Highly porous palladium nanodendrites: Wet-chemical synthesis, electron tomography and catalytic activity

Article abstract of DOI:10.1039/c9dt00107g

A simple procedure to obtain highly porous hydrophilic palladium nanodendrites in one-step is described. The synthetic strategy is based on the thermal reduction of a Pd precursor in the presence of a positively charged polyelectrolyte such as polyethylenimine (PEI). Advanced electron microscopy techniques combined with X-ray diffraction (XRD), thermogravimetry and BET analysis demonstrate the polycrystalline nature of the nanodendrites as well as their high porosity and active surface area, facilitating a better understanding of their unique morphology. Besides, catalytic studies performed using Raman scattering and UV-Vis spectroscopies revealed that the nanodendrites exhibit a superior performance as recyclable catalysts towards hydrogenation reaction compared to other noble metal nanoparticles.

Full text of DOI:10.1039/c9dt00107g

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V. Montes García, S. Rodal-Cedeira, N. Naomi Winckelmans, I. Perez-Juste, H. Wu, S. Bals, J. Perez-Juste
and I. Pastoriza-Santos, Dalton Trans., 2019, DOI: 10.1039/C9DT00107G.
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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Highly Porous Palladium Nanodendrites: Wet-chemical Synthesis,
Electron Tomography and Catalytic Activity
Stefanos Mourdikoudis,a,b,c Verónica Montes-García,a Sergio Rodal-Cedeira,a Naomi
Winckelmans,d Ignacio Pérez-Juste,a Han Wu,e Sara Bals,d Jorge Pérez-Juste,a* and Isabel
Pastoriza-Santosa*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A simple procedure to obtain highly porous hydrophilic palladium nanodendrites in one-step is described. The synthetic
strategy is based on the thermal reduction of a Pd precursor in the presence of a positively charged polyelectrolyte such as
polyethylenimine (PEI). Advanced electron microscopy techniques combined with X-ray diffraction (XRD), thermogravimety
and BET analysis demonstrate the polycrystalline nature of the nanodendrites as well as their high porosity and active
surface area, facilitating a better understanding of their unique morphology. Besides, catalytic studies performed using
Raman scattering and UV-Vis spectroscopies revealed that the nanodendrites exhibit a superior performance as recyclable
catalysts towards hydrogenation reaction compared to other noble metal nanoparticles.
www.rsc.org/
crystallite diameter exhibited the highest catalytic activity for CO2
reduction to CO.9 This catalytic effect was explained, based on DFT
calculations, due to the presence of an optimum ratio of edge sites
Introduction
Metals are one of the most important catalytic systems for
that favours the adsorption of reaction intermediates.
chemical transformation.1, Petroleum refining, fuel cells, and
production of chemical intermediates, pharmaceuticals and
agrochemicals are some examples of using metals as catalysts. As
most of the catalytic reactions occur on metal surfaces, metals at
nanometer scale are preferred due to their high surface/volume
ratio. Over the years, lots of studies have been focused on the
improvement of the catalytic activity of metallic nanoparticles.3-5 The
dominant factors that may affect the catalytic efficiency can be
divided into four categories: composition, size and morphology of
the nanoparticles and nature of the active sites. The development of
different nanotechnology areas has contributed to the fast advance
in the fabrication of nanoparticles (monometallic, alloys and hybrids)
with full control over their properties. In the field of catalysis, Pd, Au,
Pt, Ru and Ag nanoparticles (NPs) have been already extensively
2
Non-spherical nanoparticles are enclosed by a certain number of
facets and presenting edges (the intersection of facets) and corners,
the intersection of two edges. The edges and corners are constituted
by atoms of low-coordination number; therefore those sites have
larger surface energies. It is generally accepted that nanoparticles
with low coordinated surface atoms show a higher catalytic activity
than those with a higher coordination number. Consequently, it is
important to optimize not only the size of the nanoparticle to obtain
a high surface area but also the number of low-coordination sites
(such as edges and corners).10, 11 Alternatively, metal nanoparticles
with high index facets, meaning a set of Miller indices {hkl} with at
least one index greater than 1, present a high density of terraces,
steps and kinks (constituted as well by low-coordination sites).12, 13
These high index facets can serve as intrinsically more active sites for
catalysis.14 For example, single-crystal surfaces of bulk Pt possessing
high index planes exhibit higher catalytic activity than other single-
crystal surfaces with more stable, low-index planes like {111} and
{100}.15 So far most of the metal nanoparticle catalysts reported
presents a solid core. But from a structural point of view a porous
particle will lead to an increase in the surface area, in comparison
with a solid one, due to the presence of an internal surface in
addition to the external surface. Furthermore, such configuration
could produce an increase in the surface atoms with a low
coordination number in comparison to that for a solid structure,
leading generally to an improved catalytic activity.10 For instance,
dendritic-like nanoparticles which consist of highly branched arm-
composed structure, and therefore can be considered as porous
particles, potentially exhibit high catalytic activity thanks to their high
surface area and to the presence of low coordinated sites in high
densities on their branches.16-20 In summary, both parameters, that
exploited, achieving good performance in a large variety of organic
transformations.1,
dimensions possess a higher number of surface atoms, they do not
always show the best performance.9 For example, Zhu et al. found
that among 4, 6, 8, and 10 nm Au NPs, 8 nm Au NPs with 4 nm
6-8
Although nanoparticles with smaller
a. Department of Physical Chemistry and Biomedical Research Center (CINBIO),
Universidade de Vigo, 36310 Vigo, Spain. E-mail: juste@uvigo.es,
pastoriza@uvigo.es
b. Biophysics Group, Department of Physics and Astronomy, University College
London, London, WC1E 6BT, UK.
