CN110404548B

CN110404548B - high-pH-adaptability carbon nanotube catalyst and application thereof

Info

Publication number: CN110404548B
Application number: CN201910775488.8A
Authority: CN
Inventors: 许柯; 张翼飞; 任洪强; 耿金菊
Original assignee: Nanjing University
Current assignee: Nanjing University
Priority date: 2019-08-21
Filing date: 2019-08-21
Publication date: 2020-12-04
Anticipated expiration: 2039-08-21
Also published as: JP2021030217A; JP6706404B1; CN110404548A

Abstract

The invention relates to the technical field of catalytic degradation, in particular to a carbon nano tube catalyst with high pH adaptability and application thereof, wherein the carbon nano tube catalyst is prepared from the following components in a molar ratio of 1: 2: 2 Zn (NO) is weighed₃)₂·6H₂O、Fe(NO₃)₃·9H₂O and citric acid (C)₆H₈O₇) As a modifier, the modifier was mixed with CNT-P in a ratio of 1: (5-6) adding the mixture into absolute ethyl alcohol according to the mass ratio for dissolving, performing ultrasonic treatment for 30min, adding urea when no obvious water exists through water bath evaporation, and stirring and mixing; putting the obtained mixture into a tubular furnace, and calcining the mixture in a nitrogen atmosphere with the flow rate of 50mL/min at the calcining temperature of 400-800 ℃ for 4 h; and cooling the calcined product to room temperature, and freeze-drying to obtain the CNT-Fe/Zn. The invention adds ZnFe by a dipping and calcining method₂O₄The method is simple and easy to obtain, the prepared CNT-Fe/Zn catalyst has magnetism, can be separated, improves the reuse rate, is economic and environment-friendly, and particularly has good catalytic activity on dibutyl phthalate (DBP) under acid-base conditions.

Description

high-pH-adaptability carbon nanotube catalyst and application thereof

Technical Field

The invention relates to the technical field of catalytic degradation, in particular to a carbon nano tube catalyst with high pH adaptability and application thereof.

Background

The discharge amount of industrial wastewater in China is huge, and the types of pollutants in the wastewater are very complicated. The current advanced treatment technology of wastewater mainly comprises an adsorption technology, a membrane separation technology, a biotechnology, an advanced oxidation technology and the like.

Catalytic ozonation is a research hotspot which is concerned in recent years as an effective way of advanced wastewater treatment technology. However, the catalytic oxidation of ozone is limited by various factors in practical application, wherein the influence of pH is particularly significant, when the pH of a solution is greater than the isoelectric point of the catalyst, the positively charged catalyst effectively reacts with anionic pollutants, and when the pH of the solution is less than the isoelectric point of the catalyst, the negatively charged catalyst effectively reacts with cationic pollutants, so that the catalytic ozone oxidation effect is significant when PKa is greater than pH > PZC or PZC is greater than pH > PKa, in other words, the catalytic reaction is hindered under other conditions, and therefore, the development of a catalyst with high pH adaptability is crucial.

The carbon nano tube is a potential catalyst carrier, and has a certain catalytic capacity due to the excellent physicochemical property, and other documents indicate that the pretreated carbon nano tube can generate a good complexing effect with metal ions such as zinc, iron and the like, but the single carbon nano tube has limited catalytic activity and cannot simultaneously adapt to a catalytic reaction under an acid-base condition, so that the surface of the carbon nano tube is modified, and the carbon nano tube catalyst with high pH adaptability for an ozone catalytic oxidation technology is provided.

Disclosure of Invention

In order to achieve the above purpose, the invention provides a carbon nanotube catalyst with high pH adaptability, and the specific technical scheme is as follows:

at present, few researches on pH adaptability of the catalyst are carried out, the existing researches can improve the pH adaptability to a certain extent, but the existing researches have the unavoidable defects, and by combining the research backgrounds, the catalyst is prepared by using a carbon nano tube subjected to physicochemical pretreatment as a carrier and using zinc-iron spinel formed by doping iron and zinc as an active component, DBP (dibutyl phthalate) is used as a target organic matter, the catalytic activity of the DBP is tested, the mechanism is formulated, and the support is provided for deeply treating actual industrial wastewater by ozone catalytic oxidation.

