Catalytic properties of intermetallic platinum-tin nanoparticles with non-stoichiometric compositions

Article abstract of DOI:10.1016/j.jcat.2019.04.013

Intermetallic compounds are unique catalyst platforms for mechanistic studies and industrial applications, because of their ordered structures in comparison to random alloys. Despite the intrinsically defined stoichiometry of intermetallic compounds, compositional deviations can still occur in intermetallic catalysts. The location of the extra metal atoms could differ the catalytic properties of intermetallic compounds with non-stoichiometric composition if those metal atoms end up on/near the surface. In this study, we synthesized PtSn intermetallic compounds with accurate stoichiometry and slightly Pt-/Sn-rich compositions. We used furfural hydrogenation and acetylene semi-hydrogenation as probe reactions to investigate the surface structures of PtSn intermetallic catalysts after reduction at different temperatures. Even though the intermetallic PtSn is the major bulk phase among non-stoichiometric compositions, the intermetallic PtSn surface can only be observed under the high-temperature reduction in Sn-rich PtSn intermetallic nanoparticles (iNPs), while the Pt-rich PtSn iNPs show Pt-rich-surfaces regardless of reduction temperatures. Four structural models were constructed based on the comprehensive surface and bulk characterizations. This work extends the understanding of intermetallic catalysts with non-stoichiometric compositions to tailor the intermetallic surface structures for catalysis.

Full text of DOI:10.1016/j.jcat.2019.04.013

Journal of Catalysis 374 (2019) 136–142
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Catalytic properties of intermetallic platinum-tin nanoparticles with
non-stoichiometric compositions
Yuchen Pei a,b, Biying Zhang , Raghu V. Maligal-Ganesh , Pranjali J. Naik a,b, Tian Wei Goh a,b,
a
a
a
a
a,b
a,b
a,b
Heather L. MacMurdo , Zhiyuan Qi , Minda Chen , Ranjan K. Behera , Igor I. Slowing , Wenyu
a,b,
⇑
Huang
a
Department of Chemistry, Iowa State University, Ames, IA 50011, USA
Ames Laboratory, U.S. Department of Energy, Ames, IA 50011, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Intermetallic compounds are unique catalyst platforms for mechanistic studies and industrial applica-
tions, because of their ordered structures in comparison to random alloys. Despite the intrinsically
defined stoichiometry of intermetallic compounds, compositional deviations can still occur in intermetal-
lic catalysts. The location of the extra metal atoms could differ the catalytic properties of intermetallic
compounds with non-stoichiometric composition if those metal atoms end up on/near the surface. In this
study, we synthesized PtSn intermetallic compounds with accurate stoichiometry and slightly Pt-/Sn-rich
compositions. We used furfural hydrogenation and acetylene semi-hydrogenation as probe reactions to
investigate the surface structures of PtSn intermetallic catalysts after reduction at different temperatures.
Even though the intermetallic PtSn is the major bulk phase among non-stoichiometric compositions, the
intermetallic PtSn surface can only be observed under the high-temperature reduction in Sn-rich PtSn
intermetallic nanoparticles (iNPs), while the Pt-rich PtSn iNPs show Pt-rich-surfaces regardless of
reduction temperatures. Four structural models were constructed based on the comprehensive surface
and bulk characterizations. This work extends the understanding of intermetallic catalysts with non-
stoichiometric compositions to tailor the intermetallic surface structures for catalysis.
Received 26 February 2019
Revised 8 April 2019
Accepted 10 April 2019
Keywords:
Platinum
Tin
Furfural hydrogenation
Acetylene hydrogenation
Surface structure
Ó 2019 Elsevier Inc. All rights reserved.
1
. Introduction
Surface structure is essential in affecting the catalytic properties
Alloy nanoparticles are mostly studied bimetallic catalysts with
easily tunable bulk compositions leading to high catalytic activity
and selectivity [4–7]. In certain cases, the bulk composition of
bimetallic nanoparticles can be synthetically related to their sur-
face structures [8,9]. Additionally, direct modification of the alloy
surfaces has been developed to provide more surface structural
controls, such as colloidal galvanic replacement [10–12], electro-
chemical under-potential deposition [13–15], and reaction-driven
gas pretreatment [2,9]. However, atoms in alloys can migrate from
the surface to bulk phase and vice versa [16], leading to the surface
segregation and compositional inhomogeneity through alloy
nanoparticles [17–19]. The random structures of alloy catalysts
complicate the establishment of their structure-property
relationships.
