Reduction of carbon dioxide during the synthesis of metal nano-particles in water

Article abstract of DOI:10.1016/S1387-7003(03)00247-8

An effort was made to synthesize "carbon-free" metal (Fe ⁰, Co⁰, Ni⁰) nano-particles via the reduction of their salts with BH₄^- in aqueous solutions. Surprisingly it was found that when the synthesis

Full text of DOI:10.1016/S1387-7003(03)00247-8

Inorganic Chemistry Communications 6 (2003) 1266–1268
www.elsevier.com/locate/inoche
Reduction of carbon dioxide during the synthesis of metal
nano-particles in water
a
a,d,
*
Irena Rusonik , Hurriyet Polat a,b, Haim Cohen a,c, Dan Meyerstein
a
Department of Chemistry, Ben-Gurion University of the Negev, P.O. Box 653, 84105 Beer-Sheva, Israel
b
Department of Chemistry, Ismir Institute of Technology, Ulra-Izmir, Turkey
c
Nuclear Research Centre Negev, Beer-Sheva, Israel
d
Department of Biological Chemistry, College of Judea and Samaria, Ariel, Israel
Received 14 July 2003; accepted 19 July 2003
Published online: 8 August 2003
Abstract
An effort was made to synthesize ‘‘carbon-free’’ metal (Fe0, Co0, Ni0) nano-particles via the reduction of their salts with BHꢀ4 in
aqueous solutions. Surprisingly it was found that when the synthesis is carried out in the presence of CO2, e.g., in aerated solutions,
the CO2 is catalytically reduced by BHꢀ4 on the surface of the metal particles. Carbon-free metals can be prepared by reduction
under an inert atmosphere. Thus metal surfaces might have acted as catalysts for CO2 fixation, probably via the initial formation of
carbon clusters, in the reductive atmosphere in the prebiotic era.
Ó 2003 Elsevier B.V. All right reserved.
Keywords: Carbon dioxide; Metal nano-particles; Reduction; Sodium borohydride
1. Introduction
2. Results and discussion
In a parallel study [1] the formation of CH4, C2H4,
C2H6, C3H6 and C3H8 during the partial dissolution
of analytical iron powder, and other metal powders, in
slightly acidic aqueous solutions was observed. These
organic gases stem from the reduction of carbon at-
oms, or clusters, present in the analytical metal pow-
ders. The evolution of light alkanes and alkenes in
this process complicates the study of the mechanism
of reduction of halogenated organic compounds by
iron powder, which was our original aim. Therefore it
was decided to synthesize pure metal powders that will
not contain any carbon. The synthesis chosen was the
reduction of FeIðIaqÞ, or other Mð2aþqÞ ions by sodium
borohydride in aqueous solution, a method used pre-
viously [2,3] for the preparation of iron nano-particles.
Borohydride reduction of metal ions is currently used
extensively for the production of nanoscale metal and
metal boride particles [4–8].
Iron nano-particles were synthesized as described in
Section 3 in a vessel open to air. Surprisingly it was
found that when the reduction is carried out in the
presence of CO2, e.g., in air which is the common
practice [6–8], carbon dioxide is reduced during the re-
2þ
duction of the M ions.
aq
Partially dissolving the iron particles thus obtained
in phosphate buffer at pH 4.0 results surprisingly in
the emission of the organic gases methane, ethane,
ethylene and propene (Table 1), determinated by GC
of the gaseous phase. As the solutions contained no
organic compounds this result suggested that CO2,
present in the FeCl3 solutions, is the source of the
organic gases.
In order to verify this assumption the reductions were
repeated under CO2 and under He continuous bubbling
during the synthesis. The reactions were performed in a
closed three-neck round bottom flask at room temper-
ature. The iron particles thus prepared were immersed in
the phosphate buffer and the system was shaken for 24
h. After that the gas concentration and composition
* Corresponding author. Tel.: +972-3-9066153; fax: +972-3-9067440.
E-mail address: danmeyer@bgumail.bgu.ac.il (D. Meyerstein).
1387-7003/$ - see front matter Ó 2003 Elsevier B.V. All right reserved.
doi:10.1016/S1387-7003(03)00247-8

