Synthesis, characterization and application of pure and decorated with palladium mesoporous cobalt hydroxide hexagonal nanorings

Article abstract of DOI:10.1016/j.jallcom.2020.156422

For most of the catalytic reactions dealing with noble metals, a replacement of pure noble metals with hybrid noble metal-nanomaterials or nanocomposites is a well-stablished strategy for reducing the cost of the catalytic process. The application of this strategy is challenging and of interest for the scientific community. In this context, we report the design of Pd-anchored mesoporous cobalt hydroxide nanoring composites which, used as catalysts, can efficiently catalyze Suzuki C–C coupling reactions with excellent activity, versatility and selectivity due to the strong synergetic effect standing from its structure and composition. Hybrid systems based on mesoporous Co(OH)₂ nanoparticles and Pd are able to reach up to 88percent conversion of iodobenzene and 37percent yield of the biphenyl product at 24 h. Additionally, the photocatalytic performance of the hybrid system has been investigated in selective oxidation of benzylalcohol and organic dye degradation. This nanostructured material was able to selectively photo-oxidize benzyl alcohol to benzaldehyde under LED light with moderate catalytic activity (turnover numbers and turnover frequencies were of 4.62 and 0.78 h^?1 respectively with 95percent selectivity). In addition, the studied hybrid system was able to degrade methylene blue in water with good performance via a photocatalytic reaction by irradiation with UV light and LED light (the kinetic constant for a first order degradation was 1.03·10^?4 s^?1 and 1.24·10^?5 s^?1, respectively). Thus, our preliminary catalytic results show that these nanomaterials, based on Co(OH)₂ nanoparticles with low content of noble-metal, are promising candidates for their potential use in different industrial catalytic applications.

Full text of DOI:10.1016/j.jallcom.2020.156422

Journal Pre-proof
Synthesis, characterization and application of pure and decorated with palladium
mesoporous cobalt hydroxide hexagonal nanorings
Masih Darbandi, Ehsan Narimani, Diana Díaz-García, Santiago Gómez-Ruiz
PII:
S0925-8388(20)32786-9
DOI:
https://doi.org/10.1016/j.jallcom.2020.156422
JALCOM 156422
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 3 June 2020
Revised Date: 14 July 2020
Accepted Date: 15 July 2020
Please cite this article as: M. Darbandi, E. Narimani, D. Díaz-García, S. Gómez-Ruiz, Synthesis,
characterization and application of pure and decorated with palladium mesoporous cobalt hydroxide
hexagonal nanorings, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/
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Synthesis, characterization and application of pure and
decorated with palladium mesoporous cobalt hydroxide
hexagonal nanorings
Masih Darbandi 1,*, Ehsan Narimani 1, Diana Díaz-García 2 and Santiago Gómez-Ruiz 2,*
1 Nanomaterials research laboratory, Department of Physical Chemistry, Faculty of Chemistry, University
of Tabriz, Tabriz, Iran
2 COMET-NANO Group, Departamento de Biología y Geología, Física y Química Inorgánica, ESCET,
Universidad Rey Juan Carlos, Calle Tulipán s/n, E-28933 Móstoles (Madrid), Spain
*
Corresponding
authors:
Masih
Darbandi
email:
Masih.Darbandi@gmx.de,
masih.darbandi@tabrizu.ac.ir
Santiago Gómez-Ruiz email:
s
antiago.gomez@urjc.es
Credit authors Statement:
Masih Darbandi: Conceptualization, methodology, resources, supervision, writing - original draft and
writing - review & editing
Ehsan Narimani: Methodology, investigation, visualization, writing - original draft.
Diana Díaz-García: Methodology, investigation, visualization, writing - original draft.
Santiago Gómez-Ruiz: Conceptualization, methodology, resources, supervision, writing - original draft
and writing - review & editing.
All the authors have supervised and agreed with the final version of this manuscript.

