| dc.description.abstract | Hydrodesulfurization (HDS) is an important catalytic process in oil refining, in which sulfur is removed from organo sulfur compounds in crude oil. Conventional HDS catalysts consist of γ-alumina supported molybdenum sulfide slabs (MoS2) promoted by either cobalt, nickel or both cobalt and nickel. Although the HDS catalysts are industrially used for a long time, the promoting effect of cobalt and nickel is still not fully understood. In this research X-ray absorption spectroscopy and microscopy were used to obtain deeper insight into the oxidation and reduction sensitivity of cobalt and nickel species present in promoted HDS catalysts.
First, cobalt and nickel promoted MoS2 catalysts were synthesized using incipient wetness impregnation methods. Samples were prepared by both simultaneous and sequential impregnation in order to study differences in morphology and activity between the impregnation methods. The impregnation was followed by a drying step, calcination procedure and sulfidation reaction to obtain the active sulfide catalysts. Although the catalyst samples were very sensitive to beam damage by an electron beam, STEM-EELS and TEM images showed that there was no difference in morphology between for samples promoted by different elements or samples prepared by different impregnation sequences. Furthermore, STEM-EELS and TEM showed the formation of typical MoS2
slabs for all sulfide samples. Catalytic tests showed that all synthesized catalyst were active in thiophene desulfurization and verified the promoting effect by cobalt and nickel. However, a synergistic effect for double promotion could not be verified. An in-situ X-ray absorption experiment of an oxidation reaction was performed using a CoNiMoS sample and confirmed the oxidation of cobalt and nickel by analysis the Co- and Ni K-edges. CTM4XAS calculations indicated a transition from a Co 2+ Td system to a mixture of Co 2+ Oh and Co 3+ Oh species. Furthermore an in-situ X-ray absorption experiment of a H2 reduction reaction was performed using a CoNiMoO sample and confirmed the reduction of cobalt and nickel by analysis the Co- and Ni K-edges. CTM4XAS calculations were used to simulate the pre-edge structures of the Co K-edge and indicated a transition from a Co 2+ Oh system to a Co 2+ Td system during the H2 reduction reaction. From spectral elemental maps obtained from STXM measurements it was shown that molybdenum, cobalt and nickel are not homogeneously distributed on the aluminum support particle. | |
