Tin-Assisted Fully Exposed Platinum Clusters Stabilized on Defect-Rich Graphene for Dehydrogenation Reaction

Article abstract of DOI:10.1021/acscatal.9b00601

Tin-assisted fully exposed Pt clusters are fabricated on the core-shell nanodiamond@graphene (ND@G) hybrid support (a-PtSn/ND@G). The obtained atomically dispersed Pt clusters, with an average Pt atom number of 3, were anchored over the ND@G support by the assistance of Sn atoms as a partition agent and through the Pt-C bond between Pt clusters and defect-rich graphene nanoshell. The atomically dispersed Pt clusters guaranteed a full metal availability to the reactants, a high thermal stability, and an optimized adsorption/desorption behavior. It inhibits the side reactions and enhances catalytic performance in direct dehydrogenation of n-butane at a low temperature of 450 °C, leading to >98% selectivity toward olefin products, and the turnover frequency (TOF) of a-PtSn/ND@G is ～3.9 times higher than that of the traditional Pt₃Sn alloy catalyst supported on Al₂O₃ (Pt₃Sn/Al₂O₃).

Full text of DOI:10.1021/acscatal.9b00601

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Article
Tin Assisted Fully Exposed Platinum Clusters Stabilized
on Defect-Rich Graphene for Dehydrogenation Reaction
Jiayun Zhang, Yuchen Deng, Xiangbin Cai, Yunlei Chen, Mi Peng, Zhimin Jia, Zheng Jiang, Pengju
Ren, Siyu Yao, Jinglin Xie, Dequan Xiao, Xiao-Dong Wen, Ning Wang, Hongyang Liu, and Ding Ma
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019
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Tin Assisted Fully Exposed Platinum Clusters
Stabilized on Defect-Rich Graphene for
Dehydrogenation Reaction
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Jiayun Zhang,1, 2 # Yuchen Deng,3# Xiangbin Cai,4 # Yunlei Chen,5, 6 # Mi Peng,3
1, 2
Zhimin Jia,
Zheng Jiang,7 Pengju Ren,5, 6 Siyu Yao,3 Jinglin Xie,3 Dequan Xiao,8
Xiaodong Wen, 5, 6 Ning Wang, 4 * Hongyang Liu, 1, 2 * and Ding Ma, 3*
1
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, Shenyang 110016, P. R. China.
2 School of Materials Science and Engineering, University of Science and Technology
of China, Hefei 230026, P. R. China.
3
Beijing National Laboratory for Molecular Sciences, College of Chemistry and
Molecular Engineering and College of Engineering, and BIC-ESAT, Peking University,
Beijing 100871, P. R. China.
4 Department of Physics and Center for Quantum Materials, Hong Kong University of
Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, P. R. China.
5
State Key Laboratory of Coal Conversion, Institute Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, P. R. China.
6 University of Chinese Academy of Science, No. 19A Yuanquan Road, Beijing 100049,
P. R. China.
7
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai
201204, P. R. China.
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8 Center for Integrative Materials Discovery, Department of Chemistry and Chemical
Engineering, University of New Haven, 300 Boston Post Road, West Haven,
Connecticut 06516, United States.
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# These authors contributed equally to this work.
Corresponding authors: *Hongyang Liu (liuhy@imr.ac.cn), *Ning Wang
(phwang@ust.hk) and *Ding Ma (dma@pku.edu.cn)
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ABSTRACT: Tin assisted fully exposed Pt clusters are fabricated on the core-shell
nanodiamond@graphene (ND@G) hybrid support (a-PtSn/ND@G). The obtained
atomically dispersed Pt clusters, with an average Pt atom number of 3, were anchored
over the ND@G support by the assistance of Sn atoms as a partition agent and through
the Pt-C bond between Pt clusters and defect-rich graphene nanoshell. The atomically
dispersed Pt clusters has guaranteed a full metal availability to the reactants, a high
thermal stability as well as an optimized adsorption/desorption behavior. It inhibits the
side reactions and enhances catalytic performance in direct dehydrogenation of n-
butane at a low temperature of 450 °C, leading to >98% selectivity toward olefin
products, and the TOF of a-PtSn/ND@G is approximately 3.9 times higher than that of
the traditional Pt3Sn alloy catalyst supported on Al2O3 (Pt3Sn/Al2O3).
