JP2001246264A - Photocatalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a photocatalyst for decomposing and removing toxic halides, such as dioxin, in exhaust gas and waste water discharged from a municipal waste incinerator, or the like. SOLUTION: This photocatalyst is formed by adding an Ru noble metal selected from Pt, Pd, Ru, and Ir and one kind of transition metal selected from Fe, Ni, Co, Cu, V, and Mn to the fine particles of TiO2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a photocatalyst and a method for decomposing harmful substances using the photocatalyst.

[0002]

BACKGROUND OF THE INVENTION Exhaust gas or wastewater discharged from various incinerators such as municipal garbage incinerators, industrial waste incinerators, and sludge incinerators include, in addition to nitrogen oxides, Harmful halogenated aromatic compounds represented by dioxins and PCBs, highly condensed aromatic hydrocarbons,
In some cases, harmful substances such as environmental hormones are contained, causing damage to the human body, animals and plants, and destroying the natural environment, which has become a serious social problem.

A photocatalyst can oxidize and decompose various kinds of hydrocarbons adsorbed by using light such as ultraviolet light, so that there is no need to apply energy from the outside, and it is one of the earth-friendly environmental protection means. Particularly, in recent years, suppression of emission of environmental hormone substances such as dioxins has been called for, and various methods for detoxification by using a photocatalyst or the like have been studied.

However, anatase-type TiO ₂ generally used as a current photocatalyst has a particle size of 2 μm.
Since it is as large as 00 ° or more, it has not reached the particle size level having the quantum size effect. For this reason, attempts have been made to reduce the particle size of TiO ₂ to exhibit the quantum size effect and improve the catalytic activity. However, if the particle size is simply reduced, the problem that TiO ₂ is re-agglomerated and coarsened. is there. As a result, there is a demand for the development of a photocatalyst that does not become coarse due to re-aggregation or the like even if it is miniaturized and that can effectively exhibit the catalytic activity effect.

Further, ultraviolet light is absorbed by a photocatalyst, and the oxidation of organic substances and the like is promoted by active oxygen and hydroxyl radicals generated by the decomposition of water. However, since the oxidizing power is insufficient, the decomposed organic substances are completely eliminated. There is also a risk that it cannot be converted to CO ₂ or H ₂ O and is emitted as another incomplete harmful substance.

[0006] In view of the above problems, an object of the present invention is to provide a photocatalyst that enhances the oxidizing ability of TiO _{2 and} a method for decomposing harmful substances using the photocatalyst.

[0007]

Means for Solving the Problems The invention of claim 1 for solving the above-mentioned problems is characterized in that a transition metal is added to TiO ₂ fine particles.

According to a second aspect of the present invention, in the first aspect, the transition metal is any one of Pt, Pd, Ru, Ir, and Rh, and one of Fe, Ni, Co, Cu, V, and Mn. It is characterized by.

The invention of claim 3 is the invention according to claim 2, wherein when the transition metal is a noble metal of Pt, Pd, Ru, Ir, Rh, the content is 0.001 to 10%, and other Fe,
In the case of a transition metal of Ni, Co, Cu, V, and Mn, 0.1
It is characterized in that it is from 01 to 30%.

[0010] The invention of claim 4 is characterized in that, in any one of claims 1 to 3, at least one of organometallic compounds of Si, Al, Zr, P and B is added.

The invention of claim 5 is characterized in that harmful substances in exhaust gas or aqueous solution are brought into contact with the photocatalysts of claims 1 to 4 to decompose the harmful substances.

The invention of claim 6 is characterized in that, in claim 5, ozone is added to the decomposition of the photocatalyst.

The invention of claim 7 is the invention according to claim 5, wherein the harmful substance is at least one selected from dioxins, polyhalogenated biphenyls, halogenated benzenes, halogenated phenols and halogenated toluenes. It is a halogenated aromatic compound, a highly condensed aromatic hydrocarbon, and an environmental hormone.

The invention of claim 8 is the invention according to claim 5, wherein the dioxins are polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polybrominated dibenzo-p-dioxins. (PBDDs), polybrominated dibenzofurans (PB
DFs), polyfluorinated dibenzo-p-dioxins (P
FDDs), polyfluorinated dibenzofurans (PFDF)
s), polyiodinated dibenzo-p-dioxins (PI
DDs), polyiodinated dibenzofurans (PIDFs)
It is characterized by being.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described, but the present invention is not limited thereto.

