TWI670112B - Catalyst and method for removing nox from combustion exhaust - Google Patents

Abstract

The present invention provides a catalyst for removing NOx from combustion exhaust gas, especially low NOx combustion exhaust gas, which is less likely to accumulate ceria and which does not cause a decrease in denitration performance even if the amount of ruthenium dioxide is increased. In the pore volume distribution in the range of the pore diameter of 10 ⁵ Å or less, the ratio of the pore volume of the pore diameter of 500 Å or more and 3,000 Å or less to the total pore volume is 15% or more and 40% or less, preferably fine. The ratio of the pore volume in the range of the pore diameter of 1000 Å or more to the total pore volume is 10% or more and 45% or less. Further, there is provided a method of removing NOx by contacting a low NOx combustion exhaust gas with the catalyst in the presence of ammonia or an ammonia precursor.

Description

Catalyst and method for removing NOx from combustion exhaust gas

The present invention relates to a catalyst and method for removing NOx from combustion exhaust gases. More specifically, the present invention relates to a type of cerium oxide which is less likely to accumulate, and which does not cause a decrease in denitration performance even if the amount of cerium oxide is increased, and is used for removing NOx from combustion exhaust gas, preferably low NOx combustion exhaust gas. Catalyst and method.

The NOx generated by the combustion of fuels such as coal and biomass is roughly classified into fuel NOx generated by oxidation of nitrogen components in the fuel and thermal NOx generated by oxidation of nitrogen in the air. As a combustion method (low NOx combustion method) for reducing NOx contained in combustion exhaust gas, a low air ratio combustion method, an air multi-stage combustion method, a fuel multi-stage combustion method, an exhaust gas recirculation combustion method, a lean premixed combustion method are known. Law and so on. The low NOx combustion exhaust gas has a lower NOx content than the normal combustion exhaust gas, but even so, in order to reduce the environmental load, it is desirable to carry out the denitration treatment. As a prior denitration catalyst, for example, Patent Document 1 discloses an ammonia contact reduction denitration catalyst containing an oxide of titanium and molybdenum and having a range of about 1 × 10 ³ to 1 × 10 ⁴ Å. The first pore group having an average diameter and the second pore group having an average diameter in a range of about 10 ² to 10 ³ Å, and the first pore group is in the range of 10 to 50% of the total pore volume. Patent Document 2 discloses a denitration catalyst for reducing and detoxifying nitrogen oxides in exhaust gas, which contains titanium oxide as a main component, and has a total pore volume of 0.20 to 0.45 ml/g and a diameter of 1000 Å or more. The pore volume occupied by the pores is 0.05 to 0.2 ml/g. Patent Document 3 discloses an ammonia contact reduction denitration catalyst which is based on a volume of pores having an average pore diameter of 10000 Å or less and having a pore diameter of 400 to 5000 Å with respect to the total pore volume. The titanium oxide having a ratio of 50% or more is supported by a denitration active ingredient. Patent Document 4 discloses a catalyst for removing nitrogen oxides, which is characterized by being formed by a secondary aggregate of mesoporous ceria having a mesopores having an average pore diameter of 5 nm or less and 100 to 10000 nm. The large pores are impregnated and supported with a titanium oxide sol having a particle diameter larger than the diameter of the mesopores, and the resultant further carries the denitration catalyst component. However, if the denitration treatment of the low NOx combustion exhaust gas is carried out by using the prior denitration catalyst as described above, there is a case where the cerium oxide is deposited on the surface of the catalyst (here, the "cerium oxide" system The meaning of including all the states of Si; the following meanings are the same), leading to deterioration of the denitrification catalyst. It is considered that the cause of the deterioration is that Si contained in coal or the like is not sufficiently oxidized in low NOx combustion, and a large amount of gaseous cerium oxide such as a siloxane is contained in the combustion exhaust gas. [PRIOR ART DOCUMENT] [Patent Document 1] Japanese Patent Laid-Open No. Hei 5-66175 (Patent Document 2) Japanese Patent Laid-Open Publication No. SHO63-147547 (Patent Document 3) Japanese Patent Laid-Open No. Hei 1-130720 [Patent Document 4] Japanese Patent Laid-Open Publication No. 2009-219980 [Non-Patent Document] [Non-Patent Document 1] Matsumoto et al. "Development of Ultra Low NOx Coal-fired M-PM Burner" Mitsubishi Heavy Industries Technology Research Report Vol. .50 No.3 (2013) Power Generation Technology P18 [Non-Patent Document 2] Naito "Reduction of the environmental load of the furnace obtained by the ultra-low NOx burner" PETROTEC Vol. 31, No. 9 (2008) [Non-patent literature 3] Watanabe "Coal combustion technology for thermal power generation" 31 (2012) 339-344

