Lanthanide metal doped Co3O4 catalyst for synergistically eliminating NO and N2O as well as preparation method and application of lanthanide metal doped Co3O4 catalyst

Abstract

The invention belongs to the technical field of environmental catalytic materials, and particularly relates to a lanthanide metal doped Co3O4 catalyst for synergistically eliminating NO and N2O and a preparation method and application thereof.The preparation method comprises the steps that 1, lanthanide metal salt and cobalt salt are dissolved in water, and a mixed salt solution is obtained; (2) dissolving soluble carbonate or bicarbonate in water to obtain a precipitant solution; (3) stirring and adding the precipitant solution into the mixed salt solution to enable the pH value of a reaction system to be 9-11, and stirring while adding to obtain coprecipitation slurry; (4) performing hydrothermal reaction on the coprecipitation slurry prepared in the step (3) to obtain a hydrothermal product; and (5) cooling the hydrothermal product, washing, drying, grinding, calcining, and naturally cooling to room temperature to obtain the lanthanide metal doped Co3O4 catalyst. The electronic structure of the cobalt-based catalyst is regulated and controlled through lanthanide series metal doping, lattice distortion and a surface isolation effect are introduced, and the catalytic performance of the catalyst is effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of environmental catalytic materials technology, specifically relating to a lanthanide metal-doped Co3O4 catalyst for synergistic elimination of NO and N2O, its preparation method, and its application. Background Technology

[0002] Nitrogen oxides (NO) and nitrous oxide (N2O) in industrial exhaust gases are major air pollutants. NO is a precursor to photochemical smog and acid rain, while N2O is not only a potent greenhouse gas but also detrimental to the ozone layer. Therefore, the synergistic elimination of these two nitrogen oxides is of great significance for environmental protection.

[0003] Currently, the technical pathways for the synergistic elimination of N2O and NO are mainly divided into one-stage and two-stage processes. One-stage processes achieve simultaneous removal within a single reaction unit, including selective catalytic reduction (SCR) technology (in the presence of a reducing agent such as NH3, using catalysts such as vanadium-titanium or iron molecular sieves to reduce NO to N2 and simultaneously catalytically decompose or reduce N2O) and composite catalyst technologies capable of synergistically catalyzing NO reduction and N2O decomposition. Two-stage processes employ a stepwise treatment approach, with a typical flow being "SCR denitrification followed by catalytic decomposition of N2O": first, flue gas is passed into an SCR unit to reduce NOx to N2, and then in a subsequent reactor, a specific catalyst (such as cobalt-based spinel oxide or molecular sieve catalysts) is used to decompose residual N2O into N2 and O2. This process demonstrates advantages in nitric acid tail gas treatment, including low reducing agent consumption and high N2O removal efficiency. Nevertheless, existing technologies still face challenges such as temperature window matching and insufficient resistance to O2 and water vapor interference. Future research will focus more on precisely controlling the electronic structure and active sites of catalysts through atomic-scale doping (such as lanthanide metal doping Co3O4) to develop new catalytic materials and processes with higher low-temperature activity, better stability, and the ability to synergistically remove multiple pollutants.

[0004] CN106076112A discloses a method for simultaneously removing N2O and NOx. This technology employs a catalyst composed of noble metals (0.1–5%), transition metal oxides (3–15%), and alkali metal hydroxides (0.5–1%) supported on an alumina support. Within a temperature range of 200–600°C, it synergistically catalyzes the decomposition of N2O (generating N2 and O2) and the reduction reaction between NOx and NH3 (generating N2), thereby achieving simultaneous removal of N2O and NOx in a single bed. While this technology achieves integrated removal of N2O and NOx, its main drawbacks include reliance on high-cost noble metals, the introduction of potentially hazardous NH3, questionable long-term catalyst stability, and potential insufficient adaptability to complex flue gas conditions. These factors limit its potential for large-scale industrial application.

[0005] CN105396460A discloses a highly efficient method for the combined removal of N2O and NO. x The method involves using carbon materials such as coke / activated carbon at high temperatures of 500–1100℃ to precipitate NO. x The nitrogen is completely reduced to N2, and N2O is partially decomposed. Subsequently, in a medium-temperature range of 200–600℃, a specialized catalyst is used to decompose the remaining N2O into N2 and O2, ultimately achieving the desired N2O and NO concentration at the outlet. x The total concentration is below 100 ppm. Although the process has a certain removal effect, it is constrained by key issues such as high energy consumption, material consumption, system complexity, and insufficient adaptability to operating conditions, posing a serious challenge to its industrial promotion.

