Atomically thin Pt shells on Au nanoparticle cores: Facile synthesis and efficient synergetic catalysis

Article abstract of DOI:10.1039/c5ta08922k

We present a facile synthesis protocol for atomically thin platinum (Pt) shells on top of gold (Au) nanoparticles (NPs) (Au@PtNPs) in one pot under mild conditions. The Au@PtNPs exhibited remarkable stability (> 2 years) at room temperature. The synthesis, bimetallic nanostructures and catalytic properties were thoroughly characterized by ultraviolet-visible light spectrophotometry, transmission electron microscopy, nanoparticle tracking analysis and electrochemistry. The 8 ± 2 nm Au@PtNPs contained 24 ± 1 mol% Pt and 76 ± 1 mol% Au corresponding to an atomically thin Pt shell. Electrochemical data clearly show that the active surface is dominated by Pt with a specific surface area above 45 m² per gram of Pt. Interactions with the Au core increase the activity of the Pt shell by up to 55% and improve catalytic selectivity compared to pure Pt. The Au@Pt NPs show exciting catalytic activity in electrooxidation of sustainable fuels (i.e. formic acid, methanol and ethanol), and selective hydrogenation of benzene derivatives. Especially high activity was achieved for formic acid oxidation, 549 mA (mg_Pt)^-1 (at 0.6 V vs. SCE), which is 3.5 fold higher than a commercial < 5 nm PtNP catalyst. Excellent activity for the direct production of γ-valerolactone, an alternative biofuel/fuel additive, from levulinic acid and methyl levulinate was finally demonstrated.

Full text of DOI:10.1039/c5ta08922k

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10.1039/C5TA08922K.
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DOI: 10.1039/C5TA08922K
Journal Name
ARTICLE
Atomically Thin Pt Shells on Au Nanoparticle Cores: Facile
Synthesis and Efficient Synergetic Catalysis
C. Engelbrekta, N. Šešelja, R. Poreddyb, A. Riisagerb, J. Ulstrupa, J. Zhanga*
Received 00th January 20xx,
Accepted 00th January 20xx
We present a facile synthesis protocol for atomically thin platinum (Pt) shells on top of gold (Au) nanoparticles (NPs)
(Au@PtNPs) in one pot under mild conditions. The Au@PtNPs exhibited remarkable stability (>2 years) at room
temperature. The synthesis, bimetallic nanostructures and catalytic properties were thoroughly characterized by
ultraviolet-visible light spectrophotometry, transmission electron microscopy, nanoparticle tracking analysis and
electrochemistry. The 8 ± 2 nm Au@PtNPs contained 24 ± 1 mol% Pt and 76 ± 1 mol% Au corresponding to an atomically
thin Pt shell. Electrochemical data clearly show that the active surface is dominated by Pt with a specific surface area
above 45 m2 per gram of Pt. Interactions with the Au core increase the activity of the Pt shell by up to 55% and improve
catalytic selectivity compared to pure Pt. The Au@Pt NPs show exciting catalytic activity in electrooxidation of sustainable
fuels (i.e. formic acid, methanol and ethanol), and selective hydrogenation of benzene derivatives. Especially high activity
was achieved for formic acid oxidation, 549 mA(mgPt)-1 (at 0.6 V vs SCE), which is 3.5 fold higher than a commercial <5 nm
PtNP catalyst. Excellent activity for the direct production of γ-valerolactone, an alternative biofuel/fuel additive, from
levulinic acid and methyl levulinate was finally demonstrated.
DOI: 10.1039/x0xx00000x
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activity of Pt, and a performance boost from core-shell
synergy.5 Au-Pt core-shell NPs can be prepared by precursor
co-reduction followed by thermal treatment,6 by
Introduction
Platinum (Pt) catalysts are crucial for a range of chemical
reactions in automotive, petroleum and energy conversion
industries.1 Pt has excellent catalytic properties but low
availability and high cost have shifted focus from pure Pt to
advanced bimetallic Pt-based nanostructures.
Nanomaterial properties depend critically on chemical
composition, size, shape, crystallinity, and surface structure.
