Two-step synthesis of Ir-decorated Pd nanocubes and their impact on the glycerol electrooxidation

Article abstract of DOI:10.1016/j.jcat.2019.07.042

Despite intense research to synthesize well-ordered noble-metals nanoparticles to understand catalyzed electrochemical reactions, the synthesis and application of multi-metallic well-ordered nanomaterials is still rare. Here, we report a facile strategy for synthesizing Ir-decorated Pd nanocubes (NCs) in one-batch through two sequential chemical reduction processes. Afterwards, the activity of these materials was investigated for glycerol electrooxidation in alkaline medium by cyclic voltammetry and in situ FTIR. Microscopy images and compositional mapping reveal that Ir deposition barely changes the size of the cubes. The bi-metallic nanocubes shifts the onset potential toward lower values, but decreases the current density. The start potential displacement is rationalized as a consequence of a facilitated CO removal and an anticipated carbonate production; while the decrease in current may be a consequence of a reaction with multiple steps, each one extracting a few electrons. FTIR in situ spectra reveal CO₂ production only for Ir-Pd NCs, which is due to a massive production of carbonate, which ultimately acidifies the interfacial pH. The protocol of synthesis showed here may be used to study well-ordered multimetallic nanostructures in several applications, not limited for electrocatalysis.

Full text of DOI:10.1016/j.jcat.2019.07.042

Journal of Catalysis 377 (2019) 358–366
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Two-step synthesis of Ir-decorated Pd nanocubes and their impact on
the glycerol electrooxidation
c
a
a,
Cinthia R. Zanata a, Cauê A. Martins b, , Érico Teixeira-Neto , Martha J. Giz , Giuseppe A. Camara
⇑
⇑
a Institute of Chemistry, Universidade Federal do Mato Grosso do Sul, CP 549, 79070-900 Campo Grande, MS, Brazil
b Institute of Physics, Universidade Federal do Mato Grosso do Sul, CP 549, 79070-900 Campo Grande, MS, Brazil
c Laboratorio Nacional de Nanotecnologia, CNPEM, C.P. 6192, CEP 13083-970 Campinas, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Despite intense research to synthesize well-ordered noble-metals nanoparticles to understand catalyzed
electrochemical reactions, the synthesis and application of multi-metallic well-ordered nanomaterials is
still rare. Here, we report a facile strategy for synthesizing Ir-decorated Pd nanocubes (NCs) in one-batch
through two sequential chemical reduction processes. Afterwards, the activity of these materials was
investigated for glycerol electrooxidation in alkaline medium by cyclic voltammetry and in situ FTIR.
Microscopy images and compositional mapping reveal that Ir deposition barely changes the size of the
cubes. The bi-metallic nanocubes shifts the onset potential toward lower values, but decreases the cur-
rent density. The start potential displacement is rationalized as a consequence of a facilitated CO removal
and an anticipated carbonate production; while the decrease in current may be a consequence of a reac-
tion with multiple steps, each one extracting a few electrons. FTIR in situ spectra reveal CO2 production
only for Ir-Pd NCs, which is due to a massive production of carbonate, which ultimately acidifies the
interfacial pH. The protocol of synthesis showed here may be used to study well-ordered multimetallic
nanostructures in several applications, not limited for electrocatalysis.
Received 10 May 2019
Revised 16 July 2019
Accepted 21 July 2019
Keywords:
Two-step synthesis
Pd nanocubes
Iridium
Glycerol electrooxidation
Interfacial pH
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
chemical composition applied in electrocatalysis. Special attention
has been payed to the development of nanocatalysts for glycerol
The possibility to control the size and shape of nanometer-sized
colloidal crystals enabled a variety of fields in applied science, from
nanomedicine [1] to electrocatalysis [2–4]. The size-controlled
synthesis allows for the production of specific surfaces, possibly
mimicking bulk crystal surfaces [5]; meaning that an active crystal
may be reproduced in nanoscale as nanocrystals. The different
atomic surface arrangement leads toward different pathways in
terms of surface reaction [6–8], specially in electrocatalysis of
small-chain organic molecules, in which the adsorption and further
electron transfer are highly dependent on the surface structure.
