Highly selective solar-driven methanol from CO2 by a photocatalyst/biocatalyst integrated system

Article abstract of DOI:10.1021/ja509650r

The successful development of a photocatalyst/biocatalyst integrated system that carries out selective methanol production from CO₂ is reported herein. The fine-tuned system was derived from a judicious combination of graphene-based visible light active photocatalyst (CCG-IP) and sequentially coupled enzymes. The covalent attachment of isatin-porphyrin (IP) chromophore to chemically converted graphene (CCG) afforded newly developed CCG-IP photocatalyst for this research endeavor. The current work represents a new benchmark for carrying out highly selective methanol formation from CO₂ in an environmentally benign manner.

Full text of DOI:10.1021/ja509650r

Communication
pubs.acs.org/JACS
Highly Selective Solar-Driven Methanol from CO by a Photocatalyst/
2
Biocatalyst Integrated System
Rajesh K. Yadav, Gyu Hwan Oh, No-Joong Park, Abhishek Kumar, Ki-jeong Kong, and Jin-Ook Baeg*
Division of Green Chemistry and Engineering Research, Korea Research Institute of Chemical Technology (KRICT), 100 Jang-dong,
Yuseong, Daejeon 305 600, Republic of Korea
*
S Supporting Information
Although intensively researched, it continues to suffer from
serious drawbacks such as slow electron-transfer rate, low yields,
and poor selectivity. These limitations can be overcome by a
photocatalyst/biocatalyst integrated system wherein clean and
abundantly available solar light is directly used by the
photocatalyst for continuous NADH regeneration, which is
utilized by the enzymes to produce solar fuel. Quite surprisingly,
the potential of such an environmentally benign strategy to
ABSTRACT: The successful development of a photo-
catalyst/biocatalyst integrated system that carries out
selective methanol production from CO2 is reported
herein. The fine-tuned system was derived from a judicious
combination of graphene-based visible light active photo-
catalyst (CCG-IP) and sequentially coupled enzymes. The
covalent attachment of isatin-porphyrin (IP) chromophore
to chemically converted graphene (CCG) afforded newly
developed CCG-IP photocatalyst for this research
endeavor. The current work represents a new benchmark
for carrying out highly selective methanol formation from
obtain methanol exclusively from CO remains unexplored. We
2
now herein report the development of a photocatalyst/
20
biocatalyst integrated system to achieve this aim. The system
for this study was obtained by combining our newly developed
graphene-based photocatalyst with sequentially coupled en-
zymes (formate dehydrogenase, formaldehyde dehydrogenase,
and alcohol dehydrogenase) as depicted in Scheme 1. To the best
of our knowledge, this is the first report on exclusive methanol
CO in an environmentally benign manner.
2
he ever increasing fossil fuel demand and a concomitant rise
T
in atmospheric CO levels demands concerted carbon
2
formation from CO directly by visible light-driven photo-
2
management. Therefore, conversion of CO into fuels and
2
1
catalyst/biocatalyst integrated system.
chemicals has been envisaged. In this context, development of
A visible light photocatalyst with broad spectral absorption for
efficient light harvesting and concomitant electron transfer is a
prerequisite for this research work. One may therefore assume
systems for the fixation of CO into methanol is particularly
2
interesting. Consequently, a number of studies on multi-electron
reduction of CO to methanol have being carried out so far. For
2
instance, chemical reduction of CO to methanol by organo-
2
Scheme 1. Schematic Illustration of the CCG-IP
metallic and inorganic catalysts has been reported. Despite the
use of hydrogen gas or hydride ion along with high temperature
Photocatalyst/Biocatalyst Integrated System for Carrying out
a
1
−7
Methanol Formation From CO2
and/or high pressure, they afford methanol in poor selectivity.
The electrocatalytic reduction of CO2 leads to methanol
formation in higher selectivity. However, it requires a continuous
supply of electricity which is primarily obtained from fossil fuels
1
,8
and hence environmentally unfavorable.
Solar light is the ideal energy source from a sustainable
perspective. Therefore, the use of photocatalysts for solar-driven
methanol from CO is a very attractive approach. Hence a variety
2
of inorganic semiconductors and metal complexes as photo-
catalysts or photoelectrocatalysts have been evaluated. However,
most of them provide low yields with poor product selectivity
7
,9−14
along with the requirement of UV light for photoreduction.
It is quite evident from literature that obtaining methanol from
CO in high selectivity continues to be a major challenge. The
2
biocatalysts or enzymes are known to carry out highly selective
transformations at ambient conditions. Therefore, the reduction
15−17
of CO to methanol using enzymes has also been explored.
