CN104950014A - Chlorinated hydrocarbons volatile gas sensing material and preparation method thereof - Google Patents

Abstract

The invention discloses a composite oxide sensing material for chlorinated hydrocarbon volatile gases and a preparation method thereof. The present invention first adopts the co-precipitation method to prepare the composite metal hydroxide precursor; then the precursor is placed in a muffle furnace to roast to form a composite metal oxide material; finally, the composite metal oxide material is prepared into a gas sensor for use in Detection of low concentration chlorinated hydrocarbon gases. Composite oxide materials are prepared with cheap raw materials and a simple synthesis process, and the controllable preparation of a series of composite oxides is realized through micro-control of the preparation conditions, and then the macro-control of its gas-sensing performance is realized, and the chlorinated hydrocarbons are screened out. The composite material with low detection limit, high sensitivity, excellent selectivity and repeatability of inert gas has great development potential and is conducive to promotion. Especially for ZnO/Al ₂ O ₃ /CuO composite oxide materials, CuO as a reactant and Al ₂ O ₃ as a dispersant during the gas sensing response process significantly improved the response and selectivity of ZnO materials to low-concentration chlorinated hydrocarbons.

Description

A kind of chlorinated hydrocarbon volatile gas sensing material and preparation method thereof

technical field

The invention belongs to the technical field of gas sensing, and in particular relates to a composite oxide sensing material for chlorinated hydrocarbon volatile gases and a preparation method thereof.

Background technique

Volatile organic compounds (VOCs) not only cause indoor air pollution, but also lead to increased concentrations of photochemical smog, secondary organic aerosols, and atmospheric organic acids under the action of light, affecting human health and the atmospheric environment. Volatile organic compounds of chlorinated hydrocarbons are one of VOCs, which refer to organic compounds obtained by replacing hydrogen atoms in alkanes with chlorine atoms. They are often used as important organic solvents and product intermediates in industrial production, and are carcinogenic and mutagenic. , Mutagenic hazards. For example, chloroform is an organic synthetic raw material for the production of freon, dyes and medicines. It is a toxic and suspected carcinogen, and it can produce highly toxic phosgene when exposed to light. At present, there are many technologies for the detection of chlorinated hydrocarbons in water, such as confocal laser-induced fluorescence, gas chromatography, high-performance liquid chromatography, and mid-infrared detection. The above instruments are expensive and cannot meet the needs of portable detection. At present, among the gas detection methods, the most important and common method is gas sensor detection. The gas sensor is simple, easy to carry, and low in price, and is very suitable for the routine detection of toxic and harmful gases. Among them, metal oxide gas sensors are widely used in gas sensors with fast response, high sensitivity and good repeatability. But it still has inherent shortcomings, such as low detection sensitivity at low concentrations, high working temperature and so on. At present, the gas-sensing performance is mainly improved by compounding, doping and changing the morphology.

Among semiconductor materials, ZnO is a basic gas-sensing sensing material that has been studied more. It has high sensitivity to gases and fast response, but has poor selectivity and responds to most gases. Al ₂ O ₃ is mainly used as a dispersing and carrying agent, which can well prevent material agglomeration. We propose to prepare composite metal oxide materials through a simple hydrothermal method and co-precipitation followed by high-temperature sintering to improve the gas-sensing properties of the target. Therefore, it is of far-reaching significance to develop and improve the preparation technology of simple hydrothermal composite materials.

Contents of the invention

The purpose of the present invention is to provide a method for preparing a composite oxide material for detecting chlorinated hydrocarbon volatile gases, prepare the material into a gas sensor, and explore its sensitivity to chlorinated hydrocarbon gases. The element ratio screens out the sensing material with the best gas-sensing performance.

The technical scheme of the present invention is as follows: firstly, the composite metal hydroxide precursor is prepared by co-precipitation; then the precursor is placed in a muffle furnace for roasting to form a composite metal oxide material; Sensitive element, used in the detection of low concentration chlorinated hydrocarbon gases.

