Copper-catalyzed radiolytic reduction of CO2 to CO in aqueous solutions

Article abstract of DOI:10.1021/jp004567d

Radiolysis of aqueous solutions containing CO₂ and Cu(II) ions under reducing conditions leads first to reduction of the Cu(II) and then to formation of CO. Experiments under various conditions show that although Cu(II) was often reduced to colloidal Cu(O) particles, formation of CO requires the presence of Cu(I). It also requires that CO₂ be first reduced to the ì?CO₂^- radical. The ì?CO₂^- radical was produced radiolytically by reaction of CO₂ with e_aq^- or by reaction of formate ions with Hì? and ì?OH radicals and photochemically by reaction of formate with the acetone triplet. The ì?CO₂^- radicals are reduced to CO via addition to Cu(I) and subsequent reaction of the product with another Cu(I). The first reaction produces CuCO₂, which undergoes protonation at pH < 4. The reaction of the neutral CuCO₂ with Cu(I) leads to reduction of the copper to form Cu₂⁺ and subsequently Cu(0) particles. However, the reaction of the protonated form, CuCO₂H⁺, with Cu(I) leads to oxidation of the copper and formation of CO in the form of the CuCO⁺ complex. After most of the copper is converted into CuCO⁺, subsequently reactions involve this species instead of Cu⁺ and lead to further production of CO. From pulse radiolysis measurements, the rate constants for the reactions of the ì?CO₂^- radicals with Cu⁺ and CuCO⁺ were found to be a?? 1 ?? 10⁹ and (1.5 ?± 0.4) ?? 10⁸ L mol^-1 s^-1, respectively. The protonated adduct formed by the latter reaction at pH 3.4, Cu(CO)CO₂H⁺, reacts with CuCO⁺ with a rate constant a?? 5 ?? 10⁵ L mol^-1 s^-1 to produce more CO. It also undergoes first-order decomposition and second-order decay reactions.

Full text of DOI:10.1021/jp004567d

J. Phys. Chem. B 2001, 105, 4967-4972
4967
Copper-Catalyzed Radiolytic Reduction of CO2 to CO in Aqueous Solutions
†
J. Grodkowski and P. Neta*
Physical and Chemical Properties DiVision, National Institute of Standards and Technology,
Gaithersburg, Maryland 20899-8381
ReceiVed: December 21, 2000; In Final Form: March 6, 2001
2
Radiolysis of aqueous solutions containing CO and Cu(II) ions under reducing conditions leads first to reduction
of the Cu(II) and then to formation of CO. Experiments under various conditions show that although Cu(II)
was often reduced to colloidal Cu(0) particles, formation of CO requires the presence of Cu(I). It also requires
•
-
•
-
that CO
CO
2
be first reduced to the CO
2
radical. The CO
2
radical was produced radiolytically by reaction of
-
•
•
2
with eaq or by reaction of formate ions with H and OH radicals and photochemically by reaction of
•
-
formate with the acetone triplet. The CO
reaction of the product with another Cu(I). The first reaction produces CuCO
at pH < 4. The reaction of the neutral CuCO with Cu(I) leads to reduction of the copper to form Cu
subsequently Cu(0) particles. However, the reaction of the protonated form, CuCO
2
radicals are reduced to CO via addition to Cu(I) and subsequent
2
, which undergoes protonation
+
2
2
and
H , with Cu(I) leads to
oxidation of the copper and formation of CO in the form of the CuCO complex. After most of the copper
+
2
+
+
+
is converted into CuCO , subsequent reactions involve this species instead of Cu and lead to further production
of CO. From pulse radiolysis measurements, the rate constants for the reactions of the CO
•
-
2
radicals with
+
+
9
8
-1 -1
Cu and CuCO were found to be ≈ 1 × 10 and (1.5 ( 0.4) × 10 L mol s , respectively. The protonated
+
+
adduct formed by the latter reaction at pH 3.4, Cu(CO)CO
2
H , reacts with CuCO with a rate constant ≈ 5
5
-1 -1
×
10 L mol
s
to produce more CO. It also undergoes first-order decomposition and second-order decay
reactions.
Introduction
Experimental Section12
Photochemical reduction of CO2 has been explored as a means
of energy storage.1 The process was catalyzed in either
homogeneous or heterogeneous systems and the CO2 was
Most of the compounds used were analytical grade reagents
from Baker or Mallinckrodt. Poly(vinyl alcohol) (PVA, M.W.
