Basic Reaction Steps in the Sulfidation of Crystalline MoO₃to MoS₂, As Studied by X-ray Photoelectron and Infrared Emission Spectroscopy

Abstract

The sulfidation of crystalline MoO 3 and the thermal decomposition of (NH 4 ) 2 MoO 2 S 2 to MoS 2 via an {MoOS 2 } oxysulfide intermediate have been studied by means of monochromatic X-ray photoelectron spectroscopy (XPS) and infrared emission spectroscopy (IRES). Several basic steps of the sulfidation reaction could be resolved and explained in terms of the structure of crystalline MoO 3 . The sulfidation reaction starts at low temperatures with an exchange of terminal O 2- ligands of the oxide for S 2- by reaction with H 2 S from the sulfiding atmosphere. In subsequent Mo−S redox reactions, bridging S 2 2- ligands and Mo 5+ centers are formed. Lattice relaxation and further sulfur uptake are the main processes before, at temperatures above 200 °C, direct reactions with H 2 occur, during which the Mo 5+ centers are converted into the 4+ oxidation state. The decomposition experiments with (NH 4 ) 2 MoO 2 S 2 show that terminal O 2- ligands serve as the reactive sites. The conversion of terminal Mo O t to Mo S entities and the subsequent generation of Mo O t from μ 2 and μ 3 oxygen atoms in the MoO 3 lattice appear as the general principle of the sulfidation reaction.