Alkali-etched Co0. 98Pd0. 02Al2O4-delta-alkali catalyst for catalytic oxidation of methane and preparation

Abstract

The invention relates to an alkali-etched Co0. 98Pd0. 02Al2O4-delta-alkali catalyst for catalytic oxidation of methane and a preparation method thereof, and belongs to the field of catalytic chemistry and environmental chemistry. The preparation method comprises the following steps: doping palladium into cobalt aluminate crystal lattices based on a one-step synthesis sol-gel method, carrying out high-temperature roasting treatment to obtain a Co < 0.98 > Pd < 0.02 > Al < 2 > O < 4-delta > catalyst, modifying the surface of the catalyst through NaOH, selectively dissolving a part of Al ions on the surface, and carrying out high-temperature roasting again to obtain an etching modified Co < 0.98 > Pd < 0.02 > Al < 2 > O < 4-delta >-alkali catalyst, the catalyst shows excellent low-temperature catalytic activity on methane combustion and has excellent thermal stability. The method has a good application prospect in the field of air pollution control.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of low temperature methane catalytic oxidation of alkali etching Co 0.98 Pd 0.02 Al2O 4-δ Catalyst and its preparation method, the catalyst shows good methane low temperature catalytic oxidation activity.It belongs to catalytic chemistry and environmental chemistry field. BACKGROUND

[0002] Methane is a strong greenhouse gas, the greenhouse effect potential value of every kg CH4 in atmosphere is 120 times (i.e. instantaneous influence) and 28 times (100 years comprehensive influence) of CO2, is one of important reasons of atmospheric warming. IPCC sixth report points out that from 1850 to now human activity emission pollutant influence on climate: methane emission contribution is more than 25 %;Global temperature rises 1.1 ℃ compared with pre-industrialization level, about 0.5 ℃ is derived from methane emission.

[0003] Methane has highly symmetrical tetrahedral stable structure, C-H bond breaking needs high energy, and it is difficult to activate at low temperature.For low concentration CH4, catalytic oxidation method is the most effective and industrialized elimination method. The deceleration step of methane catalytic combustion is the activation of C-H bond;Fast activation of gas phase oxygen to supplement the consumption of surface active oxygen species is the key to realize low temperature deep oxidation. However, in the process of catalytic combustion, metal active site thermodynamics is unstable, easy to agglomerate and deactivate, and it is difficult to maintain low carbon alkane stable and efficient purification.

[0004] Defect engineering is a very effective strategy to increase the density of oxygen defects on the catalyst surface by adjusting the electronic and geometric structure of the catalyst, and to change the kinetic activity of the reaction. Interface defects are defects that destroy the symmetry of ordered arrangement in one-dimensional direction. The interface constructed can directly affect the surface adsorption and desorption of active species. The most common method of defect construction is etching. Etching mainly uses the chemical reaction between added chemicals and materials according to the chemical properties of the catalyst to manufacture defects. For example, amphoteric metal ions can be etched by alkaline solution to selectively construct specific cation defects. Oxygen defects in oxides can be generated by reduction etching with hydrogen, ethylene glycol, etc. Atomic sulfur defects can be constructed by oxidative etching with hydrogen peroxide. The defects generated by etching can directly act as adsorption sites of the catalyst. Oxygen vacancies, as the most common defect type in metal oxides, can effectively enhance the ability of the catalyst to adsorb and activate O2. The formation of defects usually changes the local electronic structure distribution of the surrounding atoms, regulates the electronic structure and adsorption energy of the catalyst, and directly affects the adsorption energy of the reactant species on the active site, thereby affecting the kinetics and selectivity of the catalytic reaction. In fact, the high reactivity of the cation defects generated by etching also comes from the change of the surface electronic structure, which can regulate the adsorption energy of the reaction molecules and intermediates. Defects can also regulate the catalytic activity and selectivity through interaction with the catalytically active center. Studies have found that the adsorption energy of metal ions on the defect site is much higher than that on the non-defect site. Therefore, defects not only can adsorb and activate reaction molecules to improve catalytic activity, but also can anchor other heteroatoms or groups at the defect site to form new active centers. Due to the unique electronic interaction between defects and atoms or groups, the new active center may have better catalytic activity than other non-defect anchoring sites.

[0005] Inspired by this, the abundant defect sites and oxygen vacancies on the oxide surface may be an effective strategy to enhance the activity of oxidation reactions. Studies have found that Pd σ+ species embedded in the oxide lattice have better methane activation activity. The structure and properties of the support will affect the coordination environment of the active Pd center, thereby determining the catalytic activity of the catalyst. Co is a single active site in CoAl2O4 catalyst and Al is easy to etch, so CoAl2O4 is considered an ideal model for studying material structure, defects and performance. The etching effect of plasma in alkaline environment induces the formation of Al 3+ cavities. Cation cavities are conducive to exposing more Co active sites. Due to the etching effect of NaOH, a relatively rough surface with abundant Al 3+ defects is formed. Moreover, an amorphous structure is formed in situ on the surface. Surface amorphization can increase the flexibility of the structure and the number of active sites.

