Article pubs.acs.org/est
No Catalyst Addition and Highly Efficient Dissociation of H2O for the Reduction of CO2 to Formic Acid with Mn Lingyun Lyu, Xu Zeng, Jun Yun, Feng Wei, and Fangming Jin* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: The “greenhouse effect” caused by the increasing atmospheric CO2 level is becoming extremely serious, and thus, the reduction of CO2 emissions has become an extensive, urgent, and long-term task. The dissociation of water for CO2 reduction with solar energy is regarded as one of the most promising methods for the sustainable development of the environment and energy. However, a high solarto-fuel efficiency keeps a great challenge. In this work, the first observation of a highly effective, highly selective, and robust system of dissociating water for the reduction of carbon dioxide (CO2) into formic acid with metallic manganese (Mn) is reported. A considerably high formic acid yield of more than 75% on a carbon basis from NaHCO3 was achieved with 98% selectivity in the presence of simple commercially available Mn powder without the addition of any catalyst, and the proposed process is exothermic. Thus, this study may provide a promising method for the highly efficient dissociation of water for CO2 reduction by combining solar-driven thermochemistry with the reduction of MnO into Mn.
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INTRODUCTION The Earth’s surface temperature has risen by approximately 0.85 °C from 1880 to 2012 according to the Intergovernmental Panel on Climate Change (IPPC 2013), which is a phenomenon caused by CO2 emissions from fossil carbon to meet the energy demand of economic growth and civilization of the human society.1−5 Presently, the concentration of CO2 is increasing markedly than any other time, in which senses, all sectors, such as agriculture, food production, industry, tourism, and health, are affected by this important phenomenon on the agenda.6−9 Melting of glaciers, rising of the water level in the oceans, and vaporization in the fresh water resource as the heat increases all harm the natural balance and threaten the ecological environment. A great deal of effort has been expended to reduce the CO2 concentration in the atmosphere, among which the solar technologies are the ideal solution for the “greenhouse effect” problem.10−12 Artificial photosynthesis, in which solar energy is converted into chemical energy of renewable, non-polluting fuels and chemicals, is regarded as one of the most promising methods for the solar energy technologies. However, direct conversion of solar energy into chemical energy retains many problems, such as low conversion efficiencies and low product selectivity. Developing an efficient solar-to-fuel conversion process is a great and fascinating challenge.13−16 An integrated system should be expected to improve artificial solar-to-fuel efficiency. Recently, some interesting integrated technologies of a solar two-step water-splitting thermochemical cycle based on © 2014 American Chemical Society
the redox of metals/metal oxides, such as Fe/Fe3O4, Zn/ZnO, Mn(III)/Mn(II), Mn(IV)/Mn(II), and even MgxOy/Mg, using solar energy have been achieved,17−19 and dissociation of water to hydrogen production20 was significantly higher than that with directive use of solar energy. Consequently, the dissociation of water with metal for the reduction of CO2 would represent one of the most promising approaches to increase artificial photosynthetic efficiency. Previous research has shown potential of the dissociation of water for CO2 reduction with Fe. However, the yield of product formic acid was relatively low, even with the addition of a nickel catalyst, and the highest formic acid yield on a carbon basis was approximately 16%.21−23 Mn, as a first-row transition metal, has an extraordinarily appealing coordination chemistry because of its reactive redox nature.24 As a key element in photosynthesis, Mn can mediate the splitting of water to provide the necessary electrons for photosynthesis.25−27 In addition, Mn plays a significant role in the synthesis of catalysts for CO2 hydrogenation and Fischer−Tropsch (FT) synthesis.28 Moreover, many researchers have reported a redox process of ZnO/Zn, Mn(III)/Mn(II), and Mn(IV)/Mn(II) reactions using solar energy.18,19 Recently, Uchida et al. have also demonstrated that MgO/Mg can be circulated by solar power concentration using Received: Revised: Accepted: Published: 6003
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laser technology.17 The redox of MgO/Mg suggests that the reduction of MnxOy should be much easier than MgO using solar energy because Mg is more active that Mn. Thus, a circulation of Mn should be achieved. Therefore, Mn may have a much more significant implication than Fe in water splitting for the conversion of CO2. Thus far, no study has reported the use of metallic Mn as an efficient reductant of hydrogen production for the conversion of CO2. With the goal of highly efficient dissociation of water based on the redox of metals/metal oxides for the CO2 reduction with a high yield, the use of Mn as a reductant to produce hydrogen for CO2 reduction was investigated. We found that Mn performed very well in the CO2 conversion process. These new findings are reported in this paper.
used to investigate other possible chemicals in liquid samples. The system used a 2 mmol/L HClO4 solution as the mobile phase at a flow rate of 1.0 mL/min. The solid samples were washed with deionized water 3 times to remove impurities and ethanol 3 times to make the solid sample quickly dry. The samples were then dried in an isothermal oven at 40 °C for 3−5 h and characterized using X-ray diffraction (XRD). XRD analyses were performed on a Bruker D8 Advance X-ray diffractometer. The step scan covered angles of 10−80° (2θ) at a rate of 2°/s.
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RESULTS AND DISCUSSION Potential of CO2 Reduction with Mn. In a previous study in which CO2 was reduced using Fe, the main product was formic acid.20 To investigate whether CO2 could be reduced to useful chemicals or organics and to determine the reduction products in the presence of Mn, experiments with NaHCO3 and Mn powder (200 mesh) were conducted at 300 °C for 2 h with a water filling of 35%, which is a good condition for obtaining a high yield of formic acid when using Fe as a reductant. Liquid samples were analyzed by HPLC and GC/ MS. As shown in Figure 1, HPLC analysis showed that the
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MATERIALS AND METHODS Materials. Mn powder was obtained from Aladdin Chemical Reagent, and NaHCO3 was obtained from Sinopharm Chemical Reagent Co., Ltd. In this study, NaHCO3 was used as a CO2 source to simplify handling. Gaseous CO2 and H2 (>99.995%) were purchased from Shanghai Poly-Gas Technology Co., Ltd. Deionized water was used throughout the study. Experimental Procedure. Experiments were conducted using a series of batch SUS 316 tubing reactors [9.525 mm (3/8 in.) outer diameter, 1 mm wall thickness, and 120 mm long] with end fittings, providing an inner volume of 5.7 mL. Teflonlined reactors were used to examine the effect of the reactor wall on the reaction for CO2 reduction. The schematic drawing can be found elsewhere.29,30 The experimental procedure was conducted as follows. The desired amounts of Mn, NaHCO3, and deionized water were added to the reactor chamber. The reactor was then sealed and put into a salt bath that had been preheated to the desired temperature. After the preset reaction time, the reactor was removed from the salt bath and then placed into a cold water bath to quench the reaction. After cooling to room temperature, the reaction mixture was collected and filtered through a 0.22 μm syringe for analysis. The water filling was defined as the ratio of the volume of the water put into the reactor to the inner volume of the reactor, and the reaction time was defined as the duration of time that the reactor was kept in the salt bath. Product Analysis. The yield of formic acid was defined as the percentage of formic acid and the initial NaHCO3 on a carbon basis as follows: Y=
CF CS
Figure 1. HPLC and GC/MS chromatogram of the liquid sample after reaction (temperature, 300 °C; time, 2 h; NaHCO3, 1 mmol; Mn, 8 mmol; water filling, 35%).
