“This document is the Accepted Manuscript version of a Published Work that appeared in final form
in Applied Catalysis B: Environmental, www.elsevier.com/locate/apcatb, copyright ©2017 Elsevier
B.V.after peer review and technical editing by the publisher. To access the final edited and published
work see http://www.sciencedirect.com/science/article/pii/S0926337317304976

Magnetically-actuated mesoporous nanowires for
enhanced heterogeneous catalysis
Albert Serrà ‖,@*, Sergi Grau $, Carolina Gimbert-Suriñach $, Jordi Sort ┴,#, Josep Nogués &,#,*, Elisa
Vallés ‖,@*
‖

Grup d’Electrodeposició de Capes Primes i Nanoestructures (GE-CPN), Departament de Ciència de

Materials i Química Física, Universitat de Barcelona, Martí i Franquès, 1, E-08028, Barcelona,
Catalonia, Spain.
@
$

Institute of Nanoscience and Nanotechnology (IN2UB), Universitat de Barcelona

Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology,

Avinguda Països Catalans 16, 43007 Tarragona, Catalonia, Spain.
┴

Departament de Física, Universitat Autònoma de Barcelona, E-08193, Bellaterra, Catalonia, Spain.

#

ICREA, Pg. Lluís Companys 23, 08010, Barcelona, Spain.

&

Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, The Barcelona Institute of

Science and Technology, Campus UAB, Bellaterra, E-08193 Barcelona, Spain.
*

Corresponding Authors: a.serra@ub.edu; josep.nogues@uab.cat; e.valles@ub.edu.

ABSTRACT: We study the optimization of the catalytic properties of entirely magnetic CoPt compact
and mesoporous nanowires of different diameters (25 – 200 nm) by using magnetic actuation. The
nanowires are a single-entity, robust, magnetic-catalyst with a huge catalytically-active surface area.
We show that apart from conventional parameters, like the size and morphology of the nanowires, other

factors can be optimized to enhance their catalytic activity. In particular, given the magnetic character
of the nanowires, rotating magnetic fields are a very powerful approach to boost the performance of the
catalyst. In particular, the magnetic field induces them to act as nano-stirrers, improving the local flow
of material towards the active sites of the catalyst. We demonstrate the versatility of the procedure by
optimizing (i) the degradation of different types of pollutants (4-nitrophenol and methylene blue) and
(ii) hydrogen production. For example, by using as little as 0.1 mg mL-1 of 25 nm wide CoPt mesoporous
nanowires (with 3 nm pore size) as catalysts, kinetic normalized constants knor as high as 20667 and
21750 s-1g-1 for 4-nitrophenol and methylene blue reduction, respectively, are obtained. In addition,
activity values for hydrogen production from borohydride are as high as 25.0 L H2 g-1 min-1, even at
room temperature. These values outperform any current state-of-the-art proposed catalysis strategies
for water remediation reactions by at least 10-times and are superior to most advanced approaches to
generate hydrogen from borohydride. The recyclability of the nanowires together with the simplicity of
the synthetic method makes this approach (using not only CoPt, but also other mesoporous magnetic
catalysts) very appealing for very diverse types of catalytic applications.

Graphical Abstract

Highlights


Magnetic actuation of purely magnetic mesoporous nanowires (NWs) is revealed as a novel
and efficient catalysis procedure in reactions of heterogeneous catalysis in environmental and
energy fields.



Mesoporous CoPt NWs are simultaneously effective catalysts itself and the stirring device,
using magnetic fields, to catalyse reactions in solution.



The procedure allows ultra-fast degradation of different types of pollutants.

2



The NWs show very high effectivity for hydrogen generation (25.0 L H2 g-1 min-1) by catalysing
the hydrolysis of sodium borohydride using magnetic actuation.

KEYWORDS: mesoporous magnetic nanowires; magnetically-actuated catalysis; water remediation;
hydrogen production; catalyst recyclability.

1. Introduction
It is well-known that catalysis plays a crucial role in our everyday life and it is one of the pillars of
modern chemistry. Most industrial chemical processes (e.g., energy, pharmaceutical, food or material
production industries) are based on catalytic processes [1-6]. Similarly, catalysis plays a strategic role
in many environmental and energy issues such as air pollution control (e.g., car exhausts), wastewater
remediation or hydrogen production [7-12]. Thus, any improvement on the performance of catalytic
processes could result in a huge economic and social impact. Consequently, many different types of
processes such as microwave-, plasma-, ultrasound-, electric field-, acoustic-, light- or temperatureassisted catalysis, among others, are continuously emerging as promising strategies to enhance these
type of reactions [13-22]. Another aspect which is being actively explored to improve heterogeneous
catalysis is to increase the surface area of the catalyst, using for example nanostructured materials [2329]. In particular, mesoporous materials, which possess an extraordinarily large specific surface area,
have attracted great attention for heterogeneous catalysis in liquid media [30, 31]. Additionally, of all
the variables involved in heterogeneously catalysed reactions, effective mass transfer is fundamental to
enhance the reaction rate, magnetic stirring bars or mechanical stirrers are usually used to promote the
mass transportation efficiency [32, 33]. However, delicate catalysts might be damaged under vigorous
mechanical stirring or catalysts involving magnetic materials may get stuck to the magnetic stirring bars
[34].
Strikingly, an aspect that has been somewhat underexploited in catalysis is magnetic actuation on the
catalyst itself. In fact, during the 90s and early 2000s it was shown that magnetic fields could actually
control the mass transport in some chemical reactions [35]. In recent years, the important role of
magnetic materials in the control of magnetic field induced convection has been established [36, 37].
Other attractive aspects of magnetic fields in catalysis are their influence on the oxidation process of
magnetic ions or their effect on the electronic structure of the catalysts [38, 39]. Although lately there
has been an increase of the use of magnetic materials in catalytic processes, usually the magnetic
counterpart does not play an active role in the catalysis, but it is rather used as a simple means to recover
3

