Platinum-tin oxide core-shell catalysts for efficient electro-oxidation of ethanol

Article abstract of DOI:10.1021/ja505456w

Platinum-tin (Pt/Sn) binary nanoparticles are active electrocatalysts for the ethanol oxidation reaction (EOR), but inactive for splitting the C-C bond of ethanol to CO₂. Here we studied detailed structure properties of Pt/Sn catalysts for the EOR, especially CO₂ generation in situ using a CO₂ microelectrode. We found that composition and crystalline structure of the tin element played important roles in the CO₂ generation: non-alloyed Pt₄₆-(SnO₂)₅₄ core-shell particles demonstrated a strong capability for C-C bond breaking of ethanol than pure Pt and intermetallic Pt/Sn, showing 4.1 times higher CO ₂ peak partial pressure generated from EOR than commercial Pt/C.

Full text of DOI:10.1021/ja505456w

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Communication
Platinum–Tin Oxide Core–Shell Catalysts
for Efficient Electro-Oxidation of Ethanol
Wenxin Du, Guangxing Yang, Emily Wong, N. Aaron Deskins, Anatoly I Frenkel, Dong Su, and Xiaowei Teng
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 17 Jul 2014
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Platinum–Tin Oxide Core–Shell Catalysts for Efficient Electro-
Oxidation of Ethanol
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Wenxin Du,† Guangxing Yang, † Emily Wong, † N. Aaron Deskins,‡ Anatoly I. Frenkel,£ Dong Su,δ
Xiaowei Teng†*
†Department of Chemical Engineering, University of New Hampshire, Durham, New Hampshire 03824, United States
‡Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, United States
£Department of Physics, Yeshiva University, New York, New York 10016, United States
δ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
Supporting Information Placeholder
ciation of ethanol primarily happen at Pt sites, while Sn or SnOx
provide oxophilic species (OHads) to remove COads and CHx,ads
ABSTRACT: Platinum–tin (Pt/Sn) binary nanoparticles are
active electrocatalysts for the ethanol oxidation reaction (EOR),
but inactive for splitting the C–C bond of ethanol to CO2. Here we
studied detailed structure–properties of Pt/Sn catalysts for the
EOR, especially CO2 generation in situ using a CO2 microelecꢀ
trode. We found that composition and crystalline structure of tin
element played important roles in the CO2 generation: non–
alloyed Pt46–(SnO2)54 core–shell particles demonstrated strong
capability for CꢀC bond breaking of ethanol than pure Pt and inꢀ
termetallic Pt/Sn, showing 4.1 times higher CO2 peak partial presꢀ
sure generated from EOR than commercial Pt/C.
.
The concentration and distribution of Sn needs to be optimized so
that the resulting Pt/Sn catalysts have strong capability for ethanol
dissociation (from Pt) and CO/CHx removal (from Sn) concurrentꢀ
ly.3, 13ꢀ17 Second, whether Pt–SnO2 (nonalloys) or PtSn (alloys)
would be the best catalyst for EOR kinetics and/or CO2 generation
is still controversial: Antolini, Silva and Jiang reported that Pt–
SnO2 (non–alloys) showed faster EOR kinetics than PtSn (alꢀ
loys),3, 15, 18, 19 while De Souza, Zhu and Godoi reported the oppoꢀ
site results.14, 20ꢀ22
To fundamentally understand the EOR, especially CO2 generaꢀ
tion via C–C splitting, increasing attention has been paid to specꢀ
troscopic techniques, e.g., in–situ Fourier transform infrared specꢀ
troscopy (FTIR) and on–line differential electrochemical mass
spectrometry (DEMS).2, 4, 12, 13, 17, 23ꢀ27 The former measurement is
usually conducted on a thin–layer catalyst surface in stagnant
fluid, while the latter uses a flow cell. The experimental results
from both techniques were not identical, adding further controverꢀ
sy. For example, FTIR measurements showed that Pt3Sn catalysts
favored the C–C bond splitting in EOR,2, 28, 29 in contrast DEMS
Ethanol is one of the most hopeful fuels renewable energy apꢀ
plications due to its low toxicity, high availability from biomass
production, and high energy density due to the twelve–electron
charge transfer upon complete oxidation.1 During the ethanol
oxidation reaction (EOR), strongly adsorbed intermediates such as
CO and CHX poison the catalyst (e.g., Pt) surface and slow reacꢀ
tion kinetics considerably. Moreover, complete oxidation of ethaꢀ
nol into CO2 via C–C bond cleavage is mechanistically difficult.
