Acidic and Basic Properties on Solid Surfaces

A complete description of acidic and basic properties on solid surfaces requires the determination of the acid and base strength, and of the amount and nature (Bronsted or Lewis type) of the acidic and basic sites. 

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Determination of Acidic and Basic Properties on Solid Surfaces... 

Beside its use for the direct characterization of solids, PL has been applied in combination with probe molecules, luminescent or not, to investigate the acidic and basic properties of oxide surfaces (Section 3). In relation to catalysis, the investigation of the formation of hydroxyl groups from hydrogen-containing reactants (such as hydrocarbons or alcohols) or reaction products (notably water) is an important step forward. Various types of hydroxyl group can be formed upon adsorption of such molecules. In the case of water adsorption on MgO, this can be illustrated schematically by the following reaction ... 

In addition to the acidic and basic properties mentioned previously, oxides and halides can possess redox properties. This is particularly true for solids containing transition metal ions because the interactions with probe molecules such as CO, H2, and O2 can lead to electron transfer from the surface to the adsorbed species and to the modification of the valence state of the metal centers. An important role in surface redox processes involving CO is played by the most reactive oxygen ions on the surface (e.g., those located at the most exposed positions such as corners), which can react with CO as follows ... 

For solid acids and bases, the acidic and basic properties can also be expressed by a similar equation, but it incorporates the concentrations of acidic or basic sites present on the surface instead of concentrations of hydrogen or hydroxyl ions ... 

The second approach, was proposed by Schultz and Lavielle. It is based on the Gutmann s acid-base theory. The authors elaborated a semiquantitative approach to characterize the acidic/basic properties of solid surfaces... 

The principle of this method is the same as that of gaseous base adsorption method (2.1.1.B) and all of the latter method can be applied. As sidsorbates, acidic molecules such as carbon dioxide, nitric oxide and phenol vapor have been used. The adsorption of phenol is not necessarily good for the measurement of basic property, because phenol is easily dissociated to adsorb on both acidic and basic sites and hence acidic property affects the adsorption of phenol. Nitric oxide is used for the measurement of unusually strong basic sites. The amount of carbon dioxide irreversibly adsorbed is a good measure of the amount of basic sites on solid surfaces. The TPD profiles of carbon dioxide desorbed from alkaline earth oxides are shown in Fig. 2.9. Since acidic carbon dioxide desorbs at higher temperature from stronger base sites. 

Nineteen years have passed since the monograph Solid Acids and Bases was published in 1970. During this period many new kinds of solid acids and bases have been found and synthesized. The surface properties (in particular, acidic and basic properties) and the structures of the new solids have been clarified by newly developed measurement methods using modern instruments and techniques. The characterized solid acids and bases have been applied as catalysts for diversified reactions, many good correlations being obtained between the acid-base properties and the catalytic activities or selectivities. Recently, acid-base bifunctional catalysis on solid surfaces is becoming an ever more important and intriguing field of study. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

The snrface acid-base properties of supported oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. As shown, acidic and basic sites are often present simultaneously on solid surfaces. The knowledge of the detailed amphoteric character of supported metal oxides is of extreme interest due to the possibility of using them as catalysts in different reactions in which acidity governs the reaction mechanism. 

Since solid acid catalysts are used extensively in chemical industry, particularly in the petroleum field, a reliable method for measuring the acidity of solids would be extremely useful. The main difficulty to start with is that the activity coefficients for solid species are unknown and thus no thermodynamic acidity function can be properly defined. On the other hand, because the solid by definition is heterogeneous, acidic and basic sites can coexist with variable strength. The surface area available for colorimetric determinations may have widely different acidic properties from the bulk material this is especially true for well-structured solids like zeolites. It is also not possible to establish a true acid-base equilibrium. 

The chemical behaviour of a given species strongly depends on the nature of the other molecules involved in the interaction. For a specific type of reaction, an appropriate model is needed to simulate the chemical environment of the species of interest. In the present work, the interest is focused on the initial response of the molecule to a particular type of chemical situation, independent of the value of those parameters that characterize one specific reaction. In other words, the intrinsic capabilities of the chemical species are studied and modelled as derivatives of the electronic properties with respect to an appropriate independent variable. For example, in those processes where charge transfer is involved (such as Lewis acidity and basicity, electrophile-nucleophile interactions and coordination compounds), the number of electrons must be an independent variable when a small molecule interacts with a very large counterpart (such as enzyme-substrate interaction and adsorption on solid surfaces), the chemical potential of the large partner will be imposed on the small molecule, and its number of electrons will not be independent. 

