JPH11137995A - Photocatalyst for reduction, decomposition and removal of nitrogen monoxide in gas phase, and purifying method using the photocatalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photocatalyst material which reduces, decomposes and removes nitrogen oxides in a gas phase or nitrogen oxides produced in a refuse disposal facility by using energy such as UV rays. SOLUTION: This photocatalyst to reduce, decompose and remove nitrogen oxides in a gas phase consists of a holladite crystalline phase expressed by formula of Ax My Sn8-y O16 . In the formula, A is one or more elements selected from K, Rb, Cs, Ca, Ba and Na, M is a bivalent or tervalent metal, however, Na is used only when M is Cr, and (x) and (y) (satisfy 0.7<2.0<=2.0 and 0.7<y<=2.0. In this purifying method of nitrogen oxides in a gas phase by using this photocatalyst, the catalyst is brought into contact with nitrogen oxides while irradiated with light in the presence of a reducing agent. Thus, nitrogen oxides are decomposed and removed under reducing conditions to be converted into nitrogen.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing and decomposing harmful nitrogen oxides generated inside a highway tunnel with heavy traffic and nitrogen oxides generated at a garbage disposal site by using energy such as ultraviolet rays. The present invention relates to a photocatalytic material to be removed and an efficient purification method thereof.

[0002]

2. Description of the Related Art Conventionally, as a method for purifying nitrogen oxides in a gas phase, a method of reducing by heating using ammonia or urea is generally performed (for example, "Chemical and Industrial",
47, No. 2, pp. 152-155 (1994), etc. Other methods include using a reducing agent such as a hydrocarbon,
3O on an alumina-based catalyst carrying a platinum-based noble metal catalyst
A method of reducing by heating at a temperature of O ° C. or higher (for example, “Chemistry and Industry”, Vol. 47, No. 2, 152 to 15 as in the previous example)
5 (1994)).

[0003] However, in the former case of the prior art, the processing apparatus becomes large-scale from the viewpoint of safety, and also in the latter case, the space to be processed is limited because heating is required. It is not practically preferable because of its processing cost and complicated processing technology.
More recently, to overcome these problems, Ti
Attempts have been made to oxidatively remove nitrogen oxides using an O ₂ -based photocatalyst (see, for example, “Chemistry and Industry”, Vol. 46, No. 12, pp. 1839-1841 (199).
This method, which utilizes the high oxidation ability of TiO ₂ -based photocatalyst, turns nitric oxide into nitric acid and is fixed on the catalyst surface from the atmosphere. It was not a sufficiently effective method for protecting the global environment because it would lead to groundwater pollution.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a photocatalyst which is simpler than conventional techniques, and has high energy efficiency and high nitrogen oxide decomposition efficiency. It is an object of the present invention to provide a material and an efficient decomposition and purification method using a reducing agent.

[0005]

Means for Solving the Problems The present inventors irradiate nitrogen oxides contained in the gas phase with light energy such as ultraviolet rays in the coexistence of a reducing agent such as a hydrocarbon, thereby forming a nitrogen oxide on the catalyst surface. As the reducing agent is gently oxidized, the reduction reaction of nitrogen oxides progresses, and by balancing this oxidation-reduction reaction, we develop photocatalytic materials that have the property of decomposing and purifying hard-to-decompose nitrogen oxides. As a result of studying for the purpose, the present invention has been completed.

The invention of claim 1 of the present application has a general formula: A _x M
_y Sn _8-y O ₁₆ (where A is K, Rb, Cs, Ca, B
One or more elements selected from the group consisting of a and Na, and M represents a divalent or trivalent metal. However, the Na element is limited only when M is Cr. x and y are 0.7 <x ≦
2.0 and 0.7 <y ≦ 2.0. ) And a photocatalyst for reductive decomposition and purification of nitrogen oxides in a gaseous phase, comprising a hollandite-type crystal phase.
The invention according to claim 1, wherein in the catalyst according to claim 1, x
And y satisfy 1.6 <x ≦ 2.0 and 1.6 <y ≦ 2.0, whereby nitrous oxide (N ₂ O) as a by-product
Reductive decomposition of nitrogen oxides in the gas phase, characterized by suppressing the generation of nitrogen and increasing the selectivity of nitrogen generation to 50% or more.
It is a purification photocatalyst, and the invention of claim 3 further comprises:
A photocatalyst characterized in that the hollandite compound is brought into contact with a nitrogen oxide and a reducing agent in the presence of a reducing agent while irradiating with light, whereby the nitrogen oxide is reductively decomposed and purified and converted to nitrogen with high efficiency. It is a method of purifying nitrogen oxides in the vapor phase.

