Synthesis of coordination polymers of cobalt meso-pyridylporphyrins and its oxygen reduction properties

Article abstract of DOI:10.1016/j.molstruc.2021.130032

The present work sheds light on the role of coordinating side groups on the porphine ring in deciding the extent of the metalation reaction as well as the formation of coordination polymers of porphyrin. Electrocatalytic activity of coordination polymers of isomers of pyridylporphyrins has been investigated after pyrolysing them at different temperatures. The pyrolysed materials were characterized by XRD, XPS, SEM, TEM and BET techniques. The electrocatalytic behaviour of the sample were investigated by modifying the glassy carbon electrode with the sample (in both normal and pyrolysed form). The electrode modified using the pyrolysed sample of polymerized pyridylporphyrin at 800 °C acted as a good electrocatalyst for the reduction of dioxygen. The studies on kinetics of oxygen reduction using rotating disk electrode shows the reduction through 2-electron pathway forming H₂O₂ in alkaline medium.

Full text of DOI:10.1016/j.molstruc.2021.130032

Journal of Molecular Structure 1232 (2021) 130032
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Synthesis of coordination polymers of cobalt meso-pyridylporphyrins
and its oxygen reduction properties
∗
Neha Saran, Tincy Lis Thomas, Purushothaman Bhavana
Birla Institute of Technology and Science (BITS), Pilani, K. K. Birla Goa Campus, Goa, 403726, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The present work sheds light on the role of coordinating side groups on the porphine ring in deciding
the extent of the metalation reaction as well as the formation of coordination polymers of porphyrin.
Electrocatalytic activity of coordination polymers of isomers of pyridylporphyrins has been investigated
after pyrolysing them at different temperatures. The pyrolysed materials were characterized by XRD, XPS,
SEM, TEM and BET techniques. The electrocatalytic behaviour of the sample were investigated by mod-
ifying the glassy carbon electrode with the sample (in both normal and pyrolysed form). The electrode
modified using the pyrolysed sample of polymerized pyridylporphyrin at 800 °C acted as a good elec-
trocatalyst for the reduction of dioxygen. The studies on kinetics of oxygen reduction using rotating disk
electrode shows the reduction through 2-electron pathway forming H2O2 in alkaline medium.
Received 16 November 2020
Revised 16 January 2021
Accepted 27 January 2021
Available online 31 January 2021
Keywords:
Pyridylporphyrins
Coordination polymer
O2 reduction
Pyrolysis
Metalation
© 2021 Elsevier B.V. All rights reserved.
RDE
1. Introduction
also on the heteroatom present on it, the self-assembling proper-
ties/supramolecular architecture are decided [8].
Porphyrins with aromatic groups at the meso position are
macrocyclic molecules with aromatic π- system that has differ-
ent types of sites available for further functionalization. It forms
complexes with a variety of metal ions of varying sizes, oxida-
tion number and coordination properties. Depending on the above
mentioned properties, it forms planar [1], siting-a-top [2] or sand-
wich complexes [3]. Based on its coordination properties, the metal
centre in planar porphyrin can be tetra (with only porphyrin lig-
and) or, penta or hexa (with porphyrin and, one or two axial lig-
ands respectively) coordinated. Though the coordination chemistry
of the centre part of this class of molecules is extensively investi-
gated, the same at the periphery of the molecule has received less
attention [4,5].
One of the special classes of heteroaromatic porphyrins is meso-
pyridylporphyrins. In this, the substituents at the meso-positions
have nearly same size as that of phenyl in the tetraphenylpor-
phyrin [compared to the other side groups of bigger (naphthyl, an-
thracenyl, quinolinyl etc.) or smaller (imidazolyl, thienyl etc.) size
generally seen in synthetic porphyrins]. The presence of nitrogen
on the side group makes this porphyrin very interesting in its coor-
dination behaviour (as in imidazolylporphyrins). Pyridylporphyrins
are known for sensing [9–11] and catalytic [12,13] activities. They
are known for their tendency to form coordination polymers with
different metal ions [14–18]. Their hydrogen bonding properties
also have been reported [19]. Pyridine molecules are known for the
coordination properties [20]. One of the nonameric porphyrin re-
ported in literature has a tetrapyridylporphyrin as one of the con-
stituents [21]. The metalation reactions of pyridylporphyrins are
generally carried out using the solvent dimethylformamide (DMF)
under normal metalation condition [22–24], hydrothermally [16] or
in presence of acid or organic base [12,25]. The compounds derived
from these reactions are further studied in DMF for their applica-
tions.
