Structural characterization of Ni–W hydrocracking catalysts using in situ EXAFS and HRTEM

The detailed structural characterization of the metal function of two fully formulated Ni–W hydrocracking catalysts was investigated by time resolved in situ X-ray absorption spectroscopy (EXAFS and XANES) at both the Ni K-edge and W L3-edge, and by high resolution transmission electron microscopy. These two hydrocracking catalysts (designated as HCA and HCB) contained the same wt% of Ni and W, the same wt% of the other constituents (γ-Al2O3/silico-aluminate), and were calcined at the same temperature, but were prepared by different methods in order to highlight the sensitivity of the characterization techniques to the structural differences. The morphology of the WS2 particles in the sulfided catalyst is substantially different between the two catalysts: in the HCA catalyst there are many more particles with multiple WS2 layers whereas in HCB there are more single layers. The average diameter of the WS2 plates is similar in both. The catalysts show a difference in the amount of W sulfidation after the 410 °C sulfidation treatment in 10% H2S/H2. The W of HCA catalyst was completely sulfided whereas 16% W of HCB catalyst was unsulfided. Similarly there is a difference in the amount of sulfided Ni: 16% Ni in HCA and 60% Ni in HCB remain unsulfided. In the sulfided form of both catalysts the Ni exists in three different morphologies: oxidized Ni (six-fold coordinate as a nickel aluminate), nanoparticulate Ni3S2, and Ni decorating the edge sites of the WS2. In both the Ni3S2, and Ni decorating the edge sites of the WS2, the Ni is in a tetrahedral coordination with sulfur. In both catalysts the sulfidation of the Ni and W occurs essentially simultaneously over most of the temperature range and the sulfidation of the W proceeds through the same tungsten oxysulfide in both samples. The characterization methodology presented here is a powerful one for elucidating the structural differences of complex heterogeneous catalysts.

Schematic models of the two sulfided Ni–W catalysts.Figure optionsDownload high-quality image (37 K)Download as PowerPoint slide