the catalyst to recycle and reuse it [40-42]. An appealing aspect of magnetic materials in catalysis, that
has been proposed recently, is the concept of magnetically-actuated remote micro-stirrers [43-47]. The
use of these magnetic stirrers has been shown to improve diverse types of catalytic processes by
favouring the local flow of matter. However, again, in these catalysts the magnetic counterpart does not
play an active role in the reaction [43-47]. Moreover, these micro-stirrers are often composites (with
both magnetic and catalytic moieties and usually other components), which could make them rather
fragile. Moreover, often they require complex synthesis procedures, often involving many steps, making
them less attractive for large scale production [43, 47].
It is important to highlight that catalysts based on magnetic ions (e.g., Fe, Co or Ni) have also started
to attract a great deal of attention, although not necessarily due to their magnetic nature but due to their
transition-metal character. Namely, they have been proposed as suitable candidates to partially
substitute the expensive noble-metal catalysts (e.g., Pt, Au or Pd) by inexpensive alternatives, which in
general improve the catalytic performance [48-51]. In particular, the partial substitution of Pt by Co has
been shown to result in excellent alternatives for many different types of processes [52-55].
Here we demonstrate that easily-synthesized mesoporous CoPt magnetic nanowires (NWs) can be used
as effective magnetically-actuated micro-stirrers to boost diverse types of catalytic reactions (e.g., for
4-nitrophenol and methylene blue degradation or hydrolysis of alkaline sodium borohydride for
hydrogen production). These mesoporous NWs constitute a novel simple, all-in-one catalyst (i.e., a
strongly catalytically active magnetic material) with a large active area, which can be easily manipulated
by magnetic fields. The role of different parameters in the improved catalytic performance is
investigated, such as the magnetic field rotation speed, the nanowire morphology and diameter and the
reaction geometry. The results show that narrow mesoporous NWs magnetically actuated at high
rotation speeds in planar reaction geometry is the optimum combination to achieve outstanding
performances in both types of reactions.
2.

Experimental

2.1. Electrochemical media.
The electrochemical media (Table 1) used for synthesising compact or mesoporous NWs were an
aqueous solution (W) of 2.5 mM CoCl2·6H2O (Carlo Erba, > 98.0 %) + 1.2 mM Na2PtCl6·6H2O (Alfa
Aesar, 98 %) + 0.1 M NH4Cl (Fluka, 99.5 %) + 10 g L-1 H3BO3 (Merck, > 98 %) and an ionic liquidin-water (IL/W) microemulsion, respectively. IL/W microemulsions were prepared by mixing the
adequate proportion of each component (W – aqueous solution –, S – Surfactant, p-octyl poly
(ethyleneglycol) phenyl ether a.k.a. Triton X-100 (Acros Organics, 98 %) –, and IL – Ionic Liquid, 1-

4

butyl-3-methylimidazolium hexafluorophosphate a.k.a. bmimPF6 (Solvionic, 99 %) –) during 5 min
under magnetic stirring (300 rpm) and argon bubbling, leading to a transparent, isotropic, and
thermodynamically stable microemulsion [56, 57]. All solutions were prepared using deionized water
(Millipore Q-System) with a resistivity of 18.2 MΩ cm.
Electrochemical media

W / wt. %

S / wt. %

IL / wt. %

Aqueous solution (W)

100

-

-

83.8

15.1

1.1

Ionic Liquid-in-water
microemulsion (IL/W)

Table 1: Electrochemical media selected for synthesising Co–Pt NWs.

2.2. Electrochemical synthesis.
The shape-controlled electrodeposition of compact or mesoporous (double-template electrodeposition)
Co–Pt NWs was carried out using a microcomputer-controlled potentiostat/galvanostat Autolab with a
PGSTAT30 equipment and GPES software. Commercial polycarbonate (PC) membranes (Millipore
Company) with nominal pore sizes of 100 or 200 nm and alumina membranes (SmartPor Membranes
GmBH) with a nominal pore size of 25 nm were used as working electrodes. The pores of the
membranes act as templates for the shape-controlled growth of nanostructured wires, where aqueous or
IL/W microemulsion media lead to either compact or mesoporous NWs. The use of microemulsions
allows the formation of a three-dimensionally interconnected network of open mesopores along the
entire area of NWs [31, 56]. However, in order to use the membranes as a working electrode a
conductive layer on one side is necessary. Therefore, vacuum evaporation was used to coat the
membrane on one side with around a 100 nm-thick gold layer. The electrodeposition process was
performed at 25 oC, using a three-electrode electrochemical system – polycarbonate/Alumina
membranes, Pt spiral, and Ag/AgCl/KCl (3 M) electrodes, as a working, counter and reference
electrodes, respectively – , in potentiostatic mode (at – 1.0 V). The electrochemical media was deaerated by argon bubbling before each experiment and maintained under argon atmosphere during it.
After the deposition, the samples were dried and weighted several times until a constant weight in
vacuum conditions was attained in order to determine the total mass of synthesized NWs. Then, the
NWs were released from the membrane first removing the Au layer using a saturated solution of I 2/Iand subsequently dissolving polycarbonate or alumina membrane with either chloroform (x10) or
NaOH (1 M), and then washing with ethanol (x5) and water (x5) under flashing ultrasound stirring.

