CN110732335B

CN110732335B - Transition metal @ BO for methane dry gas reforming reactionxCore-shell structure nano catalyst and preparation method thereof

Info

Publication number: CN110732335B
Application number: CN201810803133.0A
Authority: CN
Inventors: 傅强; 董金虎; 包信和
Original assignee: Dalian Institute of Chemical Physics of CAS
Current assignee: Dalian Institute of Chemical Physics of CAS
Priority date: 2018-07-20
Filing date: 2018-07-20
Publication date: 2020-07-07
Anticipated expiration: 2038-07-20
Also published as: CN110732335A

Abstract

The invention discloses a transition metal @ boron oxide core-shell structure nano catalyst for methane dry gas reforming reaction (DRM) and a preparation method thereof. The mass fraction of transition metal in the catalyst is 0.4-16%, and the carrier is boron nitride; supported metal nanoparticles having a particle diameter of 3nm to 50nm and an ultra-thin Boron Oxide (BO) on the surface_x) Covered with a core-shell structure, denoted TM @ BO_xand/BN. The catalyst can be used for the dry gas reforming reaction of methane and carbon dioxide at high temperature, and can be operated for a long time without obvious inactivation. The catalyst shows excellent carbon deposition resistance and sintering resistance. The preparation method of the catalyst is simple, efficient and reliable, has easily obtained raw materials, is easy for mass preparation, has universality, and can be used for various transition metals @ BO_xAnd (3) preparing a core-shell catalyst.

Description

Transition metal @ BO for methane dry gas reforming reactionxCore-shell structure nano catalyst and preparation method thereof

Background

Since the human beings entered the industrial era through the industrial revolution, the global climate has been continuously warmed due to the emission of a large amount of greenhouse gases, and has become an important factor influencing the sustainable development of the human society. Wherein CH₄And CO₂As two important greenhouse gases, how to treat and use them to suppress climate warming is becoming a focus of increasing attention. Methane dry gas reforming (DRM) is the reaction of CH at high temperature₄And CO₂Conversion to syngas (CO + H)₂) The reaction not only eliminates two greenhouse gases, but also generates synthesis gas with economic value, can be used for further synthesizing hydrocarbons (such as olefin, oil products, alcohols and the like) with high added values through the Fischer-Tropsch reaction, is a well-known green and sustainable chemical process, and many practitioners are dedicated to industrial application of the related process of methane dry gas reforming.

Among the numerous metal catalysts having methane dry gas reforming activity, Ni-based catalysts are considered to have the most industrial application prospect due to their low cost and high reactivity. However, the development of high performance Ni-based catalysts for commercial methane dry gas reforming reactions still faces many challenges. First of all, the problem of stability at high temperatures. Since the methane dry gas reforming reaction is a reaction with strong heat absorption in thermodynamics, the actual operation needs to be carried out at a high temperature, and after considering a plurality of thermodynamics processes, 700 ℃ to 900 ℃ is a suitable reaction interval. At such severe reaction temperatures, the active component of the catalyst is very susceptible to sintering, reducing the surface area of the active component and thereby losing reactivity. Secondly, catalyst deactivation by carbon deposition. Due to the existence of CH in the dry gas reforming reaction system₄If the carbon deposits generated by the two reactions cannot be removed from the metal surface in time and accumulate, the active sites of the metal are easy to block, and the catalyst is inactivated. Therefore, the development of Ni-based catalysts resistant to sintering and carbon deposition under high temperature dry gas reforming reaction conditions is a key and difficult point of research.

