CN114984967B - Cobalt oxide catalyst, method for preparing the same, and method for decomposing ammonia - Google Patents

Abstract

The invention provides a cobalt oxide catalyst and its preparation method and a method for ammonia decomposition. The preparation method of the cobalt oxide catalyst comprises: dissolving an alkaline reagent containing Na ions or K ions in water to obtain a precipitate agent solution; the cobalt precursor was dissolved in water to obtain a precursor solution; the precipitant solution was added dropwise to the precursor solution until the pH reached a certain value, and aging, suction filtration, washing, and drying were performed to obtain a crude catalyst; The cobalt oxide catalyst is obtained after calcining the crude catalyst; wherein the cobalt oxide catalyst includes Na or K doped cobalt oxide. Cobalt oxide doped with Na or K can economically decompose NH ₃ , has high activity, and the catalyst itself is not loaded on a carrier or composited with other metal elements, and the cost is low, and the cobalt oxide catalyst prepared by the present invention Obviously has higher ammonia decomposition activity.

Description

Cobalt oxide catalyst and its preparation method and method for ammonia decomposition

technical field

The invention relates to the field of environmental catalysis, in particular to a cobalt oxide catalyst, a preparation method thereof and a method for ammonia decomposition.

Background technique

Ammonia has high energy density and octane number, and is also a carrier of hydrogen energy. When it is used as a fuel alone, there are problems such as high energy required for ignition, low flame propagation velocity, and high exhaust _NOx emissions, so it is often necessary to introduce additives. One of the solutions is to use H ₂ , the product of catalytic decomposition of ammonia, which can improve the combustion performance of ammonia and reduce the emission of tail gas NO _x . It can be seen that whether it is directly used as a fuel or used to purify hydrogen, the key reaction of ammonia decomposition is involved. In recent years, research cases continue to prove that ammonia decomposition for hydrogen production is economically feasible. Due to the high activation energy required for the ammonia decomposition reaction, it is necessary to use a suitable catalyst to reduce the temperature required for the reaction to reduce energy consumption. At the same time, the cost and economic benefits of catalyst preparation must also be considered.

Contents of the invention

Based on this, the present invention proposes a method for preparing a cobalt oxide catalyst and a method for ammonia decomposition. Cobalt oxide doped with Na or K can economically decompose NH ₃ , has high activity, and the catalyst itself It is not loaded on the carrier or compounded with other metal elements, and the cost is low.

According to one aspect of the present invention, a kind of preparation method of cobalt oxide catalyst is provided, comprising:

Dissolving an alkaline reagent containing Na ions or K ions in water to obtain a precipitant solution;

Dissolving the cobalt precursor in water to obtain a precursor solution;

Add the above precipitant solution dropwise to the above precursor solution until the pH reaches a certain value, then perform aging, suction filtration, washing, and drying to obtain the crude catalyst;

The above-mentioned cobalt oxide catalyst is obtained after the above-mentioned crude catalyst is calcined in a muffle furnace;

Wherein, the above cobalt oxide catalyst includes Na or K doped cobalt oxide, preferably Co(NO ₃ ) ₂ ·6H ₂ O, CoSO ₄ ·7H ₂ O or CoCl ₂ ·6H ₂ O;

The alkaline reagent is preferably NaOH or K ₂ CO ₃ or KOH.

According to an embodiment of the present invention, wherein the cobalt precursor is a hydrate of divalent cobalt salt.

According to an embodiment of the present invention, wherein, the range of the pH value is 9.0-10.0.

According to an embodiment of the present invention, the above-mentioned calcination temperature is 550-600° C., the calcination time is 3-4 hours, and the heating rate is 5-10° C./min.

According to an embodiment of the present invention, wherein, the aging time is 3-5 hours.

According to another aspect of the present invention, there is provided a cobalt oxide catalyst prepared by the above preparation method, wherein the cobalt oxide catalyst includes Na or K doped cobalt oxide.

