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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 19836 Received 30th July 2014, Accepted 5th August 2014 DOI: 10.1039/c4cp03388d www.rsc.org/pccp
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New insights into highly efficient reduction of CO2 to formic acid by using zinc under mild hydrothermal conditions: a joint experimental and theoretical study† Xu Zeng,a Makoto Hatakeyama,b Koji Ogata,b Jianke Liu,c Yuanqing Wang,b Qi Gao,d Katsushi Fujii,e Masamichi Fujihira,d Fangming Jin*a and Shinichiro Nakamura*b
We report here a theoretical study with quantum chemical calculations based on experimental results to understand highly efficient reduction of CO2 to formic acid by using zinc under hydrothermal conditions. Results showed that zinc hydride (Zn–H) is a key intermediate species in the reduction of CO2 to formic acid, which demonstrates that the formation of formic acid is through an SN2-like mechanism.
The conversion of CO2 into chemicals or fuels has the potential to decrease the environmental impacts of greenhouse gas emissions and relieve the shortage of fossil-fuels.1 The hydrogenation of CO2 with gaseous hydrogen is currently regarded as the most commercially feasible method.2,3 However, this approach is an energyintensive process due to the high kinetic and thermodynamic stability of CO2, which requires either exotic catalysts or highpurity hydrogen. Recently, some more straightforward approaches have been investigated extensively, such as electrochemical and photocatalytic methods.4,5 Unfortunately, these strategies are still highly costly or less efficient. Therefore, the development of a novel highly efficient and economical approach for the conversion of CO2 into chemicals or fuels remains a big challenge. Hydrothermal reactions have played an important role in the formation of fossil fuels6,7 because of some unique inherent properties of high-temperature water, such as a high ion product and a low dielectric constant.8–10 High-temperature water acts not only as a reaction medium but also as a hydrogen source.
a
School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China. E-mail: [email protected] b RIKEN Research Cluster for Innovation Nakamura Laboratory, Wako, Saitama, 351-0198, Japan. E-mail: [email protected] c College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, China d Science and Technology Research Center, Mistsubishi Chemical Group, Yokohama, 227-8502, Japan e Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, 153-8904, Japan † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cp03388d
19836 | Phys. Chem. Chem. Phys., 2014, 16, 19836--19840
Hydrothermal reduction of CO2 to formic acid with iron has been reported;11–14 however, the yield is relatively low. Recently, we reported the production of formic acid from CO2 by using a series of metals, such as Zn, Al, Mn, etc.15 Especially, a high yield of formic acid (ca. 75 C%) with Zn was obtained. Considering that Zn could be regenerated from its oxidation state, ZnO, by using concentrated solar energy,16–19 which has shown high potential for large-scale demonstration of a novel solar chemical reactor,20 a highly efficient method for CO2 utilization with zinc through a Zn–ZnO cycle system by using solar energy could be realized. Although highly efficient reduction of CO2 to formic acid could be achieved, the detailed mechanism is still unknown because the reaction intermediates are difficult to be experimentally identified under hydrothermal conditions. Density functional theory (DFT) has always been used in the theoretical study of various types of chemical reactions, including the reaction of Zn with H2O.21–24 Therefore, in this study, a theoretical study was performed based on experimental results obtained by DFT. Unlike our previous study, which mainly reported the initial experimental data, this theoretical study provides basic information for understanding the reaction mechanism of highly efficient reduction of CO2 to formic acid, because it is hard to experimentally study the reaction mechanism, e.g. by an in situ technique, due to the reactions proceeding in high-temperature water. DFT calculations were adopted with the RPW91 density functional by using Gaussian 09,25 which considered van der Waals (VDW) interactions. Initially, experiments were conducted to study the effect of reaction time on the yield of formic acid. NaHCO3 was used as a CO2 resource to simplify handling. Zinc (powder, 200-mesh, of analytical grade) and NaHCO3 (99%, of analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. The detailed experimental information can be found in the ESI.† As shown in Fig. 1, the yield of formic acid increased promptly as the reaction proceeded, and the yield reached up to 50% after only 10 min. It is probably because Zn has a strong thermodynamic driving force for oxide formation in high-temperature water. The morphology of zinc powder and the solid products
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PCCP Table 1
Yield of formic acid with NaHCO3 and gaseous CO2a
Entry NaHCO3/mmol CO2/mmol NaOH/mmol H2O
YHCOOH/%
1 2 3 4
64 0.3 0.1 6
1 1 — —
— — 1 1
— — — 1
With Without With With
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a
The reaction was carried out with 6 mmol Zn, Temp. 573 K, time 2 h, water filling 35%.
