A local proton source from carboxylic acid functionalized metal porphyrins for enhanced electrocatalytic CO2reduction

Article abstract of DOI:10.1039/d0nj02900a

The effect of intramolecular carboxylic groups on the activity of metal porphyrins for CO₂reduction has been investigated. Metal porphyrins functionalized with aromatic carboxylic acid (benzoic acid) at thepara-position exhibited CO₂

Full text of DOI:10.1039/d0nj02900a

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A local proton source from carboxylic acid functionalized metal
porphyrins for enhanced electrocatalytic CO reduction
2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
Yiwei Zhou, Yunheng Xiao and Jian Zhao *
DOI: 10.1039/x0xx00000x
The effect of intramolecular carboxylic group on the activity of metal porphyrins for CO
Metal porphyrins functionalized with aromatic carboxylic acid (benzoic acid) at the para-position only exhibited CO
catalytic current in the presence of external proton sources and Fe metal center obtained the highest CO catalytic current
value among Cu, Co, Ni metal centers. In the absence of external proton donors, Fe porphyrins grafted with aliphatic
2
reduction has been investigated.
2
2
carboxylic acid (propanoic acid) as local proton source could achieve comparable CO
counterpart in the presence of H O and Brønsted acid. Online electrocatalytic CO
2
catalytic activity with “benzoic acid”
2
2
results show that the local proton
2
donor-“propanoic acid” indeed enhance the conversion of CO to CO.
14
15
rhenium and manganese bipyridine catalysts is dependent
Introduction
on co-substrates that provide protons. The efficiency of
2+
rhenium tricarbonyl bipyridyl complex [Re(bpy)(CO) (py)]
3
(
bpy=2,2’-bipyridine, py=pyridine) increases with the acidity of
The catalytic reduction of carbon dioxide (CO ) to fuels and
valuable industry chemicals is one of the most important
2
the weak Brønsted acids used, trifluoroethanol > phenol >
methanol > H O. In the case of manganese tricarbonyl
complex [Mn(bpy-R)(CO) X] (bpy-R = 2,2’-bipyridine or its 4,4’-
disubstituted derivatives; X=Cl, Br) the selective catalytic
reduction of CO to CO occurs only in the presence of H O.
Mn(bpy-R)(CO) Br and [Mn(bpy-R)(CO) (MeCN)](OTf) show
high selectivity, efficiency, and stability for the reduction of
CO2 to CO in the presence of H O, methanol and
14
2
contemporary energy and environmental challenges. Since CO2
3
is the most oxidized state of carbon, it is a highly inert
high energy input is required for its
transformation. For example, one-electron injection to CO2
molecule.
A
16
1
-5
2
2
3
3

-
(CO to CO ) occurs at very negative potential, -1.97 V vs. SHE
2
2
0
in an aprotic solvent like N,N -dimethylformamide (DMF) and -
2
3
19
1
.9 V vs. normal hydrogen electrode (NHE) in water. Reactions
trifluoroethanol. The addition of H O and weak Brønsted
2
pathway that would go through this anion radical intermediate
acids such as trifluoroethanol, phenol, and acetic acid also
is therefore quite unreasonable in terms of energy and boosts the catalysis of CO2 reduction to CO with iron

