Rare earth oxide modified supported palladium catalyst for catalyzing methane oxidation at low temperature and preparation method of rare earth oxide modified supported palladium catalyst

Abstract

The invention discloses a rare earth oxide modified supported palladium catalyst for low-temperature catalytic methane oxidation and a preparation method thereof, and belongs to the field of catalysis. The preparation method comprises the following steps: synthesizing KCC-1 with a high specific surface area by adopting a hydrothermal method, impregnating CeO2 on the surface of a KCC-1 carrier to form CeO2 / KCC-1 with small particle size and rich oxygen species, and loading Pd on the CeO2 / KCC-1 by adopting a sodium borohydride reduction method to prepare the Pd / CeO2 / KCC-1 catalyst. The process is simple, reaction gases comprise 10000 ppm of methane, 20 vol% of O2 and N2 (equilibrium gas) under normal pressure, the space velocity (SV) is 20000 mL / (g.h), T50% and T90% of the Pd / CeO2 / KCC-1 catalyst are 302 DEG C and 353 DEG C respectively, and the catalyst has excellent low-temperature methane catalytic oxidation activity and stability.

Description

Technical Field

[0001] The present invention relates to a rare earth oxide modified supported palladium catalyst for low-temperature catalytic methane oxidation and a preparation method thereof. The catalyst exhibits excellent catalytic methane oxidation activity and belongs to the field of nanomaterial preparation. Background Art

[0002] Methane is the second largest greenhouse gas globally, characterized by a high global warming potential and a short lifespan. Strictly controlling methane emissions has the climate benefit of mitigating global warming, the economic benefit of energy resource utilization, and the environmental benefit of co-controlling pollutants. Among various methods for treating methane, catalytic oxidation is considered one of the greenest and cleanest methods due to its low energy consumption, high purification efficiency, etc. The key to this method is to develop a catalyst with good low-temperature oxidation activity, high-temperature stability, and low cost. Common catalysts include noble metal catalysts and non-noble metal catalysts.

[0003] Pd-based catalysts are widely used and have excellent hydrothermal stability. Preparing Pd-based catalysts with a low loading and high Pd dispersion can achieve the industrial application of noble metal catalysts. Li et al. (C.S. Li et al. ACS Appl. Nano Mater. 2020, 3, 12130 - 12138) synthesized an alumina with a two-dimensional feather-like lamellar structure, which is easy to anchor and disperse Pd nanoparticles. It was found that the catalyst with a Pd loading of 1.0 wt% had the best catalytic effect, and the methane conversion rate could reach over 90% at 400 °C under a space velocity of 35000 mL / (g·h). After reacting at 450 °C for 50 h, the conversion rate only decreased by 8.6%.

[0004] Due to the limited and expensive noble metal resources, improving the utilization rate of noble metals and synthesizing supported noble metal catalysts with higher activity are important research topics currently. Chen et al. (S.Y. Chen et al. ACS Catal. 2021, 11, 5666 - 5677) studied the catalytic activity of Pd catalysts supported on CeO2 carriers with different morphologies for methane oxidation and found that octahedral CeO2 with a Pd loading of 1.4 wt% had the best catalytic performance, and the methane conversion rate could reach over 90% at 400 °C under a space velocity of 60000 mL / (g·h). Enhancing the adsorption and activation of O2 by CeO2 is beneficial to improving the activity of the catalyst.

[0005] In addition, in terms of support selection, mesoporous silica has been widely used as a catalyst support due to its high specific surface area, uniform pore structure, good thermal stability, and appropriate surface acidity and alkalinity. KCC-1 is a kind of silica with a unique flower-like porous structure. Its unique morphology and excellent physical and chemical properties endow it with broad application potential in many fields. Compared with other conventional silicas, KCC-1 has a larger specific surface area and better stability.

[0006] So far, there has been no literature reporting the use of rare earth oxide-modified KCC-1 supported Pd (Pd / CeO2 / KCC-1) catalyst for methane oxidation reaction. The present invention discloses a controllable preparation method of Pd / CeO2 / KCC-1 catalyst, and it is found that the Pd / CeO2 / KCC-1 catalyst has excellent low-temperature catalytic activity and thermal stability for methane oxidation. Summary of the Invention

[0007] The purpose of the present invention is to prepare a rare earth oxide-modified KCC-1 supported Pd catalyst by hydrothermal method, impregnation method and sodium borohydride reduction method. That is, KCC-1 with a high specific surface area is synthesized by hydrothermal method, CeO2 is impregnated on the surface of KCC-1 support to form CeO2 / KCC-1 with small particle size and rich oxygen species, and then Pd is loaded on CeO2 / KCC-1 by sodium borohydride reduction method, thereby preparing Pd / CeO2 / KCC-1 catalyst. Pd particles are evenly distributed on the surface of the catalyst, and the prepared catalyst has excellent and stable catalytic activity for methane oxidation at low temperature. The reaction conditions are: under normal pressure, the reaction gas composition is 10000 ppm CH4 + 20 vol% O2 + N2 (balance gas), and the space velocity is 20000 mL / (g·h).

