Acknowledgement

The invention described hereafter has been generated within the research projects entitled "Multiphase CNT/Green silica nanofillers for advanced tires, Green NanONano" and "Environmental health Risk Evaluation for nano-Structures (NSs) Developed for Water-Remediation, NanEAU II", supported by the National Research Fund, Luxembourg (Ref. C10/MS/852466 and C10/SR/825684).

Description

Technical field

[0001] The invention is directed to the development of a catalyst for the production of methane from carbon dioxide.

(Background art

[0002] Sabatier's reaction

[0003] The methanation of carbon dioxide was discovered by the chemist Paul

Sabatier in 1910s. It involves the reaction of hydrogen with carbon dioxide at elevated temperatures (about 300°C-400°C) and pressures in the presence of catalyst to produce methane, water and energy. It is described in the following exothermic reaction:

CO ₂ + 4H ₂ → CH ₄ + 2H ₂ O + energy (heat)

[0004] Implementation of this chemical process has been achieved for instance by the NASA for cabin atmosphere regeneration.

[0005] As CO2 is a worldwide polluting greenhouse effect gas, its elimination and valorisation into fuels appears as promising possibility for numerous industrial applications.

[0006] However, this chemical process is sometimes impaired by the fact that it also leads to by-product such as carbon monoxide (CO), notably in reverse water gas shift reaction:

CO ₂ + H ₂ + energy→ CO + H ₂ O

[0007] Carbon monoxide is a well-known dangerous gas. [0008] A second possible impairment of the Sabatier's reaction is the modified Fisher-Tropsch synthesis, where CO2 is used instead of CO, leading to the synthesis of hydrocarbons in accordance with the following system:

CO ₂ + H ₂ + energy→ CO + H ₂ O

CO + 2 H ₂ → -CH2- + H ₂ O + energy

[0009] In order to solve these problems, several catalysts have been developed. It is known that catalysts based on copper favours the carbon monoxide formation while the nickel-based catalyst rather favours the methane production. Highly dispersed nickel catalysts supported on various oxides have been described. Generally speaking, the performance of nickel-based catalysts toward CO2 methanation is dependent on various parameters, such as the effect of the support {e.g. S1O2, CeO2, 01-AI2O3, ΤΊΟ2, MgO, ZrO2), the effect of nickel loading, the effect of a second metal (e.g. Fe, Zr, Co, La, Y, Mg) and the effect of the preparation method (Wang W. et al., Chem. Soc. Rev., 2011 , 40, 3703- 3722; and Aziz M. A. A., et al., Green Chem., 2015, 17, 2647-2663).

[0010] In particular, it was shown that cobalt metal addition (Ni being the main metal) can significantly improve the catalytic stability of the catalysts.

[001 1] Other metals (e.g. ruthenium, rhodium, cobalt) than nickel have been investigated. The bimetallic Co-Ru ultrathin film is known to enhance the yield of methane (Zhu Y. et al., ACS Catal., 2012, 2, 2403-2408).

[0012] One study has shown that the selectivity of CO2 methanation over the conversion of CO2 to hydrocarbons, carried out with a catalyst made of nickel is 99.8%, with a nickel catalyst supported on alumina is 99.4%, with a nickel catalyst supported on silica is 99.8 % and with a ruthenium catalyst supported on silica is 99.4%. Nothing is described about the selectivity of the formation of CH ₄ over the formation of CO. The same catalysts have however a poorer selectivity in the conversion of CO to CH ₄ , i.e. between 40% and 90% (Fujita S.-l., Takezawa N., Chem. Eng. J., 1997, 68, 63-68).

[0013] Other catalyst, such as highly porous gallium oxide with a high CO2 affinity for the photocatalytic conversion of CO2 into CH ₄ has been developed (Park H-A., et al., J. Mater. Chem., 2012, 22, 5304-5307). [0014] A drawback of the Sabatier's reaction is that the known catalysts cannot produce the methane without being totally selective over the desired products, leading to toxic by-product (such as carbon monoxide) or to unwanted product (such as hydrocarbon).

Summary of invention

Technical Problem

[0015] The invention has for technical problem to alleviate the drawbacks of present in the art, notably by the lack of fully selective catalyst in the formation of methane from carbon dioxide.

