Cobalt porphyrin catalyzed reduction of CO2. Radiation chemical, photochemical, and electrochemical studies

Article abstract of DOI:10.1021/jp9807017

Several cobalt porphyrins (CoP) have been reduced by radiation chemical, photochemical, and electrochemical methods, in aqueous and organic solvents. In aqueous solutions, the Co^IP state is stable at high pH but is shorter lived in neutral and acidic solutions. Stable Co^IP is also observed in organic solvents and is unreactive toward CO₂. One-electron reduction of Co^IP leads to formation of a species that is observed as a transient intermediate by pulse radiolysis in aqueous solutions and as a stable product following reduction by Na in tetrahydrofuran solutions. The spectrum of this species is not the characteristic spectrum of a metalloporphyrin π-radical anion and is ascribed to Co⁰P. This species binds and reduces CO₂. Catalytic formation of CO and HCO₂^- is confirmed by photochemical experiments in acetonitrile solutions containing triethylamine as a reductive quencher. Catalytic reduction of CO₂ is also confirmed by cyclic voltammetry in acetonitrile and butyronitrile solutions and is shown to occur at the potential at which Co_IP is reduced to Co₀P. As compared with CoTPP, fluorinated derivatives are reduced, and catalyze CO₂ reduction, at less negative potentials.

Full text of DOI:10.1021/jp9807017

2870
J. Phys. Chem. A 1998, 102, 2870-2877
Cobalt Porphyrin Catalyzed Reduction of CO2. Radiation Chemical, Photochemical, and
Electrochemical Studies
D. Behar,† T. Dhanasekaran, and P. Neta*
Physical and Chemical Properties DiVision, National Institute of Standards and Technology,
Gaithersburg, Maryland 20899
C. M. Hosten, D. Ejeh, and P. Hambright
Department of Chemistry, Howard UniVersity, Washington, D.C. 20059
Etsuko Fujita
Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973
ReceiVed: January 7, 1998; In Final Form: February 25, 1998
Several cobalt porphyrins (CoP) have been reduced by radiation chemical, photochemical, and electrochemical
methods, in aqueous and organic solvents. In aqueous solutions, the CoIP state is stable at high pH but is
shorter lived in neutral and acidic solutions. Stable CoIP is also observed in organic solvents and is unreactive
toward CO2. One-electron reduction of CoIP leads to formation of a species that is observed as a transient
intermediate by pulse radiolysis in aqueous solutions and as a stable product following reduction by Na in
tetrahydrofuran solutions. The spectrum of this species is not the characteristic spectrum of a metalloporphyrin
π-radical anion and is ascribed to Co0P. This species binds and reduces CO2. Catalytic formation of CO
and HCO2- is confirmed by photochemical experiments in acetonitrile solutions containing triethylamine as
a reductive quencher. Catalytic reduction of CO2 is also confirmed by cyclic voltammetry in acetonitrile and
butyronitrile solutions and is shown to occur at the potential at which CoIP is reduced to Co0P. As compared
with CoTPP, fluorinated derivatives are reduced, and catalyze CO2 reduction, at less negative potentials.
Introduction
corresponding to those for reduction of the CoI state. The
present study is aimed at characterizing the species produced
by this reduction, which has been assumed to be reduced at the
ligand,12 and to examining its role in electrochemical and
photochemical reduction of CO2.
Catalyzed reduction of CO2 has been studied extensively.1
A number of transition-metal complexes have been utilized as
electron-transfer mediators to achieve electrochemical2,3 or
photochemical4-6 reduction of CO2. Among the porphyrin
complexes studied, iron porphyrins have been demonstrated to
be the most efficient homogeneous catalysts in the electro-
chemical reduction of CO2 to CO.7 The mechanism was
suggested to involve formation of an intermediate Fe-CO2
complex after the initial FeIIIP is reduced to the Fe0P oxidation
state. Recent photochemical studies also demonstrated reduction
of CO2 catalyzed by reduced iron porphyrins,8 although the
turnover numbers for the porphyrins were not very high due to
reductive decomposition of the macrocycle. In the present study
we extend this work to cobalt porphyrins. We utilize radiation
chemical, photochemical, and electrochemical techniques to
examine several cobalt porphyrins for their one-electron reduc-
tion steps and their possible role as catalysts for photoreduction
of CO2.
Experimental Section13
Most photochemical and electrochemical experiments in
organic solvents were carried out with cobalt tetraphenylpor-
phyrin (TPP), which was supplied in the form of CoIITPP.
Several experiments were carried out with derivatives of CoII-
TPP, where each phenyl ring contained either a 3-F (T3FPP)
or a 3-CF3 group (T3CF3PP) or was perfluorinated (TF5PP).