2
a
FDH = formate dehydrogenase, F DH = formaldehyde dehydrogen-
However, enzymes require a stoichiometric amount of expensive
NADH cofactor to carry out biocatalysis. It is therefore necessary
to develop economical methods for NADH cofactor regener-
ation. To achieve this goal, electrochemical regeneration of
ald
ase, ADH = alcohol dehydrogenase.
Received: September 18, 2014
1
8,19
NADH has been developed over the last few decades.
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja509650r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
that any visible light active photocatalyst might be suitable for the
followed by exposure to visible light (1 sun). As shown in Figure
2a, CCG-IP was successful in photoregeneration with constant
task. We recently reported a system for CO reduction to formic
2
acid utilizing visible light by developing chemically converted
graphene coupled multi-anthraquinone-substituted porphyrin
21
(
CCGCMAQSP) photocatalyst. Thus, CCGCMAQSP was
initially evaluated for this research endeavor. However, it
provided rather poor results (vide infra). For improved
performance, two highly versatile chromophoric motifs, isatin
22
and porphyrin, were combined together. The trisubstitution of
isatin onto porphyrin ring provided 1,1′,1″-((20-(2-((7-amino-
9
,10-dioxo-9,10-dihydroanthracen-2-yl)amino)quinolin-3-yl)
porphyrin-5,10,15-triyl) tris (quinoline-3,2-diyl)) tris(indoline-
,3-dione) (hereinafter IP) as a suitable chromophore for our
2
research work. The subsequent covalent attachment of IP
chromophores with chemically converted graphene (CCG) led
to the robust CCG-IP photocatalyst (Figure 1) which was
Figure 2. (a) Photocatalytic 1,4-NADH regeneration activity of CCG-
+
IP, CCGCMAQSP, IP, and CCG photocatalyst [β-NAD (1.24 μmol),
2+
rhodium complex [Cp*Rh(bpy)H O] Rh (0.62 μmol), TEOA (1.24
2
mmol), and photocatalyst (0.5 mg) in 3.1 mL of sodium phosphate
buffer (100 mM, pH 7.0)]. (b) Exclusive production of methanol from
CO2 (flow rate: 0.5 mL/min) under visible light by CCG-IP,
CCGCMAQSP, IP, and CCG upon integration with enzymes [β-
+
NAD (1.24 μmol), Rh (0.62 μmol), TEOA (1.24 mmol), photocatalyst
(0.5 mg), and 9 units of each enzyme (formate dehydrogenase,
formaldehyde dehydrogenase, and alcohol dehydrogenase) in 3.1 mL of
sodium phosphate buffer (100 mM, pH 7.0)].
Figure 1. 3D structure of CCG-IP photocatalyst alongwith detailed
chemical structure of IP chromophore.
NADH accumulation of 38.99% linearly over a period of the next
6
0 min. On the other hand, CCGCMAQSP photocatalyst and IP
chromophore provided 28.46 and 7.03% of NADH regeneration
in the same time frame, respectively. CCG failed to carry out any
NADH regeneration. We then evaluated CCG-IP and
CCGCMAQSP as photocatalyst of the integrated system for
methanol formation from CO . Following the bubbling of CO
in dark for 30 min, methanol concentration of 11.21 μM was
obtained on exposure to visible light over the next 60 min when
CCG-IP was used as the photocatalyst (Figure 2b). On the other
hand, only 5.62 μM of methanol concentration was obtained
upon the use of CCGCMAQSP as photocatalyst. The experi-
ment with IP chromophore provided trace amount of methanol
which could not be quantified, while CCG failed to produce any
methanol. From these experiments, the notably higher photo-
catalytic performance of CCG-IP in carrying out exclusive
characterized by various techniques such as atomic force
microscopy, X-ray photoelectron spectroscopy, energy-disper-
sive X-ray spectroscopy, Fourier transform infrared spectrosco-
py, Raman spectroscopy, and thermogravimetric analysis. The
details of all these analysis are provided in the Supporting
Information (SI).
2
2
The absorption spectrum of CCG-IP (Figure S6, SI) exhibits a
strong soret band at 480 nm. The IP chromophore also exhibits
absorption band at similar wavelength albeit with lower
2
3
absorption. On the other hand, isatin and 2,6-diaminoan-
thraquinone (AAQ) exhibit much lower absorption in 400−500
nm range. These observations indicate that CCG-IP is capable of
efficient visible light harvesting and thus provides photocatalytic
energy for regeneration of enzymatically active 1,4-NADH in
high efficiency, which is indispensable for methanol formation
methanol production from CO is clearly evident.
