Environmental Engineering Reference
In-Depth Information
either fresh sulfided catalysts or spent catalysts under significantly different conditions than
those encountered during industrial operations
[55]
. Thus, little information is available on the
form of catalyst during the steady state operation, i.e., under in situ conditions. Inevitably,
under hydroprocessing conditions (e.g., 600-700 K and 5-15MPa of H
2
), some properties of
catalysts, i.e., interaction of active phase with support, lattice vibrations, interaction of
promoting metal with base metal of active phase, etc., will differ from those observed under
conditions employed during catalyst characterization. Therefore, it is desirable that a testing
protocol, which could closely simulate practical situation, is developed, although this would
appear to be rather challenging task.
3.3.1.1 Co(Ni)-Mo(W)-S Phase
Several research groups have been involved in determining the structure of hydroprocessing
catalysts. The contributions of Topsoe et al.
[53]
to the understanding of these issues should be
noted. In the case of the CoMo/Al
2
O
3
catalyst, several species could be detected on the
-Al
2
O
3
surface. Thus, presence of the species, such as MoS
2
,Co
9
S
8
, and Co/Al
2
O
3
,was
clearly confirmed. Moreover, the Mossbauer emission spectroscopy provided clear evidence
for the presence of the phase in which Co was associated with MoS
2
, i.e., Co-Mo-S phase.
Similar structures were also found in the NiMo/Al
2
O
3
, CoW/Al
2
O
3
, and NiW/Al
2
O
3
catalysts,
e.g., Ni-Mo-S, Co-W-S, and Ni-W-S, respectively. In this phase, enhanced concentrations of
Co and/or Ni promoters at the edge planes of MoS
2
crystals have been confirmed. The
occurrence of these promoters in the same plane as that of Mo ruled out the intercalation of the
former between the layers of MoS
2
. In the Co-Mo-S phase, the Mo S bond is weaker than in
the unpromoted MoS
2
. Then, the CUS required for hydroprocessing reactions can be
facilitated more readily. Temperature and the H
2
S/H
2
ratio are among the important operating
parameters for controlling the CUS concentration.
The structure of the Co-Mo-S phase is temperature-dependent
[53,58,59]
. Thus, the type I
phase formed at lower temperatures was still chemically bound with the support, as it was
evidenced by the presence of the Al-O-Mo entities. This phase was favored at low Mo loading
on the
-Al
2
O
3
. The occurrence of this phase was an indication of the incomplete sulfiding.
The sulfiding at higher temperatures facilitated the transformation of the type I phase into type
II phase. Consequently, the Al-O-Mo entities were not present indicating a diminished
interaction of the active phase with the
-Al
2
O
3
support. The existence of the type II phase
was further confirmed in the unsupported Co/MoS
2
system as well as in the CoMo catalyst
supported on carbon
[58]
, suggesting that type I phase requires the presence of oxygen on the
support to facilitate the interaction with the active phase. Because of a lesser interaction with
the support, the structure of type II phase is dominated by the multiple stacks of slabs
compared with more or less monolayer-like distribution occurring in type I phase. Generally,
the former phase exhibits a higher catalytic activity. This suggests that the active sites are
present at the edges and corners of the Mo(W)S crystallites. The proportion of such sites in the
