CuMnOS Nanoflowers with Different Cu+/Cu2+ Ratios for the CO2-to-CH3OH and the CH3OH-to-H2 Redox Reactions
A conservative CO 2 -Methanol (CH 3 OH) regeneration cycle, to capture and reutilize the greenhouse gas of CO 2 by aqueous hydrogenation for industry-useful CH 3 OH and to convert aqueous CH 3 OH solution by dehydrogenation for the clean energy of hydrogen (H 2 ), is demonstrated at normal temperature and pressure (NTP) with two kinds of CuMnOS nanoflower catalysts. The [Cu + ]-high CuMnOS led to a CH 3 OH yield of 21.1 mmol•g −1 catal.•h −1 in the CuMnOS-CO 2 -H 2 O system and the other [Cu + ]-low one had a H 2 yield of 7.65 mmol•g −1 catal.•h −1 in the CuMnOS-CH 3 OH-H 2 O system. The successful redox reactions at NTP rely on active lattice oxygen of CuMnOS catalysts and its charge (hole or electron) transfer ability between Cu + and Cu 2+ . The CO 2 -hydrogenated CH 3 OH in aqueous solution is not only a fuel but also an ideal liquid hydrogen storage system for transportation application.Clean and renewable energies such as solar, wind, hydraulic, hydrogen, biomass energies can partially substitute for fossil fuels for the sakes of the depletion in oil resources and the emissions in greenhouse gas. The methanol economy advocated by Nobel prize winner George A. Olah in 1990 s has been promising as CH 3 OH is not only a good hydrogen liquid carrier but also a key industrial chemical feedstock 1,2 . The success in methanol economy is related to the reversible CO 2 hydrogenation-CH 3 OH dehydrogenation or the CO 2 -CH 3 OH cycle. In this way, the underground fossil carbon can be conservatively exploited.CO 2 hydrogenation by the thermochemical conversion of the mixture of CO/CO 2 /H 2 (termed "syngas") for CH 3 OH operates at high temperature of ~250 °C and high pressures of ~50 bar over copper-based heterogeneous catalysts in an industrial production scale 3-5 . Studt et al. discovered a stable Ni-Ga alloy catalyst that reduced CO 2 to methanol at ambient pressure 6 . As CO 2 conversion to methanol is a thermodynamically exergonic process, the Ni-Ga catalyzed conversion of CO 2 to methanol needs to be operated at and above 200 °C. To process at a mild condition, the homogeneous catalysis mainly with the use of ruthenium-and iridium-based complexes for CO 2 hydrogenation is being developed below 145 °C under a high pressure of 60 bars [7][8][9] . Photocatalysis for CO 2 conversion to methanol in water under high power lamp with sulfide/sulfate reagents is still an active research area [10][11][12] . Liu et al. demonstrated the good performance with a Cu/CeO 2 /doped graphene catalyst, but they needed the 250 W lamp illumination and Na 2 SiO 3 reagent.CH 3 OH dehydrogenation for hydrogen (H 2 ) currently perform through steam reforming over the heterogeneous CuO/ZnO/Al 2 O 3 catalyst at 200-300 °C13-15 . The lower reaction temperature of 65-150 °C has been achieved with the active homogeneous ruthenium-complex catalysts, but additional reagents are needed 4,16,17 . Photocatalysis for aqueous alcohol conversion with a large amount of hydrogen has been achieved under high power lamp, Pt, CdS, or sulfate reagent [18][19][20] . A low operation temperature for alcohol dehydrogenation can be key factor to make CH 3 OH suitable as energy liquid carrier for portable and vehicle applications.High temperature and pressure conversion reaction, high illumination intensity, external electrical energy input, and complex reagents have been the major approaches to make parts of the CO 2 -CH 3 OH cycle feasible for effective energy and environment managements. Therefore, the energy supplies from the perennial sources such as sun, wind, and geothermal to execute the reactions for fuels and chemicals have been widely pursued. Recently, metallic AgPd hollow sphere anchored on graphene demonstrated the dehydrogenation of formic acid for H 2 at room temperature 21 . It is encouraging that the catalytic reaction with AgPd for H 2 production without thermal,
