Hydroprocessing of Petroleum - Spent Hydroprocessing Catalysts - page 25

type II phase is much greater than in the type I phase because the latter may still be attached to
-Al 2 O 3 via Mo O bonds.
The study on the effect of support on the structure of active phase conducted by Bouwens et al.
[59] revealed that type II phase on carbon supports resembled type I phase on SiO 2 and
-Al 2 O 3 supports, i.e., in the former case, type II phase approached a monolayer-like form.
This was consistent with the significant dispersion of active metals on some carbon supports.
In this regard, the presence of surface defects on carbons may play an important role. For
example, much more efficient dispersion of active metals should be achieved on activated
carbon (AC) compared with that on pristine graphite [55] . For both NiMo/AC and
NiMo/Al 2 O 3 catalysts, only two forms of metal sulfides were detected [60] . One was type II
form, such as Ni-Mo-S, and the other Ni 3 S 2 . The latter was detected after the Ni/Mo ratio
exceeded 0.48 and 0.56 for the NiMo/AC and NiMo/Al 2 O 3 catalysts, respectively.
The evolution of the Co-Mo-S phase in the AC supported catalysts appeared to be H 2
pressure-dependent, as it was observed by Dugulan et al. [61] . These authors reported that the
Mossbauer spectra of the CoMo/AC catalyst sulfided at 573 K under high H 2 pressure (e.g.,
4MPa) differed from those obtained at atmospheric pressure. Under high H 2 pressure, the
stability of the Co sulfide species as part of the Co-Mo-S phase was affected compared with
the CoMo/Al 2 O 3 catalyst. This suggests that under high H 2 pressure conditions, properties of
the Co-Mo-S phase on carbon supports may differ from those on the -Al 2 O 3 support.
3.3.1.2 Brim Sites Model
Further insight into the structure, morphology and activity of MoS 2 , Co-Mo-S, and Ni-Mo-S
phases were obtained by Topsoe et al. [62,63] using a combination of novel experimental and
theoretical methods like STM, DFT, and HAAD-STEM. The STM method showed the
atom-resolved images of the catalytically active edges of MoS 2 , Co-Mo-S, and Ni-Mo-S
nanoclusters. The edge was found to exhibit a special electronic edge state identified as brim
sites. Detailed analysis using DFT revealed that the brim sites have metallic character. It was
postulated that because of metallic character, brim sites may bind sulfur-containing molecules,
and when hydrogen is available at the neighboring edge sites in the form of SH groups,
hydrogen transfer and HYD reactions can take place. The brim sites are thus catalytically
active for HYD reactions. But, the brim sites are not CUS. It was generally accepted for a long
time that CUS were the key sites involving in both HYD and hydrogenolysis reactions. It was
believed that MoS 2 or Co-Mo-S structure with higher ( > 2) sulfur vacancies at the corners are
primarily responsible for HYD by adsorption and that hydrogenolysis site could be edge site
with lower (1 or 2) sulfur vacancies [64,65] . The new “brim site” model, proposed by Topsoe
et al. [62,63] , is consistent with many inhibition steric and poisoning effects, which have been
difficult to interpret using “vacancy” model. DFT calculations have helped to gain detailed
insight into the HDS of thiophene under industrial conditions. Thus, it was suggested that the
HYD reactions take place on brim sites, whereas the direct sulfur removal can take place at