Bimetallic nanostructures provide a strategy for design of
advanced functional nanomaterials. A second element can, for
example, reduce the concentration of surface sites for catalytic
poisons - the so-called third-body effect.2 Metal monolayers
also have dramatically altered properties by electronic
interaction with the underlying core.2 Facile synthesis of thin
underpotential deposition (UPD) of copper (Cu) on Au followed
by galvanic displacement by Pt precursors7, or reduction via
seed-mediated growth.8 Thermally driven methods require
high temperature, strong reducing agents, controlled
atmosphere, or non-aqueous solvents.6 UPD is limited by the
need for immobilization on conducting supports. UPD methods
are superior for atomically thin Pt shells, while wet syntheses
often provide thicknesses >1 nm.8 Seed-based wet chemical
syntheses offer potential for large-scale one-pot, green
preparation, but the complexity of the processes gives poor
yield and control of product morphology. Shell precursor
reduction, diffusion through seed coatings, and deposition on
seed surfaces all have to be controlled.9
metal shells on top of
a metal core offers attractive
Non-toxic procedures and sustainability have emerged as
criteria in nanomaterial synthesis and applications.10 In our
previous work, a starch and glucose based recipe, the
saccharide-based approach to metal nanostructure synthesis
(SAMENS), for the synthesis of gold nanoparticles (AuNPs)11
and other metal nanostructures was developed. A dynamics
study showed that glucose acts as reducing agent and starch as
capping agent in phosphate buffer. The Good’s buffer 2-(N-
morpholino)ethanesulfonic acid (MES) participates in the
reduction of HAuCl4 and adsorbs on the AuNP surface as well
as controlling pH.12 Very small (1.6-1.8 nm) catalytically active
starch coated platinum nanoparticles (PtNPs) have also been
prepared by the SAMENS recipe.13 In the present study we
have explored SAMENS for the synthesis of stable and uniform
approaches to tuning the catalytic activity as well as efficiently
utilizing Pt.3 Gold (Au) nanostructures have been among the
most studied systems due to unique optical and electronic
properties, and application as catalysts for CO oxidation.4
Core-shell Au-Pt NPs show improved electrocatalysis,
benefitting from electrochemical stability of Au, high catalytic
a.NanoChemistry, Department of Chemistry, Technical University of Denmark,
Kemitorvet 207, 2800, Kgs. Lyngby, Denmark
b.Centre for Catalysis and Sustainable Chemistry, Department of Chemistry,
Technical University of Denmark, Kemitorvet 207, 2800, Kgs. Lyngby, Denmark
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Au-Pt bimetallic nanostructures with high catalytic activity. been employed to hydrogenate LA to GVL with Ru-based
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Uniform AuNPs serve as seeds for Au-Pt core-shell (Au@Pt) catalysts being superior.27,28 Pt is active DfoOrI:L1A0.1h0y3d9r/Co5gTeAn0a8t9io22nK,
NPs with atomically thin Pt shells. Ultraviolet-visible light (UV- but activity comparable to that of Ru using green technologies
Vis) spectroscopy, NP tracking analysis (NTA), transmission is yet to be obtained.
electron microscopy (TEM) and electrochemical methods were The Au@Pt catalyst introduced in this work efficiently and
employed to characterize the synthesis, structure, and selectively catalyzes the ARH of BA and benzamide (BM), with
properties. A catalyst of well dispersed Au@PtNPs with 12 wt% high activity (TOF = 62 h-1) for GVL synthesis from both LA and
metal (3 wt% Pt, 9 wt% Au) on graphitized carbon black (G-CB) MLA. To the best of our knowledge, ARH of BA and its
was prepared and comprehensively characterized. Starch derivatives, and catalytic production of GVL from LA/MLA, over
capping was successfully removed thermally and the catalytic Au-Pt catalysts has not been reported before.
activity investigated by electrocatalytic oxidation of small
organic fuel molecules, and by selective aromatic ring
hydrogenation (ARH) processes and hydrocyclization of
Experimental section
levulinic acid (LA) and methyl levulinate (MLA).
Au-Pt core-shell nanoparticles (Au@Pt NPs) with atomically thin Pt
shells
Small organic molecules such as formic acid (FA), methanol
(MeOH) and ethanol (EtOH), have been widely used for fuel
cell (FC) systems as these fuels can be produced from
renewable sources without harmful waste. FA is attractive
since it is non-toxic and can be used at high concentrations
due to low rates of crossing commonly used membranes. This
leads to good FC performance despite the low energy density
compared to MeOH and EtOH. FA oxidation entails no C-C
bond breaking which is a serious obstacle for EtOH oxidation.
MeOH has received much attention as a C1 fuel with high
energy density but faces challenges of toxicity and membrane
crossing. EtOH exhibits low crossover and as a potential 12-
electron oxidation process has a very high energy density.14
The Au@PtNP catalysts exhibit high specific electrochemical Pt
surface area (ECSA) and excellent catalytic activity for all the
FA (FAOR), MeOH (MOR) and EtOH (EOR) oxidation reactions.
Currents reach 549, 219 and 229 mA(mgPt)-1 for FA, MeOH and
EtOH, respectively, in 0.1 M fuel (scan rate 50 mVs-1), or 3.5,
2.0 and 3.1 times that of a <5 nm commercial PtNP catalyst
(Sigma). Apart from improved Pt utilization in the atomically
thin shell, synergy between the Pt shell and the Au core
improves the Pt surface activity by as much as 55% relative to
pure Pt.
5-200 mL AuNP seed solution was prepared by pre-heating a
MES buffered mixture of glucose and starch for 5-10 min in a
95 °C water or oil bath, adding gold precursor, and allowing 30
min for AuNP formation. The AuNP seed solution contained 2
mM HAuCl4, 10 mM glucose, 0.6 wt% starch and 10 mM MES
(pH 7). Following AuNP formation, the seed solution was
diluted 4 times with water and 20 mM H2PtCl6 was added to
obtain a Au:Pt mol ratio of 5:2 while heating was maintained
for another 1-2 h. Complete reduction of the Pt precursor was
confirmed with UV-vis. The original seed solution had a deep
red colour whereas the final core-shell nanoparticle solution
was dark brown.