<!>Methods<!>Characterization of CuMnOS. The photoelectron spectrometry (XPS) was proceeded with VG Scientific<!>Reduction/hydrogenation reactions. Reduction of Cr(VI) by the catalytic reduction reaction of<!>Reduction conversion of CO 2 by aqueous hydrogenation with CuMnOS catalyst.<!>Oxidation/dehydrogenation reactions. Degradation of methylene blue by dye oxidation reaction with<!>XPS analysis.<!>UV-Vis absorption and photoluminescence. The optical absorption property of CuMnOS was charac-<!>Reduction activity of CuMnOS on Cr(VI).<!>Degradation of methylene blue by oxidation reaction.<!>Hydrogen production by aqueous CH 3 OH dehydrogenation.<!>Discussion
<p>electrical, and photo energies is not impossible to occur. Here, we demonstrate two kinds of inorganic CuMnOS catalysts with low cost: the [Cu + ]-high CuMnOS acts as catalyst to accelerate the reduction reaction of the CO 2 hydrogenation to methanol and the [Cu + ]-low CuMnOS to speed up the oxidation reaction of the methanol dehydrogenation into hydrogen and carbon dioxide, to complete the conservative CO 2 -CH 3 OH cycle at normal temperature and pressure without additional reagents.</p><!><p>Synthesis of CuMnOS. To prepare CuMnOS powder, 1.5 g thioacetamide (CH 3 CSNH 2 ) was added into a 500 ml solution with cupric nitrate (Cu(NO 3 ) 2 •2.5H 2 O) and manganese (II) chloride (MnCl 2 ) in the weight ratio of 1: 1, followed by 30 min stirring. Then the mixture solution was steadily heated to 95 °C and 0.0, 0.1, 0.2, 0.3 and 0.4 ml hydrazine (N 2 H 4 ) were added to prepare the powders at different redox conditions. After stirring for 2 h, the precipitates were collected after centrifugation and washing procedures. The precipitates were dried in oven at 80 °C for 24 h. The obtained catalysts were labeled as CuMnOS-0, CuMnOS-1, CuMnOS-2, CuMnOS-3, and CuMnOS-4, depending upon their N 2 H 4 content. For comparative purpose, the CuOS was prepared at the same procedure without adding MnCl 2 .</p><!><p>ESCALAB 250 XPS under the Al Kα X-rays (hv = 1486.6 eV) radiation and calibrated with carbon C1s (Ea = 284.62 eV). The X-ray powder diffraction (XRD) study was conducted on Bruker D2 phaser X-ray diffractometry at 10 kV using Cu Kα radiation at a scanning step size of 0.05° and with residence time of 0.5 min. SEM images were obtained from JSM-7610F field-emission scanning electron microscope (FE-SEM) operated at an accelerating voltage of 15 kV. A Tecnai F20 G2 instrument was used to obtain the TEM images and microstructural information. To obtain the specific surface area (S BET ), N 2 adsorption-desorption experiments were performed on Micromeritics ASAP 2020 porosity and specific surface area analyzer after the sample degassed at 150 °C for 2 h. UV-Vis DRS was evaluated on a JASCD V-670 UV-Vis spectrophotometer with an integrating sphere of 60 mm and BaSO 4 as a reference material. Photoluminescence (PL) emission spectrum was measured at room temperature on JASCD FB-8500 fluorescence spectrophotometer with a laser beam at 330 nm emission wavelength.</p><!><p>CuMnOS. To execute the reduction of Cr(VI), the 50 mg catalyst was added into the reactor filled with 100 mL Cr(VI) solution of 50 mg/L. The reactor also was wrapped with aluminum foil. After reaction for 2 min, approximately 8 mL sample was taken out and passed through a 0.45 μ m membrane filter syringe to immediately separate catalysts from the solution. The diphenycarbazide (DPC) colorimetric method 22 with JASCD V-670 spectrophotometer and the ion chromatography (IC) method with Thermo ICS-5000 spectrophotometer were used to determine the Cr(VI) concentration in filtrate. To evaluate the reusability, the catalysts after the first run were re-used for the second run at the same condition after re-filling with a fresh Cr(VI) solution without being washed. For this reusability purpose, a larger amount of 50 mg catalyst was used to avoid the larger deviation caused by the weight loss.</p><!><p>Reduction conversion of CO 2 to CH 3 OH with CuMnOS was carried out in a home-made and jacketed quartz reactor wrapped by aluminum foil. For each run, the 0.1 g catalyst was added into the reactor with 70 mL distilled water, then CO 2 gas, released from NaHCO 3 solution by controlling the addition of dilute HNO 3 aqueous solution, was passed into the reactor under the ambient laboratory condition. The whole procedure lasted for 18 h. The products were collected and analyzed by GC with flame ionization detector (FID).