Atom-resolved scanning tunneling microscopy investigations of molecular adsorption on MoS2 and CoMoS hydrodesulfurization catalysts

We present an atomic-scale investigation of the molecular interactions relevant to hydrodesulfurization (HDS) catalysis at the catalytically active edge sites of single-layer MoS2 and cobalt-promoted MoS2 (CoMoS) nanoparticles synthesized as model catalysts. Atom-resolved scanning tunneling microscopy observations of the adsorption of thiophene, pyridine, and various refractory alkyl-substituted dibenzothiophenes have allowed us to reveal their predominant adsorption modes, study possible active sites, and provide explanations for the observed selectivity in HDS reactions, which are relevant to the two identified HDS pathways - the direct desulfurization route and the hydrogenation route. Hydrogenation is proposed to occur through interaction with edge S-H groups when the molecules are adsorbed in a flat-lying configuration near the nanoparticle edges of single-layer MOS2 and CoMoS. This adsorption mode is directly observed with STM at low temperatures and is realized due to the interaction of the aromatic pi-system of the adsorbing molecule with metallic edge states (brim states) in MOS2 and CoMoS which change the adsorption properties of the edges compared to the inert basal plane. The model studies also demonstrate the strong effect of sterical hindrance on the adsorption onto sulfur vacancies on MOS2 edges in direct desulfurization of the most refractory compounds, but it is also established that certain corner sites may exist for MoS2 and the promoted CoMoS structures, which directly facilitate strong adsorption. (C) 2015 Elsevier Inc. All rights reserved.