JP5582545B2 - Photocatalyst containing carbon nitride, method for producing the same, and air purification method using the photocatalyst - Google Patents

Description

  The present invention relates to a photocatalyst, a method for producing the same, and an air purification method using the photocatalyst, and more particularly, to a photocatalyst having improved visible light response.

In recent years, air purification techniques have been studied, and photocatalysts capable of decomposing and removing environmental pollutants by sunlight and room light have attracted attention, and their research has been vigorously conducted.
Titanium oxide is a typical example and exhibits a strong photocatalytic activity. However, since titanium oxide has a large band gap and does not absorb visible light that occupies most of the sunlight and is active only to ultraviolet light, it cannot fully utilize sunlight, and ultraviolet light is extremely weak. There were issues such as not functioning indoors. Therefore, various improvements have been made so that visible light can be used.
For example, Patent Document 1 describes that a photocatalyst exhibiting a high photocatalytic action can be obtained by irradiation with visible light by mixing and baking ammonia or a substance that generates ammonia by heating and a titanium compound. Patent Document 2 describes titanium hydroxide containing 3.3% or more of nitrogen, and a photocatalyst obtained by firing the titanium hydroxide.

  On the other hand, semiconductors such as tungsten oxide are capable of absorbing visible light because they have a smaller band gap than titanium oxide, and are expected as visible light active photocatalysts (visible light responsive photocatalysts) ( Patent Documents 3 and 4). These visible light responsive photocatalysts often promote their activity using a promoter such as platinum, palladium, or copper compound.

  In recent years, graphite-like carbon nitride has been proposed (Non-patent Document 1). It has been known since the 1980s that this graphite-like carbon nitride powder can be synthesized by pyrolyzing melamine or cyanamide, but the catalytic action has not been studied until recently, and in fact, the graphite-like powder synthesized from melamine Even if carbon nitride is used as it is, there is almost no catalytic activity. However, the literature 1 reports that the activity is improved by supporting platinum or ruthenium and it can be used for water photolysis (hydrogen generation, oxygen generation). ing.

  On the other hand, as for graphite-like carbon nitride, non-patent document 2 synthesizes ultrafine-grained graphite-like carbon nitride using silica as a template, and synthesizes graphite-like carbon nitride with a high specific surface area by removing silica with hydrofluoric acid. However, the photocatalytic activity is not evaluated. Patent Documents 5 and 6 describe that a solution treated with an MOH aqueous solution (M = K, Na, Li) or a mineral acid exhibits excellent fluorescence characteristics or lubrication characteristics. Is not described at all.

JP 2002-166179 A JP 2001-335321 A JP 2009-61426 A JP 2008-149312 A Japanese Patent Laid-Open No. 02-206619 Japanese Patent Laid-Open No. 02-300273

Maeda Kazuhiko, Domen Kazunari, WANG Xinchen, THOMAS Arne, ANTONIETTI Markus, Nishihara Yasushi Under photocatalytic activity " Matthijs Groenewolt, Markus Antonietti; Adbanced Materials 2005,17,1789-1792, Synthesis of g-C3N4 Nanoparticles in Mesoporous Silica Host Matrices

  As described above, in the photocatalyst using titanium oxide, it is possible to use visible light by doping nitrogen, but its efficiency is still low and it cannot be said that construction materials and filters, When processing into a thin film, there exists a problem that activity falls remarkably. In addition, since titanium oxide decomposes organic substances contained in the adhesive and paint, it is necessary to use an inorganic adhesive or paint that does not contain organic substances when titanium oxide is used. Furthermore, titanium oxide has good compatibility with inorganic materials such as glass and concrete, but has a problem that it has poor compatibility with organic materials.

Tungsten oxide-based semiconductors are expected to be visible light-responsive photocatalysts, but their price is about 10 times that of titanium oxide, and the supply is unstable because they are strategic substances. There is also.
Further, the graphite-like carbon nitride powder does not have catalytic activity as it is, and some activation means is necessary. However, the activation means using platinum or ruthenium described in Non-Patent Document 1 is expensive, and hydrofluoric acid is used. There is a problem that the synthesis method of Non-Patent Document 2 used is dangerous.

