HK1068300B - Photocatalyst producing method, photocatalyst, and gas purifier - Google Patents

Description

Method for producing photocatalyst, and gas purifier

Technical Field

The present invention relates to a method for preparing a photocatalyst having properties showing photocatalytic activity and thermocatalytic activity, and a photocatalyst prepared by such method, and a gas purifier for decomposing Volatile Organic Compounds (VOCs) in air or the like using such photocatalyst.

Background

In recent years, environmental problems caused by volatile organic compounds contained in, for example, air have been worsened. Specific examples thereof include: it is pointed out that formaldehyde generated from a binder or a preservative for construction materials causes building sickness syndrome in an indoor space, acetaldehyde generated from, for example, residues of food factories causes industrial odor, and the like.

Conventionally, a treatment has been performed in which air is purified using an air filter containing an adsorbent such as activated carbon, thereby adsorbing volatile organic compounds. However, in the case of using activated carbon, a problem has been pointed out that the adsorption activity of the adsorbed organic compound gradually decreases as it covers the active sites of the activated carbon. In view of this, there has been proposed a method of purifying air by decomposing volatile organic compounds using a photocatalyst, which is activated to exhibit redox activity when light is applied, so-called photocatalytic activity, as a purification means instead of using activated carbon.

As an example of the above-mentioned photocatalyst, it has been known that such a photocatalyst is composed of titania having a property of exhibiting photocatalytic activity and a support which is silica having titania supported thereon (hereinafter, such a photocatalyst will be referred to as "titania/silica catalyst"). The titania/silica catalyst has a strong oxidizing ability, but its reducing ability is slightly inferior to the oxidizing ability. It is known that when platinum (Pt) is further supported on a titania/silica catalyst, a photocatalyst in which the redox activity is further enhanced (hereinafter referred to simply as "platinum-titania/silica catalyst") can be obtained (see, for example, non-patent document 1).

When the above platinum-titania/silica catalyst is used for purifying air containing volatile organic compounds such as acetaldehyde, light from, for example, invisible light is applied to a reactor filled with a layer of photocatalyst, thereby activating the photocatalyst, and at the same time, air is fed onto the photocatalyst by means of a feeding and discharging device such as a fan. In this case, acetaldehyde is decomposed into, for example, carbon dioxide and water by photocatalytic activity, thereby purifying the air.

[ non-patent document 1]

E.obuchi, T.Sakamoto, and K.Nakano, "Photocatalytical Decomposition of Acetaldehyde over TiO₂/SiO₂Catalyst ", chemical engineering Science, 54(1999), page 1525-.

However, there is a problem that acetaldehyde is not sufficiently decomposed by the above photocatalyst. Specifically, although most of acetaldehyde may be decomposed by the photocatalyst, even if several tens ppm remain, it may cause an uncomfortable feeling due to its strong offensive odor. In addition, when the catalyst activity is low, acetaldehyde is not decomposed into carbon dioxide and water in some cases, and thus another odorous substance such as formic acid or acetic acid, which are intermediate substances, is produced. Therefore, it is a fact that when purifying air of, for example, an indoor space, the indoor air is circulated onto the photocatalyst to reduce the concentration of acetaldehyde or an intermediate substance for a certain long period of time. For this reason, a method that can rapidly purify air at a higher decomposition rate has been demanded, and as one of them, a need for a photocatalyst of higher activity has been considered.

Another problem is that if coke (coke), a so-called residue generated upon decomposition of acetaldehyde, adheres to the surface of the catalyst, in this case, the active sites of the catalyst are covered with coke, thereby reducing the decomposition rate of acetaldehyde. Coke poisons the catalyst but can be removed by a catalyst recovery process by heating it in a furnace at, for example, 200-. However, in order to carry out such a catalyst recovery process, the photocatalyst is taken out of the reactor, and the gas purification process is interrupted, which is not only time-consuming but also labor-consuming. In addition, in order to maintain the decomposition rate of acetaldehyde at a certain level, it is possible to increase the frequency of performing the catalyst recovery process to suppress the adhesion of coke to the catalyst surface.

Summary of The Invention

The present invention has been made under such circumstances, and an object thereof is to provide a highly active photocatalyst which can purify a gas at a high decomposition rate, for example.

It is another object of the present invention to provide a method for preparing such a photocatalyst.

It is a further object of the present invention to provide a gas purifier that can purify gas at high decomposition rates using such a photocatalyst.

