CO2 METHANATION USING PLASMA CATALYSIS

Abstract

An apparatus for forming methane from carbon dioxide and hydrogen is described. The apparatus comprises: a dielectric barrier discharge, DBD, device arranged to generate a plasma; and a passageway having an inlet for the carbon dioxide and the hydrogen and an outlet for the methane and including therein a catalyst comprising nickel and alumina. The passageway extends, at least in part, through the DBD device wherein, in use, the carbon dioxide is exposed to the catalyst in the presence of the hydrogen in the generated plasma, thereby forming the methane from at least some of the carbon dioxide and the hydrogen. A method, a use and a catalyst are also described.

Claims

1. An apparatus for forming methane from carbon dioxide and hydrogen, the apparatus comprising: a dielectric barrier discharge, DBD, device arranged to generate a plasma; and a passageway having an inlet for the carbon dioxide and the hydrogen and an outlet for the methane and including therein a catalyst comprising nickel on a support comprising alumina, wherein the passageway extends, at least in part, through the DBD device wherein, in use, the carbon dioxide is exposed to the catalyst in the presence of the hydrogen in the generated plasma, thereby forming the methane from at least some of the carbon dioxide and the hydrogen.

2. The apparatus according to claim 1, wherein the catalyst comprises nickel in a range from 1 to 35 wt. %.

3. The apparatus according to claim 1, wherein the catalyst comprises metallic nickel.

4. The apparatus according to claim 1, wherein the catalyst comprises one or more rare earth elements, one or more first row and/or second row transition metals or one or more alkaline metals or mixtures thereof.

5. The apparatus according to claim 4, wherein the catalyst comprises manganese in a range from 0.01 to 10 wt. % by weight of the alumina.

6. The apparatus according to claim 1, wherein the catalyst comprises nickel particles having a mean particle diameter in a range from 1 nm to 10 nm.

7. A method of forming methane from carbon dioxide and hydrogen, the method comprising: generating a plasma using a dielectric barrier discharge device, DBD; and exposing the carbon dioxide to a catalyst comprising nickel on a support comprising alumina in the presence of hydrogen in the generated plasma, thereby forming the methane from at least some of the carbon dioxide and hydrogen.

8. The method according to claim 7, wherein the reaction temperature is in a range from 35 to 180° C.

9. The method according to claim 7, wherein generating the plasma using the DBD device comprises generating a stable plasma in a time in a range of from 1 to 60 minutes.

10. The method according to claim 7, wherein the method comprises activating the catalyst using, at least in part, the generated plasma by supplying an electrical power of in a range of 0.72 to 20 kJ/L.

11. The method according to claim 7, having a conversion of carbon dioxide to methane of at least 70%.

12. The method according to claim 7, having a selectivity of methane of at least 85%.

13. The method according to claim 7, having a yield of methane of at least 70%.

14. The method according to claim 7, wherein exposing the carbon dioxide to the catalyst in the presence of hydrogen in the generated plasma comprises exposing the carbon dioxide to the catalyst in the presence of hydrogen in the generated plasma at approximately ambient pressure.

15. The method according to claim 7, wherein hydrogen and carbon dioxide are provided in a stoichiometric H.sub.2/CO.sub.2 ratio of 4:1.

16. (canceled)

17. A catalyst comprising nickel on a support comprising alumina.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0093] For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

[0094] FIG. 1A schematically depicts the experimental setup; and FIG. 1B schematically depicts an axial cross-section and a transverse cross-section of a DBD reactor;

[0095] FIG. 2A shows XRD patterns of the fresh catalysts after calcination; and FIG. 2B shows XRD patterns of the fresh catalysts after reduction;

[0096] FIG. 3A shows conversion of CO.sub.2 and H.sub.2; FIG. 3B shows gaseous product selectivities;

[0097] FIG. 3C shows gaseous product yields; and FIG. 3D shows energy efficiency for CO.sub.2 conversion and CH.sub.4 production (H.sub.2/CO.sub.2=4:1, GHSV=1792 h.sup.−1, 1 atm, SEI=17 kJ/L);

[0098] FIG. 4A schematically depicts the experimental setup; and FIG. 4B schematically depicts an axial cross-section and a transverse cross-section of a DBD reactor;

[0099] FIG. 5A shows XRD patterns of fresh catalysts after calcination; and FIG. 5B shows XRD patterns of spent catalysts;

[0100] FIGS. 6A to 6F show metal particle size dispersion of fresh catalysts after reduction on: FIG. 6A Fe/Al.sub.2O.sub.3; FIG. 6B FeMn/Al.sub.2O.sub.3; FIG. 6C Cu/Al.sub.2O.sub.3; FIG. 6D CuMn/Al.sub.2O.sub.3; FIG. 6E Ni/Al.sub.2O.sub.3; FIG. 6F NiMn/Al.sub.2O.sub.3;

[0101] FIG. 7A shows CO.sub.2 and H.sub.2 conversions; FIG. 7B gaseous product selectivities; FIG. 7C shows gaseous product yields; and FIG. 7D shows CO.sub.2 and CH.sub.4 energy efficiencies (H.sub.2/CO.sub.2=2:1, GHSV=2684 h.sup.−1, 1 atm, SEI=57 kJ/L);

[0102] FIG. 8 shows reaction performance over the NiMn/Al.sub.2O.sub.3 catalyst as a function of time (H.sub.2/CO.sub.2=2:1, GHSV=2684 h.sup.−1, 1 atm, SEI=57 kJ/L);