  The present invention has been made in view of the above circumstances, and provides a photocatalyst obtained by a cheaper and safer method and responsive to not only ultraviolet light but also visible light, and a method for producing the same. It is intended.

As a result of intensive studies to achieve the above object, the present inventors have found that the photocatalytic activity can be improved by subjecting the graphite-like carbon nitride powder to an alkali treatment or an acid treatment.
The present invention has been completed based on these findings, and according to the present invention, the following inventions are provided.
[1] A photocatalyst comprising, as an active ingredient, powder obtained by subjecting graphite-like carbon nitride powder to an alkali treatment or an acid treatment.
[2] The photocatalyst according to [1], wherein the treatment is a heat treatment in an alkaline aqueous solution or an acidic aqueous solution.
[3] The photocatalyst according to the above [1] to [2], wherein the specific surface area is 20 m ² / g or more.
[4] The photocatalyst according to the above [1] to [3], which has visible light responsiveness.
[5] A method for producing a photocatalyst comprising graphite-like carbon nitride as a main component, wherein the graphite-like carbon nitride powder is subjected to alkali treatment or acid treatment to improve photocatalytic activity.
[6] The method for producing a photocatalyst according to [5], wherein the treatment is a heat treatment in an alkaline aqueous solution or an acidic aqueous solution.
[7] A method for purifying air, comprising using the photocatalyst of any one of [1] to [4] to decompose and remove pollutants and / or bad odors in the air.

  According to the present invention, a photocatalyst can be obtained by an inexpensive and safe method, and the obtained photocatalyst can utilize a photocatalytic action using visible light even if the ultraviolet light is insufficient in the room. Therefore, titanium oxide and tungsten oxide currently used as photocatalysts can be replaced with inexpensive materials.

BET area of graphite-like carbon nitride obtained by firing raw material melamine for 2 hours, and BET area of graphite-like carbon nitride obtained by further adding NaOH to heat treatment at 90 ° C. for 20 hours, and graphite The figure which shows the relationship of the calcination temperature at the time of the synthesis | combination of a shape carbon nitride. NOx removal rate of graphite-like carbon nitride obtained by firing raw material melamine for 2 hours, and NOx removal rate of graphite-like carbon nitride obtained by adding NaOH and heating at 90 ° C. for 20 hours The figure which shows the relationship of the calcination temperature at the time of the synthesis | combination of graphite-like carbon nitride. The figure which shows the electron spin resonance (ESR) spectrum of the graphite-like carbon nitride obtained by adding sodium hydroxide aqueous solution and heat-processing at 90 degreeC for 20 hours. The figure which shows the relationship between the integrated intensity | strength of the ESR signal observed in g = 2.004 vicinity of the ESR spectrum of graphite-like carbon nitride, and the calcination temperature at the time of the synthesis | combination of graphite-like carbon nitride. The figure which shows the visible ultraviolet diffuse reflection spectrum of graphite-like carbon nitride and titanium oxide. Fig. 3 is a powder X-ray diffraction diagram of graphitic carbon nitride. The figure which shows the enlarged part of the main peak in FIG. The figure which shows the profile of the NOx removal test by the graphite-like carbon nitride powder which heat-processed for 90 hours at 130 degreeC by adding sodium hydroxide aqueous solution. The figure which shows the profile of the NOx removal test by the graphite-like carbon nitride powder which added hydrochloric acid and heat-processed at 150 degreeC for 2 hours. The figure which shows the relationship between the NOx removal rate of the graphite-like carbon nitride obtained by adding sodium hydroxide aqueous solution and heat-processing at 90 degreeC for 20 hours, and the wavelength of the irradiated light. The figure which shows the relationship between heating temperature when adding sodium hydroxide aqueous solution, and heat-processing for 20 hours, and a NOx removal rate. The figure which shows the relationship between heating time when adding sodium hydroxide aqueous solution, and heat-processing at 110 degreeC, and a NOx removal rate. The figure which shows the relationship between heating temperature when a sodium hydroxide aqueous solution is added and heat-processed for 20 hours, and a recovery rate. The figure which shows the photocatalyst purification test result of acetaldehyde by the untreated graphite-like carbon nitride powder and the graphite-like carbon nitride powder obtained by adding sodium hydroxide aqueous solution and heat-processing at 90 degreeC for 20 hours. The figure which shows the photocatalyst purification test result of toluene by the graphite-like carbon nitride powder obtained by adding hydrochloric acid and heat-processing at 150 degreeC for 2 hours.