According to an aspect of the present invention, there is provided a method for producing a photocatalyst including a carrier on which a titanium oxide and a metal having a property showing a thermocatalytic activity are supported, the method including a metal supporting step of causing the titanium oxide-supported carrier to support the metal compound; a reduction step of reducing the metal compound supported by the carrier in the metal supporting step with hydrogen at a first treatment temperature in a heating atmosphere; and an oxidation step of oxidizing the metal obtained by the hydrogen reduction in the reduction step at a second treatment temperature in a heating atmosphere.

According to the photocatalyst preparation method of the present invention, the oxidation process is performed after the hydrogen reduction process, thereby releasing the metal from an extremely strong reduction state, and therefore, the metal is highly dispersed on the surface of the catalyst in the form of very fine crystals. As a result, since the thermal catalytic activity of the metal is improved in addition to the photocatalytic activity of titanium oxide, a highly active photocatalyst can be obtained.

The second process temperature may be equal to or lower than the first process temperature. The second treatment temperature may fall within the range of 300 ℃ to 600 ℃, or within the range of 500 ℃ to 600 ℃.

The photocatalyst preparation method may further include a temperature adjustment step of setting the temperature of the support at the second treatment temperature in an inert gas atmosphere after the reduction step.

The metal may be at least one of platinum, rhodium, ruthenium and nickel. In the case of platinum, the content thereof may be 0.04 wt% to 0.5 wt% with respect to the weight of the photocatalyst.

The content of titanium dioxide may be 10 wt% or more based on the weight of the photocatalyst.

The support may be silica spheres.

The photocatalyst preparation method may further include, before the metal supporting step, a step of infiltrating a first treatment liquid containing titanium tetraisopropoxide and isopropanol into the carrier; a step of hydrolyzing the titanium compound permeated into the carrier to load the carrier with titanium dioxide; and a step of calcining the titania-loaded support.

According to another aspect of the present invention, there is provided a procatalyst prepared by the above photocatalyst preparation method.

According to still another aspect of the present invention, there is provided a gas purifier for purifying a volatile organic compound-containing gas, the gas purifier comprising a reactor filled with the above photocatalyst; a light emitting device for emitting light on the photocatalyst in the reactor; and a supply and discharge means for supplying a gas to the photocatalyst, wherein the temperature of the photocatalyst is 100 ℃ to 200 ℃ when purifying the gas.

The volatile organic compound may be at least one of acetaldehyde, formaldehyde, paraffin, olefin, and aromatic compound.

Brief Description of Drawings

FIG. 1 is a flow chart illustrating the preparation of a photocatalyst according to a preferred embodiment of the present invention.

FIG. 2 is a longitudinal sectional view illustrating a gas purifier according to a preferred embodiment of the present invention;

FIG. 3 is a transverse cross-sectional view taken along line P-P in FIG. 2;

FIG. 4A is a transverse sectional view showing a modification of the gas scrubber shown in FIGS. 2 and 3;

FIG. 4B is a transverse sectional view showing another modification of the gas purifier shown in FIGS. 2 and 3;

FIG. 4C is a transverse sectional view showing still another modification of the gas purifier shown in FIGS. 2 and 3;

FIG. 5 is a characteristic graph showing the results of tests conducted to demonstrate the effects of the present invention;

FIG. 6 is a characteristic graph showing the results of tests conducted to demonstrate the effects of the present invention; and

fig. 7 is a characteristic graph showing the results of tests performed to demonstrate the effects of the present invention.