[0103] FIG. 9A schematically depicts the experimental setup; and FIG. 9B schematically depicts an axial cross-section and a transverse cross-section of a DBD reactor;

[0104] FIG. 10A shows XRD patterns of the fresh catalysts after calcination; FIG. 10B shows XRD spectra of the fresh catalysts after reduction; and FIG. 10C shows XRD patterns of the spent catalysts;

[0105] FIGS. 11A to 11D show metal particle size distributions on fresh catalysts after reduction;

[0106] FIG. 11A NiZrAl; FIG. 11B NiMgAl; FIG. 11C NiCeAl; and FIG. 11D NiMnAl;

[0107] FIG. 12A shows conversions of CO.sub.2 and H.sub.2; FIG. 12B shows selectivity of gas products;

[0108] FIG. 12C shows yield of gas products (H.sub.2/CO.sub.2=4:1, GHSV=1792 h.sup.−1, 1 atm); and FIG. 12D shows energy efficiency for CO.sub.2 conversion and CH.sub.4 production at different gas flow rates;

[0109] FIG. 13 shows CO.sub.2 conversion and products selectivities over NiMnAl as a function of time;

[0110] FIG. 14A shows conversions of H.sub.2 and CO.sub.2; FIG. 14B shows selectivity of gas products;

[0111] FIG. 14C shows energy efficiency for CO.sub.2 conversion and CH.sub.4 production at different gas flow rates; FIG. 14D shows conversion of H.sub.2 and CO.sub.2 at different reaction temperatures over NiMnAl (H.sub.2/CO.sub.2=4:1, GHSV=29857 h.sup.−1, 1 atm); and FIG. 14E shows the converted amounts of CO.sub.2 and H.sub.2 over NiMnAl at different H.sub.2/CO.sub.2 molar ratios (GHSV=29857 h.sup.−1, 1 atm, SEI=1.2 kJ/L);

[0112] FIG. 15A shows CO.sub.2 conversion and products selectivities over NiMnAl as a function of time; and FIG. 15B shows CO.sub.2 conversion and CH.sub.4 space time yield (STY) versus recycle times over NiMnAl (H.sub.2/CO.sub.2=4:1, GHSV=29857 h.sup.−1, 1 atm, SEI=1.2 kJ/L);

[0113] FIG. 16 schematically depicts an axial cross-section and a transverse cross-section of a big DBD reactor; and

[0114] FIG. 17 shows conversions of H.sub.2 and CO.sub.2, and selectivity of CH.sub.4 at different gas hourly space velocity (GHSV, h.sup.−1) and different discharge powers over NiMnAl (H.sub.2/CO.sub.2=4:1, GHSV=100-900 h.sup.−1, power=30-40 W, 1 atm).

DETAILED DESCRIPTION OF THE DRAWINGS

[0115] Experimental

[0116] The following procedures were used in the examples which follow.

[0117] N.sub.2 adsorption-desorption was conducted using a Micromeritics ASAP 2020 Sorptometer at −196° C. Prior to measurement, the samples were degassed at 300° C. for 6 h. The specific surface areas were estimated via the Brunauer-Emmett-Teller (BET) method in the P/P.sub.0 values ranging from 0.05 to 0.3 of the fresh catalysts.

[0118] Transmission electron microscopy (TEM) analysis was performed with a FEI Tecnai G2 f20 s-twin microscope (200 kV). The average particle size was determined by the Nano Measurer software through more than 5 micrographs and around 200 particles for each catalyst. Moreover, an X-ray energy dispersive spectrometer (EDS) was utilized in conjunction with a STEM HAADF detector for elemental mapping.

[0119] Powder X-ray diffraction (XRD) patterns were recorded in a Bruker AXS Advance D.sub.8 diffractometer (40 kV, 40 mA) using Cu K.sub.α radiation as the X-ray source. For each sample Bragg angle was set between 5° and 80° (2θ) with a scan speed of 0.05°/s. The JCPDS standard cards were used for the identification of different phases.

[0120] X-ray photoelectron spectroscopy (XPS) was used to study both chemical composition and oxidation state of the catalyst surfaces. Photoelectron spectra were recorded with a Thermo Scientific ESCALAB 250Xi spectrometer equipped with Al—K.sub.α radiation (hv=1486.6 eV). The corresponding binding energies were calibrated with the C 1s line at 284.8 eV as a reference.

[0121] The H.sub.2 temperature-programmed reduction (H.sub.2-TPR) was performed in a Micromeritics AutoChem 2920 instrument. About 50 mg fresh sample was firstly pretreated under pure Ar at 300° C. for 1 h to remove the impurities on the catalyst surface, and then cooled down to 30° C. After that, the reduction of the catalysts was carried out over a 10% H.sub.2/Argon flow in 30 mL/min with the temperature raised to 900° C. (10° C./min).

[0122] The consumed hydrogen was monitored by a thermal conductivity detector (TCD). The ability of CO.sub.2 adsorption and the acid site density on each catalyst was evaluated by CO.sub.2 temperature programmed desorption (CO.sub.2-TPD) in the same equipment. Previously to CO.sub.2 adsorption, around 50 mg of the samples were reduced at 650° C. with 10 vol. % H.sub.2/Ar for 60 min firstly, then purified with Ar for 40 min, and cooled to 50° C. After exposed to CO.sub.2 for 1 h, the sample was flushed in Ar flow. The TPD profile was recorded in Ar from 50 to 900° C. (10° C./min).

