Structure Determination of a Surface Tetragonal Pt1Sb1 Phase on Pt Nanoparticles

Article abstract of DOI:10.1021/acs.chemmater.8b02071

A new tetragonal Pt₁Sb₁ phase not known in phase diagram was evidenced in 2-3 nm core-shell nanoparticle catalysts by in situ synchrotron XAS, XRD, HAADF imaging, and EDS analysis. A kinetically controlled diffusion of Sb(0) into Pt(0) nanoparticles with minimum atomic rearrangement of the FCC Pt structure was proposed to explain the formation of this unique phase on top of a Pt core at the nanoscale. Each Pt atom is isolated by Sb atoms, and thus the Pt₁Sb₁ alloy exhibits high selectivity in propane dehydrogenation reaction resulting from the geometric effect that minimizes the hydrogenolysis side reaction.

Full text of DOI:10.1021/acs.chemmater.8b02071

Communication
pubs.acs.org/cm
Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Structure Determination of a Surface Tetragonal Pt Sb Phase on Pt
1
1
Nanoparticles
Chenliang Ye,†,‡,∥ Zhenwei Wu, Wei Liu, Yang Ren, Guanghui Zhang,* and Jeffrey T. Miller*,†
†,∥
§
⊥
,†
†Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907, United
States
‡
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
§
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China
⊥
X-ray Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, Illinois 60439, United States
*
S Supporting Information
ntermetallic compounds offer the opportunity to obtain new
catalysts with enhanced performance in a broad range of
known in the bulk materials, was identified using synchrotron
in situ XRD and X-ray absorption spectroscopy (XAS). Precise
determination of the crystal phase led to in-depth under-
standing toward the structure−function relationship. The high
selectivity for propane dehydrogenation of the core−shell
nanocatalysts with a Pt core and Pt Sb shell (Pt@Pt Sb ) can
be attributed to the isolation of the active Pt atoms, i.e., a
geometric effect that minimizes the hydrogenolysis side
reaction. High turnover rates were also observed likely due
to the weakened Pt-alkene bonds resulting from the lower
energy level of the filled Pt 5d states and higher energy level of
the unfilled Pt 5d states compared with monometallic Pt.
A series of Pt−Sb bimetallic nanoparticles were synthesized
using sequential impregnation with Sb/Pt molar ratios of 0.5:1,
I
reactions compared with their parent monometallic counter-
parts. The unique properties have inspired increasing
investigations into their controlled synthesis, structure−activity
relationship, and applications in various areas. Significant
advances have been achieved in the past decades, and
currently, intermetallic catalysts are widely utilized in many
catalytic applications with the Pt−Sn catalyst for naphtha
reforming and paraffin dehydrogenation being one successful
1
1
1
1
1
−10
example.
Intermetallic nanoparticles usually share the same
crystal phases as bulk intermetallic compounds, and phase
diagrams can be used to help guide the design and
understanding of intermetallic nanocatalysts. However, it has
been very challenging to determine the crystal structure of
supported intermetallic nanocatalysts due to their small
1:1, 2:1, and 4:1. A monometallic Pt catalyst was synthesized
for comparison. The particle sizes are around 2.5 nm with a
distribution of ±0.5 nm (Figure S1). Their Pt L3 X-ray
absorption near edge structure (XANES) edge energies range
from 11564.0 to 11565.5 eV, suggesting that the Pt presents as
Pt(0) (Figure 1A). The edge and white line shifted to higher
energy with increasing the Sb/Pt ratio indicating the increase
of the energy level of the empty Pt 5d states and intermetallic
interactions. Three main peaks were observed in the extended
X-ray absorption fine structure (EXAFS) spectrum of Pt/SiO2,
1
1−18
size.
Recently, with the help of synchrotron X-ray
diffraction (XRD), Pt Zn , Pt In and PtIn phases have been
1
1
3
2
identified in Pt−Zn and Pt−In dehydrogenation catalysts with
11,12
∼
2 nm particle size.
It was proposed that the formation of
these intermetallic nanoparticles is kinetically controlled, and it
favors the Au Cu and AuCu type structures that have relatively
3
high symmetry and require least rearrangement from the
parent face-centered cubic (FCC) structure of Pt. For example,
Pt−Zn alloys have several possible intermetallic phases, but the
Pt Zn phase with the AuCu type structure is preferentially
1
1
formed. Pt−In, Pd−Zn, and Pd−In have also shown similar
19−21
preferential phase formation in nanoparticles.
