ZnS photocatalyst, preparation therefor and method for producing hydrogen by use of the same

The present invention relates to a novel photocatalyst, to preparing the photocatalyst and to a method for producing hydrogen in the presence of the photocatalyst. The novel photocatalyst of this present invention is characterized by the following formula IV: EQU Pt(x)/Zn[M(y)]S IV wherein "x" represents a percentage by weight of Pt in the photocatalyst ranging from 0.05 to 2.50; "M" is a promoter selected from the group consisting of V, Cr, Mo, Mn, Re, Ru, Os, Rh, Ir, Cu, Al and Ga; and "y" represents a mole % of M/(M+Zn) ranging from 0.01 to 20.00. The method for preparing the photocatalyst comprises the steps of dissolving Zn-containing and M-containing compounds in water in such a way that M/(M+Zn) ranges, in mole %, from 0.01 to 20.00; adding H.sub.2 S or Na.sub.2 S as a reactant in the solution with stirring to precipitate Zn[M]S; washing the precipitate with water until pH reaches 7 and vacuum-drying the precipitate in a nitrogen(stream) atmosphere; adding Pt-containing compound to this precipitate in such a way that Pt ranges, in percentage by weight, from 0.05 to 2.50; illuminating the platinum-enhanced precipitate with ultraviolet light in a nitrogen atmosphere; washing the Pt-impregnated Zn[M]S thus obtained with water until the wash water pH reaches 7; vacuum-drying and then oxidation sintering the dried precipitate at from 280 to 420.degree. C. for 1 to 3 hours, reduction sintering at the previously-sintered product at from 280 to 420 .degree.C for from 1 to 3 hours.

TECHNICAL FIELD
 The present invention relates, in general, to a novel photocatalyst and
 more particularly, to a photoreaction in which hydrogen is efficiently and
 economically produced from water in the presence of the photocatalyst. The
 present invention is also concerned with a method for preparing the
 photocatalyst and a method for producing hydrogen.
 BACKGROUND ART
 Hydrogen is a very important material in the chemical industry. For
 example, it is used to produce ammonia and methanol. Also, it is an
 essential material for hydrogenation in which unsaturated compounds are
 converted into saturated ones and also for hydrotreating processes,
 including hydrogen addition, desulfurization, denitrogenation and
 demetallization. Another example for the use of hydrogen is contact
 hydrogenation of carbon dioxide in which carbon dioxide, which causes
 global waning, is reclaimed, immobilized and reused. In addition, hydrogen
 is viewed as a pollution-free, clear energy source substituting for
 existing fossil fuels.
 Conventional techniques for obtaining hydrogen include extraction from
 fossil fuels, such as naphtha, modification of natural gas, reaction of
 vapor with iron at a high temperature, reaction of water with alkaline
 metal, electrolysis of water, etc.
 However, these techniques are economically unfavorable because immense heat
 or electric energy is required. Regarding modification of fossil fuels,
 the conventional techniques have another disadvantage of generating a
 large quantity of by-products, such as carbon dioxide. In case of
 electrolysis, problems, such as electrode lifetime and generation of
 by-products, need to be solved to purify hydrogen more easily. Thus, the
 cost of facilities for hydrogen production is economically unfavorable due
 to the noted problems.
 Hydrogen gas readily escapes from the earth's gravity because it is of low
 specific gravity. Most of X exists in water or in inorganic forms. For
 these reasons, only a small quantity of hydrogen exists in the atmosphere.
 It is also very difficult and economically unfavorable to purify hydrogen
 existing in inorganic forms. The development of techniques to obtain high
 purity hydrogen efficiently from water is very important and urgently
 needed to exploit substitute energy sources.
 Recently, hydrogen producing techniques have been developed in which a
 photocatalyst is used to decompose water into hydrogen and oxygen.
 However, little has been published in prior art relating to photocatalysts
 for producing hydrogen. Representative examples are: Japanese Pat.
 Laid-Open Publication Nos. Sho 62-191045 and Sho 63-107815.
 Japanese Pat. Laid-Open Publication No. Sho 62-191045 relates to generating
 hydrogen from an aqueous Na.sub.2 S solution in the presence of a
 rare-earth element compound by a photolysis reaction The rarer element
 compound has an advantage of exhibiting optical catalytic activity in the
 range of visible light.
 Japanese Pat. Laid-Open Publication No. Sho 63-107815 concerns a photolysis
 reaction in which a composite oxide of niobium and alkaline earth metal is
 used as a photocatalyst to generate hydrogen from a methanol solution in
 water. This photocatalyst likewise has an advantage of being active in the
 range of visible light.