Many efforts have been made to improve the sintering and carbon deposition resistance of Ni-based catalysts. On the one hand, the use of suitable oxide supports, such as Ni/Al₂O₃NiAl formed after high temperature calcination₂O₄The spinel structure has high mechanical strength, relatively high specific surface area and good thermal stability, and the strong interaction between the metal Ni and the spinel under the reaction condition prevents the growth of Ni particles, so that the spinel structure has good stability, but the activity is not good enough. On the other hand, the catalytic performance of the Ni-based catalytic material can be improved by adding a proper auxiliary material. Basic oxide assistants, e.g. La₂O₃，K₂O，MgO，Ga₂O₃And CaO, can intensify CO₂The adsorption of (2) is activated to generate more adsorbed oxygen atoms. More oxygen atoms adsorbed on Ni can oxidize hydrocarbon species to generate CO, thereby reducing the formation of carbon deposition and stabilizing the catalyst. In addition, the surface of the Ni particle can be modified or covered with an oxide, a molecular sieve or a two-dimensional material to form a core-shell structure of a metal @ shell layerAnd (5) forming. The method can effectively prevent the growth and aggregation of metal particles under the action of the confinement effect of a surface shell layer, ensures the catalytic activity and the carbon deposition resistance due to good dispersity, and is proved to be an effective method for improving the Ni-based catalyst. Conventionally used capping layers include oxides and molecular sieves, such as SiO₂，Al₂O₃SBA-15, KIT-6 and the like, the preparation methods of the materials are complex, the materials can be realized by a multi-step process, the cost of raw materials is high, the macro preparation of the catalyst is difficult to realize, and the method is not suitable for industrial popularization and application.

Disclosure of Invention

The problems solved by the invention are as follows: considering the current development situation of a methane dry gas reforming reaction catalyst, overcoming the defects of the prior art, and developing a catalyst which is applicable to various transition metals and has metal @ BO_xThe preparation method of the core-shell structure nano material is simple and feasible, and can be used as a catalyst for catalyzing the methane and carbon dioxide dry gas reforming reaction. The unique structure of the catalyst can prevent the sintering of the active components of the catalyst, and the catalyst has no activity loss after long-time reaction and excellent carbon deposition resistance.

In one aspect, the invention provides a transition metal @ BO for use in methane dry gas reforming reactions_xA core-shell structured nanocatalyst comprising a core-shell structure and a support: the core structure is transition metal nano particles, and the mass fraction of the transition metal nano particles is 0.4-16%; the shell structure is boron oxide; the carrier is boron nitride.

The metal particles in the core-shell structure are one of Ni, Fe, Co, Ru and Cu, and Ni is the transition metal of the optimal methane dry gas reforming core-shell structure nano catalyst.

Boron oxide composition is denoted BO_xThe thickness is controllable and on a nanoscale scale, typically within 2 nm.

The loading capacity is mass loading capacity, and the calculation mode is metal mass/carrier mass. The particle size of the catalyst is directly related to the loading amount of metal, the mass fraction of the Ni catalyst is between 0.4 and 16 percent, and the particle size of the metal is between 5 and 50 nm.

Another aspect of the invention provides forTM @ BO in methane dry gas reforming reaction_xThe preparation method of the core-shell structure nano catalyst comprises the following steps: (1) dissolving a metal precursor by using an alcohol solvent, adding a carrier, uniformly stirring to obtain a mixed solution, soaking and stirring the mixed solution at room temperature for 12-24h, and then drying in an oven at 60-100 ℃ for 12-24h to obtain a TM/BN catalyst; (2) introducing H into the TM/BN catalyst obtained in the above (1)₂Treating at 500 deg.c for 2 hr at flow rate of 80-120ml/min, and treating at 600-900 deg.c in weak oxidizing atmosphere for 1-10 hr at flow rate of 80-120ml/min to obtain the core-shell structure catalyst.

In the step (1), the alcohol solvent is at least one of ethanol, propanol and isopropanol, and the metal precursor can be at least one of nitrate, sulfate, chloride or acetate of the metal;

the weak oxidizing atmosphere in the step (2) is pure Ar, pure He or CO diluted by inert gas with different concentrations₂。

The preparation process is characterized in that the thickness of a shell layer is adjusted in a nanometer scale by changing the temperature and the type of the treatment atmosphere: the higher the temperature, the more oxidizing the atmosphere and the thicker the shell.

The invention also provides application of the core-shell structure nano catalyst in a methane dry gas reforming reaction.

Compared with the prior art, the invention has the advantages that: the boron nitride supported metal core-shell structure nano catalyst with unique structure is obtained by a simple and easy method. The catalyst shows good sintering resistance and carbon deposition resistance in the methane dry gas reforming reaction, and the reaction activity is still stable for a long time. This is the first public report of nano BO_xThe preparation method of the layer-modified metal nano catalyst is simple and feasible, has simple raw materials and strong universality, and has great potential for other catalytic reactions.

