CN118217997A - An integral VOCs catalyst with multiple active component layers and its preparation method and application - Google Patents

Abstract

The invention provides an integral VOCs catalyst with multiple active component layers, and a preparation method and application thereof. The preparation method comprises the following steps: dissolving metal salt, a precipitator and a template agent in a solvent, stirring to obtain a mixed solution, carrying out primary hydrothermal reaction on the mixed solution and foam metal, and washing, ultrasonic treatment, drying and roasting a product of the primary hydrothermal reaction to obtain a roasting product; the metal salt comprises cobalt nitrate hexahydrate; and carrying out secondary hydrothermal reaction on KMnO ₄ and the roasting product, and washing and drying the product of the secondary hydrothermal reaction to obtain the monolithic VOCs catalyst with multiple active component layers. The invention selects foam metal as a carrier, and the prepared multi-active-component layered monolithic catalyst has higher catalytic activity.

Description

An integral VOCs catalyst with multiple active component layers and its preparation method and application

Technical Field

The present invention belongs to the technical field of catalyst preparation, and in particular relates to an integral VOCs catalyst with multiple active component layers, and a preparation method and application thereof.

Background technique

Volatile organic compounds (VOCs) are pollutants that are commonly found in outdoor and indoor air. They are volatile, easy to diffuse, and toxic. They can not only harm human health through contact and inhalation, but can also react photochemically with some gaseous pollutants (such as NO ₂ and SO ₂ ) to form photochemical smog. In addition, they can also react with strong oxidants in the atmosphere to generate secondary organic aerosols, which cause serious harm to the ecological environment.

There are many methods for purifying VOCs, which are mainly divided into recovery technology (absorption method, adsorption method, membrane separation method, condensation method, etc.) and destruction technology (plasma method, biological method, incineration method, catalytic oxidation method, photocatalytic method, etc.). Among them, catalytic oxidation method can oxidize VOCs into carbon dioxide and water or other relatively harmless substances. It has become the most effective and feasible technology with its advantages of high efficiency, low energy consumption, low secondary pollution and simple operation. In catalytic oxidation, the rate of degradation of VOCs depends largely on the physical and chemical properties of the catalytic material.

The current catalyst research is usually in powder or granular form, which has problems such as large pressure drop, high bed resistance, difficult separation, and poor recyclability, which is not conducive to practical application. Therefore, researchers are very concerned about the development and application of monolithic catalysts. Most of the monolithic catalysts currently used are prepared by extrusion molding or impregnation. However, both methods have the disadvantages of complex sample preparation, easy agglomeration of surfactants, poor stability between nanoparticles and carriers, which leads to easy desulfurization of active ingredients, and it is difficult to ensure catalytic performance. Although precious metal catalysts have good catalytic activity, they have problems such as poisoning and deactivation, high price, resource shortage, and easy sintering and poisoning.

CN114984942A discloses a method for preparing a catalyst for catalytic combustion of VOCs. First, two types of metal oxides, cerium and aluminum, are stirred, dissolved, centrifuged, washed, dried and roasted to prepare a metal oxide combined carrier, and then the carrier is modified by sodium hypophosphite, and finally Pt is impregnated and loaded onto the carrier. After centrifugation, washing, drying and roasting, the final noble metal powder catalyst with metal oxide as the carrier is obtained. However, the preparation process of this technology is complicated. Whether it is the preparation, modification or loading of Pt, operations such as washing, drying and roasting are required, and roasting requires high requirements for atmosphere and requires hydrogen/argon, which is costly. In addition, this technology selects noble metal Pt as the active component. Although noble metals have good catalytic activity, they are expensive, have high economic cost, and are easy to sinter. At the same time, the catalyst prepared by this technology is in powder form. There are problems such as large pressure drop, difficulty in separation, and poor recyclability in the application of powder catalysts, which is not conducive to industrial application. If the powder catalyst is prepared into a monolithic catalyst by methods such as extrusion molding or coating, subsequent complex operations are also required.

