Probing the Size and Shape Effects of Cubic- and Spherical-Shaped Palladium Nanoparticles in the Electrooxidation of Formic Acid

Article abstract of DOI:10.1002/cctc.201500774

A systematic study of size and shape effects in the electrooxidation of formic acid over our controlled synthesized Pd nanospheres and nanocubes as material models by using infrared spectroscopy and density functional calculations was undertaken. The bridge formate intermediate on the metal terrace was identified as the main contributor to the electrooxidation activity of formic acid, whereas linear formate with much weaker adsorption energy was unstable and could not proceed to the product. It was also demonstrated that Pd(1 0 0) cubes could stabilize two bridge formate species, and thus, they show higher electrooxidation activity than Pd spheres at comparable sizes. DRIFTing away: Various structures of adsorbed formate species on a Pd cubic nanoparticle surface are identified by diffuse reflectance infrared Fourier transform spectroscopy analysis. Their coverage and adsorption strength can affect the activity of the electrooxidation of formic acid. Pd(1 0 0) cubes are able to stabilize two bridge formate species and thus show higher electrooxidation activity than Pd spheres at comparable sizes.

Full text of DOI:10.1002/cctc.201500774

DOI: 10.1002/cctc.201500774
Communications
Probing the Size and Shape Effects of Cubic- and
Spherical-Shaped Palladium Nanoparticles in the
Electrooxidation of Formic Acid
Weiran Zheng,[a, b] Jin Qu,[b] Xinlin Hong,[a] Karaked Tedsree,[b, c] and Shik Chi Edman Tsang*[b]
A systematic study of size and shape effects in the electrooxi-
dation of formic acid over our controlled synthesized Pd nano-
spheres and nanocubes as material models by using infrared
spectroscopy and density functional calculations was undertak-
en. The bridge formate intermediate on the metal terrace was
identified as the main contributor to the electrooxidation activ-
ity of formic acid, whereas linear formate with much weaker
adsorption energy was unstable and could not proceed to the
product. It was also demonstrated that Pd(100) cubes could
stabilize two bridge formate species, and thus, they show
higher electrooxidation activity than Pd spheres at comparable
sizes.
can be made available in aqueous solution.[3] Unlike the study
of geometric effects by using single crystals, the study of size
effects on well-shaped NPs is more relevant to industry for cat-
alyst design, because factors such as ligand effects, solvent ef-
fects, lattice strain, and adsorbate–adsorbate interactions that
cannot be easily handled in ultrahigh vacuum can be investi-
gated with nanocatalysts.[4]
Well-synthesized nonspherical Pd NPs as catalysts can exhib-
it dramatic performance in both activity and selectivity. In par-
ticular, size and shape issues of NPs are always considered to
be important in catalysis.[5] However, the degrees of their im-
portance and their rational control in the synthesis for a given
catalytic process are not widely explored. The decomposition
of formic acid has recently attracted growing attention owing
to its potential applications in hydrogen storage and fuel cells.
It has been reported that the reaction is sensitive to the size,
surface structure, and lattice strain of the metal nanoparticles
as catalysts.[5a,6] However, their degree of importance and their
effect on the nature of the surface species with regard to the
mechanism of decomposition are still unclear. Herein, we
report a systematic study of these parameters in this catalyzed
reaction over Pd nanospheres and nanocubes.
