CuxO Hydrogen Storage Composite with Defect-Rich

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Energy, Environmental, and Catalysis Applications 2

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MgH/CuO hydrogen storage composite with defectrich surfaces for carbon dioxide hydrogenation Haipeng Chen, Pei Liu, Jiaqi Li, Yuanjie Wang, Chenxing She, JinQiang Liu, Lin-Bao Zhang, Qingfeng Yang, Shixue Zhou, and Xun Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11285 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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ACS Applied Materials & Interfaces

MgH2/CuxO hydrogen storage composite with defectrich surfaces for carbon dioxide hydrogenation Haipeng Chen,†,# Pei Liu,§,# Jiaqi Li,† Yuanjie Wang,† Chenxing She,† Jinqiang Liu,† Linbao Zhang,‡ Qingfeng Yang,¶ Shixue Zhou*,§ and Xun Feng*,†

†

Henan Key Laboratory of Function-Oriented Porous Materials, Luoyang Normal

University, Luoyang 471934, China ‡

State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and

Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China §

State Key Laboratory of Mining Disaster Prevention and Control Co-founded by

Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, Qingdao 266590, China ¶

State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical

Engineering, Ningxia University, Yinchuan 750021, China

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ABSTRACT Thermal conversion of CO2 to value-added chemicals is challenging by the extreme inertness of CO2 molecule and the low selectivity of products. We reported a defect-rich MgH2/CuxO hydrogen storage composite from mechanochemical ball milling for catalytic hydrogenation of CO2 to lower olefins. The defect-rich MgH2/CuxO hydrogen storage composite achieves a C2= - C4= selectivity of 54.8% and a CO2 conversion of 20.7% at 350 oC under a low H2/CO2 ratio of 1/5, which increases the efficiency of H2 utilization by offering lattice H- species for hydrogenation. Density functional theory (DFT) calculations show that the defective structure of MgH2/CuxO can promote CO2 molecule adsorption and activation, while the electronic structure of MgH2 is beneficial for offering lattice H- for CO2 molecule hydrogenation. The lattice H- can combine the C site of CO2 molecule promoting the formation of Mg formate, which can be further hydrogenated to lower olefins under a low H- concentration. This work for CO2 conversion by the defect-rich MgH2/CuxO hydrogen storage composite can inspire the catalysts design for hydrogenation CO2 to lower olefins. KEYWORDS

Carbon dioxide, Hydrogenation, Hydrogen storage composite, Lower

olefins, Defective structure


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1. INTRODUCTION To convert CO2 from a detrimental gas into value-added chemicals, e.g., CH3OH, C2H5OH, lower olefins and aromatic hydrocarbons not only contributes to mitigating CO2 emissions, but also provides a feasible avenue for carbon recycling and energy storage.12

Current methods for CO2 conversion mainly include thermal-, electro- or photo-

reduction by heterogeneous or homogeneous catalysis.3-5 As a common method for catalytic conversion,6-13 the heterogeneous catalysis is the most in-depth investigated and developed one for CO2 conversion due to its high handling capacity and easy operation.14,15 However, this thermal conversion of CO2 to value-added chemicals is a challenging task, because it requires high Gibbs energy input (the Gibbs formation energy ΔG0298.15 K = -394.4 kJ mol-1) and has to overcome high kinetic energy barriers.16,17 To activate this thermodynamically stable molecule, an electron donor is usually used when considering CO2 as an electrophile or a Lewis acid.18 The CO2 conversion can be increased using catalysts with electron-donating ligands on the metal active sites.19-21 The hydrogen source for CO2 conversion is usually gaseous H2, which can be cracked into H ions or H radicals. The activation of H2 is of importance for CO2 hydrogenation by their different influences on intermediate products. Negative H- tends to combine with the C atom of CO2 molecule, positive H+ tends to combine with the O atom, while H radicals have no specific selectivity. The activation of H2 molecule can also improve its utilization efficiency during CO2 hydrogenation, which is of importance for saving H2 resources, increasing the economy of operation. If the catalyst can provide active sites for the adsorption and activation of CO2, and also can activate H2 molecule to specific H- or H+, the hydrogenation performance of CO2 should be improved. For CO2 adsorption, the introduction of defective structure on



nanoscale catalyst should be desirable, because this can decrease the coordination of metals facilitating gaseous molecules adsorption and activation.22-29 As to H2 activation, an option is the utilization of metal-based hydrogen storage material with lattice H- for hydrogenation.30,31 The metal hydrides often have a high catalytic activity in the hydrogenation of ethene, synthesis of ammonia, and hydrogenation of organic liquids.3234

Using Lattice H- as the hydrogen source for CO2 hydrogenation is different to that using

molecular H2 of traditional methods, which should exhibit new hydrogenation characteristics and be of importance for exploring efficient catalysis for CO2 conversion. Herein, we report a defect-rich MgH2/CuxO composite from mechanochemical ball milling for CO2 hydrogenation. This composite is composed of CuxO and MgH2, in which the former is full of oxygen-vacancy (Ovac) defects and Mg/O-heteroatom defects which act as active sites for CO2 capture and activation, while the latter offers lattice H- for CO2 hydrogenation. The structure and adsorption reaction of this composite for CO2 hydrogenation are studied using a combination of experimental measurements and density functional theory (DFT) calculations. 2. EXPERIMENTAL SECTION 2.1. Composite preparation. The MgH2/CuxO hydrogen storage composite was prepared by a three-step ball-milling method. All ball-milling processes were performed on a ND81L model adjustable swing ball-mill (Nanda Tianzun Instrument Company, China) with four 250 ml vials. Typically, Mg powders were hydrided by reactive ball milling under H2 atmosphere with carbon as milling aid. The carbon preparation follows the reported method.31 Each vial of the ball-mill was charged with 8.0 g Mg (>99.0%), 2.0 g carbon and 1.0 MPa H2 (>99.99 vol%), and milling conditions were ball to sample mass ratio 28:1, milling speed 270 rpm and milling time 3.0 h. The second step was CuxO