Intermetallic compounds, featuring atomically-ordered struc-
tures and precise stoichiometry, are ideal platforms for studying
the structure-catalysis relationship in comparison to random
alloys [20–32]. Intermetallic PdZn [25], PdIn [33], and PdGa [34]
catalysts have been reported to show high selectivity in the
semi-hydrogenation of acetylene due to the geometric dilution
of heterogeneous catalysts because the geometric and electronic
properties of surface active sites change the adsorption configura-
tion and strength of molecules [1,2]. The rational architecture of
heterogeneous catalysts thus relies on the fundamental under-
standing of the surface structure-catalytic property relationships.
However, the near-surface structures of many heterogeneous cata-
lysts are different from their bulk ones, and state-of-the-art tech-
niques cannot easily characterize these thin surfaces under
reaction conditions [3]. Therefore, designing well-ordered catalyst
platforms and developing in situ structural probes are two strate-
gies to elucidate the dependence of catalytic properties on the sur-
face structures at molecular level.
⇑
Corresponding author.
E-mail address: whuang@iastate.edu (W. Huang), whuang@iastate.edu
(
W. Huang).
https://doi.org/10.1016/j.jcat.2019.04.013
021-9517/Ó 2019 Elsevier Inc. All rights reserved.
0

Y. Pei et al. / Journal of Catalysis 374 (2019) 136–142
137
À1
and electronic modification of the precious metal sites at their sur-
faces. Similar isolation effects of intermetallic compounds have
also been observed on intermetallic PtZn [35] and PtSn [36] in
the hydrogenation of nitroarenes. Due to the intrinsic stoichiome-
try of intermetallic compounds, a slight compositional deviation
can dramatically affect their catalytic properties if these extra met-
als remain on/near the surface. Because the surface structure
determines the catalytic properties of heterogeneous catalysts,
interesting and important questions for intermetallic catalysts
are where the extra metal atoms will be located and how the com-
positional deviation from the stoichiometric intermetallic com-
pounds can affect their catalytic properties. These questions are
crucial for the rational design of intermetallic catalysts.
(a total flow of 50 mL min ). The actual Pt/Sn ratios of as-
prepared PtSn iNPs were confirmed by inductively coupled
plasma-mass spectroscopy (ICP-MS).
2.2. Catalytic probe reactions for PtSn iNPs
The catalytic property of PtSn iNPs was studied using a plug
flow reactor. For furfural hydrogenation, 0.3–0.5 mg PtSn iNPs
were diluted with 200 mg quartz sand and placed into
U-shaped quartz tube attached to the reactor. A gas feed of furfural
was maintained as 0.023/11.4 mL min
8.6 mL min He flow were used to carry furfural from a bubbler
a
À1
(He)/H
2
,
where
À1
into the reactor. PtSn iNPs were calcined at 500 °C in air for 4 h,
PtSn intermetallic nanoparticles (iNPs) are chosen as the model
catalyst platform to investigate the effect of non-stoichiometric
compositions on the catalytic properties of intermetallic catalysts.
Because intermetallic PtSn is a line compound [37], Pt/Sn ratio can
maintain a mere 1:1 stoichiometry to form the accurate inter-
metallic phase. In a recent report, we have revealed that the extra
Pt in PtSn iNPs can significantly lower their furfuryl alcohol selec-
tivity in the gas-phase hydrogenation of furfural [24]. The surface-
dependent catalytic properties have also been briefly studied over
and reduced in situ at either 300 or 500 °C for 2 h in 10% H
(a total flow of 50 mL min ) before furfural hydrogenation. The
gas contents were monitored and quantified using gas chromatog-
2
/He
À1
raphy (GC) equipped with
30 m  0.32 mm  0.25 m) and
a
capillary column (EC-5,
l
a flame ionization detector
(FID). For acetylene semi-hydrogenation, 2 mg PtSn iNPs were
used, and gas feed of /C /H /He was controlled as
C
2
H
2
2
H
4
2
À1
0.15/15/1.5/13.5 mL min . The gas contents were monitored and
quantified using a GC equipped with a capillary column (HP
PLOT/Q, 30 m  0.32 mm  0.2 mm) and a FID. For the mass
3
Pt NPs, Pt Sn iNPs, and PtSn iNPs in a liquid-phase nitrostyrene
hydrogenation reaction [36], and the presence of contiguous Pt
sites can lead to the poor selectivity to hydrogenate nitro groups.
However, a systematic study has not been conducted to investigate
how the extra Pt or Sn in PtSn iNPs can affect their catalytic
properties.
2
diffusion limit test, we used monometallic Pt@mSiO to test the
flow reactors over furfural hydrogenation at the above reaction
2
conditions. The catalyst amount of Pt@mSiO was varied from 0.3
to 1.2 mg.
