Publisher Summary The properties of a surface are influenced by the surface groups and the knowledge of their existence and of their chemistry are important for many technological processes This chapter examines the surface compounds on carbon—microcrystalline carbon, graphite, and diamond—on silica, on titania, and on alumina and silica–alumina Most important and best known among the surface compounds of carbon are those with oxygen and with sulfur Thus, two kinds of surface oxides are known Basic surface oxides are formed always when a carbon surface is freed from all surface compounds by heating in a vacuum or in an inert atmosphere and comes into contact with oxygen only after cooling to low temperatures Acidic surface oxides are formed when carbon is treated with oxygen a temperatures near its ignition point Unambiguous identification of carboxyl groups has been achieved by two reactions of the acyl chlorides: Friedel–Crafts reaction and Schmidt rearrangement

Chemical Identification of Surface Groups

A new method of theoretical analysis for temperature-programmed desorption (TPD) of ammonia to determine the acid amount and strength and its distribution from a one-time experiment is proposed on the basis of the equilibrium between gaseous and adsorbed ammonia, i.e., free readsorption of ammonia. The entropy change was assumed to consist of the constant phase-transformation term and the gaseous mixing term as a function of gaseous concentration of ammonia. The enthalpy change, namely adsorption heat, was assumed to have several kilojoules per mole of the distribution. Thus a simulated TPD curve could be fitted well with the experimental data observed on mordenite and ZSM-5 zeolites. From the parameter set that gave the best fitted curve, the acidic properties of zeolite were determined. The determined acid amount was close to the difference between the aluminum and sodium contents, [Al] − [Na], in most cases. This confirms a simple principle that one acid site is generated by isomorphous substitution of...

Determination of the Acidic Properties of Zeolite by Theoretical Analysis of Temperature-Programmed Desorption of Ammonia Based on Adsorption Equilibrium

Publisher Summary This chapter discusses the specific poisoning of a catalyst surface. A probe molecule other than the reactant is preadsorbed as a poison and its effect on the catalytic activity studied. From the nature of interaction of this probe molecule with the surface sites one may obtain information regarding the chemical nature of these sites and from the number of poisoning molecules necessary to bring the activity to zero one may estimate an upper limit of the active site density. Besides the obvious difficulty to discriminate between surface sites that purely absorb the probe molecule and those that are involved in the catalytic process as active sites, there are a number of other limitations making it difficult to obtain clear-cut information on the nature of active sites. The chapter describes the proposals that are indispensable for a proper application of this method and the precautions that have to be kept in mind while drawing conclusions from specific poisoning experiments. The chapter illustrates two examples—namely, the dehydration of alcohols and the isomerization of olefins on alumina.

Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces

The acid properties of H-ZSM-5 zeolites with different SiO2/Al2O3 ratios (30, 50, 80, 150, 280 and 1000) were examined by means of the temperature programmed desorption of ammonia (NH3-TPD). Different pretreatments together with a comparison of different curve-fitting methods after desorption of ammonia were used to differentiate between different adsorption sites. 27Al-MAS-NMR spectroscopy was applied to distinguish the framework (AlF) from the extra-framework aluminum (AlEF). Both techniques reveal that the concentration of Bronsted acid sites is lower than expected when considering the aluminum content. The strength of the Bronsted acid sites, determined by a theoretical model assuming the free readsorption of ammonia, is found to be in the range of ΔH = 129–161 kJ mol−1 and shows variations in the acid strength to be less than σ = 12 kJ mol−1.

The acid properties of H-ZSM-5 as studied by NH3-TPD and 27Al-MAS-NMR spectroscopy

Nearly all atoms contributing to the local structure of Br⊘nsted acid sites in zeolites exhibit isotopes accessible for multinuclear solid-state nuclear magnetic resonance (NMR) investigations. Therefore, in the last 15 years, NMR spectroscopy has found numerous applications for the determination of the types of hydroxyl proton in zeolites, of their concentration, accessibility, and mobility, and for the characterization of their acid strength and local structure. It allows the study of the role of hydroxyl groups in the formation of adsorbate complexes and in heterogeneously catalyzed reactions. Meanwhile, NMR spectroscopy belongs to the most powerful techniques for the characterization of Br⊘nsted acid sites in zeolites and related materials. The basis of this success is the invention of new sample preparation techniques, external magnetic fields with high-flux densities, effective line-narrowing methods, and new two-dimensional experiments, making the detection of highly resolved solid-state N...

Br⊘nsted Acid Sites in Zeolites Characterized by Multinuclear Solid-State NMR Spectroscopy

The measurement of catalyst acidity using indicators forming stable surface carbonium ions

Proton acids as electron acceptors on aluminosilicates

The effect of ammonia adsorption on the acidity of silica-alumina and alumina catalysts: The nature of the acid sites☆

The effect of faujasite cations on acid sites

On the generation of carbonium ions by electrophilic sites as proposed by porter and hall

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