5

2.3. Materials characterization.
The morphology of the Co–Pt NWs was examined by using transmission electron microscopy (Jeol
2100) and the elemental composition was determined by energy-dispersive X-ray analysis incorporated
in a JSM–7100 scanning electron microscopy.
Hysteresis loops of the compact and mesoporous NWs (while still embedded in the membrane) were
measured at room temperature using a vibrating sample magnetometer from Micro-Sense (LOTQuantum Design), with a maximum external magnetic field of 20 kOe, applied either along or
perpendicular to the NWs axis.
The electrochemical surface area (ECSA) values of the CoPt NWs were determined by using cyclic
voltammetries in 0.5 M H2SO4 at 20 mV s-1 at 25 oC. For each NW type, 5 μL of a NWs ink (0.1 mg
mL-1) was dropped on the surface of a glassy carbon (GC) rod (0.031 cm2 in diameter) and dried under
nitrogen. It is well known that for Pt base nanostructures, the ECSA can be obtained from the total
charge passed during hydrogen adsorption/desorption (after accounting for the double layer capacity
and assuming that the charge required to oxidise a mono-layer of hydrogen on bright Pt is 210 µC cm1

) by using the following equation (Eq. 1) [31, 56]:
=

·

(Eq. 1)

Where Qmeasured is the charge measured by integration of a voltammetric peak associated with an
adsorption process, Qtheoretical is the charge required for monolayer coverage of 1 cm2 of electrode surface
by the adsorbed species and mCoPt is the mass of catalyst.

X-ray powder diffraction (XRPD) measurements were acquired with a PANalytical X’Pert Pro
MPD diffractometer in the Bragg-Brentano reflection θ/2θ geometry, at 45 kV, 40 mA, and λ =
1.5406 Å (Cu Kα1). The samples were prepared by drop casting and solvent evaporation of the
catalyst ink on a monocrystalline Si holder. XRPD patterns were obtained after θ/2θ scans from
20 to 120 ° 2 θ, with a step size of 0.0172 °, a measuring time of 200 seconds per step. Note that
the measurements were repeated to acquire sufficient statistics. The data were analysed using
the X’Pert HighScore Plus software.
X-Ray photoemission spectroscopy (XPS) measurements were carried out in ultra-high vacuum using
a PHI ESCA-550 multi-technique system (Physical Electronics), with a monochromatic x-ray source
(Al Kα line of 1486.6 eV energy and 350 W) placed perpendicular to the analyser axis and calibrated
using the Ag3d5/2 line with a full width at half maximum of 0.8 eV. The analysed area was a circle 0.8

6

mm in diameter, and the selected resolution for the spectra was 187.85 eV of pass energy and 0.8 eV
per step for the general spectra.

2.4. Water remediation.
The reduction of 4-nitrophenol and methylene blue was chosen as model reactions to test the catalytic
activity of the compact and mesoporous Co–Pt NWs. In both reactions the temporal evolution of the
catalytic reaction was tracked ex-situ by UV–visible spectroscopy in a quartz cuvette with an optical
length of 1 cm using a UV-1800 Shimadzu UV-Visible spectrophotometer. All of the experiments were
conducted at room temperature. For these experiments, two different pollutants were selected:
(i) 4-nitrophenol: 300 μL of 4-nitrophenol (0.7 mM solution) was diluted with 1 mL of Millipore
water, followed by the addition of 1 mL of freshly prepared ice-cold NaBH4 solution (0.5 M),
and a given amount of the prepared catalyst ink (0.1 mg mL-1). As a result, the bright yellow
solution gradually fades as the reaction progresses. The absorption peak intensity at 400 nm was
used to quantify the temporal evolution of the 4-nitrophenol concentration.
(ii) Methylene blue: 300 μL of methylene blue (0.4 mM solution) was diluted with 1 mL of Millipore
water, followed by the addition of 1 mL of freshly prepared ice-cold NaBH4 solution (2.3 mM),
and a given amount of the prepared catalyst ink (0.1 mg mL-1). As a result, the dark blue solution
gradually fades as the reaction progresses. The absorption peak intensity at 662 nm was used to
quantify the temporal evolution of the concentration of methylene blue.

2.5. Optimization of the catalytic performance.
Importantly, four different reaction conditions were pursued:
(i) Silent, where no specific actuation was carried out during the reduction reaction.
(ii) Magnetic actuation, where the NWs were influenced by an external rotatory magnetic field
(~400 Oe – magnetic stirrer RH digital – IKA) at various ration speeds.
(iii) Mechanical actuation, where the NWs were mechanically stirred at 100 rpm using a Metrohm
622 rod stirrer.
(iv) Magneto-mechanical actuation, where the NWs were simultaneously mechanically (100 rpm)
and magnetically (100 rpm) stirred.
Moreover, the effects of the reaction geometry was also assessed:

7

(i)

(i) Vertical geometry, where the container used was “rectangular vertical” with 10 x 10 x
26 mm3 shape (see Fig. 2).