Experiments in aqueous solutions were carried out with the
water-soluble cobalt porphyrins, which were supplied in the form
of ClCoIIIP. These included tetrakis(4-sulfonatophenyl)porphy-
rin (TSPP), tetrakis(N-methyl-4-pyridyl)porphyrin (TM4PyP),
tetrakis(N-methyl-3-pyridyl)porphyrin (TM3PyP), tetrakis(N-
methyl-2-pyridyl)porphyrin (TM2PyP), and tetrakis(N,N,N-
trimethyl-4-aminophenyl)porphyrin (TAP). These water-soluble
porphyrins bear four negative or positive charges at the meso
substituents. Because the effect of these charges on the redox
reactions is secondary, and to have similar notations for all the
porphyrins in terms of the ligand or metal oxidation state, we
do not include these peripheral charges in our abbreviated
notations. The porphyrins were obtained from Mid-Century
Chemicals (Posen, IL). Triethylamine (TEA) was from Aldrich;
acetonitrile, N,N-dimethylformamide (DMF), 2-propanol (2-
Electrocatalytic reduction of CO2 by cobalt porphyrins9 and
phthalocyanines10 in aqueous solutions has been reported to yield
CO and formic acid. More efficient electrocatalysis has been
found with cobalt phthalocyanines deposited on the surface of
carbon electrodes.11 Reduction of the Co center in these
complexes to the CoI state did not lead to catalytic reduction of
CO2. Catalytic reduction was apparent only at potentials
† On leave from the Soreq Nuclear Research Center, Yavne, Israel.
S1089-5639(98)00701-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/02/1998

Co Porphyrin Catalyzed Reduction of CO2
J. Phys. Chem. A, Vol. 102, No. 17, 1998 2871
PrOH), sodium formate, and the inorganic compounds were
analytical grade reagents from Mallinckrodt. Butyronitrile
(BuN), TEA, and tetrahydrofuran (THF) were purified by
published methods14 and stored in a vacuum over activated
molecular sieves, NaK, and NaK, respectively. Tetrapropyl-
ammonium perchlorate (TPAP) was recrystallized twice from
a mixture of water and ethanol and was dried under vacuum.
(Warning: the perchlorate salts may be explosive and potentially
hazardous.) Water was purified with a Millipore Super-Q
system.
Fresh solutions were prepared before each experiment.
Irradiations were performed with solutions that were bubbled
with Ar or He to remove oxygen or were saturated with CO2.
Radiolysis was performed in a Gammacell 220 60Co source with
a dose rate of either 0.8 Gy s-1 or 2.7 Gy s-1. Photolysis was
performed with a 300 W xenon lamp, using water filters to
absorb the IR and cutoff filters to absorb at λ < 320 nm.
Changes in porphyrin structure were derived from spectropho-
tometric measurements, and the CO evolved was determined
by gas chromatography (Carboxen-1000 column, thermal con-
ductivity detector). Formate ions were analyzed by a Dionex
DX-500 ion chromatograph using an AS-11 column and NaOH
solutions as eluent. To observe short-lived intermediates, pulse
radiolysis was carried out with the apparatus described before,15
which utilizes 50 ns pulses of 2 MeV electrons from a Febetron
model 705 accelerator. Reaction rate constants are reported with
their estimated standard uncertainties. All experiments were
performed at room temperature, (20 ( 2) °C. Some of the
porphyrins were also reduced by using a sodium mirror in THF
by standard vacuum line procedures using home-built glassware
at 25 °C.16 An excess of Na was generally used, and the end
point of each reduction was carefully monitored by the loss of
isosbestic points. The Na chamber was removed from the cell
before the addition of research grade CO2.
Cyclic voltammograms were recorded with a BAS100B
electrochemical analyzer with scan rates ranging from 20 mV
s-1 to 1 V s-1. A conventional three-electrode system consisting
of a Au, Pt, or glassy carbon working electrode, a Pt counter
electrode, and a standard calomel reference electrode was used.
Ferrocene was added as an internal standard at the end of all
experiments. All potentials are given with reference to the
standard calomel electrode. Thin-layer spectroelectrochemistry
was performed using an in-house designed cell. The electro-
chemical system was similar to that used in the cyclic voltam-
metric studies except that the working electrode was a gold mesh
with 60% transmission obtained from Buckbee-Mears. For the
Figure 1. Radiolytic reduction of CoIIITSPP in deoxygenated aqueous
solutions containing 0.01 mol L-1 HCO2 and 0.2 mol L-1 KOH,
-
showing the spectra of CoIIITSPP (solid line), CoIITSPP (dashed line),
and CoITSPP (dotted line).
equivalent to the total yield of radicals under these conditions,
-
•
-
indicating reduction of the porphyrin by all eaq and CO2
radicals.
H2O Df eaq-, H•, •OH, H+, H2, H2O2
(1)
(2)
(3)
(4)
H•/•OH + HCO2- f H2/H2O + •CO2
-
ClCoIIIP + eaq- f Cl- + CoIIP
ClCoIIIP + •CO2- f Cl- + CoIIP + CO2
After complete reduction of CoIIIP to CoIIP, further irradiation
of the solution led to formation of CoIP. The radiolytic
reduction yield for this step under alkaline conditions, where
CoIP is stable, was 0.68 µmol J-1, indicating that reactions 5
and 6 are also quantitative.
