Location and coordination of promoter atoms in Co- and Ni-promoted MoS2-based hydrotreating catalysts

In this study, we used scanning tunneling microscopy (STM) and density functional theory (DFT) to investigate the atomic-scale structure of the active Co- or Ni-promoted MoS2 nanoclusters in hydrotreating catalysts. Co-promoted MoS2 nanoclusters (Co–Mo–S) are found to adopt a hexagonal shape, with Co atoms preferentially located at (image) edges with a 50% sulfur coverage. The first atom-resolved STM images of the Ni-promoted MoS2 nanoclusters (Ni–Mo–S) reveal that the addition of Ni also leads to truncated morphologies, but the degree of truncation and the Ni sites are observed to depend on the nanocluster size. Larger clusters (type A) are structurally similar to Co–Mo–S exposing fully Ni-substituted (image) edges with a 50% S coverage. Smaller clusters (type B) show dodecagonal shapes terminated by three different edges, all of which contain Ni-promoter atoms fully or partially substituting the Mo atoms. The findings may shed more light on the different selectivities observed for the Co- and Ni-promoted hydrotreating catalysts.