According to another aspect of the present invention, there is provided a method for using the above-mentioned cobalt oxide catalyst for ammonia decomposition, comprising:

Under NH ₃ /Ar atmosphere, the cobalt oxide catalyst is heated and activated to reduce the cobalt oxide catalyst to metallic Co, and decompose NH ₃ to H ₂ on the active sites of the metallic Co and N ₂ and desorption;

Wherein, the above-mentioned cobalt oxide catalyst includes Na or K doped cobalt oxide.

According to an embodiment of the present invention, wherein, in the above NH ₃ /Ar atmosphere, NH ₃ accounts for 5-100% of the total volume.

According to an embodiment of the present invention, wherein, when the above-mentioned cobalt oxide catalyst is reduced to metallic Co, the above-mentioned Na or K is in a highly dispersed state.

According to an embodiment of the present invention, there is electronic interaction between the above-mentioned Na or K and the metallic Co, which is used to suppress the sintering and agglomeration of the active sites of the metallic Co.

According to an embodiment of the present invention, wherein, the activation temperature of the cobalt oxide catalyst is 480-520°C.

As can be seen from the foregoing technical scheme, the cobalt oxide catalyst for ammonia decomposition provided by the present invention has the following beneficial effects:

The cobalt oxide doped with Na or K can decompose NH ₃ economically and has high activity, and the catalyst itself is not loaded on a carrier or compounded with other metal elements, so the cost is low. Compared with existing catalysts used for ammonia decomposition (the conversion rate of ammonia decomposition below 600° C. is lower than 50%), the cobalt oxide catalyst prepared by the present invention obviously has higher ammonia decomposition activity (the ammonia decomposition rate of 500° C. The conversion can reach almost 100%). Under the same test conditions, the activity of the cobalt oxide catalyst prepared by the invention is higher than that of the cobalt oxide catalyst prepared by other methods. The preparation process is simple, the cost is low, and industrial production can be realized.

Description of drawings

Fig. 1 is the comparison chart of the ammonia conversion rate of the cobalt oxide catalyst of the embodiment of the present invention and the pure cobalt oxide catalyst;

Fig. 2 is the comparative figure of the ammonia conversion rate of cobalt oxide catalyst sample in the embodiment of the present invention;

Fig. 3 is the XRD diffraction spectrogram before the catalysis of cobalt oxide catalyst sample in the embodiment of the present invention;

Fig. 4 is a catalyzed XRD diffraction spectrum of a cobalt oxide catalyst sample in an example of the present invention.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

In related technologies, noble metal catalysts represented by Ru-based catalysts have high ammonia decomposition activity, but are difficult to be put into large-scale application due to their high price. Transition metal catalysts are relatively environmentally friendly, cheap and easy to obtain, and are one of the catalyst systems that are expected to replace noble metals.

Among transition metal catalysts, ammonia decomposition catalysts with cobalt as the main active component have high ammonia decomposition activity and have attracted attention in recent years. At present, pure cobalt oxide is used as a catalyst. Due to the small specific surface area of pure cobalt oxide, high temperature and easy sintering, etc., the ammonia decomposition activity is low, and it often needs to be loaded on a carrier or compounded with other metal elements. The complicated preparation process increases the cost. , which is not conducive to industrial application.

According to an overall inventive concept of the present invention, a method for preparing a cobalt oxide catalyst is provided, comprising:

Step 1: dissolving an alkaline reagent containing Na ions or K ions in water to obtain a precipitant solution;

Step 2: dissolving the cobalt precursor in water to obtain a precursor solution;

Step 3: Add the precipitant solution dropwise to the precursor solution until the pH reaches a certain value, then perform aging, suction filtration, washing, and drying to obtain a crude catalyst;

Step 4: The cobalt oxide catalyst is obtained after the crude catalyst is calcined;

Wherein, the cobalt oxide catalyst includes Na or K doped cobalt oxide.

The cobalt oxide doped with Na or K can decompose NH ₃ economically and has high activity, and the catalyst itself is not loaded on a carrier or compounded with other metal elements, so the cost is low. Compared with the existing pure cobalt oxide catalysts used for ammonia decomposition (the ammonia decomposition conversion rate below 600 ° C is lower than 50%), the cobalt oxide catalyst prepared by the present invention obviously has higher ammonia decomposition activity (500 °C ammonia decomposition conversion can reach almost 100%). Under the same test conditions, the activity of the cobalt oxide catalyst prepared by the invention is higher than that of the cobalt oxide catalyst prepared by other methods. The preparation process is simple, the cost is low, and industrial production can be realized.