The preparation method of chlorinated hydrocarbons volatile gas sensor of the present invention, its specific process steps are as follows:

Prepare a mixed salt solution of soluble divalent metal salt and soluble trivalent aluminum salt, wherein the molar ratio of divalent metal cation M ²⁺ to Al ³⁺ is 1-4, and the concentration of divalent metal cation M ²⁺ is 0.01-0.1 mol/L; prepare an alkali solution with a molar ratio of NaOH and Na ₂ CO ₃ of 1-16, and the concentration of NaOH is 1-5mol/L; drop the alkali solution and mixed salt solution into the reaction bottle at the same time, and control the The pH is between 6-10; after the dropwise addition, transfer to a high-pressure reactor, hydrothermally crystallize at 80-150°C for 1-24 hours, wash by centrifugation, and dry at 20-100°C;

(2) Roasting the precursor prepared in step (1), raising the temperature to 500-1000°C at a heating rate of 5-10°C/min, and keeping it warm for 1-5 hours to obtain a composite metal oxide;

(3) Preparation of gas sensor:

Grind the composite metal oxide obtained in step (2), add a solvent to make a paste, and then apply it on the ceramic tube sensor or interdigital electrode sensor with a coating thickness of 0.1-1 mm; finally place it on the sensor aging table , Aging for 12-120 hours at a current of 80-120mA.

The M ²⁺ is selected from one or both of Cu ²⁺ , Zn ²⁺ , Fe ²⁺ and Cd ²⁺ . Zn ²⁺ and Cu ²⁺ in a molar ratio of 1-10 are preferred.

Described solvent is water, ethanol or ethylene glycol.

The volatile gas of chlorinated hydrocarbons is one or more of monochloromethane, dichloromethane, trichloromethane and tetrachloromethane.

The advantages of the present invention are: the composite oxide material is prepared with cheap raw materials and simple synthesis process, and the controllable preparation of a series of composite oxides is realized through the microscopic regulation of the preparation conditions, and then the macroscopic regulation of its gas-sensing performance is realized, and the A composite material with low detection limit, high sensitivity, excellent selectivity and repeatability for chlorinated hydrocarbon volatile gases has great development potential and is conducive to popularization. Especially for ZnO/Al ₂ O ₃ /CuO composite oxide materials, CuO as a reactant and Al ₂ O ₃ as a dispersant during the gas sensing response process significantly improved the response and selectivity of ZnO materials to low-concentration chlorinated hydrocarbons.

Description of drawings

Fig. 1 is the response value to chloroform gas at different working temperatures of the gas sensor made of the composite oxide after roasting obtained in Examples 1, 2, 3 and 4.

Fig. 2 is the response curve of the gas sensor made of the calcined composite oxide material obtained in Example 1 of the present invention to different concentrations of chloroform.

Fig. 3 is the response recovery curve of the gas sensor made of the composite oxide obtained in Example 1 to 5 ppm concentration of chloroform.

Detailed ways

【Example 1】

1. Preparation of composite metal oxide materials

Weigh Zn(NO ₃ ) ₂ 6H ₂ O (9.66g, 0.04mol), Al(NO ₃ ) ₃ 9H ₂ O (7.5g, 0.02mol), Cu(NO ₃ ) ₂ .3H ₂ O (5.95 g, 0.02mol) was dissolved in 200ml of deionized water and stirred evenly to obtain a mixed salt solution, which was recorded as solution A; NaOH (6.4g, 0.16mol), Na ₂ CO ₃ (1.06g, 0.01mol) were dissolved in 80ml of deionized The alkali solution obtained in water is recorded as solution B; solution A and solution B are respectively put into burettes, and double-dropped into the four-necked flask at a uniform speed, and the pH is controlled during the dropping process = 8, and 30 rpm is continuously stirred; the mixed solution is transferred to the reaction In the kettle, react at 100°C for 12 hours; transfer the reacted solution to a centrifuge bucket, centrifuge at 3500 rpm, wash with deionized water until pH = 7; dry the precipitate at 60°C; weigh an appropriate amount of product in a crucible , the heating rate of 5°C/min was raised to 500°C and calcined for 5 hours to obtain a composite oxide material ZnO/Al ₂ O ₃ /CuO (Zn:Al:Cu=1:0.5:0.5). The gas sensitivity response to 50ppm chloroform was measured at the optimum working temperature of 200°C, the sensitivity value was 8.5, and the response time was 20s. In addition, the gas sensor can detect 1ppm chloroform, has a lower detection limit and good repeatability.

