PdPt Alloy Nanocatalysts Supported on TiO2: Maneuvering Metal–Hydrogen Interactions for Light-Driven and Water-Donating Selective Alkyne Semihydrogenation

Article abstract of DOI:10.1002/smll.201604173

Low power consumption and minimal potential hazards are ultimate goals for the modern development of chemical manufact? however, it often reduces the selectivity of chemical reactions by implementing a new reaction system. A nanocatalyst design is reported for achieving efficient and selective alkyne semihydrogenation through the photocatalytic hydrogen transfer from water, which avoids the use of a heat source and explosive H₂. The PdPt catalytic sites that are implemented on the TiO₂ photocatalyst hold the key to achieving both high activity and selectivity. As compared with pure Pd or Pt, the alloy cocatalysts can better harness H diffusion/desorption for selective semihydrogenation as well as suppress competitive H₂ evolution. This work opens up new possibilities for green and selective alkyne semihydrogenation and highlights the importance of lattice engineering to catalytic selectivity.

Full text of DOI:10.1002/smll.201604173

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Nanocatalysis
PdPt Alloy Nanocatalysts Supported on TiO2:
Maneuvering Metal–Hydrogen Interactions for
Light-Driven and Water-Donating Selective Alkyne
Semihydrogenation
Mengqiao Li, Ning Zhang, Ran Long,* Wei Ye, Chengming Wang, and Yujie Xiong*
L
ow power consumption and minimal potential hazards are ultimate goals for
the modern development of chemical manufacturing; however, it often reduces
the selectivity of chemical reactions by implementing a new reaction system.
A nanocatalyst design is reported for achieving efficient and selective alkyne
semihydrogenation through the photocatalytic hydrogen transfer from water, which
avoids the use of a heat source and explosive H . The PdPt catalytic sites that are
2
implemented on the TiO photocatalyst hold the key to achieving both high activity
2
and selectivity. As compared with pure Pd or Pt, the alloy cocatalysts can better
harness H diffusion/desorption for selective semihydrogenation as well as suppress
competitive H evolution.This work opens up new possibilities for green and selective
2
alkyne semihydrogenation and highlights the importance of lattice engineering to
catalytic selectivity.
Selective semihydrogenation of alkynes is a class of highly using flammable H , with steric or electronic effects imple-
2
[
1–6]
important organic reactions producing alkenes—the key mented.
Given the ultimate goal of developing efficient
building blocks in various natural products, pharmaceuticals, and safe processes in chemical manufacturing, the heteroge-
and agrochemicals—which requires to suppress the over- neous catalysis readily for product separation, combined with
[
1]
hydrogenation to alkanes. To achieve both high activity a substantial reduction in power consumption and potential
and selectivity, the alkyne semihydrogenation is generally hazards, would be an ideal solution.
accomplished through a thermal-based chemical process
To minimize power consumption, photocatalysis is a
promising approach to replace the conventional thermal
process by solar energy in which the photogenerated elec-
trons and holes can participate in organic reactions through
M. Li, N. Zhang, Dr. R. Long, Dr. W. Ye, Dr. C. Wang,
Prof. Y. Xiong
Hefei National Laboratory for Physical Sciences
at the Microscale
iChEM (Collaborative Innovation Center
of Chemistry for Energy Materials)
School of Chemistry and Materials Science
Hefei Science Center (CAS), and National Synchrotron
Radiation Laboratory
[7,8]
various mechanisms.
Meanwhile, the potential danger of
explosion from high-pressure H2 can be avoided through
[
9–13]
catalytic hydrogen transfer in hydrogenation.
Although
water should be a clean and sustainable hydrogen source
[
14]
for the hydrogen-transfer hydrogenation,
the hydrogen
sources in this approach have been mainly donated by
organic molecules (e.g., isopropanol, triethanolamine, orga-
nosilanes, and formic acid) despite the involvement of water
University of Science and Technology of China
Hefei, Anhui 230026, P. R. China
E-mail: longran@ustc.edu.cn; yjxiong@ustc.edu.cn
[9,13]
in reaction systems.
