CN103143361A - Graphene-promoted hydrotalcite-based denitration catalyst and preparation method thereof - Google Patents

Abstract

A graphene-promoted hydrotalcite-based denitration catalyst and a preparation method thereof belong to the technical field of environmental protection catalysts. The chemical formula of the catalyst is M ²⁺ ₂ Mg ²⁺ Al ³⁺ (O); wherein, M ²⁺ is one or a combination of Cu ²⁺ , Co ²⁺ , Ni ²⁺ , Mn ²⁺ , and M ²⁺ The molar ratio of ⁺ : Mg ²⁺ : Al ³⁺ is 2:1:1; the specific surface area of the catalyst is 80-110 m ² /g, the pore volume is 0.3-0.4 cc/g, and the pore size distribution is mesopore distribution. The catalyst supports hydrotalcite (LDHs) nanosheets on the surface of graphene by co-precipitation method, and M ²⁺ ₂ Mg ²⁺ Al ³⁺ (O) is obtained after calcination. The advantage is that the catalyst has high catalytic activity and good stability for the storage of _NOx , which is much better than the non-graphene-promoted hydrotalcite-based catalyst, and it also has enhanced NO direct decomposition ability.

Description

A kind of graphene-promoted hydrotalcite-based denitration catalyst and preparation method thereof

technical field

The invention belongs to the technical field of environmental protection catalysts, and in particular provides a graphene-promoted hydrotalcite-based denitration catalyst and a preparation method thereof.

technical background

Since the beginning of the 21st century, with the continuous development of the national economy and the transportation industry, it has directly driven the motor vehicle industry forward, resulting in a rapid increase in the production and ownership of motor vehicles, which has brought great benefits to our lives. While convenient, it has also caused serious pollution to the environment on which we live. Among them, the emission of _NOx (mainly NO) from motor vehicle exhaust has become one of the main sources of air pollution, and the resulting photochemical smog and acid rain pose a serious threat to the urban environment and public health. Considering the efficient use of fossil fuels, lean-burn engines will be the development trend of future motor vehicles. Therefore, the removal of NO _x under lean-burn conditions has become a research hotspot and difficulty in the field of environmental protection. Among many denitrification technologies, NO _x Storage-Reduction (NSR) technology is the most promising development technology for mobile source NO _x removal. Storage performance is a very important characteristic of NSR catalysts, which directly affects the complete removal of NO _x under appropriate conditions, so its nitrogen oxide storage capacity (NO _x Storage Capacity, NSC) is an indicator of the activity of the catalyst. important aspect.

Hydrotalcites (LDHs) are layered compounds formed by the orderly assembly of positively charged laminates and interlayer anions. Its chemical composition can be expressed by the general formula: [M ²⁺ _1-x M ³⁺ _x (OH) ₂ ] ^x+ (A ^n- ) _x/n mH ₂ O, where M ²⁺ and M ³⁺ are divalent and trivalent metal cations on the main laminate, A ^n- is an interlayer anion, x (= M ³⁺ /(M ²⁺ +M ³⁺ )) is a molar ratio. The composite metal oxides obtained by roasting LDHs as precursors have both uniform distribution of metal elements and intrinsic alkalinity, and have direct decomposition, storage and reduction functions in the denitrification reaction, so they have received extensive attention in the research of NSR technology. However, the particle size of LDHs prepared by traditional co-precipitation method is usually large, and it is easy to sinter during high-temperature calcination. Therefore, the composite metal oxides obtained by calcination of LDHs precursors usually have the disadvantages of small specific surface area and poor particle dispersion, thus reducing the catalytic activity of the catalyst. At the same time, some studies have shown that the higher the calcination temperature, the more obvious this shortcoming will be. For example, in 2009, Li et al. reported in Applied Catalysis B: Environmental, Vol. 91, pages 406-415, that the LDHs-based Co _2.5 Mg _0.5 AlO composite metal oxide catalyst prepared by the co-precipitation method increased the nitrogen oxide storage of the catalyst with the increase of the calcination temperature. The amount decreased rapidly, which was mainly due to the reduction of the specific surface area and the reduction of the number of surface active centers caused by catalyst sintering.

