Preparation method and application of Ni-CeO2 catalyst

Abstract

The invention relates to a preparation method and application of a Ni-CeO2 catalyst. The method comprises the following steps: (1) dissolving nickel precursor salt, cerium oxide precursor salt and citric acid monohydrate in deionized water to prepare a mixed solution; (2) refluxing, stirring and aging the mixed solution in a water bath at 50-90 DEG C for 1-5 hours; (3) performing rotary evaporation on the aged sol to obtain gel; after drying, grinding into powder, and calcining at 300-500 DEG C to obtain a NiO-CeO2 precursor; (4) reducing the obtained NiO-CeO2 precursor, so as to obtain a Ni-CeO2 catalyst A; the prepared Ni-CeO2 catalyst is used for methanation reaction. The catalyst has excellent carbon dioxide conversion rate and methane selectivity at low temperature, and meanwhile, the catalyst has long-period stability and good carbon deposition resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of catalyst preparation and carbon dioxide resource utilization, and specifically relates to a method for preparing a Ni-CeO2 carbon dioxide methanation reaction catalyst. Background Art

[0002] Since the Industrial Revolution, human activities, especially the extensive use of fossil fuels, have increased the content of CO2 in the atmosphere. The greenhouse effect has been increasingly enhanced, causing global warming and other phenomena, leading to a series of climate problems such as melting glaciers, rising sea levels, and frequent extreme weather, which seriously threaten human survival. However, fossil energy still occupies a dominant position in the world's energy. Carbon dioxide emissions are difficult to control in a short period of time. At the same time, the extensive use of fossil fuels has also caused increased energy consumption. Therefore, reducing the concentration of carbon dioxide in the atmosphere and developing renewable energy are of great significance to improving the living environment and climate of mankind. At present, carbon dioxide methanation is one of the most effective technologies for the effective utilization of carbon dioxide. Its product methane is an important basic chemical raw material, which can be used to prepare a variety of high-value-added organic chemicals. At the same time, as an energy source, the combustion value of methane is also higher than that of other fossil fuels, and the combustion process is safe and clean.

[0003] Patent CN 106268858B discloses a high-performance carbon dioxide hydrogenation methanation catalyst, but its active ingredients are precious metals such as ruthenium, platinum, and palladium, and its industrialization process will have the problem of high cost. Considering the activity and cost factors, metal Ni is an ideal substitute for precious metals. Among them, the catalyst composed of nickel as the active center loaded on the CeO2 carrier has received great attention. This is because after the cerium oxide carrier is pre-treated by reduction, there will be more oxygen vacancy structures on the surface, which can activate carbon dioxide and inhibit carbon deposition. However, this catalyst also has the problem of low activity at low temperatures (<350°C).

[0004] Kikuchi et al. (Energy & Fuels, 2021, 35, 5241-5251) investigated the key factors affecting the catalytic performance of Ni / CeO2 catalysts in the methanation reaction of carbon dioxide. The results show that the heterogeneous oxygen vacancy Ni-ν0-Ce structure plays a key role in the adsorption and activation of carbon dioxide, but the authors did not provide an effective method to adjust the content of the Ni-ν0-Ce structure (ensuring a certain Ni loading amount). Guo et al. (Appl. Catal. B, 2020, 277, 119278) prepared CeO by hydrothermal crystallization combined with isoelectric point adsorption. 2-x Interface confinement and nickel-based catalysts derived from silicate nickel nanotubes revealed Si δ+ with Ni 2+It will enter the cerium oxide lattice structure and form a unique Ni-ν0-Ce and / or Si-ν0-Ce (ν0 represents oxygen vacancies) external defect structure during the reduction process. The researchers also found that the oxygen vacancy structure formed by the insertion of this heterogeneous element (hereinafter referred to as heterogeneous oxygen vacancies) changed the electronic environment of Ni and promoted the activation ability of oxygen vacancies to carbon dioxide molecules, which significantly improved the activity of the catalyst. However, the preparation process of this catalyst is complicated, which is not conducive to industrial application, and inactive Si components are introduced, which reduces the concentration of active sites. Therefore, it is urgent to develop a simple, efficient, and non-introduced catalyst preparation method to increase the content of Ni-ν0-Ce heterogeneous oxygen vacancies in Ni-CeO2 catalysts, thereby improving their catalytic performance in carbon dioxide methanation reactions. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a Ni-CeO2 catalyst and its application in view of the limitations of the current technology. The method regulates the content of Ni-ν0-Ce heterogeneous oxygen vacancies on the reduced Ni-CeO2 catalyst by changing the reducing atmosphere (ratio of H2, CO and CO2) of the NiO-CeO2 precursor; the catalyst is used in the carbon dioxide methanation reaction. The present invention has excellent carbon dioxide conversion rate and methane selectivity at low temperature, and the catalyst has long-term stability and good anti-carbon deposition performance.