14 000, 100% hydrolyzed) was from Aldrich. Water was purified
with a Millipore Super-Q system. The solutions were saturated
with Ar (ultrahigh purity), CO2 (Matheson, 99.99%) (∼36 mmol
2
,3
reduced in most cases to CO and/or formic acid. A few
experiments demonstrated photoreduction of CO2 to methane
-
1
-1
4
L ), or CO (Air Products, C.P. Grade) (∼1.1 mmol L ).
on the surface of metal colloids. Electrochemical reduction of
Gamma radiolysis was carried out in one of two Gammacell
CO2 led to production of formate on various metallic electrodes,
while CO was formed on Ag and Au, and hydrocarbons were
6
0
2
20 Co sources with dose rates of 2.1 and 7.0 kGy/h.
formed on Cu electrodes.5,6 More recently, additional reduction
products were identified following electrolysis on Cu, including
Photolysis was performed with an ILC Technology 300 W
xenon lamp, using a water filter to absorb the IR. Absorption
spectra before and after irradiation were recorded with a Cary
7
C2 and C3 alcohols and aldehydes. Along with the electro-
3
spectrophotometer. The gases (CO2, CO, H2, CH4) were
chemical reduction of CO2, copious amounts of H2 were also
produced. In recent studies, using alkaline methanol as the
electrolyte, the production of H2 was suppressed while CO2 was
analyzed by gas chromatography (Carboxen-1000 column,
thermal conductivity detector) by sampling the headspace. The
total yields were calculated from the known volumes of the
solution and the headspace and the known solubilities.13 Formate
ions were analyzed by a Dionex DX-500 ion chromatograph
using an AS-11 column and NaOH solutions as eluent. Pulse
radiolysis experiments were carried out with 1 µs pulses of 6
MeV electrons from a Varian linear accelerator; other details
8
efficiently reduced to CO, formate, methane, and ethylene.
The present study was undertaken to examine whether CO2
can be reduced on the surface of colloidal copper particles in
aqueous solutions and whether reduction can be carried out
beyond the CO stage. Photochemical and radiolytic reduction
methods were used. In the first experiments, colloidal copper
particles were prepared by radiolytic reduction of CuSO4 in
deoxygenated aqueous solutions containing 2-propanol, to create
reducing conditions, and poly(vinyl alcohol), to stabilize the
14
were as described before. All experiments were performed at
room temperature, (21 ( 2) °C. The values of the yields, molar
absorption coefficients, and rate constants are reported with their
estimated overall standard uncertainties.
9-11
metal colloid, similar to the conditions used in earlier studies.
In subsequent experiments, the conditions were varied to include
CO2 and to study its reduction mechanism.
Results and Discussion
Preliminary experiments with CO2-saturated aqueous solu-
tions containing CuSO4, 2-PrOH, and PVA showed production
of a reddish copper colloid and then production of CO. To
†
On leave from the Institute of Nuclear Chemistry and Technology,
Warsaw, Poland.
1
0.1021/jp004567d CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/04/2001

4968 J. Phys. Chem. B, Vol. 105, No. 21, 2001
Grodkowski and Neta
understand the relation between these two observations, we
examined the spectral evolution during the formation of copper
colloids under various conditions and their dependence on the
presence of CO2.
Colloidal particles of metallic copper were prepared by
radiolytic reduction of CuSO4 in Ar-saturated aqueous solutions
•
•
containing 2-PrOH, to convert the H and OH radicals into
reducing species, and PVA to stabilize the colloid (using [PVA
monomer]/[Cu ions] ) 20). This procedure is similar to that
used in previous studies.9 The copper colloid exhibits a broad
absorption between 200 and 800 nm with the characteristic
plasmon absorption peak at 590 nm (Figure 1a). The absorption
increases with irradiation dose, after an initial delay, and levels
off at doses above 3 kGy (Figure 1b). A similar experiment
with a CO2-saturated solution, however, led to the development
of a different spectrum (Figure 1c), with a weaker peak at 590
nm and nearly flat absorption between 300 and 800 nm, but
with a higher peak at 275 nm developing with a different time
profile (Figure 1d). The final absorption in Figure 1a remained
practically unchanged after prolonged irradiation but that in
Figure 1c decreased and the copper precipitated. However, upon
exposure to air, all the final spectra reverted to the original
spectra.
