CN112108145B - A kind of alumina supported iridium cluster catalyst and its preparation and application - Google Patents

Abstract

The invention relates to an alumina supported iridium cluster catalyst and preparation and application thereof. Specifically, the content of iridium is 0.1-5% of the total mass of the catalyst, the iridium is highly dispersed on alumina in the form of clusters, and the cluster size is 0.5-5 nm. The iridium cluster as the only catalytic activity center is suitable for room-temperature formaldehyde elimination and catalytic decomposition of NJ-DT-3, shows excellent catalytic activity, can completely catalyze and oxidize high-concentration formaldehyde (180 ppm) into carbon dioxide and water at room temperature, and has higher reaction rate when being used for the decomposition reaction of NJ-DT-3.

Description

A kind of alumina supported iridium cluster catalyst and its preparation and application

technical field

The invention relates to an alumina-loaded iridium cluster catalyst and its preparation and application, which can be used for room temperature elimination of typical volatile organic pollutant formaldehyde and NJ-DT-3 decomposition reaction.

Background technique

Volatile Organic Compounds (Volatile Organic Compounds, VOCs) are the main air pollutants, and formaldehyde (HCHO), as one of the typical VOCs, has caused increasingly serious air pollution problems and has attracted people's attention. On the one hand, outdoor formaldehyde mainly comes from industrial waste gas and vehicle exhaust emissions. The latest research shows that the emitted formaldehyde can react with sulfur dioxide to form hydroxymethanesulfonate, which is an important source of PM2.5. Therefore, formaldehyde is the "culprit" that causes my country's winter haze weather. On the other hand, indoor formaldehyde mainly comes from the release of construction, decoration and decoration materials, such as adhesives, plywood, paint and thinner. As a highly toxic substance, formaldehyde is the main indoor air pollutant. Long-term exposure to formaldehyde can cause headaches, nausea, allergies and other symptoms, and even cause teratogenic and carcinogenic effects. Therefore, the elimination of formaldehyde, especially at room temperature, is of great significance for effectively improving air quality, solving air pollution problems, and protecting human health.

At present, the commonly used methods for eliminating formaldehyde at room temperature mainly include adsorption, photocatalysis and catalytic oxidation [ChemSusChem, 2013, 6, 578-592]. Adsorption methods generally use porous materials as adsorbents, and the common ones are carbon-based adsorbents (such as activated carbon, carbon fiber, etc.) and porous oxides (such as activated alumina, silica gel, etc.). Although the price is cheap and the adsorption capacity is good, the adsorption capacity is limited and it is easy to reach saturation, so it cannot continuously eliminate formaldehyde. The photocatalytic method mostly uses photocatalysts such as ZnO and TiO ₂ to oxidize the adsorbed formaldehyde into CO ₂ and H ₂ O through the action of light, but the catalytic process is often accompanied by the generation of by-products such as CO, and the cost of the ultraviolet light source used is high. , short life, which limits the wide application of this method. The catalytic oxidation method is to use metal or its oxide as a catalyst, and oxygen in the air as an oxidant to convert formaldehyde into non-toxic CO ₂ and H ₂ O. Due to the advantages of low energy consumption, high efficiency and environmental friendliness, this method has attracted the attention of researchers.