Other important characteristic of a surface reaction is the chemical
composition, which is easily tuned by classic batch reaction of non-
controlled shape, but it is still a drawback for scaled multimetallic
well-ordered nanoparticles, since the composition is sensitive to
thermal gradients [9].
electrooxidation. This alcohol is a major (and unavoidable) bypro-
duct of biodiesel production, and has been investigated as potential
fuel for fuel cell [10–14] and a possible matrix for electrosynthesis
[15–19]. The potential use of glycerol to feed anodes of fuel cells
relies on the presence of three hydrated carbons, which are already
at the first stage of the oxidation process; however, the complete
conversion into CO2 is limited under practical applied potential
[12,20]. Still, strategies have been developed to use such alcohol
as a fuel, for instance in low-power energy converters microfluidic
fuel cells (mFC) and [21,22] high power mixed-media mFC
[10,11,13]. The use of glycerol to feed electrolyzers is economically
advantageous, since this low-market-priced matrix can be turned
into high valuable compounds, as dihydroxyacetone, pyruvic acid
and others [23–26], whose prices may be orders of magnitude
higher [27]. Regardless the use of glycerol in electrochemical sys-
tems (even in chemical systems, i.e. reforming), the activity or
selectivity is highly dependent on the surface crystallinity and
chemical composition.
Well-ordered bulk surfaces have been used as platforms to pre-
dict the design of nanoparticles in terms of shape and surface
In this context, there is a variety of possible metallic catalysts,
usually based on a noble one, such as Pt, Pd and Au. Among these
metals, Pd-based materials have attracted attention due to its high
⇑
Corresponding authors.
E-mail addresses: caue.martins@ufms.br (C.A. Martins), giuseppe.silva@ufms.br
(G.A. Camara).
https://doi.org/10.1016/j.jcat.2019.07.042
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

C.R. Zanata et al. / Journal of Catalysis 377 (2019) 358–366
359
activity in alkaline medium [28–31]. However, so far a few works
reported the use of Pd NCs as catalysts for glycerol electrooxidation
reaction [19,28,29]. Zalineeva et al. reported the synthesis and
application of Bi-decorated Pd NCs through electrochemical [28]
and spectroelectrochemical measurements [29]. The authors found
the activity of Pd is improved after Bi spontaneous adsorption
while the production of hydroxyacetone is favored at low poten-
tials [29]. Guima et al. synthesized Pd NCs and used them as elec-
trocatalysts for glycerol electrooxidation in alkaline medium [19].
These authors found Pd NCs are selective to the production of tar-
tronate after long-time electrolysis, whose path was induced by
extended (1 0 0) surface domains. In other work, Zhou et al. syn-
thesized Pd NCs and Pt@Pd NPs as catalysts for electrooxidation
of glycerol in alkaline solutions [32]. Pd NCs led the reaction
toward glyceraldehyde, while glycolic acid was produced on Pt@Pd
[32]. Xu et al. prepared binary PdPb alloy NCs with improved elec-
trocatalytic activity compared to commercial Pd/C [33].
Ir is a metal with with alleged electrocatalytic properties, simi-
lar to those observed for Pd and Pt [34] and its use as a cocatalyst
improves the activity and stability of carbon-supported Pt NPs
[35]. Nevertheless, there is no works reporting the synthesis of
well-ordered Ir NPs nor their use for glycerol electrooxidation. A
possible strategy to investigate Ir as a cocatalyst is through the
decoration of Pd seeds with Ir [36,37] but these binary nanostruc-
tures have never been used in electrocatalysis of alcohols.
of a CO monolayer. For such experiments, CO was bubbled into
the solution during 5 min with the working electrode polarized
at 0.1 V. The excess of CO was removed by bubbling N2 during
30 min, when the CO was finally stripped by sweeping the applied
potential toward 1.1 V.
FTIR in situ experiments were carried out in a Perkin-Elmer
spectrometer with a MCT detector, equipped with a optic accessory
VeeMax II. An electrochemical cell specially designed to accommo-
date an CaF2 window at the bottom was used with the same three
electrodes. The experiments were performed in reflectance mode
in the range of 3000–1000 cmꢀ1 collected as the ratio (R/R0) where
R is the spectrum at a given potential and R0 is the reference one at
0.1 V. A complete series of spectra was collected from 100 to
1200 mV at 5 mV sꢀ1 at intervals of 50 mV with resolution set to
8 cmꢀ1
.