(ii)

Planar geometry, where the container used was “cylindrical planar” with a

x 172 x 3 mm3

shape or a microdroplet (~ 0.1 mL) (see Fig. 2).
In the catalytic test, the concentrations of NaBH4 in all the reactions are fixed at ~2380–times and ~19–
times higher that of 4-nitrophenol and methylene blue, respectively. Therefore, an excess amount of
reductant NaBH4 was used and, consequently, the reduction rate constant can be calculated based on
pseudo-first-order kinetics. The apparent kinetic rate constant (kapp) is proportional to the total surface
area of the catalyst, which can be estimated based on the regression of the slope from the logarithm plot
(Eq. 2)
=−

→

=

=−

(Eq. 2)

Moreover, in order to quantitatively compare with other catalysts, the normalized rate constant (
was also determined, which is associated with the amount of catalyst, i.e.,

=

⁄

)
. In

some cases, the catalytic activity was evaluated by the degradation efficiency after 60 s by the relation
between (

/

). Lastly, in the microscopic systems, the colour change of blue (methylene blue) or

yellow (4-nitrophneol) to colourless was a direct indicator of the reaction progress, i.e., the catalytic
activity.

2.6. Hydrogen production.
The hydrolysis of sodium borohydride (Eq. 3) was selected to test the catalytic activity of the compact
and mesoporous Co–Pt NWs towards hydrogen generation.
+ 2

→

+ 4

(Eq. 3)

An alkaline-stabilized solution 4.2 mM sodium borohydride (adjusted to pH = 13 using NaOH 1M),
was prepared. Under these conditions, the self-hydrolysis of borohydride is minimized. Then the CoPt
catalyst - 24 µL of a catalyst ink (0.1 mg mL-1) - was used for hydrogen production in a controlled
manner. The generated hydrogen volume and the catalytic activity were measured using a Clark-type
hydrogen electrode (Unisense) in the headspace of a gas-tight chamber containing 1 mL of total solution
at 25 ºC, which was calibrated with known amounts of hydrogen gas. The experimental conditions were
selected to allow a quantitative comparison with previously reported results. Following the results of
the catalyst optimization for water remediation (see Results section), both silent and magnetic actuation

8

(1200 rpm) in both vertical and planar geometry reactor conditions for compact and mesoporous Co–
Pt NWs were attempted.

3.

Results

3.1. CoPt nanowires.
To evaluate the role of catalyst morphology, two types of Co-rich, Co66±3Pt34±3, NWs, 1.2 ± 0.3 µm in
length and with three different diameters (25, 100 and 200 nm), were electrochemically synthesized
(see Experimental) [54, 55]. As can be seen in Figure 1a, b and Table S1, for the 100 nm NWs, while
one type of nanowire is compact (with a smooth surface), the other one has a mesoporous morphology
with an interconnected network of pores (with a pore size of about 3.1 ± 0.9 nm and an extraordinary
high surface area [electrochemically active surface area (ECSA)] of 255 m2 g-1– Figure 1d and Table
S1). Importantly, the elemental maps of the CoPt NWs (Figures 1a, b) show that Co and Pt are
homogeneously distributed along the area of both types of NWs.
Both types of NWs are magnetic at room temperature with somewhat similar magnetic characteristics,
with the easy axis along the nanowire axis (Figure 1c).
The crystal structure of the CoPt NWs was probed by using x-ray diffraction (XRD) (Figure 1e). From
the patterns three families of peaks can be identified: fcc Au, corresponding to some remaining gold in
the NWs from the working electrode (highlighted by dotted green arrows), fcc CoPt3 (Pm-3m)
(highlighted by dotted red arrows) and a hexagonal hcp CoPt phase (P63mmc) (marked by its Miller
indices). Note that this latter phase corresponds to hcp Co but very distorted due to the incorporation of
Pt atoms in the crystalline lattice. Since the patterns of compact and mesoporous NWs are almost
identical, it is clear that they have analogous crystalline structures, composed of polycrystalline cubic
and hcp phases. Note that similar crystalline structures have been reported for electrodeposited CoPt
films and nanostructured materials [58, 59].

The bimetallic nature of the prepared NWs is further

confirmed by XPS analysis. The high-resolution scans of the XPS spectra (Figure 1f) show for Pt double
peaks with binding energies of 71.80 eV (4f7/2) and 75.21 eV (4f5/2), clearly shifted from the standard
values for pure Pt (i.e., 71.20 and 74.53 eV). These values are consistent with the formation of a CoPt
allow since Pt 4f peaks are known to change to higher energies when alloying with Co [60]. On the
other hand, for Co two peaks at 778.7 eV (Co 2p3/2) and 794.0 eV (Co 2p1/2) are observed, which deviate
from the standard values of pure Co (778.30 eV and 793.27 eV). In concordance to the Pt peaks, the
shift in energy of the Co peaks is also consistent with the formation of a CoPt alloy [60]. Note also that
although the Co 2p spectra also shows the formation of some cobalt oxides the relative intensity of the
9

oxide peaks indicates that only a small fraction of Co is oxidized. It is important to emphasize that the
XPS spectra for both types of NWs is almost identical in agreement with the EDX results.