The invention relates to a Zn and Fe spinel loaded carbon nanotube and application thereof in catalytic ozonation. Carbon nanotube and Zn (NO)₃)₂·6H₂O、Fe(NO₃)₃·9H₂O, citric acid and the like are used as raw materials, and the catalyst is prepared by a method of impregnation and calcination. And (3) verifying the catalytic performance of the organic matter in the presence of ozone by taking DBP as a mode organic matter. The preparation method comprises the following steps:

s1 pretreatment of the carbon nanotubes:

s11 ball milling: putting the commercial carbon nano tube into a ball mill for ball milling for 5 hours to obtain the carbon nano tube with the average length of 1 mu m for later use; the carbon nano tube after ball milling has increased amorphous broken tube structure, and the specific surface area active site is obviously improved.

S12 impurity removal and oxygen-containing group addition: weighing the ground Carbon nano tube, putting the Carbon nano tube into concentrated nitric acid, stirring for 14h at 135 ℃, then cooling to room temperature, washing until the pH value is 6.2, and freeze-drying to obtain CNT-P (Carbon Nanotube-Prepare); the CNT-P side wall forms active functional groups such as-OH, -COOH and C ═ O, and the activity of the carbon nano tube is improved.

S2 modification of CNT-P with Zn, Fe spinel:

s21 is prepared by mixing the following components in a molar ratio of 1: 2: 2 Zn (NO) is weighed₃)₂·6H₂O、Fe(NO₃)₃·9H₂O and citric acid (C)₆H₈O₇) As a modifier; fe³⁺、Zn²⁺A large number of active sites are provided on the CNT surface. Citric acid is used as a complexing agent, so that the loading of metal ions on the surface of the CNT-P can be enhanced, and the stability of the catalyst is improved; on the other hand, citric acid can be used as a combustion agent, so that the prepared oxide precursor has small particle size and good dispersity.

S22, adding the modifier and the CNT-P into absolute ethyl alcohol according to a certain proportion for dissolving, carrying out ultrasonic treatment for 30min, adding urea when no obvious water exists through water bath evaporation, and stirring and mixing; the urea is used as a pore-foaming agent, so that the unnecessary reduction of the specific surface area in the calcining process can be reduced.

S23, putting the mixture into a tube furnace, calcining in nitrogen atmosphere, cooling the calcined product to room temperature, and freeze-drying to obtain the CNT-Fe/Zn.

Further, the mass ratio of the CNT-P dissolved in the absolute ethyl alcohol to the modifier is 1: (5-6). The amount of the modifier loaded on the CNT-P directly influences the catalytic performance of the final product CNT-Fe/Zn: the catalytic performance of the CNT-Fe/Zn does not reach the standard due to insufficient load; when the loading amount is excessive, active sites on the wall of the carbon nanotube can be covered, and the metal ions are not firmly loaded and are easy to fall off.

Further, the nitrogen flow rate was 50 mL/min. In the calcining process, the waste gas generated in the calcining process can not be taken away in time due to too low nitrogen flow rate; the nitrogen flow rate is too high, which disturbs the calcined product and affects the final yield.

Furthermore, the temperature rise rate during the calcination in the tubular furnace is 5 ℃/min, and the calcination time is 4h when the calcination temperature range is 400-800 ℃.

Further, the specific application method of the high-pH adaptive carbon nanotube catalyst is as follows:

preparing organic wastewater with a certain concentration, adding the organic wastewater into a reactor with the volume of 1L, and adding a certain mass of the carbon nano tube catalyst; the oxygen passes through an ozone generator to generate ozone with a certain concentration, and the mixed gas flow of the ozone and the ocean gas is introduced into the bottom of the reactor to carry out a catalytic ozone oxidation experiment; and exhausting tail gas from the top of the reactor, and introducing the tail gas into a 2% KI solution for absorption treatment.

