CN109954507B

CN109954507B - Ni-Rh/αβ-MoXC composite catalyst, preparation and application

Info

Publication number: CN109954507B
Application number: CN201910304432.4A
Authority: CN
Inventors: 马保军; 杨旭
Original assignee: Ningxia University
Current assignee: Ningxia University
Priority date: 2019-04-16
Filing date: 2019-04-16
Publication date: 2021-09-10
Anticipated expiration: 2039-04-16
Also published as: CN109954507A

Abstract

The invention discloses a preparation method and application of a carbon dioxide hydrogenation methanation catalyst, wherein the catalyst takes alpha-phase molybdenum carbide and beta-phase molybdenum carbide coexisting molybdenum carbide as a carrier and Ni and Rh bimetal as active components to form Ni-Rh/alpha beta-Mo_XC, the composite catalyst is used for the reaction of preparing methane by carbon dioxide hydrogenation. Ni-Rh/alpha beta-Mo of the invention_XThe catalyst C has higher activity of preparing methane by carbon dioxide hydrogenation. Wherein, at the best loading, the composite catalyst Ni-Rh/alpha beta-Mo_XThe methane selectivity of C is as high as 86%, which is higher than that of alpha beta-Mo in a single coexisting phase_XC is increased by about 16%, and the catalyst shows excellent catalytic performance. The preparation and modification methods of the catalyst are cheap and simple, and the obtained catalyst has uniform and controllable morphology, good stability and good application prospect.

Description

Ni-Rh/αβ-MoXC composite catalyst, preparation and application

Technical Field

The invention relates to Ni-Rh/alpha beta-Mo_XPreparation of the C composite catalyst and application thereof in preparing methane by carbon dioxide hydrogenation.

Background

As the concentration of carbon dioxide in the atmosphere increases year by year, the problems of global warming, climate change, etc. brought by the increase seriously affect the activities and ecological environment of human beings. In order to reduce the concentration of carbon dioxide in the atmosphere, a great deal of scientists have led to research on the separation, storage and conversion utilization of carbon dioxide, and particularly in the field of catalytic hydrogenation of carbon dioxide, how to realize efficient chemical conversion of carbon dioxide has become a hot research spot. Natural gas is used as an efficient and clean fuel, the proportion of energy utilization is continuously increased, the development of coal-based natural gas is one of important methods for solving the energy problem in China, and the natural gas industry in China will enter a period of accelerating development.

In the traditional carbon dioxide methanation catalyst, most of the catalyst is composed of alumina as a carrier and single nickel or nickel plus a rare earth element as an active component. These types of catalysts have relatively low catalytic activity, requiring relatively high pressures, low space velocities, excess hydrogen, and the likeThe harsh reaction conditions cause the results of high input and low output, and the development of the novel methanation catalyst is a hot point of research of researchers. In recent years, molybdenum carbide has been applied to the reaction of preparing methanol by hydrogenating carbon dioxide and reverse water gas, and shows excellent and stable conversion rate and stability, such as: pororsoff et al prepared beta-Mo₂C catalyst, and loading metal Co to beta-Mo by evaporation deposition impregnation method₂On the C catalyst, the catalytic performance of the catalyst in RWGS reaction is examined under the conditions of normal pressure and 300 ℃, and the research finds that beta-Mo₂The C catalyst has higher catalytic activity and is far higher than a bimetallic catalyst containing noble metal under the same condition; after a small amount of Co is loaded, the activity of the catalyst is further improved. However, the application of molybdenum carbide in the preparation of methane from carbon dioxide is only rarely reported, and Ni-Rh/alpha beta-Mo is synthesized by using a dipping reduction method_XThe C composite catalyst has higher activity when being applied to the methanation reaction of carbon dioxide.

Disclosure of Invention

The invention aims to provide Ni-Rh/alpha beta-Mo_XThe method is simple and easy to implement, and does not need complex and expensive equipment. The prepared catalyst has high methane selectivity.

