Catalysis by manganese oxide monolayers part 1: alumina and magnesia supports

Abstract

Manganese oxide deposited on alumina and magnesia by different preparation methods has been studied with respect to its surface structure and reactivity. The controlled deposition of mono-atomic layers is effected by a grafting reaction between surface hydroxyl groups and manganese ethoxide. Repeated grafting allows multiple layers to be built up in a controlled manner. The properties of these catalysts differ from those prepared by conventional wet impregnation techniques and mechanical mixing. Test reactions involving carbon monoxide oxidation and isopropanol decomposition reveal significant differences in the activity and selectivity of the catalysts prepared by grafting, impregnation and mechanical mixing. The manganese oxide grafted to alumina shows bulk-like properties after a full monolayer is built up. XPS studies show that the grafting reaction leads to continuous films of the manganate covering the entire surface with monolayer thickness, whereas impregnation most likely results in small crystallites which cover only part of the surface. The calcination temperature is an important parameter determining the oxidation state of the manganese and the crystallinity of the oxide. Ammonia and isopropanol temperature-programmed desorption indicates that the acid/base and redox properties of the catalysts are considerably modified by a strong interaction between the support and the overlayer.