Ultra-Low Loading Pt/CeO2 Catalysts: Ceria Facet Effect Affords Improved Pairwise Selectivity for Parahydrogen Enhanced NMR Spectroscopy

Article abstract of DOI:10.1002/anie.202012469

Oxide supports with well-defined shapes enable investigations on the effects of surface structure on metal–support interactions and correlations to catalytic activity and selectivity. Here, a modified atomic layer deposition technique was developed to achieve ultra-low loadings (8–16 ppm) of Pt on shaped ceria nanocrystals. Using octahedra and cubes, which expose exclusively (111) and (100) surfaces, respectively, the effect of CeO₂ surface facet on Pt-CeO₂ interactions under reducing conditions was revealed. Strong electronic interactions result in electron-deficient Pt species on CeO₂ (111) after reduction, which increased the stability of the atomically dispersed Pt. This afforded significantly higher NMR signal enhancement in parahydrogen-induced polarization experiments compared with the electron-rich platinum on CeO₂ (100), and a factor of two higher pairwise selectivity (6.1 %) in the hydrogenation of propene than any previously reported monometallic heterogeneous Pt catalyst.

Full text of DOI:10.1002/anie.202012469

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Accepted Article
Title: Ultra-Low Loading Pt/CeO2 Catalysts: Ceria Facet Effect Affords
Improved Pairwise Selectivity for Parahydrogen Enhanced NMR
Authors: Bochuan Song, Diana Choi, Yan Xin, Clifford R Bowers, and
Helena Hagelin Weaver
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202012469
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Ultra-Low Loading Pt/CeO2 Catalysts: Ceria Facet Effect Affords
Improved Pairwise Selectivity for Parahydrogen Enhanced NMR
Bochuan Song,[a] Diana Choi,[b] Yan Xin,[c] Clifford R. Bowers*[b] and Helena Hagelin-Weaver*[a]
[a]
B. Song, Prof. H. E. Hagelin-Weaver
Department of Chemical Engineering
University of Florida
Gainesville, FL 32611 (USA)
E-mail: hweaver@che.ufl.edu
D. Choi, Prof. C. R. Bowers
Department of Chemistry
[b]
University of Florida
Gainesville, FL 32611 (USA) University of Florida
E-mail: bowers@chem.ufl.edu
Gainesville, FL 32611 (USA)
Y. Xin
[c]
National High Magnetic Field Laboratory
Florida State University
Tallahassee, FL 32310, USA
Supporting information for this article is given via a link at the end of the document.
Abstract: Oxide supports with well-defined shapes are beneficial in
catalysis studies as they enable investigations on the effects of
surface structure on metal-support interactions as well as correlations
to catalytic activity and selectivity. Here, a modified atomic layer
deposition technique was developed to achieve ultra-low loadings (8-
16 ppm) of Pt on shaped ceria nanocrystals. By using octahedra and
cubes, which expose exclusively (111) and (100) surfaces
respectively, the effect of CeO2 surface facet on the Pt-CeO2
interactions under reducing conditions was revealed. Strong
electronic interactions were found to result in electron-deficient Pt
species on the CeO2 (111) after reduction, which increased the
stability of the atomically dispersed Pt and afforded significantly higher
NMR signal enhancement compared with the electron-rich platinum
on the CeO2 (100), and a factor of two higher pairwise selectivity
(6.1 %) than previously reported for a monometallic heterogeneous Pt
catalyst.
evaluated in the hydrogenation of propene, a standard probe
reaction for characterizing the PHIP performance.
The Pt/CeO2 catalysts were synthesized by carefully controlling
the dose time (5 s) during ALD (see Supporting Information). At
the same deposition temperature (180 °C), a smaller Pt loading is
observed on the CeO2 cubes compared with the octahedra,
despite the slightly higher specific surface area of the cubes
(Table 1). To match the Pt loadings and allow direct comparisons
between the catalysts, a lower deposition temperature (120 °C)
was also used for the CeO2 octahedra (Table 1). The Pt loadings
in this study (8-16 ppm) are an order of magnitude lower than
previously reported ultra-low loading catalysts.[5] Only isolated Pt
ions are present on the (111) and (100) surfaces of the CeO2
octahedra and cubes after deposition and a calcination treatment
with static air at 350 °C (Figures 1c,f and S1), consistent with the
stability of isolated Pt under oxidative conditions.[2b, 3c] The surface
Pt ions are located in the same positions as Ce ions since they
coordinate to oxygen ions, either by replacing a cerium ion in the
lattice or by bonding to the top of the oxygen-terminated CeO2
surfaces.[3a]
Catalysts with atomically dispersed active metals have attracted
significant attention as they afford maximum atomic efficiency,
which is especially important when using precious metals such as
Pt.[1] While isolated Pt ions are remarkably stable under oxidative
conditions on supports like CeO2,[2] improving stability under
reducing conditions remains a challenge that must be addressed
for hydrogenation reactions.[3] We hypothesize that atomically
dispersed Pt on a CeO2 support (either as isolated Pt+ ions or
Ptδx+ clusters interacting strongly with the CeO2) can improve the
pairwise selective addition of H2 to unsaturated substrates, the
latter a necessary property in parahydrogen-induced polarization
(PHIP) Nuclear Magnetic Resonance (NMR) (vide infra). To limit
Pt agglomeration to the greatest extent possible, we prepared
ultra-low Pt loadings using a modified atomic layer deposition
(ALD) process to maximize the distance between Pt atoms or ions.