The photocatalyst having visible light responsiveness of the present invention is characterized in that its photocatalytic activity is improved by treating graphite-like carbon nitride powder in an alkaline aqueous solution or an acidic aqueous solution.
That is, by treating the graphite-like carbon nitride in an alkaline solution or an acidic solution, the specific surface area is increased and the photocatalytic activity is improved.
Hereinafter, the present invention will be described in more detail using specific measurement results.

(Production of graphite-like carbon nitride powder)
Graphite-like carbon nitride was synthesized as follows.
30 g of melamine (manufactured by Wako Pure Chemical Industries, Ltd.) is placed in an alumina crucible, covered, baked in an electric furnace at 550 ° C. for 1 hour, the product is ground in a mortar, and then placed in a crucible again for 550 hours. Baked at ℃. The resulting yellow powder was ground in a mortar to obtain graphite-like carbon nitride powder.

(Alkali treatment of graphitic carbon nitride)
The alkali treatment of graphite-like carbon nitride was performed as follows.
1.0 g of the graphite-like carbon nitride powder and 100 ml of a 0.10 mol / l aqueous solution of sodium hydroxide (Wako Pure Chemical Industries) are placed in a crucible made of Teflon (registered trademark), and sodium hydroxide is utilized using an ultrasonic generator. Was dissolved. At this time, the sodium hydroxide concentration was 0.1 mol / l, and the pH at room temperature was 13. When changing the pH of the solution, the concentration of sodium hydroxide was appropriately changed. A Teflon crucible was placed in a stainless steel jacket and heated while stirring with a magnetic stirrer. The temperature was measured using a thermocouple at the top of the stainless steel jacket, and the temperature was adjusted using a temperature controller, slidac, and mantle heater. After heating at a predetermined temperature for 20 hours, it was allowed to cool to room temperature. The suspension in the Teflon crucible was centrifuged to obtain a precipitate. Add 30 ml of water to the precipitate, stir, and repeat the process of washing the precipitate with water several times by centrifuging with an ultracentrifuge (Kubota micro-cooled centrifuge model 3700, 20000 g for 10 minutes), and then alkali treatment Graphitic carbon nitride was obtained.

(Acid treatment of graphite-like carbon nitride)
The acid treatment of graphite-like carbon nitride was performed as follows.
1.0 g of graphite-like carbon nitride powder obtained by baking the melamine and 100 ml of 0.2 mol / l hydrochloric acid (Wako Pure Chemical Industries) were placed in a Teflon crucible and stirred using an ultrasonic generator. . When changing the pH of the solution, the amount of concentrated hydrochloric acid was appropriately changed. When using an acid other than hydrochloric acid, for example, sulfuric acid or nitric acid, the concentration of an acid reagent (manufactured by Wako Pure Chemical Industries, Ltd.) added as appropriate was adjusted so that the hydrogen ion concentration was comparable. A Teflon crucible was placed in a stainless steel jacket and heated while stirring with a magnetic stirrer. The temperature was measured using a thermocouple at the top of the stainless steel jacket, and the temperature was adjusted using a temperature controller, slidac, and mantle heater. After heating at a predetermined temperature for 20 hours, it was allowed to cool to room temperature. The suspension in the Teflon crucible was centrifuged to obtain a precipitate. The process of washing the precipitate with water by adding 30 ml of water to the precipitate and stirring and centrifuging was repeated several times to obtain acid-treated graphite-like carbon nitride.