At present, the study of improving the dispersion of nano-catalyst particles and improving the catalytic activity by selecting some suitable supports has aroused widespread interest. For example, in recent years, there have been many reports on novel carbon supports such as carbon fibers (CF), activated carbon (AC), and carbon nanotubes (CNTs) to support simple metals/metal oxides. In 2011, Li et al. reported carbon nanotube (CNT) supported V ₂ O ₅ /TiO ₂ catalysts on pages 915-921 of Volume 192 of Journal of Hazardous Materials. The promotion effect of carbon nanotubes is reflected in the increase of the specific surface area of the catalyst. and pore volume, thus providing more active sites for catalytic reactions. In 2012, the graphene (Graphene) supported Au, Pt, Pd nanoparticle catalyst reported by Xu et al. in The Journal of Physical Chemistry C, Volume 112, 19841-19845, showed great application potential in the field of catalysis, among which it is very important The reason is the dispersion effect of the graphene carrier on the active component. The above examples show that choosing a suitable carbon support can increase the dispersion of metal element or metal oxide, thereby improving the catalytic activity.

Contents of the invention

The object of the present invention is to provide a graphene-promoted hydrotalcite-based denitration catalyst and a preparation method thereof. The preparation process is simple and the conditions are mild, and no organic reagent is required, and the hydrotalcite nanosheets in the obtained graphene-loaded hydrotalcite composite material are uniform in size and highly dispersed. The obtained graphene-promoted hydrotalcite-based catalysts after calcination showed excellent NO _x storage capacity and decomposition performance in the denitration reaction, which was attributed to the high dispersion promotion effect of graphene on the active components.

In the initial preparation, graphene oxide first adsorbs metal cations in the salt solution by electrostatic force, and inhibits the growth and agglomeration of hydrotalcite nuclei during the nucleation process, thereby obtaining a highly dispersed and uniformly sized hydrotalcite-based catalyst, thereby overcoming the existing Hydrotalcite-based catalysts generally have a shortcoming of low storage capacity of nitrogen oxides in the technology. The catalyst exhibits high storage capacity and good direct decomposition performance for _NOx removal under lean-burn conditions, which is much better than graphene-free promoted hydrotalcite-based catalysts, and the preparation process is simple and the conditions are mild. The catalyst was recycled 4 times, and the storage capacity and decomposition rate remained basically unchanged, indicating that the catalyst had high stability.

The chemical formula of the graphene-promoted hydrotalcite-based denitration catalyst of the present invention is M ²⁺ ₂ Mg ²⁺ Al ³⁺ (O); wherein, M ²⁺ is Cu ²⁺ , Co ²⁺ , Ni ²⁺ , Mn ²⁺ One or several combinations of them, the molar ratio of M ²⁺ : Mg ²⁺ : Al ³⁺ is 2:1:1; the specific surface area of the catalyst is 80-110 m ² /g, and the pore volume is 0.3-0.4 cc/ g, The pore size distribution is mesopore distribution.

The catalyst loads hydrotalcite (LDHs) nanosheets on the surface of graphene (Graphene) by co-precipitation method, and obtains M ²⁺ ₂ Mg ²⁺ Al ³⁺ (O) after calcination.

The preparation method of the graphene-promoted hydrotalcite-based catalyst of the present invention is to firstly mix a certain proportion of M ²⁺ (one or more combinations of Cu ²⁺ , Co ²⁺ , Ni ²⁺ , Mn ²⁺ ), Mg ^{2 +} , Al ³⁺ salt solution is mixed with graphene oxide colloidal solution, metal cations are electrostatically adsorbed on the surface of graphene oxide, and then small-sized, highly dispersed hydrotalcite nanosheets are generated in situ in an alkaline environment, and obtained after roasting corresponding catalyst. Specifically include the following steps:

(1) Pre-oxidation of graphite: Add concentrated H ₂ SO ₄ , K ₂ S ₂ O ₄ and P ₂ O ₅ into the flask in sequence, add graphite powder under stirring, and finally react in a 60-80 ^o C water bath for 6 ~8 hours, washed to neutral, dried, and set aside; wherein the mass ratio of graphite:concentrated H ₂ SO ₄ :K ₂ S ₂ O ₄ :P ₂ O ₅ is 1:4~8:0.5:0.5.