Figure 1: HR-TEM images and elemental mapping of (a) compact and (b) mesoporous 100 nm NWs.
Scale bar 50 nm. (c) Room-temperature parallel (red) and perpendicular (black) to the wire axis
hysteresis loop of mesoporous 100 nm NWs (inset: compact NWs). (d) Cyclic voltammetries of the
compact (blue) and mesoporous (black) NWs. (e) XRD patterns of compact (blue) and mesoporous
(black) NWs. The index miller indexes of the main peaks of the hexagonal CoPt phase are indicated
next to the corresponding peaks. The fcc Au and fcc CoPt3 phases are highlighted by green and red

10

dotted arrows, respectively. At the bottom, Si holder background for each sample is also shown. (f) Pt
4f and Co 2p XPS spectra from compact (blue) and mesoporous (black) NWs.

3.2. Water remediation.
3.2.1. Basic catalytic activity.
Before evaluating the catalytic activity, it is important to analyse the adsorption of the two compounds
since this may interfere in the degradation process. It is well-known that some materials, particularly
when nanostructured, have a strong physical adsorption of pollutants, such as 4-nitrophenol and
methylene blue, as a consequence of the surface affinity [61-70]. Consequently, the absorbance of
various solutions of each pollutant (without the presence of sodium borohydride) was measured before
and after 24 and 48 h posteriorly to the addition of the NWs (either compact or mesoporous), both in
silent mode or in the presence of an external rotatory magnetic field (~400 Oe and 1200 rpm). In all
cases, the absorbance remains virtually constant even after 48 h. Therefore we can conclude that the
adsorption of these pollutants on the CoPt NWs is not significant. It is also plausible to assume that in
the presence of borohydride the capture of pollutants should become even more negligible due to the
formation of hydrogen bubbles on the catalyst surface and pollutant degradation.
To test the catalytic activity, the 100 nm diameter NWs are used to degrade two different types of wellknown wastewater pollutants: 4-nitrophenol and methylene blue. Both types of NWs can trigger the
reduction of 4-nitrophenol and methylene blue, confirming the good catalytic properties of the CoPt
alloys (see Figure S1, Table S2 and Video S1-S2). Moreover, the mesoporous NWs are clearly more
effective than the compact ones in promoting the degradation of 4-nitrophenol and methylene blue,
reducing the reaction time by 4 (4-nitrophenol) or 2 (methylene blue), respectively. This is expected
from the high surface area of the mesoporous wires, which exposes more active sites to carry out the
reaction. In fact, the apparent kinetic constants kapp (and the mass normalized constant, knor) of both
reactions for the mesoporous wires (i.e., without mechanical or magnetic stirring), are already rather
large, knor = 3807 s-1g-1 (4-nitrophenol) and knor = 3916 s-1g-1 (methylene blue) (Table S2) and
comparable with other types of catalysis proposed for these reactions (see Table S4) [43, 71-19].
Importantly, the release of H2 gas in solution by the catalysed hydrolysis of sodium borohydride (see
Video S1-2) implies the self-pumping of the fluid medium at the micro and nanoscale levels and,
therefore, the enhancement of the mass transport flow. However, the efficiency of the reaction is
somewhat restricted by the limited flow of material towards the catalyst, which is basically driven by
diffusion. Thus, to favour the mass flow, the same reactions were carried out by applying mechanical
stirring at 100 rpm (see Figure S2 and Video S1-2). As expected, mechanical stirring improves the

11

reaction kinetics; nevertheless, the gain is somewhat moderate, with a 15-30% improvement in knor for
the two types of reactions (see Table S2).

3.2.2.

Magnetically-assisted catalysis.

As a first assessment of the role of the magnetic field in the catalysis, the reduction of 4-nitrophenol
and methylene blue was carried out in the presence of an out-of-plane static field of 200 Oe for the 100
nm NWs. Remarkably, the mere presence of a magnetic field already improves the efficiency of the
reaction (Table S2, and Video S3-S4), with an enhanced knor by 10-15%. This is in agreement with
earlier studies in other types of catalysts where the magnetic fields (and/or field gradients generated by
the magnetic catalysis or magnetic objects in close proximity to the catalytic front) enhanced the
catalytic activity [47, 80]. However, based on the chemical nature of both pollutants, the origin of the
effect of magnetic fields on the kinetics of degradation is not straightforward. Thus, several experiments
were carried out where different out-of-plane static magnetic field intensities (200, 110 and 45 Oe) were
applied directly to the reactor (i.e. on the catalyst surface) – similar magnetic field distributions – and
an out-of-plane static of 200 Oe was applied at different distances to the reactor base (0 cm – 200 Oe
and 1.52 cm – 105 Oe due to the magnetic reluctance of air) – different magnetic field intensity and
different magnetic field distribution. Surprisingly, the presence of a magnetic field increases the
efficiency of the reaction, obtaining a similar increase independently to the applied field intensity.
However, the distance—i.e. the distribution of the field—seems more determinant, since the
improvement of the kinetics is slightly lower (2-4 %) in comparison to a similar magnetic intensity
value but directly applied on the catalyst. Actually, due to the nanometric morphology of the
mesoporous wires, with multiple edges (which should locally enhance the magnetic field gradients) [36,
37], this latter effect is probably magnified for this type of investigated structure.
To asses a second aspect of the magnetic field actuation, the reactions were performed using a rotating
magnetic field (400 Oe) at different speeds. Note that the experiment was carried out using a magnetic
stirring set up; however, no macroscopic magnetic bar was introduced in the container. Instead, the
magnetic NWs themselves act as micro-stirring bars by the effect of the rotating field (see Figure 2).
As can be seen in Fig 3 (and Figure S3 to S9, Table S2 and Videos S1-2), the rotating magnetic field
has an outstanding effect on the effectiveness of the reaction, drastically reducing the reaction time to
degrade 4-nitrophenol and methylene blue.