main product was formic acid, and GC/MS analysis validated this result, with very little acetic acid detected. Additionally, the TOC analysis showed that the selectivity of the production of formic acid was over 98%, which was defined as the percentage of formic acid and the TOC in liquid sample based on the carbon. Analysis of gas samples by GC/TCD showed that no organic product was produced, and only hydrogen and a small amount of CO2 were detected. These results indicated that formic acid was the main product from CO2 in the presence of Mn. Quantitative analysis of the products obtained under this condition showed that the formic acid yield was 43%. In comparison to the reaction that used Fe as a reductant, in which the highest formic acid yield was only 16% with nickel as a catalyst,20,22 the yield of formic acid in the presence of Mn was much higher. The results demonstrated that Mn was more efficient in reducing CO 2 into formic acid than Fe. Subsequently, experiments with different sizes of Mn powder were also conducted by changing the size of Mn powder from 50 to 1400 mesh, and the results indicated that the formic acid yield has no evident change with different size Mn (see Figure S1 of the Supporting Information). Thus, 200-mesh Mn
(1)
where CF and CS are the amounts of carbon in formic acid and in the initial NaHCO3 added to the reactor. Liquid samples were filtered (0.22 μm filter film) and then analyzed by highperformance liquid chromatography (HPLC), total organic carbon (TOC), and gas chromatography/mass spectroscopy (GC/MS). HPLC analysis was performed on KC-811 columns (SHODEX) with an Agilent Technologies 1200 system, which was equipped with a tunable ultraviolet/visible (UV/vis) absorbance detector adjusted to 210 nm and a differential refractometer detector. TOC was analyzed using a Shimadzu TOC 5000A. Gas samples were analyzed by gas chromatography/thermal conductivity detector (GC/TCD). The Agilent 7890 GC/MS system, which was equipped with a 5985C inert mass selective detector (MSD) and a triple-axis detector, was 6004
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shown in eqs 3 and 4, and the decarboxylation of formic acid is the predominant pathway in HTW.31,32
powder was chosen in this study. Further, the effect of the reactor wall of SUS 316 was also investigated with a Teflonlined reactor, and the results demonstrated that no significant catalytic role of the reactor wall of SUS 316 was observed (see Table S1 of the Supporting Information). In addition, an energy assessment for CO2 reduction into formic acid with Mn was also examined. According to the proposed mechanism, the overall reaction could be described by eq 2, and then the calculated reaction heat and free energy using available thermodynamic date are negative. Thus, the dissociation of water for CO2 reduction using Mn as reductant is not only spontaneous but also exothermic. It is known that, for an exothermic reaction, its equilibrium constant (Keq) will decrease with an increase in the temperature. As expected, the calculated Keq (600 K) is significantly lower than Keq (298 K) (the details can be found in the Supporting Information). Mn + CO2 + H 2O → MnO + HCOOH ° = −23.01 kJ/mol ΔG298
° = −114.75 kJ/mol ΔH298
° = −10.78 kJ/mol ΔG600
° = −115.78 kJ/mol ΔH600
Keq (298 K) = e9.29 = 10829
HCOOH ↔ CO + H 2O
(3)
HCOOH ↔ CO2 + H 2
(4)
Thus, high reductant conditions may also inhibit the decomposition of the formed formic acid. The GC/TCD results confirmed this assumption, showing that only a small amount of CO2 and no CO were present in the gas samples. To examine the effect of the initial amount of NaHCO3 (carbon source) on the formic acid formation from CO2, experiments were conducted at 300 °C for 2 h by fixing the Mn amount at 8 mmol. As shown in Figure 2b, when the amount of NaHCO3 was varied from 0.5 to 1.0 mmol, no obvious change on the yield of formic acid was observed. However, as the amount of NaHCO3 further increased, the formic acid yield decreased. No obvious change in the formic acid yield in the range between 0.5 and 1 mmol could be explained as more than 8:1 of the corresponding ratio of Mn/NaHCO3 because the formic acid yield remained nearly constant when the ratio of Mn/NaHCO3 was above 8 mmol (see Figure 2a). For the decrease in the formic acid with a further increase in NaHCO3, it should be due to a decrease of the ratio of Mn/NaHCO3 to below 8 when NaHCO3 increased to above 1.0 mmol. Therefore, the reactant amounts of 8 mmol of Mn and 1 mmol of NaHCO3 were chosen for the following study. The effect of the initial solution pH should be an important factor in the reduction of CO2 to formic acid because pH can affect the decomposition equilibrium of NaHCO3. The decomposition of formic acid is also related to the pH of the solution because alkaline conditions are generally not favorable for the decomposition of formic acid.33 Takahashi et al.34 have also reported that formic acid can be selectively formed by CO2 reduction in a weak alkaline solution under HTW conditions when using Fe. Experiments to investigate the effect of pH were conducted by adjusting the initial pH with NaOH or HCl. As shown in Figure 3, the highest formic acid yield of 43% occurred at the initial pH of 8.3, which is the same pH value as that observed at 1 mmol of NaHCO3 with no additional NaOH or HCl. In the cases of lower acidity, pH 6.6, and higher alkalinity, pH 13.0, the yield of formic acid decreased to 34 and 10%, respectively. These results suggested that a weak alkaline pH value of about 8.3 is favorable for the formation of formic
(2)
Keq (600 K) = e 2.16 = 8.67
Characteristics of the Reaction of Dissociation of Water for CO2 Reduction with Mn and the Parameter Design of the High Yield of Formic Acid. First, the effects of the initial amounts of Mn and NaHCO3 were studied to investigate characteristics of dissociation of water for CO2 reduction. As shown in Figure 2a, the initial amount of Mn
Figure 2. Effect of the amount of Mn and NaHCO3 on the formic acid yield (temperature, 300 °C; time, 2 h; water filling, 35%; NaHCO3, 1 mmol for the effect of the amount Mn; Mn, 8 mmol for the effect of NaHCO3).