-
17, 18, 22
activation. A possible approach to avoid the CO2 anion radical tetraphenylporphyrin catalysts (1, Scheme 1).
is to catalyze CO reduction through proton-assisted multiple-
2
6-13
electron transfer (PMET) pathway (Table 1),
which presents Table 1. The standard potentials for various CO reduction
2
reactions.
much favourable potentials. Toward this goal, a great variety
of organometallic molecular systems have been investigated
because they have accessible multiple redox states to promote
such a PMET pathway.
0
0
Reactions
E (V) vs. NHE
E
(V) vs.
(pH=7)
-1.9
RHE
-
∙-
CO + e  CO
-1.48
-0.19
-0.11
-0.06
+0.04
+0.18
2
2
+
-
In this context, external proton donor such as H O and weak CO + 2H + 2e  HCOOH
Brønsted acid has been added as proton donor into these
metal complexes to enhance the redox catalytic activity for
-0.61
-0.53
-0.48
-0.38
-0.24
2
2
+
-
CO + 2H + 2e  CO + H O
2
2
14-23
+
-
CO2 reduction.
For example, the CO2 reduction by CO + 4H + 4e  HCHO + H O
2
2
+
-
CO + 6H + 6e  CH OH + H O
2
3
2
+
-
CO + 8H + 8e  CH + 2H O
2
4
2
The next thought was to install internal functionalities to
provide proton to catalyze CO reduction instead of Brønsted
2
2
4-27
acids addition.
Phenolic group on porphyrin ligands, the
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availability of local proton source has been reported greatly Eletrocatalysts fabrication.
enhance the activity for CO reduction to CO for copper- and
iron-metal
tetraphenylporphyrin with phenolic groups (
ortho and ortho’ positions of the phenyl groups lead to a
considerable increase of catalytic activity. Acidic OH groups
on polypyridyl ligands have also demonstrated an excellent *4,4’,4’’,4’’’-(Porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid)]
performance in the reduction of CO , such as [fac- or TPP~COOH [hematoporphyrin] was added and bubbled for
Mn(dhbpy)(CO) Br]
View Article Online
DOI: 10.1039/D0NJ02900A
Metal porphyrin electrocatalysts were synthesized according
70 ml N, N’-dimethylformamide
DMF, anhydrous) was bubbled with argon for 0.5 h. 1.82
mmol metal (II, Cu/Fe/Co/Ni) chloride was added and bubbled
with argon for another 0.5 h. 0.33 mmol TPP_COOH
2
2
4-26
center.
Modification
of
iron
28, 29
to literature methods.
(
2
, Scheme 1) in all
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2
o
(dhbpy
=
4-phenyl-6-(1,3- another 1 h. The mixture solution was refluxed at 160-170 C
3
dihydroxybenzen-2-yl) 2,20-bipyridine), containing two acidic overnight. After cooling and DMF removal by distillation, the
OH groups in the proximity of the Mn center (
2
7
3
, Scheme 1).
obtained electrocatalysts M(Cu/Fe/Co/Ni)TPP_COOH or
M(Cu/Fe/Co/Ni)TPP~COOH were precipitated by adding water.
The precipitate was then dissolved in 0.1 M NaOH solution and
re-precipitated by adding 1 M HCl solution. Finally, the metal
porphyrin was dissolved in ethanol and recrystallized by
solvent evaporation.
Here, we modified metal tetraphenylporphyrin (TPP)
through the introduction of aromatic carboxylic acid ligands (
MTPP_COOH, M: Cu, Fe, Co, Ni, Scheme 1) and aliphatic
carboxylic acid ligands ( , MTPP~COOH, Scheme 1) to the
phenyl groups. Then we have demonstrated that the ligands
with aromatic carboxylic acid at the para-positions, which
cannot act as intramolecular proton source for metal center
protonation, could promote CO2 reduction only in the
presence of Brønsted acids. Among these catalysts, the
catalyst with iron metal center has shown the highest