In meso-tetraphenylporphyrins, the four phenyl groups present
at the meso position of the molecule are known to be oriented per-
pendicular to the centre part of the molecule [6,7]. These groups
do not influence the coordination chemistry of the molecule to a
great extent. Other than phenyl groups, porphyrinic molecules with
alkyl and other aromatic (homo- or hetero-) groups are well re-
ported. Depending on the size and shape of the meso-groups and
In the research on fuel cells, the cathodic process attained more
attention due to the slow kinetics of dioxygen reduction on con-
ventional electrodes. The use of platinum as the electrode, though
the most effective found till date, is limited in application due to
its high cost and scarcity. One of the aims of research on fuel cells
is to replace the Pt electrode with an economically viable elec-
∗
Corresponding author at: Department of Chemistry, Birla Institute of Technology
and Science (BITS), Pilani, K. K. Birla Goa Campus, Zuarinagar, Goa, 403 726, India.
E-mail addresses: tincy@goa.bits-pilani.ac.in (T.L. Thomas), pbhavana@goa.bits-
pilani.ac.in (P. Bhavana).
https://doi.org/10.1016/j.molstruc.2021.130032
0
022-2860/© 2021 Elsevier B.V. All rights reserved.

N. Saran, T.L. Thomas and P. Bhavana
Journal of Molecular Structure 1232 (2021) 130032
The metal content in each catalyst was analysed by induc-
tively coupled plasma-atomic emission spectrometry (ICP-AES, AR-
COS Simultaneous ICP Spectrometer SPECTRO Analytical Instru-
ments GmbH, Germany). A FEG 250 (Quanta) scanning electron mi-
croscope (SEM) was used to investigate the morphology of catalyst.
Transmission electron microscopy (TEM) images were recorded on
Tecnai G2, F30 transmission electron microscope at an acceleration
voltage of 300 kV. Powder X-ray diffraction patterns (XRD) were
recorded on Bruker D8 Advance diffractometer. X-ray photoelec-
tron spectra (XPS) of the synthesized catalysts were recorded on
a PHI 5000 Versa Prob II (FEI Inc.) spectrometer using Al Kα radi-
ation (1486.6 eV). A Microtrac BEL Corp mini-II surface area anal-
yser was used to measure the surface area, pore size distribution
and N2 physisorption isotherms of the materials at 77 K. Degassing
was carried out in vacuum at 100 °C for 2 h on the synthesized
materials before starting the sorption measurements.
The linear sweep voltammetry experiments and the experi-
ments using rotating disc electrode (RDE-2, Bioanalytical systems)
were carried out using BASi EPSILON model electrochemical work
station. Modified glassy carbon electrode, Ag/AgCl electrode and
spiral platinum wire were used, respectively, as working, reference
and auxiliary electrodes. All measurements related to reduction
of dioxygen were performed in 0.1 M KOH solution purged with
dioxygen for 30 min (unless otherwise mentioned).
Fig. 1. Molecular structure of the compounds used for the study.
trode. In this scenario, non-noble metal catalysts have gained im-
portance in oxygen reduction reactions (ORR). Extensive research
has been carried out using porphyrin as catalyst (electrode mod-
ifying agent) in the electrochemical reduction of dioxygen. Por-
phyrins, either used as such [26,27] or used in the form of hy-
brid [28] or composite [29] with other suitable materials, are in-
vestigated as the electrode modifying agents. The electrochemical
dioxygen reduction properties of pyridylporphyrins in its water sol-
uble form is widely studied [22,30–38]. Water insoluble pyridyl-
porphyrins are relatively less explored for the same application
2.2. Synthesis
2.2.1. Synthesis of free base meso-tetrapyridylporphyrin (H2APyP,
A = 2, 3 or 4)
The free base porphyrins were synthesized by following the
general procedure for the synthesis of free base porphyrins by
Adler et al. with modifications in it, using the respective aldehydes
[45,46]. Typically, to 120 mL of propionic acid under refluxing con-
ditions, 2.7 mL (29.8 mmol) of pyridine-4-carboxaldehyde followed
by 2 mL (29.8 mmol) of pyrrole were added. This was followed
by the addition of 4.8 mL of acetic anhydride in the reaction mix-
ture. Refluxing the reaction mixture was continued for 30 min. At
the end of this period, the crude product was collected by distilla-
tion under reduced pressure. The free base porphyrin was purified
by column chromatography using neutral alumina as the packing
material and chloroform - methanol as the eluting mixture. The
product obtained was purple in colour. It was characterized by UV
visible, 1H NMR and mass spectroscopic techniques.
[
39–44].