[0006] The Pd is doped into the cobalt aluminate lattice by a sol-gel method in the application, and then the surface of the catalyst is modified by etching with NaOH to selectively dissolve the surface Al ions. The activation ability of the catalyst to O2 is significantly improved, and efficient catalytic oxidation of methane can be realized at low temperature (the temperatures (T 50％ and T 90％ ) at which the methane conversion rates are 50% and 90% are 339 DEG C and 381 DEG C, respectively; and after being treated at 750 DEG C for 10 h, the T 0.98 of the Co 0.02 Pd 4-δ Al2O 90％ -alkali catalyst only rises by 41 DEG C, showing excellent high-temperature thermal stability. Embedding Pd into the CoAl2O4 lattice makes Pd in a highly dispersed state, limits the migration of Pd, and hinders sintering of Pd due to high temperature in the process of catalytic combustion of methane, so that the Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst maintains excellent high-temperature thermal stability under high-temperature aging conditions.

[0007] To the best of our knowledge, there is no literature and patent reporting a catalyst doped with Pd into a cobalt aluminate lattice by a sol-gel method and treated with alkali and the catalytic performance of the catalyst on methane oxidation. The application discloses a controllable preparation method of a Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst, and research finds that the Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst has excellent catalytic activity and high-temperature thermal stability on methane oxidation, and reduces the amount of noble metal. SUMMARY

[0008] The application aims to prepare a Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst by a sol-gel method and an alkali etching strategy, and use the catalyst for low-temperature catalytic oxidation of methane to improve the O2 activation ability at low temperature.

[0009] The application is a preparation method of an alkali-etched Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst for low-temperature catalytic oxidation of methane, and specifically includes the following steps:

[0010] (1) Take cobalt nitrate and aluminum nitrate powder into a crucible, add deionized water to stir to form a uniform mixture of cobalt-aluminum precursor salt aqueous solution, then add palladium nitrate powder and stir again to obtain a clear solution; add glycine powder according to a certain stoichiometric ratio, stir for 1 h to obtain a clear solution; transfer the mixed solution to a 80℃ water bath, continue to stir until a viscous gel is formed; cover the crucible containing the gel and place it in a muffle furnace, raise the temperature from room temperature to 300℃ at a rate of 3℃ / min and keep it for 10 min, so that the viscous gel foams to release decomposition gas; grind the obtained composite into powder, then transfer it to a muffle furnace, raise the temperature from room temperature to 550℃ at a rate of 3℃ / min and keep it for 4 h, to obtain Co 0.98 Pd 0.02 Al2O 4-δ catalyst;

[0011] (2) Take sodium hydroxide powder, add deionized water to stir and dissolve to form a basic solution with a concentration of 1 mol / L, take Co 0.98 Pd 0.02 Al2O 4-δ catalyst, add to the lye and stir evenly; then add ethylenediaminetetraacetic acid powder, continue to stir for 5 h, then wash and dry to obtain a composite; transfer the obtained composite to a muffle furnace, raise the temperature from room temperature to 450℃ at a rate of 3℃ / min and keep it for 3 h, to obtain Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst.

[0012] In step (1), the molar ratio of metal Co:Al:Pd is 0.98:2.00:0.02, and the doping amount of Pd is 1.0wt%.

[0013] In step (2), 80mL of 1mol / L NaOH solution corresponds to 0.5g of catalyst, and the molar ratio of NaOH to total metal ions (Co+Al+Pd) is 4:1, and the alkali solution is excessive; the molar ratio of sodium hydroxide to ethylenediaminetetraacetic acid is controlled to be 1:1.

[0014] The catalyst obtained by the method can be used for low-temperature and high-efficiency catalytic oxidation of methane at low temperature, and T 50％ and T 90％ are 339℃ and 381℃, respectively.