Electrocatalytic oxidation
Details about electrochemical measurements are given in the
supplementary information. After potential stabilization and
CV in 0.1 M H2SO4, formic acid (FA), methanol (MeOH) or
ethanol (EtOH) were admitted into the cell to obtain a
concentration of 0.1 M. All the experiments were performed at
room temperature (r.t.).
Catalytic hydrogenation
ARH to cyclohexane derivatives is important in petrochemical
and pharmaceutical industry.15,16 Chemoselective ARH of
benzoic acid (BA) to cyclohexane carboxylic acid (CCA) is
crucial in the synthesis of ɛ-caprolactam,17 but ARH requires
A detailed description of hydrogenation experiments is given
in the supplementary information. In brief, chemoselective
aromatic
hydrogenation
of
BA
and
BM,
and
harsh conditions with
a suitable catalyst to selectively
cyclohydrogenation of LA and MLA was conducted in a 100 mL
stainless steel autoclave reactor (Fig. S1). Substrate, solvent,
and catalyst were mixed in the autoclave, the reactor
evacuated and pressurized with dihydrogen, heated to the
target temperature, and maintained for 0.5 to 24 h. After
cooling to r.t., aliquots of the reaction mixture were submitted
to gas chromatography (GC), and the structure of the products
identified using GC coupled with mass spectrometry (GC-MS).
hydrogenate the ring. Noble metal catalysts (Ru-Pt,18 Rh/C,19
Pd/C,17,20 Pd-Ru21) have been extensively studied. Pd/C is
traditionally used though there are a number of limitations for
ARH of BA to CCA, i.e. fast deactivation, and need for high
temperature (423 K) and high H2 pressure (15 MPa).22-24
Recently, Pt was reported as an active AHR catalyst with
activity/selectivity depending strongly on NP shape and size.25
Catalytic conversion of cellulosic biomass to various platform
molecules, e.g. γ-valerolactone (GVL), is a key step in
methyltetrahydrofuran production. GVL can be obtained by
hydrocyclization of LA, an inexpensive starting material
obtainable from biomass. Multiple catalytic strategies to
produce LA via biomass transformation are available,26 but
efficient conversion of LA to GVL using green technology is
warranted. To date, numerous supported metal catalysts have
Results and Discussion
Controlling the size of the Au core allows tuning of the core-
shell nanostructure properties. AuNPs in the range of 8-80 nm
were successfully prepared using a seeded growth approach
inspired by Bastus et al29. The synthesis procedure and AuNP
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DOI: 10.1039/C5TA08922K
Fig. 1 Size control of AuNP cores. 3D plots of (a) extinction, (b) core diameter and (c) hydrodynamic diameter of the 10 partitions. Plots for individual partition are
stacked along the y-axis with extinction (in a) and frequency (in b and c) represented with the colour scheme from 0 (black) to max (purple). The UV-vis spectra were
normalized at the LSPRmax. The y-axis labels represent the size of the AuNPs derived from TEM images. UV-vis spectra and size distributions for the individual fractions
are provided in Figure S2-S3.
characterization are provided in the supplementary heating or even after complete Pt precursor reduction. Under
information, Fig. S2-S6 and Table S1-S2. Fractions were these conditions, separate PtNPs rather than core-shell NPs
obtained at various phases of the synthesis and characterized were formed and the colloids retained the characteristic red
by UV-vis, TEM and NTA (Fig. 1a, -b and -c, respectively). A colour of the AuNPs, Fig. S8. All NP dispersions remain stable
systematic increase in size was confirmed with stable NP after more than 2 years at r.t. owing to the efficient steric
dispersions in the range of 8-80 nm. With increasing particle stabilization by the starch coating. The LSPR peak of the AuNP
size, the LSPR band initially narrowed followed by red-shifting core grew slightly over time due to rearrangement of the Pt
and broadening. Above 80 nm the dispersions became shell, Fig. S9.
unstable leading to aggregation due to a thinning of the starch
capping with increasing size as seen by comparing TEM and
NTA data.
Synthesis of Au@Pt core-shell NPs
The two-step, one-pot synthesis strategy for the preparation
of Au@PtNPs from the SAMENS method is illustrated in Fig.
2a. The AuNP cores were first prepared by chemical reduction
of HAuCl4 in buffered glucose-starch solution.12 Secondly, an
atomically thin Pt shell was formed by reduction of H2PtCl6 to
provide Au@Pt core-shell NPs. The precursor was reduced
directly after the formation of AuNPs with the temperature
maintained at 90 °C. The structures of the synthesis
components are given in Scheme S1. Formation of Au and Pt
bimetallic NPs requires control of precursor reduction. Too fast
reduction of the Pt precursor leads to formation of separate
PtNPs. Characteristic UV-vis spectra of the SAMENS prepared
Au@PtNPs, PtNPs and AuNPs are shown in Fig. 2b. The
extinction from the Au@PtNPs (a continuum with a weak
shoulder around 500 nm) resembles that of PtNPs with an
indication of spectral features from AuNPs, indicating a drastic
change in the AuNP surface structure. No unreacted metal
precursors could be detected.