CN112958110A discloses a composite catalyst for low-concentration VOCs, firstly modifying γ- _Al2O3 , then mixing the modified γ _{- Al2O3} with copper oxide and manganese oxide, ball milling to prepare coating slurry, and finally applying the coating slurry to ceramic or iron-chromium- _aluminum carrier for drying and roasting. However, the preparation of this technology is complicated, the modification steps of γ- _Al2O3 are numerous, and the sources of _copper oxide and manganese oxide as active components are not explained, and the synthesis preparation also requires other steps. In addition, the mixing mode of γ- _Al2O3 and copper _oxide and manganese oxide is random, and uneven phenomenon is prone to occur. In addition, this technology adopts the coating mode to load the active component, and the coating process is cumbersome and difficult to control the thickness and uniformity of the coating, the active phase components are prone to agglomeration, and the bonding property of the active component and the carrier is poor, and it is easy to fall off.

Therefore, there is a need for a non-precious metal monolithic catalyst that is highly efficient, low-cost, and suitable for industrial production and application.

Summary of the invention

In order to solve the above technical problems, the purpose of the present invention is to provide a monolithic VOCs catalyst with multiple active component layers and a preparation method and application thereof. The catalyst is a monolithic catalyst with multiple active component layers using foam metal as a carrier.

To achieve the above object, the present invention provides a method for preparing a monolithic VOCs catalyst having a multi-active component layer, wherein the preparation method comprises:

(1) dissolving a metal salt, a precipitant, and a template agent in a solvent, stirring to obtain a mixed solution, subjecting the mixed solution to a hydrothermal reaction with a foamed metal, and washing the product of the hydrothermal reaction with water, ultrasonicating, drying, and roasting to obtain a roasted product; the metal salt comprises cobalt nitrate hexahydrate;

(2) _KMnO4 is subjected to a secondary hydrothermal reaction with the calcined product, and the product of the secondary hydrothermal reaction is rinsed with water and dried to obtain an integral VOCs catalyst having multiple active component layers.

According to a specific embodiment of the present invention, preferably, in step (1), the molar ratio of the metal salt:precipitant:template is 2-8:10-40:0-32, more preferably 3-6:10-20:1-16.

According to a specific embodiment of the present invention, preferably, in a hydrothermal reaction, the mass ratio of the sum of the mass of the metal salt, the precipitant and the template to the mass of the pretreated foamed metal is 1.6-8.1:1.

According to a specific embodiment of the present invention, preferably, in the secondary hydrothermal reaction, the mass ratio of the _KMnO4 to the calcined product is 0.22-2.2:1.

According to a specific embodiment of the present invention, preferably, in step (1), the ultrasonic time is 2-10 min, preferably 5 min.

According to a specific embodiment of the present invention, preferably, in step (1), the stirring time is 10-40 min, preferably 30 min.

According to a specific embodiment of the present invention, preferably, the KMnO ₄ is reacted in the form of a KMnO ₄ solution, which is obtained by dissolving KMnO ₄ in water and stirring for 30 minutes. In the secondary hydrothermal reaction, the use of the KMnO ₄ solution can form _{MnO 2 without} calcination, and there is no need to add a precipitant again, which simplifies the preparation steps.

In some specific embodiments, preferably, the solubility of the _KMnO4 solution is 0.01-0.1 mol/L.

According to a specific embodiment of the present invention, preferably, the foam metal is foam nickel and/or foam copper.

According to a specific embodiment of the present invention, preferably, the foam metal has a thickness of 1-2 mm and a pore density of 80-120 ppi.

According to a specific embodiment of the present invention, preferably, the metal salt may further include other metal salts, that is, the metal salt may be cobalt nitrate hexahydrate or a combination of cobalt nitrate hexahydrate and other metal salts.

According to a specific embodiment of the present invention, preferably, the other metal salts include one or a combination of two or more of ferric nitrate nonahydrate, aluminum nitrate nonahydrate, nickel nitrate nonahydrate, copper nitrate trihydrate, zinc nitrate hexahydrate, magnesium nitrate hexahydrate, manganese nitrate, cerium nitrate hexahydrate, and lanthanum nitrate hexahydrate.

According to a specific embodiment of the present invention, preferably, when the metal salt is a combination of cobalt nitrate hexahydrate and other metal salts, the molar ratio of the cobalt nitrate hexahydrate to other metal salts is 2-4:1-2, more preferably 2:1.