Examples: Plasma-Catalytic CO.SUB.2 .Hydrogenation to Methane

Example 1: Ni/Al.SUB.2.O.SUB.3 .Catalysts with Different Ni Loadings (1-20 wt. % Ni/Al.SUB.2.O.SUB.3., Denoted as xNiAl, x=1, 4, 7, 10, 15, 20)

[0123] FIG. 1A schematically depicts the experimental setup; and FIG. 1B schematically depicts an axial cross-section and a transverse cross-section of a DBD reactor.

[0124] Coaxial DBD plasma reactor without external heating or cooling (electrode gap: 2 mm; discharge length 6 cm; inner electrode (high voltage electrode): stainless steel (SS) rod, 2 mm diameter; outer electrode (ground electrode): SS mesh); specific energy input (SEI): 17 kJ/L; discharge power: 8.5 W.

[0125] Gas: H.sub.2/CO.sub.2=4:1 (total 30 ml/min); GHSV (gas hourly space velocity) 1792 h.sup.−1.

[0126] Catalyst: 0.5 g; 40-60 mesh; packing length 4 cm; reaction temperature: 125-155° C.

TABLE-US-00001 TABLE 1 Physical properties of the fresh catalysts (after reduction) Surface area Pore volume Catalysts (m.sup.2/g) .sup.a (cm.sup.3/g) .sup.a Al.sub.2O.sub.3 (comp) 221 0.45 1NiAl 209 0.41 4NiAl 195 0.41 7NiAl 174 0.36 10NiAl 171 0.34 15NiAl 166 0.33 20NiAl 145 0.31 .sup.a Measured by N.sub.2-adsorption

[0127] FIG. 2A shows XRD patterns of the fresh catalysts after calcination; and FIG. 2B shows XRD patterns of the fresh catalysts after reduction.

TABLE-US-00002 TABLE 2 Chemical properties of the fresh catalysts H.sub.2 consumed CO.sub.2 desorbed Catalysts (mmol/g) .sup.a (mmol/g) .sup.b Al.sub.2O.sub.3 (comp) — 2.55 1NiAl 0.11 2.40 4NiAl 0.32 2.33 7NiAl 0.43 2.24 10NiAl 0.51 1.83 15NiAl 1.51 1.74 20NiAl 1.88 1.56 .sup.a Calculated via H.sub.2-TPR analysis of the fresh catalysts after calcination .sup.b Calculated via CO.sub.2-TPD analysis of the fresh catalysts after reduction

[0128] FIG. 3A shows conversion of CO.sub.2 and H.sub.2; FIG. 3B shows gaseous product selectivities;

[0129] FIG. 3C shows gaseous product yields; and FIG. 3D shows energy efficiency for CO.sub.2 conversion and CH.sub.4 production (H.sub.2/CO.sub.2=4:1, GHSV=1792 h.sup.−1, 1 atm, SEI=17 kJ/L).

[0130] Effect of Ni loading of Ni/Al.sub.2O.sub.3 catalysts (1 wt. %-20 wt. %): the 15NiAl catalyst (15 wt. % Ni/Al.sub.2O.sub.3) shows the highest conversion of CO.sub.2 (90%) and selectivity of CH.sub.4 (91%), providing the highest methane yield of ˜81%.

Example 2: X/Al.SUB.2.O.SUB.3 .and X/Mn/Al.SUB.2.O.SUB.3 .(X═Fe, Cu and Ni) Catalysts

[0131] γ-Al.sub.2O.sub.3 supported metal (7 wt. %) catalysts were prepared by the incipient wetness impregnation method using iron nitrate (Fe(NO.sub.3).sub.3.Math.9H.sub.2O), cupric nitrate (Cu(NO.sub.3).sub.2.Math.2.5H.sub.2O) and nickel nitrate (Ni(NO.sub.3).sub.2.Math.6H.sub.2O) as the metal precursors respectively. Manganous nitrate (Mn(NO.sub.3).sub.2.Math.4H.sub.2O) also was added to the solution for 1 wt. % Mn containing catalysts. The impregnated samples and bare Al.sub.2O.sub.3 were dried in the air overnight before being put into the oven at 120° C. for 4 h, then calcined at 400° C. for 5 h in the muffle. After calcination, the samples were pre-reduced by the mixed gas of 30 mL/min H.sub.2 and 170 mL/min Ar at 450° C. for 5 h. The obtained catalysts are denoted as X (X═Fe, Cu and Ni) and XMn.

[0132] FIG. 4A schematically depicts the experimental setup; and FIG. 4B schematically depicts an axial cross-section and a transverse cross-section of a DBD reactor.

[0133] As shown in FIG. 4B, a coaxial DBD reactor was employed in this study. An 8 cm long stainless-steel mesh was wrapped outside the quartz tube (14 mm external diameter and 10 mm inner diameter) and was grounded through an external capacitor C.sub.ext (0.47 ρF). A stainless-steel rod (diameter 5 mm) was fixed in the centre of the quartz tube as the high voltage electrode. The discharge gap was 2.5 mm. The DBD reactor was connected to a high voltage AC power supply (Suman). The total discharge power was fixed at 30 W under different experimental conditions. The specific energy input was fixed at 57 kJ/L. Soap-film flowmeter was used to measure the gas flow rate after the reaction. Catalyst (0.5 g) was packed in the center of the discharge area. The feed gas was composed of 20 ml/min H.sub.2 and 10 ml/min CO.sub.2 (H.sub.2/CO.sub.2=2:1).