c. UCL Healthcare Biomagnetic and Nanomaterials Laboratories, 21 Albemarle
Street, London W1S 4BS, UK.
d. EMAT-University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
e. Centre for Nature Inspired Engineering (CNIE), Department of Chemical
Engineering, University College London, Torrington Place, London, WC1E 7JE, UK
Electronic Supplementary Information (ESI) available: Additional TEM images and
EDX characterization, FTIR, catalytic studies and supporting information videos. See
DOI: 10.1039/x0xx00000x
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Figure 1. Representative TEM images of PdND1 (A) and PdND2 (B).
is, surface area and density of low-coordination sites are strongly Pd nanocrystals synthesis and characterization. Two different types
intertwined, and unless the particle shape and crystallinity is of Pd nanodendrites were fabricated using a one-pot wet chemical
perfectly defined both should be taken into consideration at the method. The methodology developed is based on our previous
same time.
experience on the synthesis of platinum nanodendrites and
nanoflowers related to the use of polymers and high boiling point
In this work, we In this work, we present a one-pot wet chemical
strategy to fabricate hydrophilic and highly porous Pd nanodendrites,
based on the thermolytic reduction of palladium salts in a high
boiling point solvent, such as N, N-dimethylformamide (DMF) or
diethylene glycol (DEG), in the presence of polyethylenimine (PEI).
Two types of Pd nanodendrites with different porosity and surface
area were obtained. The obtained nanodendrites have been
analysed by a number of techniques such as high resolution angle
annular dark field scanning transmission electron microscopy
(HAADF-STEM), 3D tomography, X-ray diffraction (XRD),
thermogravimetry and BET analysis in order to obtain information
about their morphology, crystallinity as well as the porous structure.
Furthermore, we have performed a detailed study of their catalytic
activity based on the hydrogenation of 4-nitrophenol (4-NP) to 4-
aminophenol (4-AP). Interestingly, we have demonstrated that the
reaction process could be followed by Raman spectroscopy which
solvents which allows to apply
a wide range of synthesis
temperatures, thus modifying their reducing capacity.16, 21 In the
present case we studied the reduction of palladium acetylacetonate
(Pd(acac)2) or potassium tetrachloropalladate (K2PdCl4) in DMF or
DEG, respectively, at 150 ºC in the presence of a positively charged
polyelectrolyte, such as PEI (see experimental section for details).
While the combination of Pd(acac)2 and DMF led to Pd
nanodendrites, hereafter PdND1, with an average diameter of 37.4 ±
5.3 nm (see representative TEM images in Figure 1A and Figure S1 in
the Supporting information), the reduction of K2PdCl4 in DEG resulted
in larger nanodendrites (51.8 ± 7.5 nm), PdND2, with an apparently
more compact structure (see Figure 1B and Figure S2 in the SI).
Moreover, the ζ-potential analysis shows highly positive ζ-potentials
for both samples (47.0 ± 0.8 mV for PdND1 and 54.9 ± 0.9 mV for
PdND2) indicating the presence of PEI as capping ligand at the
nanoparticle surface.
allowed to monitor the reaction on
a preparative scale
demonstrating the applicability of our catalytic platform in In order to perform a 3D structural analysis, the Pd nanodendrites
synthetically useful procedures. The results illustrate that the were characterized by electron tomography based on high angle
reported Pd nanodendrites present a higher catalytic performance annular dark field scanning transmission electron microscopy
towards hydrogenation reactions compared to other noble metal (HAADF-STEM), revealing interconnected arms branching in various
nanoparticles, such as, Pt, Ag or Au. Additionally, the recyclability orientations for both samples. Figures 2A-B show high resolution
tests after several cycles reveal no loss in catalytic activity.