Most of the charge generated is from partial oxidation of ethanol
to acetaldehyde and/or acetic acid, which only involve two– or
four–electron transfer.2–7 One common method to improve the
performance of Pt for EOR is to use binary catalysts by adding an
oxophilic metal, such as Sn.8, 9 Oxophilic metals can facilitate the
formation of adsorbed OH species (OHads) via dissociative adꢀ
sorption of water, which in turn helps the removal of adsorbed
intermediates (COads, CHX,ads) on adjacent Pt sites (bifunctional
effect). Sn also weakens the Pt–COads bond by altering d–band
properties of Pt orbitals (the ligand effect). Although Pt/Sn binary
catalysts show improved EOR kinetics, its selectivity for complete
oxidation of ethanol to CO2 is inferior to Pt. Addition of noble
metals such as Rh or Ir to Pt has also been studied. The resulting
ternary PtRhSn, PtRh–SnO2 and PtIr–SnO2 catalysts showed imꢀ
proved reaction kinetics and C–C splitting ability.5, 10ꢀ12 Howevꢀ
er, the scarcity and/or expense of Ir and Rh metals, even comꢀ
pared with Pt, impede their practical applications for fuel cell
technology. Therefore, improving C–C splitting of ethanol on
the surface of Pt/Sn catalysts becomes imperative for ethanol fuel
cell technology. Many studies investigated Pt/Sn binary catalysts
for the EOR; however, factors that may control CO2 generation on
Pt/Sn surface are still not well understood. First, the influence of
Sn or SnO2 on C–C splitting is ambiguous. Adsorption and dissoꢀ
data showed that CO2 formation on Pt3Sn was similar to that on
13, 17, 30
Pt.3,
Ion–selective potentiometric electrodes have been
used in aqueous solutions for decades in blood gas analysis and
31
the measurement of CO2 during fermentation.24,
The subject
solution and buffer within the electrode are separated by a Teflon
membrane which is permeable to dissolved CO2 gas in reaction
solution. The partial pressure of CO2 gas in the reaction solution
can be analyzed via the measurement of the pH of a thin layer of a
NaHCO3/NaCl buffer solution within the electrode in equilibrium
with CO2. McGuire reported that a CO2 microelectrode can enaꢀ
ble rapid and continuous measurements of CO2 concentration in
xylem sap of trees.32 However, implementation of a CO2 microeꢀ
lectrode to measure the CO2 generation in–situ during electroꢀ
chemical reactions has not been reported until the current report.
In this work, Pt/Sn nanocrystals were synthesized via a “surfacꢀ
tant–free” Polyol process described by the authors elsewhere.33
Pt/Sn nanocrystals with an average diameter of less than 5 nm
were harvested as shown from the transmission electron microsꢀ
copy (TEM) images in Figure 1 (Figure S1 and Table S1). Two
types of Pt/Sn nanoparticles were synthesized with atomic ratios
of Pt to Sn as 70/30 and 46/54, respectively (see EDS spectra in
Figure S2). The X–ray Diffraction (XRD) pattern shown in Figꢀ
ure S3 indicates mixed phases of pure face centered cubic Pt and
tetragonal SnO2 (JCPDS 00–041–1445). To further confirm the
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structure of the as–made Pt/Sn nanocrystals, high–angle annular
lysts. Slightly decreased white line intensity of both intermetallic
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dark–field scanning TEM (HAADF–STEM) images (Figure 1c)
and electron energyꢀloss spectroscopy (EELS) line scans (Figure
1d) were generated from an individual Pt–SnO2 nanoparticle.
Intensity profiles of Sn and Pt from EELS show that Sn appears to
have higher signal intensity at the edges of the particle, in contrast
to Pt showing a “volcano” shape of intensity profile. Both the
XRD and EELS line scan strongly suggest that as–made Pt/Sn
particles were non–alloyed (Pt−SnO2) with a SnO2 shell–rich and
Pt core–rich phase–segregated core–shell structure.