CoIIP + eaq- f CoIP
(5)
(6)
CoIIP + •CO2- f CoIP + CO2
The optical absorption spectra of CoIIITSPP and its CoII and
CoI states, produced by radiolytic reduction at high pH, are
shown in Figure 1. Similar results were obtained with CoIII-
TAP, CoIIITM2PyP, and CoIIITM3PyP. CoIIITM4PyP, however,
was reduced to a mixture of CoITM4PyP and the phlorin anion
CoIITM4PyPH- (with broad 820 nm absorption). The CoIP
products were stable for hours (in the absence of O2) at pH >
12 but disappeared more rapidly as the pH was decreased; they
disappeared within minutes at neutral pH and were not observed
in acidic solutions. The products of these decays were not
examined in detail; CoTSPP in neutral solutions gave the chlorin
(sharp peak at -620 nm), and CoTM4PyP gave some phlorin
anion and related products but little chlorin. These results show
that clean and stable CoIP species in aqueous solutions can be
prepared from four of the above five porphyrins but only at
high pH. We can utilize these preparations to carry out pulse
radiolysis experiments with these solutions to observe the
transient species produced by the next one-electron reduction
step.
electrochemical studies the porphyrin, (0.1-1) × 10-3 mol L-1
,
was dissolved in anhydrous acetonitrile or butyronitrile, and the
supporting electrolyte was tetrabutylammonium perchlorate or
tetrapropylammonium perchlorate. The solutions were purged
with Ar or CO2.
Results and Discussion
Radiolytic Reduction of Cobalt Porphyrins. Radiolytic
reduction of Co(III) porphyrins in aqueous17 and in alcohol18
solutions has been shown to lead to efficient production of Co-
(II) porphyrins, i.e., with a radiolytic yield corresponding to
quantitative reaction of all reducing radicals with the porphyrin.
We obtained the same results with the water-soluble porphyrins
upon γ-irradiation in aqueous solutions containing formate ions
as scavengers for H• and •OH. The radiolytic yields for
reduction of CoIIIP to CoIIP in deaerated solutions containing 5
× 10-5 mol L-1 porphyrin, 1 × 10-2 mol L-1 HCO2-, and 0.2
mol L-1 KOH were ∼0.66 µmol J-1 for the various porphyrins,
To observe the short-lived species produced from CoIP, we
prepared solutions of CoIP by γ-radiolysis of CoIIIP in alkaline
media, as described above, and then irradiated samples of these

2872 J. Phys. Chem. A, Vol. 102, No. 17, 1998
Behar et al.
840 nm, with molar absorptivities of about 1.7 × 103 L mol-1
cm-1 and with no clear peak. These absorptions are different
than the typical spectra of π-radical anions produced from the
corresponding complexes of Zn, Al, or Sn,17,18,20 where an
intense peak is found near 700 nm with molar absorptivity near
1 × 104 L mol-1 cm-1. From this difference we suggest that
the species formed from CoIP may be Co0P. The differential
spectra monitored upon one-electron reduction of CoITSPP and
CoITAP also exhibit broad absorptions in the 600-800 nm
region, with no intense peaks, and are also ascribed to the Co0P
species. These species are expected to be the reactive inter-
mediates which bind and reduce CO2.
CoIP + eaq- f Co0P
Co0P + CO2 f (CO2-CoP)
(7)
(8)
(9)
(CO2-CoP) + H2O f CO + CoIIP + 2OH-
These reactions, however, could not be observed by pulse
radiolysis because it was not possible to prepare stable CoIP
for these experiments in the presence of CO2. Although CoIP
does not react with CO2 (see below), the presence of CO2 in
water requires a pH near neutral where the CoIP is relatively
short-lived (decays within minutes).21
The product of reaction 8 may be produced also by reaction
•
-
of CoIP with CO2 radicals. This reaction is expected to be
slower than reaction 7 and may take place by two routes: either
to produce the same adduct as in reaction 8
CoIP + •CO2- f (CO2-CoP)
or via electron transfer, reaction 11.
CoIP + •CO2- f Co0P + CO2
(10)
(11)
-
For CoITM3PyP and CoITM2PyP the rapid decay of the eaq
absorption was followed by a slower increase of absorbance
over the next 10-20 µs, due to reaction with CO2-. From
•
this slower formation, a rate constant of ∼5 × 109 L mol-1 s-1
is estimated for reaction of •CO2- with CoITM3PyP. This value
is in the same range as the rate constants reported for several
other TMPyP metal complexes.22 For CoITSPP and CoITAP,
however, no slower formation step was observed, indicating that
the •CO2- radicals decay by self-reaction rather than react with
these porphyrins. From the concentration of the porphyrins,
the concentration of •CO2- formed in the pulse, and the known
rate constant for the self-decay of •CO2-, we estimate an upper
Figure 2. Differential absorption spectra recorded by pulse radiolysis
of deoxygenated aqueous solutions containing 3 × 10-5 mol L-1 CoIP,
0.01 mol L-1 HCO2-, and 0.2 mol L-1 KOH: (a) CoITM2PyP, 3 µs
after the pulse; (b) CoITM3PyP, 3 µs (O) and 15 µs (b) after the pulse;
(c) the difference between the two spectra shown in (b). The solutions
of CoIP were prepared by γ-radiolysis of CoIIIP solutions in the above
medium.
