Preparation method and application of copper-aluminum composite oxide rich in acid sites

Abstract

The invention discloses a preparation method and application of a copper-aluminum composite oxide rich in acid sites. The copper-aluminum composite oxide is used for selective catalytic oxidation of NH3. The catalyst obtained by calcining the aluminum-doped malachite precursor synthesized by taking urea as an additive has more acidic sites and higher catalytic activity; the method is mainly characterized in that in the process of synthesizing aluminum malachite by using urea as a precipitator, the surface of Al2O3 is more easily hydroxylated, and then hydroxyl is reversibly adsorbed on the surface of Al2O3, so that the adsorption sites of NH3 are increased, and the acidic sites of the catalyst are enhanced; secondly, an organic solvent is used for soaking, the organic solvent in the supported catalyst promotes CuO species to exist in a multi-layer covering mode, interaction between the CuO species and gamma-Al2O3 support is weakened, and therefore reduction of the CuO species is promoted, the oxidation-reduction performance of the catalyst is enhanced, and the overall catalytic activity is improved.

Description

Technical Field

[0001] The invention is used in the technical field of environmentally friendly catalytic treatment, and specifically relates to a copper-aluminum composite metal oxide with abundant acid sites, and its application in the selective catalytic oxidation of NH3 to generate N2 and H2O. Background Art

[0002] As a green renewable fuel, NH3 has been widely studied in the energy system; however, the use of NH3 fuel has aggravated the problem of NH3 leakage, causing pollution such as haze and photochemical smog. This makes the purification of NH3 particularly important; among NH3 treatment technologies, NH3 selective catalytic oxidation technology is considered to be the most promising technology, and the core of this technology lies in the selection of catalyst materials. Traditional NH3-SCO catalysts are mainly precious metal-based catalysts, transition metal oxide-based catalysts and molecular sieve catalysts; among them, copper-aluminum-based catalysts in transition metal-based catalysts have good development prospects due to their low price and excellent N2 selectivity. In previous studies, Al2O3 is usually used as a carrier to load CuO. The CuAl-based catalyst prepared by this method has a relatively high N2 selectivity, but the complete conversion temperature of NH3 is relatively high, and it can only be completely converted at 300 ℃; at this stage, a large number of studies are aimed at developing copper-aluminum-based catalysts with higher activity. The main means is to adjust the interaction between metal oxides and carriers and the interface effect by doping precious metals to enhance the catalytic activity, but this increases the preparation cost of the catalyst. Therefore, it is very necessary to develop a high-performance, low-cost CuAl-based catalyst. Summary of the invention

[0003] The present invention provides a method for preparing a copper-aluminum composite metal oxide rich in acidic sites. The copper-aluminum composite metal oxide catalyst rich in acidic sites can completely convert NH3 at 240°C, while the N2 selectivity can still reach more than 90%; the performance is better than most of the reported copper-aluminum-based catalysts, and this synthesis method significantly improves the low-temperature NH3 oxidation activity.

[0004] The purpose of the present invention is to provide a preparation method and application of a copper-aluminum composite oxide catalyst rich in acidic sites: aluminum malachite is first synthesized as a precursor by a hydrothermal method, and the precursor is calcined to synthesize a copper-aluminum composite metal oxide. Urea is used as a precipitant, and a large amount of hydroxylation occurs on the surface of Al2O3 during the synthesis process. The hydroxyl groups are reversibly adsorbed on the surface of Al2O3, which enhances the acidic sites on the catalyst surface and increases the adsorption sites of NH3; the organic solvent promotes the existence of CuO species in the form of multi-layer coverage, and weakens the interaction between CuO species and γ-Al2O3 support, thereby enhancing the redox performance; the copper-aluminum composite oxide catalyst rich in acidic sites prepared by the present invention can completely convert NH3 at 240°C, significantly improving the selective catalytic oxidation activity of NH3 at low temperature; compared with the existing copper-aluminum composite oxide catalysts at this stage, it has more excellent catalytic activity and good application prospects.

[0005] To achieve the above object, the present invention adopts the following technical solution: A method for preparing a copper-aluminum composite oxide catalyst rich in acidic sites comprises the following steps: (1) Mix the copper source, aluminum source and water, then add appropriate amounts of surfactant and precipitant, and stir at room temperature to allow the materials to fully contact; (2) transferring the material obtained in step (1) into a hydrothermal kettle and placing it in an oven for hydrothermal reaction; (3) After the hydrothermal autoclave is cooled, the solid product obtained after the reaction in step (2) is centrifuged, and then soaked in an organic reagent. After soaking, the solid product is centrifuged, washed, and dried to obtain an aluminum malachite precursor.