[0006] The present invention provides the following technical solutions: A method for preparing a Ni-CeO2 catalyst, the main body of which is a sol-gel method, the method comprising the following steps: (1) dissolving a nickel precursor salt, a cerium oxide precursor salt and citric acid monohydrate in deionized water to prepare a mixed solution; Among them, 0.872-3.781 g of nickel precursor salt, 3.040-7.382 g of cerium oxide precursor salt, and 3.541-8.642 g of citric acid monohydrate were added to every 20 mL of deionized water; The nickel precursor salt is nickel nitrate hexahydrate; the cerium oxide precursor salt is cerium nitrate hexahydrate; (2) Stir and age the mixed solution in a water bath at 50-90°C for 1-5 h under normal pressure; (3) The aged sol is subjected to rotary evaporation at 50-80°C to obtain a viscous gel; after taking out the gel, it is dried at 80-120°C for 12-36 h. After drying, the fluffy substance is ground into powder and placed in a crucible, and then calcined in a muffle furnace at a heating rate of 1-5°C / min to 300-500°C and calcined for 2-6 h to obtain a NiO-CeO2 precursor; (4) reducing the obtained NiO-CeO2 precursor in a reducing atmosphere at 300-500°C for 1-3 h to obtain a Ni-CeO2 catalyst; The nickel loading in the catalyst may be 5-40 wt.%, preferably 15-25 wt.%; The reducing atmosphere is one or more of hydrogen, carbon monoxide, and carbon dioxide, and must contain hydrogen; wherein the volume proportion of hydrogen is greater than 20%.

[0007] Preferably, the preparation method of the above-mentioned Ni-CeO2 methanation reaction catalyst adjusts the gas composition, reduction temperature and reduction time during the reduction process to optimize the content ratio of intrinsic oxygen vacancies (Ce-ν0-Ce) and heterogeneous oxygen vacancies (Ni-ν0-Ce).

[0008] The Ni-CeO2 catalyst prepared by the method is used for methanation reaction.

[0009] The specific steps include: introducing a mixed gas composed of CO2 and H2 into a fixed bed reactor loaded with a Ni-CeO2 catalyst, reacting at 250-350°C and normal pressure to obtain methane; Among them, the airspeed is 10-120 L·g ﹣1 ·h ﹣1 ; The volume ratio of CO2 and H2 is 1:4.

[0010] The essential features of the present invention are: The preparation method of the Ni-CeO2 methanation reaction catalyst of the present invention is reasonable optimized and screened, and the obtained catalyst has a high proportion of specific heterogeneous oxygen vacancy structure (Ni-ν0-Ce). The existence of this structure first enhances the interaction between the metal activity and the carrier, and inhibits the sintering of Ni nanoparticles. At the same time, the spacing between the hydrogen decomposition site (Ni) and the carbon dioxide activation site (oxygen vacancy ν0) in the methanation reaction is spatially pulled closer, which is conducive to the rapid reaction of the active hydrogen species H* with the key intermediate activated by carbon dioxide to generate methane, and inhibits the formation and growth of by-product carbon deposition species. In summary, the Ni-CeO2 catalyst obtained by this method is a catalyst with high and low temperature activity and high stability in the methanation reaction.

[0011] The beneficial effects of the present invention are: 1 Without using other components as additives, the present invention innovatively provides a method to significantly change the metal-support interaction (MSI) by adjusting the reduction atmosphere of the oxide precursor. For Ni-CeO2 catalyst, this change significantly increases the content of heterogeneous oxygen vacancy structure (Ni-ν0-Ce) on it, promotes carbon dioxide activation, and enhances the performance of the catalyst.

[0012] 2 For non-precious metal nickel-based catalysts, the present invention significantly improves its low-temperature catalytic activity while ensuring methane selectivity (>99%). Compared with conventional hydrogen-reduced Ni-CeO2 catalysts, its carbon dioxide conversion rate is increased by nearly 7 times (from 11% to 71%). At the same time, the catalyst has high stability and can be used for 100 hours (300°C, atmospheric pressure, mass space velocity 40 L·g ﹣1 ·h ﹣1 ) The carbon dioxide conversion rate is always maintained at around 70%, while the methane selectivity is always >99%. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 These are the Raman spectra after reduction with pure hydrogen and a mixture of hydrogen, carbon monoxide and carbon dioxide in a volume ratio of 5:1:1.

