Electrochemical and in situ ATR-SEIRAS investigations of methanol and CO electro-oxidation on PVP-free cubic and octahedral/tetrahedral Pt nanoparticles

Article abstract of DOI:10.1039/c4ra00815d

Most wet-chemical methods that synthesize metal nanoparticles (NPs) of a particular size and desired shape include the use of a stabilizing surfactant, such as poly(vinylpyrrolidone) (PVP). The latter has the ability to prohibit and/or promote surface sites from participating in electrocatalytic reactions, i.e. the methanol oxidation reaction (MOR). In light of our recent findings that adsorbed PVP can enhance the MOR on Pt NPs, a strong effort was made herein to separate the NP surface orientation effect from that of surface-bound PVP. We report the in situ ATR-SEIRAS (attenuated total reflection-surface enhanced infrared reflection absorption spectroscopy) and electrochemical (EC) studies of MOR and CO oxidation reaction (COR) performed on PVP-free cubic and octahedral/tetrahedral (O/T) Pt NPs that were obtained using an adapted liquid phase UV photo-oxidation (UVPO) technique. Transmission electron microscope (TEM) images showed no observable changes of shape and size after the elimination of the PVP, while the integrity of the atomic surface structure was further confirmed by the orientation-dependent EC stripping analysis of irreversibly adsorbed adatoms, i.e. Bi and Ge. The MOR activity was enhanced by the preferential surface orientation of the O/T Pt NPs compared to commercial Pt black and the cubic Pt NPs. The in situ ATR-SEIRAS measurements showed that the PVP-free Pt NPs adsorbed more bridge-bound gaseous CO than those with residual PVP and the weakly hydrogen-bound interfacial water played an important role for the orientation dependent enhancement in MOR activity. Additionally, the findings of this work are coupled with previously published investigations regarding the influence that PVP exerts on the ultimate activity of the Pt NPs, i.e., the adsorbed PVP enhances further the MOR activity on the O/T but suppresses it on the cubic Pt NPs. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra00815d

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Electrochemical and in situ ATR-SEIRAS
investigations of methanol and CO electro-
oxidation on PVP-free cubic and octahedral/
tetrahedral Pt nanoparticles
Cite this: RSC Adv., 2014, 4, 21284
Augusta M. Levendorf, De-Jun Chen, Christopher L. Rom, Yangwei Liu
*
and YuYe J. Tong
Most wet-chemical methods that synthesize metal nanoparticles (NPs) of a particular size and desired shape
include the use of a stabilizing surfactant, such as poly(vinylpyrrolidone) (PVP). The latter has the ability to
prohibit and/or promote surface sites from participating in electrocatalytic reactions, i.e. the methanol
oxidation reaction (MOR). In light of our recent findings that adsorbed PVP can enhance the MOR on Pt
NPs, a strong effort was made herein to separate the NP surface orientation effect from that of surface-
bound PVP. We report the in situ ATR-SEIRAS (attenuated total reflection-surface enhanced infrared
reflection absorption spectroscopy) and electrochemical (EC) studies of MOR and CO oxidation reaction
(COR) performed on PVP-free cubic and octahedral/tetrahedral (O/T) Pt NPs that were obtained using
an adapted liquid phase UV photo-oxidation (UVPO) technique. Transmission electron microscope (TEM)
images showed no observable changes of shape and size after the elimination of the PVP, while the
integrity of the atomic surface structure was further confirmed by the orientation-dependent EC
stripping analysis of irreversibly adsorbed adatoms, i.e. Bi and Ge. The MOR activity was enhanced by the
preferential surface orientation of the O/T Pt NPs compared to commercial Pt black and the cubic Pt
NPs. The in situ ATR-SEIRAS measurements showed that the PVP-free Pt NPs adsorbed more bridge-
bound gaseous CO than those with residual PVP and the weakly hydrogen-bound interfacial water
played an important role for the orientation dependent enhancement in MOR activity. Additionally, the
findings of this work are coupled with previously published investigations regarding the influence that
PVP exerts on the ultimate activity of the Pt NPs, i.e., the adsorbed PVP enhances further the MOR
activity on the O/T but suppresses it on the cubic Pt NPs.
