A novel method of NaHCO3 reduction into formic acid with N2H4·H2O over Ni catalyst

Article abstract of DOI:10.1039/c4ra14326d

A novel Ni-catalysed CO₂ reduction with N₂H₄·H₂O was proposed. A 50% yield of formic acid with 99% selectivity was achieved by using a commercially available Ni powder catalyst. The proposed method is simple and environmentally friendly because it requires neither elaborately prepared catalysts, nor high-purity, high-pressure hydrogen.

Full text of DOI:10.1039/c4ra14326d

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A novel method of NaHCO3 reduction into formic
acid with N2H4$H2O over Ni catalyst†
Cite this: RSC Adv., 2015, 5, 11257
*
*
Feiyan Chen, Guodong Yao, Zhibao Huo and Fangming Jin
Received 11th November 2014
Accepted 9th January 2015
DOI: 10.1039/c4ra14326d
www.rsc.org/advances
A novel Ni-catalysed CO2 reduction with N2H4$H2O was proposed. A medium.6 For example, HTW has a lower dielectric constant,
50% yield of formic acid with 99% selectivity was achieved by using a fewer and weaker hydrogen bonds, and a higher isothermal
commercially available Ni powder catalyst. The proposed method is compressibility than ambient liquid water.7 Thus, some reac-
simple and environmentally friendly because it requires neither elab- tions that can hardly proceed at low temperatures can occur in
orately prepared catalysts, nor high-purity, high-pressure hydrogen.
HTW.8 Recently, there is also an increasing interest in CO2
reduction under hydrothermal conditions. For example, our
group and others have demonstrated the reduction of CO2 into
value-added chemicals using metals as the reductant without
expensive or complex catalysts in HTW.9 As such, it is of
great potential for efficient reduction of CO2 into chemicals
using N2H4$H2O as the reductant together with a simple cata-
lyst in HTW.
In this study, we focused on the use of hydrothermal reac-
tions for the hydrogenation of CO2 into formic acid by
employing N2H4$H2O as an in situ liquid hydrogen source with
Ni powder as the catalyst.
CO2 reduction into chemicals or fuels has attracted consider-
able attention as an important method to reduce greenhouse
gas emissions and also to alleviate the current dependence on
fossil fuels.1 In the past several decades, many promising
methods for CO2 reduction have been proposed.2 Among these
methods, the photochemical reduction of CO2 is regarded as
the most promising approach, but its efficiency is very low, and
expensive catalysts or complex catalysts are needed.3 The
hydrogenation of CO2 with gaseous hydrogen is currently
considered to be the most commercially feasible synthetic
route. However, this method requires either elaborately
prepared and expensive (or complex) catalysts, such as noble-
metal (Ir, Ru, Rh, etc.) complexes, or high-purity, high pres-
sure hydrogen.4 Therefore, developing a new method for the
hydrogenation of CO2 with alternative hydrogen sources and
simple catalysts is urgently required.
Hydrous hydrazine, N2H4$H2O, might be a promising
reductant for the reduction of CO2. Recently, several studies
on hydrogen production by the catalytic decomposition of
N2H4$H2O have been reported.5 To the best of our knowledge,
CO2 reduction with N2H4$H2O as an in situ, liquid hydrogen
source was not yet reported previously.
Hydrothermal reactions have received an increasing atten-
tion in organic chemical synthesis because high-temperature
water (HTW) is an environmentally benign solvent compared
to organic solvents and has suitable properties as a reaction
Initially, to test the possibility of CO2 reduction into chem-
icals with N2H4$H2O, experiments with N2H4$H2O and
NaHCO3, which was used as the CO2 source in order to simplify
the experiments, were conducted at 300 ꢀC. 300 ꢀC was selected
ꢀ
because our previous studies have shown that 300 C was the
optimal temperature for CO2 reduction into chemicals.10 Anal-
yses of the liquid samples with GC-MS and HPLC showed that
the major product was formic acid (see Fig. SI-1 and SI-2). It can
be seen from Table 1 that no formation of formic acid was
observed in the absence of N2H4$H2O (entry 1). These results
indicated that CO2 was selectively reduced into formic acid and
that N2H4$H2O acted as the reductant. Considering that the real
product should be formate due to the alkalinity of N2H4$H2O,
the formate yield, which was dened as the percentage of
formate to the initial amount of NaHCO3 based on carbon, was
used to assess CO2 reduction in this study. As shown in Table 1,
a formate yield of 18% was obtained (entry 2).
