Preparation method of graphene grid supported iron-cobalt bimetallic catalyst with double active sites

Abstract

The preparation method of the double active site graphene fence loaded iron cobalt bimetallic catalyst aims to improve the catalytic activity of the catalyst for carbon dioxide, the stability of the catalyst and the selectivity of the target product. The graphene oxide is prepared by improving the hummers method, and then the sheet layer dry graphene oxide is obtained by adopting the freeze drying method. The sheet layer of the graphene oxide is folded and curved when being hydrothermally treated at high temperature, so as to form the graphene fence, and the inside and outside double active sites are formed through the action of the graphene fence. Therefore, the double metal catalyst with graphene as the carrier and iron cobalt as the active site is prepared by adopting the high-temperature hydrothermal and equal-amount impregnation method. The hydrothermal metal and the impregnation metal are separated through the action of the graphene fence, so as to form the inside and outside iron cobalt double metal active sites. The catalyst prepared by the application has the stable structure, the catalytic active center is easily formed in the reaction, the conversion rate of carbon dioxide is improved, and the selectivity of the target product is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of catalysts and relates to a carbon dioxide hydrogenation catalyst, in particular to a graphene-supported iron-cobalt catalyst. Background Art

[0002] While the combustion of fossil fuels such as coal, oil, and natural gas has driven socioeconomic development, the massive emission of carbon dioxide has also led to a series of environmental problems, including rising sea levels, global warming, and acid rain. Global warming and the massive consumption of traditional energy sources are two major issues that severely constrain sustainable economic, social, and environmental development.

[0003] Graphene, a novel carbon material that has been extensively studied in recent years, consists of a tightly packed monolayer of carbon atoms forming a two-dimensional honeycomb lattice. Graphene has attracted increasing attention due to its large surface area and unique two-dimensional (2D) structure. Graphene also offers advantages such as large surface area, excellent thermal stability, and good dispersibility. These excellent properties have made graphene a promising support material for iron-cobalt FTS catalysts.

[0004] Iron and cobalt are both commonly used metal catalysts in carbon dioxide hydrogenation reactions. When iron and cobalt are used together, the catalyst's performance is more than simply the sum of the performance of either iron or cobalt alone. Their synergistic effect can effectively improve carbon dioxide conversion and target product selectivity. Using graphene as a carrier can enhance the dispersion of the active components and extend the catalyst's lifespan.

[0005] Based on the above ideas, a dual-active site graphene fence-loaded iron-cobalt bimetallic catalyst was invented, which showed high catalytic activity and good catalytic stability. Summary of the Invention

[0006] The present invention discloses a method for preparing a dual-active-site graphene fence-supported iron-cobalt bimetallic catalyst. The method uses graphene as a carrier and utilizes the fence formed by graphene oxide (GO) during a high-temperature hydrothermal process. Metal elements added during the hydrothermal process are segregated within the carrier by the graphene fence, while impregnated metals are isolated externally by the graphene fence. This results in the formation of internal and external bimetallic active sites in the catalyst, improving carbon dioxide conversion and target product selectivity.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] The invention discloses a preparation method of a double-active-site graphene fence-loaded iron-cobalt bimetallic catalyst, wherein the graphene is loaded with 15-30% iron, 2-10% cobalt, and 0.1-1.5% potassium through a high-temperature hydrothermal method and an equal amount impregnation method.

[0009] The preparation method of the catalyst of the present invention comprises the following steps:

[0010] (1) preparing a graphene oxide solution by the Hummers method, and freeze-drying to obtain dry graphene oxide;

[0011] (2) adding freeze-dried graphene oxide, urea, and a cobalt source to a mixed solution of ethylene glycol and deionized water to prepare a graphene oxide solution, stirring and ultrasonically treating the solution, and finally rotating a pot for hydrothermal reaction;

[0012] (3) The product obtained in step (2) is washed with deionized water until neutral and then freeze-dried, and then the dried product is calcined in a nitrogen atmosphere. The calcined product is immersed in a solution of a potassium source and an iron source and then freeze-dried. Finally, the dried product is calcined in a nitrogen atmosphere to obtain the target catalyst.

