Nanoceria-Supported Single-Atom Platinum Catalysts for Direct Methane Conversion

Article abstract of DOI:10.1021/acscatal.8b00004

Nanoceria-supported atomic Pt catalysts (denoted as Pt₁@CeO₂) have been synthesized and demonstrated with advanced catalytic performance for the nonoxidative, direct conversion of methane. These catalysts were synthesized by calcination of Pt-impregnated porous ceria nanoparticles at high temperature (ca. 1000 °C), with the atomic dispersion of Pt characterized by combining aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses. The Pt₁@CeO₂ catalysts exhibited much superior catalytic performance to its nanoparticulated counterpart, achieving 14.4% of methane conversion at 975 °C and 74.6% selectivity toward C₂ products (ethane, ethylene, and acetylene). Comparative studies of the Pt₁@CeO₂ catalysts with different loadings as well as the nanoparticulated counterpart reveal the single-atom Pt to be the active sites for selective conversion of methane into C₂ hydrocarbons.

Full text of DOI:10.1021/acscatal.8b00004

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Nanoceria Supported Single-Atom Platinum
Catalysts for Direct Methane Conversion
Pengfei Xie, Tiancheng Pu, Anmin Nie, Sooyeon Hwang, Stephen
C. Purdy, Wenjian Yu, Dong Su, Jeff T. Miller, and Chao Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00004 • Publication Date (Web): 03 Apr 2018
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Nanoceria Supported Single‐Atom Platinum Catalysts for Direct Me‐
thane Conversion
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Pengfei Xie,† Tiancheng Pu,† Anmin Nie,‡ Sooyeon Hwang,# Stephen C. Purdy,ǁ Wenjian Yu,†
Dong Su,# Jeffrey T. Miller,ǁ Chao Wang†,*
†Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States
‡Shanghai University Materials Genome Institute and Shanghai Materials Genome Institute, Shanghai University, Shanghai
200444, China
ǁSchool of Chemical Engineering, Purdue University, West Lafayette, Indianapolis 47907, United States
#Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
Keywords: single atom catalysts, direct methane conversion, platinum, ceria nanocrystals, carbon coking
Supporting Information Placeholder
for many reactions, including CO oxidation,13‐14 water‐gas
ABSTRACT: Nanoceria‐supported atomic Pt catalysts (de‐
shift,15‐16 methane steam reforming,17 selective hydrogenation
noted as Pt1@CeO2) have been synthesized and demonstrat‐
ed with advanced catalytic performance for the non‐
of alkynes and dienes18‐19 and so on. The superior catalytic
performance are usually attributed to the atomic dispersion
of metal atoms with low coordination number, quantum
confinement and/or strong metal‐support (mostly metal
oxides) interactions.20‐22 It has also been reported that atom‐
ic Fe sites embedded in a silica matrix give rise to high cata‐
lytic selectivity for the non‐oxidative conversion of methane
to ethylene, aromatics and hydrogen; the absence of metal
ensembles suppresses C‐C coupling and carbon coking, giv‐
ing rise to long‐term stability under the high‐temperature
reaction conditions.10 Despite the progress, it remains elusive
in many cases whether the single‐atom sites offer distinct
catalytic mechanisms from their ensembled counterparts
with continuous surfaces.21‐22 One main obstacle is the lack of
robust control over the dispersion of metals during the syn‐
thesis, and sub‐nanometer clusters or nanoscale metal parti‐
cles usually co‐exist with single metal atoms in the compo‐
site materials. The other challenge is the stability of the SACs
under reaction conditions, especially considering the poten‐
tial atomic aggregation and agglomeration at high tempera‐
tures.
oxidative, direct conversion of methane. These catalysts were
synthesized by calcination of Pt‐impregnated porous ceria
nanoparticles at high temperature (ca. 1,000 oC), with the
atomic dispersion of Pt characterized by combining aberra‐
tion‐corrected high‐angle annular dark‐field scanning
transmission electron microscopy (HAADF‐STEM), X‐ray
photoelectron spectroscopy (XPS), X‐ray absorption spec‐
troscopy (XAS) and diffuse reflectance infrared Fourier trans‐
form spectroscopy (DRIFTS) analyses. The Pt1@CeO2 cata‐
lysts exhibited much superior catalytic performance to its
nanoparticulated counterpart, achieving 14.4% of methane
o
conversion at 975 C and 74.6% selectivity toward C2 prod‐
ucts (ethane, ethylene and acetylene). Comparative studies of
the Pt1@CeO2 catalysts with different loadings as well as the
nanoparticulated counterpart reveal the single‐atom Pt to be
the active sites for selective conversion of methane into C2
hydrocarbons.