That is, the present invention provides a catalyst comprising a hollandite-type crystal phase represented by the following general formula:
Nitrogen oxides and a reducing agent coexist in the catalyst and are brought into contact with each other while irradiating light. It purifies. General _{formula: A x M y Sn 8-} y O 16 ( wherein, A represents K, Rb, Cs, Ca, 1 or two or more elements selected from the group consisting of Ba and Na, M is a divalent Or a trivalent metal, provided that the Na element is limited to the case where M is Cr. X and y are 0.7 <x ≦ 2.0 and 0.7.
<Y ≦ 2.0. )

[0008]

The composition of the nitrogen oxide decomposing optical catalyst in the gas phase in the Detailed Description of the Invention The present invention has the general _{formula: A x M y Sn 8-} y O 16 ( wherein, A represents K, Rb, Cs, Ca , Ba and Na, one or more elements selected from the group consisting of M, M represents a divalent or trivalent metal, provided that the Na element is limited to the case where M is Cr. 0.7 <x ≦ 2.0 and 0.7
<Y ≦ 2.0. ) And must be a catalyst comprising a hollandite-type crystal phase.

The hollandite type crystal is a compound having a one-dimensional tunnel structure. Examples of the tunnel ion include an alkali metal ion such as K or an alkaline earth metal ion such as Ba. In the case of Na, a hollandite-type crystal structure can be obtained only when used in combination with Cr. However, when combined with other elements, the tunnel structure is lost, resulting in a Freudenbergite-type crystal structure or the like. It is not preferable.

The values of x and y are respectively 0.7 <x ≦ 2.
0 and 0.7 <y ≦ 2.0, and above or below this range, an alkali metal oxide or carbonate, an alkaline earth metal oxide or carbonate, 2
An oxide of a trivalent or trivalent metal and an oxide of Sn are precipitated, and the activity is remarkably reduced. Further, when Ba is used, M = 2 due to the electric neutral condition in the compound.
It is preferable that y = x for valence and y = 2x for M = 3. When an alkali metal and an alkaline earth metal element are combined, 0.7 <x <
1.2 is preferred.

More preferably, x in the above composition formula
And y are set to 1.6 <x ≦ 2.0 and 1.6 <, respectively.
By setting y ≦ 2.0, the generation of by-product nitrous oxide (N ₂ O), which may be generated in the process of reductively decomposing nitrogen oxides into nitrogen, is suppressed, and nitrogen generation is performed. Can be increased to 50% or more. As the divalent or trivalent metal element used in the photocatalyst of the present invention, A1, Ga, Cr, Mg, or the like is preferable for forming a hollandite-type crystal structure.

[0012] General _{formula: A x M y Sn 8-} y O 16 ( wherein, A represents K, Rb, Cs, Ca, 1 or two or more elements selected from the group consisting of Ba and Na, M Represents a divalent or trivalent metal element, where Na is M for Cr
Only if. x and y indicate 0.7 <x ≦ 2.0 and 0.7 <y ≦ 2.0. It is known that the hollandite-type crystal phase represented by the formula (1) can be synthesized by various methods, but the method for producing the hollandite-type catalyst constituting the photocatalyst of the present invention is not particularly limited.

For example, the solid phase synthesis method includes a carbonate of an alkali metal or an alkaline earth metal, tin oxide and
After mixing the trivalent or trivalent metal element oxide,
The method of firing at a temperature of 500 degrees or less, as a liquid phase method,
Using an aqueous solution of an inorganic salt such as a nitrate of an alkali metal or alkaline earth metal element, tin chloride and a nitrate of a divalent or trivalent metal element, the mixed solution is dropped into aqueous ammonia or aqueous ammonia and aqueous ammonium oxalate, The precipitate is washed, filtered, dried, and then calcined at a temperature of 500 ° C to 1200 ° C. Examples of the coprecipitation method and the alkoxide method include alkali metal or alkaline earth metal elements, tin and divalent or trivalent. Alkoxides such as methoxide, ethoxide and butoxide of a valent metal element are mixed in a non-aqueous solution, hydrolyzed and dried,
It can be obtained by firing at a temperature not higher than the temperature.