(2) Preparation of graphene oxide: measure concentrated H ₂ SO ₄ in a four-neck flask, add pre-oxidized graphite powder and NaNO ₃ to the concentrated H ₂ SO ₄ in sequence under stirring at 0-10 ^o C, and Slowly add KMnO ₄ , stir and react for 60-90 minutes; then transfer the flask to a constant temperature water bath at 35 ^o C, and continue stirring for 30-60 minutes; finally add deionized water while stirring, and control the reaction temperature at 95-98 ^o C, continue to stir for 30-60 minutes; treat the reaction solution with 30-50ml of 3% H ₂ O ₂ until golden yellow, then filter while hot, and then fully wash with 5% HCl and deionized water until there is no SO4 in the filtrate ^2- ; wherein the mass ratio of graphite: concentrated H ₂ SO ₄ : NaNO ₃ : KMnO ₄ is 1:42:0.5:3, and the volume ratio of concentrated H ₂ SO ₄ : H ₂ O is 1:2.

(3) Preparation of M ²⁺ ₂ Mg ²⁺ Al ³⁺ (O) catalyst: Transfer 1 g of graphite oxide to a 1L four-neck flask, dilute it with deionized water to 500 ml, and stir at 100W ～1000W frequency ultrasound for 0.5～2 hours to obtain a uniformly dispersed graphene oxide colloidal solution with a mass concentration of 2mg/ml; weigh 0.007～0.021 mol M ²⁺ (NO ₃ ) ₂ xH ₂ O, M ²⁺ is One or more combinations of Cu ²⁺ , Co ²⁺ , Ni ²⁺ , Mn ²⁺ , 0.0035～0.0105 mol Mg (NO ₃ ) ₂ ·6H ₂ O, 0.0035～0.0105 mol Al(NO ₃ ) ₃ · 9H ₂ O was dissolved in 100 ml deionized water and poured into 500 ml graphene oxide colloidal solution, stirred for 30-60 minutes and assisted by ultrasound; then the molar ratio [Na ₂ CO ₃ ]/[Al(NO ₃ ) ₃ · 9H ₂ O]=2, [NaOH]/[Na ₂ CO ₃ ]=3.2 The mixed lye is dropped into the flask and kept for 30-50 minutes, the pH of the solution is stable at 10±0.5 when the lye is added; ^o C was crystallized for 4 hours, centrifuged, washed, and dried at 60-80 ^o C, and the product was calcined in a muffle furnace in an air atmosphere at 500 ^o C for 4 hours to obtain a catalyst expressed as M ²⁺ ₂ Mg ²⁺ Al ³⁺ (O).

Compared with the prior art, the present invention has the following advantages and characteristics:

(1) The hydrotalcite/graphene (LDH/Graphene) composite material was prepared by a simple synthetic route. The obtained carrier has high stability, easy dispersion of hydrotalcite nanosheets and uniform size.

(2) The spinel particles of the hydrotalcite-based denitration catalyst prepared by the present invention are highly dispersed and small in particle size, and they still have a high NO _x storage capacity (NSC) after repeated recycling, and have good commercial application prospects , provide a high-storage hydrotalcite-based denitration catalyst and a preparation method thereof.

Description of drawings

Fig. 1 is the SEM spectrogram of embodiment 1 precursor.

Fig. 2 is the TEM spectrogram of embodiment 1 precursor.

Fig. 3 is the SEM spectrogram of the catalyst Co ²⁺ ₂ Mg ²⁺ Al ³⁺ (O) obtained after the precursor of Example 1 was roasted.

Detailed ways

The present invention will be further described in detail below in conjunction with the examples, but the embodiments of the present invention are not limited thereto.

Example 1:

(1) Pre-oxidation of graphite: add 24 ml ^of concentrated H ₂ SO ₄ , 5 g of K ₂ S ₂ O ₄ and 5 g of P ₂ O ₅ into the flask in turn, add 10 g of graphite powder under stirring, and finally React in a water bath for 6 hours, wash until neutral, dry, and set aside.