More details about the change in optical properties of the
Au@PtNPs were obtained by varying the concentration of the
Pt precursor. 0.2, 0.5 and 1.0 mM Pt precursor (AuPt-0.2, -0.5
and -1.0, respectively) was employed while keeping the
concentration of Au fixed at 0.5 mM. Fig. S7 shows UV-vis
spectra of the three samples after terminated heating (solid
lines) and 2 years of storage at r.t. (dotted lines). The lowest Pt
precursor concentration led to a dramatic broadening of the
LSPR peak of the AuNP core strongly indicative of formation of
a Pt shell around the AuNPs. This damping by deposited Pt was
previously reported by Henglein when reducing Pt precursor
with hydrogen gas.30 No significant LSPR damping was
observed with higher Pt concentrations after terminated
Fig. 2
(a) Principle of synthesis of Au@Pt NPs. AuNP cores (red) coated by starch
(green) are synthesized and the Pt shell (orange) of the Au@Pt NPs subsequently
formed in the same pot. (b) Extinction spectra of as-synthesized Au@Pt NPs (red),
PtNPs (black) and AuNPs (blue) from the SAMENS method showing LSPR quenching by
the Pt shell. (c) TEM micrograph of as-synthesized Au@Pt NPs. No evidence of separate
PtNPs was observed. Insert in (c) shows the size distribution of the NPs measured TEM
images; the scale is nm. (d) HR-TEM of as-synthesized Au@Pt nanostructures showing
no indication of a Pt shell despite consistent Pt signal in EDX measurements.
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Fig. 3
Identification of the Pt shell. (a) EDX spectrum of as-synthesized Au@Pt NPs. (b) Estimated Pt shell thickness based on EDX and HR-TEM images of individual
Au@Pt NPs. Red band shows mean ± SD and the green dashed line the diameter of metallic Pt. CVs of (c) as-synthesized Au@Pt NPs, single-crystal Pt and Au, (d) as-
synthesized monometallic NPs and bare glassy carbon (0.1 M H2SO4, RE = RHE, CE = Pt, scan rate = 50 mV/s). Pt and Au oxide reduction and hydrogen
adsorption/desorption are highlighted.
TEM was used to evaluate the size and elemental composition This ratio was also found for Au@PtNP samples in which the Pt
of the Au@PtNPs, Fig. 2c-d. The size of the AuNP seeds was 8 ± shell was produced immediately after AuNP formation. The Pt
2 nm, which was also found for Au@PtNPs. The size of the shell thickness was estimated based on ensemble EDX
freshly formed AuNPs and AuNPs which had undergone measurements assuming perfectly spherical 8 nm particles (as
cooling and reheating (to simulate the Pt shell formation measured from TEM) with ideal core-shell geometry to 3 Å.
process) was measured to 8 ± 2 and 7 ± 2 nm, respectively (Fig. This was substantiated by of individual NP EDX data providing
S10). In all cases, no significant change in size was identified. a shell thickness of 2.9 ± 0.7 Å, Fig. 3b, which fits with a single
Energy dispersive X-ray spectroscopy (EDX) confirmed that the Pt atom (2.77 Å). In fact, STEM-EDX of single NPs shows evenly
Au@PtNPs contained both Au and Pt, Fig. 3a, at 76 ± 1 and 24 distributed Pt and that Pt signals dominate around the NP
± 1 mol%, respectively (based on 12 EDX measurements).
perimeters, Fig. 4.
Further evidence of a Pt shell was found in the electrochemical
response of the bimetallic NPs. Cyclic voltammetry (CV) of the
Au@PtNPs drop cast on glassy carbon electrodes revealed
broad anodic signals from 0.6 to 1.3 V from oxidation of the Pt
shell and uncovered areas of the Au core, and clear cathodic
peaks corresponding to the reduction of PtOx (0.36 V) and
AuOx (0.86 V) showing that both metals are present at the
surface, Fig. 3c. The oxide reductions in the bimetallic NPs
accord with observations for pure SAMENS Au and Pt NPs
tested under the same conditions (Fig. 3d) though the
reduction potentials are shifted slightly, Table S3. The Au oxide
reduction is shifted 40 mV negatively suggesting that Au oxide
is stabilized by the Pt shell whereas the Pt oxide reduction is
shifted 30 mV positively. The hydrogen adsorption/desorption
region at low potentials is characteristic of Pt and further
supports the abundance of exposed metallic Pt.
Catalyst preparation and characterization
Fig. 4
Elemental mapping of individual Au@PtNPs by STEM-EDX. High-resolution
The Au@PtNPs were immobilized on a graphitized carbon
black support (G-CB), which is robust against degradation due
STEM images are shown in the left column, elemental maps of Au and Pt in the middle
columns, and a Au/Pt signal ratio map in the right column (white/yellow = high Pt/Au,
red = low Pt/Au).