Technical solution

[0016] The first object of the invention is directed to the use of zinc oxide support covered by cobalt nanoparticles as a catalytic system for the Sabatier's reaction.

[0017] According to one embodiment, the zinc oxide support is a macroporous, mesoporous or microporous support, preferentially nanorods.

[0018] According to one embodiment, the step of reacting carbon dioxide gas and hydrogen gas at a pressure comprises between atmospheric pressure and 6 bar, and at a temperature comprises between 450°C and 500°C.

[0019] According to one embodiment, said pressure is 6 bar and said temperature is 500°C.

[0020] According to one embodiment, said Sabatier's reaction is carried out during 4 hours.

[0021] According to one embodiment, the surface coverage of zinc oxide nanorods by cobalt nanoparticles is equal to 12%.

[0022] The second objection of the invention is directed to a method for a controlled-deposition of cobalt onto zinc oxide, forming a zinc oxide support covered by cobalt nanoparticles for use as a catalytic system for the Sabatier's reaction in accordance with the first object of the invention. Said method is carried out with a fluidized bed reactor and comprises the steps of (a) fluidization of zinc oxide, (b) impregnation of the fluidized zinc oxide of step (a) by sublimated cobalt (II) acetylacetonate powder, wherein said fluidized bed reactor comprises a gas inlet and a gas outlet being located downstream from said gas inlet, a tube inserted between said gas inlet and said gas outlet, a heating part connected to said tube, wherein said tube comprises an upstream zone and a downstream zone, and wherein said upstream zone and said downstream zone are separated by a separation filter.

[0023] According to one embodiment, said tube is made of material which is resistant to temperature of at least up to 1000°C and said tube is transparent.

[0024] According to one embodiment, said tube is made of quartz.

[0025] According to one embodiment, said heating part is a heating cable, a heating jacket and/or any thermal activation source.

[0026] According to one embodiment, said zinc oxide has been synthesized by forming an equimolar solution of zinc acetate dihydrate and hydrazine hydrate followed by the subsequent step of heating at 150°C under a pressure of 3 bar during 30 minutes.

[0027] According to one embodiment, step (b) is carried out with a flow of gas.

[0028] According to one embodiment, said flow of gas is composed of 80 seem of nitrogen gas and 20 seem of hydrogen gas.

[0029] According to one embodiment, said step (b) is carried out during 30 minutes.

[0030] According to one embodiment, said sublimated cobalt(ll) acetylacetonate powder is formed by thermal decomposition of cobalt(ll) acetylacetonate powder at 600°C under a vacuum of 7 mbar.

Advantages of the invention

[0031] The invention is particularly interesting in that the supported catalyst of the present invention has a huge surface to volume ratio, is really cheap to synthesize and promotes an excellent selectivity (superior to 99%) into the methanation of carbon dioxide. No by-product gases are formed during the chemical process, resulting subsequently in no production at all of toxic carbon monoxide. As well, no hydrocarbons by-products have been observed when performing the Sabatier's reaction.

Brief description of the drawings

[0032] Figure 1 : SEM picture of ZnO nanorods covered by Co nanoparticles.

[0033] Figure 2: Bar chart indicating the conversion rate of CO2 into CH ₄ .

[0034] Figure 3: Bar chart indicating the conversion selectivity for CH ₄ formation.

[0035] Figure 4: Scheme of the fluidized bed reactor used for developing the

ZnO nanorods covered by Co nanoparticles.

Description of an embodiment

[0036] Zinc oxide is a cheap metal oxide material that is known by the skilled person in the art in numerous implementations, such as photocatalysis, application as transparent conducting oxides and application as piezoelectricity or gas sensing.

[0037] ZnO is further known to have good thermal properties.

[0038] ZnO can be synthesized under many different shapes and sizes.

[0039] Copper catalyst supported on ZnO has been described in the literature, notably for the reverse water-gas shift reaction (Stone. F. S., Waller D.,

Topics in Cata., 2003, 22, 3-4).

[0040] In the field of ferromagnetic composite materials, cobalt nanoparticles onto ZnO nanorods have been developed (Lee M. L, et al., Vacuum,

2015, 118, 90-93).

[0041] All chemicals were purchased from Sigma-Aldrich and used as received.