PtSn catalysts compared to Pt foil indicated a lower–lying d–band
center relative to the Fermi level, pointing to a more filled d–
band. This modified d–band feature may result from electron
back–donation from Sn upon alloy formation, which has been
previously observed in various Sn–containing noble metal alꢀ
loys.33–35 The Sn K–edge XANES features in intermetallic PtSn
catalysts are located between those of a Sn foil and non–alloyed
Pt–SnO2/C, indicating a mixture of metallic and oxidized phases.
EXAFS analyses are shown in Figure S7 and fitting results of the
intermetallic alloy are given in Table S2. The Pt atoms showed
strong coordination with Sn in Pt46Sn54 (NPt–Sn = 2.2 ± 0.3) and
Pt70Sn30 (NPt–Sn = 1.8 ± 0.3), and the Sn showed strong Sn–Pt
coordination and noticeable Sn–O coordination in Pt46Sn54 (NSn–Pt
= 5.3 ± 0.9, NSn–O = 3.3 ± 0.8) and Pt70Sn30 (NSn–Pt = 9.5 ± 2.1,
NSn–O = 2.9 ± 1.3). Fitting results indicates metallic Sn (in a form
of Pt–Sn intermetallic alloy) and trace oxidized Sn (in a form of
SnO2) bi–phase in both intermetallic catalysts.
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Figure 1. (a) Bright–field TEM, (b) dark–field STEM and (c)
HAADF images of the carbon–supported Pt–SnO2 catalyst. (d)
EELS line scan across Pt46–(SnO2)54 particle as indicated by the
arrow in (c).
Figure S4 shows the XANES spectra of Sn K–edge of the non–
alloyed Pt–SnO2 catalysts. Pt46–(SnO2)54 and Pt70–(SnO2)30 have
almost identical spectra that differ strongly from bulk Sn and
correspond to the predominantly oxidized state of Sn. This is in
good agreement with the results derived from XRD measureꢀ
ments, where the segregated Pt and SnO2 bi–phases coexisted
within Pt–SnO2 non–alloyed samples. EXAFS analysis was also
conducted as shown in Figure S5. The best fit values of coordinaꢀ
tion numbers and bond distances are summarized in Table S2.
EXAFS results show that Pt existed mainly in the metallic phase
in both non–alloyed Pt–SnO2 samples, as evident in the relatively
high Pt–Pt coordination compared to the Pt–O obtained in both
samples. Table S2 also shows that Sn is predominantly present in
the form of SnO2 in both Pt–SnO2 catalysts, since strong Sn–O
coordination number can be observed (NSn–O = 6.7 ± 1.1 in Pt46–
(SnO2)54 and NSn–O = 6.0 ± 0.5 in Pt70–(SnO2)30). Strong Sn–O
coordination and nonꢀdetectable Sn–Sn and Pt–Sn coordinations
corroborate well the XRD and STEM–EELS analyses concluding
the as–made Pt–SnO2 catalysts were comprised of partially oxiꢀ
dized (due to the detectable PtꢀO contribution, Table S2) Pt nanoꢀ
particles with phase–segregated SnO2 clusters on the surface.
Figure 2. (a) Representative CVs (forward scans) and (b)
ECASA–averaged CV current densities of carbon supported Pt/Sn
and Pt (ETEK) catalysts at 0.35V in 0.5 M H2SO4 and 0.5 M ethꢀ
anol solution. (c) Representative CAs and (d) ECASA–averaged
CA current densities Pt/Sn and Pt/C catalysts at oneꢀhour reaction
at 0.35 V in 0.5 M H2SO4 and 0.5 M ethanol solution.
The electrochemically active surface areas (ECASAs) of non–
alloyed and alloyed Pt/Sn catalysts, commercial Pt/C (ETEK), and
a mixture of commercial Pt/C with SnO2 nanoparticles are shown
in Figure S8, measured using cyclic voltammograms (CVs) in 0.5
M H2SO4. The EOR electroactivities of all Pt/Sn and Pt/C cataꢀ
lysts were measured by CVs through potential sweepings from –
0.1~0.5 V vs. Ag/AgCl in a mixture of 0.5 M H2SO4 and 0.5 M
ethanol, and current densities were normalized by the ECASAs.