Exposure of metal nanoparticles (NPs) with a particular crystal-
lographic plane through their controlled growth by using a mo-
lecular stabilizer in solution and leading to higher catalytic per-
formance has been gaining attention over the past decade.[1]
This takes a different approach to traditional surface catalysis,
for which ultrahigh vacuum technologies are commonly used
for the synthesis and testing of large single-crystal model sur-
faces for primarily gas-phase reactions under artificial condi-
tions. On the other hand, the controlled growth of nanoparti-
cles with a particular size and shape as nanocatalysts adapta-
ble to a wide range of catalytic reactions represents a recent
and new direction in catalysis. Noble-metal NPs such as Pd can
be synthesized in different morphologies including octahe-
drons, cubes, and tetrahedrons.[2] For nonspherical NPs with
generally unstable and active surfaces, surfactants or capping
agents are used to lower the surface energies for their synthe-
sis. For example, by using poly(vinylpyrrolidone) (PVP) as a cap-
ping agent, octahedral Pd NPs and cubic and plate-like NPs
To disentangle the size and shape effect of Pd NPs from
each other, Pd NPs with various sizes were carefully synthe-
sized in both spherical and cubic forms by using a modified lit-
erature method (see the Supporting Information).[7] The size
was controlled through the adjustment of the ratio of the
metal precursor and the amount of PVP used; the shape was
controlled by using Brꢀ ions. The surfaces of spherical NPs are
considered as a combination of low- and high-index facets
(terraces: {111}, {100}; steps: {331}, {443}, etc.; kinks: {10,8,7},
{13,11,9}, etc., and corner sites and adatoms, etc.); spherical
NPs have a higher surface roughness[8] than cubic NPs, which
are enclosed exclusively with six low-index Pd(100) facets,[2] as
shown in our crystal models (Figure S1, Supporting Informa-
tion). The transmission electron microscopy (TEM) images are
shown in Figure 1. For the synthesis of spherical NPs, it is inter-
esting to see that the spherical shape can be assumed if the
average size is below 4 nm. However, above this critical size,
the particles start to deviate from a spherical shape to irregular
polyhedrons with sharp edges. In contrast, for the synthesis of
cube NPs, no well-defined cubic shape was observed below
4 nm owing to the higher surface energy of the facet sites.
However, the uniform cubic shape is well maintained at larger
sizes from 4.73 to 10.24 nm, which indicates that the Brꢀ ions
work more effectively in larger size regimes. It is known that
stabilizers may bind to particles to block access of metal sur-
[a] W. Zheng, Prof. X. Hong
College of Chemistry and Molecular Sciences
Wuhan University
Wuhan, 430072 (P.R. China)
[b] W. Zheng, Dr. J. Qu, Dr. K. Tedsree, Prof. S. C. E. Tsang
Wolfson Catalysis Centre, Department of Chemistry
University of Oxford
Oxford, OX1 3QR (UK)
E-mail: edman.tsang@chem.ox.ac.uk
[c] Dr. K. Tedsree
Nanocatalysis Laboratory, Department of Chemistry
Faculty of Science
Burapha University
Chonburi, 20131 (Thailand)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500774.
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band identified as multidentate
formate is observed.[12] It is inter-
esting to note an unusually
broad bridge formate region in
Figure 2a-ii,a-iii,a-iv on the cubic
NPs surfaces, which could be fur-
ther resolved into two bands:
one at approximately n˜ =
1606 cmꢀ1 and another at n˜ =
1572 cmꢀ1 (Supporting Informa-
tion). The latter matches with
the bridge formate position on
our Pd spheres (n˜ =1568 cmꢀ1)
and the reported Pd(111) sur-
face of typical band width.
Figure 1. TEM of PVP-Pd NPs: a) spherical shape and b) cubic shape with increasing size; average diameter (for
spherical NPs) or diagonal (for cubic NPs) D is shown below the figures; insets are the corresponding size-distribu-
tion histograms, scale bar is 30 nm.