According to an embodiment of the present invention, in step 1, the alkaline reagent containing Na ions or K ions may be NaOH or K ₂ CO ₃ or KOH.

According to an embodiment of the present invention, in the first step, the precipitating agent (NaOH, KOH, etc.) simultaneously acts as an ammonia decomposition coagent. The content of Na/K in cobalt oxide can be effectively adjusted by changing the amount of precipitant added, the number of times of suction filtration and washing, etc.

According to an embodiment of the present invention, in step 1, the water is high-purity water.

According to an embodiment of the present invention, in step 2, the cobalt precursor is a hydrate of divalent cobalt salt.

According to an embodiment of the present invention, in step 2, the cobalt precursor may be Co(NO ₃ ) ₂ ·6H ₂ O, CoSO ₄ ·7H ₂ O or CoCl ₂ ·6H ₂ O and other divalent cobalt salt hydrates.

According to an embodiment of the present invention, in Step 3, the pH range is 9.0-10.0. For example, in step three, the pH value can be 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0 or selected according to actual conditions.

According to an embodiment of the present invention, in step 3, the aging time is 3-5 hours.

According to an embodiment of the present invention, for example, in step 3, the aging time may be 3h, 3.5h, 4h, 4.5h, 5h or be selected according to actual conditions.

According to an embodiment of the present invention, in step 4, the firing temperature is 550-600° C., the firing time is 3-4 hours, and the heating rate is 5-10° C./min.

According to an embodiment of the present invention, for example, in step 4, the firing temperature may be 550°C, 560°C, 570°C, 580°C, 590°C, or 600°C.

According to an embodiment of the present invention, for example, in step 4, the firing time may be 3h, 3.1h, 3.2h, 3.3h, 3.4h, 3.5h, 3.6h, 3.7h, 3.8h, 3.9h, 4h.

According to an embodiment of the present invention, for example, in step 4, the heating rate may be 5°C/min, 6°C/min, 7°C/min, 8°C/min, 9°C/min, 10°C/min.

According to another aspect of the present invention, a cobalt oxide catalyst prepared by the preparation method is provided, wherein the cobalt oxide catalyst includes Na or K doped cobalt oxide.

According to the concept of another aspect of the present invention, a method for using the cobalt oxide catalyst for ammonia decomposition is provided, including:

Under NH ₃ /Ar atmosphere, the cobalt oxide catalyst is heated and activated, the cobalt oxide catalyst is reduced to metallic Co, and NH ₃ is decomposed into H ₂ and N ₂ on the active site of metallic Co and desorbed ;

Wherein, the cobalt oxide catalyst includes Na or K doped cobalt oxide.

According to an embodiment of the present invention, the method for ammonia decomposition is as follows: firstly, under a 15% NH ₃ /Ar atmosphere, the catalyst is activated at high temperature, and Co ₃ O ₄ is reduced to metallic Co, which plays a role in the ammonia decomposition reaction. The role of the main active site. _NH3 is decomposed into _H2 and _N2 on the active site and desorbed.

For unsupported catalysts, the sintering and agglomeration of active sites are important factors affecting the ammonia decomposition activity. On the one hand, the electronic interaction between alkali metals Na _and K and Co can effectively inhibit the sintering and agglomeration of active sites; desorption, thereby improving the ammonia decomposition activity.

According to an embodiment of the present invention, wherein, in the NH ₃ /Ar atmosphere, NH ₃ accounts for 5-100% of the total volume.

According to an embodiment of the present invention, in the NH ₃ /Ar atmosphere, NH ₃ may account for 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% of the total volume.

According to an embodiment of the present invention, preferably, in the NH ₃ /Ar atmosphere, NH ₃ accounts for 12-17% of the total volume.

According to an embodiment of the present invention, in the NH ₃ /Ar atmosphere, NH ₃ may account for 12%, 13%, 14%, 15%, 16%, and 17% of the total volume.