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KEYWORDS: Atomic dispersion, Defect-rich graphene, Pt nanocluster,
Dehydrogenation, Heterogeneous catalysis
INTRODUCTION
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Light olefins, a widely used feedstock, are important building blocks for the synthesis
of polymers and other value-added chemicals.1-3 Direct dehydrogenation (DDH) of
light alkane is a typically industrial process for production of olefin which is an
endothermic reaction thus requires high temperatures to obtain satisfactory conversion
rates and olefin yields. However, DDH could also lead to serious catalyst deactivation
by sintering of active sites and coking at high reaction temperatures.1, 4 To date, Pt3Sn
alloy catalyst (Pt3Sn/Al2O3) is widely recognized as one of the best catalysts for this
reaction, but rapid deactivation is still a main problem because the sintering of Pt3Sn
nanoparticles (NPs) is unavoidable during dehydrogenation process. Meanwhile, only
the surface Pt atoms in Pt3Sn NPs can participate in the catalytic reaction which is
uneconomical for Pt utilization. Therefore, developing a better dispersed and more
stabilized Pt-based catalyst is pivotal for the DDH of alkanes.
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Among all the strategies to increase metal dispersion on support, the most extreme
cases aim to create isolated sites or atomically dispersed species on the surface.5-14 In
those cases, each metal atom could be utilized for the reaction and the absence of
nearest-neighbor atoms may inhibit the carbonaceous deposition in the reaction
according to some reports.15 In general, the support of catalyst plays an important role
in Pt-based catalysts.11, 16-18 Among various supports, nanocarbons, especially those
with functional groups, have drawn much attention, as they can provide abundant
surface sites to host metal atoms, forming single-site catalysts7-9, 19-22 and exhibiting
distinct catalytic performance in various reactions.23-25 The nanodiamond (ND) is a
unique class of nanocarbon materials. The native ND or those after various treatments
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is an ideal platform for the host of metal NPs.26 We have demonstrated recently that
after a thermal treatment, the surface of ND can be reconstructed into an ultrathin,
curved and defect-rich sp2 graphene nanoshell reinforced by a sp3 diamond nanocore
(ND@G) which could anchor isolated Pd atoms through Pd-C bond to obtain
atomically dispersed Pd catalysts.27
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Herein, we report a method to construct Pt-Sn catalyst with unique Pt species, i.e.,
fully exposed Pt clusters anchoring over the ND@G support (a-PtSn/ND@G). By the
strong interaction with the support and also the partitioning role of homogenously
dispersed Sn species, Pt atoms were atomically dispersed over the surface of ND@G.
The stable Pt clusters with average Pt atom number of 3 were constructed. The as-
synthesized a-PtSn/ND@G catalyst shows excellent catalytic performance in DDH of
n-butane at a relatively low temperature (i.e., 450 oC), benefiting from the efficient use
of Pt atoms and easy desorption of butene from these stable fully exposed Pt clusters.
RESULTS and DISCUSSION
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Figure 1. HAADF-STEM images of the a-PtSn/ND@G catalyst, A) Showing the homogeneous
distribution of Pt clusters, scale bar, 10 nm, and B-E) the atomic dispersion of Pt and Sn, scale bar,
0.5 nm. Some Pt and Sn atoms in image B and C are marked by red and green circles according to
the Z-contrast mechanism, respectively. F-I) Energy-dispersive X-ray (EDX) mapping spectroscopy
showing the composition maps of the a-PtSn/ND@G catalyst, scale bar, 10 nm. J) HAADF-STEM
image of a-PtSn/ND@G and the extracted line profiles K, L) along red and blue directions in J),
demonstrating the pronounced intensity difference between Pt and Sn, consistent well with their
distinct atomic numbers (Z), together with the single-atomic-layer thickness of a typical PtSn
cluster, scale bar, 0.5 nm.