Synthesis, characterization and application of pure and
decorated with palladium mesoporous cobalt hydroxide
hexagonal nanorings
Masih Darbandi 1,*, Ehsan Narimani 1, Diana Díaz-García 2 and Santiago Gómez-Ruiz 2,*
1 Nanomaterials research laboratory, Department of Physical Chemistry, Faculty of Chemistry, University
of Tabriz, Tabriz, Iran
2 COMET-NANO Group, Departamento de Biología y Geología, Física y Química Inorgánica, ESCET,
Universidad Rey Juan Carlos, Calle Tulipán s/n, E-28933 Móstoles (Madrid), Spain
*
Corresponding
authors:
Masih
Darbandi
email:
Masih.Darbandi@gmx.de,
masih.darbandi@tabrizu.ac.ir
Santiago Gómez-Ruiz email:
s
antiago.gomez@urjc.es
ABSTRACT
For most of the catalytic reactions dealing with noble metals, a replacement of pure
noble metals with hybrid noble metal-nanomaterials or nanocomposites is a well-
stablished strategy for reducing the cost of the catalytic process. The application of this
strategy is challenging and of interest for the scientific community. In this context, we
report the design of Pd-anchored mesoporous cobalt hydroxide nanoring composites
which, used as catalysts, can efficiently catalyze Suzuki C-C coupling reactions with
excellent activity, versatility and selectivity due to the strong synergetic effect standing
from its structure and composition. Hybrid systems based on mesoporous Co(OH)2
nanoparticles and Pd are able to reach up to 88% conversion of iodobenzene and 37%
yield of the biphenyl product at 24 h. Additionally, the photocatalytic performance of
the hybrid system has been investigated in selective oxidation of benzylalcohol and
organic dye degradation. This nanostructured material was able to selectively photo-
oxidize benzyl alcohol to benzaldehyde under LED light with moderate catalytic
activity (turnover numbers and and turnover frequencies were of 4.62 and 0.78 h-1
respectively with 95% selectivity). In addition, the studied hybrid system was able to
degrade methylene blue in water with good performance via a photocatalytic reaction by
irradiation with UV light and LED light (the kinetic constant for a first order
1

degradation
was
1.03·10-4
s-1
and
1.24·10-5
s-1, respectively). Thus, our preliminary catalytic results show that these nanomaterials,
based on Co(OH)2 nanoparticles with low content of noble-metal, are promising
candidates for their potential use in different industrial catalytic applications.
Keywords
Nanostructured materials; Composite materials; Chemical synthesis; Catalysis; Kinetics
1. Introduction
Systems based on noble metals (such as Pt, Pd, etc) are one of the most active
materials with good catalytic performance and have extensively been used for different
catalytic reactions ranging from organic processes such as C-C couplings, carbonylation
or oxidation reactions[1–9] to inorganic processes such as water splitting [10,11].
The main problem regarding the use of noble metal-based catalysts is the high
price and low abundance of some of the metals, which confines their applications. In
order to solve the problems associated with the use of noble metals in some important
reactions, in recent years a different strategy has been developed by depositing or by
doping with noble metals into the structure of nanoparticles (NPs).
Although considerable efforts have been carried out to reduce the cost of
fabrication and to improve the resistance to deactivation in oxidation reactions and the
development of other non-noble-metal catalysts containing abundant elements has
increased over the past decade [12–17], the support of noble metal on nanostructured
materials is still the most widely used strategy in catalysis.
Nowadays, the loading of noble metal nanoparticles is carried out via two
different methods: a) directly during the synthetic process of the supporting
nanomaterial [18–21], b) by post-synthesis procedures on the surface of NPs or metal
ions with a noble-metal precursor [4,22–29].
In this context, the presence of noble metals on porous nanomaterials acting as
supporting systems can enhance the catalytic performance of the materials, by providing
them with higher active surface, enhancing the catalytic interactions. Therefore, a high
number of research groups have already reported studies on the support of noble-metal
ions [30–34] or metal nanoparticles [31,35–39] onto high-surface nanomaterials for
various applications.
2