[Problem to be Solved by the Invention] A first object of the present invention is to provide a catalyst and method for removing NOx from combustion exhaust gas. A second object of the present invention is to provide a catalyst and method for removing NOx from combustion exhaust gas, especially low NOx combustion exhaust gas, which is less likely to accumulate ceria and which does not cause a decrease in denitration performance even if the amount of ruthenium dioxide is increased. . [Means for Solving the Problems] In order to solve the above problems, efforts have been made to study the present invention. As a result, the present invention including the following aspects has been completed. [1] A catalyst for removing NOx from combustion exhaust gas, which is in a pore volume distribution in a range of pore diameters of 10 ⁵ Å or less, and pore volume in a range of pore diameters of 500 Å or more and 3000 Å or less with respect to The ratio of the total pore volume is 15% or more and 40% or less. [2] The catalyst according to [1], wherein in the pore volume distribution in the range of pore diameters of 10 ⁵ Å or less, the ratio of the pore volume in the range of pore diameters of 1000 Å or more to the total pore volume is 10% or more and 45% or less. [3] The catalyst according to [1] or [2], which comprises an oxide of titanium, an oxide of molybdenum and/or tungsten, and an oxide of vanadium. [4] The catalyst according to any one of [1] to [3] wherein the combustion exhaust gas is a low NOx combustion exhaust gas. [5] The catalyst according to any one of [1] to [4] wherein, in the pore volume distribution in the range of pore diameters of 10 ⁵ Å or less, the pore volume in the range of pore diameters of 40 Å or more is relative to The ratio of the total pore volume is 90% or more. [6] A method for removing NOx from a combustion exhaust gas, comprising: contacting a combustion exhaust gas with a catalyst according to any one of [1] to [5] above in the presence of ammonia, and removing NOx from the combustion exhaust gas . [7] The method according to [6], wherein the combustion exhaust gas is a low NOx combustion exhaust gas. The present invention includes the following as a more preferred form. [8] A catalyst for removing NOx from combustion exhaust gas, wherein pore volume in a range of pore diameters of 40 Å or more and 3,000 Å or less is relative to total pore volume distribution in a range of pore diameters of 10 ⁵ Å or less The ratio of the pore volume is 80% or more, and the ratio of the pore volume in the range of 500 Å or more and 3,000 Å or less to the total pore volume is 15% or more and 40% or less. [9] A catalyst for removing NOx from combustion exhaust gas, wherein a pore volume distribution in a range of pore diameters of 3000 Å or more with respect to a total pore volume is in a pore volume distribution in a range of pore diameters of 10 ⁵ Å or less The ratio is 10% or less, and the ratio of the pore volume in the range of 500 Å or more with respect to the total pore volume is 25% or more and 50% or less, and the pore volume in the range of pore diameter of 40 Å or more is relative to the total fineness. The ratio of the pore volume is 90% or more. [10] The catalyst according to [8] or [9], wherein in the pore volume distribution in the range of pore diameters of 10 ⁵ Å or less, the pore volume in the range of pore diameters of 1000 Å or more is relative to the total pores. The ratio of the volume is 10% or more and 45% or less. [11] A method of removing NOx from a combustion exhaust gas, comprising: contacting a combustion exhaust gas with a catalyst according to any one of [8] to [10] above in the presence of ammonia, and removing NOx from the combustion exhaust gas. . [12] The method according to [11], wherein the combustion exhaust gas is a low NOx combustion exhaust gas. [13] The method according to [11], wherein the NOx concentration of the combustion exhaust gas is 350 ppm or less. [Effects of the Invention] The catalyst of the present invention is suitable for an ammonia contact reduction reaction for removing NOx from combustion exhaust gas. In the catalyst of the present invention, cerium oxide is less likely to accumulate on the surface of the catalyst, and even if cerium oxide adheres and accumulates, the denitration performance hardly decreases, and the resistance to cerium oxide deposition is high. The catalyst of the present invention is suitable for use in the ammonia contact reduction reaction for removing NOx from low NOx combustion exhaust gases. According to the method of the present invention, even if cerium oxide adheres to the surface of the catalyst, the denitration performance hardly decreases, so that the frequency of exchange of the catalyst can be reduced, and the cost of purification of the combustion exhaust gas can be reduced. Since the pore diameter of less than 500 Å is smaller than the average free path of the gas, the gas easily collides with the inner wall of the pore. Therefore, it is considered that gaseous cerium oxide such as a siloxane is reacted on the surface of the pores of the catalyst to form and deposit cerium oxide, so that the denitration performance of the catalyst having a small pore diameter is lowered. On the other hand, it is considered that the pore diameter of 500 Å or more is close to or larger than the average free path of the gas, so that the collision of gaseous molecules in the pores is relatively small, and the deposition of cerium oxide is reduced, and the degradation of the denitration performance can be suppressed.