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The a-PtSn/ND@G catalyst was prepared by impregnation of a solution of Pt and Sn
precursors over the ND@G hybrid support with a diamond core and a thin graphene
shell decorated with abundant defects formed during the annealing of nanodiamond
(TEM images see fig. S1A-B). Meanwhile, as the control, a Pt/ND@G with the same
Pt loading but without addition of Sn was prepared. It is clear from XRD profiles of
both catalysts that the Pt and Sn species did not form large NPs over the surface of
ND@G (fig. S2A). The aberration-corrected high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) was then employed to study the
detailed structure of Pt and Sn on ND@G (Fig. 1). Notably, lots of island-like
aggregates (1-2 nm in size, marked in red circles) were observed on ND@G support as
shown in Fig. 1A, while no crystalized Pt NPs were observed. The zoomed-in images
(Fig. 1B-E) of these island-like aggregates showed that the Pt clusters were randomly
distributed on the ND@G surface with irregular shapes (marked in red circles). The
mono-dispersed Sn species were also observed on adjacent area (marked in green
circles). The co-existence of Pt clusters and mono-dispersed Sn atoms was further
confirmed by the EDX mapping (Fig. 1F-I), as distinct distribution forms of Pt (red)
and Sn (green) were clearly resolved on the carbon support (blue). These results
demonstrated that the irregular island-like Pt aggregates were composed of irregular
ultra-small Pt clusters with Pt atom number ranging from 1 to few, partitioned by
surrounding mono-dispersed Sn atoms. Significantly, these ultra-small Pt clusters were
one-atomic-layer thick (Fig. 1J-L and fig. S4 A-F), indicating a maximum dispersion
of all Pt atoms, so we termed it as “fully exposed Pt clusters”. A further H2/O2 titration
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measurement (Table S1) showed that for the a-PtSn/ND@G catalyst, the dispersion of
Pt is 99.2%, further confirming that almost all the Pt atoms were fully exposed on the
ND@G surface, consistent well with the HAADF-STEM results.
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In order to investigate the dispersion of Sn species on ND@G, HAADF-STEM
images of the Sn/ND@G were recorded (fig. S1C-F), where the loading amount of Sn
is identical to that of a-PtSn/ND@G, but without adding Pt. Notably, the Sn species are
prone to atomically disperse on the defect-rich graphene nanoshell as marked in fig.
S1F, which has the same feature as that in a-PtSn/ND@G catalyst. However, for Pt
catalyst without adding Sn in the preparation step, the structure of the obtained catalyst,
Pt/ND@G, changed significantly. Different from that of a-PtSn/ND@G with only fully
exposed clusters, Pt species on Pt/ND@G has a different distribution on the surface of
the support. The co-existence of the fully exposed Pt clusters and Pt nanoparticles with
3-dimensional lattice structure were clearly resolved on Pt/ND@G, as marked in fig.
S3B. The aggregation of Pt atoms also caused a decrease in Pt dispersion (Table S1).
The results indicate that the co-existence of atomically dispersed Sn species has a
critical influence over the dispersion of Pt on ND@G support. Without the assistance
of the mono-dispersed Sn species, Pt species tend to aggregate to form crystalized NPs
and cannot achieve the high dispersion as that of a-PtSn/ND@G catalyst, which
inevitably lead to significant difference on the corresponding catalytic performance.
In addition, a typical previous reported PtSn alloy (Pt3Sn alloy) catalyst (Pt3Sn/Al2O3)
with high activity in DDH of alkane reaction was prepared as well.16 Fig. S3C-K shows
typical HAADF-STEM images of Pt3Sn/Al2O3 catalyst. A relatively broad distribution
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of metal NPs on Al2O3 was observed (fig. S3D). No diffraction peaks related with Pt,
Sn or Pt-Sn species were resolved on alumina (fig. S2A), due to the overlapping of the
diffraction peaks. EDX mapping analysis (fig. S3H-K) illustrate that Pt and Sn species
are uniformly existed in the same nanoparticles. Pt3Sn alloy was further confirmed by
lattice distance of 0.406nm (the lattice distance of Pt3Sn(100) is 0.4004 nm PDF#65-
0958) (fig. S3E-G), providing the further evidence that Sn was alloyed with Pt to form
Pt3Sn alloy nanoparticles on Al2O3 support.
