Pd/C-catalyzed reduction of NaHCO3 into CH3COOH with water as a hydrogen source

Article abstract of DOI:10.1016/j.cattod.2016.05.010

In this paper, a simple method of NaHCO₃ reduction to CH₃COOH in water with Al powder as a reductant on a commercially available Pd/C catalyst is described that producing CH₃COOH with a good yield of 8% from NaHCO₃ on a carbon basis at 300 °C under hydrothermal conditions. A high adsorption enthalpy of C₁ intermediates on the Pd catalyst may act as an important role in the reduction of NaHCO₃ into CH₃COOH on Pd/C catalyst. This research provides a simple way to convert the CO₂ into a C₂ compound.

Full text of DOI:10.1016/j.cattod.2016.05.010

Catalysis Today 274 (2016) 28–34
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Pd/C-catalyzed reduction of NaHCO3 into CH3COOH with water as a
hydrogen source
Heng Zhonga, Hansong Yaob, Jia Duoc, Guodong Yaoc, Fangming Jina,c,∗
a Graduate School of Environmental Studies, Tohoku University, 6-6 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
b State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science & Engineering, Tongji University, 1239 Siping Road,
Shanghai 200092, China
c School of Environmental Science and Engineering, State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai
200240, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, a simple method of NaHCO3 reduction to CH3COOH in water with Al powder as a reductant
on a commercially available Pd/C catalyst is described that producing CH3COOH with a good yield of 8%
from NaHCO3 on a carbon basis at 300 ◦C under hydrothermal conditions. A high adsorption enthalpy
of C1 intermediates on the Pd catalyst may act as an important role in the reduction of NaHCO3 into
CH3COOH on Pd/C catalyst. This research provides a simple way to convert the CO2 into a C2 compound.
© 2016 Elsevier B.V. All rights reserved.
Received 29 October 2015
Received in revised form 16 May 2016
Accepted 19 May 2016
Available online 28 May 2016
Keywords:
CO2 reduction
CH3COOH
Hydrothermal reactions
Pd/C catalyst
C
C bond formation
Global warming
1. Introduction
can be reduced into organic chemicals such as CH4, HCOOH and
CH3OH effectively and efficiently under hydrothermal condition
Global warming caused due to the increase in atmospheric CO2
concentration is one of the most serious problems faced by human
beings in the 21st century [1,2]. A great deal of efforts has been
expended to reduce the CO2 concentration in the atmosphere [3–7].
The hydrogenation of CO2 into organic chemicals with gaseous
hydrogen is one of the most developed methods to reduce CO2
[8–11]. However, high-purity of gaseous hydrogen and/or noble
metal catalysts is needed, which increase the cost. Recently, pho-
tocatalytic, photoelectrochemical and electrochemical methods for
CO2 reductions have been widely reported because renewable
energies can be used [12–14]. However, their CO2 conversion rates
and efficiencies are still very low, and a highly efficient conversion
still remains a great challenge.
with earth abundant metallic materials such as Fe, Al, and Zn
[17–20]. In such hydrothermal CO2 reduction, the required hydro-
gen source comes from the in-situ reaction of high temperature
water (HTW). Therefore, the disadvantages of hydrogen gas prepa-
ration, transportation and storage in traditional hydrogenation of
CO2 reactions can be avoided. Although the metal reductants are
oxidized during the reaction in HTW, these metal oxides can be
reduced into its zero-valent state by using solar energy, renew-
able energies derived electricity or biomass (for example: glycerol)
[17,21,22]. Therefore, the metal reductants can be recycled with
renewable energies. Although hydrothermal reactions are usually
operated under high temperatures (200–350 ◦C), the reactions of
CO2 reduction are exothermic with a metal reductant [18], which
means they can be potentially self-supported. Thus, the hydrother-
mal reduction of CO2 into organic chemicals could be one promising
and efficient method to help alleviating the problem of global
warming and provide alternative and potentially sustainable chem-
ical feedstock instead of the traditional fossil fuel-based feedstock.
In previous research of hydrothermal reduction of CO2, the
products were mainly C1 compounds such as HCOOH and CH3OH
[18–20]. Although there have been reports of synthesis of long
carbon-chain organics from CO2 for hydrogenation and electro-
Hydrothermal reactions have played an important role in the
abiotic conversion of CO2 into hydrocarbons in the Earth crust
[15,16]. Previous research have demonstrated that CO2 or NaHCO3
∗
Corresponding author at: School of Environmental Science and Engineering,
State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 800
Dongchuan RD, Shanghai 200240, China.