Fig. 1 Effect of reaction time on the yield of formic acid (1 mmol NaHCO3, 10 mmol Zn, 111 mmol H2O, Temp. 573 K).
after the reaction were characterized by SEM, as shown in Fig. 2. The reagent zinc powder before reactions was an aggregation of many small particles and had an irregular shape (Fig. 2A). Zinc powder was used for the reaction without any pre-treatment. However, the solid products after reactions were small flower-like tetrapod whiskers with a size of less than 10 mm (Fig. 2B). Thus, results of SEM analyses can further evidence that zinc powder was converted to ZnO after the reaction. Zinc was oxidised to ZnO after the reaction during the production of formic acid, which was determined by XRD analysis (see Fig. S2 in the ESI†). It should be noted that almost all of the Zn was oxidised to ZnO within 10 min. After 10 min, the increased rate of formic acid yield decreased significantly. When the reaction of Zn with H2O was almost finished, the yield of formic acid increased slowly. Thus, it can be concluded that the efficient reduction of CO2 should be primarily attributed to the reaction of Zn with H2O.
Fig. 2 SEM images of zinc powder before the reaction (A) and solid products after the reaction (B) (at 573 K for 10 min, 1 mmol NaHCO3; 6 mmol Zn; 111 mmol H2O).
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Furthermore, experiments with NaHCO3 or gaseous CO2 were performed to investigate the role of H2O and the effect of different carbon sources. As shown in Table 1, a yield of 64% formic acid was achieved in the presence of H2O (entry 1). However, only a small amount of formic acid was produced in the absence of H2O, (entry 2). When NaHCO3 was replaced by gaseous CO2, only a very small amount of formic acid was produced in the absence of NaOH (entry 3). If NaOH was added, a 6% yield was obtained (entry 4). These results suggested that formic acid cannot be produced directly from gaseous CO2, but can be produced easily in the presence of NaOH, i.e. under alkaline conditions. Subsequently, experiments were done with an additional dissolution process of gaseous CO2 at room temperature before the reaction started, to confirm the hypothesis that the dissolution of gaseous CO2 in NaOH solution is necessary to obtain a high yield of formic acid. As shown in Fig. 3, the yield of formic acid increased with the decrease of pH values, which were measured when the dissolution process finished. The pH value of the solution with 1 mmol NaOH and 1 mmol gaseous CO2 changed to 9.2, which was very close to the pH value of the 1 mmol NaHCO3 solution. Because the ratio balance of CO2, carbonate and bicarbonate in the solution changed depending on the pH value,26–28 1 mmol gaseous CO2, dissolved in 1 mmol NaOH solution, produced a similar amount of formic acid with 1 mmol NaHCO3. These results indicated that high yield of formic acid is not from gaseous CO2 directly, but from HCO3 . To investigate the effects of HCO3 and CO32 and metal cations on the yield of formic acid, experiments with KHCO3 and K2CO3 were performed. As shown in Fig. S3 (ESI†), formic acid can be produced in a high yield from HCO3 , and not from CO32 .
Fig. 3 Yield of formic acid with the dissolution of gaseous CO2 (1 mmol gaseous CO2, 1 mmol NaOH, 111 mmol H2O, 6 mmol Zn, 573 K, 2 h); dissolution of gaseous CO2 at room temperature.