Received 28th January 2014
Accepted 29th April 2014
DOI: 10.1039/c4ra00815d
www.rsc.org/advances
DMFC reactions in fuel cells because they offer an efficient way
of utilizing expensive materials due to large surface area to
1. Introduction
volume ratios and unique catalytic properties due to their
reduced dimensions.6 Extrapolating the results obtained on
macroscopic single crystal surfaces to the nanoscale electro-
catalysts, however, is not as straightforward as it appears since
the reduced dimensions of metal NPs can introduce unexpected
geometric and electronic effects that can alter substantially the
catalytic behavior of the exposed nanofacets.7–9 For example, the
MOR trends in acidic media on Pt NPs are different from the
trends observed on bulk crystals, i.e. the activity increases in the
order of Pt NPs enclosed by Pt(100) < Pt(110) < Pt(111)
nanofacets.8,10
Since the pioneering work of El-Sayed et al. in 1996 to
synthesize preferentially shape-controlled Pt NPs, sophisticated
investigations by Feliu et al., Sun et al. and others continue to
elucidate the relationships between structure and catalytic
activity at the nanoscale as well as correlations between
macroscopic and nanoscopic systems.9–15 The advances in NP
Among the various small alcohol fuel cells, which efficiently
convert chemical energy into electrical energy, the direct
methanol fuel cell (DMFC) offers promising technical advan-
tages in terms of energy density and facile fuel distribution for
future implementation.1–3 Single crystal Pt studies have served
as the foundation for understanding the methanol oxidation
reaction (MOR) occurring at the anode, which is sensitive to the
surface morphology as dened by the exposed crystallographic
planes.2,4,5 These single crystal studies have established that the
activity for MOR in acidic media, i.e. HClO4 or H2SO4, increases
in the order of Pt(111) < Pt (110) < Pt(100) for the bulk surfaces.
In practice, however, metallic nanoparticles (NPs) are the
economically viable form of electrocatalysts for catalyzing
Department of Chemistry, Georgetown University, 37th & O Street NW, Washington DC,
20057, USA. E-mail: yyt@georgetown.edu; Fax: +1-202-687-5591; Tel: +1-202-687-
5872
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research allow NPs of unique shapes and sizes to be synthesized Pt (111) bound NPs (octahedrons/tetrahedrons), respectively.
from a diverse array of techniques ranging from thermal Alternating 94 mL of 0.375 M PVP (55 000 Mw – 3 mL total) and
decomposition to potentiometric methods.9,14,16 In the polyol- 47 mL of 0.0675 M H2PtCl6$6H2O (1.5 mL total) in EG were
based synthetic approaches, surfactants are frequently used as added to the boiling EG solution at 30 s intervals. The reaction
size stabilizing and/or shape-controlling agents, where poly(- solution was reuxed for an additional 10 min then allowed to
vinylpyrrolidone) (PVP) is among the most popular used. cool to room temperature.
Following the synthesis, the remaining surface-site-blocking
In general, the reaction solution was centrifuged for 20 min
surfactants are usually considered reaction inhibitors and at 6000 rpm to remove the AgCl precipitate if required. In order
consequently become the targets of elimination. The eradica- to remove the excess EG and PVP, the supernatant was centri-
tion of PVP without damaging the structural integrity of the NPs fuged with a triple volume of acetone for 20 min at 6000 rpm
has been a challenging endeavor for the community.17–19
until the NCs were clearly separated. The resulting Pt NPs was
In addition to synthetic control of the shape that has been dispersed into ca. 5 mL EtOH via sonication and then 20 mL of
correlated qualitatively to the exposed crystallographic planes hexanes were added before centrifugation for 20 min at 6000
via TEM, evaluating the atomic arrangement at the surface rpm, which was repeated at least 3 times until nal dispersion
quantitatively is of paramount importance.6 Among the detailed into EtOH.
electrochemical (EC) investigations reported by Feliu et al., they
A modied procedure of liquid phase UV combined with
have developed methods that allow for in situ characterization oxygenated H2O2 (UVPO) was optimized to remove the residual
of the surface order using irreversible adatom adsorption PVP from the surface.17,19 Briey, a small proportion of the
that more accurately describes the surface atomic ethanolic-suspension of NPs was dispersed into about 2 mL of
arrangement.10,12,20
H2O with 200–250 mg NaOH to yield a basic environment.
In view of the recent investigations that adsorbed PVP can Separately, a basic solution of 10 mL of H2O2 was prepared by
improve the MOR on Pt through surface water activation and an adding 200–250 mg NaOH and then the solution was bubbled
enhanced direct pathway,21,22 more specically, an unexpected for at least 10 min with O2 gas. The basic solution of NPs
large enhancement in MOR was observed on the octahedral/ underwent UV irradiation (254 nm) for 1 h with the addition of
tetrahedral (O/T) Pt NPs that had ꢀ16 wt% residual PVP aer a ca. 1 mL of O2-saturated H2O2 solution at specic time intervals,
strong base treatment.8 Our subsequent studies21,22 have also i.e. every 10 min. Aer UV exposure, the resultant solution was
shown that adsorbing PVP onto Pt black and carbon-supported puried by the repetitive centrifugation and precipitation
Pt NPs can substantially enhance the MOR and formic acid process with EtOH and ultimately dispersed in EtOH before
ꢁ
oxidation reaction activities. In this follow-up study to recently drying overnight in the oven at 60 C.