FIG. 1 shows the specific surface area (□) of graphite-like carbon nitride powder obtained by firing melamine at different temperatures for 2 hours in the production of the above-mentioned graphite-like carbon nitride powder, and the graphite-like carbon nitride powder is hydroxylated. It is a figure which shows the relationship between the specific surface area ((circle)) of the graphite-like carbon nitride powder obtained by adding sodium aqueous solution (concentration 0.1 mol / L), and heat-processing at 90 degreeC for 20 hours, and a calcination temperature.
The specific surface area was measured by a multipoint BET method using nitrogen as an adsorbate. For the measurement, Autosorb-1 manufactured by Cantachrome was used. About 0.2 g of a sample was placed in a sample holder, deaerated at 120 ° C. for 1 hour, and then the BET area was measured.
From FIG. 1, it can be seen that a graphite-like carbon nitride powder having a large BET area can be prepared by firing at a higher temperature.

FIG. 2 shows the NOx removal rate (□) of the graphite-like carbon nitride powder obtained by baking the melamine at different temperatures for 2 hours, and the aqueous solution of sodium hydroxide (concentration 0.1 mol / wt). It is a figure which shows the relationship between NOx removal rate ((circle)) of the graphite-like carbon nitride powder obtained by adding L) and heat-processing at 90 degreeC for 20 hours, and a calcination temperature.
The NOx removal rate was measured as follows.
0.2 g of the treated g-C ₃ N ₄ was suspended in a small amount of water, the whole amount was applied to a glass plate having a width of 50 mm and a length of 100 mm, and dried at 50 ° C. to prepare a photocatalyst test piece. The test piece was placed in a photocatalytic reaction vessel shown in JIS R1701-1, covered with a Pyrex (registered trademark) lid, and simulated polluted air containing 1.0 ppm of NO gas was circulated at 1.0 L / min. . The humidity was 6% at 25 ° C. The NO and NO ₂ gas concentrations in the simulated contaminated air coming out of the reaction vessel were measured with a chemiluminescent NOx measuring device (manufactured by MonitorLabs, 8840). Light from a white fluorescent lamp (Toshiba FL10W) was irradiated to the photocatalyst sample piece at an intensity of 6000 Lx through an ultraviolet light removal filter (Sumitex LF-39 manufactured by Sumitomo Chemical), and the photocatalytic action was observed. NOx concentration (the sum of NO gas concentration and NO ₂ gas concentration) is determined and {[NOx concentration when light is not irradiated] − [NOx concentration when light is irradiated]} / {[light is irradiated NOx concentration when not present]} × 100 was defined as the NOx removal rate.

As shown in FIG. 2, the sample that was not treated with an aqueous sodium hydroxide solution (hereinafter referred to as “NaOH treatment”) showed a low NOx removal rate regardless of the firing temperature. In the case of the sample after the NaOH treatment, the NOx removal rate was higher than that before the NaOH treatment, and the NOx removal rate was the highest especially around 550 ° C.
From these results, in the method for producing graphite-like carbon nitride described above, graphite-like carbon nitride obtained by firing a carbon nitride raw material in the range of 450 ° C. to 650 ° C., preferably 500 ° C. to 600 ° C. It can be seen that the graphite-like carbon nitride having a high photocatalytic activity can be obtained by treating this in an alkaline aqueous solution.
Further, as apparent from the results of FIGS. 1 and 2, it was found that the photocatalytic activity does not increase only by increasing the BET area. This is considered to be related to the radical generation ability described below.

(Electron spin resonance (ESR) spectrum of graphitic carbon nitride)
An electron spin resonance (ESR) spectrum of graphite-like carbon nitride obtained by adding a sodium hydroxide aqueous solution (concentration 0.1 mol / L) and heat-treating at 90 ° C. for 20 hours was measured using a JEOL TE300 ESR spectrometer. And measured at liquid nitrogen temperature.
FIG. 3 shows the result, and an ESR signal intensity indicating the presence of unpaired electrons was observed in the vicinity of g = 2.003 to 2.005 even in the dark. Irradiation with visible light for 5 minutes increased the ESR signal intensity, indicating an increased amount of unpaired electrons. Furthermore, when the light irradiation was stopped, the ESR signal returned to the intensity in the dark place. Thus, it was confirmed that the amount of unpaired electrons for initiating the photocatalytic reaction is increased by irradiation with visible light.