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Figure 2. FT-EXAFS profiles of different catalysts, a-PtSn/ND@G, Pt/ND@G, Pt3Sn/Al2O3, Pt foil
and PtO2.
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The structure and the local environment of Pt and Sn species were further
investigated by the extended X-ray absorption fine structure (EXAFS) spectroscopy.
The Fourier-transformed (FT) k3-weighted EXAFS at the Pt L3-edge is shown in Fig.
2. For the a-PtSn/ND@G catalyst, a distinct peak at 1.6 Å and a relatively weaker peak
at 2.6 Å were observed, corresponding to Pt-C/O and Pt-Pt first coordination shells,
respectively. For the Pt/ND@G catalyst, the Pt-Pt coordination peak at 2.6 Å were
relatively stronger than Pt-C/O coordination peak.The relative intensity of two peaks
in a-PtSn/ND@G and Pt/ND@G is more clearly presented in wavelet transformation
(WT) of Pt L3-edge EXAFS oscillations in Fig. 3A-B. For Pt/ND@G (Fig 3B), the
relatively stronger signal at 2.6 Å (corresponds to Pt-Pt coordination, fig. S5) than the
signal at 1.6 Å (corresponds to Pt-O/C coordination, fig. S5) implying the existence of
Pt NPs with larger average Pt-Pt coordination number (C.N.), which agrees well with
the results of HAADF-STEM and H2/O2 titration measurement. The structure of Sn
species was also confirmed by XAFS characterization. As shown in XANES spectra of
Sn K-edge (fig S6), the adsorption edge of Sn in a-PtSn/ND@G is close to SnO2,
indicating that the high dispersion of Sn lead to an increased average chemical valance.
The Fourier-transformed (FT) k2-weighted EXAFS at the Sn K-edge (fig S7) also
showed that no Sn-O-Sn coordination signal was observed, which again confirmed the
atomic dispersion of Sn. As for Pt3Sn/Al2O3, the only peak at around 2.6 Å belongs to
the first coordination shell of Pt-Pt/Sn (Fig.2). The strong intensity of the peak implies
the formation of PtSn alloy nanoparticles which agrees well with HAADF-STEM
results
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Figure 3. Wavelet transform (WT) analysis of different catalysts, A) a-PtSn/ND@G and B)
Pt/ND@G. C) The optimized structure of Pt3 cluster embedded into graphene (Pt3-Gr, through Pt-
C bond) from top and side view.
The detailed EXAFS fitting parameters of these catalysts are shown in Table S2 and
fig. S8-9. For a-PtSn/ND@G, the average C.N. of Pt-C/O is 2.2, and the average C.N.
of Pt-Pt is only 1.8. In comparison, the average C.N. of Pt-Pt in Pt/ND@G is 4.8. The
result confirmed the presence of ultra-small Pt clusters in the a-PtSn/ND@G catalyst
(Pt-Pt coordination number around 2 (1.8)), revealing that each ultra-small Pt clusters
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has an average Pt atom number of 3. This means all the Pt atoms in the catalyst are
available for adsorptions and reactions. To further investigate the local coordination
structure of Pt species, DFT calculations were used to investigate different bonding
models of atomically dispersed Pt3-(C/O)x clusters on ND@G. We use a cluster with 3
Pt atoms to represent the atomically dispersed Pt cluster species. The optimized
geometry of the Pt3 cluster is shown in Fig. 3C. It was clear that a Pt3 cluster was bound
to the graphene layer through Pt-C bonds (termed as Pt3-Gr). Clearly, the optimized
structure of the triangular Pt3 cluster was not in parallel with carbon support surface,
which may explain the irregular atomic structures of Pt clusters under STEM due to the
projection nature of TEM imaging (fig. S4G-J). The average Pt-C bond length was 2.05
Å, in consistent with the experimental observation of the Pt-C bond length (2.02±0.02
Å) in the first coordination shell (Table S2), suggesting the validity of current model.
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Figure 4. Direct dehydrogenation of n-butane over the different catalysts, A) Conversion and
o
selectivity by time-on-stream during DDH of n-butane at 450 C. GHSV=18000 mL∙gcat-1∙h-1,
nC4:H2=1:1 with He balance. B) The turnover frequency (TOF) for a-PtSn/ND@G, Pt3Sn/Al2O3
and Pt/ND@G. C) Stability test at 450 oC.