Description of the preferred embodiments

Referring to fig. 1, a description will be made regarding a method for preparing a photocatalyst according to a preferred embodiment of the present invention. First, as shown in step S1 in FIG. 1, a porous support made of, for example, silica, such as silica pellets having an average particle size of 2 to 4mm, is placed on a supportE.g., 500 c, to dryness, thereby removing water. Then, as shown in step S2, a first treatment liquid obtained by mixing together titanium tetraalkoxide, for example, titanium Tetraisopropoxide (TIP), and alcohol, for example, Isopropanol (IPA), in a predetermined ratio, for example, 1: 1 ratio is infiltrated into the silica pellets at, for example, normal temperature. This state is maintained, for example, overnight a day to allow the first treatment liquid to spread uniformly on the surface and to penetrate into the pores of the silica beads. Thereafter, as shown in step S3, the silica pellets are separated from the mother liquor and placed in a humid atmosphere, such as the atmosphere, to promote hydrolysis of TIP, thereby preparing titanium dioxide (TiO)₂). Then, as shown in step S4, the silica pellets are calcined at, for example, 500 ℃ for 2 hours, thereby obtaining a titania/silica catalyst. This titanium dioxide is, for example, anatase crystalline. If the ratio of titania supported on silica is low, the photocatalytic activity of titania cannot be sufficiently obtained, and if it is too high, the micropores of the silica spheres are blocked, thereby decreasing the specific surface area thereof. For this reason, it is desirable that the surface of the silica pellets be uniformly covered with titanium dioxide. In this embodiment, the content of titania is preferably adjusted to, for example, 10 wt% or more, particularly 10 to 25 wt% based on the weight of the platinum-titania/silica catalyst obtained later.

Subsequently, as shown in step S5, a second treatment liquid containing a platinum compound, such as an aqueous chloroplatinic acid solution, is impregnated into the above titania/silica catalyst at, for example, a normal temperature. Thereafter, as shown in step S6, the titania/silica catalyst separated from the solution is dried under reduced pressure by using, for example, a vacuum drier to remove water until the weight of the titania/silica catalyst reaches an approximately constant weight. Thereafter, an inert gas, such as argon, is fed to the titanium dioxide/silica catalyst to heat it in an argon atmosphere. Then, as shown in step S7, when the temperature reaches the first processing temperature, such as 300 ℃ or higher, preferably 600 ℃, hydrogen gas is supplied instead of argon gas to perform a hydrogen reduction process in a hydrogen atmosphere, which lasts for, for example, 2 hours, so that chloroplatinic acid, which is a platinum compound, is reduced to, for example, platinum. Thereafter, argon is again supplied instead of hydrogen, and the temperature of the titania/silica catalyst is controlled in an argon atmosphere to adjust the temperature to a second treatment temperature, such as 300-600 deg.C, preferably 500-600 deg.C, as shown in step S8. Subsequently, as shown in step S9, air, for example, is supplied instead of argon gas, and an oxidation process is performed in the gas, which is performed for 1 hour, for example, to obtain a platinum-titania/silica catalyst. At this time, if the supported platinum ratio is too low, the catalytic activity of platinum cannot be obtained, whereas if it is too high, not only the photocatalytic activity of titanium oxide is lowered but also the cost becomes relatively high. For this reason, it is preferable that the platinum content is set, for example, at 0.04 to 0.5 wt%, particularly at 0.1 wt%, based on the weight of the platinum-titania/silica catalyst obtained later. Incidentally, the oxidation process is not limited to the use of air, and oxygen, ozone, or an oxidizing gas, for example, may be used.

Now, the surface state of the catalyst when the above-described oxidation process is carried out will be described as follows. First, when the hydrogen reduction process is performed before the oxidation process, the platinum compound is reduced to platinum as described above. In this case, platinum was dispersed on the catalyst in the form of having a raft-like crystal structure when observed with a microscope. In the present invention, this state is referred to as "extremely reduced state". Then, by performing the oxidation process at, for example, a second treatment temperature not higher than the hydrogen reduction process temperature, the junctions (joining lanes) in the raft-like crystal structure are cut off, and therefore, platinum is now highly dispersed in the form of very fine grains on the surface of the catalyst. That is, platinum is not oxidized during the oxidation process to the extent that it is converted to an oxide, but rather is released from the very strongly reduced state described above.

Referring now to fig. 2 and 3, a description will be made regarding a case of a gas purifier for purifying a gas containing volatile organic compounds, for example, a gas containing at least one of acetaldehyde, formaldehyde, paraffin, olefin, and aromatic compounds, using the platinum-titania/silica catalyst obtained by the above-described method. Fig. 2 is a sectional view in side view of the gas purifier, and fig. 3 is a transverse sectional view taken along the line P-P of fig. 2. In both figures, numeral 10 indicates a housing forming part of the body of the gas purifier. In the housing 10, a reaction tube 11, i.e. a tubular reactor, is provided, which is made of a light-permeable material at least over a part thereof and has, for example, a diameter of 10mm, a thickness of 0.5mm and a length of 200 mm. In the reaction tube 11, the photocatalyst obtained by the above-described method is packed, for example, 170mm high to form the catalyst layer 12. The upper and lower sides of the catalyst layer 12 are supported by support members 13, and each support member 13 is formed with gas flow holes each having a size not allowing the photocatalyst to pass therethrough. The light-permeable material is selected from heat-resistant glass (trade name: Pyrex glass), quartz glass, acrylic resin, polycarbonate, and the like.