HAADF-STEM images of PdND1 (Figure 2A) and PdND2 (Figure 2B) and
their corresponding 3D surface visualizations (in yellow). At first sight
it can be observed that the dendritic morphology in the PdND2
sample is larger in size and more compact compared to those in the
PdND1 sample, which are smaller and more porous. Supporting
information videos, SV1 and SV2, show a movie of the 3D
reconstructions of both type of particles. The 3D reconstructions
were performed using the Discrete Algebraic Reconstruction
Technique (DART) as implemented in the ASTRA toolbox.22-24 Using
this technique, segmentation is performed during the reconstruction
and as a result, the so-called missing wedge artefacts that limit the
amount of information obtainable from the object are significantly
reduced in the reconstruction, yielding more reliable quantitative
results.25 Thus the segmented volume can be used to estimate the
Results and discussion
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active surface area and the porosity. For this purpose, a part in the difference in weight loss between both samples (35% and 10% at 600
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middle of the reconstructed volume was considered, an example is ºC for PdND1 and PdND2, respectively) suggDesOtIi:n1g0.t1h0e39p/rCe9sDeTn0c0e1o0f7Ga
provided at Figure 2C. This segmented volume contains two different higher amount of PEI electrolyte at the first type of dendrites. The
types of voxels: those that belong to Pd and empty voxels or pores. different porosities obtained by electron tomography were also
The surface area of the dendrite was calculated as well as the volume reflected by their corresponding pore volume and surface area
of the dendrite and the pores. The porosity is defined as:
values obtained by BET (see Table 1 and Figures S4 and S5). Thus,
PdND1 present a higher pore volume than PdND2 (0.096 vs 0.067 cm3
g-1). Additionally, the BET measurements yielded surface area values
of 35.3 m2 g-1 and 17.3 m2 g-1 for PdND1 and PdND2, respectively. The
higher BET-surface area for PdND1 than PdND2 is in agreement with
what one would expect for a material with higher porosity and pore
volume. In fact, the considerably lower BET-surface area for PdND2
in comparison to its electron tomography-calculated surface area
may be attributed to the following reasons: in BET analysis, the N2
sorption technique requires the accessibility of the pores/surfaces
that allows transport of the gas molecules; therefore the results can
only be interpreted to the accessible pores and surfaces. This can be
in principle different from the TEM analysis when closed
(inaccessible) pores can also be identified. For BET, mesopores
(2<d<50 nm) can account for much of the surface measured by this
technique. Also BET won’t be able to include macropores during
measuring. Finally, BET, despite its limitations, is a macroscopic
technique, running using around 100 mg of powder of each sample.
This includes a vast number of nanodendrites. Electron tomoraphy,
pore
porosity =
pore + dendrite
This procedure was repeated for different volumes for the same
dendrites. Electron tomography was performed on five different
nanodendrites for each type of nanodendrites (see Table S1 in the SI
for a detailed description of the porosity and surface area measured
for each of the analyzed particles). Table 1 shows the average
porosity and surface area measured for PdND1 and PdNPD2. The
difference in porosity between the nanodendrites is also reflected by
the active surface area values estimated per gram of Pd (Table 1).
Thus, an active surface area per gram of ca. 3.7 times and ca. 5.3
times larger for PdND1 and PdND2, respectively, than their
corresponding spherical and compact (solid) counterparts is derived
(see Table 1 and SI for a detailed description of the surface area
estimation). Further insights on the surface area of the samples at a
macroscopic level were acquired through thermogravimetric analysis
(TGA) and BET analysis. The TGA analysis (Figure S3) reveals a clear
B
A
C
Figure 2. Representative high resolution HAADF-STEM images of PdND1 (A) and PdND2 (B). On the right side in yellow: 3D
visualization of the tomographic reconstruction along different viewing directions of the two types of Pd nanodendrites. See
supporting information videos for animated versions. (C) Representative 3D visualisation of a central part of the tomographic
reconstructed volume used for the calculation of the active surface area and the porosity (see text for details).
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Figure 3. High resolution HAADF-STEM images showing the branches of different Pd nanodendrites oriented along [100] (A)
and [110] and [111] (B) directions.
being precise in certain aspects, is a microscopic technique, thus half-maximum using the (111) peak in all cases. The obtained values
limited in a small number of individual dendrites.
were 7.8 nm and 4.8 nm for PdND1 and PdND2, respectively. These
values are much smaller than those derived by TEM, which implies
that the Pd nanocrystals are composed of multiple crystallographic
domains, in accordance with the high resolution HAADF-STEM
observations (Figure 3A-B). All peaks depicted at the XRD
measurements were found to shift by ~ 0.5o toward lower angles as
compared to bulk Pd. This indicates that the Pd-Pd interatomic
distance increases after formation of the Pd nanocrystals (a sign of
lattice expansion), which is in agreement with previous reports.29
By inspecting the dendrites at the atomic scale, it can be observed
that their branches are oriented along multiple crystallographic
directions, thus reflecting some degree of polycrystallinity (Figure 3).
However, by analyzing different particles it was observed that there
is a preferential orientation of the branches along the [111] direction.