[0006] CN115739172A discloses a catalyst for the synergistic removal of N2O and NO and its preparation method, belonging to the field of denitrification catalyst technology. It uses Beta or ZSM-5 molecular sieves as supports, loading non-precious metal active components such as Co, Fe, and Cu. This catalyst exhibits high activity over a wide temperature window of 400–600℃ and is considered to have economic and environmental advantages due to its simple preparation and low cost. However, this technology still faces key challenges in transitioning from laboratory to industrial application. First, in actual flue gas environments, water vapor and sulfur dioxide easily lead to hydrolysis of the molecular sieve structure or sulfur poisoning of active sites, resulting in insufficient chemical stability. Second, under the long-term impact of high temperature and exothermic reaction, the active metal is prone to sintering, and the molecular sieve framework may collapse; its hydrothermal stability and long-term durability need to be verified.

[0007] CN106000420A utilizes a catalyst prepared from noble metals (such as palladium, platinum, rhodium, and ruthenium) and transition metal oxides (such as vanadium pentoxide, copper oxide, and zinc monoxide) combined with alkali metal hydroxides (such as sodium hydroxide and potassium hydroxide). Through specific impregnation, calcination, and liquid-phase reduction steps, a catalyst for the integrated removal of N2O and NOx is prepared. This catalyst is used for gas treatment at 200–600°C. It achieves simultaneous removal efficiencies of over 95% for both N2O and NOx, avoids the oxidation of ammonia by noble metals, lowers the process temperature, ensures the stability of each component, and simplifies the process flow. While it may perform excellently under laboratory conditions, its large-scale industrial application faces multiple challenges in terms of economic cost, long-term stability, and adaptability to complex real-world operating conditions.

[0008] Therefore, developing a catalyst for the synergistic elimination of NO and N2O is a technical problem that urgently needs to be solved and breakthroughs achieved in this field. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides a lanthanide metal-doped Co3O4 catalyst for the synergistic elimination of NO and N2O, and its preparation method.

[0010] To achieve the objectives of this invention, the technical solution adopted is as follows: The first aspect of this invention provides a method for preparing a lanthanide metal-doped Co3O4 catalyst for synergistic elimination of NO and N2O, comprising the following steps: (1) Dissolve lanthanide metal salts and cobalt salts in water to obtain a mixed salt solution; (2) Dissolve soluble carbonates or bicarbonates in water to obtain a precipitant solution; (3) At 30-50℃, the precipitant solution prepared in step (2) is stirred and added to the mixed salt solution prepared in step (1) to make the pH value of the reaction system 9-11. Stirring is carried out while adding the solution to obtain a coprecipitated slurry. (4) Place the coprecipitated slurry prepared in step (3) into a reaction vessel and stir at 180-220°C for 2-4 hours to obtain hydrothermal products; (5) After the hydrothermal product prepared in step (4) is cooled, washed, dried and ground, it is calcined at 480-520℃ for 2-4 h and then naturally cooled to room temperature to obtain the lanthanide metal-doped Co3O4 catalyst.

[0011] Preferably, the molar ratio of lanthanide metals to cobalt in the mixed salt solution in step (1) is 0.05 to 0.2.

[0012] Preferably, the concentration of the mixed salt solution in step (1) is 0.5 to 1.0 mol / L, and the concentration of the precipitant solution in step (2) is 0.4 to 0.6 mol / L.

[0013] Preferably, the lanthanide metal salt in step (1) is at least one of Sm(NO3)3·6H2O and Eu(NO3)3·6H2O.

[0014] Preferably, the cobalt salt in step (1) is one of Co(NO3)2·6H2O, CoSO4·6H2O, or CoCl2·6H2O.

[0015] Preferably, the soluble carbonate in step (2) is at least one of Na2CO3 and K2CO3, and the bicarbonate is at least one of NaHCO3 and KHCO3.

[0016] Preferably, the stirring speed in step (4) is 350-450 r / min.