</p><!><p>CuMnOS catalyst. To proceed the MB degradation experiments, the 25 mg catalyst was added into the reactor filled with 100 mL MB solution of 10 mg/L. The reactor was wrapped with aluminum foil to exclude the effects of UV and visible light irradiations. The 3 mL sample was taken out from the reactor every 5 min, followed by instant centrifugation in 1 min. The supernatant absorbance was measured with a JASCD V-670 UV-Vis spectrophotometer for peak located at 663 nm. Their concentration was calculated based on the Lambert-Beer law. To evaluate the reusability, the catalysts after the first run were re-used for the second run at the same condition after re-filling with a fresh MB solution without being washed.</p><p>Hydrogen generation by catalytic dehydrogenation (aqueous oxidation) with CuMnOS. Hydrogen generation was conducted in a home-made and jacketed quartz reactor equipped with the input and output valves to control the gas flow. To exclude the visible light irradiation, the reactor was wrapped by aluminum foil. One CuMnOS sample was compared by exposure under the 150 W Halogen lamp illumination. The input valve was connected to a gas tank of 99.99% Ar and the output one to a well-callibrated gas chromatography (GC) with thermal conduction detector (TCD) system. The hydrogen evolution experiment was carried out with the well-dispersed 225 mg catalyst in 450 mL pure ethanol (C 2 H 5 OH), water, ethanoic acid, or the methanol (CH 3 OH), ethanol, or ethanoic acid aqueous solution (20% v/v). The gas sampling was taken for each time interval of 20 min. Gas sampling was conducted by flowing Ar gas through the reactor to GC-TCD system for several minutes. A hydrogen calibration line was built to quantitatively measure the H 2 generation rate.</p><!><p>The compositions of CuMnOS-0 and CuMnOS-3 are listed in Table 1 after the XPS analyses. The Cu, Mn, O, and S contents are close for the two kinds of catalysts prepared at different N 2 H 4 contents.</p><p>The substitutional Mn has a Mn/(Mn + Cu) molar ratio of ~0.038, a much lower content than the Cu content. Figure 1a shows the high resolution Cu2p XPS spectra of CuMnOS-0 and CuMnOS-3. The asymmetric Cu2p peaks were contributed to the different chemical states of Cu in CuMnOS. The peak separation of 20.0 eV between Cu2p3/2 and Cu2p1/2 located at 933.8 eV and 953.8 eV, respectively, indicates that copper is for the monovalent Cu + 23 . The peaks of 2p3/2 and 2p1/2 located at 935.0 eV and 955.3 eV, respectively, were attributed to the spin-orbit splitting of the bivalent Cu 2+ 24,25 . According to the quantitative analysis by integrating the peak area, both of catalysts are richer in Cu + than Cu 2+ and the Cu + /Cu 2+ molar ratios were calculated to be 1.49 for CuMnOS-0 and 2.39 for CuMnOS-3. With increasing the reducing N 2 H 4 content, the Cu + /Cu 2+ molar ratio increased or the Cu 2+ → Cu + transition was accelerated. CuMnOS-0 without adding N 2 H 4 has a lower Cu + content and it is labeled as [Cu + ]-low CuMnOS. CuMnOS-3 had a higher Cu + content after adding N 2 H 4 during the preparation stage and it is labeled as [Cu + ]-high CuMnOS.</p><p>Figure 1b shows the high resolution Mn2p XPS spectra of CuMnOS-0 and CuMnOS-3. The peak separation of 11.5 eV between Mn2p3/2 and Mn2p1/2 located at 640.0 eV and 651.5 eV, respectively, indicates that copper is for the bivalent Mn 2+ 26,27 . Figure 1c shows the high resolution O1s XPS spectra of CuMnOS. The asymmetric O1s peak was convoluted into three kinds of peaks at 531.4 eV contributing from the hydroxyl oxygen 28 , at 530.5 eV from the Mn-O and monovalent Cu + -O 29,30 , and 529.7 eV from the bivalent Cu 2+ -O 31 . Figure 1d shows the high resolution S2p XPS spectra of CuMnOS. The S2p peaks at 163.6 eV belongs to the S 33 and at 170.3 eV to S 6+ 34 . With increasing the reducing N 2 H 4 content, the S 6+ content in CuMnOS, for the sake of charge neutrality, increases