Furthermore, to investigate the dependence of the metal-support
interactions on the surface facet exposed in Pt-CeO2 catalysts,[4]
two different well-defined CeO2 shapes were used as supports,
namely, octahedra and cubes, which exclusively expose (111)
and (100) surfaces, respectively. The resulting catalysts were
After temperature programmed reduction (TPR, Figure S2), the
Pt species on the surfaces of the CeO2 shapes have a very
different appearance (Figures 1, S1), revealing a highly facet
dependent stability under reducing conditions. While the isolated
Pt atoms are essentially preserved on the CeO2 octahedra, only
Pt nanoparticles (no isolated Pt atoms) are present on the CeO2
cubes after reduction (Figures 1d,g). Reduction of Pt2+ to Pt0 on
CeO2(100) is known to destabilize the Pt-CeO2 interactions, and
this is exacerbated by hydrogen spillover which results in CeO2
reduction and facile migration of Pt across the CeO2 (100)
surface.[3c, 4b] The CeO2 (100) surface is easier to reduce than the
CeO2 (111) surface facet,[6] which may account for the higher
stability of Pt on CeO2 (111). Additionally, strong electronic metal
support interactions (EMSI) have also been observed for small Pt
nanoparticles supported on CeO2 (111) surfaces.[7] The minimal
.
1
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Figure 1. (a) Scheme of ultra-low
loading catalysts on ceria octahedra
and cubes synthesized via atomic
layer deposition illustrating the
different behavior of Pt following
temperature programmed reduction
(TPR) to 350 °C before PHIP
experiments. The high angle annular
dark field scanning transmission
electron
microscopy
(HAADF-
STEM) images of the catalysts after
calcination reveal very well defined
CeO2 surface facets, the expected
(111) for the octahedra (b) and (100)
for the cubes (e). High-resolution
STEM images of fresh (c) Oct-
16ppm and (f) Cub-10ppm catalysts
reveal only isolated Pt ions after the
calcination
treatment.
After
temperature programmed reduction
(TPR) the Oct-16ppm catalyst (d)
still reveal visible isolated Pt. The
Cub-10ppm catalyst after TPR (g) in
contrast contains exclusively Pt
nanoparticles (as indicated by the
yellow arrow). Yellow circles in (c),
(d) and (f) indicate individual Pt
atoms and insets display the line
intensity profile along the line
between the two * symbols.
formation of small flat Pt clusters (Figure S3) on the octahedra
further supports the presence of strong interactions with the CeO2
(111) facet. Such EMSI have been shown to increase the pairwise
selectivity in hydrogenation reactions.[8]
hyperpolarized gas or liquid phase products.[11] Unfortunately,
these catalysts exhibit a low pairwise selectivity of parahydrogen
addition to unsaturated substrates. The low pairwise selectivity,
and thus low NMR signal enhancement, is due to a combination
of (i) the high mobility of hydrogen atoms after facile dissociation
on metal surfaces and (ii) the step-wise addition of hydrogen to
the substrate in the Horiuti−Polanyi mechanism leading to a loss
of the singlet spin order of parahydrogen.[10c] To test our
hypothesis that atomically dispersed Pt/CeO2 catalysts can
improve the pairwise selectivity, continuous-flow propene
hydrogenation reactions were conducted with either 50%
enriched parahydrogen (p-H2) or normal hydrogen (n-H2). The
PHIP NMR spectra (Figure 2), collected in ALTADENA mode,[8]
reveal that a reaction temperature of 200 °C is necessary to
observe significant activity and pairwise selectivity (Figures 3, S4-
S6). Under the reducing conditions used in this study, neither the
bare CeO2 octahedra nor the cubes contribute to the reactivity, as
shown in Figures 3, S7.[12]
PHIP[9] is a technique for generating nuclear-spin-hyperpolarized
fluids for sensitivity enhancement of NMR and MRI signals.
Compared to the dynamic nuclear polarization method,
parahydrogen based techniques use relatively uncomplicated
instrumentation with lower cost and shorter polarization times.[10]
A key advantage of heterogeneous hydrogenation over supported
metals is the facile separation of the solid catalysts from the
Table 1. Properties of Pt catalysts supported on CeO2 octahedra (Oct) and
cubes (Cub). Support surface facet and specific surface area, Pt loading, and
ALD deposition temperature of ultra-low loading catalysts.