The bottleneck for utilizing water in
photocatalytic hydrogen-transfer hydrogenation originates
from the challenges for catalyst design. In the lack of cata-
lytic sites, protons can be reluctantly reduced to participate
The ORCID identification number(s) for the author(s) of this article
can be found under http://dx.doi.org/10.1002/smll.201604173.
[
15]
in hydrogenation.
However, too active catalytic sites may
[
16,17]
[2,3]
DOI: 10.1002/smll.201604173
undergo H evolution
or overhydrogenation,
which
2
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Figure 1. a) TEM, b) HRTEM, and c) STEM images of TiO -Pd0.5Pt0.5 hybrid structures. d,e) EDS line scan profiles of Pd and Pt elements.
2
compromises the activity and selectivity of alkyne semihy- Pd Pt
NPs possess comparable particle sizes and loading
x
1-x
drogenation. It thus seems an irreconcilable issue between densities (Table S1, Supporting Information). Figure 1a
the efficient utilization of water hydrogen and the high per- shows the transmission electron microscopy (TEM) image
formance of semihydrogenation.
of a typical sample—TiO -Pd0.5Pt0.5 NPs, indicating that the
2
The PdPt alloy nanoparticles supported on semicon- metal nanoparticles with an average size of ≈2.8–3.3 nm have
ductor TiO can serve as ideal catalytic sites for reducing the been deposited on the surface of TiO nanosheets. The lat-
2
2
protons in water toward efficient and selective light-driven tice fringes with a spacing of 2.3 Å in high-resolution TEM
alkyne semihydrogenation. The key factor to our design is the (HRTEM) image (Figure 1b) can be assigned to the (111)
adsorption of H (H ) to metal (M) sites upon the reduction planes of PdPt alloy. As determined by inductively coupled
ad
of protons to Had atoms. The M–H binding strength deter- plasma mass spectrometry (Table S1, Supporting Informa-
mines the rates for Had diffusion and desorption—the rate- tion), the loading of Pd0.5Pt0.5 NPs on TiO2 nanosheets is
determining steps in H-participating reactions.[2] As a proof about 1.99 mol.%. The elemental distributions of Pd and
of concept, we select Pd and Pt as the metals for forming Pt are further characterized by scanning TEM (STEM) and
alloy lattice. As both metals are active for proton reduction energy-dispersive X-ray spectroscopy (EDS) line scan anal-
but have different M–H binding strengths, the concentrations yses. Figure 1c,d shows that Pd and Pt elements are nearly
of Pd and Pt in lattice provide a knob for maneuvering the distributed at the same locations, suggesting the formation of
diffusion and desorption of Had atoms. Based on this plat- alloy.
form, we reveal that Pd0.5Pt0.5 is an optimal configuration
The TiO -Pd Pt
hybrid structures with controllable
2
x
1-x
to significantly improve the activity and selectivity in water- Pd/Pt ratios set up a platform for correlating hydrogena-
donating alkyne semihydrogenation by suppressing H evolu- tion activity and selectivity with catalytic sites in the system
2
tion and alkyne overhydrogenation, as compared with pure of photocatalytic H transfer from water. The addition of
Pd and Pt surfaces.
Pd Pt NPs does not affect the light absorption at λ < 400 nm
x 1-x
The anatase TiO nanosheets with an average edge length corresponding to the bandgap of anatase TiO (Figure S7,
2
2
of ≈50 nm and thickness of 6–7 nm (Figures S1 and S2, Sup- Supporting Information). Under light illumination, TiO sup-
2
porting Information), which are first prepared through a ports can thus efficiently supply photoexcited electrons to
[18]
modified hydrothermal method,
ductor supports. As shown in Figures S3–S6 (Supporting Schottky junction.