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that of immobilized, unsupported Au@Pt NPs but provide six
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times higher currents attributed to the opDtOimI: 1i0z.e1d03d9i/sCp5eTArs0i8o9n22inK
three dimensions on the G-CB. The commercial catalyst
exhibits characteristic Pt response throughout the applied
potential range. The well-defined features of the supported
catalysts allow for deeper evaluation of their electrochemical
properties. The electrochemically active surface areas (ECSAs)
of the Au@Pt/G-CB catalysts and a commercial Pt/G-CB
catalyst were evaluated by integrating the charge in the
hydrogen adsorption/desorption region (H-UPD) for Pt and the
metal oxide reduction peaks for Pt and Au, Table 1 and Fig.
S15. The specific ECSA of Pt in the commercial catalyst was
21.2 and 21.8 m2g-1 from H-UPD and PtOx, respectively, which
is consistent with previous reports.32 The specific ECSA of Pt in
Au@Pt/G-CP was as high as 45.3-52.5 m2g-1, 2.1-2.4 times that
of the commercial catalyst depending on whether hydrogen
adsorption or PtOx reduction is considered, Table 1. The Pt
ECSA decreased drastically with thermal treatment (39% from
H-UPD and 33% from PtOx) which was unexpected from the
XRD and TEM results. Indeed, the Au ECSA dropped by only
10% suggesting that agglomeration is not the only effect of
heating. It is likely that the Pt shell rearranges forming thicker
areas during heating. Considering the ECSAs from metal oxide
Fig. 5
TEM micrographs of (a) as-synthesized Au@Pt/G-CB catalyst and (b)
Au@Pt/G-CB catalyst after thermal treatment to remove the starch coating.
to a high content of sp2 carbon.31 Good wetting of the support
was ensured by ultrasonication, prolonged stirring and heating
in water prior to mixing with as-synthesized Au@PtNPs. The G-
CB suspension was added to the Au@PtNP colloid shortly after
Pt shell formation, and adsorption completed within 1 h, Fig.
S11. The starch coating used in this work was not covalently
bound to the NP surface and could be removed via oxidation
by heating in air at 300 °C for 1 h. Fig. S12a shows
thermogravimetric analysis (TGA) confirming that starch began
decomposing/oxidizing around 200 °C, while the bare G-CB
support was stable up to 700 °C. TGA of the Au@Pt/G-CB
before and after thermal treatment are identical except for the
low-temperature region corresponding to mass loss from
starch and no indication of starch was found after thermal
treatment. Oxidation of the carbon support was catalyzed by
the Au@PtNPs and the onset of oxidation shifted from 730 to
570 °C in the presence of the bimetallic NPs. The oxidation of
the starch capping was also catalyzed by the Au@PtNPs as
indicated by shifts of the two exothermic peaks in the
differential thermal analysis (DTA) of starch from 300/390 °C
to 190/260 °C, Fig. S12b. The finally obtained masses were
assigned to the adsorbed Au@PtNPs and corresponded to 12-
13 wt% of the catalyst (excluding starch).
X-ray photoelectron spectroscopy (XPS) based on the Au (93-
81 eV) and Pt (80-68 eV) 4f signals provided 67 ± 3 wt% of
Au@Pt/G-CB, Fig. S13, with Pt comprising 21.8 ± 0.7 wt% and
Au 45 ± 3 wt%. More Pt was found with XPS, a surface specific
technique, than EDX due to Pt-rich NP surface. The thermal
treatment did not affect the Au and Pt 4f spectra, but the
shoulder in the C 1s spectrum at 286.5 eV corresponding to C-
O and the O 1s signal disappeared after treatment. X-ray
powder diffraction (XRD) and TEM were used to verify that the
thermal treatment did not compromise the bimetallic NP
nanostructure as no change in the diffraction pattern was
found, Fig. S14. Evaluation of the crystallite size using the
Scherrer equation on the metal peaks at 38 and 65° confirmed
that no line narrowing and crystallite growth had occurred.
TEM further supported that the overall catalyst structure was
stable during the starch removal process, Fig. 5, though slight
agglomeration of the NPs was observed.
reduction, the Pt coverage can be estimated by ECSAPt
/
ECSAPt+Au. The Pt coverage decreases from 73 to 66% during
thermal treatment due to contraction of the Pt shell. Braidy et
al. showed with detailed EDX mapping how Au-Pt core-shell
nanoparticles separate into pure phases joined by a flat
interface at 600 °C in inert atmosphere.33 They reported that
the core-shell structure is stable at 300 °C. The smaller NP,
thinner Pt shell, air atmosphere and catalysed starch oxidation
in the current system could thus facilitate shell rearrangement
at lower temperatures, e.g. 300 °C. The increase in the Au:Pt
ratio after thermal treatment observed in XPS further supports
that more Au is found at the particle surface. The greater
reduction in Pt ECSA relative to Au ECSA could also be caused
by partial dissolution of the Pt shell in the bulk of the NP. Both
as-synthesized and thermally treated Au@Pt/G-CB catalysts
were further tested in catalytic systems.
