Cobalt oxide and copper-based molecular sieve mixed catalyst and preparation method and application thereof

Abstract

The invention relates to a cobalt oxide and copper-based molecular sieve mixed catalyst, a preparation method thereof and application of the cobalt oxide and copper-based molecular sieve mixed catalyst in low-concentration ammonia gas selective catalytic oxidation, and belongs to the field of catalytic technologies and environmental protection. The preparation method of the catalyst comprises the following steps: (1) mechanically grinding cobalt oxide and a copper-based molecular sieve into powder to obtain a catalyst precursor; and (2) continuously carrying out dry granulation on the catalyst precursor obtained in the step (1) under the pressure of 10-40Mpa for 10-30 minutes, crushing, and sieving with a 20-80-mesh sieve to obtain the catalyst. The method is simple, the preparation cost is low, the copper-based molecular sieve catalyst and the cobalt oxide are mixed in a grinding mode, the strong oxidizing catalyst and the acid catalyst can make full contact, it can be guaranteed that the oxygen activating capacity of the catalyst is effectively improved, meanwhile, the catalyst has the corresponding ammonia gas adsorbing and activating capacity, and the service life of the catalyst is prolonged. The catalyst has high activity under a low-temperature condition, can ensure high nitrogen selectivity, and is expected to achieve the performance of noble metal.

Description

Technical Field

[0001] The invention relates to a cobalt oxide and copper-based molecular sieve mixed catalyst and a preparation method thereof and application in selective catalytic oxidation of low-concentration ammonia, belonging to the fields of catalytic technology and environmental protection. Background Art

[0002] Ammonia is a colorless, toxic, corrosive gas that is emitted into the atmosphere from various sources, especially nitrogen fertilizers used in agricultural production, volatilized ammonia from livestock manure, and low-concentration ammonia from coal-fired power plants and diesel vehicles and ships during NH3-SCR treatment of flue gas. This irritating gas is very harmful to human health and the environment. Existing methods for removing ammonia include liquid washing, traditional adsorption, biological treatment, chemical treatment, and ammonia selective catalytic oxidation (NH3-SCO). Among them, NH3-SCO is a method for selectively catalytically oxidizing ammonia into nitrogen and water, as shown in equation (1). It is an effective and environmentally friendly method for treating low-concentration ammonia. However, this method will also produce toxic byproducts such as NO, NO2, and N2O to a certain extent during the reaction process, as shown in equations (2)-(4). How to develop a catalyst with high catalytic activity and selectivity is a difficult point in the research of NH3-SCO. Therefore, from the perspective of saving energy and not generating secondary pollution, it is urgent to develop a catalytic oxidation catalyst that has both high activity and excellent nitrogen selectivity.

[0003] 4NH3 + 3O2 → 2N2 + 6H2O (1)

[0004] 4NH3 + 5O2 → 4NO + 6H2O (2)

[0005] 4NH3 + 7O2 → 4NO2 + 6H2O (3)

[0006] 2NH3 + 2O2 → N2O + 3H2O (4)

[0007] At present, the catalysts used for NH3-SCO are mainly divided into three categories: one is precious metal catalysts, one is molecular sieve catalysts, and the other is transition metal oxide catalysts. Precious metal catalysts often have high activity, but their nitrogen selectivity is not high and they produce more by-products; molecular sieve catalysts are generally obtained by loading precious metals or transition metals onto molecular sieves, and generally have good nitrogen selectivity and high-temperature activity; transition metal oxides are less active than precious metals, but have higher nitrogen selectivity. Summary of the invention

[0008] The purpose of the present invention is to provide a cobalt oxide and copper-based molecular sieve mixed catalyst and its preparation method and application in the selective catalytic oxidation of low-concentration ammonia. By adjusting the mixing ratio of cobalt oxide and copper-based molecular sieve, a catalyst with both high activity and high nitrogen selectivity is obtained.

[0009] In order to achieve the above object, the technical solution of the present invention is as follows:

[0010] In one aspect, the present invention provides a method for preparing a mixed catalyst of cobalt oxide and copper-based molecular sieve, the method comprising the following steps:

[0011] (1) mechanically grinding cobalt oxide and copper-based molecular sieve into powder to obtain a catalyst precursor;

[0012] (2) Dry granulating the catalyst precursor obtained in step (1) at a pressure of 10-40 MPa for 10-30 min, crushing it and passing it through a 20-80 mesh sieve to obtain the catalyst.

