It is a general assumption that organic sulfur compounds are
converted to hydrogen sulfide in a reducing environment, and that
dissociation of thiophene (or any other organic sulfur compound)
proceeds via sulfur compound adsorption followed by hydrogenation
and then reduction by hydrogen leading to the formation of
hydrogen sulfide [35]. In the present study, it was seen that highly
active and stable catalyst formulations gave high sulfur yields. It is
proposed that there exists a strong correlation between the tolerance
to sulfur and hydrogen concentration during reforming.
Catalysts not containing Rh produced less hydrogen and relatively
low sulfur yields. Since hydrogen is required for reduction of the
hydrogenated sulfur compound, site-blocking sulfides may accumulate
on the less active Ni sites and cause a continuous drop in
activity over time. STEM/EDS indicated that sulfur was preferentially
adsorbed on the surface of Ni crystallites, thus blocking the
reaction sites. Bulk sulfur absorption was not observed and only
surface adsorption was seen. No detectable sulfur was found on
the surface of the small crystallites. While this may be due to higher
intrinsic activity of the Rh particles, it is also possible that any sulfur
adsorbed on Rh sites may migrate to nearby Ni sites, particularly
when Ni and Rh particles were present in close proximity. Based
on the difference in electronegativities of the elements forming the
bonds (S = 2.58, Rh = 2.28, and Ni = 1.91), the Rh–S bond is less
stable than would be a Ni–S bond and thus it is more likely for sulfur
to migrate from Rh to Ni than the reverse path. H2 may assist in
sulfur migration via H2S species, as described by Strohm et al. [31].
The potential migration of sulfur from Rh to Ni may further act to
retain the reforming activity of the Rh sites.