Using PtSn iNPs as a model intermetallic catalyst, we endeavor
to study the effect of extra Pt or Sn on their structural and catalytic
properties. To synthesize the PtSn intermetallic candidates, our
group has developed a ship-in-a-bottle method of preparing Pt-
2.3. Characterization
Powder X-ray diffractions (PXRD) patterns of the samples were
acquired by a STOE Stadi P powder diffractometer or a Bruker D8
based iNPs encapsulated in mesoporous silica shells (mSiO
2
)
a
Advance Twin diffractometer, Cu K radiation (40 kV, 40 mA,
[
24,38,39]. As-prepared PtSn iNPs can maintain the capping free
k = 0.1541 nm). Transmission electron microscopy (TEM), high-
angle annular dark field scanning TEM (HAADF-STEM), and
elemental mapping analysis were investigated using a Tecnai G2
F20 electron microscope equipped with energy-dispersive X-ray
spectroscopy (EDS) detector (Oxford INCA EDS) and a Titan Themis
300 probe corrected TEM with a Super-X EDS detector. The actual
metal loadings in the PtSn iNPs were measured by ICP–MS (X Ser-
ies II, Thermo Scientific). All metals in samples were digested com-
pletely using aqua regia after removing the mesoporous silica.
Diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) was conducted using an Agilent Cary 670 Fourier trans-
form infrared spectrometer equipped with a linearized HgCdTe
nature, high thermal stability and ordered structure. This ship-in-
a-bottle method also enables the precise composition control of
the iNPs by adjusting the amount of the secondary metal precur-
sors. Herein, we demonstrated a structure-catalysis relationship
over PtSn iNPs with Pt/Sn ratios that slightly deviate from the
1
:1 stoichiometry. We also demonstrated that the reduction tem-
perature was a crucial factor to alter the surface structure of the
non-stoichiometric PtSn iNPs. Four structural models were sug-
gested by two probe reactions and different characterization tech-
niques to correlate the structure evolution of extra metals in
intermetallic PtSn catalysts induced by reduction temperatures.
(
MCT) detector, a Harrick diffuse reflectance accessory, and a Pray-
ing Mantis high-temperature reaction chamber. CO was the probe
molecule. 10–20 wt.% sample was diluted with KBr before the
2
. Experimental section
À1
measurement. 5% CO/He mixture (50 mL min in total flow) was
À1
2.1. Synthesis of PtSn iNPs with various ratios
used to saturate PtSn iNPs surface and 50 mL min He was used
to remove gas CO remnants. The spectra were obtained at the
steady state of CO adsorption peak after removing all gas CO.
X-ray absorption fine structure spectroscopy (XAFS) and X-ray
absorption near edge spectroscopy (XANES) spectra were
A proper amount of Pt@mSiO
Information) was taken out from methanol by centrifugation
12,000 rpm, 10 min) and re-dispersed in 2.5 mL acetone. The
Pt@mSiO seeds-acetone solution was transferred to 80 mL tetra-
ethylene glycol (TEG) with the addition of a calculated amount of
SnCl O based on the desired Pt/Sn molar ratios in the PtSn iNPs
Á2H
0.9, 1.0 and 1.2). The resulting TEG solution was protected in
argon, heated to 260 °C in 45 min with stirring, and then to
80 °C in 20 min. The solution was maintained at 280 °C for 2 h.
2
seeds (ca. 10 mg, Supporting
(
2
3
measured in transmission mode (Pt L edge = 11,564 eV and Sn K
edge = 23,220 eV) at 9-BM-B and 20-BM-B beamlines of the
Advanced Photon Source in Argonne National Laboratory. XAFS of
reference samples were collected using pure finely ground pow-
ders homogeneously dispersed on polyimide Kapton tape. While
the PtSn iNPs (diluted with boron nitride) were pressed into a pel-
let fit to a hole embedded in a sample holder, reference platinum
was acquired simultaneously with each measurement for energy
calibration. The Athena program, which is an interface to IFEFFIT
and FEFFIT, was used for smoothing raw data and converting spec-
tra from energy to k space. All the extended X-ray absorption fine
2
2
(
2
After cooling to room temperature, 80 mL acetone was added to
the TEG solution, and PtSn iNPs were centrifuged down at
1
2,000 rpm for 10 min. After washing 3 times with ethanol, PtSn
iNPs were dried in vacuum at room temperature and calcined at
00 °C in air for 4 h to remove organic capping agents. The PtSn
iNPs were then reduced at 300 or 500 °C for 2 h in 10% H /Ar
5
3
2
structures (EXAFS) data fittings were performed with a k weight in

1
38
Y. Pei et al. / Journal of Catalysis 374 (2019) 136–142
R space by Artemis program from the same package using the
quick shell theory of related Pt and Sn coordination. The Pt disper-
sions of catalysts were measured over Micromeritics AutoChem II
access the PtSn intermetallic cores to initiate chemical reactions,
such as furfural and nitroarenes [24,36]. The detailed catalyst char-
acterization was included in the Supporting Information as well as
in our previous reports [24,36,38].