•
-
limit of E1 × 108 L mol-1 s-1 for the reaction of CO2 with
these two porphyrins. This upper limit is lower than the rate
constant reported for reduction of CoIITSPP by •CO2- radicals,17b
but it does not rule out an efficient reaction under low-dose
rate conditions, as in the γ-radiolysis experiments discussed
below.
solutions with a short electron pulse and monitored the
differential absorption after the pulse. In all cases we observed
-
the rapid decay of the eaq absorption at 700 nm, indicating
that the porphyrins were reduced by eaq-. The rate constant
for this reaction was determined by following the decay of the
The reaction of •CO2- with CoITM2PyP, observed following
the reaction of eaq-, results in increased absorbance (or
bleaching) at all wavelengths examined, with nearly the same
ratio, suggesting that the reaction leads to reduction of the
porphyrin (reaction 11). The yield is quite low, indicating that
-
eaq absorption as a function of porphyrin concentration and
found to be (2.1 ( 0.4) × 1010 L mol-1 s-1 for the negatively
charged CoITSPP and (4 ( 1) × 1010 L mol-1 s-1 for the
positively charged CoITM3PyP, in line with previous values
for different porphyrins.19
•
-
only about 20% of the CO2 radicals lead to reduction. The
rest probably decay by other modes. On the other hand, the
reaction of •CO2- with CoITM3PyP results in increased absor-
bance in the 580-720 nm range but little increase in the 740-
840 nm range, and the ratios of absorbances at the other
wavelengths also vary (Figure 2b). This suggests that the
-
The absorbance remaining after the decay of eaq can be
ascribed to the porphyrin reduction product. The differential
absorption spectra monitored with CoITM2PyP (Figure 2a) and
CoITM3PyP (Figure 2b) show bleaching of the CoIP absorption
at 550 nm and formation of very broad absorptions at 600-

Co Porphyrin Catalyzed Reduction of CO2
J. Phys. Chem. A, Vol. 102, No. 17, 1998 2873
potential of +0.60 V using a Pt working electrode. The
transformation of CoIITPP (412 nm peak) to CoITPP (peaks at
361 and 421 nm) at a potential of -1.0 V was also confirmed
by spectroelectrochemical experiments in CH3CN.
When the CoIITPP (5 × 10-4 mol L-1) solution was saturated
with CO2, the electrochemical wave for the CoIIP/CoIP step was
unaffected but the wave for the CoIP/Co0P step was increased
by a factor of 8 (at a scan rate of 100 mV s-1). A catalytic
current of 240 µA was observed, compared with only 30 µA
for the CoIIP/CoIP wave. This catalytic reduction of CO2 took
place at a potential ∼300 mV less negative than that required
for direct reduction of CO2 in a similar solution without
porphyrin (Figure 3). These findings are in line with recent
electrochemical results25 and with the photochemical experi-
ments described below.
To confirm that CO2 reduction is indeed effected by the Co0P
species, and to enhance the electrochemical catalysis by shifting
the wave to less negative potentials, we examined several
derivatives of CoIITPP that are expected to be more readily
reduced. First, we attempted to use the perfluoro derivative,
CoIITF5PP. This compound was reduced in two reversible
waves at potentials of -0.61 V and -1.61 V vs SCE, i.e., 0.25
and 0.41 V less negative than CoIITPP. However, in the
presence of CO2, the reduction waves were similar to those
under Ar, indicating no catalysis of CO2 reduction. The reason
for this lack of catalysis may be the reduction of the perfluori-
nated rings (see below). To prevent this route, we studied two
other porphyrins with a lower degree of fluorination, and thus
with reduction potentials less negative than those of CoIITPP
but more negative than those of CoIITF5PP. These were CoII-
T3CF3PP, with reduction potentials of -0.75 V and -1.81 V
vs SCE, and CoIIT3FPP, with potentials of -0.79 V and -1.89
V (Table 1). The catalyzed reduction of CO2 took place at the
CoIP/Co0P potential for both of these porphyrins and occurred
at -1.81 V (Figure 3) and -1.89 V, respectively. These
potentials are less negative than that required for direct reduction
of CO2 in the absence of porphyrin, and the currents were
increased 4 and 6 times, respectively, using 5 × 10-4 mol L-1
porphyrin solutions and 100 mV s-1 scan rate. Thus, CoII-
T3CF3PP and CoIIT3FPP catalyze CO2 reduction at less negative
potentials than does CoIITPP, but the catalytic current is smaller.
The catalytic current, ip, is given by eq 12 if a reversible
electron-transfer reaction is followed by a fast catalytic reaction26
(assuming that the reaction is first order in catalyst and that
[CO2] . [cat.]).
Figure 3. Cyclic voltammograms of CoIITPP (center) and CoIIT3CF3-
PP (top) in butyronitrile solutions saturated with Ar (dotted line) and
with CO2 (solid line). The solutions contained 5 × 10-4 mol L-1
porphyrin and 0.1 mol L-1 tetrapropylammonium perchlorate, and the
scan rate was 100 mV s-1. The bottom voltammogram is with the same
solution in the absence of porphyrin. The current scale is 100 µA/
division.
product of this reaction is not the same as the product of reaction
7, and the difference spectrum (Figure 2c) may be ascribed to
the adduct formed by reaction 10, which is observed at high
pH before it decays to final products. This adduct may
decompose to yield CoIIP, as in reaction 9. Alternatively, at
high pH, it may decompose to CO2 and the reduced porphyrin,
leading to the same product as reaction 7. The relative
contributions of these two routes may be estimated from the
yields of products in γ-radiolysis experiments.