face sites (PVP with Pd by oxygen atoms[9]); thus, the synthe-
sized Pd NPs were extensively washed with water and ethanol
to reduce their surface influence. Thermogravimetric analysis
(TGA) showed no significant carbon contamination and X-ray
photoelectron spectroscopy (XPS) showed no detectable resid-
ual Br (Figure S2). The adsorption of formic acid (vapor in N2
gas was passed to saturate the coverage of the metal surface
in an environmental cell) as a chemical probe on these Pd
nanoparticles was studied by insitu diffuse reflectance infrared
Fourier transform (DRIFT) spectroscopy. This characterization
method enables comparative analysis of surface species and
their distribution on different metal sites.[10]
We discounted this new band at n˜ =1606 cmꢀ1 observed on
the cube surface resulting from small residue PVP–formic acid
intermolecular interactions, as no such band was detected for
the spherical Pd NPs by deliberately adding and varying the
PVP/Pd ratio (Figure S3). Dipole–dipole interactions between
neighboring adsorbents could affect the position of the ad-
sorption band, and this is commonly observed for small adsor-
bates such as CO with high dipole moments.[13] Although the
dipole–dipole effect was reported to affect the position of the
symmetric OꢀCꢀO stretching mode of formate, the asymmetric
stretching mode was found to show no effect[14] (Supporting
Information). We believe that the surface structure of Pd(100)
enclosing the cubic Pd NPs is to give rise to different bridge
formate species, as previously discussed on Cu(100) surfa-
ces.[15] The difference in bridge distance of the formate is
mainly caused by the intrinsic difference in the Pd–Pd distan-
ces on the Pd(100) surface, as illustrated in Figure 3. The two
closest bridge configurations were considered on Pd(10 0), al-
though more bridge formate species could be envisaged. Their
adsorption energies and band positions were calculated by
DFT and are shown in Table 1. Notably, both bridge formates
at adsorption energies of ꢀ0.59 and ꢀ0.71 eV (calculated Oꢀ
CꢀO asymmetric stretch wavenumbers match the experimental
The DRIFT spectra in the formate region of n˜ =1500 to
1800 cmꢀ1 on various PVP-Pd NPs is shown in Figure 2a. Ac-
cording to the assignment of formate species,[11] bands located
at n˜ =1657 (Figure 2a-i), 1691 (Figure 2a-ii), 1685 (Figure 2a-iii),
and 1683 cmꢀ1 (Figure 2a-iv) are attributed to a linear mode of
adsorbed formate (termed “linear formate”); the band at n˜ =
1568 cmꢀ1 (Figure 2a-i) is derived from an asymmetric bridge
mode of adsorbed formate (termed “bridge formate”), but no
Figure 3. Illustration of formate on the surface of ideal Pd cubes enclosed
with six Pd(100) surfaces, Pd atoms in gray, C atoms in darker gray, H atoms
in white, O atoms in red: a) multidentate formate; b) short and long bridge
formate species on Pd(100) with two closest surface PdꢀPd distances,
namely, 2.75 and 3.89 ꢁ, respectively (as compared to Pd(111), which shows
only a PdꢀPd distance of 2.75 ꢁ); c) linear formate. One oxygen atom of the
formic acid bonds with the top site: d) Pd(100), ꢀ0.29 eV; e) Pd(221),
ꢀ0.24 eV and Pd(33 1) (not shown), ꢀ0.02 eV to produce linear formate but
at different adsorption energies dependent on other interaction(s) of adsor-
bate moieties with Pd atoms in proximity.
Figure 2. a) DRIFT spectra of adsorbed formate species on the surface of
PVP-Pd NPs: i) Pd spheres (D=6.80 nm), ii) Pd cubes (D=4.73 nm), iii) Pd
cubes (D=6.92 nm), iv) Pd cubes (D=10.24 nm); b) DRIFT peak position of
linear formate on Pd spheres and cubes with comparable sizes; the progres-
sive change in linear and bridge formate species on Pd nanoparticles with
increasing size: c) Pd spheres and d) Pd cubes.
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cially on well-defined Pd cubes of large size, gives an
overall over 75% bridge species (two bridge forms).
Cyclic voltammetry (CV) was used to characterize
the samples. The results of CO stripping voltammetry
are shown in Figure 4a,b. The peak between 0.50
and 0.90 V was assigned to the oxidation peak of
preadsorbed CO on the Pd surface. The peak current
and peak area were found to decrease with increas-
ing size for both the spherical and cubic NPs. This is
not unexpected because of the larger surface area
Table 1. Adsorption energy (Ead) of formate and frequencies of the OꢀCꢀO asymmet-
ric stretching mode calculated on the basis of DFT modeling of formate species on
Pd(100).[a]
Adsorption
on Pd(100)
Ead
[eV]
OꢀCꢀO
Adsorption
on Pd(100)
Ead
[eV]
OꢀCꢀO
[cmꢀ1
]
[cmꢀ1
]
short-bridge formate 1
long-bridge formate 2
free formic acid
ꢀ0.59
ꢀ0.71
0
1587
1601
1747
linear formate 1
linear formate 2
linear formate 3
ꢀ0.29
0.04
ꢀ0.01
1637
1738
1733
[a] The corresponding structures can be found in the Supporting Information.
data) are reasonably stable at comparable energies. This sug-
gests their likely higher coverage and coexistence (Figure S4).