E-mail address: fmjin@sjtu.edu.cn (F. Jin).
http://dx.doi.org/10.1016/j.cattod.2016.05.010
0920-5861/© 2016 Elsevier B.V. All rights reserved.

H. Zhong et al. / Catalysis Today 274 (2016) 28–34
29
chemical reduction [23–25], synthesis of C2 compounds using
•
hydrothermal methods has not been well studied. He et al. reported
that CO2 can be reduced into CH3COOH under hydrothermal condi-
tions, however, a very long reaction time (ca. 72 h) is required [26].
Therefore, more efficient methods and higher active catalysts are
needed to be developed. Unlike production of C1 compound, the
synthesis of C2 compounds requires C C bond formation which
is usually difficult to realize in CO2 reduction. The reduction of
CO2 into C2 compounds is a key step for further synthesizing long-
chain organics [24,27]. It has been reported that palladium (Pd)
based catalysts have the activity to catalyze C C bond formation
in the hydrogenation of CO2 [28,29]. However, the activity of Pd
catalyst for CO2 reduction under hydrothermal conditions is still
unknown. In the present article, we report the hydrothermal reduc-
tion of CO2 into CH3COOH by using commercially available Pd/C
catalyst and Al (powder) reductant. In the process, water acts not
only as an excellent solvent, but also as a source of hydrogen gen-
erated by reduction of metal Al. The proposed system is simple and
efficient for C2 compound (CH3COOH) formation from CO2. Based
on the experimental results and thermodynamic data of the reac-
tion intermediates, a reaction mechanism for HCO3− reduction into
CH3COOH is proposed.
•
AlO(OH)
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
20
40
60
80
2-theta (deg)
Fig. 1. XRD pattern of solid precipitate after the reaction catalyzed by Pd/C (reac-
tion conditions: 1.0 mmol NaHCO3, 6.0 mmol Al, 0.1 g Pd/C, water filling ratio: 35%,
300 ◦C, 2 h).
ble chemicals in liquid phase. The solid samples were dried in an
isothermal oven (50 ◦C) for 24 h after washing with distilled water.
Then it was examined by X-ray diffractometer (XRD) using a Bruker
D8 Advance X-ray Diffractometer equipped with Cu K␣ radiation
(␭ = 1.5406 Å, scanning rate: 0.02◦ s−1, 2␪ ranges: 10–90◦).
The Pd/C catalyst was analyzed by a high-resolution scanning
transmission electron microscopy (HRSTEM) with a JEM-2100F
field-emission electron microscope equipped with a high-angle
annular dark field (HAADF) detector. Samples for HRSTEM analysis
were ultrasonically dispersed in ethanol and then suspended on a
copper grid before the analysis. Surface area of the Pd/C catalyst was
also determined by nitrogen sorption experiments on an ASAP2010
instrument (Micromeritics, USA). The samples were degassed at
160 ◦C for 2 h under vacuum prior to the adsorption measurements.
Thermal stability of Pd/C was examined on a SII Exstar 6000 (TG-
DTA 6200) system between RT and 500 ◦C in N2 at a heating rate of
5 ◦C/min.
2. Experimental
Since CO2 can be effectively absorbed in NaOH solutions to form
NaHCO3 [30], NaHCO3 was used as the source of CO2 in this research
to simplify the operation and to allow for quantification of CO2.
Chemicals of NaHCO3 (99.5%, Sinopharm Chemical Reagent Co. Ltd,
China), Al powder (200-mesh, 99%, Sinopharm Chemical Reagent
Co. Ltd, China) and Pd/C powder (200-mesh, 5% Pd, Sinopharm
Chemical Reagent Co. Ltd, China) were used directly without any
further purification. Standard reagents of HCOOH (puriss. p.a.,
∼98%, Fluka) and CH3COOH (99.7%, Sigma-Aldrich Co. LLC.) were
used for HPLC quantitative analysis. A stainless steel (SUS316) tube
with a Swagelok cap and a gas valve (Autoclave Engineering Inc.)
with a reducing union at each end was used as the hydrothermal
reactor. The inner volume of the reactor was 5.7 mL. The schematic
drawing of the hydrothermal reactor can be found elsewhere [31].
Experimental procedures were employed and described as follows.