strongly affected the formic acid yield at a fixed amount of NaHCO3 (1 mmol). As the initial amount of Mn increased from 2 to 10 mmol, the formic acid yield increased clearly from 13 to 43%, which was then remained nearly constant when the amount of Mn was 8 mmol. The increase in the formic acid yield with the increase in Mn most likely occurred because stronger reduction conditions or a larger amount of hydrogen improves the conversion of CO2. Additionally, it has been reported that formic acid decomposes under high-temperature water (HTW) via dehydration and/or decarboxylation, as
Figure 3. Effect of the pH on the formic acid yield (temperature, 300 °C; time, 2 h; NaHCO3, 1 mmol; Mn, 8 mmol; water filling, 35%). 6005
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In addition, considering that the increase in water filling lead to a decrease in the initial concentration of NaHCO3, which may increase costs for formic acid separation, further experiments were also conducted in the different water filling from 25 to 55% by fixing the initial concentration of NaHCO3 and Mn as the same as the optimal concentration (1 mmol of NaHCO3 and 8 mmol of Mn with 55% water filling) at 325 °C for 1 h. As shown in Figure 5, the yield of formic acid increased clearly with the increases in water filling when the ratio of NaHCO3/ Mn/H2O was constant, which showed that the increase in the formic acid yield was directly related to the pressure but not the NaHCO3 concentration. CO2 Role of Improving Hydrogen Production from Water. In our previous study, it was found that, in the absence of NaHCO3, no hydrogen was produced when using Fe as the reductant. However, a substantial quantity of hydrogen was produced in the presence of NaHCO3, which indicated that an increase in the initial NaHCO3 could lead to an increase in hydrogen production. To determine whether NaHCO3 also affects hydrogen production when using Mn as the reductant, gas samples with and without NaHCO3 were collected and analyzed by GC/TCD. As shown in Table 1, in the absence of
acid, and thus, no additional alkali was used in the following experiments. Subsequently, the effects of the reaction temperature and time on the conversion of CO2 were studied. As shown in Figure 4, the yield of formic acid was very low and increased
Figure 4. Effect of the temperature and time on the formic acid yield (NaHCO3, 1 mmol; Mn, 8 mmol; water filling, 35%).
Table 1. Amount of H2 and CO2 for Gas Samples and the Yield of Formic Acida
unconspicuously with the reaction time increasing at 250 and 275 °C. However, the formic acid yield increased obviously when the temperature reached 300 °C, and the yield was as high as 60% when the temperature reached 325 °C. Hence, a high temperature was favorable for CO2 conversion. As shown in Figure 4, the time profile indicated that the formic acid yield increased rapidly at first, and a further increase in the reaction time did not result in a significant increase in the yield of formic acid. Therefore, 2 and 1 h were the optimal reaction times at 300 and 325 °C, respectively. Finally, the effect of an important parameter of water filling was investigated using constant initial amounts of NaHCO3 (1 mmol) and Mn (8 mmol), with the water filling varying from 25 to 55%. As shown in Figure 5, the formic acid yield
entry
Mn (mmol)
NaHCO3 (mmol)
H2 (mL)
CO2 (mL)
yield of formic acid (%)
1 2 3
4 4 8
0 1 1
80.5 89.5 140.0
0.5
23 43
At 300 °C for 120 min. The volume of total gas was measured at room temperature of 20 ± 1 °C and pressure of 1 atm.
a
NaHCO3, 80.5 mL of hydrogen was produced when 4 mmol of Mn was used, whereas 89.5 mL of hydrogen was produced in the presence of NaHCO3. Additionally, a formic acid yield of 23% (0.23 mmol of formic acid) was obtained in the presence of NaHCO3. Thus, the total hydrogen production in the presence of NaHCO3 was higher than the total hydrogen production without NaHCO3 because the formic acid formed from CO2 hydrogenation consumed some amount of hydrogen. Namely, CO2 can also promote hydrogen generation when using Mn as a reductant, which is probably because the oxidation of Mn shifts the reaction to the right because of the consumption of hydrogen (CO2 hydrogenation) in the presence of CO2, as shown in eq 5. Thus, CO2 provides an additional benefit of improving hydrogen production from water.
Possible Mechanism of Mn Oxidation in Water and Role of MnxOy in the Formation of Formic Acid. The mechanism of Mn oxidation was investigated by collecting the solid residues after reactions and analyzing them by XRD. Interestingly, the rapid color change of the solid residues from green to brown was observed during collection (see Figure 6a). The color of MnO is green among oxides of Mn, and MnO is easily oxidized to Mn3O4. Thus, Mn is most likely oxidized into MnO in the reactions, and the color change is attributed to the further oxidation of MnO during collection in air. As expected, XRD analysis showed that MnO and Mn3O4 were detected, as
Figure 5. Effect of the water filling on the yield of formic acid (NaHCO3, 1 mmol; Mn, 8 mmol).
increased evidently with the increase in water filling, and the yields of 76 and 61% were attained as the water filling increased to 55% at 325 and 300 °C, respectively. The results indicated that the increase of water filling was favorable for CO2 reduction, which may be attributed to the increase in the pressure of the system because of the increase in water filling. 6006
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Figure 6. (a) Photographs of solid samples and (b) XRD patterns of the solid samples (NaHCO3, 1 mmol; time, 2 h; Mn, 8 mmol; temperature, 300 °C; water filling, 35%).
shown in Figure 6b, and Mn(OH)2 was also formed. Thus, there are two possible pathways for the oxidation of Mn to MnO. One is that MnO is formed directly by the oxidation of Mn, and the other is via the formation of Mn(OH)2, as shown in eq 6.
Figure 7. (a) Photographs of solid samples and (b) XRD patterns of the solid samples (NaHCO3, 1 mmol; Mn, 8 mmol; temperature, 300 °C; water filling, 35%).