published works focused on PVP-protected NPs, we investigate
whether the MOR activity enhancement observed on these PVP- 2.2. Transmission electron microscopy (TEM)
protected systems is purely a PVP induced effect or a combi-
nation of the former with the surface orientation effect at
nanoscale.8,33 This paper reports the results obtained from the
EC and in situ SEIRAS (surface enhanced IR reection absorp-
tion spectroscopy) investigations of MOR and carbon monoxide
(CO) oxidation reaction (COR) performed on the PVP-free cubic
The resultant NPs were characterized by a JEOL-2100 LaB6
microscope at the Nanoscale Imaging and Spectroscopy Labo-
ratory at UMD College Park. Shape and size distributions were
based on a minimum of 150 counts with the aid of the ImageJ
soware.
and O/T Pt NPs. The shape-controlled Pt NPs were synthesized
2.3. Thermogravimetric analysis (TGA)
employing an established polyol method that uses PVP as the
TGA experiments were performed on a SDTQ600 or Q50
size stabilizer as done previously, however we were able to
TA Instrument. The data was analyzed with Universal TA
Analysis 2000 soware to determine weight percent of PVP on
achieve complete removal of surface-bound PVP with an opti-
mized liquid phase UV treatment combined with oxygenated
the particle surface. Typically, at least 2 mg of sample was
H2O2 (UVPO).17,19 TEM and adatom adsorption that will be
placed into a ceramic pan at room temperature (RÀT1)
discussed later, were used to qualitatively and quantitatively
before the temperature was increased at a rate of 10 ꢁC min
probe the atomic surface arrangement of the Pt NPs to ensure
to a minimum of 600 ꢁC under a steady ow of nitrogen
structural integrity prior to the EC and in situ SEIRAS
(20 mL minÀ1).
measurements.
2.4. Electrochemical (EC) analysis
2. Materials and methods
EC measurements were performed in an Ar-blanketed conven-
tional three electrode EC cell using a CHI potentiostat (CH
2.1. Synthesis of shape-controlled Pt NPs
The shape-controlled Pt NPs of ꢀ11 nm were synthesized in a Instrument, Inc) that was controlled with CHI soware. The
one-step procedure according to the established polyol based supporting electrolyte solution was 0.5 M H2SO4 (GFS Chem-
procedures as done previously.23,24 In a round bottom ask, 2.5 icals, double distilled). Commercial Ag/AgCl (1 M) (CH Instru-
mL of ethylene glycol (EG) was reuxed for ca. 20 min. Subse- ment, Inc) and Pt gauze electrodes were used as the reference
quently, 0.5 mL of 0.002 M or 0.06 M AgNO3 in EG were added to and counter electrodes, respectively. The working electrode was
the boiling EG in order to achieve Pt (100) bound NPs (cubes) or comprised of NPs that were deposited onto a well-polished 3
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mm commercial glassy carbon electrode (GCE) (BioAnalytical Pt gauze electrodes were used as the reference and counter
Systems, Inc). The NP deposition involved a dilute suspension electrodes, respectively. The working electrode was comprised
of NPs in water, usually 2 mg mLÀ1 that was drop cast onto the of a well-polished Si prism that had a thin Au lm chemically
GCE and allowed to air dry.
deposited onto one surface.29 The NP catalysts were deposited
Cyclic voltammetry (CV) and chronoamperometry (CA) were directly onto the Au lm as onto the GCE described above. The
commonly used to assess the electrocatalytic activity of the potential difference spectra were collected during a potential
samples. CVs were recorded with a 50 mV sÀ1 scan rate unless stair step experiment over the designated potential range. The
otherwise stated. The electrode potentials are given in reference spectra are shown in the absorbance units dened as Àlog(I À
to the reversible hydrogen electrode (RHE), although all data IR)/IR, where I and IR are the spectra taken at the measured and
were collected using the Ag/AgCl reference electrode. The the reference potentials, respectively. The electrocatalysts were
currents reported are normalized with respect to the Pt elec- monitored between the potential-step experiments via CV in
trochemical surface area (ECSA) determined by the hydrogen order to ensure that the system was stable.
desorption charge per area. The charge densities, based on Pt
single crystal measurements, were 220, 208 and 240 mC cmÀ2 for
polycrystalline, (100) and (111) atomic arrangements,
respectively.25
3. Results and discussion
3.1. Physical characterization
The atomic surface structure was also investigated using the Size and shape distributions of the synthesized Pt NPs are dis-
aforementioned irreversible adsorption of Bi or Ge, the details played in Fig. 1 along with representative TEM images. The
of which can be found elsewhere.10,20 In brief, Bi was sponta- shape of the NPs suggests the existence of preferential surface
neously adsorbed onto a freshly rinsed Pt sample by immersion planes, i.e. the cubic NPs display (100) planes, whereas O/T NPs
into a 0.001 M Bi2O3 in 0.6 M HClO4 then placed into the EC cell exhibit (111) planes. The average size of the cubic and O/T Pt
at 0.3 V aer being rinsed. Similarly, the rinsed sample was NPs were 10.9 Æ 1.4 nm and 11.6 Æ 2.0 nm, respectively, with
immersed into a 0.01 M GeO2 in 1.0 M NaOH solution to adsorb over 70% yield of the desired shape obtained. More specically,
Ge, but the electrode was not rinsed following adsorption. In 71.2% of the NPs from the cubic sample were cubes with (100)
each case, the sample was rst cycled in 0.5 M H2SO4 below 0.9 facets, while 73.2% of the O/T sample were either octahedrons
V to achieve a stable CV, then cycled up to 1.0 V for 2 cycles and or tetrahedrons with (111) facets.
