Unraveling the Redox Behavior of a CoMoS Hydrodesulfurization Catalyst: A Scanning Transmission X-ray Microscopy Study in the Tender X-ray Range

We visualize the elemental zoning in an alumina-supported cobalt molybdenum sulfide (CoMoS) catalyst with scanning transmission X-ray microscopy (STXM). We use the Canadian Light Source beamline 10-ID’s (SM) unique combination of soft X-ray and tender X-ray STXM to determine the spatial variation of Co, Al, Mo, and S species. The results clearly show the correlation between cobalt and molybdenum and the anticorrelation between cobalt and alumina, confirming that Co is closely associated with the MoS 2 phase. The tender X-ray STXM images show that the fresh catalyst particle contains a 50 nm ring of molybdenum oxide encapsulating the supported MoS 2 phase. After the reduction at 200 °C with H 2, this oxide rim disappears and a uniform MoS 2 distribution is found. Upon oxidation at 400 °C, the sulfur disappears from the catalyst sample and molybdenum is oxidized from a Mo IV sulfide to a mainly tetrahedral Mo VI oxide, while cobalt keeps its divalent nature and changes from a Co II sulfide to a Co II oxide.