Surface acid sites, characterization

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. 

Another possibility for characterizing zeolite acid sites is the adsorption of basic probe molecules and subsequent spectroscopic investigation of the adsorbed species. Phosphines or phosphine oxides have been quite attractive candidates due to the high chemical shift sensitivity of 31P, when surface interactions take place [218-222]. This allows one to obtain information on the intrinsic accessibility and acidity behavior, as well as the existence of different sites in zeolite catalysts. 

Characterization of Surface Acid Sites. Characterization of surface acidity was one of the first successful applications of infrared in catalyst studies (1). Catalyst acidity is typically a function of surface hydration, and water is relatively easy to remove. 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

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. 

Temperature-prograimned reduction, oxidation and desorption (TPR, TPO, TPD), belong probably to the most widely used in situ techiuques for the characterization of oxidation catalysts and are discussed in more detail in Section 19.4. While TPD (with ammonia as the probe molecule) is frequently used to examine surface acid sites, TPR and TPO (with H2 or O2, respectively) provide information on the redox properties of oxide catalysts being crucial for their performance in catalytic oxidation reactions. Important information on reaction mechanisms can be obtained when the catalysts are heated in the presence of reactants combined with mass spectrometric product analysis. This is called temperature-programmed reaction spectroscopy (TPRS). As far as reaction mechanisms and kinetics are concerned, transient techniques which reflect the response of the catalytic system to a sudden change of reactant are inevitable tools. Two such techiuques, namely the temporal analysis of products (TAP) reactor and steady-state isotopic transient kinetic analysis (SSITKA) will be described in more detail in Section 19.5. 

Zeolites as cracking catalysts are characterized hy higher activity and better selectivity toward middle distillates than amorphous silica-alumina catalysts. This is attrihuted to a greater acid sites density and a higher adsorption power for the reactants on the catalyst surface. 

Acid catalysis by titanium silicate molecular sieves another area characterized by recent major progress. Whereas only two categories of acid-catalyzed reactions (the Beckmann rearrangement and MTBE synthesis) were included in the review by Notari in 1996 (33), the list has grown significantly since then. In view of the presence of weak Lewis acid sites on the surfaces of these catalysts, they can be used for reactions that require such weak acidity. 

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. 

Figure 4.35 IR spectrum of pyridine adsorbed on acidic sites of H-Y zeolite. (Reprinted from Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis, Vol. 58, J.H.C. van Hooff, J.W. Roelofsen, Techniques of Zeolite Characterization, pp. 241-283. Copyright 1991. With permission from Elsevier.)...

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