In the housing 10, a plurality of (4 in this embodiment) invisible lights 14, such as ultraviolet lights, are provided so as to surround the reaction tubes 11, and such invisible lights 14 serve as light sources of light emitting means for emitting light. Such invisible light is configured to raise the temperature of the catalyst layer 12 to a temperature of a purification process, such as not higher than a lower ignition temperature limit of a coking substance (coking material) described below, by the radiant heat of the invisible light 14, and to raise the temperature to, for example, 100-. As a specific example, when the invisible light 14 of 6W each is used, the gap distance L between the reaction tube 11 and each beam of the invisible light 14 is set to 3 mm. On the upstream side of the reaction tube 11, a filter 15 is disposed for separating suspended solid matter such as dust contained in the gas to be treated, and on the downstream side of the reaction tube 11, a fan 16 is disposed as a supply and discharge means for feeding the gas to be treated to the catalyst layer 12.

When the above-described gas cleaner is used to clean gas to be treated, such as air containing acetaldehyde, the fan 16 is first activated to introduce the air into the filter 15, where suspended solid matter such as dust is separated. Then, air is fed to the catalyst layer 12. In this case, when light such as ultraviolet light from the invisible light 14 is irradiated on the photocatalyst, electrons jump over the titanium dioxide band gap to jump to its conduction band, thereby activating the photocatalyst. Here, when acetaldehyde in the air comes into contact with the surface of the activated photocatalyst, the acetaldehyde is decomposed into, for example, carbon dioxide and water, and coke substances and/or coke by-produced are attached to the surface of the catalyst. Coke species represent volatile precursors that later become coke. Therefore, the active sites of the catalyst are lowered, thereby gradually lowering the decomposition rate. However, as the radiant heat from the invisible light 14 activates the catalytic combustion reaction of the platinum, the coke material is ignited to burn and even further burn the coke located therearound. The burnt coke and/or coke is separated from the surface of the catalyst in the form of, for example, carbon dioxide gas, so that the active sites of the catalyst covered with coke and/or coke are then re-present, which contributes to the decomposition of acetaldehyde. In addition, due to the combustion heat in this case, the catalytic combustion reaction of platinum is further activated. In this way, acetaldehyde is decomposed and the air is purified. Incidentally, in addition to the above reaction, it is considered that a reaction occurs in which active oxygen is generated when the activated photocatalyst, oxygen and an aqueous phase in the air are brought into contact, and the generated active oxygen and acetaldehyde react with each other to generate carbon dioxide and water.

According to the above embodiment, in the preparation process of the catalyst, the oxidation process is performed at a predetermined temperature after the hydrogen reduction process. Therefore, platinum is released from a very strong reduction state, so that platinum is highly dispersed on the catalyst in the form of very fine grains. Thus, a large number of dissociated oxygen or hydrogen molecules (which are the metallic characteristic of platinum but which are lost due to the raft-like crystal form) are utilized, and thus high activity can be obtained. Thereby, the catalytic combustion reaction of platinum is promoted to ignite the coke substance, for example. Therefore, as will be clear also from the examples described later, the activity of the catalyst is rapidly increased by the synergistic effect between the photocatalytic activity and the thermal catalytic activity, and thereafter, a high state can be maintained. Incidentally, the second treatment temperature may be set higher than the first treatment temperature as long as the second treatment temperature is lower than the temperature at which calcination is performed in the catalyst preparation process.

Further, according to the above embodiment, since the invisible light 14 as the light source provides heat to the photocatalyst to perform the purification process at a predetermined temperature. This heat thus promotes the catalytic combustion reaction of the platinum, ensuring ignition of the coke-causing material. Thereby burning the coke-formed material and/or coke adhered to the catalyst to suppress the decrease in the activity of the catalyst. As a result, it is possible to eliminate the need for a catalyst recovery process or to greatly reduce the frequency thereof. Incidentally, if the temperature of the catalyst layer 12, i.e., the temperature of the gas purification method, is too high, the coking substance may solidify into coke, making ignition thereof difficult. For this reason, it is desirable to control the state of the coking substance so that it is a volatile precursor. Further, it is preferable to determine the temperature of the purification process according to the kind of the coking substance to be generated. In view of this, in the present embodiment, it is set to the lower limit of the ignition temperature of the coking substance as described above.

