Tunable, light-assisted co-generation of CO and H2 from CO 2 and H2O by Re(bipy-tbu)(CO)3Cl and p-Si in non-aqueous medium

Article abstract of DOI:10.1039/c1cc16024a

The light-assisted co-generation of carbon monoxide and hydrogen from carbon dioxide and water is reported. The combination of a homogeneous CO-evolving electrocatalyst and a heterogeneous H₂-evolving photoelectrode surface provides for tunability of the H₂/CO ratio. A total Faradaic efficiency of 102 ± 5% and a H₂/CO ratio of 2:1 were achieved at a low homogeneous catalyst concentration (0.5 mM) in acetonitrile/water mixtures.

Full text of DOI:10.1039/c1cc16024a

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COMMUNICATION
Tunable, light-assisted co-generation of CO and H from CO and H O
2
2
2
by Re(bipy-tbu)(CO) Cl and p-Si in non-aqueous mediumwz
3
a
b
b
ab
Bhupendra Kumar, Jonathan M. Smieja, Alissa F. Sasayama and Clifford P. Kubiak*
Received 27th September 2011, Accepted 4th November 2011
DOI: 10.1039/c1cc16024a
The light-assisted co-generation of carbon monoxide and hydrogen
from carbon dioxide and water is reported. The combination of a
homogeneous CO-evolving electrocatalyst and a heterogeneous
consists of two steps: (1) thermal dissociation of ZnO to Zn
(or Fe O to FeO) using concentrated solar radiation (endothermic
3
4
process) and (2) the reduced Zn (or FeO) reacts with H
2 2
O and CO
H
H
H
2
-evolving photoelectrode surface provides for tunability of the
2
/CO ratio. A total Faradaic efficiency of 102 ꢀ 5% and a
2
/CO ratio of 2 : 1 were achieved at a low homogeneous
3 4
to generate ZnO (or Fe O ) together with syngas (exothermic
11,12
process).
Previously we reported the two-electron electrocatalytic
catalyst concentration (0.5 mM) in acetonitrile/water mixtures.
reduction of CO
0
2
3
to CO by Re(bipy-tBu)(CO) Cl (where
0
bipy-tBu = 4,4 -di-tert-butyl-2,2 -bipyridine) in acetonitrile
with high turnover frequency, high turnover number, and a
2
The co-generation of carbon monoxide (CO) and hydrogen (H )
1
3
1,2
Faradaic efficiency of 99 ꢀ 2%. That work, reported on a
glassy carbon electrode, was then followed up by work with
the same catalyst on a p-Si photoelectrode surface. In that
is an important step in the synthesis of renewable fuels. This
mixture, commonly referred to as synthesis gas (or syngas), can
be fed into the Fischer–Tropsch process to make synthetic liquid
3
study we showed the photoelectrochemical reduction of CO2
fuels. These fuels can be used in the transportation sector or for
to CO with a photovoltage of more than 500 mV and a
storage of intermittent energy from renewable sources such as
solar and wind. Over the past three decades considerable research
1
4
Faradaic efficiency 97 ꢀ 3%.
Here we report the successful coupling of a homogeneous
catalyst for the reduction of CO with heterogeneous water
reduction by illuminated p-Si (Fig. 1). In this system, the
homogeneous CO reduction and heterogeneous water
2
has been performed on both carbon dioxide (CO ) reduction
4
–6
7–9
electrocatalysts and proton reduction electrocatalysts. This
research, however, has been devoid of significant findings on the
2
co-generation of CO and H
for the reduction of protons to H
produced by the reduction of CO
2
. It is often the case that a catalyst
will be poisoned by the CO
, or that a catalyst for the
2
reduction are in direct competition for photo-generated electrons
from the cathode. This feature provides for a tunable co-generation
2
2
of CO and H through variation of homogeneous catalyst concen-
2
reduction of CO2 to CO will not be able to compete with
hydrogen production in the presence of proton sources.
Although syngas can be produced from CO via the water-gas-
tration and through addition of water. This system has the ability
to produce synthesis gas in the desired H /CO ratio of 2 : 1 with
2
high overall Faradaic efficiency. The photoelectrochemical (PEC)
shift (WGS) reaction or from H via the reverse water-gas-shift
2
14
cell used in these experiments has been described.
The characteristic behavior of the p-Si/Re(bipy-tBu)(CO)
(rWGS) reaction, it is desirable to employ a system to achieve the
3
Cl
intended ratio of gases in one step and thus eliminate the
inefficiencies of multiple process integration.
molecular catalyst junction, where both the first and second
There are a very few reports of simultaneous CO
2
and H
2
O
10
reduction to syngas. Recently, electrochemical and thermo-
1
1,12
chemical
simultaneously. In the electrochemical method, water and CO
were electrochemically reduced at a silver cathode at a potential of
2 2
methods have been reported to reduce CO and H O
2
10
approximately ꢁ2.00 V vs. SCE. The thermochemical method
a
Materials Science and Engineering Program, University of California
San Diego, 9500 Gilman Drive MC 0418, La Jolla, California
92093, USA
b
Department of Chemistry and Biochemistry, University of California
San Diego, 9500 Gilman Drive MC 0358, La Jolla, California
92093, USA. E-mail: ckubiak@ucsd.edu; Fax: +1 858 534 5383;
Tel: +1 858 822 2665
w This article is part of the ChemComm ‘Artificial photosynthesis’ web
themed issue.
Fig. 1 Schematic representation for light-assisted co-generation of
2 2
H and CO at biased illuminated p-Si by heterogeneous H O reduction
(
depicted by orange arrows) at the p-Si surface and homogeneous
0
reduction of CO
0
2
by Re(bipy-tBu)(CO)
3
Cl (where bipy-tBu = 4,4 -
z Electronic supplementary information (ESI) available: See DOI:
1
0.1039/c1cc16024a
di-tert-butyl-2,2 -bipyridine) (depicted by red arrows), respectively.
2
72 Chem. Commun., 2012, 48, 272–274
This journal is c The Royal Society of Chemistry 2012

