Hybrid technologies for an enhanced carbon recycling based on the enzymatic reduction of CO2 to methanol in water: Chemical and photochemical NADH regeneration

Article abstract of DOI:10.1002/cssc.201100484

Rocking chair enzyme: Chemical reducing agents (sodium dithionite) or bioglycerol (as H and e^--donor under irradiation in the presence of ZnS-A as photocatalyst) are able to back-convert NADP⁺ into NADPH, which is used as e^--donor in the enzymatic reduction of CO ₂ into CH₃OH. In doing so, the molar ratio CH ₃OH/CO₂ has been increased (without recycling of NADP ⁺) using the photocatalyst. Copyright

Full text of DOI:10.1002/cssc.201100484

DOI: 10.1002/cssc.201100484
Hybrid Technologies for an Enhanced Carbon Recycling Based on the
Enzymatic Reduction of CO to Methanol in Water: Chemical and
2
Photochemical NADH Regeneration
[
a]
[a]
[b]
[b]
[a]
[c]
Angela Dibenedetto,* Paolo Stufano, Wojciech Macyk, Tomasz Baran, Carlo Fragale, Mirco Costa, and
Michele Aresta
[
a]
Our society lives the dilemma of the expansion of the demand
for energy, while a dramatic reduction of CO2 emission is
urged for limiting the climate change. A substantial part (at
least 80%) of the needed energy is obtained today from fossil
carbon and, additionally, there are no alternatives available for
application, such a reduction must be somehow performed by
implementing the most energetically and economically con-
venient technologies.
Recently, we have faced the problems of: i) developing
[
7]
+
robust enzymes and ii) reducing NAD in situ to determine
[
1]
+
[8]
next twenty years. This is rising serious worries about the
future of our Planet should the correlation of the increase of
the extent to which NAD can be recycled to increase the
ratio CH OH/NADH. This ratio is a key issue in defining whether
3
the temperature to the CO emission continue. Technologies
a biotechnological production of methanol would be possible.
In this paper, we report the preliminary result on the behaviour
2
for the reduction of the immission of CO into the atmosphere
2
[8]
have been proposed. They encompass a number of possibili-
ties, such as the improvement of the efficiency in the produc-
tion (that requires large investment capitals) and use of
of some chemical and photochemical reducing systems and
+
show their potential for an effective recycling of NAD .
The enzymatic reduction of CO2 in water, represented in
Scheme 1, was first reported by Obert and Dave, who also re-
ported the properties of the enzymes and showed that the
[
2]
energy, the capture and storage of emitted carbon dioxide
[
3]
(
CCS), the use of perennial energy sources (solar, wind, water)
[
4]
[9]
and the use of renewables (biomass). CCS has risen large ex-
pectations in the last ten years, but it is potentially site-related
and energy intensive: it will have a net effect on expanding
the extraction of fossil carbon, the intensity of which depends
three enzymes were deactivated in a short time. An attempt
on the local conditions for CO transportation to the disposal
2
site and housing.
Besides such technologies, the utilisation of carbon dioxide
as a carbon source for the synthesis of chemicals and fuels is
under evaluation for its contribution to recycling carbon to
mimic nature, with a consequent reduction of the extraction of
Scheme 1. CO
and ADH.
2
reduction to methanol in water promoted by FateDH, FaldDH
[
5]
[10]
fossil fuels. Such an approach merges biological and chemical
technologies for the development of new hybrid nanotechnol-
ogies.