-11
Radiolytic production of copper colloids in CuSO4/2-PrOH/
PVA solutions could be carried out in acidic solutions (2.5 <
pH < 6) but at higher pH the intermediate Cu(I) precipitated.
To overcome this difficulty we used ammonia solutions. Figures
2
L
a and 2b show the production of the copper colloid in 0.1 mol
-1
NH4OH solution at pH 11. The starting material is in the
2+
form of Cu(NH3)4 , which has a UV absorption. Radiolytic
reduction led first to reduction of all the Cu(II) to Cu(I), as
monitored by the decrease in absorbance at 263 nm (G ) 5.8
-
7
-1
×
10 mol J ), and only then was the Cu(I) reduced to form
the metallic particles, with an absorption peak at 561 nm. This
spectrum (Figure 2a) remained unchanged upon prolonged
irradiation (with a dose >10 times greater than the dose required
to produce the colloid). Exposure to air, however, led to
oxidation of the colloidal copper in two stages, first to Cu(I)
and more slowly to Cu(II). When the Cu(NH3)42+/2-PrOH/PVA
solution was bubbled with CO2 until its pH decreased to 7, the
peak of the Cu(II) shifted to 276 nm and it decreased upon
-7
-1
radiolysis with a similar yield (G ) 5.7 × 10 mol J ) (Figure
2
c,d). Further irradiation led to production of colloidal Cu
particles but the spectrum was quite different from that observed
under Ar.
Prolonged irradiation of the CO2-saturated solutions (at pH
3
.8 without ammonia and at pH 7 with ammonia) led to
production of CO. This process began after the solution was
irradiated with the dose required to reduce all the Cu(II), it
continued with a slope corresponding to a radiolytic yield G ≈
-
7
-1
2
× 10 mol J , but after a large dose it stopped (Figure 3).
Figure 1. Growth of optical absorption upon γ-irradiation of aqueous
-
1
-1
In the same solution at pH 3.8, H2 production began with no
4
solutions containing CuSO (0.5 mmol L ), PVA (10 mmol L ,
-
1
-
7
molarity in terms of monomer units), and 2-PrOH (0.7 mol L ); (a)
Ar-saturated, pH 5, spectra with increasing absorbance in the visible
range were recorded after irradiation with the following doses: 0, 450,
delay and continued linearly with dose, G ≈ 1.3 × 10 mol
-
1
J
(Figure 3). This radiolytic yield of H2 is the sum of the
-
7
primary yield from the radiolysis of water (GH ) 0.45 × 10
2
6
00, 900, 1360, 1820, and 4550 Gy; (b) dose dependence of the 590
-
1
•
mol J ), the yield of H atoms, which react with 2-PrOH to
nm absorbance for the experiment in a; (c) CO -saturated, pH 3.8,
2
-
7
-1
-
form H2, (GH ) 0.6 × 10 mol J ), about 6% of the eaq that
spectra with increasing absorbance at 270 nm were recorded after
irradiation with the following doses: 0, 300, 600, 900, 1360, 1820,
and 4550 Gy; (d) dose dependence of the absorbance at 275 nm (O)
and 590 nm (b) for the experiment in (c).
+
•
-7
-1
react with H to form H atoms (GH ≈ 0.17 × 10 mol J ),
and possibly small amounts from the radiolysis of the 2-PrOH
component. Small amounts of CH4 were also detected (G ≈
-
7
-1
-7
-1
0
.08 × 10 mol J ) but the yield was not increased in the
radiolytic yield G ≈ 0.1 × 10 mol J , i.e., < 10% of the
presence of Cu and is assumed to be due totally to the radiolysis
of 2-PrOH.15 In addition to these gaseous products, small
amounts of formic acid were detected in the solution, with a
yield of CO. However, in the absence of copper, the yield of
-7
-1
formic acid was G ) 0.5 × 10 mol J but the yield of CO
-
7
-1
was negligible (≈0.01 × 10 mol J ). These results indicate

Cu-Catalyzed Radiolytic Reduction of CO2 to CO
J. Phys. Chem. B, Vol. 105, No. 21, 2001 4969
Figure 3. Radiolytic production of CO (b) and H
2
(O) in CO
2
-saturated
-
1
aqueous solution containing CuSO
4
(0.5 mmol L ), PVA (10 mmol
-
1
-1
-1
L
), and 2-PrOH (0.7 mol L ). The yields are given in mmol L
and represent the total amount of the gas as if it remained completely
in the aqueous phase.
Cu, production of formic acid is strongly diminished and CO
is formed instead.