FIG. 4 shows the integrated intensity of the ESR signal observed around g = 2.004, measured in the same manner as above for the graphite-like carbon nitride powder obtained by firing the melamine at different temperatures for 2 hours. This shows the relationship of the firing temperature.
It was confirmed that the graphite-like carbon nitride obtained by firing at around 550 ° C. has a relatively large number of unpaired electrons and has the property that unpaired electrons are likely to exist.

(Elemental analysis of graphitic carbon nitride)
Elemental analysis of graphitic carbon nitride obtained by firing a carbon nitride raw material at 550 ° C. was performed. As a result, when the composition is expressed as C ₃ N _{4 + x} H _y O _z , x = 0.52, y = 1.74. It was determined that z = 0.17. The C / N ratio was 0.66, which was smaller than the theoretical value 0.75 of perfect C ₃ N ₄ (x = y = z = 0).
When the firing temperature of the carbon nitride raw material is changed between 520 ° C. and 650 ° C., the composition of the obtained graphite-like carbon nitride varies depending on the firing temperature, but 0.4 <x <0.6. 1.4 <y <3.1 and 0.1 <z <2.5.
In either case, the results of X-ray diffraction described later (see FIGS. 6 and 7) showed that the layered compound was like a graphite-like carbon nitride-like, so that incomplete C ₃ with defects such as amino groups was present. N ₄ was identified.
Also, when cyanamide is used as a raw material for carbon nitride, or when cyanamide, melamine, and urea are mixed, almost the same results are obtained, so a raw material for carbon nitride other than melamine may be used. A plurality of carbon nitride raw materials may be mixed.

Next, each graphite-like carbon nitride obtained by firing at 520 ° C. to 650 ° C. is subjected to the above-mentioned NaOH treatment to obtain graphite-like carbon nitride having an enhanced photocatalytic activity. As a result, 0.48 <x <0.55, 1.4 <y <2.7, and 0.3 <z <0.7.
From these results, the graphite-like carbon nitride-like compound of the composition containing hydrogen and oxygen is slightly higher than the complete composition of C ₃ N ₄ (x = y = z = 0). It was found to have photocatalytic activity.

From the above, in each Example described below, graphite-like carbon nitride obtained by firing a carbon nitride raw material at 550 ° C. was used.
Hereinafter, in the present specification, the graphitic carbon nitride is simply referred to as “g-C ₃ N ₄ ”, but includes g-C ₃ N ₄ having the incomplete composition described above.

Table 1 below shows the measurement results of the NOx removal rate and the BET area by the g-C ₃ N ₄ powder obtained by the various alkali treatments and acid treatments.

As is evident from Table 1, are obtained in a conventional manner, NOx removal efficiency of the untreated _g-C 3 _{N 4} (no additives) was 3.9%. Even when only water was added and heated, the NOx removal rate did not improve. Details of the results of the NOx removal test will be shown later in the drawings.
Various additives were added, heat treatment was performed for about 20 hours, and the resulting precipitate was washed with water to obtain a photocatalyst sample. As a result, a high NOx removal rate was obtained. In particular, when a sodium hydroxide aqueous solution was added and heat-treated at 90 ° C. to 130 ° C., a high NOx removal activity of 30% or more was obtained.
After the NaOH treatment, a higher NOx removal rate was obtained by washing the sample with distilled water. This is considered that oxidation of NOx is inhibited when a large amount of Na ions remain. However, even if the number of times of cleaning is small, the NOx removal rate is clearly higher than before the NaOH treatment, and thus cleaning is not considered essential. Moreover, since the removal rate of the sample washed with water 8 times was low, it seems that the removal rate is lowered if it is washed too much.
Treatment with hydrochloric acid (hereinafter referred to as “HCl treatment”) was effective when performed at a hydrochloric acid concentration of 0.2 mol / l (pH <1), but was effective at a hydrochloric acid concentration of 0.02 mol / l (pH of about 2). Was not seen. Even at pH <1, the effect was small at 110 ° C. From this, it is considered that a high hydrogen ion concentration and a temperature exceeding 110 ° C. are necessary.
When the NOx removal rate is increased by these treatments, the specific surface area is increased to 20 m ² / g or more regardless of either the alkali treatment or the acid treatment. It is considered that g-C ₃ N ₄ particles were miniaturized by acid treatment or alkali treatment, and the photocatalytic activity was significantly improved.

