Sulfidation state of alumina-supported molybdenum catalysts estimated by a 35Sradioisotope tracer method

Abstract

The sulfidation state in Mo/Al₂O₃ and Co-Mo/Al₂O₃ catalysts have been investigated using a ³⁵S pulse tracer method. [³⁵S]H₂S pulses were introduced into Mo/Al₂O₃ or Co-Mo/Al₂O₃ in a nitrogen stream until the amount of radioactivity in one pulse was constant, i.e. the catalyst no longer reacted with [³⁵S]H₂S. From the amount of reacted [³⁵S]H₂S, the amount of sulfur accumulated on the catalyst was estimated. The amounts of sulfur accumulated on the Mo/Al₂O₃ and Co-Mo/Al₂O₃ catalysts increased with increasing temperature. The amounts of sulfur accumulated on Mo/Al₂O₃ with 12 wt.% MoO₃ at 100, 200, 300 and 400°C were 34.0, 38.5, 50.7 and 60.5 mg S (g cat)^-1, respectively and the value at 400°C corresponded to a sulfidation state of MoS_2.2/Al₂O₃. At the same temperature, the amount of sulfur accumulated on Mo/Al₂O₃ increased with increasing the amount of molybdenum loaded. Below 300°C, the amount of sulfur accumulated on Mo/Al₂O₃ was almost same as that on CoMo/Al₂O₃. The amount of sulfur accumulated on CoMo/Al₂O₃ (CoO: 3.8 wt.%, MoO₃: 12.3 wt.%) at 400°C was 73.4 mgS (g cat)^-1, larger than that on Mo/Al₂O₃ with 12 wt.% MoO₃. These results suggest that only molybdenum is sulfided in both Mo/Al₂O₃ and CoMo/Al₂O₃ catalysts below 300°C and that only at 400°C is cobalt on CoMo/Al₂O₃ sulfided.