Note that two bands were also observed during temperature-
programmed decomposition (TPDe) of Pd cubes preadsorbed
with formic acid (Figure S6).
As shown in Figure 2a,b, the wavenumber of the OꢀCꢀO
stretching mode of formate in the linear mode is higher than
that of the bridge species for the two forms of the Pd nano-
particles. This is a result of a weaker degree of electron back-
donation, for which one oxygen atom of formate interacts
with a top surface metal atom hence to give a weaker adsorp-
tion energy. We note that there is a rapid shift to lower wave-
numbers as the size of the Pd spherical NPs rises from 3 to
5 nm, which is indicative of a stronger degree of electron back-
donation from the metal surface to the formate adsorbate.
However, the shift progressively levels off as the average size
of the spherical particle becomes greater than 5 nm. This
agrees with the fact that smaller nanoparticles significantly de-
viate from bulk metallic properties at sizes below 5 nm.[8,16]
Similarly, the open structure of the Pd(100) surface with fewer
Pd neighbors could reduce the local electron density and,
hence, give a wavenumber that is higher for the linear formate
on the Pd cubes than for the linear formate on the Pd spheres
in a comparable size range. The populations of linear formate
and bridge formate with increasing size of the spheres and
cubes are plotted in Figure 2c,d; it is revealed that the popula-
tion of bridge formate increases at the expense of the popula-
tion of linear formate with increasing size, despite their shape.
This fact is consistent with the TEM images shown in Figure 1,
for which both particles give higher degrees of facets at in-
creasing size. Comparatively, there appears to be a higher cov-
erage of linear formate on the spheres than on the cubes for
the same size range. According to our DFT calculations and
Figure 4. COads stripping voltammograms of a) Pd spheres, b) Pd cubes with
various sizes in 0.5m H2SO4 aqueous solution. Scan rate was 10 mVsꢀ1 and
only oxidation curve of the first cycles are shown; c) formic acid electrooxi-
dation activity on Pd spheres and cubes with various sizes; d) electrooxida-
tion peak position on NPs with various sizes in 0.5m H2SO4 and 1m HCOOH
aqueous solution, scan rate was 10 mVsꢀ1 and the oxidation current was
normalized to the ECSA of the Pd nanoparticles.
per gram of sample with smaller metal NPs.[17] The electro-
chemically active surface areas (ECSAs) were calculated on the
basis of the COad oxidation peaks and were used to normalize
the corresponding electrooxidation activities of formic acid.[18]
For spherical NPs, it is noted from Figure 4a that the broad
peak potential shifts from 0.65 to 0.67 V with increasing pro-
portion of higher voltage at increasing size. This is attributed
to the progressive change from linear to bridge CO at increas-
ing nanoparticle size.[6a] Similar to the adsorbed formate spe-
cies, electron backdonation from the metal sites results in
models, the adsorption energy and the wavenumber of the Oꢀ bridge CO species that are more strongly bound to the surface
CꢀO stretching mode of linear formate depend critically on
the metal site and environment (Figure 3 and the Supporting
Information). The additional interaction(s) of neighbor metal
sites in a specific plane, edge, or corner could interact with
other moieties of the linear formate, which would give a range
of linear modes of different stabilities (see modeling in Sup-
porting Information). The higher degree of surface roughness
and the higher proportion of high index sites on the spherical
form thus give a higher proportion of linear formate than
Pd(100) cubes with weaker adsorption energies. Conversely,
the higher proportion of Pd(100) terrace in the cubes, espe-
than the linear form, which hence gives a longer CꢀO stretch-
ing wavenumber[19] (Figure S8). Thus, the peak at 0.65 V pri-
marily corresponds to the electrooxidation of linear adsorbed
CO, and the peak at 0.67 V primarily corresponds to the elec-
trooxidation of the bridge form.[20] Similarly, the CO stripping
peak on the Pd cubic NP surface shows a significant greater
positive shift in sweeping potential with increasing NPs size.