Although CO2 can be reduced into formic acid without the
addition of a catalyst, the formate yield was not high—only
18%. Thus, various metals were screened as the catalyst in
order to enhance the yield of formate. As shown in Table 1,
among the metals investigated, only Ni catalyst showed better
School of Environmental Science and Engineering, State key lab of metal matrix
composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240,
China. E-mail: fmjin@sjtu.edu.cn; Fax: +86-21-54742283; Tel: +86-21-54742283
† Electronic supplementary information (ESI) available: Experimental details,
characterization data. See DOI: 10.1039/c4ra14326d
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Table 1 The yields of formate from CO2 with N2H4$H2Oa
Entry
Reductant
Catalyst
Yieldb (%)
1
2
3
4
5
6
7
8
9
—
—
—
Ni
Ni
Cu
Fe
Co
AlNi
WC
0
18
1
23
9
13
3
N2H4$H2O
—
N2H4$H2O
N2H4$H2O
N2H4$H2O
N2H4$H2O
N2H4$H2O
N2H4$H2O
4
6
a
Reaction condition: NaHCO3: 0.5 M; N2H4$H2O: 2 M; Ni: 4 mmol;
b
water lling: 35%; time: 120 min; temp.: 300 ꢀC. The formate yield
is dened as the percentage of formate to initial NaHCO3 on the
carbon basis.
performance, affording formate with increased yield (entry 4).
XRD analyses of the solid residues aer the reaction showed
that Ni was still in the metallic state (see Fig. SI-3), suggesting
that the role of Ni metal was the catalyst in the reduction of CO2
into formate with N2H4$H2O.
Subsequently, a series of experiments were conducted over
a wide range of conditions by changing the concentration
of reactants, water lling, reaction temperature and time in
order to study the reaction characteristics and then design the
optimal synthetic conditions for a higher yield of formate. First,
the effect of the Ni amount was studied. As shown in Fig. 1, the
formate yield increased to 27% with the addition of 2 mmol Ni.
However, the formate yield decreased when the amount of Ni
further increased to 3 mmol. The decrease in the formate yield
with overdose of Ni can be probably ascribed to the decompo-
sition of formate into CH4, which was supported by the fact that
CH4 was detected as the side product when using 3 mmol of Ni.
The pressure of the reaction system usually has a strong
inuence on the product yield. The effect of the pressure was
then investigated by varying water lling in a batch reactor
system. As shown in Fig. 2 (a), the increase in the water lling/
pressure facilitated the CO2 reduction into formate, and the
yield of formate signicantly increased to 50% as the water
lling increased to 60%. A possible reason for the increase in
the formate yield with an increase in water lling is that more
Fig. 2 (a) Effects of water filling and the amount of N2H4$H2O on the
yields of formate (time: 60 min; temp.: 300 ꢀC.). (b) Effects of reaction
time and temperature on the yields of formate (N2H4$H2O: 5 M; water
filling: 55%.).
hydrogen was dissolved in water under a higher pressure. Since
similar yields of formate were obtained with 55% and 60%
water lling, the following experiments were conducted with
55% water lling. The amount of N2H4$H2O has also been
optimized. It is expected that the increase amount of N2H4$H2O
could enhance the product yield whereas the capital cost would
increase as well. Thus, to obtain the optimal concentration of
N2H4$H2O (or the ratio of N2H4$H2O to CO2), the amount of
N2H4$H2O was varied with a xed amount of NaHCO3 (0.5 M).