2
920 equipped with a thermal conductivity detector. All Pt-/Sn-
rich PtSn catalysts are freshly reduced at the targeted tempera-
tures, and in situ reduced again (150 °C) before the measurement
at 30 °C. For Pt and Pt1.0Sn@mSiO , the Pt dispersion was measured
2
over Micromeritics 3Flex.
The ship-in-a-bottle strategy can provide precise composition
control of the individual bimetallic particle by altering the ratios
of two metal precursors [38]. Bulk Pt/Sn ratios of PtSn iNPs can
thus be finely tuned with extra Pt or Sn departing from the intrinsic
1
2
:1 stoichiometry. We varied the extra metal ratios from 10 to
0 mol.% capable of covering the whole surfaces of a 20 nm PtSn
3
. Results and discussion
spherical particle (theoretical dispersion: 7 mol.%) [40]. Due to
the high incorporation efficiency of secondary metals using the
ship-in-a-bottle method, the molar ratios of Pt seed to the sec-
ondary metal precursors determine the compositions of the final
bimetallic NPs. We thus prepared Pt1.2Sn, Pt1.0Sn and Pt0.9Sn iNPs
with precise Pt/Sn ratios, and their bulk compositions were con-
firmed by ICP-MS as 1.16 ± 0.03, 0.99 ± 0.04 and 0.91 ± 0.03, close
to the respective precursor ratios (Table S1 in Supporting Informa-
tion). In the ship-in-a-bottle method, Pt seeds are essential to facil-
itate the reduction of secondary metal precursors in the TEG
solvent [38]. For random alloy NPs, PtPd NPs synthesized by this
method could have flexible Pd/Pt ratios from 0.5 to 2.0 [38]. How-
ever, we observed a restricted bimetallic ratio in intermetallic PtSn
NPs likely due to the high thermodynamic stability and restricted
stoichiometry of PtSn intermetallic phase (i.e. the exothermic for-
mation of PtSn phase) [41]. We found that we could not deposit
The PtSn iNPs were the model catalysts synthesized via the
ship-in-a-bottle strategy as reported in our previous work
24,36,38]. PtSn model iNPs have a core-shell structure where PtSn
intermetallic cores (ca. 18 nm) are encapsulated inside the mSiO
shells (ca. 10 nm in thickness) as shown in Fig. 1a. Fig. 1b and c
are TEM images of the PtSn iNPs encapsulated in mSiO . Noting
that the formation of accurate PtSn intermetallic phase was com-
pleted and confined within the mSiO shells, this synthesis archi-
[
2
2
2
tecture ensures the atomic level homogeneity of the PtSn
composition in individual NPs [24,36]. The homogeneity of the
intermetallic phase at single particle level renders the PtSn iNPs
in this study as model catalysts with structural clarification in
comparison to intermetallic compounds prepared by general
2
impregnation method. The surface composition of these mSiO -
encapsulated PtSn iNPs is also stable in the reducing environment
up to 500 °C as evidenced by in situ ambient-pressure X-ray photo-
electron spectroscopy in our previous work [36]. The average pore
more than 10% extra Sn on Pt seeds for Sn-rich PtSn iNPs, so Pt0.9
Sn is the highest Sn content we could reach. For Pt-rich PtSn iNPs,
the major crystal phase can only fall into intermetallic Pt Sn when
Pt/Sn ratio approaches 3:1. Even though the composition of PtSn
-
3
2
size of mSiO shells is 2.2–2.5 nm where small molecules can
Fig. 1. (a) Structural illustration of our model core-shell PtSn iNPs. (b, c) TEM images of Pt1.0Sn iNPs after 500 °C calcination and 300 °C reduction with different
magnifications. No aggregation of iNPs was observed after the thermal treatment. (d) PXRD patterns of Pt0.9Sn, Pt1.0Sn, and Pt1.2Sn iNPs reduced at 300 and 500 °C. Blue/red
triangles indicate the characteristic diffraction peaks in Pt/intermetallic PtSn standard patterns. (For interpretation of the references to colour in this figure legend, the reader
is referred to the web version of this article.)

Y. Pei et al. / Journal of Catalysis 374 (2019) 136–142
139
iNPs cannot be freely tuned, this restricted composition of PtSn
iNPs exemplifies the ordered and thermodynamically stable struc-
tures of intermetallic compounds in comparison to random alloys.
Due to the protection of the mesoporous silica shells, these PtSn
iNPs have high thermal stability against aggregation (Fig. 1a–c).