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preparation from raw CuO by mechanochemical reduction with Mg as reducing agent under Ar atmosphere. Each vial of the ball-mill was charged with 9.0 g CuO (>99.0%) and 1.0 g Mg (>99.0%), and milling conditions were ball to sample mass ratio 28:1, milling speed 180 rpm and milling time 1.0 h. The third step was to compose MgH2 and CuxO under Ar atmosphere by a low-energy ball milling, in which the two materials were mixed together under mass ratio MgH2/CuxO = 8/2, and then ball milled for 10 min with ball to sample mass ratio 14:1 and milling speed 90 rpm. 2.2. CO2 hydrogenation. The hydrogenation of CO2 was performed using a fixed-bed reactor consisting of a stainless-steel tube with an internal diameter of 8 mm, and typically 0.5 g of MgH2/CuxO composite were charged in the reactor. Prior to CO2 hydrogenation, the feed gas H2/CO2/N2 (containing 75 vol% N2 as the balance gas) was charged into the reactor with a total flow rate of 40 cm3 min-1 (GHSV = 2400 h-1) and pressured gradually to 1.0 MPa, and the reactor system was then heated up at a ramp rate of 10 oC min-1 to the reaction temperature. All the products after reaction were introduced in a gaseous state and analyzed with two online gas chromatograms (GC). The hydrogenation performance of the MgH2/CuxO composite after at least 2.0 h on stream was used for discussion. For CO2 hydrogenation over bulk MgH2 without H2 atmosphere, the reaction conditions were 0.5 g of MgH2, CO2/N2 = 1/4, total flow rate of 40 cm3 min-1, pressure of 1.0 MPa, GHSV = 2400 h-1 and 2.0 h. 2.3. Characterization methods. X-ray diffraction (XRD) was performed on a Rigaku D/Max-rB X-ray diffractometer (Cu Kα radiation) operating at 40 kV and 40 mA using a scanning speed of 2

o

min-1 and in steps of 0.02o. Transmission electron microscopy

(TEM) and selected area electron diffraction (SAED) were performed on a JEOL JEM-



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2100F electron microscope operating at 200 kV. Differential scanning calorimetry (DSC) was performed on a TA Q2000 instrument with a N2 flow rate 50 ml min-1 and a heating rate 10 °C min-1. De-/re-hydriding of MgH2 were carried out on a Setaram PCTPro-2000 instrument at 310 oC with H2 pressure of 1.5 MPa (hydriding) and 0.01 MPa (dehydriding).

Temperature-programmed

desorption

of

CO2

(CO2-TPD)

and

temperature-programmed reduction of H2 (H2-TPR) experiments were carried out on an AutoChem II 2720 chemisorption analyser equipped with a thermal conductivity detector (TCD). For the CO2-TPD experiment, 50.0 mg of sample were loaded in a quartz U-tube and pre-treated with He flow at 100 oC for 0.5 h and then cooled down to 50 oC to clean moisture and other adsorbed gases. After that, the sample was saturated in pure CO2 for 1.0 h for adsorption and then purged with He flow for 1.0 h. Then, the sample was heated up with a heating rate of 10 oC min-1 in a He flow of 25 ml min-1 for detection. For the H2-TPR experiment, 50 mg of sample were loaded in a quartz U-tube and heated with 10 oC

min-1 under 10 vol.% H2 in Ar. X-ray photoelectron spectroscopy (XPS) was

performed on a Thermo Escalab 250Xi spectrometer with monochromatic Al Kα excitation (1486.6 eV). Hydrocarbons were analyzed using a GC system equipped with a KB-Al2O3/Na2SO4 capillary column and a flame ionization detector (FID), while CO2 was analyzed using a TDX-01 packed column and a thermal conductivity detector (TCD). 2.4. Computational models and methods. All of the DFT calculations were carried out with a CASTEP package in Materials Studio 7.0 (Accelrys Software Inc.).35 Both Cu2O and Cu supercells were constructed with (2  2  6) (111) slabs. The Ovac defective Cu2O(111) was constructed by removing an O atom on the outmost layer of the clean one, while the Mg/O-heteroatom defective Cu(111) was constructed by replacing two neighbouring Cu atoms with Mg and O atoms on the outmost layer of the clean one. The


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vacuum space was set at 20 Å as the periodic boundary condition. A 4 × 4 × 4 MonkhorstPack k-mesh was used for Cu2O and Cu unit cells optimization, while a 4 × 4 × 1 Monkhorst-Pack k-mesh was set for Cu2O(111) and Cu(111) supercells optimization. The MgH2 supercell was constructed with a (2  2  6) (001) slabs. Mg-vacancy (Mgvac) defective MgH2(001) was constructed by removing a Mg atom on the outmost layer of the clean one, and vacuum space was set at 20 Å as the periodic boundary condition. A 7 × 7 × 7 Monkhorst-Pack k-mesh was used for MgH2 unit cell optimization, while a 4 × 4 × 1 Monkhorst-Pack k-mesh was set for MgH2(001) supercell optimization. The core electrons of Cu and O were treated with the ultrasoft pseudopotential, while the exchange-correlation effects of valence electrons were described via the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) function.36 The Hubbard model from the DFT + U method was used to describe the correlated electrons of Cu2O,37,38 and the calculated band structures of Cu2O satisfy the computational requirements (Figure S1, supporting information). The Kohn-Sham wave functions were expanded with 440 eV for cutoff energy and a (48 × 48 × 48) mesh for fast Fourier transformation. For all calculations, the convergence criteria were set as energy change per atom 5.0  10-6 eV, maximum force 0.01 eV Å-1, maximum stress 0.02 GPa, and maximum displacement 5.0  10-4 Å. Transition states were calculated by using the complete LST/QST method, and the minimum energy paths (MEPs) for CO2 on MgH2(001) was calculated by using the Nudged-Elastic-Band (NEB) method.39,40 3. RESULTS AND DISCUSSION To explore the possibility of lattice H- for CO2 hydrogenation, bulk MgH2 was prepared by thermal hydriding of Mg at 330 oC under 2.0 MPa H2, then used for CO2 hydrogenation. Typically, bulk MgH2 has a high dehydriding temperature. For the MgH2 from Mg



hydriding, as shown in Figure 1A, the dehydriding peak temperature is 428.2 oC and initial temperature is 419.8 oC, which are consistent with other reports with similar conditions.41,42 The XRD pattern of bulk MgH2 before dehydriding is shown in the inset of Figure 1A, in which Mg (ICDD 35-0821) and MgH2 (ICDD 12-0697) are obvious and their abundances are calculated to be, respectively, 30.5 wt% and 69.5 wt% by the Rietveld refinement (Table S1, supporting information). This result suggests that the MgH2 crystal is the main phase while the Mg core is hard to be hydrided under this condition. CO2 hydrogenation by bulk MgH2 was performed in temperature range 225350 oC, which are lower than its dehydriding temperature, insuring the lattice H- for CO2 hydrogenation rather than gaseous H2 from MgH2 decomposition. As shown in Figure 1B, three hydrocarbons, i.e., CH4, C2H6 and C2H4, are obtained. With the increase of temperature, CH4 decreases while C2H6 and C2H4 selectivity increase, suggesting that the C2 hydrocarbons dominate in products at high temperatures. Although the selectivity is high, the CO2 conversion is low at these reaction temperatures, which is only 3.7% at 350 oC.