enhancement of current density for CO reduction. Compared
with aromatic carboxylic acid (ligand
porphyrin groups with aliphatic carboxylic acid (ligand
achieve the best performance in the absence of external
proton source such as Brønsted acids, the same as the activity
4
,
5
Electrochemical measurements.
Cyclic voltammetry. The cyclic voltammetry (CV) test was
conducted in one compartment cell (CHI instruments). Glassy
carbon electrode (GC electrode, 3mm) was utilizedd as
working electrode, Pt wire as counter electrode and Ag/AgCl as
reference electrode for quick cyclic voltammetry (CV) test
while Ag/Ag (0.1 M AgNO in electrolyte) was used in long
time CO2 reduction test. The scan rate for all cyclic
voltammograms was 100 mV/sec unless otherwise noted. The
electrolyte contained 1 mM metal porphyrin electrocatalyst
and 0.1 M tetrabutyl ammonium tetrafluoroborate (TBATFB)
as supporting electrolyte in anhydrous DMF. Argon and CO2
saturated electrolyte was obtained by bubbling the
corresponding gas into electrolyte for 15 min.
2
+
3
4
), the modification of
) could
5
with the addition of acids for ligand 4. Through online CO2
reduction investigation, we further confirm that the
introduction of local proton donor promote CO production and
the most obvious promotion effect is observed for Fe metal
center.
Reference electrode was calibrated by scanning in
ferrocenium/ferrocene couple Fc /Fc at the end of each
experiment, which allowed the conversion of all potentials to
Fc /Fc. The corresponding CV curves of Fc /Fc are provided in
Figure S1.
+
+
+
Electrocatalytic CO2 reduction. Electrocatalytic CO test was
2
conducted in a two compartment H-type cell connected with
online Gas Chromatography (Shimazu 2014) and Autolab
Potentiostat (M204). Graphite rod electrode was utilized as
working electrode (working area dia. 6 mm
 1.5 cm), Pt plate
+
as counter electrode and Ag/Ag (0.1 M AgNO in 0.1 M
3
tetrabutyl ammonium tetrafluoroborate (TBATFB) as
supporting electrolyte in anhydrous DMF) as reference
electrode for electrocatalytic CO2 test. The electrolyte
contained 1 mM metal porphyrin electrocatalyst and 0.1 M
tetrabutyl ammonium tetrafluoroborate (TBATFB) as
supporting electrolyte in anhydrous DMF. The test was
conducted at -1.8 V to -2.3 V vs. Ag/Ag (-1.2 V to -1.7 V vs.
Fc /Fc as shown in Figure S1). Gas products were analyzed on
online Gas chromatography (GC) Shimadzu GC-2014 with a
thermal conductivity detector (TCD) and a flame ionization
detector (FID). The sample was tested at certain applied
potential after 1 h electrolysis to make sure that the GC system
+
+
Scheme 1. Molecular structure of
M=Cu, Fe, Co, Ni),
1
,
2
,
3
,
4
(MTPP_COOH,
5 (MTPP~COOH, M=Cu, Fe, Co, Ni).
Experimental
and Cell system are full with CO reduction product.
2
2
| J. Name., 2012, 00, 1-3
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The online Faradaic Efficiency (FE) was calculated based on the Role of water or weak acid addition.
following equation.
FE=nZF/Q=nZF/It=nZF/(IV/v), where I is recorded current (A); t
is the time required to fill the sampling loop (s); V is the
volume of sampling loop (cm ); v is recorded flow rate (mL
View Article Online
DOI: 10.1039/D0NJ02900A
We investigated the effect of water (H O, pKa=14), acetic acid
MeCOOH, pKa=4.76) and benzoic acid (PhCOOH, pKa=4.19) on
the CO reduction. CV curves of Cu AH system with different
concentrations of these Brønsted acids are provided in Figure
2
(
3
-1

