Finally, it should be mentioned that in the literature there is
another possible explanation for the negative effect of Na ions on
the hydrogenation activity of the HDS catalysts. Thus, for the
Ni-W/MCM-41 catalysts modified with Na+ and K+, it was assumed
that Na+ and K+ species may act as a trap for dissociated hydrogen
species, significantly reducing hydrogen spillover and hydrogen
mobility on the catalyst surface [57]. It was suggested that spillover
hydrogen may play an important role in the activity and
selectivity of HDS catalysts [57,67]. Therefore, the low catalytic
activity of the NiMo/TNT(7.1) catalyst obtained in the present work
(Table 4) could be attributed to the fact that, even if the presence of
sodium in the catalyst and its electron donation to the catalytically
active MoS2 phase facilitate formation of a significant number of
CUS sites, the rate of DBT desulfurization is limited by a lack of dissociated
hydrogen required to form a biphenyl product. In contrast,
a decrease in the support’s sodium content should result in an
increase in the number and mobility of dissociated hydrogen, promoting
in this way both the DDS and the HYD routes of
hydrodesulfurization. Our catalytic activity results confirm this
supposition, since an increase in catalyst activity and hydrogenation
ability was observed with a decrease in the Na content.