The photocatalyst according to the present invention is obtained by supporting a transition metal having an oxidizing ability such as Pt around TiO ₂ . As a result, it has been found that the organic compound to be decomposed is completely decomposed and can be converted into stable CO ₂ or H ₂ O. The transition metal to be added is a noble metal other than Pt,
Examples thereof include Pd, Ru, Ir, and Rh. In addition to noble metals, for example, Fe, Ni, Co, Cu,
V, Mn, and the like.

As a method of addition, a method in which fine transition metal is supported on a photocatalyst is preferred, and examples thereof include an impregnation method, a vapor deposition method, and a coprecipitation method. In addition, since noble metals are expensive and have a small amount of performance, a method of supporting them in a colloidal state is also effective.

The loading amount is preferably such that the absorption of ultraviolet light into the TiO ₂ photocatalyst is not hindered. Specifically, in the case of a noble metal, about 0.001 to 10%, and in the case of another transition metal, , 0.01 to 30%.

The material used as a photocatalyst is TiO.
₂ is mainly used, but other metals besides Ti include Si, A
By using at least one of the organometallic compounds of 1, Zr, P and B as a composite oxide photocatalyst, fine particles (approximately 100 ° or less) can be obtained.

As described above, the photocatalyst in the form of fine particles has an enhanced photocatalytic function due to the quantum size effect, and the ability to oxidize organic substances is dramatically increased.

Here, the photocatalyst forms a hydroxyl radical from water using ultraviolet light, and since the hydroxyl radical has a high-performance oxidizing ability, an organic compound or the like can be oxidatively decomposed at room temperature. According to the present invention, the fine particle TiO ₂ also has an increased specific surface area, so that the ability to adsorb organic substances and the like increases, and the quantum size effect works, so that the photocatalytic action is dramatically improved and the oxidizing action of Pt and the like increases. Therefore, for example, harmful substances can be completely decomposed.

Further, a transition metal such as a noble metal to be added is more effective when coordinated near the photocatalyst. This is because an oxidation reaction of an organic substance is promoted by a hydroxyl radical considered as an active species of the photocatalyst.

The fixing substrate of the photocatalyst according to the present invention includes:
Glass that transmits ultraviolet light is preferred. This is for efficiently irradiating the photocatalyst with ultraviolet light. The photocatalyst with improved oxidizing power of the present invention is more easily handled by fixing it to a quartz glass substrate or the like.

As a method for immobilization, for example, a dipping method, a spraying method, a spin coating method and the like are preferable.

In the above dipping method, titanium alkoxide is preferable as a raw material of Ti, and there is a method of immersing a substrate such as glass in a titanium alkoxide solution.

The spraying method is a method in which a finely divided liquid photocatalyst raw material is sprayed onto a base material and adhered thereto.

In the spin coating method, in order to make the film thickness of the photocatalyst uniform and as thin as possible, the substrate is rotated, and excess photocatalyst raw material slurry is scattered by centrifugal force.

The raw material of the photocatalyst is preferably in a liquid phase using a metal alkoxide (such as an ethyl group, a methyl group, a propyl group, and a buty group).

As a TiO ₂ raw material, a TiCl ₄ solution,
A solution such as a TiSO ₄ solution, a Ti (SO ₄ ) ₂ solution, or a titania sol may be used. In the case of immobilization, an NH ₃ solution may be added to the above solution to form TiO ₂ , and then the TiO ₂ may be supported on a substrate.

The effect of the photocatalyst according to the present invention is further enhanced by utilizing ultraviolet light. Further, the oxidizing ability is increased by adding ozone. Furthermore, a synergistic effect of decomposition of harmful substances of dioxins is exhibited by the combination of these.

The harmful substances decomposed by the photocatalyst of the present invention include nitrogen oxides, dioxins and PXB.
(X represents a halogen.) Hazardous substances such as harmful halogenated aromatic compounds and high-condensation degree aromatic hydrocarbons represented by the class. It is not limited to these if it is a hormone.