[0013] The freeze-drying time in step (1) is 24-72 hours.

[0014] In step (2), the hydrothermal reaction temperature is controlled between 150-200° C., and the hydrothermal reaction time is 10-20 h.

[0015] In step (2), the cobalt source is selected from cobalt acetylacetonate, cobalt nitrate and cobalt chloride.

[0016] In step (3), the potassium source is selected from anhydrous potassium carbonate and potassium nitrate, and the iron source is selected from one of ferric nitrate nonahydrate, ferric chloride and ferric sulfate.

[0017] In step (3), the two calcination temperatures are both controlled at 300-600° C., and the calcination time is both controlled at 1-5 h.

[0018] or

[0019] The preparation method may also be as follows: no cobalt source is added in step (2), and in step (3), the product of the first roasting is immersed in a solution of a potassium source, an iron source and a cobalt source.

[0020] The preparation method may also be as follows: in step (2), an iron source is added at the same time as the potassium source, and in step (3), the product of the first roasting is immersed in a potassium source solution.

[0021] The preparation method may also be as follows: in step (2), instead of adding a cobalt source, an iron source is added; and in step (3), the product of the first roasting is immersed in a potassium source and a cobalt source solution.

[0022] The concentration of the graphene oxide solution in step (2) is 3-12 mg / ml.

[0023] After the second roasting in step (3), the content ratio of each main component element satisfies the following relationship: iron content 15-30%, cobalt content 2-10%, potassium content 0.1-1.5%.

[0024] Evaluation conditions of the catalyst of the present invention

[0025] The reaction was carried out in a micro fixed bed reactor with a gas flow rate of 30 mL min -1 The hydrogen was reduced at 400℃ for 12h. Then the temperature was lowered and the reaction was switched to the raw gas. The reaction conditions were: 20℃, 3.0MPa, TOS=8h, W / F=4.5gh mol -1 , H2 / CO2 / Ar=68:27:5.

[0026] Beneficial effects of the present invention:

[0027] The present invention aims to improve the catalytic activity of a catalyst for carbon dioxide, the stability of the catalyst, and the selectivity of the target product. The present invention prepares graphene oxide by improving the Hummers method, and then obtains flaky dried graphene oxide by freeze-drying. During high-temperature hydrothermal treatment, graphene oxide undergoes folding and bending of the lamellae, thereby forming graphene fences, which form inside and outside dual active sites through the action of the graphene fences. The present invention uses high-temperature hydrothermal and equal-amount impregnation methods to prepare a bimetallic catalyst with graphene as a carrier and iron and cobalt as active sites. The action of the graphene fence separates the hydrothermal metal and the impregnated metal, thereby forming inside and outside iron and cobalt bimetallic active sites. The catalyst prepared by the present invention has a stable structure, easily forms catalytic active centers during the reaction, and improves the conversion rate of carbon dioxide and the selectivity of the target product. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 XRD spectra of the catalysts prepared in Examples 1-4; wherein, Example 1: GO-Co / K-Fe, Example 2: GO / K-Fe-Co, Example 3: GO-Fe-Co / K, Example 4: GO-Fe / K-Co;

[0029] Figure 2 SEM and mapping images of the four catalysts;

[0030] Figure 3 Transmission electron microscope images of Examples 1-4; a, b, c, and d are TEM images of GO-Co / K-Fe, GO / K-Fe-Co, GO-Fe-Co / K, and GO-Fe / K-Co, respectively. DETAILED DESCRIPTION

[0031] The present invention will be further described below through specific examples.