[0017] Preferably, the specific process of adding the precipitant solution prepared in step (2) to the mixed salt solution prepared in step (1) is as follows: first, the precipitant solution is added dropwise to the mixed salt solution at a rate of 50-75 mL / h for a time of 0.5-1.5 h; then, the precipitant solution is added dropwise to the mixed salt solution at a rate of 95-145 mL / h for a time of 30-40 min.

[0018] Preferably, the heating rate during calcination in step (5) is 2-5℃ / min; the drying temperature in step (5) is 100-120℃ and the drying time is 4-6 h; the washing process in step (5) is: washing with water until the filtrate is neutral.

[0019] Preferably, step (1) stirring is performed at room temperature for 15 to 30 minutes; step (2) stirring is performed at room temperature for 30 to 45 minutes.

[0020] Preferably, the stirring speed in step (3) is 15 to 30 r / min.

[0021] A second aspect of the present invention provides a lanthanide metal-doped Co3O4 catalyst prepared using the preparation method described in the first aspect.

[0022] A third aspect of the present invention provides the application of the lanthanide metal-doped Co3O4 catalyst described in the second aspect in the elimination of NO and / or N2O.

[0023] Preferably, the specific operation of the lanthanide metal-doped Co3O4 catalyst synergistically eliminating NO and N2O is as follows: the catalyst is packed into a quartz tube fixed-bed reactor, and simulated flue gas with high-purity He as the balance gas is introduced, the specific composition of which includes: 5-10% N2O, 3-10% O2, 0.1%-0.2% NO, and 0-2% H2O. The total gas flow rate is 25-200 mL / min, and the space velocity is 5000-20000 h⁻¹. -1 The reaction temperature is controlled within the range of 150 to 550℃, and the reaction is stable for 30 to 60 minutes at each temperature point.

[0024] Compared with the prior art, the present invention has the following advantages: (1) This invention modulates the electronic structure of cobalt-based catalysts through lanthanide metal doping, introducing lattice distortion and surface isolation effects, effectively improving the catalyst's resistance to sintering and hydrothermal stability, and ensuring its activity and structural stability during long-term reactions. The catalyst of this invention can still maintain high activity in simulated flue gas atmospheres, demonstrating good adaptability to operating conditions.

[0025] (2) The catalyst of the present invention is based on non-precious metals and has low raw material cost; its preparation method is simple, controllable and reproducible, suitable for large-scale production, and avoids the use of dangerous reducing agents such as NH3, thus ensuring high safety.

[0026] (3) The catalyst of this invention can achieve simultaneous catalytic decomposition of NO and N2O over a wide temperature range, realizing the synergistic and efficient removal of the two pollutants, simplifying the process flow, and overcoming the defects of complex and energy-intensive multi-stage process systems. The catalyst can achieve a decomposition efficiency of 80% at 250℃ and maintain an ultra-high decomposition efficiency of over 98% over a wide temperature range of 300 to 550℃, adapting to the actual working conditions of fluctuating industrial flue gas temperature. Attached Figure Description

[0027] Figure 1 The efficiency curves of N2O decomposition of the catalysts prepared in Examples 1 to 4 and Comparative Examples 1 to 3 of this invention are shown. Figure 2 The above are efficiency curves of the catalysts prepared in Examples 1 to 4 and Comparative Examples 1 to 3 of this invention for decomposing NO. Detailed Implementation

[0028] The embodiments described herein are merely illustrative of the technical content of the invention and are not intended to limit the scope of protection of the invention. The invention can be implemented in many forms and should not be construed as limited to the specific examples listed below.

