Platinum-modulated cobalt nanocatalysts for low-temperature aqueous-phase fischer-tropsch synthesis

Article abstract of DOI:10.1021/ja400771a

Fischer-Tropsch synthesis (FTS) is an important catalytic process for liquid fuel generation, which converts coal/shale gas/biomass-derived syngas (a mixture of CO and H₂) to oil. While FTS is thermodynamically favored at low temperature, it is desirable to develop a new catalytic system that could allow working at a relatively low reaction temperature. In this article, we present a one-step hydrogenation-reduction route for the synthesis of Pt-Co nanoparticles (NPs) which were found to be excellent catalysts for aqueous-phase FTS at 433 K. Coupling with atomic-resolution scanning transmission electron microscopy (STEM) and theoretical calculations, the outstanding activity is rationalized by the formation of Co overlayer structures on Pt NPs or Pt-Co alloy NPs. The improved energetics and kinetics from the change of the transition states imposed by the lattice mismatch between the two metals are concluded to be the key factors responsible for the dramatically improved FTS performance.

Full text of DOI:10.1021/ja400771a

Article
pubs.acs.org/JACS
Platinum-Modulated Cobalt Nanocatalysts for Low-Temperature
Aqueous-Phase Fischer−Tropsch Synthesis
Hang Wang,†,# Wu Zhou,§,∥,# Jin-Xun Liu,‡,# Rui Si,⊥ Geng Sun,† Meng-Qi Zhong,† Hai-Yan Su,‡
Hua-Bo Zhao,† Jose A. Rodriguez,⊥ Stephen J. Pennycook,∥ Juan-Carlos Idrobo,∥ Wei-Xue Li,*,‡
Yuan Kou,*,† and Ding Ma*,†
†College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China
‡State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academic of Sciences, Dalian, 116023, China
§Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, United States
∥Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
⊥Department of Chemistry, Brookhaven National Laboratory, Upton, New York11973, United States
S
* Supporting Information
ABSTRACT: Fischer−Tropsch synthesis (FTS) is an important catalytic
process for liquid fuel generation, which converts coal/shale gas/biomass-derived
syngas (a mixture of CO and H2) to oil. While FTS is thermodynamically favored
at low temperature, it is desirable to develop a new catalytic system that could
allow working at a relatively low reaction temperature. In this article, we present a
one-step hydrogenation−reduction route for the synthesis of Pt−Co nano-
particles (NPs) which were found to be excellent catalysts for aqueous-phase
FTS at 433 K. Coupling with atomic-resolution scanning transmission electron microscopy (STEM) and theoretical calculations,
the outstanding activity is rationalized by the formation of Co overlayer structures on Pt NPs or Pt−Co alloy NPs. The improved
energetics and kinetics from the change of the transition states imposed by the lattice mismatch between the two metals are
concluded to be the key factors responsible for the dramatically improved FTS performance.
easily separated from the reaction mixture after the reaction.
The reduction of the catalyst particle size, the high dispersion
of the NP catalysts, and maintaining the three-dimensional
freedom of the particles in water were believed to be the key
factors for the significant increase of the catalytic activity at
relatively lower temperatures.11−14 Very recently, Hensen et al.
reported that by controlling the reaction parameters, oxygenate
selectivity up to 70% can be realized over Ru NP catalyst in
aqueous-phase FTS.15 It was suggested that the reaction
temperature strongly affected the relative rates of different
termination mechanisms and thus led to unexpectedly high
oxygenate selectivity. While the low-temperature aqueous-
phase FTS is attractive and has potential for various industrial
applications, it is of great importance to find non-noble metal
alternatives for the Ru NP catalyst. We began with the
conventional FTS element, Fe; however, it was proven that Fe
is not stable in aqueous phase and only with the protection of a
solvent with strong reduction ability, such as polyethylene
glycol,16 can the low temperature FTS activity be observed on
Fe catalysts.
INTRODUCTION
■
Fischer−Tropsch synthesis (FTS), which converts fossil fuel-
based syngas to liquid fuel products over Ru, Co, or Fe
catalysts, was used by Germany and South Africa to circumvent
oil embargos during the last century. Since it is now possible to
use diverse resources, such as coal, biomass, and shale gas as the
source for the production of syngas, with the surging
consumption of fossil fuels, FTS is once again of essential
economic interest as a gas to liquid (GTL) process.1−8 During
the last 80 years, conventional supported and unsupported
catalysts have been widely investigated. Research has shown
that Ru, Fe, and Co catalysts working in the temperature range
of 473−623 K have the best performance.2−6 Among these
catalysts, less expensive Co-based catalysts represent the
optimal choice for industrial synthesis of long-chain hydro-
carbons due to the higher hydrocarbon productivity, good
stability, and commercial availability.5,6,9
As the FTS is highly exothermic and thermodynamically
favored at low temperature, it is obviously desirable to develop
a catalyst system that could facilitate working at low reaction
temperature while maintaining excellent catalytic performance.
We have recently demonstrated that poly(N-vinyl-2-pyrroli-
done) (PVP) stabilized Ru nanoparticles (NPs) in aqueous-
phase FTS10 show a 35-fold increase in activity over traditional
supported Ru catalyst at an operating temperature of 423 K and
a 16-fold increase at only 373 K. As the products of FTS,
hydrocarbons, are insoluble in the solvent, water, they can be
Co-based materials represent another important catalyst for
FTS.17−21 However, traditional Co-based catalysts usually
operate at a temperature >473 K in order to get acceptable
activity and selectivity, especially for those working under
Received: January 23, 2013
Published: February 21, 2013
© 2013 American Chemical Society
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liquid-phase or slurry-phase reaction conditions. Moreover, it is
rather challenging to realize FTS in liquid, especially in aqueous
phase. Indeed, there have been tremendous endeavors to realize
low-temperature FTS over Co-based catalysts in the past
decade. For example, Dupont et al.22 conducted FTS in an
ionic liquid [Bmim][NTf2] with Co NPs as catalysts, and they
observed an activity of 0.04 molCO·molCo−1·h−1 at 483 K. Yan et
al.23 dispersed Co NPs in squalane and obtained an activity of
1.3 molCO·molCo−1·h−1 at 473 K. Fan et al.16 prepared Co NPs
by the NaBH4 reduction method, but the obtained Co NPs
show very poor performance in aqueous-phase FTS. Until now,
however, there are no reports on the successful construction of
a Co-based FTS catalyst system operating at relatively low
reaction temperature. Therefore, it is important to improve the
activity of Co-based aqueous-phase FTS catalysts at lower
temperatures.