The technical scheme of the invention is as follows:

alpha beta-Mo for preparing methane by carbon dioxide hydrogenation_XA composite catalyst in which Ni and Rh are supported on the surface of C (X-1-2), wherein a transition metal Ni and a noble metal Rh are simultaneously supported on alpha beta-Mo_XAnd C, adding C. Wherein, alpha beta-Mo_XC is a coexisting phase of alpha-phase molybdenum carbide and beta-phase molybdenum carbide, the loading capacity of the Ni auxiliary agent is 1-10 wt%, and the loading capacity of the Rh auxiliary agent is 0.1-5 wt%.

Preferably, the loading of the Ni promoter is 3% wt and the loading of the Rh promoter is 1% wt.

The alpha beta-Mo prepared by the invention_XThe C (X ═ 1-2) nanowire (with the length of 100-In the process, the carbon dioxide is adsorbed on the surface of the active site of the catalyst and activated by C-O bonds, so that the conversion rate of the reaction raw materials is improved.

Further, the α β -Mo_XThe C nanowire needs to be subjected to surface pretreatment before Ni and Rh auxiliary agents are uniformly loaded by adopting a dip precipitation method, and the specific pretreatment steps are as follows: taking alpha beta-Mo_XDispersing the C in a solvent, carrying out ultrasonic treatment for 1-3h, washing with water and absolute ethyl alcohol, then drying in an oven at 120 ℃ for 6-24 h, taking out and grinding for later use.

Further, the α β -Mo_XAfter the surface of C (X is 1-2), uniformly loading Ni and Rh auxiliaries by adopting a dipping precipitation method, wherein the dipping method comprises the following specific steps: preparing nickel nitrate solution and rhodium chloride solution according to the required Ni and Rh loading amount, and carrying out surface pretreatment on the alpha beta-Mo_XDispersing C in a mixed solution of a nickel precursor and a rhodium precursor, uniformly dispersing Ni and Rh ions on the outer surface of the nanowire through repeated processes of 'ultrasonic stirring and standing', drying the obtained dispersion in a water bath kettle at 50-100 ℃, grinding and reducing with hydrogen to obtain the catalyst.

The ultrasonic stirring-standing operation is carried out repeatedly, specifically after 5-30 minutes of ultrasonic treatment, the ultrasonic frequency is 40KHz, the dispersion is stirred for 5-30 minutes by adopting mechanical stirring, the stirring speed is 1200r/min, the dispersion is stirred for 5-30 minutes by adopting mechanical stirring, then standing is carried out for 20-60 minutes, and then the ultrasonic treatment of the next cycle is carried out, and the operation is repeated for 3-5 times. Can ensure that Ni and Rh ions are uniformly adsorbed and dispersed in alpha beta-Mo_XThe depressed positions on the surface of C are dried and reduced, and the particle diameters of Ni and Rh are small and are uniformly distributed on alpha beta-Mo_XThe surface of C is beneficial to increasing the number of effective synergistic action sites.

Further, the hydrogenation of the carbon dioxide to prepare Ni-Rh/alpha beta-Mo in the methane_XC, the preparation steps of the composite catalyst are as follows:

(1) synthesis of alpha and beta coexisting phase alpha beta-Mo_XPrecursor of C: ammonium molybdate was dissolved in distilled water and aniline was added with stirring. Slowly dripping 1mol/L HCl solution into the above solution, transferring into 60-100 deg.C water bath, and stirring for 5-12 hrWashing with ethanol, filtering, and drying. Thus obtaining alpha beta-Mo_XAnd C, precursor.

(2) Synthesis of noble metal alpha beta-Mo_XC, catalyst: grinding the dried sample in the step (1), putting the ground sample into a quartz boat, transferring the quartz boat into a tube furnace, and obtaining the sample with the concentration of 5% H₂/N₂And (3) calcining by controlling the temperature through a program and setting a target temperature under the atmosphere. The calcination time is 3-6h, the temperature is 740 and 850 ℃, and finally the temperature is 1 percent of O₂/N₂Passivating for 1-5h to obtain alpha beta-Mo_XAnd C (X is 1-2) catalyst.