[0006] (4) The aluminum malachite precursor is calcined in a muffle furnace to obtain copper-aluminum composite oxide.

[0007] Furthermore, the aluminum source in step (1) is aluminum nitrate nonahydrate (Al(NO3)3·9H2O).

[0008] Furthermore, the copper source in step (1) is copper nitrate trihydrate (Cu(NO3)2·3H2O).

[0009] Furthermore, the precipitant in step (1) is urea (CH4N2O).

[0010] Furthermore, the surfactant described in step (1) is PEG-4000.

[0011] Furthermore, the molar ratio of the copper source (calculated as Cu(NO3)2·3H2O) used in step (1) to the aluminum source (calculated as Al(NO3)3·9H2O) plus the copper source is 40%; and the molar amount of the precipitant is 1.5-2 times the total molar amount of the aluminum source and the copper source.

[0012] Furthermore, the temperature of the hydrothermal reaction in step (2) is 110° C. and the time is 12 h.

[0013] Furthermore, the drying temperature in step (3) is 80°C, the drying time is 12 h, and the centrifugal speed is 4000 r / min.

[0014] Furthermore, the organic reagent in step (3) is methanol, and the soaking time is 4-6 h.

[0015] Furthermore, the calcination temperature in step (4) is 400°C, the calcination time is 6 h, and the heating rate is 2°C / min.

[0016] The acidic site-rich copper-aluminum composite metal oxide catalyst prepared by the above method has relatively excellent NH3-SCO performance, and can completely convert NH3 at 240°C, while the N2 selectivity can still reach more than 90%; this has surpassed most of the reported copper-aluminum-based catalysts.

[0017] The application of the copper-aluminum composite oxide in the selective catalytic oxidation of NH3 is as follows: the copper-aluminum composite oxide is used as a catalyst, NH3 and O2 are used as reaction gases, and high-purity Ar is used as a balance gas; NH3 is converted into N2 and H2O by O2 on a fixed bed; the volume concentration of NH3 in the reaction gas is 0.3%, the volume concentration of O2 is 2.5%, and the volume space velocity is 60000 h -1 .

[0018] The significant advantages of the present invention are: The preparation method of the present invention is unique, and a copper-aluminum composite oxide catalyst with abundant acidic sites is synthesized; the catalyst is synthesized using an aluminum malachite precursor to promote electron transfer between divalent copper and monovalent copper, and organic reagents promote the existence of CuO species in the form of multi-layer coverage, weakening the interaction between CuO species and γ-Al2O3 support, and having stronger redox performance; at the same time, hydroxylation occurs on the surface of Al2O3, and hydroxyl groups are reversibly adsorbed on the surface of Al2O3, increasing the adsorption sites of NH3, and the catalyst prepared by this method can achieve complete conversion of NH3 at 240°C, and the N2 selectivity performance reaches more than 90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 NH3 conversion rate spectrum of the samples prepared in Examples 1-3.

[0020] Figure 2 This is the N2 selectivity spectrum of the samples prepared in Examples 1-3.

[0021] Figure 3This is the XRD pattern of the precursor of the sample prepared in Example 1.

[0022] Figure 4 The XRD patterns of the samples prepared in Examples 1-3 after calcination.

[0023] Figure 5 The following are SEM images of the samples prepared in Examples 1-3.

[0024] Figure 6 NH3-TPD diagram of the samples prepared in Examples 1-3.

[0025] Figure 7 NH3-TPD acid site content distribution diagram of the samples prepared in Examples 1-3. DETAILED DESCRIPTION

[0026] In order to make the contents of the present invention easier to understand, the technical solution of the present invention is further described below in conjunction with specific implementation methods, but the present invention is not limited thereto.

[0027] Example 1 The synthesis of a copper-aluminum composite oxide catalyst rich in acidic sites specifically comprises the following steps: Aluminum-doped malachite precursor was prepared by hydrothermal method, and aluminum-doped malachite was calcined to obtain copper-aluminum composite oxide. 12 mmol copper nitrate trihydrate and 18 mmol aluminum nitrate nonahydrate were weighed and dissolved in 60 mL deionized water at a molar ratio of Cu:(Cu+Al)=40%. Urea was added as a precipitant after the metal salt was completely dissolved, and the molar ratio of urea to total metal ions (Cu+Al) was 1.5:1; 0.5 g of PEG-4000 was added as a surfactant. Stir for half an hour until the solid was completely dissolved, transfer to a polytetrafluoroethylene liner and hydrothermally heat at 110 °C for 12 h. After the hydrothermal treatment, centrifuge at 4000 r / min; after centrifugation, 25 mL of methanol solvent was added to the obtained precipitate, stirred and soaked for 4 h, and then centrifuged again. The separated precipitate was washed with deionized water. The washed sample was dried at 80 °C for 12 h to obtain the aluminum-doped malachite precursor, denoted as CuAlCCB. CuAlCCB was then calcined at 400 °C for 6 h to obtain the target product, denoted as CuAlO. x CCB-Z (Z=4 is 40%).

