Effect of citric acid addition on the hydrodesulfurization activity of MoO3/Al2O3 catalysts

A series of MoO3/Al2O3 catalysts was prepared using citric acid (CA) as a chelating agent by a simultaneous impregnation method or by a post-treatment method. The catalyst was tested for the HDS of thiophene. The catalysts were characterized by means of LRS, XRD, Mo K-edge EXAFS and XANES, NO adsorption capacity, and surface area measurements. The catalysts were also subjected to a chemical vapor deposition (CVD) technique using CO(CO)(3)NO as a precursor of Co to prepare CVD-Co/MoS2 catalysts. The addition of citric acid to 8.7 wt% Mo/Al2O3 increased neither the HDS activity nor the edge dispersion of MoS2 particles. On the other hand, the addition of citric acid by the post-treatment to MoO3/Al2O3 catalysts having a higher Mo content (>20 wt% Mo) significantly enhanced the edge dispersion of MoS2 particles and thus the catalytic activity of MoS2/Al2O3 and CVD-Co/MoS2/Al2O3. The addition of citric acid by the post-treatment resulted in the formation of well-dispersed Mo species on the catalyst surface at the expense of crystalline MoO3. The Mo K-edge XAFS and XANES showed that the sulfidation degree of Mo is greatly improved by the addition of citric acid at a high Mo content regardless of the addition procedure. (C) 2009 Elsevier B.V. All rights reserved.