The catalyst of the present invention is used to remove NOx from combustion exhaust gases. The catalyst of the present invention has a pore volume distribution as described below. The catalyst of the present invention is in a pore volume distribution in the range of pore diameters of 10 ⁵ Å or less, and the ratio of the pore volume to the total pore volume in the range of pore diameters of 40 Å or more and 3,000 Å or less is preferably 80%. More preferably, it is 85% or more. The catalyst of the present invention preferably has pores having a pore diameter of more than 3,000 Å and pores having a pore diameter of less than 40 Å. Specifically, in the pore volume distribution in the range of the pore diameter of 10 ⁵ Å or less, the ratio of the pore volume in the range of the pore diameter of 3000 Å or more to the total pore volume is preferably 10% or less, more preferably 8 % or less, the ratio of the pore volume in the range of the pore diameter of 40 Å or more to the total pore volume is preferably 90% or more, more preferably 93% or more.

The catalyst of the present invention is in a pore volume distribution in the range of pore diameters of 10 ⁵ Å or less, and the ratio of the pore volume in the range of pore diameters of 500 Å or more and 3,000 Å or less to the total pore volume is preferably 15% or more. 40% or less, more preferably 19% or more and 39% or less.

Further, the ratio of the pore volume in the range of the pore diameter of 500 Å or more to the total pore volume is preferably 25% or more and 50% or less, more preferably 26% or more and 49% or less, still more preferably 23% or more. 45% or less.

Further, the ratio of the pore volume in the range of the pore diameter of 1000 Å or more to the total pore volume is preferably 10% or more and 45% or less, more preferably 11% or more and 42% or less, still more preferably 7.5% or more. 28% or less.

With regard to such a catalyst having a novel pore volume distribution, it is difficult to deposit cerium oxide on the surface of the catalyst, and even if cerium oxide is deposited, the denitration performance is hardly lowered.

Further, the pore volume distribution of the present invention is measured by a mercury infiltration method.

The catalyst of the present invention preferably comprises an oxide of titanium, an oxide of molybdenum and/or tungsten, and an oxide of vanadium. As a catalyst of a preferred aspect of the present invention, for example, TiO ₂ -V ₂ O ₅ -WO ₃ , TiO ₂ -V ₂ O ₅ -MoO ₃ , TiO ₂ -V ₂ O ₅ -WO ₃ -MoO ₃ and so on.