Further, the treatment object in the organic wastewater includes oxalic acid, ciprofloxacin, indigo, methyl orange, phenol, dimethyl phthalate, and sulfamethoxazole.

Furthermore, dibutyl phthalate (DBP) is an environmental hormone and endocrine disrupting substance, is included in a list of pollutants controlled by China, is common in printing and dyeing wastewater, is slightly soluble and difficult to remove, and can be used as a simulated organic matter for testing the performance of the catalyst, and the concentration of the DHP is 2 mg/L.

Compared with the existing ozone oxidation catalyst, the invention has the beneficial effects that:

1) the traditional ozone oxidation catalyst can only be used under an acidic condition or an alkaline condition, and the CNT-Fe/Zn catalyst prepared by the invention has excellent catalytic performance under the acidic condition and also has good catalytic performance under the alkaline environment.

2) The invention adds ZnFe by a dipping and calcining method₂O₄The CNT-Fe/Zn catalyst is synthesized with the outer wall of the carbon nano tube, the prepared CNT-Fe/Zn catalyst has magnetism, is convenient to separate and has excellent recovery rate, and after 10 experimentsThe removal rate is only reduced by 10 percent, and the cost can be effectively reduced.

3) The CNT-Fe/Zn catalyst prepared by the invention has excellent hydroxyl radical generation rate and still has good catalytic effect under the premise of the existence of the hydroxyl trapping agent.

Drawings

FIG. 1 is a scanning electron microscope image of the present invention;

FIG. 2 is a graph of the catalytic performance of CNT-Fe/Zn at different pH's according to the present invention;

FIG. 3 is a graph of the catalytic performance of CNT-Fe/Zn at different ozone dosing levels according to the present invention;

FIG. 4 is a graph comparing the synergy of zinc iron spinel and carbon nanotubes in catalytic ozonation processes according to the present invention;

FIG. 5 is a comparative graph of the carbon nanotube adsorption experiment of the present invention;

FIG. 6 is a graph of data from a CNT-Fe/Zn free radical experiment of the present invention;

FIG. 7 is a data plot of the results of a hydroxyl quenching experiment of the present invention;

FIG. 8 is a graph of Boehm titration experimental results for CNT-Fe/Zn of the present invention;

FIG. 9 is a schematic diagram of the flow of the application of CNT-Fe/Zn of the present invention.

In the figure: the device comprises a 1-oxygen cylinder, a 2-ozone generator, a 3-reactor, a 31-aeration head, a 32-microporous filter membrane, a 33-CNT-Fe/Zn catalyst and a 4-KI solution.

Detailed Description

To further illustrate the manner in which the present invention is made and the effects achieved, the following description of the present invention will be made in detail and completely with reference to the accompanying drawings.

The first embodiment is as follows: preparation of high pH adaptive carbon nanotube catalyst

(ii) Material preparation and Instrument preparation

The commercial carbon nanotube is purchased from Chongqing technology Co Ltd, and the specific parameters are shown in Table 1:

TABLE 1 CNT parameters

Model number	Average pipe diameter (nm)	Average length (μm)	Specific surface area (m)²/g)
				NC700	9.5	1.5	250-300

The raw materials and reagents used are shown in table 2:

TABLE 2 Main raw materials and reagents

The apparatus and equipment used are shown in Table 3:

TABLE 3 Main instruments and Equipment

(II) preparation of CNT-P

1. Ball milling: putting the commercial carbon nano tube into a planetary ball mill for ball milling to obtain the carbon nano tube with the average length of 1 mu m for later use; the untreated carbon nano tube has obvious entanglement phenomenon and longer length; the carbon nano-tube treated by ball milling has less entanglement, shorter length and more amorphous structures, and the physical changes can improve the catalytic performance of the carbon nano-tube.