(3) Simultaneously loading transition metal Ni and noble metal Rh on a noble metal-like catalyst alpha beta-Mo_XC, on: alpha beta-Mo after pretreatment by dipping precipitation method_XLoading Ni and Rh auxiliary agents with different contents on the surface of C, configuring a mixed solution of a nickel precursor and a rhodium precursor according to the required Ni and Rh loading amount, and carrying out surface pretreatment on the alpha beta-Mo obtained in the step (2)_XDispersing the C nanowire in the mixed solution of the nickel-rhodium precursor, and uniformly dispersing Ni and Rh ions in alpha beta-Mo through repeated 'ultrasonic-stirring-standing' process_XAnd C, placing the obtained dispersion liquid in a 50-100 ℃ water bath for drying, grinding, and reducing by hydrogen (350-500 ℃) to obtain the catalyst.

Preferably, the α β -Mo is_XC (X ═ 1-2) catalyst, wherein the ratio of alpha-phase molybdenum carbide to beta-phase molybdenum carbide is 6: 2.5-6: 3.2

In addition, the coexisting phase of the present invention, α β -Mo_XThe application of the composite catalyst of C with Ni and Rh loaded on the surface in the preparation of methane by carbon dioxide hydrogenation is characterized by comprising the following steps: the reaction for synthesizing methane by hydrogenation of carbon dioxide is carried out on a pressurized fixed bed continuous flow reactor. Tabletting and granulating the catalyst, uniformly mixing the catalyst with 20-40 mesh quartz sand, filling the mixture into a fixed bed reactor, and reacting the mixture in a reactor H₂Reducing at 400 ℃ for 1H, and then switching the gas to H₂/CO₂The raw material gas is reacted at a reaction condition of 0.5-2MPa, 200-temperature and 600-temperature, and an airspeed of 6000-temperature and 8000mg g^-1h^-1Introduction of CO into₂And (4) hydrogenation to prepare the methane.

Further, H in the raw material gas₂And CO₂Is 4: 1.

Preferably, the conditions for synthesizing methane by hydrogenating carbon dioxide are as follows: the reaction pressure is 1Mpa, the reaction temperature is 400-^-1h^-1。

Further, the feed gas volume space velocity was varied by varying the gas flow rate of the catalyst bed. In particular, a small amount of inert gas is added to the raw material gas as an internal standard substance to determine the relative content of each gas component in the product, and nitrogen is used as the internal standard substance in the invention.

The invention has the advantages of

The invention relates to Ni-Rh/alpha beta-Mo_XC composite catalyst, alpha beta-Mo_XC is a noble metal-like catalyst, and a small amount of noble metal Rh and transition metal Ni are loaded on the alpha beta-Mo_XOn C, the selectivity of methane can be obviously enhanced, and the reduction of the thermocatalysis cost is promoted.

The invention relates to Ni-Rh/alpha beta-Mo_XThe preparation method of the C composite catalyst is simple and easy to implement, complex and expensive equipment is not needed, large-scale production is facilitated, the obtained catalyst is uniform and controllable in appearance, good in stability and good in application prospect.

Ni-Rh/alpha beta-Mo of the invention_XThe C composite catalyst has higher methane selectivity. Wherein, at the best loading, the composite catalyst Ni-Rh/alpha beta-Mo_XThe methane selectivity of C is as high as 86%, which is higher than that of alpha beta-Mo in a single coexisting phase_XC is improved by about 16%, and the catalyst shows excellent catalytic performance.

Drawings

Figure 1 XRD patterns of the precursors at different calcination temperatures.

FIG. 2.Ni-Rh/Mo_XMapping graph of the C composite hydrogenation catalyst.

FIG. 3.Ni-Rh/Mo_XC, a catalytic activity diagram of the composite hydrogenation catalyst.

Detailed Description

The present invention is further illustrated by the following specific examples.

Example 1:

dioxide oxidationCarbon hydromethanation catalyst (Ni-Rh/alpha beta-Mo)_XC) The preparation method comprises the following steps:

(1) 2.48g of ammonium heptamolybdate was dissolved in 40mL of distilled water, and 3.2g of aniline was added thereto with continuous stirring. Slowly dropwise adding 1mol/L HCl solution into the solution, dropwise adding the solution to a pH value of 4.0-5.0, moving the solution into a 60 ℃ water bath kettle, continuously stirring for 5h, washing with water, washing with ethanol, performing suction filtration, and drying overnight. Thus obtaining alpha beta-Mo_XC precursor (length 100-250nm, diameter 2-10 nm).

(2) Grinding the dried sample in the step (1), putting the ground sample into a quartz boat, transferring the quartz boat into a tube furnace, and obtaining the sample with the concentration of 5% H₂/N₂And (3) calcining by controlling the temperature through a program and setting a target temperature under the atmosphere. Wherein the heating rate is 3 ℃/min, the calcination time is 4h, the temperature is 800 ℃, and finally the temperature is 1 percent O₂/N₂Passivating in passivating gas for 1h to obtain alpha beta-Mo_XAnd C (X ═ 1-2) catalyst, wherein the ratio of the alpha-phase molybdenum carbide to the beta-phase molybdenum carbide is 6: 2.5-6: 3.2.

(3) First 0.077g of nickel nitrate hexahydrate is weighed into 30mL of deionized water, then a rhodium chloride solution with a mass concentration of 5mg/mL (5 mg of rhodium in 5mL of rhodium chloride solution) is added, and 0.5g of alpha beta-Mo is added with stirring_XAnd C, repeatedly carrying out 'ultrasound-stirring-standing' operation, putting the solution into an ultrasonic machine for ultrasonic treatment for 30 minutes at the ultrasonic frequency of 40KHz, stirring the dispersion liquid for 30 minutes by adopting mechanical stirring at the stirring speed of 1200r/min, standing for 30 minutes, then carrying out next cycle of ultrasonic treatment, and repeating the operation for 3 times. Then transferring the mixture into a water bath kettle, evaporating the mixture in a water bath at 80 ℃ to dryness, and grinding the powdery catalyst in a mortar for 3 minutes. Putting the sample into a quartz boat, transferring the quartz boat into a tube furnace, calcining the sample for 3h at 400 ℃ in a hydrogen atmosphere at a gas flow rate of 30ml/min and a temperature rise rate of 5 ℃/min, and transferring the sample into 1% O after the temperature is reduced to room temperature₂/N₂Passivating for 1h by using passivating gas to obtain 3 percent of Ni-1 percent of Rh/alpha beta-Mo_XC, a composite hydrogenation catalyst.

The application of the catalyst comprises the following steps:

weighing the catalyst prepared in the step (3), tabletting and granulating the catalyst, uniformly mixing the catalyst with quartz sand with the particle size of 20-40 meshes, filling the mixture into a fixed bed reactor,at H₂Reducing at 400 deg.C for 0.5H, and then switching gas to H₂/CO₂The pressure of the raw material gas is 1MPa, the reaction temperature is 400-600 ℃, and the space velocity of the raw material is 6000mL g^-1h^-1The catalytic carbon dioxide hydrogenation reaction is carried out under the condition of (1) to prepare the methane.

Example 2

Preparation of the catalyst with reference to example 1, except that 0.5ml/mg of rhodium chloride was added in step (3), the final rhodium loading in the catalyst was 0.1% wt, reported as 3% Ni-0.1% Rh/α β -Mo_XC, the rest steps are the same as the example 1.

Example 3

Preparation of the catalyst with reference to example 1, except that in step (3) rhodium chloride was added at 15ml/mg, the final rhodium loading in the catalyst was 3% wt, reported as 3% Ni-3% Rh/α β -Mo_XC, the rest steps are the same as the example 1.

Example 4

Preparation of the catalyst with reference to example 1, except that 25ml/mg of rhodium chloride was added in step (3), the final rhodium loading in the catalyst was 5 wt%, reported as 3% Ni-5% Rh/α β -Mo_XC, the rest steps are the same as the example 1.

Example 5

Catalyst preparation reference example 1 was made except that 0.025g of nickel nitrate was added in step (3) and the final nickel loading in the catalyst was 1% wt, reported as 1% Ni-1% Rh/α β -Mo_XC, the rest steps are the same as the example 1.

Example 6

Catalyst preparation reference example 1 was made except that 0.13g of nickel nitrate was added in step (3) and the final nickel loading in the catalyst was 5% wt, reported as 5% Ni-1% Rh/α β -Mo_XC, the rest steps are the same as the example 1.

Example 7

Catalyst preparation reference was made to example 1 except that 0.275g of nickel nitrate was added in step (3) to finally obtain a catalyst having a nickel loading of 10% wt, recorded as 10% Ni-1% Rh, and the remaining steps were the same as in example 1.

Example 8

Catalyst preparation referring to example 1, except that the calcination temperature in step (2) was 760 ℃, catalyst 3% Ni-1% Rh/α β -Mo was finally obtained_XC_6:1.5(molar ratio α: β ═ 6:1.5), the rest of the procedure was the same as in example 1.

Example 9

Catalyst preparation referring to example 1, except that the calcination temperature in step (2) was 820 ℃, catalyst 3% Ni-1% Rh/α β -Mo was finally obtained_XC_1.2:1(molar ratio α: β ═ 1.2:1), the rest of the procedure was the same as in example 1.

Example 10

Catalyst preparation referring to example 1, except that the calcination temperature in step (2) was 850 ℃, the catalyst 3% Ni-1% Rh/α β -Mo was finally obtained_XC_1:5.8(molar ratio α: β ═ 1:5.8), and the rest of the procedure was the same as in example 1.

Comparative example 1

Preparation of the catalyst with reference to example 1, except that 35ml/mg of rhodium chloride was added in step (3), the final rhodium loading in the catalyst was 7 wt%, reported as 3% Ni-7% Rh/α β -Mo_XC, the rest steps are the same as the example 1.

Comparative example 2

Catalyst preparation reference example 1 was made except that 0.012g of nickel nitrate was added in step (3) to finally obtain a catalyst having a nickel loading of 0.5% wt, noted as 0.5% Ni-1% Rh/α β -Mo_XC, the rest steps are the same as the example 1.

Comparative example 3

Catalyst preparation referring to example 1, except that the calcination temperature in step (2) was 880 ℃, the catalyst 3% Ni-1% Rh/α β -Mo was finally obtained_XC_0.5:6(molar ratio α: β ═ 0.5:6), the rest of the procedure was the same as in example 1.

Comparative example 4

Preparation of catalyst referring to example 1, except that the calcination temperature in step (2) was 600 deg.C, pure α -MoC was obtained_1-xFinally, the catalyst 3% Ni-1% Rh/α -MoC was prepared, and the remaining steps were the same as in example 1.

Comparative example 5

Preparation of catalyst referring to example 1, except that the calcination temperature in step (2) was 900 deg.C, pure β -Mo was obtained₂C, finally preparing the catalyst 3% Ni-1% Rh/beta-Mo₂C, the rest steps are the same as the example 1.

Sample evaluation

The finished catalysts obtained in examples 1-10 and comparative examples 1-5 were respectively loaded in a fixed bed reactor (the reduction process was the same as in example 1) under the condition that the space velocity of the raw material was 6000mL g^-1h^-1The pressure is 1MPa, the reaction temperature is 400-₂/CO₂4: 1; the CO of each catalyst at 500 ℃ is respectively considered₂Conversion rate, CH₄The results of selectivity and methane space-time yield evaluations are shown in tables 1 and 2.

Table 1 examples 1-10 catalyst catalysis results

Table 2 comparative examples 1-5 catalyst catalysis results

From the results of the carbon dioxide hydrogenation reactions of the catalysts of examples 1-7, it can be seen that CO increases with the loading of nickel and rhodium under the same reaction conditions₂Conversion rate, CH₄The selectivity and the methane space-time yield both show a trend of increasing and then decreasing, and the optimal values are reached when the addition amount of nickel is 3 wt% and the addition amount of rhodium is 1 wt%, which shows that the addition amounts of Ni and Rh addition agents are not more and better, namely

3％Ni-1％Rh/αβ-Mo_XThe C catalyst has higher reaction activity and methane selectivity.

The results of the carbon dioxide hydrogenation reactions over the catalysts of examples 8-10It is known that under the same reaction conditions for α β -Mo_XAnd the catalyst C has different proportions of alpha-phase molybdenum carbide and beta-phase molybdenum carbide due to different calcination temperatures, wherein the optimal proportion is that alpha to beta is 6: 2.5-3.5.

From the results of the carbon dioxide hydrogenation reaction of the catalysts of comparative examples 1 to 3, it can be seen that the catalysts outside the claims exhibit low methanation activity under the same reaction conditions.

As can be seen from the results of the carbon dioxide hydrogenation reaction of the catalysts of comparative examples 4 to 5, under the same reaction conditions, α β -Mo was obtained_XThe catalytic performance of C as a carrier is obviously higher than that of pure alpha-phase molybdenum carbide and beta-phase molybdenum carbide as carriers, and further proves that the alpha beta-Mo_XC has good methanation activity.

Fig. 1 is XRD of the precursor at different calcination temperatures. As can be seen in the figure, the peak at 700 ℃ shifts to the beta phase molybdenum carbide at 600 ℃ for alpha phase molybdenum carbide (standard card PDF #10-0457), and the peak at 800 ℃ for two phase mixed alpha beta-Mo_XC, 900 ℃ is pure beta-phase molybdenum carbide (standard card PDF #35-0787), and elementary Mo appears at 1000 ℃. Proves that the alpha beta-Mo with two coexisting phases is successfully synthesized at 800 DEG C_XC。

FIG. 2 shows Ni-Rh/α β -Mo_XMapping graph of the C composite hydrogenation catalyst. It can be seen from the figure that Ni and Rh have been successfully loaded on alpha beta-Mo_XC, and the dispersion is very uniform.

FIG. 3 shows Ni-Rh/α β -Mo_XC, a catalytic activity diagram of the composite hydrogenation catalyst. Reaction conditions are as follows: the airspeed of the raw material is 6000mL g^-1h^-1The pressure is 1MPa, the feed gas is H₂/CO₂4: 1. It can be seen from the figure that Ni-Rh/alpha beta-Mo occurs at reaction temperatures between 200 ℃ and 600 ℃_XCO of C composite hydrogenation catalyst₂Conversion and CH₄The selectivity is obviously higher than that of the coexisting phase alpha beta-Mo_XC and higher than single metal loaded alpha beta-Mo_XC。

Claims

1. Ni-Rh/alpha beta-Mo_XC composite catalyst, characterized in that: the Ni-Rh/alpha beta-Mo_XThe C composite catalyst is prepared by loading transition metal Ni and noble metal Rh onto alpha beta-Mo_XC, wherein X = 1-2; wherein, the carrier is alpha beta-Mo_XC is a coexisting phase of alpha-phase molybdenum carbide and beta-phase molybdenum carbide, the loading capacity of the Ni auxiliary agent in the composite catalyst is 1-10 wt%, and the loading capacity of the Rh auxiliary agent is 0.1-5 wt%; carrier alpha beta-Mo_XC is a nanowire with the length of 100-250nm and the diameter of 2-10 nm;

the alpha beta-Mo_XThe molar ratio of the alpha-phase molybdenum carbide to the beta-phase molybdenum carbide in the C carrier is 6: 1.5-1: 5.8.

2. A method for preparing the composite catalyst of claim 1, wherein:

the preparation steps are as follows,

(1) synthesis of alpha and beta coexisting phase alpha beta-Mo_XPrecursor of C: dissolving ammonium molybdate in distilled water, and adding one or two or three of aniline, ethylenediamine and hexadecylamine under stirring; dropwise adding 1-2 mol/L HCl solution into the solution until the pH is =4.0-5.0, then moving the solution into a water bath kettle at the temperature of 60-100 ℃, continuously stirring for 5-12h, washing with water, washing with ethanol, filtering, and drying; thus obtaining alpha beta-Mo_XC, precursor;

(2) synthesis of noble metal alpha beta-Mo_XC, catalyst: grinding the dried sample in the step (1), putting the ground sample into a quartz boat, and transferring the quartz boat into a tube furnace, wherein the volume concentration of H is 5-10 percent₂/N₂Setting a target temperature for calcination under an atmosphere; the calcination time is 3-6h, the temperature is 740-850 ℃, and finally the volume concentration is 1-5 percent of O₂/N₂Passivating for 1-5h to obtain alpha beta-Mo_XC, a catalyst;

(3) simultaneously loading transition metal Ni and noble metal Rh on a noble metal-like catalyst alpha beta-Mo_XC, on: alpha beta-Mo after pretreatment by dipping precipitation method_XLoading Ni and Rh auxiliaries with different contents on the surface of the C, and preparing a mixed solution of a nickel precursor and a rhodium precursor according to the required Ni and Rh loading amounts; dispersing in nickel-rhodium precursor mixed solution, and uniformly dispersing Ni-Rh ions in alpha beta-Mo through repeated 'ultrasonic-stirring-standing' process_XC outer surface, placing the obtained dispersion liquid in 50-Drying in a water bath kettle at 100 ℃, grinding, and reducing with hydrogen to obtain the catalyst.

3. The method for preparing the composite catalyst according to claim 2, wherein: the alpha beta-Mo_XThe molar ratio of the alpha-phase molybdenum carbide to the beta-phase molybdenum carbide in the C carrier is 6: 1.5-1: 5.8.

4. The method for preparing the composite catalyst according to claim 2, wherein: the precursor of Ni is one or more of nickel sulfate, nickel nitrate, nickel chloride and nickel hydroxide; the precursor of Rh is one or more of rhodium chloride, ammonium chlororhodate, rhodium carbonyl and rhodium iodide.

5. The method for preparing the composite catalyst as claimed in claim 2, wherein the repeated "ultrasound-stirring-standing" process means that after ultrasound for 5-30 minutes, the ultrasound frequency is 40-60KHz, the dispersion is stirred by mechanical stirring for 5-30 minutes at a stirring speed of 600-1200r/min, then standing for 20-60 minutes, and then performing the next cycle of ultrasound, and repeating for 3-5 times.

6. The Ni-Rh/α β -Mo of claim 1_XApplication of C composite catalyst, namely Ni-Rh/alpha beta-Mo_XThe C composite catalyst is used in the reaction of preparing methane by carbon dioxide hydrogenation.

7. Use according to claim 6, characterized in that: the reaction for synthesizing methane by carbon dioxide hydrogenation is carried out on a pressurized fixed bed continuous flow reactor;

the conditions for synthesizing methane by hydrogenating carbon dioxide are as follows: the reaction pressure is 0.5-2Mpa, the reaction temperature is 400-600 ℃, and the airspeed is 6000-8000 mL g^-1 h^-1，n (CO₂) : n (H₂) The molar ratio is 1-4.

8. Use according to claim 6 or 7, characterized in that: tabletting and screening the catalyst by 20-40 meshes, wherein the catalyst is used after being reduced before reaction, and the reduction conditions of the catalyst are as follows: reducing with pure hydrogen at 300-400 ℃ for 1-3 h.