Effect of the preparation method on the reducibility of molybdena-alumina catalysts

Two series of molybdena-alumina catalysts were prepared. The first series was prepared by the incipient wetness procedure (1.7-8.8 wt.% Mo) (series IW). A second series was obtained using the equilibrium adsorption (1.3-8.2 wt.% Mo) (series EA) method. X-ray photoelectron spectroscopy (XPS) was used to study the distribution of Mo oxidation states. The average Mo oxidation state estimated from XPS agreed with that obtained by measuring the O-2 consumption on reoxidation, for a given catalyst. The chemisorption of oxygen at -78 degrees C and the isomerization of 1-butene were performed as probes for assaying the number of coordinatively unsaturated sites created by the reduction process. These studies were conducted in order to study possible changes in Mo speciation or distribution, relevant to catalytic reactions, which may occur as a result of the specific features of the preparation method (EA vs. IW). The results showed that for both series of solids, the reducibility of the molybdenum species increases as the metal loading increases, in agreement with the literature. For catalysts with similar loading, the IW series showed a higher degree of reduction with a concomitant increase in the relative abundance of Mo(IV) species attributed to paired double-bonded Mo(IV) entities (as in MoO2). The reducibility data were consistent with the catalytic results and the oxygen chemisorption results. IW preparations (with similar Mo loading) were more active towards the isomerization of 1-butene and chemisorbed larger amounts of oxygen than their EA counterparts. The differences in reducibility can be ascribed to a nonuniform repartition of the molybdenum species between the external and internal surfaces of the alumina, for the IW preparations. Another possible explanation may stem from a decoration effect of the molybdenum species by Al(III) ions. The latter may arise from dissolution of the alumina, which is favored on the EA series due to the long contact time between the solution of the Mo promoter and the support. (C) 1999 Elsevier Science B.V. All rights reserved.