FTIR Studies of Mo/Al2O3-Based Catalysts

Abstract

Quantitative FTIR studies of the alumina surface hydroxyl groups of unpromoted and promoted Mo/Al2O3 catalysts have been used to get new insight into the structure/morphology of calcined and sulfided catalysts. The interaction of Mo with surface OH groups in the calcined state is in agreement with previous studies seen to involve all the different types of OH groups, but the extent of interaction and the resulting Mo structural changes depend on the nature of these groups. At low Mo concentration, the interaction involves mainly the most basic OH groups, and as a result the relative concentration of the remaining OH groups in calcined Mo/Al2O3 catalyst is quite different from that observed for the alumina support. For the present calcined catalysts, monolayer-type structures dominate up to 12% Mo. The concentration of the OH groups determined by IR rather than the surface area of the alumina has been found to determine the upper limit for formation of the monolayer structures. CO2 adsorption, which has previously been used to determine the coverage of the alumina surface by supported metals, was shown to have some inherent limitations in that too high coverages are typically obtained. Upon sulfiding, substantial regeneration of surface hydroxyl groups occurs involving predominantly the most basic OH groups. The resulting relative abundance of the various hydroxyl groups becomes quite similar to that observed for the alumina support. The results indicate that sulfiding leads to substantial rearrangement and lateral agglomeration of the supported Mo structures. However, for catalysts with up to 12% Mo, the monolayer dispersion is maintained and the MoS2 phase appears to be predominantly present as 2D single-slab structures oriented flat-wise on the alumina support (c-axis perpendicular to alumina surface). The decrease upon sulfiding in the apparent coverages of the alumina by Mo is explained by the strong preference for Mo to interact with certain OH sites in the calcined catalysts, rather than due to contraction of the Mo phase upon sulfiding as suggested in the literature. For unpromoted catalysts, single-slab structures appear to be stable even after sulfiding at 873 K.