The reaction chemicals (Pd/C: 0–0.3 g; Al: 1–6 mmol; NaHCO3:
0.5–4 mmol) and deionized water (1.4–3.1 mL) were first added to
the reactor, and then the reactor was sealed. Next, the reactor was
put into a salt bath (mixture of NaNO3 and KNO3) which was pre-
heated to target reaction temperature (260–320 ◦C). At the desired
reaction time (0.5–3 h), the reactor was taken out from the salt bath
and was put into a cold-water bath (20 ◦C) immediately to quench
the reaction. The reaction time is defined as the duration of the
reactor remaining in the salt bath. A water filling ratio, which is
defined as the ratio of the volume of water added into the reac-
tor to the total inner volume of the reactor, is used to describe the
used amount of water. At the reaction temperature of 260–320 ◦C,
the water exists in a mixture of liquid and vapor state. The pressure
of the water vapor is 4.7, 8.6 and 11.3 MPa at 260, 300 and 320 ◦C,
respectively [32].
The yields of organic acids were defined as:
Carbon in the produced organic acids, mol
Yield, mol% =
× 100%
(1)
Carbon in the initial NaHCO3, mol
Each experiment in this study was repeated at least three times
and the average of the experimental data with a deviation less than
10% was used in this report.
3. Results and discussion
3.1. Investigation of the catalytic activity of Pd/C in the CO2
reduction into CH3COOH
Firstly, the effect of Pd/C catalyst on the CO2 reduction was
investigated by reacting NaHCO3 (1.0 mmol) with Al (6.0 mmol),
Pd/C (0.1 g) and H2O (2 mL) at 300 ◦C for 2 h. Results showed that
CH3COOH and HCOOH were produced. The XRD analysis of the
solid precipitate after the reaction showed that only AlO(OH) was
detected in the solid precipitate (See Fig. 1), demonstrating a reduc-
tant role of Al. However, Pd was not observed in the XRD pattern,
which is probably because the particle size of Pd is very small and
is well distributed in the activated carbon support. To verify the
catalytic activity of Pd, reactions with activated carbon support
instead of Pd/C or in the absence of Pd/C were further conducted,
and the results showed that no formation of CH3COOH in both
cases, suggesting that the Pd/C acts as a catalyst in the reduction of
After the reactions, gas samples were collected through the gas
valve and analyzed by a Hewlett-Packard model 5890 Series II
gas chromatograph equipped with a thermal conductivity detector
(GC/TCD). The liquid samples were filtered (0.22 ␮m filter film) and
analyzed by an Agilent 1200 high-performance liquid chromatog-
raphy (HPLC) with a UV-detector (210 nm), one Shodex RSpak KC-G
and two RSpak KC-811 columns using 2 mmol L−1 HClO4 as flowing
solvent. An Agilent 7890 gas chromatography-mass spectrometry
(GC–MS) equipped with 5985C inert mass selective detector (MSD)
with triple-axis detector was also used to investigate other possi-

30
H. Zhong et al. / Catalysis Today 274 (2016) 28–34
Fig. 2. High-resolution scanning transmission electron microscopy (HRSTEM) images of Pd/C catalyst ((a) and (b)) and particle size distribution of Pd (c) (average particle
size: 2.82 1.03 nm).
NaHCO3 into CH3COOH. These results can be evident that the Pd/C
can catalyze the C C bond formation in the CO2 reduction under
hydrothermal condition.
The effect of the Pd/C amount on the CH3COOH production
was investigated with 1.0 mmol NaHCO3, 6.0 mmol Al and 0–0.3 g
Pd/C at 300 ◦C for 2 h. As shown in Fig. 3a, an increase in the
Pd/C amount could promote the formation of CH3COOH, and the
CH3COOH production rate increased from 5.8 ␮mol·g−1(Pd)·min−1
to 9.7 ␮mol·g−1(Pd)·min−1 when the Pd/C amount increased from
0.05 to 0.3 g. This is probably because the increase in the amount of
catalyst causes the increase in the formation of reaction interme-
diates that were adsorbed on the Pd/C as discussed in Section 3.3.