[0028] Example 2 The copper-aluminum composite oxide catalyst is synthesized by a sol-gel method, which specifically comprises the following steps: 12 mmol of copper nitrate trihydrate, 18 mmol of aluminum nitrate nonahydrate, and 45 mmol of citric acid were weighed and dissolved in 60 mL of deionized water at a molar ratio of Cu:(Cu+Al) = 40%, and recorded as solution A. The molar ratio of citric acid to the total amount of metal cations was 1.5:1. Solution A was placed in a water bath and stirred and heated. The temperature was set to 80 °C. Stir and heat until the water in the solution evaporated to form a transparent gel. The gel was then dried in an oven at 80 °C for 12 h. During the drying process, the gel gradually expanded. After drying, it was heated to 400 °C in a muffle furnace at a heating rate of 2 °C / min and calcined for 6 h. The calcined catalyst was recorded as CuAlO x -SG.

[0029] Example 3 The copper-aluminum composite oxide catalyst is synthesized by a coprecipitation method, which specifically includes the following steps: In the molar ratio of Cu:(Cu+Al)=40%, 12 mmol of copper nitrate trihydrate and 18 mmol of aluminum nitrate nonahydrate were weighed and dissolved in 60 mL of deionized water. The mixture was stirred until the solid was completely dissolved and recorded as solution A. The stirring time was 30 min. 1.8 g of NaOH was weighed and dissolved in water to make a 1.5 mol / L solution. The alkaline solution was slowly added dropwise to solution A, and the pH was adjusted to 9. During this process, the pH value of the reaction solution was kept constant. After the reaction was complete, the mixture was stirred and crystallized for 12 h. The precipitated material was centrifuged at a speed of 4000 r / min and washed with deionized water. The washed solid material was dried in an oven at 80 °C for 12 h, and then calcined in a muffle furnace, and the temperature was raised to 400 °C at a heating rate of 2 °C / min and maintained for 6 h. The calcined catalyst was recorded as CuAlO x CP.

[0030] Application Examples The copper-aluminum composite oxide obtained in Examples 1-3 was used as a catalyst, NH3 and O2 were used as reaction gases, high-purity Ar was used as a balance gas, and the total gas flow rate was set to 100 mL / min. The three mixed gases were introduced into the fixed bed, and the reaction temperature was controlled by a program temperature device to convert NH3 into N2, H2O, N2O, and NO2. The concentrations of NH3, NO, N2O, and NO2 at different temperatures during the reaction were detected by an infrared gas detector to obtain the conversion rate of NH3 and the selectivity of N2; the volume concentration of NH3 in the reaction gas was 0.3%, the volume concentration of O2 was 2.5%, and the volume space velocity was 60000 h -1 ; During the test, the mass of the catalyst was weighed as 0.1 g.

[0031] Figure 1The NH3 conversion rate spectra of the samples prepared in Examples 1-3 are shown. As can be seen from the figure, the sample synthesized in Example 1 has the highest NH3 conversion rate, T 100% = 240 °C. The NH3 complete conversion temperature of Example 2 is about 300 °C, and the NH3 complete conversion temperature of Example 3 is about 320 °C, both of which are significantly higher than that of Example 1.

[0032] Figure 2 The N2 selectivity diagram of the samples prepared in Examples 1-3 is shown. In Example 1, the N2 selectivity can basically reach about 93% when NH3 is completely converted. Although the N2 selectivity is slightly lower than that of Examples 2 and 3 when NH3 is completely converted, when the test temperature reaches 400°C, the N2 selectivity of Example 1 is basically the same as that of Examples 2 and 3, and can still reach an N2 selectivity of more than 80%.

[0033] Figure 3 The XRD spectrum of the precursor of the sample prepared in Example 1 is shown. As can be seen from the figure, the characteristic peaks of the precursor of Example 1 are basically consistent with the PDF standard card of pure basic copper carbonate (JCPSD NO.41-1390), indicating that the malachite structure is successfully prepared, and aluminum exists in an amorphous form; forming an aluminum-doped malachite structure.