[0008] A preparation method of Pd / CeO2 / KCC-1 catalyst for catalytic methane oxidation, the present invention specifically includes the following steps:

[0009] First, KCC-1 with a specific morphology is synthesized by hydrothermal method; cetyltrimethylammonium bromide and urea are dissolved in deionized water, which is called solution A; at the same time, 1-pentanol and tetraethoxysilane are mixed in cyclohexane and stirred for 30 minutes, which is called solution B; then, solution B is dropped into solution A, stirred for 30 minutes, and the obtained solution is put into a polytetrafluoroethylene-sealed autoclave and placed in an oven at 120 °C for heating for 4 hours; through centrifugal separation, the obtained product is dried in an oven at 80 °C for 12 hours, and then transferred to a muffle furnace for calcination in air atmosphere, heated to 550 °C at a heating rate of 5 °C / min, and kept warm for 6 hours, thereby obtaining KCC-1;

[0010] In the further A solution, for every 1.6 g of cetyltrimethylammonium bromide, there are 0.96 g of urea and 48 mL of deionized water; in the B solution, the volume ratio of 1-pentanol, tetraethoxysilane, and cyclohexane is: 2.4:4.3:48.

[0011] When the B solution is dropped into the A solution, for every 1.6 g of cetyltrimethylammonium bromide, there are 4.3 mL of tetraethoxysilane;

[0012] Secondly, weigh KCC-1 and add it to deionized water, add Ce(NO3)3·6H2O and stir for 6 hours. After rotary evaporation of the mixture, transfer it to a muffle furnace and calcine it in an air atmosphere. Heat it to 400 °C at a heating rate of 5 °C / min and hold for 2 hours to obtain CeO2 / KCC-1; use the NaBH4 reduction method protected by polyvinyl alcohol (PVA) to load Pd. First, stir the PVA solution until dense foam is generated, then add the PdCl2 aqueous solution and stir evenly. Finally, quickly add the NaBH4 solution under ice bath conditions to form a mixed solution to ensure the uniformity of the solution color; then, add CeO2 / KCC-1 to the above solution and stir for 6 hours; filter the above mixture by suction and dry it overnight in an 80 °C oven, then transfer it to a muffle furnace and calcine it in an air atmosphere. Heat it to 550 °C at a heating rate of 5 °C / min and hold for 2 hours to obtain the Pd / CeO2 / KCC-1 catalyst.

[0013] For every 0.5 g of KCC-1, there are 0.3 - 0.6 g (preferably 0.4 - 0.5 g) of Ce(NO3)3·6H2O; for every 0.5 g of CeO2 / KCC-1, there are 0.004 - 0.016 g (preferably 0.008 - 0.010 g) of PdCl2; preferably, the concentrations of NaBH4, PVA, and PdCl2 in the mixed solution of PVA, PdCl2, and NaBH4 are: 0.74 g / l: 0.5 g / l: 0.69 g / l.

[0014] Catalyst performance evaluation:

[0015] Select methane as the probe molecule for catalyst activity evaluation. The reaction gas mixture composition is 10,000 ppm methane + 20 vol% O2 + N2 (balance gas), and the space velocity is 20,000 mL / (g·h). On the Pd / CeO2 / KCC-1 catalyst, the reaction temperatures (T 50％ and T 90％ ) required for the methane conversion rate to reach 50% and 90% are 302 °C and 353 °C respectively. The Pd / CeO2 / KCC-1 catalyst has excellent catalytic activity. After 50 hours of continuous methane reaction (reaction temperature is 290 °C and space velocity is 20,000 mL / (g·h)) testing, the catalyst still maintains a 40% methane conversion rate.

[0016] The microstructure of the Pd / CeO2 / KCC-1 catalyst was determined by using high-resolution FE-STEM SU9000 super-resolution scanning transmission electron microscopy technology. It can be clearly seen that KCC-1 with a flower-like porous structure was successfully synthesized by the hydrothermal method, which has a high specific surface area and a large pore volume.

[0017] The catalyst preparation process of the present invention is simple, and rare earth oxide modified supported palladium catalysts for highly efficient catalytic oxidation of methane can be prepared on a large scale. They show excellent low-temperature catalytic activity for methane oxidation, have a long catalyst life, and have good application prospects in the field of methane emission control. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is the XRD pattern of the prepared catalyst.