to make more robust enzymes was made later by encapsu-
lating the enzymes into an hybrid matrix made of alginate and
silicate obtained by reacting Ca-alginate with tetramethoxysi-
lane (TMOS) in water. Hydrolysis of TMOS produced methanol
and a cage of Ca-silicate, which blocked the enzymes without
An interesting case is the biotechnological reduction of CO2
to methanol, which has applications as a raw chemical and
[
6]
[10]
fuel. Such reduction occurs in water at room temperature
and is promoted by three enzymes, namely formate dehydro-
genase (F DH), formaldehyde dehydrogenase (F DH), and al-
any apparent modification of their activity. However, if TMOS
is not completely hydrolysed before NADH is added, methanol
can be formed that can be counted as methanol produced in
ate
ald
cohol dehydrogenase (ADH), while the necessary energy is pro-
vided by nicotinamide adenine dinucleotide phosphate
the reduction of CO . We have verified that this may be the
2
13
12
case by using CO and Si(O CH ) in the encapsulation of the
2
3 4
+
+
12
13
(NADH), which is oxidised to NAD . In nature, NAD is re-
enzymes and measuring the amount of CH OH and CH OH
3 3
13
12
duced back to NADH through solar light; in a biotechnological
collected after reduction of CO . Variable amounts of CH OH
2
3
(
2–10% of the total) have been detected that cannot be
13
formed from CO . To eliminate such parasitic formation of
2
[
a] Prof. A. Dibenedetto, P. Stufano, Prof. C. Fragale, Prof. M. Aresta
Department of Chemistry and CIRCC, University of Bari
Campus Universitario, via Orabona 4-70126 Bari (Italy)
Fax: (+39)080-5443606
methanol, we have used Si(OEt) (TEOS) as starting material for
4
the formation of the encapsulating cage of the enzymes so
that if incomplete hydrolysis of TEOS occurs ethanol is formed,
which can be easily distinguished from methanol produced en-
a.dibenedetto@chimica.uniba.it
[
b] Dr. W. Macyk, T. Baran
zymatically from CO . Figure 1 shows the beads formed by co-
2
Faculty of Chemistry
Jagiellonian University, Krakow (Poland)
encapsulation of the three enzymes.
We have also investigated, whether the co-encapsulation of
the enzymes gives better results than the single encapsulation,
or not. We have found that the use of singly encapsulated en-
[
c] Prof. M. Costa
Department of Organic and Industrial Chemistry
University of Parma, Science Area, 17 A, 43124 Parma (Italy)
ChemSusChem 2012, 5, 373 – 378
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
373

Figure 1. Beads produced from Ca-alginate and TEOS containing co-encap-
sulated FateDH, FaldDH and ADH.
zymes reduces the rate of formation of methanol by 10–40%
with respect to co-encapsulated enzymes, depending on the
conditions and the amount of beads used.
2
Figure 2. Methanol yield for CO reduction by co-encapsulated enzymes,
with NADH regeneration by NAD reduction using SDT, conducting the two
reactions in “one-pot” (*) and separately (~). SDT for in situ NADH regenera-
+
Such co-encapsulated enzymes were used in experiments of
tion was added in a 1:1 molar ratio with respect to NADH initially added.
The reaction was conducted in 0.5m TRIS·HCl [TRIS=tris(hydroxymethyl)ami-
nomethane] buffer at pH 7 and 378C.
+
the reduction of CO coupled to regenerate NAD . The artifi-
2
+
cial reduction of NAD to NADH can be performed in different
ways by using: i) chemicals that act as reducing agents,
ii) transition metal complexes coupled to various reducing
agents including H , iii) semiconductors or metal systems that
Evidently, although the addition of SDT increases the
amount of CH OH formed per mol of NADH (the ratio CH OH/
2
may use (solar) light as a source of energy for the reduction
3
3
+
and iv) electrocatalysts that may reduce NAD under electro-
NADH grows from 0.33 to more than 2), the catalytic system is
deactivated and after seven cycles it is dead. We have verified
that the contact of SDT with the enzymes causes their rapid
deactivation.
chemical conditions.
For class (i) reagents inexpensive and easily affordable chem-
icals must be used. We have tested both hydrazine and
[
7]
sodium dithionite (SDT) and have found that the latter is
much more efficient than the former (Scheme 2).
To avoid such detrimental contact, we have separated the
+
+
oxidation of NADH and the reduction of NAD . After NAD
formation, the solution was separated from the beads, reacted
with stoichiometric amounts of SDT and re-added to the
beads: the life of the enzymes was prolonged, and CH OH was
3
formed in higher amounts (Figure 2, ~); more than 4.14 mol of
CH OH per mole of NADH were formed.