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Furthermore, experiments with ZnO and gaseous H2 were conducted. Compared with the high yield of formic acid with Zn, the yield with ZnO and gaseous H2 was very low, only 13% (see Table S1 in the ESI†). It indicated that the reaction with ZnO and gaseous H2 was totally different with Zn. Next, two reactions with Zn and ZnO + H2 were compared from a thermodynamic point of view using available thermodynamic data,29 as shown in eqn (1) and (2). In eqn (1), the negative DG value suggests that a high yield of formic acid can be expected. However, in eqn (2), DG is positive, which may be the reason why the production of formic acid was not easy when using ZnO and gaseous hydrogen. Zn + H2O + NaHCO3 - ZnO + HCOONa + H2O DG (573 K) =
1
103.79 kJ mol
(1)
ZnO + H2 + NaHCO3 - ZnO + HCOONa + H2O DG (573 K) =
5.37 kJ mol
1
(2)
It has been reported that zinc hydride species (Zn–H) can be produced in the reaction of Zn with H2O at 700 K,30 which can reduce CO2 to formic acid as an active reducing agent.31–33 Herein, we speculated that Zn–H is a plausible intermediate that plays a crucial role in the highly efficient reduction of CO2 to formic acid. For ZnO, the formation of H–Zn O–H due to the chemisorption of H2 should be responsible for the production of formic acid. However, the coverage is only approximately 5–10% surface sites of ZnO.34–36 Therefore, it is easy to understand why the yield of formic acid with ZnO and gaseous H2 is very low. Herein, a possible reaction mechanism for the highly efficient production of formic acid with Zn is proposed. As shown in Scheme 1, in the first step, the Zn–Hd species is produced via the reaction of Zn and H2O. In the second step, Hd attacks the carbon centre Cd+, forming a bond with carbon as a transition state. In the final step, the OH species leave, forming HCOO (an SN2-like mechanism). To verify the proposed mechanism, we performed a theoretical DFT study focusing on the crucial intermediate (Zn–H) and the mechanism of the formic acid formation. We initially adopted a model system consisting of a Zn5 cluster with two H2O molecules for all the theoretical investigations, then by using a large cluster (Zn20) we confirmed the critical energetics (see Fig. S12 in ESI†). The Zn5 model contains all the three site types, i.e., step (linear),
Scheme 1
Proposed SN2-like mechanism for the formation of HCOO .
19838 | Phys. Chem. Chem. Phys., 2014, 16, 19836--19840
terrace (surface) and kink (vertex), which are essential for catalytic function from a geometrical point of view (see Scheme S1 in the ESI†). This model is of a suitable size to perform a systematic and exhaustive screening study. We performed two steps of theoretical calculations: (1) the formation of the plausible Zn–H intermediate, and (2) the formation of formic acid from this intermediate. We first optimized all possible conformations of the Zn5 cluster with all the possible fragments {H+, OH , H2O} generated through the fragmentation of two H2O molecules (see Table S2 in the ESI†). Adopting the free energy as the screening criteria, relative to the reference consisting of the optimized Zn5 cluster and the two non-fragmented H2O molecules, we showed the existence of the energetically favourite Zn–H intermediate at 573 K as a product of the Zn5 + 2H2O reactions. Details of geometries are reported in the ESI.† Next, the feasibility of the formic acid formation through this Zn–H intermediate was determined. The geometry of the transition state and the energy for the formic acid formation were calculated. The most stable form of Zn–H (see Fig. S7 in ESI†) was used in the TS search. Calculations starting from the other patterns are similar (see Fig. S8 in the ESI†). As shown in Fig. 4, the activation energy of the TS from the initial state (Zn–H and HCO3 ) is 24.1 kcal mol 1. The geometry and the Mulliken charge on the TS reflect the production of HCOO from HCO3 . The Zn–H bond distance is approximately 1.94 Å. The charge of H in the Zn–H species is 0.221. This charge is assigned to be the hydride and not to a proton. An important implication of the TS geometry and charge distribution is that this is an SN2-like reaction. As the hydride of Zn–H approaches the carbon of HCO3 , the OH separates from the carbon atom. Thus, as HCOO (i.e., formic acid) is formed, the OH is drawn to the Zn5 cluster, as shown in Scheme 1. The results of IRC calculations and the shapes of the HOMO and LUMO of the TS are shown in Fig. 5. As shown in Fig. 5(A), the IRC of the TS leads smoothly to the reactant Zn–H + HCO3 and to the product formic acid. The reaction energy barrier for the formation of formic acid from HCO3 was not very high, which indicates that once the Zn–H species was produced, formic acid could be easily produced. In Fig. 5(B), the occupied HOMO of the TS shows a bonding interaction between the carbon (C) atom of HCO3 and the hydrogen (H) atom of the
Fig. 4 The geometries of the TS (left) and the activation energy of Zn–H + HCO3 - Zn5–OH + HCOO (right).