2. Preparation of gas sensor

Take a small amount of roasted samples in an agate mortar, add appropriate amount of deionized water to make a paste. Apply the adjusted slurry on the ceramic tube with a brush, and the thickness of the coating is 0.5 mm. Use a soldering iron to connect the metal wire to the base, and pass through the Ni-Cr alloy heating wire in the middle and fix it. Place the welded gas sensor on a special aging table, set 80mA current, and age for 12 hours. After aging, its gas sensitivity to chlorinated hydrocarbon gas was measured.

3. Gas sensitivity performance test

3.1 Test the optimum working temperature of the gas sensor

a. Use a hair dryer to blow clean the 5L gas distribution bottle, vacuum it with a vacuum pump, then use a micro-sampler to extract the chlorinated hydrocarbon liquid, put it into the gas distribution bottle to prepare a 50ppm chlorinated hydrocarbon gas atmosphere, fully volatilize it, and set it aside.

b. Change the heating temperature of the device by adjusting the current through the heating wire. Test the resistance change of the device under different currents, and record the corresponding R _a and R _g values. The gas response value of the device S=R _a /R _g , according to the recorded changes of _Ra and R _g , use Origin to draw a graph to obtain the change curve of the device response value with the heating current, so as to obtain the best working temperature of the gas sensor. As shown in Figure 1.

3.2 Test the relationship between gas sensitivity response value and concentration

Under the optimum working current of the device, a series of concentrations of chlorinated hydrocarbon gases were sequentially prepared in a 5L gas cylinder, and the relationship curve between the gas sensor response value and the concentration of the gas sensor to chlorinated hydrocarbon gases at different concentrations was measured, as shown in Figure 2 It is the relationship curve between gas sensor response value and concentration of gas sensor to chloroform volatile gas.

3.3 Test repeatability curve for chlorinated hydrocarbon gases

a. Use a hair dryer to blow clean the 5L gas distribution bottle, vacuum it with a vacuum pump, then extract the chlorinated hydrocarbon liquid with a 1μL micro-sampler, inject it into the gas distribution bottle to prepare a 5 ppm chlorinated hydrocarbon atmosphere, fully volatilize it, and set it aside.

b. Adjust the current to the best working condition of the device, the stable resistance value of the gas sensor in the air, and then place the element in a 5ppm chlorinated hydrocarbon atmosphere, measure the resistance value of the element in the target gas, and when the resistance value is stable, Re-exposed to the air, the resistance value became stable again, repeated 4 times, measured the corresponding R _a , R _g values, and imported the measured data into the Origin software for drawing. Figure 3 is the response recovery curve of the component against the volatile gas of chloroform .

The gas sensing performance evaluation method above adopts the chemical gas sensing-8 (CGS-8) testing system of Beijing ELITE TECH to measure the response performance of the prepared device to chlorinated hydrocarbon volatile gases. Device Sensitivity S= _Ra / _Rg , _Ra - base resistance in air, _Rg - resistance in target gas. Response time is defined as the time when the sensor is exposed to the target gas, and the resistance changes from R _a to R _a -90%×(R _a -R _g ), and the recovery time is defined as the time from R _g to R _g when the sensor removes the target gas and is placed in air +90% x (R _a - R _g ) time.