Electrocatalysis
The Au@PtNP performance in FC anode electrocatalysis was
evaluated by studying electrooxidation of small organic
molecules used in direct alcohol/carboxylic acid FCs. The three
tested systems vary in complexity from simple 2-electron
oxidation of FA to the oxidation of EtOH in a 12-electron
process. Fig. 6b-d shows CVs recorded at 50 mVs-1 in pure 0.1
M
H2SO4 electrolyte with 0.1 M FA, MeOH or EtOH,
respectively, Table 1. As-synthesized Au@Pt/G-CB, thermally
treated Au@Pt/G-CB without starch (t-Au@Pt/G-CB) and
commercial PtNPs (Sigma-Aldrich) on G-CB (Pt/G-CB) were
tested and are presented as red, blue and black lines,
respectively. All data were recorded at identical Pt loadings
and are shown as Pt mass specific activities.
Further electrochemical analysis of the Au@Pt/G-CB surface
was undertaken, Fig. 6a. The CVs of bimetallic catalysts match
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Fig. 6
Cyclic voltammograms of Au@Pt/G-CB (red), t-Au@Pt/G-CB where starch has been removed (blue) and commercial Pt/G-CB (black) in (a) pure electrolyte (0.1 M H2SO4),
or electrolyte with (b) 0.1 M formic acid, (c) 0.1 M methanol or (d) 0.1 M ethanol. In all cases RE = RHE, CE = Pt and scan rate = 50 mVs-1.
Table 1 Electrochemical characterization and fuel oxidation efficiency of Pt-based catalysts.
Mass activity [mAmgPt-1]
Surface area activity [mAcm-2]b)
ECSA [m2g-1]
Coverage
ΓPt
Catalyst
FAOR
0.61 V
556
MOR
0.54 V
219
EOR
0.62 V
229
FAOR
0.61 V
1.14
MOR
0.54 V
0.45
EOR
0.62 V
0.47
a)
H-UPD
45.3
27.5
PtOx
52.5
35.1
21.8
AuOx
6.50
5.85
-
Au@Pt/G-CB
t-Au@Pt/G-CB
Pt/G-CB
73 %
66 %
(100 %)
315
197
162
1.02
0.64
0.53
21.2
158
110
74
0.74
0.51
0.34
a) Pt surface area relative to total metal surface area based on metal oxide reduction. b) Data normalized to the average ECSA from H-UPD and PtOx.
Formic acid (FA) electrooxidation Electrocatalytic oxidation of improves the Pt utilization not only by increasing active surface
FA to CO2 on a metal surface may proceed via a triple path area but also by modifying the activity itself of that surface.
mechanism though the exact mechanism is under debate.2
Apart from direct decomposition of FA to CO2, oxidation can Methanol (MeOH) electrooxidation The electrochemical
occur through dehydrogenation and adsorption of formate, or oxidation of MeOH on Pt involves several intermediate steps,
dehydration and subsequent oxidation of strongly adsorbed i.e. dehydrogenation, CO-like species chemisorption, OH- (or
CO.34 CVs of electrochemical oxidation of FA are shown in Fig. H2O) adsorption, chemical interaction between adsorbed CO
6b and display two anodic peaks at 0.30 and 0.61 V vs SCE and -OH compounds, and CO2 evolution. The primary by-
corresponding to oxidation of FA via adsorbed formate and products in methanol oxidation are formaldehyde and FA.37
CO, respectively.32 The peak ratio, the pathway factor, Electrooxidation of MeOH is a 6-electron process which gives
indicates the catalyst selectivity towards direct oxidation rise to the formation of strongly adsorbed CO species in linear
through non-poisoning dehydrogenation. The bimetallic or bridge-bonded form, i.e. catalyst poisoning.37 The
catalysts exhibit higher pathway factors than the commercial electrocatalytic activity of Pt can be promoted by the presence
PtNP catalyst and improved selectivity towards the formate of a second metal (e.g. Ru, Sn, Au)38 acting either as an adatom
pathway relative to pure Pt. The largest factor was observed or a bimetal. CVs in Fig. 6c show the electrooxidation of MeOH
for t-Au@Pt/G-CB more than 2.5 times that of Pt/G-CB. The on the three catalysts tested. One main cathodic peak is
mass activities estimated from the peak currents around 0.6 V observed at 0.54 V in the forward scan and represents
were 556, 315 and 158 mA(mgPt)-1 for Au@Pt/G-CB, t- oxidation of MeOH through dehydrogenation and adsorption
Au@Pt/G-CB and Pt/G-CB, respectively, Table 1. The mass of residues.39 Again, the as-synthesized Au@Pt/G-CB
activity of the Au@Pt/G-CB is improved 3.5 times compared to demonstrated the highest mass-specific peak-current density
the commercial catalyst and is higher than or comparable to of 219 mA(mgPt)-1, or twice that of the commercial Pt catalyst
several recently reported Pt-based catalysts such as Pt (110 mA(mgPt)-1). This value is higher than obtained previously
nanotetrahedra,35 PtCu trigonal bipyramid nanoframes36 and at 5-times higher fuel concentration with Au-Pt core-shell
PtPb nanoflowers32, which were even obtained at higher FA systems.40 Recently, currents around 500 mA(mgPt)-1 in 1 M
concentrations (0.25-1 M) than those employed here. With a MeOH with Au-Pt core-shell NPs on carbon support41, PtRu
higher specific surface area of the bimetallic structures than of NPs on N-doped graphene42 and Pt NPs on TiO2/N-doped
pure PtNPs, a higher mass activity is expected. Normalizing the carbon nanocomposite were obtained.43 Peak-currents
CVs to Pt ECSA discloses a synergetic contribution from the Au increase with MeOH concentration.44 In contrast to FA
core, Table 1, and a surface 55% more active on the Au@Pt/G- oxidation, the thermally treated catalyst showed only slight
CB compared to the Pt/G-CB. The surface of the thermally reduction in activity (197 mA(mgPt)-1) indicating that reduction
treated bimetallic catalyst was slightly less active (1.02 mAcm- of ECSA induced by heating was counteracted by increased