In the photocatalyst of the present invention, the metal supported on the carrier and having the property of exhibiting thermocatalytic activity is not limited to platinum, but may be selected from rhodium, ruthenium, palladium, nickel and the like. Also in this case, a very strong reduction state is released by performing an oxidation process after the hydrogen reduction process, so that the metallic characteristics of the selected metal are reproduced, and therefore, the same effects as in the above case can be obtained. In addition, in the photocatalyst of the present invention, titanium dioxide is not limited to the anatase type, but may be the rutile type or the rutile-anatase type. Further, niobium oxide or the like may be supported on the carrier instead of titanium dioxide as long as it has a property of exhibiting photocatalytic activity. Further, in the procatalyst of the present invention, the carrier is not limited to silica, but may be a porous body such as alumina, preferably gamma-alumina, diatomaceous earth, stucco and the like.

In the gas purifier of the present invention, the heating means for supplying heat to the photocatalyst may be independently equipped. However, in view of the cost of the equipment, a preferred configuration is to have both light and heat from a common light source as described above. In addition, in the gas purifier of the present invention, the light source is not limited to the invisible light 14, but may be a fluorescent lamp, an ultraviolet lamp, a mercury lamp, an ozone lamp, a germicidal lamp, or the like, as long as it emits light in the ultraviolet region. Also in this case, the same effects as in the above case can be obtained.

Further, in the gas purifier of the present invention, a plurality of reaction units each including the reaction tube 11 and the invisible light 14 may be equipped as shown in fig. 4A. In addition, a plurality of reaction tubes 11 may be provided in the housing 10. An example of which is shown in fig. 4B. As will be appreciated, the invention is not so limited. Preferably, the number and arrangement of the reaction tubes 11 and the invisible light 14 are determined according to the supply flow rate of the gas to be treated, the kind of the volatile organic compound, the kind of the light source, and the like. Further, the present invention is not limited to a structure in which light is emitted from the surface of the reaction tube 11. For example, as shown in fig. 4C, the invisible light 14 may be provided inside the reaction tube 11, so that light is emitted from the inside of the reaction tube 11.

Examples

Now, description will be made regarding embodiments performed to demonstrate the effects of the present invention.

Example 1

In this example, a platinum-titania/silica catalyst of the invention was prepared. Silica beads (CARiACT 30 manufactured by Fuji Silysia Chemical ltd.) having an average particle size of 2 to 3mm and having micropores with an average pore diameter of 30nm were used as the silica support. First, silica pellets were heated to dryness at 500 ℃ and then 100g of such silica pellets were weighed into a beaker. Subsequently, while shaking the beaker with the silica pellets therein, the first treated liquid obtained by mixing TIP and IPA together was dropped into the beaker using a dropper, and the beaker was left to stand for 24 hours in a state where the silica pellets were all immersed. Thereafter, the silica pellets were taken out and calcined at 500 ℃ for 2 hours in the atmosphere, thereby obtaining a titania/silica catalyst. Subsequently, the titania/silica catalyst was placed in a beaker, impregnated with a chloroplatinic acid solution, left to stand overnight so as to be loaded with 0.1 wt% of platinum, and then dried under reduced pressure using a vacuum drier. After the titania/silica catalyst reached approximately constant weight, it was removed from the vacuum dryer. Then, the catalyst was heated to 600 ℃ under an argon atmosphere while feeding argon. Thereafter, argon was changed to hydrogen, and the catalyst was subjected to hydrogen reduction in hydrogen at 600 ℃ (first treatment temperature), which lasted for 2 hours. Thereafter, hydrogen was changed to argon, and the catalyst was maintained under an argon atmosphere for 1 hour. Thereafter, argon was changed to air, and the catalyst was oxidized in the atmosphere at 600 ℃ (second treatment temperature), which lasted for 1 hour. Thereafter, the temperature of the catalyst was lowered in an argon atmosphere, thereby obtaining a platinum-titania/silica catalyst. This platinum-titania/silica catalyst is referred to as catalyst a.

Example 2

In this example, the same process as in example 1 was carried out except that the oxidation was carried out at 500 ℃ (second treatment temperature), thereby obtaining a platinum-titania/silica catalyst. This platinum-titania/silica catalyst is referred to as catalyst B.