[0134] Coaxial DBD plasma reactor without external heating or cooling (electrode gap: 2.5 mm; discharge length 8 cm; inner electrode: SS rod, 5 mm diameter; outer electrode SS mesh); SEI:

[0135] 57 kJ/L; discharge power: 30 W.

[0136] Gas: H.sub.2/CO.sub.2=2:1 (total 30 ml/min); GHSV 2684 h.sup.−1.

[0137] Catalyst: 0.5 g; 40-60 mesh; packing length 1.2 cm; reaction temperature: 85-120° C.

[0138] The gas products were analyzed by a gas chromatograph (Agilent GC 7820A) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). Each measurement was repeated three times and had a high reproducibility with a measurement error of less than 1%.

TABLE-US-00003 TABLE 3 Physical properties of the fresh catalysts (after reduction) Surface area Pore volume Pore size Catalysts (m.sup.2/g) .sup.a (cm.sup.3/g) .sup.a (nm) .sup.a Al.sub.2O.sub.3 (comp) 221 0.45 3.8 Fe/Al.sub.2O.sub.3 (comp) 181 0.38 3.6 FeMn/Al.sub.2O.sub.3 (comp) 193 0.37 3.5 Cu/Al.sub.2O.sub.3 (comp) 194 0.40 3.9 CuMn/Al.sub.2O.sub.3 (comp) 201 0.38 3.9 Ni/Al.sub.2O.sub.3 199 0.40 3.8 NiMn/Al.sub.2O.sub.3 202 0.39 3.8 .sup.a Measured by N.sub.2-adsorption

[0139] The Brunaur-Emmett-Teller (BET) specific surface area, pore volume and average pore diameter of the catalysts are listed in Table 3. The γ-Al.sub.2O.sub.3 support exhibits a surface area of 221 m.sup.2.Math.g.sup.−1 which is larger than other catalysts. It is probably due to the part of dopant blocking of the pore structures. Also, by the addition of Mn promoter, the surface area is slightly larger than that of the catalysts without Mn.

[0140] FIG. 5A shows XRD patterns of fresh catalysts after calcination; and FIG. 5B shows XRD patterns of spent catalysts.

[0141] The crystallite structures of the fresh and spent catalysts are presented in FIGS. 5A and 5B. The diffraction pattern for the Al.sub.2O.sub.3 has a number of peaks that can be well-indexed to γ-Al.sub.2O.sub.3(JCPDS79-1558). Sharp and intense peaks appearing in the Fe can be indexed to Fe.sub.2O.sub.3 (JCPDS33-0664) and those in the Cu-based catalysts can be indexed to CuO (JCPDS44-0706). Meanwhile, NiO characteristic peaks in 37.2°, 43.3° and 62.9° (JCPDS47-1049), corresponding to the NiO (111), (200) and (220) crystal planes respectively, were detected on Ni. However, after Mn was added to the catalysts, no characteristic diffraction peaks of MnO.sub.x or metal-salt could be observed because of its lower loading content and weak crystallization. Furthermore, note that the diffraction peaks of the metallic oxides became weaker and broader, and even disappeared. These indicate that the particles on the supporter were smaller and evenly distributed.

[0142] According to the XRD patterns of the catalysts after the reaction, metallic Fe, Cu and Ni are detected, implying the different degrees of reduction for the metallic oxides during the reaction. This can be ascribed to the part of thermal H.sub.2-reduction treatment before the plasma and interaction with the H species in the plasma.

[0143] FIGS. 6A to 6F show metal particle size dispersion of fresh catalysts after reduction on: FIG. 6A Fe/Al.sub.2O.sub.3; FIG. 6B FeMn/Al.sub.2O.sub.3; FIG. 6C Cu/Al.sub.2O.sub.3; FIG. 6D CuMn/Al.sub.2O.sub.3; FIG. 6E Ni/Al.sub.2O.sub.3; FIG. 6F NiMn/Al.sub.2O.sub.3.

TABLE-US-00004 TABLE 4 Chemical properties of the catalysts H.sub.2 consumed CO.sub.2 desorbed Metal/Al Catalysts (mmol/g) .sup.b (mmol/g) .sup.c (%) .sup.d Al.sub.2O.sub.3 (comp) — 2.32 — Fe/Al.sub.2O.sub.3 (comp) 0.40 2.23 3.32 FeMn/Al.sub.2O.sub.3 (comp) 0.54 1.48 3.04 Cu/Al.sub.2O.sub.3 (comp) 0.73 1.90 2.72 CuMn/Al.sub.2O.sub.3 (comp) 0.75 1.96 2.77 Ni/Al.sub.2O.sub.3 0.62 1.67 2.76 NiMn/Al.sub.2O.sub.3 0.76 1.98 3.69 a. Calculated via H.sub.2-TPR analysis of the fresh catalysts after calcination .sup.b Calculated via CO.sub.2-TPD analysis of the fresh catalysts after reduction .sup.c Calculated via XPS analysis of the fresh catalysts after reduction

[0144] Effect of 7 wt. % X/Al.sub.2O.sub.3 and 7 wt. % X1 wt. % Mn/Al.sub.2O.sub.3 catalysts (X═Fe, Cu and Ni)

[0145] Note the DBD reactor and operating conditions used in this experiment are different to those used in example 1 and 3, especially H.sub.2/CO.sub.2 molar ratio. The NiMn catalyst (7 wt. % Ni/1 wt. % Mn/Al.sub.2O.sub.3) shows the highest conversion of CO.sub.2 and CH.sub.4 selectivity. However, the performance of this catalyst under these conditions is lower than that shown in example 1 and 3. The NiMn catalyst is still very stable after running the reaction for 8 hours.

