Investigation of sulfur behavior on CoMo-based HDS catalysts supported on high surface area TiO 2 by 35S radioisotope tracer method

Abstract

Three series of CoMo catalysts (MoO 3: 6, 11, 16 wt.%) with various Co/Mo molar ratios were prepared by successive incipient wetness impregnations of a titania sample previously prepared by the pH swing method, which provides a TiO 2 carrier with a high SSA (134 m 2 g −1) and excellent mechanical properties. DBT HDS activity of the catalysts increased with addition of cobalt up to Co/Mo = 0.4 and then decreased for higher ratios, irrespective of the Mo loading. The results of a [ 35S]DBT HDS method showed that S 0, the amount of labile sulfur atoms, increased in parallel with the activity when adding Co up to a molar ratio of 0.4. In contrast, unlike on CoMo/Al 2O 3 catalysts, only a slight increase in k RE, the H 2S release rate constant, was observed upon Co addition. This was due to formation of the TiMoS phase: while formation of Ti S * Mo bonds favorably induces an increase in sulfur mobility on Mo/TiO 2 catalysts, electronic density on Mo atoms increases, which limits the promoting effect of Co on mobility of S a sulfur atoms bridged between Mo and Co. Further, while the increase in HDS activity upon Co addition on a uniform MoS 2 monolayer on TiO 2 was in rather good agreement with the quantitative and kinetics data of the 35S tracer method, over MoS 2 slabs on TiO 2 this increase was larger than that expected from the results of the 35S tracer method. This was attributed to the presence of □ * and □ a CUS, i.e. catalytic sites that are not replenished with sulfur, at the steady state. A larger number of □ * CUS was present at low Mo wt.% or for low Co/Mo. As these particular catalytic sites are in the form of CUS at the steady state, they are not accounted by the [ 35S] radiotracer method. This explains the differences observed between the experimental DBT HDS promotion and the promotion deduced from the increase in k RE and S 0.