Addition of 0.1 mol L- acetone to the CuSO4/2-PrOH/PVA/
CO2 solution suppressed the production of CO completely. Since
acetone reacts rapidly with the hydrated electron and thus
prevents its reaction with CO2, this result indicates that
1
•
-
production of CO2 radicals is necessary for the formation of
CO. Further evidence for this conclusion was obtained from
photochemical experiments. A mixture of acetone and 2-PrOH
1
6
is known to produce the (CH3)2 C˙ OH radical upon UV
photolysis. Photolysis of deoxygenated CuSO4 solutions con-
-
1
-1
taining 0.7 mol L acetone, 0.7 mol L 2-PrOH, and 10 mmol
L
-
1
PVA led to the production of colloidal copper. The spectra
monitored under Ar and under CO2 were identical and no CO
•
-
was detected. This indicates that formation of CO2 radicals
•
-
is necessary for the production of CO. To produce CO2
radicals by photolysis we added formic acid. The acetone triplet
•
-
abstracts hydrogen from formic acid to form CO2 . Under these
conditions, CO was detected as a product of photolysis and the
absorption spectrum of the copper was similar to that observed
in γ-irradiated CO2-saturated solutions.
•
-
The above results lead to the conclusion that CO2 radicals
are a necessary intermediate for the production of CO but do
not shed light on the copper species involved in this process. It
is clear from Figures 1 and 2 that the spectra of the copper
species observed under CO2 are different from those observed
under Ar. To ascertain that the spectral differences are not due
solely to the change in pH and to characterize the various
absorbing species we carried out additional experiments in the
presence and absence of PVA.
Figure 2. Changes in optical absorption upon γ-irradiation of aqueous
A clear difference between the spectra in the presence and
absence of CO2 (in both Figures 1 and 2) is the flat absorbance
extending to 800 nm that is observed only under CO2. Such a
flat absorbance is typically due to light scattering by fine
particles. Since this is observed only in the presence of CO2 it
is probably due to insoluble Cu(I) carbonate. In fact, its level
was more pronounced at increased bicarbonate concentration
or increased pH and in all CO2-saturated solutions a precipitate
appeared after prolonged irradiations.
-1
-1
4 4
solutions containing CuSO (0.5 mmol L ), PVA (10 mmol L ), NH -
-
1
-1
OH (0.1 mol L ), and 2-PrOH (0.7 mol L ); (a) Ar-saturated, pH 11,
spectra recorded after irradiation with the following doses: 1, 0; 2,
5
20; 3, 870; 4, 1340; 5, 1800; 6, 2270; 7, 3140 Gy; (b) dose dependence
of the absorbance at 263 (O) and 561 (b) nm for the experiment in
a); (c) CO -saturated, pH 7, spectra recorded after irradiation with the
following doses: 1, 0; 2, 290; 3, 580; 4, 1160; 5, 1590; 6, 2310; 7,
370; 8, 6100; 9, 16 300 Gy; (d) dose dependence of the absorbance
(
2
4
at 276 nm (O) and 578 nm (b) for the experiment in (c).
Another spectral feature that is observed only under CO2 is
the relatively sharp peak at 275 nm. This peak was found to be
due to a Cu(I)CO complex. This assignment was confirmed by
an experiment with CO-saturated solutions containing 0.5 mmol
that Cu affects the yields of CO and formic acid but is not
involved in the production of H2 and CH4. Formic acid is
produced by disproportionation of CO2 radicals in acidic
aqueous solutions, where no CO is formed. In the presence of
•
-
-
1
-1
-1
L
CuSO4, 10 mmol L PVA, and 0.7 mol L 2-PrOH.

4970 J. Phys. Chem. B, Vol. 105, No. 21, 2001
Grodkowski and Neta
Irradiation of such solutions led to production of the 275-nm
1
7
absorption but no absorption at all from 330 to 800 nm. The
absorbance at 275 nm reached its maximum after a dose of 1000
Gy, when all the Cu(II) was reduced to Cu(I). From the
absorbance at this level we calculate a molar absorption
+
3
-1
coefficient for the CuCO species ꢀ275 ) 2.4 × 10 L mol
-
1
cm . By using this ꢀ value, and from the absorbance buildup
vs dose, we calculate the radiolytic yield for formation of this
-
7
-1
product G ) 6.0 × 10 mol J . This value is in line with the
-
expected reduction of Cu(II) by all eaq and (CH3)2 C˙ OH radicals
and indicates that the Cu(I) produced is efficiently coordinated
to CO. Production of the species absorbing at 275 nm was also
evident when the CO-saturated solution was reduced with zinc
amalgam instead of radiolysis.