 However, both of these prior art methods are disadvantageous because the
 amount of hydrogen generated by them is as little as 10 ml/0.5 g hr.
 Korean Pat. Appl'n. No. 95-7721, No. 95-30416, and No. 96-44214 solve the
 above problems to some degree by suggesting a photocatalyst represented by
 the following formula I:
EQU Cs(a)/K.sub.4 Nb.sub.6 O.sub.7 I
 This technique has little effect on the environment and generates hydrogen
 at room temperature. However, the oxygen-containing organic compounds,
 which act as hydrogen-generating promoters, make it impossible to reuse
 required reactants.
 Korean Pat. Appl'n No.95-30416 suggests a photocatalyst represented by the
 following formula II
EQU Cs(a)H(c)/S(b) II
 This technique has little affect on the environment and generates hydrogen
 without an oxygen-containing organic compound as a hydrogen-generating
 promoter at room temperature, but encounters a problem with the lifetime
 and the stability of the photocatalyst.
 For example, when an alkali metal, such as cesium is impregnated into a
 photocarrier, the amount of generated hydrogen is increased outstandingly
 but the stability of the catalyst is decreased.
 In addition, Korean Pat. Appl'n No. 96544214 suggests a photocatalyst
 represented by the following formula III
EQU Pt(a)/Zn[M(b)]S III
 wherein "a" represents % by weight of Pt in the photocatalyst, ranging from
 0.1 to 3.5; "M" represents a promoter selected from a group consisting of
 Co, Fe, Ni and P; and "b" represents mole % of M.
 Similarly, this technique also has little affect on the environment. This
 compound shows not only the optical activity of photocatalyst in some
 degree but also the preparation is relatively simple and the stability of
 photocatalyst is superior. The lifetime of said compound is longer which
 depends on electron donors and reducing agents and the amount of generated
 hydrogen is larger than that of prior arts.
 When doping with Pt instead of Cs the stability of the catalyst is improved
 but the choice for a promoter is less, and the amount of generated
 hydrogen is too little. In addition, there are some problems in the
 preparation of said photocatalyst. It needs sintering and rewashing twice
 followed by etching with an acid after primary sintering.
 DISCLOSURE OF THE INVENTION
 Therefore, it is an object of the present invention to overcome the
 previously-noted problems encountered in prior art, and to provide a novel
 photocatalyst for producing hydrogen, which shows optical activity in the
 range of visible light (adjusted by a light filter) and also efficiently
 produces a large quantity of hydrogen.
 It is another object of the present invention to provide a photocatalyst
 which has a semi-permanent lifetime.
 It is a further object of the present invention to provide a simpler method
 for a photocatalyst to produce hydrogen.
 In accordance with an aspect of the present invention, there is provided a
 photocatalyst represented by the following formula IV:
 Pt(x)/Zn[M(y)]S IV
 wherein "x" represents % by weight of Pt, ranging from 0.05 to 2.50; "M" is
 a metal element selected from the group consisting of V, Cr, Mo, MA, Re,
 Ru, Os, Rh, Ir, Cu, Al, and Ga; and "y" is mole % of MN(M+Zn) in the range
 of 0.01 to 20.00.
 Another aspect of the present invention is a method for preparing a
 photocatalyst comprising the steps of: dissolving a Zn-containing and an
 M-containing compound in water in such a way that the mol percent of M
 ranges from 0.01 to 20; adding sufficient H.sub.2 S and/or Na.sub.2 S as a
 reactant to the solution (with stirring) to precipitate Zn[M]S; washing
 the resulting precipitate with water until the pH of the wash water
 reaches 7 and then drying the precipitate; adding a liquid Pt-containing
 compound to this resulting precipitate to obtain a precipitate with from
 0.05 to 2.50% by weight Pt; doping the Pt to Zn[M]S, e.g., by irradiation
 with UV light in a nitrogen atmosphere; washing the Pt-doped precipitate
 with wash water until the pH of the wash water reaches 7, and drying it;
 oxidation sintering in air at from 280 to 420.degree. C. for from 1 to 3
 hours; and reduction sintering (e.g., in a 95/5 nitrogen/hydrogen
 atmosphere) at from 280 to 420.degree. C. for 1 to 3 hours.
 In accordance with a farther aspect of the present invention, hydrogen is
 produced by a method in which UV or visible light (adjusted by a light
 filter) is irradiated onto a suspension of the photocatalyst in water to
 which Na.sub.2 S (as an electron donor) and NaH.sub.2 PO.sub.2 (as a
 reduction agent) have been added.