12

Figure 2: Scheme of the different reaction conditions.

Importantly, the effect of the field increases for higher rotating speeds. For example, the time for a full
degradation of 4-nitrophenol and methylene blue is reduced by an excellent 2-fold factor when
comparing the experiment in absence of magnetic field with that in which a 1200 rpm rotating field is
applied. In fact, the knor values for the mesoporous NWs for both types of reactions (Figure 3a and Table
S2) are exceptionally high, outperforming all state-of-the-art catalysts proposed to date to degrade these
pollutants (see Table S4). It is likely that the local magnetic field (and field gradients) promote a better
13

local circulation of the liquid containing the pollutant, allowing it to easily reach the catalyst. However,
as can be seen in the evolution of knor with rotating speed (Figure 3) there is a tendency towards
saturation for the highest speeds. This is probably caused by the fact that for sufficiently high speeds
the NWs will no longer be able to follow the field. If fact, for even higher speeds other effects, like the
heating of the wires due to hysteresis losses, could lead to new consequences in the reaction [47, 80].
In an attempt to further enhance the catalytic activity, magnetic and mechanical stirring were combined.
The experiment was carried out using mechanical stirring at 100 rpm together with a rotating magnetic
field at the same frequency (Figure S7 and Video S1-S2). However, surprisingly, although the
combined approach resulted in a better performance than mechanical stirring alone, it was worse than
pure magnetic stirring. This probably stems from the fact that mechanical stirring not only displaces the
liquid but also the catalyst NWs. As the NWs are moved away from the bottom of the container by the
mechanical stirring, the magnetic field they feel becomes considerably weaker, thus the advantageous
effects of the magnetic field (local stirring and magnetic field gradient effects) are progressively lost.
In a more classical role of catalysts with magnetic materials, the recyclability of the nanowire catalysts
was also evaluated. As can be seen in Figures S10-11, both compact and mesoporous NWs showed
excellent stability during cycling tests, where no obvious decrease of activity even after eight cycles
was observed.

3.2.3.

Role of the reaction geometry.

The above experiments evidence the importance of the mass flow to improve the time needed to fully
degrade the 4-nitrophenol and methylene blue pollutants. Moreover, it is clear that the optimum
performance of the NWs is attained when they are close to the magnet. Thus, for an additional
optimization of the magnetic actuation in the catalytic reaction, the role of the shape of the reaction
vessel from vertical to planar geometry was evaluated using the 100 nm diameter wires. Two types of
approaches were investigated (Figure 2): (i) microdroplets and (ii) a wide container with low height
(with the same reaction volume as the vertical case). The former experiment is important for lab-in-achip and microfluidics type of applications, while the latter would be more relevant for large-scale
applications like waterwaste remediation.
Due to their small size, the quantitative evolution of the reaction in the microdroplets is rather complex.
Thus, to evaluate the performance of the mesoporous NWs in the microdroplets, the discoloration in
the different configurations were compared (see Video S5-S6). Indeed, the degradation of 4-nitrophenol
and methylene blue proceeds considerably faster in the microdroplets than in the conventional container

14

under the same magnetic field conditions. This is in concordance with the results obtained with
composite microstirrers, which were demonstrated to efficiently enhance the catalysis of diverse types
of reactions in microdroplet conditions [32, 33].
Similar effects are obtained when the experiment is carried out in a macroscopic container. Remarkably,
just by changing the geometry of the reaction container from “rectangular vertical” to “cylindrical
planar” geometry (see Figure 2) and maintaining the same reaction volume and the same magnetic
stirring conditions (100 rpm and 1200 rpm), the efficiency of the reactions almost doubles leading to
knor as high as 14058 s-1g-1 and 14600 s-1g-1 for 4-nitrophenol and methylene blue, respectively (see
Figure 3b, Figure S12, Table S3 and Videos S7-S8). These results clearly highlight the importance of
the geometry to optimize the role of the magnetic field.

3.2.4.

Role of the catalyst size.