+
The CuCO complex was produced at much lower yields in
irradiated ammonia solutions because the large excess of
+
ammonia shifts the equilibrium toward [Cu(NH3)2] . Aside from
+
that case, CuCO was formed in all irradiated solutions in which
CO was formed, i.e., solutions containing either CO2 or formate,
independent of the presence of PVA. In all these solutions where
+
-7
CuCO was formed, its radiolytic yield was G ≈ 2 × 10
-
1
mol J .
Previous results10 with Ar-saturated formate solutions at pH
+
7
.3 did not indicate formation of CuCO . To find the cause for
this apparent discrepancy, we examined the effect of formate
+
concentration and pH. We found that production of CuCO was
pronounced whether the concentration of formate was 5 mmol
-
1
-1
L , as in ref 10, or 0.1 mol L , as in the present study.
However, it was pronounced only at pH e 4 while at pH g 5
radiolysis led predominantly to production of colloidal copper
Figure 4. Radiolysis of CO
2
-saturated aqueous solutions containing
-1
-
-1
CuSO
a) spectra with increasing absorbance at 275 nm were recorded after
4
(0.25 mmol L ) and HCO H/HCO (0.1 mol L ) at pH 3.4;
2
2
(
+
with only intermediate formation of CuCO . Production of
irradiation with the following doses: 0, 210, 250, 830, 1250, 1660,
and 16,650 Gy; the insert shows the growth of absorbance at 275 nm
with dose; (b) yield of H (O) and CO (b) as a function of dose.
2
colloidal copper was also evident in the absence of PVA,
although its long-term stability may be limited and was not
examined in these experiments.
water radiolysis react with 2-PrOH or formate, while eaq- reacts
Radiolysis experiments with CO2-saturated solutions contain-
2+
mostly (>90%) with CO2 and partially with copper ions. Cu
-
1
-
ing CuSO4 and 0.1 mol L HCO2H/HCO2 at pH 3.4 in the
absence of PVA show the development of the optical absorption
at 275 nm (Figure 4a) and the production of CO and H2 (Figure
-
ions are reduced by eaq , by the (CH3)2 C˙ OH radicals derived
from 2-PrOH, and by the CO2 radicals formed from formate
and CO2, to produce Cu ions. These ions are unstable in the
•
-
+
4
3
b) as a function of dose. Unlike the results in Figures 1c and
, radiolytic production of CO in this experiment continued
absence of suitable ligands. They are much more reactive than
2+
•
-
Cu toward (CH3)2 C˙ OH and CO2 radicals and react to form
linearly with dose up to very high doses and no copper colloid
10
+
0
adducts. Direct reduction of Cu to Cu by these radicals is
not possible, because of the very negative potential of the latter
species. Therefore, it has been suggested that metallic copper
(
i.e., no absorbance at 300 to 800 nm) was formed. In the early
+
stages of radiolysis Cu(II) was reduced to Cu(I), then CuCO
was formed, and then CO was produced in excess of copper
and was released into the solution and gas phases. Production
of CO began after the Cu(II) was converted into CuCO+ and
10
is formed via a reaction of the organo-copper species with an
+
additional Cu . For example,
-7
+
•
-
continued linearly with dose, with a yield G(CO) ) 3.3 × 10
Cu + CO f CuCO2
(1)
(2)
2
-
1
-7
-1
mol J (Figure 4b). GH was ≈ 1 × 10 mol J (Figure 4b).
2
After a considerable amount of CO was produced, we analyzed
the remaining amounts of formate and CO2 and found that both
of these compounds were consumed approximately to the same
extent. This is in line with the expected reactions of formate
+
+
CuCO + Cu f Cu + CO2
2
2
The Cu2+ grows into metallic particles by subsequent dispro-
10
•
-
portionation and reduction reactions. The reaction of CO2
•
•
-7
-1
with H and OH radicals (G ) 3.4 × 10 mol J ) and of
radicals with Cu(I) was reported to be extremely rapid to form
-
-7
-1
CO2 with eaq (G ) 2.8 × 10 mol J ), all reactions leading
10
CuCO2, which absorbs at 480 nm at pH 7. At pH 4 the peak
•
-
to formation of CO2 . Moreover, in these experiments, the pH
of the solution increased, in line with the overall consumption
of two protons upon reduction of CO2 to CO. In contrast, when
of the adduct absorption was shifted to 385 nm and the adduct
+
was presumably protonated, CuCO2H . From the pH depen-
dence of colloidal copper production we conclude that reaction
2
-PrOH was used instead of formate, and metallic copper was
2
takes place when the adduct is in its neutral form, but when
produced along with CO, the pH decreased, in line with the
+
the adduct is protonated Cu is oxidized rather than reduced,
leading to direct formation of the stable CuCO .
release of two protons upon reduction of Cu2 to Cu .