ZnO nanorods were synthesized using a microwave assisted hydrothermal method in a Monowave 300 scientific microwave from Anton Parr. An equimolar solution of 25 mM of zinc acetate dihydrate Zn(COOCH ₃ ) ₂ .2H ₂ O 98% and hydrazine hydrate N ₂ H ₄ .H ₂ O 50-60% was prepared and stirred for 15 min in order to form a [Zn(CH3COO) ₂ ]m[N ₂ H ₄ ]n complex. The pH of the solution was measured to be around 10. [0042] In order to generate the nanorods of ZnO, heating under pressure is required. Generally, the temperature is comprised between 130°C and 170°C, preferentially between 140°C and 160°C. The pressure is comprise between 2 bar and 4 bar, preferentially between 2.5 bar and 3.5 bar. The reaction time is comprised between 20 minutes and 40 minutes, preferentially between 25 minutes and 35 minutes.

[0043] More specifically, a quantity of the complex solution of

[Zn(CH3COO)2]m[N2H ₄ ]n is introduced in hermetically closed vials. In this case, 20 ml of the prepared solution was introduced in 30 ml hermetically closed vials.

[0044] Then, 30 minutes of reaction at 150 °C under a pressure of 3 bar have allowed the synthesis of dumbbell-shaped nanorods in the solution.

[0045] Generally speaking, in order to develop the synthesis, different supports can be envisioned. Such supports are usually macroporous support, mesoporous support or microporous support.

[0046] Nanorods, nanoparticles, nanofilms, nanourchins or nanowires can also be envisioned as support.

[0047] Once the support has been prepared (here, a support made of ZnO nanorods), such support has to be functionalized with the metal of interest (here cobalt nanoparticles). Such functionalisation can be achieved by gas phase impregnation, for example in a fluidized bed reactor as depicted in figure 4.

[0048] The fluidized bed reactor 100 is equipped with a tube 2 which is transparent and resistant to high temperature (up to 1000°C, preferentially up to 1500°C or even more). Said tube 2 is preferably made of quartz. Said tube 2 is inserted between a gas inlet 4 and a gas outlet 6. The gas outlet 6 is located downstream from the gas inlet 4. The connection between the tube 2 and the gas inlet 4 is sealed by a first seal 8 whose the diameter is comprised between 50 mm and 60 mm. This diameter is sufficient to insert the tube 2 inside the first seal 8. The connection between the tube 2 and the gas outlet 6 is sealed by a second seal 10 whose the diameter is comprised between 50 mm and 60 mm. This diameter is sufficient to insert the tube 2 inside the second seal 10.

[0049] In addition to be transparent and resistant to high temperature, the tube

2 is designed for being resistant to high vacuum and is electrically insulated.

[0050] The tube 2 is divided in two zones, an upstream zone and a downstream zone, the terms upstream and downstream being defined according to the direction of the gas.

[0051] The upstream zone is adapted to comprise a solid precursor, or a precursor powder to be sublimated 26.

[0052] The downstream zone is adapted to comprise a solid support 28 onto which the precursor powder to be sublimated 26 must be impregnated.

[0053] The gas phase impregnation is only possible when the precursor powder to be sublimated 26 has been sublimated. This is possible by the fact that this precursor powder to be sublimated 26 is placed within the upstream zone of the tube 2 of the fluidized bed reactor 100, said upstream zone of the tube being connected to a heating part 18 of the fluidized bed reactor.

[0054] The upstream zone and the downstream zone are physically separated by a separation filter 14 which is preferentially porous.

[0055] The tube 2 is further closed by a first porous filter 12 and a second porous filter 16. Those first and second porous filter are needed for containing the different materials present in the tube 2. When the first porous filter 12 and the second porous filter 16 are placed on the tube, they thus prevent the materials to exit the tube 2.

[0056] The separation filter 14, the first porous filter 12 and the second porous filter 16 are completely inserted within the tube 2.

[0057] The tube 2 is surrounded by a heating part 18, which is adapted to increase the temperature. The heating part 18 may be a heating cable, a heating jacket and/or any thermal activation source. The heating part 18 is further adjusted to the upstream zone which contains the powder precursor.