As shown in Fig. 2(a), the yield of formate increased greatly with
increased concentration of N2H4$H2O, and reached a plateau
when the concentration of N2H4$H2O further increased to 6 M.
The effects of reaction time and temperature on the yield of
formate were further investigated under the optimal conditions
obtained above. As shown in Fig. 2(b), the formate yield
increased rst and then decreased with an increase in the
reaction time, and the highest yield was obtained aer 60 min.
The decrease in the formate yield aer 60 min indicated that the
rate of decomposition of formate was faster than its formation
rate. For the effect of temperature, as shown in Fig. 2(b), the
formate yield drastically increased as the reaction temperature
increased from 250 to 300 ꢀC and reached a maximum at 300 ꢀC.
The observed decrease in the formate yield at temperatures
above 300 ꢀC could be caused by the decomposition of formate.
The organic carbon in the liquid samples was also deter-
mined by TOC analysis, and the amount of carbon in the
formate was comparable to the total carbon in the samples,
which indicated that formate was the dominant product in the
Fig. 1 Effect of the amount of Ni on the yields of formate (NaHCO3:
0.5 M; N2H4$H2O: 2 M; water filling: 35%; time: 120 min; temp.:
300 ꢀC).
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absence of Ni were conducted. Analyses of the gaseous samples
with GC-TCD showed that the amounts of both H2 and total gas
were much higher in the presence of Ni compared to those in
the absence of Ni. These results indicated that Ni can improve
the decomposition of N2H4 to H2 and N2 by N–H bond cleavage
(Route 1), while avoiding the undesired decomposition of N2H4
to ammonia (NH3) (Route 2). Similar results were reported in H2
production from N2H4, in which Raney Ni and Ni-based bime-
tallic catalysts, such as Ni–Rh, Ni–Ir and Ni–Pt, were used.12
Obviously, in this study, the common Ni powder can selectively
decompose N2H4 into H2, which is most likely that HTW can
activate the Ni catalyst surface and make the H in N2H4 more
easily adsorbed onto Ni, leading to N–H bond cleavage and the
formation of H2. Then, the experiment without Ni under the
optimum condition (300 ꢀC, 55% water lling, 5 M N2H4$H2O)
was conducted and the yield of formate was 15%, which was
much lower than that with 3.135 mmol Ni (49% yield). The
result further conrmed the catalytic role of Ni. In addition, to
study whether the surface of Ni can be oxidised in HTW and
then affect the catalyst activity, XPS of the Ni before and aer
the reaction were measured, and trace amounts of Ni(OH)2 and
NiO before the reaction and Ni(OH)2 aer the reaction were
observed (see Fig. SI-4). However, reactions under the catalysis
of Ni(OH)2 or NiO gave low yields of formate yield (8% and 7%
respectively), much lower than that even without Ni. These
results suggested that Ni, rather NiO or Ni(OH)2, acted as the
catalyst in the CO2 reduction with N2H4$H2O. This phenom-
enon likely occurred because Ni2+ was reduced into Ni(0) by
N2H4$H2O, which was supported by the fact that Ni(0) was
observed in the reaction with NiO or Ni(OH)2 in the presence of
N2H4$H2O. Finally, an experiment with NaHCO3 and ammonia
(NH3$H2O) was conducted to examine whether CO2 can be
reduced by ammonia. The result showed that almost no
formation of formate was observed, indicating that the forma-
tion of formate should be contributed to the hydrogen from
N2H4 rather than NH3.
Fig. 3 Effect of reused Ni on the yields of formic acid (NaHCO3: 0.5 M;
N2H4$H2O: 5 M; Ni: 3.135 mmol; water filling: 55%; time: 60 min;
temp.: 300 ꢀC).
hydrogenation of CO2. The main by-product was acetic acid with
a yield of 0.4%, and therefore, the selectivity for the production
of formate was approximately 99%.