to compensate the loss in the positive charge due to the Cu 2+ → Cu + transition. That is to say, [Cu + ]-low  CuMnOS-0 has a lower S 6+ content and [Cu + ]-high CuMnOS-3 a higher one. The Cu + content is proportional to the S 6+ content in CuMnOS. The lattice O −2 /S −2 molar ratio, removing the contribution from the hydroxyl oxygen, is ~0.466 for both of catalysts with a slightly higher S 2ratio. SEM and TEM microstructural and structural analyses. Figure 3a shows the FE-SEM images of CuMnOS-3. CuMnOS looks like the petal-gathered nanoflower particles with its size of 300~500 nm. Similar to CuMnOS-3 in FE-SEM image, CuMnOS-0 was not displayed. Figure 3b shows the TEM image of CuMnOS to further verify its nanoflower-like microstructure. The inset in Fig. 3b shows the image at higher magnification. Figure 3c shows the HR-TEM image of CuMnOS. Different lattice fringes belonging to different grains were observed, indicating the nature of nanoparticles. Figure 3d shows the selected area electron diffraction (SAED) pattern of CuMnOS-3. The ring patterns from the (102), ( 103), ( 110) and (203) planes explain its polycrystalline nature. The scattered ring pattern explains the solid solution nature of CuMnOS. Figure 3e gives the HAADF-STEM image, which reveals many pores with different sizes inside the nanoflower-like CuMnOS particles. Figure 3f shows the FE-SEM-EDS spectrum, which verifies that aggregates are composed of Cu, Mn, S, and O. Figure 3g-j show the HAADF-STEM-EDX elemental maps of Cu, Mn, O, and S. From these element mappings, we can confirm the composition uniformity in samples.</p><!><p>terized by UV-Vis absorption spectroscopy. CuMnOS had a better visible light absorbance than CuOS. From the UV-Vis spectra, the direct band gap was measured with the equation versus photon energy (hν) 35 :</p><p>where α is the absorbance coefficient, h the Planck constant, k the absorption constant for a direct transition, hν the absorption energy, and E g the band gap. Figure 4a shows the (α hν) 2 -hν curves of CuMnOS together with the comparative CuOS. The E g values were determined to be 2.0 eV for CuOS and 1.5~1.6 eV for CuMnOS with the higher value at the higher Cu + content in CuMnOS. The variation of energy band gap further indicates that CuMnOS is a bimetal oxysulfide solid solution instead of monocrystalline CuO with band gap of E g = 1.2-1.4 eV, Cu 2 O of 2.0-2.2 eV, CuS of 2.15-2.36 eV, and Cu 2 S of 1.2-1.25 eV.</p><p>Figure 4b shows PL spectra of the CuMnOS catalysts. Under a laser beam at wavelength of 330 nm, catalysts were excited with PL spectra at about 593 nm. The peak at 660 nm is originated from the laser contribution. It is observed that the 593 nm peak intensity increases with the N 2 H 4 -adding content or the Cu + content. The more Cu + content in CuMnOS-3 can contribute the more defect levels to lead to the higher emission intensity.</p><p>BET and pore size analyses. Figure 5a shows the N 2 adsorption-desorption isotherm of CuMnOS, which displays the type IV isotherm with the hysteresis loop at relative pressure (P/P 0 ) between 0.75 and 1.0, indicating its mesoporous feature 36 . Figure 5b shows the pore size distribution of CuMnOS. CuMnOS-0 and CuMnOS-3 had the surface area (S BET ) of 20.3 and 18.6 m 2 /g, the total pore volumes of 0.151 and 0.141 cm 3 /g, and the average pore diameters of 30.5 and 30.4 nm, respectively. The large pore diameter is attributed to the aggregation of the petal-gathered nanoflower particles.</p><!><p>Table 2 shows the reduction of Cr(VI) over CuMnOS and CuOS catalysts in the dark. The different CuMnOS catalysts performed differently in Cr(VI) reduction with the efficiency in the order: CuMnOS-4 ≈ CuMnOS-3 > CuMnOS-2 > CuMnOS-1 > CuMnOS-0 > CuOS. The CuMnOS-3 and CuMnOS-4 catalysts completed the Cr(VI) reduction in 2 min, while CuOS only completed 8.5%. As tested by IC method, the Cr(VI) in solution was confirmed to be reduced to Cr 0 without the existence of Cr 3+ . In order to test the catalytic capability and their reusability, CuMnOS was continuously tested for three runs. After the 3 rd run, the CuMnOS-3 still maintained the good catalytic activity to reduce more than 97.4% of Cr(VI). The results indicate that the bimetal [Cu + ]-high CuMnOS oxysulfide catalysts prepared with a higher N 2 H 4 amount show excellent catalytic activity without the needs of other chemicals and photo energy. The [Cu + ]-high CuMnOS is promising for industrial applications in Cr(VI) waste water treatment.