It is well documented that the addition of a small amount of
noble metals into the transition-metal catalysts may greatly
promote the catalytic activity of the latter. For example, the gas-
phase FTS performance of Co-based catalysts can be improved
greatly by the addition of small amount of Pt, Ru, or Re.24−27
The noble metals added were thought to facilitate the reduction
of cobalt oxide to the active metal state or help to stabilize the
metallic Co species through hydrogen spillover and hence
accelerate the FTS rates.5 Besides the reported reduction−
improvement effect, it is interesting to know whether those
bimetallic catalysts could activate the reactants/intermediates
more efficiently because of the altered electronic properties and
geometrical structures.28−34 For the bimetallic catalysts,
depending on the preparation and reaction conditions, the
two components may form distinct structural motifs, such as an
alloy30,34 or core−shell31,33−35 structure (both in metallic
states) or metal/metal oxide interfaces.36 How the two metal
components interact with each other may affect significantly the
reaction performance.
In this work, we report a new hydrogen-reduction route, a Pt
seed method, for the aqueous one-step synthesis of Pt−Co
bimetallic NP catalysts. The obtained bimetallic Pt−Co NP
catalysts have exceptional high aqueous-phase FTS activity at
low reaction temperature (433 K). The structure of the
bimetallic catalysts was characterized in detail by X-ray
absorption fine structure (XAFS) and aberration-corrected
scanning transmission electron microscopy (STEM). Various
structures, including monometallic (Co or Pt) particles, Co
particles with single Pt atoms on the surface, and Co layers
decorated on Pt or Pt−Co alloy particles, were observed. Based
on density functional theory (DFT) calculations, it is found
that the outstanding activity comes from the formation of
strained Co layers on Pt or Pt−Co alloy particles, which
improve the overall energetics and kinetics by forming favorable
transition states (TSs) due to the lattice expansion of the
supported Co layers.
Scheme 1. Illustration of One-Step Hydrogen Reduction of
Pt−Co Bimetallic NPs
cobalt acetylacetone at a mild temperature, resulting in the
simultaneous formation of the Pt−Co bimetallic NPs. As a control,
we synthesized Pt NPs in the autoclave by the hydrogen-reduction
method first, and then Co precursors (Co/Pt = 10:1) were added. The
autoclave was sealed with 2.0 MPa hydrogen, and the reduction/
nucleation process was conducted at 333 K for 4 h to obtain 10% Pt−
Co−S NPs (hydrogen-sequence-reduction method). As-prepared NPs
are magnetically separable (Figures S1 and S2). Pure Co NPs were
prepared by the NaBH4 reduction method.37 Typically, 0.500 g (2.0 ×
10−3 mol) Co(OAc)2·4H2O was dissolved in 20 mL water containing
2.2 g (2.0 × 10−2 mol) PVP. Then 0.4 g (1 × 10−2 mol) NaBH4 was
added quickly under vigorous stirring. The mixture turned from red to
black immediately, suggesting the formation of Co NPs. Subsequently,
the NPs were separated by a magnet and washed with nitrogen-
saturated water two times to get Co−NaBH4 NPs. The NPs were then
dispersed in 40 mL water containing 2.2 g (2.0 × 10−2 mol) PVP for
subsequent FTS reaction. 10% Pt−Co−NaBH4 was prepared by a
similar procedure except 0.084 g (2.0 × 10−4 mol) K2PtCl4 was added
together with Co(OAc)2·4H2O in the beginning of the synthesis.
Catalytic Performance Testing. In a typical experiment, the
freshly prepared catalyst (2.0 × 10−3 mol, in 40 mL water) was placed
in a 140 mL stainless steel autoclave. The reactor was purged three
times with N2 (99.99%) and was then sealed with 3.0 MPa syngas with
CO:H2:Ar = 32:64:4. FTS performance was measured at designated
temperatures under stirring (800 r/min) until the total pressure
decreased by about 1−1.5 MPa (corresponding to a CO conversion of
about 30−50%). Except for the cases of very slow reaction rates, the
reaction time required was in the range of 8−24 h. After reaction, the
autoclave was cooled down to room temperature, and the products
were collected and analyzed. After reaction, the autoclave was linked
with gas chromatography (GC) to give a full analysis of the gas
products. Porapark Q and 5A packed column with thermal
conductivity detector was used to analyze CO2, CO, Ar, H2, and
CH4. HP-AL/M column with flame ionization detector (FID) was
used to analyze hydrocarbons in the gas phase. Twenty mL
cyclohexane was injected into the autoclave through a 20 mL syringe
with 20 μL decalin (18.0 mg) as the internal standard. The products
were analyzed immediately by Agilent 6820 GC. HP-5 column with
FID was used to analyze products in the upper phase (mainly alkanes
and olefins, Figure S3). HP-INNOWAX column with FID was used to
analyze products in the aqueous phase (no obvious products
detected). The calculated carbon balance for a normal FTS analysis
is >85%. The products were also qualitatively analyzed by Agilent 7890
GC-5975 MS with HP-5 MS and HP-INNOWAX MS columns.
