Concept of acid–base catalysis by metal sulfides

The structure of the active component of the sulfide hydrodesulfurization (HDS) catalysts is considered. The active component consists of a single slab of MoS2 (WS2) with Ni or Co atoms being located in its edge plane. The essential element of the active component is hydrogen occluded into the MoS2 matrix, which localizes in the center of “empty” trigonal prism of a MoS2 matrix made from six sulfur atoms under Ni (Co) atom at the distance of 1.5 Å. Occluded hydrogen appeared in the active component during the catalyst sulfidation as a result of its oxidative addition to the Ni (Co) atoms. This occluded hydrogen creates an electronic d6 configuration of Ni(IV) or Co(III) atoms, which are the Lewis acid sites with two vacant dz2 and dxy orbitals. Therefore, these active centers can adsorb donor molecules of thiophene. The activation of hydrogen occurs on the couplet sulfur atoms at the edge planes of a single MoS2 (WS2) slab. The driving force of this process is the presence of the SS chemical bond at the terminal sulfur atoms that restrict an electrically neutral macromolecule of the active component and act as the Lewis basic sites. An excess of electron density is transferred from these atoms to a hydrogen molecule, which results in its homolytic dissociation. The high rate of catalytic transformation is ensured by the fast transfer of electrons and protons along the system of chemical bonds within an electrically neutral macromolecule of the active component.