nally cycled with an upper potential of 0.9 V before adsorption
The high degree of monodispersity of the synthesized NPs in
of Bi or Ge. The adsorption experiments were performed in Fig. 1 is attributed to the presence of PVP during the synthesis.
triplicate on the Pt black, cubic NPs and O/T NPs for each Using thermal analysis, the weight loss due to the decomposi-
adatom.
tion and/or desorption of the PVP as a function of increasing
In a typical catalytic experiment, the blank CV was recorded temperature provides insight into the amount of PVP present on
in a potential window until a stable curve was obtained in the the NP, i.e. surface-bound and vicinal units of the polymer.
supporting electrolyte. For the shape controlled NPs, it was Fig. 2 summarizes the TGA measurements performed on the as-
important to minimize the surface restructuring due to oxide synthesized and UVPO treated Pt NPs. Pure PVP desorbs
formation, thus the potential limit was 0.9 VRHE prior to further between 400 and 450 ꢁC in the TGA as indicated by the inset of
measurements. Oxidation of adsorbed gaseous CO was per- Fig. 2B. Thus, the surfactant accounts for 7 wt% and 12 wt% on
formed by holding the potential at 0.3 V while rst saturating the as-synthesized cubic and O/T Pt NPs as seen by the weight
the electrolyte with CO gas for 5–10 min, then purging the loss from 375 to 425 ꢁC in Fig. 2A. Following the adapted liquid
solution with Ar gas for 15–25 min. Finally, a CV was recorded phase UVPO treatment, the NPs in Fig. 2B exhibit no major
within a specic potential window to oxidize or strip the CO weight loss in this temperature range expected for the desorp-
from the Pt surface. CV was employed to measure the intrinsic tion of PVP, which strongly suggests that no polymer was
activity of the Pt NPs for the MOR at 0.5 M CH3OH concentra- present on the surfaces and were similar to as-received
tions in the supporting electrolytes described above. Following commercial Pt black.
the MOR CV, the CA current was measured over a 30 min period
Despite the effectiveness of the UVPO method at removing
with the potential held at 0.5 V in the CH3OH-containing elec- the surface-bound PVP, the adverse effect of UV exposure on the
trolyte solution. Lastly, a CV was collected in pure electrolyte to surface atomic arrangement has been cautioned in the litera-
ensure catalyst stability.
ture.18,30 There were no observable changes of the size and shape
of the NPs before and aer the UVPO treatment as noted by the
TEM measurements, although the particles became more
agglomerated. As mentioned shape suggests surface arrange-
ment of the NPs only qualitatively and is by no means a quan-
2.5. Attenuated total reection (ATR)-surface enhanced
infrared reection adsorption spectroscopy (ATR-SEIRAS)
The ATR-SEIRAS measurements were collected on a Bruker titative description for the surface atomic arrangement.30
Vector-22 IR Spectrometer that was modied to house an EC Moreover, TEM is an ex situ method with a sample size that is
apparatus.26–28 The in situ EC measurements were performed in much smaller than the number of NPs that are used for elec-
a modied three electrode EC cell using a potentiostat (CH trocatalysis. In order to measure the Pt surface structure more
Instrument, Inc) that was controlled by a computer with CHI quantitatively, we adopted the irreversible adsorption/EC
soware. Commercial Ag/AgCl (1 M) (CH Instrument, Inc) and stripping methods developed by Feliu et al.10,12,20
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Fig. 1 Representative TEM images of the cubic Pt NPs (A) and O/T NPs (B) synthesized along with size and shape distributions based on a
minimum of 150 counts. S, C, O and T denote spherical and random shapes, cubes, octahedrons and tetrahedrons, respectively.
charge density values of 220, 208 and 240 mC cmÀ2 for poly-
crystalline, (100) and (111) atomic arrangements of Pt, respec-
tively.25 Foremost, the symmetry and clearly dened peaks on
each sample in Fig. 3A further supports the cleanliness of the
samples, especially the synthesized NPs because surfactants
tend to eliminate the sharpness of the associated peaks as
previously reported.22,24,31
Upon visual inspection of Fig. 3A, there are four principal
peaks, whose charges depend on the exposed surface struc-
ture.20,24 The dominant peaks at ca. 0.09 V and ca. 0.22 V on Pt
Fig. 2 TGA curves of as-synthesized (A) and UVPO treated (B) cubic
(red) and O/T (blue) Pt NPs along with the unaltered Pt black (black).