We first calcined these PtSn iNPs at 500 °C in air to remove surface
capping agents. During the calcination, PtSn iNPs transform to
obvious Pt PXRD peak from Pt1.2Sn-500 iNPs (Fig. 1d). Pt0.9Sn-300
iNPs also have a Pt/Pt-rich core with a PtSn shell structure
(Fig. 2f) that is less obvious in comparison to Pt1.2Sn-500 iNPs.
Upon the reduction at 500 °C, Pt0.9Sn-500 iNPs show a relatively
homogeneous distribution of Pt and Sn signals through the entire
NP (Fig. 2h). The inserted HAADF-STEM images in Fig. 2 also show
the heterogeneity of Pt and Sn distributions in Pt-rich Pt1.2Sn iNPs
(a bright core) in comparison to Sn-rich Pt0.9Sn iNPs.
phase-segregated Pt and SnO
Our previous studies have demonstrated the recovery of ordered
intermetallic PtSn phase from Pt-SnO particles (Pt:Sn = 1:1) by a
2 2
particles (Pt-SnO particles) [24].
3 x
We also acquired EXAFS spectra at Pt L and Sn K edges for Pt Sn
iNPs as shown in Fig. S1-2 and Table S2-3 to obtain the ensemble
information and validate the HAADF-STEM results. The white lines
2
reduction at 300 °C [36]. When the Pt:Sn ratio deviates from 1:1,
the recovery of the intermetallic PtSn phase could lead to different
surface structures depending on the Pt/Sn ratio after we reduced
3
in all PtSn iNPs at Pt L edge show a shift to high-energy in compar-
ison to that of Pt foil, due to the formation of PtSn intermetallic
phase (Fig. S1a). The white lines of all PtSn iNPs at Sn K edge are
positioned at similar energy in comparison to that of a Sn foil
(Fig. S1b) [42,43]. Pt1.0Sn and Sn-rich Pt0.9Sn iNPs have the Pt-Pt
these Pt-SnO
two reduction temperatures for the reconstruction of PtSn iNPs at
00 and 500 °C. These PtSn iNPs reduced at the two temperatures
were denoted as Pt Sn-300 and Pt Sn-500 (x = 0.9, 1.0 and 1.2).
2
particles at different temperatures. We thus applied
3
x
x
3
and Pt-Sn coordination numbers close to 2 and 4 at Pt L edge
PXRD patterns of Pt1.0Sn iNPs reduced under different tempera-
tures show that the intermetallic PtSn phase was formed after
either 300 or 500 °C reductions (Fig. 1d). The PXRD pattern of Pt-
rich Pt1.2Sn-300 iNPs majorly displays the PtSn intermetallic phase
despite a small amount of monometallic Pt phase indicated by the
blue triangle in Fig. 1d. The Pt phase in Pt1.2Sn-500 was suppressed
after reducing at 500 °C. For stoichiometric Pt1.0Sn and Sn-rich
Pt0.9Sn iNPs, we only observed the intermetallic PtSn phase from
the PXRD patterns regardless of reduction at 300 or 500 °C. Unlike
alloys, Pt phase can be clearly distinguished from the intermetallic
PtSn phase in Pt1.2Sn with only 10–20% extra Pt than the 1:1
stoichiometry.
(Table S2-3), as well as the Sn-Pt coordination number close to 6
at Sn K edge [44]. Although we noticed that the fitted Pt-Sn coor-
dination number (4) deviates from the theoretical value (6), which
might need to be calibrated by using a pure phase bulk PtSn inter-
metallic compounds. However, the trend in coordination number
could still be used to understand the structure change of the inter-
metallic NPs. The Pt-Pt, Pt-Sn, and Sn-Pt coordination numbers of
Pt1.0Sn and Sn-rich Pt0.9Sn iNPs are very close, indicating the for-
mation of ordered PtSn phase in Pt1.0Sn and Pt0.9Sn iNPs regardless
of the different reduction temperatures. On the other hand, we
observed a significant deviation in the coordination numbers for
the Pt-rich Pt1.2Sn-300, in which the Pt-Pt, Pt-Sn, and Sn-Pt coordi-
nations were fitted as 4.6 (0.3), 3.3 (0.1) and 4.5 (0.2), respectively.
The large deviation of Pt-Pt and Sn-Pt coordination numbers from
their theoretical values of 2 and 6 indicates the significant Pt aggre-
gates in the bulk phase of Pt1.2Sn-300 iNPs. This observation agrees
with the presence of concentrated Pt cores as observed in the
HAADF-STEM images and EDS line scan profiles (Fig. 2a and b).