In the present work, metalation of pyridylporphyrin using
cobalt acetate was carried out using the commonly used chloro-
form – methanol solvent system. The primary aim of the study
was to investigate the coordination behaviour of the meso-groups
of different isomers of porphyrin. So we have selected pyridylpor-
phyrins as its coordination chemistry depends on the position of
the pyridinic-N. The study also intends to find out a method of in-
creasing the cobalt content in the synthesized (metalated) product
for ORR. The prepared compounds were pyrolysed under different
temperature and their electrocatalytic oxygen reduction properties
were investigated. Fig. 1 shows the molecular structure of the por-
phyrins used for the present investigation and the abbreviations.
The yield of the product was 650 mg (14.59%), 810 mg (18.2%)
and 950 mg (21.3%) for H22PyP, H23PyP and H24PyP, respectively.
2
.2.2. Synthesis (of polymeric form) of cobalt containing meso-
2
. Experimental
tetrapyridylporphyrin (p-CoAPyP, A = 2, 3 or 4). Insertion of cobalt
ion in free base porphyrin was carried out using two different
equivalents of cobalt(II) acetate tetrahydrate.
2
.1. Materials and methods
(
a) Using five equivalence of cobalt acetate tetrahydrate
Pyrrole, 2-pyridinecarboxaldehyde, 3-pyridinecarboxaldehyde,
-pyridinecarboxaldehyde cobalt(II) acetate tetrahydrate and
4
To 50 mg (0.08 mM) of H24PyP in 20 mL of chloroform,
100.64 mg (0.4 mM) of cobalt(II) acetate tetrahydrate in 6 mL of
methanol was added and refluxed for 4 h. The precipitate formed
during the course of the reaction was collected by centrifugation.
The crude product obtained was treated with water repeatedly (till
the washing did not show the presence of cobalt acetate by UV vis-
ible spectroscopy). The solid was dried under vacuum at 100 °C for
12 h. The yield of the product was 51.7 mg.
Nafion® 117 solution (~ 5% in a mixture of lower aliphatic alcohols
and water) were purchased from Sigma–Aldrich. Chloroform,
methanol, propionic acid, acetic anhydride, potassium hydroxide
and DMF were purchased from SD Fine Chemicals, India. Neutral
alumina was obtained from Fischer scientific. Milli-Q water was
used for preparation of all the electrolyte solutions.
Optical absorption spectra were recorded on a JASCO V-570
model UV/VIS/NIR spectrophotometer using quartz cells of 1 cm
path length. 1H NMR spectra were recorded on a DRX-500 spec-
trometer in deuterated chloroform using tetramethylsilane as the
internal standard. Mass spectrum was recorded on a Agilent Tech-
nology Model 6460 Triple Quadrupole LC/MS consisting 1290 in-
finity II Binary Pump, auto sampler and Diode array detector (ESI).
(
a) Using one equivalence of cobalt acetate tetrahydrate
Same procedure given above was followed in this reaction (us-
ing 50 mg of porphyrin and 20.12 mg of cobalt acetate tetrahy-
drate). The yield of the product (precipitate after treating with wa-
ter and drying) was 22.9 mg.
2

N. Saran, T.L. Thomas and P. Bhavana
Journal of Molecular Structure 1232 (2021) 130032
Same procedure (a and b) was followed for the metalation of
H 3PyP and H 2PyP and the yield obtained are as follows.
2
2
Procedure a: 33.5 mg for H 3PyP and 28.0 mg for H 2PyP
2
2
Procedure b: 11.8 mg for H 3PyP and 5.1 mg for H 2PyP
2
2
2
.3. Preparation of catalyst
Electrocatalyst was prepared by pyrolysing the porphyrin using
alumina crucible in a tubular furnace. The p-Co4PyP sample was
heated from room temperature to 400 °C (p-Co4PyP-400), 600 °C
(
p-Co4PyP-600) and 800 °C (p-Co4PyP-800) separately with a heat-
ing rate of 3 °C/min under nitrogen atmosphere. Similar pyrolysis
treatment was done for p-Co3PyP and the naming was done ac-
cordingly (p-Co3PyP-400, p-Co3PyP-600 and p-Co3PyP-800).
2
.4. Electrode preparation and electrochemical measurements
All electrochemical studies were carried out by linear sweep
voltammetry (using rotating disk electrode (RDE)) technique. For
modifying the electrode 1.0 mg catalyst was suspended in 470 μL
of ethanol and 500 μL of water containing 30 μL of Nafion®.
This solution was sonicated for two hours to form stable suspen-
sion. 2.5μL of the solution was dropped on glassy carbon electrode
Fig. 2. UV visible absorption spectrum of H24PyP and p-Co4PyP recorded in DMF.
