Mechanochemical activation of MoS2—Surface properties and catalytic activities in hydrogenation and isomerization of alkenes and in H2/D2 exchange

High-energy ball milling has been employed to convert a microcrystalline, stoichiometric (S/Mo = 2), catalytically completely inactive MoS2 (prepared by high-temperature decomposition of ammonium tetrathiomolybdate, ATM) into an active catalyst, the activity profile of which was studied with test reactions (ethylene hydrogenation, H2/D2 scrambling, cis–trans isomerization of cis-but-2-ene, double-bond isomerization of 2-methyl-1-butene). Structural and surface properties of the materials were studied by XRD, SEM, TEM, XPS, nitrogen physisorption, oxygen chemisorption, and isotope exchange with D2 (quantity of exchangeable surface hydrogen). The reaction rates obtained after mechanochemical activation were compared with data from a reference MoS2 made by low-temperature decomposition of ATM. Ball-milled MoS2 became active for most of the test reactions (except double-bond isomerization) only after reductive treatments that were effective also with the reference MoS2. After identical treatments, the ball-milled MoS2 was much more active in hydrogenation than the reference MoS2, whereas H2/D2 scrambling proceeded more slowly, and cis–trans isomerization not at all. On the basis of earlier conclusions about the site selectivity of these reactions, the activity pattern indicates that mechanochemical activation led to a site structure dominated by sites with multiple vacancies whereas single-vacancy sites were not present at all, which is in agreement with TEM results showing highly defective bent nanoslab structures after ball milling. The results confirm that ethylene hydrogenation, H2/D2 scrambling and cis–trans isomerization of cis-but-2-ene may be employed as test reactions for sites on MoS2 surfaces whereas data for the double-bond shift in 2-methyl-1-butene suggest that the Brønsted sites catalyzing this reaction are related to structural defects rather than to the regular MoS2 structure.