R.A. van Santen

Surface composition and selectivity of alloy catalysts

Publisher Summary Alloying of a catalytically active metal with an inert component changes the selectivity in hydrocarbon reactions such that C–C bond fission is suppressed compared with C–H bond breaking. Upon alloying, the selectivity for cracking reactions decreases. This chapter shows that the surface composition of macroscopic alloys often differs from the bulk composition. Selectivity changes can be understood in terms of the primary ensemble effect—that is, the dilution of the metal surface with inactive atoms diminishes the probability for metal-adsorbate complexes containing several neighboring metal atoms as required for cracking. The concept of an ensemble of metal atoms, crucial to the understanding of selectivity patterns on alloys, implies that the metal atoms in the surface of the alloy keep their individuality and are only influenced by their immediate environment. The chapter discusses the secondary ensemble effect that ascribes changes in heat of chemisorption of multiply bonded atoms to a decrease in the coordination of these atoms to the surface metal atoms. This effect leads to a decrease in heat of adsorption upon alloying.

On the Mechanism of Ethylene Epoxidation

Abstract The epoxidation of olefins, while of considerable economic importance, forms a challenging problem to fundamental research in catalysis. When surveying the vast quantity of published material [1-6] two facts appear relevant.

Rigid ION and Shell Model Calculations of IR Spectra of Zeolitic Silica Polymorphs

Rigid ion and Shell model calculations of the frequencies and intensities of the infrared active modes of aluminiumfree sodalite and faujasite are presented. Results of Shell model calculations agree better with experiment than those of Rigid ion calculations. A good agreement of the frequencies in the bending and torsional mode regions is found. One concludes from comparing experimental and calculated frequencies in the stretching modes region that the short range potential for the Si-O stretch frequency is too weak. The improvement of the Shell model versus the Rigid ion model results can be ascribed to screening of the long range electrostatic interactions by polarization of the O 2- -anions.

15-O-03 - A theoretical study of the methylation of toluene by methanol over acid mordenite

Publisher Summary This chapter presents a theoretical study of the alkylation of toluene by methanol catalyzed over acid mordenite. Cluster density functional theory (DFT) as well as periodic structure DFT calculations have been performed to obtain full reaction energy diagrams of the elementary reaction steps leading to the formation of the three xylene isomers. The use of periodic structure calculations allows taking into account the framework electrostatic contributions and steric constraints that are important in zeolite catalysis.