[0014] Figure 2 CO2-TPD spectra of samples of Example 1 and Example 3.

[0015] Figure 3 Long-term activity evaluation results of the samples in Example 3.

[0016] Figure 4 TG spectrum of the sample after reaction in Example 3. DETAILED DESCRIPTION

[0017] To further illustrate the present invention, the present invention is now described in detail through several specific implementation cases. The general equipment, drugs, consumables, etc. used in the examples can be purchased through commercial channels unless otherwise specified.

[0018] Example 1 2.461 g of nickel nitrate hexahydrate, 5.022 g of cerium nitrate hexahydrate, and 8.40 g of citric acid monohydrate were placed in a single-necked flask, and 20 mL of deionized water was measured with a measuring cylinder and poured into the single-necked flask. Then the mixed solution was stirred at 70 ° C and normal pressure for 3 h. After that, the mixed solution was rotary evaporated at 65 ° C until a uniform and transparent grass-green gel was obtained. Then it was dried at 100 ° C for 24 h, and the dried fluffy foam material was taken out and ground into powder and placed in a crucible, and then calcined in a muffle furnace, and the temperature was increased to 500 ° C at a heating rate of 2 ° C / min and calcined for 2 h to obtain a NiO-CeO2 precursor. The NiO-CeO2 precursor obtained in the above steps was programmed to increase the temperature (2 ° C / min) to 500 ° C under pure H2 conditions for 1 h, and the total gas flow rate was 100 ml / min, and the Ni-CeO2 methanation reaction catalyst was obtained, and the nickel loading was 20%.

[0019] Example 2 The preparation method is exactly the same as that in Example 1, except that the reducing gas is replaced by a mixture of hydrogen, carbon monoxide and carbon dioxide in a volume ratio of 8:1:1.

[0020] Example 3 The preparation method is exactly the same as that in Example 1, except that the reducing gas is replaced by a mixture of hydrogen, carbon monoxide and carbon dioxide in a volume ratio of 5:1:1.

[0021] Figure 1 The Raman spectra of Example 1 and Example 3 are compared. After the peak separation, it can be observed that the external defect content of the sample reduced in a mixed atmosphere of hydrogen, carbon monoxide and carbon dioxide with a volume ratio of 5:1:1 is significantly increased, which can be judged that there are a large number of heterogeneous oxygen vacancy Ni-ν0-Ce structures on it. The CO2-TPD spectra of the two further ( Figure 2 ) shows that after mixed gas reduction, the adsorption amount (peak intensity) of CO2 gas by the sample of Example 3 with a large number of heterogeneous oxygen vacancies is significantly increased, and it can be observed that the content of strong alkaline sites is significantly increased. This may be the main reason for the significant improvement of its low-temperature activity (see Table 1).

[0022] Example 4 The preparation method is exactly the same as that in Example 1, except that the reducing gas is replaced by a mixture of hydrogen, carbon monoxide and carbon dioxide in a volume ratio of 1:1:1.

[0023] Example 5 The preparation method is exactly the same as that in Example 1, except that the reducing gas is replaced by a mixture of hydrogen, carbon monoxide and carbon dioxide in a volume ratio of 1:1:2.

[0024] From the comparison of Examples 1-5, it can be seen that the activity results of Ni-CeO2 catalyst in CO2 methanation reaction (see Table 2) are strongly correlated with the change of reducing atmosphere. As CO and CO2 are added to the reducing gas, the catalyst activity shows a significant increasing trend (Examples 2 and 3). However, when CO and CO2 components are excessive (Examples 4 and 5), the catalyst activity decreases.

[0025] Example 6 0.872 g of nickel nitrate hexahydrate, 7.382 g of cerium nitrate hexahydrate, and 8.40 g of citric acid monohydrate were placed in a single-necked flask, and 20 mL of deionized water was measured with a measuring cylinder and poured into the single-necked flask. The mixed solution was then stirred at 70°C and normal pressure for 3 h. The mixed solution was then rotary evaporated at 65°C until a uniform and transparent grass-green gel was obtained. Subsequently, it was dried at 100°C for 24 h, and the dried fluffy foamy substance was taken out and ground into powder and placed in a crucible, and then calcined in a muffle furnace, and the temperature was raised to 500°C at a heating rate of 2°C / min and calcined for 2 h to obtain a NiO-CeO2 precursor. The NiO-CeO2 precursor obtained in the above steps was reduced at 500°C for 1 h at a temperature programmed at 2°C / min in a mixture of hydrogen, carbon monoxide and carbon dioxide in a volume ratio of 5:1:1, with a total gas flow rate of 100 ml / min, to obtain the Ni-CeO2 methanation reaction catalyst with a nickel loading of 6%.

