Sulphidation of an oxidic CoMo/Al2O3 catalyst under practical conditions: different kinds of sulphur species

The sulphidation of a CoMo/Al2O3 hydrotreating catalyst was studied under p ressure and flow conditions close to industrial practice. The sulphiding mi xture contained equal amounts of H2S and CH4 diluted in hydrogen under 3.9 MPa total pressure. The oxidic precursor was flushed by the reagent gas at 40 degreesC for 10 h, then the temperature was raised to 400 degreesC withi n 2 h. Mass variation was continuously recorded by means of a suspension ma gnetic balance, and the gas phase was analysed by mass spectrometry. Sulphu r contents were also determined at intermediate temperatures by chemical an alyses. Thus, mass variations due to the presence of adsorbed species or to the formation of CoMo sulphides could be distinguished. The rapid gain in mass noted at 40 degreesC was mostly due to adsorption of H2S over the oxid ic solid, since it increased with H2S partial pressure. At this stage, howe ver, a small amount of the reactive oxygen was already exchanged for sulphu r. Upon raising the temperature, the mass decreased due to a combination of desorption and sulphiding steps. Above 140 degreesC, H2S consumption was e videnced, together with water production, and the rate of sulphiding increa sed with the H2S partial pressure. At 300 degreesC, the mass variation was close to that expected for complete transformation into Co9S8 and MoS2. Abo ve 350 degreesC, the mass further increased due to the replacement of adsor bed water by H2S. At 400 degreesC, an important excess mass was observed at all H2S partial pressures. Thus, the adsorption sites on the metal sulphid es are essentially saturated by H2S species under practical conditions.