Characterization of Mo additions in iron-based Fischer-Tropsch catalysts using X-ray absorption spectroscopy and X-ray diffraction

An iron-based Fischer-Tropsch catalyst with a low concentration of molybdenum (90Fe/10Mo/5Cu/17Si) used as a promoter was characterized by X-ray absorption spectroscopy (XAS) and X-ray diffractometry (XRD). The catalyst was prepared using coprecipitation, pretreated in CO, then one sample passivated and one calcined. The XRD data show that after CO pretreatment the calcined and passivated catalysts are almost amorphous with respect to Fe2O3 with nanoparticle size of 10 and 100 angstrom for Fe3C (only present in the passivated sample). Least squares fitting of the XANES region show that the calcined and passivated samples were similar in the bulk and surface structures, with the calcined samples completely oxidized. As expected, K and L-III edges Mo-XANES shows only small molybdenum carbide formation compared to iron carbide. (C) 2007 Elsevier B.V. All rights reserved.