ALUMINA-SUPPORTED MANGANESE OXIDE CATALYSTS .1. CHARACTERIZATION - EFFECT OF PRECURSOR AND LOADING

Alumina-supported manganese oxide catalysts were prepared by two different routes, from Mn acetate and from Mn nitrate, and characterized by temperature-programmed reduction (TPR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy. The characterization studies clearly showed that the two different preparations result in significantly different supported manganese oxide phases. The nitrate precursor resulted primarily in manganese oxide [Mn(III) and Mn(IV)] microcrystals 2-4 nn in size deposited mainly at the outer region of the alumina particles, while the acetate precursor resulted mainly in a highly dispersed surface manganese oxide phase [Mn(III)], homogeneously distributed throughout the alumina particles. These different states of the manganese oxide are reflected in the reduction properties of the alumina-supported manganese oxide phases (peak positions, sharpness, and extent of reduction). The origin of these differences is related, on the one hand, to the presence of the nitrate, which leads to agglomeration and oxidation of the Mn, and on the other hand, to the ion-exchange mechanism of Mn with surface hydroxyl groups in the acetate preparation route. (C) 1994 Academic Press, Inc.