The low CO2 conversion should be ascribed to the low H- concentration on MgH2

surface, because bulk MgH2 usually has a big crystallite size that causes a long distance for H- diffusion from the inner to the crystal surface. A hypothesis is proposed that the CO2 conversion can be increased by increasing lattice H- concentration on MgH2 surface.


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Figure 1. (A) DSC curve of bulk MgH2 from Mg hydriding at 330 oC under 2.0 MPa H2, and inset shows its XRD pattern. (B) GC curves from 2.0 h CO2 hydrogenation over bulk MgH2 in temperature range 225-350 oC under 1.0 MPa CO2/N2 gas flow (CO2/N2 = 1/4), and inset shows the corresponding product distribution and CO2 conversion.

To verify this hypothesis, H2 atmosphere is introduced to increase the Hconcentration on MgH2 surface on the fact that the dissociation of H2 molecule on Mg surface can offer lattice H- (Figure S2, supporting information). Due to H2 introduction, CO2 conversion shows an obvious increasing trend, and even achieves 29.1% at 350 oC (Figure S3A, supporting information), which is 25.4% higher than that without H2 atmosphere. However, the product selectivity has changed when using H2



atmosphere, in which C2 species decreases, while CH4 increases and dominates in the products. Herein, the increase of H- concentration on MgH2 surface by introducing H2 can be attributed to the asymmetric conditions of de-/re-hydriding. For MgH2, the hydriding is usually prior to dehydriding. For Mg hydriding, both hydriding time and cycling times can increase the H- concentration in Mg lattice (Figure 2A), and the H2 content even achieves >1.0 wt% after 9 cycles. However, at the same temperature (310 oC), MgH

2 dehydriding reactions are negligible, and the H2 released from MgH2 are below

0.15 wt% (Figure 2B), verifying that the hydriding reaction is prior to dehydriding. The partial pressure of H2 in CO2 hydrogenation is 0.2 MPa, under which the H- concentration on MgH2 surface can be maintained by the solid solution α-MgH2 and crystalline βMgH2.43-45 This can be verified by the XRD pattern of the MgH2 after CO2 hydrogenation (Figure S3B, supporting information). Above results verify that CO2 can be converted into hydrocarbons using lattice H- as hydrogen source.


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Figure 2. (A) Hydriding performance of Mg from 1-9 cycles at 310 oC under 1.50 MPa H2. (B) Dehydriding performance of MgH2 from 1-9 cycles at 310 oC under 0.01 MPa H2.

Although the lattice H- can achieve CO2 hydrogenation, the selectivity and conversion is unsatisfactory comparing with traditional methods. To improve this, a defect-rich MgH2/CuxO hydrogen storage composite is designed, in which CuxO from mechanochemical reduction is expected to offer active sites for CO2 capture and activation, while MgH2 from reactive ball milling gives lattice H- for CO2 hydrogenation. The DSC curve of the composite shows that the dehydriding peak temperature is 348.3 oC

and initial temperature is 312.8 oC (Figure 3A). The XRD pattern of the composite

before dehydriding is shown in the inset of Figure 3A, in which MgH2 (ICDD 12-0697),



Cu (ICDD 04-0836) and Cu2O (ICDD 05-0667) are obvious and their abundances are calculated to be, respectively, 82.2, 3.4 and 14.4 wt% by the Rietveld refinement (Table S1, supporting information). HRTEM observation of Figure 3B shows that several small and irregular crystal domains are obvious. These crystal domains with lattice fringes of 0.22 and 0.24 nm can be indexed to MgH2(200) and Cu2O(111) planes, respectively. The wide diffraction rings of SAED patterns (inset of Figure 3B) suggest that the crystals have a nano-crystalline structure, confirming the HRTEM observation result. The performance of CO2 hydrogenation over the MgH2/CuxO hydrogen storage composite under different H2/CO2 ratios are shown in Figure 3C. Under a high H2/CO2 ratio 10:1, the product is mainly CH4 with a selectivity of 93.6%. By reducing H2/CO2 ratios, the selectivity for CH4 decreases, while the selectivity for lower olefins (C2= - C4=) shows an obvious increasing trend. Under a low H2/CO2 ratio 1:10, the selectivity for C2= - C4= achieves 53.9% while C10 - C30 is 46.1%, showing a better selectivity for lower olefins. As to CO2 conversion, it achieves 27.2% under H2/CO2 ratio 10:1, while decreases to 7.2% when H2/CO2 ratio reduces to 1:10, suggesting an obvious decreasing trend. This performance of CO2 hydrogenation with different H2/CO2 ratios can be ascribed to the different concentration of lattice H- on MgH2 surface. Under a high H2 partial pressure, it can offer sufficient lattice H- for CO2 conversion, thus the saturated hydrocarbons dominate in the products. When the lattice H- concentration is at a low level, the carbon chain can grow, thus C2= - C4= gradually increases. Moreover, this low concentration of lattice H- together with the low Cu content of composite weakens the reverse water-gas shift reaction46-48 and the generation of alcohol or carboxylic products.49,50 Besides, the temperature can influence the product selectivity and CO2


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conversion. With the increase of temperature, as shown in Figure 3D, the selectivity for C2= - C4= shows an increasing trend, and it achieves 54.8% at 350 oC. The CO2 conversion increases from 7.3% (250 oC) to 20.7% (350 oC) under H2/CO2 =1/5. Comparing with metal oxide/zeolite composite catalyst, this method shows a comparable conversion performance but a lower C2= - C4= selectivity.16 Comparing with the Fe-based catalyst, although the C2= - C4= selectivity is comparability, the CO2 conversion is still at a low level.51 However, traditional methods usually require a H2/CO2 ratio higher than 1/1. If it is taken H2/CO2 = 3/1 for calculation, the utilization efficiency of H2 over the MgH2/CuxO composite under H2/CO2 = 1/5 should be 4.5 times that of the traditional methods, suggesting that the lattice H- can significantly increase the efficiency of H2 utilization.

Figure 3. (A) DSC curve and (B) HRTEM image of MgH2/CuxO composite, and insets of (A, B) show its XRD pattern and SAED pattern, respectively. CO2 conversion and product distribution of CO2 hydrogenation for (C) different H2/CO2 ratios at 300 oC and (D) different temperatures under H2/CO2 = 1/5 over MgH2/CuxO composite with gas flow



40 cm3 min-1, GHSV = 2400 h-1 and 1.0 MPa.