+
0
+
On the basis of all the above results and the mechanism
proposed for the production of colloidal copper from Cu2+
1
0
+
+
+
-
2+
CuCO H + Cu f CuCO + OH + Cu
(3)
2
ions we can outline the mechanism of CO formation in the
•
•
present systems. The H atoms and OH radicals produced by
This mechanism is also in line with the observed radiolytic yield

Cu-Catalyzed Radiolytic Reduction of CO2 to CO
J. Phys. Chem. B, Vol. 105, No. 21, 2001 4971
+
-7
-1
of this species, G(CuCO ) ≈ 2 × 10 mol J , i.e., 3 reducing
+
equivalents are required to form one CuCO .
After most of the copper is converted into CuCO , the CO2
radicals will react with this product via reaction 4 or 5.
+
•
-
+
•
-
CuCO + CO f Cu(CO)CO2
(4)
(5)
2
+
•
-
CuCO + CO f CO + CuCO2
2
The results outlined below suggest that reaction 4 predominates.
The adduct undergoes protonation at pH < 4,
+
+
Cu(CO)CO + H / Cu(CO)CO H
(6)
2
2
and the product reacts with another CuCO+.
+
+
Cu(CO)CO H + CuCO f
2
Cu(CO) + + OH + Cu + CO (7)
-
2+
2
Cu(CO) + / CuCO + CO
+
(8)
2
Under these conditions, where all the initial Cu(II) is already
reduced to Cu(I), production of CO requires only two reducing
-
7
equivalents, in line with the finding that G(CO) ) 3.3 × 10
-
1
mol J (i.e., half the total yield of reducing species).
To determine the rate constants of the above reactions, pulse
radiolysis experiments were carried out with CO2-saturated
-
aqueous solutions containing CuSO4 and HCO2 /HCO2H at pH
•
-
3
.4, where CO2 is the predominant species formed by the
2
+
18
-1
pulse. Cu forms a complex with formate. At 0.1 mol L
formate ∼90% of the Cu(II) is present as formate complexes
(
a mixture of complexes containing 1-4 formate ions). Upon
•
-
-
pulse irradiation, Cu(II) is reduced to Cu(I) by the CO2
2+
•
radicals. The rate constant for reduction of Cuaq by CO2
radicals, measured in the presence of 5 mmol L formate, was
-
1
9
-1 -1 10
2
L
× 10 L mol s . The rate constant measured at 0.1 mol
-1
19
8
-1 -1
formate was lower, 1.5 × 10 L mol s , and this low
value may be ascribed to the Cu(II)-formate complexes (with
2+
a contribution from the remaining Cuaq ). On the basis of these
•
-
rate constants, the reaction of CO2 radicals with Cu(II) to form
-
4
Cu(I) has a half-life t1/2 < 50 µs at [Cu(II)] > 1 × 10 mol
-1
•
-
10
L . Since Cu(I) reacts with CO2 radicals much more rapidly
than Cu(II), the accumulated Cu(I) becomes the dominant
reactant and forms the adduct (reaction 1). We followed the
formation and decay of this adduct at 385 nm (the protonated
form at pH 3.4) in consecutive pulses. As observed before under
1
0
2
Figure 5. Pulse radiolysis of CO -saturated aqueous solutions contain-
similar conditions, the decay of the adduct was rapid in the
first several pulses, due to reaction with the remaining Cu(II),
but became slower as Cu(II) was progressively reduced to Cu(I).
In these experiments, it was not possible to monitor the decay
-
-1
ing CuSO and HCO H/HCO (0.1 mol L ) at pH 3.4. The solutions
4 2 2
2+
+
were pre-pulsed to convert the Cu into CuCO ; (a) transient
absorption spectrum recorded 100 µ after the pulse, the solution
-
1
contained 0.1 mmol L copper; (b) the observed rate of formation at
385 nm as a function of copper concentration, the insert shows a typical
+
of CuCO2H via reaction 3 since a large fraction of the Cu(I)
-
1
+
trace taken at 0.3 mmol L copper; (c) decay at 385 nm recorded at
was present as the CuCO complex.