In a final attempt to further optimize the efficiency of the catalytic reactions we investigate the effect
of the diameter of the mesoporous CoPt NWs. The idea of using different diameters is two-fold: (i) to
assess the effect that the geometry of the NWs may have in the microstirring process and (ii) to evaluate
the role of the surface area of the NWs, which increases as the diameter is decreased (Table S3).
The degradation of the water pollutants was carried out at 1200 rpm both in vertical and planar
containers. The results, shown in Figure 3c, Table S3 and Figure S13, unambiguously demonstrate that
in all the conditions the narrower 25 nm wires have a significantly better performance for both types of
reactions than the wider ones. The best performance is obtained for mesoporous 25 nm NWs at 1200
rpm in planar geometry, with knor = 20667 and 21750 s-1 g-1, for 4-nitrophenol and methylene blue,
respectively.
Interestingly, when comparing the increase in surface area with the enhanced catalytic performance it
is clear that apart from the surface area, the magnetic actuation also plays an important role in the
catalytic activity. Namely, when decreasing the diameter from 200 nm to 25 nm, the surface area only
increases a factor ECSA(25 nm)/ECSA(200 nm) = 1.56, while the kinetic constant improves
substantially more knor(25 nm)/knor(200 nm) = 2.2.

15

Figure 3: Values of the normalized rate constants, knor, for the pollutants degradation. (a) Actuation
type for CoPt 100 nm NWs. (b) Influence of the reactor geometry for CoPt 100 nm NWs at different
magnetic actuation (c) Influence of the catalyst diameter of CoPt NWs (200, 100 and 25 nm) at 1200
rpm of magnetic actuation.
Remarkably, when comparing the somewhat standard procedure of 100 nm compact NWs with
mechanical stirring in a vertical container with the novel approach of 25 nm mesoporous NWs with
1200 rpm magnetic microstirring in a planar container the gain in efficiency is simply impressive,
16

reducing the degradation time by a factor 5.5 (Figure 4). This results in an improved mass normalized
apparent kinetic constant of knor = 20667 and 21750 s-1 g-1, which considerably exceeds all standard and
advanced procedures for 4-nitrophenol and methylene blue degradation.

Figure 4: Time-dependent UV–Visible spectra of 4-nitrophenol (a, c) and methylene blue (b, d)
catalysed reduction in (dark yellow) mechanical actuation (catalyst diameter: 100 nm; actuation: 100
rpm; geometry: vertical) and (green) magnetic actuation (catalyst diameter: 100 nm; actuation: 1200
rpm; geometry: planar) conditions, in which 12 µL of a suspension of mesoporous Co–Pt NWs (0.1
mg mL-1) were added. Plots of − ln(

/

) against reaction time for the catalyzed reduction of (e) 4-

nitrophenol or (f) methylene blue under (dark yellow) mechanical actuation (catalyst diameter: 100 nm;
actuation: 100 rpm; geometry: vertical) and (green) magnetic actuation (catalyst diameter: 100 nm;
actuation: 1200 rpm; geometry: planar) conditions, in which 12 µL of a suspension of mesoporous Co–
Pt NWs (0.1 mg mL-1) were added.
17

3.3. Hydrogen production
It is well known that sodium borohydride (NaBH4) is a preeminent candidate for pure hydrogen
generation by means of its irreversible hydrolysis (Eq. 3), with the significant advantage that 50% of
the produced hydrogen originates from water itself, since sodium borohydride is a water-splitting agent
[81-84]. Importantly, the formation of hydrogen from borohydride decomposition produces pure H 2
without carbon contaminations such as CO or CO2. Moreover, NaBH4 is stable, non-flammable, nontoxic and it is able to store hydrogen with a capacity of 10.8 wt. %. [84-86]. Sodium metaborate
(NaBO2), which is formed simultaneously to hydrogen in the reaction described in Eq. 3, is also
environmentally clean and it can actually be recycled by electrosynthesizing it back to sodium
borohydride (Eq. 4) [81]:
+2

→

+ 2

(Eq. 4)

Hence, in these conditions, only water would be consumed for hydrogen production. Consequently,
borohydride is a great candidate for direct fuel cells [87-89]. By extension, the development of
competitive catalysts for this type of reaction is of major importance; especially using catalysts viable
for industrial applications (i.e. low cost and good catalytic activity), since the non-catalysed hydrolysis
is extremely slow (see Figure S14).
The magnetically-actuated catalysis for hydrogen production was determined by measuring the
hydrogen generation yield as a function of time in both vertical ( x 72 x 7.0 mm3) and planar ( x 122
x 2.4 mm3) reaction conditions using compact and mesoporous 25 nm NWs. Figure 5 and S15 and
Table 2 show that the catalytic performance dramatically depends on three factors: (i) catalyst nature;
(ii) reactor geometry; and (iii) use or not of magnetic actuation. In agreement with the pollutant
degradation reactions the combination of mesoporosity-planar geometry and magnetic actuation renders
the best hydrogen generation activity, leading to remarkable 25.0 L H2 g-1 min-1. In fact, in these
conditions the reaction reaches its maximum yield in less than 1000 s (Figure 5 and S15 and Table 2).
Hence, the three factors together play a synergetic role in optimizing the kinetics of heterogeneous
catalysis. For example, compared to the reaction using magnetically actuated compact nanowires in
vertical geometry, an impressive 312x improvement is obtained (compare entries i and vi Table 2).
Importantly, the maximum values of hydrogen generation rate were obtained by a differential procedure
and correspond to the instantaneous rate at t = 0 s. However, these values do not necessarily match
exactly the maximum reaction rate for mesoporous NWs as a consequence of the pore diffusion
resistance [60, 61]. Despite this limitation, the values for the hydrogen generation activity are far better
than in many other catalysts and are actually comparable to the more competitive state-of-the-art solid
catalysts based on noble metals [47, 81-97] (Table S4). In addition, this approach offers the relevant

18

advantage of reducing the Pt content (i.e., cost-effectiveness) and the important benefits from the
magnetic behaviour of the catalysts, i.e., easy manipulation, recyclability and improved catalytic
performance. Nevertheless, it should be emphasized that the hydrogen generation activity values in
Table 2 are based on the use of only 4.2 mM of NaBH4, while most of the reactions reported in Table
S4 use between 50 and 1000 times more NaBH4. Since the hydrolysis of NaBH4 cannot be clearly
considered a zero-order reaction [98, 99], the catalytic activity should be affected by the amount of
NaBH4. Taking this fact into account, using magnetically actuated mesoporous CoPt nanowires in
planar geometry should far surpass all current hydrogen generation approaches based on NaBH4.