1
6
CO
2
to CO on p-Si. This system achieved its best results with
II 2+
1
8 mM [(Me
6
[14]aneN
4
)Ni ]
in a 1 : 1 acetonitrile–water
solution with 0.1 M LiClO4 as the supporting electrolyte.
Under these conditions, a H /CO ratio of 0.5 and a Faradaic
2
efficiency of 95 ꢀ 5% were obtained. In our work, the ratio of
H /CO can be increased from 0.4 to 1.2 as the water content is
2
increased from 1% to 5% (by volume) with 5 mM Re(bipy-tBu)-
(
CO)
0 ꢀ 5% and 100 ꢀ 5% for co-generation. Further addition of
water did not change the H to CO ratio at a catalyst
concentration of 5 mM. It should be noted that the concen-
tration of CO in acetonitrile/water stays constant up to a
3
Cl. The Faradaic efficiencies in those cases are between
9
2
2
water concentration of 3 M, but for concentrations of water
higher than that the CO solubility in acetonitrile decreases
2
1
7
considerably. To fully utilize the activity of Re(bipy-tBu)-
CO) Cl for reduction of CO to CO, therefore, the water
(
3
2
concentration must be kept at or below 3 M. In addition, at
higher concentrations of water both tetrabutylammonium
hexafluorophosphate (TBAH, electrolyte) and Re(bipy-tBu)-
(
CO)
3
Cl are not fully soluble.
At a high Re(bipy-tBu)(CO) Cl concentration, the molecular
3
catalyst is able to compete with the heterogeneous water reduction
at illuminated p-Si even at water concentrations (2.5 M) much
Fig. 2 (a) Re(bipy-tBu)(CO)
3
Cl electrochemistry under a CO
2
atmo-
higher than the CO
2
concentration (0.28 M in acetonitrile at
17
sphere on both glassy carbon and p-Si working electrodes showing
more than 500 mV of photovoltage at illuminated p-Si 0.5 mM
one atmosphere of CO2). To achieve a H /CO ratio of 2 : 1
2
ꢁ1
catalyst, 0.1 M TBAH electrolyte, Pt counter electrode, 100 mV s
scan rate. (b) Cyclic voltammograms of 1 mM Re(bipy-tBu)(CO) Cl in
0 mL of acetonitrile on the illuminated p-Si photocathode without
3
3
water addition (black), 0.4 M (0.2 mL) water (red) and 2.6 M (1.5 mL)
water (blue).
reductions of the Re complex on illuminated p-Si are 530 mV
more positive than at metal electrodes (Pt or glassy carbon)
under an argon atmosphere, is shown in Fig. S1 (ESIz). In the
presence of CO , there is a large increase in current in the second
2
reduction of the complex, which is assigned to the homogeneous
reduction of CO to CO at both the p-Si photocathode and the
2
glassy carbon cathode as shown in Fig. 2a. The kinetically limited
2
photocurrent density for homogeneous CO reduction at p-Si is
modulated by incoming light intensity (Fig. S2, ESIz). Upon
addition of deoxygenated water under an argon atmosphere, the
3
cyclic voltammogram of the p-Si/Re(bipy-tBu)(CO) Cl molecular
catalyst junction (under illumination) loses its characteristic
reduction peaks (Fig. 2b). Under these conditions the
Re(bipy-tBu)(CO) Cl reduction peaks are obscured by a broad
3
+
reduction feature in the range of ꢁ1.38 to ꢁ1.78 V vs. Fc/Fc . It
was also observed, as expected, that the addition of water
narrowed the electrochemical window due to the reduction of
2
protons from water to H . A similar feature was observed on p-Si
under illumination in acetonitrile upon addition of water with no
Fig. 3 (a) Water addition to 0.5 mM Re(bipy-tBu)(CO)
3
Cl in 40 mL
of acetonitrile under CO at illuminated p-Si results in a distortion of
2
3
Re(bipy-tBu)(CO) Cl. Gas chromatography of the head space
+
the voltammogram’s shape and a redistribution of current as a result
of heterogeneous water reduction to hydrogen gas; without water
after a bulk electrolysis experiment run at ꢁ1.65 V vs. Fc/Fc
3
with and without Re(bipy-tBu)(CO) Cl under an argon atmo-
(
black) and 2.6 M (2 mL) water added (red). Intermediate additions of
sphere showed only hydrogen production. This confirms that the
water reduction to hydrogen in this system is in fact heterogeneous
reduction of water in a non-aqueous medium at illuminated p-Si.
Bradley et al. reported the photoelectrochemical reduction
water are omitted for clarity in this figure, but show a gradual
distortion of the CV trace. (b) Chart shows the ratio of H
with increase in charge passed during bulk electrolysis. 2 : 1 H
2
to CO
/CO is
2
near the ideal ratio for most cobalt-based Fischer–Tropsch systems
and provides for the highest possible percentage of liquid fuel
molecules.
1
5
of tetraazomacrocyclic metal complexes on illuminated p-Si
and they were later shown to catalyze the photoreduction of
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 272–274 273