After reduction at 500 °C, the Pt-Pt and Sn-Pt coordinations of
Pt1.2Sn-500 iNPs were fitted as respective 2.7 (0.3) and 5.5 (0.2),
To visualize the location of extra Pt and Sn in PtSn iNPs, we
acquired elemental mappings and EDS line scans for PtSn iNPs
(Fig. 2) by HAADF-STEM. A Pt/Pt-rich core with a PtSn shell struc-
ture was likely observed in Pt1.2Sn-300 iNPs (Fig. 2b). After the
reduction at 500 °C, Pt1.2Sn-500 iNPs have a more homogeneous
mixing of Pt and Sn than Pt1.2Sn-300 iNPs, while a spike of Pt signal
was still observed, indicating the presence of Pt domains in the NP
(Fig. 2d). This observation is consistent with the suppressed but
Fig. 2. Elemental mappings and EDS line scan of representative Pt0.9Sn and Pt1.2Sn iNPs reduced at 300 and 500 °C. The scale bars in Fig. 2a, c, e, and g are 5 nm. The EDS line
scans of (b) Pt1.2Sn-300, (d) Pt1.2Sn-500, (f) Pt0.9Sn-300, and (h) Pt0.9Sn-500. The red dots and green triangles are Pt and Sn signals of PtSn iNPs. The inserts in Fig. 2b, d, f, and h
are HAADF-STEM images of (a) Pt1.2Sn-300, (c) Pt1.2Sn-500, (e) Pt0.9Sn-300, and (g) Pt0.9Sn-500. (For interpretation of the references to colour in this figure legend, the reader
is referred to the web version of this article.)

1
40
Y. Pei et al. / Journal of Catalysis 374 (2019) 136–142
approaching their corresponding theoretical values of 2 and 6, indi-
cating the bulk structure of the Pt-rich particles transforms to the
major PtSn intermetallic phase at 500 °C.
showed a high selectivity of >99%; whereas only 30% selectivity
was observed on the monometallic Pt@mSiO catalyst. We summa-
rized the apparent activation energy of Pt-Sn iNPs for furfural
hydrogenation in Fig. 3a and Fig. S4-5 to compare the different cat-
alytic properties. To exclude the mass transfer limitation and
obtain accurate activation energy values, we varied the amount
2
The PXRD, EDS, and EXAFS results demonstrate the structure
evolution of PtSn intermetallic compounds with non-
stoichiometry compositions upon reduction. However, these char-
acterization techniques cannot provide detailed information on
their surface structures. Despite that Pt/Pt-rich core was observed
in Pt1.2Sn-300, the other catalysts did not show a major structural
difference to trace the location of the extra Pt and Sn. We further
used furfural hydrogenation as a probe reaction to investigate
the surface structures of these PtSn iNPs reduced at different tem-
peratures. To our surprise, these four catalysts beheld different cat-
alytic properties as shown in Fig. 3 and Fig. S3. In a previous report,
we had demonstrated that Pt1.0Sn-300 iNPs are highly active and
selective (>99%) in the hydrogenation of furfural to furfuryl alcohol
due to the tiled end-on adsorption geometry of furfural molecules
on the intermetallic PtSn(110) facet [24]. Herein, Pt1.2Sn and
Pt0.9Sn iNPs show different catalytic properties in furfural
hydrogenation depending on their compositions and reduction
temperatures. Using catalysts that contain similar amounts of Pt
and comparing their performance at the 220 °C reaction tempera-
ture, Pt-rich Pt1.2Sn iNPs have high activity (20% conversion) and
2
of Pt@mSiO and observed a linear relationship between the con-
version and the amount of used catalyst as shown in Fig. S6-7.