Characterization. Transmission electron microscopy (TEM)
measurements were carried out on a Tecnai F-30 electron microscope
operated at 300 kV. The NPs were diluted with deionized water
(∼0.01 mol·L−1) and dispersed by ultrasonication for ∼15 min. Then
one drop of solution was placed on a copper grid coated with a carbon
film. The average particle sizes and distributions of the NPs were
determined from more than 200 particles for each sample. STEM
imaging and spectroscopy analysis were performed on an aberration-
corrected Nion UltraSTEM-100 operating at 60 and 100 kV.38 The
convergence semiangle for the incident probe is 31 mrad. Annular
dark-field (ADF) images are collected from a half-angle range of ∼86−
200 mrad. Electron energy loss spectroscopy (EELS) was performed
using a Gatan Enfina spectrometer, with a collection semi-angle of 48
mrad. The ADF images presented in this manuscript are low-pass
filtered in order to reduce the random noise in the images. Co K and
EXPERIMENTAL SECTION
■
Preparation of the Catalysts. Scheme 1 shows the general
protocol for the fabrication of Pt−Co bimetallic NPs by one-step
hydrogen reduction. In a typical experiment, 0.586 g (2.0 × 10−3 mol)
Co(acac)2·2H2O and 0.084 g (2.0 × 10−4 mol) K2PtCl4 are dissolved
in 40 mL water containing 2.2 g (2.0 × 10−2 mol) PVP. The mixture
was placed in a stainless steel autoclave and treated with 2.0 MPa H2 at
333 K for 4 h. The obtained material is a black colloidal solution. The
as-prepared NPs are termed 10% Pt−Co NPs (analogous procedures
are used to form x% Pt−Co NPs). It was believed that the rapidly
formed Pt seeds catalyzed the hydrogenation of the carbonyl groups of
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a
Table 1. Catalytic Performances of Various Catalysts
selectivity (wt%)
b
entry
1
catalysts
CH4
CO2
C2−4
C5−12
C13−20
C21+
activity (molCO·molCo−1·h−1
)
blank
−
0
−
100
2
−
0
−
0
−
0
−
0
3
2
0
0
0
0
c
d
2
Pt NPs
0
3
4
5
6
7
5% Pt−Co NPs
10% Pt−Co NPs
10% Pt−Co−S NPs
Co−NaBH4 NPs
10% Pt−Co NaBH4 NPs
10
11
21
33
36
17
17
19
41
32
48
49
16
19
20
20
19
6
0.60
1.1
2
38
5
0.10
0.09
0.15
2
10
2
b
a
Reaction were conducted at 160 °C, CO/H2/Ar = 32/64/4 (initial pressure = 3.0 MPa). CO2 not included when calculating the activity.
c
d
Reduction of K2PtCl4 by H2 at 333 K. Activity of WGSR is 0.44 molCO·molPt−1·h−1.
Pt LIII edge X-ray absorption fine structure (XAFS) spectra were
(entry 2). Instead, the water−gas shift reaction (WGSR)
collected at beamline X19A of the National Synchrotron Light Source
at Brookhaven National Laboratory. The sample was mounted onto an
adhesive Kapton tape. The XAS spectra were taken repeatedly in the
“transmission mode”. The monochromator was detuned 35% for Co
and 20% for Pt in order to reduce the amount of higher harmonics in
the beam. The photon energy was calibrated for each scan with the
first inflection point of the Co K (7709 eV) and Pt LIII (11564 eV)
edge in Co and Pt metal foils, respectively. Extended XAFS (EXAFS)
data have been analyzed using the Athena and Artimis programs.39
Calculations. DFT spin-polarized calculations have been per-
formed using Vienna Ab Initio Simulation Package (VASP).40
Throughout the calculations, projector augmented wave (PAW)41
potentials and the generalized gradient approximation with the
Perdew−Burke−Ernzerhof (PBE) exchange−correlation functionals
were adopted.42 The planewave cutoff energy was 400 eV, and the
energy and force convergence were 1 × 10−4 eV and 0.02 eV/Å,
respectively. Monkhorst−Pack k-points sampling of 8 × 8 × 8 was
applied for calculating the crystal parameters of bulk Pt and Co with
face-centered cubic (fcc) crystal structures, and calculated lattice
constants 3.98 and 3.52 Å are used. For Pt3Co, the optimized lattice
constant is 3.89 Å.
occurred, which suggests that pure Pt NP catalysts are not
active for FTS but are good catalysts for the WGSR, as is well-
known for the case of pure Pt NPs.45 When using pure Co NP
catalysts prepared by the NaBH4 reduction method (Table 1,
entry 6), the formation of hydrocarbons was detected at 433 K
but with poor activity (0.09 molCO·molCo−1·h−1). At the same
time, high methane formation rate and relatively low selectivity
toward longer-chain hydrocarbons (C2−4, 41%; C5+, 21%) were
observed, which is far less than those in the high-temperature
gas-phase FTS process.17−19 However, when a small amount of
Pt (molar ratio of Pt:Co = 0.05) was introduced during the
one-step hydrogen-reduction preparation, a dramatic change of
the catalytic behavior was observed (Table 1, entry 3). The
activity increased to 0.6 molCO·molCo−1·h−1, around 1 order of
magnitude higher than that of the pure Co catalyst, even
comparable to Co catalysts working at higher reaction
temperatures.17 More importantly, the selectivity toward
unwanted products, methane and CO2, dropped dramatically
(methane: 10%; CO2: 2%), while those toward C2−5 and C5+
changed to 17% and 70%, respectively. This indicates that the
addition of Pt over the Co NPs could greatly improve the
reaction rate of the catalysts.
The (111) surface was simulated by a four-layer slab, whereas the
vicinal (311) surface observed by STEM was simulated by a four
equivalent (111) layer slabs. For 1 ML Co/Pt(111), 2 ML Co/
Pt(111), and 2 ML Co/Pt(311), the top-most surface Pt atoms were
replaced by 1 or 2 ML Co atoms. To construct 1 ML Co/Pt3Co(111),
the top-most surface layer of a five layer Pt3Co(111) slab was replaced
by Co atoms. For Co(311) and 2 ML Co/Pt(311) surfaces, four
bottom layers were fixed, whereas for the other surfaces the bottom
two layers were fixed. A p(3 × 3) unit cell was used for the (111)
surface, while p(2 × 2) unit cell was applied for the stepped (311)
surface. The single Co atom decorated Pt(111) surface, labeled as
Co1@Pt(111), was simulated by substituting one top-most Pt atom of
the Pt(111) surface by a single Co atom. Pt1@Co(111) and Pt1@
Co(311) systems were modeled similarly. We used Monkhorst−Pack
mesh k-points of 5 × 5 × 1 for Co(311) and 2 ML Co/Pt(311)
surfaces, 3 × 3 × 1 for 1 ML Co/Pt3Co(111), and 4 × 4 × 1 for the
other (111) surfaces. The vacuum region along the z direction was 15
Å at least, and dipole correction was adopted.