2. Removing impurities and adding oxygen-containing groups: weighing the ground carbon nano tube, putting the carbon nano tube into concentrated nitric acid, stirring for 14h at 130 ℃, then cooling to room temperature, washing until the pH value is 6.2, and freeze-drying to obtain CNT-P; the CNT-P side wall forms active functional groups such as-OH, -COOH and C ═ O, and the activity of the carbon nano tube is improved.

(III) preparation of CNT-Fe/Zn

1.1 g of CNT-P is weighed for use, and the molar ratio is 1: 2: 2 Zn (NO) is weighed₃)₂·6H₂O、Fe(NO₃)₃·9H₂O and citric acid were mixed as a modifier, and the mass ratio of CNT-P to the modifier is shown in Table 4:

TABLE 4 mass ratio of CNT-P to modifier

CNT-P quality (g)	Modifier Mass (g)	Degradation rate at pH 7 (%)
			1	5	50
1	5.4	56
			1	5.7	60
1	6	57

As can be seen from Table 4, the catalytic performance of CNT-Fe/Zn increases and then decreases as the amount of modifier increases. This phenomenon occurs because, after the amount of the modifier reaches the upper limit of the theoretical loading value of the carbon nanotube, the active sites on the outer wall of the carbon nanotube are covered by increasing the amount of the modifier, so that the catalytic performance of the CNT-Fe/Zn is rather decreased. Therefore, the mass ratio of the CNT-P to the modifier is preferably 10: 57, that is, in this example, 1g of CNT-P and 1.14g of Zn (NO) were weighed₃)₂·6H₂O, 3.09g of Fe (NO)₃)₃·9H₂O and 1.47g of citric acid.

Then adding the weighed CNT-P and the modifier into absolute ethyl alcohol for dissolving, carrying out ultrasonic treatment for 30min, adding urea when no obvious water exists in the CNT-P and the modifier through water bath evaporation, and stirring and mixing;

Fe³⁺、Zn²⁺a large number of active sites can be provided on the surface of the CNT, and the citric acid can strengthen the loading of metal ions on the surface of the CNT-P and increase the stability of the catalyst; the urea is added as a pore-foaming agent, so that the unnecessary reduction of the specific surface area in the calcining process can be reduced.

The resulting mixture was placed in a tube furnace and calcined under nitrogen at a flow rate of 50mL/min, the calcination temperature and the effect of the CNT-Fe/Zn catalytic ozonation on DBP removal performance (degradation rate) are shown in Table 5.

Percent degradation (%) - (C)₀–C)/C₀×100％

TABLE 5 influence of calcination temperature at pH4 on DBP removal by CNT-Fe/Zn catalyzed ozonation

Calcination temperature (. degree.C.)	Degradation Rate (%)
		400	46
500	65
		600	73
700	66
		800	58

TABLE 6 influence of calcination temperature at pH 9 on DBP removal by CNT-Fe/Zn catalyzed ozonation

As can be seen from tables 5 and 6, the calcination temperature was increased from 400 ℃ to 600 ℃ and the catalytic performance of CNT-Fe/Zn was gradually improved; when the calcination temperature is increased to be more than 600 ℃, the catalytic performance of the CNT-Fe/Zn is basically kept unchanged; however, when the calcination temperature is increased to 800 ℃, the catalytic performance of CNT-Fe/Zn is obviously reduced. As can be seen from the graph, the specific surface area and pore volume of the carbon nanotubes tend to increase and decrease with the increase of the calcination temperature, and the catalyst obtained at the calcination temperature of 600 ℃ has the best performance. The reason for this is that the microstructure of the carbon nanotubes is destroyed due to an excessively high calcination temperature, and the phenomena of collapse of the pore structure and surface sintering occur, which reduces the specific surface area thereof, resulting in a reduction in the prepayment and capacity for organic substances. Thus, the preferred calcination temperature for this application is 600 ℃.