[0026] Example 7 Except that the amount of nickel nitrate hexahydrate used was 3.781 g and the amount of cerium nitrate hexahydrate used was 3.040 g, the rest of the preparation method was exactly the same as that of Example 6, and the nickel loading was 40%.

[0027] Example 8 2.461g nickel nitrate hexahydrate, 5.022g cerium nitrate hexahydrate, and 8.40g citric acid monohydrate were placed in a single-necked flask, and 20mL deionized water was measured with a measuring cylinder and poured into the single-necked flask. The mixed solution was then stirred at 70℃ and normal pressure for 3h. The mixed solution was then rotary evaporated at 65℃ until a uniform and transparent grass-green gel was obtained. It was then dried at 100℃ for 24h, and the dried fluffy foamy substance was taken out and ground into powder and placed in a crucible, and then calcined in a muffle furnace, and the temperature was raised to 300℃ at a heating rate of 2℃ / min and calcined for 2h to obtain a NiO-CeO2 precursor. The NiO-CeO2 precursor obtained in the above steps was reduced at 300°C for 1 h at a total gas flow rate of 100 ml / min under the conditions of a mixture of hydrogen, carbon monoxide and carbon dioxide in a volume ratio of 5:1:1, with a programmed temperature increase (2°C / min) and a total gas flow rate of 100 ml / min to obtain the Ni-CeO2 methanation reaction catalyst with a nickel loading of 20%.

[0028] Example 9 2.461g nickel nitrate hexahydrate, 5.022g cerium nitrate hexahydrate, and 8.40g citric acid monohydrate were placed in a single-necked flask, and 20mL deionized water was measured with a measuring cylinder and poured into the single-necked flask. The mixed solution was then stirred at 70℃ and normal pressure for 3h. The mixed solution was then rotary evaporated at 65℃ until a uniform and transparent grass-green gel was obtained. It was then dried at 100℃ for 24h, and the dried fluffy foamy substance was taken out and ground into powder and placed in a crucible, and then calcined in a muffle furnace, and the temperature was raised to 300℃ at a heating rate of 2℃ / min and calcined for 2h to obtain a NiO-CeO2 precursor. The NiO-CeO2 precursor obtained in the above steps was reduced at 500°C for 3 h at a temperature programmed at 2°C / min under a mixture of hydrogen, carbon monoxide and carbon dioxide in a volume ratio of 5:1:1, with a total gas flow rate of 100 ml / min, to obtain the Ni-CeO2 methanation reaction catalyst with a nickel loading of 20%.

[0029] Compared with the Ni / CeO2 catalyst reported in the existing literature, the catalyst prepared by the method of this patent has a more obvious activity advantage in the carbon dioxide methanation reaction, which is reflected in that at a higher space velocity, the catalyst still has a higher carbon dioxide conversion rate and product methane selectivity. The specific comparison is shown in Table 1.

[0030]

[0031] Comparative Example 1 The supported Ni / CeO2 catalyst was prepared by an equal volume impregnation method. The specific preparation method is as follows: 2.473g nickel nitrate hexahydrate was dissolved in 2ml deionized water, 2g commercial nano CeO2 powder was added dropwise and stirred thoroughly, dried at room temperature for 24h, and then dried in an oven at 100℃ for 24h. Then it was ground into powder and placed in a crucible, and then calcined in a muffle furnace, and the temperature was raised to 300℃ at a heating rate of 2℃ / min and calcined for 2h to obtain NiO-CeO2 precursor. The NiO-CeO2 precursor obtained in the above steps was programmed to increase the temperature (2℃ / min) to 500℃ under pure hydrogen conditions for 1h, and the total gas flow rate was 100ml / min to obtain a supported Ni / CeO2 methanation reaction catalyst.

[0032] Comparative Example 2 The preparation method is exactly the same as that of Comparative Example 1, except that the reducing gas is replaced by a mixture of hydrogen, carbon monoxide and carbon dioxide in a volume ratio of 5:1:1.

