Effects of the H2S partial pressure on the performance of bimetallic noble-metal molybdenum catalysts in simultaneous hydrogenation and hydrodesulfurization reactions

The influence of the H2S concentration on the activity and selectivity to hydrodesulfurization (HDS) of bimetallic noble-metal molybdenum (NM-Mo, NM = Pt, Ru, Pd) supported on gamma-Al2O3 catalysts was studied. The effect of H2S on the modification of the HDS pathway and the influence of the Mo loading on the sulfur resistance of catalysts were also analyzed. NM-Mo/gamma-Al2O3 and conventional CoMo/gamma-Al2O3 catalysts were prepared and evaluated in simultaneous dibenzothiophene (DBT) HDS and naphthalene hydrogenation (HYD) reaction cycles, in the presence of dimethyl disulfide (DMDS) in order to obtain H2S partial pressures of 10, 30 and 50 kPa. Catalysts were characterized by atomic absorption spectroscopy and temperature-programmed reduction of sulfides (TPRS). TPRS experiments after exposing catalysts to simulated reaction conditions (TPRS-SRC) were also performed. Results show that the H2S does not poison the studied catalysts, but an inhibition of both HYD and HDS reactions is observed. The inhibition of catalyst activities is attributed to the interconversion of HDS, HYD and inactive sites; this interconversion is influenced by the H2S/H-2 ratio, the nature of the NM and the composition of catalysts. Activity, TPRS and TPRS-SRC results confirmed this scheme. A modification of the reaction pathway was observed when the H2S partial pressure was increased; the direct desulfurization was principally affected. The interconversion of sites could explain this modification. Moreover, our results show that the Mo loading and specifically the Pt/Mo ratio, which determine the Pt dispersion, influence the sulfur resistance of the catalyst. (c) 2006 Elsevier B.V. All rights reserved.