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(No. 14JC1403100) and China Postdoctoral Science Foundation (No. 2013M541520).
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Notes and references
Fig. 5 IRC calculation (A) and HOMO and LUMO orbital shapes (B) of the TS.
Zn–H intermediate, whereas the unoccupied LUMO of the TS shows an anti-bonding character between the carbon of HCO3 and the OH (there is also an anti-bonding character in the LUMO of the Zn–H bond). The HOMO and LUMO of the TS clearly demonstrated that the CO2 reduction in the HCO3 dominant reactions followed an SN2-like mechanism. In conclusion, we provide new insights into highly efficient reduction of CO2 to formic acid with zinc under hydrothermal conditions by a combined experimental and computational study. The experimental investigation speculated that the Zn–H species, produced from the reaction of Zn and H2O, might be a plausible intermediate in the formation of HCOO due to its high reduction activity. The proposed mechanism was further validated by a DFT study. The quantum chemical calculations identified Zn–H species as a key intermediate. The newly elucidated reaction mechanism can be used to explain the highly efficient formation of the C–H bond of HCOO . From the mechanistic and kinetic points of view, the present findings provide new chemical insight into understanding the reduction of CO2 to formic acid with a metal under hydrothermal conditions. We anticipate that this study will promote the generation of a new concept in the field of CO2 utilization.
Acknowledgements The authors thank the financial support of the National Science Foundation of China (No. 21277091), Key Basic Research Projects of Science and Technology Commission of Shanghai
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1 W. Wang, S. P. Wang, X. B. Ma and J. L. Gong, Chem. Soc. Rev., 2011, 40, 3703. 2 C. A. Huff and M. S. Sanford, J. Am. Chem. Soc., 2011, 133, 18122. 3 B. Chan and L. Radom, J. Am. Chem. Soc., 2006, 128, 5322. 4 T. W. Woolerton, S. Sheard, E. Pierce, S. W. Ragsdale and F. A. Armstrong, Energy Environ. Sci., 2011, 4, 2393. 5 E. E. Barton, D. M. Rampulla and A. B. Bocarsly, J. Am. Chem. Soc., 2008, 130, 6342. 6 J. Horita and M. E. Berndt, Science, 1999, 285, 1055. 7 T. M. McCollom and J. S. Seewald, Geochim. Cosmochim. Acta, 2001, 65, 3769. 8 F. M. Jin and H. Enomoto, Energy Environ. Sci., 2011, 4, 382. 9 P. E. Savage, R. B. Levine and C. M. Huelsman, Thermochemical Conversion of Biomass to Liquid Fuels and Chemicals, ed. M. Crocker, Royal Society of Chemistry, 2009. 10 F. M. Jin, T. Moriya and H. Enomoto, Environ. Sci. Technol., 2003, 37, 3220. 11 G. Martin and L. Gabor, Energy Environ. Sci., 2012, 5, 8171. 12 J. Ferenc, ChemSusChem, 2008, 1, 805. 13 G. Tian, C. He, Y. Chen, H. M. Yuan, Z. W. Liu, Z. Shi and S. H. Feng, ChemSusChem, 2010, 3, 323. 14 H. Takahashi, T. Kori, T. Onoki, K. Tohji and N. Yamasaki, J. Mater. Sci., 2008, 43, 2487. 15 F. M. Jin, Y. Gao, Y. J. Jin, Y. L. Zhang, J. L. Cao, Z. Wei and R. L. Smith, Energy Environ. Sci., 2011, 4, 881. 16 A. Steinfeld, A. Meier, A. Weidenkaff and D. Wuillemin, Energy, 1998, 23, 803. 17 A. Steinfeld, A. Reller, R. Palumbo, J. Murray and Y. Tamaura, Int. J. Hydrogen Energy, 1998, 23, 767. 18 A. Steinfeld, Int. J. Hydrogen Energy, 2002, 27, 611. 19 F. He, J. Trainham, G. Parsons, J. S. Newman and F. X. Li, Energy Environ. Sci., 2014, 7, 2033. 