[0146] FIG. 7A shows CO.sub.2 and H.sub.2 conversions; FIG. 7B gaseous product selectivities; FIG. 7C shows gaseous product yields; and FIG. 7D shows CO.sub.2 and CH.sub.4 energy efficiencies (H.sub.2/CO.sub.2=2:1, GHSV=2684 h.sup.−1, 1 atm, SEI=57 kJ/L);

[0147] FIG. 7A shows CO.sub.2 and H.sub.2 conversion on Al.sub.2O.sub.3 and metal-loaded catalysts with the plasma. For the comparison, all of the catalysts were investigated in the same experimental conditions at low temperatures (85-120° C.) in a furnace and without plasma, which result in no conversion. With the presence of plasma, the CO.sub.2 conversion for the catalysts without Mn follows the order Fe<plasma alone<Al.sub.2O.sub.3<Cu<Ni. After the addition of Mn, the H.sub.2 conversions were slightly increased in Fe and Cu-based catalysts which are in good agreement with H.sub.2-TPR results. However, there is no obvious promotion for CO.sub.2 which indicates that the reactions still mainly occurred in the plasma gaseous phase. Interestingly, as to NiMn, H.sub.2 and CO.sub.2 conversions were considerably increased after Mn promotion, reaching nearly 50% for CO.sub.2 and surpassing 80% for H.sub.2 which are very closed to the limitation of the reaction equilibrium towards CO.sub.2 methanation.

[0148] As presented in FIG. 7B, the product selectivities were also significantly affected combining with the catalysts, especially Ni-based catalysts. However, when it comes to Cu-based and Fe-based catalysts, their results are very similar with that using plasma only, most of the CO.sub.2 conversion was triggered by CO.sub.2 deoxygenation because of the electron collisions and decomposition to CO in the plasma. The CH.sub.4 selectivity (around 45% for Ni) was further enhanced by the presence of Mn and reached a maximum of 60%. In this case, CH.sub.4 becomes the main product whilst olefins, ethane, propane and butane as by-products. Other carbons in the liquid phase was dominated with methanol. Moreover, the CH.sub.4 yield of NiMn was much higher than the sum of solo Mn and Ni catalysts. This might be a result of the stronger interaction and electron transfer between Ni and Mn metals rather than Fe or Cu, which contributed to the path of CO.sub.2 methanation.

[0149] FIG. 8 shows reaction performance over the NiMn/Al.sub.2O.sub.3 catalyst as a function of time (H.sub.2/CO.sub.2=2:1, GHSV=2684 h.sup.−1, 1 atm, SEI=57 kJ/L).

[0150] With the recent booming development of renewable technologies, the supply of hydrogen via water electrolysis has become economically available option (to some extent) and thus plasma-catalytic CO.sub.2 methanation. In terms of energy efficiency of our work, the figures obtained with NiMn exceeded 0.4 mol/kWh. Thus, for each kWh supplied, more than 0.43 mol of carbon dioxide was converted and nearly 0.26 mol of CH.sub.4 was produced, enhancing the CH.sub.4 production achieved with plasma alone by a factor of 31.

[0151] Moreover, the stability of NiMn catalyst in the plasma under the same reaction conditions was investigated, as shown in FIG. 8. The high stability and effectively catalytic ability indicate that the carbon deposit and catalyst poisoning could be suppressed consummately. This low energy consumption technology makes it possible to produce synthetic CH.sub.4 together with CO.sub.2 emissions reduction without any additional heating or adiabatic apparatus, which could significantly reduce of negative investment and extend the service life of catalysts. Indeed, the flexibility of the production system allows the use of electrical energy produced off-peak (<30 €/MWh) or from the excess production coming from renewable sources, which is possible for an industrial scale.

TABLE-US-00005 TABLE 5 Summary of results for different catalysts. CO.sub.2 Conversion CH.sub.4 Selectivity CH.sub.4 Yield Catalyst (%) (%) (%) Plasma only 30.8 3.1 0.9 Al.sub.2O.sub.3 33.5 3.1 1.0 1 wt % Mn/Al.sub.2O.sub.3 35.6 2.7 0.9 Fe/Al.sub.2O.sub.3 28.7 2.8 0.8 FeMn/Al.sub.2O.sub.3 28.5 2.6 0.7 Cu/Al.sub.2O.sub.3 36.5 2.9 1.0 CuMn/Al.sub.2O.sub.3 35.9 2.5 0.9 Ni/Al.sub.2O.sub.3 44.8 45.3 20.2 NiMn/Al.sub.2O.sub.3 50.3 59.7 30.0

[0152] In summary, our work further demonstrates that the combination of plasma and different catalysts offers very interesting opportunities for CO.sub.2 methanation. The totally different catalytic ability and reaction mechanism in the plasma for Fe, Cu, Ni are significantly divergent from that in thermal catalysis field. Apart from Fe and Cu, the promoter Mn successfully enhanced the CO.sub.2 conversion and CH.sub.4 selectivity by the promotion the dispersion of Ni particles with the formation of smaller Ni sizes, the decrease of interaction effect between nickel and aluminum, and increment of weak-moderated basic sites for carbon oxide species adsorption. The excellent performance of NiMn catalyst coupling with the plasma makes it possible to obtain high CH.sub.4 yield by less energy consumption (electrical and hydrogen supply) without any other heating equipment.

