Silica-Encapsulated Pt-Sn Intermetallic Nanoparticles: A Robust Catalytic Platform for Parahydrogen-Induced Polarization of Gases and Liquids

Article abstract of DOI:10.1002/anie.201701314

Recently, a facile method for the synthesis of size-monodisperse Pt, Pt₃Sn, and PtSn intermetallic nanoparticles (iNPs) that are confined within a thermally robust mesoporous silica (mSiO₂) shell was introduced. These nanomaterials offer improved selectivity, activity, and stability for large-scale catalytic applications. Here we present the first study of parahydrogen-induced polarization NMR on these Pt-Sn catalysts. A 3000-fold increase in the pairwise selectivity, relative to the monometallic Pt, was observed using the PtSn@mSiO₂ catalyst. The results are explained by the elimination of the three-fold Pt sites on the Pt(111) surface. Furthermore, Pt-Sn iNPs are shown to be a robust catalytic platform for parahydrogen-induced polarization for in vivo magnetic resonance imaging.

Full text of DOI:10.1002/anie.201701314

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201701314
German Edition: DOI: 10.1002/ange.201701314
Hydrogenation
Silica-Encapsulated Pt-Sn Intermetallic Nanoparticles: A Robust
Catalytic Platform for Parahydrogen-Induced Polarization of Gases
and Liquids
Evan W. Zhao, Raghu Maligal-Ganesh, Chaoxian Xiao, Tian-Wei Goh, Zhiyuan Qi, Yuchen Pei,
Helena E. Hagelin-Weaver, Wenyu Huang,* and Clifford R. Bowers*
Abstract: Recently, a facile method for the synthesis of size-
monodisperse Pt, Pt3Sn, and PtSn intermetallic nanoparticles
(iNPs) that are confined within a thermally robust mesoporous
silica (mSiO2) shell was introduced. These nanomaterials offer
improved selectivity, activity, and stability for large-scale
catalytic applications. Here we present the first study of
parahydrogen-induced polarization NMR on these Pt-Sn
catalysts. A 3000-fold increase in the pairwise selectivity,
relative to the monometallic Pt, was observed using the
PtSn@mSiO2 catalyst. The results are explained by the
elimination of the three-fold Pt sites on the Pt(111) surface.
Furthermore, Pt-Sn iNPs are shown to be a robust catalytic
platform for parahydrogen-induced polarization for in vivo
magnetic resonance imaging.
Platinum is among the most versatile and active metals in
heterogeneous catalysis. With applications to clean and
renewable energy, Pt has been the subject of many funda-
mental studies, but its low selectivity, high cost, and suscept-
ibility to poisoning drive the search for alternatives. Pt-Sn
bimetallic catalysts offer key advantages for many types of
reactions.[18–20] In hydrogenation, Pt-Sn catalysts can provide
higher selectivity while maintaining activity.[21,22] Surface-
science studies of adsorption of H2 on Pt-Sn surface alloys by
Koel et al. in the 1990s revealed the importance of contiguous
three-fold Pt(111) sites in dissociative H2 chemisorption.[23–26]
Elimination of three-fold Pt sites upon incorporation of Sn is
illustrated in Figure 1. However, aggregation of Pt-Sn iNPs
during the high-temperature annealing treatment has ham-
pered the elucidation of the role of these sites in hydro-
genation catalysis.
P
arahydrogen-induced polarization[1,2] by heterogeneous
hydrogenation catalysis (hetPHIP) offers a fast, scalable,
and inexpensive method for the production of hyperpolarized
gases[3,4] and liquids.[5–9] Metal nanoparticle (NP) catalysts are
particularly attractive for in vivo applications since they tend
not to leach into the products, but unfortunately, the low
pairwise selectivity of hydrogenation has up-to-now limited
the hyperpolarization levels to a few percent.[10–14] To become
a viable alternative to dissolution dynamic nuclear polar-
ization (dDNP), the dominant hyperpolarization method-
ology for biomedical imaging,[15–17] higher polarization levels
need to be realized and efficient protocols to extend hetPHIP
to biomolecular substrates in water need to be developed.
Herein we show how these challenges can be addressed with
silica-protected intermetallic nanoparticles (iNPs). The
results also illustrate how hetPHIP can probe previously
hidden details of the hydrogenation mechanisms for these
advanced catalytic nanomaterials.