Upon increasing the Pt loading to 10% (entry 4), the activity
increased further, whereas the product distribution remained
almost unchanged with 70% selectivity toward C5+. The catalyst
was very stable. We conducted recycling experiments for five
times on 10% Pt−Co NPs catalyst (Figure 1). The activity
dropped only slightly in the second run. After that, it kept
Force reversed method43 was used to determine the TSs, and a
force tolerance of 0.03 eV/Å was utilized without the zero point
correction. We also searched the TSs of some of the minimum-energy
reaction pathways by using the climbing image nudged elastic band
method.44 For the initial and final states, we adopt the separate
adsorption of the species involved in the reactions.
RESULTS AND DISCUSSION
■
Catalytic Performance. The FTS was conducted in
aqueous phase at a relatively low temperature of 433 K. As
shown in Table 1, without using catalysts, no activity was
observed by GC over a 4 h run (Table 1, entry 1). No activity
toward hydrocarbon formation was observed over pure Pt NPs
Figure 1. FTS performance of 10% Pt−Co NPs recycling experiments
for five times.
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stable during the remaining tests with an activity of 0.85−0.95
molCO·molCo−1·h−1 (Table S1), which shows that Pt−Co NP
catalyst prepared with the one-step hydrogen reduction could
be reused. The chain length distribution of the reaction
products follows the Anderson−Schulz−Flory statistics, with
the growth factor (α) of hydrocarbon products of 0.8 (Figure
2). The product distributions using 10% Pt−Co NPs and Co−
relatively low reaction temperature may favor the production of
olefins by decreasing the capability of α-hydrogenation in the
chain termination step for the Pt−Co NPs catalyst.
It should be mentioned that these highly active Pt−Co
bimetallic catalysts were prepared through the one-step
hydrogen-reduction method, i.e., adding Pt and Co precursors
simultaneously into the synthetic mixture and then reducing
with H2 in a stainless steel autoclave (Scheme 1). The Pt and
Co precursors are reduced and nucleated simultaneously. When
we prepared the bimetallic sample by the NaBH4 reduction
method (Pt−Co−NaBH4 NPs) or by just changing the order of
the two metal precursors added in the hydrogen-reduction
synthesis process, catalysts with very poor performance were
always obtained. As shown in Table 1, entry 7, the 10% Pt−
Co−NaBH4 NP catalyst shows activity of 0.15
molCO·molCo−1·h−1, while the selectivities toward C2−5 and
C5+ are only 32% and 22%, respectively (Figure 3). For Pt−Co
catalysts prepared by the hydrogen sequence reduction method,
i.e., 10% Pt−Co−S NPs catalyst, the catalytic performance is
similar to that of the Co NP catalyst. It should be noticed that
the selectivity toward CO2 is 38%, suggesting that the water−
gas shift activity of this catalyst is emerging, even though not as
high as the pure Pt NP catalyst. These results suggest that
preparing the bimetallic catalyst by the one-step hydrogen-
reduction method is critical for obtaining good catalytic
performance for low-temperature aqueous-phase FTS. As the
structures of the catalysts with the same composition but
different preparation methods may be different, this indicates
that the structure of Pt−Co catalysts has a decisive influence on
their FTS performance.
Figure 2. Anderson−Schulz−Flory distribution of products for 10%
Pt−Co NP catalyst.
Structural Characterization. To identify the origin of the
higher activity of the bimetallic catalysts synthesized and the
active sites, a series of structural characterizations was carried
out. The bimetallic Pt−Co NPs as prepared were first studied
by TEM. Two types of particles with different bright-field
image contrast were observed in this sample, and their
corresponding TEM micrographs and size distributions are
shown in Figure 4. It can be found that both low- and high-
contrast NPs have a similar average size of 3.4 0.5 and 3.5
0.5 nm with relatively narrow size distribution, respectively.
EDX analysis indicates that the low-contrast NPs are composed
mainly of Co, whereas the high-contrast NPs are composed
mainly of Pt. Similar behavior was found for 5% Pt−Co NPs as
prepared (Figure S4). Both the low- and high-contrast NPs give
similar average sizes of 3.6 0.7 and 3.6 0.5 nm, although
the size distribution is slightly broader in the low-contrast case.
This indicates that the different amount of Pt added has little
influence on the average size of the catalysts synthesized. After
the FT reaction, both the average sizes of low- and high-
contrast NPs for 10% Pt−Co NPs (Figure S5) show only slight
increases, to 3.6 0.5 and 3.5 0.5 nm. This suggests that the
Pt−Co NPs synthesized are rather stable and do not suffer
pronounced sintering. For comparison, we show the TEM
image for 10% Pt−Co−S NPs in Figure 5. Compared to the
10% Pt−Co NPs, it can be found that the low-contrast particles
have a similar average size of 3.3 0.7 nm (versus 3.4 0.5
nm), whereas the high-contrast particles give a smaller average
size of 2.9 0.4 nm (versus 3.5 0.5 nm). Although a size-
dependent catalytic behavior has been reported on Co-based
FTS catalysts,19 the small size variation observed in the current
case cannot explain the huge difference in catalytic performance
of the 10% Pt−Co and 10% Pt−Co−S NPs catalysts.
Figure 3. FTS product selectivities for 10% Pt−Co, Co−NaBH4, and
10% Pt−Co−S NPs as catalysts.