3. Results and discussion
This work is divided in two parts, the synthesis and characteri-
zation of Ir-decorated Pd NCs and the application of such materials
for glycerol electrooxidation.
3.1. Synthesis of Ir-decorated Pd nanocubes
Considering the lack of reports about well-ordered Pd-based
nanoparticles contrasting with its potential application, here we
thoroughly detailed the process of a reproducible two-step synthe-
sis of Ir-decorated Pd NCs. Moreover, to move a step forward
toward the use of well-ordered multimetallic nanocrystals, we
investigate the activity of such materials for glycerol electrooxida-
tion reaction in terms of electrocatalytic parameters and by in situ
FTIR to correlate the extent of the reaction with the chemical com-
position. Our results show that Ir-decorated Pd NCs facilitate the
reaction, as a consequence of CO removal and carbonate produc-
tion at lower potentials. The production of carbonate is fast enough
to consume large amounts of hydroxyls and change the interfacial
pH (from alkaline to acidic).
The Pd NCs were synthesized in two successive steps at the
same batch, without transfer or waiting time. The first step was
the growth of Pd NCs, followed by its decoration with Ir. The syn-
thesis of Pd NCs was adapted from a previous report [38]. Namely,
a 10 mM H2PdCl4 aqueous solution was prepared by adding
0.1443 g PdCl2 to 10 mL 0.2 M HCl in 100 mL D.I. water, and used
as precursor solution. Next, 5.0 mL of this solution were added to
a round-bottom flask containing 100 mL 12.5 mM cetrimonium
bromide (CTAB), which acted as capping agent. The mixture was
kept under reflux and stirred for 5 min after reaching 95 °C. At this
stage, some cations were already reducing, starting the formation
of a few Pd seeds, and the solution kept the initial brown color,
as illustrated in the scheme of Fig. 1. Afterwards, 1 mL of
100 mM ascorbic acid was added to grow the Pd NCs during
30 min. At this stage, the cubes were formed and the solution
turned into black.
2. Experimental
To decorate the Pd NCs with Ir, an additional 1 mL 100 mM
ascorbic acid was added to the reactional flask, followed by
2 mM IrCl3 aqueous solution. It is important to state that the con-
trolled decoration is only achieved if the IrCl3 solution is carefully
added in aliquots of 100 mL each 5 min, totalizing 50 min. After-
wards, the solution was kept at 95 °C for additional 10 min, before
cooling to room temperature. The volume of IrCl3 solution was
The composition of the materials was investigated by an EDX
detector coupled to a scanning electron microscopy JEOL model
JSM 6380-LV. The morphology was investigated by transmission
electron microscopy JEOL JEM2100 with a LaB6 filament operating
at 200 kV. The Ir-adatom deposition was studied on Pd nanocubes
by compositional maps using a JEOL-JEM 2100F transmission elec-
tron microscope from the Nanotechnology National Laboratory
(LNNano) (CNPEM, Campinas, SP). The materials were imaged
using the high angle annular dark field (HAADF) technique. X-ray
energy dispersive spectrum imaging technique (XEDS-SI) was
employed with a Digital Micrograph 1.8 system (Gatan, Inc.) con-
trolling a Thermo-Noran XEDS.
adjusted to obtain nominal atomic mass compositions of Pd90Ir10
,
Pd70Ir30 and Pd NCs mostly covered by Ir, here nominally called
PdxIry. Fig. 1 illustrates the decoration of the Pd NCs through a
selected image of a NC and its elemental mapping using XEDS-SI.
The image clearly shows that Ir is deposited over the Pd surface.
After being synthesized, metallic NPs contain excess of capping
agent and remnants of the synthesis on their surface, which com-
promises its further use in (electro)catalysis, since such adsorbates
preclude the adsorption of reactants [39,40]. Therefore, one of the
main steps during the synthesis of useable nanomaterials in (elec-
tro)catalysis is the cleaning process, which is often neglected in sci-
entific reports. After cooled, NaOH pellets were added into the
solution in order to precipitate the NCs, as reported by Vidal-
Iglesias et al. [41]. It is important to note that NaOH pellets must
be added until the precipitation, although there is not a precise
amount of those. Hence, some pellets must be added at a time,
while the solution is kept at rest, until total precipitation. Finally,
the NCs were collected and washed with D.I. water and kept
The electrochemical measurements were performed in a con-
ventional three-electrodes electrochemical cell.