Recently, a facile approach to the synthesis of size-
monodisperse nanoscale intermetallic compounds has been
introduced.[22,27] The well-defined Pt-Sn iNPs are encapsu-
[*] E. W. Zhao, Prof. H. E. Hagelin-Weaver, Prof. C. R. Bowers
Department of Chemistry and Department of Chemical Engineering
University of Florida, Gainesville, FL 32611 (USA)
E-mail: bowers@chem.ufl.edu
R. Maligal-Ganesh, Dr. C. Xiao, T. W. Goh, Z. Qi, Y. Pei,
Prof. W. Huang
Department of Chemistry, Iowa State University
Ames Laboratory, U.S. Department of Energy
Ames, IA 50011 (USA)
Figure 1. TEM images, structural models and PXRD patterns for
A) Pt@mSiO2, B) Pt3Sn@mSiO2, and C) PtSn@mSiO2. Metal core
diameters [nm], determined from TEM, are 14.3ꢀ0.8 (Pt), 16.6ꢀ0.9
(Pt3Sn), and 20.6ꢀ0.9 (PtSn). Sn atoms are shown in orange. The
blue triangle indicates the three-fold Pt sites discussed in the text.
E-mail: whuang@iastate.edu
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201701314.
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lated in a mesoporous silica (mSiO2) shell (Figure 1) that
stabilizes them at high temperatures.[28,29] We employed
hetPHIP NMR to examine the effects of incorporating Sn
on the pairwise selectivity (f) of hydrogenation with these
iNPs. Additionally, the Pt-Sn iNPs are shown to be a promising
catalytic platform for hyperpolarization of biocompatible
gases and liquids for in vivo magnetic resonance imaging.
When two protons originating from the same parahydro-
gen molecule are simultaneously incorporated into the same
hydrogenation adduct molecule, a portion of the singlet spin
order inherent to parahydrogen is carried with them. The
efficiency of the process depends on the reaction rate and spin
relaxation losses. Symmetry breaking chemical shift and spin–
spin interactions can subsequently transform the singlet spin
order into NMR-observable hyperpolarization, resulting in
NMR signal enhancements of 4–8 orders of magnitude,
depending on magnetic field strength. This powerful NMR
sensitivity enhancement technique is compatible with the
preparation of long-lived states,[30,31] spin-transfer catalysis by
reversible exchange,[32–34] magnetic resonance imaging,[35–38]
and remote detection.[39] However, signal enhancements
obtained by hetPHIP have been disappointing due to the
low pairwise selectivity for supported metals like Ir, Pt, or
Rh.[10–14] In view of the known effect of Sn on the dissociation
of molecular H2 on Pt-Sn surface alloys, we hypothesized that
pairwise addition—a fundamental requirement for the obser-
vation of PHIP—should be strongly enhanced in Pt-Sn iNPs.
The mSiO2-protected Pt-Sn iNPs provide the first opportunity
to test this hypothesis. We also investigated the efficacy of
these catalysts for hetPHIP in an aqueous slurry.
contained in a modified 10 mm NMR tube (see the Support-
ing Information). Both types of reactors were operated in the
“ALTADENA” mode, which yields characteristic net align-
ment multiplet patterns. The slurry-type reactor is suitable for
non-volatile, higher mass substrates, and the design facilitated
hydrogenation studies at elevated pressures, which increases
the concentration of dissolved p-H2, increases the boiling
point of the solvent, and allows reactions to be run at higher
temperatures. Slurry reactions were performed with the NMR
tube located in the earthꢀs field followed by quick manual
transfer into the high field of the 9.4 T NMR spectrometer.
1
The H NMR spectra produced by the reaction of p-H2
with propene (PE) over the mSiO2-encapsulated catalysts in
the fixed bed reactor are presented in the lower panels of
Figure 2 and Figure S5 for bed temperatures of 100, 200, and
The Pt3Sn@mSiO2 and PtSn@mSiO2 iNPs were prepared
by an encapsulation-mediated synthesis starting with
Pt@mSiO2 NPs that were synthesized as described previ-
ously,[22] with a few minor modifications, as described in the
Supporting Information.[22,27] PtSn(110) was included in this
study because of its low surface energy compared to other
facets of PtSn.[22] Representative TEM images and PXRD
patterns of the NPs are presented in Figure 1. The metal
loading, particle size, and Pt dispersion of the mSiO2-
encapsulated iNPs are reported in Table 1. Characterization
details are provided in the Supporting Information.