Comparing table 5 and table 6, it can be seen that the catalyst CNT-Fe/Zn prepared by the present invention has excellent catalytic performance in an acidic environment, good catalytic performance in an alkaline environment, and good pH adaptability, which improves the defect that the catalyst used in the conventional ozone catalysis can only adapt to an acidic or alkaline environment, greatly expands the application range, and has good market prospects.

And cooling the calcined product to room temperature, and freeze-drying to obtain the CNT-Fe/Zn.

Example two: application of high pH adaptive carbon nano tube catalyst

As shown in FIG. 9, 2mg/L DBP solution was prepared and added to a reactor with a volume of 1L, followed by 50mg CNT-Fe/Zn; adjusting an oxygen partial pressure valve of an oxygen cylinder 1 to ensure that the air inlet speed is 0.4L/min, introducing into an ozone reactor 2, controlling through a gear to ensure that the ozone yield is 8mg/min, introducing the mixed airflow of ozone and oxygen into the bottom of a reactor 3 through an aerator 31, and carrying out a catalytic ozone oxidation experiment; and tail gas is discharged from the top end of the reactor, and is introduced into a 2% KI solution 4 for absorption treatment.

As shown in fig. 1, the untreated carbon nanotubes have a significant entanglement phenomenon and a long length; the treated carbon nano tube has less entanglement, short length and increased amorphous structure, metal ions grow on the surface of the carbon nano tube in a crystallization way, a large number of active sites are provided, and the changes can improve the catalytic performance of the CNT-Fe/Zn.

1. Catalytic performance of CNT-Fe/Zn at different pH

As shown in FIG. 2, experiments at different pH also confirmed the high pH adaptation of CNT-Fe/Zn: the catalytic performance of the CNT-Fe/Zn is higher than that of other two modified carbon nanotubes under neutral, acidic and alkaline (pH4-9), and the removal rate of DBP catalyzed by ozone for 30min is 52-73%, which is improved by about 40% compared with that under the condition of ozone alone.

2. Influence of different ozone dosage

As shown in FIG. 3, the catalytic performance of CNT-Fe/Zn, the removal rate for DBP, gradually increased with increasing ozone concentration of the inlet gas, but the growth rate gradually decreased and stabilized at an ozone concentration of 20mg/L of the inlet gas. Therefore, the preferred embodiment of the present invention is to operate under the condition that the ozone concentration is 20mg/L, pH-4.

3. Synergistic effect

As shown in fig. 4, the effect of the zinc-iron spinel supported nanocarbon catalyst (CNT-Fe/Zn) on the catalytic degradation of DBP is superior to that of iron and zinc in an ionic state and also superior to that of carbon nanotubes physically doped with zinc-iron spinel. Therefore, the zinc-iron spinel and the carbon nano tube show excellent synergy in the catalytic oxidation process of ozone by the immersion calcination method.

4. Adsorption experiments

The adsorption of the carbon nanotubes on organic substances is influenced by their own characteristics and properties of the organic substances. The carbon nano tube is a nonpolar molecule, and organic matters with small molecular weight and strong polarity and macromolecular organic matters have weaker adsorption capacity. In order to make the carbon nano tube easily reach adsorption balance, the target pollutant for ozone oxidation with smaller saturated adsorption quantity is adopted.

As shown in FIG. 5, the removal rate of DBP by the carbon nanotube within 30min is about 15%, which is about 20% of the total removal rate, so that the oxidation of free radicals is the main mechanism for removing DBP by the catalytic oxidation of CNT-Fe/Zn.

5. Free radical experiments

As shown in FIG. 6, by setting the ozone oxidation with pure ozone oxidation and the ozone oxidation with the addition of CNT-Fe/Zn catalyst as the control group, hydroxyl radicals and H in the presence of CNT-Fe/Zn can be obtained₂O₂The concentration and the consumption of the carbon dioxide are obviously increased, and the fact that the CNT-Fe/Zn can promote the conversion of ozone into hydroxyl radicals and H is proved₂O₂。

6. Hydroxyl quenching experiments

The principle is as follows: chloride ions are common anions in water and have the capability of capturing hydroxyl radicals like TBA, and phosphate, organic matters and ozone generate competitive adsorption in the reaction process so as to inhibit the reaction.