[0032] Example 1

[0033] Preparation method of dual-active site graphene fence supported iron-cobalt bimetallic catalyst, specific steps:

[0034] (1) Preparation of graphene oxide: In an 80°C water bath, 7.5g of potassium persulfate and 7.5g of phosphorus pentoxide were stirred with 40ml of concentrated H2SO4 for 15min, and then 10g of graphite powder was added. After reacting for 4.5h, the mixture was filtered and rinsed until the pH value of the supernatant reached 7. It was then air-dried for 12h. The dried pre-oxidized graphite was transferred to a reaction flask. Concentrated H2SO4 was added to the reaction flask in an ice-water bath. Under the above conditions, 50g of potassium permanganate was added in stages. The mixture was stirred at 35°C for 3h, then mixed with deionized water and 30% hydrogen peroxide, and aged for 12h. The bottom slurry of the solution was transferred to 3% hydrochloric acid for acidification. After centrifugation and washing until neutral, the graphene oxide was transferred to deionized water and ultrasonically stirred for 5h. Finally, it was freeze-dried for 24h to obtain dry graphene oxide.

[0035] (2) 2 g of dry graphene oxide, 2 g of urea, and 0.3 g of cobalt acetylacetonate were dissolved in a mixed solution of 40 ml of ethylene glycol and 293 ml of deionized water, followed by stirring and ultrasonication for 2 h. The resulting liquid was transferred to a stainless steel autoclave lined with polytetrafluoroethylene and subjected to rotary one-pot hydrothermal synthesis at 200 °C for 10 h.

[0036] (3) The hydrothermal product was washed and filtered until neutral, freeze-dried, and then calcined at 500°C in a nitrogen atmosphere for 2 h. 2.1 g of ferric nitrate nonahydrate and 0.1 g of potassium carbonate were dissolved in deionized water, and then the calcined catalyst was dissolved in the mixed solution by impregnation. After freeze-drying again, the resulting catalyst was dried and calcined at 500°C in a nitrogen atmosphere for 2 h.

[0037] Example 2

[0038] (1) Same as Example 1

[0039] (2) 3 g of dry graphene oxide and 2 g of urea were dissolved in a mixed solution of 40 ml of ethylene glycol and 293 ml of deionized water, followed by stirring and ultrasonication for 2 h. The resulting liquid was transferred to a stainless steel autoclave lined with polytetrafluoroethylene and subjected to rotary one-pot hydrothermal synthesis at 180 °C for 12 h.

[0040] (3) The hydrothermal product was washed and filtered until neutral, freeze-dried, and then calcined at 400°C in a nitrogen atmosphere for 4 h. 0.2 g of cobalt chloride, 1.2 g of ferric sulfate, and 0.1 g of potassium carbonate were dissolved in deionized water, and then the calcined catalyst was dissolved in the mixed solution by impregnation. After freeze-drying again, the resulting catalyst was dried and calcined at 500°C in a nitrogen atmosphere for 4 h.

[0041] Example 3

[0042] (1) Same as Example 1

[0043] (2) 3 g of dry graphene oxide, 2 g of urea, 0.4 g of cobalt acetylacetonate, and 2.5 g of ferric nitrate nonahydrate were dissolved in 40 ml of ethylene glycol and 293 ml of deionized water, followed by stirring and ultrasonication for 2 h. The resulting liquid was transferred to a stainless steel autoclave lined with polytetrafluoroethylene and subjected to a rotary one-pot hydrothermal synthesis at 180 °C for 12 h.

[0044] (3) The hydrothermal product was washed and filtered until neutral, freeze-dried, and then calcined at 550°C in a nitrogen atmosphere for 2 h. 0.2 g of potassium carbonate was dissolved in deionized water, and then the calcined catalyst was dissolved in the mixed solution by impregnation. After freeze-drying again, the resulting catalyst was dried and calcined at 550°C in a nitrogen atmosphere for 2 h.

[0045] Example 4

[0046] (1) Same as Example 1

[0047] (2) 3 g of dry graphene oxide, 2 g of urea, and 1.8 g of ferric chloride were dissolved in 40 ml of ethylene glycol and 293 ml of deionized water, followed by stirring and ultrasonication for 2 h. The resulting liquid was transferred to a stainless steel autoclave lined with polytetrafluoroethylene and subjected to rotary one-pot hydrothermal synthesis at 200 °C for 10 h.

[0048] (3) The hydrothermal product was washed and filtered until neutral, freeze-dried, and then calcined at 500°C in a nitrogen atmosphere for 2 h. 0.3 g of cobalt nitrate and 0.1 g of potassium nitrate were dissolved in deionized water, and then the calcined catalyst was dissolved in the mixed solution by impregnation. After freeze-drying again, the resulting catalyst was dried and calcined at 400°C in a nitrogen atmosphere for 3 h.