20 http://www.nrel.gov/hydrogen/pdfs/development_solar-thermal_ zno.pdf. 21 H. Tachikawa, K. Iokibe, K. Azumia and H. Kawabatab, Phys. Chem. Chem. Phys., 2007, 9, 3978. 22 K. Iokibe, H. Tachikawa and K. Azumi, J. Phys. B: At., Mol. Opt. Phys., 2007, 40, 427. 23 J. P. Perdew, K. Burke and Y. Wang, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 16533. 24 C. Peng and H. B. Schlegel, Isr. J. Chem., 1993, 33, 449. 25 M. J. Frisch, et al., Gaussian 09, Revision B.1, Gaussian, Inc., Wallingford, CT, 2009. 26 Y. Maenaka, T. Suenobu and S. Fukuzumi, Energy Environ. Sci., 2012, 5, 7360. 27 T. M. Mccollom and J. S. Seewald, Geochim. Cosmochim. Acta, 2001, 65, 3769. 28 D. R. Lide, Handbook of Chemistry and Physics, CRC Press, BocaRaton, FL, 84th edn, 2004.
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¨thi, J. Chem. Phys., 2001, 114, 3949. 33 S. Tsuzuki and H. P. Lu 34 H. Hao, C. Cui, H. W. Roesky, G. Bai, H. G. Schmidt and M. Noltemeyer, Chem. Commun., 2001, 1118. 35 M. Uchiyama, S. Furumoto, M. Saito, Y. Kondo and T. Sakamoto, J. Am. Chem. Soc., 1997, 119, 11425. 36 A. B. Anderson and J. A. Nichols, J. Am. Chem. Soc., 1986, 108, 4742.
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19840 | Phys. Chem. Chem. Phys., 2014, 16, 19836--19840
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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 19836 Received 30th July 2014, Accepted 5th August 2014 DOI: 10.1039/c4cp03388d www.rsc.org/pccp
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New insights into highly efficient reduction of CO2 to formic acid by using zinc under mild hydrothermal conditions: a joint experimental and theoretical study† Xu Zeng,a Makoto Hatakeyama,b Koji Ogata,b Jianke Liu,c Yuanqing Wang,b Qi Gao,d Katsushi Fujii,e Masamichi Fujihira,d Fangming Jin*a and Shinichiro Nakamura*b
We report here a theoretical study with quantum chemical calculations based on experimental results to understand highly efficient reduction of CO2 to formic acid by using zinc under hydrothermal conditions. Results showed that zinc hydride (Zn–H) is a key intermediate species in the reduction of CO2 to formic acid, which demonstrates that the formation of formic acid is through an SN2-like mechanism.
The conversion of CO2 into chemicals or fuels has the potential to decrease the environmental impacts of greenhouse gas emissions and relieve the shortage of fossil-fuels.1 The hydrogenation of CO2 with gaseous hydrogen is currently regarded as the most commercially feasible method.2,3 However, this approach is an energyintensive process due to the high kinetic and thermodynamic stability of CO2, which requires either exotic catalysts or highpurity hydrogen. Recently, some more straightforward approaches have been investigated extensively, such as electrochemical and photocatalytic methods.4,5 Unfortunately, these strategies are still highly costly or less efficient. Therefore, the development of a novel highly efficient and economical approach for the conversion of CO2 into chemicals or fuels remains a big challenge. Hydrothermal reactions have played an important role in the formation of fossil fuels6,7 because of some unique inherent properties of high-temperature water, such as a high ion product and a low dielectric constant.8–10 High-temperature water acts not only as a reaction medium but also as a hydrogen source.