The ratio of the V element to the Ti element is expressed by weight percentage of V ₂ O ₅ /TiO ₂ , preferably 2% by weight or less, more preferably 1% by weight or less. The ratio of the Mo element and/or the W element to the Ti element is expressed by the weight percentage of (MoO ₃ + WO ₃ ) / TiO ₂ when the oxide of molybdenum is used in combination with the oxide of tungsten, preferably 10 parts by weight. % or less, more preferably 5% by weight or less.

In the preparation of the catalyst, as a raw material of the oxide of titanium, a titanium oxide powder or a titanium oxide precursor can be used. Examples of the titanium oxide precursor include a titanium oxide slurry and a titanium oxide sol; titanium sulfate, titanium tetrachloride, and titanate.

In the present invention, as the raw material of the oxide of titanium, those which form anatase type titanium oxide can be preferably used.

As a raw material of the vanadium oxide, a vanadium compound such as vanadium pentoxide, ammonium metavanadate or vanadyl sulfate can be used.

As a raw material of the tungsten oxide, ammonium paratungstate, ammonium metatungstate, tungsten trioxide, tungsten chloride, or the like can be used.

As a raw material of the molybdenum oxide, ammonium molybdate, molybdenum trioxide or the like can be used.

The catalyst of the present invention may further comprise an oxide of P, an oxide of S, an oxide of Al (for example, alumina), an oxide of Si (for example, glass fiber), an oxide of Zr (for example, zirconia), Gypsum (for example, gypsum or the like), zeolite or the like is used as a co-catalyst component or an additive, and these may be used in the form of a catalyst, a sol, a slurry, a fiber or the like in the preparation of a catalyst.

The catalyst of the present invention is not limited by its shape, and may be in the form of, for example, a pellet, a sphere, a cylinder, a honeycomb, a plate, a mesh, or a wave.

The catalyst of the present invention is not particularly limited by its method of manufacture. The catalyst of the present invention can be produced using a method of controlling the pore volume distribution known in the art. For example, the titanium oxide powder or the titanium oxide precursor may be pre-baked, and then a catalyst component such as V, W or Mo may be added, and if necessary, a catalyst component or an additive may be molded and then calcined, or By adding an organic polymer compound such as polyvinyl alcohol or polyethylene oxide to a titanium oxide powder or a titanium oxide precursor, a catalyst component such as V, W or Mo, and optionally a catalyst component or an additive. It is obtained by performing formal baking after molding.

The method of the present invention comprises a method of removing NOx from a combustion exhaust gas by contacting a combustion exhaust gas with a catalyst of the present invention in the presence of ammonia.