In the formaldehyde oxidation reaction, supported noble metal catalyst systems generally exhibit excellent catalytic performance, and Pt, Pd, Au, etc. are currently studied more. He et al. reported the Pt/TiO ₂ catalyst modified by alkali metal ions (Li, Na, K), and found that the 2% Na-1% Pt/TiO ₂ catalyst had excellent catalytic performance in the formaldehyde oxidation reaction. 600ppm formaldehyde is completely oxidized [Angew.Chem.Int.Ed., 2012, 51, 9628-9632]. Zhou et al. conducted a comparative study on the performance of Pd catalysts supported on CeO ₂ with different shapes in the formaldehyde oxidation reaction, and found that the catalytic activity of noble metal Pd was supported when CeO ₂ was cubic and the exposed crystal plane was (100) crystal plane. It is more active in catalytic formaldehyde oxidation than octahedral and rod-shaped CeO ₂ as supports [Environ.Sci.Technol.2015,49,8675-8682]. As an important member of the platinum group metals, the noble metal iridium shows excellent catalytic activity in reactions such as PROX and water vapor shift, but less research has been done in the oxidation of formaldehyde. Recently, Li et al. prepared Ir/TiO ₂ catalyst by excessive impregnation method, which has low activity in catalyzing formaldehyde oxidation reaction. After further modifying the catalyst with Na ions, the catalytic activity is greatly improved, and formaldehyde can be completely eliminated at room temperature. It is believed that the addition of Na ions promotes the reduction of titanium oxide at the metal-support interface, increases the oxygen vacancies on the surface of the support, promotes the activation of water, and improves the catalytic performance [ACS Catal.2018, 8, 11377-11385]. Gao et al. used hydrogenated TiO ₂ (that is, NaBH ₄ pre-reduction treatment) as a carrier, and loaded Ir to catalyze the oxidation reaction of formaldehyde. They found that the hydrogenated TiO ₂ had abundant oxygen vacancies and surface hydroxyl groups so that the catalytic activity was significantly improved. , but the conversion rate at room temperature is still less than 20%, and the activity is low [New J.Chem., 2018, 42, 18381-18387]. At present, in the reported Ir catalyst system, reducible TiO ₂ is used as the carrier, and inert Al ₂ O ₃ is used as the carrier to support Ir-based catalysts for formaldehyde oxidation.

The propellant provides the energy source for the thrust of the rocket engine, which will have a direct impact on the flight performance of the aircraft, so it is very important for the rocket engine. Traditional propellants are mainly divided into solid propellants and liquid propellants. Solid propellant exists in the rocket engine in the form of solid, which is easy to store and has a high density, but has low specific impulse, poor controllability, and cannot be started and stopped repeatedly. Although the flow rate of liquid propellant is easy to control, it is prone to leakage. The safety factor is low, so solid propellants and liquid propellants cannot fully meet the requirements for propellant safety and high efficiency. In the continuous exploration of new propellants, gel propellants have emerged as the times require. Because they have both the advantages of solid propellants and liquid propellants, they are considered to be very promising propellants in the aerospace field in the future. The gel propellant can be quickly decomposed under the action of a catalyst, producing a large amount of gas and releasing heat, thereby realizing the rapid conversion of chemical energy to kinetic energy. At present, the most typical catalyst for catalyzing the decomposition of gel propellants is Al ₂ O ₃ supported Ir catalyst, such as the catalyst code-named shell405 [USPat. As a rare precious metal and high-grade strategic material, Ir needs to be optimized through a new catalyst synthesis method to reduce the amount of Ir and reduce the dependence on Ir reserves. Dispersing single atoms is an effective method [ZL201218006496.5]. However, in the process of propellant decomposition, Ir particles may aggregate and grow due to the high specific surface energy of single atoms [Angew.Chem., Int. Ed.2012, 51, 5929]. Up to now, Ir-based catalysts mainly adopt the impregnation method, the precursors are initially dispersed on the surface of the carrier in the form of ions, and it is difficult to avoid the formation of unevenly dispersed catalytic centers in the post-treatment process [Catalysis Today, 2012, 185, 198], resulting in a low proportion of effective components , wasting most of the Ir metal.

The present invention first applies the Ir cluster catalyst with uniform size supported by inert carrier Al ₂ O ₃ to the catalytic decomposition of propellant NJ-DT-3.

Contents of the invention

The object of the present invention is to provide a cluster Ir catalyst supported by an inert carrier Al ₂ O ₃ and its preparation and application.

To achieve the above object, the technical solution adopted in the present invention is:

A cluster Ir catalyst, with non-reducible Al ₂ O ₃ as the carrier, noble metal Ir as the active component, the Ir content accounts for 0.1-5% of the total mass of the catalyst, and is highly dispersed in the form of clusters on Al ₂ O ₃ , The cluster size is 0.5-5nm.