Figure 2 shows EOR electroactivities for Pt/Sn and Pt catalysts in
both CV and chronoamperometric (CA) analysis both at 0.35 V.
Table S1 and Figure 2b show that the current densities from CV
measurements follow the order as: Pt70Sn30 > Pt46Sn54 > Pt70–
(SnO2)30 > Pt46–(SnO2)54 > Pt/C (ETEK) ≈ Pt/C + SnO2. Specifiꢀ
cally, the Pt70Sn30/C intermetallic catalyst showed about 21.5
times higher current density at 0.35V compared to commercial
Pt/C. Meanwhile, the ECASA–specific current densities from
one–hour CA measurements follow the same order, and the
Pt70Sn30/C intermetallic showed best long–term stability.
As–made Pt–SnO2 particles were converted into intermetallic
compounds after being treated in an Ar/H2 flow at 450°C. Figure
S1 and S3 show their TEM images and XRD spectra. XRD
shows that Pt70Sn30/C has a pure cubic Pt3Sn intermetallic phase
(JCPDS 00–035–1360), while the Pt46Sn54/C catalysts show a
mixture of intermetallic cubic Pt3Sn (JCPDS 00–035–1360) and
hexagonal Pt1Sn1 alloyed phases (JCPDS 00–025–0614), respecꢀ
tively. No SnO2 reflection was observed from both samples. Figꢀ
ures S6 show the XANES spectra of intermetallic PtSn/C cataꢀ
Figure S9 shows the schematic of the electrochemical cell for
in–situ CO2 measurement, consisting of a CO2 microelectrode, Pt
foils (working and counter electrode) and Ag/AgCl reference
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microelectrode. Pure argon and nitrogen gases mixed with 0.5, 1
Previous theoretical work indicated over Pt (111) that C–C bond
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and 5% CO2 were used to calibrate the CO2 microelectrodes in an
electrolyte solution at 25 °C (Figure 3a). The generation of CO2
on Pt/Sn and commercial Pt/C catalysts were evaluated in–situ
using an electrochemical cell applied with a constant potential of
0.35 V. Figure 3b shows the plot of the partial pressure of CO2
(PCO2), normalized by ECASA values, as a function of reaction
time. The results are also summarized in Table S1. Pt46–
(SnO2)54/C demonstrated the best CO2 generation ability comꢀ
pared to the rest of Pt/Sn and Pt catalysts. In particular, maximum
CO2 generation found in Pt46–(SnO2)54/C were 2.0 and 4.1 times
higher than those found in its intermetallic counterpart and Pt/C
breaking of ethanol has one of the lowest activation barriers
through a CHCO intermediate.36 Other pathways may be possible
for C–C breaking, but this species was chosen as a representative
intermediate to assess ethanol oxidation ability. For each surface
several adsorption sites were considered. For instance, adsorption
at the Pt/SnO2 surface was modeled over SnO2, over the (111)
facets of the Pt, and in the Pt/SnO2 interfacial region. Only the
most stable adsorption sites are reported herein and were considꢀ
ered for evaluating the reaction energy for C–C scission. Figure 4
shows the DFT results for CHCO decomposition to CO and CH
over the various surfaces of interest. We calculated reaction enerꢀ
gies (energies of product states minus reactant states) for CHCO
decomposition to be –1.04 eV over Pt (111), –0.19 eV over
Pt/SnO2, and 0.36 eV over PtSn. These results indicate that breakꢀ
ing C–C bond is the easiest over Pt, followed by Pt/SnO2, and
then PtSn. These simple calculations indicate that Pt/SnO2 is betꢀ
ter able to break C–C bond compared to PtSn, in agreement with
experiment that shows greater CO2 production for Pt/SnO2 cataꢀ
lysts. Thus, we expect PtSn to behave much different than
Pt/SnO2 as an ethanol oxidation catalyst. While Pt performs best
for C–C bond breaking, a full reaction path analysis is needed to
explain its overall activity, which is experimentally inferior.
