Acid-base surface sites, adsorption

Acid-base surface sites, adsorption of organic molecules, 23... 

Adsorption of aniline on Cui xZnxFc204 at <100°C indicates a simple molecular adsorption through N-atom on an acid-base pair site. However, above 100°C, N-H bond dissociates and aniline chemisorbed strongly on the catalyst surface. Chemisorbed aniline... 

The pyridone surface species has a C=0 stretching band at 1634 cm-1,3 Hydrogen gas has been detected by mass spectrometry (210), and the formation of this surface compound has been established by chemical methods by Boehm (215). This surface reaction points to the existence of strongly basic OH" ions held to certain sites on alumina surfaces, their number being of the order of magnitude of 1013/cm2 (121). Additional evidence for the existence of these reactive and strongly basic OH" ions on aluminas comes from surface reactions observed on adsorption of nitriles and ketones (see Section IV.F) and of carbon dioxide (see Section IV.G). These reactions may, thus, be valuable for the detection of the corresponding sites that most probably have to be considered as acid-base pair sites. 

Pulse chemisorption in flow Active sites surface area Degree of dispersion Determination of strong gas-solid interaction Acid/base surface properties Isostheric heat of adsorption... 

The acid-base interactions in adsorption of polymers onto surfaces such as fillers involve the acidic or basic surface sites of the adsorbent, the acidic or basic sites of the polymer, and also the acidic or basic sites of the solvent, if present. The acid-base contribution to the work of adhesion, Wab, can be related to the interface population of acid-base pairs, and the heat of acid base interaction ... 

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. 

Hydroxylated groups, which arc the cause of the surface charge (see Chapter 6), may al.so act as coordination sites for di.s.solved cations, or may be substituted by anions (surface coordination). They may also act as nucleation Sites for a solid phase in the case of surface precipitation. Therefore, several possible mechanisms should be considered in the overall phenomena of adsorption, and most of them may be described in terms of the classical concepts of coordination chemistry. However, adsorption exhibits some specific characteristics because one of the partners is a solid with structural and physical properties that may play an important role. Although adsorption often involves only the surface of the particles, if the solid exhibits appreciable ionic mobility (in the case of an ion-exchange material), or electronic mobility (for a mixed-valence material), acid-base surface reactions may trigger various phenomena in the core of the particles, reactions which in turn may cause ionic and/or electron transfers through the. solid-.solution interface. 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

Ammonia and pyridine are frequently used as probe molecules for the characterization of acidic surfaces, but they also adsorb on strongly basic sites. Tsyganenko et al. (54) proposed various species resulting from NH3 adsorption on basic solids (Scheme 1). The formation of species I corresponds to hydrogen bonding to a basic surface oxygen, and species II, formed by dissociation to give NH2 and hydroxyl species, involves an acid-base site. Such adsorption requires... 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

X ray photoelectron spectroscopy (XPS) is powerful in identifying species present at the surface/interface and atoms or functional groups involved in acid-base interactions [116]. Since XPS measures the kinetic energy of photoelectrons emitted from the core levels of surface atoms upon X ray irradiation of the uppermost atomic layers, it can be used to characterize surface acid sites, in combination with base probe molecules adsorption. 

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. 

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 . 

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