(
GCE) and dried in air. Scanning was done in the potential range
has resulted in relatively less amount of precipitate (22.9 mg) and
an intensely coloured supernatant liquid (which showed the pres-
ence of free base porphyrin; indicating relatively higher amount of
unreacted free base (case b)). This has indicated that the incorpo-
ration of cobalt at the core of the porphyrin is incomplete in case
b. The precipitate formed in both reactions showed similar solubil-
ity characteristics.
0
.0 V to −1.2 V at a scan speed of 10 mV/s at different rotating
speed ranging from 100 to 2600 rpm in the oxygen saturated so-
lution of 0.1 M KOH.
Analysis of the results obtained using RDE measurements was
done using Koutecky-Levich (K-L) equation [47]
ꢀ
ꢁ
1
/2
1
/J = 1/ Bω
+ Jk
Similar metalation procedure was followed for 2- and 3-
pyridylporphyrins. In the case of H 2PyP, the amount of metalated
2
)
/3 −1/6
2
B = 0.2nFCO2(DO2
In this equation, J is the measured current density, Jk is the
kinetic current density of the oxygen reduction reaction, n is the
number of electrons, F is the Faraday constant, D is the diffusion
υ
product (precipitate) obtained was very less (from 50 mg of the
starting material, 11.8 mg in case a and 5.1 mg in case b). The
supernatant liquid contain large amount of free base porphyrin
even after refluxing for extended duration (8 h). Similar reaction
of H23PyP has resulted in the product to unreacted starting mate-
rial ratio in between that of H22PyP and H24PyP.
coefficient of O2 (DO2 = 1.9 × 10− cm2s ), υ is the kinetic vis-
5
−1
2
−1
cosity of the electrolyte (υ = 0.01 cm s ), ω is the rate of ro-
O2
tation of the electrode and C
is the bulk concentration of O2
The dependence on the quantity of precipitate (the product) to
the amount of cobalt acetate added and the solubility characteris-
tics have led to the conclusion that the pyridyl group of the por-
phyrins is acting as ligand to interact with cobalt metal ion at the
periphery of the porphyrin (along with the insertion at the centre
of the molecule as evidenced from UV visible spectra). The ligation
has resulted in a polymerised structure. As the coordination is easy
due to the favored symmetric structure of the 4-pyridylporphyrin,
the amount of polymerized product formed is maximum (superna-
tent liquid showed presence of only small amount of freebase por-
phyrin in case a). This is supported by the relatively less amount of
−6
−3
(
CO2 = 1.2 × 10
molcm ) in 0.1 M aqueous KOH. Slope of K-
−1/2
) plot gives the value of B.
L (J 1 vs ω
−
3
. Results and discussion
3
.1. Synthesis
The incorporation of cobalt ion at the core of H APyP (A = 2,
2
3
or 4) using cobalt acetate tetrahydrate was carried out in chlo-
roform – methanol solvent system. Metal insertion reactions were
carried out with different ratio of free base porphyrin to cobalt ac-
etate (1:5 and 1:1). The addition of 5 equivalents of cobalt acetate
has resulted in the nearly complete precipitation of the product
the precipitate formed in the cases of H 3PyP and H 2PyP. Among
2
2
H 3PyP and H 2PyP, the amount of metalated (polymerized) prod-
2
2
uct is more for H 3PyP. This can be attributed to the less steric
2
in the case of H 4PyP. The supernatant liquid in the reaction mix-
constraints present in H 3PyP compared to that in H 2PyP in form-
2 2
2
ture turned almost colourless at the end of the reaction (case a).
The UV visible spectrum indicated the presence of trace amount
of freebase porphyrin in the mother liquor. After refluxing for 4 h,
the product was washed repeatedly with water (till the washings
showed the absence of cobalt acetate as evidenced by UV visible
spectroscopy). In all the commonly used organic solvents (except
DMF, in which the compound dissolved to a great extent over a
period of time), the product was insoluble. UV visible spectrum of
the product recorded in DMF showed the spectrum correspond-
ing to metalated porphyrin suggesting the incorporation of cobalt
metal ion at the centre of the porphyrin (one Soret and two Q
bands with second Q band appearing as a shoulder, Fig. 2). Sim-
ilar reaction carried out with only one equivalent of cobalt acetate
ing a coordinate bond through the pyridyl-N to the cobalt metal
ion (Fig. 3). Since the amount of precipitate formed in H 2PyP is
2
very less, further investigation was not carried out with this por-
phyrin.