Here we report the synthesis of ceria (CeO2)‐supported
atomic Pt catalysts for direct conversion of methane into
light hydrocarbons. Pt has been widely used to active the C‐
H bond in hydrocarbons,23‐25 but carbon coking usually takes
place on the conventional catalysts composed of Pt clusters
Introduction
Over the recent years, natural gas has risen as a clean and
cost‐effective source of hydrocarbons, with great potential
for replacing coal and crude oil in many sectors of energy
and chemical industries.1‐2 The conventional approaches for
methane conversion via syngas (a mixture of CO and H2) is
however challenged by the low carbon efficiency, large loss of
exergy and high capital cost associated with the complex,
multistage processes.3‐5 Alternatively, direct conversion of
methane can be achieved via oxidative coupling6‐7 or non‐
oxidative dehydrogenation8‐10 to produce olefins or aromat‐
ics. These approaches are believed to be more economical
and environment‐friendly than via the syngas.11‐12
o
or nanoparticles at high temperatures (e.g., >800 C), which
has limited the application of Pt‐based catalysts for methane
conversion26‐27. In this study, nanoceria‐supported atomic Pt
catalysts were synthesized by calcination of Pt‐impregnated
porous CeO2 nanoparticles (Figure S1) at ca. 1,000 oC (see the
Supporting Information for experimental details). The ob‐
tained Pt1@CeO2 catalysts were characterized by using
HAADF‐STEM and XPS, and the absence of Pt ensembles
was further confirmed by DRIFTS analysis using CO as a
molecular probe. The Pt1@CeO2 catalysts of various loadings
Single atom catalysts (SACs) represent a new frontier of
heterogeneous catalysis and have been extensively studied
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(with 0.5 – 1.0 wt% of Pt) were then evaluated for the me‐
Pt0 was estimated to be ~1.5 in the PtNPs/CeO2 catalyst, with
the oxidized Pt likely coming from surface oxidation of the Pt
nanoparticles during calcination (see the Supporting Infor‐
mation for the details of synthesis).
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thane conversion reaction, and the catalytic performance was
further compared to their nanoparticulated counterpart to
reveal the single‐atom active sites.
The XPS spectra collected at the Ce 3d edge are shown in
Figures 2 b and d. The spectra can be deconvoluted on the
basis of two multiplets that correspond to the 3d3/2 and 3d5/2
core holes of Ce (denoted as u and v, respectively) and have a
spin‐orbit splitting of ~18.6 eV:30‐31 u0 (898 eV) and v0 (880
eV) for Ce(3d94f1)‐O(2p6), u (901 eV) and v (882 eV) for
Ce(3d94f2)‐O(2p4), uI (904 eV) and vI (885 eV) for Ce(3d94f2)‐
O(2p5), uII (906 eV) and vII (889 eV) for Ce(3d94f1)‐O(2p5)
and uIII (916 eV) and vIII (897 eV) for Ce(3d94f0)‐O(2p6). The
states marked with u0/v0 and uI/vI are features of Ce3+, which
was estimated to occupy ~46% and 33% of the Ce species in
the 0.5% Pt1@CeO2 and PtNPs/CeO2 catalysts, respectively
(Table S1). These results indicate that the CeO2 nanoslabs
employed as support here are rich in Ce3+ defects and oxygen
vacancies, which is likely a result of oxygen evolution during
the high‐temperature (1000 oC) treatment.
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Results and Discussion
Figure 1a shows the representative transmission electron
microscopy (TEM) image of the Pt1@CeO2 catalyst with ~0.5
wt% of Pt. They exhibit a slab‐like morphology with the size
varying from ~15 to ~40 nm, which were likely transformed
from the porous nanospheres. High‐resolution HAADF‐
STEM images reveal that Pt is dispersed on the CeO2
nanoslabs at the atomic scale (Figures 1 b‐d). In these images,
individual Pt atoms are exhibited as bright dots with higher
contrast than the surrounding CeO2 lattice (Figures 1 e, f).
