30th Annual SSRL Users' Meeting — October 9-10, 2003

HRTEM, Synchrotron, and Simulation Techniques Applied to Activity and Selectivity Correlation in Hydrodesulfurization Catalysts

M. Perez De La Rosa,1 G. Berhault,2 M. J. Yácaman,3 A. Mehta,4 S. Fuentes,5 and R. R. Chianelli1

1M.R.T.I., University of Texas at El Paso, El Paso, TX
2LACCO-CNRS, Poitiers, France
3Department of Chemical Engineering, University of Texas at Austin, Austin, TX
4Stanford Synchrotron Radiation Laboratory, Stanford, CA
5Centro de Ciencias de la Materia Condensada, Ensenada, B. C., México

The highly anisotropic character and inherent disorder in the structure of supported MoS2-based catalysts, used extensively to perform hydrotreating reactions for the removal of heteroatoms (S, N, and O), aromatics, and metals, makes characterization of the active catalyst a difficult challenge. XAS (x-ray adsorption spectroscopy), XRD (X-ray diffraction) and HRTEM (high resolution transmission electron microscopy) have been widely used to study these catalysts in an attempt to understand the structure and origin active phases in these catalysts. However, all these techniques have limitations in determining the structure of the active MoS2 phase and the associated Co promoter when used individually. Current techniques are not able to provide information of both lateral dimensions along the basal direction and of stacking height of MoS2 slabs without ambiguity. We report here the use of a synchrotron source for X-ray scattering measurements of supported MoS2 and cobalt-promoted MoS2 catalysts that strongly increases the signal to noise ratio resulting in the detection of diffraction features providing information on the dispersion of the active phase in combination with HRTEM giving a more complete picture of catalyst structure and the of the active phases present. Furthermore supported industrial catalysts have been studied that have operated under refinery conditions for more than four years leading to new understanding of the catalytic phase that are stabilized under these conditions. Industrial hydrotreating conditions induce a "destacking" process resulting in the stabilization of single-layered nanoparticles with the MoS2 structure. This effect has been confirmed on a freshly sulfided model CoMo/Al2O3 catalyst that underwent substantial morphological change leading to the formation of single slabs under HDS conditions as determined by XRD. Other structural effects are also reported. This study emphasizes the importance of determining the catalytically stabilized phases under operating hydrotreating conditions as a basis for understanding the activity and selectivity of this class of catalysts.