Finally, the stability of the Ni catalyst was examined, as
shown in Fig. 3. Although slight decrease in the yield of formate
was observed in the 2nd cycle, further decrease was not
observed, indicating that Ni can maintain the activity aer 2
cycles. The specic surface area of the Ni powder was measured
by BET, which showed that the specic surface area of Ni
decreased from 2290 m2 kgÀ1 before the reaction to 2196 m2
kgÀ1 aer the rst cycle, which may explain the slight decrease
in the formate yield. Moreover, to conrm whether the leaching
of the Ni catalyst occurred and the possible contamination of
main compositions (Fe, Mn) of the SUS316 reactor wall, the ion
concentration of Ni, as well as Fe and Mn, in the solution aer
the reactions were measured by ICP; small amount of these ions
was found in the liquid sample with Ni at 2.126 ppm, Fe at
0.5366 ppm, and Mn at 0.1642 ppm, respectively. The results
indicated that the leaching of these metals was negligible. To
further rule out the possible catalytic activity of Fe or Mn from
the reactor wall, control experiments were conducted in the
absence of Fe or Mn and in the presence of the two with a 104-
fold concentration as the values detected by ICP. The results
were comparable, indicating that Fe or Mn are not able to
catalyse the reaction.
Aer understanding the catalytic activity of Ni for decom-
posing N2H4 into H2, an important question is whether the
formed hydrogen reacts with CO2 or HCO3À. As a result, reac-
tions with gaseous CO2 instead of NaHCO3 were conducted. The
yield of formate was only 1%, suggesting that the hydrogenation
can be considered to occur mainly with HCO3À rather than CO2.
Furthermore, to explore the difference between the activity of
the in situ hydrogen from N2H4$H2O and gaseous hydrogen, an
experiment with gaseous hydrogen as the hydrogen source
The possible mechanism of CO2 reduction into formic acid
with N2H4$H2O was investigated. It is generally known that the
decomposition of N2H4 takes place in two typical reaction
routes:11 N2H4 / N2 (g) + 2H2 (g) and 3N2H4 / 4NH3 + N2 (g),
and the ways of decomposition can be explained by Scheme 1.
To investigate the main route of N2H4 decomposition in this
study, reactions with only N2H4$H2O in the presence and
Scheme 2 Proposed mechanism of reduction of HCO3À into formate
with N2H4$H2O over Ni.
Scheme 1 Decomposition ways of N2H4$H2O.
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instead of N2H4$H2O was performed. Only a 2% yield of formate
was obtained. Although further evidence is needed, from these
results it is suggested that in situ hydrogen formed from
N2H4$H2O may act as a higher activity than the gaseous
hydrogen in CO2 reduction into formic acid.
Based on the above results, a possible mechanism was
proposed, as shown in Scheme 2. First, the H in N2H4$H2O is
adsorbed onto the Ni surface and the N2H4$H2O prefers the
cleavage of the N–H bond, leading to H2 formation. Subse-
quently, the in situ generated H2 is also adsorbed onto the Ni
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surface and is activated by Ni. Furthermore, the active H attacks
À
the C]O, followed by the leaving of hydroxyl group of HCO3
.
Finally, formate was obtained together with H2O, which was
formed by the combination ofÀanother active H of H2 and
leaving hydroxyl group of HCO3
.
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Conclusions
We have, for the rst time, demonstrated the highly selective
reduction of CO2 into formic acid using N2H4$H2O over a
common Ni powder in HTW. Nearly 99% selectivity and
approximately 50% yield of formate were achieved. The recy-
cling of Ni displayed stable activity. This technique provides a
simple method for reducing CO2 into value-added chemicals
and also provides an example for a multitude of possible
chemical reactions for CO2 conversion and utilization.
Acknowledgements
The authors thank the nancial support of the National Natural
Science Foundation of China (no. 21277091), the State Key
Program of National Natural Science Foundation of China (no.
21436007), Key Basic Research Projects of Science and Tech-
nology Commission of Shanghai (no. 14JC1403100) and China
Postdoctoral Science Foundation (no. 2013 M541520). We
gratefully acknowledge nancial support from the ENN
Institute.
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11260 | RSC Adv., 2015, 5, 11257–11260
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