Synthesis of cobalt complex of 4-pyridylporphyrin is reported
in literature [22–24] with DMF or others [12,25] as the solvent for
metal insertion reaction. In any of the report, formation of precip-
itate during the metalation reaction is not mentioned. In order to
confirm the polymerized structure of the metalated product un-
der the reaction condition of the present work, ICP-AES of meta-
lated products of both H 4PyP and H 3PyP (named p-Co4PyP and
2
2
p-Co3PyP, respectively) was carried out. The percentage of cobalt is
13.01% and 12%, respectively, for p-Co4PyP and p-Co3PyP. If it was
3

N. Saran, T.L. Thomas and P. Bhavana
Journal of Molecular Structure 1232 (2021) 130032
Fig. 4. Linear sweep voltammogram of (i) p-Co4PyP and (ii) p-Co3PyP under nitro-
gen; (iii) p-Co4PyP and (iv) p-Co3PyP under oxygen (all in 0.1 M KOH, 2600 rpm).
Fig. 3. Incorporation of cobalt metal ion in meso-3-pyridylporphyrin.
a monomeric metalated product, the percentage of cobalt present
in the sample should have been 8.55%. So the obtained ICP-AES re-
sults (i.e., the presence of more than one cobalt ion per porphyrin
ring) confirm the formation of coordination polymers of pyridyl-
porphyrins in presence of cobalt metal ion carrier in the solvent
system chloroform – methanol. The relatively lesser amount of
cobalt in p-Co3PyP is attributed to the unfavourable geometry of 3-
pyridyl group of porphyrin in interacting with the cobalt metal ion
through N of the pyridyl group. In the cobalt insertion reactions re-
ported in literature using DMF as the solvent, no coordination with
cobalt by the pyridyl group at the periphery is reported to the best
of our knowledge. In the reported crystal structures of Co4PyP and
Co3PyP, pyridyl group of one molecule appeared as an axial lig-
and on cobalt of another molecule [5,8,16]. This axial coordination
leads to the stoichiometry of 1:1 for cobalt ion to porphyrin ligand
in these reported cases.
Fig. 5. SEM images (with scale bar given in figure) of (a) p-Co4PyP (500 nm), (b)
p-Co4PyP-800 (1 μm), (c) p-Co3PyP (500 nm) and (d) p-Co3PyP-800 (1 μm).
The synthesized porphyrin p-Co4PyP dispersed (1 mg in 1 mL)
in water:ethanol (1:1) was used to modify the surface of the
glassy carbon electrode by drop-casting (using nafion as the bind-
ing agent). This modified electrode was used for the dioxygen re-
duction in alkaline medium (0.1 M KOH). Fig. 4 shows the reduc-
tion of dioxygen using modified electrode. p-Co3PyP also showed
a similar activity, but to a lesser extent. The current generated was
less in both the cases, though the onset potential was promising.
In order to increase the activity, these cobalt containing coordina-
tion polymers were pyrolysed under different temperature and in-
vestigated as GCE modifying agent. Pyrolysis of the metalloorganic
compounds is known for its increased activity towards dioxygen
reduction [48].
unpyrolysed samples, the surface is smooth for the pyrolysed sam-
ple in all the cases indicating the formation of ordered carbona-
ceous matrix on heating.
3
.2.2. Transmission electron microscopy (TEM & HRTEM)
Cobalt nanoparticles are found to be embedded in the matrix
(
Fig. 6a and b) [49] in the TEM images of p-Co4PyP-800. High res-
olution TEM images of p-Co4PyP-800 and p-Co3PyP-800 (Fig. 6c-
f) and the XRD pattern (discussed below) indicate that the cobalt
nanoparticles with a highly crystalline structure are encapsulated
within graphitic carbon matrix
3
.2.3. Powder X-ray diffraction analysis (XRD)
3
3
.2. Characterisation of the pyrolysed samples
The XRD patterns of pyrolysed samples of both p-Co4PyP and p-
Co3PyP at different temperatures are shown in Fig. 7. A very broad
°
.2.1. Scanning electron microscopy (SEM)
peak at 25 was observed in all the pyrolysed samples, which is as-
SEM image of p-Co4PyP-800 showed a ribbon-like morphology
signed to the diffraction of carbon (002). The diffraction at 44° in
the XRD pattern of all samples is attributed to cobalt (111) plane
(PDF#89–4307). The sharpness of the diffraction peaks increased
with temperature. The peak corresponding to cobalt (200) plane
also appears on pyrolysis. These results indicate the formation of
metallic Co during pyrolysis of the porphyrinic polymer. The mate-
rial shows a weak diffraction feature of metallic Co (111) after heat
treatment at 400 °C. On heat treatment at 600 °C and 800 °C both
with a length in the range 3.68 μm to 5.8 μm and width 375 nm
to 500 nm. It has a highly porous structure (Fig. 5a and b). Sim-
ilar morphology on pyrolysed porphyrins is reported for cobalt
porphyrin conjugated polymers [49]. p-Co3PyP-800 showed small
granular structure (Fig. 5c and d) [53]. SEM images of p-Co4PyP-
6
00, p-Co4PyP-400, p-Co3PyP-600 and p-Co3PyP-400 are given in
supplementary information (Figure S1). Compared to that of the
4

N. Saran, T.L. Thomas and P. Bhavana
Journal of Molecular Structure 1232 (2021) 130032
Fig. 6. TEM (a,b) and high-resolution TEM (c,d) images of the p-Co4PyP-800 and high-resolution TEM images of p-Co3PyP-800 (e-h).