[0033] In the comparative example, since the Ni active sites were loaded by the impregnation method, a strong interaction between Ni and the carrier CeO2 was not formed. Therefore, even if a mixture of hydrogen, carbon monoxide and carbon dioxide with a volume ratio of 5:1:1 was used for reduction pretreatment, a large number of heterogeneous oxygen vacancy structures were not formed in the sample, resulting in a significantly lower catalytic activity than the Ni-CeO2 methanation catalyst prepared by the sol-gel method.

[0034] The present invention adopts a micro fixed bed reactor to evaluate the activity of the catalyst.

[0035] 0.15 g of the catalyst was loaded into a vertical reaction tube with an inner diameter of 6 mm and a length of 40 mm, and quartz sand was loaded at both ends; the reaction tube was placed in a micro fixed bed reaction experimental device.

[0036] The test conditions are: 250℃, 300℃ and 350℃, normal pressure, and air velocity of 40L·g ﹣1 ·h ﹣1 The activity of each catalyst was evaluated at 300 °C, atmospheric pressure, and mass space velocity of 40 L·g ﹣1 ·h ﹣1 The long-term stability of the catalyst with the best performance was evaluated under the following conditions.

[0037] The reactants were analyzed and the results are shown in Table 2 below:

[0038] Under the same evaluation conditions, the catalyst obtained in Example 3 was tested for a long period of time ( Figure 3 The results show that the sample is at 300℃ and 40L·g ﹣1 ·h ﹣1 After 50 hours of reaction under the same conditions, the CO2 conversion rate and CH4 selectivity did not change significantly, which indicates that the sample has good catalytic stability. The catalyst after the reaction was characterized by TG. The results are as follows Figure 4 As shown, no obvious mass loss was found in the sample in the spectrum, which indicates that no carbon deposition occurred on the sample after a long reaction period, indicating that it has good anti-carbon deposition performance.

[0039] Matters not covered by the present invention are known technologies.

Claims

1. A method for preparing a Ni-CeO2 catalyst, characterized in that the method comprises the following steps: (1) dissolving a nickel precursor salt, a cerium oxide precursor salt and citric acid monohydrate in deionized water to prepare a mixed solution; in, Add 0.872-3.781 g of nickel precursor salt, 3.040-7.382 g of cerium oxide precursor salt, and 3.541-8.642 g of citric acid monohydrate per 20 mL of deionized water; (2) Stir and age the mixed solution in a water bath at 50-90°C for 1-5 h under normal pressure; (3) Rotary evaporating the aged sol at 50-80°C to obtain a viscous gel; taking out the gel, drying it at 80-120°C for 12-36 h, grinding it after drying, and then heating it to 300-500°C and calcining it for 2-6 h to obtain a NiO-CeO2 precursor; (4) The obtained NiO-CeO2 precursor is reduced in a reducing atmosphere at 300-500°C for 1-3 h to obtain a Ni-CeO2 catalyst.

2. The method for preparing a Ni-CeO2 catalyst as claimed in claim 1, wherein the nickel loading in the catalyst is 5-40 wt.%.

3. The method for preparing a Ni-CeO2 catalyst as claimed in claim 1, wherein the nickel precursor salt is nickel nitrate hexahydrate; and the cerium oxide precursor salt is cerium nitrate hexahydrate.

4. The method for preparing a Ni-CeO2 catalyst as claimed in claim 1, characterized in that the loading amount is 15-25 wt.%.

5. The method for preparing a Ni-CeO2 catalyst according to claim 1, wherein the reducing atmosphere is one or more of hydrogen, carbon monoxide, and carbon dioxide, and must contain hydrogen; wherein, The volume proportion of hydrogen is more than 20%.

6. The method for preparing a Ni-CeO2 catalyst as claimed in claim 1, wherein the size of Ni particles on the prepared catalyst is in the range of 3-10 nm.

7. The method for preparing a Ni-CeO catalyst as claimed in claim 1, characterized in that The heating rate in step (3) is 1-5°C / min.

8. Use of the Ni-CeO2 catalyst prepared by the method as claimed in claim 1, characterized in that it is used for methanation reaction.

9. The use as claimed in claim 8, characterized in that it comprises the following steps: introducing a mixed gas consisting of CO2 and H2 into a fixed bed reactor loaded with a Ni-CeO2 catalyst, reacting at 250-350°C and normal pressure to obtain methane; in, Air velocity is 10-120 L·g ﹣1 ·h ﹣1 ; The volume ratio of CO2 and H2 is 1:4.

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