Cobalt tungsten oxide hydroxide hydrate (CTOHH) on DNA scaffold: An excellent bi-functional catalyst for oxygen evolution reaction (OER) and aromatic alcohol oxidation

Article abstract of DOI:10.1039/c9dt03941d

A material with interdisciplinary properties is of wide interest for use in environmental applications. Currently, hydrogen generation by electrolysis and formation of carbonyl derivatives from alcohols are two different fields that focus on energy and environmental applications. In this work, a new material, Cobalt Tungsten Oxide Hydroxide Hydrate (CTOHH) on deoxyribonucleic acid (DNA) scaffold having chain-like morphology has been prepared for the first time by a facile microwave heating method. The same CTOHH was also prepared without the DNA scaffold and resulted in irregular aggregated molecular structures. Further, both CTOHH-DNA and CTOHH were converted into CoWO₄-DNA and CoWO₄, respectively by annealing them at a temperature of 600 °C. All the four catalysts were used for electrocatalytic oxygen evolution reaction (OER) and for oxidation of aromatic alcohols. In OER, CTOHH-DNA delivered fruitful results compared to all other electrocatalysts. For attaining a current density of 10 mA cm^-2, it just required an overpotential of 355 mV with a Tafel slope value of 47.5 mV dec^-1. Similarly, all four catalysts were also analyzed for selective and controlled oxidation of aromatic alcohols to their respective aldehydes and ketones using molecular oxygen as a green oxidant where CTOHH-DNA showed better results. Chemo-selectivity has been observed for CTOHH-DNA in the co-presence of hydroxyl and cyano functional groups. The durability of CTOHH-DNA was analyzed and it showed excellent catalytic activity retention up to five cycles.

Full text of DOI:10.1039/c9dt03941d

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Cobalt tungsten oxide hydroxide hydrate (CTOHH)
on DNA scaffold: an excellent bi-functional
catalyst for oxygen evolution reaction (OER) and
aromatic alcohol oxidation†
Cite this: Dalton Trans., 2019, 48,
17117
c
Sangeetha Kumaravel,
‡
a,b Prabaharan Thiruvengetam,
‡
a,b
Sivasankara Rao Ede, a,b K. Karthick, a,b S. Anantharaj,
Selvasundarasekar Sam Sankar a,b and Subrata Kundu
a,b
*
A material with interdisciplinary properties is of wide interest for use in environmental applications.
Currently, hydrogen generation by electrolysis and formation of carbonyl derivatives from alcohols are
two different fields that focus on energy and environmental applications. In this work, a new material,
Cobalt Tungsten Oxide Hydroxide Hydrate (CTOHH) on deoxyribonucleic acid (DNA) scaffold having
chain-like morphology has been prepared for the first time by a facile microwave heating method. The
same CTOHH was also prepared without the DNA scaffold and resulted in irregular aggregated molecular
structures. Further, both CTOHH-DNA and CTOHH were converted into CoWO4-DNA and CoWO4,
respectively by annealing them at a temperature of 600 °C. All the four catalysts were used for electro-
catalytic oxygen evolution reaction (OER) and for oxidation of aromatic alcohols. In OER, CTOHH-DNA
delivered fruitful results compared to all other electrocatalysts. For attaining a current density of 10 mA
cm−2, it just required an overpotential of 355 mV with a Tafel slope value of 47.5 mV dec−1. Similarly, all
four catalysts were also analyzed for selective and controlled oxidation of aromatic alcohols to their
respective aldehydes and ketones using molecular oxygen as a green oxidant where CTOHH-DNA
showed better results. Chemo-selectivity has been observed for CTOHH-DNA in the co-presence of
hydroxyl and cyano functional groups. The durability of CTOHH-DNA was analyzed and it showed excel-
lent catalytic activity retention up to five cycles.