To determine the reaction pathway of the oxidation of Mn into MnO, experiments were conducted using short reaction times of 1, 5, and 60 min. No green solid was observed during the collection of solid samples, and XRD analysis showed that only Mn(OH)2 was formed when the reaction times were l and 5 min, as shown in Figure 7. When the reaction time was increased to 60 min, MnO and Mn3O4 were observed, whereas the amount of Mn(OH)2 and Mn decreased gradually. Simultaneously, the color change from green to brown in a solid sample was observed during the collection of solid samples. These results suggested that the oxidation of Mn to MnO occurs via the formation of Mn(OH)2. It is generally known that the use of a catalyst is needed for activating hydrogen in the hydrogenation of CO2. However, interestingly, there is a high formic acid yield without the addition of any catalyst in the present study, which suggests that some intermediates, such as MnxOy, formed in situ may act as a catalyst in the presence of Mn. To investigate this topic, experiments with NaHCO3 and gaseous hydrogen were conducted by changing the amount of gaseous hydrogen from 5 to 18 mmol. As shown in Figure 8, all of the formic acid yields with gaseous hydrogen were very low, keeping in only about 2%, while a considerably high yield of formic acid can be obtained when using Mn. These results suggested that MnxOy may act as a catalyst in the reduction of CO2 with Mn. To further provide evidence, experiments with gaseous hydrogen and MnO or Mn3O4 additive were conducted. As shown in Figure 8, the formic acid increased to 9% when MnO was
Figure 8. Formic acid yields obtained with different amounts of gaseous H2 and additive MnO or Mn3O4 (temperature, 325 °C; time, 1 h; NaHCO3, 1 mmol; water filling, 55%; Mn, 8 mmol; gaseous H2, 6 mmol; MnO, 6 mmol; Mn3O4, 6 mmol).
added (run 2); however, the formic acid was less than 2% (run 3) when Mn3O4 was added. The results indicated that MnO can provide a catalytic activity in the reduction of CO2 into formic acid. However, the yield of 9% with MnO was much lower than that with Mn. One of the possible explanations is because MnO formed in situ when using Mn is more active than the added MnO. Investigation of Formic Acid Formation via HCO3− or Gaseous CO2. After understanding the promotion of hydrogen production in the presence of NaHCO3 and the mechanism of Mn oxidation, we also wanted to know whether the formation of formic acid is via gas CO2 or HCO3−. To achieve this 6007
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the Supporting Information, very little formic acid was produced without water (entry 4), which indicated that water is necessary for the reduction of CO2 to formic acid in the presence of Mn. Furthermore, from a thermodynamic point of view, the standard free energy change for CO2 hydrogenation with H into formic acid in the aqueous phase is −399.48 kJ/mol, whereas the reaction between H and CO2 in the gas phase is −373.48 kJ/mol (eqs 9 and 10). The details were presented in the Supporting Information.
objective, experiments with NaHCO3 and gaseous CO2 in the presence of Mn were conducted. As shown in Table S2 of the Supporting Information, a formic acid yield of 43% was achieved with 1 mmol of NaHCO3 (entry 1). However, 0.2 and 5.5% formic acid yields were produced, respectively, when using gaseous CO2 as a reactant instead of NaHCO3 in the absence of NaOH and in the presence of NaOH (entries 2 and 3), which suggested that the formation of formic acid is closely related to the concentration of CO2 in the solvent, and the low concentration of CO2 in the solvent is not favorable for the formic acid formation. If this hypothesis is true, increasing the dissolution time of CO2 before the reactions should lead to an increase in the formic acid yield. To test this hypothesis, experiments were performed with an additional dissolution process at room temperature before the hydrothermal reaction in an attempt to increase the dissolution of CO2 in NaOH solution. As expected, the yield of formic acid clearly increased and the initial pH of solution decreased with the prolongation of the dissolution time; the best formic acid yield was achieved at a weak alkaline pH value, as shown in Figure 9. This
° = −399.48 kJ/mol ΔG298 HCO3− + 2H ⇌ HCOO− ° = −411.35 kJ/mol ΔH298 + H 2O (9)
CO2 + 2H ⇌ HCOOH
° = −373.48 kJ/mol ΔG298 ° = −467.18 kJ/mol ΔH298 (10)
The results indicated that the reaction is exothermic and HCO3− is preferred over CO2 for the interconversion between hydrogen and formic acid. In conclusion, a novel method of no catalyst addition and highly efficient dissociation of water for highly selective conversion of CO2 into formic acid with Mn powder as a reductant was developed. A considerably high formic acid yield of more than 75% was obtained from CO2 with simple commercially available Mn powder. CO2 not only acts as a carbon source but also improves hydrogen production from water. The present study is helpful for providing a promising method for highly efficient dissociation of H2O for CO2 reduction combined with MnO/Mn using solar energy.
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Figure 9. Yield of formic acid with the dissolution of CO2 (Mn, 8 mmol; CO2, 1 mmol; NaOH, 1 mmol; water filling, 35%; dissolution of CO2, at room temperature; temperature, 300 °C; time, 2 h).
Formic acid yield with a Teflon-lined reactor (Table S1), difference in the yields of formic acid between NaHCO3 and CO2 gases (Table S2), effect of the amount of metal reductant ratio and Mn size on the formic acid yield (Figure S1), and energy assessment for reduction of CO2 into formic acid with Mn. This material is available free of charge via the Internet at http://pubs.acs.org.
observation is in agreement with the fact that the best formic acid yield was achieved using a weak alkaline pH. CO2 can exist as different forms of hydrogen carbonate and carbonate at different pH values, as shown in eq 7, and the distribution coefficient of HCO3− is more than 0.9 when the pH value is 8.3, as shown in eq 8. pK1= 6.3
CO2 + H 2O XoooooooY
HCO3−
+ pK 2 = 10.3
+ H XoooooooooY CO3
2−
+ 2H
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pH = p
1+
[HCO3−] K1
AUTHOR INFORMATION
Corresponding Author
*Telephone/Fax: +86-21-54742283. E-mail: [email protected].
+
Notes
(7)
K 2[HCO3−] + KW
ASSOCIATED CONTENT
S Supporting Information *
The authors declare no competing financial interest.
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= p K 2K1
ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants 21077078 and 21277091).