-1
0
.3 mmol L copper, the line was calculated from the model (see text).
To monitor the spectrum and decay kinetics of the Cu(CO)-
+
CO2H adduct in the absence of Cu(II), the solution was
subsequent protonation. The present value of ꢀ385, however, is
1
0
irradiated with repetitive pulses until all the Cu(II) was converted
twice as high as that observed before, suggesting that under
the present conditions the adduct is different, possibly indicating
that reaction 4 is more important than reaction 5.
+
into CuCO . The transient spectrum observed by pulse radiolysis
of this solution (Figure 5a) has a peak at 385 nm and is ascribed
+
to the Cu(CO)CO2H adduct formed via reactions 4 and 6. The
The rate of formation of the absorption at 385 nm was
+
+
molar absorption coefficient of Cu(CO)CO2H was calculated
determined at various CuCO concentrations, in each case
-
1
-1
2+
to be ꢀ385 ) (1700 ( 100) L mol cm by correcting the
beginning with Cu and pre-pulsing the solution until all the
•
-
+
measured absorbance for the efficiency of reaction of CO2
with the copper species. The 385 nm peak is similar to that
copper was converted into CuCO . From the concentration
dependence shown in Figure 5b we derive a rate constant k4 )
10
8
-1 -1
observed before at the same pH at low formate concentrations,
(1.5 ( 0.4) × 10 L mol s . This value is lower than k1,
which we ascribe to CuCO2H+ formed via reaction 1 and
which we estimate from our experiments as ≈ 1 × 10 L mol
9
-1

4972 J. Phys. Chem. B, Vol. 105, No. 21, 2001
Grodkowski and Neta
-
1
-
•
-
s . This estimate for k1 is an order of magnitude lower than
that estimated before10 and the difference may be due to the
large difference (20 times) in the formate concentrations used.
can CO react with eaq . Therefore, the reaction between CO
radicals and Cu(I) cannot be evaluated under these conditions.
Acknowledgment. This research was supported in part by
the Division of Chemical Sciences, Office of Basic Energy
Sciences, U.S. Department of Energy.
+
The decay of Cu(CO)CO2H was followed at several
wavelengths at different copper concentrations. The decay traces
an example is shown in Figure 5c) did not fit either first-order
(
or second-order rate law but could be best fitted to a combination
of a second-order and a first-order process. A plot of the first-
order rate vs [CuCO ] shows a linear dependence from which
References and Notes
(1) Sutin, N.; Creutz, C.; Fujita, E. Comments Inorg. Chem. 1997, 19,
+
67.
5
-1 -1
(2) (a) Fujita, E.; Brunschwig, B. S.; Ogata, T.; Yanagida, S. Coord.
we derive a rate constant k7 ) (5 ( 2) × 10 L mol s . The
Chem. ReV. 1994, 132, 195. (b) Ogata, T.; Yanagida, S.; Brunschwig, B.
S.; Fujita, E. J. Am. Chem. Soc. 1995, 117, 6708. (c) Matsuoka, S.;
Yamamoto, K.; Ogata, T.; Kusaba, M.; Nakashima, N.; Fujita, E.; Yanagida,
S. J. Am. Chem. Soc. 1993, 115, 601. (d) Ogata, T.; Yamamoto, Y.; Wada,
Y.; Murakoshi, K.; Kusaba, M.; Nakashima, N.; Ishida, A.; Takamuku, S.;
Yanagida, S. J. Phys. Chem. 1995, 99, 11916.
(3) Behar, D.; Dhanasekaran, T.; Neta, P.; Hosten, C. M.; Ejeh, D.;
Hambright, P.; Fujita, E. J. Phys. Chem. 1998, 102, 2870. Dhanasekaran,
T.; Grodkowski, J.; Neta, P.; Hambright, P.; Fujita, E. J. Phys. Chem. A
plot has a large intercept which may be due to a first-order
2
-1
decomposition of the adduct with k ≈ 3 × 10 s . The second-
order component of the kinetic trace leads to an estimated rate
8
-1 -1
constant of 2k ≈ 1 × 10 L mol
s
for the self-decay of
Cu(CO)CO2H . The latter reaction is important in the pulse
radiolysis experiments, which were done with an initial CO2
concentration of (0.7 to 3) × 10 mol L , but is negligible in
the γ-radiolysis experiments, where the steady-state concentra-
tion of the radicals is many orders of magnitude lower.