The combustion exhaust gas is preferably a low NOx combustion exhaust gas, more preferably a combustion exhaust gas having a NOx concentration of 350 ppm or less. Ammonia can be added to the combustion exhaust gas according to methods known in the art. The amount of ammonia is not particularly limited as long as it is a range in which the reduction reaction proceeds smoothly. If the combustion exhaust gas is brought into contact with the catalyst of the present invention in the presence of ammonia, for example, the reduction reaction represented by the formula (1) proceeds, and the nitrogen oxides are converted into non-toxic nitrogen gas and water. NO + NH ₃ + 1/4 O ₂ → N ₂ + H ₂ O (1) [Examples] Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples. Example 1 820 kg of titanium dioxide was put into a kneader, and thereafter, a monoethanol aqueous solution obtained by dissolving 8.9 kg of ammonium metavanadate and 69.6 kg of ammonium paratungstate was added, and 46.7 kg of glass fiber, 46.7 kg of activated clay, and polyepoxy were added. The ethane was 9.3 kg, which was kneaded by a kneader. Thereafter, microcrystalline cellulose was added so as to be 15% by weight based on the dry weight of the catalyst, and the mixture was kneaded while performing moisture adjustment. Thereafter, the kneaded product was extrusion-molded by a vacuum extruder equipped with a screw for a honeycomb extrusion nozzle to obtain a honeycomb formed body. The honeycomb shaped body was naturally dried, and then dried under airing at 100 ° C for 5 hours. Thereafter, both ends of the axial direction are cut and aligned, and baked at 600 ° C in an electric furnace to obtain an outer diameter of 150 mm × 150 mm, an axial length of 800 mm, a cell pitch of 7.4 mm, an inner wall thickness of 1.15 mm, and The honeycomb shaped body A of the pore volume distribution shown in Fig. 1 . The ratio of the pore volume of the pore diameter of 40 Å or more and 3,000 Å or less to the total pore volume is 80% or more, and the ratio of the pore volume of the pore diameter of 500 Å or more to the total pore volume is 33%. The ratio of the pore volume of the pore diameter of 500 Å or more to 3,000 Å or less to the total pore volume is 24%, and the ratio of the pore volume of the pore diameter of 1000 Å or more to the total pore volume is 19%. . The honeycomb formed body A was placed in a denitration device of a low-NOx combustion boiler, and NOx removal from the combustion exhaust gas (NOx concentration: 200 ppm) was carried out for 56,000 hours. The amount of cerium oxide deposited on the surface of the catalyst and the rate of denitration were measured. The relationship (×) between the amount of ruthenium deposited on the surface of the catalyst and the reaction rate constant ratio k/ko is shown in Fig. 4 . Example 2 An outer diameter of 150 mm × 150 mm and an axial length of 800 mm were obtained by the same method as in Example 1 except that the amount of the microcrystalline cellulose was changed to 10% by weight based on the dry weight of the catalyst. The honeycomb forming body B having a cell pitch of 7.4 mm, an inner wall thickness of 1.15 mm, and a pore volume distribution obtained in Fig. 2 . The ratio of the pore volume of the pore diameter of 40 Å or more to 3,000 Å or less to the total pore volume is 80% or more, and the pore volume of the pore diameter of 500 Å or more and 3000 Å or less is relative to the total pore volume. The ratio is 26%, the ratio of the pore volume in the range of 500 Å or more and 3,000 Å or less to the total pore volume is 18%, and the pore volume in the range of pore diameter of 1000 Å or more is relative to the total pore volume. The ratio is 11%. The honeycomb formed body B was placed in a denitration device of a low-NOx combustion boiler, and NOx removal from the combustion exhaust gas (NOx concentration: 350 ppm) was performed for 15,000 hours. The amount of cerium oxide deposited on the surface of the catalyst and the rate of denitration were measured. The relationship (◆) between the amount of ruthenium deposited on the surface of the catalyst and the reaction rate constant ratio k/ko is shown in Fig. 4 . Comparative Example 1 An outer diameter of 150 mm × 150 mm and an axial length of 800 mm were obtained by the same method as in Example 1 except that the amount of the microcrystalline cellulose was 0% by weight based on the dry weight of the catalyst. The cell body has a cell pitch of 7.4 mm, an inner wall thickness of 1.15 mm, and a honeycomb formed body C having a pore volume distribution as shown in FIG. The ratio of the pore volume of the pore diameter of 500 Å or more to the total pore volume is 9%, and the ratio of the pore volume of the pore diameter of 500 Å or more to 3000 Å or less with respect to the total pore volume is 4%. The ratio of the pore volume in the range of pore diameters of 1000 Å or more to the total pore volume is 7%. The honeycomb formed body C was placed in a denitration device of a low-NOx combustion boiler, and NOx removal from the combustion exhaust gas (NOx concentration: 270 ppm) was performed for 30,000 hours. The amount of cerium oxide deposited on the surface of the catalyst and the rate of denitration were measured. The relationship (■) between the amount of ruthenium dioxide deposited on the surface of the catalyst and the reaction rate constant ratio k/ko is shown in Fig. 4 . According to the above, if the catalyst of the present invention is used, even if the amount of cerium oxide is increased on the surface of the catalyst, the denitration performance is hardly lowered, and the exhaust gas, preferably low NOx combustion