The slab‐like nanocrystals exhibit lattice fringes with the
spacing measured to be ca. 0.31 nm, which can be assigned to
the (111) planes of CeO2 in the fluorite phase (Figure 1b). Fur‐
ther synthesis shows that the ratio of Pt can be tuned from
ca. 0.25 – 1.0 wt%, although Pt clusters start to appear in the
1.0% Pt1@CeO2 (Figure S2). For comparison, 3 nm Pt nano‐
particles were also synthesized and deposited on similar
CeO2 nanoslabs (with 0.5 wt% of Pt, denoted as PtNPs/CeO2)
(Figures S3 and S4). X‐ray diffraction (XRD) patterns collect‐
ed for the Pt1@CeO2 catalyst only show the CeO2 peaks in the
fluorite (Fm3തm) phase (Figure S5), where the absence of Pt‐
phase peaks is consistent with the atomic dispersion of Pt as
observed in the STEM images. The single‐atom dispersion of
Pt in the catalysts was further confirmed by CO chemisorp‐
tion (Table S2) and extended X‐ray absorption fine structure
(EXAFS) analyses (Figure S8a). In particular, the EXAFS spec‐
trum for 0.5% Pt1@CeO2 only exhibits one pronounced peak
associated with the first‐shell Pt‐O bond (~2.03 Å), where the
absence of Pt‐Pt and higher‐shell Pt‐O‐Pt scattering demon‐
strates atomic dispersion of Pt in the catalyst.
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Figure 2. XPS spectra collected on the (a, b) 0.5% Pt1@CeO2
and (c, d) PtNPs/CeO2 catalysts at the (a, c) Pt 4f and (b, d)
Ce 3d edges.
The XPS analysis shows that, in the Pt1@CeO2 catalysts, Pt
was dispersed on the CeO2 support in the oxidized form
(Pt2+). It was reported that Pt can be emitted as volatile PtOx
above 800 oC in air.14 The porous nanospheres (Figure S1)
employed as precursor in the present study could have
helped trap the PtOx vapor, while Ce(III) and oxygen vacan‐
cies enriched on the formed CeO2 nanoslabs represent coor‐
dinatively unsaturated, electrophilic sites and could have
attracted and stabilized the atomic platinum oxides, e.g., in
the form of planar Pt2+O4 clusters32. Thereby Pt was favorably
dispersed as single‐atom species in the derived Pt1@CeO2
catalysts.
To gain a more extensive evaluation of the atomic disper‐
sion of Pt, we have further performed diffuse reflectance in‐
frared Fourier transform spectroscopy (DRIFTS) analysis of
CO adsorption on the Pt1@CeO2 catalysts. This method has
previously been demonstrated to be effective in identifica‐
tion of single Pt atoms on oxide supports.13 Figure 3 com‐
pares the absorption spectra recorded on the Pt1@CeO2 cata‐
lysts with various loadings of Pt (0.25, 0.5 and 1.0 wt%) and
PtNPs/CeO2 (0.5 wt%), where CO was pre‐adsorbed at differ‐
ent partial pressures. Only one peak was observed at 2,089
Figure 1. Representative (a) TEM and (b‐d) high‐resolution
HAADF‐STEM images of the Pt1@CeO2 catalyst with 0.5 wt%
of Pt. (e, f) Intensity profiles of the scans along the dash lines
marked in (d).
Oxidation state of Pt in the catalysts was characterized by
using XPS (Figure 2). The spectrum collected for the 0.5%
Pt1@CeO2 catalyst shows two peaks at the Pt 4f edge with
binding energies of 73.7 eV and 76.9 eV, which are assigned
to the 4f7/2 and 4f5/2 states of Pt2+, respectively (Figure 2a).28‐29
For PtNPs/CeO2, the Pt 4f doublet exhibits downshift by ~1
eV in binding energy (Figure 2c). Deconvolution analysis
reveals the presence of two additional peaks at 72.2 and 75.7
eV, in addition to the aforementioned two peaks associated
with Pt2+, which can be assigned to the same spin‐orbital
split of metallic Pt (Pt0). The atomic ratio between Pt2+ and
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cm‐1 for the Pt1@CeO2 catalysts with 0.25% and 0.5% of Pt
Figure 4c provides the comparison of methane conversion
and product selectivity for the three Pt1@CeO2 catalysts with
different Pt loadings as well as the PtNPs@CeO2 catalysts at
975 oC (see the calculated turnover frequencies (TOFs) in
Table S2). It is noted that even in the blank reaction tube,
CH4 had a conversion of 1.1% at this temperature due to the
non‐catalytic, thermal activation and dehydrogenation, but
no hydrocarbons were detected at significant amounts (albeit
with some ethane at 3.2% selectivity), suggesting that the
converted methane mostly became coke (Figure S10) and
deposited on the tube wall. The bare CeO2 support exhibited
somewhat higher (6.9%) CH4 conversion, but coke was still
the dominant (88.3%) product. The PtNPs@CeO2 catalyst
had a CH4 conversion of 9.7%, with 8.3% and 6.3% selectivi‐
ties toward ethylene and acetylene, respectively, whereas the
majority (79.8%) of carbon also ended up in coke deposition.