According to a specific embodiment of the present invention, preferably, when the other metal salts are two or more metal salts, the total molar amount of the metal salts used can be kept consistent, and a suitable molar ratio can be selected, such as cobalt nitrate hexahydrate: iron nitrate nonahydrate: cerium nitrate hexahydrate = 1-3:0.5-1:0.1-0.5, more preferably 2:0.5:0.5.

According to a specific embodiment of the present invention, preferably, the template is ammonium fluoride and/or cetyltrimethylammonium bromide (CTAB).

According to a specific embodiment of the present invention, preferably, the precipitant is urea. Selecting urea as the precipitant can make the active component in situ deposited (in situ grown) on the surface of the nickel foam; at the same time, the amount of urea added will also affect the deposition effect. If urea is not added, the loading amount of the active component on the foam metal will be reduced.

According to a specific embodiment of the present invention, preferably, the solvent of the mixed solution includes one or a combination of two or more of water, ethanol, and methanol. More preferably, when the added amount of the metal salt is 2-8 mmol, the volume of the solvent is 50-150 ml.

According to a specific embodiment of the present invention, preferably, the preparation method further comprises the step of soaking the foam metal in dilute hydrochloric acid and ethanol in sequence for pretreatment; more preferably, the pretreatment step is soaking the foam metal in dilute hydrochloric acid and ethanol in sequence for ultrasonic treatment, and then washing with water and drying to obtain the pretreated foam metal. Further preferably, the solubility of the dilute hydrochloric acid can be 0.5-3 mol/L; the ultrasonic time of the pretreatment step can be controlled to 10-20 min, preferably 15 min.

According to a specific embodiment of the present invention, preferably, the temperature of the primary hydrothermal reaction is 100-160° C., and the time of the primary hydrothermal reaction is 6-16 h.

According to a specific embodiment of the present invention, preferably, the temperature of the secondary hydrothermal reaction is 140-180° C., and the time of the secondary hydrothermal reaction is 12-24 h.

According to a specific embodiment of the present invention, preferably, in step (1), the calcination temperature is 300-500° C., and the calcination time is 1-3 h.

According to a specific embodiment of the present invention, preferably, a recovery process is further included after the primary hydrothermal reaction.

In some specific embodiments, preferably, the recovery process comprises: taking out and mixing the liquid after the first hydrothermal reaction, the liquid for washing the product of the first hydrothermal reaction, and the liquid after ultrasound, and obtaining the Co-based oxide powder after centrifugation, drying, and roasting. More preferably, in the recovery process, the roasting temperature is 300-500° C., and the roasting time is 1-3 hours.

In the above preparation method, preferably, the specific steps of the preparation method include:

(1) Pretreatment of the support: soaking the foam metal (preferably foam nickel) in dilute hydrochloric acid and ethanol in turn and performing ultrasonic treatment for 10-20 min (preferably 15 min), then rinsing with pure water to remove the oxide layer and impurities on the surface, and drying at room temperature for later use to obtain the pretreated foam metal;

(2) Loading metal-based oxides: dissolving a metal salt (preferably Co salt), a precipitant (preferably urea), and a template (preferably ammonium fluoride) in a solvent (preferably purified water), stirring for 10-40 min (preferably 30 min) to fully mix the solutions to obtain a mixed solution, placing the mixed solution and the pretreated foamed metal into a reactor to perform a hydrothermal reaction, taking out the product after the hydrothermal reaction, rinsing with deionized water, ultrasonicating for 2-10 min (preferably 5 min), removing the active components that are not firmly loaded, drying in an oven, and finally placing in a muffle furnace for calcination to obtain a calcined product, i.e., a metal-based oxide monolithic catalyst;

(3) Extraction of Co-based oxide powder: The liquid in the reactor after the first hydrothermal reaction in step (2), the liquid after washing the product, and the liquid after ultrasonication are taken out and placed in a container for mixing, and then centrifuged and dried. Finally, the powder is placed in a muffle furnace for calcination to obtain a calcined product, i.e., Co-based oxide powder;

(4) Loading MnO ₂ : Putting KMnO ₄ solution and the calcined product obtained in step (2) into a reactor for secondary hydrothermal reaction, the product after the secondary hydrothermal reaction is rinsed with deionized water and dried to obtain a MnO ₂ composite Co-based oxide monolithic VOCs catalyst with multiple active component layers.