s ).
II
2
Characterization.
Attenuated total reflectance Fourier transform infrared S3 and Figure 2a. The current under CO and argon is similar,
spectroscopy (ATR-FTIR) was performed using a PerkinELmer not enlarged with the addition of low concentration of
FT-IR/NIR spectrometer frontier with a universal ATR sampling PhCOOH and MeCOOH. As given in Figure 2a, there is a
accessory. XRD spectra were obtained on a Rigaku UItima IV dramatic increase of current under CO compared with that
diffractometer using Cu Kα radiation (λ= 1.5418 Å). The Fe under argon atmosphere when 1 M to 3 M H O is added. In
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metal states were examined via X-ray photoelectron the presence of 3 M H O, the current under CO is 2.3 times
2
2
+
spectroscopy (XPS) measurements (Thermo Scientific Escalab the current under argon at -1.52 V vs. Fc /Fc. With the same
50Xi). To investigate the interaction between the catalysts concentration (1 M), PhCOOH and MeCOOH show higher H FE
2
2
and CO , UV-Vis spectra were tested on a UV-2600 and lower CO FE than H O (Figure 2b). This indicates that
2
2
spectrophotometer (Shimadzu).
stronger acid leads to proton reduction for hydrogen evolution,
competing with CO reduction. Thus, H O is considered as the
2
2
suitable external proton source for CO reduction.
2
Results and discussion
Aromatic carboxylic group functionalized porphyrins with
different metal centers for CO reduction.
2
Take Cu as examples for the peak analysis. Cyclic voltammetry
(CV) curves of CuTPP under argon (black curve in Figure 1)
II
I
I
0
shows two subsequent reductions, Cu /Cu and Cu /Cu
reversible. After grafting aromatic carboxylic group on the
porphyrin ligands, CuTPP_COOH (Cu AH) shows two
II
subsequent reductions under argon (green curve in Figure 1),
II
I
I
0
II
Cu /Cu reversible, but Cu /Cu irreversible. The irreversibility Figure 2. a) Cyclic voltammetry of Cu AH under argon (dash)
I
0
of the Cu /Cu process is because the reduction of proton to and CO (solid) at 100 mV/s with various concentration of H O.
2
2
hydrogen or CO to products (under CO , red curve in Figure 1) b) FE of CO reduction products with the addition of 1 M H O,
happened in the presence of aromatic carboxylic group. MeCOOH and PhCOOH. 1 mM Cu AH in DMF (0.1 M TBATFB).
Simultaneously, Cu is oxidized to Cu .
The catalytic current for CO reduction is negligible. This
result is different from phenolic group functionalized metal compounds in Scheme 1, H O was chose as a model external
porphyrins as local proton donor reported by J. M. Savéant proton donor for CO reduction. For FeTPP_COOH (M AH for
and co-workers,
reduction increased dramatically. These confirm that the its following CV analysis. 2, 3, 4 and 5 M H O was added
aromatic carboxylic group at the para-position does not respectively and corresponding CV curves are given in Figure 3.
improve the catalytic performance since it cannot act as local The catalytic current under CO dramatically increases. At -
proton source. The same results are obtained for Fe, Co and Ni 1.52 V, the catalytic current is up to ~3.5, 4.0, 5.0 and 4.6 times
center, provided in Figure S2. Thus, the external proton donors the current under argon for 2, 3, 4 and 5 M H O respectively.
are added into the systems as given in the following part.
2
2
2
2
II
0
I
2
To further investigate the catalytic activity of other
2
II
2
2
4-26
where the catalytic current for CO2 active states), the XPS analysis (Figure S4) is in agreement with
2
2
2
More importantly, the onset potential for CO reduction shifts
2
to -0.92 V from -1.42 V under argon. This onset potential is
II
much lower than the potential required when using Cu AH as
electrocatalyst. This is probably caused by the interaction of Fe
metal center with CO . A peak shift is only observed for Fe
metal center in UV-Vis spectra (Figure S5). To exclude other
factors on the catalytic activity, the ATR-FTIR and XRD spectra
of Cu AH and Fe AH were tested (Figure 4). A strong peak at
2
II
II
-1
1
000 cm ascribed to Fe-N and Cu-N confirms the catalyst
II
II
structure of Fe AH and Cu AH in Scheme 1. The spectra of all
these prepared samples are similar to the spectrum of original
ligand TPP_COOH, which indicates that no other crystal phases
were formed.
Figure 1. Cyclic voltammetry of 1 mM CuTPP (black) under
argon and 1 mM Cu AH under argon (green) and CO (red) in
anhydrous DMF (0.1 M TBATFB) at 100 mV/s.
II
2
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I
-
II
④
⑤
⑥
Fe AH - e