Comparative γ-radiolysis experiments were carried out with
CoITSPP and CoITM3PyP in aqueous solutions containing 0.01
-
mol L-1 HCO2 and 0.2 mol L-1 KOH. We found that CoI-
TM3PyP is removed by the radiolysis with a yield of ∼0.18
µmol J-1 to produce a mixture of products (chlorin, phlorin
anion, and further reduced products), which upon exposure to
O2 recovers ∼70% of the CoIIITM3PyP present in the original
solution. (This result indicates that ∼70% of the products were
reduced and protonated at meso positions, since these products
are known to be readily oxidized to the original porphyrins,
whereas chlorins are not oxidized by O2.) By considering that
the radiolytic conversion of CoITM3PyP into stable products
probably involves more than one-electron reduction, the ob-
served radiolytic yield indicates at least partial reduction of CoI-
ip ) nFA[cat.](Dk[CO2])1/2
(12)
•
-
TM3PyP by CO2 with possibly some oxidation via reaction
9, though the contribution of the latter route is uncertain. On
the other hand, removal of CoITSPP by the radiolysis took place
with a yield of only 0.04 µmol J-1, i.e., at least 4 times lower
than that for reduction of CoITM3PyP, suggesting that reaction
9 plays a more important role in this case, though it does not
occur quantitatively. The products formed upon reduction of
CoITSPP after long irradiations did not yield any CoIIITSPP
upon exposure to O2, indicating that the reduction products were
protonated at pyrrole rings rather than at meso positions.23
Electrochemistry of Cobalt Porphyrins. Cyclic voltam-
mograms of Ar-saturated solutions of CoIITPP in freshly distilled
and dry butyronitrile, using a glassy carbon working electrode,
show two successive reversible reduction steps, to CoITPP and
then to Co0TPP, separated by 1.15 V (Figure 3). The reduction
potentials for these processes are -0.86 and -2.02 V vs SCE,
in agreement with previous results in several solvents.24 The
oxidation of CoIITPP to CoIIITPP was also observed at a
In this equation, n is the number of electrons per molecule
reduced, F is the Faraday constant, A is the area of the electrode,
D is the diffusion coefficient of the catalyst, and k is the rate
constant for the homogeneous reaction. Plots of the observed
catalytic current vs [CoT3FPP] (Figure 4a) and vs [CO2]1/2
(Figure 4b) fit straight lines. These indicate that the reaction
is homogeneous, not an electrode-surface reaction, and the
reaction is first-order in CO2. These results suggest a mecha-
nism for the electrochemical reduction of CO2 involving
reactions 7 and 8.
Chemical Reduction of Cobalt Porphyrins. The short
lifetimes of the reduced cobalt porphyrins studied by radiolytic
and photochemical methods may be due to the presence of water
or traces of oxygen or impurities. To overcome this limitation,
we reduced several cobalt porphyrins in highly purified THF
solutions under vacuum by successive exposures to distilled
sodium metal. The spectra of the solutions were monitored after

2874 J. Phys. Chem. A, Vol. 102, No. 17, 1998
Behar et al.
TABLE 1: Electrochemical and Spectral Properties of Cobalt Porphyrins
E1/2
,
b
porphyrin
V vs SCEa
λ
max, nm (10-3ꢀ, L mol-1 cm-1
)
CoIITPP
-0.86
-2.02
282 (28.6), 413 (271), 529 (16.6), 606 (3.2)
260 (27.4), 300 (33.9), 364 (53.1), 426 (59.7), 510 (12.1), 544 sh (8.4), 620 (3.7), 778 (2.5)
314 (30.9), 420 (33.2), 498 (14.6), 528 sh (12.2), 650 (5.2)
[CoITPP]-
[Co0TPP]2-
CoIIT3FPP
-0.79
-1.89
240 (27.7), 282 (27.6), 414 (284), 528 (16.6), 600 sh (2.0)
[CoIT3FPP]-
[Co0T3FPP]2-
CoIIT3CF3PP
[CoIT3CF3PP]-
[Co0T3CF3PP]2-
CoIITF5PP
260 (27.8), 302 (39.0), 366 (69.4), 424 (75.4), 514 (14.9), 536 (14.2), 622 (3.5), 682 (1.6), 776 (1.1)
318 (37.3), 390 sh (39.3), 422 (44.7), 442 sh (41.6), 500 (19.8), 522 sh (18.4), 638 sh (4.6)
284 (26.0), 416 (296), 512 sh (10.7), 528 (16.1), 600 sh (2.5)
260 (29.5), 302 (42.7), 366 (67.3), 422 (86.5), 510 (17.2), 536 sh (16.0), 628 (4.3), 778 (1.8)
320 (33.8), 424 (40.5), 442 sh (36.6), 494 (19.9), 638 sh (7.4)
288 (21.5), 408 (274), 524 (13.5), 544 sh (9.5)
300 (25.3), 366 (44.4), 422 (48.0), 500 sh (7.7), 544 (11.0), 630 (2.3)
-0.75
-1.81
-0.61
-1.61
[CoITF5PP]-
a In butyronitrile. b Molar absorptivities were calculated by assuming 100% conversion from the CoIIP species in THF.
Figure 4. (a) Observed catalytic and noncatalytic currents vs the
concentration of CoIIT3FPP in butyronitrile solutions saturated with
CO2: (b) catalytic current with 100 mV s-1 scan rate at the CoIP/
Figure 5. (a) Spectral changes upon sodium reduction of CoIITPP in
Co0P potential, (2) catalytic current with 20 mV s-1 scan rate at the
CoIP/Co0P potential, (O) noncatalytic current with 100 mV s-1 scan
rate at the CoIIP/CoIP potential, (4) noncatalytic current with 20 mV
s-1 scan rate at the CoIIP/CoIP potential. (b) Observed catalytic current
vs relative [CO2]1/2 for a 5 × 10-4 mol L-1 solution of CoIITPP; 100
mV s-1 scan rate; 10%, 30%, and 100% CO2 were used.