(
a ratio suitable for liquid fuel formation by the Fischer–Tro
¨
psch
efficiency of 32%. In the case of water reduction, the thermo-
dynamic potential of proton reduction to hydrogen in aceto-
nitrile with water as the proton source is not available in the
literature. Therefore, the thermodynamic potential (ꢁ1.46 V
process) the Re complex concentration was decreased from 5 mM
to 0.5 mM. Fig. 3a shows the effect of water in an illuminated
p-Si/Re(bipy-tBu)(CO) Cl junction system under an atmosphere
3
+
of CO . Initial addition of a small amount of water lowers the
2
vs. Fc/Fc ) of proton reduction with acetic acid (weak acid) as
catalytic current density at the 2nd reduction of Re(bipy-tBu)-
the proton source in acetonitrile was used in the conversion
efficiency calculation as the thermodynamic potential for water
(
CO)
homogeneous CO
sated for by heterogeneous water reduction at illuminated p-Si so
that the catalytic current density recovers. Co-generation of H
and CO was confirmed by simultaneous detection in a split
column gas chromatograph (GC). The H to CO ratio could then
be calculated quantitatively from the number of moles of H and
3
Cl. At higher concentrations of water, the decrease in
20
2
reduction by Re(bipy-tBu)(CO) Cl is compen-
3
2
reduction. Therefore, the overpotential for H generation is
0.44 V. The co-generation process has a Faradaic efficiency of
nearly 100%, so IR loss is zero in this case. With a monochromatic
2
ꢁ
2
light (661 nm) illumination intensity of 95 mW cm on the
ꢁ2
2
photocathode surface and a catalytic current density of 5.6 mA cm ,
2
the total light-to-chemical energy conversion efficiency for the
cathodic half cell reaction calculated by eqn (1) is 4.6%.
Here we have shown the efficient co-generation of CO and H2
CO produced by comparing GC peak areas with calibration
curves. Fig. 3b shows consistent co-generation of H and CO
2
with a ratio near 2 : 1 during bulk electrolysis experiments run at a
+
from CO2 and H O in a mixed homogeneous/heterogeneous
2
potential of ꢁ1900 mV vs. Fc/Fc .
system that allows for tunability in product distribution. We are
Control experiments were conducted with glassy carbon as
the working electrode under the same conditions as those
able to tune the H
and variation of Re(bipy-tBu)(CO)
knowledge this is the first example of co-generation of this type
and may provide for opportunities to co-generate H and CO in
2
/CO ratio from 0 to 2 : 1 by addition of water
3
Cl concentration. To our
described above and H
observed. This experiment confirms the role of p-Si as the
heterogeneous catalyst for H production from water and the
Cl as the only CO reduction
2
/CO ratios of only 1 : 7 (or 0.14) were
2
2
the future at lower overpotential and higher overall light-to-
chemical energy conversion efficiency. Further studies are ongoing
in our laboratory to further optimize the system through catalyst
and surface optimization.
role of Re(bipy-tBu)(CO)
catalyst.
3
2
The gas chromatographs of CO and H are shown in Fig. S3
2
and S4 (ESIz). As the amount of charge passed during bulk
electrolysis is increased from 0 to 13.7 C, the total area of both
Notes and references
the CO and H peaks increased linearly. Fig. S5a (ESIz) shows
2
1
2
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of the curve in Fig. S5a (ESIz) by the area of the electrode gives a
3 A. J. V. Underwood, Ind. Eng. Chem., 1940, 32, 449–454.
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2
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of moles of gas (H + CO) with a slope 1.94 (indicative of both
2
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2
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o
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1
2
mainly as thermal combustion fuels. The DH
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ꢁ
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ꢁ1
2
2
74 Chem. Commun., 2012, 48, 272–274
This journal is c The Royal Society of Chemistry 2012