CN102626640B - A monolithic catalyst for low-temperature oxidation of methane and its preparation - Google Patents

Abstract

A monolithic catalyst for low temperature oxidation of methane comprises cordierite honeycomb ceramic carrier and a coating thereon. The coating has an expression of Pd / CoxCry-M / Al2O3, wherein Pd is noble metal palladium; CoxCry is cobalt chromium compound oxide, and a molar ratio of cobalt to chromium is between 0.001-100; M represents one or more doped metal selected from cerium, zirconium, iron, lanthanum, nickel and manganese, and a dopping amount (molar ratio) of M to (Co+Cr+M) is less than 0.10; and Al2O3 is gamma-Al2O3 with high thermal stability. The preparation method includes the following steps: step one, preparing the composite metal oxide CoxCry-M; step two, preparing the coating slurry; step three, dipping; step four, drying and calcining to obtain the monolithic catalyst. The catalyst of the invention can be used in methane purification of natural gas and vehicle tail gas and has good methane oxidation activity and thermal stability.

Description

A monolithic catalyst for low-temperature oxidation of methane and its preparation

technical field

The invention belongs to the technical field of environmental protection engineering and relates to the treatment of methane in the tail gas of natural gas vehicles, in particular to an integral catalyst for methane low-temperature oxidation reaction and its preparation.

Background technique

Environmental and energy security requirements are driving the rapid development of natural gas vehicles (NGVs) worldwide. According to the data of the International Gas Vehicle Association, the average annual growth rate of the number of gas vehicles in the world has reached 24% since 2001, and it is expected to reach 65 million by 2020. In 2010, the number of natural gas vehicles (CNG and LNG) in China's 16 key demonstration cities (regions) for clean vehicles exceeded 600,000.

With the rapid development of natural gas vehicles in the world, the environmental problems caused by natural gas vehicles have gradually attracted people's attention, which is mainly the methane emitted by the tail gas. More than 80% of the HC in the exhaust gas of natural gas vehicles is unburned methane, and more than 95% of the remaining non-methane hydrocarbons (NMHC) are C1-C4 hydrocarbons; while methane in the exhaust gas of gasoline vehicles only accounts for 5-8%, mainly The composition is C4-C8 hydrocarbons; the methane emitted by the tail gas of natural gas vehicles is increased, and the emission is 10 times that of gasoline vehicles; and methane is the most difficult to be oxidized hydrocarbons, so the catalyst for purification of exhaust gas of natural gas vehicles must have a higher resistance to methane conversion efficiency.

At present, the main active components of catalytic converters are noble metals such as Pt, Pd, and Rh. The purification effect of noble metals, especially Pd, on methane is much higher than that of other metal catalysts. However, the cost and large-scale application of catalysts are greatly restricted due to the high price and limited resources of noble metals. Therefore, reducing the amount of noble metals and even developing non-noble metal catalysts has been a research hotspot both at home and abroad. It is generally believed that transition metal elements such as Co, Cr, Mn, Cu, and Ni have high methane oxidation ability, and can be considered as active components of potential methane oxidation reactions.

J.R.Paredes et al. (Studies in Surface Science and Catalysis, 2002, 144, 427) disclosed a Co-Cr spinel oxide prepared by co-precipitation method, which was used for methane oxidation reaction, and it was found that it had Methane oxidation reaction has very good catalytic performance, and has good sulfur resistance.

Doping can change the properties of a certain aspect of the catalyst, such as activity, stability, anti-poisoning ability, etc., and is an important research method in the research of catalyst formulation. The inventor of the present invention has reported the Co-Cr double metal oxide catalyst of cerium doping prepared by co-precipitation method, and found that an appropriate amount of cerium doping has effectively improved methane catalytic oxidation activity (Catal.Today, 2011,175,216 ). However, the above studies are all based on metal oxides, and the specific surface area of oxides is relatively small, which limits their practical application. Loading metal oxides on a certain carrier can overcome the above disadvantages.