NaBH4 NP catalysts in FTS are shown in Figure 3. For 10%
Pt−Co NPs, there were only 13% undesired C1 (CH4 and
CO2). However, for the Co−NaBH4 NP catalyst, the selectivity
toward undesired C1 products is as high as 38%. Moreover,
using the Pt−Co NP catalyst, the more useful olefins take up
49% of the whole C2−4 products. For the desired C5−12
products, it is clear from Figure 3 that the Pt−Co catalyst
has a good selectivity of 49% (with 47% olefins), while it is only
19% (with 46% olefins) over the pure Co NP catalyst. In
addition, the share of C13−20 products on Pt−Co is 19% while
only 2% for Co−NaBH4 NPs. Clearly, beside the possible role
of stabilizing the metallic Co species through hydrogen
spillover, the addition of Pt with one-step hydrogen-reduction
method has a notable effect on increasing the chain growth
probability and gives more desired products. Moreover, the
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Figure 6. Absorption spectra for different Pt−Co samples collected
near the Co K edge: (a) XANES and (b) EXAFS Fourier transforms.
coordination number of Co, N(Co−Co), as well as other structural
parameters (e.g., interatomic distance, Debye−Waller factor)
can be extracted from EXAFS refinement. The average first-
shell coordination number decreased with decreasing particle
size. The corresponding N(Co−Co) is 5.4 for 10% Pt−Co NPs
and 6.1 for 10% Pt−Co−S NPs. Although N(Co−Co) found here
is only half of that for Co foil (N(Co−Co) = 12), our results are in
agreement with the results reported by Cheon et al.,30 where
the extracted N(Co−Co) and Co−Co bond distance from Co K
edge EXAFS were reported to be 5.2 and 2.49 Å for 4 nm Co
particles but 7.1 and 2.49 Å for much larger 7 nm Co particles,
respectively. From the Co K edge EXAFS spectra (Figure 6b),
there are no pronounced characteristic peaks from Pt−Co
coordination found in either 10% Pt−Co or 10% Pt−Co−S
NPs. This is due to the fact that Co−Co scattering remains
dominant in both samples, which is understandable because
only a small amount of Pt (10%) was introduced. Interestingly,
at the same experimental condition, we observed a slight
smaller N(Co−Co) for 10% Pt−Co NPs (5.4 versus 6.1, 10% Pt−
Co−S NPs), although both the catalysts have similar average
size (3.4 versus 3.3 nm) for the low-contrast NPs (Co-rich
NPs) under TEM. This indicates that the decreased Co−Co
coordination for 10% Pt−Co NPs may be due to the Pt−Co
coordination, although this is not clearly visible by Co K edge
EXAFS experiments.
Figure 4. TEM micrographs and size distributions of the (a) low-
contrast particles, with size of 3.4
0.5 nm, and (b) high-contrast
particles, with size of 3.5 0.5 nm, in the 10% Pt−Co NP catalyst.
a
Table 2. EXAFS Data on the Co K Edge
sample
Co foil
shell
N
R (Å)
Δσ2 (Å2)
ΔE (eV)
Co−Co
Co−Co
Co−Pt
Co−Co
Co−Pt
12
5.4
−
2.49
2.50
−
−
0.007
−
−
0.5
10% Pt−Co
6.1
2.49
0.007
0.1
10% Pt−Co−S
−
−
−
a
Figure 5. TEM micrographs and size distributions of the (a) low-
contrast particles, with size of 3.3 0.7 nm, and (b) high-contrast
particles, with size of 2.9 0.4 nm, in the 10% Pt−Co−S NP catalyst.
N is the coordination number; Δσ2 is the change in the Debye−
Waller factor value relative to the reference factor; and ΔE is inner
potential correction to account for the difference in the inner potential
between the sample and the reference.
To get further microscopic structural information of the
active Pt−Co NPs, XAFS spectra for as prepared 10% Pt−Co
NPs were measured and are shown in Figure 6 (Co K edge).
For comparison, the results from 10% Pt−Co−S NPs are also
included. For both samples, the normalized X-ray absorption
near-edge structure (XANES) spectra (Figure 6a) show four
characteristic peaks (A−D), similar to those observed for Co
foil. The Fourier transforms of the EXAFS spectra (Figure 6b)
indicate a common dominant peak at ∼2.50 Å for both the 10%
Pt−Co NPs and 10% Pt−Co−S NPs, a typical bond distance
from Co−Co scattering as found in Co foil. The first-shell
To better see the Pt−Co and Pt−Pt coordination, XAFS
spectra on the Pt LIII edge were collected and are shown in
Figure 7. A pronounced difference in XANES (Figure 7a)
between the 10% Pt−Co and 10% Pt−Co−S NPs can be
found. For the former one, after the white lines (peak A), peak
B is weak, and peaks C and D shift to slightly higher energies
with respect to Pt foil, whereas for the latter one all peaks occur
at the similar position as that of Pt foil. A more clear difference
can be seen in the Fourier transforms of the EXAFS (Figure
7b). For 10% Pt−Co−S NPs, the dominant feature at around
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a
To get more insights into the structures of the bimetallic
NPs, aberration-corrected STEM was applied to characterize
the structure and composition of the Pt−Co NP catalysts (i.e.,
5% and 10% Pt−Co, Figure 8). Since Pt is much heavier, it can
Table 3. EXAFS Data on the Pt LIII Edge
sample
Pt foil
shell
N
R (Å)
Δσ2 (Å2)
ΔE (eV)
Pt−Pt
Pt−Co
Pt−Pt
Pt−Co
Pt−Pt
12
3.6
3.7
1.3
8.6
2.76
2.58
2.75
2.57
2.75
−
−
0.004
0.004
0.006
0.005
10% Pt−Co
7.3
10% Pt−Co−S
8.0
a
N is the coordination number; Δσ2 is the change in the Debye−
Waller factor value relative to the reference factor; and ΔE is inner
potential correction to account for the difference in the inner potential
between the sample and the reference.