THF. (b) Spectral changes upon sodium reduction of CoITPP in THF.
(c) The CoITPP/Co0TPP difference spectrum in THF.
porphyrin systems. This reduced species, with a large splitting
of the Soret band (314 and 420 nm), may be ascribed to Co0P.
Such a large splitting (360 and 452 nm) was observed in the
spectrum of Fe0TPP in DMSO28a and THF28b together with
peaks in the visible region. The molar absorptivities (in 103 L
mol-1 cm-1) of the peaks for Fe0TPP are as follows: in
DMSO,28b 362 nm (44.8), 459 nm (55.7), 526 nm (21.6), 556
nm (12.6), 760 nm (4.1), and 840 nm (5.7); and in THF,28b 358
nm (66.8), 450 nm (68.6), 514 nm (25.2), 710 nm (5.4), and
778 nm (7.8). The molar absorptivities of the peaks at 500-
800 nm for Fe0TPP are similar to those for Co0TPP, indicating
a similar amount of mixing with the porphyrin π-radical anion.
Further reduction of Co0TPP and Fe0TPP or reduction in a wet
solvent seems to produce a species absorbing at ∼840 nm,
indicating formation of a phlorin anion. Both CoITPP and Co0-
each reduction step. In all cases, gradual reduction of the CoIIP
to the CoIP state was readily achieved, as demonstrated by the
spectral changes (Figure 5a and Table 1), which are similar to
those reported before.27 The spectrum is consistent with
formation of CoITPP with some back-donation from the metal
into the porphyrin system (see small absorptions at 620 and
778 nm with molar absorptivities of 3700 and 2500 L mol-1
cm-1, respectively). Subsequent reduction of CoIP led to
formation of a product that does not exhibit the typical spectrum
of a π-radical anion, as seen from Figure 5b. The molar
absorptivity of the peak at 650 nm is only 5200 L mol-1 cm-1
consistent with some back-donation from the metal into the
,

Co Porphyrin Catalyzed Reduction of CO2
J. Phys. Chem. A, Vol. 102, No. 17, 1998 2875
TPP are very sensitive to oxygen and moisture, and opening
the solution to air leads to their immediate oxidation to CoII-
TPP.
these conditions. To prevent these side reactions, we studied
the spectral changes associated with reduction of CoIIT3FPP
and CoIIT3CF3PP and found them (Table 1) to be quite similar
to those observed with CoIITPP. In both of these porphyrins,
the CoIP species do not react with CO2 but the Co0P species
do.
Photochemical Reduction of Cobalt Porphyrins. Photo-
chemical reduction of CoIIIP and CoIIP was studied mainly in
acetonitrile solutions containing 5% TEA (0.36 mol L-1) as a
reductive quencher. The reduction mechanism is similar to that
discussed for iron porphyrins.8
The differential absorption spectrum shown in Figure 5c is
obtained by subtracting the spectrum of CoITPP from that of
Co0TPP. This is in general agreement with those obtained by
pulse radiolysis of water-soluble porphyrins (Figure 2).
Since TEA is used as reductive quencher in the photochemical
experiments described below, we determined whether TEA
binds to CoIITPP, CoITPP, and Co0TPP by repeating the above
experiments with solutions containing 5% TEA in THF. The
spectra of all the species were found to be identical to those
observed without TEA. This is consistent with the results that
the cyclic voltammograms of CoIITPP in Ar-saturated buty-
ronitrile solutions with and without TEA are identical. It seems
that, if TEA binds to any these species, the binding constant is
quite small.
Et3N(Cl)CoIIIP
9
hν8 Et3N•+ + Cl- + CoIIP
(15)
(16)
Et3N•+ + Et3N f Et3NH+ + Et2NC˙ HCH3
Et2NC˙ HCH3 + ClCoIIIP f Et2N+)CHCH3 + CoIIP + Cl-
Addition of CO2 to a THF solution containing CoITPP did
not cause any spectral change. However, addition of CO2 to a
solution containing Co0TPP produced the spectrum of CoITPP
instantly, and this gradually changed to CoIITPP within several
hours. The presence of TEA in the solution accelerated the
reaction to complete the change to CoIITPP within 1 h. We
did not detect any formation of CoIIITPP or (CO)CoIIITPP in
the solution.29 The reaction of CO2 with Co0TPP is expected
to form the complex (CO2-CoTPP) (reaction 8), which will
decompose to CO and CoIITPP (reaction 9). This CO2 complex
was not observed under the present experimental conditions.
Its decomposition may be facilitated by any acid that can accept
the oxide ion (e.g., reaction 9); in this case excess CO2 may
serve as the oxide acceptor.
(17)
Photolysis of various CoIII porphyrins under these conditions
produced clean CoIIP solutions in all cases. Reduction of CoIIP
to CoIP took place with lower quantum yields, due to the fact
that TEA does not bind to the cobalt in these reduced states, as
indicated by the electrochemical and spectroscopic results
discussed above. Clean CoIP was formed from all the Co(II)
porphyrins examined, except for CoIITM4PyP, which formed
directly a mixture of CoIP and the phlorin anion CoIIPH- (with
a broad peak at 820 nm).31 The water-soluble porphyrins were
also photolyzed in aqueous TEA solutions and gave essentially
the same results. CoIITPP was also photolyzed in DMF/TEA
solutions and formed the same product. Photolysis was then
carried out in CO2-saturated solutions. The CoIP state was
readily obtained and was stable in organic solvents in the
presence of CO2 but was not observed in aqueous solutions due
to the lower pH under CO2.