Soviet patent SU410806-A (1974) discloses a SiO ₂ supported Cr-Co bimetallic catalyst for the complete oxidation of methane, changing the mass ratio of Cr ₂ O ₃ , Co ₃ O ₄ and the carrier in the catalyst They are: 35~42, 20~23 and 35~45. When the mass ratio of Cr ₂ O ₃ and Co ₃ O ₄ is between 0.3-1.8-2.2, the activity and stability of the catalyst can be improved.

D. Fino et al. (Catal.Today, 2006, 117, 559) prepared a CoCr ₂ O ₄ monolithic catalyst by urea combustion method (SCS), and used the prepared material to purify methane from CNG vehicle exhaust. The preparation process is as follows: γ-Al ₂ O ₃ is first synthesized in situ on the honeycomb ceramic carrier by urea combustion method, and then CoCr ₂ O ₄ is synthesized on the carrier by the same method, and then the noble metal Pd is loaded by impregnation method, γ- The mass ratio of Al ₂ O ₃ , CoCr ₂ O ₄ and Pd is 100:15:1, and the CNG vehicle exhaust gas purification catalyst is prepared after heat treatment. Although the urea combustion method is easy to operate and does not require roasting, there is a potential explosion hazard in the reaction, and harmful gases such as NO _x will be released during the reaction.

Chinese patent CN102039146A discloses a catalyst for exhaust purification of natural gas engines and a preparation method thereof. The catalyst consists of a carrier, a coating coated on the carrier and a catalytically active component loaded on the coating. The active components of the catalyst include transition metal composite oxides MnO _x -MO _x (M=Co, Sn, Fe), CoO _x -MO _x (Mn, Sn, Fe), SnO _x -MO _x (Mn, Co, One or one or more of Fe), FeO _x -MO _x (M=Mn, Co, Sn), or transition metal-rare earth composite oxide M-Ce (M=Mn, Co, Ni, Fe , Sn, Cu). .

Contents of the invention

In order to overcome the deficiencies of the above-mentioned prior art, the object of the present invention is to provide a monolithic catalyst for methane low-temperature oxidation reaction and its preparation, which can be used for catalytic purification of methane from natural gas vehicle tail gas, and has high stability.

In order to achieve the above object, the technical scheme adopted in the present invention is:

A monolithic catalyst for low-temperature oxidation of methane, consisting of a cordierite honeycomb ceramic carrier and a coating coated on the carrier, the expression of the coating is Pd/CoxCry-M/Al ₂ O ₃ , the formula Among them, Pd is noble metal palladium; CoxCry is a cobalt-chromium composite oxide, wherein the cobalt-chromium molar ratio is between 0.001 and 100; M is a doped metal, and M is one of cerium, zirconium, lanthanum, iron, nickel, manganese or More than one kind, when there are two or more kinds, the ratio between them is arbitrary, and the doping amount (molar ratio) M: (Co+Cr+M)<0.10; Al ₂ O ₃ is γ- Al ₂ O ₃ .

The mesh number of the cordierite honeycomb ceramic carrier is 300-600 mesh.

When the molar ratio of cobalt and chromium in the CoxCry is x:y=1:2, the catalyst activity is optimal .

The weight of the coating accounts for 20-40% of the weight of the honeycomb carrier. In the coating, the weight percentage of each component is Pd: CoxCry-M: Al ₂ O ₃ =70-100: 10-20: 0.2-1 .

The present invention also provides a method for preparing the monolithic catalyst, comprising the steps of:

Step 1, preparation of composite metal oxide CoxCry-M: prepare a mixed solution of soluble cobalt ion (Co ²⁺ ) and chromium ion (Cr ³⁺ ) or soluble cobalt ion (Co ²⁺ ), chromium ion (Cr ^{3+ +} ) and doped metal ion M ⁿ⁺ mixed solution, n+ is the valence state of doped metal, [Co ²⁺ ]+[Cr ³⁺ ]=1～2mol/L or [Co ²⁺ ]+[Cr ³⁺ ]+[M ⁿ⁺ ]=1～2mol/L; adjust the pH of the mixed solution at 7～9.5 with 10wt.% ammonia solution, then age under stirring; vacuum filter and then dry at 100～120°C, and then Calcining at 450-600°C for 2 hours or more to prepare metal-doped cobalt-chromium composite metal oxide CoxCry-M;

Step 2, preparation of coating slurry: mix γ-Al ₂ O ₃ and CoxCry-M in proportion to form a mixture, then add deionized water, stir evenly to obtain a suspension, adjust the amount of deionized water added, so that The solid content of the suspension is between 30% and 40%; the suspension is ball milled to control the particle size D90 to be less than 50 μm to obtain a slurry; then a Pd(NO ₃ ) ₂ solution with a Pd mass fraction of 10% is added in proportion, Use nitric acid or acetic acid to adjust the pH between 3 and 7 to obtain the required coating slurry;

Step 3, impregnation: immerse the cordierite honeycomb ceramic carrier in the coating slurry prepared in step 2, pull out at a uniform speed, and blow off;

Step 4, drying and roasting: After drying the cordierite honeycomb ceramic carrier coated with the coating slurry in step 3 at 100-120°C, repeat step 3 until the coating amount of the cordierite honeycomb ceramic carrier reaches 20-40%, followed by roasting at 450-700° C. for 2-5 hours to obtain monolithic catalysts.

In step 1, when the doped metal M is cerium, lanthanum, iron, nickel, zirconium, the metal salt solution is its nitrate, and when the doped metal M is manganese, the metal salt solution is manganese acetate.

In step one, cobalt and chromium are their nitrates in the metal salt solution.

In the second step, when Pd(NO ₃ ) ₂ is added, it is directly added to the coating slurry, and coated on the cordierite honeycomb carrier together with the slurry.

In step 2, when Pd(NO ₃ ) ₂ is added, the coating slurry is first coated on the cordierite honeycomb support, and then the Pd(NO ₃ ) ₂ solution is impregnated into the composite metal oxide CoxCry -M surface.

In step 3, when the honeycomb ceramic carrier is immersed in the slurry, it stays for 1-3 seconds, and then is lifted out at a constant speed of 30-60 mm per second.

Compared with the prior art, the molecular sieve catalyst of the present invention has good catalytic oxidation ability for natural gas vehicle tail gas methane, and better resistance to water vapor and sulfur poisoning, and has the advantage of strong adaptability to harsh environments. At the same time, the catalyst prepared by the invention has abundant raw materials, low cost and has application prospects.

Detailed ways

The present invention will be described in further detail below in conjunction with the examples.

Embodiment one

Dissolve 25.648g Co(NO ₃ ) ₂ ·6H ₂ O and 70.536g Cr(NO ₃ ) ₃ ·9H ₂ O in 200mL deionized water to obtain a cobalt-chromium salt solution, in which [Co ²⁺ ]: [Cr ³⁺ ]=1:2, [Co ²⁺ ]+[Cr ³⁺ ]=1.32mol/L; under vigorous stirring, add the above-mentioned salt solution dropwise with 10wt.% ammonia solution until the mixed solution pH=9.0, Then aging under stirring; vacuum filtration, and washing with deionized water for 3 times, then drying at 110°C, and then roasting at 500°C for 2 hours to obtain a composite metal oxide Co ₁ Cr ₂ ;

Put 50g of γ-Al ₂ O ₃ and 5.5g of oxide Co ₁ Cr ₂ into a beaker, add 140g of deionized water, and stir at room temperature for 0.5h; ball mill the suspension to control the particle size D ₉₀ =30μm, Obtain the slurry; then add 8.5g Pd(NO ₃ ) ₂ (the mass fraction of Pd is 10%), wherein the addition of Pd accounts for about 1.5% of the catalyst coating weight, and finally adjust the pH at 4 with nitric acid to obtain the coating slurry material;

Dip the 400 mesh, Φ83×60 cordierite honeycomb ceramic carrier into the coating slurry, stay for 2 seconds, pull out at a constant speed, and then blow it with air, and the cordierite honeycomb ceramic carrier coated with the coating slurry in the step Dry at 110° C. for 1 hour, repeat the steps of impregnation and drying until the coating amount of the cordierite honeycomb ceramic carrier reaches 30%, and then bake at 600° C. for 4 hours to obtain a monolithic catalyst.

Embodiment two

The preparation method is the same as in Example 1, except that 4.5g Pd(NO ₃ ) ₂ (mass fraction of Pd is about 10%) is added, wherein the amount of Pd added accounts for about 0.8% of the catalyst coating weight, and catalyst 2 of the present invention is obtained. .

Embodiment Three

The preparation method is the same as in Example 1, except that in the salt solution, 49.791g Co(NO ₃ ) ₂ 6H ₂ O and 34.230g Cr(NO ₃ ) ₃ 9H ₂ O are dissolved in 200mL deionized water to obtain cobalt chromium Salt solution, wherein [Co ²⁺ ]:[Cr ³⁺ ]=2:1, the catalyst 3 of the present invention was obtained.

Embodiment Four

The preparation method is the same as in Example 1, except that in the salt solution, 24.370g Co(NO ₃ ) ₂ 6H ₂ O and 67.015g Cr(NO ₃ ) ₃ 9H ₂ O are dissolved in 200mL deionized water to obtain cobalt chromium Salt solution, where [Co ²⁺ ]:[Cr ³⁺ ]=1:2, and 5.741g Ce(NO ₃ ) ₃ ·6H ₂ O was added so that [Ce ³⁺ ]/([Co ²⁺ ]+[ Cr ³⁺ ]+[Ce ³⁺ ])=0.05, and the catalyst 4 of the present invention was obtained.

Catalyst activity tests were carried out in a stainless steel fixed bed reactor. Methane, oxygen and nitrogen are thoroughly mixed in a mixer before entering the reactor. The simulated gas composition (volume fraction) in the catalyst activity evaluation experiment is: CH ₄ 0.5%, O ₂ 10%, N ₂ is the reaction equilibrium gas, the total flow rate of the gas is 150mL/min, and the corresponding reaction space velocity (GHSV) is 40,000h ^-1 ; the reaction pressure is normal pressure; the reactor is placed in a temperature-controlled electric furnace, and the reaction temperature is 200-500°C. The reaction products were analyzed by on-line chromatography, and the catalyst activity was expressed as methane conversion (%). The simulation experiment results are as follows:

Embodiment five

The preparation method is the same as in Example 1, except that in the salt solution, 24.370g Co(NO ₃ ) ₂ 6H ₂ O and 67.015g Cr(NO ₃ ) ₃ 9H ₂ O are dissolved in 200mL deionized water to obtain cobalt chromium Salt solution, where [Co ²⁺ ]:[Cr ³⁺ ]=1:2, and 4.178g La(NO ₃ ) ₃ ·6H ₂ O was added so that [La ³⁺ ]/([Co ²⁺ ]+[ Cr ³⁺ ]+[La ³⁺ ])=0.05, the catalyst Pd/Co ₁ Cr ₂ -La/Al ₂ O ₃ of the present invention is obtained.

Embodiment six

The preparation method is the same as in Example 1, except that in the salt solution, 24.370g Co(NO ₃ ) ₂ 6H ₂ O and 67.015g Cr(NO ₃ ) ₃ 9H ₂ O are dissolved in 200mL deionized water to obtain cobalt chromium Salt solution, wherein [Co ²⁺ ]:[Cr ³⁺ ]=1:2, and 3.240 g C ₄ H ₆ MnO ₄ ·4H ₂ O was added so that [Mn ²⁺ ]/([Co ²⁺ ]+[ Cr ³⁺ ]+[Mn ²⁺ ])=0.05, the catalyst Pd/Co ₁ Cr ₂ -Mn/Al ₂ O ₃ of the present invention is obtained.

Embodiment seven

The preparation method is the same as in Example 1, except that in the salt solution, 24.370g Co(NO ₃ ) ₂ 6H ₂ O and 67.015g Cr(NO ₃ ) ₃ 9H ₂ O are dissolved in 200mL deionized water to obtain cobalt chromium Salt solution, where [Co ²⁺ ]:[Cr ³⁺ ] = 1:2, and 2.671 g Fe(NO ₃ ) ₃ .9H ₂ O and 1.922 g Ni(NO ₃ ) ₂ .6H ₂ O were added, where [ Fe ³⁺ ]:[Ni ²⁺ ]=1:1, such that ([Fe ³⁺ ]+[Ni ²⁺ ])/([Co ²⁺ ]+[Cr ³⁺ ]+[Fe ³⁺ ]+[ Ni ²⁺ ])=0.05, the catalyst Pd/Co ₁ Cr ₂ -FeNi/Al ₂ O ₃ of the present invention is obtained.

Claims (5)

1. preparation method for the integral catalyzer of methane low-temperature oxidation reaction, described integral catalyzer is comprised of cordierite honeycomb ceramic carrier and the coating that is coated on carrier, and the expression formula of described coating is Pd/CoxCry-M/Al ₂O ₃, in formula, Pd is precious metal palladium; CoxCry is cobalt chromium composite oxides, and wherein cobalt chromium mol ratio is between 0.001～100; M is the metal of doping, and M is more than one in cerium, zirconium, lanthanum, iron, nickel, manganese, and when being two or more, ratio is any therebetween, and with molar ratio computing, doping M:(Co+Cr+M)<0.10; Al ₂O ₃For having the γ-Al of high thermal stability ₂O ₃

It is characterized in that, comprise the steps:

Step 1, the preparation of composite metal oxide CoxCry-M: cobalt ions, chromium ion and the doped metal ion M of preparation solubility ^N+Mixed solution, n+ is the valence state of doping metals, [Co ²⁺]+[Cr ³⁺]+[M ^N+]=1～2mol/L; With the ammonia spirit of 10wt.%, regulate mixed solution pH 7～9.5, then under agitation aging; Vacuum filtration is then dried under 100～120 ℃, then 450～600 ℃ of lower roastings, more than 2 hours or 2 hours, make the cobalt chromic composite metal oxide CoxCry-M of doping metals;

Step 2, the preparation of coating paste: in proportion by γ-Al ₂O ₃With CoxCry-M, mix, form mixture, then add deionized water, stir and obtain suspension, regulate the addition of deionized water, make the solid content of suspension between 30～40%; By the suspension ball milling, control granularity D90 and be less than 50 μ m, obtain slurries; Then adding in proportion the Pd mass fraction is 10% Pd (NO ₃) ₂Solution, 3～7, obtain required coating paste with nitric acid or vinegar acid for adjusting pH;

Step 3, dipping: cordierite honeycomb ceramic carrier be impregnated in coating paste prepared by step 2, at the uniform velocity pull out, blow stifled;

Step 4, dry and roasting: by the cordierite honeycomb ceramic carrier after step 3 applying coating slurry after under 100～120 ℃, drying, repeating step three again, coated weight to cordierite honeycomb ceramic carrier reaches 20～40%, then, 450～700 ℃ of roastings 2～5 hours, obtain integral catalyzer.

2. preparation method according to claim 1, is characterized in that, when in step 1, doping metals M was cerium, lanthanum, iron, nickel, zirconium, metal salt solution was its nitrate, and when doping metals M was manganese, metal salt solution was manganese acetate.

3. preparation method according to claim 1, is characterized in that, in step 1, cobalt and chromium all add with its nitrate form.

4. preparation method according to claim 1, is characterized in that, in step 2, adding Pd (NO ₃) ₂The time, directly join in the slurries of coating, together with slurries, be coated on the cordierite honeycomb carrier.

5. preparation method according to claim 1, is characterized in that, in step 3, when honeycomb ceramic carrier impregnated in slurries, stopped 1～3 second, and then the speed with per second 30～60mm at the uniform velocity proposes.