ACS Catalysis
Letter
transition state might inhibit dechlorination. Thus, the
enhanced chemoselectivity of PtZn can be explained by the
unfavored adsorption (or activation) of the C−Cl moiety.
To demonstrate this feature, periodic DFT calculations were
performed using Pt(111) and PtZn(111) slab models (see
Figure 5).
limiting H2 adsorption (activation) is accelerated on electron-
enriched Pt catalysts. A possible interpretation is that the
enhanced σ back-donation from Pt to the H−H antibonding
orbital reduced the activation energy of H2 adsorption.
Alternatively, the electron-enriched Pt might have weakened
the adsorption of the nitro group that was competitive to H2
adsorption. Thus, the electron-enriched Pt not only inhibited
undesired dechlorination but also promoted the desired nitro-
hydrogenation, each of which concertedly improved the
chemoselectivity.
In conclusion, intermetallic PtZn exhibited high catalytic
activity, high chemoselectivity, and a wide substrate scope in
HNB hydrogenation to HAN. This highly efficient heteroge-
neous catalyst worked well under the atmospheric pressure of
H2 at near room temperature. The excellent chemoselectivity of
PtZn was attributed to unfavored C−Cl dissociation and the
promotion of nitro-hydrogenation on electron-enriched Pt
sites.
Figure 5. Energy diagram of 4-CAN dechlorination over PtZn(111)
and Pt(111) slabs. The sum of 4-CAN and bare slab energies was set
to zero for each plane. Side and top views for the transition state (TS)
and the final state are shown. [Legend: Pt, black; Zn, pink; C, gray; Cl,
light green; N, blue; H, white.]
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
■
S
Experimental details, XRD patterns, and TOF data
Geometry optimization revealed that the dissociated aryl ring
displayed an interfacial angle of ∼45° toward each slab surface.
On Pt(111), the dissociation formed aryl C−Pt and Cl−Pt
bonds at a hexagonal closely packed (hcp) hollow and top sites,
respectively, with an adsorption energy (ΔEad) of −18.6 kJ
mol−1. A similar DFT result has also been reported for the
Pd(111) plane.21 Conversely, on PtZn(111), the corresponding
bond formations occurred at a Pt top and a Pt1Zn2 hollow sites,
respectively, with ΔEad = +27.8 kJ mol−1. The dechlorination
activation energy for PtZn(111) (134 kJ mol−1) was much
higher than that for Pt(111) (78.2 kJ mol−1). Thus, the
calculation demonstrates that the C−Cl bond activation on
PtZn(111) is unfavorable kinetically and thermodynamically,
compared with that on Pt(111).
AUTHOR INFORMATION
Corresponding Authors
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This work was supported by JSPS Kakenhi (Grant No.
23360353). We thank Center for Advanced Materials Analysis
Tokyo Institute of Technology for the aid of TEM analysis.
A kinetic study was also performed for Pt/SiO2 and Pt−M/
SiO2, revealing that a first-order dependence of the 4-CNB
conversion rate on hydrogen pressure was observed. This
indicates that nitro-hydrogenation is much faster than the rate-
limiting H2 adsorption. We confirmed that hydrogen diffusion
is not rate-limiting by control experiments with different
stirring rates (125 and 375 rpm), which showed the same
reaction rate. On the other hand, a kinetic study using 4-CAN
as a reactant exhibited that dechlorination showed a first-order
dependence on the 4-CAN concentration for each catalyst,
suggesting that dissociative adsorption of 4-CAN is the rate-
determining step of dechlorination. Note that the rate-
determining step differs between nitro-hydrogenation and
dechlorination. The reaction rate order is as follows: nitro-
hydrogenation > H2 adsorption > dechlorination, consistent
with the observation that 4-CAN was obtained as a main
product over most catalysts.
We also investigated the effect of the electron-rich nature of
Pt on the nitro-hydrogenation of 4-CNB to 4-CAN. A strong
positive correlation between the turnover frequency (TOF) of
the nitro-hydrogenation of 4-CNB and the electron-rich nature
of Pt (Pt 4f7/2 binding energy) was observed (see Figure S4 in
the Supporting Information). Note that PtZn afforded a TOF
7-fold higher than that of Pt. This trend suggests that the rate-
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