a
School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China. E-mail: [email protected] b RIKEN Research Cluster for Innovation Nakamura Laboratory, Wako, Saitama, 351-0198, Japan. E-mail: [email protected] c College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, China d Science and Technology Research Center, Mistsubishi Chemical Group, Yokohama, 227-8502, Japan e Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, 153-8904, Japan † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cp03388d
19836 | Phys. Chem. Chem. Phys., 2014, 16, 19836--19840
Hydrothermal reduction of CO2 to formic acid with iron has been reported;11–14 however, the yield is relatively low. Recently, we reported the production of formic acid from CO2 by using a series of metals, such as Zn, Al, Mn, etc.15 Especially, a high yield of formic acid (ca. 75 C%) with Zn was obtained. Considering that Zn could be regenerated from its oxidation state, ZnO, by using concentrated solar energy,16–19 which has shown high potential for large-scale demonstration of a novel solar chemical reactor,20 a highly efficient method for CO2 utilization with zinc through a Zn–ZnO cycle system by using solar energy could be realized. Although highly efficient reduction of CO2 to formic acid could be achieved, the detailed mechanism is still unknown because the reaction intermediates are difficult to be experimentally identified under hydrothermal conditions. Density functional theory (DFT) has always been used in the theoretical study of various types of chemical reactions, including the reaction of Zn with H2O.21–24 Therefore, in this study, a theoretical study was performed based on experimental results obtained by DFT. Unlike our previous study, which mainly reported the initial experimental data, this theoretical study provides basic information for understanding the reaction mechanism of highly efficient reduction of CO2 to formic acid, because it is hard to experimentally study the reaction mechanism, e.g. by an in situ technique, due to the reactions proceeding in high-temperature water. DFT calculations were adopted with the RPW91 density functional by using Gaussian 09,25 which considered van der Waals (VDW) interactions. Initially, experiments were conducted to study the effect of reaction time on the yield of formic acid. NaHCO3 was used as a CO2 resource to simplify handling. Zinc (powder, 200-mesh, of analytical grade) and NaHCO3 (99%, of analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. The detailed experimental information can be found in the ESI.† As shown in Fig. 1, the yield of formic acid increased promptly as the reaction proceeded, and the yield reached up to 50% after only 10 min. It is probably because Zn has a strong thermodynamic driving force for oxide formation in high-temperature water. The morphology of zinc powder and the solid products
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Yield of formic acid with NaHCO3 and gaseous CO2a
Entry NaHCO3/mmol CO2/mmol NaOH/mmol H2O
YHCOOH/%
1 2 3 4
64 0.3 0.1 6
1 1 — —
— — 1 1
— — — 1
With Without With With
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a
The reaction was carried out with 6 mmol Zn, Temp. 573 K, time 2 h, water filling 35%.
Fig. 1 Effect of reaction time on the yield of formic acid (1 mmol NaHCO3, 10 mmol Zn, 111 mmol H2O, Temp. 573 K).
after the reaction were characterized by SEM, as shown in Fig. 2. The reagent zinc powder before reactions was an aggregation of many small particles and had an irregular shape (Fig. 2A). Zinc powder was used for the reaction without any pre-treatment. However, the solid products after reactions were small flower-like tetrapod whiskers with a size of less than 10 mm (Fig. 2B). Thus, results of SEM analyses can further evidence that zinc powder was converted to ZnO after the reaction. Zinc was oxidised to ZnO after the reaction during the production of formic acid, which was determined by XRD analysis (see Fig. S2 in the ESI†). It should be noted that almost all of the Zn was oxidised to ZnO within 10 min. After 10 min, the increased rate of formic acid yield decreased significantly. When the reaction of Zn with H2O was almost finished, the yield of formic acid increased slowly. Thus, it can be concluded that the efficient reduction of CO2 should be primarily attributed to the reaction of Zn with H2O.
Fig. 2 SEM images of zinc powder before the reaction (A) and solid products after the reaction (B) (at 573 K for 10 min, 1 mmol NaHCO3; 6 mmol Zn; 111 mmol H2O).