(CO2-CoTPP) + CO2 f CO + CoIITPP + CO3 (13)
2-
It appears that the CoIITPP formed by this reaction is rapidly
reduced by the remaining Co0TPP in a comproportionation
reaction to form CoITPP.
Photolysis of CoIIP in organic solvents saturated with CO2
was found to lead first to formation of CoIP, then to production
of CO and HCO2-, and finally to reduction and destruction of
the porphyrin macrocycle. The concentrations of CO and
Co0TPP + CoIITPP a 2CoITPP
(14)
Subsequent decay of CoITPP may be via a slow reverse reaction
14 followed by the rapid reaction 8.
HCO2 measured after photolysis were much greater than the
-
initial concentrations of the porphyrins, indicating catalytic
Since Co0TPP is formed at a very negative potential (-2.02
V vs SCE), it seemed worthwhile to investigate the interaction
of CO2 with other Co porphyrins which have less negative CoIP/
Co0P potentials. Replacing the phenyl with pyridyl groups
lowers the reduction potential of the porphyrin, but the solubility
of CoIITPyP was very low. Then we lowered the reduction
potential by using fluorinated derivatives. First we studied CoII-
TF5PP. The UV-vis spectra of the species formed by Na
reduction of CoIITF5PP in THF indicated formation of CoITF5-
PP with good isosbestic points, but further reduction showed a
decrease of all the peaks in the 300-800 nm region, with no
apparent formation of Co0TF5PP. This decay may be due to
reduction of the macrocycle or of the perfluorinated phenyl
groups. It is known that perfluorobenzene, unlike mono- or
difluorobenzenes, reacts rapidly with eaq- and eliminates F-.30
Therefore, it is likely that the electron added to CoITF5PP may
be partially delocalized on the perfluorophenyl rings and lead
to defluorination. This was confirmed by reducing CoIITF5PP
in γ-irradiated alkaline methanol solution until the absorption
of the CoI species disappeared and finding F- ions as products.
The yield corresponds approximately to only one F- ion per
molecule of porphyrin, indicating that other routes, such as
protonation of the reduced macrocycle, also play a role under
photoreduction. To estimate the amount of CO and HCO2
-
that may be produced from other sources, similar photolysis
experiments were carried out in solutions saturated with He
instead of CO2.32 The concentrations of CO and HCO2- formed
under He were much lower than those formed under CO2 in all
solvents, but the difference was largest in acetonitrile, where
the background levels were generally <1%. As discussed
before,8 photolysis of FeP in DMF solutions formed CO from
DMF via excitation of a DMF-FeP complex, and a similar
mechanism may operate with CoP. This mechanism is practi-
cally unimportant in acetonitrile solutions. Therefore, subse-
quent measurements were done with acetonitrile solutions. The
yield of CO as a function of photolysis time (Figure 6) indicates
that 1 × 10-5 mol L-1 CoIITPP leads to production of about 80
times that concentration of CO. But the porphyrin gradually
degrades to colorless products and CO production stops. The
same photolysis also produced formate at concentrations about
4-5 times greater than the concentrations of CO. Thus, the
total turnover number for CO2 reduction by CoTPP is >300.
The other porphyrins were compared to CoTPP after photolysis
under similar conditions, and the yields of CO and formate were
generally lower than those found with CoTPP. The yield of
CO was lower by about a factor of 2 for CoT3FPP, CoT3CF3-

2876 J. Phys. Chem. A, Vol. 102, No. 17, 1998
Behar et al.
eventually reduced and decomposed, and production of CO and
formate stops. Studies with other porphyrin-type complexes
are underway in an attempt to extend the catalytic efficiency
and longevity of this system.
Acknowledgment. This research was supported by the
Division of Chemical Sciences, Office of Basic Energy Sciences,
U.S. Department of Energy, under contracts DE-AI02-95ER14565
(NIST) and DE-AC02-76CH00016 (BNL). C.M.H. is indebted
to NIH for support of the preliminary electrochemical experi-
ments at Howard University under MBRS Grant No. S06GM-
08016. P.H. is indebted to NASA for support of his work under
Grant No. NCC-5-184.
References and Notes
Figure 6. Photochemical production of CO in acetonitrile solution
containing 1 × 10-5 mol L-1 CoIITPP, 5% TEA, saturated with CO2.
The solution was photolyzed in a Pyrex bulb cooled by a water jacket,
placed 10 cm away from an ILC Technology LX-300 UV lamp.
(1) Catalytic ActiVation of Carbon Dioxide; Ayers, W. M., Ed.; ACS
Symp. Ser. 1988, 363. Behr, A. Carbon Dioxide ActiVation by Metal
Complexes; VCH: Weinheim, 1988. Electrochemical and Electrocatalytic
Reactions of Carbon Dioxide; Sullivan, B. P., Ed.; Elsevier: Amsterdam,
1993. Sutin, N.; Creutz, C.; Fujita, E. Comments Inorg. Chem. 1997, 19,
67.