[Example 2]

1. Preparation of materials

Weigh (Zn(NO ₃ ) ₂ 6H ₂ O (12.08g, 0.05mol), Al(NO ₃ ) ₃ 9H ₂ O (7.5g, 0.02mol), Cu(NO ₃ ) ₂ ^. 3H ₂ O( 2.42g, 0.01mol) was dissolved in 200 milliliters of deionized water and stirred evenly to obtain a salt solution, and all the other reaction conditions were the same as in Example 1, finally obtaining ZnO/Al ₂ O ₃ /CuO (Zn:Al:Cu=1:0.2: 0.4) Composite metal oxides.

2. Prepare the gas sensor as in Example 1.

3. The test process is as in Example 1, and the results are shown in Figure 1.

[Example 3]

1. Weigh Zn(NO ₃ ) ₂ 6H ₂ O (23.80g, 0.08mol), Al(NO ₃ ) ₃ 9H ₂ O (11.26g, 0.03mol), Cu(NO ₃ ) ₂ 3H ₂ O (2.38g, 0.01mol) was dissolved in 200mL deionized water and stirred evenly to obtain a salt solution, and all the other reaction conditions were the same as in Example 1, finally obtaining ZnO/Al ₂ O ₃ /CuO (Zn:Al:Cu=1:0.375: 0.125) composite metal oxide.

2. Prepare the gas sensor as in Example 1.

3. The test process is as in Example 1, and the results are shown in Figure 1.

【Example 4】

1. Weigh Zn(NO ₃ ) ₂ 6H ₂ O (9.66g, 0.04mol), Al(NO ₃ ) ₃ 9H ₂ O (7.5g, 0.02mol) in 200ml of deionized water and stir evenly to obtain a mixture Salt solution, and other reaction conditions are the same as those in Example 1 to finally obtain ZnO/Al ₂ O ₃ (Zn:Al=1:0.5) composite metal oxide.

2. Prepare the gas sensor as in Example 1.

3. The test process is as in Example 1, and the results are shown in Figure 1.

Claims (5)

1. a preparation method for chlorinated hydrocarbon escaping gas sensor, is characterized in that, its concrete technology step is as follows:

(1) preparation of presoma:

The mixing salt solution of preparation soluble divalent metal salt and solubility trivalent aluminium salt, wherein divalent metal M ²⁺and Al ³⁺mol ratio be 1-4, divalent metal M ²⁺concentration be 0.01-0.1mol/L; Preparation NaOH and Na ₂cO ₃mol ratio is the aqueous slkali of 1-16, and the concentration of NaOH is 1-5mol/L; Aqueous slkali and mixing salt solution are instilled in reaction bulb, in dropping process, control pH is between 6-10 simultaneously; Be transferred to after being added dropwise to complete in autoclave, in 80-150 DEG C of hydrothermal crystallizing 1-24 hour, centrifuge washing, 20-100 DEG C of drying;

(2) by presoma roasting prepared by step (1), be warming up to 500-1000 DEG C with the heating rate of 5-10 DEG C/minute, insulation 1-5 hour, obtains composite metal oxide;

(3) preparation of gas sensor:

After composite metal oxide grinding step (2) obtained, solubilizer furnishing starchiness, is then coated on ceramic pipe sensor or interdigital electrode sensor, coating thickness 0.1-1 millimeter; Finally be placed on sensor ageing platform, aging 12-120 hour under the electric current of 80-120mA.

2. preparation method according to claim 1, is characterized in that, described M ²⁺be selected from Cu ²⁺, Zn ²⁺, Fe ²⁺and Cd ²⁺in one or both.

3. preparation method according to claim 1, is characterized in that, described M ²⁺for mol ratio is the Zn of 1-10 ²⁺and Cu ²⁺.

4. preparation method according to claim 1, is characterized in that, described solvent is water, ethanol or ethylene glycol.

5. preparation method according to claim 1, is characterized in that, described chlorinated hydrocarbon escaping gas is one or more in monochloro methane, methylene chloride, methenyl choloride, tetrachloromethane.