1 (pK1 + pK 2) = 8.3 (8) 2 As discussed previously, the highest formic acid yield occurred at an initial pH of 8.3. The yield of formic acid significantly decreased under more acidic or more alkaline conditions, which may be attributed to the fact that the main ion present is not HCO3− when the pH is less than 6.3 or more than 10.3. These results are further evidence that the reduction of CO2 occurs via HCO3− rather than CO2. Additionally, as shown in Table S2 of =
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REFERENCES
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No Catalyst Addition and Highly Efficient Dissociation of H2O for the Reduction of CO2 to Formic Acid with Mn Lingyun Lyu, Xu Zeng, Jun Yun, Feng Wei, and Fangming Jin* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: The “greenhouse effect” caused by the increasing atmospheric CO2 level is becoming extremely serious, and thus, the reduction of CO2 emissions has become an extensive, urgent, and long-term task. The dissociation of water for CO2 reduction with solar energy is regarded as one of the most promising methods for the sustainable development of the environment and energy. However, a high solarto-fuel efficiency keeps a great challenge. In this work, the first observation of a highly effective, highly selective, and robust system of dissociating water for the reduction of carbon dioxide (CO2) into formic acid with metallic manganese (Mn) is reported. A considerably high formic acid yield of more than 75% on a carbon basis from NaHCO3 was achieved with 98% selectivity in the presence of simple commercially available Mn powder without the addition of any catalyst, and the proposed process is exothermic. Thus, this study may provide a promising method for the highly efficient dissociation of water for CO2 reduction by combining solar-driven thermochemistry with the reduction of MnO into Mn.
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INTRODUCTION The Earth’s surface temperature has risen by approximately 0.85 °C from 1880 to 2012 according to the Intergovernmental Panel on Climate Change (IPPC 2013), which is a phenomenon caused by CO2 emissions from fossil carbon to meet the energy demand of economic growth and civilization of the human society.1−5 Presently, the concentration of CO2 is increasing markedly than any other time, in which senses, all sectors, such as agriculture, food production, industry, tourism, and health, are affected by this important phenomenon on the agenda.6−9 Melting of glaciers, rising of the water level in the oceans, and vaporization in the fresh water resource as the heat increases all harm the natural balance and threaten the ecological environment. A great deal of effort has been expended to reduce the CO2 concentration in the atmosphere, among which the solar technologies are the ideal solution for the “greenhouse effect” problem.10−12 Artificial photosynthesis, in which solar energy is converted into chemical energy of renewable, non-polluting fuels and chemicals, is regarded as one of the most promising methods for the solar energy technologies. However, direct conversion of solar energy into chemical energy retains many problems, such as low conversion efficiencies and low product selectivity. Developing an efficient solar-to-fuel conversion process is a great and fascinating challenge.13−16 An integrated system should be expected to improve artificial solar-to-fuel efficiency. Recently, some interesting integrated technologies of a solar two-step water-splitting thermochemical cycle based on © 2014 American Chemical Society
the redox of metals/metal oxides, such as Fe/Fe3O4, Zn/ZnO, Mn(III)/Mn(II), Mn(IV)/Mn(II), and even MgxOy/Mg, using solar energy have been achieved,17−19 and dissociation of water to hydrogen production20 was significantly higher than that with directive use of solar energy. Consequently, the dissociation of water with metal for the reduction of CO2 would represent one of the most promising approaches to increase artificial photosynthetic efficiency. Previous research has shown potential of the dissociation of water for CO2 reduction with Fe. However, the yield of product formic acid was relatively low, even with the addition of a nickel catalyst, and the highest formic acid yield on a carbon basis was approximately 16%.21−23 Mn, as a first-row transition metal, has an extraordinarily appealing coordination chemistry because of its reactive redox nature.24 As a key element in photosynthesis, Mn can mediate the splitting of water to provide the necessary electrons for photosynthesis.25−27 In addition, Mn plays a significant role in the synthesis of catalysts for CO2 hydrogenation and Fischer−Tropsch (FT) synthesis.28 Moreover, many researchers have reported a redox process of ZnO/Zn, Mn(III)/Mn(II), and Mn(IV)/Mn(II) reactions using solar energy.18,19 Recently, Uchida et al. have also demonstrated that MgO/Mg can be circulated by solar power concentration using Received: Revised: Accepted: Published: 6003
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laser technology.17 The redox of MgO/Mg suggests that the reduction of MnxOy should be much easier than MgO using solar energy because Mg is more active that Mn. Thus, a circulation of Mn should be achieved. Therefore, Mn may have a much more significant implication than Fe in water splitting for the conversion of CO2. Thus far, no study has reported the use of metallic Mn as an efficient reductant of hydrogen production for the conversion of CO2. With the goal of highly efficient dissociation of water based on the redox of metals/metal oxides for the CO2 reduction with a high yield, the use of Mn as a reductant to produce hydrogen for CO2 reduction was investigated. We found that Mn performed very well in the CO2 conversion process. These new findings are reported in this paper.
used to investigate other possible chemicals in liquid samples. The system used a 2 mmol/L HClO4 solution as the mobile phase at a flow rate of 1.0 mL/min. The solid samples were washed with deionized water 3 times to remove impurities and ethanol 3 times to make the solid sample quickly dry. The samples were then dried in an isothermal oven at 40 °C for 3−5 h and characterized using X-ray diffraction (XRD). XRD analyses were performed on a Bruker D8 Advance X-ray diffractometer. The step scan covered angles of 10−80° (2θ) at a rate of 2°/s.
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RESULTS AND DISCUSSION Potential of CO2 Reduction with Mn. In a previous study in which CO2 was reduced using Fe, the main product was formic acid.20 To investigate whether CO2 could be reduced to useful chemicals or organics and to determine the reduction products in the presence of Mn, experiments with NaHCO3 and Mn powder (200 mesh) were conducted at 300 °C for 2 h with a water filling of 35%, which is a good condition for obtaining a high yield of formic acid when using Fe as a reductant. Liquid samples were analyzed by HPLC and GC/ MS. As shown in Figure 1, HPLC analysis showed that the
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MATERIALS AND METHODS Materials. Mn powder was obtained from Aladdin Chemical Reagent, and NaHCO3 was obtained from Sinopharm Chemical Reagent Co., Ltd. In this study, NaHCO3 was used as a CO2 source to simplify handling. Gaseous CO2 and H2 (>99.995%) were purchased from Shanghai Poly-Gas Technology Co., Ltd. Deionized water was used throughout the study. Experimental Procedure. Experiments were conducted using a series of batch SUS 316 tubing reactors [9.525 mm (3/8 in.) outer diameter, 1 mm wall thickness, and 120 mm long] with end fittings, providing an inner volume of 5.7 mL. Teflonlined reactors were used to examine the effect of the reactor wall on the reaction for CO2 reduction. The schematic drawing can be found elsewhere.29,30 The experimental procedure was conducted as follows. The desired amounts of Mn, NaHCO3, and deionized water were added to the reactor chamber. The reactor was then sealed and put into a salt bath that had been preheated to the desired temperature. After the preset reaction time, the reactor was removed from the salt bath and then placed into a cold water bath to quench the reaction. After cooling to room temperature, the reaction mixture was collected and filtered through a 0.22 μm syringe for analysis. The water filling was defined as the ratio of the volume of the water put into the reactor to the inner volume of the reactor, and the reaction time was defined as the duration of time that the reactor was kept in the salt bath. Product Analysis. The yield of formic acid was defined as the percentage of formic acid and the initial NaHCO3 on a carbon basis as follows: Y=
CF CS
Figure 1. HPLC and GC/MS chromatogram of the liquid sample after reaction (temperature, 300 °C; time, 2 h; NaHCO3, 1 mmol; Mn, 8 mmol; water filling, 35%).
