Angewandte
Communications
Chemie
the nominal “PtNPs” (Figure 2c). When the size of the PtNPs
decreases, the intensity of the “white lines” at 11.56 keV
[40]
increases, indicating an increase in the d-band vacancy.
PtNPs of a large size (Pt-2.4) show fine XANES features
resembling those of a Pt foil, while PtNPs of small sizes (Pt-1.2
and Pt-0.7) show fine features deviating from those of the Pt
foil, with Pt-0.7 displaying similar features to those from
[
41]
PtO2,
suggesting a gradual metal-oxide transition with
decreasing size of the PtNPs. Fourier transformation of
extended X-ray absorption fine structure (FT-EXAFS) fur-
ther confirms the metal-oxide transition, showing weakening
Pt–Pt coordination shells while emerging Pt–O shells (Fig-
ure 2d).
Therefore, a decrease of crystallinity and a metal-oxide
transition were observed with the decreasing size of the
PtNPs. To further reveal their interaction with hydrogen and
quinoline molecules, the d-band electron structures of the
PtNPs were characterized by high-resolution valence-band
(
VB) XPS spectra (Figure 2e), which are proportional to the
density of states (DOS), and directly related to the strength of
[
5,42,43]
Figure 3. Catalytic activities of Pt-x (x=0.7, 1.2, 2.4, and 5.3) in
interaction between the PtNPs and guest molecules.
The
hydrogenation of quinoline at room temperature and ambient (bal-
loon) hydrogen pressure. a) Possible products of the quinoline hydro-
genation. b) Plots of conversion of quinoline and selectivity toward
spectra were recorded in an ultra-high vacuum (UHV), so
could be different from those taken under ambient reaction
conditions. Therefore, this analysis provides a general, qual-
itative trend of the d-band structure of the PtNPs varying with
their sizes. According to the d-band center theory, when
a guest molecule is adsorbed on a metal surface, hybridization
between the metal d-band and an induced state by the guest
molecule occurs to form fully filled bonding DOS and
partially filled antibonding DOS states. The bond strength is
determined by the filling degree of the antibonding states,
which can be described by the position of the d-band
1,2,3,4-tetrahydroquinoline against reaction time with Pt-x as the
catalyst. c) Plot of TOF (per surface Pt atom) against diameter of the
PtNPs. The TOF for activation of H and D to form HD is listed for
2
2
comparison. Inset: Size-dependent mass activity (per unit mass of Pt)
of the PtNPs in hydrogenation of quinoline. d) Change of conversion
and selectivity in 5 cycles of the catalysis with Pt-1.2 as the catalyst.
e) TEM images of the catalyst before and after cycling.
[
44–46]
center.
above the d electron band,
a reference for the d-band center.
Because the antibonding states lie directly
was over 99% (Figure 3b). With decreasing size of the PtNPs,
the turnover frequencies (TOFs) first increase and then
decrease, reaching a maximum when the size of the PtNPs was
1.2 nm (Figure 3c). The champion catalyst (Pt-1.2) shows an
approximately sixfold increase in the TOF compared with
PtNPs of a large size, 5.3 nm, and the increase in the catalytic
mass activity reaches as high as 28-fold (Figure 3c, inset). The
volcano-shape profile illustrates very well the Sabatier
[
44]
we chose the VBM as
[
42]
Figure 2e shows
a narrowing of the d electron bands with decreasing size of
the PtNPs, which can be attributed to the hybridization of
a lower number of the wave functions in PtNPs of a small size.
It leads to a significant shift of the d-band center towards the
VBM, resulting in an upward shift of the antibonding DOS
states, lower occupation of them, and thus stronger inter-
action with guest molecules, hydrogen and quinoline in this
case. On the other hand, the continuously increasing oxida-
tion state, that is, the d-band vacancy of the PtNPs with
decreasing size may also account for the stronger interac-
[46,48]
principle,
with optimal catalytic activity achieved at
a specific size of the PtNPs (ca. 1.2 nm), wherein the
interaction between the PtNPs and the reactants is neither
too weak (size > 1.2 nm) nor too strong (size < 1.2 nm). It is
worth noting that although Pt-5.3 is capped by PVP with
different surface chemistry, the size-dependence is already
demonstrated based on the other three catalysts. In addition,
thanks to the heterogeneous mechanism (Figure S10) and the
stabilization of the ultrasmall PtNPs by the RF resin, the
catalyst can be recovered from the reaction system without
causing dissolution, detachment or aggregation of the PtNPs
(Figure 3e, Figure S14 and Table S3 in the Supporting Infor-
mation). The recovered catalyst can be repeatedly applied in
the hydrogenation reactions, for example in 5 runs of our
demonstration, without showing discernible decrease in the
conversion of quinoline or the reaction selectivity (Fig-
ure 3d).
[
14,47]
tion,
owing to there being fewer electrons available for
filling the antibonding DOS states.
Based on the analysis of the electron structure, as well as
the prediction of increasing surface unsaturated sites, it is
expected that PtNPs of appropriately small sizes may show
significantly enhanced catalytic activity in regioselective
hydrogenation of quinoline (Figure 3a). In a typical reaction
under ambient conditions, clear differences in the catalytic
activity can be observed with PtNPs of varying sizes (Fig-
ure 3b). Among them, Pt-1.2 represents the most active
catalyst with conversion of quinoline reaching over 99%
within 80 min, showing astonishingly high catalytic activity,
[
23,28]
which substantially exceeds the best results in literature.
The size-dependent interaction between PtNPs and the
reactants can be also inferred from the HD formation when
The selectivity toward 1,2,3,4-tetrahydroquinoline (THQ)
Angew. Chem. Int. Ed. 2016, 55, 1 – 7
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