Furthermore, from the images in Figure 3, the face centered cubic
(fcc) crystal structure of Pd can be confirmed. Additional
crystallographic data were obtained by XRD measurements (Figure
S6). XRD patterns recorded for the two samples show characteristic Regarding the formation mechanism of Pd NDs, such morphology
peaks which can be assigned to the [111], [200], [220], [311], and indicates a kinetically controlled process where factors such as
[222] reflections of the fcc structure of metallic Pd (JCPDS card #05- solvent, capping agent, temperature and Pd precursor may play
0681). The relative intensities between the different peaks are critical roles. In our case, the quick appearance of black color during
similar to those reported for bulk fcc Pd due to the random synthesis evidences that the rate at which the newly formed metal
orientations of the Pd nanoparticles on the supporting substrate.27 atoms are supplied is relatively high. After the formation of the Pd
Moreover, the ‘macroscopic’ XRD screening of the different nuclei at initial stage of reaction, the high reduction rate of the Pd
palladium nanocrystals indicates an abundance of {111} planes in precursor might favour that the adatoms deposited on a specific
accordance with the high resolution HAADF-STEM analysis of the region of the Pd surface (often the most active site) do not migrate
PdNDs. The mean crystalline grain size of the Pd samples was to sites with lower free energy but they remain where deposition
28
calculated using the Scherrer’s formula from the peak width at occurs. This situation could explain the dendritic particle growth.16,
Table 1. Average porosities and surface areas obtained by electron tomography and pore volumes and surface areas
determined by BET for PdND1 and PdND2.
Porosity
(%)b
Active Surface
Area (nm²)b
Active Surface
Area (m² g-1)
Pore Volume
(BET) (cm3/g)
Surface Area
(BET) (m² g-1)
Size (nm)a
35.3
PdND1
PdND2
37.4 ± 5.3
49.4 ± 6.6
8354 ± 1915
50.1c/13.3d
0.096
17.3
51.8 ± 7.5
39.4 ± 4.0
26826 ± 4566
50.6c/9.6d
0.067
a Particle diameter estimated assuming a spherical geometry. b Average values obtained from analysing five different
particles for each type of nanodendrite.c Estimated surface area per gram of Pd nanodendrites, calculated as explained
in Ref. 26 d Estimated surface area assuming a spherical geometry and a solid particle.
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2.0
1.5
1.0
0.5
0.0
B
A
5Kcts
5Kcts
300
400
500
500
750
1000 1250 1500
Raman shift (cm-1)
Wavelength (nm)
Figure 4. (A) UV-Vis and (B) Raman scattering spectra in liquid state of 4-NP- (black line) and 4-AP- (red line) at pH 11. Raman
scattering spectra were collected with a 532 nm laser line, 15X objective, 24.5 mW and 10 s for 4-NP (10-2 M) or 30 s for 4-
AP- (0.1 M).
30, 31
Unfortunately, although both approaches give rise to All the catalytic studies were performed at pH 11-12 for two main
nanodendrites with well-defined size and different porosity, all reasons; first, to enhance the stability of sodium borohydride
efforts to control both parameters through systematic variation of avoiding its hydrolysis and formation of hydrogen as by-product. The
parameters such as metal precursor and concentration, surfactant second reason has to do with the fact that the Raman and absorption
concentration or temperature, among others, were not sucessful. It cross sections of 4-NP- are higher than those for the 4-NP form.
resulted in either very compact/aggregated structures or more
Similarly, 4-AP at basic pH evolves to 4-aminophenolate (4-AP-)
porous structures with very polydisperse, inhomogeneous sizes and
displaying an absorption band centered at 300 nm (see Figure 4A).
shapes. Thus, we restricted oursellves to the study of the catalytic
Besides, the measured Raman scattering spectrum of 4-AP- is mainly
performance of these two types of particles and then we compared
dominated by ring deformation (469 cm-1), ring twisting deformation
with results found in the literature for other catalysts.
(645 cm-1), ring breathing (848 cm-1), NH2 wagging (848 cm-1), NH2
Catalytic performance of Pd nanocrystals. The catalytic twisting (1172 cm-1), ring deformation and CCH bending (1261 cm-1)
performance of both kinds of Pd nanodendrites was studied for the and C=O stretching and CCH bending (rocking) (1619 cm-1) (see
hydrogenation of 4-NP to 4-AP by sodium borohydride. It is well Figure 4B). A detailed description of the vibrational assignments of 4-
known that 4-NP constitutes an especially harmful component of AP- can be found in the Supporting Information, Figure S8 and Table
industrial wastewater, on the other hand 4-AP is very useful and S3.
important in many applications including analgesic and antipyretic
In summary, both the reactant (4-NP-) and the product (4-AP-) can be
drugs, photographic developers, corrosion inhibitors, among
easily distinguished by either UV-Vis or Raman scattering
others.32, 33 Thus, the development of a cost effective, stable, and
spectroscopy. In order to evaluate the applicability of Raman
highly efficient catalyst for the reduction of nitro-compounds to
scattering spectroscopy to study the kinetics of the 4-NP-
amino-compounds under mild environments is greatly desirable.