 BEST MODE FOR CARRYING OUT THE INVENTION
 As a result of research to solve the previously-noted problems, it was
 found that each of V, Cr, Mo, Mn, Re, Ru, Os, Rh, Ir, Cu, Al, and Ga, as
 well as Fe, Co, Ni, and P (Korean Pat. Appl'n No. 95-30416), can be an
 effective M ingredient as a promoter in the photocatalyst of the present
 invention.
 It was found that Pt, as an electron acceptor, works well in the range of
 from 0.05 to 2.50% by weight. Below 0.05% by weight, the amount of
 generated hydrogen is decreased, and the stability of the photocatalyst is
 also decreased. On the other hand, over 2.50% by weight, the amount of
 generated hydrogen is decreased, and the cost of hydrogen production is
 increased.
 The proper content of M in said photocatalyst is from 0.01 to 20.00 mole %
 . In case of less than 0.01 mole % of M, the function of photocatalyst is
 lost, and in case of over 20.00 mole % of M, the amount of generated
 hydrogen is decreased.
 The appropriate molar ratio of Zn/S is from 1:0.1 to 1:2.8, and more
 desirably from 1:0.6 to 1:1.4. Within said molar ratio, the effectiveness
 of the photocatalyst is improved.
 The reason why sintering at oxidation and reduction states and drying after
 pH reaches 7 is to keep Pt, an electron acceptor, of said photocatalyst in
 pure state. As reportedly, Pt of H.sub.2 PtCl.sub.6 is bonded to S of ZnS
 to form PtS by irradiating UV for the reaction and is transferred to the
 Wurzite structure at a temperature over 300.degree. C. at oxidation and
 reduction states for hours and, at the same time, Pt, an electron
 acceptor, is transferred to Pt(0) by sintering at over 300.degree. C. for
 from 1 to 2 hours.
 Examples of Zn-containing compounds are ZnSO.sub.4.7H.sub.2 O and
 Zn(NO.sub.3).sub.2.6H.sub.2 O; other examples of M-containing compounds
 are VCl.sub.3, VOSO.sub.4, VOCl.sub.3, K.sub.2 Cr.sub.2 O.sub.7,
 Cr(NO.sub.3).sub.3, MnF.sub.3, ReCl.sub.3, MoCl.sub.5, FeCl.sub.3,
 Fe(NO.sub.3).sub.3, RuCl.sub.3, Co(NO.sub.3).sub.2, CoCl.sub.2,
 Co(CH.sub.3 COO).sub.2, RhCl.sub.3, Co(NO.sub.3).sub.2, CoCl.sub.2,
 Co(CH.sub.3 COO).sub.2, RhCl.sub.3, Rh(NO.sub.3).sub.3, IrCl.sub.3,
 Ni(NO.sub.3).sub.2, NiCl.sub.2, Pd(NO.sub.3).sub.2, CuCl.sub.2,
 Cu(NO.sub.3).sub.2, CuSO.sub.4, Al(NO.sub.3).sub.3, AlCl.sub.3,
 Ga(NO.sub.3).sub.2, and H.sub.3 PO.sub.2.
 Korean Pat. Appl'n No. 9644214 presents a procedure comprising washing acid
 after etching with an acid, followed by primary sintering, but in this
 present invention, only vacuuming in a nitrogen atmosphere is only needed
 without primary sintering, etching with an acid and washing the acid.
 It is preferable to impregnate Zn[M]S with Pt, comprising the steps of:
 dissolving hydrogen hexachloroplatinate (H.sub.2 PtCl.sub.6) in water,
 adding the resulting solution to the Zn[M]S, and then illuminating the
 obtained product with UV light; washing the Pt-impregnated Zn[M]S until
 the pH of the wash water reaches 7, and subjecting the Pt-impregnated
 precipitate to vacuum-drying in a nitrogen atmosphere and at from 100 to
 120.degree. C. for from 1.5 to 2.5 hours before oxidation sintering at a
 temperature from 280 to 420.degree. C. for from 1 to 3 hours, and
 reduction sintering at a temperature from 280 to 420.degree. C. for from 1
 to 3 hours.
 Actually, the sintering is preferably performed at a temperature from 320
 to 400.degree. C., beyond this temperature range the lifetime and activity
 of the obtained photocatalyst are decreased.
 According to the present invention hydrogen is produced by dissolving from
 0.15 to 0.40 mols per liter of Na.sub.2 S (as an electron donor) and from
 0.20 to 0.5 mols per liter of NaH.sub.2 PO.sub.2 (as a reducing agent) in
 primary and/or secondary distilled water or in the previously treated
 water, and adding the photocatalyst of the present invention thereto.