[0019] Figure 2 It is the SEM image of the Pd / CeO2 / KCC-1 catalyst, where (a) is a partial view of the Pd / CeO2 / KCC-1 catalyst, and (b) is the overall morphology of Pd / CeO2 / KCC-1. Its petal-shaped circular morphology can be observed.

[0020] Figure 3 It is the activity of the prepared Pd / CeO2 / KCC-1 catalyst and the Pd / KCC-1 comparative catalyst for the catalytic oxidation of methane. The reaction conditions are: 10000 ppm methane + 20 vol% O2 + N2 (balance gas), and the space velocity is 20000 mL / (g·h).

[0021] Figure 4 It is the catalytic stability of the prepared Pd / CeO2 / KCC-1 catalyst for methane oxidation. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be further described below in conjunction with the embodiments, but the present invention is not limited to the following embodiments, and the drawings are given to describe the various catalyst materials obtained by the present invention.

[0023] Example 1

[0024] (1) Preparation of the Pd / CeO2 / KCC-1 catalyst

[0025] First, KCC-1 with a specific morphology was synthesized by the hydrothermal method; cetyltrimethylammonium bromide and urea were dissolved in deionized water, which was called solution A; at the same time, 1-pentanol and tetraethoxysilane were mixed in cyclohexane and stirred for 30 minutes, which was called solution B; then, solution B was dropped into solution A and stirred for 30 minutes, and the obtained solution was put into a polytetrafluoroethylene-sealed autoclave and placed in an oven at 120 °C for heating for 4 hours; through centrifugal separation, the obtained product was dried in an oven at 80 °C for 12 hours, and then transferred to a muffle furnace for calcination in an air atmosphere, and the temperature was raised to 550 °C at a heating rate of 5 °C / min and held for 6 hours, thus obtaining KCC-1;

[0026] Further, in solution A, for every 1.6 g of cetyltrimethylammonium bromide, it corresponded to 0.96 g of urea and 48 mL of deionized water; in solution B, the volume ratio of 1-pentanol, tetraethoxysilane, and cyclohexane was: 2.4:4.3:48.

[0027] When solution B was dropped into solution A, for every 1.6 g of cetyltrimethylammonium bromide, it corresponded to 4.3 mL of tetraethoxysilane;

[0028] Secondly, a certain amount of KCC-1 was weighed and added to deionized water, Ce(NO3)3·6H2O was added and stirred for 6 hours, the mixture was rotary evaporated and then transferred to a muffle furnace for calcination in an air atmosphere, and the temperature was raised to 400 °C at a heating rate of 5 °C / min and held for 2 hours, thus obtaining CeO2 / KCC-1; the NaBH4 reduction method protected by polyvinyl alcohol (PVA) was used to load Pd. First, the PVA solution was stirred until dense foam was generated, then the PdCl2 aqueous solution was added and stirred evenly, and finally the NaBH4 solution was quickly added under ice bath conditions to ensure the uniformity of the solution color; then, CeO2 / KCC-1 was added to the above solution and stirred for 6 hours; the above mixture was filtered by suction and dried overnight in an oven at 80 °C, and then transferred to a muffle furnace for calcination in an air atmosphere, and the temperature was raised to 550 °C at a heating rate of 5 °C / min and held for 2 hours, thus obtaining the Pd / CeO2 / KCC-1 catalyst.

[0029] For every 0.5 g of KCC-1, it corresponded to 0.433 g of Ce(NO3)3·6H2O, and for every 0.5 g of CeO2 / KCC-1, it corresponded to 0.008 g of PdCl2;

[0030] Preferably, the concentrations of NaBH4, PVA, and PdCl2 in the mixed solution of PVA, PdCl2, and NaBH4 were respectively: 0.74 g / l: 0.5 g / l: 0.69 g / l.

[0031] (2) Preparation of Pd / KCC-1 catalyst

[0032] First, KCC-1 with a specific morphology was synthesized by the hydrothermal method, and then Pd was loaded by the reduction method of NaBH4 protected by polyvinyl alcohol (PVA). First, the PVA solution was stirred until a dense foam was generated, then the aqueous PdCl2 solution was added and stirred evenly, and finally the NaBH4 solution was quickly added under ice bath conditions to ensure uniform solution color; then, 0.5 g of KCC-1 was added to the above solution and stirred for 6 hours. The above mixture was filtered by suction and dried overnight in an 80 °C oven, and then transferred to a muffle furnace and calcined in an air atmosphere. It was heated to 550 °C at a heating rate of 5 °C / min and held for 2 hours to obtain the Pd / KCC-1 catalyst.