To clarify the mechanism of MgH2/CuxO composite on CO2 hydrogenation, the structure changes of the composite before and after CO2 hydrogenation were investigated. Elemental mapping of CuxO from 1.0 h ball milling shows that Cu, O and Mg are distributed evenly with contents of 73.6, 19.1 and 7.3 wt%, respectively (Figure S4A, supporting information). HRTEM image shows that CuxO has several crystal domains with lattice fringes of 0.24 and 0.21 nm, corresponding to d-spacing of Cu2O(111) and Cu(111) respectively (Figure S4C, supporting information). Comparing with that of raw CuO, these crystal domains are small and irregular, suggesting defective structures are generated on CuxO surface by ball milling. XRD patterns of CuxO in Figure S5 (supporting information) show that CuO, Cu2O and Cu crystals are obvious from mechanochemical reduction of CuO with Mg as reducing agent, but no Mg-based crystals are detected. This suggests that the oxidation products of Mg should exist in the form of amorphous structures. Elemental mapping of the spent CuxO composite after CO2 hydrogenation show that C, Cu, O and Mg are obvious, and their contents are 5.6, 75.0, 10.4 and 9.0 wt%, respectively (Figure 4A). XRD patterns of the spent composite show that MgH2 and Cu are the main crystal phase of the spent MgH2/CuxO composite, while Cu2O is negligible (Figure 4B). H2-TPR analysis shows that the initial reduction temperature of CuxO is 241.5 oC, while the TPD analysis of MgH2 shows that its decomposition temperature is as high as 368.4 oC (Figure S6, supporting information). Figure 4C shows that lattice fringes of 0.22 and 0.21 nm corresponding to d-spacing of MgH2(200) and Cu(111) are obvious. Above results can verify the reduction of CuxO during CO2 hydrogenation, in


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line with other reports.52-54 Although the CuxO has reduced to Cu during CO2 hydrogenation, the high O content on the spent MgH2/CuxO composite may suggest that there are stable oxygen-related structures in the composite. It is proposed a hypothesis that this oxygen-related structure is a kind of an intermediate structure of “Cu-O-Mg” from mechanochemical reduction when considering the high O content and the amorphous Mg in the CuxO.

Figure 4. (A) EDS mapping and elemental content of the spent CuxO from 2.0 h CO2 hydrogenation with conditions of gas flow 40 cm3 min-1, GHSV = 2400 h-1, 1.0 MPa, 350 oC

and H2/CO2 =1/5. (B) XRD patterns for the spent MgH2/CuxO composite and the raw

CuxO. (C) HRTEM image of the spent MgH2/CuxO composite.

To verify above hypothesis, XPS analysis was performed on the MgH2/CuxO composite before and after CO2 hydrogenation. For raw MgH2/CuxO, the Cu 2p3/2 peak in period of 930-938 eV can be decomposed into two peaks of 932.4 and 933.8 eV, corresponding to Cu0 species (30.8 at%) and Cu+ species (35.3 at%), respectively (Figure



5A).55 For the spent MgH2/CuxO, Cu0 species and Cu+ species are also obvious, and Cu0 species increases to 52.1 at% while Cu+ species reduces to 13.5 at%, confirming the reduction of CuxO during CO2 hydrogenation. For both raw and spent MgH2/CuxO composites, these Cu0 and Cu+ species can be confirmed by the Auger peaks from Cu LMM spectra with kinetic energies at 918.0 and 916.6 eV,56 respectively (Figure 5B). The corresponding O 1s spectra of raw and spent MgH2/CuxO composite both show three peaks (Figure 5C): lattice oxygen species (Olatt) in range of 528-532 eV, surface oxygen species (Osur) in range of 529-534 eV belonging to defective oxide or the surface oxygen ions, and adsorbed oxygen species (Oads) in range of 532-535 eV.57 Due to CO2 hydrogenation, Olatt species decreases from 29.1 at% to 15.2 at%, while Osur species increases from 66.8 at% to 83.4 at%, suggesting the Osur species dominate on the surface of spent MgH2/CuxO composite. The coexistence of Cu+ and Osur species on the surface of the spent MgH2/CuxO composite should support the “Cu-O-Mg” structure from mechanochemical reduction, which can be further verified by the Mg 2p spectra. As shown in Figure S7 (Supporting information), for the CuxO from 1.0 h ball milling with 10 wt% Mg as milling aid, the three peaks at 51.1, 49.8, and 49.1 eV are from Mg, MgO and “Cu-O-Mg” structure, respectively.58 After CO2 hydrogenation, these three peaks are also obvious especially for that of “Cu-O-Mg”. This “Cu-O-Mg” structure can be recognized as Mg/O-heteroatom defects on Cu surfaces, and this defective structure should influence the performance of CO2 hydrogenation of the composite. CO2-TPD curves show two desorption peaks (α peak and β peak) for both raw and spent MgH2/CuxO composites (Figure 5D). The α peak weakens after CO2 hydrogenation while the β peak shows no significant change, which suggest that the α peak should relate to the reduction of Cu+ to Cu0 during CO2 hydrogenation, while the β peak should relate to the defective


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structures (Mg/O-heteroatom defects) from mechanochemical ball milling. The strong β peak suggests that the spent MgH2/CuxO can still adsorb CO2 molecules, and this adsorption is mainly due to the defective structures rather than the Cu0 from Cu+ reduction.

Figure 5. XPS spectra of (A) Cu 2p, (B) Cu LMM and (C) O 1s for the raw and spent MgH2/CuxO composite from 2.0 h CO2 hydrogenation. (D) CO2-TPD profiles of the raw and spent MgH2/CuxO composite.

Based on above structural analysis, it is concluded that the MgH2/CuxO is composed of Cu2O, Cu and MgH2 crystals with defective structures for CO2 hydrogenation. Herein, three typical defective structures, i.e., the Cu2O(111) with Ovac defect, the Cu(111) with Mg/O-heteroatom defect for simulating “Cu-O-Mg” structure, and the MgH2(001) with Mgvac defect, are constructed to explore the reaction mechanism of the MgH2/CuxO composite. All simulations are based on the first-principles calculations of density functional theory.59 Comparing with that of defective structures, there is no obvious



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orbital hybridization observed for CO2 on clean Cu2O(111) (Figure 6A1). With the Ovac defect introduction, the CO2 adsorption performance is enhanced, and the hybridizations for O* 2p (CO*2) and Cu 3d orbitals at -6.46, -4.53, -3.84 and 1.01 eV are obvious (Figure 6A2). The bond length for C=O* (L1) stretches from 1.16 Å to 5.71 Å (Table S2, supporting information), while no obvious hybridizations for O* 2p (CO*2) and C 2p, C 2s are found (Figure 6A3), suggesting the activation of CO2. For the Cu(111) surface, the clean one shows weak adsorption performance, while that with Mg/O-heteroatom defect shows a strong adsorption for CO2 molecule. The hybridizations between O 2p, 2s (CO2) and Mg 2p orbitals at -10.16 and -8.91 eV are obvious, while C 2p (CO2) and O* 2p orbitals at -8.91 eV is obvious (Figure 6B2, B3), suggesting that the “Cu-O-Mg” structure on Cu(111) is beneficial for the CO2 adsorption. The adsorption energy (Eads) for CO2 on CuxO surfaces can be calculated as follows: Eads = Etotal  ECO2  Esurf