Entr
y
i
ii
iii
iv
v
vi

Catalyst
Compact
NWs
Compact
NWs
Compact
NWs
Mesoporous
NWs
Mesoporous
NWs
Mesoporous
NWs

Reactor

Magnetic

Hydrogen

Activity / L
H2 gcat-1

Actuation /

generation yield

rpm

(1200 s) / %

1

ment a

Vertical

1200

1.0

0.08

1

Planar

0

4.6

0.25

3.1

Planar

1200

24.1

1.50

18.8

Vertical

1200

41.8

3.02

37.8

Planar

0

67.2

10.00

125

Planar

1200

100.0

25.00

312.5

geometry

min

Activity
-

Enhance

Table 2: Comparison of catalytic conditions and hydrogen generation performance of compact and
mesoporous 25 nm NWs in vertical and planar reaction geometry and silent and magnetic actuated
heterogeneous catalysis. All reactions were performed using 1 mL of 4.2 mM NaBH 4 (pH 13) and 24
µL of a catalyst ink (0.1 mg mL-1) at 25 ºC.
a

Relative to experiment i.

19

Figure 5: Hydrogen generation (HG) yield as a function of reaction time for compact (i, ii, iii)
mesoporous (iv, v, vi) 25nm NWs in planar (ii, iii, v, vi) or vertical (i, iv) reaction geometry and silent
(ii and v) or magnetic actuation (i, iii, iv, and vi) conditions

4.

Conclusions

Summarizing, we have evaluated magnetic actuation of entirely magnetic NWs as a novel procedure to
enhance the catalytic activity for two significant reactions in environmental and energy fields: the
reduction reaction of 4-nitrophenol and methylene blue water pollutants with borohydride and purehydrogen production by means of borohydride decomposition. These NWs have a huge catalytically
active surface and are easily manipulated (i.e., remote stirring) using magnetic field, combined in a
simple, inexpensive, single material (in contrast to most previous multicomponent complex advanced
magnetic catalysts). The results show that there are several aspects which need to be adjusted for optimal
performance: (i) mesoporous NWs are much more effective than compact ones; (ii) high-speed (1200
rpm) magnetic actuation is shown to be far more effective than mechanical stirring in speeding the

20

degradation process; (iii) planar geometry is better than vertical one; and (iv) narrow NWs (25 nm)
evidence a considerably better catalytic performance than wider ones (200 nm) wires. In fact, 25 nm
mesoporous NWs with 1200 rpm magnetic microstirring in a planar container have an excellent
degradation activity with knor = 20667 and 21750 s-1 g-1, for 4-nitrophenol and methylene blue,
respectively, and a very effective room temperature rate constant of 25.0 L H2 g-1 min-1 for hydrogen
production. These outstanding performances demonstrate that magnetic actuation (magnetic field
induced convection and remote magnetic microstirring) of purely magnetic narrow mesoporous NWs
is an efficient way to greatly enhance the catalytic efficiency in different reactions of heterogeneous
catalysis, including energy production by fuel cells. Interestingly, the advantages of using CoPt
mesoporous NWs as an entirely magnetic catalyst are numerous: (i) it results in magnetic field-induced
convection, (ii) it can be used as a remote microstirring device, (iii) it allows easy and efficient
recyclability, (iv) it is very robust, (v) it benefits from a cost-effective and easily scalable synthesis that
does not require of hybrid multi-component catalysts and (vi) it reduces the cost of materials since the
amount of Pt precious metal is reduced. Hence, the high efficiency of magnetic actuation together with
the simplicity and cost-effective synthesis approach and the efficient recyclability of the magnetic
mesoporous NW catalysts make this approach very appealing to commercialize new viable alternatives
for ecological decontamination and clean energy production.

Acknowledgements
This work was supported by the EU ERDF (FEDER) fund and the Spanish Government grants
TEC2014-51940-C2-2-R

and

MAT2014-57960-C3-1-R

from

Ministerio

de

Economía

y

Competitividad (MINECO) –the latter cofinanced by FEDER. Partial funding from the European
Research Council (Consolidator Grant, project n. 648454, SPIN-PORICS) and the Generalitat de
Catalunya (2014-SGR-1015 project) is acknowledged. The authors thank the CCiT-UB for the use of
their equipment and A. Llobet for the hydrogen evolution experiments, funded by MINECO and
FEDER (CTQ-2016-80058-R, SEV-2013-0319, and CTQ-2014-52974-REDC) and EU COST actions
CM1202 and CM1205. ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant
SEV-2013-0295).

21

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