I. Rusonik et al. / Inorganic Chemistry Communications 6 (2003) 1266–1268
1267
Table 1
Organic gases emitted during the partial dissolution of metal nano-
particles
Table 2
The influence of the activation of the metal particles synthesized under
CO2 on gas emission
Metal powder
CH4
(ppm)
C2H4
(ppm)
C2H6
(ppm)
Metal
powder
Activation
CH4
(ppm)
C2H4
(ppm)
C2H6
(ppm)
Fe synthesized under He
Fe synthesized under Air
Fe synthesized under CO2
Ni synthesized under He
Ni synthesized under CO2
Co synthesized under He
Co synthesized under CO2
12
54.3
95.6
13
–
–
Ni
Fe
Co
–
75.8
110.7
95.6
135
–
1.8
2.0
2.3
3.3
7.1
10
0.8
2.3
–
2.3
2.3
–
+
–
–
2.3
3.2
–
+
–
75.8
7
–
1.8
–
220.8
518
–
+
–
220.8
–
7.1
3 g metal powder, 3 ml phosphate buffer solution 0.1 M, pH 4.0,
T ¼ 25 °C, t ¼ 24 h. The product concentrations in the gaseous phase
are reported. Error limit ꢁ15%.
3 g metal powder in 3 ml phosphate buffer solution 0.1 M, pH 4.0,
T ¼ 25 °C, t ¼ 24 h. The product concentrations in the gaseous phase
are reported. Error limit ꢁ15%.
process, are not adsorbed to the metal surface, but are
incorporated in the metal particles.
were determined by GC. The results (Table 1) clearly
demonstrate that the gas concentrations depend on the
availability of CO2 during the synthesis of the iron
nano-particles. (The low yield of CH4 for iron particles
prepared under He is probably due to some air pene-
tration during the addition of NaBH4 to the reaction
vessel.)
In order to check whether the CO2 reduction occurs
during the reduction of the iron ions or later on the
surface of the iron nano-particles the following experi-
ment was performed: iron nano-particles were synthe-
sized under He and then stored for a week in bulbs
saturated with air or CO2. Then these particles were
immersed in the deaerated phosphate buffer for 24 h and
the yield of the organic gases measured. The results
demonstrate that the yield of the organic gases does not
increase due to the storage under air or CO2. Thus the
results clearly demonstrate that the CO2 is reduced by
the BHꢀ4 catalytically on the iron particles as they are
formed. No reduction of CO2 was observed in the ab-
sence of iron particles in blank experiments.
The kinetics of the gas emission (Fig. 1) revealed
different behavior for the different metals. In the case of
iron the process is very quick and ends within 30 min,
but in the case of Ni and Co the process continues for
considerably longer periods. The different dissolution
rates might be due to:
1. Differences in the redox potential: Fe ()0.44 V), Co
()0.28 V), Ni ()0.25 V).
2. Differences in the pH at which the metal is covered by
hydroxides and/or phosphates, which inhibit the con-
tinuation of the process.
3. As the process is a heterogeneous one its rate depends
on the surface structure of the nano-particles.
In summary the results demonstrate that:
1. The metal particles and/or metal–boron particles [4,7]
are catalysts for CO2 reduction by NaBH4 during the
metal powder synthesis.
2. Preparation of pure metal nano-particles requires a
synthesis in the absence of CO2.
Finally it is tempting to suggest that metal surfaces,
metal clusters, might have acted as catalysts for CO2
fixation, probably via the production of carbon atoms,
carbides, or carbon clusters in the reductive atmosphere
in the prebiotic era [9]. Then these carbon atoms, car-
bides were transformed into alkanes or alkenes upon
contact with water.
Analogous experiments revealed that CO2 is also
reduced during the preparation of nickel and cobalt
nano-particles via the same procedure. The results are
summed up in Table 1. The concentrations of the
emitted organic gases increase dramatically when the
synthesis is done under CO2.
In order to check whether the organic gases stem
from carbon atoms or organic compounds, adsorbed to
the metal surface during the reduction, the following
experiment was performed: nano-particles synthesized
under CO2 were activated by immersing them in H2SO4
(0.1 M) for 2 min and then washing the particles 7–8
times by 8 ml portions of pure water. This procedure
clearly dissolves the surface of the particles and releases
any compounds adsorbed to the surface. The results
(Table 2) clearly demonstrate that the activation step
increases the concentration of the emitted organic gas-
eous products considerably. Thus the results point out
that the carbon atoms, formed in the CO2 reduction
Fig. 1. Kinetic study of the CH4 emission process. 3 g of non-activated
metal powder, 3 ml phosphate buffer solution 0.1 M, pH 4.0, T ¼
25°C, t ¼ 24 h. Error limit ꢁ15%.