This implies that the interaction between the CO and Pd sites
is further enhanced (0.72 V), as shown in Figure 4b. Binding of
bridge CO with Pd atoms on the Pd(100) terrace sites is stron-
ger than that on the corner and edge sites, and this was em-
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ployed in the shape-controlled synthesis of Pd nanostructures
under a CO atmosphere.[21] We calculated the adsorption
energy of bridge CO on Pd(100) to be ꢀ1.89 eV, which is
more stable than the adsorption energy of ꢀ1.56 eV on the
edge/corner sites. The broad peak observed in Figure 4b may
represent different bridge CO structures, particularly the forma-
tion of a shoulder at approximately 0.68 V for the largest Pd
cubes (9.63 nm), which was previously reported.[6c,22] Compar-
ing the sweeping peak potentials of both shaped NPs, that of
the Pd cubes is significantly higher than that of the spheres.
This reflects the higher adsorption energy of bridge CO on the
(100) facet of the cubes than that of the dominant stable
(111) facet associated with high-index sites (corner and edge)
in the spheres.[23]
stepwise mechanism to produce CO2 through a surface-assist-
ed dehydrogenation route [reactions (3) and (4)]. Comparing
the higher CO stripping potential required (0.70 V), the contri-
bution of the poisoning species route [indirect pathway, reac-
tions (5) and (6)] to the overall activity at 0.64 V is thought to
be minimal over the sphere samples (it is envisaged that there
is no significant quantity of CO formation from HCOOH on this
surface). However, it is interesting to note from Figure 3d that
the electrooxidation potential of formic acid on Pd cubic NPs
becomes higher than that on Pd spherical NPs at larger parti-
cle sizes, reaching 0.72 V whereby the strongly bound bridge
CO on the (100) surface of cubic NPs can also be readily oxi-
dized through reactions (5) and (6).[6f,25] Also, at comparable
bridge formate populations, the NPs adopting a cubic shape
displayed considerably higher activity than the spherical-
shaped NPs (Figures 2c,d and 4c), which is indicative of the
higher activity of the (100) surface on the cube form. It is
postulated that the long bridge formate population (IR at n˜ =
1572 cmꢀ1) showing a good correlation with the electrooxida-
tion activity on Pd cubes could be the precursor for subse-
quent indirect oxidation to CO2 by reactions (5) and (6)
through bridge CO adsorption. Further study is warranted to
investigate the structure–activity relationship of this specific
low metal terrace.
The electrooxidation of formic acid was studied to evaluate
the difference between the cubic and spherical NPs. PVP was
reported to influence the decomposition of formic acid.[19]
However, the effect of PVP was carefully kept insignificant. PVP
was removed by repeated cyclic voltammetry pretreatment
before our electrochemical experiments,[24] as shown in Fig-
ure S10. The activities of the cubic and spherical NPs were nor-
malized according to their ECSAs. It is accepted that one mole-
cule of formic acid can be electrochemically oxidized by two
pathways: a reactive intermediate [direct pathway, reaction (1)]
and poisoning species [indirect pathways, reactions (5) and
(6)], and the surface structure has a significant impact on the
reaction activity,[23,24] but the nature of the metal sites and
chemical species still remain obscure. The study of the electro-
oxidation of formic acid over our rationally grown Pd samples
was performed to shed light on these mechanisms, and the re-
sults are shown in Figure 4c,d. The activity increases with in-
creasing size for both forms. However, the activity of the
spherical NPs reaches a maximum of 252 mAcmꢀ2 and levels
off at a particle size of 5 nm. In the case of cubic Pd NPs, the
activity continuingly rises along with the particle size up to
10.2 nm (activity=396 mAcmꢀ2).[5a] These trends in activity–
size match very well with the bridge formate populations, as
shown in Figure 2c,d. This clearly suggests that bridge for-
mates on the surface of Pd NPs contribute to the activity in
a dominant way over the linear formates. As reflected from the
DFT calculations and the IR spectroscopy data, the adsorption
energies of strongly surface bound bridge formates on the ter-
races are far higher than the adsorption energies of weakly