[0029] Example 1 A method for preparing a lanthanide metal-doped Co3O4 catalyst for synergistic elimination of NO and N2O, the specific steps of which are as follows: (1) Weigh 1.784 g Eu(NO3)3·6H2O and 11.64 g Co(NO3)2·6H2O according to the molar ratio of lanthanide metals to cobalt of 0.1. Add Eu(NO3)3·6H2O and Co(NO3)2·6H2O to 50 mL of deionized water and stir at room temperature for 30 min to prepare a mixed salt solution with a concentration of 0.8 mol / L. (2) Weigh 7.95 g Na2CO3 and add it to 150 mL of deionized water. Stir at room temperature for 30 min to prepare a precipitant solution with a concentration of 0.5 mol / L. (3) Place the mixed salt solution prepared in step (1) in a constant temperature water bath at 40°C, and add the precipitant solution prepared in step (2) dropwise to the mixed salt solution at a stirring speed of 30 r / min: first add slowly at a rate of 60 mL / h for 1.5 hours, and then add rapidly at a rate of 100 mL / h for 30 min, so that the pH value of the reaction system is 9.5, and a coprecipitated slurry is obtained; (4) The coprecipitated slurry prepared in step (3) was placed in a sealed high-temperature and high-pressure reactor and hydrothermally reacted for 3 h at 200℃ and 400 r / min stirring speed to obtain hydrothermal products. (5) After cooling the hydrothermal product prepared in step (4), wash it repeatedly with deionized water until the filtrate is neutral. Dry the obtained solid in an oven at 120°C for 5 h. Grind the dried sample into powder, spread it evenly in a magnetic boat, place it in a muffle furnace, and calcine it from room temperature to 500°C at a heating rate of 5°C / min for 3 h. Then, cool it naturally to room temperature to obtain the lanthanide metal-doped Co3O4 catalyst, denoted as Eu. 0.1 / Co catalyst.

[0030] Example 2 A method for preparing a lanthanide metal-doped Co3O4 catalyst for synergistic elimination of NO and N2O is basically the same as in Example 1, except that: in step (1), 1.784 g of Eu(NO3)3·6H2O lanthanide metal salt is replaced with 1.778 g of Sm(NO3)3·6H2O lanthanide metal salt, and the prepared catalyst is denoted as Sm 0.1 / Co catalyst.

[0031] Example 3 A method for preparing a lanthanide metal-doped Co3O4 catalyst for synergistic elimination of NO and N2O is basically the same as in Example 1, except that: in step (1), 1.784 g of Eu(NO3)3·6H2O lanthanide metal salt is replaced with 3.568 g of Eu(NO3)3·6H2O lanthanide metal salt, wherein the molar ratio of Eu to Co is 0.2, and the prepared catalyst is denoted as Eu. 0.2 / Co catalyst.

[0032] Example 4 A method for preparing a lanthanide metal-doped Co3O4 catalyst for synergistic elimination of NO and N2O is basically the same as in Example 2, except that: in step (1), 1.778 g of Sm(NO3)3·6H2O lanthanide metal salt is replaced with 3.556 g of Sm(NO3)3·6H2O lanthanide metal salt, wherein the molar ratio of Sm to Co is 0.2, and the prepared catalyst is denoted as Sm 0.2 / Co catalyst.

[0033] Comparative Example 1 A method for preparing a catalyst for synergistic elimination of NO and N2O is basically the same as that in Example 1, except that: in step (1), no lanthanide metal salt is added, and 11.65 g of Co(NO3)2·6H2O is used to prepare a salt solution. The prepared catalyst is referred to as Co3O4 catalyst.

[0034] Comparative Example 2 A method for preparing a catalyst for the synergistic elimination of NO and N2O is basically the same as that in Example 1, except that: in step (1), 1.784 g of Eu(NO3)3·6H2O lanthanide metal salt is replaced with 1.164 g of Cu(NO3)3·6H2O copper salt, and the prepared catalyst is denoted as Cu. 0.1 / Co catalyst.

[0035] Comparative Example 3 A method for preparing a catalyst for the synergistic elimination of NO and N2O is basically the same as that in Example 1, except that: in step (1), 1.784 g of Eu(NO3)3·6H2O lanthanide metal salt is replaced with 0.591 g of Li2CO3 lithium salt, and the prepared catalyst is denoted as Li 0.1 / Co catalyst.

[0036] Performance testing: The catalytic activity of the catalysts prepared in Examples 1-4 and Comparative Examples 1-3 was evaluated using a fixed-bed reactor. The specific steps are as follows: 0.303 g of the prepared catalyst was packed into a quartz tube reactor. Simulated flue gas with high-purity He as the equilibrium gas was introduced into the reaction system. The simulated flue gas composition included: 7 vol.% N₂O (99.99%), 5 vol.% O₂ (99.99%), and 0.1 vol.% NO (0.994%). The total flow rate of the reaction gas was 50 mL / min, and the space velocity was 10000 h⁻¹. -1 The tests were conducted within a temperature range of 150–550°C, with the reaction temperature increasing by 50°C in increments and held constant for 30 minutes to determine the conversion rates of N2O and NO at different temperatures.