As shown in FIG. 7, the results of the hydroxyl quenching experiments indicate that the oxidation of hydroxyl radicals and the adsorption of catalyst are the main removal mechanisms of the present invention.

7. Boehm titration experiment

Boehm titration experiments are qualitative and quantitative analyses of oxides based on the possibility of different strengths of alkali reacting with acidic surface oxides. NaHCO 2₃Only the carboxyl group, Na, on the carbon surface is neutralized₂CO₃The carboxyl and lactone groups on the carbon surface can be neutralized, and the carboxyl, lactone and phenolic hydroxyl groups on the carbon surface can be neutralized by NaOH. The amount of the corresponding functional group can be calculated according to the consumption of the base.

As shown in FIG. 8, it can be derived from Boehm titration experiments that the catalyst CNT-Fe/Zn surface oxygen-containing groups (hydroxyl, carboxyl) provide a large number of active sites for the catalyst, wherein the surface hydroxyl is the main active center.

Claims

1. A carbon nanotube catalyst for high pH adaptability is characterized in that the preparation method comprises the following steps:

s1 pretreatment of the carbon nanotubes:

s11 ball milling: putting the commercial carbon nano tube into a ball mill for ball milling for 5 hours to obtain the carbon nano tube with the average length of 1 mu m for later use;

s12 impurity removal and oxygen-containing group addition: weighing the ground carbon nano tube, putting the carbon nano tube into concentrated nitric acid, stirring for 14h at 135 ℃, then cooling to room temperature, washing until the pH value is 6.2, and freeze-drying to obtain CNT-P;

s2 modification of CNT-P with Zn, Fe spinel:

s21 is prepared by mixing the following components in a molar ratio of 1: 2: 2 Zn (NO) is weighed₃)₂·6H₂O、Fe(NO₃)₃·9H₂O and citric acid as modifiers;

s22, adding the modifier and the CNT-P into absolute ethyl alcohol according to a certain proportion for dissolving, carrying out ultrasonic treatment for 30min, adding urea when no obvious water exists through water bath evaporation, and stirring and mixing;

s23, putting the mixture into a tube furnace, calcining in a nitrogen atmosphere, cooling the calcined product to room temperature, and freeze-drying to obtain CNT-Fe/Zn;

the CNT-Fe/Zn prepared by the steps has a remarkable degradation effect on treatment objects in organic wastewater, including oxalic acid, ciprofloxacin, indigo, methyl orange, phenol, dimethyl phthalate and sulfamethoxazole, in an environment with the pH of 4-9.

2. The carbon nanotube catalyst for high pH adaptation according to claim 1, wherein the mass ratio of the CNT-P dissolved in the absolute ethanol to the modifier in step S22 is 1: (5-6).

3. The carbon nanotube catalyst for high pH adaptation according to claim 1, wherein the nitrogen flow rate in step S23 is 50 mL/min.

4. The carbon nanotube catalyst for high pH adaptability according to claim 1, wherein in the step S23, the temperature rise rate during the tube furnace calcination is 5 ℃/min, the calcination temperature range is 500-.

5. The use of the carbon nanotube catalyst for high pH adaptation according to claim 1, wherein the specific method is as follows:

preparing organic wastewater with certain concentration, adding the organic wastewater into a reactor with the volume of 1L, and adding the CNT-Fe/Zn; oxygen in the oxygen cylinder (1) passes through the ozone generator (2) to generate ozone with a certain concentration, and mixed gas of the ozone and the oxygen flows through the aeration head (31) and is introduced into the bottom of the reactor (3) to carry out a catalytic ozone oxidation experiment; and tail gas is discharged from the top end of the reactor (3), and is introduced into a 2% KI solution (4) for absorption treatment.