[0049] Table 1 SEM mapping element content data of catalysts in Examples 1-4

[0050]

[0051] SEM and mapping images of the four catalysts are shown in Figure 2. Figure 2As shown in Figure 1, the SEM image shows the graphene fences formed by high-temperature hydrothermal treatment of GO, while the mapping image reveals that the loaded K, Co, and Fe are all supported on the graphene. Under rotating-pot hydrothermal conditions, the hydrothermal metal elements are encapsulated within the graphene fences as they form. Therefore, after hydrothermal treatment, the hydrothermal metals are primarily encapsulated within the graphene fences, while metals loaded by the impregnation method are more likely to be loaded on the surface of the reduced graphene. The difference in surface metal content further demonstrates the distinct distribution between the inner and outer layers. The Fe / Co ratios in Table 1 show that when Fe is loaded by the impregnation method, it is loaded on the graphene surface, making its presence easily detected by EDS elemental analysis, resulting in a relatively large Fe / Co ratio. When Fe is loaded by the hydrothermal method, the Fe is loaded within the fences formed during the hydrothermal reduction of graphene oxide, making it difficult to detect, resulting in a relatively small Fe / Co ratio. This indicates that the formed graphene fences effectively "bind" the hydrothermal metal elements, resulting in a distinct elemental distribution inside and outside the graphene.

[0052] Table 2 Evaluation results of carbon dioxide hydrogenation of catalysts of Examples 1-4

[0053]

[0054] Note: C2-C4 P Selectivity for C2-C4 alkanes; C2-C4 ＝ The selectivity of C2-C4 olefins; C2-C4 iso Selectivity for C2-C4 isomerization; C5 + It is selective for hydrocarbons above C5.

[0055] The carbon dioxide hydrogenation reaction evaluation results recorded in Table 2 show that the catalysts of the four examples all had carbon dioxide conversion rates exceeding 30%, and carbon monoxide selectivity below 15%. Due to the effect of the graphene fence, Examples 1 and 2 have weaker hydrogen adsorption capacities, making them more susceptible to reaction with carbon dioxide during the reaction, resulting in an increase in the C / H ratio and improved selectivity for light olefins in the product. In Example 3, iron and cobalt were added during the hydrothermal process. The interaction between the iron-cobalt bimetallic and the graphene support was enhanced, which also enhanced the catalyst's hydrogen adsorption capacity, making the generated light olefins more susceptible to secondary hydrogenation reactions to form alkanes. Due to the effect of the graphene fence, Example 4 forms a unique reaction path. At the internal active site of Fe, CO2 is first converted into CO through the RWGS reaction, and then generates light olefins through the Fischer-Tropsch process. The diffusion effect causes some light olefins to be transferred to the iron-cobalt active sites on the surface graphene fence. Since the addition of cobalt makes the external iron-cobalt sites have a strong adsorption capacity for hydrogen, the light olefins are hydrogenated into alkanes. Therefore, the selectivity of the catalyst product of Example 4 for LPG (liquefied petroleum gas) reaches 43.6%.

[0056] Figure 1 The following are XRD spectra of four catalysts: Example 1: GO-Co / K-Fe, Example 2: GO / K-Fe-Co, Example 3: GO-Fe-Co / K, and Example 4: GO-Fe / K-Co. Comparison with PDF#33-0664 and PDF#25-1402 reveals that Fe in all four catalysts exists primarily as Fe₂O₃. Due to the small Co particle size and good dispersion, no Co-related diffraction peaks were observed in the XRD spectra of the four graphene-supported catalysts. The low K content in the catalysts, around 1%, also resulted in no K-related diffraction peaks being detected.

[0057] Figure 3 Transmission electron microscope images of Examples 1-4; a, b, c, and d are TEM images of GO-Co / K-Fe, GO / K-Fe-Co, GO-Fe-Co / K, and GO-Fe / K-Co, respectively. Although different loading methods are used to load Fe, the Fe element after high-temperature calcination mainly exists in the form of Fe2O3. This result is consistent with the XRD spectrum results, and the 0.25nm lattice spacing is attributed to the (119) crystal plane of Fe2O3.