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Furthermore, experiments with NaHCO3 or gaseous CO2 were performed to investigate the role of H2O and the effect of different carbon sources. As shown in Table 1, a yield of 64% formic acid was achieved in the presence of H2O (entry 1). However, only a small amount of formic acid was produced in the absence of H2O, (entry 2). When NaHCO3 was replaced by gaseous CO2, only a very small amount of formic acid was produced in the absence of NaOH (entry 3). If NaOH was added, a 6% yield was obtained (entry 4). These results suggested that formic acid cannot be produced directly from gaseous CO2, but can be produced easily in the presence of NaOH, i.e. under alkaline conditions. Subsequently, experiments were done with an additional dissolution process of gaseous CO2 at room temperature before the reaction started, to confirm the hypothesis that the dissolution of gaseous CO2 in NaOH solution is necessary to obtain a high yield of formic acid. As shown in Fig. 3, the yield of formic acid increased with the decrease of pH values, which were measured when the dissolution process finished. The pH value of the solution with 1 mmol NaOH and 1 mmol gaseous CO2 changed to 9.2, which was very close to the pH value of the 1 mmol NaHCO3 solution. Because the ratio balance of CO2, carbonate and bicarbonate in the solution changed depending on the pH value,26–28 1 mmol gaseous CO2, dissolved in 1 mmol NaOH solution, produced a similar amount of formic acid with 1 mmol NaHCO3. These results indicated that high yield of formic acid is not from gaseous CO2 directly, but from HCO3 . To investigate the effects of HCO3 and CO32 and metal cations on the yield of formic acid, experiments with KHCO3 and K2CO3 were performed. As shown in Fig. S3 (ESI†), formic acid can be produced in a high yield from HCO3 , and not from CO32 .
Fig. 3 Yield of formic acid with the dissolution of gaseous CO2 (1 mmol gaseous CO2, 1 mmol NaOH, 111 mmol H2O, 6 mmol Zn, 573 K, 2 h); dissolution of gaseous CO2 at room temperature.
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Furthermore, experiments with ZnO and gaseous H2 were conducted. Compared with the high yield of formic acid with Zn, the yield with ZnO and gaseous H2 was very low, only 13% (see Table S1 in the ESI†). It indicated that the reaction with ZnO and gaseous H2 was totally different with Zn. Next, two reactions with Zn and ZnO + H2 were compared from a thermodynamic point of view using available thermodynamic data,29 as shown in eqn (1) and (2). In eqn (1), the negative DG value suggests that a high yield of formic acid can be expected. However, in eqn (2), DG is positive, which may be the reason why the production of formic acid was not easy when using ZnO and gaseous hydrogen. Zn + H2O + NaHCO3 - ZnO + HCOONa + H2O DG (573 K) =
1
103.79 kJ mol
(1)
ZnO + H2 + NaHCO3 - ZnO + HCOONa + H2O DG (573 K) =
5.37 kJ mol
1
(2)
It has been reported that zinc hydride species (Zn–H) can be produced in the reaction of Zn with H2O at 700 K,30 which can reduce CO2 to formic acid as an active reducing agent.31–33 Herein, we speculated that Zn–H is a plausible intermediate that plays a crucial role in the highly efficient reduction of CO2 to formic acid. For ZnO, the formation of H–Zn O–H due to the chemisorption of H2 should be responsible for the production of formic acid. However, the coverage is only approximately 5–10% surface sites of ZnO.34–36 Therefore, it is easy to understand why the yield of formic acid with ZnO and gaseous H2 is very low. Herein, a possible reaction mechanism for the highly efficient production of formic acid with Zn is proposed. As shown in Scheme 1, in the first step, the Zn–Hd species is produced via the reaction of Zn and H2O. In the second step, Hd attacks the carbon centre Cd+, forming a bond with carbon as a transition state. In the final step, the OH species leave, forming HCOO (an SN2-like mechanism). To verify the proposed mechanism, we performed a theoretical DFT study focusing on the crucial intermediate (Zn–H) and the mechanism of the formic acid formation. We initially adopted a model system consisting of a Zn5 cluster with two H2O molecules for all the theoretical investigations, then by using a large cluster (Zn20) we confirmed the critical energetics (see Fig. S12 in ESI†). The Zn5 model contains all the three site types, i.e., step (linear),
Scheme 1
Proposed SN2-like mechanism for the formation of HCOO .