A reversible
hydrogen electrode was used as reference (RHE), a high surface
area Pt plate as counter electrode and a modified-glassy carbon
as working electrode. 50 mL of the catalytic ink was dropped onto
a 0.8 cm2 polished glassy carbon in order to build a film with a the-
oretical loading of 6.25 mg cmꢀ2. We used a potentiostat/galvanos-
tat Metrohm Autolab (model 12874). The measurements were
performed in O2-free 1 mol Lꢀ1 NaOH in the absence or presence
of 1 mol Lꢀ1 glycerol. Stripping of CO was used for both, as reaction
probe and to determine the electrochemically active surface area
considering 420 mC cmꢀ2 as the charge involved in the desorption

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C.R. Zanata et al. / Journal of Catalysis 377 (2019) 358–366
Fig. 1. Illustrative scheme of the synthesis. Palladium nanocubes are previously formed and then modified with iridium.
stationary to precipitate again. This process is repeated 10 times.
The dispersion is concentrated to 30 mL of D.I. water and used as
a catalytic ink for electrochemical experiments. Compared to the
synthesis previously reported [36,37], the present protocol is car-
ried out in milder temperature and finished in less than two hours
(excepting the cleaning process). Moreover, our method is carried
out sequentially with no need to clean and storage the Pd NCs prior
to their decoration. On the other hand, those authors found smaller
NPs than those synthesized here (further discussed), which is also
important due to the increase in area. Besides these technical dif-
ferences, the Ir@Pd NCs synthesized by Xia et al. were not charac-
terized by electrochemical measurements, although their materials
present remarkable catalytic activity [37].
The chemical composition was estimated by MEV-EDX as an
average of two different regions of the samples. The real atomic
compositions are Pd97Ir3, Pd95Ir5 and Pd88Ir12 corresponding to
Pd90Ir10, Pd70Ir30 and PdxIry of nominal composition respectively,
detailed in Table S1. Since the protocol described here intends to
decorate already synthesized Pd NCs instead of producing alloys,
it is expected their real compositions to be different from the
respective nominal ones. Most important, the Ir coverage degree
increases with the concentration of Ir precursor. Further consider-
ations regarding the surface chemical composition will be dis-
cussed together with the electrochemical results.
The morphologies of the decorated NCs were investigated by
TEM, as shown through representative images for each material
in Fig. 2. The cubic shape is well-defined and the good efficiency
of the synthesis is evidenced due to the high density of cubes, with
very low presence of nanoparticles of other forms. Fig. 2A shows Pd
NCs, whose form is not affected by the Ir ad-atom decoration, as
seen in Fig. 2B–D. The NCs have sizes between 22 and 24 nm with
a narrow size distribution, as shown in Fig. S1. The NCs synthesized
here are twice the size of those synthesized by Lim et al. [42],
which can be a consequence of the ratio stabilizer/cations used
in the synthesis. The ratio PVP/Pd+ used by Lim et al. [42] is twice
the ratio CTAB/Pd+ used here.
The decorated NCs have similar sizes (Fig. S1), suggesting that
the Ir atoms are bound to the Pd surface to build a very thin layer
and/or to form a low-density dendritic surface. These features can
be easily realized by investigating higher resolution images, seen
by the insets in Fig. 2C and D. The Ir deposition is barely seen on
Pd97Ir3 and Pd95Ir5, as show the insets of Fig. 2B and C, respectively.
On the other hand, the ad-atom deposition is obvious in the inset
in Fig. 2D, suggesting higher amounts of Ir on Pd88Ir12. The rough-
ness estimated for Pd88Ir12 suggests the growth of dendrites.