Fe AH
View Article Online
DOI: 10.1039/D0NJ02900A
II
-
I
Fe AH(CO) + e
 Fe AH + CO
I
-
0
Fe AH + e
 Fe AH
0
-
I
Fe AH +CO + e

Fe AH + Products
2
I
II
⑦
Fe AH + CO - e-

Fe AH(CO)
Next, we compare the H O effect on the CO reduction for
2
2
II
II
CuTPP_COOH (Cu AH), FeTPP_COOH (Fe AH), CoTPP_COOH
Co AH), and NiTPP_COOH (Ni AH). UV-Vis spectra of these
0
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II
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(
samples are given in Figure S6. The ATR-FTIR spectra (Figure S7)
of Co and Ni center are also provided. According to the CV
results of Cu (Figure 2c), Fe (Figure 3), Co and Ni center (Figure
S8), the catalytic current ratio of CO reduction jCO (-1.52 V
2
2
under CO ) to hydrogen evolution jH (-1.52 V under argon) in
the presence of different H O content are plotted in Figure 5.
The ratio increases with the increase of H O content at first
2
2
2
2
and then reaches to a plateau or slightly decreases when H O
2
content continue increasing. This is probably due to the
II
favourable hydrogen evolution with higher H O content. Fe AH
2
exhibits the best activity for CO reduction with this external
2
proton donor when the added concentration of H O is higher
2
than 1 M. The onset potential for these systems is also listed in
II
Figure 3. Cyclic voltammetry of 1 mM Fe AH under argon Table 2. The shift of the onset potential to more positive
black) and CO (red) in anhydrous DMF (0.1 M TBATFB) at 100 potential indicates lower energy input for CO2 reduction,
(
2
mV/s, with various concentration of H O (2 M, 3 M, 4 M, and 5 which in the order of Fe > Co > Ni > Cu.
M).
2
II
II
Figure 5. The ratio of catalytic current under CO and argon
2
jCO /jH , at -1.52 V vs. Fc /Fc) versus water content for
aromatic carboxylic acid functionalized metal porphyrins
Figure 4. XRD and ATR-FTIR spectra of Cu AH, Fe AH and pure
TPP_COOH ligand.
+
(
2 2
II
(
M AH).
From Figure 3, we can also observe that the reduction and
oxidation waves of Fe AH/Fe AH shift to more negative
potential in the presence of CO . In the CuTPP_COOH system Table 2. Comparison of aromatic carboxylic acid functionalized
stated above, however, the addition of CO does not change
the first wave Cu /Cu . It is indeed well-known that Fe TPP
strongly bind CO after the iron porphyrin complex from Fe(III) Electrocatalysts
II
I
2
II
metal porphyrins (M AH) in the presence of optimized H O
2
2
content.
II
I
II
+
Onset potential (V vs. Fc /Fc)
jCO /jH
2 2
3
0, 31
state to an active Fe(II) center.
The shift of the
at -1.52 V
II
I
Argon
-1.22
-1.42
-1.32
-1.42
CO2
Fe AH/Fe AH waves could be attributed to the complexation of
II
II
Fe AH with CO product [Fe AH(CO)] from CO reduction during
2
CuTPP_COOH
FeTPP_COOH
CoTPP_COOH
NiTPP_COOH
-1.22
-0.92
-1.07
-1.27
2.3
5.0
2.5
1.9
CV testing. The similar phenomenon was observed in phenolic
functionalized FeTPP, where the strongly stabilized O=C:Fe TPP
complex resulted in a substantial shift of the Fe TPP oxidation
II
I
1
7
potential. The peak analysis for Figure 3 is provided in the
following equations:
II
-
I
①
②
③
Fe AH + e