 Then, the thus-obtained suspension is irradiated with UV or visible light
 (adjusted by a light filter) with string at a temperature of from 5 to
 85.degree. C. under from 0.1 up to 5 atm. to produce hydrogen in a high
 yield.
 When the concentration of the electron donor and of the reducing agent is
 less than the indicated lower limit, the amount of generated hydrogen is
 decreased. On the other hand, when the concentration of the electron donor
 and of the reducing agent is over the upper limit, the amount of generated
 hydrogen does not increase. The appropriate reaction conditions are at a
 temperature of from 10 to 60.degree. C. from in vacuo to under 2
 atmospheres.
 Preparation Example I
 To 250 ml of water add a 1 molar amount of ZnSO.sub.4.7H.sub.2 O, a 0.005
 molar amount of Al(NO.sub.3).sub.2.9H.sub.2 O, and sufficient H.sub.2 S
 (with stirring) to obtain a precipitate of Zn[Al]S. Wash the precipitate
 with wash water until the resulting wash water has a pH of 7. Then vacuum
 dry the thus-washed precipitate at a temperature of 110.degree. C. in a
 nitrogen atmosphere for 2 hours.
 Add hydrogen hexachloroplatinate (H.sub.2 PtCl.sub.6) to the dried
 precipitate Zn[Al]S to impart 0.8 weight % of Pt to the precipitate.
 Illuminate the platinum-enhanced precipitate with UV light (450 W high
 pressure mercury lamp with light source 4 cm from sample) for 0.5 hour to
 obtain Pt/Zn[Al]S. Wash the Pt/Zn[Al]S with wash water until the pH of the
 wash water is 7. Then dry the washed Pt/Zn[Al]S precipitate in a nitrogen
 atmosphere at 110.degree. C. for 2 hours. Subject the thus-washed and
 dried precipitate to oxidation sintering in air at 370.degree. C. for 1.5
 hours to obtain a final Pt(0.8 wt. %)/Zn[Al]S photocatalyst.
 Preparation Example II
 Repeat Preparation Example I with 0.01 molar (instead of 0.005 molar)
 Al(NO.sub.3).sub.2.9H.sub.2 O to obtain a final Pt(0.8 wt. %)/Zn[Al]S
 photocatalyst.
 Preparation Example III
 Repeat Preparation Example I with 0.05 molar (instead of 0.005 molar)
 Al(NO.sub.3).sub.2.9H.sub.2 O to obtain a final Pt(0.8 wt. %)/Zn[Al]S
 photo catalyst.
 Preparation Example IV
 Repeat Preparation Example I with 0.05 molar H.sub.3 PO.sub.2 {instead of
 Al(NO.sub.3).sub.2.9H.sub.2 O)} to obtain a final Pt(0.8 wt. %)/Zn[P]S
 photocatalyst.
 Preparation Example V
 Repeat Preparation Example I with 0.005 molar H.sub.3 PO.sub.2 {instead of
 Al(NO.sub.3).sub.2.9H.sub.2 O)} to obtain a final Pt(0.8 wt. %)/Zn}P]S
 photocatalyst.
 Preparation Example VI
 Repeat Preparation Example IV with sufficient hydrogen hexachloroplatinate
 to impart 0.4% by weight (instead of 0.8% by weight) of Pt to the
 precipitate to obtain a final Pt(0.4 wt. %)/Zn[P]S photocatalyst.
 Preparation Example VII
 Repeat Preparation Example IV with sufficient hydrogen hexachloroplatinate
 to impart 2.5% by weight (instead of 0.8% by weight) of Pt to the
 precipitate to obtain a final Pt(2.5 wt. %)/Zn[P]S photocatalyst.
 Preparation Examples VIII TO XXIV
 Repeat Preparation Example I varying the M-containing compound to obtain
 each of the Pt(0.8 wt. %)/Zn[M]S photocatalyst compositions of Table I.

EXAMPLES I TO XXIV
 Suspend 0.5 g of each photocatalyst, respectively, obtained in Preparation
 Examples I to XXIV in 500 ml of water which contains 0.24 molar Na.sub.2 S
 and 0.35 molar NaH.sub.2 PO.sub.2 (with stirring at a speed of 400 rpm) in
 a photo reactor of a closed gas circulation system.
 Illuminate the resulting aqueous suspension with visible light {500 W Xe
 lamp with an optical filter (which passes light having a wavelength over
 400 nm) with the light source 4 cm from the sample} at room temperature
 under one atmosphere. The amount of generated hydrogen thus produced (as
 analyzed by gas chromatography) is set forth in Table I.