The present invention also provides an integral VOCs catalyst having multiple active component layers, which is obtained by the preparation method of the integral VOCs catalyst.

The present invention also provides the use of the above-mentioned integral VOCs catalyst with multiple active component layers in the field of purifying volatile organic compound pollutants.

Compared with the prior art, the present invention has the following beneficial effects:

(1) The present invention selects porous foam metal as a catalyst carrier and adopts a simple hydrothermal method + calcination method to prepare a multilayer _MnO2 composite Co-based oxide monolithic catalyst, which has the characteristics of simple process, low cost, firm active components, convenience and high efficiency.

(2) Co-based oxides themselves have good catalytic activity for VOCs. Using Co-based oxides as the middle layer can not only increase the specific surface area of the overall catalyst, but also provide more sites for the loading of MnO _2. Moreover, during the secondary hydrothermal process, potassium permanganate hydrothermally decomposes to produce MnO ₂ , and a redox reaction also occurs between MnO ₄ ^- and the middle layer Co-based oxide, thereby increasing the content of Mn ³⁺ and improving the activity of the catalyst.

(3) The preparation method of the present invention can not only obtain the integral catalyst, but also separate and recover high-purity Co-based oxide powder. The powder can be used to prepare other integral catalysts by coating, extrusion molding and other methods, and can also be used in other fields, thereby achieving efficient utilization of raw materials and avoiding waste.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microscope image of the Co ₃ O ₄ /NF monolithic catalyst prepared in Example 3. FIG.

FIG. 2 is a scanning electron microscope image of the MnO ₂ /Co ₃ O ₄ /NF-0.05 monolithic VOCs catalyst prepared in Example 3. FIG.

Detailed ways

In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be construed as limiting the applicable scope of the present invention.

Example 1

This embodiment provides a monolithic VOCs catalyst having multiple active component layers, and the preparation method thereof comprises:

The nickel foam with a pore density of 110 ppi was immersed in 1 mol/L dilute hydrochloric acid and anhydrous ethanol in turn, and ultrasonically treated for 15 min respectively, then rinsed with pure water three times, and dried at room temperature to obtain the pretreated nickel foam.

6mmol of cobalt nitrate hexahydrate, 20mmol of urea and 16mmol of ammonium fluoride were dissolved in 75ml of pure water and stirred for 30 minutes to make the solution fully mixed. The mixed solution and a pre-treated 0.73g nickel foam (10cm×2cm×0.15cm) were placed in a 100ml reactor and maintained at 100°C for 14h in an oven (the heating rate of the oven was 2°C/min) to perform a hydrothermal reaction. The foam nickel catalyst product after the hydrothermal reaction was taken out, washed twice with deionized water, placed in a deionized water solution for ultrasonic treatment for 5min, dried at 50°C for 12h, and finally placed in a muffle furnace at a rate of 2°C/min to 400°C for calcination for 2h to obtain a Co ₃ O ₄ /NF monolithic catalyst.

The solution in the reactor after the first hydrothermal reaction, the solution after washing once, and the solution after ultrasound were mixed together, then placed in a centrifuge and centrifuged at a speed of 3000r/min for 2min, the supernatant was removed, and the bottom material was left. It was placed in an oven and dried at 100°C for 12h, and finally placed in a muffle furnace and calcined at a rate of 2°C/min to 400°C for 2h to recover the _{Co3O4 powder} catalyst.

3.75mmol _KMnO4 was dissolved in 75ml pure water and stirred for 30 minutes. The _KMnO4 solution and _{the Co3O4 /} NF monolithic catalyst obtained above were placed in a 100ml reactor and maintained at 160°C in an oven for 24h (the heating rate of the oven was 2°C/min) for a secondary hydrothermal reaction. The product after the secondary hydrothermal reaction was taken out, rinsed three times with deionized water, and dried at 50°C for 12h to obtain a monolithic VOCs catalyst with multiple active component layers, namely, a _MnO2 composite Co-based oxide monolithic VOCs catalyst ( _MnO2 / _Co3O4 /NF- _0.05 ).

Example 2

The 3.75 mmol KMnO ₄ in Example 1 was replaced by 0.75 mmol KMnO ₄ , and other reaction conditions remained unchanged, to obtain a MnO ₂ /Co ₃ O ₄ /NF-0.01 monolithic VOCs catalyst.