exhaust gas, can be removed. NOx. Further, the cerium oxide treatment was carried out for a certain period of time in a laboratory test, and the reaction rate constant ratio k/ko was measured. Example 3 Honeycomb shaped body A was exposed to simulated exhaust gas containing helium oxide at a laboratory scale. Thereafter, the denitration rate was measured. The relationship (×) between the ratio of the pore volume of the pore diameter of 500 Å or more and 3,000 Å or less to the total pore volume and the reaction rate constant ratio k/ko is shown in Fig. 5 . Further, the relationship (×) between the ratio of the pore volume of the pore diameter of 500 Å or more to the total pore volume and the reaction rate constant ratio k/ko is shown in Fig. 6 . Example 4 An outer diameter of 150 mm × 150 mm and an axial length of 800 mm were obtained by the same method as in Example 1 except that the amount of the microcrystalline cellulose was changed to 18% by weight based on the dry weight of the catalyst. The cell body has a cell pitch of 7.4 mm, an inner wall thickness of 1.15 mm, and a honeycomb formed body D having a pore volume distribution as shown in FIG. The ratio of the pore volume of the pore diameter of 40 Å or more to 3,000 Å or less to the total pore volume is 80% or more, and the ratio of the pore volume of the pore diameter of 500 Å or more to the total pore volume is 42%. The ratio of the pore volume of the pore diameter of 500 Å or more to 3,000 Å or less to the total pore volume is 38%, and the ratio of the pore volume of the pore diameter of 1000 Å or more to the total pore volume is 27%. . Honeycomb shaped body D was exposed to simulated exhaust gas containing helium oxide at a laboratory scale. Thereafter, the denitration rate was measured. The relationship between the ratio of the pore volume in the range of 500 Å or more and 3,000 Å or less to the total pore volume and the reaction rate constant ratio k/ko (●) is shown in Fig. 5 . Further, the relationship between the ratio of the pore volume of the pore diameter of 500 Å or more to the total pore volume and the reaction rate constant ratio k/ko (●) is shown in Fig. 6 . Example 5 An outer diameter of 150 mm × 150 mm and an axial length of 800 mm were obtained by the same method as in Example 1 except that the amount of the microcrystalline cellulose was changed to 16% by weight based on the dry weight of the catalyst. The cell body has a cell pitch of 7.4 mm, an inner wall thickness of 1.15 mm, and a honeycomb shaped body E having a pore volume distribution as shown in FIG. The ratio of the pore volume of the pore diameter of 40 Å or more and 3,000 Å or less to the total pore volume is 80% or more, and the ratio of the pore volume of the pore diameter of 500 Å or more to the total pore volume is 33%. The ratio of the pore volume of the pore diameter of 500 Å or more to 3,000 Å or less to the total pore volume is 30%, and the ratio of the pore volume of the pore diameter of 1000 Å or more to the total pore volume is 23%. . Honeycomb shaped body E was exposed to simulated exhaust gas containing helium oxide at a laboratory scale. Thereafter, the denitration rate was measured. The relationship between the ratio of the pore volume in the range of 500 Å or more and 3,000 Å or less to the total pore volume and the reaction rate constant ratio k/ko (〇) is shown in Fig. 5 . Further, the relationship between the ratio of the pore volume in the range of 500 Å or more in the pore diameter to the total pore volume and the reaction rate constant ratio k/ko (〇) is shown in Fig. 6 . Example 6 An outer diameter of 150 mm × 150 mm and an axial length of 800 mm were obtained by the same method as in Example 1 except that the amount of the microcrystalline cellulose was changed to 20% by weight based on the dry weight of the catalyst. The cell body has a cell pitch of 7.4 mm, an inner wall thickness of 1.15 mm, and a honeycomb shaped body F having a pore volume distribution as shown in FIG. The ratio of the pore volume of the pore diameter of 40 Å or more to 3,000 Å or less to the total pore volume is 80% or more, and the ratio of the pore volume of the pore diameter of 500 Å or more to the total pore volume is 49%. The ratio of the pore volume of the pore diameter of 500 Å or more to 3,000 Å or less to the total pore volume is 39%, and the ratio of the pore volume of the pore diameter of 1000 Å or more to the total pore volume is 42%. . Honeycomb shaped body F is exposed to simulated exhaust gas containing helium oxide at a laboratory scale. Thereafter, the denitration rate was measured. The relationship between the ratio of the pore volume in the range of 500 Å or more and 3,000 Å or less to the total pore volume and the reaction rate constant ratio k/ko (◇) is shown in Fig. 5 . Further, the relationship between the ratio of the pore volume in the range of 500 Å or more in the pore diameter to the total pore volume and the reaction rate constant ratio k/ko (◇) is shown in Fig. 6 . Comparative Example 2 Honeycomb shaped body C was exposed to a simulated exhaust gas containing helium oxide at a laboratory scale. Thereafter, the amount of cerium oxide deposited on the surface of the catalyst and the rate of denitration were measured. The relationship between the ratio of the pore volume in the range of 500 Å or more and 3,000 Å or less to the total pore volume and the reaction rate constant ratio k/ko (■) is shown in Fig. 5 . Further, the relationship between the ratio of the pore volume in the range of 500 Å or more in the pore diameter to the total pore volume and the reaction rate constant ratio k/ko (■) is shown in Fig. 6 .