Example 3

In this example, the same process as in example 1 was carried out except that the oxidation was carried out at 400 ℃ (second treatment temperature), thereby obtaining a platinum-titania/silica catalyst. This platinum-titania-silica catalyst is referred to as catalyst C.

Example 4

In this example, the same process as in example 1 was carried out except that the oxidation was carried out at 300 ℃ (second treatment temperature), thereby obtaining a platinum-titania/silica catalyst. This platinum-titania/silica catalyst is referred to as catalyst D.

Comparative example 1

In this comparative example, the same process as in example 1 was carried out except that oxidation was not carried out, thereby obtaining a platinum-titania/silica catalyst. This platinum-titania/silica catalyst is referred to as catalyst E.

The following will describe the experiments conducted to determine the decomposition properties of the catalysts A to E obtained in examples 1 to 4 and comparative example 1. Before the description thereof, the conditions of the gas purifier used in the test will be explained. As the reaction tube 11, Pyrex Glass having a diameter of 10mm, a thickness of 0.5mm and a length of 200mm was used. In the reaction tube 11, the photocatalyst loading height was 170mm to form the catalyst layer 12. 4 beams of invisible light 14 each of 6W were used as a light source, and the distance L between the reaction tube 11 and each of the beams of invisible light 14 was set to 3 mm.

Test 1

In this test, in order to prove the effect of carrying out the oxidation process after the reduction of hydrogen, the decomposition performance of the catalysts A to E obtained in examples 1 to 4 and comparative example 1 was measured. Air having a temperature of 25 ℃ and containing 3000 ppm by volume of acetaldehyde was prepared and brought into contact with the catalyst A (B-E) charged in the reaction tube 11 at a flow rate of 20ml/min, and after confirming that the adsorption equilibrium was reached, light was irradiated from the invisible light 14 onto the catalyst A (B-E) to conduct a decomposition test for 5 to 6 hours. In this case, using gas chromatography, the acetaldehyde remaining in the air at the outlet of the catalyst layer 12 was analyzed at intervals of about 10 minutes and the decomposition rate was calculated. Here, the decomposition rate indicates the decomposition rate of acetaldehyde in the reaction tube 11, which is calculated by the following expression:

(((inlet concentration (volume ppm) — outlet concentration (volume ppm))/(inlet concentration (volume ppm))) × 100.

Results and discussion regarding test 1

The results of the decomposition rates of catalysts A-E are shown in FIG. 5. As shown by the results in FIG. 5, the decomposition rates of all the catalysts A to E decreased from the start of the test up to 50 minutes. However, in catalyst a, when the time reached about 100 minutes, the decomposition rate rapidly increased and exceeded 90%. The decomposition rate of 90% or more was stably maintained even thereafter, and the average decomposition rate in a stable state was 96.5%. With respect to catalyst B, the decomposition rate began to increase rapidly at a time of about 80 minutes and reached 90% at a time of about 150 minutes. Further, as with the catalyst a, the decomposition rate was stabilized thereafter, and the average decomposition rate in the stabilized state was 92.0%. On the other hand, in the case of catalyst C, the decomposition rate started to increase proportionally when the time reached about 80 minutes, and reached 90% when the time reached about 350 minutes. In the case of catalyst D, the decomposition rate started to increase at a time of about 200 minutes, reached 30% at a time of 350 minutes, and finally exceeded 90%. In contrast, in the case of the catalyst E in which the oxidation process was not carried out, the decomposition rate did not show a sharp increase, and the decomposition rate was stable after a time of 100 minutes, with an average decomposition rate of about 15%. That is, it has been proved that the activity of the catalyst can be improved by performing the oxidation process after the hydrogen reduction process. It has also been demonstrated that the decomposition rate increases earlier as the temperature of the oxidation process (second treatment temperature) becomes higher. Further, when the catalyst A was used, the acetic acid concentration in the air after the treatment was 5 ppm by volume or less. That is, it has been demonstrated that a small amount of intermediate substance is generated.

Test 2

This test was performed to demonstrate the effect of heat from the light source. The catalyst a obtained in example 1 was used as a photocatalyst. The experiment was the same as experiment 1 except that a coolant gas was fed into the gap between the reaction tube 11 and the invisible light 14 via a feed line (not shown) at room temperature to cool the catalyst layer 12.