main product was formic acid, and GC/MS analysis validated this result, with very little acetic acid detected. Additionally, the TOC analysis showed that the selectivity of the production of formic acid was over 98%, which was defined as the percentage of formic acid and the TOC in liquid sample based on the carbon. Analysis of gas samples by GC/TCD showed that no organic product was produced, and only hydrogen and a small amount of CO2 were detected. These results indicated that formic acid was the main product from CO2 in the presence of Mn. Quantitative analysis of the products obtained under this condition showed that the formic acid yield was 43%. In comparison to the reaction that used Fe as a reductant, in which the highest formic acid yield was only 16% with nickel as a catalyst,20,22 the yield of formic acid in the presence of Mn was much higher. The results demonstrated that Mn was more efficient in reducing CO 2 into formic acid than Fe. Subsequently, experiments with different sizes of Mn powder were also conducted by changing the size of Mn powder from 50 to 1400 mesh, and the results indicated that the formic acid yield has no evident change with different size Mn (see Figure S1 of the Supporting Information). Thus, 200-mesh Mn
(1)
where CF and CS are the amounts of carbon in formic acid and in the initial NaHCO3 added to the reactor. Liquid samples were filtered (0.22 μm filter film) and then analyzed by highperformance liquid chromatography (HPLC), total organic carbon (TOC), and gas chromatography/mass spectroscopy (GC/MS). HPLC analysis was performed on KC-811 columns (SHODEX) with an Agilent Technologies 1200 system, which was equipped with a tunable ultraviolet/visible (UV/vis) absorbance detector adjusted to 210 nm and a differential refractometer detector. TOC was analyzed using a Shimadzu TOC 5000A. Gas samples were analyzed by gas chromatography/thermal conductivity detector (GC/TCD). The Agilent 7890 GC/MS system, which was equipped with a 5985C inert mass selective detector (MSD) and a triple-axis detector, was 6004
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shown in eqs 3 and 4, and the decarboxylation of formic acid is the predominant pathway in HTW.31,32
powder was chosen in this study. Further, the effect of the reactor wall of SUS 316 was also investigated with a Teflonlined reactor, and the results demonstrated that no significant catalytic role of the reactor wall of SUS 316 was observed (see Table S1 of the Supporting Information). In addition, an energy assessment for CO2 reduction into formic acid with Mn was also examined. According to the proposed mechanism, the overall reaction could be described by eq 2, and then the calculated reaction heat and free energy using available thermodynamic date are negative. Thus, the dissociation of water for CO2 reduction using Mn as reductant is not only spontaneous but also exothermic. It is known that, for an exothermic reaction, its equilibrium constant (Keq) will decrease with an increase in the temperature. As expected, the calculated Keq (600 K) is significantly lower than Keq (298 K) (the details can be found in the Supporting Information). Mn + CO2 + H 2O → MnO + HCOOH ° = −23.01 kJ/mol ΔG298
° = −114.75 kJ/mol ΔH298
° = −10.78 kJ/mol ΔG600
° = −115.78 kJ/mol ΔH600
Keq (298 K) = e9.29 = 10829
HCOOH ↔ CO + H 2O
(3)
HCOOH ↔ CO2 + H 2
(4)
Thus, high reductant conditions may also inhibit the decomposition of the formed formic acid. The GC/TCD results confirmed this assumption, showing that only a small amount of CO2 and no CO were present in the gas samples. To examine the effect of the initial amount of NaHCO3 (carbon source) on the formic acid formation from CO2, experiments were conducted at 300 °C for 2 h by fixing the Mn amount at 8 mmol. As shown in Figure 2b, when the amount of NaHCO3 was varied from 0.5 to 1.0 mmol, no obvious change on the yield of formic acid was observed. However, as the amount of NaHCO3 further increased, the formic acid yield decreased. No obvious change in the formic acid yield in the range between 0.5 and 1 mmol could be explained as more than 8:1 of the corresponding ratio of Mn/NaHCO3 because the formic acid yield remained nearly constant when the ratio of Mn/NaHCO3 was above 8 mmol (see Figure 2a). For the decrease in the formic acid with a further increase in NaHCO3, it should be due to a decrease of the ratio of Mn/NaHCO3 to below 8 when NaHCO3 increased to above 1.0 mmol. Therefore, the reactant amounts of 8 mmol of Mn and 1 mmol of NaHCO3 were chosen for the following study. The effect of the initial solution pH should be an important factor in the reduction of CO2 to formic acid because pH can affect the decomposition equilibrium of NaHCO3. The decomposition of formic acid is also related to the pH of the solution because alkaline conditions are generally not favorable for the decomposition of formic acid.33 Takahashi et al.34 have also reported that formic acid can be selectively formed by CO2 reduction in a weak alkaline solution under HTW conditions when using Fe. Experiments to investigate the effect of pH were conducted by adjusting the initial pH with NaOH or HCl. As shown in Figure 3, the highest formic acid yield of 43% occurred at the initial pH of 8.3, which is the same pH value as that observed at 1 mmol of NaHCO3 with no additional NaOH or HCl. In the cases of lower acidity, pH 6.6, and higher alkalinity, pH 13.0, the yield of formic acid decreased to 34 and 10%, respectively. These results suggested that a weak alkaline pH value of about 8.3 is favorable for the formation of formic
(2)
Keq (600 K) = e 2.16 = 8.67
Characteristics of the Reaction of Dissociation of Water for CO2 Reduction with Mn and the Parameter Design of the High Yield of Formic Acid. First, the effects of the initial amounts of Mn and NaHCO3 were studied to investigate characteristics of dissociation of water for CO2 reduction. As shown in Figure 2a, the initial amount of Mn
Figure 2. Effect of the amount of Mn and NaHCO3 on the formic acid yield (temperature, 300 °C; time, 2 h; water filling, 35%; NaHCO3, 1 mmol for the effect of the amount Mn; Mn, 8 mmol for the effect of NaHCO3).