(1)

where Etotal is the total energy of a specific supercell after CO2 adsorption; ECO2 is the total energy of a CO2 molecule; Esurf is the total energy of a specific surface. From calculation, the Eads for clean Cu2O(111), Ovac defective Cu2O(111), clean Cu(111) and Mg/O-heteroatom defective Cu(111) are -0.12, -1.70, -0.16 and -0.25 eV (Table S2, supporting information), verifying that the defective structure can enhance the CO2 adsorption performance on CuxO surfaces.


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Figure 6. DFT calculations for CO2 adsorption on clean and defective CuxO. (A1) Deformation charge density maps for CO2 adsorption on clean Cu2O(111) and that with Ovac defect. (A2, A3) Partial density of states (PDOS) for CO2 adsorption on Cu2O(111) with Ovac defect. (B1) Deformation charge density maps for CO2 adsorption on clean Cu(111) and that with Mg/O-heteroatom defect. (B2, B3) PDOS for CO2 adsorption on Cu(111) with Mg/O-heteroatom defect.

The electronic structure of clean MgH2 and that with Mgvac defect are calculated to evaluate MgH2 surface offering lattice H- for CO2 hydrogenation. As shown in Figure S8A (supporting information), MgH2(001) has a weak ability for electron transfer, in which Mulliken atomic charge values for H and Mg are, respectively, -0.42 and 0.85, and



band gap is up to 3.18 eV. By introducing Mgvac defect, as shown in Figure S8B (supporting information), band gap reduces to 2.88 eV, and Mulliken atomic charge values for H and Mg improves to, respectively, -0.34 and 0.80, suggesting the improvement of electron transfer and the weakening of Mg-H bond. This improvement should facilitate the adsorption of CO2 molecule and the promotion lattice H- for CO2 hydrogenation. The adsorption distance for C-H (D1) and O*-Mg (D2) on Mgvac defective MgH2(001) is calculated to be, respectively, 2.77 and 2.27 Å (Table S3, supporting information), both lower than those on clean surfaces. From Figure 7A1,B1, the adsorption of CO2 molecule on MgH2(001) is horizontal adsorption, in which the C atom combines with H while the O atom combines with Mg. The hybridizations for C 2p and H 1s orbitals at -7.49 eV, C 2s and H 1s orbitals at 5.46 eV, and O 2s, O 2p and Mg 3s orbitals at -9.10 eV are obvious (Figure 7A2B2). It is noted that this orbital hybridization should promote the formation of Mg formate, which is resulting from the Lewis acid characteristic of the C atom of CO2 molecule.


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Figure 7. (A1, B1) Deformation charge density isosurface maps and (A2, B2) PDOS for CO2 adsorption on MgH2(001), where (A) and (B) show clean and Mgvac defective MgH2(001), respectively. The gray and yellow indicate electron enrichment region and depletion region, respectively.

MEPs for the formation of Mg formate from CO2 on clean and defective MgH2(001) are shown in Figure 8. The calculated adsorption energies for CO2 on clean and defective MgH2(001) are -0.06 and -0.11 eV, suggesting that the defective surface is more favorable for CO2 adsorption. The energy barriers for the formation of Mg formate on clean MgH2(001) is 0.31 eV, and that is 0.17 eV on defective MgH2(001), which suggests that the defective surface can promote the formation of Mg formate. As an important intermediate product, the formate is beneficial for further hydrogenation to hydrocarbons.60-62 When the lattice H- concentration is at a low level on MgH2 surfaces, the Mg formate can be further hydrogenated to lower olefins by the growth of carbon chain. Based on the experimental and theoretical analysis, the characteristics of CO2 hydrogenation over the defective MgH2/CuxO composite can be summarized into three aspects: (i) mechanochemical ball milling can generate defective structures on MgH2/CuxO composite which promote the capture and activation of CO2 molecule; (ii) CO2 hydrogenation by lattice H- of MgH2 promoting the formation of Mg formate; (iii) the low lattice H- concentration on MgH2 surface facilitates the hydrogenation of Mg formate to lower olefins. Overall, the defect-rich MgH2/CuxO composite exhibits an outstanding performance for hydrogenation of CO2 to lower olefins with a high H2 utilization by using lattice H-.



Figure 8. MEPs for CO2 hydrogenation to Mg formate on clean and defective MgH2(001).

4. CONCLUSIONS MgH2/CuxO hydrogen storage composite for CO2 hydrogenation can be synthesized using a three-step ball-milling method, in which reactive ball milling converts metallic Mg into MgH2 under H2 atmosphere, while mechanochemical reduction converts CuO into CuxO. This defect-rich MgH2/CuxO hydrogen storage composite can achieve hydrogenation of CO2 to lower olefins with a selectivity of 54.8% and a CO2 conversion of 20.7% at 350 oC

under a low H2/CO2 ratio of 1/5, which significantly increases the efficiency of

H2 utilization comparing with traditional methods. DFT calculations provide clear evidence that Ovac and Mg/O-heteroatom defects on CuxO surfaces can act as active sites for CO2 molecule adsorption and activation, while Mgvac defects on MgH2 surfaces can facilitate the lattice H- for CO2 hydrogenation. The horizontal adsorption of CO2 on MgH2 can promote the formation of Mg formate, and low lattice H- concentration can facilitate the carbon chain growth for generating lower olefins. This improvement of CO2 conversion by the defect-rich MgH2/CuxO hydrogen storage composite can inspire the catalyst design for hydrogenation of CO2 to lower olefins.


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ASSOCIATED CONTENT Supporting Information Band structure for Cu2O; deformation charge density maps for H2 dissociation on Mg(0001); CO2 conversion and product distribution for CO2 hydrogenation over bulk MgH2; EDS mapping, elemental content and HRTEM image of CuxO and raw CuO; XRD patterns of CuxO from different milling time; H2-TPD profile for MgH2 and H2-TPR profile for CuxO from ball milling; XPS spectra of Mg 2p for raw CuxO and spent MgH2/CuxO composite; band structure and deformation charge density maps for clean and Mgvac defective MgH2(001); crystalline parameters and abundances for bulk MgH2 and MgH2/CuxO hydrogen storage composite; theoretical results for CO2 molecule on CuxO and MgH2 surfaces.