</p><p>The experimental methods for Cr(VI) depollution include photocatalysis and absorption with high surface energy nanomaterials. As the rate constant is affected by the catalyst amount, illumination light intensity etc., the quantity of K 2 Cr 2 O 7 amount (mg) divided by catalyst amount (mg), i.e. W 2,(K2Cr2O7) /W 1,(catalyst) , is used for comparison. Under the UV light, TiO 2 -CNT with a W 2 /W 1 value of 0.013 reduced 100% Cr(VI) in 40 min 37 . Under the visible light, Fe 2 O 3 /g-C 3 N 4 with a W 2 /W 1 value of 0.014 reduced 100% Cr(VI) in 15 min 38 . With no light illumination, diamino pyridine-modified grapheme oxide with a W 2 /W 1 value of 0.5 absorbed 94.5% Cr(VI) in 90 min at the help of the electrostatic force 39 . In the dark condition, our CuMnOS with a W 2 /W 1 value of 0.1 reduced Cr(VI) in 2 min. The CuMnOS catalyst had demonstrated the excellent ability in the reduction of Cr(VI). 3 shows the yields of CH 3 OH in conversion of CO 2 over CuMnOS and CuOS. It is interesting to note that pure CuOS did not produce CH 3 OH. However, the aqueous hydrogenation of CO 2 by CuMnOS to produce CH 3 OH with the yield in the order: CuMnOS-3 > CuMnOS-2 > CuMnOS-1 > CuMnOS-4 > CuMnOS-0. The CH 3 OH yield increased with the Cu + content in CuMnOS but reached the highest yield of 21.1 mmol•g −1 catal.•h −1 at CuMnOS-3. The [Cu + ]-high CuMnOS favors the aqueous hydrogenation of CO 2 . In the industrial scale, thermal conversion above 200 °C had a rate above 60 mmol•g − 1 catal.•h −1 40 . For photo conversion, the maximal rate of 0.51 mmol•g −1 catal.•h −1 was achieved by the Cu-CeO 2 system with the 250 W Xe lamp 10 . Our production in the dark with a rate of 21.1 mmol•g −1 catal.•h −1 is quite promising.</p><!><p>Figure 6 shows the degradation of MB over different catalysts in the dark. It is noted that N 2 H 4 added in processing has an important effect for preparing CuMnOS on the degradation of MB with the efficiency in the order: CuMnOS-0 ≈ CuMnOS-1 > CuMnOS -2 > CuMnOS-3 > CuMnOS-4. CuMnOS-0 and CuMnOS-1 could completely degrade MB in 5 min. However, the CuOS catalyst only removed 9.9% MB in 30 min. In order to test the catalyst reusability, the supernatant of CuMnOS-0 catalyst after the first test and gravity setting was decanted and then the fresh 100 mL MB solution of 10 ppm was added for the reuse test in the dark without washing catalysts. The 2 nd run was also completed in</p><p>5 min. After the 3 rd run, the CuMnOS remained effective to degrade 95% MB in 5 min. To differentiate the dye degradation or adsorption, the wash-out ethanol solution of CuMnOS powder after the 3 rd run was analyzed with UV-Vis spectrophotometer at 663 nm. The disappearance of the characteristic peak confirmed there was no MB absorption on CuMnOS. A comparative experiment on activated carbon with S BET above 1000 m 2 /g did show the characteristic peak at 663 nm for MB after the carbon powder was washed. Our CuMnOS has a too low S BET value of ~20 m 2 /g for adsorption to proceed, so the MB dye here looks to be degraded by the catalytic reaction in the dark. Together with the XPS data, the [Cu + ]-low CuMnOS dominates over [Cu + ]-high one for the MB oxidative degradation.</p><p>There are some reports on the MB degradation in the dark. The quantity of MB amount (mg) divided by catalyst amount (mg), W 2,(MB) /W 1,(catalyst) , is used for evaluation. Ag-In-Ni-S nanocomposites with a W 2 /W 1 value of 0.0022 degraded 98% MB in 12 min 41 . NiS nanoparticles with a W 2 /W 1 value of 0.0089 degraded 100% MB in 15 min 42 . CuS caved superstructure with a W 2 /W 1 value of 0.01 degraded 100% MB in 15 min with the help of hydrogen peroxide 43 . Our CuMnOS with a W 2 /W 1 value of 0.04 degraded 100% MB in 5 min. The CuMnOS catalyst had demonstrated the admirable ability in the degradation of MB.