strongly affected the formic acid yield at a fixed amount of NaHCO3 (1 mmol). As the initial amount of Mn increased from 2 to 10 mmol, the formic acid yield increased clearly from 13 to 43%, which was then remained nearly constant when the amount of Mn was 8 mmol. The increase in the formic acid yield with the increase in Mn most likely occurred because stronger reduction conditions or a larger amount of hydrogen improves the conversion of CO2. Additionally, it has been reported that formic acid decomposes under high-temperature water (HTW) via dehydration and/or decarboxylation, as
Figure 3. Effect of the pH on the formic acid yield (temperature, 300 °C; time, 2 h; NaHCO3, 1 mmol; Mn, 8 mmol; water filling, 35%). 6005
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In addition, considering that the increase in water filling lead to a decrease in the initial concentration of NaHCO3, which may increase costs for formic acid separation, further experiments were also conducted in the different water filling from 25 to 55% by fixing the initial concentration of NaHCO3 and Mn as the same as the optimal concentration (1 mmol of NaHCO3 and 8 mmol of Mn with 55% water filling) at 325 °C for 1 h. As shown in Figure 5, the yield of formic acid increased clearly with the increases in water filling when the ratio of NaHCO3/ Mn/H2O was constant, which showed that the increase in the formic acid yield was directly related to the pressure but not the NaHCO3 concentration. CO2 Role of Improving Hydrogen Production from Water. In our previous study, it was found that, in the absence of NaHCO3, no hydrogen was produced when using Fe as the reductant. However, a substantial quantity of hydrogen was produced in the presence of NaHCO3, which indicated that an increase in the initial NaHCO3 could lead to an increase in hydrogen production. To determine whether NaHCO3 also affects hydrogen production when using Mn as the reductant, gas samples with and without NaHCO3 were collected and analyzed by GC/TCD. As shown in Table 1, in the absence of
acid, and thus, no additional alkali was used in the following experiments. Subsequently, the effects of the reaction temperature and time on the conversion of CO2 were studied. As shown in Figure 4, the yield of formic acid was very low and increased
Figure 4. Effect of the temperature and time on the formic acid yield (NaHCO3, 1 mmol; Mn, 8 mmol; water filling, 35%).
Table 1. Amount of H2 and CO2 for Gas Samples and the Yield of Formic Acida
unconspicuously with the reaction time increasing at 250 and 275 °C. However, the formic acid yield increased obviously when the temperature reached 300 °C, and the yield was as high as 60% when the temperature reached 325 °C. Hence, a high temperature was favorable for CO2 conversion. As shown in Figure 4, the time profile indicated that the formic acid yield increased rapidly at first, and a further increase in the reaction time did not result in a significant increase in the yield of formic acid. Therefore, 2 and 1 h were the optimal reaction times at 300 and 325 °C, respectively. Finally, the effect of an important parameter of water filling was investigated using constant initial amounts of NaHCO3 (1 mmol) and Mn (8 mmol), with the water filling varying from 25 to 55%. As shown in Figure 5, the formic acid yield
entry
Mn (mmol)
NaHCO3 (mmol)
H2 (mL)
CO2 (mL)
yield of formic acid (%)
1 2 3
4 4 8
0 1 1
80.5 89.5 140.0
0.5
23 43
At 300 °C for 120 min. The volume of total gas was measured at room temperature of 20 ± 1 °C and pressure of 1 atm.
a
NaHCO3, 80.5 mL of hydrogen was produced when 4 mmol of Mn was used, whereas 89.5 mL of hydrogen was produced in the presence of NaHCO3. Additionally, a formic acid yield of 23% (0.23 mmol of formic acid) was obtained in the presence of NaHCO3. Thus, the total hydrogen production in the presence of NaHCO3 was higher than the total hydrogen production without NaHCO3 because the formic acid formed from CO2 hydrogenation consumed some amount of hydrogen. Namely, CO2 can also promote hydrogen generation when using Mn as a reductant, which is probably because the oxidation of Mn shifts the reaction to the right because of the consumption of hydrogen (CO2 hydrogenation) in the presence of CO2, as shown in eq 5. Thus, CO2 provides an additional benefit of improving hydrogen production from water.
Possible Mechanism of Mn Oxidation in Water and Role of MnxOy in the Formation of Formic Acid. The mechanism of Mn oxidation was investigated by collecting the solid residues after reactions and analyzing them by XRD. Interestingly, the rapid color change of the solid residues from green to brown was observed during collection (see Figure 6a). The color of MnO is green among oxides of Mn, and MnO is easily oxidized to Mn3O4. Thus, Mn is most likely oxidized into MnO in the reactions, and the color change is attributed to the further oxidation of MnO during collection in air. As expected, XRD analysis showed that MnO and Mn3O4 were detected, as
Figure 5. Effect of the water filling on the yield of formic acid (NaHCO3, 1 mmol; Mn, 8 mmol).
increased evidently with the increase in water filling, and the yields of 76 and 61% were attained as the water filling increased to 55% at 325 and 300 °C, respectively. The results indicated that the increase of water filling was favorable for CO2 reduction, which may be attributed to the increase in the pressure of the system because of the increase in water filling. 6006
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Figure 6. (a) Photographs of solid samples and (b) XRD patterns of the solid samples (NaHCO3, 1 mmol; time, 2 h; Mn, 8 mmol; temperature, 300 °C; water filling, 35%).
shown in Figure 6b, and Mn(OH)2 was also formed. Thus, there are two possible pathways for the oxidation of Mn to MnO. One is that MnO is formed directly by the oxidation of Mn, and the other is via the formation of Mn(OH)2, as shown in eq 6.
Figure 7. (a) Photographs of solid samples and (b) XRD patterns of the solid samples (NaHCO3, 1 mmol; Mn, 8 mmol; temperature, 300 °C; water filling, 35%).
