Understanding Ni Promotion of MoS2/-Al2O3 and its Implications for the Hydrogenation of Phenanthrene

The chemical composition and structure of NiMo sulfides supported on -Al2O3 and its properties for hydrogenation of polyaromatic compounds is explored. The presence of Ni favors the formation of disperse octahedrally coordinated Mo in the oxide precursors and facilitates its reduction during sulfidation. This decreases the particle size of MoS2 (measured by transmission electron microscopy) and increases the concentration of active sites up to a Ni/(Mo+Ni) atomic ratio of 0.33. At higher Ni loadings, the size of the MoS2 did not decrease further, although the concentration of adsorption sites and accessible Ni atoms decreased. This is attributed to the formation of NiSx clusters at the edges of MoS2. Nickel also interacts with the support, forming separated NiSx clusters, and is partially incorporated into the -Al2O3, forming a Ni-spinel. The hydrogenation of phenanthrene follows two pathways; by adding one or two H-2 molecules, 9,10-dihydrophenanthrene or 1,2,3,4-tetrahydrophenanthrene are formed as primary products. Only symmetric hydrogenation, leading to 9,10-dihydrophenanthrene, was observed on unpromoted MoS2/-Al2O3. In contrast, symmetric and deep hydrogenation (leading to 9,10-dihydrophenanthrene and 1,2,3,4-tetrahydrophenanthrene, respectively) occur with similar selectivity on Ni-promoted MoS2/-Al2O3. The rates of both pathways increase linearly with the concentration of Ni atoms in the catalyst. The higher rates for symmetric hydrogenation are attributed to increasing concentrations of reactive species at the surface, and deep hydrogenation is concluded to be catalyzed by Ni at the edge of MoS2 slabs.