An exhaustive study of the literature indicated that, to the best of our knowledge,
there is not an established chemical method available to load noble-metals (especially
Pd) on mesoporous Co(OH)2 NPs. This encouraged us to study more in detail the
potential functionalization with nobel metal ions or nanoparticles of this kind of
nanostructured Co(OH)2, as well as the potential synergistic effect of both metals.
Thus, this work reports an easy, efficient and cost effective two-step process for
the synthesis of palladium-loaded Co(OH)2 NPs. The first step is the controlled
synthesis of mesoporous nanorings of Co(OH)2 NPs with an hexagonal morphology and
subsequently the functionalization with Pd ions on the surface of Co(OH)2 NPs by the
reduction reaction of an organometallic Pd(II) precursor at mild temperature.
The resultant nanohybrids were used for the catalytic study of Suzuki C-C
coupling reactions and the catalytic results showed a good performance of the
synthesized hybrid materials with high conversions at 110 ºC in the reactions of
iodobenzene or bromobenzene and phenylboronic acid. In addition, the synthesized
materials were tested as catalysts in the selective oxidation of aromatic alcohols to
aldehydes, as this is a current challenge of the chemical industry. Furthermore, the
synthesized materials have been tested in the photo-oxidation of benzyl alcohol as a
model reaction, observing a good catalytic activity and selectivity in the formation of
benzaldehyde. Finally, the synthesized nanohybrids were also used as photocatalyst in
the decomposition of an organic dye such as methylene blue (MB) under LED and UV
light irradiation, as a model of a water decontamination reaction (as MB is widely used
as dye in cosmetics, food industries, pharmaceutical and polymeric process and is found
in waste waters of those industries).
2. Experimental part
2.1. Preparation of Co(OH)2 nanoparticles
Mesoporous hexagonal cobalt hydroxide NPs were prepared through a chemical
precipitation method following similar procedures to those described by our group [13].
An amount of cobalt precursor (CoCl2) was added in a three-necked flask and with
vigorous stirring dissolved into pure H2O at room temperature. In a subsequent step, a
certain amount of NaOH solution was slowly poured into the above-mentioned solution
3

with intense stirring, in which the pH was controlled and maintained around 9.0 and an
aqueous solution of 30% H2O2 was quickly added to the solution.
After the completion of the precipitation reaction, the NPs were separated from
o
the solution by centrifugation, and finally heated in an oven at 120 C for about three
hours.
2.1.1. Characterization
For the preparation of TEM samples, a drop of a suspension containing the final
NPs was casted on a copper TEM grid coated with an amorphous C film. The TEM
measurement was conducted using a Philips CM12 microscope working at 120 kV. The
XRD experiments were carried out using a Philips, PW1730a (commercial θ-2θ X-ray
diffractometer). The measurement was conducted at RT with the characteristic Kα X-
ray lines of Cu.
2.2. Preparation of the hybrid materials Co(OH)2-Pd-1
2.2.1. Synthesis of the Pd precursor
8.6 mmol (1.53 g) of PdCl2 was dissolved in concentrated (35%) HCl (6 mL). The
resultant mixture was diluted with 150 mL of ethanol and filtered through a filter paper.
The filter paper and the brown residue were washed twice with 10 mL of ethanol.
Subsequently, 20.4 mmol (2.5 mL) of 1,5-cyclooctadiene was added to the PdCl2
solution with stirring. The color of this solution changed immediately from brown to
orange and then a yellow-orange solid precipitated. The solution stirred for additional
20 minutes and filtered. The yellow-orange precipitated was washed with diethylether
(3 · 10 mL). Finally, the product was dried overnight under vacuum to give about 16.2
mmol (2.29 g) of [PdCl2(cod)] (yield: 96%).
2.2.2. Preparation of hybrid Pd-Co(OH)2 systems
Functionalization of Co(OH)2 was carried out using the Pd precursor [PdCl2(cod)]
and Co(OH)2 following a general procedure described previously by our research group
[40–43]. Briefly, 50 mg of Co(OH)2 and 4.2 mg of [PdCl2(cod)] (to give a theoretical
loading of Pd about 1% wt.) were added to a Schlenk tube and dried in vacuum for
about one hour at RT. Subsequently, 50 mL of dimethylformamide (DMF) was added to
this solution under an inert atmosphere. The mixture was then heated to 110 °C and
stirred for three additional hours. The product was then separated by filtration and
4