2
To rule out photocatalytic methanol formation by some
2
4,25
26
from CO2.
carbon residues in the reaction mixture, a control experiment in
The role of photocatalyst in the integrated system is to harness
visible light for carrying out continuous regeneration of 1,4-
NADH. Therefore, to begin with, CCG-IP was examined for its
visible light-driven 1,4-NADH photoregeneration ability. The
system was initially allowed to equilibriate in dark for 30 min,
the absence of CO was performed. However, no methanol
2
formation was observed in this experiment. In a related manner,
one control experiment without the CCG-IP photocatalyst (but
with enzymes) and another one without visible light irradiation
were performed. No methanol formation in these cases
B
dx.doi.org/10.1021/ja509650r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
confirmed photocatalytic NADH regeneration by CCG-IP under
visible light irradiation. A set of control experiments in the
+
absence of Rh and/or NAD also failed to produce any methanol.
Overall the control experiments clearly point toward visible light-
driven NADH regeneration and subsequent methanol formation
from CO by the photocatalyst/biocatalyst integrated system.
2
The stability and reusability of the photocatalyst were also
examined. The photocatalyst was subjected to three cycles of
NADH regeneration. In comparison to the first cycle wherein the
photocatalyst carried out 38.99% (100%) of NADH regener-
ation, 36.81% (94.41%) was obtained in the third cycle (Figure
S7, SI). This remarkably high photocatalytic activity indicates a
stable and reusable photocatalyst.
To elucidate the photoinduced electron-transfer mechanism,
electrochemical studies were performed. The reduction potential
at cathodic peak current of Rh and CCG-IP was observed at
around −0.72 V (Figure 3a) and −0.92 V (Figure S8, SI),
Figure 4. Photocurrent−time (I−T) profiles of FTO/IP and FTO/
CCG-IP electrodes under simulated solar light (1 sun) illumination
2
(
three electrodes; scan rate 50 mV/s; input power: 100 mW/cm ; and
electrolyte: 0.1 M NaCl in water, bias potential: 0−0.1 V (vs Ag/AgCl)).
The photo electrochemical test device was fabricated by drop casting the
photocatalyst dispersed in DMF (0.01g/mL) onto fluorine-doped tin
oxide (FTO)-coated glass.
photocatalyst and the electrocatalytic center Rh enables the
electron transfer from former to latter. The electrically reduced
Rh thus obtained is chemically protonated in aqueous media,
followed by its catalytic regeneration of enzymatically active-1,4-
NADH in high yield.
For a better understanding, transient photocurrent study was
also carried out. A significantly higher photocurrent was observed
with CCG-IP in comparison to IP electrode (Figure 4). The
prompt photocurrent response of CCG-IP corresponded well
with ON/OFF cycles of simulated sun light irradiation. It
indicates electron transfer from IP to CCG upon irradiation
leading to regeneration of enzymatically active 1,4-NADH and
21
subsequent methanol formation.
The origin of photocatalytic activity and activity enhancement
of the graphene-based photocatalyst was also investigated
theoretically by a series of first-principles calculations. The
geometry optimizations and electronic properties of CCG and IP
were carried out in the framework of the density functional
theory (DFT) by using the B3LYP exchange−correlation
2
7,28
3
29
potential
as implemented in DMol . Double numerical
polarized (DNP) basis set that includes all occupied atomic
orbitals plus a second set of valence atomic orbitals plus polarized
d-valence orbitals was employed. The theoretically obtained
atomic geometries and the wave functions of HOMO (highest
occupied molecular orbital) and LUMO (lowest unoccupied
molecular orbital) are shown in Figure 5.
The incident light is absorbed at IP, where photoexcitation
occurs, and the created electrons in LUMO move into CCG via
stable amide bond. The electron-transfer efficiency could be
Figure 3. Cyclic voltammograms of (a) CCG-IP (10 μM), Rh (0.2
+
mM), and Rh with NAD (0.4 mM) solutions. (b) Mixture of two
+
components (Rh and CCG-IP) in the absence and presence of NAD
(
(
−1
0.4 mM). The potential was scanned at 100 mV s using glassy carbon
working), silver-silver chloride (reference), and platinum (counter)
electrodes in sodium phosphate buffer (100 mM, pH 7.0).
respectively. However, on mixing together the two components
(
Rh and CCG-IP) in solution, reduction was observed at −0.80
V. It is a cathodic shift of Rh reduction, or in other words, an
anodic shift of CCG-IP reduction (Figure 4b) which indicates
21
electron transfer from CCG-IP to Rh. The CCG-IP-Rh system
+
revealed an increase in reduction peak current with NAD ,
implying that the system consisting of CCG-IP and Rh catalyzes
+
the reduction of NAD to NADH (Figure 3b). As per literature,
the catalytic effect of Rh results in strongly increased rate of Rh
+
21
reduction in the presence of NAD . From these observations,
one may conclude that the transfer of electrons from TEOA to
CCG-IP results in the electrical reduction of Rh. The vicinity and
potential gradient between the light harvesting CCG-IP
Figure 5. Visualization of the molecular orbitals of IP (a) HOMO and
(b) LUMO as obtained from DFT calculations.