In each of
these catalysts, the structure of the nanophase was identical to
that of a known bulk alloy.
Although Sb has been extensively used to modify Pt catalysts
in methanol/formic acid fuel cells, merely two examples are
present in the literature using Pt−Sb bimetallic catalysts in
alkane dehydrogenation reaction with controversial conclu-
sions due to the lack of precise understanding of the Pt−Sb
2
2−26
alloy phase.
Interestingly, none of the bulk Pt−Sb
Figure 1. Pt L3 edge XANES (A) and EXAFS (B) spectra. Pt/SiO
2
2
(
(
red), 0.5Sb−Pt/SiO (yellow) 1Sb−Pt/SiO (olive), 2Sb−Pt/SiO
intermetallic phases have Au Cu or AuCu type structures
2
2
3
−
1
2
cyan) and 4Sb−Pt/SiO (wine). FT k-range of 3.0−12.0 Å , k -
that are predicted to form according to the hypothesis of
kinetically controlled formation of intermetallic nanoparticles.
Here we communicate our effort on the synthesis, character-
ization, and application of a series of Pt−Sb intermetallic
nanoparticle catalysts for selective propane dehydrogenation
reaction. A AuCu type tetragonal Pt Sb phase, which is not
2
weighted. All the samples were treated with H at 550 °C.
2
Received: May 16, 2018
Revised: July 3, 2018
1
1
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.chemmater.8b02071
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Communication
typical of metallic Pt−Pt scattering (Figure 1B). As the amount
of Sb increases, the peak positions and relative intensities
change, consistent with the presence of Sb scatterers within
bonding distance. The Pt−Pt coordination number decreased
from 8.4 to 5.0, and that of Pt−Sb increased from 0.5 to 3.7
with increasing the Sb:Pt ratio from 0.5:1 to 4:1 (Table 1 and
Table 1. Summary of the EXAFS Fitting Results
a
2
2
Sample
Pt/SiO2
Scattering Pairs
C. N.
Distance (Å) σ (Å )
Pt−Pt
Pt−Pt
Pt−Sb
Pt−Pt
Pt−Sb
Pt−Pt
Pt−Sb
Pt−Pt
Pt−Sb
9.7 ± 0.5
8.4 ± 0.4
0.5 ± 0.2
7.6 ± 0.5
1.0 ± 0.3
5.2 ± 1.5
2.8 ± 0.5
5.0 ± 4.0
3.7 ± 0.7
2.74 ± 0.01
2.74 ± 0.01
2.63 ± 0.02
2.75 ± 0.01
2.63 ± 0.01
2.78 ± 0.01
2.65 ± 0.01
2.80 ± 0.02
2.65 ± 0.01
0.007
0.007
0.003
0.007
0.004
0.009
0.007
0.014
0.005
0
1
2
4
.5Sb−Pt/SiO2
Figure 2. XRD patterns (A) Pt simulation (orange), Pt/SiO (red),
2
Sb−Pt/SiO2
1Sb−Pt/SiO (olive), 2Sb−Pt/SiO (cyan), and 4Sb−Pt/SiO
2
2
2
(wine). (B) simulation of the proposed Pt Sb alloy (purple), 4Sb−
1
1
Sb−Pt/SiO2
Pt/SiO -400 (black), 4Sb−Pt/SiO -500 (navy), Sb O (blue) and Pt
2
2
2
3
simulation (orange). X-ray energy: 105.715 keV (0.11730 Å).
Sb−Pt/SiO2
Pt−Sb catalysts are, therefore, matched by comparing with
simulated patterns using the structures with Pt−Sb bond
distances from EXAFS fits. The Pt−Sb and Pt−Pt EXAFS
a
2
C. N. = coordination number. Fittings were done using k -weighted
R-space EXAFS spectra with the k-range of 3.0−12.0 Å and the R-
−
1
range of 1.7−3.2 Å.
distances in 4Sb−Pt/SiO are very similar to those of the Pt−
2
Zn (2.63 Å) and Pt−Pt (2.81 Å) in the tetragonal Pt Zn
1
1
S1). The Pt−Sb distance was ∼2.64 Å in all the samples, while
interemtallic phase, which is one of the phases expected to
11
the Pt−Pt distance increased from 2.74 Å in 0.5Sb−Pt/SiO to
form in such catalysts according to previous studies. The
2
2
.80 Å in 4Sb−Pt/SiO . The variation of the Debye−Waller
XRD pattern of 4Sb−Pt/SiO -500 further suggests possible
2
2
factor of the Pt−Sb path is relatively small (0.003−0.007),
whereas that of the Pt−Pt path increased from 0.007 to 0.014,
suggesting an increasing disorder of the Pt−Pt path as
increasing the Sb:Pt ratio. As shown in Figure S2 and Table
S2, the Sb K-edge XANES edge energies and peak intensities
of the nanoparticles were very close to those of Sb O ,
formation of a tetragonal Pt Sb phase (Figure S8).