The catalyst is prepared by a _two -step method of colloid formation-deposition loading, and the specific process is: using a low-carbon alcohol reduction method to prepare Ir sol as a precursor, and then adding it dropwise to the _Al2O3 carrier suspension under vigorous stirring, React for 3 hours, let stand for aging for 1 hour, filter and wash while hot, and dry at 80°C for 12 hours to obtain the target catalyst.

The low-carbon alcohol reduction method to prepare Ir cluster sol, the specific process is: dissolving chloroiridic acid in 50-100mL low-carbon alcohol to prepare a solution with a concentration of 3.9-39mmol L ^-1 , adding 50-100mL 0.03-0.75mol L ^-1 NaOH or KOH low-carbon alcohol solution, stirred at room temperature for 0.5-3 hours until uniformly mixed, transferred to an oil bath at 100-160°C, and reacted for 1-3 hours under the protection of argon or nitrogen atmosphere to obtain iridium cluster sol.

The low-carbon alcohol is one or more of methanol, ethylene glycol, glycerol, and 1,4-butanediol.

The catalyst is reduced under a hydrogen atmosphere, the gas composition is 20-100vol% H ₂ , He is the balance gas, and the reduction is performed at 200-300° C. for 0.5-2 hours.

The catalyst can be used to catalyze the oxidation reaction of formaldehyde at room temperature. The raw material gas composed of 180ppm formaldehyde, 20vol.% O ₂ , and He balance, the relative humidity is 50%, and the space velocity is 3×10 ⁴ mL g _cat ^-1 h ^-1 Pass it into a normal-pressure fixed-bed reactor equipped with a catalyst, and test the formaldehyde conversion rate in the range of 20-80°C.

The catalyst can be used for the NJ-DT-3 propellant to catalyze the decomposition reaction, and has a relatively high reaction rate.

Compared with prior art, the substantive characteristics that the present invention has are:

1. The catalyst prepared by the method of the present invention has the active component Ir dispersed in clusters of uniform size, which is conducive to improving the catalytic activity of the catalyst and reducing the amount of noble metal Ir.

2. The carrier used in the preparation of the catalyst in the present invention is inert alumina, which has a large specific surface area, which is conducive to the dispersion of Ir, and Ir is used as the only active center.

3. The Ir catalyst supported by the inert carrier alumina prepared by the present invention has high activity, and can completely oxidize formaldehyde with a high concentration of 180ppm at room temperature. breakthrough.

4. The rate of the NJ-DT-3 propellant decomposition reaction catalyzed by the cluster Ir catalyst prepared by the present invention is higher than that of the Ir/Al ₂ O ₃ catalyst prepared by other preparation methods.

Description of drawings

Figure 1 is the XRD pattern of the Ir/Al ₂ O ₃ catalyst prepared in Example 1 and Comparative Examples 1 and 2.

Fig. 2 is the HAADF-STEM image and the particle size statistical image of the Ir/Al ₂ O ₃ catalyst prepared in Example 1.

Fig. 3 is a comparison chart of catalytic formaldehyde oxidation performance of Ir/Al ₂ O ₃ catalysts prepared in Example 1 and Comparative Examples 1 and 2 of the present invention.

Fig. 4 is a test chart of catalytic formaldehyde oxidation performance of Ir/Al ₂ O ₃ catalysts prepared in Examples 1, 12, 13, 14 and 15 of the present invention.

Fig. 5 is a comparison chart of catalytic formaldehyde oxidation performance of the Ir/Al ₂ O ₃ catalyst prepared in Comparative Example 3 of the present invention.

Fig. 6 is a stability test diagram of the Ir/Al ₂ O ₃ catalysts prepared in Example 1 and Comparative Examples 1 and 2 of the present invention.

Fig. 7 is a graph comparing decomposition rates of Ir/Al ₂ O ₃ catalyst NJ-DT-3 prepared by different methods.

detailed description

The following examples are used to describe the present invention in detail, but do not limit the content of the present invention.