[0013] In the above technical solution, preferably, the method for preparing the cobalt oxide comprises the following steps:

[0014] 1) Using a cobalt source, urea and methanol as raw materials, fully mixing them and stirring them at room temperature;

[0015] 2) transferring the mixed solution to a hydrothermal synthesis reactor, maintaining it at 160-190° C. for 10-16 hours for hydrothermal crystallization, and centrifuging, washing, and drying the resulting precipitate;

[0016] 3) The dried powder is heated to 400-550° C. in an air atmosphere and calcined for 4-6 hours to obtain cobalt oxide.

[0017] In the above technical solution, preferably, the copper-based molecular sieve is synthesized by an impregnation method, and the copper-based molecular sieve includes any one of Cu-KFI and Cu-SSZ-13, wherein the copper loading is 5-20wt%.

[0018] In the above technical solution, preferably, the mass ratio of the cobalt oxide to the copper-based molecular sieve is 1:4-4:1.

[0019] In the above technical solution, preferably, in step 3), the heating rate is 1-5°C / min.

[0020] Another aspect of the present invention provides an application of a mixed catalyst of cobalt oxide and copper-based molecular sieve prepared by the above-mentioned preparation method, which is used for selective catalytic oxidation of low-concentration ammonia, wherein the volume concentration of the ammonia is ≤1%.

[0021] The principle of the present invention is:

[0022] Cobalt oxide has high activity, but its strong oxidizing property leads to excessive byproducts, which reduces its selectivity. Copper-based molecular sieves have good nitrogen selectivity, but their activity needs to be further improved. The present invention utilizes the synergistic effect of cobalt oxide and copper-based molecular sieves to carry out ammonia selective catalytic oxidation reaction, utilizes the high oxidizing property of cobalt oxide to fully oxidize ammonia, thereby obtaining a higher ammonia conversion rate, and utilizes copper-based molecular sieves to convert NO generated by excessive oxidation into nitrogen. x It is converted into nitrogen through catalytic reduction, thus achieving higher nitrogen selectivity.

[0023] The reaction formula is as follows:

[0024] 4NH3 + 4NO + O2 → 4N2 + 6H2O (5)

[0025] 4NH3 + 3NO2 → 3.5N2 + 6H2O (6)

[0026] The beneficial effects of the present invention are:

[0027] (1) The mechanical mixing method of the present invention synthesizes a catalyst of a mixture of cobalt oxide and copper-based molecular sieve. The method is simple and has low preparation cost.

[0028] (2) The method of mixing the copper-based molecular sieve catalyst and the cobalt oxide in a grinding manner in the present invention allows the strong oxidizing catalyst and the acid catalyst to be fully in contact, which can not only effectively increase their ability to activate oxygen, but also have the corresponding ability to adsorb and activate ammonia, so that the catalyst has both high activity and high nitrogen selectivity under low temperature conditions, and is expected to achieve the performance of precious metals.

[0029] (3) The cobalt oxide and copper-based molecular sieve mixed catalyst prepared by the present invention has high activity and nitrogen selectivity. Under the condition of space velocity of 40,000 mL / (g·h), the conversion rate can reach 100% at 250°C, and T 100 When selecting performance remains above 80%. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The NH3 conversion rate of the catalysts of Examples 1, 2 and 3 varies with temperature;

[0031] Figure 2 The N2 selectivity diagram of the catalysts of Examples 1, 2, and 3 as a function of temperature;

[0032] Figure 3 The NH3 conversion rate of the catalysts of Examples 4 and 5 varies with temperature;

[0033] Figure 4 The N2 selectivity diagram of the catalysts of Examples 4 and 5 as a function of temperature;

[0034] Figure 5 The NH3 conversion rate diagram of catalyst temperature change in Example 2 and Comparative Examples 1 and 2;

[0035] Figure 6 The N2 selectivity graph of the catalysts of Example 2 and Comparative Examples 1 and 2 as a function of temperature;

[0036] Figure 7 This is the in-situ drift spectrum of the catalyst of Example 2. DETAILED DESCRIPTION

[0037] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and cannot be interpreted as limiting the present invention.

[0038] Unless otherwise specified, the materials used in the embodiments of the present invention can be obtained through commercial channels or prepared according to conventional methods well known to those skilled in the art.