3
In a further attempt to improve the system, we have modi-
fied the reaction apparatus and built a two-compartment reac-
Scheme 2. “One-pot” conversion of CO
2
into CH
3
OH by co-encapsulated
tion cell (Figure 3) so that the reduction of CO occurs in com-
2
+
+
F
ateDH, FaldDH and ADH enzymes, with NAD reduction to NADH using SDT.
partment A, while the reduction of NAD takes place in com-
partment B. Compartments A and B are separated by a sin-
tered glass disc that reduces the back diffusion of the reagents
Adding SDT slowly up to a stoichiometric amount [Eq. (1)] to
+
+
NAD after all NADH was oxidised to NAD caused the reduc-
+
tion of NAD into NADH that could reduce more CO . The re-
2
action was followed by using UV-VIS at l=344 nm to monitor
the amount of NADH and by evaluating the amount of CH OH
3
formed.
þ
2ꢀ
þ
2ꢀ
3
NAD þ S O þ 2 H O ! NADH þ 3 H þ2 SO
ð1Þ
2
4
2
The conversion of CO into CH OH after each addition of
2
3
SDT to the pot, where CO was bubbled through a suspension
2
of beads in water in presence of NADH, is shown in Figure 2.
2
Figure 3. Two-compartment reactor for CO reduction to methanol by co-
encapsulated enzymes (compartment A) and NADH regeneration by SDT
(compartment B). P is a plug for the transfer of the solution from B to A.
After seven cycles, a total amount of 2.13 mol of CH OH per
3
mol of starting NADH was formed (Figure 2, *).
3
74
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2012, 5, 373 – 378

present in compartment B. A pump circulates the liquid from
+
compartment B to A after the reduction of NAD has occurred.
Care must be taken in avoiding any excess of SDT in compart-
ment B as it is brought to compartment A and may destroy
the enzymes. In this way, the ratio CH OH/NADH was further
3
increased up to 35, which is a proof of the concept of the recy-
+
clability of NAD under the operative conditions.
A further improvement may be brought to the system when
transition metal systems (TMs) are used as catalysts for the re-
+
duction of NAD under visible light irradiation. In this case,
the electron donor has a key importance as it determines the
[11]
cost of production of methanol. Phosphites have been used
that are of little practical application, as their cost makes the
process un-economical. We have used bioglycerol as hydrogen
source. Bioglycerol is the co-product of biodiesel in the conver-
sion of natural lipids: it is accumulating at a rate that will make
it a worrying waste if it is not converted into useful products
+
Figure 4. Comparison of the activity of ZnS-A ( ) in the reduction of NAD
&
with that of Ru-loaded ZnS-A (*), ZnS-C nanorods (!) and without a photo-
catalyst (~). The NADH concentration is based on NADH absorption mea-
sured at l=344 nm.
[
12]
[13]
or used for added-value reactions.
[RuCl (PPh ) ]
or
2
3 3
[
14]
+
[
(dppe) RuCl ] (DPPE=1,2-bis(diphenylphosphino)ethane) are
NADH from NAD was followed by concurrently using differ-
2
2
able to extract hydrogen from aqueous bioglycerol that acts as
ent techniques. The fluorescence spectrum of the NADH
(Figure 5) clearly shows an increase of the peak intensity due
to the reduced species upon irradiation. HPLC also showed the
continuous increase of the concentration of NADH during irra-
diation.
+
a reducing agent towards CO and NAD . The first attempts
2
have demonstrated that the reaction is feasible. A ratio
CH OH/NADH=3.2 has been reached that can be improved
3
considerably. Key issue is again the separation of the two pro-
+
cesses (reduction of CO and reduction of NAD ) that require
2
much engineering for an optimal exploitation of the reactive
system. Alternatively, dihydrogen can be biotechnologically
produced from glycerol and used with TM systems (Rh, Mo, Ir,
[
15]
W, Pd, Ni, Pt) as catalysts. Such attempts are still under inves-
tigation to define the catalyst behaviour.
Here, we describe the behaviour of semiconductors such as
ZnS or Ru/ZnS and show that they are interesting agents for
+
the reduction of NAD and the regeneration of NADH.
ZnS-A synthesised at room temperature has a large surface
2
ꢀ1
area (105 m g ) and a maximum of light absorption at about
l=360 nm. Therefore, it allows working at the border of UV
and Visible light. It performs considerably better than ZnS-C
(nanorods) or Ru-loaded ZnS-A (Figure 4). ZnS-A was tested
with watery-glycerol or isopropanol, selected for its known hy-
drogen-transfer properties in industrial applications.