washed twice with 10 mL of DMF and 10 mL of diethylether. The hybrid product
Co(OH)2-Pd-1 was dried under vacuum at RT for about 12 hours to remove the trace of
solvents.
2.3. Studying the catalytic performance in Suzuki C-C coupling reactions
The catalytic studied was carried out on the reaction of phenylboronic acid and
iodobenzene or bromobenzene. Two different temperatures 70 and 110 °C were tested
and the reaction mixture was 95:5 DMF:water in the presence of K2CO3 as a base. The
catalytic study was performed at different reaction times (6, 24 and 48 hours). The
experiments were carried out under the same conditions in order to compare the results
and facilitate a subsequent analysis. The reactions were conducted using bromo- or
iodobenzene as limiting reagent and the molar ratio between the boronic acid and the
halide was 1.2:1. Additionally, the molar ratio between the K2CO3 base and the halide
was 2:1. The amount of catalyst in all cases was 20 mg. To achieve higher final catalytic
performance all reactions were conducted using degassed solvents and under inert
atmosphere [44].
X
B(OH)2
K2CO3
+
Co(OH)2-Pd-1
X = Br, I
Scheme 1. C-C coupling reaction catalyzed by Co(OH)2-Pd-1
Briefly, a 0.02 M solution of 1-bromobenzene or 1-iodobenzene in a mixture of
degassed solvents (DMF:H2O 95:5) was added under N2 atmosphere to a Schlenk tube.
In a parallel step, another Schlenk flask was filled with 7.3 mg (0.060 mmol)
phenylboronic acid, and K2CO3 (13.8 mg, 0.100 mmol) and Pd catalyst (20 mg of
Co(OH)2-Pd-1, 0.2 mg of Pd, 1.87·10-3 mmol Pd, 3.7% mol Pd in the reaction). To
remove adsorbed water from the solids and oxygen from the reaction atmosphere 3
times vacuum/N2 cycles (10 min/1 min) were conducted.
Later, 0.05 mmol (2.5 mL) of the solution 0.02 M of 1-iodobenzene or 1-
bromobenzene were transferred under nitrogen to the Schlenk flask containing the solid
product. Subsequently, the solution heated to the desired temperature (110 or 70 °C)
with a condenser. Solution was then stirred for 6, 24 and 48 hours. In the next step, the
5

mixture was cooled to RT and filtered with a nylon filter (0.22 µm). Finally, the product
was analyzed with a GC-FID instrument (Perkin-Elmer GC Clarus 580) using a
Velocity® column (dimethylpolysiloxane, 30 m, 0.25 mm, 1.00 µm) and a temperature
program (Fig. 1) with an injection temperature of 240 ºC and a detector temperature of
300 ºC.
Fig. 1. Temperature program for GC quantification of catalytic and photocatalytic products
2.4. Photocatalytic study
2.4.1. Benzyl alcohol oxidation in the absence of light
Co(OH)2 NPs were used as heterogeneous catalysts for the oxidation of benzyl
alcohol in regular catalytic conditions (in the absence of light) and under LED or UV
light, in order to clarify their photocatalytic and catalytic performance and its selectivity
in the formation of benzaldehyde.
In a normal catalytic reaction, 48.1 mmol (5 mL) of benzyl alcohol, 19.6 mmol
(2.00 mL) of H2O2 solution in H2O 30 % (w/w) and 25 mg of catalyst were mixed under
N2 atmosphere in a Schlenk tube. The mixture was stirred at 110 ºC for 6 hours. Later,
the reaction product was filtered with a nylon filter (0.45 µm) and the solid product was
analysed by GC-FID (Perkin-Elmer GC Clarus 580) with a Velocity® column
(dimethylpolysiloxane, 30 m, 0.25 mm, 1.00 µm) to estimate the conversion efficiency
6

to benzaldehyde, using the temperature ramp of Fig. 1. The injection temperature was
240 ºC and the detector temperature was 300 ºC.
2.4.2. Benzyl alcohol photo-oxidation under UV light
The reaction was carried out using the same process described in 2.4.1 but at RT
and using a UV lamp of 60 W and 360 nm (Scheme 2b)
2.4.3. Benzyl alcohol photo-oxidation under LED light
The reaction was carried out using the same process described in 2.4.1 but at RT
and using a LED lamp of 60 W (6000 K) (Scheme 2c)
HO
O
H
H2O2 / 100oC
solvent-free
a)
Co(OH)2
HO
O
H
360 nm
60 W
H2O2 / rt
solvent-free
b)
Co(OH)2
HO
O
H
LED light
60 W
H2O2 / rt
solvent-free
c)
Co(OH)2
Scheme 2. Studied catalytic and photocatalytic oxidation of benzylalcohol to benzaldehyde
2.4.4. Methylene blue (MB) photodegradation under UV-light
MB photodegradation experiments were carried out under UV-light, 15 mg of
the Co(OH)2 NPs was added to 50 mL of a 0.02 mM MB solution. The reaction was
carried out in a black box with a UV lamp (365 nm, 36W) as light source. Before
7

