STRONG SUPPORT INTERACTION BETWEEN COPPER OXIDE AND HIGH-SURFACE AREA SUPPORTS : A CATALYTIC AND MICROSTRUCTURE STUDY

Abstract

The research for this thesis is to compare the catalytic activity of "monolayer"* catalysts with that of traditionally prepared highly dispersed systems. Active material in the "monolayer" is in direct contact with the support. By using different supports, a support influence is to be investigated. "Monolayer" catalysts are prepared by a specific grafting reaction between a metal alkoxide with surface hydroxyl groups of the support in a non-aqueous solvent. The thickness of the deposit is limited to one layer since reaction is only possible between the alkoxide and surface hydroxyl groups. A single grafting step will therefore generally lead to sub-monolayer coverage because the bulky precursor occupies much more space at the surface than the CuO group. After a layer has been grafted onto the surface, the organic residue is removed by heat treatment. Surface hydroxyl groups are regenerated during this step. Thus, by repeated grafting steps successively thicker layers have been prepared. Other preparation methods such as impregnation and mechanically mixing are also carried out to compare with the grafting method. Copper oxide was chosen as the catalytically active material. CuO supported on alumina, zirconia, magnesia, silica and titania was prepared by grafting from Cu(OCHMeCH2NMe2)2 precursor in hexane. The structure of the catalysts was characterized by AAS, BET surface area, XRD, XPS, IR, TPR and NH3, TPD. Catalytic activity was tested using N2O decomposition and CO oxidation as model reactions to investigate the influence of the support on catalytic activity. It was found that the grafting reaction resulted in deposition of copper oxide on the surface of the support. Each grafting step deposited about 1/8 to 1/3 of a full monolayer. The reason is that the bulky ligand blocks all possible adsorption sites within a certain radius from the site where a grafting reaction took place. In alumina and zirconia systems, the grafting reaction from a metal alkoxide precursor made it easy to synthesize overlayers of copper oxide with rnonolayer dispersion. XPS showed that CuO was deposited as one monolayer on alumina after five grafting steps and on zirconia after four steps. This was confirmed by TPR and XRD results. The catalytic activity after the first grafting step was very low. This can be understood if the catalytic reaction requires a site consisting of at least a pair of copper ions at the surface. Once pair sites have formed, the activity increases dramatically. The activity in the first monolayer is strongly influenced by the support, and is far lower on alumina than on zirconia. The activity of grafted, impregnated and mechanically mixed catalysts decreased in following order: grafted > impregnated >> mechanically mixed catalysts. This indicates wet impregnation leads also to highly dispersed systems, but not to a complete coverage of the surface with the active phase. And mechanically mixing followed by calcination did not yield a highly dispersed copper oxide system. In the magnesia system, the grafting method did not result in CuO dispersion as monolayer. CuO seemed to go sub-surface during the calcination step. The grafted catalysts and catalysts prepared by the conventional wet impregnation method showed very similar properties.