Investigation of sulfur behavior on CoMo-based HDS catalysts supported on high surface area TiO2 by 35S radioisotope tracer method

Three series of CoMo catalysts (MoO3: 6, 11, 16wt.%) 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 TiO2 carrier with a high SSA (134m2g-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 S0, 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/Al2O3 catalysts, only a slight increase in kRE, the H2S release rate constant, was observed upon Co addition. This was due to formation of the TiMoS phase: while formation of TiS*Mo bonds favorably induces an increase in sulfur mobility on Mo/TiO2 catalysts, electronic density on Mo atoms increases, which limits the promoting effect of Co on mobility of Sa sulfur atoms bridged between Mo and Co. Further, while the increase in HDS activity upon Co addition on a uniform MoS2 monolayer on TiO2 was in rather good agreement with the quantitative and kinetics data of the 35S tracer method, over MoS2 slabs on TiO2 this increase was larger than that expected from the results of the 35S tracer method. This was attributed to the presence of 2* and 2a CUS, i.e. catalytic sites that are not replenished with sulfur, at the steady state. A larger number of 2* 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 kRE and S0.