Continuous-flow reactions were performed with either
normal hydrogen (n-H2) or hydrogen enriched to 50%
parahydrogen (p-H2) using two different reactor configura-
tions: 1) a fixed bed gas/solid reactor operating at a pressure
of ca. 1 bar positioned in the fringe field of the superconduct-
ing NMR magnet, as described in our previous work,[4] and
2) a novel pressurized slurry-type gas/liquid/solid reactor
Figure 2. Thermally polarized (top) and hyperpolarized (bottom)
1H NMR spectra of reactor effluent obtained using 10 mg of
A) Pt@mSiO2, B) Pt3Sn@mSiO2, and C) PtSn@mSiO2 at 3008C. The
reactant flow rates were 120 mLminꢁ1 H2, 210 mLminꢁ1 PE, and
70 mLminꢁ1 N2. All spectra are displayed on the same vertical scale.
3008C. Propane (PA) exhibits the ALTADENA net align-
ment signal pattern (-CH3 multiplet in emission phase). The
thermally polarized spectra acquired at 3008C are presented
in the upper panels of Figure 2 and Figure S6. The intensities
of the PA peaks (labelled “a” and “b”) in the thermally
polarized spectra are proportional to total PE!PA conver-
sion (random + pairwise), denoted c. Pairwise selectivity f is
calculated from the ratio of the experimental and theoretical
signal enhancements, f = hexp/htheory [40]
.
Figure 3 and
Tables S1–S3 report c, f, and hexp obtained using 10 mg of
Pt@mSiO2, Pt3Sn@mSiO2, and PtSn@mSiO2 at 100, 200, and
3008C. Good reproducibility of the experimental data was
obtained.
Table 1: Metal Loading, Particle Size and Pt Dispersion of the Meso-
porous Silica-Encapsulated NPs.
At 3008C, monometallic Pt@mSiO2 catalyst produced
only a very weak ALTADENA NMR signal. In contrast, the
Pt3Sn@mSiO2 and PtSn@mSiO2 catalysts produced similarly
strong ALTADENA peaks. Despite the drastically lower
activity exhibited by PtSn@mSiO2 (compare the “a” peaks in
the spectra of the upper panels of Figure 2 and Figure S6), this
catalyst still gave the strongest ALTADENA signals. Thus,
PtSn@mSiO2 delivered the highest hexp and f. A remarkable
Sample
Pt
Sn
Loading
Core Size[b]
[nm]
Pt
Loading[a]
Dispersion
Pt@mSiO2
Pt3Sn@mSiO2
PtSn@mSiO2
44.5%
52.0%
36.6%
0%
9.0%
22.4%
14.3ꢀ0.8
16.6ꢀ0.9
20.6ꢀ0.9
6.5%
2.8%
1.8%
[a] Metal loadings were measured based on weight percentage.
[b] Diameter of the metal core.
2
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synthetic batch of PtSn@mSiO2, enhancement factors hexp
>
103 were observed, corresponding to f > 10%, setting a world
record for any heterogeneous NP catalyst. While our setup
did not permit these studies to be extended to higher pressure
and catalyst mass, the results clearly indicate that higher
conversions can likely be achieved. We hypothesize that the
slight differences in f and c between the two catalyst batches
are due to variations in uniformity of Pt and Sn surface atomic
distributions that exist despite the same 1:1 atomic ratio in
bulk. For the reactions at 3008C with 10 mg, the second batch
exhibited a higher f (10.4% vs. 7.5%) and a lower c (0.2% vs.
0.3%) than the first batch, consistent with anticorrelation of f
and c, and lower surface Pt/Sn ratio for the second synthetic
batch.