According to an embodiment of the present invention, wherein, when the cobalt oxide catalyst is reduced to metallic Co, Na or K is in a highly dispersed state.

According to an embodiment of the present invention, the activation temperature of the cobalt oxide catalyst is 480-520°C.

According to an embodiment of the present invention, the activation temperature of the cobalt oxide catalyst may be 480°C, 490°C, 500°C, 510°C, or 520°C.

The technical solutions of the present invention will be described in detail below through preferred embodiments. It should be noted that the specific embodiments below are only for illustration and are not intended to limit the present invention.

Example 1: Preparation of samples.

Preparation of Sample 1:

Step 1: Dissolve 8g NaOH in 200mL high-purity water to obtain a precipitant solution;

Step 2: Dissolve 5.821g Co(NO ₃ ) ₂ ·6H ₂ O in 100mL high-purity water to obtain a precursor solution;

Step 3: Add the precipitating agent solution dropwise to the precursor solution until pH=9, continue stirring for 1 hour, stop stirring, and age for 3 hours; filter and wash with suction, and dry the obtained solid to obtain a crude catalyst;

Step 4: Put the crude catalyst into a muffle furnace for calcination at 600° C. for 4 hours at a heating rate of 10° C./min to obtain a cobalt oxide catalyst as sample 1.

Preparation of sample 2:

Step 1: Dissolve 27.64g K ₂ CO in 180mL high-purity water to obtain a precipitant solution;

Step 2: Dissolve 5.622g CoSO ₄ ·7H ₂ O in 90mL high-purity water to obtain a precursor solution;

Step 3: Add the precipitating agent solution dropwise to the precursor solution until pH = 9.5, continue stirring for 2 hours, stop stirring, and age for 4 hours; filter and wash with suction, and dry the obtained solid to obtain a crude catalyst;

Step 4: Baking in a muffle furnace at 600° C. for 3 hours with a heating rate of 5° C./min to obtain a cobalt oxide catalyst as sample 2.

Preparation of sample 3:

Step 1: Dissolve 11.22g KOH in 220mL high-purity water to obtain a precipitant solution;

Step 2: Dissolve 4.759g CoCl ₂ ·6H ₂ O in 110mL high-purity water to obtain a precursor solution;

Step 3: Add the precipitating agent solution dropwise to the precursor solution until pH = 10, continue to stir for 1.5 hours, stop stirring, and age for 5 hours; filter and wash with suction, and dry the obtained solid to obtain a crude catalyst;

Step 4: Baking in a muffle furnace at 550° C. for 3.5 hours with a heating rate of 10° C./min to obtain a cobalt oxide catalyst as sample 3.

Preparation of comparative samples:

Step 1: Dissolve 5.820g Co(NO3)2·6H2O and 5.044g citric acid in 20mL of high-purity water, stir until completely evaporated to dryness, and obtain a crude product;

Step 2: Move the above crude product into an oven at 105°C overnight, then pulverize it, and roast it in a muffle furnace at 600°C for 3 hours with a heating rate of 5°C/min to obtain a pure cobalt oxide catalyst as a comparative sample.

Embodiment 2: Explore the ammonia conversion rate of the sample.

Fig. 1 is the comparison chart of the ammonia conversion rate of the cobalt oxide catalyst of the embodiment of the present invention and the pure cobalt oxide catalyst;

Under NH ₃ /Ar atmosphere, the catalyst provided by sample 1 and comparative sample is heated;

The catalytic conditions are: [NH ₃ ]=15%, Ar is the balance gas, the total flow rate is 50mL/min, GHSV=25,000mLg _cat ^-1 h ^-1 , 500°C, 1h:

As shown in Figure 1, compared with the existing catalysts used for ammonia decomposition, the ammonia decomposition conversion rate of the comparative sample below 600 ° C is lower than 50%, and the cobalt oxide catalyst prepared by the present invention obviously has higher ammonia decomposition activity , the ammonia decomposition conversion rate of sample 1 at 500 °C can reach almost 100%. Under the same test conditions, the activity of the cobalt oxide catalyst prepared by the invention is higher than that of the cobalt oxide catalyst prepared by other methods. The preparation process is simple, the cost is low, and industrial production can be realized.