We then evaluated the direct dehydrogenation of n-butane over the a-PtSn/ND@G
o
catalyst under atmospheric pressure at 450 C, with Pt/ND@G, Sn/ND@G and
Pt3Sn/Al2O3 as references. As shown in Fig. 4A and Table S3, the initial conversion of
the Pt3Sn/Al2O3 catalyst was 13.6%, and the selectivity to total C4 olefin (e.g., n-butene,
1, 3-butadiene) was 92.1%. However, the activity experienced a sharp decline to 9.5%
in 4 h and gradually decreased to 6.2% in 18 h. As to Pt/ND@G, although the initial
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conversion (24.4%) is higher than that of Pt3Sn/Al2O3 catalyst (13.6%), a quick
deactivation also happened causing conversion dropped from 24.4% to 13.4% in 18 h
test, which is agree with the reaction behavior of Pt3Sn catalyst previously reported.
Deactivation was observed for both Pt/ND@G and Pt3Sn/Al2O3 catalysts, which may
result from coke deposition and/or sintering of active centers. In addition, the
Sn/ND@G catalyst did not show any activity under the reaction condition, providing
the further evidence that mono-dispersed Sn species merely work as a partition agent
to promote the formation of fully exposed Pt clusters on ND@G. Instead, for the a-
PtSn/ND@G catalyst, the initial conversion of n-butane was as high as 40.9%, and the
selectivity of C4 olefin reached 98.2%. Significantly, only a slight deactivation after 18
h (from 40.9% to 35.6%) was observed. In a long-term reaction test, the activity of a-
PtSn/ND@G (Fig. 4C) remained at a rather high level, and the selectivity toward C4
olefin was >98% after 50 h reaction. Atomic images of the used catalyst (fig. S10)
showed the structures of atomically dispersed Pt clusters were well preserved after the
reaction (in 18 h), and the dispersion of Pt in the used a-PtSn/ND@G catalyst was
97.3% (Table S1), indicated that nearly all the Pt atoms remained atomically exposed.
Moreover, the Pt-based activity of a-PtSn/ND@G reached 114 h-1, which was
approximately 3.9 times as that of Pt3Sn/Al2O3 and 2.2 times as that of Pt/ND@G
catalyst, respectively (Fig. 4B). These results demonstrated that the atomic dispersion
of Pt clusters could promise a maximum utilization of Pt atoms during the DDH
reaction of butane, while the improved longevity of a-PtSn/ND@G might be ascribed
to the stabilization effect conferred by partition agents, Sn species.
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Another possible reason for the high activity and stability of a-PtSn/ND@G may
originate from the distinct adsorption/desorption behavior of reaction species on the
catalytic surfaces. From the temperature programmed desorption of n-butene (n-C4H8-
TPD, fig. S11), it was clear that the adsorption of n-C4H8 on atomically dispersed Pt
clusters was much weaker than that on the Pt3Sn alloy. An easier desorption of butene
benefited the high reactivity and prevented the aimed product from further deep
dehydrogenation to undesired products or coke deposition. From all the results above,
we concluded that a-PtSn/ND@G could deliver robust activity, good selectivity, and
high stability for the DDH of n-butane, and it was superior to the previously reported
Pt-based catalysts as displayed in Table S4.
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Figure 5. Gibbs free energy profile of direct butane dehydrogenation on the Pt3-Gr,The structures
for intermediates and transition states from C4H10 to 2-C4H8 (T = 450 oC).
