Garmt R. Meima

The effect of sub-surface oxygen on the catalytic properties of silver catalysts

Abstract To elucidate the influence of the pretreatment of silver catalysts on their catalytic activity, the interaction of oxygen and hydrogen with reduced and pre-oxidized samples was studied with chemisorption experiments, TPD measurements and electron microscopy. The oxidation of carbon monoxide with molecular oxygen was used as a test reaction.

The interaction of oxygen with isolated silver particles of ca. 70 nm supported on α-alumina. Part 1.—Oxygen sorption and temperature-programmed desorption measurements

The interaction of oxygen with a silver catalyst containing isolated silver particles of ca. 70 nm supported on α- Al2O3 has been studied using (combined) volumetric adsorption and temperature-programmed desorption measurements after various pretreatments. The amount of oxygen taken up is greatly influenced by the pretreatment. Considerable amounts of oxygen are able to penetrate into the silver lattice. Impurities present in or on the silver particles strongly affect the amount of oxygen taken up at 443 K. It is argued that penetrated oxygen can facilitate the reorganization of the silver surface giving rise to low-index facets.

CUMENE PRODUCTION BASED ON MODIFIED MORDENITE CATALYSTS

A novel mordenite catalyzed cumene process containing a separate alkylation and transalkylation step is discussed. The overall yield is higher than the traditional SPA (solid phosphoric acid) catalyzed route. The long lifetime of the catalyst and a high selectivity are associated with the unique characteristics of the modified mordenite catalysts.

The interaction of oxygen with isolated silver particles of ca. 70 nm supported on α-alumina. Part 2.—CO oxidation measurements

The oxidation of carbon monoxide with molecular oxygen over a catalyst containing isolated silver particles of ca. 70 nm, supported on α-Al2O3 has been studied after various pretreatment. Both the extent of oxygen sorption and the activity were greatly affected by the pretreatment. A rise in activity was observed after reduction and consecutive cooling in hydrogen, whereas a drop followed an oxidizing treatment. A severe deactivation of the catalyst occurred after reduction at 673 K followed by cooling in nitrogen. The differences in activity due to the various pretreatments are discussed in terms of changes in the surface structure of the silver particles.

Influence of pretreatment on the properties of Ag/α-Al2O3 catalysts containing large (± 1 µm) pure and Cs-promoted silver particles. Part 2.—CO oxidation measurements

The catalytic activity for the oxidation of CO with molecular oxygen of silver catalysts containing large silver particles (ca. 1 µm) has been studied. Both pure and caesium-promoted silver catalyst were investigated. The various pretreatments to which the catalysts were subjected, i.e. reduction or oxidation at different temperatures, influenced their activity. The promoted catalyst proved to be somewhat less susceptible to the various pretreatments.Arrhenius plots of the catalysts often exhibited a sharp break, giving rise to two sets of kinetic parameters. An apparent activation energy of 40 kJ mol–1 was found at low temperatures, whereas an activation energy of 60 kJ mol–1 was observed at high temperatures. It is argued that the activation energy of 40 kJ mol–1 can be ascribed to the reaction of CO with oxygen atoms in the neighbourhood of defects and the activation energy of 60 kJ mol– to the reaction of CO with oxygen penetrated into the bulk at the same sites. The variation in activity could be attributed to changes in the pre-exponential factor.A separate discussion is devoted to a comparison of the present results with the findings on silver catalysts with a smaller mean particle size we reported on previously.

Influence of pretreatment on the properties of Ag/α–Al2O3 catalysts containing large(± 1 µm) pure and Cs-promoted silver particles. Part 1.—Extent of oxygen and hydrogen sorption and TPD studies

In order to elucidate the influence of the pretreatment on the (catalytic) properties of silver catalysts containing large silver particles (± 1 µ m), the interaction of oxygen and hydrogen with reduced and pre-oxidized samples has been investigated. The effect of the presence of caesium on the silver surface has also been investigated. Volumetric (ad)sorption of oxygen and hydrogen, TPD experiments, combined with chemisorption measurements, and electron microscopy were used as characterization techniques.The combined results indicate that the penetration of oxygen into these large silver particles does not occur to any significant extent. When the catalysts are saturated with oxygen, only ca. 40% of the monolayer coverage of oxygen atoms is situated in the subsurface layers.This type of pretreatment exhibited small, but distinct, effects on the uptake of oxygen and hydrogen at 443 K. Higher uptake of oxygen was observed after reduction of the catalyst at more elevated temperatures. The presence of caesium lessens considerably the variation in the uptake.