[0039] The catalyst prepared by the present invention is prepared by mechanically mixing cobalt oxide and copper-based molecular sieve catalyst, and the mass ratio of cobalt oxide to copper-based molecular sieve is 1:4-4:1. The specific operation details of synthesizing the catalyst are shown in Example 1; the reaction is evaluated by using an ammonia selective catalytic oxidation catalyst with an ammonia concentration of 1%, as shown in Test Example 1.

[0040] The preparation method of Co3O4 comprises the following steps:

[0041] 4.36 g of cobalt nitrate hexahydrate, 1.8 g of urea and 70 ml of methanol were fully mixed and stirred at room temperature. The mixture was transferred to a hydrothermal synthesis reactor and kept at 180°C for 12 hours for hydrothermal crystallization. The obtained precipitate was centrifuged, washed and dried. The dried powder was heated to 400°C in an air atmosphere and calcined for 4 hours to obtain Co3O4.

[0042] Example 1

[0043] (1) Co3O4 and 10% Cu / KFI were mixed in a mass ratio of 1:1 (a total of 0.9 g) and ground into powder;

[0044] (2) The powder was placed in a powder tableting mold (mold diameter 15 mm), the mold was placed on a tablet press and pressurized to 20 MPa for 10 min, the extruded block catalyst (diameter 15 mm, thickness 2 mm) was taken out, crushed with a mortar and sieved to 40-60 mesh, and the obtained mixed catalyst was recorded as C:CK (1:1)-20.

[0045] Example 2

[0046] (1) Co3O4 and 10% Cu / KFI were mixed in a mass ratio of 1:3 (a total of 0.9 g) and ground into powder;

[0047] (2) The powder was placed in a powder tableting mold (mold diameter 15 mm), the mold was placed on a tablet press and pressurized to 20 MPa for 10 min, the extruded block catalyst (diameter 15 mm, thickness 2 mm) was taken out, crushed with a mortar and sieved to 40-60 mesh, and the obtained mixed catalyst was recorded as C:CK (1:3)-20.

[0048] Example 3

[0049] (1) Co3O4 and 10% Cu / KFI were mixed in a mass ratio of 1:3 (a total of 0.9 g) and ground into powder;

[0050] (2) The powder was placed in a powder tableting mold (mold diameter 15 mm), the mold was placed on a tablet press and pressurized to 40 MPa for 10 min, the extruded block catalyst (diameter 15 mm, thickness 2 mm) was taken out, crushed with a mortar and sieved to 40-60 mesh, and the obtained composite catalyst was recorded as C:CK (1:3)-40.

[0051] Example 4

[0052] (1) Co3O4 and 10% Cu / KFI were mixed in a mass ratio of 4:1 (a total of 0.9 g) and ground into powder;

[0053] (2) The powder was placed in a powder tableting mold (mold diameter 15 mm), the mold was placed on a tablet press and pressurized to 10 MPa for 10 min, the extruded block catalyst (diameter 15 mm, thickness 2 mm) was taken out, crushed with a mortar and sieved to 40-60 mesh, and the obtained composite catalyst was recorded as C:CK (4:1)-10.

[0054] Example 5

[0055] (1) Co3O4 and 10% Cu / KFI were mixed in a mass ratio of 1:4 (a total of 0.9 g) and ground into powder;

[0056] (2) The powder was placed in a powder tableting mold (mold diameter 15 mm), the mold was placed on a tablet press and pressurized to 10 MPa for 10 min, the extruded block catalyst (diameter 15 mm, thickness 2 mm) was taken out, crushed with a mortar and sieved to 40-60 mesh, and the obtained composite catalyst was recorded as C:CK (1:4)-10.

[0057] Comparative Example 1

[0058] Place the Co3O4 powder in a powder tableting mold (mold diameter 15 mm), put the mold on a tablet press and pressurize to 20 MPa for 10 min, take out the extruded block catalyst (diameter 15 mm, thickness 2 mm), crush it with a mortar and sieve out 40-60 mesh, recorded as C-20.

[0059] Comparative Example 2

[0060] Place Cu / KFI powder with a loading of 10% in a powder tableting mold (mold diameter 15 mm), place the mold on a tablet press and pressurize to 20 MPa for 10 min, take out the extruded block catalyst (diameter 15 mm, thickness 2 mm), crush it with a mortar and sieve out 40-60 mesh, recorded as Cu / KFI-20.