Example 3: 7 wt. % Ni/1 wt. % X/Al.SUB.2.O.SUB.3 .(X═Zr, Mg, Ce, Mn) Catalysts

[0153] Al.sub.2O.sub.3 supported 7 wt. % Ni and 1 wt. % promoters (Zr, Mg, Ce, Mn) were prepared using an incipient wetness impregnation technique. In a typical procedure, the metal precursors of nitrate hydrate for nickel (II), zirconium(IV), magnesium(II), cerium(III) and manganese (II) were dissolved in 4.8 mL of deionized water. The solution was then added dropwise to 2 g of γ-Al.sub.2O.sub.3(Aladdin, 40-60 mesh) and mixed. The impregnated samples and pure Al.sub.2O.sub.3 were dried overnight in the air, with further dehydration in the oven at 120° C. for 4 h, then transferred to a muffle furnace to be calcined at 450° C. for 5 h. The calcined catalysts were pre-reduced under the mixed gas of 20 mL/min H.sub.2 and 30 mL/min Ar at 600° C. for 5 h before the reaction. The obtained catalysts were labeled as NiXAI (X═Zr, Mg, Ce and Mn).

[0154] The performance of the catalysts for CO.sub.2 hydrogenation was evaluated in a coaxial DBD reactor, same to in Example 1. As is shown in FIG. 9B, a 6 cm long stainless-steel mesh was wrapped outside the quartz tube (6 mm i.d.), acting as a ground electrode through an external capacitor C.sub.ext (0.47 μF). The inner high voltage electrode was a stainless-steel rod (diameter 2 mm), fixed in the center of the quartz tube. The DBD reactor was supplied by a high voltage AC power supply with a maximum peak voltage of 30 kV and a frequency of 5-20 kHz. All the catalysts (0.5 g) were packed into the center of the plasma reactor discharge area. H.sub.2 and CO.sub.2 were used as reactant gases. The discharge power was calculated using the Lissajous method. Besides, an online power measurement system was used to monitor the discharge power of the DBD reactor in real-time. The specific energy input (SEI) was controlled from 1 kJ/L to 19 kJ/L.

[0155] Catalyst: 0.5 g; 40-60 mesh; packing length 4 cm; reaction temperature: 125-155° C.

[0156] During the CO.sub.2 hydrogenation reactions, the gas products were quantified with a gas chromatograph (Agilent GC 7820A) equipped with a flame ionisation detector (FID) and a thermal conductivity detector (TCD). Each measurement was repeated three times and had a high reproducibility with a measurement error of less than 1%.

TABLE-US-00006 TABLE 6 Physical properties of the fresh catalysts (after reduction) Surface area Catalysts (m.sup.2/g) .sup.a Al.sub.2O.sub.3 (comp) 206 NiAl 141 NiZrAl 175 NiMgAl 170 NiCeAl 173 NiMnAl 158 .sup.a Measured by N.sub.2-adsorption

[0157] The values of specific BET surface area of the reduced catalysts are listed in Table 6.

[0158] The bare γ-Al.sub.2O.sub.3 supporter exhibits a surface area of 206 m.sup.2/g which is larger than other loaded catalysts (from 141 to 175 m.sup.2/g). This may be due to the blocking of pore structures. However, by the addition of promoters, the surface areas grow with respect to single-metal catalysts, which result from the suppression of agglomeration during the reduction process. Despite these delicate differences, the physical properties of the catalysts are quite similar to each other, irrespective of what kind of promoters are used.

[0159] The crystallite structures of the calcined, reduced and spent catalysts are presented in FIGS. 10A to 10C, respectively. The diffraction pattern of Al.sub.2O.sub.3 has several peaks that can be well-indexed to γ-Al.sub.2O.sub.3(JCPDS10-0425). According to the XRD patterns of the catalysts before the reduction, only weak and inconspicuous NiO characteristic peaks at 37.2°, 43.3° and 62.9° (JCPDS47-1049) were detected among the Ni-based catalysts. After being reduced at 600° C., the metallic Ni characteristic peaks in 44.5°, 51.8° and 76.4° (JCPDS65-2865), corresponding to the Ni (111), (200) and (220) crystal planes respectively, were evidently confirmed on the catalysts. It is interesting to note that the metal diffraction peaks become weaker and broader, which indicates that the particles on the support are smaller and evenly distributed. Furthermore, after Mn was added to the Ni-based catalyst, the characteristic diffraction peaks of neither Mn nor Ni could be observed because of the weak crystallization and suppression of agglomeration during the high-temperature reduction process. After the reaction, the characteristic peaks in FIG. 10C exhibiting the same position and trend indicate the stability of the catalysts during the plasma discharge.

[0160] FIGS. 11A to 11D show metal particle size distributions on fresh catalysts after reduction;

[0161] FIG. 11A NiZrAl; FIG. 11B NiMgAl; FIG. 11C NiCeAl; and FIG. 11D NiMnAl.