The aromatic halogen compound to be decomposed in the present invention is not limited to harmful substances (for example, environmental hormones) represented by dioxins and PCBs. Here, the dioxins are polyhalogenated dibenzo-p-dioxins (P
XDDs) and polyhalogenated dibenzofurans (PX
DFs) (X represents halogen), and is said to be generated in a small amount when a halogen compound and a certain organic halogen compound are burned. Depending on the number of halogens, there are monohalides to octahalides, of which dibenzo-p-dioxin tetrachloride (T ₄ CDD) is particularly preferred.
Is known to be the most toxic. The harmful halogenated aromatic compounds include, in addition to dioxins, various organic halogen compounds which are precursors thereof (for example, aromatic compounds such as phenol and benzene (for example, halogenated benzenes, halogenated phenols and halogenated phenols). And halogenated alkyl compounds) and must be removed from the ash.
That is, dioxins represent not only chlorinated dioxins but also halogenated dioxins such as brominated dioxins. PXBs (polyhalogenated biphenyls) are a general term for compounds in which several halogen atoms are added to biphenyl, and there are isomers depending on the number and position of substitution of halogen, but in the case of PCB (polychlorinated biphenyl), , 2,6-dichlorobiphenyl, 2,2'-dichlorobiphenyl, 2,3,5-trichlorobiphenyl and the like are typical, are highly toxic, and may emit dioxins when incinerated. Known and need to be removed. Needless to say, the PXBs include the coplanar PXB.

Highly condensed aromatic hydrocarbons are a general term for polynuclear aromatic compounds, which may contain one or more OH groups, are recognized as carcinogenic substances, and need to be decomposed and removed. .

Further, in many manufacturing processes, in addition to dust, exhaust gas containing, for example, a gaseous organic compound such as formaldehyde, benzene or phenol may be generated. Since these organic compounds are also environmental pollutants and significantly impair human health, they also need to be decomposed and removed.

The nitrogen oxides to be treated in the present invention usually mean NO and NO ₂ , and a mixture thereof.
It is also called NOx. However, the NOx often contains various oxidation numbers and unstable nitrogen oxides in addition to the above. Accordingly, x is not particularly limited, but is usually a value of 1 to 2. It becomes nitric acid, nitrous acid, etc. in rainwater or NO, or NO is said to be one of the main causes of photochemical smog, and is a harmful compound to the human body.

By using the above-mentioned photocatalyst according to the present invention, the above-mentioned harmful substances such as nitrogen oxides, halogenated aromatic compounds, and highly condensed aromatic hydrocarbons, and gaseous organic compounds can be contacted. It can be detoxified by reduction or decomposition. Here, the halogenated aromatic compound, the precursor of the halogenated aromatic compound, the halogenated aromatic compound such as PXB, the highly condensed aromatic hydrocarbon, and the environmental hormone of the harmful substance of the present invention are contained in the exhaust gas of the present invention. Like P
By supporting a transition metal having a high oxidizing ability such as t,
The oxidative decomposition effect is improved, and harmful substances are completely eliminated from CO ₂ and H ₂
O and detoxification processing is performed.

A photocatalyst supporting a transition metal such as Pt according to the present invention is fixed to a carrier such as a glass substrate, and a catalyst device is formed. Harmful substances can be decomposed and removed.

The photocatalyst of the present invention can exhibit an effective photocatalytic function not only in the decomposition of harmful substances in the liquid phase into the gas phase but also in the decomposition of harmful organic compounds such as dioxins in the gas phase. .

That is, by using the photocatalyst according to the present invention, after removing soot in exhaust gas discharged from various incinerators such as municipal waste incinerator, industrial waste incinerator, sludge incinerator, etc. In addition, harmful substances such as nitrogen oxides, halogenated aromatic compounds, highly condensed aromatic hydrocarbons and environmental hormones in exhaust gas can be decomposed and removed.

Since the photocatalyst of the present invention can decompose dioxins and the like at room temperature, it is not necessary to set the treatment temperature as high as 200 to 300 ° C. unlike the conventional oxidation catalyst. Therefore, the decomposition at room temperature does not generate dioxins due to resynthesis of the precursor of dioxins, which is a preferable treatment temperature.