black are associated with hydrogen redox processes on (110)-
The inset of B displays the TGA curve for pure PVP (purple), which
and (100)-like step sites, whereas the hump around 0.5 V is
indicative of (bi)sulfate adsorption on ordered (111) terrace
domains. The increase of the hump at ca. 0.5 V on the O/T Pt
NPs corresponds nicely to the presumed increase of (111) facets
due to their shape. Moreover, the relative increase at the (110)-
like step sites, which occur at step sites nearby (111) terrace
domains, and negligible change at the (100) sites due to edges
and corners demonstrates the preferential orientation of the
surface as (111) facets. In contrast, the cubic Pt NPs show hardly
any anion adsorption current hump at ꢀ0.5 V indicating the
highlights the absence of PVP weight loss at 400 to 450 ꢁC on the
UVPO treated compared to the 7 wt% and 12 wt% presence of PVP on
the as-synthesized cubic and O/T Pt NPs, respectively (A).
CV was performed on the Pt black, cubic and O/T Pt NPs in
0.5 M H2SO4 to quantitatively infer the surface structure based
on the charge distribution of peaks related to hydrogen and the
(bi)sulfate adsorptions on various sites. The CV proles for the
samples in Fig. 3A are normalized using single crystal hydrogen
Fig. 3 (A) CVs of the Pt black (black), cubic (red) and O/T Pt NPs (blue) (A) in 0.5 M H2SO4. Comparison of CV profiles of pristine (solid) cubic (B)
and O/T (C) Pt NPs with CVs after germanium (dot) and bismuth (dash) adsorption in 0.5 M H2SO4.
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Fig. 4 A cubic (A), octahedral (B), and tetrahedral (C) Pt NP compared to the Pt black (D) with 10 nm scale bars. The tabulated results for mean
size and desired shape percentage of the NPs based on TEM images along with percentage of the (111) and (100) terrace sites on the NP
ensembles based on irreversible adsorption of Bi onto the (111) and Ge onto the (100) sites, respectively.
absence of the (111) terrace domains, which is further degree as the synthesized NPs. Moreover, a qualitative inspec-
conrmed by the decrease of the current peak related to the tion of the hydrogen charge distributions in Fig. 3 for the Pt NPs
(110)-like step sites and increase of the current assigned to the yields a similar conclusion for preferred surface arrangements.
(100)-like sites. Furthermore, the presence of the current Therefore, these UV treated NPs were used as the preferentially
shoulder at ca. 0.3 V indicates the increase in (100) terrace sites oriented, PVP-free electrocatalysts for this work due to the
that are absent on the Pt black and O/T Pt NPs. Qualitatively, the absence of PVP and strong evidence for the desired atomic
charge distribution of the CV results approximates that arrangements.
indeed the atomic surface structure corresponds with the shape
of the NPs.
3.2. Catalytic activity characterization
For a more quantitative analysis of the atomic arrangements,
the NPs were probed in situ using the irreversible adatom
adsorption of Bi and Ge.20 As described in the literature, the
charge of the Bi redox peak occurring at ca. 0.58 V is propor-
tional to the number of (111) terrace sites, and likewise, the
charge from Ge at ca. 0.46 V is related to the (100) terrace sites.
The CV results for the cubic and O/T Pt NPs are displayed in
Fig. 3B and C, respectively. The propensity for Bi to adsorb more
on the O/T Pt NPs and less on the cubic NPs, while the inverse
was observed for Ge adsorption, is the additional conrmation
to the preferential surface structure obtained. Following the
necessary adjustments, the charge of adatom adsorption/strip-
ping is correlated to the hydrogen charge of the original CV
results, which provides a percentage of the adatoms on different
surface sites, i.e. the surface structure.
Fig. 4 presents the HRTEM images of individual shaped NPs
and the quantitative results of adatom adsorption measure-
ments, clearly indicating that the shape controlled NPs were
dominantly composed of the desired facets. More specically,
the cubic Pt NPs are ca. 40% comprised of the (100) terrace sites,
which are nearly double the number of the (111) sites. Mean-
while, the O/T Pt NPs show ca. 50% being the (111) terrace sites.
For comparison, the adatoms were adsorbed on the Pt black,
which exhibited no preference for the terrace sites to the same
The normal CVs for the Pt black, cubic and O/T Pt NPs in 0.5 M
H2SO4 are displayed in Fig. 5A, respectively, which are repro-
duced from Fig. 3A for facilitating the discussion. As described
above, the dominant (110)-like and (100)-like step sites occur at
ca. 0.09 V and ca. 0.22 V on Pt in 0.5 M H2SO4, while the hump at
ca. 0.5 V is indicative of anion adsorption on the (111) terrace
domains and the bump at ca. 0.3 V is characteristic of (100)
terrace domain.
