Surface acidity studies

Recently, some techniques have been presented in the literature making use of volumetric titrations of surface sites in liquids, of different polar and protic characteristics, to determine the amount of acid sites and relevant acid strength effective acidity). Two different methods will be here discussed the first one is based on a pulse liquid chromatographic method (a dynamic method) [8] and the second one on a liquid recirculation chromatographic method (a Masi-static method) [9]. When surface acidity studies are concerned, the measurements may be performed in apolar, aprotic liquid (like cyclohexane), for the determination of the intrinsic acidity, or in several other liquids with polar/protic characteristics, for the determination of the effective acidity. Basic probes of different basicity (following the pK scale) may be used (e.g., 2-phenylethylamine, PEA, aniline, AN, pyridine, PY, etc.). Titration temperatures may be varied from room temperature (r.t.) up to the normal boiling point of the liquid used in order to calculate, from the collected isotherms of adsorption, the isosteric heats of adsorption which can be related to the acid strength of the surface sites [10, 11]. 

It was noted that the content of functional groups on the surface of studied A1,03 was 0,92-10 mol/g of acid character for (I), FOS-IO mol/g of basic character for (II). The total content of the groups of both types was 1,70-lO mol/g for (III). The absence of appreciable point deviations from a flat area of titration curves in all cases proves simultaneously charges neutralization character on the same adsoi ption centers and non-depending on their density. The isoelectric points of oxide surfaces have been detenuined from titration curves and have been confirmed by drift method. 

Spectral studies at low temperatures enable us to broaden the number of test molecules for surface acidic sites and besides ammonia pyridine and nitriles, to use CO, NO and that do not adsorb at 300 K. 

Surface science studies have generated much insight into how hydrocarbons react on the surfaces of platinum single crystals. We refer to Somorjai [G.A. Somor-jai. Introduction to Surface Chemistry and Catalysis (1994), Wiley, New York] for a detailed overview. Also, the reactions of hydrocarbons on acidic sites of alumina or on zeolites have been studied in great detail [H. van Bekkum, E.M. Flanigan and J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice (1991), Elsevier, Amsterdam],... 

Gas-phase methylation of catechol by methanol was studied on y -AI2O3 modified by the basic elements K, Li, Mg and Ca. Addition of 7.5 at.% Mg to y-AljOa was optimal and increased the 3-methyl catechol selectivity from 0.26 to 0.65. X-ray diffraction experiments showed the diffusion of Li and Mg cations into the y -AI2O3 bulk. This induces a change in the surface species (XPS data) and the surface acid-base properties (TPD experiments). Ca and K addition to y-alumina was ineffective due to formation of basic oxide layers on the sur ce. 

Sun SG, Cai WB, Wan LJ, Osawa M. 1999. Infrared absorption enhancement for CO adsorbed on Au films in perchloric acid solutions and effects of surface structure studied by cyclic voltammetry, scanning tunneling microscopy, and surface-enhanced IR spectroscopy. J Phys Chem B 103 2460-2466. 

One question addressed in the literature is the relationship between the angle of orientation of the adsorbed species within the monolayer and their amphiphilic character. The case of surfactants like fatty acids or phospholipids is deferred until Section VI, since the technique of choice is SFG in order to perform a surface vibrational study. Phenol deri-... 

Lercher, J.A., Gruendling, C. and Eder-Mirth, G. (1996) Infrared studies of the surface acidity of oxides and zeolites using adsorbed probe molecules, Catal. Today, 27, 353. 

Fluoride is also used as an anionic dopant. An early study demonstrated that F substitutes surface OH species leading to an increase in the degradation of phenol at least three times faster than an undoped sample [56]. Other anions are also effective as dopants in reducing the Ti02 bandgap. Chloride, for example, was shown to give active photocatalysts in the visible range thanks to the red shift in the absorption spectra (Eg = 3 eV) but also increased surface acidity [57]. 

Reviewed here are surface electrochemical studies of organic molecules adsorbed at well-defined Pt(lll) electrode surfaces from aqueous solution. Emphasis is placed upon studies of nicotinic acid (NA), pyridine (PYR), and nine related pyridine carboxylic acids. 

Adsorption of acetic acid on Pt(lll) surface was studied the surface concentration data were correlated with voltammetric profiles of the Pt(lll) electrode in perchloric acid electrolyte containing 0.5 mM of CHoCOOH. It is concluded that acetic acid adsorption is associative and occurs without a significant charge transfer across the interface. Instead, the recorded currents are due to adsorption/desorption processes of hydrogen, processes which are much better resolved on Pt(lll) than on polycrystalline platinum. A classification of adsorption processes on catalytic electrodes and atmospheric methods of preparation of single crystal electrodes are discussed. 

Parry, E.P. An infrared study of pyridine adsorbed on acidic solids. Characterization of surface acidity. J. Catal. 1963, 2, 371-379. 

Both acid- and base-promoted reactions may be affected by acidic surfaces and, hence, by the factors which influence the surface acidity. Kinetic evidence for increased Br nsted acidity at clay surfaces has been presented by McAuliffe and Coleman (80) who studied the hydrolysis of ethylacetate and the inversion of sucrose. They noted that potentionmetrie pH measurements did not explain the catalytically effective H+-concentration at the clay surface. 

Several studies have presented kinetic evidence that the BrjSn-sted surface acidity increases with decreasing moisture content. 

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