hydrogenation we have estimated the detection limits of 4-NP- and
Prior to the catalytic studies we have performed the characterization 4-AP- which are dependent on their corresponding Raman scattering
of 4-NP and 4-AP by UV-Vis and Raman scattering spectroscopies cross section . As shown in Figure S9 in the SI, the limit of detection
(see Figure 4 and Figures S7 and S8 in the SI). First we analyzed the for 4-NP is 10-4 M displays a good linear correlation between the
influence of the pH on the absorption and Raman scattering Raman intensity and its concentration in the 10-2 M to 10-4 M range.
spectrum of 4-NP. As shown in Figure S7A, while at pH 5 the molecule On the other hand, the estimated Raman cross section of 4-AP- is
presents the main absorption band at 320 nm, at basic pH a more significantly lower since its limit of detection is only 5 mM. Thus, the
intense band at 400 nm is observed corresponding to the formation kinetics of the 4-NP- to 4-AP- reduction can be easily followed by
of 4-nitrophenolate (4-NP-). In the case of the Raman scattering Raman spectroscopy by simply monitoring the decrease in the
spectrum of 4-NP (Figure S7B), it- is mainly dominated by ONO characteristic bands of 4-NP-.
bending (858 cm-1) and asymmetric CCH bending (1116, 1172, 1292
and 1339 cm-1) signal.34 Besides it should be pointed out that a
change in pH from 5 to 11 principally led to an inversion of the bands
located at 1292 and 1339 cm-1 (see Figure S7B) as well as to increase
of most of the Raman signals. Assignment of the observed Raman
bands was performed on the basis of DFT calculations (see
experimental section and Figure S8 and Table S2 in the SI).
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A
PdND1
NaBH4
C
B
5Kcts
3Kcts
Time
kapp= 8.12 10-3 s-1
400 600 800 1000 1200 1400
0
100 200 300 400 500
Raman shift (cm-1)
Time (s)
Figure 4. (A) Schematic representation of 4-NP- reduction to 4-AP-, (B) Spectral evolution of a mixture of 4-NP- and PdND2
upon borohydride addition. [4-NP-]= 8.2 mM, 0.13 mg of PdND2, [NaBH4]= 77 mM and [NaOH]= 10 mM , T= 25 C using
o
Raman scattering spectroscopy. (C) Kinetic trace of the Raman intensity at 1292 cm-1 during the reduction of 4-NP- by NaBH4
in the presence (open circles) and in the absence (open squares) of PdND2, the red line represents the best fit to a first order
rate constant.
Although sodium borohydride is regarded as a strong reducing agent, min for PdND1 and PdND2, respectively. It should be noted that a
it can be considered as inert for the 4-nitrophenol reduction in the large excess of NaBH4 with respect to 4-NP- was used (molar ratio of
absence of a metal catalyst (see Figure 5C). The addition of Pd NaBH4 to 4-NP- of 7:1) and it can be assumed to be constant during
nanodendrites to the reaction medium led to gradual decrease of the the whole reaction and allows us to treat the kinetics of the catalytic
characteristic 4-NP- Raman bands (see Figure 5B and S10A for PdND2 process as a pseudo-first order reaction. Therefore, the overall
and PdND1, respectively), indicating the catalytic conversion from 4- reaction rate can be written as
NP- to 4-AP-. The kinetic trace of the reaction is illustrated in Figure
d[4NP] = kobs S NaBH4 [4NP
―
[
]
―
]
4C (PdND2) and Figure S10B (PdND1) in the SI, where the Raman
intensity of the asymmetric CCH bending at 1292 cm-1 is represented
versus reaction time. Thus a rapid decrease of the Raman intensity is
observed until the complete reactant disappearance after 15 or 10
(1)
dt
where kobs is the observed pseudo-first order rate constant, S is the
metal catalyst surface, [4-NP-] is the concentration of 4-NP- at a given
time t and [NaBH4] is the sodium borohydride concentration. It can
be simplified to
[
] = kapp [4NP - ]
d 4NP
-
(2)
dt
where kapp is the apparent first-order rate constant of the reaction.
(a) Raman 4-NP-
Figure 4C and S10B show the fit of the kinetic trace to a first-order
rate equation (see the experimental section), which yields an
observed rate constant of 3.95 × 10−3 s−1 for PdND1 and 8.12 × 10−3
s−1 for PdND2.
(b) Catalysis t = 0s
Under the Raman experimental conditions used and considering the
low Raman cross section of 4-AP- the main peak of 4-AP- (1262 cm-1,
ring deformation and CCH bending) produced during the reaction
kinetics was hardly distinguished. Alternatively, once the reaction
finished, the Raman spectrum was refined by increasing the
acquisition time (60 s) and number of accumulations (10), in such a
way that the characteristic 4-AP- showed up. Figure 5 shows the
Raman spectra of commercially available 4-NP- and 4-AP- and the
corresponding Raman spectra of the reaction mixture at time 0 and
1500 s. The good agreement between the different spectra reveals
that all 4-NP- has been converted into 4-AP- since none of the
(c) Catalysis t = 1500s
(d) Raman 4-AP-
400
600
800 1000 1200 1400
Raman shift (cm-1)
Figure 5. Raman scattering spectra of commercial 4-NP- (a),
catalytic reaction at t= 0 (b) and t= 1500 s (c) and commercial
4-AP- (d).