(2) Preparation of graphene oxide: Measure 115 ml of concentrated H ₂ SO ₄ in a 500 ml flask, add 5 g of pre-oxidized graphite powder and 2.5 g of NaNO ₃ to the concentrated H ₂ SO 4 under stirring in an ice-water bath ₄ , and slowly added 15 g KMnO ₄ , stirred for 90 minutes; then transferred the flask to a constant temperature water bath at 35±3 ^o C, and continued to stir for 30 minutes; finally, added 230 ml of deionized water to control the reaction When the temperature is lower than 98 ^o C, continue to stir for 30 minutes; treat the reaction solution with 30ml of 3% H ₂ O ₂ until golden yellow, then filter while hot, and wash thoroughly with 5% HCl and deionized water until there is no SO4 ^2- in the filtrate .

(3) Preparation of Co ²⁺ ₂ Mg ²⁺ Al ³⁺ (O) catalyst: transfer 1 g of graphite oxide to a 1L four-neck flask, dilute it with deionized water to 500 ml, and ultrasonicate for 1 hour under stirring Obtain a uniformly dispersed graphene oxide colloidal solution with a mass concentration of 2 mg/ml; weigh 0.007 molCo(NO ₃ ) ₂ 6H ₂ O, 0.0035 mol Mg (NO ₃ ) ₂ 6H ₂ O, 0.0035 molAl(NO ₃ ) ₃ ·9H ₂ O was dissolved in 100 ml deionized water and poured into the graphene oxide colloidal solution, stirred for 30 minutes and assisted by ultrasound; finally the molar ratio [Na ₂ CO ₃ ]/[Al(NO ₃ ) ₃ · 9H ₂ O]=2, [NaOH]/[Na ₂ CO ₃ ]=3.2 The mixed lye is dropped into the flask, and the time for adding the lye is about 40 minutes. When the lye is added, the pH of the solution is stable at 10; Crystallize at 65 ^o C for 4 hours, centrifuge, wash, and dry at 60 ^o C, and roast the product in a muffle furnace in an air atmosphere at 500 ^o C for 4 hours to obtain the catalyst Co ²⁺ ₂ Mg ²⁺ Al ³⁺ (O) .

Example 2:

This embodiment is the same as embodiment 1 except the following features: 1 g of graphite oxide is transferred to a 1L four-necked flask, diluted with deionized water to 500 ml, and ultrasonically stirred for 1 hour to obtain uniformly dispersed graphene oxide Colloidal solution, the mass concentration is 2mg/ml; weigh 0.014 mol Co(NO ₃ ) ₂ 6H ₂ O, 0.007 mol Mg (NO ₃ ) ₂ 6H ₂ O, 0.007 mol Al(NO ₃ ) ₃ 9H ₂ O Dissolved in 100ml deionized water and poured into the graphene oxide colloidal solution, stirred for 30 minutes and supplemented with ultrasound; finally the molar ratio [Na ₂ CO ₃ ]/[Al(NO ₃ ) ₃ 9H ₂ O]=2, [NaOH]/[Na ₂ CO ₃ ]=3.2 The mixed lye is dropped into the flask, and the dropping time is kept for about 40 minutes. When the lye is added, the pH of the solution is stable at 10; finally, it is crystallized at 65 ^o C for 4 hours , centrifuged, washed, and dried at 60 ^o C. The product was calcined in a muffle furnace at 500 ^o C in an air atmosphere for 4 hours to obtain the catalyst Co ²⁺ ₂ Mg ²⁺ Al ³⁺ (O).

Example 3:

This embodiment is the same as embodiment 1 except the following features: 1 g of graphite oxide is transferred to a 1L four-necked flask, diluted with deionized water to 500 ml, and ultrasonically stirred for 1 hour to obtain uniformly dispersed graphene oxide Colloidal solution, the mass concentration of which is 2 mg/ml; weigh 0.021 mol Co(NO ₃ ) ₂ ·6H ₂ O, 0.0105 mol Mg (NO ₃ ) ₂ ·6H ₂ O, 0.0105 mol Al(NO ₃ ) ₃ ·9H ₂ O Dissolved in 100ml deionized water and poured into 500 ml graphene oxide solution, stirred for 30 minutes and assisted by ultrasound; finally the molar ratio [Na ₂ CO ₃ ]/[Al(NO ₃ ) ₃ 9H ₂ O]=2 , [NaOH]/[Na ₂ CO ₃ ]=3.2 The mixed lye was dropped into the flask, and the dropping time was kept for about 40 minutes. When the lye was added, the pH of the solution was stable at 10; finally, 65 ^o C crystallized 4 hours, centrifuged, washed, and dried at 60 ^o C, and the product was calcined in a muffle furnace at 500 ^o C in an air atmosphere for 4 hours to obtain the catalyst Co ²⁺ ₂ Mg ²⁺ Al ³⁺ (O).