Although the details are unclear, it is reasonable to assume that the
CH3COOH formation from the intermediates adsorbed on the Pd/C
has a reaction order higher than 0. Therefore, an increase in the
amount of Pd/C catalyst leads to an increase in the concentration
of adsorbed reaction intermediates, which then promotes the reac-
tion rate of CH3COOH formation. Fig. 3b shows the yield of the liquid
products after the reaction at the same condition. Only HCOOH was
produced with a yield of ca. 60% when Pd/C was not used, suggest-
To check whether a pre-reduction of the Pd/C catalyst was
needed or not, experiments of first reducing 0.1 g Pd/C at 320 ◦C
with 1.5 MPa H2 for 3 h, and then reaction at 300 ◦C for 2 h after fur-
ther addition of 1.0 mmol NaHCO3, 6.0 mmol Al and 2 mL H2O in the
same reactor was conducted. No obvious difference was observed in
the liquid products obtained with H2-reduced Pd/C or as-obtained
Pd/C, suggesting that a pre-reduction of the Pd/C catalyst is not
necessary in this reaction. Thermal stability of the Pd/C catalyst
was then investigated with thermogravimetric analysis (TGA) in
N2 atmosphere, and the result (Fig. S1, Supporting Information)
showed that the Pd/C was stable at temperatures of about 300 ◦C.
Subsequently, the surface area of the Pd/C was determined by BET
analysis, and the result showed that the surface area of Pd/C was
983 m2/g. Fig. 2 shows the HRSTEM images and particle size distri-
bution of Pd/C, and the average particle size of Pd was determined
to be 2.82 1.03 nm.
−
ing that Al has the ability to reduce HCO3 into HCOOH, which is
in accordance with our previous research [34]. However, the yield

H. Zhong et al. / Catalysis Today 274 (2016) 28–34
31
(a)
(b)
Fig. 3. Effect of the Pd/C amount on the (a) concentration of CH3COOH and its production rate; (b) yields of CH3COOH and HCOOH after the reduction of 1.0 mmol NaHCO3
with 6.0 mmol Al and 35% water filling ratio at 300 ◦C for 2 h.
500
400
300
200
100
0
500
400
300
200
100
(b)
(a)
260
270
280
290
300
310
320
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Temperature (oC)
Reaction time (h)
Fig. 4. Effects of (a) reaction temperature from 260 to 320 ◦C at a constant reaction time of 2 h and (b) reaction time from 0.5 to 3.0 h at a constant reaction temperature of
320 ◦C on the concentration of produced CH3COOH after the reactions with 0.5 mmol NaHCO3, 1.0 mmol Al, 0.3 g Pd/C and 35% water filling ratio.
of HCOOH was not obviously affected by gradually increasing the
amount of Pd/C, indicating that Pd/C mainly catalyzes the CH3COOH
formation.
600
500
400
300
200
100
0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
3.2. Effect of various reaction conditions on the CH3COOH
production
3.2.1. Effect of reaction temperature and time
The effect of reaction temperature and time on CH3COOH pro-
duction was investigated by varying the temperature from 260
to 320 ◦C and reaction time from 0.5 to 3.0 h, and the results are
shown in Fig. 4. As the reaction temperature increased from 260 to
300 ◦C, the concentration of produced CH3COOH slightly increased
from 255 to 278 ppm (Fig. 4a). When the reaction temperature fur-
ther increased to 320 ◦C, the CH3COOH concentration increased
20 25 30 35 40 45 50 55 60
Water filling ratio (%)
Fig. 5. Concentrations (᭹) and absolute amounts (᭿) of the CH3COOH obtained at
various water filling ratios after the reduction of 0.5 mmol NaHCO3 with 1.0 mmol
Al and 0.3 g Pd/C at 320 ◦C for 2 h.
−
to 398 ppm. The reduction of HCO3 into CH3COOH is exother-
mic (Section 3.3), suggesting that increasing temperature should
be adverse to the CH3COOH formation. One possible explanation
for the increase in CH3COOH production with the increase −in the
reaction temperature could be that the reduction of HCO3 into
CH3COOH has high activation energy, and then increasing the tem-
perature is advantageous to overcome the high activation energy.
Considering the pressure limitation of the used reactor, exper-
iments with temperatures over 320 ◦C were not conducted. As
illustrated in Fig. 4b, when the temperature was maintained at
320 ◦C, increasing the reaction time from 0.5 to 3.0 h promoted
the CH3COOH production from 327 to 428 ppm, which suggests
that a longer reaction time is effective to enhance the CH3COOH
production.