Acid-base properties of oxide surfaces are employed in many fields and their relationship with PZC has been often invoked. Adsorption and displacement of different organic molecules from gas phase was proposed as a tool to characterize acid-base properties of dry ZnO and MgO [341]. Hammet acidity functions were used as a measure of acid-base strength of oxides and some salts [342]. Acidity and basicity were determined by titration with 1-butylamine and trichloroacetic acid in benzene using indicators of different pAg. There is no simple correlation between these results and the PZC. Acid-base properties of surfaces have been derived from IR spectra of vapors of probe acids or bases, e.g. pyridine [343] adsorbed on these surfaces. The correlation between Gibbs energy of adsorption of organic solvents on oxides calculated from results obtained by means of inverse gas chromatography and the acceptor and donor ability of these solvents was too poor to use this method to characterize the donor-acceptor properties of the solids [344],... 

Finely divided particles in suspension can also interfere with colorimetric mea.surements if one of the indicator forms happens to be preferentially adsorbed. Lanthanum hydroxide is a very striking example of such interference. This compound is a strong base which is very slightly soluble in water. A saturated solution in water at 25° has a pH of 9.0. If the pH of a suspension (turbid solution) of the solid hydroxide is measured with thymol-phthalein, the result obtained is 10.5. The suspension is colored a dark blue, although thymolphthalein is colorless at pH 9.0. The precipitate settles after a time, leaving a colorless supernatant solution although the solid itself is dark blue. Because of the strong basic properties of solid lanthanum hydroxide, it forms on its surface a salt with the indicator acid. In other words, the adsorption of the colored indicator anion predominates, and the presence of the solid phase favors a displacement of the indicator equilibrium towards the alkaline form. Phenol-... 

The catalysts were characterized by using various techniques. X-ray diffraction (XRD) patterns were recorded on a Siemens D 500 diffractometer using CuKa radiation. The specific surface areas of the solids were determined by using the BET method on a Micromeritics ASAP 2000 analyser. Acid and basic sites were quantified from the retention isotherms for two different titrants (cyclohexylamine and phenol, of p/Ta 10.6 and 9.9, and L ,ax 226 and 271.6 nm, respectively) dissolved in cyclohexane. By using the Langmuir equation, the amount of titrant adsorbed in monolayer form, Xm, was obtained as a measure of the concentration of acid and basic sites [11]. Also, acid properties were assessed by temperature-programmed desorption of two probe molecules, that is, pyridine (pKa= 5.25) and cyclohexylamine. The composition of the catalysts was determined by energy dispersive X-ray analysis (EDAX) on a Jeol JSM-5400 instrument equipped with a Link ISI analyser and a Pentafet detector (Oxford). 

In considering solid surfaces, properties such as superficial area, porosity (number and mean diameter of pores), and the existence of acidic and/or basic (from both types Lewis and Bronsted) sites on the surfaces are of prominent importance to understand and predict the chemical behavior of such solids. 

A thermodynamic scale of surface acidity and basicity can be constructed by exploring the acid-base properties of numerous solids and comparing the heats of adsorption and the adsorption uptakes of gas-phase probe molecules (NH3, CO2, SO2). These solids, varying in their physical and chemical properties, have been selected in order to cover a wide range of acid-base behaviours representative of acidic, amphoteric and basic solids. They can be divided into three main groups according to their adsorption properties towards acidic probes (which interact with basic solids) or basic probe molecules (which adsorb on acidic solids). Amphoteric solids display an adsorption edacity towards both acidic and basic probe molecules. 

Without doubt, surface acidity and basicity are decisive for the properties and the catalytic performance of a solid. Consequendy, measuring the acidity and basicity of a given family of OH groups is of great importance. Below, we shall briefly consider the main principles of such measurements, thereby focusing on vibrational spectroscopy. Several monographs and review papers addressing this problem have recendy been pubfished (56,60,88,129-139). 

In their extensive work on the mechanochemical reactions involving hydroxide-oxide mixtures. Senna and co-workers (Liao Senna, 1992, 1993 Watanabe et al., 1995b, 1996, Avvakumov et al., 2001) showed that the mechanism in these mixtures is governed by an acid-base reaction between different hydroxyl groups on the solid surface. The driving force for these reactions is the acid-base potential, i.e., the difference in the acid-base properties between an acidic and basic surface -OH group, which is determined by the type of metal on which it is bound, and therefore, on the strength of the M-OH bond (M denotes the... 

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