TABLE OF CONTENTS It is essential that a large number of the active sites on catalyst surface are maintained for a long period of TOS. This requires the suitable H 2S/H 2 ratio to ensure a desirable size of the active site (CUS) comprising a sulfur vacancy and SH group at its proximity (63). It was established that the number of vacancies decreased with increasing H 2S/H 2 ratio. On the other hand, at low H 2S/H 2 ratios, the catalyst over-reduction may occur. Such situation favors the adsorption of N-bases as well as deposition of coke and disfavors hydrogen activation. Furthermore, during the prolonged TOS, the crystal structure of active phase may change. For example, an increase of the MoS 2/WS 2 crystallites in normal direction will increase the number of active sites. This would be an indication of the conversion of the type I active phase to a more active type II active phase. At the same time, the growth in a lateral direction would have an opposite effect. In this regard, the studies of Shimada et al. (311) showed that the lateral growth of MoS 2 crystallites in the CoMo/Al 2O 3 catalyst was partly responsible for the loss of activity during the hydroprocessing of VGO at about 660 K and 5.9 MPa.
In an effort to simulate deactivation, Tanaka et al. (312) conducted the accelerated aging in the pilot plant...