1268
I. Rusonik et al. / Inorganic Chemistry Communications 6 (2003) 1266–1268
3. Experimental
8 in. Supelco). The carrying gas was He (30 ml/min,
T ¼ 70 °C).
All the chemicals used this study were of A.R. grade
and were used without further purification. The water
used was deionized and further purified by a Millipore
Milli-Q setup with a final resistivity of >10 MX.
The metal synthesis was performed in a three-neck
round bottom flask at room temperature. The system
was continuously stirred and bubbled with He or CO2
when air presence was undesired.
Synthesis of iron nano-particles: NaBH4 (10 g) in
small portions was added to FeCl3 ꢂ 6H2O (27 g) dis-
solved in 1 liter of pure water. The FeIII ions are clearly
initially reduced to FeII ions. After that the metal
powder was filtered and dried.
Acknowledgements
This study was supported in part by a grant from the
Budgeting and Planning Committee of The Council of
Higher Education and the Israel Atomic Energy Com-
mission. D.M. wishes to express his thanks to Mrs. Irene
Evens for her ongoing interest and support.
References
Synthesis of Co and Ni nano-particles: NaBH4 (6.5 g)
in small portions was added to NiCl2/CoCl2 ꢂ 6H2O
(23.8 g) dissolved in 1 liter of pure water. After that the
powder was filtered and dried.
[1] I. Rusonik, H. Polat, H. Cohen, D. Meyerstein, submitted.
[2] S.M. Ponder, J.G. Darab, T.E. Mallouk, Environ. Sci. Technol. 34
(2000) 2564.
[3] C.B. Wang, W.X. Znang, Environ. Sci. Technol. 31 (1997) 2154.
[4] G.N. Glavee, K.J. Klabunde, C.M. Sorencen, G.C. Hadjipanayis,
Langmuir 10 (1994) 4726.
The phosphate buffer solutions (0.1 M) at pH 4.0
were deaerated by bubbling He through them for 15 min
using the syringe technique. Then the solutions (3 ml)
were introduced into He deaerated glass bulbs (15 ml)
sealed with rubber septums containing the synthesized
metal powders (3 g) which in some experiments were
previously activated by H2SO4 (0.1 M) for 2 min fol-
lowed by 7–8 washings with 8 ml portions of pure water.
The analysis of the gaseous phase was done on HP
5890 GC FID detector (Poropaq QS GC column 10 ft 1/
[5] G.N. Glavee, K.J. Klabunde, C.M. Sorencen, G.C. Hadjipanayis,
Langmuir 9 (1993) 162.
[6] Z. Hu, Y. Hsia, J. Zheng, J. Shen, Q. Yan, L. Dai, J. Appl. Phys. 70
(1991) 436.
[7] S. Linderoth, M. Steen, J. Appl. Phys. 69 (1991) 5256.
[8] S. Wells, S.W. Charles, S. Moerup, S. Linderoth, J. Van
Wonterghem, J. Larsen, M.B. Madsen, J. Phys.: Condens. Matter
1 (1989) 8199.
[9] S. Miyakawa, H. Yamanashi, K. Kobayashi, H.J. Cleaves, S.L.
Miller, PNAS 99 (2002) 14628.