irradiation, the studied suspensions containing the photocatalyst and MB were stirred
for 1 h in the dark to reach the adsorption/desorption equilibrium. Subsequently, the
suspensions were exposed to UV-light for a maximum of 3 hours. The decomposition of
MB was monitored by measuring the absorbance (every 15 min) with a UV-
spectrophotometer (Jena Analytik) at 660 nm wavelength (absorbance maximum of
MB). The experiments were carried out in duplicate and the absorbance measured in
triplicate.
2.4.5. Methylene blue photodegradation under LED light
MB photodegradation experiments were carried out using the same conditions
described in the section 2.4.4 with a visible LED lamp (4.5 W, 6000 K, AS-Schwabe
42411) as the radiation source. The reaction time was 16 h and the experiments were
conducted in duplicate and the absorbance measurement in triplicate.
3. Results and discussion
3.1. Physical characterization of NPs
Fig. 2. a) TEM image of β-Co(OH)2 NPs and b) detail of the hexagonal nanorings, including data of the
thickness and edges.
A morphological analysis of the nanostructure of the synthesized Co(OH)2 NPs
was studied by TEM. As previously reported by our group [13] and observed in Fig. 2,
the Co(OH)2 NPs present an hexagonal nanoring morphology. In addition, a few rod-
like NPs could also be observed in the TEM image, which means that some of the
synthesized NPs stand on the side of the hexagonal nanorings.
8

Interlayer spacing of the as-synthesized NPs was also characterized (Fig. 2b inset) to
confirm, as previously reported by our group, the presence of β form (not α) [13]. The
measured interlayer lattice spacing was about 0.27 nm which fit well to crystal plane of
(100) in β-Co(OH)2 structure.
The crystalline structure and phase of the as-synthesized Co(OH)2 NPs was also
characterized by XRD (Fig. 3). Diffraction peaks were in agreement with those
observed for β-Co(OH)2 NPs (JCPDS file no. 30-443). There were no additional phases
observed in the XRD pattern, indicating a high degree of purity of the obtained
crystalline phase. The pattern of Co(OH)2 NPs gave signals at 2θ 20, 39, 50, 65 and 92º
assigned to Miller indices (001), (102), (002), (111) and (200), respectively.
Fig. 3. Powder X-ray diffractogram of β- Co(OH)2 NPs
The N2 adsorption–desorption measurements (Fig. 4) gave a type IV isotherm
(according to IUPAC classification) with a considerable hysteresis loop, which confirms
the mesoporous nature of the studied NPs. The BET specific surface area of the NPs
determined from the isotherm was ca. 215 m2 g-1. To estimate the pore size distribution
of the sample a BJH approach was used, observing that the mean pore diameter of the
NPs was ca. 4.8 nm, which is also in agreement with the nature of the prepared NPs as a
mesoporous solid material [13].
9

Fig. 4. Nitrogen adsorption–desorption isotherm of the mesoporous β-Co(OH)2 nanorings in STP
condition (left) and BJH pore-size distribution of the mesoporous β-Co(OH)2 nanorings (right)
3.2. Preparation of Co(OH)2-Pd-1
A hybrid material based on Co(OH)2 and palladium was prepared by the reaction of the
nanostructured Co(OH)2 with [PdCl2(cod)] in DMF for 3 hours. This reaction allowed
the formation of a hybrid material formed by supported Pd, obtained probably by the
reduction of [PdCl2(cod)], as described previously by our group for other support
materials such as titanium oxide, silica or alumina or graphene oxide [40–43,45].
3.3. Catalytic study of Co(OH)2-Pd-1 in Suzuki C-C coupling reactions
Noble metals (especially Pd) can effectively catalyse C-C cross-coupling reactions. In
this context, Suzuki-Miyaura process is one of the most widely studied reaction for the
synthesis of biphenyl products. To evaluate the catalytic performance of the hybrid
catalyst, we used Co(OH)2-Pd-1 in two different Suzuki C-C coupling reactions,
namely, 1-bromobenzene with phenylboronic acid and 1-iodobenzene with
phenylboronic acid (Scheme 1). For this kind of reactions, we had already observed that
the most interesting temperatures to explore were 70 and 110 ºC using K2CO3 as base
and mixture of DMF/H2O (95:5) as solvent [45]. In order to determine the evolution of
the reaction kinetics, the conversion and yields were monitored at different reaction
time intervals of 6, 24 and 48 hours.
Table 1. Halide conversions and Ph-Ph yields in C-C coupling reactions using 1-bromobenzene or 1-
iodobenzene and phenylboronic acid catalyzed by Co(OH)2-Pd-1.
TOF
Halide
Time
(h)
T
TON
Ph-Ph yield
(molar %)
conversion
conversion
(molar %)
Halide
conversion
(oC)
(h -1)
10