3.2.2. Effect of water filling ratio
Since water is used not only as the reaction media, but also as
the reactant to provide hydrogen source in the hydrothermal CO2
reduction, the effect of water filling ratio on the CH3COOH produc-
tion were investigated. The concentrations of CH3COOH obtained at
various water filling ratios after the reduction of NaHCO3 at 320 ◦C
for 2 h are illustrated in Fig. 5. When the water filling ratio increased
from 25% to 45%, no significant change in the concentration of
CH3COOH was observed. However, a further increase in the water
filling ratio up to 55% led to a remarkable decrease in the CH3COOH

32
H. Zhong et al. / Catalysis Today 274 (2016) 28–34
900
800
700
600
500
400
300
200
100
600
580
560
540
520
500
0
0
1
2
3
4
0
1
2
3
4
5
6
Amount of NaHCO3 (mmol)
Amount of Al (mmol)
(a)
(b)
Fig. 6. Concentrations of produced CH3COOH as functions of (a) Al amount and (b) NaHCO3 amount after the reactions at 300 ◦C for 2 h (Pd/C: 0.3 g, water filling ratio: 35%,
(a): NaHCO3: 1.0 mmol; (b): Al: 1.0 mmol).
concentration. Since the initial amount of NaHCO3 was unchanged
in these experiments, increasing the water filling ratio should dilute
the produced CH3COOH and then affect the CH3COOH concentra-
tion. Therefore, the real effects of the water filling ratio cannot be
distinguished from the CH3COOH concentration. Then, the abso-
lute amounts of CH3COOH production with different water filling
ratios were investigated. As shown in Fig. 5, the absolute amount of
the produced CH3COOH first increased with the water filling ratio
from 0.65 to 1.2 mg and then decreased when the water filling ratio
exceeded 45%. This suggests that the water filling ratio has a posi-
tive effect on the CH3COOH production at low water filling ratios;
however, it becomes adverse to the CH3COOH formation when the
−
water filling ratio is higher than 45%. This is probably because a high
water filling ratio will reduce the empty space in the reactor, which
leads to an increase in the partial pressure of produced H2. Then,
the increased H2 partial pressure becomes unfavorable to the H2
gas generation from the water according to the Le Chatelier’s prin-
ciple. As a result, the CH3COOH production decreased at high water
filling ratios. The yield of CH3COOH from NaHCO3 was also exam-
ined, and the results showed that the highest yield of CH3COOH
was ca. 8%, which was obtained with the water filling ratio of 45%.
Scheme 1. Proposed reaction mechanism of the hydrothermal reduction of HCO3
into CH3COOH catalyzed by Pd/C.
Pd (5% of 0.3 g Pd/C), the highest production rate of CH3COOH
is 14.6 ␮mol·g−1(Pd)·min−1. Compared to the previous report, in
which 210 ppm CH3COOH was produced after a 72 h hydrother-
mal reaction catalyzed by nano-Fe, giving an average reaction rate
of 0.017 ␮mol·g−1(Fe)·min−1 [26], our results significantly reduced
the reaction time and promoted the CH3COOH production.
3.3. Reaction mechanism of the CH3COOH formation
3.2.3. Effect of the amount of reactants
The effect of the amount of Al and NaHCO3 on the CH3COOH
formation was studied by changing the amount of NaHCO3
(0.5–4.0 mmol) and Al (1.0–6.0 mmol) at 300 ◦C for 2 h with 0.3 g
Pd/Cand a water filling of 35%. As shown in Fig. 6a, when the amount
of NaHCO3 was fixed at 1.0 mmol, increasing the amount of Al from
1.0 mmol to 6.0 mmol caused a slight decrease in the CH3COOH
concentration from 571 to 527 ppm. This is probably because the
excess amount of Al promotes the conversion of CO2 into HCOOH,
which has been studied in our previous researches [34]. On the
other hand, when the amount of Al was fixed at 1.0 mmol, the
CH3COOH production ascended dramatically from 278 to 788 ppm
when the NaHCO3 amount increased from 0.5 to 4.0 mmol as illus-
trated in Fig. 6b. The CH3COOH increased clearly with the NaHCO3
amount when the amount of NaHCO3 was less than 1.0 mmol. How-
ever, the increase in CH3COOH concentration became slow when
the amount of NaHCO3 further increased. This is probably caused
by the insufficient amount of Pd/C catalyst.
Our previous research has showed that the Al has the ability
to produce H2 from H2O based on Reaction (2) and catalyze the
reduction of NaHCO3 into HCOOH under hydrothermal conditions,
in which the HCO3 is considered to be a reactant for HCOOH for-
mation [34].
