Monitoring the ambient oxidation of a NiMoSx/Al2O3 hydrotreating catalyst by a thermogravimetric method

•Each TG stage indicates specific oxidation products on NiMoSx/Al2O3 exposed to air.•Oxidation degree of NiMoSx/Al2O3 can be characterized by TG.•MoSxOy and NiSO4 are final oxidation products in ambient air. The ambient oxidation of a liquid-sulfurized commercial NiMoSx/Al2O3 hydrotreating catalyst was monitored by a thermogravimetric (TG) method. Each mass loss stage in TG was indicative of different oxidation species formed during the ambient oxidation. The mass loss is indicative of the change of oxidation products and degree with the air exposure time. The results of TG were confirmed by other characterization techniques including X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and elemental analysis. Upon the ambient oxidation, molybdenum sulfide and nickel sulfide were converted into MoSxOy and NiSO4, respectively. Almost no MoO3 and NiO are observed and little sulfur loss occurred, even if the catalyst was exposed to air for 25 days. Both the carbonaceous deposition generated by the liquid-phase sulfurization and oxidation products could form passivation layers to preclude sulfides from deep oxidation. TG in air might be a rapid, low-cost and less sample consumption method for monitoring the oxidation of sulfurized catalysts. [Display omitted]