Figure 7. Absorption spectra for different Pt−Co samples collected
near the Pt LIII edge: (a) XANES and (b) EXAFS Fourier transforms.
The fitting curves for first-shells Pt−Pt (red curve) and Pt−Co (blue
curve) are also presented.
2.74 Å from characteristic Pt−Pt scattering is clearly seen. The
two large peaks labeled as Pt−Pt are due to a single Pt−Pt path
contribution. This splitting is due to Ramsauer−Townsend
resonance at a single energy in the backscattering amplitude of
Pt.35,46 The corresponding coordination number N(Pt−Pt) is 8.6,
just slightly smaller than that of Pt foil (N = 12) due to the
small particle size. In addition, there is a small peak at 2.6 Å
from characteristic Pt−Co single scattering with a coordination
number of 1.3. For pure Pt particles at around 3 nm, the
corresponding N(Pt−Pt) was reported from 8 to 9 in the
literature.47 These results, together with Co K edge EXAFS,
suggest that most of the Pt and Co in 10% Pt−Co−S NPs are
well separated, possibly staying as discrete Pt and Co NPs.
There is less interaction/bonding between Pt and Co. The
relatively small coordination of Co (N(Pt−Co) = 1.3) may come
from highly dispersed Co atoms over Pt particles or Pt atoms
dispersed on Co particles. For 10% Pt−Co NPs, the intensity of
the peaks at 2.74 Å for Pt−Pt coordination decreases, while the
intensity of the peak at 2.6 Å for Pt−Co scattering increases
and becomes even larger than that of Pt−Pt peak. N(Pt−Co) and
N(Pt−Pt) for the 10% Pt−Co NPs are 3.7 and 3.6, respectively.
The dramatic decrease of Pt−Pt and increase of Pt−Co
coordination numbers indicates that Pt in 10% Pt−Co NPs has
intensive contact with Co, probably forming Co−Pt bimetallic
structures. It should be noted that for interfacially contacted
Pt−Co nanostructures, such as a Co−Pt core shell structure,
the Pt−Co coordination number at the Pt−Co interface was
found to be 2.6.30 The possibility for the formation of Pt−Co
core−shell-like structure may also exist in the 10% Pt−Co NPs
catalysts. Although XAFS only gives out statistical/overall
structural information, it is clear that the highly active catalyst
(10% Pt−Co NPs) has intensive Pt/Co interaction/coordina-
tion.
Figure 8. STEM ADF imaging of: (a−d) 5% Pt−Co and (e−h) 10%
Pt−Co catalysts. (a−g) ADF images. (h) Intensity line profile taken
along the two arrows in (g). Red and orange circles highlight
disordered and crystalline Co particles, respectively, decorated by Pt
single atoms (green) on the surface. Yellow circles highlight crystalline
Pt particles ∼2 nm in diameter. The surfaces of the Pt particles are
usually decorated by disordered Co layers with lower image contrast
(highlighted by the light-blue circles and identified by EELS), with
thickness of 1−3 atomic layers. The disordered Co layers on Pt
particles are more clearly shown in (d). Some Pt-containing particles
in the 10% Pt−Co sample are Pt−Co alloy particles, as shown by the
intensity line profile in (g), where abrupt contrast variation between
adjacent atomic columns can be observed, suggesting the presence of
different amounts of Co in different atomic columns.
be easily distinguished from Co by its higher intensity
compared with Co in atomic resolution high-angle annular
dark-field (HAADF) images.48 Besides monometal Pt and Co
NPs, Figure 8a−g clearly shows the presence of two additional
types of structures in the 5% and 10% Pt−Co samples. They
are (i) ordered or disordered Co particles (as identified by
EELS) decorated with surface single Pt atoms (highlighted in
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Figure 8a,b,e; denoted as Pt1@Co NPs) and (ii) Pt NPs (2−3
nm, shown in Figure 8c,d) or Pt−Co alloy NPs (4−6 nm,
shown in Figure 8f,g) with a few atomic layers (1−3 layers) of
Co decoration on the surface, which are termed Co-Layer@Pt
NPs or Co-Layer@Pt−Co alloy NPs. The formation of Pt−Co
alloy was confirmed by ADF image intensity analysis (Figure
8h) and chemical mapping with EELS (Figure S6). Most of the
surface of these Pt or Pt−Co alloy particles consists of Pt {111}
and {100} planes, with some small amount of high index
planes, like {311} (Figure S7). Importantly, the surface of the
Pt and Pt−Co alloy particles in the two active catalyst samples
is often observed with a fuzzy low-contrast layer in the ADF
images (Figure 8c,d,f), indicating the presence of a surface Co
decoration layer in the unoxidized particles. The presence of
Co in the surface of the NPs was further supported by EELS
mapping (Figure S6), where a skin layer of Co can be seen on
the Pt−Co particle surface. Due to the exposure to air during
the TEM sample preparation and handling, most of the Co
layers were oxidized, making it impossible to determine the
precise atomic structure of the Co layers on Pt or Pt−Co alloy
NPs in their pristine state. However, it should be noted that the
Co surface layers should form during the synthesis instead of
being induced by oxidation during ex-situ STEM experiment, as
previous ex-situ electron microscopy studies have already shown
that exposure to air would not change the Co and Pt
distribution in homogeneous Pt/Co alloy nanostructures.49−54
The HAADF-STEM imaging indicates that there is extensive
interaction/coordination between Pt and Co in the Pt−Co NP
sample, which is in good agreement with the XAFS
experiments, where the statistic N(Pt−Co) was calculated to be
3.6. Although atomic resolution STEM observation provides
more details about the bimetallic catalysts, the diversity of
structures has prevented us from making a direct conclusion of
the catalytically active structures. However, the possibility of
monometal Pt or Co NPs being the most active species can be
ruled out, since the pure Pt NP catalyst only has WGSR
activity, and pure Co NP catalysts alone have low FTS activity
at 433 K (Table 1, entries 2 and 6). More importantly, for the
Pt−Co catalysts with poor FTS activity, the Pt−Co−S NP
catalysts, STEM analysis found that it contains mostly pure Pt
and pure Co NPs (Figure S8) as well as some Pt1@Co NPs.