</p><!><p>The results of hydrogen production by aqueous CH 3 OH dehydrogenation with the CuMnOS catalysts prepared at different N 2 H 4 contents are shown in Table 4. It is interesting to mention that the catalyst in each pure H 2 O, alcohol, and organic acid did not generate hydrogen, but the aqueous solutions of alcohol and organic acid produced H 2 at NTP in the dark. For the mixture solution of alcohol and organic acid without water, it did not work out for H 2 generation, either. These results indicate that hydrogen generation process involves the catalytic reactions with water and alcohol, or water and organic acid. The reaction between catalyst and water is especially critical. Without the existence of water to participate, hydrogen does not produce. From Table 4, the highest H 2 yield of 7.65 mmol•g −1 catal.•h −1 in the methanol solution and 9.45 mmol•g −1 catal.•h −1 in the ethanol solution, are much higher than that of CuOS at 0.270 mmol•g −1 catal.•h −1 . The H 2 generation also occurred for CuMnOS-0 with the yield of 2.17 mmol•g − 1 catal.•h −1 in an ethanoic acid solution. Under the 150 W Halogen lamp visible illumination, the H 2 yield degraded to 2.04 mmol•g −1 catal.•h −1 in methanol solution. The ineffectiveness of photo-induced electron-hole pairs under light illumination for H 2 production at NTP indicates dehydrogenation of aqueous CH 3 OH by CuMnOS-0 is not initiated by the electron/hole charges. After CuMnOS-0 nanoflower catalyst was annealed at 200 °C, its H 2 yield further degraded to 1.20 mmol•g −1 catal.•h −1 due to the deactivation of the catalyst activity. There is a trend of lowering the H 2 yield at the higher Cu + content for preparing CuMnOS with the higher N 2 H 4 amount. To test</p><p>The other excellent catalyst was Sr-NaTaO 3 with a rate of 48.9 mmol•g −1 catal.•h −1 or 0.79 mmol•g − 1 catal.•h −1 •watt −1 45 . Ruthenium hydride complex performed the best for the homogeneous catalysis of methanol and water into CO 2 and H 2 O at 90 °C under the additive of KOH 7 . Without the precious metal, the H 2 production rate is low. Our CuMnOS-1 with a rate of 9.45 mmol•g −1 catal.•h −1 in the dark is encouraging.</p><!><p>The developments of the pure electron-transport catalyst for reduction/hydrogenation of CO 2 into methanol and the pure hole-transport one for oxidation/dehydrogenation of aqueous alcohol into H 2 without the thermal, electrical, and photo energies are our major goals. We adopt the hexavalent Cr reduction and dye degradation for screening the redox capability during our search for catalysts. Compared with the reported redox reactions for the pollutant removal, our catalytic reactions are pretty fast at NTP. To further test the redox capability with our CuMnOS system, aqueous CO 2 hydrogenation is used for testing the catalytic reduction reaction and aqueous CH 3 OH dehydrogenation for oxidation one. The first evidence for the success in the hydrogenation-dehydrogenation redox reactions is the content of the different Cu charge states. The [Cu + ]-high CuMnOS is used and good for CO 2 reduction, therefore it can transport electrons through the Cu + /Cu 2+ charge centers for the solution/catalyst interface reaction. The [Cu + ]-low CuMnOS is used for aqueous CH 3 OH dehydrogenation, therefore it can transport holes through the Cu + /Cu 2+ charge centers.</p><p>For CH 3 OH generation from the simple CuMnOS-CO 2 -H 2 O system, the formation of proton is needed, followed by the reaction with CO 2 for forming CH 3 OH. For H 2 generation from the simple CuMnOS-CH 3 OH-H 2 O system at NTP, it needs any one of CH 3 OH, H 2 O, and CuMnOS added to the mixture of the other two, otherwise there is no H 2 gas release. This observation gives a hint that a series reaction operates in this system. To logically explain the complex reactions in each of the simple systems, our catalyst has to be quite active and can react in the CO 2 -H 2 O or CH 3 OH-H 2 O solution with H 2 O existing in both situations. To make the series reaction happen and to explain the rare phenomena, the