19838 | Phys. Chem. Chem. Phys., 2014, 16, 19836--19840
terrace (surface) and kink (vertex), which are essential for catalytic function from a geometrical point of view (see Scheme S1 in the ESI†). This model is of a suitable size to perform a systematic and exhaustive screening study. We performed two steps of theoretical calculations: (1) the formation of the plausible Zn–H intermediate, and (2) the formation of formic acid from this intermediate. We first optimized all possible conformations of the Zn5 cluster with all the possible fragments {H+, OH , H2O} generated through the fragmentation of two H2O molecules (see Table S2 in the ESI†). Adopting the free energy as the screening criteria, relative to the reference consisting of the optimized Zn5 cluster and the two non-fragmented H2O molecules, we showed the existence of the energetically favourite Zn–H intermediate at 573 K as a product of the Zn5 + 2H2O reactions. Details of geometries are reported in the ESI.† Next, the feasibility of the formic acid formation through this Zn–H intermediate was determined. The geometry of the transition state and the energy for the formic acid formation were calculated. The most stable form of Zn–H (see Fig. S7 in ESI†) was used in the TS search. Calculations starting from the other patterns are similar (see Fig. S8 in the ESI†). As shown in Fig. 4, the activation energy of the TS from the initial state (Zn–H and HCO3 ) is 24.1 kcal mol 1. The geometry and the Mulliken charge on the TS reflect the production of HCOO from HCO3 . The Zn–H bond distance is approximately 1.94 Å. The charge of H in the Zn–H species is 0.221. This charge is assigned to be the hydride and not to a proton. An important implication of the TS geometry and charge distribution is that this is an SN2-like reaction. As the hydride of Zn–H approaches the carbon of HCO3 , the OH separates from the carbon atom. Thus, as HCOO (i.e., formic acid) is formed, the OH is drawn to the Zn5 cluster, as shown in Scheme 1. The results of IRC calculations and the shapes of the HOMO and LUMO of the TS are shown in Fig. 5. As shown in Fig. 5(A), the IRC of the TS leads smoothly to the reactant Zn–H + HCO3 and to the product formic acid. The reaction energy barrier for the formation of formic acid from HCO3 was not very high, which indicates that once the Zn–H species was produced, formic acid could be easily produced. In Fig. 5(B), the occupied HOMO of the TS shows a bonding interaction between the carbon (C) atom of HCO3 and the hydrogen (H) atom of the
Fig. 4 The geometries of the TS (left) and the activation energy of Zn–H + HCO3 - Zn5–OH + HCOO (right).
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Notes and references
Fig. 5 IRC calculation (A) and HOMO and LUMO orbital shapes (B) of the TS.
Zn–H intermediate, whereas the unoccupied LUMO of the TS shows an anti-bonding character between the carbon of HCO3 and the OH (there is also an anti-bonding character in the LUMO of the Zn–H bond). The HOMO and LUMO of the TS clearly demonstrated that the CO2 reduction in the HCO3 dominant reactions followed an SN2-like mechanism. In conclusion, we provide new insights into highly efficient reduction of CO2 to formic acid with zinc under hydrothermal conditions by a combined experimental and computational study. The experimental investigation speculated that the Zn–H species, produced from the reaction of Zn and H2O, might be a plausible intermediate in the formation of HCOO due to its high reduction activity. The proposed mechanism was further validated by a DFT study. The quantum chemical calculations identified Zn–H species as a key intermediate. The newly elucidated reaction mechanism can be used to explain the highly efficient formation of the C–H bond of HCOO . From the mechanistic and kinetic points of view, the present findings provide new chemical insight into understanding the reduction of CO2 to formic acid with a metal under hydrothermal conditions. We anticipate that this study will promote the generation of a new concept in the field of CO2 utilization.
Acknowledgements The authors thank the financial support of the National Science Foundation of China (No. 21277091), Key Basic Research Projects of Science and Technology Commission of Shanghai
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