Fig. 2 is a comparison chart of the ammonia conversion rate of the cobalt oxide catalyst samples in the examples of the present invention.

Under NH ₃ /Ar atmosphere, the cobalt oxide catalysts provided by sample 1, sample 2 and sample 3 were activated by heating, and the cobalt oxide catalysts provided by sample 1, sample 2 and sample 3 were reduced to metallic state Co, and the NH ₃ is decomposed into _H2 and _N2 on the active site of metallic Co and desorbed;

The catalytic conditions are: [NH ₃ ]=15%, Ar is the balance gas, the total flow rate is 50mL/min, GHSV=25,000mLg _cat ^-1 h ^-1 , 500°C, 1h:

As shown in Fig. 2, three kinds of samples all have relatively good conversion rate, and wherein the initial conversion rate of sample 1 is the highest, but the cobalt oxide catalyst provided by sample 1, sample 2 and sample 3 is to ammonia when approaching 600 degree The conversion rate can reach almost 100%;

Embodiment 3: explore the catalyst structure before and after the catalysis of the sample

Fig. 3 is the XRD diffraction spectrum of the cobalt oxide catalyst sample before catalysis in the embodiment of the present invention.

Fig. 4 is a catalyzed XRD diffraction spectrum of a cobalt oxide catalyst sample in an example of the present invention.

As shown in Figure 3, it can be seen from the XRD diffraction pattern that the fresh samples provided by Sample 1, Sample 2 and Sample 3 that did not participate in the catalytic reaction all showed diffraction peaks attributed to Co ₃ O ₄ .

As shown in Figure 4, it can be seen from the XRD diffraction pattern that the samples after the catalytic reaction provided by sample 1, sample 2 and sample 3 show diffraction peaks attributed to metal Co.

It can be seen from the XRD diffraction pattern that no Na/K-related diffraction peaks were observed in all the fresh and reacted samples provided by Sample 1, Sample 2, and Sample 3, indicating that Na/K is in a highly dispersed state.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. Within the spirit and principles of the present invention, any modifications, equivalent replacements, improvements, etc., shall be included in the protection scope of the present invention.

Claims (5)

1. A method of using a cobalt oxide catalyst for ammonia decomposition, comprising:

at NH ₃ Activating cobalt oxide catalyst by heating under Ar atmosphere to reduce the cobalt oxide catalyst to metallic Co and NH ₃ Decomposition to H at the active site of the metallic Co ₂ And N ₂ And desorbing;

wherein the cobalt oxide catalyst comprises Na or K doped cobalt oxide;

the NH is ₃ NH in Ar atmosphere ₃ Accounting for 5-100% of the total volume;

in the case where the cobalt oxide catalyst is reduced to Co in a metallic state, the Na or K is in a highly dispersed state;

the activation temperature of the cobalt oxide catalyst is 480-520 ℃;

the preparation method of the cobalt oxide catalyst comprises the following steps:

dissolving an alkaline reagent containing Na ions or K ions in water to obtain a precipitant solution;

dissolving cobalt precursor in water to obtain a precursor solution;

dropwise adding the precipitant solution into the precursor solution until the pH reaches a certain value, and performing aging, suction filtration, washing and drying treatment to obtain a crude catalyst;

roasting the crude catalyst to obtain the cobalt oxide catalyst;

wherein the cobalt oxide catalyst comprises Na or K doped cobalt oxide.

2. A method according to claim 1, wherein the cobalt precursor is a hydrate of a divalent cobalt salt, preferably Co (NO ₃ ) ₂ ·6H ₂ O、CoSO ₄ ·7H ₂ O or CoCl ₂ ·6H ₂ O；

The alkaline reagent is preferably NaOH or K ₂ CO ₃ Or KOH.

3. The method of claim 1, wherein the pH is in the range of 9.0-10.0.

4. The method of claim 1, wherein the firing temperature is 550-600 ℃, the firing time is 3-4 hours, and the temperature rise rate is 5-10 ℃/min.

5. The method of claim 1, wherein the aging time is 3-5 hours.

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