Regeneration - Spent Hydroprocessing Catalysts - page 138

Figure 6.10: Formation of CO 2 and SO 2 under N 2 and in air for spent extracted catalyst [From
ref. 391 . Reprinted with permission].
adverse effect on catalyst activity recovery. There is little information on the structure of
carbon and nitrogen in regenerated catalysts, although it was proposed that the metal carbides
and oxycarbides as well as nitrides and oxynitrides are possible phases. The conditions under
which such structures can be formed were described elsewhere [377] . The residual sulfur in
regenerated catalysts was the focus of study conducted by Yoshimura et al. [381] who
confirmed that the last amount of sulfur in regenerated catalyst was a sulfate form. Bogdanor
and Rase [392] detected NiSO 4 in the regenerated NiMo/Al 2 O 3 catalyst. This sulfate remained
in the catalyst after subsequent sulfiding. Apparently, the temperature employed during
sulfiding was not high enough to achieve the sulfate decomposition. This was in agreement
with the observations made in several studies where a sulfate presence was confirmed in
fresh-sulfided catalysts [393-395] . Although the process may be rather slow, the sulfate