Fig. 7. Powder X-ray diffraction patterns of p-Co4PyP and p-Co3PyP pyrolysed at different temperatures.
Table 1
metallic Co (111) and Co (200) peaks appear which indicates that
Surface composition of different samples evaluated from the
XPS analysis.
the catalyst is transformed to a composite of cobalt and carbon on
pyrolysis [50]. Also, with the increase in heating temperature, the
diffraction peaks becomes sharper, indicating the increase in crys-
tallanity of cobalt.
Sample
C(at%)
N(at%)
O(at%)
Co(at%)
p-Co4PyP
64.73
69.60
64.69
65.92
71.48
70.12
63.71
64.06
9.00
8.45
9.21
2.36
9.26
8.45
8.96
0.59
14.28
11.98
18.20
19.79
10.76
7.88
3.99
1.45
2.50
4.14
0.59
0.67
2.63
3.75
p-Co4PyP-400
p-Co4PyP-600
p-Co4PyP-800
p-Co3PyP
3
.2.4. X-ray photoelectron spectroscopy (XPS)
XPS results showed the presence of peaks corresponding to C
s, N 1 s (weak), O 1 s and Co 2p in the samples of p-CoAPyP, p-
1
p-Co3PyP-400
p-Co3PyP-600
p-Co3PyP-800
CoAPyP-400, p-CoAPyP-600 and p-CoAPyP-800 (A = 3 or 4). The
percentage of surface Co are 4.14 and 3.75 at%, respectively, for
p-Co4PyP-800 and p-Co3PyP-800 (Table 1). This percentage con-
tent is found to be higher than that reported previously for 4-
pyridylporphyrin (synthesized and studied differently). In the re-
ported case, the atomic percentage of cobalt is 0.6 [24]. But the
nitrogen content in this case is 6.8% which is higher than that for
the samples of present study. In the reported work, authors have
claimed that the species responsible for the electrocatalytic activ-
15.55
23.64
ity (ORR) is CoNx centre even after pyrolysis [51]. However, in the
present work, the nitrogen content is very less (2.36 atomic% for p-
Co4PyP-800 and 0.59 atomic% for p-Co3PyP-800). The less amount
5

N. Saran, T.L. Thomas and P. Bhavana
Journal of Molecular Structure 1232 (2021) 130032
Fig. 8. XPS spectra of p-Co4PyP-800 catalyst: (a) Co 2p (b) N 1 s (c) C 1 s and of p-Co3PyP-800 catalyst: (d) Co 2p (e) N 1 s (f) C 1 s.
of nitrogen indicates that CoNx centre is not being the active cen-
tre in this case.
responds to the metallic cobalt present in the pyrolysed sample,
780.4 eV to cobalt oxides, 784.8 eV to Co(CO)4 and 795.3 eV to Co
In the article reported on the oxygen reduction activity of
Co4PyP synthesized using DMF as the solvent [24] also it is given
that CoN4 centre is the cause for its ORR activity. But in the XPS
investigation of the present work, we cannot see any peak corre-
sponding to Co-N bond. Instead peak corresponding to Co nanopar-
ticle is seen (discussed below). So Co metallic nanoparticle is ex-
pected to have a contribution in the ORR activity (Fig. 8). XRD
also supported the presence of Co nanoparticle. From deconvoluted
spectrum of p-Co4PyP-800 it is understood that the N peak corre-
sponds to pyridinic (398.6 eV), pyrrolic (400.4 eV) and pyridine-N-
oxide (402.1 eV). However, in the case of p-Co3PyP-800, there are
only pyridinic (398.5 eV) and pyrrolic (400.2 eV) N peaks [24,51].
Deconvoluted spectrum of p-Co4PyP-800 indicated the presence
of metallic cobalt in the pyrolysed sample (779.7 eV) which is ab-
sent in the same of unpyrolysed one, p-Co4PyP. This is an indi-
cation on the formation of cobalt nanoparticle on pyrolysis. The
other peaks in the pyrolysed samples are at 781.2 eV (cobalt ox-
2P1/2.