To determine the reaction pathway of the oxidation of Mn into MnO, experiments were conducted using short reaction times of 1, 5, and 60 min. No green solid was observed during the collection of solid samples, and XRD analysis showed that only Mn(OH)2 was formed when the reaction times were l and 5 min, as shown in Figure 7. When the reaction time was increased to 60 min, MnO and Mn3O4 were observed, whereas the amount of Mn(OH)2 and Mn decreased gradually. Simultaneously, the color change from green to brown in a solid sample was observed during the collection of solid samples. These results suggested that the oxidation of Mn to MnO occurs via the formation of Mn(OH)2. It is generally known that the use of a catalyst is needed for activating hydrogen in the hydrogenation of CO2. However, interestingly, there is a high formic acid yield without the addition of any catalyst in the present study, which suggests that some intermediates, such as MnxOy, formed in situ may act as a catalyst in the presence of Mn. To investigate this topic, experiments with NaHCO3 and gaseous hydrogen were conducted by changing the amount of gaseous hydrogen from 5 to 18 mmol. As shown in Figure 8, all of the formic acid yields with gaseous hydrogen were very low, keeping in only about 2%, while a considerably high yield of formic acid can be obtained when using Mn. These results suggested that MnxOy may act as a catalyst in the reduction of CO2 with Mn. To further provide evidence, experiments with gaseous hydrogen and MnO or Mn3O4 additive were conducted. As shown in Figure 8, the formic acid increased to 9% when MnO was
Figure 8. Formic acid yields obtained with different amounts of gaseous H2 and additive MnO or Mn3O4 (temperature, 325 °C; time, 1 h; NaHCO3, 1 mmol; water filling, 55%; Mn, 8 mmol; gaseous H2, 6 mmol; MnO, 6 mmol; Mn3O4, 6 mmol).
added (run 2); however, the formic acid was less than 2% (run 3) when Mn3O4 was added. The results indicated that MnO can provide a catalytic activity in the reduction of CO2 into formic acid. However, the yield of 9% with MnO was much lower than that with Mn. One of the possible explanations is because MnO formed in situ when using Mn is more active than the added MnO. Investigation of Formic Acid Formation via HCO3− or Gaseous CO2. After understanding the promotion of hydrogen production in the presence of NaHCO3 and the mechanism of Mn oxidation, we also wanted to know whether the formation of formic acid is via gas CO2 or HCO3−. To achieve this 6007
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the Supporting Information, very little formic acid was produced without water (entry 4), which indicated that water is necessary for the reduction of CO2 to formic acid in the presence of Mn. Furthermore, from a thermodynamic point of view, the standard free energy change for CO2 hydrogenation with H into formic acid in the aqueous phase is −399.48 kJ/mol, whereas the reaction between H and CO2 in the gas phase is −373.48 kJ/mol (eqs 9 and 10). The details were presented in the Supporting Information.
objective, experiments with NaHCO3 and gaseous CO2 in the presence of Mn were conducted. As shown in Table S2 of the Supporting Information, a formic acid yield of 43% was achieved with 1 mmol of NaHCO3 (entry 1). However, 0.2 and 5.5% formic acid yields were produced, respectively, when using gaseous CO2 as a reactant instead of NaHCO3 in the absence of NaOH and in the presence of NaOH (entries 2 and 3), which suggested that the formation of formic acid is closely related to the concentration of CO2 in the solvent, and the low concentration of CO2 in the solvent is not favorable for the formic acid formation. If this hypothesis is true, increasing the dissolution time of CO2 before the reactions should lead to an increase in the formic acid yield. To test this hypothesis, experiments were performed with an additional dissolution process at room temperature before the hydrothermal reaction in an attempt to increase the dissolution of CO2 in NaOH solution. As expected, the yield of formic acid clearly increased and the initial pH of solution decreased with the prolongation of the dissolution time; the best formic acid yield was achieved at a weak alkaline pH value, as shown in Figure 9. This
° = −399.48 kJ/mol ΔG298 HCO3− + 2H ⇌ HCOO− ° = −411.35 kJ/mol ΔH298 + H 2O (9)
CO2 + 2H ⇌ HCOOH
° = −373.48 kJ/mol ΔG298 ° = −467.18 kJ/mol ΔH298 (10)
The results indicated that the reaction is exothermic and HCO3− is preferred over CO2 for the interconversion between hydrogen and formic acid. In conclusion, a novel method of no catalyst addition and highly efficient dissociation of water for highly selective conversion of CO2 into formic acid with Mn powder as a reductant was developed. A considerably high formic acid yield of more than 75% was obtained from CO2 with simple commercially available Mn powder. CO2 not only acts as a carbon source but also improves hydrogen production from water. The present study is helpful for providing a promising method for highly efficient dissociation of H2O for CO2 reduction combined with MnO/Mn using solar energy.
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Figure 9. Yield of formic acid with the dissolution of CO2 (Mn, 8 mmol; CO2, 1 mmol; NaOH, 1 mmol; water filling, 35%; dissolution of CO2, at room temperature; temperature, 300 °C; time, 2 h).
Formic acid yield with a Teflon-lined reactor (Table S1), difference in the yields of formic acid between NaHCO3 and CO2 gases (Table S2), effect of the amount of metal reductant ratio and Mn size on the formic acid yield (Figure S1), and energy assessment for reduction of CO2 into formic acid with Mn. This material is available free of charge via the Internet at http://pubs.acs.org.
observation is in agreement with the fact that the best formic acid yield was achieved using a weak alkaline pH. CO2 can exist as different forms of hydrogen carbonate and carbonate at different pH values, as shown in eq 7, and the distribution coefficient of HCO3− is more than 0.9 when the pH value is 8.3, as shown in eq 8. pK1= 6.3
CO2 + H 2O XoooooooY
HCO3−
+ pK 2 = 10.3
+ H XoooooooooY CO3
2−
+ 2H
■
pH = p
1+
[HCO3−] K1
AUTHOR INFORMATION
Corresponding Author
*Telephone/Fax: +86-21-54742283. E-mail: [email protected].
+
Notes
(7)
K 2[HCO3−] + KW
ASSOCIATED CONTENT
S Supporting Information *
The authors declare no competing financial interest.
■
= p K 2K1
ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants 21077078 and 21277091).
1 (pK1 + pK 2) = 8.3 (8) 2 As discussed previously, the highest formic acid yield occurred at an initial pH of 8.3. The yield of formic acid significantly decreased under more acidic or more alkaline conditions, which may be attributed to the fact that the main ion present is not HCO3− when the pH is less than 6.3 or more than 10.3. These results are further evidence that the reduction of CO2 occurs via HCO3− rather than CO2. Additionally, as shown in Table S2 of =
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