Regarding the calcination temperature, a hollandite-type crystal structure is stably formed even at a calcination temperature of 1500 ° C. or higher, but calcination at a high temperature causes a decrease in the specific surface area of the catalyst, which is not preferred. Further, even if the firing time is too long, the specific surface area is undesirably reduced. Further, if the rate of temperature rise during firing is too high, coking occurs and the catalytic activity is reduced. Therefore, the firing rate is preferably 10 degrees / minute or less, and more preferably, coking is prevented by firing in an oxygen stream. Can be If the specific surface area of the hollandite catalyst is 0.1 m ² / g or more, nitrogen oxides in the atmosphere can be decomposed and purified in the presence of a reducing agent. The nitrogen oxide decomposition and purification efficiency of this catalyst increases as the specific surface area increases. Particularly, in the case of a continuous flow type processing apparatus in which a large amount of processing gas is used, it is preferable that the specific surface area be large.

[0015] In addition to using this catalyst as a powder,
A hollandite-type catalyst can be coated on a porous catalyst carrier, a quartz glass tube, a quartz glass substrate, or the like, and used as a film of a hollandite-type crystal phase. When used as a film, a porous catalyst carrier, a quartz glass tube or a quartz glass substrate is immersed in an aqueous solution or a non-aqueous solution in which a hollandite type catalyst is dispersed, and then baked, or a method of CVD. A method of forming a film of a hollandite-type crystal phase on a surface such as a porous catalyst carrier, a quartz glass tube, or a quartz glass substrate by PVD or sputtering is used.

There is no particular limitation on the light irradiation method in the present invention.
Light irradiation can be performed from the outside as needed. In addition, the wavelength of the light to be irradiated may be light having a wavelength longer than that of ultraviolet light, and a fluorescent lamp or sunlight can be used.However, since the wavelength is longer, the reaction speed is reduced.
It is effective to use light in the ultraviolet region, particularly near 360 nm.

As a method for reductively decomposing and purifying nitrogen oxides in the atmosphere, a catalyst having a high chemical adsorption capacity for nitrogen oxides must be used, and the adsorption capacity is 1 microgram per gram of catalyst. It is desirable to have a chemisorption amount of at least mol / g. The amount of chemisorption in the present invention refers to a constant-rate degassing method in which nitrogen oxide is adsorbed on a catalyst surface for a certain period of time, and then the temperature is raised at a constant speed to measure the adsorption intensity of the adsorbed substance on the catalyst surface. Separation method (Temperature Program De
(sorption or TPD) at a temperature of 300 ° C. or higher, and corresponds to the amount of nitrogen oxides showing desorption for the first time. It is possible to reductively decompose under such high nitrogen oxide affinity.

Further, in the method for purifying nitrogen oxides using a photocatalyst of the present invention, a reducing agent is required. However, the reducing agent is not particularly limited as long as it can be oxidized and decomposed on the photocatalyst. Aldehydes, ketones, fatty acids and compounds belonging to aromatic hydrocarbons are exemplified as reducing agents. The reaction of oxidatively decomposing hardly decomposable organic chlorinated compounds (such as dioxin and trichloroethane) and this nitrogen oxide It is also possible to achieve both reductive decomposition reactions.

Further, such an oxidation-reduction reaction proceeds by the reaction of a nitrogen oxide and a reducing agent on the catalyst surface, but it is also possible to coexist oxygen present in the atmosphere at room temperature. The emission standard for nitrogen oxides in the atmosphere is about 1 ppm indoors, and tens to hundreds of ppm outdoors.
pm, and the stoichiometric amounts of the reducing agent and oxygen required to reductively decompose this concentration of nitrogen oxides are necessarily the same. For this reason, excessively dissolved oxygen usually adversely affects the balance of the oxidation-reduction reaction and may reduce the reductive decomposition efficiency of nitrogen oxides. Due to its high ability to selectively adsorb oxides, the efficiency of reductive decomposition of nitrogen oxides by light is high, so even if excessively dissolved oxygen coexists, nitrogen oxides can be decomposed and purified with high efficiency. We can expect what we can do.