The Ir deposition on the surface of Pd NCs is also demonstrated
by elemental mapping using XEDS-SI. HAADF images from Pd97Ir3
(Fig. 3A–C) and Pd95Ir5 (Fig. 3D–F) are selected to illustrate the Ir
ad-atom deposition. The elemental maps (Fig. 3) show the distribu-
tion of the measured Ir X-rays over the Pd NCs, showing successful
decoration by Ir. Moreover, the two-step method allows low
quantities of Ir, which is in agreement with the fact that the
non-decorated and decorated particles have the same size. It is
worth noticing that the changes in terms of quantitative surface
composition are very difficult to identify, since the selected
image may not be representative of the average. In this sense, these
images are only used to prove the decoration of Pd NCs with Ir and
the in situ electrochemical measurements (further commented) are
used to enlighten such surface changes.
The in situ investigation of a well-ordered metallic surface has
been considered a powerful tool to characterize surface arrange-
ments of potential catalysts [3,43,44]. The micrographs shown in
Fig. 3 highlight evidences of Ir decoration on small regions of the
samples, which is an unescapable technical condition. On the other
hand, the use of cyclic voltammetry allows the surface to be
accessed as a whole. Furthermore, the electrocatalyst is tested
under conditions similar to those of operation (i.e., in the presence
of the reactants). This widely diffused technique was used to char-
acterize the Pd NCs synthesized in this work. Fig. 4A shows a
voltammetric profile of Pd NCs in alkaline medium. In general,
there are two well-defined regions: (i) there is a region where
hydrogen is adsorbed and desorbed on the metallic catalyst (high-
lighted in yellow background); (ii) at higher potentials there is a
region where surface oxides are formed and reduced (highlighted
in cyan background). Both regions show specific signals sensitive
to different surface arrangements [45].
In general, the profile shown in Fig. 4A is similar to those for Pd
with extended (1 0 0) surface [46], corroborating the TEM images,
since cubes are mainly constituted of planes with (1 0 0) geometry.
This result confirms that the cleaning process does not modify the
crystallinity of the nanoparticles. Namely, the peak at 0.55 V dur-
ing positive potential going scan is a result of hydrogen desorption
on wide (1 0 0) terraces [28]. The formation of oxides on those sur-
face sites display peaks at 0.75 and 0.85 V [28]. In the reverse scan,

C.R. Zanata et al. / Journal of Catalysis 377 (2019) 358–366
361
Fig. 2. Representative images of TEM for nanocubes of Pd (A), Pd97Ir3 (B), Pd95Ir5 (C) and Pd88Ir12 (D).
Fig. 3. Representative HAADF-STEM image and XEDS-SI elemental mapping of Pd97Ir3 (A–C) and Pd95Ir5 (D–F).

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C.R. Zanata et al. / Journal of Catalysis 377 (2019) 358–366
3.2. The impact of Ir-decorated Pd nanocubes on the glycerol
electrooxidation
After years struggling to understand the surface reaction of
glycerol on noble metal surfaces using electrochemical methods
with isotopically labeled molecule [20,47], spectroelectrochemical
measurements [12,48,49] and DFT calculations [5,43], it seems rea-
sonable to admit that CO plays a complex, still not clear, role on the
reaction, working as an intermediate, reactional pair and also
blocking the surface for further adsorption of reactants. Based on
those studies, it seems consensual that a strong bond between
CO and the catalytic surface is deleterious for the electrooxidation
of glycerol [5,43]. Therefore, the voltammograms recorded in the
presence of glycerol (Fig. 4E–H) are interpreted in parallel to those
observed for the stripping of CO (Fig. 5A–D).
Glycerol electrooxidation on Pd NCs is shown in Fig. 4E. An oxi-
dation peak is centered at 0.9 V and other at 0.7 V during the
Fig. 4. In the left column, the electrochemical behaviour in 1 mol Lꢀ1 NaOH and in
the right, stable cyclic voltammograms obtained in the presence of 1 mol Lꢀ1 NaOH
+ 1 mol Lꢀ1 glycerol for (A, E) Pd, (B, F) Pd97Ir3, (C, G) Pd95Ir5 and (D, H) Pd88Ir12
.
Voltammetries performed in the range 0.1–1.2 V, at 0.02 V sꢀ1
.
a cathodic peak centered at 0.71 V corresponds to the reduction of
Pd oxides while a large peak centered at 0.22 V is a consequence of
hydrogen adsorption. The intense peaks at 0.55 V and 0.85 V com-
pared to that at 0.75 V suggest well-ordered NCs with low density
of defects.