Information), Pd Pt (x = 0–1) alloy nanoparticles (NPs) either evolve as molecular H or transfer to the molecules
are used as semicon- metal sites for the reduction of protons in water due to the
[
19]
The newly formed Had atoms would
x
1-x
2
are then grown in situ on the TiO nanosheets via NaBH4 with unsaturated bonds for hydrogenation. In our investiga-
2
reduction. Despite their varied chemical compositions, the tion, we employ 2-methyl-3-butyn-2-ol (MBY) as a molecule
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Figure 2. The conversion of MBY hydrogenation and the selectivity to MBE (i.e., semihydrogenation) using various TiO -Pd Pt catalysts: a) time
2
x
1-x
dependence and b) at t = 3 h. Reaction conditions: MBY, 0.5 mmol; catalysts, 10 mg; water/methanol (3:1 v/v), 10 mL; UV irradiation (λ < 400 nm,
-
2
3
mW·cm ). c) H production from the reaction system in MBY semihydrogenation (i.e., the conditions for panel (a)) at t = 1 h, in comparison with
2
that in the absence of MBY. d) The conversion of MBY hydrogenation and the selectivity to MBE using various catalysts at t = 1 h under the same
-
2
conditions as those for panel (a), except for different irradiation power densities (3, 9, and 18 mW·cm ). In panels (a) and (d), the solid and dash
lines represent the conversion and semihydrogenation selectivity, respectively. Note that the semiconductor support TiO is not denoted in the
2
sample symbols.
model which can be transformed into 2-methyl-3-buten-2-ol owing to the moderate M–H binding. Although all the PdPt
(
2
MBE) through semihydrogenation or 2-methyl-3-butan- cocatalysts exhibit the enhanced activity and selectivity, the
-ol (MBA) through overhydrogenation, respectively. To PdPt alloy with high Pd content (i.e., Pd0.75Pt0.25) exhibits the
improve the reduction of protons, we add methanol into characteristic of Pd and gives relatively low catalytic activity,
water (25 vol.%) as a sacrificial agent to consume photo- while the performance with high Pt content (i.e., Pd0.25Pt0.75
excited holes. The methanol would be oxidized to HCHO more resembles that of Pt to possess relatively low activity
Figure S8, Supporting Information) by the holes and further and selectivity. Among the catalysts, TiO -Pd0.5Pt0.5 NPs
)
(
2
to HCOOH which can be easily catalytically decomposed to exhibit the best catalytic performance with the conversion
CO (Figure S9, Supporting Information) as indicated by our of MBY up to 99.6% and the selectivity to MBE at 94.0%
2
[
20]
product analysis.
as the reaction proceeds to 3 h. By reducing the irradiation
-2
Figure 2a shows the time-dependent conversion of MBY power density from 3 to 1 mW·cm (Figure S10, Supporting
hydrogenation and selectivity to MBE (i.e., the semihydro- Information), the conversion of MBY hydrogenation achieves
genation of MBY; see the selectivity for other by-products 97.2% while the selectivity to MBE can be maintained above
in Tables S2 and S3, Supporting Information) by various 92.5% between t = 8 and 10 h. This sufficient time interval
TiO -Pd Pt hybrid structures in which H O serves as the would allow the separation of products from raw chemicals
2
x
1-x
2
hydrogen source through photocatalysis. The performance in solution.
comparison of the catalysts outlines two key findings. First,
Upon identifying the key role of PdPt alloys, the next
the conversion has a strong correlation with the chemical question would be: which factor induces the variations in cat-
compositions of metal sites. The PdPt alloys can offer dramat- alytic activity and selectivity? As hydrogenation is an addi-
ically higher activity than pure Pd and Pt. Second, the Pt sites tion reaction of hydrogen to unsaturated bonds, the activity
exhibit quite low selectivity for producing MBE. Although and selectivity of hydrogenation should be directly associated
the Pd cocatalysts can offer high selectivity for semihydro- with the behavior of Had atoms. In principle, the Had atoms
genation, their catalytic activity is relatively low due to the should undergo atomic diffusion on catalyst surface and des-
[
3]
too strong Pd–H binding. Apparently the incorporation of Pt orption from the surface toward the hydrogenation. Mean-
into Pd lattice can effectively improve the catalytic activity.
while, the Had diffusion and desorption may also induce a
This trend can be better resolved by the performance competitive process—H evolution.[16,17] Thus, the diffusion
2
comparison at t = 3 h (Figure 2b). As compared with pure Pd and desorption of Had atoms and the competitive evolution
and Pt, the PdPt alloys largely overcome the limitations in cat- of H2 molecules are the two factors affecting the perfor-
alytic activity and semihydrogenation selectivity, respectively, mance of hydrogenation.