Received 7th October 2019,
Accepted 27th October 2019
DOI: 10.1039/c9dt03941d
rsc.li/dalton
perties compared to their bulk elements.1–3 Recently, because
of wide applications in different interdisciplinary fields, metal
Introduction
In recent times, tremendous research efforts have been oxide nanomaterials have been found to have tremendous
devoted to the successful generation of zero and one-dimen- importance among others.4–7 Among the various metal oxide
sional (1-D) nanomaterials due to their physico-chemical pro- nanomaterials, metal tungstates such as cobalt tungstates
show promising applications in energy related fields.8–10
Although there are very few reports on the synthesis of cobalt
tungstates,11–14 the formation of specific structures namely
aAcademy of Scientific and Innovative Research (AcSIR),
Cobalt Tungsten Oxide Hydroxide Hydrate (CTOHH) has not
been explored before.
Currently, there are numerous methods available to syn-
thesize 1D nanomaterials such as hydrothermal, solution
phase, polyol, and sol–gel methods.15–18 However, the above
methods are incapable of reaching the addressable effect in
CSIR-Central Electrochemical Research Institute (CECRI) Campus, New Delhi, India.
E-mail: skundu@cecri.res.in, kundu.subrata@gmail.com; Tel: +91 4565-241487
bMaterials Electrochemistry Division (MED), CSIR-Central Electrochemical Research
Institute (CECRI), Karaikudi-630003, Tamil Nadu, India
cDepartment of Chemistry, Indian Institute of Technology Madras, Chennai-600036,
India
†Electronic supplementary information (ESI) available: The details of materials
used in electrode fabrication and instruments used for various characterizations constructing nanostructures due to non-uniform nucleation
during the formation of nanostructures.19 Therefore, it is
highly desired to realize a simple bottom-up approach
are available. Supplementary figures include XRD, XPS, FE-SEM elemental
mapping, FT-IR, EDLC plots, surface concentration calculation, surface concen-
tration plots, general procedure for recycling, 1H NMR data, 13C NMR data of
method to fabricate such 1D nanomaterials. The microwave
aromatic carbonyl compounds, and comparison table for organic catalysis. See
assisted method is one of the best ways of fabricating 1D
DOI: 10.1039/c9dt03941d
‡Contributed equally for this work and publication.
nanomaterials owing to its efficient heat transfer and
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uniform irradiation via dielectric dissipation.16,18 More reaction (OER) owing to their excellent optical and electronic
precisely, the collisions of ions that are able to generate properties.54–56
homogeneous “incore” volumetric heat transfer leads to
Similarly, for the industrial chemical synthesis of value-
uniform nucleation of the nanomaterials. It could provide added products, the conversion of various alcohols to their
more control over the reaction mixture and improves corresponding carbonyl derivatives has gained importance. In
the reaction rate over conventional methods. Previously, line with the goals of environmental and energy sustainability,
our group reported the application of self-assembled the oxidation of alcohols to carbonyls under mild and environ-
ZnWO4, SnWO4 chain-like nano-structures for catalysis and mentally benign conditions are most desirable.57–59 The con-
supercapacitor applications within
time.20,21
a
short reaction version of alcohols to their corresponding carbonyl derivatives
is a key step in chemical and biological systems. In convention-
For the preparation of 1D nanoscale materials, DNA al methods, oxidation reactions are carried out by the addition
(deoxyribonucleic acid), which is a well-studied bio-mole- of stoichiometric quantities of oxidants, which leads to the
cule, was found to be more advantageous compared to generation of equal amounts of environmentally hazardous
others. DNA has specific properties, viz., its size can be and toxic by-products. Therefore, to address this issue, various
easily controlled, reasonable cost, ability to provide more transition metal catalyzed oxidation reactions have been exam-
than one binding sites, and versatile programmable chemi- ined using molecular oxygen as an eco-friendly, greener
cal structure with availability in different base oxidant for the selective oxidation of different aromatic
sequences.22–24 Due to these versatile properties, DNA mole- alcohols.60–64 Niasari et al. highlighted the influence of multi-