The voltammetric profiles of the Ir-decorated Pd NCs are shown
in Fig. 4B–D. It is clear how the hydrogen underpotential deposi-
tion (HUPD) region changes for each material (yellow background).
The peak at 0.55 V from the hydrogen desorption on Pd(1 0 0) sites
is completely suppressed with low Ir content, as shown for Pd97Ir3
in Fig. 4B. Furthermore, a redox couple at ꢁ0.21 V is revealed with
the increase of Ir content, evidenced by comparing Fig. 4B and D.
This redox coupled is reported to be due to the adsorption/desorp-
tion of hydrogen on Ir(1 0 0) surface [34]. This evidence suggests
that Ir atoms grow on the Pd surface following the arrangement
of the template, meaning that the (1 0 0) order is kept in some
extent. We ascribe such phenomenon to the slow Ir decoration
process, meaning that this synthesis protocol may be a further
alternative to control ad-atoms deposition on well-ordered
surfaces.
The Ir deposition also affects the oxide formation/reduction on
Pd (cyan background in Fig. 4B–D). The peak at 0.85 V of Pd NCs
(Fig. 4A) decreases in presence of minor amounts of Ir (Fig. 4B)
and vanishes for higher coverage degrees (Fig. 4C and D). A similar
effect, but less obvious, is observed on the peak located at 0.71 V,
as a consequence of the increase in Ir content, because the forma-
tion of Ir oxides does not take place at this potential. Since Ir dec-
oration is a surface process without influence in the crystalline
structure of the NCs, it is reasonable to expect that even few
amounts of Ir lead to high coverage degrees, with huge changes
on the voltammetric profiles. Next, we show how Ir influences
the activity of Pd NCs for glycerol electrooxidation in alkaline
medium.
Fig. 5. First (black voltammogram) and second (red voltammogram) cycles of CO
electrooxidation reaction in NaOH 1 mol Lꢀ1 for Pd (A), Pd97Ir3 (B), Pd95Ir5 (C) and
Pd88Ir12 (D). Voltammetries performed in the range 0.1–1.2 V, at 0.02 V sꢀ1
.

C.R. Zanata et al. / Journal of Catalysis 377 (2019) 358–366
363
reverse scan. It is well-know that the surface is reactivated during
the negative potential going scan, after the reduction of surface
oxides, when it becomes available to adsorb remnant species (or
their fragments). Moreover, the potential in which the reactivation
occurs depends on the potential window [50]. The onset potential
(Eonset) is precisely found by the derivative voltammetry dj/dE, pro-
posed by Marthy and Manthiram [51] and used for glycerol elec-
trooxidation by some of these authors [52,53]. The comparison
between the voltammograms and their respective derivative
voltammograms is shown in Fig. S2. The Eonset is displaced towards
lower potentials whereas Ir decoration increases, from 0.71 V on
pure Pd to 0.56 V on Pd88Ir12, respectively (Fig. S2). Thus, Ir decora-
tion anticipates the reaction by 150 mV. Now, comparing these
results with the stripping of CO (Fig. 5A–D), it is clear that CO elec-
trooxidation is facilitated in presence of Ir. Namely, a low Ir content
barely changes the oxidation profile (Compare Fig. 5A and B) while
those surfaces richer in Ir depict an oxidation peak at lower poten-
tials (Fig. 5C and D). Therefore, an easier CO removal may be one of
the reasons for the anticipated Eonset on the Pd NCs modified with
Ir. On the other hand, the current density decreases with the
increase of Ir (Fig. 4E–H), suggesting that the alternative path
responsible for the displacement of Eonset involves fewer electrons
than the original one. It is worth noticing that the cyclic voltammo-
grams collected before and after the glycerol electrooxidation are
similar in some cases or present better-defined peaks. This result
indicates that either (i) the surface arrangements of the nanomate-
rials are maintained even after the reaction or (ii) that the surface
reaction cleans the surface. The second hypothesis is equally plau-
sible, since the CO, which is formed during the GEOR, cleans the
metallic surface during the stripping [54].