bound linear formates despite their variations (see models in
the Supporting Information). It is thus clear that the activity for
the electrocatalytic oxidation of formates adsorbed to CO2 de-
pends much on their stability and surface coverage for the oxi-
dation to proceed. In this case, the bridge formate(s) on low-
index terrace(s) is the main contributor for this reaction,
whereas unstable linear formates on high-index sites are well
desorbed before further surface reactions can progress. It is
also well known that Pd nanoparticles can oxidize acidified
water to produce surface ꢀOH at approximately 0.6 V [reac-
tion (2)].[6c] Thus, the direct mechanism for formate to CO2 [re-
action (1)] appears to contribute mainly to the oxidation activi-
ty below 0.60 V. On the other hand, at the leveling potential of
0.64 V, bridge formate on spheres can also undergo an indirect
Pd þ HCOOH ! Pd þ CO2 þ 2 Hþ þ 2 eꢀ
Pd þ H2O ! PdꢀOHads þ Hþ þ eꢀ
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
PdꢀOHads þ HCOOH ! PdꢀHCOOads þ H2O
PdꢀHCOOads þ PdꢀOHads ! Pd þ CO2 þ H2O
Pd þ HCOOH ! PdꢀCOads þ H2O
PdꢀOHads þ PdꢀCOads ! Pd þ CO2 þ Hþ þ eꢀ
In conclusion, in this work we probed the size and shape ef-
fects of Pd nanoparticles in formic adsorption and CO and
formic acid electrooxidation. By combining diffuse reflectance
infrared Fourier transform spectroscopy, DFT calculations, and
cyclic voltammetry, bridge formates on terraces were identified
to be the main contributors to the electrooxidation activity of
formic acid over Pd nanoparticles. Moreover, Pd(100) was
found to be able to stabilize at least two bridge formate spe-
cies on the surface of Pd cubic nanoparticles, which contribut-
ed to the higher electrooxidation activity of the cubic nanopar-
ticles relative to that of the spherical nanoparticles at compara-
ble sizes.
Experimental Section
PVP-Pd NP synthesis and process
PVP-stabilized Pd spherical NPs were synthesized by using PVP as
a capping agent, ethylene glycol as the solvent, and l-ascorbic
acid as the reducing agent. The size of these nanoparticles was
tuned by adjusting the PVP/metal ratio, reaction temperature, and
reducing agents. The metal precursor (Na2PdCl4) was fixed at
0.114 g for all samples. For a typical synthesis process, ethylene
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glycol (EG, 34 mL) was mixed with poly(vinylpyrrolidone) (PVP,
Mw=30000), and the mixture was then heated to a fixed tempera-
ture followed by fast injection of an aqueous solution of Na2PdCl4
(2 mL) containing Na2PdCl4 (0.114 g), and the whole solution was
maintained for approximately 30–120 min. PVP-stabilized Pd cubic
NPs were synthesized by using a procedure based on Xia’s
method.[7] In a typical reaction, Na2PdCl4 (0.114 g) was mixed with
ascorbic acid (0.180 g), added as a reducing agent, and PVP (see
the Supporting Information) was also added as a capping agent.
To induce the formation of {100} facets, KBr and KCl were used as
shape controllers. After completely dissolving in water (36 mL), the
solution was kept at 808C under stirring for 180 min. Upon com-
pletion of the reaction, the dark brown solution was first precipitat-
ed in acetone (volume ratio acetone/solution=3:1) by 12000 rpm
for 10 min, then redissolved in water, followed by precipitation in
acetone again; the wash process was repeated ten times to
remove PVP, Brꢀ, and Clꢀ.
mented wave (PAW) potentials were used for the core electron in-
teraction. The Perdew–Burke–Ernzerhof (PBE) functional based on
the generalized gradient approximation (GGA) was employed to
evaluate the nonlocal exchange-correlation energy. A plane wave
basis set with a cutoff energy of 350 eV was used. For structure op-
timization, the ionic positions were allowed to relax until the
forces were less than 0.05 eVꢁꢀ1. Spin polarization was included
for all the calculations.
To estimate the adsorption energy, the following expression was
considered [Eq. (7)]:
Eads ¼ ꢀ½EslabþmolꢀðEslab þ EmolÞꢁ
ð7Þ
in which Eslab and Emol are the total energies of the Pd slab and the
single adsorbate molecule in the gas phase, whereas Eslab+mol de-
notes the total energy of the slab with the adsorbed molecule.
More details of the reaction temperature, reducing agent, PVP/
metal ratio, and reaction time can be found in the Supporting In-
formation.