[0037] The conversion rates of N2O and NO at different temperatures are as follows: Figure 1 , Figure 2 As shown in Table 1, where Figure 1 The efficiency curves of N2O decomposition of the catalysts prepared in Examples 1 to 4 and Comparative Examples 1 to 3 are shown. Figure 2 Table 1 shows the efficiency curves of the catalysts prepared in Examples 1 to 4 and Comparative Examples 1 to 3 for decomposing NO; Table 1 shows the decomposition efficiency of the catalysts prepared in Examples 1 to 4 and Comparative Examples 1 to 3 for catalytic decomposition of N2O and NO.

[0038] Table 1. Decomposition efficiency of N2O and NO by the catalysts prepared in Examples 1-4 and Comparative Examples 1-3. As shown in Table 1, the Eu prepared in Example 1 of this invention... 0.1The / Co catalyst exhibits excellent catalytic decomposition performance of N2O and NO within the temperature range of 150–550℃. As shown in Table 1, the catalyst achieves an N2O decomposition efficiency of 91.79% at 250℃, and complete decomposition of N2O occurs within the 300–550℃ range, demonstrating significant wide-temperature activity. The NOx byproduct concentration in the tail gas after the reaction is below 25 ppm, and the catalyst's selectivity for N2 is above 99.99%. All indicators are superior to national emission standards, achieving highly efficient synergistic removal of N2O and NO.

[0039] N2O is directly decomposed into N2 and O2 by a catalyst (2N2O → 2N2 + O2). In the reaction system, NO can be directly decomposed (2NO → N2 + O2), and can also undergo a synergistic reaction with the active oxygen species [O] generated by the decomposition of N2O (N2O + NO → N2 + NO2), effectively avoiding the occupation of active sites of the catalyst by oxygen species, thus significantly promoting the continuous and efficient decomposition of N2O.

[0040] The catalyst prepared in Example 1 effectively reduces the heat energy consumption in the exhaust gas treatment process by lowering the reaction temperature of N2O catalytic decomposition and widening its active temperature window. This catalyst can synergistically remove N2O and NO, achieving rapid, continuous, and harmless treatment of simulated industrial exhaust gas, fully meeting environmental emission requirements.

[0041] Sm prepared in Example 2 of this invention 0.1 The / Co catalyst possesses the dual function of simultaneously removing N2O and NO. Within the reaction temperature range of 150–550℃, this catalyst achieves highly efficient N2O decomposition (conversion rate of 92.22% at 250℃) while maintaining a stable NO decomposition rate of over 95% above 300℃, demonstrating its excellent synergistic elimination capability. Furthermore, the NOx byproduct concentration in the post-reaction gas is below 25 ppm, and the N2 selectivity exceeds 99.99%, with all indicators meeting and exceeding national emission standards.

[0042] The catalysts prepared in Examples 3 and 4 of this invention have increased the amount of lanthanide metal compared to Examples 1 and 2. However, these catalysts exhibit lower efficiency in the catalytic decomposition of N₂O and NO in the low-temperature range (150–300 °C). This is because excessive lanthanide metal doping may form a capping layer on the Co₃O₄ surface, physically obscuring Co. 3+ Active sites are reduced in density and accessibility; at the same time, excessive doping can disrupt oxygen vacancy distribution and inhibit lattice oxygen migration, thereby hindering oxygen transfer kinetics in N2O decomposition.

[0043] Compared with the lanthanide-doped Co3O4 catalysts prepared in Examples 1 and 2 of this invention, the Co3O4 catalyst prepared in Comparative Example 1 showed a significant decrease in performance in the synergistic elimination of N2O and NO, with decomposition efficiencies of 85.04% and 80.15% at 350℃, respectively. This performance difference directly confirms the important role of introducing lanthanide metals in optimizing the active sites of Co3O4 catalysts and thus improving their ability to decompose N2O and NO.

[0044] Cu-doped transition metal prepared in Comparative Example 2 0.1 The / Co catalyst's performance is far inferior to that of the lanthanide metal-doped system. This catalyst only achieves a N₂O decomposition efficiency of 96.64% at 450℃, while the lanthanide catalyst prepared in this invention achieves near-complete conversion efficiency at the same temperature. Comparative Example 3 shows the alkali metal Li-doped Li₂. 0.1 The / Co catalyst, according to catalytic performance tests, shows a significant gap compared to the lanthanide metal-doped catalyst prepared in this invention in terms of synergistic decomposition efficiency of N2O and NO and low-temperature activity. This result confirms that transition metal or alkali metal doping cannot achieve the synergistic promoting effect of lanthanides, further highlighting the uniqueness and irreplaceability of this invention's choice of lanthanide metals for doping. This result also confirms the limitations of non-lanthanide metal doping schemes.