[0015] Catalyst performance evaluation:

[0016] Methane was selected as a probe molecule to evaluate the performance of the related catalysts, and the gas composition was CH4 (1.0 vol% in volume percentage), O2 (20 vol% in volume percentage) and N2 (balance gas), and the space velocity (SV) thereof was 20000 mL / (g·h). The conversion of methane on Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalysts reached 50% and 90% at 339℃ and 381℃, respectively. The conversion of methane on Co 50％ Pd 90％ Al2O 0.98 -alkali catalysts reached 50% and 90% at 339℃ and 381℃, respectively. The conversion of methane on Co 0.02 Pd 4-δ Al2O 0.98 -alkali catalysts reached 50% and 90% at 339℃ and 381℃, respectively. The conversion of methane on Co 0.02 Pd 4-δ Al2O 90％ -alkali catalysts reached 50% and 90% at 339℃ and 381℃, respectively. The conversion of methane on Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalysts reached 50% and 90% at 339℃ and 381℃, respectively. The conversion of methane on Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalysts reached 50% and 90% at 339℃ and 381℃, respectively. The conversion of methane on Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalysts reached 50% and 90% at 339℃ and 381℃, respectively. The conversion of methane on Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalysts reached 50% and 90% at 339℃ and 381℃, respectively. The conversion of methane on Co

[0017] The microstructure of the Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalysts was explored by using high-resolution HADDF-STEM and energy dispersive X-ray spectroscopy (EDX) techniques. It can be clearly seen that the edges of the Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalysts become blurred, and an ordered layer of crystallographic defects with a thickness of 1-2 nm appears; it is confirmed that new active sites are generated on the surface of the catalyst after alkali etching, which can quickly activate the gas-phase oxygen into active oxygen species.

[0018] The catalyst preparation process of the present application is simple, and the Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalysts for catalytic oxidation of methane can be successfully prepared, which exhibits good low-temperature catalytic activity for methane oxidation, good high-temperature thermal stability of the catalyst, and good application prospect in the field of reducing greenhouse effect and methane emission reduction. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1Co 0.98 Pd 0.02 Al2O 4-δ TEM image of the alkali catalyst.

[0020] Figure 2 Co 0.98 Pd 0.02 Al2O 4-δ Catalytic activity curve of the alkali catalyst for methane oxidation. Reaction conditions: 1.0 vol% CH4, 20 vol% O2 and N2 (balance), space velocity (SV) 20000 mL / (g.h).

[0021] Figure 3 Co 0.98 Pd 0.02 Al2O 4-δ Activity test results of the alkali catalyst before and after treatment at 750°C for 10 h for methane catalytic oxidation.

[0022] Figure 4 Co 0.98 Pd 0.02 Al2O 4-δ Elemental mapping of Co, Pd, Al and O of the alkali catalyst. DETAILED DESCRIPTION

[0023] The present application is further illustrated by the following examples, but the present application is not limited to the following examples, and the obtained catalyst materials are described by the following figures.

[0024] Example 1

[0025] (1) Co 0.98 Pd 0.02 Al2O 4-δ Preparation of the alkali catalyst:

[0026] A homogeneous metal precursor aqueous solution was prepared by dissolving 2.91 g of cobalt nitrate (Co(N03)2-3H20) and 7.41 g of aluminum nitrate (Al(N03)3-9H20) powders in 30 mL of deionized water in a crucible according to the molar ratio of metal Co:Al:Pd of 0.98:2.00:0.02, and then 0.06 g of palladium nitrate (Pd(N03)2-2H20) powder was added and stirred for 0.5 h to obtain a clear solution, and then 2.25 g of glycine powder was added and continuously stirred for 1 h to obtain a homogeneous mixed solution; the mixed solution was transferred to a 80 °C water bath, and continuously stirred until a viscous gel was formed; the crucible containing the gel was covered and placed in a muffle furnace, and the temperature was raised from room temperature to 300 °C at a rate of 3 °C / min and maintained for 10 min, so that the viscous gel was foamed to release decomposition gas; the obtained composite was ground into powder, which was transferred to a muffle furnace, and the temperature was raised from room temperature to 550 °C at a rate of 3 °C / min and maintained for 4 h to obtain Co 0.98 Pd 0.02 Al2O 4-δ catalyst.

[0027] A 3.2 g of NaOH powder was weighed into a beaker and added to 80 mL of deionized water, and stirred until completely dissolved to prepare a 1 mol / L NaOH solution; 0.5 g of Co 0.98 Pd 0.02 Al2O 4-δ catalyst was added to the alkali solution, and after stirring uniformly, 3.36 g of ethylenediaminetetraacetic acid (EDTA) powder was added, and continuously stirred for 5 h, and then washed and dried to obtain a composite; the obtained composite was transferred to a muffle furnace, and the temperature was raised from room temperature to 450 °C at a rate of 3 °C / min and maintained for 3 h to obtain Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst.

[0028] (2) The Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst was used for catalytic oxidation of methane. 50 mg of catalyst (mixed quartz sand particle size of 60 mesh) was weighed and packed into a quartz fixed bed reactor, and a reaction gas with a volume concentration of 1.0 vol% methane was introduced into the fixed bed reactor, and the gas composition was 1.0 vol% CH4, 20 vol% O2 and N2 (balance gas), and the total gas flow was 16.7 mL / min, and the space velocity (SV) was 20000 mL / (g-h), and the Co 0.98 Pd 0.02 Al2O 4-δ-alkali catalysts are 339°C and 381°C, respectively. 50％ -alkali catalysts are 339°C and 381°C, respectively. 90％ -alkali catalysts are 339°C and 381°C, respectively.