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The reaction mechanism of the DDH reaction over the a-PtSn/ND@G was further
studied by the quantum chemistry simulation at the DFT level. Based on structural
parameters extracted from quantitative EXAFS curve-fitting, we found that the C.N. of
Pt-C/O in the Pt cluster is 2.2, the average Pt-C/O bond length is 2.02 Å, and the C.N.
of Pt-Pt is 1.8. Experimental observation suggests that atomically dispersed Pt3 cluster
probably binding with Ox (x=0, 1, 2, 3) species are anchored on graphene due to the
indistinguishability of Pt-C or Pt-O bonds in the EXAFS data. Tables S5-7 summarized
the potential Pt3Ox structure and relative stabilities of Pt3Ox species of different
coordination environment. We calculated the change of Gibbs free energies from the
most stable Pt3Ox-graphene to Pt3-graphene under reaction condition (T = 450 °C, P
(H2) = 0.02 Bar.). Notably, Pt3O3-graphene, Pt3O2-graphene and Pt3O1-graphene will
be reduced to Pt3-graphene under the reaction condition (H2/Ar=1/9 at 450 °C).
Therefore, Pt3-Gr model is chosen to present the working catalyst to investigate reaction
activities and product selectivity.
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According to the experimental results, 2-butene was the main product in the DDH
reaction. Thus, we focused on the dehydrogenation path from n-butane to 2-butene for
the DFT calculations (The Gibbs free energy was calculated based on the temperature
of 450 oC). We used Pt3Sn-(111) as a reference (fig. S12-13) for similar calculations.
The calculation result for Pt3-Gr is shown in Fig. 5, clearly, the dehydrogenation of n-
butane to 2-butene followed two steps. Firstly, the initial activation of n-butane
occurred at the methylene group on the Pt3 cluster, leading to the 2-butyl species (2-
C4H9). Then 2-butene (2-C4H8) was formed via further dehydrogenation at methylene
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groups of the 2-butyl group. As shown in Table S8, the key energy barrier for the
formation of 2-butene was just 0.11 eV on Pt3-Gr, but 0.72 eV for the (111) surface of
Pt3Sn. The barrier for the deep dehydrogenation of 2-butene was 1.34 eV, higher 0.12
eV than desorption barrier (as shown in Table S10).More importantly, the Gibbs free
energy change of 2-butene desorption (ΔGdes = -1.54 eV) over Pt3-Gr is highly
favorable thermodynamically at 450 ° C, demonstrating the coke deposition was
unfavorable on Pt3-Gr. These results indicated that Pt3-Gr was more active than Pt3Sn-
(111) for the dehydrogenation of n-butane and apt for the product desorption and thus
guarantees a highly selective and coke-resistant catalyst at low reaction temperature.
The theoretical results agree well with our experimental observation.
CONCLUSIONS
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In summary, we synthesized fully exposed Pt clusters catalyst on ND@G support with
the partitioning role of Sn species. The catalyst exhibited excellent catalytic
performance for the DDH reaction of n-butane at a temperature as low as 450 oC. The
unique structure of the catalyst guaranteed a fully Pt exposure to the reactants and
optimal adsorption/desorption behaviour. These characteristics are pivotal for the
design of active, stable and coking-resistant alkane DDH catalysts. The strategy
provides a new way to prepare fully exposed metal catalysts and can be extended to
other metal catalysts upon further development.
EXPERIMENTAL SECTION
Materials
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Nanodiamond (ND) powders were purchased from Beijing Grish Hitech Co., China.
The γ-Al2O3 was also purchased from Aladdin. Analytical grade chloroplatinic acid
(H2PtCl6∙6H2O) and Tin (II) chloride dehydrate (SnCl2∙2H2O) were purchased from
Sinopharm Co. Ltd.
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Catalysts preparation
The nanodiamond@graphene (ND@G) hybrid carbon support was synthesized by
thermal annealing of fresh ND powder at 1100 oC under Ar flow (80 mL•min-1) for 4
h. The a-PtSn/ND@G catalyst was prepared by co-impregnation method using
H2PtCl6∙6H2O and SnCl2∙2H2O as the precursors. Typically, certain amount of H2PtCl6
(equivalent to 1.0 wt% Pt weight loading on ND@G support) and SnCl2∙2H2O (Sn/Pt
atomic ratio: 1.7/1) were dissolved in 2 ml ethanol. The solution was impregnated into
the ND@G powder (200 mg), under stirring at room temperature for 20 h. Then, the
samples were dried at 40 oC for another 6 h. Finally, the samples were calcined in Ar
(80 mL•min-1) at 500 oC for 4 h and then reduced in H2 gas (80 mL•min-1) at 500 oC
for another 1h. For reference, the Pt/ND@G with 1.0 wt% Pt weight loading was
prepared with the similar process without the addition of Sn and the Sn/ND@G with
0.5 wt% was prepared with the similar process without the addition of Pt. A γ-Al2O3
supported Pt3Sn alloy catalyst (1.0 wt% Pt, Sn/Pt atomic ratio: 0.5/1) was also prepared
using the same preparation procedure as mentioned above and named as Pt3Sn/Al2O3.