Drawings

FIGS. 1a and 1b are respectively 4 wt% Ni @ BO in example 1_xAn X-ray diffraction pattern and a transmission electron micrograph of the/h-BN sample;

FIG. 2 is the 4 wt% Ni @ BO of example 1_xh-BN samplesHigh resolution transmission electron microscopy images;

FIGS. 3a and 3b are respectively 4 wt% Ni @ BO in example 1_xA high-resolution transmission electron microscope image and a corresponding micro-area electron energy loss spectrum (eels) image of the/h-BN sample;

FIG. 4 shows 4 wt% Ni/h-BN and 4 wt% Ni @ BO of example 1_xa/h-BN sample X-ray photoelectron spectroscopy analysis chart;

FIG. 5 is 4 wt% Ni @ BO of comparative example 1_xA high-resolution transmission electron microscope image of the/H-BN-800H sample;

FIG. 6 is the 4 wt% Ni @ BO of example 4_xA high-resolution transmission electron microscope image of the/h-BN sample;

FIG. 7 is the 4 wt% Ni @ BO of example 10_xA methane dry gas reforming reaction performance and stability chart of a/h-BN sample;

FIG. 8 is the 4 wt% Ni @ BO after 40h dry gas reforming reaction in example 10_xThermogravimetric plot of/h-BN sample;

FIG. 9 is the 4 wt% Ni @ BO of example 11_xA catalytic reaction performance diagram of the methane dry gas reforming reaction of the/h-BN sample under different space velocities.

Detailed Description

The present invention is described in detail below by way of examples, but the scope of the claims of the present invention is not limited to these examples. Meanwhile, the embodiments only give some conditions for achieving the purpose, and do not mean that the conditions must be met for achieving the purpose.

Example 1

1. Ultrasonic dissolving 2g of nickel nitrate hexahydrate in 50g of ethanol, adding 10g of hexagonal boron nitride, fully stirring, and slowly volatilizing the solvent at room temperature. Drying in a drying oven at 60 ℃ for 24h to obtain 4 wt% Ni/h-BN;

2. introducing 100mL/min of H into the 4 wt% Ni/H-BN sample₂Treating at 500 deg.C for 2 h; then cut into 100mL/min of 5% CO₂He, heating to 800 ℃ for 2h to obtain 4 wt% Ni @ BO_xthe/h-BN core-shell structure nano catalyst. The Ni particle size in the catalyst is about 15 nm. The characterization of an X-ray diffraction spectrum (figure 1a) and a transmission electron microscope (figure 1b) shows that the catalyst obtained by synthesisThe medium Ni particles are supported on hexagonal boron nitride, the particle size is about 15nm, and the analysis of a high-resolution transmission electron microscope (figure 2) and a local electron energy loss spectrum (eels) (figures 3a and 3b) shows that the surface of the Ni nano particles is coated by boron-containing oxide, the thickness is about 1.3nm, the lattice spacing in the particles is 0.21nm, the particles can be assigned as Ni (111) crystal faces, and the particles are of a typical core-shell structure. X-ray photoelectron spectroscopy showed that the Ni particle surface was mostly reduced (fig. 4) and that there was little Ni loss after high pressure CO purging. This indicates that the boron oxide of the outer layer can protect the Ni particles from oxidation by air and prevent the loss of Ni element to volatile nickel carbonyl in a CO atmosphere. The Ni particle size did not change much upon prolonged calcination at high temperature (FIG. 1a), indicating that Ni @ BO_xThe core-shell structure has good sintering resistance.

Comparative example 1

1. Ultrasonic dissolving 2g of nickel nitrate hexahydrate in 50g of ethanol, adding 10g of hexagonal boron nitride, fully stirring, and slowly volatilizing at room temperature for 12 hours until the mixture is dry. Drying in a drying oven at 60 ℃ for 24h to obtain 4 wt% Ni/h-BN;

2. introducing 100mL/min of H into the 4 wt% Ni/H-BN sample₂Directly heating to 800 ℃ for treatment for 2h to obtain 4 wt% Ni @ BO_xH-BN-800H material.