[0058] The upstream zone and the downstream zone of the tube are both surrounded by the heating part 18. [0059] The tube 2 has a cylindrical shape which is featured by a length comprised at least between 300 mm and 400 mm and by a diameter comprised at least between 25 mm and 30 mm. The thickness of the quartz layer is comprised at least between 2 mm and 4 mm.

[0060] A vibrator 20 may be positioned outside the tube 2, between the gas inlet 4 and the first porous filter 12. The vibrator 20 is adapted to enhance the fluidization particles.

[0061] A first valve 22 is positioned between the gas inlet 4 and the vibrator 20 and/or the first porous filter 12. The first valve 22 is useful for controlling the amount of gas and/or the rate of gas which is injected into the quartz tube 2.

[0062] A second valve 24 is positioned between the second porous filter 16 and the gas outlet 6. The gas outlet 6 is connected to a pumping system (not shown). The second valve 24 is useful for controlling the effects of the pumping system.

[0063] In the present case, the solid support 28 is a support made of zinc oxide

(ZnO), preferentially a macroporous, mesoporous or microporous support of ZnO, more preferentially nanorods of ZnO.

[0064] The precursor powder to be sublimated 26 is cobalt (II) acetylacetonate

97% (Co(acac) ₂ 97%).

[0065] The whole system was pumped under a vacuum of 7 mbar, and heated at 600°C with heating cables 18 to allow the thermal decomposition of Co(acac)2.

[0066] A mixture of 80 seem (seem stands for standard cubic centimetres per minute) of nitrogen gas (Air Liquide, 99 %) and 20 seem of hydrogen gas (Air Liquide, 99 %) allows the impregnation and reduction of cobalt on the ZnO nanorods. The surface coverage by Co particles on the ZnO can be controlled by adjusting the reaction time. A 30 min of reaction was arbitrarily fixed.

[0067] The surface coverage of the ZnO nanorods by the Co nanoparticles has been evaluated to 12%.

[0068] Figure 1 represents a SEM (scanning electron microscope) picture of

ZnO nanorods covered by the Co nanoparticles. [0069] This ZnO/Co system is cheap and provides high specific surface area in addition to be thermally stable.

[0070] The activity and selectivity of the ZnO/Co material concerning the CO2 methanation have been evaluated in a chemical reactor. Said reactor is composed of a pressure and temperature resistive column (p < 10 Bar, T < 800 °C) where the catalysts is placed. The quantity of catalysts used depends of the size of the column. Reactive gases (H2 and CO2) are introduced into the reactive column using mass flow controller to perfectly control the amount of gases injected. A heating system surrounds the reactive column in order to control the reaction temperature (25 < T < 800°C).

[0071] The reactions were carried out in a range of pressure comprised between 1 bar (atmospheric pressure) and 6 bar and at a temperature of 450°C or 500°C.

[0072] Tests were thus realized at atmospheric pressure, 2 bar, 3 bar, 4 bar, 5 bar and 6 bar. Each test was realized at a temperature of 450°C and at a temperature of 500°C.

[0073] The product (CH ₄ ), the starting materials (CO2) and by-products (CO, hydrocarbons derivatives,...) are analysed in real-time by gas chromatography.

[0074] The Sabatier's reaction was carried out during 4 hours.

[0075] The bar chart shown in figure 2 indicates the conversion rate of CO2 into

CH ₄ . The optimum work pressure has been determined to be equal to 6 bar while the conversion rate is generally better at a temperature of

500°C.

[0076] A selectivity superior to 99 % in the conversion of CO2 into CH ₄ with the

ZnO/Co system was observed under all conditions of pressure and temperature tested. The results can be observed on the bar chart shown in figure 3.

[0077] This demonstrates the absence of any amount of CO.

[0078] Although the conversion rate may vary with the amount of particles of

Co deposited on the ZnO, it was observed that the selectivity remains superior to 99 % independently from the amount of Co. [0079] Therefore, high temperatures (500°C or even higher) and high pressure (6 bar or even higher) are the preferred way to use the system ZnO/Co as a catalyst for the conversion of CO2 into CH ₄ without formation of CO.

[0080] Moreover, as the conversion of CO2 into CH ₄ with the system ZnO/Co as a catalyst also functions at atmospheric pressure without leading to any formation of CO, there is no need to use a reactor which is resistant to high pressure. However, the conversion rates are less interesting when low pressures are used (see figure 2).