Characterizations
The crystal structure of the samples were characterized by X-ray diffraction (XRD) on
a D/MAX-2500 PC X-ray diffractometer with monochromated Cu K radiation (λ=1.54
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Å). Atomic micrographs were collected on a JEOL JEM ARM 200CF aberration-
corrected scanning transmission electron microscope at 200 kV accelerating voltage.
XPS was carried out on ESCALAB 250 with monochromated Al Kα radiation, at a pass
energy of 50 eV; the spectra were calibrated using the adventitious carbon C–C peak to
284.8 eV. The extend X-ray absorption spectra were collected at beamline (BL14W1)
at the Shanghai Synchrotron Radiation Facility. The BET surface area, BJH pore
volume and the average pore diameter of the as-prepared samples were measured by
N2 adsorption isotherm on a Micromeritics ASAP 2020 analyzer. Temperature-
programmed desorption of n-butene (n-C4H8) was conducted in He atmosphere.
Typically, the samples (50 mg) were first treated in flowing He (30 mL•min-1) at 30 oC
for 1 h. Then, He was switch to a flowing C4H8 (15 mL•min-1) and treated at the same
temperature for 30 min. After that, the system was swept in a flowing He stream (30min
mL•min-1) until a stable baseline was obtained. The temperature of the catalyst was then
increased from 30 oC to 450 oC (ramping rate: 5 oC /min) in flowing He (30 mL•min-1)
and the MS signals (mass spectrometer: Pfeiffer-omnistar) of the reactant/product were
recorded. The H2-O2 titration measurements were performed on a Micromeritics
AutoChem II 2920 equipped with a thermal conductive detector.
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Reaction evaluation
The catalytic performance for the DDH reaction of n-butane was tested in a fixed-bed
stainless steel micro-reactor with a quartz lining under atmosphere pressure at 450 oC
using 50 mg catalyst plugged in with quartz wool at the center of the quartz lining. The
reaction was carried out in a feed gas with a composition of 2% H2, 2% n-C4H10 and
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He as carrier gas, and a gas hour space velocity (GHSV) of 18000 mL∙gcat-1∙h-1 on the
basis of the whole feed gas (the total flow rate is 15 mL∙min–1). The effluent mixture
gas was analyzed by on-line gas chromatography (Agilent 7890 with a FID and a TCD
detector). The rate and conversion of n-butane and the selectivity of total C4 olefin (n-
butene and 1, 3-butadiene) were calculated using the following equations:
n-Butane Conversion: conv. = (mol of the reacted) / (mol of inlet n-butane) x 100%
(1)
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Selectivity of C4 olefin: Selectivity = {mol of (butene formed + 1, 3-butadiene formed)}
/ (mol of reacted) × 100%
(2)
n-butane rate = (flow rate of n-butane × conversion of n-butane × 60)/(weight of Pt in
the catalyst × 22.4)
(3)
The catalyst stability was described by a first-order deactivation model:
kd = {ln[(1-Cf) / Cf] - ln[(1-Ci) / Ci]}/t
(4)
Ci: initial conversion after reaction 30 min. Cf: final conversion value. Where t
represents the reaction time (h), and kd is the deactivation rate constant (h-1) which is
used to evaluate the catalyst stability (higher kd values, lower stability).
Computational details
All calculations were performed using the plane-wave based DFT method implemented
in the Vienna Ab Initio Simulation Package (VASP).29-30 We describe the electron–ion
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interaction using the projector augmented wave (PAW) method.31-32 The generalized
gradient approximation and the Perdew–Burke–Ernzerhof functional (GGA-PBE)33
describes the exchange and correlation energies for all systems. The plane wave
expansion of the wave functions adopted an energy cut-off of 400 eV. The ground-state
structure of bulk and surfaces were obtained by minimizing forces with the conjugate-
gradient algorithm until the force on each ion is below 0.02 eV/Å and the convergence
criteria for electronic self-consistent interactions is 10-5.