Analysis of a high-resolution transmission electron microscope (figure 5) shows that Ni particles in a Ni/H-BN-800H sample show that no boron oxide shell layer exists, which indicates that the reducing gas treatment cannot synthesize Ni @ BO_xA catalyst with a core-shell structure. And the Ni surface interplanar spacing is attributed to the NiO (101) plane, the grain diameter is about 20nm, and is larger than that of the sample (15nm) in the example 1, which indicates that the Ni particles have certain surface oxidation and sintering.

Example 2

1. 0.2g of nickel nitrate hexahydrate is ultrasonically dissolved by 50g of ethanol, 10g of hexagonal boron nitride is added, and the mixture is slowly volatilized for 12 hours to be dried after being fully stirred. Drying in a drying oven at 60 ℃ for 24h to obtain 0.4 wt% Ni/h-BN;

2. introducing 100mL/min of H into the sample₂Treating at 500 deg.C for 2 h; then cut into 100mL/min of 5% CO₂He, heating to 800 ℃ for 2h to obtain 0.4 wt% Ni @ BO_xthe/h-BN core-shell structure nano catalyst. The Ni particle size in the catalyst is about 5 nm.

Example 3

1. And ultrasonically dissolving 8g of nickel nitrate hexahydrate by using 100g of ethanol, adding 10g of hexagonal boron nitride, fully stirring, and slowly volatilizing for 12 hours until the mixture is dry. Drying in a drying oven at 60 ℃ for 24h to obtain 16 wt% Ni/h-BN;

2. introducing 100mL/min of H to the 16 wt% Ni/H-BN sample₂Treating at 500 deg.C for 2 h; then cut into 100mL/min of 5% CO₂He, heating to 800 ℃ for 2h to obtain 16 wt% Ni @ BO_xthe/h-BN core-shell structure nano catalyst. The Ni particle size in the catalyst is about 50 nm.

Example 4

1. Ultrasonic dissolving 2g of nickel nitrate hexahydrate in 50g of ethanol, adding 10g of hexagonal boron nitride, fully stirring, and slowly volatilizing for 12 hours until the mixture is dry. Drying in a drying oven at 60 ℃ for 24h to obtain 4 wt% Ni/h-BN;

2. introducing 100mL/min of H into the 4 wt% Ni/H-BN sample₂Treating at 500 deg.C for 2 h; then cutting into pure-grade Ar gas of 100mL/min, heating to 800 ℃ and processing for 2h to obtain 4 wt% Ni @ BO_xthe/h-BN core-shell structure nano catalyst. The Ni particle size in the catalyst is about 15 nm.

Analysis of a high-resolution transmission electron microscope (fig. 6) shows that the surface of the Ni nano particle is wrapped by the oxide containing boron, the core-shell structure is similar to that of the sample in the example 1, but the shell layer thickness is about 0.8nm, the middle crystal face is the Ni (111) face, and the shell layer thickness can be controlled by changing the pretreatment condition.

Example 5

1. Dissolving 100mg of nickel nitrate hexahydrate in 50g of ethanol by ultrasonic waves, adding 500mg of porous boron nitride (p-BN), fully stirring, and slowly volatilizing for 12 hours until the mixture is dry. Drying in a drying oven at 60 ℃ for 24h to obtain 4 wt% Ni/p-BN;

2. introducing 100mL/min H into the 4 wt% Ni/p-BN sample₂Treating at 500 deg.C for 2 h; then cutting into pure Ar gas of 100mL/min, heating to 900 ℃ and processing for 2h to obtain 4 wt% Ni @ BO_xThe p-BN nuclear shell nano catalyst. The Ni particle size in the catalyst is about 3 nm.

Example 6

1. Dissolving 2g of cobalt nitrate hexahydrate in 50g of ethanol by ultrasonic waves, adding 10g of hexagonal boron nitride, fully stirring, and slowly volatilizing at room temperature for 12 hours until the mixture is dry. Drying in a drying oven at 60 ℃ for 24h to obtain 4 wt% Co/h-BN;

2. introducing the sample into a reactor with the flow rate of 100mL/min H₂Treating at 500 deg.C for 2 h; then cutting into pure Ar gas of 100mL/min, heating to 900 ℃ and processing for 2h to obtain 4 wt% Co @ BO_xthe/h-BN core-shell structure nano catalyst.