Example 1:

Dissolve 1.0g of chloroiridic acid in 50mL of ethylene glycol, the concentration of metal iridium ions in the solution is 39mmol L ^-1 , add 50mL of sodium hydroxide ethylene glycol solution with a concentration of 0.25mol L ^-1 into it, stir at room temperature for 0.5h, then transfer to In an oil bath at 160°C, stir and react for 1 h under the protection of an argon atmosphere to obtain Ir nano-sol, measure 4.0 mL of Ir nano-sol and add dropwise to the vigorously stirred Al ₂ O ₃ carrier suspension with a particle size of 10-12 nm , reacted at 80°C for 3h, aged for 1h, filtered and washed while hot, and dried at 80°C for 12h to obtain a 1.5wt.% Ir/Al ₂ O ₃ catalyst, marked as 1.5IrAl-NP. The catalyst was characterized by XRD and STEM after being reduced for 0.5h at 300°C in an atmosphere of 20vol.% H ₂ /He (H ₂ volume fraction is 20%, and He is the balance gas, which is expressed by this method hereinafter). The results are shown in Fig. 1 and figure 2. XRD results showed that no diffraction peaks of Ir species were found in the spectrum, and Ir species showed a highly dispersed state in the carrier Al ₂ O ₃ . According to the STEM electron micrograph, the average particle size of the Ir nanoparticles is 1.1 nm, and the particle size distribution range is narrow (0.8-2 nm), and the size is uniform. The 1.5IrAl-NP catalyst was used for the evaluation of formaldehyde oxidation reaction. The test conditions are: the amount of catalyst used is 100mg, the reaction raw material gas composition is 180ppm HCHO, 20vol.% O ₂ , He is the balance gas, the relative humidity is 50%, the total flow rate is 50mL min ^-1 (STP), and the mass space velocity is 3× 10 ⁴ mL g _cat ^-1 h ^-1 , the temperature-programmed activity test was carried out on the catalyst in the temperature range of 20-80°C. Before the reaction test, the catalyst was reduced at 300° C. for 0.5 h in a 20 vol.% H ₂ /He atmosphere. The results are shown in Figure 3, which shows that the 1.5IrAl-NP catalyst prepared by this method has excellent catalytic performance in the formaldehyde oxidation reaction, and the formaldehyde conversion rate remains at 100% in the temperature range of 20-80°C.

Embodiment 2-11: The preparation method is the same as in Example 1, and the specific conditions are as shown in the table below:

Example 13:

Compared with Example 1, the difference is that the amount of Ir nano-sol used is 2.7mL Ir nano-sol, and the rest of the steps are the same, and finally a 1.0wt.% Ir/Al ₂ O ₃ catalyst is obtained, which is marked as 1.0IrAl-NP.

Example 14:

Compared with Example 1, the difference is that the amount of Ir nano-sol used is 1.4mL Ir nano-sol, and the rest of the steps are the same, and finally a 0.5wt.% Ir/Al ₂ O ₃ catalyst is obtained, which is marked as 0.5IrAl-NP.

Example 15:

Compared with Example 1, the difference is that the amount of Ir nano-sol used is 14 mL of Ir nano-sol, and the rest of the steps are the same, and finally a 5wt.% Ir/Al ₂ O ₃ catalyst is obtained, which is marked as 5IrAl-NP.

Example 16:

Compared with Example 1, the difference is that the amount of Ir nano-sol used is 0.28mL Ir nano-sol, and the rest of the steps are the same, and finally a 0.1wt.% Ir/Al ₂ O ₃ catalyst is obtained, which is marked as 0.1IrAl-NP.

Comparative example 1:

Al ₂ O ₃ supported Ir catalysts were prepared by deposition precipitation method. Disperse 1.0g of Al ₂ O ₃ with a particle size of 10-12nm in 100mL of ultrapure water to form a suspension, and drop into 91uL of 164mg mL ^-1 chloroiridic acid solution at a rate of 3.0mL min ^-1 under vigorous stirring, Adjust the pH to 9.2 with 0.2mol L ^-1 NaOH, react at 80°C for 3h, age for 1h, filter and wash while hot, and dry at 80°C for 12h to obtain a 1.5wt.% Ir/Al ₂ O ₃ catalyst, marked as 1.5IrAl-DP.