[0061] Test Example 1

[0062] 0.3 g of the catalysts prepared in Examples 1-5 and Comparative Examples 1-2 were respectively loaded into a homemade quartz fixed bed reactor, and a mixed gas (1% NH3, 10% O2, 89% N2) was introduced, the space velocity was adjusted to 40,000 mL / (g·h), and the temperature was increased during the test to test its activity; a gas analyzer and a laser gas analyzer were used to detect the concentrations of NH3, NO, NO2, and N2O in the fixed bed exhaust gas at each temperature point.

[0063] from Figure 1-4 It can be seen that the NH3 conversion rate and N2 selectivity of the catalysts with different ratios and pelletizing pressures in Examples 1 to 5 increase with increasing temperature. Among them, the catalyst with a ratio of 1:3 and a pelletizing pressure of 20 MPa has better activity and selectivity.

[0064] from Figure 5-6 It can be seen that the mixed catalyst can have better activity and nitrogen selectivity than a single catalyst. The cobalt oxide in the mixed catalyst has a strong oxidation activity, which can make ammonia and oxygen react fully and quickly, so that ammonia is fully oxidized, but due to its over-oxidation, NO, NO2 and N2O are produced. The copper-based molecular sieve has a good performance in reducing nitrogen oxides, and can convert nitrogen oxides produced by over-oxidation into nitrogen, thus obtaining a bifunctional catalyst with high activity and selectivity.

[0065] like Figure 7 1273cm marked in -1 The band at 1434 cm belongs to NH3 coordinated to Lewis acid sites (L-NH3). -1 The band at Acid site (B-NH4 + ) ion NH4 + About, at 1317cm-1 and 1520cm -1 The band at is related to the production of nitrate. With the injection of O2, L-NH3, B-NH4 + The peak of O2 gradually decreases, while the peak of nitrate becomes more obvious. When the injection of O2 is cut off and exposed to NH3 again, the peaks of these adsorbed substances begin to recover. This indicates that nitrate is a key intermediate after the dehydrogenation of adsorbed NH3 in the reaction, which can be reduced by the subsequent gas-phase NH3 through the internal selective catalytic reduction (i-SCR) mechanism, which leads to the hybrid catalyst being able to oxidize NH3 to nitrogen oxides in large quantities and then convert it into N2.

[0066] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the implementation methods. The protection scope of the present invention shall be subject to the scope defined in the claims. Other different forms of changes or modifications may be made based on the above description. Obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for preparing a mixed catalyst of cobalt oxide and copper-based molecular sieve, characterized in that: The method comprises the following steps: (1) mechanically grinding cobalt oxide and copper-based molecular sieve into powder to obtain a catalyst precursor; (2) Dry granulating the catalyst precursor obtained in step (1) at a pressure of 10-40 MPa for 10-30 min, crushing it and passing it through a 20-80 mesh sieve to obtain the catalyst.

2. The preparation method according to claim 1, characterized in that: The preparation method of the cobalt oxide comprises the following steps: 1) Using a cobalt source, urea and methanol as raw materials, fully mixing them and stirring them at room temperature; 2) transferring the mixed solution to a hydrothermal synthesis reactor, maintaining it at 160-190° C. for 10-16 hours for hydrothermal crystallization, and centrifuging, washing, and drying the resulting precipitate; 3) The dried powder is heated to 400-550° C. in an air atmosphere and calcined for 4-6 hours to obtain cobalt oxide.

3. The preparation method according to claim 1, characterized in that: The copper-based molecular sieve is synthesized by an impregnation method. The copper-based molecular sieve includes any one of Cu-KFI and Cu-SSZ-13, wherein the copper loading is 5-20wt%.

4. The preparation method according to claim 1, characterized in that: The mass ratio of the cobalt oxide to the copper-based molecular sieve is 1:4-4:

1.

5. The preparation method according to claim 2, characterized in that: In step 3), the heating rate is 1-5°C / min.

6. An application of a mixed catalyst of cobalt oxide and copper-based molecular sieve prepared by the preparation method according to any one of claims 1 to 5, characterized in that: Used for selective catalytic oxidation of low-concentration ammonia, wherein the volume concentration of the ammonia is ≤1%.

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