1
1
To assess the possibility of the tetragonal Pt Sb phase, the
1
1
XRD patterns of the tetragonal Pt Sb phase were simulated
1
1
based on the L1 ordering. A good match was obtained for a
0
bond distance of Pt−Pt at 2.97 Å and Pt−Sb at 2.64 Å in the
alloy (Figure 2B). FCC Pt also shows up in the same
diffraction pattern, however, its peaks are shifted to a lower
angle (∼0.02° for the (111) peak), which may result from
2
3
indicating that most of the Sb in the catalysts remain as Sb(III)
after H treatment at 550 °C. In all these samples, there is only
2
2
7−30
a small amount of metallic Sb associated with metallic Pt
forming Pt-rich Pt−Sb bimetallic particles, and most of the Sb
is present as Sb O . It is also worth noting that the presence of
microstrain due to formation of core−shell structure.
It
should be noted that the relative intensity of the simulated
peaks of the proposed Pt Sb alloy were different from those of
2
3
1
1
two isosbestic points at 11567.6 and 111580.0 eV in the
XANES spectra (Figure 1A) suggests the formation of a single
Pt−Sb phase with varying Sb loadings.
the experimental data. In the simulation, the peak at 3.09° has
a higher intensity than that at 3.22°, while in the experimental
spectrum, it has a lower intensity possibly due to the presence
of a preferred crystallographic orientation in the nanoparticles,
likely indicating a two-dimensional surface layer.
To confirm the core−shell structure, high angle annular dark
filed (HAADF) image (Figure 3A) was obtained for 2Sb−Pt/
Five thermodynamically stable phases of Pt−Sb intermetallic
compounds have been reported in the inorganic crystal
structure database (ICSD) including Pt Sb, Pt Sb, Pt Sb ,
7
3
3
2
PtSb, and PtSb (Table S3). The Pt−Sb EXAFS distance of
2
∼
2.64 Å in the Pt−Sb nanoparticles is not consistent with the
formation of Pt Sb, Pt Sb, or PtSb phase in which the Pt−Sb
7
3
distances are all longer than 2.75 Å. Formation of Pt Sb and/
3
2
or PtSb phases is possible based on the Pt−Sb distances,
2
2
.60−2.68 Å. These phases, however, are not evident from in
situ synchrotron XRD. As shown in Figure 2A, Pt/SiO showed
2
4
broad peaks at 2θ of 2.98°, 3.43°, 4.88°, and 5.71°
corresponding to the (111), (200), (220), and (311) planes of
Pt FCC structure in the 2.5 nm nanoparticle, respectively.
With the incorporation of Sb, new overlapping peaks appeared
at 2θ of 3.27° and 5.29°, which may be attributed to a Pt−Sb
intermetallic or other Sb species. To get narrower and more
intense X-ray diffraction peaks for better identification, samples
with larger particle sizes (4Sb−Pt/SiO -400 and 4Sb−Pt/SiO -
2
2
5
00, Figure 2B) were synthesized. The XRD patterns do not
match any of the known bulk Pt−Sb intermetallic phases, nor
metallic Sb (Figure S6 and S7), which suggests the possible
formation of a new Pt−Sb phase that has not been reported for
bulk materials. The unknown peaks in the XRD patterns of the
Figure 3. STEM image and EDS analysis of the 2Sb−Pt/SiO
catalyst.
