Titanium compound for photochemical reactions and method for producing the same

Abstract: A titanium compound for photochemical reactions having a high catalytic activity for the unit surface area and extremely good stability to heat treatment is made of a substitutional solid solution of titanium oxide in which aluminum atoms occupy titanium sites of the titanium oxide uniformly at a predetermined ratio. Aluminum atoms preferably occupy 0.01 to 0.5% of the titanium sites. The titanium compound for photochemical reactions is obtained from a complex alkoxide of titanium and aluminum.

Description: The present invention relates to a titanium compound for photochemical reactions and a method for producing the titanium compound. It has been known that titanium oxide generates electrons and holes which exert strong oxidation and reduction actions on the surrounding substance upon photoexcitation. These reactions are utilized for oxidation and reduction of water, decomposition of hazardous substances, sterilization, and inorganic and organic syntheses. Conventionally, it has been known that titanium oxide having a large specific surface area and good crystalline properties performs efficient photochemical reactions. Production methods for such titanium oxide are reported as follows: (1) J. Mater. Sci. Lett. 15, 197 (1996); a process comprising of dissolving titanium alkoxide in toluene and hydrolyzing it in an autoclave by supplying water from the gas phase, while growing crystals simultaneously. (2) Mater. Chem. Phys., 17 (1987) 87-101; a process comprising of intercalating titania between montmorillonite layers. (3) Japan Chemical Society, 71st Autumn Annual Meeting (1996), Preprints, p. 94, 4P1, b24; a process comprising of intercalating titania between montmorillonite layers (as in (2) given above) and subsequent hydrothermal treatment. (4) 15th "Symposium on photocatalyst chemistry", p. 29; a process for producing TiO 2 powder having the tunnel structure from K 2 Ti 4 O 9 . (5) Electrochemical Society, 1996 Autumn Meeting, Synopses, p. 234, 1112; a process for producing fine particles by utilizing the aqueous phase in reversed micelle. (6) Japanese Laid-Open Patent Publication No. 8-182934; a process comprising of doping or coating titanium oxide with a potassium compound and/or a phosphorus compound and an aluminum compound for improving the efficiency. The problems with conventional fine highly crystalline titanium oxide powder are that the titanium oxide powder tends to aggregate and that the reaction product is difficult to diffuse as the primary particles of the titanium oxide powder decrease in size. These cause the low catalytic activity per unit surface area and the thermal deterioration of the titanium oxide powder. These problems are solved if ultra-fine particles of titanium oxide are supported on a carrier without aggregation. This method, however, has the disadvantage of limiting the catalytic activity per unit volume or weight including the carrier. It has been proposed that the problem might be solved by using titanium oxide particles of irregular shape (such as needle) other than sphere. However, such titanium oxide shows good selectivity in reaction but poor catalytic activity. Accordingly, there arises the object to find a titanium compound used for photochemical reactions and having very high catalytic activity per unit surface area and extremely high stability to heat treatment. There also arises the object to find a method for producing such a titanium compound. The present invention is directed to a titanium compound and method for producing the titanium compound that satisfy these needs. The titanium compound for photochemical reactions is a substitutional solid solution of titanium oxide in which aluminum atoms occupy 0.01 to 0.5 % of the titanium sites of the titanium oxide uniformly. Aluminum atoms preferably occupy 0.15 to 0.3% of the titanium sites. It is also preferable that at least portion of the titanium compound has one of an anatase structure and a rutile structure and that the titanium compound is prepared from a complex alkoxide of titanium and aluminum. The method for producing the titanium compound for photochemical reactions comprises the step of synthesizing a complex alkoxide of titanium and aluminum. The method may further comprise the step of hydrolyzing the complex alkoxide of titanium and aluminum to yield a gel having a Ti-O-Al chemical bond and the step of firing the gel under the air. According to the invention, the titanium compound exhibits high catalytic activity per unit weight and unit surface area and excellent stability to heat treatment. In addition, the method yields the titanium compound with desirable properties in a simple manner without requiring any expensive complex apparatus. For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing comparison between the catalytic activity per unit weight (1g) of the titanium compound according to the example of the present invention and that of conventional titanium oxide. Fig. 2 is a diagram showing comparison between the catalytic activity per unit surface area (1m 2 ) of the titanium compound of according to the example of the present invention and that of conventional titanium oxide. Fig. 3 is a diagram showing the result of Rietveld analysis on the titanium compound according to the example of the present invention. Fig. 4 is a diagram showing the result of Rietveld analysis on the titanium compound according to a comparative example. Fig. 5 is a diagram showing comparison between the catalytic activity per unit weight (1 g) of the sample according to the example in Fig. 3 and that of the sample according to the comparative example in Fig. 4. Fig. 6 shows X-ray diffraction charts of the titanium compound according to the example of the present invention and those of the conventional titanium oxide. Fig. 7 is a diagram showing how the catalytic activity is related with the crystal phase constituting the titanium compound the example of the present invention. Fig. 8 is a diagram showing how the catalytic activity varies depending on the temperature of heat treatment performed on the titanium compound according to the example of the present invention and the conventional titanium oxide having a high specific surface area and produced by the sulfuric acid method. