Phenanthrene catalytic cracking in supercritical water: effect of the reaction medium on NiMo/SiO2 catalysts

A series of NiMo/SiO2 catalysts was synthesized by sol-gel method for heavy oil upgrading in supercritical water (SCW). Phenanthrene was used as substrate as it represents polyaromatic structures present in asphaltenes. No phenanthrene conversion was observed in a blank (non-catalytic) experiment. However, phenanthrene conversions up to 24% after 1 h of reaction in SCW at 425 degrees C and 230 bar were observed in the presence of NiMo/SiO2, underlining the role of the catalysts in the process. Conversion was found to be dependent mainly on Ni content and the Ni/Mo ratio in the catalysts. The liquid products obtained are thought to be the result of both oxidation and hydrogenation processes. Characterization of the fresh and spent catalysts using X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) was performed. It was revealed that catalysts are not completely stable in SCW, showing that NiMo intermetallic compounds formed the initial catalysts were decomposed, Mo-0 and Ni-0 were oxidised and the latter formed Ni2SiO4. In addition, MoO2 phase domain size in the catalysts increased and the surface of the spent catalysts appeared to be enriched with Ni and depleted with Mo.