Example 4:

This embodiment is the same as embodiment 1 except the following features: 1 g of graphite oxide is transferred to a 1L four-necked flask, diluted with deionized water to 500 ml, and ultrasonically stirred for 1 hour to obtain uniformly dispersed graphene oxide Colloidal solution, the mass concentration is 2 mg/ml; weigh 0.007 mol Cu(NO ₃ ) ₂ ·3H ₂ O, 0.0035 mol Mg (NO ₃ ) ₂ ·6H ₂ O, 0.0035 mol Al(NO ₃ ) ₃ ·9H ₂ O Dissolved in 100 ml deionized water and poured into the graphene oxide colloidal solution, stirred for 30 minutes and assisted by ultrasound; finally the molar ratio [Na ₂ CO ₃ ]/[Al(NO ₃ ) ₃ 9H ₂ O]=2 , [NaOH]/[Na ₂ CO ₃ ]=3.2 The mixed lye was dropped into the flask, and the dropping time was kept for about 40 minutes. When the lye was added, the pH of the solution was stable at 10; finally, 65 ^o C crystallized 4 hours, centrifuged, washed, and dried at 60 ^o C, and the product was calcined in a muffle furnace in an air atmosphere at 500 ^o C for 4 hours to obtain the catalyst Cu ²⁺ ₂ Mg ²⁺ Al ³⁺ (O).

Example 5:

This embodiment is the same as embodiment 1 except the following features: 1 g of graphite oxide is transferred to a 1L four-necked flask, diluted with deionized water to 500 ml, and ultrasonically stirred for 1 hour to obtain uniformly dispersed graphene oxide Colloidal solution, the mass concentration is 2mg/ml; Weigh 0.007 mol Ni(NO ₃ ) ₂ ·6H ₂ O, 0.0035 mol Mg (NO ₃ ) ₂ ·6H ₂ O, 0.0035 mol Al(NO ₃ ) ₃ ·9H ₂ O Dissolved in 100 ml deionized water and poured into the graphene oxide colloidal solution, stirred for 30 minutes and assisted by ultrasound; finally the molar ratio [Na ₂ CO ₃ ]/[Al(NO ₃ ) ₃ 9H ₂ O]=2 , [NaOH]/[Na ₂ CO ₃ ]=3.2 The mixed lye was dropped into the flask, and the dropping time was kept for about 40 minutes. When the lye was added, the pH of the solution was stable at 10; finally, 65 ^o C crystallized 4 hours, centrifuged, washed, and dried at 60 ^o C, and the product was roasted in a muffle furnace at 500 ^o C in an air atmosphere for 4 hours to obtain the catalyst Ni ²⁺ ₂ Mg ²⁺ Al ³⁺ (O).

Embodiment 6:

This embodiment is the same as embodiment 1 except the following features: 1 g of graphite oxide is transferred to a 1L four-necked flask, diluted with deionized water to 500 ml, and ultrasonically stirred for 1 hour to obtain uniformly dispersed graphene oxide Colloidal solution, the mass concentration is 2 mg/ml; weigh 0.007 molMn(NO ₃ ) ₂ , 0.0035 mol Mg (NO ₃ ) ₂ ·6H ₂ O, 0.0035 mol Al(NO ₃ ) ₃ ·9H ₂ O and dissolve in 100 ml Deionized water and poured into the graphene oxide colloidal solution, stirred for 30 minutes and assisted by ultrasound; finally the molar ratio [Na ₂ CO ₃ ]/[Al(NO ₃ ) ₃ 9H ₂ O]=2, [NaOH] /[Na ₂ CO ₃ ]=3.2 The mixed lye was dropped into the flask and kept for about 40 minutes. The pH of the solution was stable at 10 at the end of the lye addition; finally, it was crystallized at 65 ^o C for 4 hours, centrifuged, washed, and 60 ^o C drying, the product was calcined in a muffle furnace at 500 ^o C in an air atmosphere for 4 hours to obtain the catalyst Mn ²⁺ ₂ Mg ²⁺ Al ³⁺ (O).

