Environmental Engineering Reference
In-Depth Information
sulfidic and oxosulfidic sulfur decreased substantially. For the Co/Al
2
O
3
catalyst, almost all
Co was converted to Co
2+
, while sulfur remained as sulfate below 423 K. In CoMo/Al
2
O
3
catalyst, the Co sulfide was more oxygen tolerant than in the Co/Al
2
O
3
catalyst as confirmed
by the presence of Co sulfide in the former catalyst even after oxidation at 488 K. The gradual
conversion of sulfidic form of sulfur into oxosulfidic and sulfate forms in the CoMo/Al
2
O
3
catalyst was more pronounced than in the Mo/Al
2
O
3
catalyst. When air was used, the Co
sulfate was usually concentrated on the exterior of catalyst particles forming a layer that
slowed down the diffusion of O
2
into the particles interior. These effects were diminished when
air was replaced with 0.5% O
2
. The extended X-ray absorption fine spectroscopy (EXAFS)
data confirmed a gradual replacement of the Mo S and Mo Mo bonds with the Mo O bonds.
An intimate contact of coke with catalyst surface suggests a potential involvement of the latter
during coke oxidation. This would however require the presence of an oxidic form of the
metals in spent catalyst. This is supported by the results shown in
Fig. 6.9 [387]
. Each point in
Fig. 6.9
was obtained during the isothermal burn-off performed at the corresponding
temperatures. Thus, at 200
◦
C, about 20% of carbon (as CO
2
) was removed compared with
almost 80% of sulfur removal during the oxidative burn-off of the spent sulfided catalyst. The
CO
2
and SO
2
profiles in
Figs 6.5 and 6.6 [369,240]
provide another confirmation that a large
portion of metals sulfides was oxidized before the onset of the oxidation of carbon. However,
this observation may not be identical in the case of the spent catalyst heavily deactivated with
Figure 6.9: Effect of temperature on cumulative amount of carbon and sulfur removed during
stepwise burn-off [From ref.
387
. Reprinted with permission].