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characteristic signals of 4-NP- is present in the spectrum recorded at both types of palladium nanodendrites. Moreover, the consistency
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the end of the reaction.
of the calculated apparent rate constant, kapp, alongside the different
additions/cycles (5 in total) of 4-NP- (8.12 × 10−3 s−1, 9.93 × 10−3 s−1,
8.01 × 10−3 s−1,7.35 × 10−3 s−1 and 6.52 × 10−3 s−1 for PdND2, see Figure
7) and (2.13 × 10−3 s−1, 3.95 × 10−3 s−1, 3.69 × 10−3 s−1, 3.30 × 10−3 s−1
and 3.95 × 10−3 s−1 for PdND1, see Figure S10 in the SI) is a clear
evidence for the remarkable stability of the PdNDx in the reaction
medium, as well as the reproducibility of their catalytic activity.
Commonly, the deactivation of catalytic efficiency during reusability
of colloidal catalysts, has been attributed to the aggregation of the
metal nanoparticles causing a decrease in the active catalytic surface
area 37. In our case, the high catalytic reusability can be ascribed to
the high stability provided by the positively charged polymer PEI
even at high ionic strength. The PdND1 nanoparticles were observed
by TEM after the catalytic cycles and, as shown in Figure S13 in the SI
the reduction of 4-NP- did not affect the dendritic morphology.
Additionally, in order to check the reusability of the particles in terms
of recovery we analysed the catalytic activity after several
centrifugation and redispersion cycles (see Figure 6B and
experimental section for details). It can be observed that although
the conversion was complete, there is a slight decrease in the
efficiency, in terms of rate constant, with the different cycles which
can be attributed most likely to nanoparticle loss during the washing
cycles rather surface poisoning since the addition of more reactant
did not lead to a decrease in the observed rate constant (see Figure
6A).
As pointed out previously the in situ Raman measurements allowed
us to follow the kinetics of the reaction on a preparative scale. We
have also compared the obtained results with a most common and
standardized methodology, such as UV-Vis spectroscopy. Under our
synthetic conditions the progress of the reaction cannot be followed
by UV-Vis spectroscopy due to the high absorption cross section of
4-NP-. Alternatively, different aliquots of the reaction mixture (20 µL)
were diluted in 400 µL of Milli-Q water to quench the reaction and to
allow to measure the UV-Vis spectra. The full reduction of 4-NP- by
NaBH4 was completed within 20 min since the complete
disappearance of 4-NP- absorption band at 400 nm was observed.
Such a process can also be visually appreciated by the gradual change
of the originally yellow-green color solution to a colorless one. Figure
S11 in the SI shows the UV-Vis time-resolved spectra of the reaction
where the absorption band at 400 nm decreases while two new
bands at 230 nm and 300 nm characteristics of 4-AP- appear. Similarly
to the Raman measurements the experimental conditions allows us
to treat the kinetics of the catalytic process as a pseudo-first order
reaction (see experimental section), which yields an observed rate
constant in agreement to that determined by Raman spectroscopy
(see Figures S10 and S11 in the SI).
In order to compare the catalytic activity of the Pd nanodendrites
with previous reports, the normalized rate constant (knor) was
determined, which was obtained by normalizing kapp with respect to
the total amount of catalyst (m) and also the borohydride
concentration ([NaBH4]),
kapp
knor
=
(3)
(m [NaBH4])
In the present case the metallic Pd content was determined by ICP-
OES spectroscopy to be 0.04 mg and 0.13 mg for PdND1 and PdND2,
respectively. Thus, the knor value obtained for PdND1 and PdND2 is
1274.4 and 787.5 g−1 s−1 M−1, respectively. This value is greater than
the knor estimated for other nanoparticle-based catalysts, such as Pd
black (23 g−1 s−1 M−1), Pt nanoflowers (233.3 g−1 s−1 M−1),30 Pt black
(69 g−1 s−1 M−1),31 Au@citrate (27.6 g−1 s−1 M−1)31 or Ag (68.9 g−1 s−1
M−1)32 and Au (77.5 g−1 s−1 M−1)33 dendrites (see Table S4 in the SI),
indicating their superior performance. Interestingly, it should be
pointed out that between the two different types of Pd
nanodendrites presented here, the smaller ones (PdND1) show a
catalytic activity that is 1.62-fold times higher than the more
compact nanodendrites (PdND2). Such increase in catalytic activity
could be attributed to its higher degree of porosity and BET-surface
area as well as to the presence of more surface defects and kinks.