The pure Pt NPs in the 10% Pt−Co−S sample are highly
crystalline and are very similar to those in the pure Pt NP
sample (Figure S9). This is in good agreement with the XAFS
observation where Pt−Pt coordination dominated the Pt LIII
edge spectrum. The fact of the common presence of the Pt1@
Co NPs in the STEM images of Pt−Co NPs and Pt−Co−S
NPs samples with vastly different catalytic performance
excludes that Pt1@Co NPs cannot contribute to the different
activity. The only difference between the Pt−Co NPs and Pt−
Co−S NPs samples is that the former contains Co-layer@Pt
NPs and/or Co-layer@Pt−Co alloy NPs structures. Therefore,
it is reasonable to conclude that they are responsible for the
main FTS activity in the low-temperature aqueous-phase FTS.
DFT Calculations. To shed light on the origin of the higher
catalytic activity of the Pt−Co samples and to decisively
identify the active structures, we resort here to DFT
calculations. For simplicity, we consider here only CO
dissociation, which was suggested to be the rate-limiting step
for FTS.55 For reference, we first calculated CO direct
dissociation on pure fcc Co, since it is a stable structure at
small particle size.56 Both flat (111) and stepped (311) surfaces
observed in experiment were considered to see the structural
sensitivity. The calculated potential energy surface of CO direct
dissociation on Co(111) and Co(311) is plotted in Figure 9,
Figure 9. Potential energy surface (in eV) for CO dissociation with
respect to CO in gas phase on pristine Co surfaces and Pt supported
pseudomorphic epitaxial Co layers. (a) Flat (111) surfaces: Co(111)
(black), 1 ML Co/Pt(111) (red), 2 ML Co/Pt(111) (blue), and 1 ML
Co/Pt3Co(111) (green); and (b) stepped (311) surfaces: Co(311)
(black) and 2 ML Co/Pt(311) (blue). The energy reference is CO in
gas phase, and the calculated barriers are indicated.
Table 4. Calculated CO Adsorption Energy ECO and Barriers
Ea with Respect to CO in Gas Phase, Elementary Reaction
Energy ΔE from Adsorbed CO to Dissociated C and O, and
the Bond Length d (in Å) between C and O at the TS on
a
Various Surfaces Considered
surface
ECO
Ea
ΔE
d
Co(111)
Co(311)
−1.66
−1.71
−1.97
−1.79
−1.78
−2.04
0.74
0.75
−0.19
−0.06
−0.51
−0.42
0.05
1.86
2.16
1.76
1.67
1.92
1.77
−0.15
−0.43
−0.80
−0.44
−0.39
1 ML Co/Pt(111)
2 ML Co/Pt(111)
2 ML Co/Pt(311)
1 ML Co/Pt3Co(111)
a
The unit for energy is eV.
and details are given in Table 4. The barriers with respect to
CO in gas phase (the energy reference defined here is adopted
for subsequent discussions unless otherwise specified) are 0.74
and −0.15 eV for Co(111) and Co(311), respectively. The
higher activity of the stepped surface agrees well with previous
calculations.57−61 When one surface Co atom of Co(111) or
the step Co atom of Co(311) is replaced by a less reactive Pt
atom representative of the Co particles decorated with surface
single Pt atoms (Pt1@Co NPs), the calculated CO dissociation
barriers are at least 0.55 eV higher than those of pure Co
counterparts. Moreover, we found that CO dissociation barriers
on Pt(111) (1.89 eV) and Pt(311) (0.76 eV) are at least 0.91
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eV higher than those of pure Co counterparts too. Replacing
one surface Pt atom of Pt(111) with Co atom (Co1@Pt NPs),
the barrier remains 0.37 eV higher than that of pure Co(111).
These results (Table S2) exclude the contribution of the Pt1@
Co, Co1@Pt, and Pt NPs found by STEM to the higher activity
of Pt−Co NPs, a fact nicely in line with the experiments.
We then turn to CO dissociation on Co-Layer@Pt NPs.
Although the detailed structure cannot be obtained from
STEM, we note that ultrathin Co layers on Pt have been well
studied on crystalline surfaces by surface science in the past. It
was found that ultrathin Co films on Pt adopt the
pseudomorphic epitaxial structure,62,63 namely, Co layer
grown on Pt will adopt the in-plane lattice constant of Pt
substrate. We therefore constructed pseudomorphic epitaxial 1
ML Co/Pt(111), 2 ML Co/Pt(111), and 2 ML Co/Pt(311)
surfaces to model the Co-layer@Pt NPs. The calculated
potential energy surfaces are plotted in Figure 9 (see also
Table 4). For 1 ML Co/Pt(111), it was found that the CO
binding strength is 0.31 eV stronger than that of pure Co(111),
whereas the binding strength with the dissociated C and O is
enhanced by 0.60 and 0.52 eV (Table S3, S4 and S5),
respectively. This not only improves the CO adsorption but
also changes the reaction energy of CO dissociation from
endothermic 0.75 eV on Co(111) to exothermic −0.06 eV on 1
ML Co/Pt(111) (Figure 9a). More importantly, the corre-
sponding barrier becomes −0.43 eV, which is 1.17 eV lower
than that of the pristine Co(111) and even 0.28 eV lower than
that of the more active Co(311). For thicker Co layers on
Pt(111), i.e., 2 ML Co/Pt(111), the CO dissociation barrier
decreases further to −0.80 eV, and the reaction energy becomes
even more exothermic (−0.51 eV). The higher activity of a
supported Co layer was also found on a stepped 2 ML Co/
Pt(311) surface; compared to the pristine Co(311), the CO
dissociation barrier decreases by 0.29 eV, along with improved
reaction energy (−0.42 versus −0.19 eV). We also studied the
so-called hydrogen-assisted CO dissociation (CO + H → HCO
→ CH + O).59,64−66 It was found that the corresponding
barrier on 1 ML Co/Pt(111) (1.81 eV with respect to the
adsorbed CO and H) is 0.29 eV lower than that of Co(111)
(1.52 eV) (Figure S10), whereas on 2 ML Co/Pt(311) the
calculated barrier of 1.16 eV is 0.21 eV lower than that of 1.37
eV on Co(311) (Figure S11). These results show that
regardless of the surface structures (flat or stepped) and CO
dissociation paths (directed or H-assisted), the activity of CO
dissociation on epitaxial Co thin layers on Pt has a higher
activity than that of the Co catalysts, and this substantiates the
Co-Layer@Pt-NPs as an active structure responsible for the
higher activity of 5% Pt−Co NPs.