Further, by providing a catalyst device provided with the photocatalyst of the present invention at the opening of the chimney, the photocatalyst can be activated by sunlight without using an ultraviolet irradiation device separately.
It is possible to decompose and remove harmful substances in exhaust gas exhausted.

Further, the photocatalyst according to the present invention is directly fixed on the surface of the fluorescent tube in a thin manner, so that photodecomposition from the fluorescent tube can decompose and remove harmful substances such as volatile gas in a room or in a tunnel. Can be.

[0043]

EXAMPLES The present invention will now be described specifically with reference to examples, but the present invention is not limited to these examples.

<Example 1> [Preparation of powder photocatalyst 1] 25 cc of titanium isopropoxide (Ti (OC ₃ H ₇ ) ₄ ) was sampled, 25 cc of isopropanol was dropped at once, and the mixture was stirred at 400 rpm for 10 minutes at room temperature. Obtain solution A. Also, a solution B is obtained by mixing 100 cc of ion-exchanged water and 200 cc of isopropanol. Also, add solution B to solution A,
The mixture is stirred at 400 ° C. for 2 hours at 60 ° C. to obtain a slurry solution C in which titanium hydroxide is formed by hydrolysis. Next, the slurry C is dried and fired at 400 ° C. for 3 hours to obtain a powder 1. This powder 1 is impregnated with an aqueous solution of chloroplatinic acid, and is impregnated with platinum by 0.5 using an impregnation method.
%. Next, after evaporating and drying, baking was performed at 450 ° C. for 3 hours to obtain a photocatalyst powder 1.

[Preparation of Powder Photocatalyst 2] In the preparation of the photocatalyst 1, when preparing the solution A, 4 cc of titanium isopropoxide and silane ethoxide (Si (OC ₂ H ₅ ) ₄ ) were added dropwise to the solution A. did. Thereafter, the same operation as in the preparation method of the photocatalyst 1 was performed to obtain a TiO ₂ · SiO ₂ composite oxide, and then, similarly to the photocatalyst 1, was impregnated with an aqueous chloroplatinic acid solution to obtain a photocatalyst powder 2.

[Adjustment of Powder Photocatalysts 3 to 13] In the above-mentioned method for adjusting the photocatalyst 1, in place of the aqueous platinum chloride solution, an aqueous solution of palladium nitrate, an aqueous solution of ruthenium chloride, an aqueous solution of iridium chloride, and an aqueous solution of rhodium chloride were replaced with Pd, Ru, I
0.5% of r and Rh were supported and adjusted in the same manner as in the photocatalyst 1 to obtain photocatalyst powders 3 to 6. In the method for adjusting the photocatalyst 1, iron nitrate, nickel nitrate, cobalt nitrate, chromium nitrate, copper nitrate, vanadium oxyhydrochloride, and manganese nitrate were used instead of the aqueous chloroplatinic acid solution.
o, Cr, Cu, V, Mn 1.5% supported, photocatalyst 1
By the same operation as described above, photocatalyst powders 7 to 13 were obtained.

[Adjustment of Powdered Photocatalysts 14 to 17] In the method for adjusting the photocatalyst 2, aluminum propoxide (Al (OC ₃ H ₇ ) ₃ ) was used instead of silane ethoxide.
Zirconium propoxide (Zr (OC ₃ H ₇ ) ₄ ),
Solution A was prepared by dropping 2 cc each of boron propoxide (B (OC ₃ H ₇ ) ₃ ) and phosphorus propoxide (P (OC ₃ H ₇ ) ₃ ). Thereafter, the same operation as that of the photocatalyst 2 was performed, and the TiO ₂ .Al ₂ O ₃ composite oxide, TiO ₂ .Z
r ₂ O ₃ composite oxide, TiO ₂ · B ₂ O ₃ composite oxide,
A TiO ₂ .P ₂ O ₃ composite oxide was obtained. Similarly to the photocatalyst 1, an aqueous solution of chloroplatinic acid was impregnated to carry Pt, and photocatalyst powders 14 to 17 were obtained.