The COR of gaseously adsorbed COads, commonly referred to
as CO stripping, on the Pt black, cubic and O/T Pt NPs in 0.5 M
H2SO4 are displayed in the inset of Fig. 5A. The peak potentials
are 0.78 V, 0.76 V and 0.86 V for the Pt black, cubic and O/T Pt
NPs, respectively with the earliest onset of oxidation occurring
on the cubic Pt NPs at 0.60 V, which is ca. 200 mV earlier than
the other two samples. As observed previously, the O/T Pt NPs
required the highest potential to oxidize COads, while the cubic
Pt NPs showed the broadest peak widths.8 The low onset of the
COR on the cubic Pt NPs in contrast to the difficulty of oxidation
on the O/T Pt NPs would suggest that the cubic Pt NPs be more
active in the MOR than the O/T NPs.
Upon investigation, however, the intrinsic activity trends for
the MOR are not corresponding to the COR trend, which is
apparent from the MOR CVs displayed in Fig. 5B. Notably, both
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Fig. 5 Normal CVs in 0.5 M H2SO4 (A) and MOR CVs in 0.5 M H2SO4 + 0.5 M CH3OH (B) of the Pt black (black), cubic (red) and O/T (blue) Pt NPs.
The higher peak currents for the negative scans had to do with that the NP surface started with no CO poisoning while this was not the case for
the positive scans. Insets show the gaseous CO oxidation in 0.5 M H2SO4 (A) and the CA measurements over 1800 s at 0.5 V in 0.5 M H2SO4 + 0.5
M CH3OH (B).
the CV and CA measurements showed that the O/T Pt NPs had
the highest MOR activity, followed sequentially by the Pt black
and cubic Pt NPs. This is the same order as observed previously
on the shape-controlled Pt NPs,8,10 but different from the single
crystal surfaces.5,10,32 Notice that the higher backward potential
scan peak current is much higher than that of the forward scan
for the O/T Pt NPs. This was caused by the relatively low up
potential limit in an attempt to alter less the surface structure.
Because of lower up potential limit, the oxidized Pt surface
could be reduced at a higher potential on the backward scan,
Fig.
6 Potential dependent ATR-SEIRAS spectra for gaseously
which led to a small potential window in which the Pt surface
was largely reduced but not yet poisoned severely by the CO thus
higher peak current.
adsorbed CO on the Pt black (A), O/T (B) and cubic (C) Pt NPs in 0.5 M
H2SO4 with the reference spectra recorded at 1.2 V.
Returning briey to the activity dependence of surface
orientation on the presence or absence of PVP, we shall
compare data obtained using the Pt black as a reference (by
setting its values to 1). The activity enhancements in the order
of O/T : black : cubic Pt NPs as measured in Fig. 5B are
1.5 : 1 : 0.97 in CV (using the peak currents of positive scans)
and 4.5 : 1 : 0.32 in CA (using the currents measured at 1800 s,
inset in Fig. 5B), respectively. These PVP-free values compared
to the corresponding values for the shape-controlled Pt NPs
with residual PVP8 were 2.9 : 1 : 0.46 in CV and 12 : 1 : 0.15 in
CA, respectively. Interestingly, the MOR activity of the PVP-free
O/T Pt NPs is about half of the O/T Pt NPs with residual PVP,
while that of the PVP-free cubic Pt NPs is about twice of the PVP-
protected cubic Pt NPs. In other words, the presence of PVP
enhances the MOR activity on the O/T Pt NPs, but suppresses
the activity on the cubic Pt NPs as compared to the PVP-free
counterparts. Notice that even without residual PVP, i.e. only
based on the morphology, the O/T Pt NPs showed ꢀ50%
increase in the MOR activity as compared to the commercial Pt
black.
measurements with a 0.1 V potential step from 0 V to 1.2 V are
displayed in Fig. 6 with 1.2 V as the reference. The spectra
exhibit the characteristic linearly bound CO (COL) and bridge
bound CO (COB) with frequency ranges from ca. 2000 cmÀ1 to
2050 cmÀ1 and ca. 1850 cmÀ1 to 1900 cmÀ1, respectively,
independent of surface structure. Note the amount of bridge
bound CO, however, was much higher here on the PVP-free NPs
than that on the PVP-protected shape-controlled Pt NPs, which
is most likely a manifestation of the availability of vicinal Pt
sites on the clean PVP-free surface.33 The small band centered
about 1700 cmÀ1 on the O/T and cubic Pt NPs can be assigned to
multi-bound CO (COM).34 There is a strong bending water
vibrational band on the Pt surfaces between 1610 cmÀ1 and
1615 cmÀ1 labeled as d(HOH) that is strongest on the Pt black
and weakest on the cubic Pt NPs.