−0.39 eV, which is already 0.24 eV lower than that of the more
active stepped Co(311). This result also substantiates the Co-
layer@Pt−Co alloys NPs as an active structure for the higher
activity of 10% Pt−Co NPs found in experiment.
The calculations above show that the higher FTS activity for
both 5% and 10% Pt−Co samples obtained could be well
attributed to the formation of Co-layer structures on Pt NPs or
Pt−Co alloy NPs due to the improved energetics and decreased
barriers for CO activation. To reveal the origin of this behavior,
we note that one distinct structural variation of the supported
Co layers is the significant expansion of the in-plane lattice
constant, 13% on Pt (111) and 10% on Pt3Co(111),
respectively. The increase of activity from lattice expansion is
well documented in the literature.67 The reason was believed to
be an electronic effect, namely, the lattice expansion would
induce an upshift of the d-band center of the surface metal
atoms, which interacts more strongly with the species involved
and stabilizes the TS accordingly. In our case however,
compared to the surface metal atoms of Co(111), there are
only modest upshifts of the Co 3d center ε3d (<0.08 eV) found
for 1 ML Co/Pt(111) and 1 ML Co/Pt3Co(111). Looking
carefully at the spin-resolved projected density of states
(PDOS) of 1 ML Co/Pt(111) (Figure S13), it was found
that Co ε3d for spin-down states shifts upward by 0.28 eV due
to the lattice expansion, whereas Co ε3d for spin-up states shifts
downward by 0.09 eV due to the hybridization with Pt
underneath. Integrating the spin-resolved PDOS to the Fermi
level found that the spin-down states contain less valence
electron than the spin-up states (2.51 versus 4.61 e, and a
corresponding surface Co magnetic moment of 2.1 bohr).
These lead to an overall small upshift of Co ε3d of surface Co
atoms on Pt. This upshift is too small to account for the change
of the energetics and barriers calculated.
The origin of the higher activity of the supported Co layer
actually comes from a structural effect, specifically, the
formation of a more favorable TS. To approach the TS on
Co(111) (Figure 10a), it can be found that C−O bond has to
be stretched to 1.86 Å from 1.19 Å at the initial state, and the O
atom then sits at the two-fold bridge sites. Whereas for the TS
on 1 ML Co/Pt(111) (Figure 10c), the C−O bond is less
stretched (1.76 Å) with a lower energy cost for C−O bond
In order to see the influence of Pt−Co alloy, specifically as
Co-Layer@Pt−Co alloy NPs observed in 10% Pt−Co NPs, on
the activity, we studied the CO dissociation on Pt−Co alloys.
For simplification, the Pt−Co alloy was approximated by
ordered Pt3Co(111). On pristine Pt3Co(111), the top-most
surface is composed of three-quarters Pt and one-quarter Co
atoms surrounding the Pt atoms. The calculated barrier for CO
dissociation on this surface is 1.40 eV (Table S2), 0.66 eV
higher than that on Co(111). This surface therefore cannot
contribute to the higher activity observed for Pt−Co alloy NPs.
This is understandable since there is only one active Co atom
involved to break the C−O bond. To study the activity of Co-
Layer@Pt−Co alloy NPs observed, similar to the above, we
constructed a pseudomorphic epitaxial 1 ML Co on Pt3Co-
(111). The calculated CO dissociation barrier on this surface is
Figure 10. Optimized TSs for CO direct dissociation on: (a) Co(111),
(b) Co(311), (c) 1 ML Co/Pt(111), and (d) 2 ML Co/Pt(311). Blue,
orange, gray, and red spheres represent Co, Pt, C, and O atoms,
respectively. The C−O bond length (Å) at the TS is indicated.
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CONCLUSIONS
■
In conclusion, Pt-modulated Co NPs synthesized using a one-
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effective and efficient catalysts for aqueous-phase FTS. The
reaction can be operated at 433 K, which is a lower operational
temperature than what can be achieved with conventional
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ASSOCIATED CONTENT
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S
* Supporting Information
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AUTHOR INFORMATION
Corresponding Author
dma@pku.edu.cn; yuankou@pku.edu.cn; wxli@dicp.ac.cn
Author Contributions
These authors contributed equally.
Notes
The authors declare no competing financial interest.
■
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ACKNOWLEDGMENTS
■
This work received financial support from the Natural Science
Foundation of China (21222306, 21173009, 20923001,
21225315, 21103164) and 973 Project (2011CB201402,
2013CB933100, 2013CB834603). This research was also
supported in part by the U.S. National Science Foundation
through grant no. DMR-0938330 (W.Z.); a Wigner Fellowship
through the Laboratory Directed Research and Development
Program of Oak Ridge National Laboratory, managed by UT-
Battelle, LLC, for the U.S. Department of Energy (W.Z.); Oak
Ridge National Laboratory’s ShaRE User Facility Program (J.-
C.I.), which is sponsored by the Office of Basic Energy
Sciences, U.S. Department of Energy; and the Office of Basic
Energy Sciences, Materials Sciences and Engineering Division,
U.S. Department of Energy (S.J.P.). This research carried out in
part at the National Synchrotron Light Source at Brookhaven
National Laboratory, which is supported by the U.S. Depart-
ment of Energy, Office of Basic Energy Sciences, under contract
DE-AC02-98CH10886. Calculations were carried out at
National Supercomputing Center in Tianjin, China.
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