Claims

1. Preparation method of a dual-active-site graphene fence-supported iron-cobalt bimetallic catalyst, wherein the catalyst is supported by graphene and loaded with iron-cobalt bimetallic, characterized in that The following steps are involved: (1) preparing a graphene oxide solution by the Hummers method, and freeze-drying to obtain dry graphene oxide; (2) adding freeze-dried graphene oxide, urea, and a cobalt source to a mixed solution of ethylene glycol and deionized water to prepare a graphene oxide solution, stirring and ultrasonically treating the solution, and finally rotating a pot for hydrothermal reaction; (3) washing the product obtained in step (2) with deionized water until neutral and then freeze-drying it, then calcining the dried product in a nitrogen atmosphere, immersing the calcined product in a solution of a potassium source and an iron source and then freeze-drying it, and finally calcining the dried product in a nitrogen atmosphere to obtain the dual-active site graphene fence-loaded iron-cobalt bimetallic catalyst.

2. The method for preparing the dual-active site graphene fence supported iron-cobalt bimetallic catalyst according to claim 1, characterized in that The freeze-drying time in step (1) is 24-72 h.

3. The method for preparing the dual-active site graphene fence supported iron-cobalt bimetallic catalyst according to claim 1, characterized in that In step (2), the hydrothermal reaction temperature is controlled between 150-200°C, and the hydrothermal reaction time is 10-20 h.

4. The preparation method of the dual-active site graphene fence supported iron-cobalt bimetallic catalyst according to claim 1, characterized in that In step (2), the cobalt source is selected from cobalt acetylacetonate, cobalt nitrate and cobalt chloride.

5. The preparation method of the dual-active site graphene fence supported iron-cobalt bimetallic catalyst according to claim 1, characterized in that In step (3), the potassium source is selected from anhydrous potassium carbonate and potassium nitrate, and the iron source is selected from ferric nitrate nonahydrate, ferric chloride and ferric sulfate.

6. The method for preparing the dual-active site graphene fence supported iron-cobalt bimetallic catalyst according to claim 1, characterized in that In step (3), the two calcination temperatures are both controlled at 300-600°C, and the calcination time is both controlled at 1-5 h.

7. The method for preparing a dual-active-site graphene fence-supported iron-cobalt bimetallic catalyst according to any one of claims 1 to 6, characterized in that The concentration of the graphene oxide solution in step (2) is 3-12 mg / ml; after the second calcination in step (3), the content ratio of each main component element satisfies the following relationship: iron content 15-30%, cobalt content 2-10%, and potassium content 0.1-1.5%.

8. Preparation method of a dual-active-site graphene fence-supported iron-cobalt bimetallic catalyst, wherein the catalyst is supported by graphene and loaded with iron-cobalt bimetallic, characterized in that The following steps are involved: (1) preparing a graphene oxide solution by the Hummers method, and freeze-drying to obtain dry graphene oxide; (2) adding freeze-dried graphene oxide, urea, and an iron source to a mixed solution of ethylene glycol and deionized water to prepare a graphene oxide solution, stirring and ultrasonically treating the solution, and finally rotating a pot for hydrothermal reaction; (3) washing the product obtained in step (2) with deionized water until neutral and then freeze-drying it, then calcining the dried product in a nitrogen atmosphere, immersing the calcined product in a solution of a potassium source and a cobalt source and then freeze-drying it, and finally calcining the dried product in a nitrogen atmosphere to obtain the dual-active site graphene fence-loaded iron-cobalt bimetallic catalyst.

9. The method for preparing the dual-active site graphene fence supported iron-cobalt bimetallic catalyst according to claim 8, characterized in that The concentration of the graphene oxide solution in step (2) is 3-12 mg / ml; after the second calcination in step (3), the content ratio of each main component element satisfies the following relationship: iron content 15-30%, cobalt content 2-10%, and potassium content 0.1-1.5%.

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