Reaction 1 is the first step in the reduction of CO2 radicals
by Cu in acidic solutions. A similar reaction may take place
+
•
-
-
5
-1
1
999, 103, 7742.
(4) Willner, I.; Maidan, R.; Mandler, D.; D u¨ rr, H.; D o¨ rr, G.; Zengerle,
K. J. Am. Chem. Soc. 1987, 109, 6080. Toshima, N.; Yamagi, Y.; Teranishi,
T.; Yonezawa, T. Z. Naturforsch., A 1995, 50, 283.
•
-
(5) Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 1985, 1695. Hori,
+
Y.; Kikuchi, K.; Murata, A.; Suzuki, S. Chem. Lett. 1986, 897.
(6) Cook, R. L.; MacDuff, R. C.; Sammells, A. F. J. Electrochem. Soc.
+
with Cu(I) sites in the small copper aggregates, Cun . The adduct
1
988, 135, 1320. Kim, J. J.; Summers, D. P.; Frese, K. W., Jr. J. Electroanal.
formed by such a reaction may react with another Cu(I) in
solution to form CO. It may be speculated, however, that a Cu(0)
site within the same aggregate, if the particle is sufficiently
small, may rapidly reduce the adduct before it leaves the particle.
Such a reaction is unlikely when the copper particle is large
Chem. 1988, 245, 223. Kyriacou, G.; Anagnostopoulos, A. J. Electroanal.
Chem. 1992, 322, 233.
(7) Hori, Y.; Takahashi, R.; Yoshinami, Y.; Murata, A. J. Phys. Chem.
B 1997, 101, 7075.
(
8) Kaneco, S.; Iiba, K.; Suzuki, S.; Ohta, K.; Mizuno, T. J. Phys.
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1988, 48, 217 and 231.
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Chem. 1991, 95, 8996.
11) Khatouri, J.; Mostafavi, M.; Amblard, J.; Belloni, J. Chem. Phys.
Lett. 1992, 191, 351.
12) The mention of commercial equipment or material does not imply
1
1
because its reduction potential is much less negative. This is
evident from the finding that production of CO stopped when
all the copper was in the form of metallic particles.
(
(
Finally, experiments were carried out with CO-saturated
solutions to test whether CO can be reduced by a similar
mechanism as CO2. Photolysis of CO-saturated solutions
containing acetone, 2-PrOH, PVA, and CuSO4, led to production
of Cu(0) particles, which were stable after extended photolysis,
with no consumption of CO. However, radiolysis of CO-
saturated solutions of Cu(II) and 2-PrOH led to production of
Cu(0) particles and subsequently to a decrease in CO concentra-
(
(
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.
(13) Stephen, H.; Stephen, T. Solubilities of Inorganic and Organic
Compounds; MacMillan, New York, 1963; Vol. 1: Binary Systems, Pt. 1.
(
(
14) Grodkowski, J.; Neta, P. J. Phys. Chem. A 2000, 104, 4475.
15) Spinks, J. W. T.; Woods, R. J. Introduction to Radiation Chemistry,
-
7
-1
tion (G ≈ 0.5 × 10 mol J ). This preferential reduction of
the copper ions is in contrast with the radiolytic results in CO2-
saturated solutions and is due to the fact that both the solubility
3rd ed.; Wiley: New York, 1990; p 426.
16) Porter, G.; Dogra, S. K.; Loutfy, R. O.; Sagumori, S. E.; Yip, R.
W. Trans. Faraday Soc. 1973, 69, 1462. Paul, H. Chem. Phys. 1979, 40,
65.
(17) Up to a dose of 1800 Gy, but further irradiation leads to production
of colloidal copper with its typical absorption as in Figure 1a.
18) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum:
New York, 1976; Vol. 3, p 2.
19) Ilan, Y.; Ilan, Y. A.; Czapski, G. Biochim. Biophys. Acta 1978,
503, 399.
(
2
-
of CO in water and its reactivity toward eaq are much lower
(factors of 45 and 4.5, respectively) than those of CO2. Thus,
(
most solvated electrons and all other reducing radicals produced
by the radiolysis react with copper ions leading to production
of Cu(0) particles and only when this process is nearly complete
(