[0029] (3) The Co 0.98 Pd 0.02 Al2O 4-δ -alkali catalysts were aged at 750°C for 10h. In the activity test, the T 90％ of methane conversion only increased by 41°C, showing excellent high-temperature thermal stability.

Claims

1. A low-temperature methane-catalyzed alkaline etching method for Co 0.98 Pd 0.02 Al2O 4-δ The method for preparing the -alkali catalyst is characterized by, Specifically, the following steps are included: (1) Cobalt nitrate and aluminum nitrate powders were placed in a crucible, and deionized water was added and stirred to form a uniform aqueous solution of cobalt-aluminum precursor salts. Then, palladium nitrate powder was added and stirred again to obtain a clear solution. Glycine powder was added and stirred for 1 hour to obtain a clear solution. The mixed solution was transferred to an 80°C water bath and stirred continuously until a viscous gel was formed. The crucible containing the gel was covered and placed in a muffle furnace. The temperature was increased from room temperature to 300°C at a heating rate of 3°C / min and held for 10 minutes to allow the viscous gel to foam and release decomposition gases. The obtained composite was ground into powder and transferred to a muffle furnace. The temperature was increased from room temperature to 550°C at a heating rate of 3°C / min and held for 4 hours to obtain Co. 0.98 Pd 0.02 Al2O 4-δ catalyst; (2) Weigh out sodium hydroxide powder, add it to deionized water, stir to dissolve, and form an alkaline solution. Weigh out Co... 0.98 Pd 0.02 Al2O 4-δ The catalyst was added to the alkaline solution and stirred until homogeneous. Then, ethylenediaminetetraacetic acid powder was added, and the mixture was stirred continuously with the container open for 5 hours. After washing and drying, the composite was obtained. The resulting composite was transferred to a muffle furnace and heated from room temperature to 450°C at a heating rate of 3°C / min and held at that temperature for 3 hours to obtain Co. 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst.

2. The method according to claim 1, characterized in that, The molar ratio of metal Co:Al:Pd is 0.98:2.00:0.02; the chemical dosage of glycine and the molar ratio of total metal ions (Co+Al+Pd) are controlled at 1:

1.

3. The method according to claim 1, characterized in that, Step (2) The concentration of the alkaline solution is 1 mol / L.

4. The method according to claim 1, characterized in that, Each 0.5g catalyst corresponds to 80mL of 1mol / L NaOH solution, with excess alkali solution; the molar ratio of sodium hydroxide to ethylenediaminetetraacetic acid is controlled at 1:

1.

5. The low-temperature methane-catalyzed alkaline-etched Co prepared by the method according to any one of claims 1-4 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst.

6. The low-temperature methane-catalyzed alkaline-etched Co prepared by the method according to any one of claims 1-4 0.98 Pd 0.02 Al2O 4-δ The application of alkali catalysts for the catalytic oxidation of methane.

7. The application according to claim 6, characterized in that, Weigh 50 mg of catalyst and pack it into a 60-mesh quartz fixed-bed reactor. A 1.0 vol% methane reactant gas is introduced into the fixed-bed reactor. The reactant gas composition is 1.0 vol% CH4, 20 vol% O2, and N2 (equilibrium gas). The total gas flow rate is 16.7 mL / min, and the space velocity (SV) is 20000 mL / (g·h). Methane is reacted in Co... 0.98 Pd 0.02 Al2O 4-δ Temperatures (T5) at which conversion rates are 50% and 90% on the alkali catalyst. 50％ and T 90％ The temperatures were 339℃ and 381℃, respectively.

8. The application according to claim 6, using Co 0.98 Pd 0.02 Al2O 4-δ Co was obtained by high-temperature aging of the alkali catalyst in a muffle furnace at 750°C for 10 hours. 0.98 Pd 0.02 Al2O 4-δ -alkali-750 catalyst, 50 mg of catalyst was weighed and packed into a 60-mesh quartz fixed-bed reactor. Methane reactant gas with a concentration of 1.0 vol% was introduced into the fixed-bed reactor. The reactant gas composition was 1.0 vol% CH4, 20 vol% O2, and N2 (equilibrium gas). The total gas flow rate was 16.7 mL / min, and the space velocity (SV) was 20000 mL / (g·h). Methane was reacted in Co... 0.98 Pd 0.02 Al2O 4-δ -alkali catalyst on T 50％ and T 90％ The temperatures are 366℃ and 422℃, respectively.

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