The dramatic increase in f, accompanied by a decrease in
c, is consistent with fewer three-fold Pt sites on the ordered
PtSn(110) surface compared to Pt(111) and Pt3Sn(111). A
Pt(111) surface consists of contiguous pure-Pt three-fold sites,
while Pt3Sn(111) bears isolated pure-Pt three-fold sites, and
Pt2Sn(111) is devoid of three-fold sites. Note that the mole
fraction of Sn in our PtSn@mSiO2 catalyst (Pt/Sn = 1:1) is
even higher than in the Sn/Pt2(111) surface alloy study. The
surface alloy studies on Pt, Pt3Sn, and Pt2Sn single crystal
surfaces have shown the three-fold Pt hollow sites to be
essential for dissociative adsorption of H2.[23–26] Chemisorp-
tion of H2 on the Sn/Pt2(111) surface alloys at room temper-
ature was found to be limited to only 2% of the saturation
coverage of hydrogen on clean Pt(111). In contrast, atomic
hydrogen readily adsorbs on the alloy surfaces at temper-
atures down to 150 K,[23] indicating that H2 chemisorption is
inhibited by kinetic rather than thermodynamic factors (high
H2 dissociation barrier).[6,7]
Due to the facile dissociative adsorption of H2 and high
rate of stepwise addition of H ad-atoms to the alkene in the
Horiuti–Polanyi mechanism, conversion by direct hydroge-
nation is extremely low for monometallic Pt NPs. The non-
linear dependence of f on Sn implicates the Pt three-fold
hollow sites in the catalysis by random addition. With
increasing Sn fraction, f also increases because of decreased
H2 dissociative chemisorption. There could also be a corre-
sponding increase in the number of sites that bind molecular
H2 and serve as active sites for direct hydrogenation.
Lastly, the efficacy of the Pt-Sn iNP catalysts in the
aqueous phase is demonstrated. Hydrogenation of 2-hydrox-
yethyl acrylate (HEA) in D2O was catalyzed by Pt3Sn@mSiO2
in the earthꢀs magnetic field. After bubbling p-H2 gas through
the slurry, the sample tube was immediately transferred into
the 400 MHz NMR magnet and the ALTADENA NMR
spectrum collected with a 908 pulse. As seen in Figures 4, S10
and S11, intense NMR signals of the methyl (c) and
methylene (d) groups of 2-hydroxyethyl propionate (HEP)
were obtained. Due to the continuous reaction in the slurry,
a reliable determination of the PHIP signal enhancement was
not possible in these experiments. Moreover, an estimated
factor of 10 in loss of polarization occurs during the ca. 10 s
transfer of the sample from low to high field.
Figure 3. A,B) Temperature dependence of pairwise selectivity, f, ALTA-
DENA signal enhancement factor, hexp, and PE-to-PA conversion, c, for
Pt@mSiO2, Pt3Sn@mSiO2 and PtSn@mSiO2 (10 mg, synthetic batch
1) at 100, 200, and 3008C (dark grey, grey, and light grey bars,
respectively). C) Linear dependence of conversion and pairwise selec-
tivity on H2 partial pressure for two PE reactant pressures (50 mg
PtSn@mSiO2, batch 2). D) Linear dependence of pairwise selectivity,
enhancement factor, and conversion on PtSn@mSiO2 catalyst mass
(batch 2).
3000-fold increase in signal enhancement relative to the
monometallic Pt@mSiO2 NPs (hexp = 0.20 ꢀ 0.07 and f =
0.0020 ꢀ 0.0007%) was observed (see Table S2). Meanwhile,
c decreases precipitously upon incorporation of Sn (Fig-
ure 3B and Table S3), following the order Pt@mSiO2 (19.5%)
> Pt3Sn@mSiO2 (17%) @ PtSn@mSiO2 (0.3%) at 3008C.
As seen in Figure 3B, conversion over Pt@mSiO2 slightly
decreased with increased temperature. This could be due to
coke formation on the pure Pt NPs where a high activity of
hydrogenation may induce local heating. Koel et al. reported
a 35% PE decomposition on Pt(111) during the TPD
experiment, ca. 5–7% decomposition on Pt3Sn, and no
decomposition on Pt2Sn as substrate temperature increased.
It was proposed that PE decomposition is also catalyzed on
contiguous pure-Pt three-fold hollow sites.[24] A slight reduc-
tion in c was observed over Pt@mSiO2 and Pt3Sn@mSiO2 in
the second run of the reaction, possibly due to the blocking of
some active sites by carbonaceous deposits.
The preceding results were obtained using only 10 mg of
PtSn@mSiO2 iNPs and 300 mbar H2. The low conversion
achieved for these conditions, attributed to the weak physi-
sorption of H2 for these iNPs, would be inadequate for most
applications. Therefore, additional experiments were per-
formed to investigate the scaling of f and c with catalyst mass
and H2 partial pressure. Figures 3C, S8 and S9 show a linear
dependence of c on pH with little change in f. Figures 3D and
2
S7 show that c also scales linearly with catalyst mass from 10
to 50 mg without significant loss of pairwise selectivity. In
these experiments, which were performed on a separate
To ensure that this hydrogenation occurs heterogeneously
on the iNPs and not homogeneously by leached metal ions, as
can occur in immobilized complexes,[41] PHIP experiments
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while f was unaffected, suggesting that larger amounts of
catalyst and higher pressures would increase the conversion.