C
dx.doi.org/10.1021/ja509650r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
estimated by the energy level alignment between the Fermi level
of CCG and the LUMO of IP chromophore system. From our
DFT calculation results, we confirmed that the energy levels of
each segment are aligned for electrons to be able to transfer from
IP finally to hydrogen reduction site via CCG. The Fermi level of
graphene lies between HOMO−LUMO gap, about 0.97 eV
lower than the LUMO level of IP. Thus, photoexcited electrons
in LUMO of IP can be easily transferred into CCG by the use of
this potential difference. By virtue of high potential difference,
the transferred charge carriers in CCG are hot electrons which
have sufficient energy to overcome the hydrogen reduction
overpotential on the rhodium complex. As is well-known,
graphene exhibits remarkably high electron mobility up to
ACKNOWLEDGMENTS
This work was supported by the KRICT 2020 project program.
■
REFERENCES
1) Kondratenko, E. V.; Mul, G.; Baltrusaitis, J.; Larrazab
■
(
́
al, G. O.;
Per
́
ez-Ramírez, J. Energy Environ. Sci. 2013, 6, 3112.
2) Riduan, S. N.; Zhang, Y.; Ying, J. Y. Angew. Chem., Int. Ed. 2009, 48,
3322.
(3) Men
(4) Huff, C. A.; Sanford, M. S. J. Am. Chem. Soc. 2011, 133, 18122.
(
́
ard, G.; Stephan, D. W. J. Am. Chem. Soc. 2010, 132, 1796.
(
(
5) Jessop, P. G.; Joo,
6) Wesselbaum, S.; vom Stein, T.; Klankermayer, J.; Leitner, W.
́
F.; Tai, C.-C. Coord. Chem. Rev. 2004, 248, 2425.
Angew. Chem., Int. Ed. 2012, 51, 7499.
(7) Wang, W.; Wang, S.; Ma, X.; Gong, J. Chem. Soc. Rev. 2011, 40,
2
12
−2
2
00,000 cm /V·s at a carrier density of 10 cm at room
3
(
703.
8) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Chem. Soc.
Rev. 2009, 38, 89.
9) (a) Tseng, I.-H.; Wu, J. C. S.; Chou, H.-Y. J. Catal. 2004, 221, 432.
(b) Pathak, P.; Meziani, M. J.; Castillo, L.; Sun, Y.-P. Green Chem. 2005,
7, 667. (c) Wang, C.; Thompson, R. L.; Baltrus, J.; Matranga, C. J. Phys.
Chem. Lett. 2010, 1, 48.
10) Ahmed, N.; Shibata, Y.; Taniguchi, T.; Izumi, Y. J. Catal. 2011,
79, 123.
11) Cao, L.; Sahu, S.; Anilkumar, P.; Bunker, C. E.; Xu, J.; Fernando,
K. A. S.; Wang, P.; Guliants, E. A.; Tackett, K. N., II; Sun, Y.-P. J. Am.
Chem. Soc. 2011, 133, 4754.
12) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Angew.
Chem., Int. Ed. 2013, 52, 7372.
13) Boston, D. J.; Xu, C.; Armstrong, D. W.; MacDonnell, F. M. J. Am.
Chem. Soc. 2013, 135, 16252.
(14) Rajeshwar, K.; de Tacconi, N. R.; Ghadimkhani, G.; Chanmanee,
30
temperature. By virtue of the extraordinary high mobility of
carriers in addition to the huge surface area, the transferred
electrons can reach reduction sites far away, resulting in the
enhancement of catalytic activity. Furthermore, the graphene
also acts as an electron reservoir to transport multiple electrons.
In most molecular systems, the energy cost of electron addition is
very high because of large Coulomb repulsion between localized
electrons. In graphene, the multi-electron addition or removal
can be possible because of the delocalized nature of wave
functions over the entire graphene surface (extending several
μm).