There is a broad peak in the C 1 s region of XPS of p-Co4PyP-
800. This peak is assigned to C–C (284.7 eV), C–O (286.3 eV) and
N–C=O (288.3 eV) after deconvolution [50]. Similar behaviour is
observed in the case of p-Co3PyP-800 (284.7 eV, 286.3 eV and
288.6 eV, respectively). The presence of oxygen can be ascribed
to atmospheric O , CO2 and H O that are adsorbed onto the sur-
2
2
face of the samples as well as from the acetate ions which may be
present as the counter ions for the cobalt present at the periphery
of the porphyrins (cobalt ions to which the meso-pyridyl groups
are interacting as ligands) [49,52].
3.2.5. BET surface area analysis
N2-adsorption and desorption isotherms of p-Co4PyP-800 ex-
hibited type-IV isotherm [24,52] (Fig. 9a). This indicates that it is a
mesoporous material. Specific surface area of p-Co4PyP-800 (~109
m2g ) is higher than that of p-Co4PyP (~11.552 m2g ). The av-
erage pore size are 8.88 nm and 9.70 nm for p-Co4PyP and p-
Co4PyP-800, respectively. This shows that pyrolysis at 800 °C, the
pore sizes of the p-Co4PyP have increased, which is ascribed to
the shrinkage of the pore walls at higher temperatures (Fig. 9b,
−1
−1
ides), 785.5 eV (minor pyrolysed byproducts Co(CO) ) and 795.9 eV
4
(
Co2P1/2) as in the case of pyrolysed CoTPP [52]. Similar peak po-
sitions are seen for p-Co3PyP-800 also. The peak at 779.2 eV cor-
6

N. Saran, T.L. Thomas and P. Bhavana
Journal of Molecular Structure 1232 (2021) 130032
Fig. 9. (a) Adsorption-desorption curve and (b) BJH plot of p-Co4PyP-800.
Table 2
Surface area analysis parameters.
Surface area(m2g
−1
)
Mesopore size(nm)
Pore volume(cm3g
−1
)
Sample
p-Co4PyP
11.552
108.87
8.878
9.699
0.0562
0.3823
p-Co4PyP-800
Table 3
Catalytic parameters of ORR.
Samples
Electron transfer at −0.45V
Onset Potential (V)
Maximum Current (mA/cm2)
p-Co4PyP
1.6
−0.214
−0.200
−0.140
−0.100
−0.300
−0.270
−0.190
−0.120
–0.300
0.370
2.5
2.7
2.3
7.0
1.1
1.4
2.0
4.3
2.3
1.75
p-Co4PyP-400
p-Co4PyP-600
p-Co4PyP-800
p-Co3PyP
1.9
1.7
2.313
1.79
1.47
1.41
2.11
1.007
1.72
p-Co3PyP-400
p-Co3PyP-600
p-Co3PyP-800
Co3PyP-800–4 hrs
Co3PyP-800–24hrs
Table 2). The higher surface area and the pore size of p-Co4PyP-
00 can be attributed to the carbonization followed by structural
600 (−0.19 V), p-Co3PyP-400 (−0.27 V) and p-Co3PyP (−0.30 V)
(Fig. 10b). The diffusion-limiting current showed by p-Co3PyP-800
8
is 4.35 mAcm 2 which is superior to that of p-Co3PyP, p-Co3PyP-
400 and p-Co3PyP-600. The electron transfer number (n) of p-
Co3PyP-800 was calculated to be 2.113 at −0.45 V (Fig. 10d and f)
which is slightly lower than that of p-Co4PyP-800. Other pyrolysed
samples of p-Co3PyP also showed relatively lesser value compared
to the 4-pyridyl counterparts.
−
rearrangement of p-Co4PyP framework on heating. p-Co4PyP has
narrow pore-size distribution. The transportation of O2 to the cat-
alytically active sites is supported by the large-pore structures.
High surface area helps in the better interaction of O2 with the
active centre [50,53].