6
20
66
70
21
71
72
25
78
80
29
82
88
53
176
187
56
8.9
7.4
3.9
9.4
7.9
4.0
11
9.1
21
23
8.3
24
26
12
32
36
14
35
37
24
48
6
70
110
70
Br
Br
I
24
48
6
189
193
67
24
48
6
209
214
78
8.7
4.5
13
24
48
110
219
235
9.1
4.9
I
The obtained results of the C-C coupling reactions are given in Table 1 and Fig. 5. The
catalytic study shows that the highest conversions were found at 110 ºC. In this context,
the hybrid catalyst Co(OH)2-Pd-1 was able to reach up to 88% conversion of
iodobenzene and 37% yield of the biphenyl product at 24 h, while the conversion and
yield slightly decreased to 72% and 26% in the case of the use of bromobenzene. These
results are comparable, if not somewhat better, than those obtained for other supported
systems based on graphite, graphene oxide, or silica and Pd nanoparticles [4,40,45].
Fig. 5. Change of conversion with time in the reaction of 1-bromobenzene or 1-iodobenzene and
phenylboronic acid catalyzed by Co(OH)2-Pd-1 at 70 °C or 110 °C.
11

3.4. Catalytic and photocatalytic study of Co(OH)2-Pd-1 in benzyl alcohol oxidation
For the production of high valuable organic raw materials at industrial scale, the
selective oxidation of alkylaromatics is a crucial point. The development of catalytic
oxidation systems in the nanoscale range under mild synthesis conditions, is an
attractive research field. Therefore, the interest in this field of research arises not only
because of its importance in academic research but also because of its potential
industrial application. For example, the selective oxidation of benzyl alcohol to
benzaldehyde has remarkable importance in the food and cosmetic industry.
Thus, the catalytic study was focused on the selective oxidation of benzyl alcohol as a
model process to investigate the catalytic performance of Co(OH)2-Pd-1 in oxidation
reactions. The catalytic activity of Co(OH)2-Pd-1 in oxidation was studied with H2O2
as a green oxidant agent for the oxidation of benzyl alcohol. Normally, H2O2 is a
greener oxidant than some other organic agents such as TBHP. Hydrogen peroxide is
cheaper than most of the organic oxidants and its harmful environmental effect is much
lower. Furthermore, the reactions were carried out in the absence of solvent, using the
benzyl alcohol as the reaction medium. In addition, hydrogen peroxide was chosen as
oxidant reagent because it is a better oxidant when using inorganic materials, due to its
high mobility onto the materials surface.
At these conditions after 6 h at room temperature, the reaction using Co(OH)2-Pd-1 as
catalyst did only give traces of benzaldehyde. The reaction temperature was, therefore,
increased to 110 ºC in order to force the catalytic oxidation, however, only the
formation of traces of benzaldehyde resulting from a very low conversion of benzyl
alcohol, was again observed.
However, considering the interesting photocatalytic properties of Co(OH)2 in water
photo-oxidation [46], and that several nanostructured oxides functionalized with Pd are
very active in catalytic oxidation of alcohols [47–49], we tested the photocatalytic
benzyl alcohol oxidation by using Co(OH)2-Pd-1 under UV and LED light (Table 2).
The results show that, under UV light (360 nm) the selectivity towards benzaldehyde is
higher than 99% but the conversion is not very high (TON of 0.81 and TOF of 0.13 h-1).
However, the photocatalytic activity increased when using LED light (with the same
potency of 60 W, the TON is 4.62 and TOF 0.78 h-1). Nevertheless, when using LED
light, the selectivity towards benzaldehyde slightly decreases to 95%. This phenomenon
may rely on a slightly different, less selective, mechanism of photo-oxidation which
12