Claims (2)

1. A graphene-promoted hydrotalcite-based denitration catalyst, characterized in that the chemical formula of the catalyst is M ²⁺ ₂ Mg ²⁺ Al ³⁺ (O); wherein, M ²⁺ is Cu ²⁺ , Co ²⁺ , Ni ²⁺ , Mn ²⁺ or a combination of several, the molar ratio of M ²⁺ : Mg ²⁺ : Al ³⁺ is 2:1:1; the specific surface area of the catalyst is 80-110m ² /g, and the pores The volume is 0.3-0.4 cc/g, and the pore size distribution is mesopore distribution; The catalyst is obtained by loading hydrotalcite nanosheets on the surface of graphene through a co-precipitation method and roasting. 2. a kind of preparation method of graphene-promoted hydrotalcite-based denitration catalyst according to claim 1, is characterized in that, comprises the following steps: (1) Pre-oxidation of graphite: Add concentrated H ₂ SO ₄ , K ₂ S ₂ O ₄ and P ₂ O ₅ into the flask in sequence, add graphite powder under stirring, and finally react in a 60-80 ^o C water bath for 6 ~8 hours, washed to neutral, dried, and set aside; the mass ratio of graphite:concentrated H ₂ SO ₄ :K ₂ S ₂ O ₄ :P ₂ O ₅ is 1:4~8:0.5:0.5; (2) Preparation of graphene oxide: measure concentrated H ₂ SO ₄ in a four-neck flask, add pre-oxidized graphite powder and NaNO ₃ to the concentrated H ₂ SO ₄ in sequence under stirring at 0-10 ^o C, and Add KMnO ₄ , stir and react for 60-90 minutes; then transfer the flask to a constant temperature water bath at 35 ^o C, and continue stirring for 30-60 minutes; finally add deionized water during stirring, and control the reaction temperature between 95-98 ^o C Continue stirring for 30-60 minutes; treat the reaction liquid with 30-50 ml of 3% H ₂ O ₂ until golden yellow, then filter it while it is hot, and wash thoroughly with 5% HCl and deionized water until there is no SO4 ² in the filtrate ^- ; wherein the mass ratio of graphite: concentrated H ₂ SO ₄ : NaNO ₃ : KMnO ₄ is 1:42:0.5:3, and the volume ratio of concentrated H ₂ SO ₄ : H ₂ O is 1:2. (3) Preparation of M ²⁺ ₂ Mg ²⁺ Al ³⁺ (O) catalyst: Transfer 1 g of graphite oxide to a 1L four-neck flask, dilute it with deionized water to 500 ml, and stir at 100W ～1000W frequency ultrasound for 0.5～2 hours to obtain a uniformly dispersed graphene oxide colloidal solution with a mass concentration of 2mg/ml; weigh 0.007～0.021 mol M ²⁺ (NO ₃ ) ₂ xH ₂ O, M ²⁺ is One or more combinations of Cu ²⁺ , Co ²⁺ , Ni ²⁺ , Mn ²⁺ , 0.0035～0.0105 molMg (NO ₃ ) ₂ ·6H ₂ O, 0.0035～0.0105 mol Al(NO ₃ ) ₃ ·9H ₂ O was dissolved in 100 ml deionized water and poured into 500 ml graphene oxide colloidal solution, stirred for 30-60 minutes and assisted by ultrasound; then the molar ratio [Na ₂ CO ₃ ]/[Al(NO ₃ ) ₃ ·9H ₂ O]=2, [NaOH]/[Na ₂ CO ₃ ]=3.2 The mixed lye is dropped into the flask and kept for 30-50 minutes, the pH of the solution is stable at 10±0.5 when the lye is added ^; C was crystallized for 4 hours, centrifuged, washed, and dried at 60-80 ^o C; the product was calcined in a muffle furnace in an air atmosphere at 500 ^o C for 4 hours to obtain a catalyst expressed as M ²⁺ ₂ Mg ²⁺ Al ³⁺ ( O).

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