Test 3

This test was performed to demonstrate the effect of light from a light source. The catalyst a obtained in example 1 was used as a photocatalyst. This test was the same as test 1 except that the periphery of the reaction tube 11 was covered with aluminum foil to prevent light from being applied to the catalyst layer 12.

Results and discussion regarding test 2 and test 3

The results of the decomposition rates of test 2 and test 3 are shown in fig. 6. Figure 6 also shows the results described above in relation to catalyst a in test 1. In test 2, the average temperature of the catalyst layer 12 was 48.0 ℃, and the average decomposition rate in a steady state was 42.6%. In test 3, the average temperature of the catalyst layer 12 was 144.8 ℃, and the average decomposition rate in a steady state was 27.2%. In contrast, as described in test 1, when light and heat were supplied simultaneously, the average decomposition rate in the steady state was 96.5%, which exceeded 69.8% (-42.6% + 27.2%) obtained by simply adding up the decomposition rate when only light was supplied and the decomposition rate when only heat was supplied. That is, it has been demonstrated that their combined effect can allow the catalyst to obtain higher activity by simultaneously providing light and heat to the catalyst.

Test 4

In this test, the decomposition test as in test 1 was repeated. The catalyst a obtained in example 1 was used as a photocatalyst.

Results and discussion regarding test 4

The decomposition rate results are shown in fig. 7. As shown by the results in FIG. 7, the decomposition rate at the beginning was about 50% at the second or later times, and at the same time, the decomposition rate rapidly increased in the early time range and reached 80 to 90% at the time of 50 minutes. Thereafter, the decomposition rate becomes stable, and in the case of the second or later, the average decomposition rate in the stable state is 90% or more. That is, it has been proved that the photocatalyst of the present invention can exhibit high activity even if repeatedly used. The fact that the change in the resolution ratio at the first (fresh) time and the change in the resolution ratio at the second, third, fourth or fifth time are different from each other will now be discussed. It is considered that in the second and subsequent cases, the coking substance used in the previous case (in the second case, the coke adhered for the first time) remains on the catalyst, and therefore, when the temperature of the catalyst layer 12 is increased, the coking substance is ignited in an earlier period of time than in the case where the coking substance is ignited after being accumulated to some extent for the first time, thus avoiding the occurrence of the situation where the decomposition rate is lowered as in the first case.

As described above, according to the present invention, in the preparation process of a catalyst, an oxidation process is performed after a hydrogen reduction process so that platinum is highly dispersed on the catalyst in the form of very fine grains, and thus a highly efficient photocatalyst can be obtained. Further, by applying such a photocatalyst to a gas purifier to which light and heat can be supplied from a light source, ignition of a coking substance is promoted, thereby suppressing a decrease in the activity of the catalyst, and therefore, a volatile organic compound present in the gas can be decomposed at a high decomposition rate.

Claims (7)

1. A method for producing a photocatalyst comprising a carrier on which titanium oxide and a metal having a property showing thermocatalytic activity are supported, said method comprising:

a metal-supporting step of supporting the titania-supported carrier with the metal compound;

a reduction step of reducing the metal compound supported by the support in the metal supporting step with hydrogen in a heating atmosphere; and

an oxidation step of oxidizing the metal obtained by hydrogen reduction in the reduction step in a heated atmosphere to cut links in a crystal structure obtained by hydrogen reduction, so that the metal is highly dispersed in the form of very fine grains on the surface of the catalyst, thereby enhancing the activity of the metal.

2. The method according to claim 1, further comprising a temperature adjusting step of setting the temperature of the support at the temperature of the oxidizing step in an inert gas atmosphere after the reducing step.

3. The process according to claim 1, wherein the metal is at least one of platinum, rhodium, ruthenium and nickel.

4. A process according to claim 3, wherein the amount of platinum is from 0.04% to 0.5% by weight relative to the weight of the photocatalyst.

5. The method according to claim 1, wherein the content of titanium dioxide is 10 wt% or more with respect to the weight of the photocatalyst.

6. The process according to claim 1, wherein the support is silica spheres.

7. The method of claim 1, further comprising:

a step of infiltrating a first treatment liquid containing titanium tetraisopropoxide and isopropanol into the carrier before the metal supporting step;

a step of hydrolyzing the titanium compound permeated into the carrier to cause the carrier to carry titanium dioxide; and

a step of calcining the titania-loaded support.