[0033] (3) Catalytic activity evaluation

[0034] The Pd / CeO2 / KCC-1 catalyst was used for the methane oxidation reaction. Weigh 50 mg of the catalyst (particle size 40 - 60 mesh) and load it into a quartz fixed-bed reactor. A reaction gas composed of 10000 ppm methane + 20 vol% O2 + N2 (balance gas) was passed into the fixed-bed reactor, where the total gas flow rate was 16.7 mL / min and the space velocity was 20000 mL / (g·h). The T 50％ and T 90％ for methane oxidation on the Pd / CeO2 / KCC-1 catalyst were 302 °C and 353 °C respectively.

[0035] (4) Catalytic stability test

[0036] The catalytic activity stability test of the Pd / CeO2 / KCC-1 catalyst was carried out at a reaction temperature of 290 °C and a space velocity of 20000 mL / (g h). During the continuous methane reaction process of up to 50 hours, the catalytic activity of the catalyst remained stable.

Claims

1. A method for preparing a Pd / CeO2 / KCC-1 catalyst for catalytic methane oxidation, characterized in that: The following steps are involved: First, KCC-1 with a specific morphology was synthesized by a hydrothermal method; hexadecyltrimethylammonium bromide and urea were dissolved in deionized water, referred to as solution A; at the same time, 1-pentanol and tetraethoxysilane were mixed in cyclohexane and stirred for 30 minutes, referred to as solution B; then, solution B was added dropwise to solution A, stirred for 30 minutes, and the resulting solution was placed in a polytetrafluoroethylene-sealed autoclave and heated in an oven at 120°C for 4 hours; The product was separated by centrifugation, dried in an oven at 80°C for 12 hours, and then transferred to a muffle furnace for calcination in an air atmosphere, heated to 550°C at a heating rate of 5°C / min, and kept at this temperature for 6 hours, thereby obtaining KCC-1; Secondly, KCC-1 was weighed and added to deionized water, Ce(NO3)3·6H2O was added and stirred for 6 hours, the mixture was rotary evaporated and transferred to a muffle furnace, calcined in an air atmosphere, heated to 400°C at a heating rate of 5°C / min, and kept warm for 2 hours to obtain CeO2 / KCC-1; Pd was loaded by NaBH4 reduction method protected by polyvinyl alcohol (PVA), the PVA solution was first stirred until dense foam was produced, and then PdCl2 was added The aqueous solution was stirred evenly, and finally the NaBH4 solution was quickly added under ice bath conditions to form a mixed solution to ensure that the solution had uniform color; then, CeO2 / KCC-1 was added to the above solution and stirred for 6 hours; the above mixture was filtered and dried in an oven at 80°C overnight, and then transferred to a muffle furnace for calcination in an air atmosphere, and the temperature was increased to 550°C at a heating rate of 5°C / min, and kept warm for 2 hours, thereby preparing a Pd / CeO2 / KCC-1 catalyst.

2. The method according to claim 1, characterized in that In solution A, every 1.6 g of hexadecyltrimethylammonium bromide corresponds to 0.96 g of urea and 48 mL of deionized water; in solution B, the volume ratio of 1-pentanol, tetraethoxysilane, and cyclohexane is 2.4:4.3:

48.

3. The method according to claim 1, characterized in that When solution B is added dropwise to solution A, 4.3 mL of tetraethoxysilane corresponds to 1.6 g of hexadecyltrimethylammonium bromide.

4. The method according to claim 1, characterized in that Every 0.5 g of KCC-1 corresponds to 0.3-0.6 g (preferably 0.4-0.5 g) of Ce(NO3)3·6H2O.

5. The method according to claim 1, characterized in that Every 0.5 g of CeO2 / KCC-1 corresponds to 0.004-0.016 g (preferably 0.008-0.010 g) of PdCl2.

6. The method according to claim 1, characterized in that The concentrations of NaBH4, PVA and PdCl2 in the mixed solution of PVA, PdCl2 and NaBH4 are 0.74 g / l: 0.5 g / l: 0.69 g / l respectively.

7. The Pd / CeO2 / KCC-1 catalyst obtained by the method according to any one of claims 1 to 6.

8. Use of the Pd / CeO2 / KCC-1 catalyst obtained according to the method of any one of claims 1 to 6 for catalytic methane oxidation.

9. The use according to claim 8, wherein the reaction gas composition is 10000ppm methane + 20vol% O2 + N2 (balance gas), the space velocity is 20000mL / (g·h), and the reaction temperature (T) required for the methane conversion rate to reach 50% and 90% on the Pd / CeO2 / KCC-1 catalyst is 50％ and T 90％ ) are 302°C and 353°C respectively. Pd / CeO2 / KCC-1 catalyst has excellent catalytic activity. After 50 hours of continuous reaction of methane (reaction temperature is 290°C and space velocity is 20000mL / (gh)), the catalyst still maintains a methane conversion rate of 40%.

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