ACS Catalysis
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soned that any surface defect, such as adatoms, kinks or
steps, could facilitate C-C dissociation by stabilizing ad-
sorption of the C6H5 moiety, as illustrated in Figure 9.
Indeed, using the step model of Figure 9 as a prototypical
case, adsorption of the C6H5 moiety is favored by 0.23 eV
relative to adsorption of the C6H5 moiety on the flat sur-
face (Figure 5). Comparison of the structures in Figures 5
and 9 show a much less distorted structure if the C6H5
moiety can engage in a Ni-C bond in plane with the aro-
matic ring. Of course, the same reasoning can be applied
to dissociation of a C-H bond of BZ. As expected, low-
coordinated sites of Ni are preferentially occupied by Zn,
as confirmed by DFT calculation. The Zn occupation on
the corner site is 0.17 eV more preferred compared to that
on a flat surface (the geometry of the surface is shown in
Figure S9). It is likely that the impact of Zn alloying is
geometric site occupation of low-coordination sites of Ni
with inert Zn atoms, which prevents the unpreferred
methanation. As a result, NiZn shows improved selectivity
for TOL over monometallic Ni catalyst.
AUTHOR INFORMATION
Corresponding Authors
*E-mails: kazuhiro.takanabe@kaust.edu.sa (K. Takanabe).
luigi.cavallo@kaust.edu.sa (L. Cavallo).
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Author Contributions
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The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
ACKNOWLEDGMENT
The research reported in this work was supported by the
King Abdullah University of Science and Technology
(KAUST). A.A. acknowledges Saudi Aramco for financial
support. A.J. and L.C. are grateful to the KAUST Supercom-
puting Laboratory (KSL) for the resources provided under
the project k1087
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CONCLUSIONS
The kinetic analysis of MCH dehydrogenation reaction
revealed a fractional order with respect to MCH for the
three catalysts. In contrast, a approximately half order
with respective to H2 was observed for NiZn and Ni cata-
lysts, differing from the nearly zero order for Pt catalyst.
DFT calculation indicated that exothermic dissociative
adsorption of TOL occurs, losing a H from the methyl
functional group. This strongly adsorbed surface inter-
mediate led to the kinetics where hydrogenation is para-
doxically the rate determining step during dehydrogena-
tion of MCH. This mechanism explains the essential role
played by H2 in maintaining the activity of the metallic
surface from the H-deficient surface species produced via
the dehydrogenation of TOL. In contrast, sensitivity to
the H2 pressure was smaller in the absence of methyl
group, i.e., CH dehydrogenation to BZ. DFT calculation
suggested that low-coordinated Ni step sites were respon-
sible for C-C cracking of the methyl group from aromatics
as the H-deficient BZ was stabilized at such sites. A com-
bined experimental and theoretical study led to the hy-
pothesis that Zn preferentially occupies these low-
coordinated unselective sites (such as corner and edge
sites), which in turn avoids the C-C cracking and is thus
responsible for the improved TOL selectivity. The effects
of the alloy and the reaction mechanism presented in this
study can be generalized for various alkyl aromatic chem-
istries, which are relevant to various catalytic reform-
ing/hydrogenation/dehydrogenation/hydrogenolysis re-
actions.
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ASSOCIATED CONTENT
Supporting Information. Catalyst preparation, thermody-
namic aspects of relevant dehydrogenation reactions, catalyst
characterization, supplementary kinetic data and theoretical
data are shown in the Supporting Information. This material
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