On the multifaceted roles of NiSx in hydrodearomatization reactions catalyzed by unsupported Ni-promoted MoS2

A series of unsupported Ni-Mo sulfide catalysts with varying Ni contents (0.13-0.72 mol(Ni) mol(Ni+Mo)(-1)) was post-synthetically treated with concentrated HCl to remove large crystallites of accessible NiSx. These sulfide particles inevitably form and grow at Ni concentrations required for the synthesis, but generally have very low activities. In all cases, Ni concentrations were greatly reduced by the HCl treatment. While this 'leaching' strategy successfully improved the reaction rates of high Ni-content (0.4 mol(Ni) mol(metal)(-1)) catalysts for the hydrogenation of phenanthrene, modest to drastic decreases in catalytic rates (x0.1-0.8) were registered for catalysts with lower Ni concentrations. For the lowest Ni-loaded catalyst (0.05 mol(Ni) mol(metal)(-1)), HCl treatment caused a dramatic loss of specific surface area and catalytic activity by more than a factor of 6 and shifted the selectivity pattern to that of pure MoS2. These observations allow us to conclude that Ni atoms incorporated at the slab edges are inherently susceptible to HCl attack. NiSx, however, are the preferential sites at which HCl induces dissolution. Experiments with inter-particle mixtures and segregated beds of NiSx and MoS2 demonstrate that NiSx not only activates H-2, but also acts as a reservoir to dynamically incorporate Ni in the MoS2 slabs at reaction conditions. These beneficial effects are reduced, as nickel sulfide particles become excessively abundant as typical for high Ni-content catalysts, for which edge substitution by Ni is near or at its maximum. The areal activity and concentration of chemisorbed nitric oxide (NO) are well correlated for the leached catalysts, with the exception of the lowest Ni-containing catalyst that has a low degree of Ni edge substitution (<20% of total edge atoms) and predominantly unpromoted sites. This linear correlation shows that the Ni-promoted sites are more than five-fold as active as the unpromoted sites. (C) 2020 Elsevier Inc. All rights reserved.