PP, and CoTM2PyP, but much lower for CoTM3PyP and
CoTM4PyP. Clearly, ring reduction in the latter cases decreases
the efficiency of CO2 reduction. The yield of formate also was
lower (by a factor of about 1.5-2) with CoT3FPP and CoT3CF3-
PP than with CoTPP. It was noticed, however, that the ratio
(2) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.; Vining, W. J.; Meyer,
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-
between the yields of CO and HCO2 obtained with the
porphyrins varied by about a factor of 2. The possible
mechanisms contributing to these variations are under study.
Summary and Conclusions
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Reduction of cobalt(II) porphyrins (CoIIP) to CoIP and Co0P
can be carried out by radiolytic, photochemical, and electro-
chemical methods and by sodium reduction in THF. Co0P is
stable in aprotic solvents but short-lived in aqueous solutions.
Its optical absorption spectrum is different from that expected
for a π-radical anion, but some back-donation from the cobalt
to the ligand is indicated. Whereas CoIP is unreactive toward
CO2, Co0P reacts rapidly with CO2 to form CoIIP, CO, and
formate. The expected complex of CO2 with the cobalt
porphyrin was not observed in the current experiments at room
temperature. Since the reduction potential of CoIP/Co0P is
highly negative, -2.02 V vs SCE for the CoTPP pair, the
complex with CO2 probably undergoes very rapid electron
transfer and subsequent reactions to yield the products. The
formate is produced by protonation, and CO is produced by
loss of oxide ion. Both of these reactions may be effected by
traces of protons in the photochemical system, but in the highly
purified and dry THF excess CO2 may act as an oxide acceptor.
Catalyzed reduction of CO2 is observed in cyclic voltammetry
experiments in acetonitrile or butyronitrile solutions and in
photochemical experiments in acetonitrile solutions containing
TEA as a reductive quencher. TEA does not appear to bind to
the cobalt center in CoIIP and CoIP, and thus the quantum yield
for the reduction is very low. It may be possible to increase
the quantum yield by use of a sensitizer such as p-terphenyl.5c
These experiments are underway in an attempt to produce and
characterize the CO2-CoTPP by laser flash photolysis.
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recognition or endorsement by the National Institute of Standards and
Technology, nor does it imply that the material or equipment identified are
necessarily the best available for the purpose.
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By substituting the phenyl rings of the CoTPP with electron-
withdrawing groups, such as F and CF3, the reduction potentials
become less negative, and the electrochemical catalyzed reduc-
tion of CO2 occurs at less negative potentials, corresponding to
those for the respective CoIP/Co0P pairs. On the other hand,
the catalytic currents were smaller. In agreement with the latter
observation, the yields of CO and HCO2- in the photochemical
experiments were somewhat lower with CoT3FPP and CoT3CF3-
PP than with CoTPP. In all cases the porphyrin macrocycle is
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Co Porphyrin Catalyzed Reduction of CO2
J. Phys. Chem. A, Vol. 102, No. 17, 1998 2877
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(31) It was further noticed that as the mixture of CoITM4PyP and
CoIITM4PyPH- underwent slow oxidation in the dark, the phlorin anion
gradually disappeared and more CoIP was formed. This is in agreement
with the previous suggestion that one-electron reduction of CoITM4PyP
yields first the π-radical anion CoITM4PyP•-, and this undergoes an
intramolecular electron transfer from the cobalt to the porphyrin ligand.12a
Under the present conditions, this CoIITM4PyP2- product undergoes
reversible protonation at the meso position to form the phlorin anion,
CoIITM4PyPH-, and when this is oxidized back to the radical anion,
CoIITM4PyP•-, the reverse intramolecular electron transfer leads to produc-
tion of CoITM4PyP.
(21) Because of the instability of CoIP in water in the presence of CO2,
we attempted to prepare CoIP by γ-radiolysis of CoIIP in deaerated
2-propanol and acetonitrile solutions. This attempt did not succeed.
Radiolysis of organic solvents always produces organic radicals in addition
to the solvated electrons, and these radicals (R) are known to react with
CoIIP via combination with the metal center to yield R-CoIIIP. Potentially
these products may be reduced by solvated electrons to R-CoIIP, which
may decompose to CoIP. But, in fact, the reduction did not proceed
effectively in this manner, and we were unable to produce clean and stable
CoIP by radiolysis in organic solvents. This may be the result of the lower
yield of solvated electrons as compared with the yield of organic radicals,
but may be also due to reaction of the organic radicals with the CoIP that
is partially formed and possibly due to reduction of R-CoIIIP or R-CoIIP
at the ligand instead of at the metal (as observed for Rh porphyrins by
Grodkowski et al.: Grodkowski, J.; Neta, P.; Abdallah, Y.; Hambright, P.
J. Phys. Chem. 1996, 100, 7066).
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1996, 25, 709.
(23) It has been noted before that TSPP metal complexes are reduced
in aqueous solutions to chlorins whereas TMPyP complexes are reduced to
phlorin anions.18.
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Truxillo, L. A.; Davis, D. G. Anal. Chem. 1975, 47, 2260. Walker, F. A.;
(32) He was chosen to deoxygenate the solution, rather than Ar or N2,
because He was the carrier gas in the GC and because Ar and N2 have GC
peaks just ahead of the CO peak, which prevents accurate measurement of
low CO concentrations.