Fe AH
I
-
0
Fe AH + e
Fe AH
0
-
I
Fe AH - e
Fe AH
4
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4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
II
Enhanced activity with aliphatic carboxylic group on porphyrin
ligands.
We also add water into the Fe ~AH system and calculate the
View Article Online
DOI: 10.1039/D0NJ02900A
catalytic current ratio jCO /jH (-1.52 V). As shown in Figure 7,
2
2
when adding 1 M H O into the system, the current ratio
jCO /jH dramatically decreases (black line). Then it slightly
decreases with increasing H O content from 2 M to 5 M. This
indicates that the addition of H O in Fe ~AH system does not
have positive effect on the catalytic activity for CO reduction.
This is contrary to the Fe AH system, where enhanced the
current ratio jCO /jH in Figure 6 was observed (red line). The
decrease of the current ratio jCO /jH for Fe ~AH is impossibly
2
As stated above, the aromatic carboxylic acid functionalized
metal porphyrins only exhibit good performance for CO2
reduction in the presence of external proton source. Aliphatic
carboxylic acid grafted onto porphyrin complex was designed
2
2
2
II
2
2
as local proton source (hematoporphyrin, ligand
5 in Scheme
II
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
). The corresponding UV-Vis and ATR-FTIR spectra are
2
2
provided in Figure S9 and Figure S10 for Fe, Cu, Co and Ni
metal center. Since the iron center exhibits the highest activity,
iron hematoporphyrin FeTPP~COOH (Fe ~AH) was then
investigated as model catalysts. The XPS was also studied and
given in Figure 6 (left). The binding energy at 711.28 eV and
II
2
2
II
caused by the competing hydrogen evolution because the
catalytic current under argon kept unchanged as shown in
Figure 8 (dash line). However, the catalytic current under CO2
decreases with the increase of water content (solid line in
Figure 8). The probable reason is that aliphatic carboxylic acid
is able to provide enough protons for CO reduction and thus
7
25.18 eV are respectively corresponding to 2p3/2 and 2p1/2 of
Fe(III). Figure 6 (right) is the cyclic voltammetry curves of the
II
2
Fe ~AH in the absence of external proton donor. Two
the best performance could achieve in the absence of H O.
2
reversible reduction waves are observed under argon (black
I
Moreover, added H O could lower the CO concentration in
electrolyte from 300 mM in anhydrous DMF to 230 mM in
DMF with 2 M H O. In turn the availability of CO to catalytic
2
2
curve), corresponding successively to the generation of Fe and
0
Fe . Although in the total absence of external proton source, a
2
2
drastic increase and change in shape of the second wave was
observed. The current under CO is 6 times the current under
argon, with an onset potential at -1.02 V. This onset potential
is low compared with other systems reported (Table S1).
Moreover, this CO catalytic current, in the absence of H O and
sites decreases. This may also contributes to the decrease of
2
the catalytic current for CO reduction.
2
2
2
II
Brønsted acid, is comparable with that of Fe AH in DMF
containing 4 M H O. These indicate that aliphatic carboxylic
2
acid is able to provide enough protons for the metal center
and corresponding proton transfer for PMET pathway. The
enhanced activity of aliphatic carboxylic acid grafted on iron
porphyrin than that of aromatic counterpart further confirms
the positional effect and space substituent effects on
proton transfer for iron porphyrins.
32
33
II
Figure 8. Cyclic voltammetry of 1 mM Fe ~AH under argon
dash) and CO (solid) in anhydrous DMF (0.1 M TBATFB) at
00 mV/s with various concentration of H O (0 M, 1 M, and 2
2
(
1
M).
2
Electrocatalytic CO reduction.
2
The prepared Cu and Fe metal center samples are investigated
as model catalysts for the online electrocatalytic CO reduction.
No CO product is detected under N atmosphere. Under CO
2
atmosphere, as shown in Figure 9, FeTPP~COOH (Fe ~AH) and
II
2
Figure 6. XPS spectra of FeTPP~COOH (Fe ~AH) and cyclic
voltammetry of 1 mM Fe ~AH under argon (black) and CO
II
2
2
II
(red) in anhydrous DMF (0.1 M TBATFB) at 100 mV/s.
II
CuTPP~COOH (Cu ~AH) exhibit higher CO faradaic efficiency
II
II
than FeTPP_COOH (Fe AH) and CuTPP_COOH (Cu AH)
respectively. It confirms the proposed superior effect for the
introduction of local proton donor to the phenyl groups. The
enhancement is obvious for Fe metal center catalyst, although
Cu metal center show higher faradaic efficiency of CO than
that of Fe center.
II
The CO FE for Fe ~AH is also compared with CO FE for other
1
3, 16, 17, 24, 26, 27, 32, 34-40
systems (Table S1).
This CO FE keeps
unchanged for 3 h electrolysis, as given in Figure S11.
According to above results and literatures,
4
, 6, 9, 35, 41
we
Figure 7. The ratio of catalytic current under CO and argon (at
2
proposed possible mechanism for enhancing CO reduction to
CO by aliphatic carboxylic acid (propanoic acid) as local proton
+
II
II
2
-
1.52 V vs. Fc /Fc) versus H O content for Fe AH and Fe ~AH.
2
This journal is © The Royal Society of Chemistry 20xx
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Figure 9. Faradaic efficiency of CO for FeTPP~COOH (Fe ~AH),
FeTPP_COOH
CuTPP_COOH (Cu AH) under CO atmosphere.
II
II
a
(Fe AH),
CuTPP~COOH
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In summary, the electrochemical behaviour of porphyrin
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aliphatic carboxylic acid as internal
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2
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Thermodynamics and kinetics of CO , CO, and H binding to
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Conflicts of interest
There are no conflicts to declare.
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Financial support of this work by the National Science
Foundation of China (NSFC, 21703154) and the Natural Science
Foundation of Tianjin (18JCYBJC43100) is gratefully
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View Article Online
DOI: 10.1039/D0NJ02900A
Modified metal tetraphenylporphyrin through the introduction of propanoic acid to the phenyl
groups as local proton donor, exhibit higher CO electrocatalytic conversion to CO than that of
2
benzoic acid.
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