(cyan background). First, we need to clarify the production of
CO2 in alkaline medium, since CO23ꢀ is expected as the product
of total glycerol electrooxidation. The complete electrooxidation
of the alcohol consumes OHꢀ, following Eq. (1). Thus, a massive
production of CO23ꢀ implies in a fast consumption of OHꢀ, decreas-
ing the interfacial pH. Consequently, the reaction is led to medium
exchange, and starts to be held in acid medium, producing CO2
accordingly [55].
ꢀ
2ꢀ
ꢀ
+ 14 H2O + 14e
CH2OH-CHOH-CH2OH + 20OH ! 3CO3
ð1Þ
Additionally, the bands at 1048 and 1111 cmꢀ1 are ascribed to a
C-O stretching from primary and secondary alcohols [56]. Gomes
et al. also identified these bands by ex situ FTIR spectroscopy in a
glycerol solution, which were later associated to those bands
observed during the electrooxidation of glycerol by in situ experi-
ments [56]. Differently, in this work, the band at 1111 cmꢀ1 is neg-
ative, suggesting formation of such alcohols, which is a probable
consequence of glycerate formation [57]. Narrowly, it is also possi-
ble to conjecture the production of glycolate through the band at
1076 cmꢀ1 [55]. Other two important bands are: an intense one
located at 1585 cmꢀ1 ascribed to carboxylate ions, as a conse-
quence of carbonylic compounds in alkaline medium, and another
less intense band at 1666 cmꢀ1, result of protonated carbonyl com-
pounds, which could be due to the change in pH. It is worth notic-
ing that the identification of products requires chromatographic
techniques (or similar) and it is out of the scope of this work. Here,
we analyze the influence of the catalyst on the reaction in general.
The growing of the CO band is particularly evident for the
higher Ir content, reinforcing that Ir surfaces promote the scission
of glycerol molecules. In turn, the band ascribed to CO reaches a
peak at intermediate potentials and then decreases as the produc-
tion of CO2 accelerates, suggesting that CO is the intermediary spe-
In order to shed a light on the role of Ir on such reaction paths,
we performed in situ spectroscopy experiments. A complete series
of in situ FTIR spectra collected from 100 to 1200 mV obtained at
5 mV sꢀ1 (the scan was paused at each 50 mV for the collection
of spectra at a constant potential) is shown in Fig. 6. Here, we will
cies required for the formation of CO2, at least for Pd88Ir12
.
look closely to two main bands, one centered at 1390 cmꢀ1
,
The ratio between the bands at 1390 cmꢀ1 (CO32ꢀ) and at
1585 cmꢀ1 (carboxylates) is bigger for the catalysts containing Ir
than that found for pure Pd NCs. Hence, it suggests that
ascribed to the formation of carbonate (CO23ꢀ) (yellow background),
and another at 2343 cmꢀ1, associated to the production of CO2
Fig. 6. In situ FTIR spectra obtained in NaOH 1 mol Lꢀ1 + glycerol 1 mol Lꢀ1 on: Pd (A), Pd97Ir3 (B), Pd95Ir5 (C) and Pd88Ir12 (D).

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C.R. Zanata et al. / Journal of Catalysis 377 (2019) 358–366
Ir-decorated catalysts lead the reaction towards an advanced
oxidation state, producing more CO23ꢀ than carbonyl compounds.
However, the current density produced by the decorated catalysts
is lower than that generated on Pd NPCs, which seems controver-
sial. This empirical evidence was once found by comparing ethanol
with glycerol electrooxidation on Pt [48]. The authors found glyc-
erol is oxidized toward stepped sequential reactions, where each
step releases a few electrons [48]. It seems Ir behaves similarly.
affected by the applied potential until ꢁ0.8 V, when CO23ꢀ reaches
a maximum. After this inflexion point, the amount of CO32ꢀ
decreases whereas CO2 increases. This finding reveals that there
is a competition between alkaline and acidic medium at the inter-
facial electrode/solution until ꢁ0.8 V, oriented by the OHꢀ con-
sumption. After the peak potential, the OHꢀ consumed is so high
that the interface turns into acid. Differently from this work, Zali-
neeva et al. identified the CO2 band on Pd NCs in alkaline medium
during glycerol electrooxidation, which was achieved by using low
concentrations of NaOH and glycerol (0.1 mol Lꢀ1 for both) [29].
