ACS Catalysis
Research Article
surface, the somewhat lower order is attributed to the fact that
furfural adsorbs strongly on the surface, which limits the
available active sites. The higher order for the coated catalysts
suggests that the available sites for DC and (to a lesser degree)
AH bind furfural more weakly, suppressing its surface
accumulation. The reaction order for hydrogen in DC was
negative in all cases, consistent with the fact that hydrogen was
not involved in the DC process. Hydrogen may have acted as a
site blocker for furfural in this case, reducing the coverage of
furfural on the surface. Additionally, the hydrogen reaction
order for AH increased on the thiol-coated catalysts, indicating
that hydrogen coverage was also reduced when the SAMs were
present, similar to furfural.
The reaction order for furfuryl alcohol in ADH and HDO,
Table 2, was near unity on both the uncoated and thiol-coated
catalysts. This is consistent with the fact that the concentration
of furfuryl alcohol in these reactions was an order of magnitude
smaller than furfural to mimic the low concentration of furfuryl
alcohol produced under the furfural hydrogenation reaction
conditions, and so the surface coverage of furfuryl alcohol was
likely small on all catalysts. Correspondingly, the reaction order
for hydrogen in HDO was near zero in all cases, indicating that
additional hydrogen did not significantly assist the reaction rate
because there was enough surface hydrogen available. The
hydrogen reaction order increased slightly on the thiol-coated
catalysts, consistent with the idea that coverage of all species
was reduced by the presence of the SAM.
The reaction order in hydrogen for the ADH reaction
presents an interesting trend. One would expect, since the
reaction requires no stoichiometric hydrogen, that the reaction
order would be negative in all cases since hydrogen would act
as a site blocker. However, the reaction order actually switched
from near zero to slightly positive on the thiol-coated catalysts.
This could be due to the effect of hydrogen lowering the
concentration of strongly bound flat-lying intermediates on the
surface or with the existence of a reaction pathway for ADH
that involves dehydrogenation of furfuryl alcohol to the acyl
species before addition of a single hydrogen atom to form the
aldehyde.20
2) Comparison between site availability and reactivity
provided insight with respect to the sites required for the
remaining processes: aldehyde hydrogenation/alcohol dehy-
drogenation and hydrodeoxygenation. Previous surface science
studies showed that high coverage of these aromatic oxygenates
can cause the flat-lying and upright structures to coexist,31
suggesting that AH and ADH reactions could proceed through
either type of intermediate. Under furfuryl alcohol hydro-
genation conditions, the application of thiol coatings decreased
the ADH rate by a much smaller factor than the DC rate,
consistent with the participation of intermediates with reduced
site requirements in ADH. Under furfural hydrogenation
conditions where surface crowding is more severe, the addition
of thiols has even less of an effect on the AH rate.
Hypothetically this is due to a low coverage of more flat-
lying AH/ADH intermediates even on the uncoated catalyst. As
discussed above, comparison between reactivity and site
availability suggests that HDO occurred primarily on particle
edges and steps from an upright structure of furfuryl alcohol.
Even in the presence of high sulfur coverage, the production of
methylfuran was unaffected by the presence of the C18-
modifier and modestly reduced by the BDT-modifier. Thus,
whereas DC preferentially occurs on terraces and HDO on step
edges, AH/ADH has intermediate site requirements, and the
dominant pathway may be dictated by reaction conditions.
While reactivity/structure relationships suggest that AH and
ADH occurred on both terrace and edge sites and that HDO
occurred primarily on particle edges, comparison between
apparent activation energies on the coated and uncoated
catalysts provided evidence that the presence of the modifiers
affected the rate limiting mechanisms for these processes. This
effect may be, in part, related to the role of hydrogen since the
presence of sulfur has been shown to hinder both hydrogen
adsorption and diffusion on single crystal surfaces.32,33
3) The mechanism by which the modifiers improved
selectivity toward the hydrogenation pathways was the same
for the thiol-coated catalysts. In other words, the improved
selectivity behavior observed using the denser BDT-modified
catalyst was fundamentally the same as the C18 catalyst, but the
higher sulfur coverage essentially made this coating more
effective for restricting the undesired DC process. Apparent
activation energies were found to be statistically identical for all
reaction processes between the BDT and C18 catalysts. This
similarity likely results from the fact that both SAMs serve to
isolate Pd sites, creating a more uniform reaction environment
than on the uncoated surface.
In order to quantitatively determine the effect of thiol
coverage on the specific kinetic properties for this reaction
network (e.g., inherent rate constants and adsorption
equilibrium constants), we developed a Langmuir−Hinshel-
wood kinetic model that is included in the Supporting
Information. A primary conclusion from the modeling results
is that the dense BDT coating is associated with elementary
step reaction rates that are far higher than would be predicted
on the basis of CO chemisorption. This result is reflected in the
data reported above, where (for example) a relatively high rate
for HDO is achieved (Figure 2), even though site availability is
drastically reduced compared to the uncoated and C18 coated
catalysts. While the HDO reactivity qualitatively trends with
edge site availability, the lack of exact correlation between edge
site availabilities and HDO rates indicates that CO DRIFTS
likely is not a sensitive probe for all active sites for the reaction
DISCUSSION
■
From the results presented here, the following interpretations
were made regarding the effect of thiolate modifiers on the
furfural reaction network.
1) The decarbonylation reaction mechanism appeared to be
similar on modified and unmodified catalysts. From compar-
isons between reactivity and site availability, the rate of DC was
systematically reduced with increasing coverage of surface
sulfur, consistent with the hypothesis that furfural needed a
larger ensemble of contiguous active sites to bind in the
multicoordinated adsorption geometry required for DC.
Apparent activation energies were found to be nearly the
same for this process on all catalysts tested, and the DC
reaction order increased upon addition of thiol modifiers. This
suggested that the coverage of strongly bound furanic species
on terrace sites was reduced due to a change in average
adsorption energy; on coated catalysts, the available sites for
DC bound furfural more weakly, leading to a higher reaction
order. Though this effect cannot be definitively resolved
without direct measurement of adsorption equilibrium
constants, the data suggest that the thiols present a fairly
simple perturbation to the mechanism without altering the rate-
limiting step.
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dx.doi.org/10.1021/cs500598y | ACS Catal. 2014, 4, 3123−3131