TABLE-US-00007 TABLE 7 Chemical properties of the catalysts H.sub.2 consumed CO.sub.2 desorbed Ni/Al Catalysts (mmol/g) .sup.b (mmol/g) .sup.c (%) .sup.d Al.sub.2O.sub.3 (comp) — 2.55 — NiAl 0.43 2.24 2.70 NiZrAl 0.46 2.18 2.57 NiMgAl 0.48 2.51 2.48 NiCeAl 0.48 2.30 2.94 NiMnAl 0.51 1.80 3.79 a. Calculated via H.sub.2-TPR analysis of the fresh catalysts after calcination .sup.b Calculated via CO.sub.2-TPD analysis of the fresh catalysts after reduction .sup.c Calculated via XPS analysis of the fresh catalysts after reduction

[0162] Effect of 7 wt. % Ni/1 wt. % X/Al.sub.2O.sub.3 catalyst (X═Zr, Mg, Ce, Mn)

[0163] The NiMnAl (7 wt. % Ni/1 wt. % Mn/Al.sub.2O.sub.3) catalyst shows the best performance: highest conversion of CO.sub.2 (˜87%) and highest methane selectivity of 99%. Investigated in various gas ratio, gas flow rate and discharge power, this catalyst shows the highest space time yield of CH.sub.4 for 1500 μmol/g.sub.Ni/S. It is also very stable after running the reaction for 8 hours.

[0164] 1. Performance at Low Flow Rate [0165] Specific energy input (SEI): 5-19 kJ/L; discharge power=8.5 W [0166] Gas: H.sub.2/CO.sub.2=4:1 (total 30-100 ml/min); gas hourly space velocity (GHSV) 1792-5971 h.sup.−1 [0167] Catalyst: 0.5 g; 40-60 mesh; packing length 4 cm; reaction temperature: 125-155° C.

[0168] FIG. 12A shows conversion of CO.sub.2 and H.sub.2; FIG. 12B shows selectivity of gas products;

[0169] FIG. 12C shows yield of gas products; and FIG. 12D shows energy efficiency for CO.sub.2 conversion and CH.sub.4 production at different gas flow rates.

[0170] FIG. 12A shows CO.sub.2 and H.sub.2 conversion on the Al.sub.2O.sub.3 and metal-loaded catalysts under the plasma activation. The temperature of the reaction was 125-155° C. when the reaction reaches stability. For comparison, all of the catalysts were investigated for CO.sub.2 hydrogenation under the same experimental conditions at 150° C. without plasma. However, no conversion could be initiated by thermal catalysis.

[0171] As is shown in FIG. 12A, in the presence of plasma, promoters have different levels of improvement for the methanation process, while only 19% of CO.sub.2 converted in the absence of a catalyst. Under plasma assistance, the CO.sub.2 conversions for the catalysts follow the order: plasma alone<Al.sub.2O.sub.3<NiAl<NiZrAl<NiMgAl<NiCeAl<NiMnAl. Interestingly, as to NiMnAl, the considerable increment of H.sub.2 and CO.sub.2 conversions were encountered, reaching nearly 83% for CO.sub.2 while surpassing 81% in the case of H.sub.2.

[0172] As presented in FIG. 12B, the product selectivities were also significantly affected combining with the catalysts. For the plasma only and aluminum oxide, the CO.sub.2 molecules dissociated directly to CO via the collision of electrons in the plasma. Moreover, the CH.sub.4 selectivity (around 74% for NiAl) was further enhanced by the addition of Mn reaching a maximum of 88%. In this case, CH.sub.4 became the dominated product.

[0173] In terms of methane energy efficiency, the figures (FIG. 12C) obtained with NiMnAl exceeded 2.7 mol/kWh, which had achieved in particular for plasma alone by a factor of 423. Then the NiMnAl catalyst packed in the plasma was investigated by varying the total flow rate with a constant H.sub.2/CO.sub.2 ratio and discharge power. The EE for both CO.sub.2 and CH.sub.4 increases to 2.5 times when the flow rate rises to 100 ml/min. Moreover, the stability of NiMnAl catalyst was investigated both in the continuous plasma and over 5 cycles, as shown in FIG. 12D. The high stability and effectively catalytic ability of this catalyst within the plasma indicated that the undesirable carbon deposit and catalyst poisoning from the C, CO and H.sub.2O during the reaction could be suppressed consummately. This low energy consumption technology makes it possible to produce synthetic CH.sub.4 together with CO.sub.2 emissions reduction without any additional heating or adiabatic apparatus, which could significantly help negative investment with the service life of the catalyst extended.

[0174] 2. Performance at High Flow Rate

[0175] Specific energy input (SEI): 0.5-17 kJ/L; discharge power=5-10 W; [0176] Gas: H.sub.2/CO.sub.2=4:1 (total 30-500 ml/min); gas hourly space velocity (GHSV) 2090-29857 h.sup.−1 [0177] Catalyst: 0.5 g; 40-60 mesh; packing length 4 cm; reaction temperature: 125-155° C.

[0178] FIG. 14A shows conversion of H.sub.2 and CO.sub.2; FIG. 14B shows selectivity of gas products;

[0179] FIG. 14C shows energy efficiency for CO.sub.2 conversion and CH.sub.4 production at different gas flow rates; FIG. 14D shows conversion of H.sub.2 and CO.sub.2 at different reaction temperatures over NiMnAl (H.sub.2/CO.sub.2=4:1, GHSV=29857 h.sup.−1, 1 atm); and FIG. 14E shows the converted amounts of CO.sub.2 and H.sub.2 over NiMnAl at different H.sub.2/CO.sub.2 molar ratios (GHSV=29857 h.sup.−1, 1 atm, SEI=1.2 kJ/L).