Ru-spots deposited on the surface of the photocatalyst
reduce the performance of the latter most probably because it
subtracts light intensity useful for ZnS-A electron excitation to
the conduction band (Figure 4). It is worth to note that the
energy of the conduction band edge of metallic Ru is consider-
ably lower than that of ZnS-A, so that an electron transfer
from the excited Ru to the ZnS-A conduction band cannot
occur at all. For such reasons, the absorption of incident light
by Ru-covered ZnS-A with respect to ZnS-A itself is less effi-
cient, resulting in a lower catalytic activity. Concerning the
lower activity of ZnS-C (nanorods), it can be ascribed to its
Figure 5. Photocatalytic NADH regeneration. Fluorescence spectra were
measured before switching on the light and after 0.5, 1.5, 2.5 and 6 h irradia-
tion in the presence of ZnS-A as a photocatalyst.
1
H NMR spectroscopy also shows the formation of the active
ꢀ
isomer of monomeric NADH. Concerning the e -donors
(Figure 6), glycerol demonstrated an interesting activity, which
was also better than isopropanol. This finding adds to our pre-
[13,14]
vious observations
and further supports the potential of
bioglycerol as a reducing agent.
+
Thus, NADH was photochemically regenerated from NAD
2
ꢀ1
ꢀ
considerably lower surface area (48 m g ) with respect to
ZnS-A.
by using bioglycerol as an e -donor and working at the border
of the visible light domain. Considering that UV light is com-
ꢀ
As a source of light either a Xe-lamp was used or a lamp
with an emission in the range l=360–440 nm. In both cases,
radiation was filtered by using a common glass, which cuts off
wavelengths lower than l=350 nm. The regeneration of
monly used with triethanolamine or ascorbic acid as e -
donors, our findings considerably improve the process both
energetically and from the point of view of the cost of re-
agents. A point that still deserves investigation is the tuning of
ChemSusChem 2012, 5, 373 – 378
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
375

Gas chromatography was used for both, liquid and gas sample
analysis. Liquid phase samples were analysed by using a flame ioni-
sation detector (FID), an HP-5 (5% phenyl-95% methylsiloxane) ca-
pillary column and He as a carrier.
HPLC analyses were performed by using a BIORAD Aminex HPX-
8
7H column. During analysis, the column was kept at 608C. A 0.1m
aqueous solution of sulfuric acid was used as a mobile phase. The
volume of the samples was 45 mL. Two kinds of detectors were
used, namely a refractive index detector and a UV-Vis absorption
detector.
UV-Vis absorption spectroscopy was used as a quantitative and
qualitative method for NADH determination. Either full spectrum
or single wavelength (l=344 nm) were used. All measurements
were recorded in 1 cm quartz cuvettes. Fluorescence spectroscopy
was used for NADH detection. Emission spectra (l=350–550 nm
characteristic for NADH.) were measured with an excitation radia-
tion of l=344 nm.
Figure 6. Comparison of the performance of different electron donors (
propanol, * glycerol) in the photocatalytic regeneration of NADH from
NAD in the presence of ZnS-A.
&
iso-
+
Brunauer–Emmett–Teller surface area analysis was used to deter-
mine the specific surface area of solids by using a Chemisorb Mi-
cromeritics System. The samples were flushed with N2 for 30 min
before analysis that was performed by using a mixture of 30% N2
and 70% He.
+
the rates of the two processes: i) formation of NAD , and ii) re-
generation of NADH. The enzymatic reaction of reduction of
CO to methanol is considerably faster than the photochemical
2
+
reduction of NAD . This issue is under investigation in our lab-
Diffuse reflectance spectra of the photocatalysts were recorded by
using a UV-VIS-NIR spectrophotometer equipped with an integrat-
ing sphere. UV-Vis diffuse reflectance spectra were measured in the
wavelength range of l=250–800 nm. Samples were ground with
oratory to identify more powerful photocatalysts that may im-
ꢀ
ꢀ
+
prove the e -transfer rate from the e -donors to NAD .
The results reported above open a new horizon to the
+
NAD conversion into NADH using photocatalysts and to the
BaSO and pressed in a pellet. A BaSO pellet was used as a refer-
4 4
ꢀ
ence. Reflectance (R) was converted to F(R ) values, where F(R ) is
use of watery bioglycerol as e - and H-donor (Scheme 3).