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(ETEK).
At steady state (after oneꢀhour reaction), Pt46–
(SnO2)54/C still showed at least 1.5 and 2.5 times higher PCO2
compared to Pt46Sn54/C and Pt/C, respectively. In contrast to Pt46–
(SnO2)54/C and Pt46Sn54 catalysts, from which the coreꢀshell partiꢀ
cles outperform the intermetallic particles in CO2 generation, the
Pt70Sn30 generates more CO2 than Pt70–(SnO2)30/C during the
EOR. The results strongly indicated that CO2 generation abilities
of various Pt/Sn catalysts were more dependent on the composiꢀ
tion of Sn than crystalline structure (alloys or non–alloy).
Figure 3. (a) CO2 microelectrode responses in various CO2 gas
environments (CO2 saturated solution) as a function of time. (b)
ECASA–specific PCO2 of Pt/Sn and Pt catalysts as a function of
reaction time at applied constant potential of 0.35V vs. Ag/AgCl.
Figure S10 shows the longꢀterm stability of Pt46–(SnO2)54 and
Pt (ETEK) at lower potentials (0.1 V and 0.2 V). Pt46–(SnO2)54
shows a steady current after oneꢀhour reaction at 0.2 V, while Pt
remains negative current during the reaction due to the dominant
cathodic current from hydrogen absorption. The data indicate that
Pt46–(SnO2)54 not only has a good stability at lower potentials, but
also has a lower onset potential for the EOR compared to Pt.
Figure S11 shows CO2 generations of Pt46–(SnO2)54 and Pt
(ETEK) after one hour CA measurements conducted at 0.1 V and
0.2 V. Combined with Figure 3b, it clearly shows: (i) both cataꢀ
lysts yield more CO2 at higher potentials; (ii) Pt46–(SnO2)54 reꢀ
mains a plausible ability to generate CO2 at lower potentials,
showing 9.6, 5.3 and 2.5 times higher PCO2 compared to Pt at 0.1,
0.2 V and 0.35 V, respectively. The good stability and high CO2
generation found at lower potentials clearly indicate that Pt46–
(SnO2)54 is a better catalyst than Pt for ethanol fuel cells, which
usually operate under equilibrium condition.
Figure 4. Preferred intermediate states for C–C bond breaking of
ꢁ
ꢀ
ꢂ
adsorbed CHCO over Pt (111), alloyed PtSn 1120 and Pt/SnO2
interface. Reaction energies for CHCO formation are indicated.
Blue spheres represent Pt, red spheres O, dark grey spheres C,
white spheres H, and light grey spheres Sn.
We here discuss two possibilities to explain the physical origin
of promotional effect of Pt46–(SnO2)54/C core–shell structures on
C–C splitting observed in this study. First, concentration of SnO2
is imperative for C–C splitting. In the primary path of EOR (black
arrows in Figure S13), weakly bound AAL forms on Pt surfaces at
lower potentials. It can readily desorb from the surface of cataꢀ
lysts or it can be oxidized further to form AA as primary products
in the EOR, which are very inert in electrocatalysis. Meanwhile,
AAL can possibly be further reacted to form C1 intermediates
(CO, CHx) via C–C bond cleavage, and eventually oxidized to
CO2 (CH3CH2OH ꢀ CH3CHO ꢀ CHx + CO ꢀCO2).37 Thereꢀ
fore, OHads, formed via dissociative adsorption of water at Sn
sites, is needed in order to oxidize COads and CHX,ads at adjacent
sites to promote CO2 generation. On the other hand, complete
oxidation of ethanol to form CO2 is more facile on Pt site. Our
data suggest strongly that Pt46ꢀ(SnO2)54 is the best catalyst for CO2
generation (See Figure S14 a), so that COads and CHX,ads can be
effectively removed, and at the same time, the ethanol dissociaꢀ
tion is not compromised.
Acetic acid (AA) and acetaldehyde (AAL) have been identified
as major products during EOR. To analyze potential interferences
from AA and AAL on the CO2 measurements, AA and AAL with
concentrations of 0.0018 mol/L (see Supporting Information for
details) were used for control experiments. As shown in Figure
S12, the measured PCO2 was attributed from dissolved CO2 only
and the interference from AA and AAL generated during the EOR
to the PCO2 was negligible.
