On hydrogen adsorption by nanodispersed MoS2-based catalysts

Abstract

The nature and the amount of hydrogen species adsorbed on MoS2–based catalysts have been studied by inelastic neutron scattering (INS) as a function of preparation and treatment conditions. Unsupported MoS2 materials have been prepared from (NH4)2MoS4 in pure H2S or in H2 flow and optionally doped with Co, Ni, and Pt. The results of INS were compared with other characterizations including temperature–programmed reduction, temperature–programmed desorption, and catalytic tests in thiophene hydrodesulfurization. The results of INS suggest that the major hydrogen species is of a similar type, independent of the preparation conditions and promotion. The observed INS frequencies were compared with the results of density functional theory calculations carried out for different configurations of M- and S- edges of (doped) MoS2 slabs. The main feature for all samples near 685–692 cm−1 corresponds to the vibrations of dangling –SH groups linked to six-coordinated Mo atoms, generated by hydrogenolysis of the S–S bonds at the M edges of MoS2 slabs. A minor signal is present as a shoulder at 610–625 cm−1, most pronounced in the Co-promoted and sulfur-saturated solid, which is probably due to interaction of the SH groups with adjacent Co atoms and/or neighboring S22− groups. The total amount of adsorbed H species depends on the treatment conditions, decreasing with the degree of material reduction.