Example 7

1. 2.87g of ferric nitrate hexahydrate is ultrasonically dissolved by 50g of ethanol, 10g of hexagonal boron nitride is added, and after the mixture is fully stirred, the mixture is slowly volatilized at room temperature for 12 hours until the mixture is dried. Drying for 24h in a drying oven at 60 ℃ to obtain a 4 wt% Fe/h-BN material;

2. introducing 100mL/min H into the 4 wt% Fe/H-BN sample₂Treating at 500 deg.C for 2 h; then cutting into pure-grade Ar gas of 100mL/min, heating to 900 ℃ and processing for 2h to obtain 4 wt% Fe @ BO_xthe/h-BN core-shell structure nano catalyst.

Example 8

1. Dissolving 1g of ruthenium chloride trihydrate with 50g of ethanol by ultrasonic waves, adding 10g of hexagonal boron nitride, fully stirring, and slowly volatilizing to be dry at room temperature. Drying in a drying oven at 60 ℃ for 24h to obtain 4 wt% Ru/h-BN;

2. introducing 100mL/min of H into the 4 wt% Ru/H-BN sample₂Treating at 500 deg.C for 2 h; cutting into pure-grade Ar gas of 100mL/min, heating to 900 ℃ and processing for 2h to obtain 4 wt% Ru @ BO_xthe/h-BN core-shell structure nano catalyst.

Example 9

Two catalysts from example 1 were used 4 wt% Ni/h-BN and 4 wt% Ni @ BO_xEach 10mg of the/H-BN is measured, and H is firstly used₂Prereduction was carried out at 450 ℃ for 2 h. Introducing CO/H at 150 DEG C₂(the ratio is 1:3), increasing the pressure to 1Mpa, purging for 3 hours, and performing EDX analysis on the catalyst before and after treatment to compare the content of the Ni element in the catalyst.

Table 1 shows the results of EDX analysis of the Ni content of the sample in example 1 after the treatment in a high-pressure CO atmosphere

Elemental analysis showed that the 4 wt% Ni/h-BN sample lost almost all of the metallic nickel after being purged with high pressure CO gas, but 4 wt% Ni @ BO_xMetallic nickel of/h-BN sample due to surface BO_xThe protective effect of (B) is largely still on the support, and this result indicates BO_xThe protective effect of (1).

Example 10

1. Weighing 4 wt% of Ni @ BO_x40mg of/h-BN catalyst, 20-40 meshes of granulation and filling in a miniature vertical fixed bed quartz tube reactor. Introducing pure hydrogen gas at 20mL/min, and pretreating at 500 ℃ for 1 h. Heating to 750 deg.C in high-purity Ar, and cutting into methane dry gas reforming reaction gas (CO)₂/CH₄/N ₂4/4/2), adjusting the mass space velocity 60000mL/g_catH. The generated product is analyzed on line by Agilent GC7890 chromatography, a HayeSep D chromatographic column and a thermal conductivity cell detector are arranged, the sample continuously reacts for 40 hours, and CO is monitored₂And CH₄Change in conversion and H in product₂The ratio of/CO.

2. In order to obtain the carbon deposition condition on the surface of the catalyst after the reaction, the catalyst is subjected to thermogravimetric analysis after the reaction for 40 hours. The sample dosage is 10mg, the testing atmosphere adopts air, the heating rate is 10 ℃/min, the temperature is increased to 900 ℃, and the weight change of the catalyst is monitored.

FIG. 7 is the 4 wt% Ni @ BO of example 10_xEvaluation results of the activity and stability of the/h-BN catalyst for the methane dry gas reforming reaction. It can be seen that the activity of the sample tends to be stable after an activation period of 2h, and the initial CO of the catalyst₂Conversion 77% CH₄The conversion was 67%, and the reaction stability was very good for a long period of time with almost no loss of activity. After 40h of continuous reaction, CO₂The conversion of (C) is still more than 76%, CH₄The conversion of (c) was still close to 67%. FIG. 8 is the carbon deposition amount analysis of the sample after 40h reaction, and it can be seen that the carbon deposition amount is only 1.3%, which indicates good carbon deposition resistance of the sample. The above results illustrate Ni @ BO_xthe/h-BN catalyst can be used as an excellent catalyst for methane dry gas reforming reactionAnd has potential industrial application value.

Example 11

1. Weighing 4 wt% of Ni @ BO_x40mg of/h-BN catalyst, 20-40 meshes of granulation and filling in a miniature vertical fixed bed quartz tube reactor. Introducing pure hydrogen gas at 20mL/min, and pretreating at 500 ℃ for 1 h. Heating in Ar to 750 deg.C, and cutting into methane dry gas reforming reaction gas (CO)₂/CH₄/N ₂4/4/2), adjusting the mass space velocity of 15000mL/g_catH, after 15h of reaction the air speed was adjusted to 375000mL/g_catH. The generated product is analyzed on line by Agilent GC7890 chromatography, a HayeSep D chromatographic column and a thermal conductivity cell detector are arranged, the sample continuously reacts for 40 hours, and CO is monitored₂And CH₄Change in conversion and H in product₂The ratio of/CO.

FIG. 9 is the 4 wt% Ni @ BO of example 11_xEvaluation results of the activity and stability of the/h-BN catalyst for the methane dry gas reforming reaction. It can be seen that at different space velocities, the samples all have long-term stable catalytic activity, indicating 4 wt% Ni @ BO_xthe/h-BN catalyst is suitable for methane dry gas reforming reaction under different conditions.

In conclusion, the beneficial effects of the invention are as follows: the metal @ boron oxide core-shell structure nano catalyst is simply and directly synthesized, the defects that the previous core-shell material is complex in synthesis steps, numerous in required raw materials and incapable of modifying metal nano particles on a nano scale are overcome, the universality is high, and the method can be popularized to most common transition metals. The defects of easy sintering and carbon deposition of the Ni-based catalyst in the methane dry gas reforming reaction are effectively overcome, and the catalyst has high activity and stability for the methane dry gas reforming reaction and great potential. The boron nitride is used as a carrier for methane dry gas reforming reaction for the first time, and has great industrial application potential.

Claims

1. A core-shell structured nanocatalyst, characterized by comprising a core-shell structure and a support: the core structure is transition metal nano particles, and the transition metal nano particles are 0.4-16% of the carrier by mass fraction; the shell structure is boron oxide; the carrier is boron nitride.

2. The catalyst of claim 1, wherein: the transition metal nano particles are Ni, Fe, Co, Ru or Cu; the particle size of the nanoparticles is between 3nm and 50 nm.

3. The catalyst of claim 1, wherein: the boron nitride carrier is hexagonal boron nitride or porous boron nitride.

4. The catalyst of claim 1, wherein: the shell structure has a thickness within 2 nm.

5. The method for preparing the core-shell structured nanocatalyst of any of claims 1 to 4, characterized by comprising the steps of:

(1) dissolving a metal precursor by using an alcohol solvent, adding a carrier, uniformly stirring to obtain a mixed solution, soaking and stirring the mixed solution at room temperature for 12-24h, and then drying the mixed solution in an oven at the temperature of 60-100 ℃ for 12-24h to obtain a TM/BN catalyst;

(2) introducing H into the TM/BN catalyst obtained in the above (1)₂Treating at 500 deg.C for 1-3h at flow rate of 80-120ml/min, and treating at 600-900 deg.C in weak oxidizing atmosphere for 1-10h at flow rate of 80-120ml/min to obtain the core-shell structure catalyst.

6. The method of claim 5, wherein: the alcohol solvent is at least one of ethanol, propanol and isopropanol.

7. The method of claim 5, wherein: the metal precursor is at least one of nitrate, sulfate, chloride or acetate of the metal.

8. The method of claim 5, wherein: the weak oxidizing atmosphere is pure-grade N₂Pure and pureStage Ar and inert gas diluted CO of different concentrations₂At least one of (1).

9. The production method according to claim 8, wherein the inert gas is at least one of high-purity Ar and high-purity He.

10. Use of the core-shell structured nanocatalyst as defined in any of claims 1-4 as a catalyst for methane carbon dioxide dry gas reforming reactions.