Additional conrmation of surface cleanliness is seen by the
comparable spectral features on the Pt NPs to Pt black, i.e. the
absence of polymeric bands. PVP has been shown to interact
mostly through the carbonyl, >C]O, group on the surface of
Pt.35 If the polymer were present, the >C]O stretch vibration
would exhibit a prominent IR band between 1660 cmÀ1 and
1680 cmÀ1 as previously reported.22 No such band was observed
3.3. ATR-SEIRAS characterization
3.3.1. CO oxidation reaction. The potential difference on the PVP-free Pt surfaces studied here.
spectra for the oxidation of gaseous COads in 0.5 M H2SO4 on the
Fig. 7 summaries the COR trends for the normalized (by the
Pt black, O/T and cubic Pt NPs during potential stair-step respective ECSA) integrated band intensities for the COL, COB
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Fig. 7 Normalized (by ECSA) integrated band intensities as a function of applied potential for linear (COL), bridged (COB) and bending water
(HOHbending) during the COR on the Pt black (A), O/T (B) and cubic (C) Pt NPs in 0.5 M H2SO4. Corresponding potential dependent frequencies of
the COL (circle) and COB (square) on the Pt black (black), O/T (blue) and cubic (red) Pt NPs in 0.5 M H2SO4 (D).
and d(HOH) on the Pt NPs as a function of the applied potential
in 0.5 M H2SO4 (A–C). In contrast to the PVP-protected shape-
controlled samples studied previously, where the gaseously
adsorbed CO behaved quite differently for different samples,33
the gaseous CO adsorbed on the PVP-free samples and Pt black
showed very similar behavior (see Fig. 7) even though the three
had distinctly different surface structures (see Fig. 4). All three
samples showed the same vibrational frequency range of COads
as a function of the electrode potential and the O/T sample had
basically the same Stark tuning rates as those of the Pt black
Fig. 8 Potential dependent ATR-SEIRAS spectra recorded during the
MOR on the Pt black (A), O/T (B) and cubic (C) Pt NPs in 0.5 M H2SO4 +
0.5 M CH3OH with the reference spectra recorded at 1.2 V.
while those of the cubic sample were somewhat smaller
(Fig. 7D). The COL was more easily oxidized than COB as noted
in previous studies with Pt.36,37 The decrease in the amplitude of
d(HOH) in the stair-step experiments corresponds well with the
oxidation of COads that is expected to consume interfacial water
as has been reported.22 The COL band intensity also decreased
slightly below ca. 0.6 V in the so-called pre-ignition region
before the rapid decrease signals quick oxidation or the ignition
black, O/T and cubic Pt NPs during the stair-step measurements
from 0 V to 1.2 V to 0 V with a 0.1 V potential step. Spectra
between 1500 cmÀ1 to 2100 cmÀ1 shown in Fig. 8A–C are
region. Simultaneously, the COB intensity remained largely
referenced to the upper potential of 1.2 V, which is shown as a
unchanged until the swi oxidation occurred beyond ca. 0.6 V.
at line in the middle of the stacked spectra. In contrast to the
Interestingly, the band intensity of the COB on the cubic Pt NPs
gaseous CO stripping spectra, the CH3OH-generated COads
exhibits a lone COL band between 2000 cmÀ1 and 2060 cmÀ1
,
increased at ca. 0.4 V that was corresponding to the pre-ignition
occurring with COL, which suggests a possible CO conversion
process. Moreover, the lack of a d(HOH) band near 1630 cmÀ1
indicates that the interfacial water was substantially hydrogen
bonded.38
except on the cubic Pt NPs that display an additional minor
peak at ca. 1700 cmÀ1. This strongly implies that the methanol
carbon probably bound to the surface only as an atop congu-
ration during the dehydrogenations. Adsorbed methanol,
CH3OHads, was also detectable at ca. 1015 cmÀ1 on the Pt NPs
similar to that observed on the commercial samples previ-
ously.28 Moreover, there are two d(HOH) bands observed during
the MOR at ca. 1610 cmÀ1 and ca. 1650 cmÀ1. The former is
assigned to the surface adsorbed interfacial water as in the
previous section, while the latter has been attributed to the
bending of the bulk water.39,40 In the following, we will focus
only on the interfacial water.
Moreover, as shown in Fig. 9, the production of CO2 can be
observed albeit the molecule can diffuse away from the surface
in the ATR-SEIRAS setting. Nonetheless, a direct correlation of
the oxidized COads and CO2 was possible as shown in Fig. 10,
which is consistent with previous works using the thin-layer
approach.21 Again, the MOR-generated COads showed very
similar behavior on the PVP-free samples in terms of the
vibration frequency at a given electrode potential, overall
frequency range, and Stark tuning rates that are surprisingly so
As mentioned above, the potential dependent frequency
shis showed much more similar behavior for COL and COB on
the PVP-free surfaces, as shown in Fig. 7D, than that on the
surfaces having residual PVP.33 One noticeable difference is that
the CO adsorbed on the cubic Pt NPs showed lower tuning rates
of 37 and 47 cmÀ1 VÀ1 for COL and COB, respectively, in 0.5 M
H2SO4 than the 55 and 58 cmÀ1 VÀ1 for the Pt black and O/T Pt
NPs, respectively. Overall, despite the evidence for clearly
different surface arrangements and in turn the catalytic activi-
ties, there was little variability detected by ATR-SEIRAS for the
gaseously adsorbed species at the high coverage of COads during
the CO stripping measurements. This observation was some-
what anticipated since the NPs were composed of Pt metal with
low index planes without electronic and/or geometric effects
induced by the surfactant.