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Perfect and defective graphene have been widely use as support of noble metal
theoretical studies34-36. Here, Pt3 cluster embedded into a monovacancy at 5×5 super
cell of graphene was adopted as model to simulate the active site of butane
dehydrogenation (Pt3-Gr) through comparative investigation between potential Pt
cluster models and EXAFS data. The vacuum layer was set to 20 Å to avoid interaction
from adjacent cells. The Monkhorst-Pack k-point set to 3×3×1 in the reciprocal lattice,
and the electronic occupancies were determined according to the Gaussian smearing
method with σ = 0.1 eV. Spin−polarized calculations have been performed. For Pt3Sn
(111) surface, a four-layer slab with a (2√3 × 2√3) R30° supercell (Totally 48 atoms)
was used to represent the Pt3Sn alloyed surfaces, achieving the coverage of adsorbates
of 1/12 ML. The successive slabs were separated by a vacuum region as thick as 20 Å
to eliminate periodic interactions. The Brillouin zone is sampled with a 2×2×1 k-points
mesh by the Monkhorst–Pack algorithm. The electronic occupancies were determined
according to the Methfessel-Paxton scheme with σ = 0.2 eV. The bottom one layers of
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the slab were kept fixed to their crystal lattice positions. Spin-polarization is not
considered in Pt3Sn (111) calculation.
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The most stable configurations of the reactants and intermediates on Pt3-Gr and Pt3Sn
(111) surface were obtained by the standard minimization of density functional theory
(DFT). These configurations were used as the initial state, from which the constrained
optimization method as described by Plessow36 was used to search the transition states
(TS). The TS optimization convergence was regarded to be achieved when the force on
each atom was less than 0.05 eV/Å. All transition states have been verified to include
only one imaginary harmonic frequency corresponding to the transition vector of the
reaction. Furthermore, small distortions along the transition vector followed by
optimization toward the minima verified the connectivity of the transition states.
We have considered zero-point energies (ZPE) of reaction species and transition
states during the dehydrogenation of n-butane. Gibbs free energies for the gas-phase
species were determined by combining electronic and ZPE with entropy (T=450 °C).
For chemisorbed reaction species, only the electronic energy and ZPE was included in
the calculation of the Gibbs free energy.
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Potential Models of Pt3Ox-graphene according to EXAFS data. Based on structural
parameters extracted from quantitative EXAFS curve-fitting, we found that the C.N. of
Pt-C/O in the Pt cluster is 2.2, the average Pt-C/O bond length is 2.02 Å, and the C.N.
of Pt-Pt is 1.8. Experimental observation suggests that atomically dispersed Pt3 cluster
binding with Ox (x=0, 1, 2, 3) species are anchored on graphene due to the
indistinguishability of Pt-C or Pt-O bonds in the EXAFS data. Table S6-8 summarized
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the potential Pt3Ox structure and relative stabilities of Pt3Ox species of different
coordination environments.
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ASSOCIATED CONTENT
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Supporting Information
The Supporting Information is available free of charge on the ACS Publications
website. Supplementary characterization and activation calculation results (Figures S1-
S13, Tables S1-S10) as described in the text.
AUTHOR INFORMATION
Corresponding Authors
*Hongyang Liu (liuhy@imr.ac.cn), *Ning Wang (phwang@ust.hk) and *Ding Ma
(dma@pku.edu.cn)
ORCID
Hongyang Liu: 0000-0003-2977-2867
Ning Wang: 0000-0002-4902-5589
Ding Ma: 0000-0002-3341-2998
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This work was supported by the MOST (2016YFA0204100, 2017YFB0602200), the
NSFC of China ((91845201, 21573254, 91545110, 21725301, 91645115, 21473003
and 21821004), the Youth Innovation Promotion Association, Chinese Academy of
Science (CAS). N. W. acknowledges the funding support from the Research Grants
Council of Hong Kong (Project Nos. C6021-14E and 16306818).
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