The hollandite catalyst according to the present invention may be used as a photocatalyst by supporting a photocatalytically active metal such as platinum or an oxide such as ruthenium oxide as required. There is no particular limitation on the light irradiation method in the present invention, and light irradiation can be performed from the inside or outside of the reaction tube where the catalyst is immobilized, as necessary. In addition, the wavelength of the light to be irradiated may be light having a wavelength longer than that of ultraviolet light, and a fluorescent lamp or sunlight may be used.However, since the reaction speed is reduced due to the longer wavelength, an ultraviolet region, particularly It is effective to use light near 360 nm. Although there is no particular limitation on the concentration of nitrogen oxides targeted by the present invention, it is effective to use it in a concentration region of lppt or more and 1% or less in order to sufficiently exert the ability of the catalyst according to the present invention. .

[0021] Also in the nitrogen oxide and a gas mixture with the catalyst contact time comprised of the reducing agent is not particularly limited, since the specific surface area is not effective if the contact time is not taken large as small, one or more 10 m ² / G, 2 g · s
/ Cm is preferable, and when the specific surface area is more than that, the effect can be sufficiently exhibited even if the contact time is shortened. Therefore, it is preferable to select according to the specific surface area.

[0022]

Examples and Comparative Examples Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples and Comparative Examples. Example 1 Tin ethoxide (manufactured by Kojundo Chemical Laboratory), potassium propoxide (manufactured by Kojundo Chemical Laboratory), and gallium butoxide (manufactured by Kojundo Chemical Laboratory) so that the composition becomes K _2.0 Ga _2.0 Sn _8.0 O _16. Was weighed and dissolved in 20 ml of dehydrated 2-methoxyethanol, respectively, and then three kinds of solutions were mixed at room temperature to prepare a sol solution. Hydrolysis water (H ₂ O / C ₂ H ₅ OH = 5.2 m1 / 120) was added to this solution.
m1) was added dropwise to carry out hydrolysis. The hydrolyzed gel was dried and pulverized and calcined at 700 ° C. for 3 hours to synthesize a single-phase powder of hollandite-type K _2.0 Ga _2.0 Sn _8.0 O ₁₆ . FIG. 1 shows an X-ray diffraction pattern of the catalyst thus obtained. The specific surface area of the obtained powder was about 30 m ² / g, and the sample had pores in the mesopore region. The catalyst activity evaluation test was carried out using a closed-circulation reactor made of quartz using ethane as a reducing agent. The reaction gas concentration was such that 4,000 ppm of nitrogen oxide and 2,000 ppm of ethane were introduced into the closed circulation system, 1 g of the catalyst was charged, and 400 W, 36 nm of ultraviolet light was irradiated with stirring using an impeller. It was carried out by analyzing the decrease in the concentration and hydrocarbon concentration and the amount of carbon monoxide and nitrogen produced using a mass spectrometer. The test results are shown in Tables 1 to 3. However, the activity of removing nitrogen monoxide in the gas phase was calculated as the nitric oxide purification rate using equation (1), and the ethane conversion activity in the gas phase was calculated as the ethane conversion rate using equation (2). Further, the production rate of nitrogen as a product was obtained from the equation (3), and the production rate of carbon monoxide as a product was calculated using the equation (4).

[0023]

[Equation 1] Nitrogen oxide purification rate (%) = {(NO₀-F1_XMet₀₎−₍NO_t-F1_XMet _{t )}} × 100 /_{NO₀-F1_X-F1_XMet_0}(1 set)

Mass spectrometry relative sensitivity: fl = sensitivity of ethane having a mass number of 30 / sensitivity of ethane having a mass number of 27

[0025]

Ethane conversion rate (%) = _{( Met ₀ −Met _t) / Met _0} × 100 (Equation 2)

Here, NO ₀ : initial concentration of nitric oxide (sensitivity of mass number 30) Met ₀ : initial concentration of methane (sensitivity of mass number 27) NO _t : nitric oxide after light irradiation for a certain time concentration (mass number 30 sensitivity) Met _t: at methane concentration (sensitivity of the mass number No. 27) the nitric oxide purification rate after a predetermined time irradiation, why sensitivity ratio is used that f1 is the mass number Since the peak of No. 30 contains the sensitivity of ethane overlapping, the sensitivity of No. 27 is replaced by the sensitivity of No. 30 and the overlap is subtracted to obtain the net sensitivity of nitric oxide. Are used in equation (1).