2) than that of the as-synthesized one (1.14 mAcm-2) but still activity towards the MOR. Indeed, the surface activity was
higher than for pure Pt (0.74 mAcm-2). The Au core thus highest for t-Au@Pt/G-CB at 0.64 mAcm-2, or 25% higher than
for pure Pt. The surface of the as-synthesized Au@Pt/G-CB was
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DOI: 10.1039/C5TA08922K
Fig. 7
(a) Conversion (green), yield (black) and TOF (red) for catalytic hydrogenation of 0.5 mmol BM (I to IV), 0.5 mol MLA (V to VII). Schemes of catalyst NP structures are
shown on the columns. (b) Reaction schemes and general conditions for BM and LMA hydrogenation. Catalyst: (I-II, V) as-synthesized Au@Pt/G-CB, (III-IV, VI) thermally treated
Au@Pt/G-CB, (VII) commercial Pt/G-CB. Catalyst loading: (I-IV) 0.32 mol% Pt, (V-VII) 1.5 mol% Pt. Solvent: (I, III, V-VII) water, (II, IV) toluene. Reaction time: (I-IV) 24 h, (V-VII) 30
min.
less active for MOR than the commercial Pt catalyst. These theoretical study in which EtOH and acetyl radical were found
observations further support that the chemistry of the Pt shell to exhibit closer binding interactions on Pt/Au(111) than on
is modified by interaction with the Au core affecting the MeOH pure Pt(111).48
electrooxidation.
Catalytic hydrogenation of benzenes
Ethanol (EtOH) electrooxidation EtOH electrooxidation leads
The activity of Au@Pt NPs towards hydrogenation (reduction)
to several products, acetic acid, acetaldehyde, acetyl radical,
reactions was probed using substituted benzenes and tested in
CO and CO2. Its complete oxidation delivers 12 electrons per
EtOH molecule giving very high energy density.14 However,
the hydrocyclization of LA to GVL, Scheme S2. ARH of BM to
CCA was chosen as a standard reaction to optimize the
breaking C-C bonds is difficult and CO2 is only produced to a
small extent in electrochemical oxidation.14 Several
reaction parameters, such as solvent effects and thermal pre-
treatment, before testing BA, and LA/MLA hydrogenation. The
spectroscopic and electrochemical techniques have been
Au@Pt/G-CB was screened in its as-synthesized and thermally
applied to elucidate the mechanism of EtOH electrooxidation
on Pt-based materials.45, 46 The contributions of the different
treated forms using both protic (water) and aprotic (toluene)
pathways depend strongly on EtOH concentration.47 Above 0.1
solvents to study how the solvent environment affects the
catalyst activity. Fig. 7 (and Table S4) shows the catalysts
M, less water is present at the surface and formation of CO2
activity for chemoselective hydrogenation of BM to
and acetic acid significantly inhibited, while production of
acetaldehyde starts to increase.47
cyclohexane carboxamide (CCM) in both water and toluene
using H2 as reducing agent. The Au@Pt catalyst was active and
The activity of the three catalysts towards the EOR is
highly selective for BM hydrogenation under all reaction
illustrated by the CVs in Fig. 6d showing a main oxidation peak
conditions. The thermally treated catalyst (Fig. 7, III) showed
at 0.62 V and a second peak at 1.0 V which is due to further
oxidation of intermediate products.47 The catalyst activities
no superior activity over the as-synthesized catalyst in water
(I). Instead, a slight decrease (from 78 to 71% CCM yield) was
follow the same trend as for the FAOR with mass-specific peak
currents of Au@Pt/G-CB (229 mA(mgPt)-1) and t-Au@Pt/G-CB
noticed, most likely due to the nanostructural changes induced
(162 mA(mgPt)-1) being 3.1 and 2.2 times higher than for
by the thermal treatment, as discussed previously. The activity
commercial Pt/G-CB (74 mA(mgPt)-1). As with the MOR, The Pt
of as-synthesized Au@Pt/G-CB dropped to 28% when going
from water to toluene (II), but could be recovered to nearly
ECSA-specific activity was the highest for the thermally treated
that of water by removing starch (IV). This is due to the low
catalyst, t-Au@Pt/G-CB followed by the as-synthesized
solubility of starch in toluene, Fig. S16. The non-hydrated
Au@Pt/G-CB being 53 and 37% higher than Pt/G-CB,
starch molecules on the Pt surface in non-polar environment
respectively. Notably the ratio between the 0.62 and 1.0 V
do not extend into the solution but rather collapse resulting in
peaks for Au@Pt catalysts was opposite of Pt/G-CB indicating
limited access to the active sites. The hydrogenation rate can
modification of surface specificity for EtOH oxidation in the
be related to H-bond donation of the environment around the
presence of the Au core. This is supported by a recent
active site. A lower activation energy barrier for protic
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solvents, due to strong interaction between protic solvents removal by heating changed the chemistry of the Pt shell as
View Article Online
and substrate by hydrogen bonding, leads to higher seen in the electrochemical characterDizOaIt:io10n.1,03a9n/Cd5TsAt0ro89n2g2lKy
hydrogenation rates.49 This is of minor importance in the influenced the catalytic performance in electrochemical
present system where the activity of thermally treated catalyst oxidation and hydrogenation, which possibly relate to
is similar in both solvents (III and IV). Finally, it is worth noting structural changes of the Pt shell. Catalytic activities for several
that no hydrogenation product of toluene was observed.