[0034] Figure 4 The XRD spectra of the samples prepared in Examples 1-3 after calcination are shown. Examples 1, 2, and 3 all exhibit the structure of CuO, which is basically consistent with the characteristic peaks of the PDF standard card of CuO (JCPSD NO.89-5898), and aluminum exists in an amorphous form.

[0035] Figure 5 The SEM spectra of the samples prepared in Examples 1-3 are shown, and it is observed that the surface of Example 1 presents a stacked sheet structure and an elongated triangular pyramid structure, and there is a dense pit structure on the sheet. This special surface morphology makes Example 1 have abundant defect sites, and these defect sites can provide more adsorption sites and active centers for NH3 molecules, thereby enhancing the catalytic performance of the catalyst. The surface of Example 2 also presents a sheet structure, on which there is an aggregate shape composed of fine particles. Example 3 shows a stacked agglomeration morphology without obvious morphological structure.

[0036] Figure 6The NH3-TPD spectra of the samples prepared in Examples 1-3 are shown. In the figure, the desorption peaks at 100-200 °C are attributed to the desorption of NH3 on the weak acid sites, the desorption peaks at 200-400 °C are attributed to the desorption of NH3 on the medium-strong acid sites, and the desorption peaks at 400-600 °C are attributed to the desorption of NH3 on the strong acid sites. It can be seen from the figure that the peak intensity of Example 1 is significantly stronger than that of Example 2 and Example 3, especially the weak acid sites and medium-strong acid sites of Example 1. This shows that Example 1 has very rich acid sites. In the process of NH3 oxidation, the acid sites are the main adsorption sites of NH3. The adsorption and desorption behavior of NH3 on the catalyst surface is considered to be the key step in the NH3-SCO reaction. Example 1 has a large number of acid sites, which is conducive to the adsorption and activation of NH3 and enhances the catalytic activity of NH3.

[0037] Figure 7 The NH3-TPD acid content distribution diagram of the samples prepared in Examples 1-3 is shown. It can be seen intuitively in the figure that the amount of acid sites in Example 1 is significantly higher than that in Example 2 and Example 3, whether it is a weak acid site, a medium-strong acid site or a strong acid site. This shows that Example 1 has very abundant surface acid sites.

[0038] In summary, the acid site-rich copper-aluminum composite oxide catalyst prepared in the present invention has abundant surface acid sites, excellent performance in NH3-SCO and good N2 selectivity, and has great application potential.

[0039] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for preparing a copper-aluminum composite oxide catalyst rich in acidic sites, characterized in that: The following steps are involved: (1) Mix the copper source, aluminum source and precipitant with water, then add an appropriate amount of surfactant and stir at room temperature to ensure full contact between the materials; (2) subjecting the material obtained in step (1) to a hydrothermal reaction; (3) The solid product obtained after the hydrothermal treatment in step (2) is centrifuged at a speed of 4000 r / min, the precipitated material after separation is soaked with an organic reagent, then centrifuged again, and washed with water; The washed sample was dried at 80 °C for 12 h to obtain an aluminum malachite precursor; (4) The aluminum malachite precursor is calcined in a muffle furnace to obtain copper-aluminum composite oxide.

2. The preparation method according to claim 1, characterized in that: The copper source and aluminum source in step (1) are copper nitrate trihydrate and aluminum nitrate nonahydrate respectively.

3. The preparation method according to claim 1, characterized in that: In step (1), the precipitant is urea, and the molar amount of the precipitant is 1.5-2 times the total molar amount of the aluminum source and the copper source.

4. The preparation method according to claim 1, characterized in that: The molar amount of the copper source in step (1) accounts for 40% of the total molar amount of the aluminum source and the copper source.

5. The preparation method according to claim 1, characterized in that: The surfactant described in step (1) is PEG-4000.

6. The preparation method according to claim 1, characterized in that: The temperature of the hydrothermal reaction in step (2) is 110 °C and the time is 12 h.

7. The preparation method according to claim 1, characterized in that: The organic reagent described in step (3) is methanol, and the soaking time is 4-6 h.

8. The preparation method according to claim 1, characterized in that: In step (4), the calcination condition in the muffle furnace is to increase the temperature to 400°C at 2°C / min and calcine for 6 h.

9. A copper-aluminum composite oxide catalyst rich in acidic sites obtained according to the preparation method according to any one of claims 1 to 8.

10. The use of the copper-aluminum composite oxide in the selective catalytic oxidation of NH3 according to claim 9, characterized in that: Copper-aluminum composite oxide is used as catalyst, NH3 and O2 are used as reaction gases, and high-purity Ar is used as balance gas; NH3 is converted into N2 and H2O by O2 on a fixed bed.

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