[Adjustment of Powder Photocatalyst 18] In the method of adjusting the photocatalyst 1, Pt was impregnated using colloidal platinum (Pt particle size: 3 nm) as a Pt source supported on the powder 1. After evaporation and drying, a photocatalyst 18 was obtained.

[Adjustment of Comparative Catalysts 1 and 2] In the method for adjusting the photocatalyst 1, the powder 1 not supporting Pt was used as the comparative catalyst 1. In addition, ordinary anatase-type TiO ₂ (trade name of “MC-50” manufactured by Ishihara Sangyo) was used as a comparative catalyst 2 and subjected to the following activity evaluation test.

The specific surface area of the obtained photocatalyst and the crystallite diameter of TiO ₂ determined by the X-ray diffraction method are shown in Table 1 below.

<Activity Evaluation Test> The photocatalyst activity was evaluated using the photocatalysts 1 to 10 and the comparative catalysts 1 and 2. The test method is a liquid phase decomposition test of dibenzofuran, which is a simulated substance of dioxins. The raw material dibenzofuran aqueous solution was prepared by the following method. First, dibenzofuran 0.0
6 g was dispersed in 1 cc of methanol for 3 minutes using an ultrasonic water washer, and 70 μL of a dibenzofuran methanol solution was dissolved in 250 cc of pure water. The powder photocatalyst was added at 0.5 g / L, and the tank reactor was stirred at 30 ° C. and 600 rpm. The ultraviolet light having a UV output of 40 W was used.

FIG. 1 shows an outline of the test apparatus. As shown in FIG. 1, the test apparatus controls a reaction vessel 12 provided in a constant temperature water tank 11, an ultraviolet lamp 13 whose periphery is protected by a protective tube 12 in the reaction vessel, and controls the temperature of the constant temperature water tank 11. A constant temperature water tank control means 14 and an ultraviolet lamp control means 15 for controlling the ultraviolet lamp 13, and a dibenzofuran aqueous solution 16 containing a photocatalyst in the constant temperature water tank 11.
And stirring the dibenzofuran aqueous solution 16 with a stirrer 17 using a stirring means 17.

In the analysis, the dibenzofuran solution concentration was analyzed 15 minutes and 30 minutes after light irradiation. The analysis method was such that after irradiation with light for a predetermined time, the slurry was sampled, and the dibenzofuran concentration was determined by gas chromatography analysis using the supernatant separated by a centrifuge. The results are shown in Table 1 below.

As a result of the actual measurement, the initial dibenzofuran concentration was 5.0 mg / L, only UV (no photocatalyst),
It has been confirmed that dibenzofuran is hardly decomposed by the photocatalyst alone (no UV).

Table 1 shows that the dibenzofuran concentration (C) at each time is C / C _{0 with} respect to the initial (C ₀ ).
Indicated by.

Further, by analyzing the by-product organic matter, dibenzofuran was decomposed and converted into another organic matter, and the degree of the incomplete acid value was represented by the following formula. By-product rate (%) = (amount of other organic matter by-product / amount of dibenzofuran decomposed and removed) × 100

[0057]

[Table 1]

From the results shown in Table 1, it was confirmed that the photocatalyst according to the present invention exhibited a high-performance photocatalytic function with almost no by-product organic matter due to the effect of the added transition metal.

[0059]

As described above, according to the first aspect of the present invention, the transition metal is added to the TiO ₂ fine particles, so that the oxidizing ability is improved and the harmful substances can be completely decomposed. As a result, environmental hormones such as dioxins can be decomposed at normal temperature.

According to the invention of claim 2, in claim 1, the transition metal is any one of Pt, Pd, Ru, Ir, and Rh, and one of Fe, Ni, Co, Cu, V, and Mn. In addition, the oxidizing ability becomes more remarkable, and the harmful substances can be completely decomposed.

According to a third aspect of the present invention, in the second aspect, when the transition metal is a noble metal of Pt, Pd, Ru, Ir, and Rh, the content is 0.001 to 10%, and other Fe,
In the case of a transition metal of Ni, Co, Cu, V, and Mn, 0.1
Since it is 01 to 30%, the oxidizing ability becomes more remarkable, and complete decomposition of harmful substances becomes possible.