2
B
DOI: 10.1021/acs.chemmater.8b02071
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Communication
a
SiO . The characteristic of atomic number correlated contrast
Table 3. Catalytic Dehydrogenation Results
2
distribution in HAADF offers intuitive demonstration of the
delicate core−shell configuration. The brighter core and
relatively darker shell correspond to the metallic Pt and
Pt
C H
CH
C H
C H
4
dispersion
(%)
TOR
−1
3
6
4
2
6
2
Catalyst
Pt/SiO2
.5Sb−Pt/
(%)
(%)
(%)
(%)
(s )
tetragonal Pt Sb phases, respectively. The spacing in the core
1
1
24
97
69
2
5
1
2
31
28
0.4
0.5
is 2.20 Å consistent with the Pt(111) plane, whereas the
spacing of outermost two atomic layers of the shell is 2.33 Å,
which could be attributed to inevitable surface oxidation
during the microcopy study. The high resolution elemental
mapping (Figure 3B−F) using energy dispersive X-ray
spectroscopy (EDS) analysis provided additional evidence of
the composition in this core−shell structure. The Sb atoms are
found only in the shell, whereas Pt locates over the entire
particle, which reveals the fact of monometallic Pt core as well
as the Pt Sb alloy shell.
0
trace
trace
trace
trace
SiO
2
1Sb−Pt/
99
99
99
trace
trace
trace
trace
trace
trace
28
0.6
0.6
−
SiO
2
2
4
Sb−Pt/
11
SiO
2
Sb−Pt/
<0.5
SiO
2
a
Reaction conditions: 500 °C, 2% C H , 3% H , and balance N . H
2
3
8
2
2
was cofed to promote the hydrogenolysis side reaction, so that the
selectivity can be monitored under severe conditions. The TOR was
determined at 10% C H conversion based on the first data point at 2
1
1
With the Pt Sb intermetallic structure identified by in situ
1
1
3
8
synchrotron XRD and core−shell geometry revealed by STEM,
min of time on stream, and the selectivity was determined at 15%
C H conversion.
the EXAFS data of the 2Sb−Pt/SiO and 4Sb−Pt/SiO were
3
8
2
2
refit using a new model including a Pt core and a Pt Sb shell
1
1
These results led to an in-depth understanding of the
structure evolution of the Pt@Pt Sb core−shell nanoparticles.
Pt was first reduced to Pt(0), and SbO near Pt is reduced at
in which the Pt−Sb and Pt−Pt distances were fixed as 2.64 and
2
.97 Å, respectively. In the new fitting results summarized in
1
1
Table 2 and S4, the Debye−Waller factors of the surface layer
x
higher temperature. The Sb(0) then diffuses into Pt(0)
nanoparticles forming the surface tetragonal Pt Sb alloy that
Table 2. Summary of the EXAFS Fitting Results Using the
1
1
a
requires minimum atomic rearrangement of the Pt FCC
structure. The fraction of the Pt Sb alloy increases with
Pt@Pt Sb Core−Shell Model
1
1
1
1
2
2
Sample
Scattering pairs
C. N.
Distance (Å) σ (Å )
increasing the Sb loading. This is in contrast to the formation
of thermodynamically stable Pt Sb phases which typically
2
Sb−Pt/SiO2
Sb−Pt/SiO2
Pt−Pt
Pt−Sb
Pt−Pt
Pt−Pt
Pt−Sb
1.1 ± 0.1
2.2 ± 0.2
6.0 ± 0.6
1.6 ± 0.1
3.2 ± 0.2
4.9 ± 1.2
2.97 (fixed)
2.64 (fixed)
2.77 ± 0.01
2.97 (fixed)
2.64 (fixed)
2.79 ± 0.01
0.006
0.005
0.008
0.007
0.004
0.011
x
y
requires high temperature where both molten Pt(0) and Sb(0)
are slowly cooled. It is worth noting that high Sb loading (Sb/
Pt = 4/1) leads to a Sb-covered surface, which is similar to
what has been reported for Pd−In alloy nanoparticles where
4
19
Pt−Pt
excess In covers the nanoparticle surface.
The surface Pt Sb phase efficiently limits hydrogenolysis,
a
2
Fittings were done using k -weighted R-space spectra with a k-range
of 3.0−12.0 Å and a R-range of 1.7−3.2 Å.
1
1
−
1
which is the side reaction during propane dehydrogenation as
summarized in Table 3 and Figure 4. The propylene selectivity
scattering paths including the Pt−Pt at 2.97 Å and Pt−Sb at
2
.64 Å are all within the range of 0.004−0.007, which are
typical of nanoparticles. The Pt−Pt bond distances in the core
increases from 2.74 to2.79 Å, which is consistent with the
observed shift to a lower angle of the Pt FCC peaks in the
XRD patterns. The Debye−Waller factor of the Pt−Pt path in
the core increases from 0.006 to 0.011, consistent with the
relatively higher disorder due to the presence of the lattice
mismatch between the FCC Pt core and the surface tetragonal
Pt Sb layer. This also rationalizes the larger Debye−Waller
1
1
2
factors (σ ) and uncertainty of the coordination number in the
original EXAFS fitting results which included only one Pt−Pt
and one Pt−Sb scattering paths. Fitting the Sb K-edge EXAFS
data did not provide useful information on the Sb−Pt bonds
due to the presence of large excess of Sb O .