Moreover, it should be noted that the particle sizes of our
iNPs are relatively large (ca. > 14 nm); hence the active metal
dispersion is comparatively low. Current research is focused
on the synthesis of smaller iNPs (diameter ca. < 5 nm) which
can be expected to deliver much higher conversion.
Hyperpolarization of 2-HEP in water is significant
because this is the standard molecule for in vivo angiography
studies.[7,9] The mSiO2 encapsulated metal particles afford
spontaneous and rapid separation from the catalyst, making
this catalyst platform intrinsically safer than dissolved or
immobilized transition-metal complexes or free-radical polar-
izing agents. Agents that have previously been demonstrated
to be hyperpolarizable by homogeneous PHIP includes
acetate and pyruvate,[44,45] lactate and phospholactate,[46,47]
nicotinimide,[48] and succinate.[49,50] It can be expected that
slurry reactions containing iNPs will be effective for para-
hydrogen-induced hyperpolarization of many if not all of
these substances.
Figure 4. 400 MHz 1H NMR spectra obtained in a slurry reactor. The
spectra were obtained from a single free induction decay. ALTADENA
spectra obtained after bubbling n-H2 (top) or p-H2 (bottom) through
a slurry containing 2 mL water, 50 mg of Pt3Sn@mSiO2 catalyst, and
dissolved 2-HEA (1% by volume) at 5.7 bar and 1208C.
Acknowledgements
This work was supported by NSF grant CHE-1507230 (C.R.B.
and H.H.W.) and CHE-1607305 (W.H.). Technical facilities
and services provided by the University of Florida Physics
Department are gratefully acknowledged. The work was also
supported by start-up funds provided by Iowa State Univer-
sity (W.H.). We thank the G. J. Miller group for allowing us to
use their instrument to measure the PXRD patterns.
were attempted using the supernatant obtained by centrifu-
gation and decanting of the aqueous slurry containing the
particles. As shown in Figure S12, no conversion and no PHIP
NMR signals could be observed. Table S6 reports the Pt and
Sn concentrations in the supernatant, as determined by ICP-
MS analysis. The measured Pt and Sn solution concentrations
of 14.67 ppb (0.0001% of Pt) and 69.62 ppb (0.003% of Sn),
respectively, are too low to catalyze any reaction.
Conflict of interest
In summary, we have shown that the mSiO2-protected
iNPs prepared by the seeded growth method are effective
catalysts for producing PHIP NMR signals of hydrogenation
adducts in the gaseous or solution phases. The mSiO2 shell
endows the iNPs with stability against aggregation, sintering,
and phase separation without totally obstructing the catalytic
surface. Particle surfaces and metal compositions can be
tailored to effectuate an increase in the pairwise selectivity of
hydrogenation. The pairwise selectivity of f = 10.9 ꢀ 0.5%
obtained with 30 mg of PtSn@mSiO2 at 3008C is the highest to
be reported for any heterogeneous NP catalyst and more than
three orders of magnitude higher than for monometallic
Pt@mSiO2 NPs of similar size. This is the observed f value,
uncorrected for relaxation losses. The results are consistent
with the elimination of the three-fold Pt hollow sites where
facile dissociative adsorption of H2 occurs. Direct hydro-
genation of the alkene, by either an Eley–Rideal or Lang-
muir–Hinshelwood channel, becomes relatively more impor-
tant upon incorporation of Sn.[42,43]
The authors declare no conflict of interest.
Keywords: hydrogenation · hyperpolarization · intermetallics ·
parahydrogen · PtSn
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Manuscript received: February 6, 2017
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Hydrogenation
E. W. Zhao, R. Maligal-Ganesh, C. Xiao,
T. W. Goh, Z. Qi, Y. Pei,
H. E. Hagelin-Weaver, W. Huang,*
C. R. Bowers*
&&&& — &&&&
Silica-Encapsulated Pt-Sn Intermetallic
Nanoparticles: A Robust Catalytic
Platform for Parahydrogen-Induced
Polarization of Gases and Liquids
Parahydrogen-induced polarization using
Pt-Sn intermetallic nanoparticles (iNPs)
achieved a >3000-fold increase in the
signal enhancement. The Pt-Sn iNPs are
also shown to be a robust catalytic plat-
form for parahydrogen-induced polariza-
tion of liquids that is applicable to in vivo
imaging.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!