catalyst has to firstly react with H 2 O, followed by the reaction with CO 2 in the CuMnOS-CO 2 -H 2 O system or with CH 3 OH in the CuMnOS-CH 3 OH-H 2 O one, as we had mentioned about the critical role of H 2 O. For catalyst to be active, its lattice bonding on surface needs to be weak for the interfacial exchange reactions. The degraded performance for the 200 °C-annealed CuMnOS in Table 4 is a support related to lattice bonding. Therefore, the second key factor for the success in the redox reactions is the weakened lattice oxygen at the catalyst surface to have its active lattice oxygen easily react with water for forming the oxygen vacancy and the oxidized OHon catalyst surface. The Kröger-Vink notation originally developed for ionic compounds is used here. For the oxygen vacancy ( + V O,lattice 2</p><p>) as an example, the main body of V represents for vacancy, the subscript for the host lattice site, and the superscript for the relative charge. Here we adopted the positive charge of 2+ instead of the "• • " symbol in the original invention. For the active lattice oxygen, it is shown asO O,lattice 0 . Therefore, water oxidation reaction is shown below in term of the Kröger-Vink notation:</p><p>O,lattice 0 (adsorb ) O,lattice 2</p><p>In the above Eq. 1, the mass, charge, and lattice site are required to be conservative. The active lattice oxygen on surface and the generated oxygen vacancy become the oxidant and the reducing agents, respectively. The redox reactions by oxide defects in CeO 2 had been used for thermochemical catalytic production of solar fuels above 1000 °C46,47 . Here we just perform the similar reactions in the liquid state at much lower temperature. Before discussing the CO 2 hydrogenation and CH 3 OH dehydrogenation, a common reaction in the Cu 2+ /Cu + -coexisting compounds is listed below for the consideration of the reaction reversibility:</p><p>where − Cu Cu represents for the occupation of Cu + on the Cu 2+ lattice site with a relative negative charge of 1-and Cu Cu 0 for the Cu 2+ on the Cu 2+ lattice site.</p><p>For the [Cu + ]-high CuMnOS-CO 2 -H 2 O reaction system to form CH 3 OH at NTP, the reducing agent of oxygen vacancy can be oxidized by H 2 O to form the active lattice oxygen on catalyst and 2 H + at the solid/liquid interface (Equation 3). The protons together with the hopping electrons between Cu + and Cu 2+ in the [Cu + ]-high CuMnOS, shown in Eq. 4, can reduce the dissolved and adsorbed CO 2 into CH 3 OH by the catalyst/solution interface reaction in Eq. 5. After combining Eqs. 2, 3, 4, and 5, the net equation 6 is obtained. Consistent with the data in Table 3, the mechanism explains that the increased Cu + content favors the electron formation in Eq. 4  The kinetic reaction steps in Eqs. 1 and 3 demonstrate the lattice oxygen in and out at the catalyst/solution interface to hold the dynamic equilibrium and to keep CuMnOS behave as a catalyst for a long period of reaction and for repeated use without being exhausted. For the reaction to continuously run, the continuous supply of electrons for Eq. 4 is needed. The establishment of thermal equilibrium in Eq. 2 is also important to avoid the electron depletion.</p><p>For the [Cu + ]-low CuMnOS-CH 3 OH-H 2 O reaction system to form H 2 at NTP, the reduced oxygen vacancy reacts with H 2 O for water reduction to form H 2 , two electrical holes (2 h + ), and active lattice oxygen, as shown in Eq. 7. The hydroxyl group from Eq. 1 together with the hopping holes between Cu + and Cu 2+ in the [Cu + ]-low CuMnOS, shown in Eq. 8, can oxidize CH 3 OH into 5/2H 2 and CO 2 , as shown in Eq. 9. After combining Eqs. 1, 7, and 9, the net equation 10 is obtained. With this proposed mechanism, it can explain the fact that the aqueous methanol dehydrogenation cannot occur without the initiation of the water oxidation reaction in Eq. 1. It also explains the increased Cu + content unfavors the hole formation in Eq. 8 and the H 2 yield in Eq. 9, as supported by the data in Table 4.