cule is highly efficient in fabricating self-assembled nano- walled carbon nanotubes based cobalt(II) Schiff complex for
materials with the desired morphology. DNA based synthesis the selective conversion of aliphatic and aromatic alcohols to
of nanomaterials involves three major steps; the first one is their carbonyl derivatives in an efficient manner.64 Therefore,
the electrostatic interaction between the precursor and DNA by looking at the advancements of 1D transition metals based
functionalities such as PO4−, OH−, CvO, and NH2. The catalysts in both electrocatalytic OER and selective oxidation of
second one is the formation of nuclei by reduction and alcohols, it is highly preferred to select a material for the fast
other chemical reactions, and the third one is growth, stabi- production of hydrogen and carbonyl derivatives. With an over-
lization, and controlled formation of nanomaterials on DNA view of these two different but important methods for indus-
scaffolds by nitrogen moieties of nucleobases.25,26 Also, DNA trial production of hydrogen and carbonyl compounds, the
has more advantages in electrocatalytic water splitting; it transition metal based catalysts, such as CTOHH-DNA, would
facilitates the charge movement across the electrode and the be fruitful and economically viable novel materials for
electrolyte interface, and it shows excellent stability.27,28 commercialization.
Most importantly, the phosphorous moiety on DNA synergis-
Herein, for the first time, we have succeeded in synthesiz-
tically improves the performance in oxygen evolution ing CTOHH nanomaterials having chain-like morphology on a
reaction.29,30 Hence, DNA has potential application in cataly- DNA scaffold. The average chain diameter of CTOHH-DNA was
sis, sensors, drug delivery, environmental devices, and other ∼110 15 nm whereas in the absence of the DNA scaffold, the
energy related fields.21,31–33
CTOHH nanoparticles (NPs) were agglomerated. The CTOHH
The production of hydrogen by electrolysis of water is a material acted as an efficient heterogeneous catalyst in electro-
growing technological field to meet the energy needs in the catalytic OER and for the selective oxidation of aromatic alco-
future for a sustainable environment.34,35 In electrolysis of hols to carbonyl derivatives for the first time. From the OER
water, oxygen is evolved at the anodic region and hydrogen is study, for CTOHH-DNA to attain a current density of 10 mA
evolved at the cathodic region.36,37 As the state-of-the-art cata- cm−2, an overpotential of 355 mV with a Tafel slope value of
lysts are noble catalysts, the findings of earth abundant based 47.5 mV dec−1 was required. From the catalytic study for the
catalysts for efficient hydrogen production is inevitable.38,39 selective and controlled oxidation of aromatic alcohols using
Recent literature analyses suggest that the efficiency at a large molecular oxygen as the green oxidant, promising results were
scale can be obtained by changing the morphology and incor- obtained with catalytic retention up to five cycles (74–91%). In
poration of metal NPs with incorporation of bimetallic future, the present process can be readily extended for the
systems.28,40,41 Different types of transition metal based cata- swift formation of other mixed metal tungstates for diverse
lysts have been studied in the form of metal nanoparticles applications such as in catalysis, electrocatalysis, or other
(NPs), bimetallic systems, oxides, layered double hydroxides energy related fields.
(LDH), chalcogenides, and recently, as phosphides in water
splitting.42–47 Recently, Cao et al. reported Co based catalysts
with different pH conditions for water splitting studies with
Experimental methods
Synthesis of CTOHH-DNA and cobalt tungstate nanomaterials
considerable improvements in their activities.48,49 Comparing
oxide based materials for OER, it is the hydroxides that show
better activities due to the exposed active sites and increased The CTOHH nanomaterials were prepared via microwave
rate of electron transport.50–53 Also, 3d transition metals based assisted process with DNA and without utilizing DNA as the
1D nanomaterials give tremendous activity in oxygen evolution scaffold. In
a typical synthesis, 50 mL of 0.1 M Co
17118 | Dalton Trans., 2019, 48, 17117–17131
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Scheme 1 Schematic illustration of the formation of CTOHH-DNA and CTOHH.