AUTHOR INFORMATION Corresponding Author *E-mail

for X. Feng: [email protected]

*E-mail

for S.X. Zhou: [email protected]

Author Contributions #These

authors contributed equally to this work

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (U1610103, 21671114, U1804131), the SDUST Research Fund (2014TDJH105), the Foundation for Science & Technology Innovation Talents in Henan province (164100510012), the Scientific & Technological Project of Henan province



(182102210431, 182102310897), the Foundation of State Key Laboratory of Highefficiency Utilization of Coal and Green Chemical Engineering (2018-K32), and Shenzhen Supercomputer Center.

REFERENCES (1) Jia, J.; Qian, C.; Dong, Y.; Li, Y. F.; Wang, H.; Ghoussoub, M.; Butler, K. T.; Walsh, A.; Qzin, G. A. Heterogeneous Catalytic Hydrogenation of CO2 by Metal Oxides: Defect Engineering-perfecting Imperfection. Chem. Soc. Rev. 2017, 46, 4631-4644. (2) Larmier, K.; Liao, W. C.; Tada, S.; Lam, E.; Verel, R.; Bansode, A.; Urakawa, A.; Comas-Vives, A.; Coperet, C. CO2-to-methanol Hydrogenation on Zirconia-supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal-support Interface. Angew. Chem. Int. Ed. 2017, 56, 2318-2323. (3) Porosoff, M. D.; Yan, B.; Chen, J. G. Catalytic Reduction of CO2 by H2 for Synthesis of CO, Methanol and Hydrocarbons: Challenges and Opportunities. Energy Environ. Sci. 2016, 9, 62-73. (4) Rao, H.; Schmidt, L. C.; Bonin, J.; Robert, M. Visible-light-driven Methane Formation from CO2 with a Molecular Iron Catalyst. Nature 2017, 548, 74-77. (5) Kattel, S.; Ramírez, P. J.; Chen, J. G.; Rodriguez, J. A.; Liu, P. Active Sites for CO2 Hydrogenation to Methanol on Cu/ZnO Catalysts. Science 2017, 357, 1296-1299. (6) Hou, C.; Wang, J.; Du, W.; Wang, J.; Du, Y.; Liu, C.; Zhang, J.; Hou, H.; Dang, F.; Zhao L.; Guo, Z. One-pot Synthesized Molybdenum Dioxide-molybdenum Carbide Heterostructures Coupled with 3D Holey Carbon Nanosheets for Highly Efficient and Ultrastable Cycling Lithium-ion Storage. J. Mater. Chem. A 2019, 7, 13460-13472. (7) Sun, H.; Yang, Z.; Pu, Y.; Dou, W.; Wang, C.; Wang, W.; Hao, X.; Chen, S.; Shao,


Page 24 of 32

Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Q.; Dong, M.; Wu, S.; Ding, T.; Guo, Z. Zinc Oxide/vanadium Pentoxide Heterostructures with Enhanced Day-night Antibacterial Activities. J. Colloid Interf. Sci. 2019, 547, 40-49. (8) Zhao, W.; Li, X.; Yin, R.; Qian, L.; Huang, X.; Liu, H.; Zhang, J.; Wang, J.; Ding, T.; Guo, Z. Urchin-like NiO-NiCo2O4 Heterostructure Microsphere Catalysts for Enhanced Rechargeable Non-aqueous Li-O2 Batteries. Nanoscale, 2019, 11, 50-59. (9) Cheng, Y. J.; Wang, R.; Wang, S.; Xi, X. J.; Ma, L. F.; Zang, S. Q. Encapsulating [Mo3S13]2- Clusters in Cationic Covalent Organic Frameworks: Enhancing Stability and Recyclability by Converting a Homogeneous Photocatalyst to a Heterogeneous Photocatalyst. Chem. Commun. 2018, 54, 13563-13566. (10) Zhao, Y.; Deng, D. S.; Ma, L. F.; Ji, B. M.; Wang, L. Y. A New Copper-based MetalOrganic Framework as a Promising Heterogeneous Catalyst for Chemo- and Regioselective Enamination of β-ketoesters. Chem. Commun. 2013, 49, 10299-10301. (11) Wang, L.; Liu, J.; Guo, H.; Deng, D. S.; Yao, T.; Wang, X. W.; Wang, X. Y.; Wu, H. L. Three Naphthalenedisulfonate Polymers with Imidazole-containing Ligands: Synthesis, Structure and Heterogeneously Catalytic Performance in Reactions of Enamination of β-dicarbonyl Compounds. Inorg. Chim. Acta. 2015, 430, 253-260. (12) Li, Z. H.; Xue, L. P.; Qin, Q. P.; Zhang, J.; Wang, J. M.; Zhang, X. Y.; Zhao, B. T. A Zinc(II) Coordination Polymer Material with Lewis Basic Pyridyl Sites: Structure, Photoluminescence and Heterogeneous Catalysis. J. Solid State Chem. 2019, 274, 81-85. (13) Li, Z. H.; Chen, J.; Su, W. P.; Hong, M. C., Pd/MgO: A Recyclable, Ligand-free Heterogeneous Catalyst for Decarboxylative Cross-coupling of Arene Carboxylic Acids with Aryl Halides. Acta Chim. Sinica. 2014, 72, 552-556. (14) Guo, Y.; Mei, S.; Yuan, K.; Wang, D. J.; Liu, H. C.; Yan, C. H.; Zhang, Y. W. Low-