Catalyst[a]
Surface
Facet
Surface
area [m2/g]
Pt loading
[ppm] (a)
ALD temperature
[°C]
As expected, the higher the Pt content of these catalysts, the
higher the propene conversion (more active sites), but despite the
ultra-low Pt content, the pairwise selectivity is low. The PHIP
signal enhancement over these catalysts may be limited by the
randomization of the parahydrogen singlet state accompanying
hydrogen spillover from Pt to CeO2,[13] rather than by the rapid
hydrogen diffusion observed on larger Pt particles.[14] In all cases,
the pairwise selectivity is higher at a reaction temperature of
Oct-8ppm
111
100
111
13.7±1.4
18.4±0.7
13.7±1.4
8.2
10
16
120
180
180
Cub-10ppm
Oct-16ppm
[a] 1 ppm equals 1 mg Pt metal over 1 kg CeO2, so that 10 ppm = 0.001 wt%.
ICP-MS on a duplicate Oct-8ppm sample confirms reproducibility (Table S1).
2
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300 °C compared with 200 °C, indicating that the rate of hydrogen
addition to propene increases faster than the spillover rate (or the
rate of surface hydrogen diffusion on the Pt metal particles on the
CeO2 cube support). The propene conversion over the Oct-8ppm
catalyst also increases, resulting in an intense PHIP signal (Figure
2). The pairwise selectivity of 6.1 % obtained over the Oct-8ppm
catalyst is significantly higher than the 0.3-0.4 % obtained over a
conventionally prepared Pt/TiO2 catalyst run under similar
conditions (Figure S8). Amongst monometallic heterogenous
catalysts, only supported Rh, a metal known to give higher
pairwise selectivities than Pt, has yielded a similar pairwise
selectivity in the hydrogenation of propene.[15]
A lower pairwise selectivity (2.7 %) is observed over the Cub-
10ppm catalyst which displayed Pt nanoparticle formation, but
this is still significantly higher than that obtained with the
conventional Pt/TiO2 catalyst under the same conditions, and it is
comparable to the best supported Pt catalyst in the literature.[14]
Furthermore, the reaction rates normalized by Pt content of the
catalysts in this study are several orders of magnitude higher than
those of the Pt/TiO2 catalyst (Figure 3c), which means that the Pt
is utilized very efficiently in these ultra-low loading catalysts. The
loss in activity and increase in selectivity observed on the Cub-
10ppm and Oct-16ppm between 200 and 300 °C are inconsistent
with sintering (Pt particle growth).[14] The observed behavior could
be explained by carbon deposition (coking) under the propene-
rich reaction conditions, as the carbon would block active sites
and hinder hydrogen diffusion. However, there was no indication
of significant coking during these experiments (Figure S9). A more
likely explanation for the decreased conversion and increased
pairwise selectivity at the higher temperature is the induction of
EMSI, as has been previously reported on other reducible
supports.[8] The higher activity of the Oct-8ppm catalyst at 300
versus 200 °C may suggest that the flat clusters, which we
observed in the TEM of the Oct-8ppm catalyst following the
reaction studies (Figure S10), are more active in the
hydrogenation reaction than the isolated Pt ions on the octahedra.
Figure 2. Thermally polarized (n-H2 top) and hyperpolarized (p-H2, bottom) 1
H
NMR spectra collected with 10mg of (a) Oct-8ppm, (b) Cub-10ppm, (c) Oct-
16ppm, (d) bare octahedra and (e) bare cubes at 300 °C. The flow rates were
365/30/105 mL/min N2/H2/propene. All spectra are shown at the same scale.
on the CeO2 octahedra (Figure 4). Unambiguous assignment of
the DRIFTS peaks is challenging, but the peak near 2105 cm-1 is
due to strongly bound CO on electron poor Pt,[2a, 16] and has
16]
previously been assigned to isolated Pt+ ions,[5b,
or PtOx
clusters.[17] This is consistent with a weak peak above 2100 cm-1
being the main feature in the spectra obtained from the fresh
(unreduced) catalyst (see Figure S11). CO DRIFTS peaks at 2075
cm-1 are typically assigned to nm-size Pt particles,[3b, 17a] but no
particles of this size are observed on the ceria octahedra. It is
possible that a small electron-deficient Pt cluster could result in
CO DRIFTS peaks at the same wavenumber. However,
calculations indicate that CO adsorbed on single atom Pt (i.e. Pt0)
To investigate the nature of the Pt-CeO2 interactions on these
catalysts, diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) was performed on the reduced Pt/CeO2
catalysts after CO adsorption at room temperature. Three distinct
features are observed in IR spectra obtained from Pt supported
on CeO2(111) surface facets would exhibit a peak at 2075 cm-
1 [17b]
.