Vibrational frequencies of surface-adsorbed intermediates were cal-
culated by applying the finite-difference method to create the Hes-
sian matrix, which was diagonalized to obtain the characteristic fre-
quencies of the system. An atomic displacement of 0.05 ꢁ was
used in the three directions of space. During the frequency calcula-
tion, as the coupling between molecular vibrations and surface
phonons of adsorbate was negligible, all the other atoms were not
allowed to move except for the adsorbates. More details are avail-
able in the Supporting Information.
Characterization
The FTIR spectra of the PVP-Pd NPs were measured with a Bruker
Optics, Vertex 70. Diffuse reflectance infrared Fourier transform
(DRIFT) accessory was provided by Pike PIKE Technologies, the PIKE
MIRacle. Tests were taken at room temperature (258C) and were
protected by a N2 atmosphere during the whole process. A PVP-Pd
NPs sample (10 mg) was diluted with potassium bromide (KBr,
200 mg), and then was filled into the DRIFT Al2O3 cell. Samples
were pretreated under high-vacuum conditions (3.00ꢂ10ꢀ3 Pa) at
1008C for 1 h to remove adsorbed molecules, such as water. Hy-
drogen gas was introduced later to reduce PdO for another 1 h.
After the cell was cooled down to room temperature, formic acid
was introduced by using a flow control valve to saturate the sur-
face. During saturation, FTIR spectra were monitored to avoid mul-
tilayer adsorption. Resolution of all spectra was set at 2 cmꢀ1, and
the scan time for all samples was 128 times by using a MCT detec-
tor refrigerated with liquid nitrogen. During the measurements,
the detector was refilled with liquid nitrogen every 4 h. The data
acquired was processed by using OPUS 6.5 software to correct
baseline.
Acknowledgements
This work was supported partially by the Engineering and Physi-
cal Sciences Research Council (EPSRC), UK; Fundamental Re-
search Funds for the Central Universities of China; National Sci-
ence Foundation of China (NSFC-20903074, 21243014); and Sci-
ence Foundation of Jiangsu Province (BK2012638). W.Z. and J.Q.
are grateful for scholarships from the China Scholarship Council
(CSC) for studying at the University of Oxford.
Keywords: density functional calculations · electrooxidation ·
morphology control · palladium · surface probe
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Electrochemical experiments were done with a Compactstat elec-
trochemical interface provided by IVIUM technologies. The Pd NPs
(30 mg) were dispersed in ethanol (1 mL) and were then sonicated
for 3 h to ensure full dispersion. Meanwhile, a glassy carbon (GC)
electrode (electrode inner diameter is 3.0 mm) was polished with
0.3 mm alumina slurry and then rinsed with double distilled water
in an ultrasonic bath. Then, the above mixture (20 mL) was dropped
on the glassy carbon electrode and then fully dried in air at room
temperature. For all tests, the reference electrode was a saturated
calomel electrode (SCE), and a Pt wire was used as the counter
electrode. Before running the electrochemical test, 20 cycles of CV
were run between ꢀ0.2 and 1.0 V to remove adsorbed PVP on the
PVP-Pd NPs surface. After pretreatment, the working electrode was
used for further CO stripping and the formic acid electrooxidation
test. More details are available in the Supporting Information.
Theoretical calculations
All calculations were performed in the framework of DFT by using
the Vienna ab initio simulation package (VASP). The projector-aug-
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Communications
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Received: July 10, 2015
Published online on && &&, 0000
&
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ÝÝ These are not the final page numbers!

COMMUNICATIONS
DRIFTing away: Various structures of
adsorbed formate species on a Pd cubic
nanoparticle surface are identified by
diffuse reflectance infrared Fourier
transform spectroscopy analysis. Their
coverage and adsorption strength can
affect the activity of the electrooxida-
tion of formic acid. Pd(100) cubes are
able to stabilize two bridge formate
species and thus show higher electro-
oxidation activity than Pd spheres at
comparable sizes.
W. Zheng, J. Qu, X. Hong, K. Tedsree,
S. C. E. Tsang*
&& – &&
Probing the Size and Shape Effects of
Cubic- and Spherical-Shaped
Palladium Nanoparticles in the
Electrooxidation of Formic Acid
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ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