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The titanium compound for photochemical reactions according to the present invention is a substitutional solid solution of titanium oxide in which aluminum atoms occupy some of titanium sites in the titanium oxide uniformly at a predetermined ratio. As will be explained in detail below, it is preferable that 0.01 to 0.5% of the titanium sites are occupied by aluminum atoms. Further, the titanium compound preferably has an anatase and/or rutile structure. It is important in the present invention that aluminum atoms occupy some of titanium sites in the titanium oxide uniformly at a predetermined ratio. According to the Inventors' study, the method disclosed in Japanese Laid-Open Patent Publication No. 8-182934 gives neither a substitutional solid solution nor complete replacement of titanium in titanium oxide crystals by aluminum. To realize the uniform substitution of titanium with aluminum in the present invention, the titanium compound of the invention is prepared by a process which comprises a step of synthesizing a complex alkoxide (double alkoxide) of titanium and aluminum. The complex alkoxide then gives a gel having the Ti-O-Al chemical bond. Hereinafter, the preferred embodiments of the present invention are explained in more detail with reference to the drawings. Mixtures of titanium isopropoxide and aluminum isopropoxide at varied ratios were prepared. Each of the mixtures was reacted in a three-neck flask under reflux for 24 hours. It was confirmed that complex alkoxide of titanium and aluminum was produced by the fact that the solution changed in color during the reflux. The resulting complex alkoxide was reacted with 2-methoxyethanol to interchange the alkoxy groups. The solution of the complex alkoxide was then dropped into boiling water for hydrolysis. Thus there was obtained a titania sol (in the form of slurry) having the Al-O-Ti chemical bond. The sol was spray-dried and the dried product was heated in the air at 550°C for 2 hours. There were obtained several samples of titanium oxide compound crystals of substitutional solid solution in which titanium was uniformly replaced by aluminum in a prescribed amount i.e., one atom of titanium was replaced by one atom of aluminum. A 1g of each titanium oxide thus obtained was suspended in a 25 vol% aqueous ethanol solution. After a solution of chloroplatinic acid was added to the suspension, the suspension was irradiated with light from a 400-W high-pressure mercury lamp of internal irradiation type, so that the surface of titanium oxide compound was coated with platinum at the ratio of 0.5 wt%. Thereafter, the aqueous ethanol solution with the titanium oxide compound coated with platinum was irradiated with the mercury lamp so that the relation between the amount of hydrogen evolved and the duration of irradiation was examined. The results are shown in Fig. 1. Fig. 1 also shows the results obtained using a titanium oxide having a trade name "P-25" (Nippon Aerosil Co., Ltd, Japan) and titanium oxide prepared by the sulfuric acid method and having a high specific surface area. The P-25 is a standard product of titanium oxide powder as the high-efficiency photocatalyst. As can be seen from Fig. 1, the sample in which the amount of aluminum in the substitutional solid solution is 0 mol% has almost the same efficiency (in terms of the rate of hydrogen evolution) as P-25. The efficiency increases as the amount of the aluminum becomes greater and the sample in which the amount of aluminum in the substitutional solid solution is greater than 0.01 mol% shows the remarkable improvement in the efficiency. When the amount of the aluminum is 0.2 mol%, the efficiency becomes maximum and is as about twice great as that of P-25. Beyond this amount, the efficiency linearly decreases and the efficiency suddenly decreases at the amount exceeding 0.5 mol%. This result suggests that the amount of aluminum in the substitutional solid solution should preferably be in the range of 0.01 mol% to 0.5 mol%, or the ratio of aluminum atoms that occupy titanium sites should preferably be in the range of 0.01% to 0.5%. According to the further study by the inventors, the amount of aluminum is more preferably in the range of 0.15% to 3%. Table 1 below shows the specific surface area (m 2 /g) of each sample of titanium compound and titanium oxide powder listed in Fig. 1. Amount of aluminum in substitutional solid solution (mol%) Specific surface area (m 2 /g) 0 45 0.005 45 0.01 45 0.05 45 0.1 45 0.15 44 0.2 43 0.25 44 0.4 45 0.5 45 1.0 45 P-25 52 Sulfuric acid method 290 The amount of hydrogen evolution per unit surface area was calculated by dividing the amount of hydrogen evolution by the specific surface area shown in Table 1 for the sample of 0.2 mol% aluminum replacement, P-25, and titanium oxide prepared by the sulfuric acid method. The results are shown in Fig. 2. It is noted from Fig. 2 that the sample according to the present invention is by far superior in catalytic activity per unit surface area to the titanium oxide powder prepared by the sulfuric acid method and having a large specific surface area. The sample of titanium compound powder prepared by replacing 0.2 % of titanium sites on titanium oxide with aluminum and heating for 2 hours at 550°C in the air was analyzed by the Rietveld method. The results are shown in Fig. 3. The Rietveld method is suitable to determine the position of the constitutional elements in a crystal whose structure is known. The reliability of the result of analysis is indicated by R-factor. A smaller R-factor indicates a more reliable result. In Fig. 3, the horizontal axis represents the amount of aluminum in the substitutional solid solution, and the vertical axis represents the R-factor corresponding to the horizontal axis. The amount of aluminum was calculated by a computer assuming