FIG. 5 is a diagram showing the visible ultraviolet diffuse reflectance spectrum of g-C ₃ N ₄ and titanium oxide. In the figure, the broken line is the spectrum of titanium oxide (ST-01), and the dotted line is the alkali-treated g- spectrum of C ₃ N _4, the solid line is the spectrum of the untreated _g-C 3 _{N 4.} Measurement was performed by attaching a diffuse reflection spectrum measurement attachment (ISR-3100) to a visible ultraviolet absorption spectrometer (Shimadzu UV-3600). Barium sulfate was used as a reference substance.
As is apparent from the figure, titanium oxide absorbs light of about 400 nm or less and can be used for photocatalytic reaction, and g-C ₃ N ₄ can absorb longer wavelength visible light (up to about 500 nm). Even when NaOH was added and heat-treated, there was no significant change.

FIG. 6 is a powder X-ray diffraction pattern of g-C ₃ N ₄ , which was measured with RU-300 manufactured by RIGAKU. In the figure, in order from the bottom, untreated _g-C 3 _{N 4,} deionized water, _g-C 3 _{N 4} and 20 hours of heat treatment at 0.99 ° C., aqueous sodium hydroxide was heated at 0.99 ° C. 20 hours g- C ₃ N ₄ , g-C ₃ N ₄ that was heat-treated at 150 ° C. for 20 hours in an aqueous HCl solution is shown.
FIG. 7 is a diagram showing an enlarged portion of the main peak, the solid line indicates untreated g-C ₃ N ₄ , and the broken line indicates NaOH-treated g-C ₃ N ₄ .
6 and 7, there is a peak peculiar to the layered compound at 27 to 28 °, and it can be seen from the position of this peak that the average of the layer spacing is calculated to be about 3.3 cm, and is gC ₃ N ₄ . In FIG. 6 , even when a sodium hydroxide aqueous solution is added and heated, there is almost no change in the diffraction pattern, but in FIG. 7 where the main peak is enlarged, a slight difference is seen. The diffraction intensity on the low angle side of the peak is reduced, and the component having a large layer spacing is removed by NaOH treatment. From this result and the result of the increase in specific surface area, it can be seen that in the agglomerated g-C ₃ N _4, a portion where the layer spacing is large and weak is destroyed and refined by NaOH treatment.

As described above, as the alkaline aqueous solution used in the present invention, a sodium hydroxide solution is the best, and a potassium hydroxide solution can also be used. When a sodium hydroxide solution having a concentration of 1.4 mol / l is used, most of the g-C ₃ N ₄ is denatured and the recovery rate is lowered, so that the concentration is preferably 1.0 mol / l or less, more preferably Uses an alkaline aqueous solution having a concentration of 0.5 mol / l or less. As the acidic aqueous solution, an aqueous solution of a strong acid can be used, but the pH is preferably 1 or less.
In addition, the treatment with these aqueous solutions is effective to some extent even at room temperature, but the effect is great when performed at 70 ° C. or higher, but the recovery rate of g-C ₃ N ₄ decreases when it exceeds 130 ° C. It is preferable to process at a temperature not exceeding ° C. Furthermore, the temperature which does not exceed 110 degreeC is preferable. Since activity does not improve even when heated in pure water, it is necessary to heat-treat in an acidic or alkaline aqueous solution.