On comparing the data (Fig. 11), it is observed that p-Co4PyP
is relatively a better electrocatalyst than p-Co3PyP under all py-
rolysed conditions (both in terms of current and potential). There
might be a slight excess of Co nanoparticle formed on pyrolysing
p-Co4PyP which in turn has resulted due to the favourable coordi-
nation geometry of the 4-pyridyl group at the meso position of the
porphyrin in binding to the cobalt ion during the metalation re-
action. The supramolecular conformational aspects of meso(3- and
3
.3. Oxygen reduction reaction (ORR)
ORR was carried out using pyrolysed samples as the modify-
ing agents (electrocatalyst for ORR) for GCE in alkaline medium
0.1 M KOH). The catalytic parameters for the ORR are summa-
(
rized in Table 3. p-Co4PyP-800 displayed a more positive onset po-
tential (−0.10 V) than that of p-Co4PyP-600 (−0.14 V), p-Co4PyP-
4
00 (−0.2 V) and p-Co4PyP (−0.21 V) during ORR (Fig. 10a). p-
4
- pyridyl) isomers of tetraruthenated porphyrins on the electro-
Co4PyP-800 showed a highly stable diffusion-limiting current (7.0
mAcm−2). On the basis of the RDE currents, the electron trans-
catalytic activities have been reported [54]. Metallic nanoparticle,
especially, cobalt nanoparticles are known to be active for the re-
duction of dioxygen [55]. The carbonaceous material formed from
the porphyrinic macrocycles on pyrolysis of the coordination poly-
mer acts as the matrix for the otherwise unstable cobalt nanopar-
ticle and help in its ORR activity. The presence of cobalt oxides also
might have played a role in the ORR activity. However, as reported
in the oxygen reduction activity of Co4PyP synthesized using DMF
fer number (n) of p-Co4PyP-800 is found to be 2.313 at −0.45 V
suggesting two-electron oxygen reduction process (Fig. 10c and
e). p-Co4PyP-600, p-Co4PyP-400 and p-Co4PyP, exhibited relatively
lower n values 1.7, 1.9 and 1.6, respectively, at −0.45 V.
Similarly, ORR was carried out using samples of p-Co3PyP
pyrolysed at different temperatures. p-Co3PyP-800 displayed a
more positive onset potential (−0.12 V) than that of p-Co3PyP-
7

N. Saran, T.L. Thomas and P. Bhavana
Journal of Molecular Structure 1232 (2021) 130032
Fig. 10. LSV curves (2600 rpm) of different samples (a,b), RDE polarization curve of p-Co4PyP-800 and p-Co3PyP-800 at different rotation rates from 100 to 3600 in O2-
saturated 0.1 M KOH solution (c,d) and Koutecky-Levich plot of different samples of p-Co4PyP and p-Co3PyP at −0.45 V (e,f).
as the solvent [24], the activity cannot be due to the CoN4 centre
as no peak corresponding to Co-N bond is seen in the XPS investi-
gation.
For p-Co3PyP-800 (the present work), the n factor is 2.113 as men-
tioned above where as for Co3PyP-800–4 hrs and Co3PyP-800–
24 hrs it is, respectively, 1.007 and 1.72 at −0.45 V (Figure S2). It
is clear from the figure that samples in which DMF is used for the
metalation, there is no considerable amount of current at −0.12 V
which is the onset potential of p-Co3PyP-800 in ORR. This differ-
ence in the n factor as well as in the amount of current also gives
an indication on the increased amount of Co content resulted dur-
ing the synthesis (leading to the formation of coordination poly-
mer) on using CHCl –CH OH as the solvent system in the present
Co3PyP prepared using DMF as the solvent [16] has also been
investigated for ORR after pyrolysing at 800 °C (Fig. 12, Table 3)
for the comparison of its activity towards ORR. For this, we have
carried out metalation reactions in DMF for two different refluxing
durations. In the first case, the refluxing of the reaction mixture
was carried out for 4 h (for comparison with our reaction condi-
tion) and in the second case the refluxing period was extended to
3
3
24 h (as reported). Both the samples were pyrolysed at 800 °C and
work.
named accordingly (Co3PyP-800–4 hrs and Co3PyP-800–24 hrs).
8

N. Saran, T.L. Thomas and P. Bhavana
Journal of Molecular Structure 1232 (2021) 130032
nation polymer helps in increasing the cobalt content in the pyrol-
ysed samples.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
CRediT authorship contribution statement
Neha Saran: Methodology, Validation, Investigation. Tincy Lis
Thomas: Methodology, Resources. Purushothaman Bhavana: Con-
ceptualization, Methodology, Supervision, Writing - review & edit-
ing.
Acknowledgements
Fig. 11. (i) p-Co4PyP and (ii) p-Co3PyP before pyrolysis under oxygen atmosphere;
(
iii) p-Co4PyP and (iv) p-Co3PyP under pyrolysis at 800 °C under oxygen atmo-
PB gratefully acknowledges Science & Engineering Research
Board (SERB), India for the research grant (EMR/2016/002367). Dr.
Richa Singhal and Ms. Ishita of Department of Chemical Engineer-
ing, BITS, Pilani – K K Birla Goa campus is gratefully acknowledged
for their help in pyrolysing samples.
sphere (all in 0.1 M KOH, 2600 rpm).
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.130032.
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