Comparative example 2:

The Al ₂ O ₃ supported Ir catalyst was prepared by isometric impregnation method. Dilute 91uL 164mg mL of chloroiridic acid solution to 0.6mL with ultrapure water, add dropwise to 1.0g of Al ₂ O ₃ powder with a particle size of 10-12nm, stir with a glass rod until uniform, and place in an oven at 80°C Dry 12h 1.5wt.% Ir/ _Al2O3 catalyst, labeled as _1.5IrAl -IMP.

Examples 17-20 are to investigate the influence of different influencing factors on the performance and stability of the prepared catalyst. The formaldehyde elimination performance test of the catalyst was carried out using a fixed-bed micro-reaction evaluation device under normal pressure. The test conditions are: the amount of catalyst used is 100mg, the reaction raw material gas composition is 180ppm HCHO, 20vol.% O ₂ , He is the balance gas, the relative humidity is 50%, the total flow rate is 50mL min ^-1 (STP), and the mass space velocity is 3× 10 ⁴ mL g _cat ^-1 h ^-1 . The temperature-programmed activity test was carried out on the catalyst in the temperature range of 20-80°C. Before the reaction test, the catalyst was reduced at 300° C. for 0.5 h in an atmosphere of 20 vol.% H ₂ /He, and then purged with He gas to cool down to room temperature. During the test, samples were taken every 20 minutes, each temperature point was kept for 1 hour, and samples were taken 3 times. The concentration of the reaction balance gas is detected by the FID detector in the chromatogram. Since the concentration of HCHO in the feed gas is at the ppm level, the obtained trace amount of _CO2 is hydrogenated in a nickel reformer before entering the FID detector, and all of it is converted into _CH4 before detection.

The calculation method of HCHO conversion rate is as follows:

HCHO Conversion (%) = [CO ₂ ]/[CO ₂ ] ^A × 100%

Where: [CO ₂ ] ^A is the corresponding _CH chromatographic peak area when formaldehyde in the feed gas is completely converted into CO ₂

[CO ₂ ] is the CH ₄ chromatographic peak area corresponding to CO ₂ in the equilibrium gas under different reaction temperature conditions.

Example 17: Investigating the influence of the preparation method on the catalytic formaldehyde elimination performance of the catalyst

100 mg of the catalysts prepared in Example 1 and Comparative Examples 1 and 2 were packed in a quartz reaction tube. Before the reaction, the catalyst was reduced at 300°C for 0.5h in an atmosphere of 20vol.% H ₂ /He, and helium was purged to room temperature , the pretreated catalyst was used for the evaluation of formaldehyde oxidation reaction. The results are shown in Figure 3, showing that the 1.5Ir/Al-NP catalyst prepared by the colloid-deposition method in Example 1 exhibits the highest formaldehyde elimination performance, and the formaldehyde conversion rate remains at 100% in the temperature test range of 20 to 80°C. However, the conversion rate of the 1.5Ir/Al-DP catalyst with the same Ir loading prepared by the deposition precipitation method was only 57% at room temperature, and the conversion rate was only 83% at 80 ° C. The 1.5Ir/Al-IMP catalyst prepared by the impregnation method The formaldehyde elimination performance is the worst, and the formaldehyde conversion rate is still below 40% at 80°C. This shows that the 1.5Ir/Al-NP catalyst prepared by the catalyst preparation method described in the right—the colloid-deposition method has obvious advantages in elimination performance in the formaldehyde elimination reaction.

Example 18: Investigating the effect of noble metal Ir loading on catalyst catalytic formaldehyde removal performance

Pack 100mg Ir loading of 0.1-5.wt% Ir/Al ₂ O ₃ catalyst in a quartz reaction tube. Before the reaction, the catalyst is reduced at 300°C for 0.5h in an atmosphere of 20vol.% H ₂ /He, helium After purging to room temperature, the pretreated catalyst was used for evaluation of formaldehyde oxidation reaction. The results are shown in Figure 4, which shows that with the increase of Ir loading, the catalytic formaldehyde oxidation performance also increases, indicating that the Ir nanoclusters are the catalytic reaction active centers in the process of catalytic formaldehyde elimination.