(
(
2
(
(
In order to gain some insight into the electron transfer
between CCG and IP moieties, time-resolved fluorescence study
on CCG-IP was performed by time-correlated single photon
counting control technique. From the decay time profiles, data of
(
the photoexcited IP in CCG-IP (λ = 480 nm) were curve-fitted
W.; Janak
́
y, C. ChemPhysChem 2013, 14, 2251.
15) Obert, R.; Dave, B. C. J. Am. Chem. Soc. 1999, 121, 12192.
16) Xu, S.-w.; Lu, Y.; Li, J.; Jiang, Z.-y.; Wu, H. Ind. Eng. Chem. Res.
ex
(
(
2
(
1
(
2
(
using biexponential decay kinetics, from which lifetimes of 1.06
ns (10.58%) and 8.96 ns (89.42%) were deduced. On the other
hand, lifetime of intact IP chromophore (λex = 479 nm) was
estimated to be 3.71 ns (100%). Thus, it is reasonable to assume a
charge separation and charge recombination scenario in CCG-IP
006, 45, 4567.
17) Kuwabata, S.; Tsuda, R.; Yoneyama, H. J. Am. Chem. Soc. 1994,
16, 5431.
18) Hollmann, F.; Schmid, A.; Steckhan, E. Angew. Chem., Int. Ed.
001, 40, 169.
19) Wu, H.; Tian, C.; Song, X.; Liu, C.; Yang, D.; Jiang, Z. Green Chem.
8
8
−1
via IP* with rate constants of 1.9 × 10 and 0.7 × 10 s ,
3
1
respectively.
In summary, we have successfully demonstrated a photo-
catalyst/biocatalyst integrated system for highly selective
2013, 15, 1773 and references cited therein..
(20) There is a report on photochemical/biocatalytic methanol
synthesis from formaldehyde by the photoreduction of toxic methyl
viologen. Amao, Y.; Watanabe, T. Appl. Catal., B 2009, 86, 109.
methanol production from CO directly. Our newly developed
2
photocatalyst (CCG-IP) is capable of harvesting sufficient visible
(21) Yadav, R. K.; Baeg, J.-O.; Oh, G. H.; Park, N.-J.; Kong, K.-j.; Kim,
light for carrying out the multi-electron reduction of CO to
2
J.; Hwang, D. W.; Biswas, S. K. J. Am. Chem. Soc. 2012, 134, 11455.
methanol at ambient conditions upon integration with the
sequentially coupled enzymes. To the best of our knowledge, this
(22) Sumpter, W. C. Chem. Rev. 1944, 35, 393.
(23) Niyogi, S.; Bekyarova, E.; Itkis, M. E.; McWilliams, J. L.; Hamon,
is the first report on CO fixation exclusively as methanol by a
2
M. A.; Haddon, R. C. J. Am. Chem. Soc. 2006, 128, 7720.
24) Xu, Y.; Liu, Z.; Zhang, X.; Wang, Y.; Tian, J.; Huang, Y.; Ma, Y.;
photocatalyst/biocatalyst integrated system. On the whole, our
approach provides an appealing strategy for selective methanol
(
Chen, Y. Adv. Mater. 2009, 21, 1275.
(25) Guo, Z.; Du, F.; Ren, D.; Chen, Y.; Zheng, J.; Liu, Z.; Tian, J. J.
Mater. Chem. 2006, 16, 3021.
production from CO by the use of inexpensive and abundantly
2
available solar energy.
(26) Yang, C.-C.; Vernimmen, J.; Meynen, V.; Cool, P.; Mul, G. J.
Catal. 2011, 284, 1.
(27) Becke, A. D. Phys. Rev. A 1988, 38, 3098.
ASSOCIATED CONTENT
■
*
S
Supporting Information
(
(
28) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 27, 785.
29) (a) Delley, B. J. Chem. Phys. 1990, 92, 508. (b) Delley, B. J. Chem.
Synthesis details and characterization data for CCG-IP photo-
catalyst. This material is available free of charge via the Internet at
http://pubs.acs.org.
Phys. 2000, 113, 7756.
30) Chen, J.; Jang, C.; Xiao, S.; Ishigami, M.; Fuhrer, M. S. Nat.
Nanotechnol. 2008, 3, 206.
31) Kodis, G.; Terazono, Y.; Liddell, P. A.; Andrea
Hambourger, M.; Moore, T. A.; Moore, A. L.; Gust, D. J. Am. Chem. Soc.
006, 128, 1818.
(
(
́
sson, J.; Garg, V.;
AUTHOR INFORMATION
■
2
Corresponding Author
jobaeg@krict.re.kr
Notes
The authors declare no competing financial interest.
D
dx.doi.org/10.1021/ja509650r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX