Highly Selective and Efficient Reduction of CO2 to Methane by

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Highly Selective and Efficient Reduction of CO2 to Methane by Activated Alkaline Earth Metal Hydrides Without a Catalyst Juan Zhao, Yin-Fan Wei, Yue-Ling Cai, Long-Zheng Wang, Ju Xie, Yun-Lei Teng, Wei Zhu, Ming Shen, and Bao-Xia Dong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05177 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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ACS Sustainable Chemistry & Engineering

Highly Selective and Efficient Reduction of CO2 to Methane by Activated Alkaline Earth Metal Hydrides Without a Catalyst Juan Zhao, Yin-Fan Wei, Yue-Ling Cai, Long-Zheng Wang, Ju Xie, Yun-Lei Teng*, Wei Zhu, Ming Shen and Bao-Xia Dong* School of Chemistry and Chemical Engineering, Yangzhou University, 180 Siwangting Avenue, Yangzhou, Jiangsu, 225002, P. R. China. *Corresponding

author. E-mail address: [email protected] (Y.-L. Teng); [email protected]

(B.-X. Dong).

ABSTRACT: Achieving highly selective and efficient reduction of carbon dioxide into methane will significantly affect the resolution of two of the current crucial issues facing humanity, namely environmental problems due to excess CO 2 and the increasing demand for clean energy. In this paper, the thermochemical reduction of carbon dioxide into methane by the activated alkaline-earth metal hydrides was reported. The results of the correlational experiments show that the reduction of carbon dioxide by the activated nanosized alkaline-earth metal hydrides is highly selective and efficient in the absence of a catalyst under moderate conditions. Only a hydrocarbon species, namely methane, is produced and the methane yield can reach 88% for the reactions between CaH 2 and carbon dioxide. The mole percentage and yield of CH 4 in the gas-state products depend largely on the type of alkaline-earth metal hydride, reaction temperature, and reaction time. KEYWORDS: Carbon dioxide; Methanation; Alkaline earth metal hydrides; Methane; Hydrogen

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INTRODUCTION For almost a century, with the development of industrialization, the burning of fossil fuels has produced large amounts of carbon dioxide (CO2) that exceeds the ability of the natural carbon cycle. The excess CO2, as a greenhouse gas, is accompanied with a series of ecological and environmental problems such as ocean acidification, the rise in sea-level, global warming, abnormal climatic changes, etc.1 These environmental issues and the growing need for clean energy render the capture and utilization of CO 2 imperative. CO2 can be used as an abundant, renewable, nontoxic, nonflammable, economical, and environmental C1 raw material that can be transformed into useful chemicals or fuels. 2-5 The artificial carbon cycle can decrease the dependence on fossil fuels, control the amount of CO2 being discharged into the air, and alleviate the greenhouse effect. The reduction of carbon dioxide into hydrocarbon fuels, which is considered one of the ideal alternative resources to fossil fuels, is a very promising method for dealing with excess CO2. It is well known that methane (CH4) carries a large amount of energy per mass, compared to gasoline. Moreover, since CH4 has the smallest C/H ratio, it releases the smallest amount CO 2 per unit of heat.6,7 Thus, CH4 is considered a promising clean fuel. However, the combustion efficiency of CH 4 is low, resulting in a narrow range of flammability, slow combustion rate, and high ignition temperature.8 These factors render the practical application of CH 4 in vehicles difficult. The disadvantages of CH4 as a fuel can be overcome by adding hydrogen to form hythane, which has a high flame speed and a short quenching distance.9,10 Hence, the mixture of hydrogen and methane (hythane) is a promising clean and efficient fuel. Achieving a highly selective and efficient conversion of carbon dioxide into methane will remarkable affect the resolution of two of the current crucial issues facing humanity, namely environmental problems due to excess CO2 and the increasing demand for clean energy. Many technologies including thermochemical, electrochemical, photochemical, and biological methods have been proposed for CO2 methanation.11–18 Most of the current approaches for achieving the thermochemical conversion of carbon dioxide to methane are based on the use of molecular hydrogen or the covalent hydrogen atoms of silane or hydrosilane as the hydrogen source in combination with a catalyst.19–23 Although the methanation of CO2 by molecular hydrogen is thermodynamically favorable, eight electrons are transferred in the process of the full reduction 2


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of carbon dioxide to methane, which results in a kinetic limitation and thus a high temperature as well as a noble metal-based catalyst to achieve an acceptable rate and selectivity. Recently, silane or hydrosilane has been used as a reductant, primarily because their polar and weaker Si‒H bond is easier to activate, compared with the H‒H bond of hydrogen.24–27 CO2 methanation by using reducing agents such as silane or hydrosilane under moderate conditions is an attractive synthesis method if it can be performed in a simple and easy-available apparatus. However, the availability of the reductant is a problem, and the downstream separation and potential recycling is needed for handling the by-products. Therefore, the development of technologically feasible and economically competitive methods to convert highly stable and inert CO2 to CH4 remains challenging. Compared with molecular hydrogen or the covalent hydrogen atom of silane, the negatively charged hydrogen atom of metal hydrides has a stronger reducibility, which may make the reduction of carbon dioxide to methane easier. Among the metal hydrides, magnesium and calcium hydrides, as excellent hydrogen storage materials, have gained much attention because they are abundant, cheap, and have high hydrogen capacities (7.6 and 4.7 wt%).28–31 Recently, our group found that, although the mechanochemical conversion of carbon dioxide to methane by magnesium and calcium hydrides at room temperature is highly selective, the productivity of methane is very low (6–15%).32 Therefore, it is necessary to explore a new method by which CO2 can be effectively reduced to CH4 by alkaline-earth metal hydrides. It has been widely reported that as-milled metal hydrides often show higher reactivity because the grain size of the as-milled metal hydrides becomes smaller (even to nanosize) and the surface becomes larger and fresh.33,34 However, little attention has been devoted to the thermochemical conversion of carbon dioxide into useful chemicals or fuels employing activated alkaline-earth metal hydrides as the hydrogen source. Therefore, it is interesting to investigate whether the activated alkaline-earth metal hydrides (MgH2 and CaH2) can effectively reduce carbon dioxide under heat treatment condition to produce hydrocarbon fuel or not. Herein, we, for the first time, report the thermochemical conversion of carbon dioxide into methane using the negatively charged hydrogen atom of activated alkaline-earth metal hydrides instead of molecular hydrogen or a covalent hydrogen atom as the hydrogen source under moderate conditions. The results of the correlational experiments show that the reduction of 3



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carbon dioxide by the activated alkaline-earth metal hydrides in the absence of a catalyst is highly selective and efficient. This work offers a novel, highly selective, and efficient approach for carbon dioxide methanation as well as in-situ preparation of the CO x-free mixture of hydrogen and methane fuel without catalysts under moderate conditions.

RESULTS AND DISCUSSION Dependence in temperature of reactions between activated alkaline-earth metal hydrides and CO2. Ball milling, which is a well-known novel synthetic technique for the preparation of nanomaterials in a non-equilibrium state, was employed to activate the alkaline-earth metal hydrides. As shown in the SEM images (Figure 1), the as-milled particles of the activated alkaline-earth metal hydrides are nearly oval, more uniform, and smaller than those of the raw samples. The average particle sizes of the as-milled MgH2 and CaH2 are reduced to 381.5 and 298.5 nm, respectively. It is believed that for the chemisorbed material, the smaller the grains, the higher is the activity. The XRD profile shows that there are no impurities in the activated alkaline-earth metal hydrides (Figure 1). The diffraction peaks of the activated alkaline-earth metal hydrides are broader and weaker than those of the raw samples, which disclose that the grain size of the activated alkaline-earth metal hydrides is in the nanometer scale. Further, BET measurements reveal that the specific surface areas of MgH2 and CaH2 after activation increased from 14.8 and 10.8 m2 g-1 to 27.1 and 56.4 m2 g-1, respectively (Figure 1), indicating that the activated alkaline-earth metal hydrides may display a better reactivity. To study the dependence in temperature of the reactions, the mechanically activated magnesium or calcium hydride was heated under a carbon dioxide atmosphere for 48 h at 200, 350, 450, and 550 ℃, respectively. Figure 2 presents the gas chromatograms and FTIR characterization of the gas-state products collected from the reactions. There is only a clear hydrogen chromatography peak at approximately 0.53 min in the gas chromatograms of the gaseous products collected from the reaction between MgH2 and CO2 at 200 ℃ (Figure 2a), whereas two gas chromatography peaks corresponding to hydrogen and methane are observed in the gas chromatograms of the gas-state products collected from the reaction between CaH2 and CO2 at 200 ℃ (Figure 2c). Although the peak of CH4 in the gas chromatograms of the gas-state products collected from the reaction of CaH2 with CO2 at 200 ℃ is weak, it is very wide. These 4


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results show that the activated CaH2 reacted with CO 2 at 200 ℃ to produce a mixture of hydrogen and methane, while the activated MgH2 reacted with CO2 at 200 ℃ to produce H2, indicating that the activated CaH2 has a higher activity for the conversion of carbon dioxide to methane compared with activated MgH2. When the reaction temperature was raised from 200 to 450 ℃, for both the MgH2–CO2 and CaH2–CO2 systems, the chromatography peaks of CH4 became increasingly stronger. The experimental results showed that the mole percentage and productivity of CH4 in the gas-state products increased with increasing reaction temperature from 200 to 450 ℃. However, the intensity and area of the CH 4 peaks became weaker and smaller when the reaction temperature was raised from 450 to 550 ℃ for the two systems, which indicates that the mole percentage and productivity of CH4 in the gas-state products do not increase with an increase in reaction temperature above 450 ℃. In other words, at 450 ℃, the maximum mole percentage and yield of CH4 were obtained from the reactions; therefore, the best result for methanation was achieved at 450 ℃ for both the MgH2–CO2 and CaH2–CO2 systems. To further analyze the components of the gas-state products obtained from the reactions between alkaline-earth metal hydrides and carbon dioxide at different temperatures, the gas-state products of the reactions between XH2 (X = Mg or Ca) and carbon dioxide at 200, 350, 450, and 550 ℃ for 48 h were characterized by gas-FTIR spectrometry, which can be used to determine whether CO2 reacted completely and whether CO or CxHy compounds were generated. Figure 2b shows the gas-FTIR characterization of the gas-state products collected from the reactions between MgH2 and carbon dioxide at different temperatures. For the gas-state products of the reaction of MgH2 with CO2 at 200 ℃, the absorption peaks as a result of CO2 are clearly detected at 3726, 3596, 2346, and 667 cm-1, while the absorption peaks of CH4 are very weak. This indicates that only hydrogen was produced, and a large amount of unreacted CO 2 was present in the gas-state products. For the gas-state products obtained at the reaction temperature of 350 ℃, the FTIR spectrum shows characteristic absorption peaks of CO2 as well as CH4 absorptions bands at 3084, 3013, 2936, 1344, 1300, and 1266 cm-1. Moreover, the characteristic absorptions of CO at 2172 cm-1 and 2115 cm-1 were also observed. These results obtained from FTIR characterization reveal that for the MgH2–CO2 system, although a large amount of CH 4 was produced from reactions taking place at 350 ℃, CO2 still remained and a by-product of CO was also produced. When the reaction temperature reached 450 or 550 ℃, only the absorptions of CH4 5



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were observed, which indicates that the gas-state products contain only a pure mixture of hydrogen and CH4 and that the reduction of carbon dioxide to methane by MgH2 at 450 or 550 ℃ is highly selective. Figure 2d shows the gas-FTIR characterization of the gas-state products collected from the reactions between CaH2 and carbon dioxide at different temperatures. In addition to the FTIR absorptions due to CO 2, CH4 FTIR absorptions were clearly observed at 200 ℃. When the reaction temperature reached 350 ℃ or above, only the characteristic infrared absorption peaks of methane were detected, indicating that the gas-state products were a pure mixture of hydrogen and CH4 and the reduction of carbon dioxide to methane by CaH 2 is highly selective. The selectivity for CH4 is nearly 100%. It can thus be seen that compared with the MgH2–CO2 system, the methanation reaction of the CaH2–CO2 system can proceed at a more moderate temperature, which is consistent with the results of GC analysis. The mole percentage and productivity of CH4 in the gas-state products collected from the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide at different temperatures for 48 h were calculated. As shown in Figure 3a and b, for the MgH2–CO2 system, the mole percentages of CH4 in the gas-state products are ca. 0, 23, 81, and 66%, and the productivities of CH4 are ca. 0, 17, 68, and 52% when the reaction temperatures are 200, 350, 450, and 550 ℃, respectively. As can be seen from Figure 3a and b, almost no CH4 generated at 200 ℃, and the mole percentage and yield of CH4 reached their maximum values at 450 ℃, which is consistent with the results of GC and gas-FTIR analyses. For the CaH2–CO2 system, the mole percentages of CH4 are ca. 22, 53, 89, and 64%, and the productivities of CH4 are ca. 20, 40, 88, and 60% when the reaction temperatures are 200, 350, 450, and 550 ℃, respectively (Figure 3c and d), which is also consistent with the results of GC and gas-FTIR analyses. Experiments prove that methane will decompose at 550 oC in our reaction system (Figure S2), which leads to the decrease of methane yield at 550 oC. The iron in the stainless steel tube might act as a catalyst, promoting the decomposition of methane.35 Moreover, the presence of part hydrogen is beneficial for methane decomposition.36 Therefore, the productivity of methane passes over a maximum at 450 ℃. According to the results, it can be seen that the mole percentage and productivity of CH4 for the CaH2–CO2 system are higher than those of the MgH 2–CO2 system at each of the temperatures, which indicates that the methanation effect of the CaH 2–CO2 system is superior to that of the MgH2–CO2 system. Therefore, the activated alkaline-earth metal hydrides 6


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can efficiently and selectively reduce CO 2 to CH4 producing H2 and CH4, which indicates that the MgH2/CaH2–CO2 reaction systems are auspicious for the storage and in situ preparation of fuels that are H2–CH4 mixtures. What is really exciting is that the mole percentage and productivity of CH4 in the gas-state products of the CaH2–CO2 system at 450 ℃ are 89% and 88%, respectively, which indicate that this system has excellent methanation performance. It is worth noting that the methanation effects of the two systems are greatly affected by the reaction temperature. Compared with the mechanochemical reactions between XH2 (X = Mg or Ca) and carbon dioxide, the thermochemical reactions between XH2 (X = Mg or Ca) and carbon dioxide gave much higher productivity of CH4.32 Dependence in time of reactions between activated alkaline-earth metal hydrides and CO2. The reactions of the activated magnesium and calcium hydride with carbon dioxide at 450 ℃ for various times were studied to understand the dependence of the reactions on time. The mechanically activated MgH2 or CaH2 was heated under a 0.25-MPa carbon dioxide pressure at 450 ℃ for 1, 24, and 48 h. The chromatography peaks of H2 and CH4 were clearly observed in the gas chromatograms of the gas-state products collected from each methanation reaction, as shown in Figure 4a and c. Additionally, the intensity of the CH4 peak increased with increasing reaction time, which indicated that the effect of methanation is proportional to the reaction time for the two systems (MgH2–CO2 and CaH2–CO2). The gas-state products collected from the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide at 450 ℃ for 1, 24, and 48 h were also characterized by gas-FTIR spectrometry. In addition to FTIR absorptions due to CH4, weak absorptions due to CO were observed at 2172 and 2115 cm-1 in the FTIR characterization of the gas-state products collected from the reaction between MgH2 and carbon dioxide at 450 ℃ for 1 h; however, no absorption due to CO was detected in the FTIR spectrum of the gas-state products collected from the reaction between CaH2 and carbon dioxide at 450 ℃, as shown in Figure 4b and d. This further demonstrates that the methanation effect of the CaH2–CO2 system is superior to that of the MgH 2–CO2 system. It should be noted that no CO2 absorptions were detected for all the reactions carried out at 450 ℃ for 1–48 h, which discloses that CO2 reacted completely after 1 h of reaction. For the reaction time of 24 h or above, only the absorption bands of CH 4 can be observed. This indicates that a pure H2 and CH4 mixture was produced in the MgH2–CO2 and CaH2–CO2 systems and the 7



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reduction of carbon dioxide to methane by the mechanically activated XH2 (X = Mg or Ca) is highly selective. The mole percentages and productivities of CH4 in the gas-state products collected from the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide at 450 ℃ as a function of reaction time are shown in Figure 5. The methane mole percentages in the gas-state products collected from the reactions between MgH2 and carbon dioxide at 450 ℃ for 1, 24, and 48 h are ca. 41, 70, and 81% and the productivities of CH4 are ca. 43, 63, and 68%, respectively. For the CaH2–CO2 system, the mole percentages of CH4 in the gas-state products obtained within 1, 24, and 48 h are 61, 85, and 89%, and the productivities of CH4 are 52, 85, and 88%, respectively. These results are consistent with the results of GC and gas-FTIR analyses, which indicate that the mole percentage values and productivities of CH4 for the MgH2–CO2 and CaH2–CO2 systems increase with increasing reaction time. However, the mole percentages and productivities of CH4 in the gas-state products of the CaH2–CO2 system are higher than those in the gas-state products of the MgH2–CO2 system under the same reaction conditions, which suggests that the methanation effect of the CaH2–CO2 system is superior to that of the MgH2–CO2 system. Dependence in pressure of reactions between activated alkaline-earth metal hydrides and CO2. Finally, the reactions between the activated XH2 (X = Mg or Ca) and carbon dioxide under different pressures were investigated to understand the effect of carbon dioxide pressure on the methanation reaction. The gas chromatograms and FTIR characterization of the gas-state products collected from the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide at 450 ℃ for 48 h under 0.1, 0.25, and 0.5 MPa are presented in Figure 6a and c, in which only the chromatography peaks of hydrogen and methane are clearly observed. Additionally, the areas of the CH4 peaks have no obvious differences. As shown in the FTIR spectra of the gas-state products (Figure 6b and d), only obvious methane absorption peaks are observed, which further confirms that the methanation reactions were highly selective. The mole percentages and productivity of CH4 in the gas-state products of the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide at 450 ℃ as a function of reaction pressure are presented in Figure 7. The mole percentages of CH4 in the gas-state products obtained from the reactions of activated MgH 2 with CO2 at 450 ℃ for 48 h under 0.1, 0.25, and 0.5 MPa are 79, 81, and 79%, and the productivities of CH4 are 63, 68, and 67%, respectively. 8


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For the CaH2-CO2 system, the mole percentages of CH4 in the gas-state products collected from the reactions at 450 ℃ for 48 h under 0.1, 0.25, and 0.5 MPa are 82, 89, and 82%, and the productivities of CH4 are 83, 88, and 86%，respectively. The results suggest that the influence of pressure on the methanation reaction is not very obvious for the MgH 2–CO2 and CaH2–CO2 systems. These experimental data also display that although both the MgH2–CO2 system and CaH2–CO2 system exhibit excellent methanation performances under different CO 2 pressures, the CaH2–CO2 system gives higher productivities of CH4 than the MgH2–CO2 system does under the same reaction conditions. Methanation mechanism of the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide. To explaining the thermochemical methanation reaction mechanisms of the MgH2–CO2 and CaH2–CO2 systems, the solid-state products collected during the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide were characterized by XRD. Figure 8a and c show the XRD characterization of the solid-state products collected from the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide at 200, 350, 450, and 550 ℃ for 48 h. Only the diffraction peaks of MgH2 were detected after the reaction of MgH2 with CO2 at 200 ℃ (Figure 8a), which indicates that for the MgH2–CO2 system, a reaction temperature below 200 ℃ is unfavorable for the methanation reaction. These results are consistent with those of the GC and gas-FTIR analyses. For the reaction temperature of 350 ℃, the diffraction peaks assigning to MgH2 are weaker, and new diffraction peaks corresponding to Mg and MgO are detected. Moreover, only the diffraction peaks of MgO were observed in the XRD characterization of the solid-state products collected from the reactions between MgH2 and carbon dioxide at 450 or 550 ℃. This indicates that MgH2 was completely consumed in the reaction performed at 450 ℃ for 48 h. The generated Mg at 350 ℃ may result from the decomposition of MgH 2, and may further react with CO2 to generate MgO with an increase in reaction temperature. The diffraction peaks of CaH2 along with new diffraction peaks of CaO were clearly observed in the XRD profile of the solid-state product collected during the reaction between CaH2 and carbon dioxide at 200 ℃ (Figure 8c), which indicates that CaH2 can react with CO2 to produce CaO at 200 ℃. As mentioned above, the GC and gas-FTIR spectra prove that CaH2 can react with CO2 at 200 ℃ to generate CH4. New weak diffraction peaks corresponding to CaCO 3 appeared at 350 ℃; however, only diffraction peaks of CaO were observed when the reaction temperature reached 9



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450 ℃ or above. The XRD profiles of the solid-state products collected from the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide at 450 ℃ for 1, 24, and 48 h are shown in Figure 8b and d. Only the diffraction peaks of Mg and MgO can be observed in the XRD profiles of the solid products obtained from the reaction of MgH 2 with CO2 at 450 ℃ for 1 h, which indicates that MgH2 was completely consumed in the reaction performed at 450 ℃ for 1 h (Figure 8b). At the reaction time of 24 h or above, only diffraction peaks belonging to MgO were observed, disclosing that MgO is the main component in the solid-state products. For the CaH2–CO2 system, the diffraction peaks of CaH2, CaCO3, and CaO were observed in the XRD profile of the solid-state product collected from the reaction performed at 450 ℃ for 1 h (Figure 8d). Further, only diffraction peaks corresponding to CaO were observed in the XRD profiles of the solid-state products collected from the reactions between CaH2 and carbon dioxide at 450 ℃ for 24 or 48 h, displaying that CaH2 was completely consumed and CaO is the main component in the solid-state products. Figure S3 shows the XRD characterization of the solid-state products collected from the reactions between activated XH 2 (X = Mg or Ca) and carbon dioxide at 450 ℃ for 48 h under 0.1, 0.25, and 0.5 MPa. Only diffraction peaks corresponding to MgO and CaO appeared, which further confirmed that the activated alkaline-earth metal hydride was completely consumed and alkaline-earth metal oxides (white in color) are the main component in the solid-state products collected during the reactions between XH2 (X = Mg or Ca) and carbon dioxide at 450 ℃ for 24-48 h. Figure 9a and b show the colors of the raw XH2 (M = Mg or Ca) samples, the activated XH2 samples, and the solid products obtained from the reactions of activated XH2 with carbon dioxide at 450 ℃ for 48 h. The raw samples are white or gray, but become black after the reaction, indicating the existence of amorphous carbon in the final solid-state products. Therefore, XPS was used to investigate the chemical environment of carbon in the solid-state products collected from the reactions between XH2 (X = Mg or Ca) and carbon dioxide. As shown in Figure 9c and d, the presence of C 1s peaks at 284.5 eV belonging to agraphitic carbon verifies the formation of agraphitic carbon in the solid-state products collected from the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide. Therefore, H2 and CH4 are the main components in the gas-state products and amorphous carbon-doped XO is the main component in the solid-state products collected during the thermochemical reactions between activated XH2 (X = Mg or Ca) 10


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and carbon dioxide. The gas-state products (H2 and CH4 fuel mixture) can be used as a conventional fuel or for vehicular application, and the solid products (amorphous carbon-doped XO) are good candidates for CO2 capture37–39 (Figure 10). Based on the above-mentioned experimental results, the methanation mechanism for the thermochemical reactions of the activated XH2 (M = Mg or Ca) with CO2 is proposed as follows: 2XH2 (X = Mg or Ca) + CO2 → 2XO + C + 2H2 → 2XO + CH4

(1)

In the MgH2–CO2 and CaH2–CO2 systems, the methanation mechanism is basically the same, which is shown in Eq. (1). CO2 was first reduced by XH2 (X = Mg or Ca) to produce elemental C, XO, and hydrogen. Then, elemental C reacted with hydrogen to generate methane. The reactions of CaH2 with 13C-labbelled CO2 at 450 ℃ for 48 h were performed and the gas products were analyzed by gas FTIR. As shown in Figure S4, the FTIR spectra show that the 13C-labelled methane is obtained because the absorption of the gas product shows red-shifts compared with the 12C-labelled methane, which proves the carbon of methane produced is from CO2. It has been proved that carbon and H2 can react to produce CH4.40,41 Calderón et al. reported that carbon radical may act as the active species for the hydrogenation of carbon to methane. 42 According to the literatures, hydrogen may be activated by the carbon radical or metal surface of the stainless steel tube.43,44 Therefore, it is proposed that CH4 is produced by the reaction of elemental C with hydrogen produced in situ. We tentatively describe how the XO is formed from XH2 on an elementary level. Figure S5 shows the schematic diagram of possible reaction steps from XH2 to XO. Firstly, two calcium atoms of the calcium hydride molecules simultaneously interact with the two oxygen atoms of the CO 2 molecule, forming intermediate 1. Then, two hydrogen atoms of the calcium hydride molecules interact with the carbon atom of the CO2 molecule, forming intermediate 2. At last, the left two hydrogen atoms continuously interact with the carbon atom of the CO2 molecule, forming XO, amorphous carbon, and hydrogen. To understanding the roles of XO in the process of the hydrogenation of carbon to methane, the reactions of C with H2 at 450 ℃ for 48 h in the absence or presence of CaO were performed. The experimental results indicate that the presence of XO is helpful to the formation of methane (Figure S6). The methanation mechanism for the thermochemical reactions of the activated XH2 (X = Mg or Ca) with CO2 is similar to that of their mechanochemical reactions reported previously.32 11



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It was turned out that carbon dioxide methanation can be occurred by thermochemical reduction employing activated alkaline-earth metal hydrides as the hydrogen source, while some previous research demonstrated that CO 2 can be converted into CH 4 utilizing H 2 gas as the hydrogen source (Sabatier reaction). 3,12 The mechanism of carbon dioxide methanation by H 2 gas has been established, which involves the conversion of carbon dioxide to carbon monoxide and the subsequent carbon monoxide methanation reaction. The reaction we proposed includes two steps: CO 2 reduced to C by activated alkaline earth metal hydride with strong reducibility, then carbon reacts with hydrogen to produce methane. The carbon dioxide methanation mechanisms of the two approach may be varying because two forms of the hydrogen source were used (one is the negatively charged hydrogen atom and the other is molecular hydrogen.). Fig ure 10 compares the mechanisms of CO 2 methanation for the two methods. Both the methods require two steps to reduce CO 2 to CH 4 completing the transmission of eight electrons. Due to the different reducibilities of the two hydrogen sources, in the first step, CO 2 was reduced at different levels. For the XH 2 –CO 2 system, in the first step, CO 2 was reduced from +4 to 0 valence producing amorphous carbon, but for the H 2 –CO 2 system, CO 2 was reduced from +4 to +2 valence producing CO 2 because molecular hydrogen has a lower reducibility. For the XH 2-CO 2 system, in the second step, amorphous carbon was reduced by H 2 from 0 to -4 valence producing CH 4 with the transmission of four electrons, whereas for the H 2 –CO 2 system, CO was reduced by H 2 from +2 to -4 valence producing CH 4 with the transmission of six electrons; this indicates that the methanation reaction in the XH 2 –CO 2 system is easier than that in the H 2 –CO 2 system because in the second step, the transmission of fewer electrons is needed to achieve methanation in the XH 2 –CO 2 system. From the point of view of CO 2 methanation, in comparison of the Sabatier reaction, carbon dioxide methanation through solid-gas thermochemical reactions between activated alkaline-earth metal hydrides and carbon dioxide possesses the following merits: (i) H 2 is replaced by safe metal hydrides; (ii) the catalyst-free CO 2 methanation reaction is highly selective and efficient; (iii) inexpensive reaction equipment is required; (iv) no useless products are generated, that is, both the gas and solid products are very useful. 12


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CONCLUSIONS We have demonstrated an efficient method for carbon dioxide methanation and synthesizing the COx-free mixture of methane and hydrogen fuel using cheap, abundant, and easily available alkaline-earth metal hydrides as the hydrogen source. The results of the correlational experiments shows that the reduction of carbon dioxide by the activated nanosized magnesium and calcium hydride is highly selective and efficient in the absence of a catalyst under moderate conditions because CH4 is the sole hydrocarbon product, and the productivity of CH4 can reach 88% for the reactions carried out at 450 ℃ for 48 h. The mole percentage and productivity of CH4 is largely determined by the type of alkaline-earth metal hydride, reaction temperature, and reaction time. The methanation effect of the CaH 2–CO2 system is superior to that of the MgH2–CO2 system. Further, analyses of the gas and solid products produced indicate that CH4 is produced by the reaction of elemental carbon with hydrogen produced in situ. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. Experimental section; Energy density, production cost and energy efficiency analysis of the XH2 (X = Mg or Ca)-CO2 system; Figure S1, schematic diagram of the reaction between the activated XH2 (X = Mg or Ca) and CO2; Figure S2, gas chromatogram of the gas-state products collected from the decomposition of methane at 550 ℃ for 48 h; Figure S3, XRD characterization of the solid products for the reactions between the XH2 (X = Mg or Ca) and CO2 (XH2/CO2 = 2) at 450 ℃ for 48 h under 0.1, 0.25 and 0.5 MPa; Figure S4, FTIR spectra of pure CH4 (red) and the gas products collected from the reaction of CaH 2 with 13C-labbelled CO2 at 450 ℃ for 48 h (green); Figure S5, The schematic diagram of possible reaction steps from XH 2 to XO on an elementary level; Figure S6, FTIR spectra of the gas products collected from the reaction of C with H2 at 450 ℃ for 48 h in the absence or presence of CaO; Figure S7, the path of CaH2 reacting with CO2 and regeneration cycle; Table S1, enthalpy change of each reaction involved in the reduction of CO2 and the CaH2 regeneration (PDF) 13