Thus, the absence of CO2 production on Pd NCs found here is a con-
sequence of the high NaOH concentration (1 mol Lꢀ1), so the excess
of OHꢀ at the thin layer is only completely consumed with an
active catalyst, such as Ir-decorated Pd NCs. In parallel, high con-
centrations of glycerol inhibit the formation of oxygen species on
the surface of catalyst, delaying the advance of the reaction [49].
Primarily, this result shows that the interfacial pH is not
affected for the reaction on Pd NCs, but is highly sensitive to Ir
sites. The implication of such findings may enlighten the under-
standing of the stability of the catalyst, since an alkaline electrolyte
is a more severe environment than the acidic one for a metallic NP
[58]. Therefore, the lack of ability of Pd NCs to lead the reaction
toward the complete oxidation may lead to instability of nano-
cubes in alkaline medium, as previously reported in measurements
in supporting electrolyte [59]. In this sense, the Ir decoration may
be a further strategy to change the interfacial pH. Regarding some
assessments on the pathway of reaction, and according to the pro-
posed routes for the oxidation of glycerol [27,56], the presence of Ir
seems to favor the initial formation of glyceraldehyde through the
adsorption and oxidation of a carbon atom containing a primary
alcohol, which can lead to the formation of CO and CO23ꢀ as final
reaction product. On the other hand, Pd seems to induce dihydrox-
yacetone formation by preferential oxidation of the secondary
alcohol. Thus, the presence of Ir seems to influence on the adsorp-
tion step of glycerol on the surface of the catalysts.
Fig.
7 shows the comparison of production of carbonate
(1390 cmꢀ1), carboxylate (1580 cmꢀ1) and CO2 (2343 cmꢀ1) for
the different materials. Ir-decorated Pd NCs turned glycerol into
carbonate at lower potentials than pure Pd NCs, which is the prob-
able reason for the shift on the Eonset. Moreover, CO2 is not pro-
duced in detectable quantities on Pd NCs, whereas it is detected
(although the signals are low) at low potentials on Ir-decorated
materials. For these materials, the production of CO2 is barely
In summary, regarding the role of Ir covering Pd NCs applied as
catalyst in glycerol electrooxidation, the ad-atom facilitates the
reaction by shifting the onset potential towards lower values,
which is a consequence of an improved CO tolerance and an antic-
ipation of carbonate formation. The carbonate formation is high
enough to consume OHꢀ and to turn acid the interfacial pH accord-
ingly. However, the current density decreases, which may be a con-
sequence of stepped oxidation paths, extracting few electrons per
step. More generally, we have demonstrated how to easily design
Ir-decorated Pd NCs, allowing investigation of Ir deposited on
well-ordered metallic templates. The synthesis in batch reactors
reported here allows for the multiple use of the materials, not only
applied in electrocatalysis; differently from those decoration
methods in which an electrode previously modified with well-
ordered nanoparticles is decorated in situ. Hence, this protocol
may be used to decorate Pd NCs with other metals, advancing in
nanodesign techniques.
4. Conclusions
We proposed a simple two-step synthesis of Ir-decorated Pd
nanocubes carried out at the same batch, without the need to
transfer the reaction content nor waiting time. The protocol
involves mild temperature (ꢁ90 °C) and short time (<2h)
-
excepting the time of the cleaning process. The decoration induces
a thin layer of deposition, since the average size is barely changed
compared to the well-ordered template, evidenced by ex situ
microscopy investigation. The in situ electrochemical characteriza-
tion of the materials shows that the Ir deposition on the Pd(1 0 0)
Fig. 7. Band height as a function of the potential. Data obtained from FTIR spectra
recorded in NaOH 1 mol Lꢀ1 + glycerol 1 mol Lꢀ1
.

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indirectly determined by following the carbonate and CO2 band
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C. R. Zanata is indebted to CAPES for a fellowship. The authors
acknowledge CNPq (Grants # 308821/2016-5, 309176/2015-8,
406779/2016-3 and 454516/2014-2), FUNDECT (Grants
#
026/2015 and #099/2016), and FINEP. We also thank LNNano-
CNPEM (Campinas-SP, Brazil) for the use of the electron micro-
scopy facilities. This study was financed in part by the Coordenação
de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brasil,
finance code 001.
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Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.07.042.
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