[0045] In summary, lanthanide metal (Sm, Eu) doping significantly enhances the catalytic decomposition performance of Co3O4 catalyst for N2O, outperforming undoped, transition metal (Cu), and alkali metal (Li) doped comparative samples. The catalytic activity order is: Sm 0.1 / Co>Eu 0.1 / Co>Eu 0.2 / Co>Co3O4>Sm 0.2 / Co>Li 0.1 / Co>Cu 0.1 / Co. Sm at 250℃ 0.1 The / Co catalyst achieves an N2O conversion rate of up to 98.46%, an improvement of 87.42% compared to the Co3O4 catalyst, and complete conversion is achieved at 300℃. This catalyst also possesses highly efficient NO decomposition capabilities, enabling synergistic purification of N2O and NO, resulting in clean products and low processing costs, providing a high-performance solution for industrial exhaust gas treatment.

[0046] The above embodiments are merely specific examples of the present invention and are intended to illustrate the invention, not to limit it. Any obvious modifications or equivalent alternatives guided by the core technical concept of the present invention fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a lanthanide metal-doped Co3O4 catalyst for synergistic elimination of NO and N2O, characterized in that, Includes the following steps: (1) Dissolve lanthanide metal salts and cobalt salts in water to obtain a mixed salt solution; (2) Dissolve soluble carbonates or bicarbonates in water to obtain a precipitant solution; (3) At 30-50℃, the precipitant solution prepared in step (2) is added to the mixed salt solution prepared in step (1) to make the pH value of the reaction system 9-11 while stirring, to obtain a coprecipitated slurry; (4) Place the coprecipitated slurry prepared in step (3) into a reaction vessel and stir at 180-220°C for 2-4 h to obtain hydrothermal products; (5) After cooling, the hydrothermal product prepared in step (4) is washed, dried and ground, and then calcined at 480-520℃ for 2-4 hours. After calcination, it is cooled to room temperature to obtain lanthanide metal-doped Co3O4 catalyst.

2. The preparation method according to claim 1, characterized in that, In step (1), the molar ratio of lanthanide metals to cobalt in the mixed salt solution is 0.05 to 0.

2.

3. The preparation method according to claim 2, characterized in that, The concentration of the mixed salt solution in step (1) is 0.5–1.0 mol / L, and the concentration of the precipitant solution in step (2) is 0.4–0.6 mol / L.

4. The preparation method according to claim 2, characterized in that, The lanthanide metal salt mentioned in step (1) is at least one of Sm(NO3)3·6H2O and Eu(NO3)3·6H2O.

5. The preparation method according to claim 4, characterized in that, The cobalt salt mentioned in step (1) is one of Co(NO3)2·6H2O, CoSO4·6H2O, or CoCl2·6H2O.

6. The preparation method according to claim 5, characterized in that, The soluble carbonate in step (2) is at least one of Na2CO3 and K2CO3, and the bicarbonate is at least one of NaHCO3 and KHCO3.

7. The preparation method according to claim 6, characterized in that, The specific process of adding the precipitant solution prepared in step (2) to the mixed salt solution prepared in step (1) is as follows: first, the precipitant solution is added dropwise to the mixed salt solution at a rate of 50-75 mL / h for a time of 0.5-1.5 h; then, the precipitant solution is added dropwise to the mixed salt solution at a rate of 95-145 mL / h for a time of 30-40 min.

8. The preparation method according to claim 7, characterized in that, The heating rate during calcination in step (5) is 2-5℃ / min; the drying temperature in step (5) is 100-120℃ and the drying time is 4-6 h; the washing process in step (5) is: wash with water until the filtrate is neutral.

9. A lanthanide metal-doped Co3O4 catalyst prepared by the preparation method described in claims 1 to 8.

10. The application of the lanthanide metal-doped Co3O4 catalyst according to claim 9 in the elimination of NO and / or N2O.

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