(NOx removal test)
The results of the NOx removal test are shown in the figure below.
FIG. 8 is a view showing a profile of a NOx removal test using g-C ₃ N ₄ powder that was heated for 90 hours at 130 ° C. with an aqueous sodium hydroxide solution (concentration 0.1 mol / L). it is added hydrochloric acid (concentration 0.2 mol / L), a diagram showing the profile of a NOx removal test by _g-C 3 _{N 4} powder heated for 2 hours at 0.99 ° C..
In the figure, the dotted line indicates the NO concentration, the dashed line indicates the NO ₂ concentration, the solid line indicates the NO _x (sum of NO and NO ₂ ) concentration, and the horizontal dotted line indicates the initial concentration of NO _x .
In either case, the concentration of NO decreases while the light is applied, and the concentration returns to the original level when the light irradiation is stopped, indicating that a photocatalytic reaction is occurring. Part of NO becomes NO ₂ and further becomes HNO ₃ and is adsorbed on the photocatalyst and removed from the circulating gas. The area of the portion surrounded by the initial concentration and NOx line corresponds to the removed NOx amount.

FIG. 10 shows the NOx removal rate of g-C ₃ N ₄ obtained by adding a sodium hydroxide aqueous solution (concentration 0.1 mol / L) and heat-treating at 90 ° C. for 20 hours, and the wavelength of the irradiated light. FIG. Using a 200 W xenon lamp as the light source, the NOx removal rate was measured while changing the cutoff wavelength of a filter that cuts off short-wavelength light. The horizontal axis represents the transmission limit wavelength of the filter. It was confirmed that g-C ₃ N ₄ can use visible light having a wavelength near 500 nm. Moreover, since the NOx removal rate with a 345 nm filter is higher than the NOx removal rate with a 400 nm filter, it was confirmed that ultraviolet rays can also be used.

FIG. 11 is a diagram showing the relationship between the heating temperature and the NOx removal rate when a sodium hydroxide aqueous solution (concentration 0.1 mol / L) is added and heat-treated for 20 hours. The dotted line in the drawing indicates the NO _x removal rate if untreated.
As is apparent from the figure, a high NOx removal rate is exhibited when the temperature is about 90 ° C. to 130 ° C., and may decrease when the temperature reaches 150 ° C.

FIG. 12 is a diagram showing the relationship between the heating time and the NOx removal rate when a sodium hydroxide aqueous solution (concentration 0.1 mol / L) is added and heat treatment is performed at 110 ° C.
A high NOx removal rate was exhibited in about 20 hours, and decreased when heated for 90 hours.

FIG. 13 is a diagram showing the relationship between the heating temperature and the recovery rate when a sodium hydroxide aqueous solution (concentration: 0.1 mol / L) is added and heat-treated for 20 hours. The recovery rate was defined as (g-C ₃ N ₄ weight after heat treatment) / (weight before heat treatment) × 100.
A part of g-C ₃ N ₄ is dissolved by the heat treatment, and since it is refined and cannot be precipitated in the centrifuge, the recoverable amount decreases. Above 110 ° C., the recovery rate was significantly reduced.