(CH3COO)2·4H2O solution was added to 25 mL of (0.12 M) chronopotentiometry (CP) study was done at a current density
DNA. Then, the mixture was kept stirred for 30 min for feasible of 10 mA cm−2 for 12 h. All the LSV data were 100% iR com-
electrostatic interaction between the positively charged metal pensated (except the long term CP analysis and LSV curves
ions and the negatively charged phosphate moieties of DNA. before and after CP analysis) manually with Rs acquired from
Initially, the mixture was found to be pink in color. Further, it the electrochemical impedance spectroscopic (EIS) analysis.
was placed in the microwave oven and irradiated for 10 s, fol- EIS analysis was conducted in the frequency range of 0.1 Hz to
lowed by the addition of 1 mL of 0.1 M, Na2WO4·2H2O into the 1000 Hz with 5 mV amplitude at potentials where the working
solution mixture. The microwave heating was stopped every 10 electrode could reach the benchmarking current density of
s and the setup was kept out from the oven for stirring. By 10 mA cm−2. The potential was converted to RHE in accord-
repeating the same, 50 mL of Na2WO4·2H2O was added gradu- ance with the earlier reports.65
ally and the total reaction time was calculated to be 8 min.
General procedure for the oxidation of alcohols to carbonyl
compounds
Following the same protocol, CTOHH was also prepared
without DNA. The prepared sample was centrifuged, washed
with ethanol and water several times, and dried at 70 °C for
12 h. Finally, the observed color of the as-synthesized
sample was purple. Further, the as-synthesized samples of
CTOHH-DNA and CTOHH were converted into CoWO4-DNA
and CoWO4, respectively, by annealing them at a temperature
of 600 °C. The synthetic scheme has been portrayed below in
Scheme 1.
The aromatic alcohols (0.75 mmol) were taken in a 15 mL
pressure tube containing 50 mg of catalyst and K2CO3
(2 equivalents) in 0.5 mL of toluene. The reaction was
degassed with nitrogen and pressurized by oxygen at 1 atm.
The resulting mixture was stirred for 24 hours at 110 °C. After
the completion of the reaction, the solvent was evaporated and
1 : 3 ratio of water–ethyl acetate solvent mixture was added.
The organic layer was phase separated and the process was
repeated 2–3 times. The resulting organic solvent was dried
Electrochemical measurements
All electrochemical experiments were conducted in a typical with anhydrous sodium sulfate and the solvent was removed
three electrode system that comprised of Pt foil as the counter under reduced pressure conditions. The crude mixture was
electrode, Hg/HgO reference electrode, and the working elec- purified by column chromatography using ethyl acetate–
trode coated with CTOHH material in 1 M KOH solution as the hexane as the eluent. The purified product was further con-
electrolyte. Linear sweep voltammetric (LSV) curves were firmed by 1H NMR and 13C NMR spectroscopic techniques,
obtained at the scan rate of 1 mV s−1 after three cycles of cyclic and has been discussed in detail under the results and discus-
voltammetry (CV) at the scan rate of 125 mV s−1. Long term sion section.
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X-ray diffraction (XRD) analysis
Results and discussion
The powder X-ray diffraction (XRD) studies of the as-syn-
thesized nanomaterials are shown in Fig. 2. Here, we stacked
the XRD patterns of both CTOHH-DNA (a) and CTOHH (b).