According to a fourth aspect of the present invention, since at least one of the organometallic compounds of Si, Al, Zr, P and B is added in any one of the first to third aspects, a finely divided photocatalyst is obtained. The quantum effect is developed, and the harmful substance can be completely decomposed.

In the invention of claim 5, the harmful substances in the exhaust gas or the aqueous solution are brought into contact with the photocatalysts of claims 1 to 4, and the harmful substances are decomposed, so that environmental hormones and the like can be decomposed.

According to the sixth aspect of the present invention, since ozone is added to the decomposition of the photocatalyst according to the fifth aspect, the oxidizing ability by ozone is increased, and a synergistic effect of the decomposition of harmful substances of dioxins is obtained by using these together. Is expressed.

According to the invention of claim 5, at least one halogenated aromatic compound selected from dioxins, polyhalogenated biphenyls, halogenated benzenes, halogenated phenols and halogenated toluenes, and at least one halogenated aromatic compound. Harmful substances such as aromatic hydrocarbons and environmental hormones can be decomposed.

According to the invention of claim 6, polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polybrominated dibenzo-
p-Dioxins (PBDDs), polybrominated dibenzofurans (PBDFs), polyfluorinated dibenzo-p-dioxins (PFDDs), polyfluorinated dibenzofurans (PFDFs), polyiodinated dibenzo-p-dioxins (PIDDs) ), Polyiodinated dibenzofurans (P
Dioxins such as IDFs) can be decomposed.

[Brief description of the drawings]

FIG. 1 is a schematic view of a powder photocatalyst test apparatus.

[Explanation of symbols]

 Reference Signs List 11 constant temperature water tank 12 reaction vessel 13 ultraviolet lamp 13a protective tube 14 constant temperature water tank control means 15 ultraviolet lamp control means 16 dibenzofuran aqueous solution 17a stirrer 17 stirring means

Continued on the front page F-term (reference) 4D037 AA13 AA15 AB14 BA18 BB02 BB06 4D048 AA11 AB03 AC07 BA03Y BA04Y BA06Y BA07X BA07Y BA23X BA23Y BA28X BA28Y BA30X BA30Y BA31Y BA33Y BA35X BA35Y BA36X A37A BAY BA37A BAY BA37A BA04A BA04B BA48A BB06A BB06B BC16A BC16B BC31A BC31B BC51A BC54A BC54B BC62A BC62B BC66A BC66B BC67A BC67B BC68A BC68B BC70A BC70B BC71A BC72A BC74A BC75A BC75B BD02A BD03A BD05 CA05A BE19 BE35

Claims (8)

[Claims] 1. A photocatalyst comprising a transition metal added to TiO ₂ fine particles. 2. The method according to claim 1, wherein the transition metal is a noble metal of Pt, Pd, Ru, Ir, and Rh.
e, Ni, Co, Cu, V, or Mn. 3. The method according to claim 2, wherein when the transition metal is a noble metal of Pt, Pd, Ru, Ir, Rh, the content is 0.001 to 10%, and the other Fe, N
In the case of a transition metal of i, Co, Cu, V, Mn, 0.0
Photocatalyst characterized by being 1 to 30%. 4. The photocatalyst according to claim 1, wherein at least one of organometallic compounds of Si, Al, Zr, P and B is added. 5. A method for decomposing harmful substances, comprising contacting harmful substances in exhaust gas or aqueous solution with the photocatalyst according to claim 1 to decompose harmful substances. 6. The method for decomposing harmful substances according to claim 5, wherein ozone is added to decompose the photocatalyst. 7. The method according to claim 5, wherein the harmful substance is at least one halogenated aromatic compound selected from dioxins, polyhalogenated biphenyls, halogenated benzenes, halogenated phenols and halogenated toluenes. A method for decomposing harmful substances characterized by being aromatic hydrocarbons and environmental hormones. 8. The method according to claim 7, wherein the dioxins are polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polybrominated dibenzo-p-dioxins (PBDDs), and polyodors. Dibenzofurans (PBD
Fs), polyfluorinated dibenzo-p-dioxins (PF
DDs), polyfluorinated dibenzofurans (PFDFs),
Polyiodinated dibenzo-p-dioxins (PIDD
s), a method for decomposing harmful substances, which is polyiodinated dibenzofurans (PIDFs).