All of the three atomic Pt catalysts performed higher activi‐
ties than their nanoparticulated counterpart, achieving >12%
conversion of methane and >95% of total selectivity toward
hydrocarbons. Among the three Pt1@CeO2 catalysts, the 0.5%
one demonstrates the highest conversion (14.4%) of methane
and selectivity (74.3%) toward C2 products, where ethylene
(33.2%) and acetylene (35.1%) represent the major products.
The slightly lower methane conversion and hydrocarbon
selectivity from 1.0% Pt1@CeO2 can be ascribed to the pres‐
ence of a small amount of Pt clusters in this catalyst, as can
be seen from the STEM images (Figure S2) and DRIFTS anal‐
ysis (Figure 3c).
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(Figures 3 a, b), which can be assigned to the linearly bonded
CO (COL) on Ptδ+.13 An additional peaks at 1,991 cm‐1 appears
in 1.0% Pt1@CeO2 (Figure 3c), as well as PtNPs/CeO2 (Figure
3d), which can be ascribed to the bridge bonded CO (COB)
on Pt, a typical feature of Pt ensembles with continual sur‐
faces.13, 33 PtNPs/CeO2 exhibits another peak at 2078 cm‐1 in
addition to the two peaks observed on 1.0% Pt1@CeO2, which
could be assigned to COL on the Pt nanoparticles with, for
example, metallic Pt sites.34 The absence of the COB peak
thereby confirms the isolation of Pt sites in the Pt1@CeO2
catalysts at relatively low Pt ratios (e.g., <1%), whereas Pt
clusters have formed in the case with higher loadings.
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After demonstrating the atomic dispersion of Pt, the
Pt1@CeO2 catalysts were further evaluated for nonoxidative
conversion of methane at 900 – 1000 oC with a space velocity
of 6 L/(gcat·h). Figure 4a summarizes the temperature‐
dependent methane conversion and product distribution for
the 0.5% Pt1@CeO2 catalyst. The methane conversion in‐
o
creased with temperature and reached 23.1% at 1,000 C. The
selectivity of C2 hydrocarbons exhibited gradual decrease
from 98.4% at 900 oC to 66.7% at 1,000 oC. The amount of C3
product was rather small and always <10% throughout the
investigated temperature range. At temperatures ≥950 oC,
aromatic products started to appear and the selectivities in‐
creased with temperature, achieving 26.6% for benzene and
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2.1% for naphthalene at 1,000 C. Breakdown of the C2 prod‐
uct distributions is further elucidated in Figure 4b. At rela‐
tive low temperatures, ethylene and ethane were the two
dominant products, with the selectivity measured to be 51.1%
o
and 43.6% at 900 C, respectively. At elevated temperatures,
acetylene became more abundant and its selectivity achieved
41.7% at 1,000 oC, whereas only 19.8% of ethylene and 5.1% of
ethane were left at this temperature. It is noticed that the
amount of hydrogen generated from the methane conversion
matches well with the concentrations calculated from the
reaction stoichiometries and mass balance by taking the var‐
ious hydrocarbon products into account (Figure S9).
Figure 4. Catalytic performance for the nonoxidative CH4
conversion evaluated at 6 L/(gcat·h). (a) Catalytic activities
and selectivities of the 0.5% Pt1@CeO2 catalyst as functions of
the reaction temperature. Black squares represent CH4 con‐
version, and the colored histograms for product distribu‐
tions. Here the light hydrocarbons are categorized as C2
(ethane, ethylene and acetylene) and C3 (propane, propylene
and propyne) hydrocarbons, with further breakdown of the
C2 products shown in (b). (c) Comparison of methane con‐
version and product distributions at 975 oC over the different
catalysts and the controls. (d) Stability test of the 0.5%
Pt1@CeO2 catalyst performed at 975 oC.
Figure 3. DRIFTS of CO chemisorption at different CO par‐
tial pressures on Pt1@CeO2 with various weight percentages
of Pt: (a) 0.25%, (b) 0.5% and (c) 1.0%. (d) PtNPs/CeO2 (0.5
wt%) was also shown for comparison.
The above observations clearly point to the single‐atom Pt
as active sites, which are not only able to activate the C‐H
bond by dehydrogenation, but also direct the C‐C coupling
toward favorable formation of C2 hydrocarbons. In the cases
of Pt nanoparticles and clusters, although the continuous
metal surface is also activate for dehydrogenation, it lacks
The atomic Pt catalyst is much superior to its nanopar‐
ticulated counterpart for the methane conversion reaction.