that the positions of titanium atoms and an oxygen atoms are fixed and that aluminum atoms occupies titanium sites. Fig. 3 shows that the value of R-factor is smallest when the amount of aluminum is 0.2 mol%. This result indicates that all aluminum atoms in the titanium oxide compound position at titanium sites to form a perfect substitutional solid solution of titanium oxide in which some of the titanium sites of the titanium oxide are occupied by aluminum. For the purpose of comparison, samples of titanium compound powder were prepared in the same manner as mentioned above, except that titanium alkoxide and aluminum alkoxide were simply mixed without the step for complexing. The samples were analyzed by the Rietveld method. The results are shown in Fig. 4. It is noted from Fig. 4 that the value of R-factor is smallest when the titanium sites are not replaced by aluminum at all. This result indicates that aluminum atoms doped do not form the substitutional solid solution but form another phase or an interstitial solid solution. Fig. 5 is a diagram showing comparison between the catalytic activity per unit weight (1 g) of the sample according to the example in Fig. 3 and that of the sample according to the comparative example in Fig. 4. As explained above, both of the two samples include 0.2 mol% of aluminum. The amount of hydrogen evolution measured in the same manner as mentioned above with reference to Fig. 1 is plotted against the duration of irradiation. Fig. 5 shows that the effect of the invention is not obtained even though the amount of doping is the same unless aluminum atoms surely occupy the titanium sites to form the substitutional solid solution. Fig. 6 shows X-ray diffraction charts of the titanium compounds according to the example of the present invention and those of the conventional titanium oxide. As shown in Fig. 6, in the case where the titanium compound powder is prepared from complex alkoxide and fired under an air according to the aforementioned method has the anatase structure or the rutile/anatase mixed structure. That is, at least portion of the titanium compound of according to the present invention has either the anatase structure or the rutile structure. On the other hand, in the case the complex alkoxide is subjected to hydrothermal treatment instead of firing in the air, the obtained titanium compound powder has the anatase/brookite mixed structure partly containing brookite phase. Fig. 7 is a diagram showing how the catalytic activity is related with the crystal phase constituting the titanium compound according to the example of the present invention. The titanium compound powder having the anatase structure or rutile/anatase mixed structure is superior to the titanium compound powder having the anatase/brookite mixed structure in the ability to evolve hydrogen. This result indicates that the desired crystal structure of titanium compound powder is anatase or rutile. Fig. 8 is a diagram showing how the catalytic activity degrades under the heat treatment performed on the titanium compound according to the example of the present invention and the conventional titanium oxide having a high specific surface area and produced by the sulfuric acid method. As is clear form Fig. 8, the conventional titanium oxide powder prepared by the sulfuric acid method and having a high specific surface area deteriorates by heating above 200°C, whereas the titanium compound powder according to the present invention remains unchanged in the ability to evolve hydrogen even by heating at 575°C. As explained above, the titanium compound for photochemical reactions according to the present invention is a substitutional solid solution of the titanium oxide in which aluminum atoms uniformly occupy the titanium sites in titanium oxide crystal. Therefore, it has high catalytic activity per unit weight and unit surface area and it is extremely stable to heat treatment. In addition, the method of the present invention yields the above-mentioned titanium compound with desirable properties in a simple manner without requiring any expensive complex apparatus, because the method includes the step of synthesizing a compound alkoxide of titanium and aluminum, thereby giving a gel having the Ti-O-Al chemical bond. In the process, lower alkoxides (C 1-10 ) are preferably reacted to form the complex but other alkoxides can also be used. The Ti and Al alkoxides need not be the same. Also, alkoxide exchange reaction illustrated can be omitted, if desired. While preferred embodiments of the invention have been disclosed, various modes of carrying out the principles disclosed herein are contemplated as being within the scope of the following claims. Therefore, it is understood that the scope of the invention is not to be limited except as otherwise set forth in the claims.

Claims: A titanium compound for photochemical reactions, the titanium compound being a substitutional solid solution of titanium oxide in which aluminum atoms occupy 0.01 to 0.5% of the titanium sites of the titanium oxide uniformly. The titanium compound for photochemical reactions according to claim 1, wherein aluminum atoms occupy 0.15 to 0.3% of the titanium sites. The titanium compound for photochemical reactions according to one of the claims 1 to 2, wherein at least portion of the titanium compound has one of an anatase structure and a rutile structure. The titanium compound for photochemical reactions according to one of the claims 1 to 3, wherein the titanium compound is prepared from a complex alkoxide of titanium and aluminum. A method for producing the titanium compound for photochemical reactions according to one of the claims 1 to 4, the method comprising the steps of: a) synthesizing a complex alkoxide of titanium and aluminum, b) hydrolyzing the complex alkoxide of titanium and aluminum to yield a gel having a Ti-O-Al chemical bond, c) firing the gel under the air.

Inventors: ABE YOSHIO, OKABE SHINSEI, ABE, YOSHIO, OKABE, SHINSEI
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1998
Main classification: C01G23
IPC codes: C01G23, B01J37, B01J35, B01J21