To test the recyclability of the Pd nanodendrites as catalysts, the
nitro to amine conversion was repeated several times using the same
colloidal dispersion, through the sequential addition of 4-NP- to an
aqueous solution containing excess sodium borohydride and a
constant concentration of the catalyst. Figure 6 and Figure S12 in the
SI show the plot of the Raman intensity at 1292 cm-1 vs. reaction time
for PdND2 and PdND1, respectively, as well as a fit to a first-order rate
equation for each 4-NP- addition. As it can be seen from the plots,
the first-order kinetic behavior was perfectly reproducible in both
cases, as predicted by equation 2. The catalyst reusability with a yield
of 100% 4-NP- hydrogenation indicated no loss in catalytic activity for
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sodium borohydride (NaBH4, 99%) were purchased from Sigma-
View Article Online
DOI: 10.1039/C9DT00107G
Aldrich. Diethylene glycol (DEG, 99.98%) was provided from Fischer
Sci. 4-aminophenol (4-AP, 98%) was supported by Alfa Aesar. 4-
nitrophenol was purchased from Fluka. Pure grade ethanol, acetone
and Milli-Q water were used as solvents. All glassware was rigorously
cleaned in aqua regia before use.
Synthesis of Pd nanocrystals. The palladium nanocrystals utilized in
this work were prepared as follows:
a) Pd nanodendrites, PdND1, were prepared as follows: 860 mg of
PEI were introduced in a three-neck flask containing 30 ml of DMF
under stirring. After 20-min stay at room temperature, the mixture
o
was heated up to 130 C. At this point, 0.1 mmol of solid Pd(acac)2
were rapidly added in the above solution. The reaction mixture
turned black at once, indicating the formation of nanoparticles. The
flask was then heated to 150 oC for 1 h in air. After cooling to room
temperature, the product was precipitated with excess ethanol, and
washed three times with ethanol, acetone and water successively
(8500 rpm, 15 min). The nanodendrites were finally dispersed into
deionized water (30 mL).
b) Pd nanodendrites, PdND2, were synthesized with the next
protocol: in
a three-neck flask and under magnetic stirring
conditions, 15 ml of DEG and 15 ml of OAm were inserted followed
by the addition of 860 mg of PEI. The mixture was stirred for 20 min
at room temperature. Afterwards, 0.3 mmol of K2PdCl4 were
introduced. The resulting mixture was further stirred for a few
minutes, before being rapidly heated (~12 oC/min) to 150 oC for 1 h,
in air. The product was then left to cool naturally at room
temperature. Precipitation and washing steps were carried out
similarly to the case of the PDND1. The final product was stored in
water, 30 mL.
Figure 6. (A) Raman kinetic traces at 1292 cm-1, registered
during the sequential reduction of 4-AP- using PdND2 as
catalyst. The arrows indicate the times at which 4-NP- was
added to obtain [4-NP-]= 2.75 mM and [NaOH]= 10 mM. The
line represents the best fit to a first-order rate equation in
each cycle. (B) Reusability test of both type of Pd
nanoparticles upon centrifugation and redispersion cycles.
Conclusions
Characterization of Pd nanocrystals. Conventional TEM images were
recorded using a JEOL JEM 1010 microscope operating at an
acceleration voltage of 100 kV. Electron tomography experiments
were carried out using a Tecnai G2 Electron Microscope, operated at
200 kV. All tilt series were acquired in HAADF-STEM mode, in order
to avoid any unwanted diffraction contrast. For the acquisition, a
Fischione model 2020 single tomography holder was used and the
series were acquired over a tilt range from -76° to +76° and using an
increment of 2°. Tomographic reconstructions were performed using
In summary, highly porous multi-crystalline Pd nanodendrites with a
relatively narrow size distribution have been synthesized through a
one-step procedure via the thermally assisted reduction of palladium
salt precursors in the presence of PEI as surfactant. The nanoparticles
were readily dispersed into water after synthesis and washing stages.
HAADF-STEM and electron tomography analysis allowed to discern
their particular three-dimensional morphology revealing a high
degree of porosity and active surface area. The obtained Pd
nanodendrites present a higher catalytic performance towards
hydrogenation reactions compared to other noble metal
nanoparticles, such as, Pt, Ag or Au. Finally, the recyclability tests
after several cycles reveal no loss in catalytic activity.
the Discrete Algebraic Reconstruction Technique (DART) as
23
implemented in ASTRA toolbox.22,
Using this technique,
segmentation is performed during the reconstruction and as a result,
the so-called “missing wedge artefacts” are significantly reduced in
the reconstruction, yielding more reliable quantitative results25 High
resolution HAADF-STEM images of the particles were acquired using
a double aberration corrected cubed FEI Titan 50-80 electron
microscope operated at 300 kV. XRD powder measurements were
carried out after drying some quantities of the colloidal solutions of
the samples. A Siemens D-5000 diffractometer with a CuKα radiation
(1.54059 Å) was used for these measurements. FTIR spectra were
recorded using a Nicolet 6700 FTIR spectrometer with a resolution of
4 cm-1 using previously dried powder of palladium nanocrystals. The
surface charge (reflected in ζ-potentials) of Pd nanocrystals was
evaluated through electrophoretic mobility measurements by taking
the average of five measurements at the stationary level, using a
Conflicts of interest
There are no conflicts to declare.