temperature CO2 Methanation over CeO2-supported Ru Single Atoms, Nanoclusters and Nanoparticles Competitively Tuned by Strong Metal-support Interactions and H-spillover Effect. ACS Catal. 2018, 8, 6203-6215. (15) Satthawong, R.; Koizumi, N.; Song, C. S.; Prasassarakich, P. Light Olefin Synthesis from CO2 Hydrogenation over K-promoted Fe-Co Bimetallic Catalysts. Catal. Today 2015, 251, 34-40. (16) Li, Z. L.; Wang, J. J.; Qu, Y. Z.; Liu, H. L.; Tang, C. Z.; Miao, S.; Feng, Z. C.; An, H. Y.; Li, C. Highly Selective Conversion of Carbon Dioxide to Lower Olefins. ACS Catal. 2017, 7, 8544-8548. (17) Wei, J.; Ge, Q. J.; Yao, R. W.; Wen, Z. Y.; Fang, C. Y.; Guo, L. S.; Xu, H. Y.; Sun, J. Directly Converting CO2 into a Gasoline Fuel. Nat. Commun. 2017, 8, 15174. (18) Liu, Q. G.; Yang, X. F.; Li, L.; Miao, S.; Li, Y.; Li, Y. Q.; Wang, X. K.; Huang, Y. Q.; Zhang, T. Direct Catalytic Hydrogenation of CO2 to Formate over a Schiff-basemediated Gold Nanocatalyst. Nat. Commun. 2017, 8, 1407. (19) Ohnishi, Y. Y.; Matsunaga, T.; Nakao, Y.; Sato, H.; Sakaki, S. Ruthenium (II)Catalyzed Hydrogenation of Carbon Dioxide to Formic Acid. Theoretical Study of Real Catalyst, Ligand Effects, and Solvation Effects. J. Am. Chem. Soc. 2005, 127, 4021-4032. (20) Li, H. L.; Wang, L. B.; Dai, Y. Z.; Pu, Z. T.; Lao, Z. H.; Chen, Y. W.; Wang, M. L.; Zheng, X. S.; Zhu, J. F.; Zhang, W. H.; Si, R.; Ma, C.; Zeng, J. Synergetic Interaction between Neighbouring Platinum Monomers in CO2 Hydrogenation. Nat. Nanotechnol. 2018, 13, 411-417. (21) Larmier, K.; Liao, W. C.; Tada, S.; Lam, E.; Verel, R.; Bansode, A.; Urakawa, A.; Comas-Vives, A.; Copéret, C. CO2-to-methanol Hydrogenation on Zirconia-supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal-support


Page 26 of 32

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Interface. Angew. Chem., Intl. Ed. 2017, 56, 1-7. (22) Tan, H.; Zhao, Z.; Zhu, W.; Coker, E. N.; Li, B.; Zheng, M.; Yu, W.; Fan, H.; Sun, Z. Oxygen Vacancy Enhanced Photocatalytic Activity of Pervoskite SrTiO3. ACS Appl. Mater. Interfaces 2014, 6, 19184-19190. (23) Wang, L.; Dash, S.; Chang, L.; You, L.; Feng, Y.; He, X.; Jin, K.; Zhou, Y.; Ong, H. G.; Ren, P.; Wang, S.; Chen, L.; Wang. J. Oxygen Vacancy Induced Room-temperature Metal-insulator Transition in Nickelate Films and its Potential Application in Photovoltaics. ACS Appl. Mater. Interfaces 2016, 8, 9769-9776. (24) Hu, S. Q.; Li, G. F.; Chang, X. H.; Ma, L. F., Low-temperature Synthesis and Adsorption Property of Rutile TiO2 Nanoparticles with Mesoporous Structure. Inorg. Nano-Met. Chem. 2017, 47, 328-331. (25) Duan, Y.; Zheng, M.; Li, D.; Deng, D.; Ma, L. F.; Yang, Y. Conversion of HMF to Methyl Cyclopentenolone Using Pd/Nb2O5 and Ca-Al Catalysts via a Two-step Procedure. Green Chem. 2017, 19, 5103-5113. (26) Lin, B.; Lin, Z.; Chen, S.; Yu, M.; Li, W.; Gao, Q.; Dong, M.; Shao, Q.; Wu, S.; Ding T.; Guo, Z. Surface Intercalated Spherical MoS2xSe2(1-x) Nanocatalysts for Highly Efficient and Durable Hydrogen Evolution Reactions. Dalton T. 2019, 48, 8279-8287. (27) Lin, Z.; Lin, B.; Wang, Z.; Chen, S.; Wang, C.; Dong, M.; Gao, Q.; Shao, Q.; Ding, T.; Liu, H.; Wu, S.; Guo, Z. Facile Preparation of 1T/2H-Mo(S1-xSex)2 Nanoparticles for Boosting Hydrogen Evolution Reaction. Chemcatchem, 2019, 11, 2217-2222. (28) Chen, H.; Zhou, S.; Han, Z.; Jiang, Y.; Yu, H.; Zhou, X.; Jiang, R.; Liu, X.; Li, X. A Copper-based Sorbent with Oxygen-vacancy Defects from Mechanochemical Reduction for Carbon Disulfide Absorption. J. Mater. Chem. A 2016, 4, 17207-17214. (29) Yang, Y.; Ma, J.; Jia, X.; Du, Z.; Duan, Y.; Xu, J. Aqueous Phase Hydrogenation of



Furfural to Tetrahydrofurfuryl Alcohol on Alkaline Earth Metal Modified Ni/Al2O3. RSC Adv. 2016, 6, 51221-51228. (30) Zhao, Y.; Liu, F.; Tan, J.; Li, P.; Wang, Z.; Zhu, K.; Mai, X.; Liu, H.; Wang, X.; Ma, Y.; Guo, Z. Preparation and Hydrogen Storage of Pd/MIL-101 Nanocomposites. J. Alloys Compd. 2019, 772, 186-192. (31) Chen, H.; Han, Z.; Feng, X.; Liu, P.; Liu, J.; Liu, G.; Yuan, W.; Ren, T.; Zhou, S. Solid-phase Hydrogen in a Magnesium-carbon Composite for Efficient Hydrogenation of Carbon Disulfide. J. Mater. Chem. A 2018, 6, 3055-3062. (32) Soga, K.; Imamura, H.; Ikeda, S. Hydrogenation of Ethylene over Lanthanum-nickel (LaNi5) Alloy. J. Phys. Chem. 1977, 81, 1762-1766. (33) Wang, P. K.; Chang, F.; Gao, W. B.; Guo, J. P.; Wu, G. T.; He, T.; Chen, P. Breaking Scaling Relations to Achieve Low-temperature Ammonia Synthesis through LiHmediated Nitrogen Transfer and Hydrogenation. Nat. Chem. 2016, 9, 64-70. (34) Ulmer, U.; Cholewa, M.; Diemant, T.; Minella, C. B.; Dittmeyer, R.; Behm R. J.; Fichtner, M. Thermochemical Energy Storage through De/hydrogenation of Organic Liquids: Reactions of Organic Liquids on Metal Hydrides. ACS Appl. Mater. Interfaces 2016, 8, 13993-14003. (35) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. First Principles Methods Using CASTEP. Z. kristallogr. 2005, 220, 567570. (36) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (37) Anisimov, V. I.; Zaanen, J.; Andersen, O. K. Band Theory and Mott Insulators: Hubbard U Instead of Stoner I. Phys. Rev. B 1991, 44, 943-954.