3.3.2. Methanol oxidation reaction. The potential depen-
dent spectra displayed in Fig. 8 summarize the MOR on the Pt
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potentials of CO2 production meaning that the COR took place
mainly at the peripheries of the COads islands as it implies that
the local coverage of the remaining COads was maintained.
Alternatively, the peak potential for the CO2 production for
the O/T Pt NPs was about 50 mV and 90 mV less positive than
those for the Pt black and cubic Pt NPs respectively (red squares
and the vertical dashed lines in Fig. 10A to C), which is in
agreement with the EC observation of the enhanced MOR on
the former (Fig. 5B). The in situ SEIRAS data, however, do not
provide a clearly distinguishable evidence as to what is the exact
mechanism for the MOR enhancement observed on the PVP-
free O/T Pt NPs. One plausible indication may be gleaned by the
rate of decrease of the interfacial water during the MOR by
following the band at 1610 cmÀ1 as presented in Fig. 10A to C
(blue triangles). The O/T Pt NPs showed the fastest decrease as
the electrode potential scanned positively, followed in the
respective order by Pt black and cubic Pt NPs, suggesting that
the interfacial water may play a differentiating role here.
It was observed previously27 that among the three possible
types of interracial water, i.e. the strongly hydrogen-bound
(ꢀ3200 cmÀ1), weakly hydrogen-bound (ꢀ3400 cmÀ1), and non-
hydrogen-bound water (ꢀ3600 cmÀ1), the weakly hydrogen-
bound water was identied as the active water species that
participates the MOR as the oxygen-containing species. Fig. 11
presents the water stretching vibrational bands for the Pt black
(A), O/T (B), and cubic Pt NPs observed during the MOR stair-
Fig. 9 Potential dependent ATR-SEIRAS spectra of CO2 produced
during the MOR on the Pt black (A), O/T (B) and cubic (C) Pt NPs in 0.5
M H2SO4 + 0.5 M CH3OH with the reference spectra recorded at 0.0 V.
close to those for the gaseous COads (Fig. 7D). Again, this is in
contrast to PVP-protected,33 where different vibrational
frequencies at a given electrode potential were observed for the
three samples and different corresponding Stark tuning rates
were noted.33 Since the Stark tuning rates for the anodic (Ano)
and cathodic (Cath) directions are almost the same, this indi-
cates that the COads was generated in the same way independent
of the direction of electrode potential scan during the MOR.
Moreover, since they are also very close to those of gaseously
adsorbed CO (Fig. 7D) also implies that the MOR-generated
COads formed closely packed islands. This suggests that the
linearity of the Stark tuning was maintained beyond the peak
Fig. 10 Normalized peak intensities as a function of applied potential for linear CO (black circles), interfacial bending water (blue triangles) and
CO2 (red squares) during the MOR on the Pt black (A), O/T (B) and cubic (C) Pt NPs in 0.5 M H2SO4. Corresponding potential dependent
frequencies of the anodic COL (solid-circle) and cathodic COL (open-circle) on the Pt black (black), O/T (blue) and cubic (red) Pt NPs (red) in 0.5 M
H2SO4 (D).
Fig. 11 Potential dependent ATR–SEIRAS spectra of the water stretching vibrations (n(OH)) during the MOR on Pt black (A), Pt (111) NCs (B) and Pt
(100) NCs (C) in 0.5 M H2SO4 + 0.5 M CH3OH with the reference spectra recorded at 0.0 V. (D) The SEIRAIS amplitude as a function of electrode
potential at the respective frequencies indicated by the dashed arrows for Pt black (circles), O/T (diamonds), and cubic (cubes) Pt NPs.
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step measurements as those in Fig. 8 and 9. For the O/T and Pt MOR enhancement on the O/T, but suppresses activity on the
black samples, there was indeed a well-dened, negative going cubic Pt NPs is warranted and being pursued in our laboratory.
peak at ꢀ3435 cmÀ1 (indicated by the corresponding dashed
arrows in Fig. 11A and B) during the rst positive potential scan.
By the end of the negative potential scan, this peak eventually
Acknowledgements
became a positive-going peak for the Pt black, but a less well- This research has been supported by the NSF (CHE-0923910)
dened shoulder for the O/T Pt NPs. For lack of a such well- and the DOE (DE-FG02-07ER15895). A.M.L. received support
dened peak for the cubic sample during the positive potential from the NSF (OISE-1130454) and was a Washington Metro
scan, the spectral shoulder at 3388 cmÀ1 in the IR spectrum Scholar (2012-2013) by the ARCS Foundation and C.L.R. was
recorded at the end of the negative potential scan, as indicated supported by a GUROP 2013 summer research grant. TEM
by the dashed arrow in Fig. 11C, was used to dene and gauge images were taken at the NISP Lab at UMD at College Park, MD.
the behavior of the weakly hydrogen-bound water in this
sample.
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