[0027]

Equation 3 nitrogen production rate _{(%) = {(N2 t} -N2 0) / (N2fin -N2 0)} × 100 (3 type)

[0028]

Equation 4] carbon monoxide production rate _{(%) = {(CO t} -CO 0) -f2 × (N2fin -N2 0) × nitrogen product ratio × 0.01} × 100 / (COfin -CO 0) -f2 × (N2fin −N2 ₀ ) (Equation 4)

Here, N 2 ₀ : initial concentration of nitrogen peak (sensitivity of mass number 14) CO ₀ : initial concentration of carbon monoxide (sensitivity of mass number 28) N 2 _t : nitrogen generation after light irradiation for a certain time concentration (sensitivity of the mass number 14 number) CO _t: carbon monoxide concentration after a certain time of light irradiation (sensitivity of the mass number 28th) N2fin: hydrocarbons, N2 generation concentration at the time of nitric oxide has been all converted ( COfin: CO production concentration when all hydrocarbons and nitric oxide are converted (sensitivity of mass number 28) f2: (Sensitivity of nitrogen peak of mass number 28) / (mass (Sensitivity of nitrogen peak of number 14) In the above equation (3), mass number 14 does not indicate sensitivity of only nitrogen, but slightly includes sensitivity of hydrocarbon.
It is defined to determine the amount derived from net nitrogen. The above equation (4) also has no mass number indicating the sensitivity of carbon monoxide alone, and the mass number 28 includes the sensitivity of nitrogen. Is defined to know the amount derived from net carbon monoxide. The selectivity of nitrogen generation is as follows (5)
Equation).

[0030]

[Expression 5] Selectivity of nitrogen generation = {Nitrogen generation rate / (Nitrogen generation rate + Nitrous oxide generation rate)} (Equation 5)

From the results shown in Table 1, it can be seen that the catalyst of this example completely absorbs nitric oxide by taking a process in which nitric oxide is rapidly adsorbed on the catalyst surface after light irradiation and ethane is slowly converted. It was found that it decomposed to nitrogen and at the same time ethane was converted to carbon monoxide. From these results, it was confirmed that the use of the photocatalyst according to the present example allowed nitrogen monoxide to be completely decomposed into nitrogen.

Example 2 Tin oxide (manufactured by Wako Pure Chemical Industries), gallium oxide (manufactured by Wako Pure Chemical Industries) and potassium carbonate (manufactured by Wako Pure Chemical Industries) were weighed so that the compositions would be the same as in Example 1 above. Then, the mixture was mixed in an agate mortar for 30 minutes, and then baked at 1200 ° C. for 2 hours to obtain a powder. The obtained powder showed the same type of X-ray diffraction pattern as that of Example 1 and was found to be a hollandite single phase, having a specific surface area of about 0.3 m ² / g. The evaluation of the catalyst activity was performed according to Example 1. The results are shown in Tables 1 to 3. From the results in the above table, it was confirmed that the catalyst according to the present example had the ability to decompose nitrogen oxides as in Example 1, even if the specific surface area was small.

Example 3 Tin ethoxide (manufactured by Kojundo Kagaku Kenkyusho), potassium propoxide (manufactured by Kojundo Kagaku Kenkyusho), and gallium butoxide (manufactured by High Purity Kagaku Kenkyu) so that the composition becomes K _1.5 Ga _1.5 Sn _6.5 O ₁₆ Was weighed and dissolved in 20 ml of dehydrated 2-methoxyethanol, respectively, and three kinds of solutions were mixed at room temperature to prepare a sol solution. Hydrolysis water (H ₂ O / C ₂ H ₅ OH = 5.2 m1 / 120) was added to this solution.
m1) was added dropwise to carry out hydrolysis. The hydrolyzed gel was dried and pulverized and calcined at 700 ° C. for 3 hours to synthesize a single-phase powder of hollandite-type K _1.5 Ga _1.5 Sn _6.5 O ₁₆ . The obtained compound showed an X-ray diffraction pattern similar to that of FIG. 1, and the powder had a specific surface area of about 30 m ² / g and was a sample having pores in the mesopore region. The evaluation of the catalyst activity was performed according to Example 1. The results are shown in Tables 1 to 3. As a result, the ability to decompose nitric oxide into nitrogen was confirmed as in Examples 1 and 2. At the same time, however, there was a process in which the production of N ₂ O, a by-product, was remarkably observed in the reaction process. Apparently, the selectivity of nitrogen production was reduced. The product was also decomposed, and it was confirmed that the selectivity for nitrogen generation was improved.