Page 28 of 32

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Capdevila-Cortada, M.; García-Melchor, M.; López, N. Unraveling the Structure Sensitivity in Methanol Conversion on CeO2: A DFT+U Study. J. Catal. 2015, 327, 5864. (39) Halgren, T. A.; Lipscomb, W. N. The Synchronous-transit Method for Determining Reaction Pathways and Locating Molecular Transition States. Chem. Phys. Lett. 1977, 49, 225-232. (40) Henkelman, G.; Jónsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978-9985. (41) Lototskyy, M.; Sibanyoni, J. M.; Denys, R. V.; Williams, M.; Pollet, B. G.; Yartys, V. A. Magnesium-carbon Hydrogen Storage Hybrid Materials Produced by Reactive Ball Milling in Hydrogen. Carbon 2013, 57, 146-160. (42) Au, Y. S.; Obbink, M. K.; Srinivasan, S.; Magusin, P. C. M. M.; De Jong, K. P.; De Jongh, P. E. The Size Dependence of Hydrogen Mobility and Sorption Kinetics for Carbon-supported MgH2 Particles. Adv. Funct. Mater. 2014, 24, 3604-3611. (43) Jain, I. P.; Lal, C.; Jain, A. Hydrogen Storage in Mg: A most Promising Material. Int. J. Hydrogen Energy 2010, 35, 5133-5144. (44) Schlapbach, L.; Züttel, A. Hydrogen-storage Materials for Mobile Applications. Nature 2001, 414, 353-358. (45) Zhou, S.; Chen, H.; Ding, C.; Niu, H.; Zhang, T.; Wang, N.; Zhang, Q.; Liu, D.; Han, S.; Yu, H. Effectiveness of Crystallitic Carbon from Coal as Milling aid and for Hydrogen Storage during Milling with Magnesium. Fuel 2013, 109, 68-75. (46) Witoon, T.; Chalorngtham, J.; Dumrongbunditkul, P.; Chareonpanich, M.; Limtrakul, J. CO2 Hydrogenation to Methanol over Cu/ZrO2 Catalysts: Effects of Zirconia Phases.



Chem. Eng. J. 2016, 293, 327-336. (47) Phongamwong, T.; Chantaprasertporn, U.; Witoon, T.; Numpilai, T.; Poo-Arporn, Y.; Limphirat, W.; Donphai, W.; Dittanet, P.; Chareonpanich, M.; Limtrakul, J. CO2 Hydrogenation to Methanol over CuO-ZnO-ZrO2-SiO2 Catalysts: Effects of SiO2 Contents. Chem. Eng. J. 2017, 316, 692-703. (48) Akkharaphatthawon, N.; Chanlek, N.; Cheng, C. K.; Chareonpanich, M.; Limtrakul, J.; Witoon, T. Tuning Adsorption Properties of GaxIn2−xO3 Catalysts for Enhancement of Methanol Synthesis Activity from CO2 Hydrogenation at High Reaction Temperature. Appl. Surf. Sci. 2019, 489, 278-286. (49) Yang, C.; Mu, R.; Wang, G.; Song, J.; Tian, H.; Zhao, Z.; Gong, J.; Hydroxylmediated Ethanol Selectivity of CO2 Hydrogenation. Chem. Sci. 2019, 10, 3161-3167. (50) Swapnesh, A.; Srivastava, V. C.; Mall, I. D. Comparative Study on Thermodynamic Analysis of CO2 Utilization Reactions. Chem. Eng. Tech. 2014, 37, 1765-1777. (51) Al-Dossary, M. M.; Ismail, A. A.; Fierro, J. L. G.; Bouzid, H.; Al-Sayari, S. A. Effect of Mn Loading onto MnFeO Nanocomposites for the CO2 Hydrogenation Reaction. Appl. Catal. B Environ. 2015, 165, 651-660. (52) Clausen, B. S.; Topsøe, H. In Situ High Pressure, High Temperature XAFS Studies of Cu-based Catalysts during Methanol Synthesis, Catal. Today 1991, 9, 189-196. (53) Kurtz, M.; Wilmer, H.; Genger, T.; Hinrichsen, O.; Muhler, M. Deactivation of Supported Copper Catalysts for Methanol Synthesis, Catal. Lett. 2003, 86, 77-80. (54) Martin, O.; Pérez-Ramírez, J. New and Revisited Insights into the Promotion of Methanol Synthesis Catalysts by CO2, Catal. Sci. Technol. 2013, 3, 3343-3352. (55) Xie, Y.; Gao, M.; Zhang, H.; Zeng, S.; Zhao, X.; Zhao, Y.; Su, H.; Song, J.; Li, X.; Jia, Q. Improvement Role of CNTs on Catalytic Performance in the CeO2/xCNTs-CuO


Page 30 of 32

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Catalysts. Int. J. Hydrogen Energy 2016, 41, 21979-21989. (56) Ding, J.; Li, L.; Li, H.; Chen, S.; Fang, S.; Feng, T.; Li, G. Optimum Preferential Oxidation Performance of CeO2-CuOx-RGO Composites through Interfacial Regulation. ACS Appl. Mater. Interfaces 2018, 10, 7935-4795. (57) Lin, X.; Li, S.; He, H.; Wu, Z.; Wu, J.; Chen, L.; Ye, D.; Fu, M. Evolution of Oxygen Vacancies in MnOx-CeO2 Mixed Oxides for Soot Oxidation. Appl. Catal. B Environ. 2018, 223, 91-102. (58) Yao, H. B.; Li, Y.; Wee, A. T. S. An XPS Investigation of the Oxidation/corrosion of Melt-spun Mg. Appl. Surf. Sci. 2000, 158, 112-119. (59) Zhang, Y.; Ji, B.; Tian, A.; Wang, W. Competition between π···π Interaction and Halogen Bond in Solution: A Combined 13C NMR and Density Functional Theory Study. J. Chem. Phys. 2012, 136, 141101. (60) Kattel, S.; Liu, P.; Chen, J. G. Tuning Selectivity of CO2 Hydrogenation Reactions at the Metal/oxide Interface. J. Am. Chem. Soc. 2017, 139, 9739-9754. (61) Martin, O.; Martín, A. J.; Mondelli, C.; Mitchell, S.; Segawa, T. F.; Hauert, R.; Drouilly, C. Indium Oxide as a Superior Catalyst for Methanol Synthesis by CO2 Hydrogenation. Angew. Chem. Int. Ed. 2016, 55, 6261-6265. (62) Gao, P.; Li, S. G.; Bu, X. N.; Dang, S. S.; Liu, Z. Y.; Wang, H.; Zhong, L. S.; Qiu, M. H.; Yang, C. G.; Cai, J.; Wei, W.; Sun, Y. H. Direct Conversion of CO2 into Liquid Fuels with High Selectivity over a Bifunctional Catalyst. Nat. Chem. 2017, 9, 1019-1024.



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