We also modeled the decomposition of CHCO over Pt (111),
ꢁ
ꢀ
ꢂ
PtSn 1120 and a Pt/SnO2 interface using density functional
theory (DFT). Details on the calculations are found in the Supꢀ
porting Information. C–C bond breaking can occur through a
variety of intermediates that form after C–H/C–O bond breaking.
Second, size or coverage of SnO2 clusters may play an imꢀ
portant role in CO2 generation. It was found that 2 nm SnO2 naꢀ
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noparticles on Pt electrode surfaces showed much higher enꢀ
REFERENCES
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hancement of methanol oxidation reaction (MOR) current densiꢀ
ties compared to Pt decorated with larger SnO2 nanoparticles.38
DFT calculations suggest that the binding energy of OHads on the
surface of SnO2 nanoparticles depended on the sizes of SnO2.
Small–sized SnO2 nanoparticles on Pt surface would have a
weaker OH–Sn interaction, and favor the release of OHads from
the SnO2 surface for effectively oxidizing COads. We found that
pure Pt/C and the mixture of Pt/C and SnO2 nanoparticles (comꢀ
mercial product) had very similar EOR kinetics and C–C splitting
ability, both inferior to Pt–SnO2 and PtSn as shown in Figures 2
and 3 (labeled as Pt/C + SnO2), suggesting that close proximity of
Pt and SnO2 is critical to enhance the CO2 generation. To further
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various PtꢀSnO2 catalysts using ECASAs (See detailed description
in supporting information). Figure S14 (b, c) shows the CO2 genꢀ
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age and SnO2 coverage/Pt coverage on surface. We found that
SnO2 coverage/Pt coverage on surface of Pt46ꢀ(SnO2)54 is around
2.4, accounting for 71% SnO2 on surface.
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The results from our work, including timeꢀresolved CO2 generꢀ
ation using a CO2 microelectrode, the influences of structures
(coreꢀshell and alloy) and compositions on the CO2 generation,
and DFT modeling of CꢀC splitting on various Pt/Sn structures,
have demonstrated that, for the first time, Pt46–(SnO2)54 core–shell
nanoparticles can effectively generate CO2 via splitting C–C bond
of ethanol. Our data show that Pt46ꢀ(SnO2)54 electrocatalysts have
9.6, 5.3 and 2.5 times higher amount of CO2 generation compared
to Pt (ETEK) after one hour reaction at 0.1, 0.2, and 0.35 V, reꢀ
spectively. We notice that sample environments for in situ CO2
determination are very different in CO2 microelectrode, FTIR and
DEMS techniques. Comparison of CO2 generation between these
three techniques will be challenging, but will be the focus of our
future work. Nevertheless, our results unambiguous demonstrate
the potential of CO2 microelectrode as a low–cost, easyꢀtoꢀuse,
high–performance in–situ tool for the CO2 detection in electroꢀ
chemical processes, compared to other optical– and mass spec–
electrochemical techniques. Our results will have a significant
impact on direct ethanol fuel cell technology by replacing pure Pt
with a lowꢀcost, more efficient PtꢀSnO2 coreꢀshell electrocatalyst
with high selectivity and activity for the EOR.
22. Silva, J.C.M., Parreira, L.S., De Souza, R.F.B., Calegaro, M.L.,
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Detailed description of the materials syntheses and characterizaꢀ
tions, EDS, XRD, XANES and EXAFS spectra, and design of in
situ cell for CO2 measurements are included. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Xiaowei Teng, xw.teng@unh.edu
ACKNOWLEDGMENT
This material is based upon work supported by the National Sciꢀ
ence Foundation (CBET–1159662 and CHE–115277) and the US
Department of Energy (DE–AC02–98CH10886). Use of the
NSLS was supported by the U.S. Department of Energy (DE–
AC02–98CH10886). Beam lines X19A/X18B are partly supportꢀ
ed by Synchrotron Catalysis Consortium through the U.S. Deꢀ
partment of Energy grant (DE–FG02–05ER15688).
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