 

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]. 

FTIR and NMR spectroscopies have been used to study the surface basicity of solids by adsorbing different probe molecules such as pyrrole, but-l-yne, acetonitrile, chloroform, CO, C02 and thiols.[19,22] Limitations arise from the formation of various adsorbate structures leading to complicated patterns, or complete dissociation of the molecule with the disappearance of the signal, or polymerization of the molecule upon heating. 

Nesterenko, N., Lima, E., Graffin, P., de Menorval, L. C., Lasperas, M., Tichit, D. and Lajula, L. Probing the basicity of oxide surfaces by FTIR spectroscopy of isocyanic acid generated in situ by thermal decomposition of nitromethane. New J. Chem., 1999, 23, 6654)66. 

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. 

Table 3.12 Molecular probes applied for surface basicity characterization.

Lavalley has recently compared the suitability of various IR-probe molecules for the surface basicity of oxides [55] and found that pyrrole (PYH) which would decompose to pyrrolate ions (PY ) over stronger bases is suitable for zeolites [56]. 

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. 

As IR-spectroscopic probes for basic centers in various catalysts, carbon dioxide, pyrrole, or chloroform were used [7-9]. Howerer, in most cases, these molecules strongly interacted with the surface and formed strong complexes that could be destroged only at elevated temperatures, which sometimes causes irreversible changes at the surface. 

By analogy to the use of small bases (amines, phosphines, phosphine oxides, NiO) as probes of acidic sites on surfaces, one might expect that small acidic molecules could serve as probes of basic sites on surfaces. For this purpose BR compounds show some promise, based on the "B nuclide (/ = 3/2, 80.4% natural abundance) (P. Marchetti and G. E. Maciel, unpublished results). 

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. 

Similarly, the use of acidic molecules allows the probing of surface basicity, perhaps with less success. We have reported a great deal of data concerning the characterisation of acidity and basicity of catalysts in recent review papers. Acidity and basicity have a complex relationship with catalytic activity in oxidation, depending also upon the natme of the reaction considered. 

Alternatively, deprotonation enthalpies can be evaluated from probe adsorption calorimetric data or from temperature programmed desorption (TPD) measurements. The strengths of surface Lewis acid sites and of surface basic sites can also be evaluated, in principle, by the heat of probes adsorption or desorption. In all cases, however, probes adsorption on solids can result in multiple interactions for example, van der Waals interactions can be superimposed to true acid-base interactions, which can also be multiple and finally give rise to some kind of solvation effects, in particular, in the zeolite cavities [23-25]. Thus, the pure acidity/basicity... 

Molecular Probes Applied for Surface Basicity Characterization... 

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