electrooxidation and hydrogenation reactions were
The Au@Pt/G-CB catalyst showed also excellent catalytic investigated and compared to commercial Pt/G-CB. The high
activity for chemoselective hydrogenation of BA under mild specific Pt surface area of the thin Pt shell led to superior
conditions. Quantitative yields (100%) of CCA were achieved activity of Au@Pt/G-CB over Pt/G-CB for electrochemical
with >99% selectivity in 24 h at 1 atm H2 pressure and 80 °C. N- oxidation of important FC fuels (FA, MeOH and EtOH).
doped carbon supported Pd nanoparticles was recently shown Au@Pt/G-CB mass activities (in mA(mgPt)-1) of 556 for FAOR,
to bring
a nine-fold increase in activity compared to 219 for MOR and 229 for EOR were 3.5, 2.0 and 3.1 times
commercial Pd on activated carbon for the hydrogenation of higher than for the commercial Pt/G-CB catalyst. The core-
BA under similar conditions.49 The Au@Pt/G-CB introduced in shell structure gave synergy between the Pt shell and the Au
this work provided as much as 15-fold increased turn-over core, as well as limiting inactive Pt. The activity of the Pt
frequency (TOF) at 0.3 mol% Pt with Au (1.0 mol%). No traces surface increased with up to 55%, depending on the structure
of BA hydrogenation to benzyl alcohol were observed under of the Pt shell.
the given reaction conditions and CCA was the only product.
The presence of the starch coating had insignificant effects on
the catalytic activity for ARH of BM in water. Removal of starch
prompted a 2.5-fold increase in activity in toluene in which the
hydrophilic starch chains collapse passivating the surface. The
Au@Pt/G-CB catalyst was active for GVL production from
LA/MLA via hydrogenation/cyclization. For this system, the
highest activity (TOF = 62 h-1) was exhibited by the thermally
treated catalyst.
The facile, green and robust synthesis presented here
produces highly active and well-defined bimetallic
nanostructures and offers prospect of further tuning catalytic
activity/selectivity by the Au core size, Pt shell thickness and
thermal treatment.
Hydrocyclization of LA to GVL
Carbon supported Pt catalysts have previously shown poor
activity and over-hydrogenation of GVL resulting in low
selectivity.27, 50 Improved activity was reported by Kon et al.
with Pt/C resulting in 14% yield of GVL in 1 h at relatively low
pressure (8 bars) and high temperature (200 °C).51 Au@Pt/G-
CB was screened for LA hydrogenation followed by cyclization
to GVL. Quantitative yields of GVL were attained with excellent
selectivity (>99%) (Table S5, VIII). The commercial Pt/G-CB
catalyst was applied as a reference providing similar results. To
determine the TOF, shorter reaction times were chosen.
Reaction with commercial Pt/G-CB for 5 h gave quantitative
yields of GVL at a TOF of 13 h-1 (Table S5, IX). The activities of
Au@Pt NP and commercial Pt catalysts (Fig. 7, V-VII) were
compared at 30 min reaction times. Commercial Pt/G-CB
catalyst yielded 40% more GVL than as-synthesized Au@Pt/G-
CB but the highest yields were obtained with thermally treated
Au@Pt/G-CB. The removal of the starch capping thus resulted
Acknowledgements
Assistance with STEM-EDX experiments from Wilhelmus
Huyzer (DTU CEN), TGA measurements from Larisa Seerup
(DTU Energy Conversion and Storage) and the experimental
hydrogenation setup from Eduardo J. García-Suárez (DTU
Chemistry) are appreciated. Financial support from Danish
Council for Independent Research Technology and Production
Sciences (DFF – 1335-00330) and the Lundbeck Foundation
(R141-2013-13273) are acknowledged.
in
a significantly higher catalytic activity than that of
commercial Pt/G-CB (48% yield vs 42%) at a TOF of 62 h-1. This
observation highlights the need for starch removal for LA
hydrocyclization. The larger molecular size of LA relative to
other tested substrates (FA, MeOH, EtOH, BA, BM) may hinder
diffusion through the starch layer and limit the activity. The
surface structure changes induced by the heating might also
increase the activity towards LA hydrocyclization. All the
catalysts showed excellent selectivity to GVL with no
detectable over-hydrogenation (formation of valeric acid, 2-
Me-THF).
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