 EXAMPLE XXV
 Following the procedure of Examples I with a photocatalyst obtained from
 Preparation Example IV, except for using UV (450 W high pressure mercury
 lamp with light source 4 cm from sample) instead of visible light, results
 are provided in Table I.
 Comparative Example I
 Repeat the procedure of Preparation Example XIX, but without the
 oxidation/reduction sintering steps, to obtain the photocatalyst.
 Following the method of Examples I to XXIV with that photocatalyst
 generates the amount of hydrogen indicated in Table I.
 Comparative Example II
 Following the method disclosed in Korean Pat. Appl'n No. 96-44214 with the
 same composition as that of Preparation Example X to obtain dried
 precipitate, treat the precipitate further as follows: primary sintering,
 etching with nitric acid and secondary sintering to obtain a
 photocatalyst. Using that photocatalyst in the method of Examples I to
 XXIV yields the amount of generated hydrogen reflected in Table I.
 TABLE I
 Exam. No. Catalyst Amount of Gas (ml/hr)
 I Pt(0.8 wt. %)/Zn[Al(0.5)]S 1329
 II Pt(0.8 wt. %)/Zn[Al(0.99)]S 1410
 III Pt(0.8 wt. %)/Zn[Al(4.76)]S 875
 IV Pt(0.8 wt. %)/Zn[P(4.76)]S 1529
 V Pt(0.8 wt. %)/Zn[P(0.5)]S 1293
 VI Pt(0.4 wt. %)/Zn[P(4.76)]S 1070
 VII Pt(2.5 wt. %)/Zn[(4.76)]S 966
 VIII Pt(0.8 wt. %)/Zn[P(9.09)]S 1370
 IX Pt(0.8 wt. %)/Zn[Cu(0.5)]S 914
 X Pt(0.8 wt. %)/Zn[Cu(0.99)]S 614
 XI Pt(0.8 wt. %)/Zn[Cr(0.2)]S 1358
 XII Pt(0.8 wt. %)/Zn[Cr(0.5)]S 914
 XIII Pt(0.8 wt. %)/Zn[Cr(4.76)]S 491
 XIV Pt(0.8 wt. %)/Zn[V(0.5)]S 1085
 XV Pt(0.8 wt. %)/Zn[Mo(0.5)]S 606
 XVI Pt(0.8 wt. %)/Zn[Mn(0.5)]S 1251
 XVII Pt(0.8 wt. %)/Zn[Re(0.5)]S 1044
 XVIII Pt(0.8 wt. %)/Zn[Fe(0.5)]S 1005
 XIX Pt(0.8 wt. %)/Zn[Ru(0.5)]S 1149
 C.I Pt(0.8 wt. %)/Zn[Ru(0.5)]S 314*
 XX Pt(0.8 wt. %)/Zn[Co(0.5)]S 725
 XXI Pt(0.8 wt. %)/Zn[Rh(0.5)]S 1227
 XXII Pt(0.8 wt. %)/Zn[Ir(0.5)]S 1058
 XXIII Pt(0.8 wt. %)/Zn[Ni(0.5)]S 966
 XXIV Pt(0.8 wt. %)/Zn[Ga(0.5)]S 954
 XXV Pt(0.8 wt. %)/Zn[P(4.76)]S 3240**
 C.II Pt(0.8 wt. %)/Zn[Cu(0.99)]S 420***
 *Without oxidation/reduction sintering steps.
 **The UV light was used for illumination.
 ***Etching with an acid after primary sintering.
 Industrial Applicability
 As apparent from the data in Table I, the photocatalyst for the hydrogen
 production in accordance with the present invention can be used with
 various kinds of promoters, and the amount of generated hydrogen with this
 novel catalyst is greater than that of conventional techniques.
 Furthermore, the photocatalyst has a longer lifetime and the method of
 photocatalyst preparation is much simpler than conventional procedures.
 The present invention for novel photocatalysts, by introducing various
 kinds of doping metal elements and various catalyst's applications and its
 adding techniques, not only overcomes previous restricted activity of
 photocatalysts to light sources, but also simplifies preparing
 photocatalysts which are superior in life expectancy as well as hydrogen
 production yield.
 The invention and its advantages are readily understood from the foregoing
 description. It is apparent that various changes may be made in the
 processes and compositions without departing from the spirit and scope of
 the invention or sacrificing its material advantages. The processes and
 compositions hereinbefore described are merely illustrative of preferred
 embodiments of the invention.