Two sets of catalysts can be classified based on their apparent acti-
vation energy values: (i) Pt-rich Pt1.2Sn-300, Pt1.2Sn-500, and Sn-
rich Pt0.9Sn iNPs-300 iNPs have activation energies close to that
À1
of Pt control (16–18 kJ mol ), and they show higher selectivity
to furfuryl alcohol (90% at 220 °C) than monometallic Pt@mSiO
2
NPs but not as high as Pt1.0Sn iNPs. Pt1.2Sn-500 iNPs show the low-
est activation energy among all PtSn iNPs samples. (ii) Differently,
Sn-rich Pt0.9Sn-500 iNPs have similar activation energy and high
selectivity in comparison to Pt1.0Sn-300 iNPs with well-ordered
À1
intermetallic structures (23–28 kJ mol ). The TOFs of these Pt-Sn
iNPs in furfural hydrogenation were also calculated as shown in
Fig. S8. The Pt-Sn iNPs have 2 to 7-fold higher TOFs than pure Pt
exemplifying the promotion effect of bimetallic Pt-Sn in furfural
hydrogenation. The Pt-rich Pt1.2Sn catalysts have similar TOFs
À1
(0.20 s ) regardless of the reduction temperatures. The Sn-rich
9
5
5% selectivity to furfuryl alcohol after reduction at either 300 or
00 °C, while Sn-rich Pt0.9Sn iNPs have variable catalytic properties
Pt0.9Sn iNPs show dramatically lower TOFs after the reduction at
À1
À1
500 °C (0.37 s for Pt0.9Sn-300 iNPs, and 0.11 s for Pt0.9Sn-500
iNPs). The highest TOF value was obtained over Sn-rich Pt0.9Sn-
300 iNPs among all catalysts. We thus speculate that the reduction
temperature can induce the reconstruction of the extra metal
atoms in Pt-Sn iNPs endowing different surface structures. The sur-
face of Pt0.9Sn-500 iNPs can reflect the ideal PtSn intermetallic
structure and is largely different from that of Pt1.2Sn-300,
Pt1.2Sn-500, and Pt0.9Sn-300 iNPs with more Pt-rich surfaces. It is
depending on the reduction temperatures. Pt0.9Sn-300 has high
activity (20% conversion) and 95% selectivity similarly to that of
Pt1.2Sn iNPs. However, the 500 °C reduction significantly depresses
the activity of Pt0.9Sn-500 iNPs (ca. 5% conversion) with enhanced
selectivity (>99%) as shown in Fig. S3a. To obtain the catalytic
benchmark, we also tested monometallic Pt@mSiO
chiometric Pt1.0Sn iNPs for the furfural hydrogenation. Pt1.0Sn iNPs
2
NPs and stoi-
Fig. 3. (a) Apparent activation energy of furfural hydrogenation over Pt@mSiO
2
, Pt0.9Sn, Pt1.0Sn, and Pt1.2Sn iNPs reduced at 300 and 500 °C. 0.3–0.5 mg catalyst was used in
À1
À1
furfural hydrogenation with a gas feed of furfural/H
spectra for Pt@mSiO
00 iNPs. The yellow/white dots represent Pt/Sn atoms, and the blue dots exclusively represent the Pt atoms in the core area. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
2
= 0.023/11.4 mL min . A He flow of 8.6 mL min carries furfural vapor from a bubbler into the reactor. (b) CO-DRIFTS
2
, Pt0.9Sn, Pt1.0Sn, and Pt1.2Sn iNPs reduced at 300 and 500 °C. (c) Proposed structural models for Pt1.2Sn-300, Pt1.2Sn-500, Pt1.0Sn, Pt0.9Sn-300, and Pt0.9Sn-
5

Y. Pei et al. / Journal of Catalysis 374 (2019) 136–142
141
Fig. 4. Temperature-dependent conversion and selectivity trends of acetylene semi-hydrogenation over (a) Pt0.9Sn and (b) Pt1.2Sn iNPs reduced at 300 and 500 °C. 2 mg
À1
catalyst was used in acetylene semi-hydrogenation with a gas feed of C
2
H
2
/C
H
2 4
/H
2
/He = 0.15/15/1.5/13.5 mL min . Each conversion and selectivity value was calculated by
averaging 10 injections.
worthwhile to note that we cannot exclude the possibility of PtSn
surface reconstruction under reaction conditions due to the inter-
actions between reactant molecules and PtSn surface atoms.
DRIFTS study was used to investigate the surface structures of
PtSn iNPs with CO as the probe molecule (Fig. 3b). In all DRIFTS
spectra of PtSn iNPs, only one mode of atop CO band was observed
after the catalyst surface was saturated with CO molecules. The
respective peak shifts of atop CO mode can be readily divided into
two categories matching well to the two activation energy sets in
the furfural hydrogenation. Due to the electron donation from Sn
to Pt, CO molecules adsorbed on Pt1.0Sn-300 iNPs display a shift
(i.e. 500 °C) can be attributed to the higher surface energy of PtSn
than monometallic Pt [47]. For Sn-rich samples, Pt0.9Sn-300 iNPs
display similar contiguous Pt sites on the surface. However, the
500 °C reduction can homogenize the structures of Pt0.9Sn-500
iNPs and lead to a well-ordered PtSn surface partially covered by
Sn, agreeing to their similar activation energies but lower activity
in comparison to stoichiometric Pt1.0Sn iNPs (Fig. 3a) in the furfural
hydrogenation. Moreover, the high mobility and low surface
energy of Sn could explain that extra Sn would prefer to stay on
the PtSn surface of Pt0.9Sn-500 iNPs [48]. We also acquired the Pt
dispersion values to validate these four models following the order
À1
to a low wavenumber (2068 cm ) in comparison to those on Pt
of
Pt ꢀ (Pt0.9Sn-300 ꢁ Pt1.2Sn-500 ꢁ Pt1.0Sn-300 ꢁ Pt1.2Sn-300)
À1
(
2073 cm ) in concurrence with our previous studies [36]. We
> Pt0.9Sn-500 (Table S4). Pt dispersion of Pt1.2Sn iNPs did not vary
regardless of the reduction temperature. However, the Pt disper-
sion was significantly decreased over the Sn-rich Pt0.9Sn iNPs after
500 °C reduction correlated to the higher surface Sn coverage.
