In the present work, a series of NiMo catalysts supported on
titania nanotubes with different sodium content were prepared,
characterized, and tested in DBT hydrodesulfurization. It was
observed that the presence of a certain amount of residual sodium
in the nanotubular support strongly affected the catalytic performance
of the NiMo catalysts, namely, their activity and selectivity.
First, the presence of sodium determines the characteristics of the
deposited Mo oxide species in calcined catalysts and the ease of
their reduction. In the catalysts supported on TNT(x) materials
with high sodium content, TPR results suggest the presence of
tetrahedral molybdate species (Na2MoO4 and Na2Mo2O7) and dispersed
octahedral ones. In the case of NiMo catalysts supported
on low-sodium nanotubes, only the presence of octahedrally coordinated
Mo oxide species, easily reduced, was detected. HRTEM
characterization of the sulfided MoS2 active phase also showed
that Na content in the support affected its morphology (length
and stacking degree). A decrease in the Na content in the support
leads, in general, to an increase in the proportion of Mo atoms
located on the catalytically active surface of the MoS2 crystallites
(fMo, Table 3) as well as of their stacking degree. The proportion
of the CUS active sites and a fully sulfided MoS2 species was also
influenced by the presence of sodium. An increase in the Na+ content
resulted in an increase in the number of CUS sites and a
decrease of their acidity, and vice versa. Considering that vacant
(CUS) sites are active for S removal from dibenzothiophene
through the DDS pathway, whereas fully sulfided MoS2 species
are responsible for the HYD route, results from IR characterization
of CO adsorbed onto the sulfided catalysts are in line with the
modifications in the selectivity patterns of the catalysts as a function
of their sodium content. Low catalytic activity of
high-sodium-content NiMo/TNT(x) catalysts may also be attributed
to the fact that Na+ species may act as a trap for dissociated
hydrogen species, significantly reducing hydrogen spillover and
hydrogen mobility on the catalyst surface.