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AUTHOR INFORMATION Corresponding author *E-mail for Y.-L. Teng: [email protected]. *E-mail for B.-X. Dong: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors appreciate the financial support by grants from the National Natural Science Foundation of China (Nos. 21573192 and 21671169), SRF of SEM for ROCS, RFDP (No. 20133250120008), the Six Talent Peaks Project in Jiangsu Province (No. 2015-XNY-011), and the Foundation from the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Figure 1. Characterizations of XH2 (Mg or Ca): SEM micrographs of raw MgH2 (a), as-milled MgH2 (b), raw CaH2 (c) and as-milled CaH2 (d); XRD profiles of raw and as-milled XH2 (Mg or Ca) samples (e); N2 sorption isotherms at 77 K of raw MgH 2 (f), as-milled MgH2 (g), raw CaH2 (h) and as-milled CaH2 (i).

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Figure 2. Gas chromatograms and FTIR characterization of the gas-state products collected during the reactions between activated XH2 (X = Mg or Ca) and CO2 (0.25 MPa, XH2/CO2 mole ratio = 2) at 200, 350, 450, and 550 ℃ for 48 h.

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Figure 3. Mole percentages and productivity of CH4 in the gas-state products collected from the reactions between activated MgH2 (a and b) or CaH2 (c and d) and carbon dioxide at different temperature for 48 h.

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Figure 4. Gas chromatograms and FTIR characterization of the gas-state products collected during the reactions between activated XH 2 (X = Mg or Ca) and CO2 (0.25 MPa, XH2/CO2 mole ratio = 2) at 450 ℃ for 1, 24, and 48 h.

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Figure 5. Mole percentages and productivity of CH4 in the gas-state products collected from the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide at 450 ℃ for various reaction time.

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Figure 6. Gas chromatograms and FTIR characterization of the gas-state products collected during the reactions between activated XH 2 (X = Mg or Ca) and CO2 (XH2/CO2 mole ratio = 2) at 450 ℃ for 48 h under 0.1, 0.25, and 0.5 MPa.

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Figure 7. Mole percentages and productivity of CH4 in the gas-state products collected from the reactions between activated XH2 (X = Mg or Ca) and carbon dioxide at 450 ℃ for 48 h under different carbon dioxide pressure.

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Figure 8. XRD characterization of solid-state products collected from the reactions between MgH2 (a) or CaH2 (c) and CO2 at different temperature for 48 h and the reactions between MgH2 (b) or CaH2 (d) and CO2 at 450 ℃ for various reaction time.

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Figure 9. Color of raw XH2 (X = Mg or Ca) samples, as-milled XH2 samples, and solid-state products collected from the reactions between as-milled XH2 and carbon dioxide at 450 ℃ for 48 h (a,b). XPS characterization of elemental C in the solid-state products collected from the reactions between as-milled XH2 and carbon dioxide at 450 ℃ for 48 h (c, d).

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Figure 10. Methanation reactions of alkaline-earth metal hydrides with CO2 (a) and methanation reactions of molecular hydrogen with CO2 (b).

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CO2 is selectively and efficiently reduced to methane by activated nanosized alkaline-earth metal hydrides without a catalyst.

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Highly Selective and Efficient Reduction of CO2 to Methane by

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