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Figure 3. Mass spectra of a) MBE with 2 D atoms (m/z = 73) and b) MBA with 4 D atoms (m/z = 77) produced over TiO -Pd0.5Pt0.5 in photocatalytic
2
hydrogenation of MBY using D O instead of H O. Mass spectra of c) MBE (m/z = 71) and d) MBA (m/z = 73) without D atoms produced over TiO -
2
2
2
Pd0.5Pt0.5 in photocatalytic hydrogenation of MBY using CD OD instead of CH OH. The isotopic labeling experiments are performed under identical
3
3
photocatalytic reaction conditions to those for Figure 2a. The standard spectra for MBE and MBA are provided in Figure S9 (Supporting Information).
To decode the mechanism, we first investigate the H evo- density, as the relatively low activity of TiO -Pd is mainly
2
2
lution as shown in Figure 2c. In the absence of MBY, no Had ascribed to strong Pd-H binding.
atoms would be consumed for hydrogenation so H evolution
significantly takes place. Similarly to our previous studies,
In addition to catalytic activity, we observe that the
2
[
17]
semihydrogenation selectivity of TiO -Pt is kept low when
2
the H evolution activity depends on the chemical compo- the power density is altered (see Figure 2d). This indicates
2
sitions of Pd Pt NPs. In particular, pure Pd possesses the that the relatively low semihydrogenation selectivity (80%
x
1-x
lowest activity for H evolution as too strong Pd-H binding in Figure 2b) is an intrinsic property of Pt active sites.
2
[
21–25]
slows Had diffusion and desorption.
It is anticipated The weak Pt-H binding and low diffusion barrier enable
that such strong Pd-H binding would also make it difficult to the fast diffusion of Had atoms to the sites where MBY
[
21–25]
transfer Had atoms to MBY, limiting the conversion (55.1% molecules are hydrogenated,
resulting in the ove-
at t = 3 h in Figure 2b) of MBY hydrogenation by TiO -Pd rhydrogenation. In sharp contrast, the TiO -Pd catalyst
2
2
catalyst. As MBY is added into the system, all the samples that possesses strong binding with Had atoms offers high
except pure Pt become inactive in H production, indicating MBE selectivity (97% in Figure 2b) owing to slow atomic
2
[
25]
that Had atoms have been mostly transferred to MBY hydro- diffusion.
genation. Nevertheless, the TiO -Pt catalyst still produce The low rate of MBE hydrogenation relative to MBY
a large amount of H due to the weaker Pt-H binding and hydrogenation is the reason for which the MBY semihydro-
2
2
[
25]
lower diffusion barrier, which largely consumes Had atoms genation can be improved by governing Had atomic diffu-
to limit the activity of MBY hydrogenation (refer to the sion. As shown in Figure S11 (Supporting Information), the
5
0.5% conversion at t = 3 h in Figure 2b).
This argument is further supported by a control experi- 0.114 mmol·h while that for MBY hydrogenation to MBE
average conversion rate of MBE hydrogenation to MBA is
-
1
-
1
ment performed under the same conditions except for the is 0.166 mmol·h . As the difference between the hydrogena-
use of higher irradiation power density.The higher irradiation tion rates of MBY and MBE is not so significant, the atomic
power density that increases the number of incident photons diffusion of Had should be tightly controlled; otherwise, MBE
would form more Had atoms at metal surface through proton would be still hydrogenated to MBA with the abundant H
reduction. As displayed in Figure 2d, the high concentration source. It is worth pointing out that the direct use of H as a
2
of Had atoms substantially promotes the conversion by TiO - hydrogen source instead of H-transfer scheme leads to low
2
-2
-2
Pt from 12.3% (3 mW·cm ) to 42.4% (18 mW·cm ) at t = semihydrogenation selectivity (Figure S12, Supporting Infor-
h. This suggests that the low activity by TiO -Pt is indeed mation), most likely because the dissociation of molecular H2
1
2
limited by the concentration of Had atoms, caused by the may locally induce too high concentrations of Had atoms. This
fast H evolution. In comparison, the activity by TiO -Pd is well manifests the niche of our water-donating H-transfer
2
2
only improved from 16.7% to 28.5% by increasing the power scheme.