To further validate these structural models, especially to
investigate the surface homogeneity, we used acetylene
semi-hydrogenation as another probe reaction. Acetylene
semi-hydrogenation is sensitive to the presence of ensemble (con-
tiguous) Pt sites that have been widely studied over single-site and
intermetallic catalysts [25,49,50]. The desired product in the
acetylene semi-hydrogenation is ethylene. However, contiguous
Pt sites favor the further hydrogenation of ethylene to ethane
and decrease the selectivity in the acetylene hydrogenation [51].
Our structure model in Fig. 3c conjectures that the presence of Pt
contiguous sites on Pt1.2Sn-300, Pt1.2Sn-500, and Pt0.9Sn-300 iNP
would lead to higher conversion and lower selectivity to ethylene,
in agreement to our catalytic observation as shown in Fig. 4. On the
contrary, the Sn-rich Pt0.9Sn-500 shows a dramatically enhanced
selectivity due to the homogeneous dilution of Pt in the inter-
metallic PtSn structures. The low activity in Pt0.9Sn-500 also con-
firms that the intermetallic surface is partially covered with Sn
(Fig. 4a). Pt1.0Sn-300 catalyst also shows high selectivity to ethy-
lene due to its ordered PtSn intermetallic surface (Fig. S9). The
acetylene semi-hydrogenation results readily correspond to the
proposed structural models. Using acetylene semi-hydrogenation
and furfural hydrogenation, we demonstrate that structure-
sensitive reactions can be used as catalytic probes to investigate
the surface structures of heterogeneous catalysts and to under-
stand their structure-property relationships. However again, we
emphasize the possibility of the PtSn surface reconstruction under
reaction conditions, which may not be observed in many surface
also reduced Pt1.0Sn iNPs at 500 °C, and the peak position of atop
CO on Pt1.0Sn-500 iNPs remains the same despite a decrease in
intensity, which indicates that Pt1.0Sn iNPs can have a relatively
stable surface structure derived from the well-ordered intermetal-
lic phase. The peak positions of atop CO adsorbed on Pt and Pt1.0Sn
can serve as two benchmarks to evaluate the surface structures of
Pt- and Sn-rich Pt-Sn iNPs. The CO adsorption features of Sn-rich
Pt0.9Sn-500 iNPs resemble that of Pt1.0Sn iNPs, indicating the pres-
ence of an ideal PtSn intermetallic surface due to ordering effect
after the 500 °C reduction. Different from Pt1.0Sn iNPs, CO mole-
cules adsorbed on Pt-rich Pt1.2Sn-300, Pt1.2Sn-500, and Sn-rich
Pt0.9Sn-300 iNPs show shifts to high wavenumbers, demonstrating
the presence of contiguous surface Pt sites. The contiguous surface
Pt sites could also explain their high activity but lower selectivity
in furfural hydrogenation.
Combining EDS mapping and line scan, EXAFS, DRIFTS, and fur-
fural hydrogenation results, we proposed the structural models of
the Pt-Sn iNPs with non-stoichiometric compositions as shown in
Fig. 3c. The stoichiometric Pt1.0Sn iNPs show homogeneous dis-
tributed Pt and Sn atoms because of the intrinsic intermetallic
structures of PtSn. Pt1.2Sn-300 iNPs have contiguous Pt sites on
the surface with a large Pt core, which can explain their similar cat-
2
alytic properties as monometallic Pt@mSiO NPs. After reduction at
5
00 °C, smaller Pt cores and more homogeneous distribution of Pt
can be induced in Pt1.2Sn-500 iNPs likely leading to the formation
of Pt-rich top-layer structures over a PtSn intermetallic sublayer.
Similar Pt/PtSn core/shell structures [32] have been reported with
enhanced electrocatalytic activities, as well as other highly active
intermetallic PtZn with a Pt-rich shell [29] and Pt skinned Pt/Pd
NPs [45,46]. The surface Pt enrichment at the high temperature

1
42
Y. Pei et al. / Journal of Catalysis 374 (2019) 136–142
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and Sn-rich compositions. The reduction temperature has a strong
influence on the structure and catalytic properties of these PtSn
iNPs, especially for the Sn-rich Pt0.9Sn iNPs. Furfural hydrogenation
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