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control over the extent of C‐C coupling and cannot suppress
AUTHOR INFORMATION
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the coking, which is consistent with the reported behavior of
conventional catalysts with ensembles of metal atoms.26‐27, 35
Besides suppression of carbon coking, the performance of the
Pt1@CeO2 catalysts is noticeably different from the previously
reported atomic Fe@SiO2 catalysts, albeit with similar me‐
thane conversions (e.g., 12.7% for Pt1@CeO2 versus ~8% for
Corresponding Author
*Email: chaowang@jhu.edu
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
for submission.
o
Fe@SiO2 at 950 C).10 The single‐atom Pt catalysts reported
here give rise to much higher C2 product selectivity than the
Fe@SiO2, e.g., ~84% versus ~47% at 950 oC. In the latter case,
the rest products are mainly aromatics (consistently at ~50%
independent of the reaction temperature) and nearly equally
distributed between benzene and naphthalene. While the
Pt1@CeO2 catalysts produce all the three kinds of C2 species,
ethylene is the only C2 product obtained with the Fe@SiO2
catalyst. These differences suggest that the Pt1@CeO2 cata‐
lysts may possess distinct catalytic mechanisms, particularly
in the C‐C coupling steps, from the Fe@SiO2 catalyst, where
multi‐carbon species were believed to form from gas‐phase
methyl (∙CH3) radicals via noncatalytic thermochemical pro‐
cesses.10
We have performed thermodynamic calculations for the
equilibria of the involved conversion reactions toward differ‐
ent products (see the Supporting Information, section 3). The
ratio of C2 species in the product was found to be much
higher than at equilibrium (Table S4). DRIFTS analysis of the
Pt1@CeO2 catalyst after methane activation at 900 oC re‐
vealed the presence of π‐bonded ethylene and acetylene,
suggesting that the single Pt sites may be capable of stabiliz‐
ing C2 adsorbates (Figure S11). We thus propose that the me‐
thane conversion on the Pt1@CeO2 catalysts involves adsorb‐
ing C2 intermediates, possibly formed from catalytic coupling
of two dehydrogenated C1 adsorbates (such as *CH3 and
*CH2) on the single Pt sites (other than adjacent Pt sites as
on the surface of Pt ensembles). As further coupling to form
C3 adsorbates may be inhibited on the atomically dispersed
Pt, these C2 intermediates must desorb and thus favor the
production of C2 hydrocarbons. The minor C3 and aromatic
products obtained at higher temperatures can be ascribed to
the thermochemical dehydrogenation and oligomerization of
the C2 molecules in gas phase.
As discussed above, the primary role of the CeO2 support
in the Pt1@CeO2 catalysts is the stabilization of the single‐
atom Pt species, which could also be true under the reaction
conditions.32, 36 However, synergetic effects may also be pre‐
sent between CeO2 and Pt on the activation of CH4. Active
sites have been shown to be present at the interface between
Pd clusters and CeO2 with low methane activation barriers.37
Although the situation may be different in the single‐atom
catalysts reported here, it is noticed that the onset tempera‐
ture of methane activation on Pt1@CeO2, i.e., ≤900 oC, is low‐
er than that (≤950 oC) previously reported for single‐atom
Fe@SiO2.10 This is indicative of the active role of CeO2 in the
methane conversion process other than only being an inert
support.
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The authors declare no competing financial interests.
ASSOCIATED CONTENT
Supporting Information
More experimental details, characterizations and catalytic results.
This material is available free of charge via the Internet at
http://pubs.acs.org.
ACKNOWLEDGMENT
This work was supported by the IDEAS Program of the
Advanced Research Projects Agency – Energy (ARPA-E),
Department of Energy (DOE) and the Johns Hopkins
Catalyst Award. Partial funding for SCP and JTM was
provided as part of the National Science Foundation’s
Energy Resarch Center for Strategic Transformation of
Alkane Resources (CISTAR) under Cooperative
Agreement No. EEC-1647722. Electron microscopy work
was performed at the Center for Functional Nanomaterials,
Brookhaven National Laboratory, which is supported by
the U.S. Department of Energy (DOE), Office of Basic
Energy Science, under contract DE-SC0012704. Use of the
Advanced Photon Source was supported by the U.S.
Department of Energy, Office of Basic Energy Sciences,
under contract No. DE-AC02-06CH11357. MRCAT
operations are supported by the Department of Energy and
the MRCAT member institutions.
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