−
2Al + 4H2O ꢀ 2AlO(OH) + 3H2
(2)
When Pd/C catalyst is added, not only HCOOH but also CH3COOH
and trace amount of CH4 are formed, suggesting that the Pd/C
can catalyze the CH3COOH formation from HCO3−. Based on our
experimental results and the reported adsorption enthalpies of
various CO2 reduction intermediates on the Pd catalyst [28], a pos-
−
sible reaction mechanism of the hydrothermal reduction of HCO3
into CH3COOH was propose−d as shown in Scheme 1. At the begin-
ning of the reaction, HCO3 is adsorbed onto the Pd catalyst to
form intermediate II, which is then transformed into intermedi-
ate III by reacting with the in-situ formed H2 from H2O. Since the
adsorption enthalpy of the intermediate III on Pd is relatively high
(56.4 kcal/mol) [28], the intermediate III is not easy to desorb from
Pd to form formate, which then leads to a further hydrogenation
Based on the results above, the highest CH3COOH produc-
tion in this research was 788 ppm, which was obtained at 300 ◦C
for 2 h with 4.0 mmol NaHCO3, 1.0 mmol Al, 0.3 g Pd/C, and 35%
water filling. Combined with the reaction time and amount of

H. Zhong et al. / Catalysis Today 274 (2016) 28–34
33
of the intermediate III into the intermediates IV and V on Pd. The
4. Conclusions
adsorption enthalpy of the intermediate V on Pd is also relatively
high (42.7 kcal/mol) [28], and then the desorption of the interme-
diate V from Pd to form CH3OH is unfavorable. As a result, the firm
adsorptions of the intermediates III and V on Pd make the reac-
tion of the intermediate III with the V to form the intermediate
VI available. The adsorption enthalpy of the intermediate VI on Pd
is reported to be only 12.8 kcal/mol [28], which is much smaller
than those of the intermediates III and V on Pd. Therefore, the
intermediate VI is easy to desorb from Pd to produce CH3COOH.
To further examine the above proposed reaction mechanism,
the adsorption enthalpies of the intermediate of formate on the
Pd, Cu and Ni were compared by referring the data from litera-
In this work, the production of CH3COOH, a C2 compound, was
achieved through a highly exothermic reaction of hydrothermal
reduction of HCO3− catalyzed by a commercial Pd/C catalyst with Al
powder as a reductant. The Pd/C acted as a good catalyst in the C
C
bond formation and CH3COOH generation. The highest production
rate of CH3COOH from HCO3− was 14.6 ␮mol·g−1(Pd)·min−1, which
was obtained at 300 ◦C for 2 h. High adsorption enthalpies of the C1
intermediates on the Pd catalyst may be a key factor in C C bond
formation and CH3COOH production.
Acknowledgements
−
ture, and our previous research, in which the HCO3 was mainly
reduced into C1 products when Cu or Ni as the catalyst [18,20,33].
It has been reported that the formate adsorption enthalpy on the
Pd, Cu and Ni follows the order of Cu (110) < Pd (110) < Ni (110)
[35], which suggest that the formate adsorption enthalpy on Cu is
the lowest, that is Cu can catalyze the HCOOH formation effectively.
This is in accordance with our previous research, in which a HCOOH
yield of 76% was obtained with the catalyst Cu [18]. On the other
hand, Ni has the largest formate adsorption enthalpy among Cu, Pd
and Ni, suggesting that more reduced products than HCOOH can be
obtained on the Ni catalyst according to the proposed mechanism
(Scheme 1). Ni has been reported to be a high active catalyst in the
formation of CH4 from CO2 [36], which requires a firm adsorption
of the intermediate of formate on Ni, and this is in accordance with
the above formate adsorption enthalpy sequence and the proposed
mechanism. Thus, the proposed mechanism in Scheme 1 is reason-
able. However, no C2 compounds were produced in our previous
research when Ni was used as the catalyst even Ni has a higher
formate adsorption enthalpy than Pd. One possible explanation
could be that although relatively firm adsorption of formate can
be achieved on Ni, further reaction of the intermediates to form
CH3COOH or other C2 compounds was not favorable on Ni. The
exact mechanism of CO2 reduction into CH3COOH is still not clear
at this stage and is under further investigation.
The authors gratefully acknowledge the financial support from
the State Key Program of National Natural Science Foundation of
China (No. 21436007), the National Natural Science Foundation of
China (Grants 21277091 and 51472159), and Key Basic Research
Projects of Science and Technology Commission of Shanghai (No.
14JC1403100).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.05.
010.
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(3)
␪
The calculated standard enthalpy (ꢀHr ) of Reaction (3) at
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(4)
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34
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