Both of them show similar diffraction peaks with semi-crystal-
line nature. The diffraction peaks observed at 2θ values of 9.6°,
23.8°, 29.4°, 35.5°, 40.8°, and 53.1° corresponded to the (110),
(222), (330), (510), (530) and (642) planes, respectively. The
peaks observed for both materials CTOHH-DNA (a) and
CTOHH (b) exactly matched with the JCPDS file no. 00-047-
0142. We carried out the XRD analysis for CTOHH-DNA and
CTOHH materials after annealing at 600 °C, which formed
CoWO4, as observed from the XRD analysis seen in Fig. S1 in
the ESI.†
From Fig. S1 in the ESI,† we observed that both
CTOHH-DNA and CTOHH materials were converted into poly-
crystalline CoWO4 on the DNA scaffold and CoWO4, respect-
ively. Also, we stacked the diffraction pattern of the annealed
materials CoWO4 on DNA (a) and CoWO4 (b). Both the pat-
terns, CoWO4 on DNA (a) and CoWO4 (b), showed similar diffr-
action peaks corresponding to the planes: (010), (001), (−110),
(011), (−111), (120), (002), (−201), (−211), (−112), (−220), (022),
(031), (−122), (−311), (−140), and (−123) (Fig. S1†). The crystal-
line planes of both the patterns were in accordance with the
earlier reports and perfectly matched with JCPDS file no. 00-
015-0867.11,12,67 By analyzing the XRD patterns in Fig. 2 and
Fig. S1,† we can understand that only microwave heating
resulted in the formation of semi-crystalline CTOHH
materials, and the combination of both microwave heating
and thermal annealing led to the formation of polycrystalline
CoWO4 material. Also, the JCPDS reference for Fig. 2 and
Fig. S1† describes the hydroxide nature of the microwave
heated room temperature sample and the pure oxide nature of
the annealed samples, which are different with respect to the
UV-Visible (UV-Vis) spectroscopic analysis
The UV-Visible analysis in Fig. 1 depicts the various stages
involved during the synthesis of CTOHH-DNA chain-like nano-
assemblies. As we can see in curve a, which is for DNA only,
showed a sharp peak at ∼260 nm due to the π–π* transitions of
aromatic nucleobases in the backbone. Aqueous cobalt acetate
solution (curve b) showed a broad peak in the range of
420–580 nm, which was assigned to the d–d transitions of the
cobalt complex. In case of the aqueous sodium tungstate solu-
tion, we did not notice any peak in the UV-Vis region (curve c).
Afterwards, we evaluated the mixture of DNA and cobalt
acetate solutions (curve d), and noticed a shift and decrease in
the intensity. The shift and change in the intensity confirms
the successful interaction between the cobalt ions and DNA.
The addition of sodium tungstate to the mixture of DNA along
with cobalt acetate mixture (curve e) resulted in the elimin-
ation of d–d transition peak of cobalt and the DNA peak was
shifted towards lower wavelength, which signified the success-
ful formation of CTOHH on DNA.66 Further, we examined the
adsorption spectra of CTOHH-DNA and CTOHH shown in
curve f and g, respectively. A broad hump observed in the
range of ∼230–310 nm for CTOHH-DNA (curve f) was due to
the presence and the interaction of DNA, and no significant
optical absorption peak was observed for CTOHH (curve g).
Also, the annealed samples of CoWO4-DNA and CoWO4
showed no significant peak in curve h and i, respectively.
The UV-Vis analysis clearly highlights the absorption
spectra of different reagents used for the synthesis of CTOHH
and further confirmed the successful interaction of CTOHH
with DNA.
Fig. 1 UV–Vis analysis of CTOHH-DNA chain-like nano-assemblies.
Curve a: Absorption band of pristine DNA, curve b: absorption band of
cobalt acetate, curve c: absorption band of sodium tungstate, curve d:
absorption band for the mixture of DNA and cobalt acetate, curve e:
absorption band for the mixture of DNA, cobalt acetate, and sodium
tungstate, curve f: absorption band of CTOHH-DNA, curve g: absorption
band of CTOHH, curve h: absorption band of CoWO4-DNA, and curve i:
absorption band of CoWO4.
Fig. 2 XRD patterns of CTOHH-DNA (a) and CTOHH (b).
17120 | Dalton Trans., 2019, 48, 17117–17131
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