Figure 14 is the untreated _g-C 3 _{N 4} powder, and an aqueous solution of sodium hydroxide (concentration 0.1 mol / L) was added to the obtained by 20 hours of heat treatment at 90 ℃ _g-C 3 _{N 4} It is a figure which shows the photocatalyst purification test result of acetaldehyde by powder.
The measurement was performed as follows. 0.2 g of NaOH-treated g-C ₃ N ₄ was suspended in a small amount of water, the whole amount was applied to a glass plate having a width of 50 mm and a length of 100 mm, and dried at 50 ° C. to prepare a photocatalyst test piece. The test piece was placed in a photocatalytic reaction vessel shown in JIS R1701-1, covered with a Pyrex lid, and simulated contaminated air containing about 2 ppm of acetaldehyde was circulated at 0.5 L / min. The humidity was 6% at 25 ° C. The concentration of acetaldehyde in the simulated contaminated air coming out of the reaction vessel was measured with a gas chromatograph (GC-14B manufactured by Shimadzu) equipped with an FID type detector. For calibration of the gas chromatograph, 5 ppm acetaldehyde standard gas was used. The CO ₂ concentration was measured with an infrared absorption CO ₂ meter (41C manufactured by Thermoelectron). Light from a white fluorescent lamp (Toshiba FL10W) was irradiated to the photocatalyst sample piece at an intensity of 6000 Lx through an ultraviolet light removal filter (Sumitex LF-39 manufactured by Sumitomo Chemical), and the photocatalytic action was observed. With this filter, the intensity of ultraviolet light of less than 380 nm is less than 0.1%. {[Acetaldehyde concentration when not irradiating light]-[Acetaldehyde concentration when irradiating light]} / [Acetaldehyde concentration when not irradiating light] × 100 was defined as an acetaldehyde removal rate.
When light irradiation is started at time zero, the concentration of acetaldehyde decreases when g-C ₃ N ₄ powder obtained by adding sodium hydroxide aqueous solution and heat-treating (Δ), and at the same time, CO ₂ is reduced. Acetaldehyde was oxidized to CO ₂ by photocatalysis. On the other hand, when untreated g-C ₃ N ₄ was used (◯), the acetaldehyde concentration was hardly lowered and the amount of CO ₂ generated was small.

Similarly, hydrochloric acid (concentration 0.2 mol / L) was added, and a photocatalytic purification test of acetaldehyde with g-C ₃ N ₄ obtained by heat treatment at 150 ° C. for 20 hours was conducted. The results are summarized in Table 2. It was.
Acetaldehyde removal rate, 4.9% in the samples in which the NaOH treatment, 14.9% for the sample to the HCl treatment was 0.3% in _g-C 3 _{N 4} sample untreated. The amount of CO ₂ generated was also significantly higher in the sample to which an aqueous sodium hydroxide solution or hydrochloric acid was added, indicating that the photocatalytic activity was improved.

FIG. 15 is a diagram showing a photocatalytic purification test result of toluene using g-C ₃ N ₄ powder obtained by adding HCl and heat-treating at 150 ° C. for 2 hours.
The measurement was performed in the same manner as the above-mentioned photocatalytic purification test for acetaldehyde.
When a gas containing toluene was brought into contact with the photocatalyst without exposure to light, the concentration decreased due to adsorption. The concentration gradually approached the introduced concentration, so when exposed to light, the concentration decreased slightly, and at the same time the generation of CO ₂ was confirmed (not shown), indicating that toluene was decomposed.
Table 3 shows the results of the toluene removal test.
_G-C 3 _{N 4} powder of untreated could not remove the toluene (decomposition). On the other hand, the NaOH-treated g-C ₃ N ₄ powder and the HCl-treated g-C ₃ N ₄ powder decomposed toluene to generate CO ₂ .

  The graphite-like carbon nitride powder with improved photocatalytic activity of the present invention can be used as a photocatalyst material by applying it to a certain substrate. Can be purified. This material can also be used to decompose acetaldehyde, toluene, and NOx, and can also be used to decompose similar compounds.

Claims (4)

A photocatalyst comprising, as an active ingredient, a powder having a specific surface area of 20 m ² / g or more obtained by heat-treating a graphite-like carbon nitride powder containing no silica as a template in an alkaline aqueous solution or an acidic aqueous solution . The photocatalyst according to claim 1, which has visible light responsiveness. It is characterized by improving the photocatalytic activity by heat-treating a graphite-like carbon nitride powder not containing silica as a template in an alkaline aqueous solution or an acidic aqueous solution to give a powder having a specific surface area of 20 m ² / g or more. A method for producing a photocatalyst comprising graphite-like carbon nitride as a main component. Using a photocatalyst according to claim 1 or 2, air purification method characterized by decomposing and removing contaminants and / or odorous air.

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