[0180] FIG. 14A shows CO.sub.2 and H.sub.2 conversion on the bare Al.sub.2O.sub.3 and metal-loaded catalysts under the plasma activation. For comparison, all of the catalysts were investigated under the same experimental conditions at 180° C. without plasma. However, no conversion could be initiated by the thermal catalysis.

[0181] As is shown in FIG. 14A, in the presence of plasma, promoters have different levels of improvement for the methanation process, while only 6% of CO.sub.2 converted in the absence of a catalyst at a high gas flow rate. Under plasma assistance, the CO.sub.2 conversions for the catalysts follow the order: Al.sub.2O.sub.3<plasma alone<NiAl<NiZrAl<NiMgAl<NiCeAl<NiMnAl. Most catalysts have a significant conversion decrease due to the increased gas flow rate, which could reduce the reaction residence time. Interestingly, as to NiMnAl, H.sub.2 and CO.sub.2 conversions were still substantial, reaching nearly 75% for CO.sub.2 while surpassing 70% in the case of H.sub.2.

[0182] As presented in FIG. 14B, the product selectivities were also significantly affected combining with the catalysts. For the plasma only and aluminum oxide, the CO.sub.2 molecules dissociated directly to CO. Moreover, the CH.sub.4 selectivity (around 74% for NiAl) was further enhanced by the addition of Mn reaching a maximum of 99.6%. In this case, CH.sub.4 became the dominated product.

[0183] In terms of the energy efficiency for methane production, the figures (FIG. 14C) obtained with NiMnAl exceeded 17.7 mol/kWh, which had achieved in particular for plasma alone by a factor of 1397. Then the NiMnAl catalyst packed in the plasma was investigated by varying the total flow rate with a constant H.sub.2/CO.sub.2 ratio and discharge power. The EE for both CO.sub.2 and CH.sub.4 is increased by 13 times when the flow rate rises to 500 ml/min. Moreover, the stability of the NiMnAl catalyst was investigated both in the continuous plasma and over 5 cycles, as shown in FIGS. 15A and 15B. The high stability and catalytic activity of this catalyst within the plasma indicated that the undesirable carbon deposit and catalyst poisoning from the C, CO and H.sub.2O during the reaction could be suppressed consummately.

[0184] This low energy consumption technology makes it possible to produce synthetic CH.sub.4 together with CO.sub.2 emissions reduction at a more flexible reaction condition. Comparing with other promoters at the higher gas low rate, the excellent performance of NiMnAl catalyst coupling with the plasma makes it possible to obtain high CH.sub.4 yield by less energy consumption (electrical and hydrogen supply) without any other heating or adiabatic equipment. Indeed, the flexibility of the production system allows the use of electrical energy produced off-peak (<30 €/MWh) or from the excess production coming from renewable sources, which strengthens the prospects for commercial application.

Example 4: 7 wt. % Ni/1 wt. % Mn/Al.SUB.2.O.SUB.3 .Catalyst (NiMnAl)

[0185] The NiMnAl catalyst (7 wt. % Ni/1 wt. % Mn/Al.sub.2O.sub.3) was also tested in a scaled-up DBD reactor (with a larger dimension and discharge volume) for plasma-catalytic CO.sub.2 methanation at 1 atm. As is shown in FIG. 16, a coaxial DBD reactor was employed in this study. An 8 cm long stainless-steel mesh was wrapped outside a quartz tube (inner diameter 22 mm, outer diameter 25 mm) and was grounded through an external capacitor C.sub.ext(0.47 μF). A stainless-steel rod (outer diameter 16 mm) was fixed in the centre of the quartz tube as the high voltage electrode. The discharge gap was 3 mm. The DBD reactor was connected to a high voltage AC power supply. No additional heating or cooling was used for this DBD reactor. The discharge power was 30 W or 40 W. The SEI was controlled from 10 kJ/L to 60 kJ/L. Catalyst (5.8 g, catalyst particle size 40-60 mesh, packing length 8 cm) was packed in the center of the discharge area. H.sub.2 and CO.sub.2 were used as reactant gases at a H.sub.2/CO.sub.2 molar ratio of 4:1 and a total flow rate of 30-200 ml/min. The GHSV was 100-900 h.sup.−1. The reaction temperature was 125-175° C.

[0186] FIG. 17 shows the reaction performance of plasma-catalytic CO.sub.2 methanation using the NiMnAl catalyst in a scaled-up DBD reactor. Compared to the same catalyst used in Example 3, similar CO.sub.2 conversions and CH.sub.4 selectivity were achieved using the scaled-up reactor. At a total flow rate of 30 ml to 90 ml/min (GHSV 134-402 h.sup.−1), the highest CO.sub.2 conversion was 68% at a discharge power of 30 W. At a discharge power of 40 W, the highest CO.sub.2 conversion (79%) and CH.sub.4 selectivity (97%) were achieved at a high total gas flow rate of 200 ml/min (GHSV 898 h.sup.−1). These results show the potential to scale-up this plasma-catalytic process for CO.sub.2 methanation.

[0187] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0188] All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and/or steps are mutually exclusive.

[0189] Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0190] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.