1
1
the notation for the Kubelka–Munk function and R ist the abso-
1
lute reflectance of the sampled layer, by applying the Kubelka–
Munk theory. The band-gap energy was accurately determined by
means of transformed diffuse reflectance spectra. The extrapola-
2
tion of the plots to [F(R )E] =0 (E is the energy of photon) allowed
1
estimating the band-gap energy of ZnS, which is a direct semicon-
ductor. The band-gap energies were determined by using a plot of
2
[
F(R )E] vs. E.
1
Preparation of co-encapsulated enzymes
2 3
Scheme 3. CO conversion into CH OH at room temperature using light to
Tetraethyl orthosilicate (TEOS, 1.6 mL) was added to a solution
(4 mL) of sodium alginate in water (2% w/w) while stirring the re-
sulting mixture vigorously for several minutes. A buffer solution
regenerate the enzyme cofactor (NADH) by ZnS-A photocatalysis, with bio-
ꢀ
glycerol as H- and e - donor.
(
1 mL) of TRIS·HCl [0.5m, TRIS=tris(hydroxymethyl)aminome-
thane], degassed and neutralised at pH 7 by using HCl, contained
0.0 mg of each enzyme (FateDH, F DH and ADH). After homoge-
In conclusion, this preliminary work done in the field of
1
ald
+
NAD reduction to NADH coupled with CO2 reduction to
neisation of the mixture, it was dropped by using syringe in a solu-
methanol has given interesting results that encourage search-
tion of CaCl (0.2m) for the formation of 1.5 mm beads. The gelifi-
2
+
ing for more reactive photo-systems that may reduce NAD at
cation of alginate was complete after 0.5 h. The beads formed
were then filtrated on paper and washed several times to eliminate
the excess of calcium ions and ethanol produced by TEOS hydroly-
sis.
a rate compatible with the CO reduction by enzymes, so to
2
feature a possible application.
Reduction of CO to methanol by co-encapsulated enzymes
and NADH
2
Experimental Section
Materials and characterisation
The reduction of CO was performed either in a batch reactor or in
2
All starting reagents and solvents were commercial products pur-
chased from Aldrich. Solvents were dried, distilled, and stored
a flow system while bubbling CO through the solution. The reac-
tion in batch was performed in a low pressure stainless steel auto-
2
[18]
under N atmosphere. All manipulations were performed under a
clave at 0.5 MPa of CO , placing a glass reactor filled with the reac-
2
2
dry N atmosphere by means of vacuum-line techniques. N and
tion mixture in the autoclave, and flushing with CO before start-
2
2
2
CO (99.999%) were supplied by Rivoira IP.
ing. For the reaction in CO flow, a sealed glass tube was used in a
2
2
3
76
www.chemsuschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2012, 5, 373 – 378

thermostated bath at 378C with a septum through wich CO was
prepared using an aqueous solution of ethylenediamine (ratio eth-
ylenediamine/water=1:2). This solution was added drop by drop
to a solution of thiourea (1.6m). The mixture was transferred into a
glass tube containing a magnetic stirring bar that was placed into
an autoclave. The reaction was run at 2008C for 20 h. The pink-
white powder was washed three times with ethanol, six times with
water and then once again with ethanol. The white powder was
dried at 808C for 17 h. The whole procedure was performed under
2
bubbled by means of a needle. The exit was connected to a 08C
trap by using a tube to avoid loss of methanol produced. The
methanol produced was collected and determined by analysis of
both the reaction mixture and the condensed phase in the trap. In
a typical reaction run, NADH (200 mg) was dissolved in TRIS·HCl
(
5 mL of the 0.5m buffer at pH 7) and degassed. The beads pre-
pared as above were suspended in the solution, and the system
kept at 378C for 3 h while slowly bubbling CO2 through the
system. Afterwards, the methanol produced was determined, and
the beads filtered, washed and reused in a new reaction cycle.
an N atmosphere. The size of the final particles was in the range
2
20–45 nm.