[0034] As Example 4 composition becomes _{K 1.8 Ga 1.8 Sn 6.2 O 16} , Suzuetokishido (manufactured by Kojundo Chemical Laboratory), potassium propoxide (manufactured by Kojundo Chemical Laboratory) and gallium butoxide (purity Chemistry Was weighed and dissolved in 20 ml of dehydrated 2-methoxyethanol, respectively, and three kinds of solutions were mixed at room temperature to prepare a sol solution. Hydrolysis water (H ₂ O / C ₂ H ₅ OH = 5.2 m1 / 120) was added to this solution.
m1) was added dropwise to carry out hydrolysis. The hydrolyzed gel was dried and pulverized and calcined at 700 ° C. for 3 hours to synthesize a hollandite-type K _1.8 Ga _1.8 Sn _6.2 O ₁₆ single-phase powder. The obtained compound showed an X-ray diffraction pattern similar to that of FIG. 1, and the powder had a specific surface area of about 30 m ² / g and was a sample having pores in the mesopore region. The evaluation of the catalyst activity was performed according to Example 1. The results are shown in Table 1. As a result, as in Example 1, the ability to decompose nitric oxide into nitrogen was confirmed, and the selectivity for nitrogen generation was high.

It should be noted that in Table 1 above, 1
The experiment was performed without light for about 2 hours. As a result of this operation, the reaction gas was slightly adsorbed on the catalyst surface, so that a conversion was generated from the reaction time of 0 hour. However, on the other hand, in this case, since no product was confirmed,
It is considered that the product was changed as shown in Table 1 by photoexcitation of the catalyst after light irradiation, and the function of decomposing nitric oxide was confirmed.

Comparative Example 1 Tin oxide (manufactured by Kishida Chemical Co., Ltd.), gallium oxide (manufactured by Kishida Chemical Co., Ltd.) and potassium carbonate (manufactured by Kishida Chemical Co., Ltd.) so that the composition becomes K _0.3 Ga _0.3 Sn _7.7 O _16. Are weighed and mixed in a Meneau mortar for 30 minutes.
The powder was fired at 00 degrees for 2 hours to obtain a powder. The obtained powder was a mixed phase comprising a trace amount of a hollandite-type crystal phase and tin oxide and gallium oxide. The activity of the catalyst was evaluated in accordance with Example 1. The results are shown in Tables 1 to 3. For the powder having the composition of Comparative Example 1, the light irradiation time was 30
By 0 hours, although a slight conversion rate due to adsorption of nitric oxide and ethane was observed, products such as nitrogen and carbon monoxide were not confirmed, and the function of decomposing nitric oxide could not be confirmed.

[Brief description of the drawings]

FIG. 1 is an X-ray diffraction diagram of a catalyst prepared in Example 1.

[Table 1]

[Table 2]

[Table 3]

Claims (3)

[Claims] 1. A general _{formula: A x M y Sn 8-} y O 16 ( wherein,
A represents one or more elements selected from the group consisting of K, Rb, Cs, Ca, Ba and Na, and M represents a divalent or trivalent metal. However, the Na element is limited only when M is Cr. x and y are 0.7 <x ≦ 2.0 and 0.7 <y ≦
2.0 is shown. A photocatalyst for reductive decomposition and purification of nitrogen oxides in a gaseous phase, characterized by comprising a hollandite-type crystal phase. 2. The catalyst according to claim 1, wherein
x and y are 1.6 <x ≦ 2.0 and 1.6 <y ≦ 2.0
By doing so, by-product nitrous oxide (N ₂
A photocatalyst for reductive decomposition and purification of nitrogen oxides in a gaseous phase, which suppresses the production of O) and increases the selectivity of nitrogen production to 50% or more. 3. The hollandite compound according to claim 1 is brought into contact with a nitrogen oxide and a reducing agent in the presence of a reducing agent while irradiating with light, whereby the nitrogen oxide is decomposed and removed reductively and converted to nitrogen with high efficiency. A method for purifying nitrogen oxides in a gas phase using a photocatalyst.