Dual Effect of H$_2$S on Volcano Curves in Hydrotreating Sulfide Catalysis

Recent progresses achieved by, quantum molecular modeling techniques enabled the rational interpretation of catalytic trends of series of transition metal sulfide catalysts. Empirical volcano curves can be explained by microkinetic models including chemical descriptors calculated at an ab initio level. This approach was successfully applied in the field of hydrotreating catalysis using the metal-sulfur bond energy descriptor. The purpose of the present work was to extend this approach by exploring the effect of reaction conditions (partial pressure of H2S) on the volcano curve. On the one hand, high resolution transmission electron microscopy (HRTEM) images combined with molecular modeling of morphologies and surfaces exposed by catalysts provide an estimate of the number of potential active sites. This approach is illustrated for the relevant case of unsupported or alumina supported Co9S8 sulfide. On the other hand, an improved microkinetic model is proposed in order to reflect the dual effects of H2S observed in the hydrogenation of toluene : an inhibiting effect for MoS2, Rh2S3, RuS2, NiMoS and a promoting effect for Cr2S3 et Co9S8. The experimental results and kinetic modeling reveal that the maximum of the volcano curve and thus the optimal sulfide catalyst depends closely on the partial pressure of H2S.