Ruthenium-loaded ZnS catalyst preparation
Test of release of methanol from beads produced from
TMOS
ZnS-A powder (400 mg) was suspended in an aqueous solution
containing the desired loading amount of RuCl ·3H O (3 mL). A so-
3
2
Beads prepared from TMOS (tetramethoxysilane, prepared as re-
lution of NaBH (excess) was added dropwise to the suspension
4
while sonicating. After 2 h, the powder was filtered and washed a
few times with water and acetone. The powder was dried at room
13
ported for TEOS) were used in the reduction of CO in a batch re-
2
actor. The formed CH OH was analysed by using GC–MS and
3
12
shown to contain 2–10% CH OH produced from residual TMOS
temperature under vacuum. The dry product was stored in an N
2
3
hydrolysed during the reduction of CO2.
atmosphere. The Ru content was determined by using atomic ab-
sorption spectroscopy and found to be 0.5%. The properties of the
catalysts are reported in Table 1.
+
NADH regeneration by reduction of NAD with SDT
+
a) In a typical experiment, NAD (200 mg) was dissolved in the
buffer TRIS HCl (5 mL, 0.5m, pH 7). The solution was transferred
.
Table 1. Band gap energies of photocatalysts.
into a glass reactor and thermostated at 378C in an N flow. After
2
Photocatalyst
ZnS-A
Band gap energy [eVꢁ0.02]
Onset [nm]
several minutes, a water solution of SDT was slowly added (in 1:1
+
3.47
3.57
3.60
358
347
344
molar ratio with respect to NAD ). The reaction was complete
0.5%Ru@ZnS-A
after 20–25 min as demonstrated by HPLC and spectrophotometric
analysis. The yield of the reaction was determined by measuring
the absorbance at l=344 nm using a calibration curve. The addi-
ZnS-C
tion was repeated until no further production of CH OH was ob-
3
served. A total molar ratio CH OH/NADH equal to 2.13 was
observed.
Photocatalytic tests for the regeneration of NADH from
NAD
3
+
+
b) The same reaction was performed by separating the solution
from the beads after oxidation of NADH and performing the reduc-
tion of NAD with SDT in a separate vessel. The solution contain-
ing NADH was added again to the beads, and CO2 bubbled
through the solution. This cycle was repeated ten times, and a
Photocatalytic tests of NAD regeneration were performed in a
glass cylindrical reactor (Pyrex, V=10 mL). The nanophotocatalyst
powder was suspended in distilled, deoxygenated water. The con-
centration of the photocatalyst was 1 gmL . Isopropanol or glyc-
erol was added as a sacrificial electron donor (5 vol%). NAD
+
ꢀ
1
+
ratio CH OH/NADH=4.14 was observed.
(1.5 mm) was added. Degassed water was used in the experiments.
3
The suspension in the closed reactor was illuminated using a
c) The cycle of reactions was performed in the equipment shown
1
25 W Xe lamp (full spectrum) or a 8 W lamp (l=365 nm) as light
in Figure 3, reaching a CH OH/NADH ratio higher than 35.
3
sources. The full light of lamps was used. During the illumination
run, the mixture was stirred magnetically at constant temperature.
The necessary volume of the reaction solution was sampled at
given intervals of time. The samples were filtered through syringe
filters (Rotilabo, 0.22 mm) and analysed by using UV-Vis absorption
spectroscopy (detection at l=344 nm), fluorescence spectroscopy
and HPLC methods.
ZnS-A preparation
ZnS-A powder was prepared according H. Kisch’s method with
minor modifications.
formed by using Schlenk techniques. A solution of Na S (4.8 g,
[16]
All steps of this preparation were per-
2
0
.2 mol) under N2 atmosphere in deionised and degassed water
50 mL) was added dropwise to an aqueous solution (50 mL) of
ZnSO ·H O (3.58 g, 0.2 mol). The mixture was stirred for 24 h at
(
4
2
Acknowledgements
room temperature. The white precipitate obtained was carefully fil-
trated under N atmosphere by using a sintered glass filter, and
then washed several times with water, until a filtrate at neutral pH
2
The financial support by MiUR and the University of Bari is ac-
knowledged.
(
7.00) was obtained. The product was dried under vacuum for 6 h.
The final product particles had dimensions in the range 10–25 nm.
Keywords: carbon dioxide storage
technologies · methanol · reduction
· enzymes · hybrid
ZnS-C preparation
ZnS-C nanorods were prepared according to M. Wu’s method with
some modifications. A solution of zinc acetate (0.8m, 0.8 L) was
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Received: August 23, 2011
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