Figure 4. Catalytic performance. (A) C H selectivity as a function of
3
6
C H conversion. (B) C H selectivity (at 15% C H conversion) and
3
8
3
6
3
8
dehydrogenation TOR (at 10% C H conversion) as a function of
3
8
shell thickness.
2
3
The above results together confirmed the formation of a
surface tetragonal Pt Sb phase that has not been reported for
of Pt/SiO was 24% at 15% propane conversion. While the
1
1
2
bulk intermetallic compounds. The fraction of surface Pt in the
Pt−Sb bimetallic nanoparticles was determined using CO
chemisorption, and the results are summarized in Table 3. The
Pt dispersions of Pt/SiO , 0.5Sb−Pt/SiO , and 1Sb−Pt/SiO
propylene selectivity of all the three Pt−Sb catalysts are >97%
at 15% propane conversion. The selectivity of 0.5Sb−Pt/SiO
2
maintained ∼91%, while catalysts with higher Sb loadings
show ∼100% selectivity at even higher propane conversion, for
2
2
2
are about 30%. Further incorporation of Sb led to a sharp
example 2Sb−Pt/SiO shows ∼100% selectivity even at 36%
2
decrease of the Pt dispersion, 11% of 2Sb−Pt/SiO and <0.5%
C H conversion (the thermodynamic equilibrium conversion
2
3
8
of 4Sb−Pt/SiO . Pt dispersions of 2Sb−Pt/SiO and 4Sb−Pt/
is 46%). The main side reaction, alkane hydrogenolysis, has
been suggested to occur on large Pt ensembles. The increased
selectivity is consistent with a Pt Sb surface alloy, in which
2
2
SiO were lower than the expectation for 2−3 nm particles,
2
suggesting that the surface of the nanoparticles may be covered
by excess Sb.
1
1
each Pt atom is well isolated by surrounding Sb atoms at 2.64
C
DOI: 10.1021/acs.chemmater.8b02071
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Communication
Å, and the shortest Pt−Pt distance is 2.97 Å, which is ∼0.2 Å
longer than that in metallic Pt. The hydrogenolysis has been
minimized to achieve ∼100% propylene selectivity.
ACKNOWLEDGMENTS
■
Z.W., G.Z., and J.T.M. were supported in part by the National
Science Foundation under Cooperative Agreement No. EEC-
647722. Any opinions, findings, and conclusions or
recommendations expressed in this material are those of the
author(s) and do not necessarily reflect the views of the
National Science Foundation. C.Y. thanks the financial support
from the Chinese Scholarship Council (CSC). Use of the
Advanced Photon Source was supported by the U.S.
Department of Energy, Office of Basic Energy Sciences
Increasing the Sb/Pt ratio from 0.5:1 to 1:1 further increases
1
−1
the dehydrogenation TOR from 0.5 to 0.6 s . Resonant
inelastic X-ray scattering (RIXS) and density functional theory
(
DFT) results suggest higher dehydrogenation TORs can be
achieved by weakening the bonds between alkenes and Pt in
alloy catalysts with lower energy level of the occupied Pt 5d
states and higher energy level of the unoccupied Pt 5d states.
In the Pt Sb phase, the Pt−Sb distance is 2.64 Å which is
1
1
[
grant number DE-AC02-06CH11357]. MRCAT operations,
much shorter than the sum of the Pt and Sb atomic radius,
.79 Å, suggesting the presence of strong Pt−Sb bonds. Lower
beamline 10-BM, are supported by the Department of Energy
and the MRCAT member institutions. The authors also
acknowledge the use of beamline 11-ID-C.
2
energy level of the filled Pt 5d states and higher energy level of
the unfilled 5d states would be expected, latter of which is
consistent with the XANES shift to higher energy. The change
of the energy levels in the Pt Sb surface alloy is believed to be
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ORCID
Guanghui Zhang: 0000-0002-5854-6909
Jeffrey T. Miller: 0000-0002-6269-0620
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Author Contributions
These authors contributed equally.
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∥
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