Example 3

In this example, a Co ₃ O ₄ /NF monolithic catalyst was obtained according to the formula and preparation process of the cobalt-based oxide loaded in Example 1. The scanning electron microscope image of the Co ₃ O ₄ /NF monolithic catalyst is shown in FIG1 , and it can be seen that Co ₃ O ₄ is evenly distributed on the surface of NF in the shape of nano needles, and the nano needles grow vertically to form a nano spherical structure.

Then, 3.75 mmol of KMnO ₄ in Example 1 was replaced with 7.5 mmol of KMnO ₄ , and other reaction conditions remained unchanged to obtain MnO ₂ /Co ₃ O ₄ /NF-0.1 monolithic VOCs catalyst. The scanning electron microscope image of the MnO ₂ /Co ₃ O ₄ /NF-0.05 monolithic VOCs catalyst is shown in FIG2 , and it can be seen that MnO ₂ successfully encapsulates Co ₃ O ₄ and is distributed in the outermost layer, presenting a nanoflower shape composed of nanosheets.

Example 4

The 6 mmol of cobalt nitrate hexahydrate in Example 3 was replaced by a mixed metal salt of 4 mmol of cobalt nitrate hexahydrate and 2 mmol of aluminum nitrate nonahydrate, and other reaction conditions remained unchanged, to obtain a MnO ₂ /Co ₂ AlO ₄ /NF monolithic VOCs catalyst.

Comparative Example 1

This comparative example provides a monolithic catalyst having a single active component layer, and the preparation method thereof comprises:

6mmol of cobalt nitrate hexahydrate, 20mmol of urea and 16mmol of ammonium fluoride were dissolved in 75ml of pure water and stirred for 30 minutes to fully mix the solution. The mixed solution and a piece of pretreated nickel foam (10cm×2cm×0.15cm) were placed in a 100ml reactor and maintained at 100°C for 14h in an oven (the heating rate of the oven was 2°C/min) for hydrothermal reaction. After the reaction, the nickel foam catalyst was taken out, washed twice with deionized water, placed in a deionized water solution for ultrasonic treatment for 5min, dried at 50°C for 12h, and finally placed in a muffle furnace at a rate of 2°C/min to 400°C for calcination for 2h to obtain a Co ₃ O ₄ /NF monolithic catalyst.

Comparative Example 2

This comparative example provides a monolithic catalyst having a single active component layer, and the preparation method thereof comprises:

3.75mmol _KMnO4 was dissolved in 75ml pure water and stirred for 30 minutes. The _KMnO4 solution and a pretreated nickel foam (10cm×2cm×0.15cm) were placed in a 100ml reactor and maintained at 160°C in an oven for 24h (the heating rate of the oven was 2°C/min) for hydrothermal reaction. After the reaction, the catalyst was taken out, rinsed with deionized water three times, and dried at 50°C for 12h to obtain a _MnO2 /NF monolithic catalyst.

The catalytic activity of the catalysts obtained in Examples 1-4 and Comparative Examples 1-2 was tested for VOCs pollutants:

n-heptane was selected as the test pollutant of VOCs for catalytic activity test, the pollutant concentration was 500ppm, the oxygen content was 21%, the balance gas was nitrogen, the experimental space velocity was 20000ml/g·h, and the monolithic catalysts prepared in Examples 1-4 and Comparative Examples 1-2 were cut into catalysts with specifications of 10cm×0.3cm×0.15cm and a mass of 0.14g for activity test. The specific test results are shown in Table 1. Among them, T ₅₀ represents the temperature when the n-heptane removal efficiency reaches 50%; T ₉₀ represents the temperature when the n-heptane removal efficiency reaches 90%. The lower the reaction temperature, the better the catalytic activity of the catalyst.