Example 19: Investigating the influence of the pretreatment atmosphere on the catalytic formaldehyde removal performance of the catalyst

The difference from Example 1 is that the catalyst 1.5IrAl-NP was pre-reduced at 300°C for 0.5h in an atmosphere of 20vol.% O ₂ /He, and purged with helium to room temperature, and then used for formaldehyde oxidation reaction evaluation. The results are shown in Figure 5, which shows that the catalytic activity is greatly reduced after the oxygen atmosphere treatment, and the conversion rate is less than 20% at room temperature, indicating that the pretreatment atmosphere of the catalyst has an important impact on the catalytic activity.

Example 20: Investigating the influence of the catalyst preparation method on the stability of the catalytic formaldehyde oxidation reaction

80 mg of the catalysts prepared in Example 1 and Comparative Examples 1 and 2 were packed in a quartz reaction tube. Before the reaction, the catalyst was reduced at 300° C. for 0.5 h in an atmosphere of 20 vol.% H ₂ /He, and helium was purged to room temperature. The pretreated catalyst was used for the evaluation of formaldehyde oxidation stability. The results are shown in Figure 6, which shows that the three catalysts prepared by the colloid-deposition method, the deposition-precipitation method and the equal-volume impregnation method all have stable reaction performance in the formaldehyde oxidation reaction, but the catalytic activity shows obvious differences, at 1.5Ir/ The formaldehyde elimination performance on the Al-NP catalyst is the highest, followed by the 1.5Ir/Al-DP catalyst, and the worst on the 1.5Ir/Al-IMP catalyst. The order of activity is consistent with the results in Figure 3.

Embodiment 21: catalytic NJ-DT-3 decomposition reaction rate test

The catalysts of Example 1 and Comparative Examples 1 and 2 were reduced at 300°C for 0.5h under a hydrogen atmosphere of 20vol.% H ₂ /He, then filled in the reactor, and a certain amount of NJ-DT-3 raw material was added to reach the test temperature Then record the reaction time. The decomposition reaction rate of NJ-DT-3 can be expressed by TOF, that is, the amount of reactants converted by a unit active site in a unit time, and the calculation formula is as follows:

Wherein, C _DT-3 is the conversion rate of NJ-DT-3, t is the reaction time after reaching the reaction temperature, and M _Ir/DT-3 is the molar ratio of Ir and raw material NJ-DT-3 in the catalyst added.

The results are shown in Figure 7, which shows that the reaction rate of the 1.5IrAl-NP catalyst prepared by the preparation method of the present invention catalyzed the decomposition of NJ-DT-3 is higher than that of the catalyst prepared by the traditional deposition precipitation method and equal volume impregnation method.

Claims (1)

1. The application of the iridium cluster catalyst loaded on alumina is characterized in that: the catalyst is used for formaldehyde elimination reaction, the dosage of the catalyst is 100mg, the composition of raw material gas for reaction is 180ppm HCHO, and 20vol.% O ₂ He is balance gas, relative humidity is 50%, and total flow is 50mL min ^-1 STP, mass space velocity of 3' \ 10005Ohio 10 ⁴ mL g _cat ^-1 h ^-1 The testing temperature is 20 ℃; catalyst before reaction testing at 20vol.% H ₂ Reducing for 0.5h at 300 ℃ in a/He atmosphere; the preparation method of the catalyst comprises the following steps of dissolving 1.0g of chloroiridic acid in 50mL of ethylene glycol, wherein the concentration of metallic iridium ions in the solution is 39mmol L ^-1 50mL of the solution was added at a concentration of 0.25mol L ^-1 Adding sodium hydroxide and ethylene glycol solution into the solution, stirring the solution at room temperature for 0.5h, transferring the solution into an oil bath at 160 ℃, stirring the solution for reaction for 1h under the protection of argon atmosphere to obtain Ir nano sol, measuring 4.0mL of the Ir nano sol, and dropwise adding the Ir nano sol into the Al with the particle size of 10 to 12nm which is stirred vigorously ₂ O ₃ Reacting in the carrier suspension at 80 ℃ for 3h, aging for 1h, filtering and washing while the solution is hot, and drying at 80 ℃ for 12h to obtain 1.5wt.% Ir/Al ₂ O ₃ A catalyst.

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