</p><p>Thermodynamic consideration for the CO 2 hydrogenation is evaluated to support the feasibility of the reaction in Eq. 6, which can be divided into Eqs. 11 and 12:</p><p>. + = +</p><p>. , related to a thermodynamic uphill and unfavorable reaction. For the reaction in Eq. 12, it behaves as the oxygen from the gas state to the metal oxide solid state with the release energy equal to the bond energy. Cu-O bond energy of 272.86 kJ/mol is reported 49  . Therefore, the net standard Gibbs free energy of the reaction in Eq. 6 is ∆</p><p>, favorable for the reaction in Eq. 6 to occur. From this explanation, the consideration of the chemical potential of lattice oxygen is very important. The schematic diagram for the chemical reaction paths for CO 2 and H 2 O to form methanol w/o catalyst is shown in Figure 7.</p><p>Similar to CO 2 hydrogenation, the catalytic reaction for aqueous methanol dehydrogenation can be calculated to be ∆ = . for Eq. 15. If P 1 is 0.01 atm for CO 2(g) and P 2 0.03 atm for H 2(g) , Δ G 4 for Eq. 14 is − 11.41 kJ/mol and Δ G 5 for Eq. 15 − 26.06 kJ/mol. The net standard Gibbs free energy of the reaction in Eq. 10 is Δ G 3 + Δ G 4 + Δ G 5 = − 28.41 kJ/mol, favorable for the reaction in Eq. 10 to occur.</p><p>The achievements in CO 2 hydrogenation and aqueous CH 3 OH dehydrogenation at 25 °C indicate our [Cu + ]-high and [Cu + ]-low CuMnOS catalysts have provided the different interfacial reactions to alter the reaction paths and to obtain fuels without additional inputs of energies and reagents.</p><p>The charge transfer between Cu + and Cu 2+ in semiconductor to provide the electron transport for the n-type or the hole transport for the p-type is understandable. The active lattice oxygen is the key factor for the success of the aqueous CO 2 reduction and aqueous CH 3 OH dehydrogenation. Basically, ceramic catalysts have long been viewed to be activated at high temperature but cannot at NTP in water. Here our proposed reaction mechanism of CuMnOS in water for redox reactions at NTP is similar to that of CeO 2 in water vapor at high temperature with the basis of oxygen vacancy 46 . The thermodynamic calculation also supports the occurrence of the redox reactions. The realization of our CO 2 -CH 3 OH cycle at NTP is strongly related to the reactions between catalyst and water (Equation 1), which are attributed to the low processing temperature for CuMnOS, the S 6+ -O bond formation, and the substitution of Mn for Cu to distort the lattice, to weaken the lattice O bonds, and to form the active lattice oxygen. The degraded performance in the H 2 yield for the 200 °C-annealed CuMnOS in Table 4 is related to the stronger bonding to deactivate the lattice oxygen for Eq. 1. The photo-excitation result in Table 4 also supports the water oxidation by catalyst as the first reaction step instead of the electron/hole-activated reaction. The weakening of the oxygen bonding to initiate catalytic reactions can be the design strategy for inorganic or heterogeneous catalysts to increase their catalytic activity at mild condition.   In summary, we demonstrate the nanoflower-like CuMnOS catalyst system to complete the conservative CO 2 -CH 3 OH hydrogenation-dehydrogenation cycle in an aqueous solution at normal temperature and pressure without additional energy inputs and reagents. This catalyst system has two different forms. The [Cu + ]-high CuMnOS can transport electrons and is used for the aqueous CO 2 hydrogenation to CH 3 OH at a yield of 21.1 mmol•g −1 catal.•h −1 . It is the [Cu + ]-low CuMnOS to transport holes and to instantaneously dehydrogenize aqueous CH 3 OH solution into H 2 at a yield of 7.65 mmol•g −1 catal.•h −1 . In additional to the electron and hole charges, the key factor in completing the CO 2 -CH 3 OH cycle is the active lattice oxygen of CuMnOS to firstly initiate water oxidation at the catalyst-water interface. The bond weakening concept in forming the active lattice oxygen opens a route to increase the catalytic activity of inorganic catalysts for redox reactions at mild condition. The H 2 liquid carrier of aqueous CH 3 OH solution with instantaneous H 2 liberation can provide wide applications in portable appliance, vehicle transportation, power plant etc.</p>