Experimental
Materials.
Dimethylformamide (DMF, >98%), branched polyethyleneimine (PEI,
Mw~ 25000), palladium (II) acetylacetonate (Pd(acac)2, 99%),
potassium tetrachloropalladate (K2PdCl4, 99.99%), oleylamine (OAm,
70% - technical grade), sodium hydroxide (NaOH, ≥97.0%, pellets),
8 | J. Name., 2012, 00, 1-3
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Malvern Zetasizer 2000 instrument (Worcestershire, U.K.), equipped Recyclability test. When the catalytic reduction of 4-NP to 4-AP was
View Article Online
with a 4 mW helium-neon laser (633 nm) as a source of incident light deemed completed, 0.1mL of 4-NP (70mM)DwOaI:s1a0d.1d0e3d9/Cag9aDiTn0t0o10t7hGe
and operating at a scattering angle of 17° and 25 °C. Diluted aqueous reaction solution, and the kinetic studies were continued after
solutions of Pd nanocrystals were injected in disposable ζ cells and mixing. This procedure was repeated several times.
successively inserted in the apparatus. The obtained data were
analysed using the software provided with the instrument.
Computational methods. Geometrical optimization of the
structures of 4NP- and 4AP- were carried out employing density
Thermogravimetric analyses were performed using a TG-DSC Setsys
functional theory (DFT) at the M062X/6-311++G** level,
evolution 16/18 (Setaram) with a heating ramp rate of 10ºC min-1
followed by analysis of their vibrational frequencies to
from 20 to 800 ºC and in a nitrogen flow. The surface feature and
characterize the structures obtained as energy minima. All
pore structure of PdND samples were determined and analysed using
computations were performed using Gaussian09.34 Assignment
adsorption/desorption isotherms of N2 at 77 K with a QUADRASORB
of the theoretical Raman spectrum to molecular vibrational
evo (Quantachrome Instrument, Florida, USA). Prior to the N2
modes was performed by visual inspection of the atomic
sorption measurements, the samples were outgassed at 383 K under
displacements for each vibrational mode in combination with
high vacuum for 20 h. The specific surface area of PdNd samples was
the results from the VEDA program, which generates an
calculated using multi-point BET (Brunauer, Emmett and Teller)
optimized set of internal coordinates based on the molecular
analysis from the adsorption isotherm. The pore size distributions
structure and provides a potential energy distribution for the
were obtained using the BJH model from the desorption isotherm.
quantitative analysis of vibrational spectra.35
Finally, ICP-OES elemental analyses were performed after digestion
of the samples with aqua regia.
Acknowledgements
4-Nitrophenol to 4-aminophenol reduction. In a 1 cm length path
quartz cuvette, 2 mL of an aqueous solution containing 10 mM of 4-
NP (10mM in NaOH) were mixed with 0.25 mL of an aqueous
dispersion of PdNDx containing either 0.04 mg of PdND1 or 0.13 mg
of PdND2, followed by the addition 0.188 mL of a freshly prepared
NaBH4 solution 0.1 M. Raman experiments were conducted with a
Renishaw InVia Reflex confocal system. The spectrograph used a
high-resolution grating (1800 grooves per mm) with additional band-
pass filter optics, a confocal microscope, and a 2D-CCD camera.
Excitation was carried out at 532 nm with a laser power at the sample
of 25 mW. Raman characterization of the hydrogenation reaction
was done using a macrosampler accessory to measure in liquid state.
The catalytic reaction was monitored by the decrease with time of
the 4-NP main signal (1292 cm-1, ring deformation). For the
hydrogenation reaction subsequent spectra were taken with one
accumulation and an acquisition time of 30 seconds per spectra. For
the last spectra of the catalysis, in order to detect the 4-AP, ten
accumulation and an acquisition time of 60 seconds were used. The
apparent observed rate constants kapp were calculated from the plots
of Raman intensity of 4-NP- (1292 cm-1) vs. time, using the first-order
rate equation:
This work was supported by the Ministerio de Economía y
Competitividad (MINECO, Spain) under the Grant MAT2016-77809-
R, Xunta de Galicia (GRC ED431C 2016-048 and Centro Singular de
Investigación de Galicia (ED431G/02)) and Fundación Ramón Areces
(SERSforSafety). S. M. acknowledges funding from the General
Secretariat for Research and Technology in Greece (Project PE4
(1546)). S.B. and N. W. acknowledge financial support by the
European Research Council (ERC Starting Grant #335078-
COLOURATOMS). We thank the EPSRC CNIE Research Facility (EPSRC
Award, EP/K038656/1) at the University College London for the
collection of the BET data. Authors thank J. Millos for the XRD
measurements.
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It = If +ΔI e -kt
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