Table 1. Catalytic activity test data of VOCs pollutants

catalyst Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 _T50 /℃ 176.4 173.6 163.9 160.9 188.7 225.0 _T90 /℃ 211.9 200.0 196.9 195.5 217.8 262.7

As can be seen from Table 1, the catalytic activity of the _MnO2 composite Co-based oxide catalyst with multiple active component layers is significantly better than that of _{the Co3O4} /NF monolithic catalyst (Comparative Example 1) and the _MnO2 /NF monolithic catalyst (Comparative Example 2) with a single active component layer. The content of the surface _MnO2 and the type of the Co-based oxide in the middle layer have _an important influence on the activity of the composite catalyst. Among them, the activity of Mn/ _Co3O4 /NF has no linear relationship with the loading amount of the surface _MnO2 , which may be attributed to the difference in the active center of the catalyst when the content of _MnO2 is different. When the Mn content is high, Mn is the main active center, and when the Mn content is low, Co or Mn-Co is the main active center.

Claims (12)

1. A method for preparing monolithic VOCs catalysts having multiple active component layers, wherein the method comprises:

Dissolving metal salt, a precipitator and a template agent in a solvent, stirring to obtain a mixed solution, carrying out primary hydrothermal reaction on the mixed solution and foam metal, and washing, ultrasonic treatment, drying and roasting a product of the primary hydrothermal reaction to obtain a roasting product; the metal salt comprises cobalt nitrate hexahydrate; carrying out a secondary hydrothermal reaction on KMnO ₄ and the roasting product, and washing and drying the product of the secondary hydrothermal reaction to obtain an integral VOCs catalyst with multiple active component layers;

wherein the metal salt: and (3) a precipitant: the molar ratio of the template agent is 2-8:10-40:0-32;

in the primary hydrothermal reaction, the mass ratio of the sum of the mass of the metal salt, the mass of the precipitant and the mass of the template agent to the mass of the foam metal is 1.6-8.1:1;

In the secondary hydrothermal reaction, the mass ratio of KMnO ₄ to the calcined product is 0.22-2.2:1.

2. The method of claim 1, wherein the KMnO ₄ is reacted as KMnO ₄ solution; the solubility of the KMnO ₄ solution is 0.01-0.1mol/L.

3. The preparation method according to claim 1, wherein the metal foam is nickel foam and/or copper foam;

Preferably, the metal foam has a thickness of 1-2mm and a cell density of 80-120ppi.

4. The preparation method of claim 1, wherein the metal salt further comprises other metal salts, the other metal salts comprising one or a combination of two or more of ferric nitrate nonahydrate, aluminum nitrate nonahydrate, nickel nitrate nonahydrate, cupric nitrate trihydrate, zinc nitrate hexahydrate, magnesium nitrate hexahydrate, manganese nitrate, cerium nitrate hexahydrate, lanthanum nitrate hexahydrate;

preferably, when the metal salt is a combination of cobalt nitrate hexahydrate with other metal salts, the cobalt nitrate hexahydrate: the molar ratio of other metal salts is 2-4:1-2.

5. The method of claim 1, wherein the template is ammonium fluoride and/or cetyltrimethylammonium bromide;

Preferably, the precipitant is urea.

6. The preparation method according to claim 1, further comprising a step of pre-treating the foam metal by immersing the foam metal in dilute hydrochloric acid and ethanol in sequence;

preferably, the solubility of the dilute hydrochloric acid is 0.5-3mol/L.

7. The preparation method according to claim 1, wherein the temperature of the primary hydrothermal reaction is 100-160 ℃, and the time of the primary hydrothermal reaction is 6-16h.

8. The preparation method according to claim 1, wherein the temperature of the secondary hydrothermal reaction is 140-180 ℃, and the time of the secondary hydrothermal reaction is 12-24 hours.

9. The production method according to claim 1, wherein the baking temperature is 300 to 500 ℃ and the baking time is 1 to 3 hours.

10. The production method according to claim 1, wherein a recovery process is further included after the one-time hydrothermal reaction;

preferably, the recovery process comprises: taking out and mixing the liquid after the primary hydrothermal reaction, the liquid for flushing the product of the primary hydrothermal reaction and the liquid after the ultrasonic treatment, and obtaining Co-based oxide powder after centrifugation, drying and roasting;

More preferably, the temperature of the calcination is 300-500 ℃ and the time of the calcination is 1-3 hours during the recovery process.

11. A monolithic VOCs catalyst having multiple active component layers, obtained by the process of preparing the monolithic VOCs catalyst of any one of claims 1-10.

12. Use of monolithic VOCs catalysts having multiple active component layers according to claim 11 in the field of purifying volatile organic compounds contaminants.