Fig. 1 is a view showing a pore volume distribution of a catalyst (honeycomb shaped body A) obtained in Example 1. Fig. 2 is a view showing the pore volume distribution of the catalyst (honeycomb shaped body B) obtained in Example 2. Fig. 3 is a graph showing the pore volume distribution of the catalyst (honeycomb molded body C) obtained in Comparative Example 1. Fig. 4 is a graph showing the relationship between the amount of ruthenium dioxide deposited on the surface of the catalyst and the reaction rate constant ratio k/ko. Fig. 5 is a graph showing the relationship between the ratio of the pore volume to the total pore volume in the range of 500 Å or more and 3,000 Å or less in relation to the reaction rate constant ratio k/ko. Fig. 6 is a graph showing the relationship between the ratio of the pore volume to the total pore volume in the range of 500 Å or more in the pore diameter and the reaction rate constant ratio k/ko. Fig. 7 is a view showing the pore volume distribution of the catalyst (honeycomb shaped body D) obtained in Example 4. Fig. 8 is a view showing the pore volume distribution of the catalyst (honeycomb molded body E) obtained in Example 5. Fig. 9 is a view showing the pore volume distribution of the catalyst (honeycomb shaped body F) obtained in Example 6.

Claims (7)

A catalyst for removing NOx from combustion exhaust gas, which comprises an oxide of titanium, an oxide of molybdenum and/or tungsten, and an oxide of vanadium, and the catalyst is in a range of pore diameters below 10 ⁵ Å. In the pore volume distribution, the ratio of the pore volume in the range of 500 Å or more and 3,000 Å or less to the total pore volume is 15% or more and 40% or less. The catalyst according to claim 1, wherein in the pore volume distribution in the range of pore diameters of 10 ⁵ Å or less, the ratio of the pore volume in the range of 40 Å or more and 3,000 Å or less to the total pore volume is 80% or more. . In the catalyst of claim 1 or 2, in the pore volume distribution in the range of pore diameters of 10 ⁵ Å or less, the ratio of the pore volume in the range of pore diameters of 40 Å or more to the total pore volume is 90% or more. The catalyst according to claim 1 or 2, wherein in the pore volume distribution in the range of pore diameters of 10 ⁵ Å or less, the ratio of the pore volume in the range of pore diameters of 1000 Å or more to the total pore volume is 10% or more and 45% or less. A method of removing NOx from a combustion exhaust gas, comprising: contacting a combustion exhaust gas with a catalyst according to any one of claims 1 to 4 in the presence of ammonia to remove NOx from the combustion exhaust gas. The method of claim 5, wherein the combustion exhaust gas is a low NOx combustion exhaust gas. The method of claim 6, wherein the NOx concentration of the combustion exhaust gas is 350 ppm or less.