may be promoted by the use of different light sources and, therefore, different activation
environment [50].
Table 2. Catalytic and photocatalytic results of the oxidation of benzyl alcohol in 6 hours of reaction with
Co(OH)2-Pd-1 as catalyst.
TON
TOF
Selectivity to
benzaldehyde
(%)
T
Light
TON
TOF
(benzyl
benzoate) benzoate, h-1)
(benzyl
(oC) source (benzaldehyde) (benzaldehyde, h -1)
none
UV
-
-
-
-
-
0.81
4.6
0.1
0.8
3.4·10-3
0.28
5.7·10-4
0.1
>99
95
rt
LED
3.5. Photocatalytic degradation of methylene blue with Co(OH)2-Pd-1
Additionally, the photocatalytic activity of the synthesized NPs was investigated in
organic dye decomposition. At this photodegradation experiment, MB was taken as a
model of water contaminant. To reach the adsorption-desorption equilibrium, in a first
step, the mixture of the solution of MB in water and Co(OH)2-Pd-1 was stirred in the
dark for one hour. Later, the degradation was studied with a 36W UV lamp (365 nm)
irradiation for 2 hours. Fig. 6 shows the results that confirm that the decomposition of
MB rules with a first order kinetics. Therefore, the apparent rate constant for the
decomposition (k, s-1) was estimated from the regression curve of ln(C/C0) vs the time
of irradiation. The value of k for Co(OH)2-Pd-1 was 1.03·10-4 s-1.
13

Fig. 6. Kinetic study of the MB degradation by Co(OH)2-Pd-1 during 2 h under UV light (36 W).
Finally, a photocatalytic degradation experiment using visible LED source (4.5 W) for
16 h was carried out (Fig. 7). The results show that the degradation of MB was slower
than when using UV light, the kinetic constant (for a first order degradation) was
calculated from the degradation curve and its value was 1.24×10-5 s-1. In this case, the
change of UV light to LED light did not lead to an increase of the photoactivity but to a
considerable decrease. It is important to note that this experiment was designed for the
same consumption of W·h (4.5 W for 16 hours) than in the case of UV light (36 W for 2
hours) in order to compare the final photoactivity. In this context, the differences in the
photocatalytic degradation activity may probably be to the extension of time in the LED
degradation, which increases the possibility the recombination of electron-hole pair,
precluding the decomposition process and decreasing, therefore, the photocatalytic
performance.
14

Fig. 7. Kinetic study of the MB degradation by Co(OH)2 NPs during 16 h under LED light (4.5 W).
4. Conclusions
Herein we have provided a promising and facile strategy for the preparation and
functionalization with a simple doping of the material with 1% Pd nanoparticles, of NPs
with mesoporous structure of hexagonal nanorings for different catalytic reactions. In
this study we have demonstrated the high versatility of the nanostructured system,
which is able to selectively photooxidize benzyl alcohol to benzaldehyde under LED
and UV lights and to degrade methylene blue in water via a photocatalytic reaction.
This could be attributed to the unique mesoporous structure and the effect of the
multiple active catalytic sites. In addition, the hybrid system catalyzed Suzuki cross-
coupling reactions, confirming that these cobalt-based materials may be further
explored in other catalytic and photocatalytic reactions for improving their potential
applicability in other industrial processes. This study also confirmed that such
mesoporous hexagonal nanorings of Co(OH)2 may be helpful in the future treatment of
environmental problems such as water purification or high value organic materials
production. Future work of our group will now be focused on the use and determination
of the optimal properties for the catalytic application of other systems based on other
cheaper metals such as copper.
Acknowledgment
15

This work has been supported by the Center for International Scientific Studies &
Collaboration (CISSC), Ministry of Science Research and Technology. S. G-R. and
D.D-G. would also like to thank Ministerio de Ciencia, Innovación y Universidades of
Spain (Grant no. RTI2018-094322-B-I00).
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18

• Pd-anchored mesoporous beta-cobalt hydroxide hexagonal nanorings have been
synthesized.
• Catalytic activity for Suzuki C-C coupling reactions has been tested.
• Photocatalytic and photodegradation performance of the nanosystems have been
investigated.
• Nanosystems were able to degrade organic dyes in water with good performance.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: