CN111215122A

CN111215122A - Palladium-based methane catalytic combustion catalyst, preparation and application

Info

Publication number: CN111215122A
Application number: CN201811416878.8A
Authority: CN
Inventors: 李为臻; 拉玛纳穆尔蒂帕勒; 任国庆; 张景才; 张涛
Original assignee: Dalian Institute of Chemical Physics of CAS
Current assignee: Dalian Institute of Chemical Physics of CAS
Priority date: 2018-11-26
Filing date: 2018-11-26
Publication date: 2020-06-02
Anticipated expiration: 2038-11-26
Also published as: CN111215122B

Abstract

The invention belongs to the field of catalysts and preparation thereof, and relates to a mesoporous molecular sieve supported noble metal palladium catalyst and a preparation method thereof. The catalyst takes mesoporous molecular sieve as a carrier, and noble metal Pd with the mass fraction (based on the total mass of the catalyst) of 0.01-20 wt.% as a catalytic active component. The preparation method is a sol precipitation method, and the specific operation steps comprise the steps of firstly preparing uniformly dispersed 0.5-15nm Pd nano particles by using a sol-gel method, then adding a mesoporous molecular sieve carrier, confining the noble metal Pd nano particles into a carrier pore channel, and finally washing, drying and roasting. The catalyst shows excellent low-temperature catalytic activity and high-temperature sintering resistance to methane catalytic combustion reaction.

Description

Palladium-based methane catalytic combustion catalyst, preparation and application

Technical Field

The invention relates to an anti-sintering noble metal Pd-based catalyst for methane catalytic combustion, a preparation method thereof and a method for catalyzing methane combustion reaction by adopting the catalyst.

Background

With the increasing severity of the problem of energy shortage, natural gas has the advantages of abundant reserves, low price, convenient use, high thermal efficiency and the like, and is widely applied to the fields of natural gas automobiles, urban heating and the like. However, methane, which is a main component of natural gas, is not only an energy gas, but also a greenhouse gas which seriously pollutes the environment, and the greenhouse effect of methane is 23 times that of carbon dioxide, and the damage capability of methane to the ozone layer is 7 times that of carbon dioxide. Therefore, the complete elimination of methane in the tail gas of natural gas automobiles or city gas has important research significance. Because of the high chemical inertia of methane molecules, the traditional direct combustion mode is carried out at a high temperature of more than 1500 ℃, and at the high reaction temperature, nitrogen in the air is easy to react with oxygen to generate NO_xFurther causing environmental pollution. The low temperature catalytic combustion of methane is therefore of particular importance.

The current catalyst for methane catalytic combustion mainly comprises: (1) supported noble metal catalysts including Rh, Pd, Pt, and the like; (2) transition metal oxide catalysts, e.g. NiO, Mn₂O₃CuO and Fe₂O₃Etc.; (3) a rare earth perovskite-type composite metal oxide catalyst; (4) hexaaluminate type catalysts, and the like. The noble metal Pd-based catalyst has the highest low-temperature catalytic activity and better anti-poisoning capability for methane catalytic combustion reaction, thereby gaining wide attention. However, the thermal stability of the supported Pd-based catalyst is poor, and the Pd species with high dispersion at the temperature higher than 600 ℃ is easy to sinter and aggregate to generate large particles with poor catalytic activity, so that the catalyst is permanently inactivated. Therefore, the development of the high-temperature sintering-resistant Pd-based noble metal catalyst is the key point for promoting the industrial application of the catalyst in the field of methane catalytic combustion.

At present, the main method for inhibiting the sintering of the Pd-based methane combustion catalyst is realized by constructing a core-shell structure of noble metal Pd nano particles wrapped by oxides. For example, CN106492824A discloses silica-coated Pd-based noble metal nanoparticles and their application in methane catalytic combustion reaction, the catalyst is subjected to 900 deg.cThe catalyst still has relatively excellent catalytic activity after being roasted; preparation of Al by Cargnello et Al₂O₃Carrier-supported CeO₂The Pd nano-particles are wrapped and applied to a methane catalytic combustion reaction, and the catalyst can still show relatively high catalytic activity after being roasted at a high temperature of 750 ℃ (Science,337(2012) 713-717); similarly, Chen et Al Si-modified Al₂O₃Preparation of ZrO for supports₂Coated Pd nanoparticles, which also exhibit relatively excellent resistance to sintering to methane combustion reactions (ACSCatalysis,4(2014) 3902-3909.). However, two important problems of the wrapped core-shell structure limit the wide application of the wrapped core-shell structure, namely: the existence of the coating material limits the direct contact of reactant molecules and the noble metal Pd particles, thereby reducing the intrinsic catalytic activity of the Pd particles; second, the thermal stability of the coating material is generally poor, and the thermal stability of the Pd nanoparticles is heavily dependent on the thermal stability of the coating support, so the use temperature thereof is greatly limited. The mesoporous molecular sieve material (such as SBA-15, MCM-41 and the like) with high silicon or full silicon has higher thermal stability, and meanwhile, abundant mesoporous channels can provide enough diffusion space for reactant molecules on one hand, and the limit function of the channels can also inhibit the sintering and aggregation of noble metal nano particles on the other hand. Therefore, the catalyst is expected to be an ideal carrier for preparing the methane catalytic combustion catalyst with high thermal stability and high activity. Currently, Dai et al use SBA-15 as the carrier, CeO₂Pd-based methane combustion catalysts with high activity at low temperature are prepared for the auxiliary agent, but because of the limitation of preparation, the dispersion uniformity degree of Pd nano-particles is relatively poor, so that the thermal stability is limited (ACS Applied Materials)&Interfaces,10(2018)477-487.)。

Disclosure of Invention

In view of the above, the present invention aims to provide a method for preparing a Pd-based methane combustion catalyst with ultrahigh thermal stability, in which mesoporous molecular sieve is used as a carrier, and Pd particles are uniformly dispersed in the pore channels of the mesoporous molecular sieve, so as to solve the problem that the Pd-based methane combustion catalyst in the prior art is easy to sinter at high temperature.

In order to achieve the purpose, the invention provides the following technical scheme:

the invention provides a preparation method of a palladium-based methane catalytic combustion catalyst, wherein a carrier of the catalyst is a mesoporous molecular sieve; the active component is metal Pd, and the weight percentage of the metal Pd is 0.1-10 wt.% based on the total weight of the catalyst; the preparation method of the catalyst specifically comprises the following steps:

(1) pd salt is dissolved in water to prepare the solution with the concentration of 1 × 10^-6-1×10^-1Adding a surfactant into a mol/L aqueous solution, and adding a reducing agent under a stirring state to obtain Pd sol nano-particles protected by the surfactant;

(2) adding a certain mass of mesoporous molecular sieve into the Pd sol nanoparticle solution, adjusting the pH value of the solution to 1-5 by using acid, and stirring for 1-12 hours;

(3) and (3) filtering, washing, drying and roasting the mixture obtained in the step (2) to obtain the mesoporous molecular sieve supported Pd nano-catalyst with the Pd mass fraction of 0.1-10%.

Wherein the mesoporous molecular sieve carrier comprises one or more of mesoporous molecular sieves such as SBA (such as SBA-15, SBA-16 and the like), MCM (such as MCM-41, MCM-48, MCM-50 and the like) and KIT (such as KIT-6 and the like); the size range of the Pd nano-particles is 0.5-15 nm.

The Pd salt in the 1) comprises PdCl₂，Pd(NO₃)₂、Pd(OAc)₂Or Na₂PdCl₄One or more than two of Pd²⁺Is preferably 1X 10^-5～1×10^-3mol/L; the surfactant comprises polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), Cetyl Trimethyl Ammonium Bromide (CTAB), ethylene glycol and sodium citrate, and the mass ratio of the surfactant to Pd is 0.01-100, preferably 0.1-10; the reducing agent comprises NaBH₄Tert-butylaminoborane, hydrazine hydrate (N)₂H₄·H₂O), sodium citrate and formaldehyde, the molar ratio of reducing agent to Pd being 0.01 to 100, preferably 0.1 to 10.

The acid used for adjusting the pH value in the step 2) is sulfuric acid or hydrochloric acid, and the pH value is preferably 2-5;

the drying temperature in the step 2) is 60-150 ℃, and preferably 80-120 ℃;

the roasting temperature in the step 2) is 300-1000 ℃, and preferably 500-800 ℃;

in addition, the invention also provides application of the catalyst to methane catalytic combustion reaction. The method specifically comprises the following steps: the catalyst is adopted, and the content of the catalyst is 0.05-10 vol.% at the space velocity of 0.5-500L/(h.g)_cat) The catalyst has excellent catalytic activity and high-temperature sintering resistance to methane catalytic combustion, and the ignition temperature range is 200-400 ℃.

The method adopts a sol-gel method to prepare and obtain uniformly dispersed Pd sol nano particles firstly, and then the Pd sol nano particles are deposited into the pore channels of the mesoporous molecular sieve, and the Pd nano particles with limited domains in the pore channels can stably exist in a high-temperature roasting process (300-1000 ℃) without sintering and aggregation. Therefore, the catalyst can show higher low-temperature catalytic activity and high-temperature anti-sintering performance for methane catalytic combustion reaction. Compared with the traditional impregnation method or coprecipitation method, the Pd nano-particles prepared by the method can effectively enter the inside of the mesoporous molecular sieve pore channel, and the proportion of the Pd particles on the outer surface of the pore channel is reduced, so that the thermal stability of the Pd nano-particles is improved, and the problem that the methane catalytic combustion catalyst in the prior art is easy to sinter at high temperature is solved.

Drawings

FIG. 1 shows Pd/SBA-15-SI-500-5H, SI-800-5H, SI-800-3d and SI-750-H prepared by examples 1-4 of the process of the present invention₂The X-ray diffraction (XRD) patterns of four catalyst samples of O-3d and Pd/SAB-15-C-IWI-500-5h, -C-IWI-800-5h and Pd/SAB-15-A-IWI-500-5h prepared in comparative examples 1-4 by adopting an equal volume impregnation method, wherein (a) is a small-angle XRD pattern, and (b) is a wide-angle XRD pattern. As can be seen from the small-angle XRD spectrogram in the figure, the pore structure of the SBA-15 is still maintained after high-temperature roasting treatment; the wide angle XRD spectrogram shows that the particle size of Pd in a sample prepared by the method provided by the invention is not obviously changed after high-temperature roasting treatment along with roasting, while the sample prepared by the traditional method is obviously sintered by Pd particles.

FIG. 2 shows Pd/SBA-15-SI-50 prepared by examples 1 to 4 according to the method of the present invention0-5H, SI-800-5H, SI-800-3d and SI-750-H₂Electron microscope photos of four O-3d catalysts and Pd/SAB-15-C-IWI-500-5h, -C-IWI-800-5h and Pd/SAB-15-A-IWI-500-5h samples prepared in comparative examples 1-4 by adopting an equal volume impregnation method, and-A-IWI-800-5 h samples. It can be seen from the figure that the particle size of Pd in the sample prepared by the method provided by the present invention is not significantly changed after the high temperature roasting treatment, while the sample prepared by the conventional method has significant sintering of Pd particles.

FIG. 3 is a plot of the light-off of eight catalyst samples prepared in catalyst test examples 1-8 for examples 1-4 and comparative examples 1-4 for a methane catalyzed combustion reaction. As can be seen from the figure, the sample prepared by the method provided by the invention has higher catalytic activity, high-temperature sintering resistance and hydrothermal resistance on methane combustion reaction.

Detailed Description

The present invention will be further described with reference to the accompanying drawings and detailed description, but the invention is not limited thereto.

Examples 1 to 4

Examples 1-4 Using the method provided by the present invention, polyvinyl alcohol (PVA) was selected as the protective agent, NaBH₄Is a reducing agent, PdCl₂Is Pd²⁺Precursor, SBA-15 is mesoporous molecular sieve carrier, and Pd/SBA-15 catalyst is prepared. The specific operation steps are as follows: weighing by m_PVA/m_PdWeigh a mass of PVA to 1.26 × 10 ═ 0.65^-4mol/L of PdCl₂Stirring the solution until the solution is dissolved; then adding 0.1mol/L NaBH4 as a reducing agent into an ice water bath at 1 ℃, wherein NaBH₄and/Pd (mol/mol) ═ 5, and stirred for 0.5 hour to give surfactant-protected Pd sol nanoparticles. And (2) weighing a certain mass of SBA-15 according to the theoretical loading capacity of 1 wt.% of Pd, adding the SBA-15 into the Pd sol nanoparticle solution, adjusting the pH value of the solution to 3.0 by using concentrated sulfuric acid, and stirring for 1 hour. And then filtering, washing and drying at 120 ℃ to obtain the SBA-15 loaded fresh Pd nano catalyst. The freshly prepared catalyst was passed under air conditions for 500-5H, 800-3d and 10 vol.% H₂Carrying out the reaction at 750 ℃ under the water vapor condition of OAnd 3d, roasting treatment. The obtained samples are named as Pd/SBA-15-SI-500-5H, SI-800-5H, SI-800-3d and SI-750-H₂O-3d。

Examples 5 to 8

Examples 1-4 Using the method provided by the present invention, polyvinyl alcohol (PVP) was selected as the protectant, hydrazine hydrate was selected as the reducing agent, Pd (NO)₃)₂Is Pd²⁺Precursor MCM-41 is mesoporous molecular sieve carrier, preparing Pd/MCM-41 catalyst. The specific operation steps are as follows: weighing by m_PVP/m_PdWeigh a mass of PVA to 1.5 × 10 ═ 0.65^-3mol/L Pd (NO)₃)₂Stirring the solution until the solution is dissolved; then adding 0.1mol/L hydrazine hydrate as a reducing agent into an ice water bath at 5 ℃, wherein N is₂H₄·H₂O/Pd (mol/mol) ═ 5, and stirred for 1 hour to give surfactant-protected Pd sol nanoparticles. Weighing MCM-41 with a certain mass according to the theoretical loading capacity of 1 wt.% of Pd, adding the MCM-41 into the Pd sol nanoparticle solution, adjusting the pH value of the solution to 5.0 by using concentrated sulfuric acid, and stirring for 5 hours. Then filtering, washing and drying at 80 ℃ to obtain the fresh Pd nano catalyst loaded by MCM-41. The freshly prepared catalyst was passed under air conditions for 500-5H, 800-3d and 10 vol.% H₂And roasting for 750-3 d under the water vapor condition of O. The obtained samples are named as Pd/MCM-41-SI-500-5H, -SI-800-5H, -SI-800-3d and-SI-750-H respectively₂O-3d。

Comparative examples 1 to 2

Comparative example 1-2 Pd/SBA-15 catalyst was prepared by conventional equivalent-volume impregnation method using SBA-15 as carrier and PdCl2 aqueous solution as Pd precursor solution. The specific operation steps are as follows: 5.0g of SBA-15 carrier was weighed into PdCl containing 0.05g of palladium, formulated according to its water uptake₂In the aqueous solution, the mixture was ultrasonically shaken and stirred for 1 hour, and then dried at room temperature for 24 hours and subsequently at 80 ℃ for 12 hours to obtain a catalyst precursor. The catalyst precursor is roasted for 500-5h and 800-5h under the air condition respectively. The obtained samples are named as Pd/SAB-15-C-IWI-500-5h and-C-IWI-800-5 h respectively.

Comparative examples 3 to 4

Comparative examples 3-4 conventional isovolumetric impregnation with SBA-15 as support, Pd (OAC)₂The water solution is a precursor solution of Pd to prepare the Pd/SBA-15 catalyst. The specific operation steps are as follows: 5.0g of SBA-15 carrier was weighed out and added to Pd (OAC) containing 0.05g of palladium, formulated according to its water uptake₂In the aqueous solution, the mixture was ultrasonically shaken and stirred for 1 hour, and then dried at room temperature for 24 hours and subsequently at 80 ℃ for 12 hours to obtain a catalyst precursor. The catalyst precursor is roasted for 500-5h and 800-5h under the air condition respectively. The obtained samples are respectively named as Pd/SAB-15-A-IWI-500-5h and-A-IWI-800-5 h.

Catalyst test example 1

The methane catalytic combustion reaction was evaluated using a fixed bed reactor at atmospheric pressure. Weighing 50mg of Pd/SBA-15-SI-500-5h of catalyst powder, placing the catalyst powder at the bottom of a quartz reaction tube with the inner diameter of 10mm, and sealing quartz wool at two ends of the catalyst. A k-type thermocouple with the outer diameter of about 1mm, which is sleeved with a quartz tube, is inserted into the catalyst powder for temperature control and measurement. The raw material gases for the reaction are respectively 0.5 vol% CH₄+10vol％O₂(N₂Equilibrium) and space velocity of 40L/(h.g)_cat). The starting material and product were separated and detected on-line using an ECHROM A90 chromatograph equipped with a Plot-Q, 5A molecular sieve chromatography column, TCD detector and FID detector.

Catalyst test examples 2 to 8

According to the method of test example 1, except that the Pd/SBA-15-SI-500-5H catalyst was replaced with Pd/SBA-15-SI-800-5H, Pd/SBA-15-SI-800-3d, Pd/SBA-15-SI-750-H₂O-3 d; Pd/SAB-15-C-IWI-500-5h, Pd/SBA-15-C-IWI-800-5h, Pd/SAB-15-A-IWI-500-5h and Pd/SAB-15-A-IWI-800-5h, respectively carrying out methane catalytic combustion reaction.

Claims

1. A preparation method of a palladium-based methane catalytic combustion catalyst is characterized by comprising the following steps: (1) pd salt is dissolved in water to prepare the solution with the concentration of 1 × 10^-6-1×10^-1Adding a surfactant into a mol/L aqueous solution, and adding a reducing agent under a stirring state to obtain Pd sol nano-particles protected by the surfactant;

(2) adding a mesoporous molecular sieve into the Pd sol nanoparticle solution, adjusting the pH value of the solution to 1-5 by using acid, and stirring for 1-12 hours;

2. The method of preparing a palladium-based methane catalyst for catalytic combustion as set forth in claim 1, wherein: the Pd salt comprises PdCl₂，Pd(NO₃)₂、Pd(OAc)₂Or Na₂PdCl₄One or more than two of them, Pd in aqueous solution²⁺Is preferably 1X 10^-5～1×10^-3mol/L; the surfactant comprises one or more of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), Cetyl Trimethyl Ammonium Bromide (CTAB), ethylene glycol and sodium citrate, and the mass ratio of the surfactant to the Pd is 0.01-100, preferably 0.1-10; the reducing agent comprises NaBH₄Tert-butylaminoborane, hydrazine hydrate (N)₂H₄·H₂O), sodium citrate and formaldehyde, and the molar ratio of the reducing agent to Pd is 0.01-100, preferably 0.1-10.

3. The method of preparing a palladium-based methane catalyst for catalytic combustion as set forth in claim 1, wherein: the mesoporous molecular sieve comprises one or more of SBA type (such as SBA-15 and SBA-16), MCM type (such as MCM-41, MCM-48 and MCM-50) and KIT type (such as KIT-6) mesoporous molecular sieves; the acid for adjusting the pH value is sulfuric acid or hydrochloric acid, and the pH value of the adjusted solution is preferably 2-5.

4. The method of preparing a palladium-based methane catalyst for catalytic combustion as set forth in claim 1, wherein: the drying temperature in the step (3) is 60-150 ℃, and preferably 80-120 ℃; the calcination temperature is 300-1000 deg.C, preferably 500-800 deg.C.

5. A palladium-based methane catalytic combustion catalyst prepared by the preparation method of any one of claims 1 to 4.

6. Use of the catalyst of claim 5 in the catalytic combustion of palladium-based methane.

7. Use according to claim 6, characterized in that: the catalyst is adopted to treat gas with methane content of 0.05-10 vol.% and oxygen content of 0.1-20 vol.% at a space velocity of 0.5-500L/(h.g)_cat) The ignition temperature range of the catalytic combustion is 200-400 ℃.

8. Use according to claim 7, characterized in that: the other gas except methane and oxygen in the methane-containing atmosphere is one or more than two of nitrogen, argon or helium.