Impact of Ni promotion on the hydrogenation pathways of phenanthrene on MoS₂/Γ-Al₂O₃

The reaction network and elementary steps of the hydrogenation of phenanthrene are explored on parent and Ni-promoted MoS₂/γ-Al₂O₃. Two pathways were identified, i.e., Path 1: Phenanthrene ⇌ 9,10-dihydrophenanthrene (DiHPhe) → 1,2,3,4,4a,9,10,10a-octahydro-phenanthrene (asymOHPhe), and Path 2: Phenanthrene → 1,2,3,4-tetrahydrophenanthrene (TetHPhe) → 1,2,3,4,5,6,7,8-octahydrophenanthrene. The steps TetHPhe → asymOHPhe (hydrogenation), and DiHPhe → TetHPhe (hydrogenation-isomerization) become notable at phenanthrene conversions above 20%. The reaction preferentially proceeds via Path 1 (90% selectivity) on MoS₂/Al₂O₃. Ni promotion (Ni/(Ni + Mo) molar ratio of 0.3 at the edges on MoS₂) increases the hydrogenation activity per active edge twofold and leads to 50% selectivity to both pathways. The reaction orders in H₂ vary from ∼0.8 on MoS₂/Al₂O₃ to ∼1.2 on Ni-MoS₂/Al₂O₃, whereas the reaction orders in phenanthrene (∼0.6) hardly depend on Ni promotion. The reaction orders in H₂S are zero on MoS₂/Al₂O₃ and slightly negative on Ni-MoS₂/Al₂O₃. DFT calculations indicate that phenanthrene is preferentially adsorbed parallel to the basal planes, while H is located at the edges perpendicular to the basal planes. Theory also suggests that Ni atoms, incorporated preferentially on the S-edges, increase the stability of hydrogenated intermediates. Hydrogenation of phenanthrene proceeds through quasi-equilibrated adsorption of the reactants followed by consecutive addition of hydrogen pairs to the adsorbed hydrocarbon. The rate determining steps for the formation of DiHPhe and TetHPhe are the addition of the first and second hydrogen pair, respectively. The concentration of SH groups (activated H at the edges) increases with Ni promotion linearly correlating the rates of Path 1 and Path 2, albeit with different functions. The enhancing effect of Ni on Path 2 is attributed to accelerated hydrogen addition to adsorbed hydrocarbons without important changes in their coverages.