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Figure 4. Schematics illustrating the photocatalytic hydrogen transfer and evolution at a) TiO -Pt NPs (fast diffusion and desorption as well as
2
competitive H evolution), b) TiO -Pd NPs (slow diffusion and desorption), and c) TiO -Pd0.5Pt0.5 NPs (medium diffusion and desorption). Note that
2
2
2
2D metal models are used to simplify schematic illustration here.
Prior to reaching a conclusion on mechanisms, we have
The information gleaned above demonstrates that the
to further validate our hypothesis that the hydrogena- photocatalytic hydrogenation takes place through a sequent
+
tion is accomplished through the H transfer from water. process—H reduction in from water, H diffusion at metal
ad
+
Although methanol may provide H to interact with H O surface and desorption from metal surface, and addition of
to form H O , this process should be significantly slower H atoms to unsaturated bonds. Given the comparable light
than the release of H from water—the major process for absorption and H reduction, the performance is mainly
forming H O in the mixture of water and methanol. As varied by the steps of Had diffusion and desorption. As indi-
2
+
3
+
+
+
3
+
[25]
H O is the hydrogen source for photocatalytic hydrogena- cated by the literature,
Had atoms are bound to Pt sites
3
tion, only a very limited number of hydrogen atoms from more weakly in either hollow-site or top-site configurations
methanol can participate in the formation of MBE and with barely no surface diffusion barrier so that the Had can
MBA upon the complete consumption of MBY. In other spill over the surface easily (Figure 4a). As a result, the
words, the hydrogen source for hydrogenation mainly orig- weakly bound Had atoms can readily evolve into molecular
inates from water. To prove our hypothesis, we perform a H for substantial consumption, limiting the conversion of
2
series of isotope labeling experiments to explore whether MBY hydrogenation. Meanwhile, they rapidly diffuse over
the Had originates from water or from methanol in our Pt surface to reach the nearby sites for overhydrogena-
semihydrogenation reactions. As H O is replaced by D O tion, reducing the selectivity of semihydrogenation. In com-
2
2
in the reaction system (Figure 3a,b), the reaction produces parison, the adsorption of Had atoms to Pd surface takes a
[
25,26]
MBE (m/z = 73 with 2 D atoms) and MBA (m/z = 77 with hollow-site configuration for strong binding (Figure 4b).
D atoms) products with the MBE selectivity of 96.2%, For this reason, the slow Had diffusion and desorption would
4
both of which are absolutely deuterated as compared with form the bottleneck for improving MBY conversion although
standard mass spectra of MBE (m/z = 71) and MBA (m/z = the selectivity of semihydrogenation can be maintained high.
7
3) (Figure S13, Supporting Information). In contrast, both Upon analyzing the limitations for pure Pd and Pt surfaces,
MBE and MBA are scarcely deuterated (Figure 3c,d) when we can envision that the integration of Pd with Pt in lat-
we substitute CD OD for CH OH. Only a trace amount of tice would offer Pd-Pt hollow sites for Had adsorption. As
3
3
D atoms has been detected in the formed MBE (m/z = 72 or such, the M–H binding can be tuned to a suitable range for
3) and MBA (m/z = 74 or more). This unambiguously dem- appropriate diffusion and desorption rates (Figure 4c). Con-
onstrates that the hydrogen source for hydrogenation sequently, the selectivity by TiO -Pd0.5Pt0.5 NPs remains high
7
2
mainly originates from water.
when the conversion reaches nearly 100%.
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Received: December 16, 2016
Revised: March 8, 2017
Published online:
J. Am. Chem. Soc. 2011, 133, 12787.
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