Experimentally, isolated Pt atoms, i.e. Pt0, have not been
observed at this wavenumber for Pt/CeO2 catalysts, which could
be because they are difficult to generate and require CeO2
octahedra with exclusive (111) surface facets. Isolated Pt ions
Figure 3. Temperature dependence of (a) propene conversion and (b) pairwise selectivity and corresponding 휂푒푥푝 (signal enhancement) in the hydrogenation of
propene to propane over Pt supported on CeO2 shapes determined using PHIP NMR spectra. The flow rates were 365/30/105 mL/min N2/H2/propene. (c) Reaction
rate of propene hydrogenation at 300 °C per unit weight of Pt over the ultra-low loading Pt/CeO2 catalysts and a reference Pt/TiO2 catalyst with a 0.76% Pt loading
synthesized via precipitation. Reactant flow rates are the same as in (a) and (b).
3
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atomically dispersed Pt+ on a CeO2 support can yield a higher
pairwise selectivity. This is likely due to a combined size and
electronic effect, which together with the high turnover frequency,
indicate that atomically dispersed catalysts have outstanding
potential as atom-efficient catalysts for parahydrogen induced
polarization. Such catalysts afford cost-effective scaling-up of
continuous-flow hetPHIP, which is important for applications (e.g.
sensitivity-enhanced medical MRI) requiring large quantities of
hyperpolarized gases.
Acknowledgements
Funding for this work was provided by NSF CHE-1507230 and
CBET-1933723, and the National High Magnetic Field Laboratory,
which is supported by the NSF Cooperative Agreement DMR-
1644779 and the State of Florida. Funding from the University of
Florida is also gratefully acknowledged.
Figure 4. CO DRIFTS (diffuse reflectance infrared Fourier transform
spectroscopy) data obtained from (a) Oct-8ppm, Cub-10ppm and Oct-16ppm
and after reduction in H2 (5% in N2) at 350 °C and a CO exposure at 25 °C. Data
collected after desorption of CO in a flow of He at 25, 50, 75, and 100 °C are
shown for Oct-8ppm (b) and Cub-10ppm CO (c).
Keywords: Ceria nanoshapes • Platinum • Hyperpolarization
NMR spectroscopy • Atomic layer deposition • Ultra-low loading
(Pt2+) on conventional CeO2 supports bind very strongly at defects
or step edges (sites that are not present on the CeO2 octahedra),
and reduction of these ions therefore requires reduction
temperatures above 400 ºC.[3a, 18] At these temperatures the Pt
atoms are unstable and highly mobile on the support, which
results in agglomeration and formation of Pt nanoparticles. The
temperature programmed desorption profile (Figure 4b) reveals
no redshift in the IR peaks, and thus a lack of dipole-dipole
coupling between CO molecules on the octahedra-supported
catalysts, and this further supports the assignment of the most
intense IR peak to isolated atoms and it is also consistent with Pt
atoms being the majority species in the STEM data.[16] A small
shoulder due to a peak around 2050 cm-1 is also present, which
is likely due to CO adsorbed on trace amounts of Pt
nanoparticles.[19]
In contrast, CO adsorbed on the Cub-10ppm catalyst yields two
features at low wavenumber, likely due to metallic Pt particles of
different sizes,[20] or potentially Pt in different locations on the
CeO2 cubes. The 2050 cm-1 peak is consistent with previous
reported CO adsorbed on small Pt particles on ceria support.[21]
The more intense feature at 2025 cm-1 is due to electron-rich Pt
particles, likely as a result of weaker interactions with the CeO2
cubes. The absence of peaks at higher wave number (>2070 cm-
1) on the Cub-10ppm catalyst is consistent with the lack of visible
isolated Pt atoms on this support (Figure 1g).
The DRIFTS data illustrate the difference in electron density of Pt
on the two CeO2 surface facets. EMSI results in electron-poor Pt
species on the CeO2 (111) surfaces of the octahedra, which
stabilizes isolated Pt against severe sintering (only a few small Pt
clusters are formed). In contrast, reducing conditions destabilize
the Pt-CeO2 interactions on the CeO2 (100) surfaces of the cubes
and, despite the ultra-low loading, larger electron-rich Pt
nanoparticle are formed. The observation of higher PHIP signals
for the electron-poor catalyst supports our hypothesis that
4
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Entry for the Table of Contents
Stronger interactions are observed between Pt and the (111) surface facets of the CeO2 octahedra compared with the (100) surfaces
of the CeO2 cubes. These interactions result in more stable and electron-poor Pt species on the CeO2 octahedra, which translates to
higher pairwise selective addition of parahydrogen to propene and enhanced NMR signals compared with the electron-rich nanoparticle
Pt on the CeO2 cubes.
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