Production of Hydrogen From Water Using Photocatalyst-Electrolysis Hybrid System

This invention relates to a process for the production of hydrogen gas from water.

Hydrogen is now expected to be an important energy source utilizable for batteries, fuels for engines, etc. One known method of producing hydrogen from water is catalytic decomposition of water into hydrogen and oxygen using sunlight and a photocatalyst. Since the photocatalyst used in the known method is active only in UV light, visible light of sunlight can not be utilized. A method is also known in which water is subjected to electrolysis to obtain hydrogen. In order to effectively produce electrolysis, however, it is necessary to use a higher bath voltage than the theoretical bath voltage of 1.23 V. Therefore, when a solar energy is utilized for the electrolysis, high performance solar batteries are needed. Short service life and high manufacturing and maintenance costs of such solar batteries make the conventional electrolysis method inapplicable in actual practice.

It is, therefore, an object of the present invention to provide a process which can produce hydrogen from water at reduced costs.

Another object of the present invention is to provide a process of the above-mentioned type which requires a reduced electrical energy in electrolysis of water.

It is a further object of the present invention to provide a process of the above-mentioned type which can effectively convert solar energy into hydrogen energy.

In accomplishing the above objects, there is provided in accordance with the present invention a process for the production of a hydrogen gas from water utilizing a hybrid system including a catalytic reaction using a photocatalyst and electrolysis. The process includes the steps of:

(a) irradiating an aqueous solution containing ferric ions with a light while maintaining the aqueous solution in contact with a photocatalyst to convert the ferric ions into ferrous ions; and

(b) electrolyzing the ferrous ion containing aqueous solution obtained in step (a) to yield a hydrogen gas.

The reactions involved in the above steps (a) and (b) are respectively as follows:

Thus, the overall reaction is:

The process for the production of hydrogen from water according to the present invention includes the two steps of (a) energy accumulation by catalytic reduction of Fe.sup.3+ into Fe.sup.2+ (.DELTA.G>0) and (b) utilization of the accumulated chemical energy for the production of hydrogen by electrolysis with reduced electrical energy.

Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the present invention which follows, when considered in light of the accompanying drawings, in which:

the sole FIGURE is a graph showing a relationship between the current and the electrode potential under electrolysis in Example 5 (dotted line) and Comparative Example 1.

Any photocatalyst may be used for the purpose of the present invention. The use of a semiconductor photocatalyst, especially a semiconductor photocatalyst having a band gap of not greater than 3.3 eV, is desirable for reasons of effective accumulation of energy. Most preferred photocatalysts are those which meet with all of the following criteria: (a) to have a band gap of not greater than 3.3 eV, (b) to have a conduction band potential higher than (as absolute value) the Redox level of Fe(III)/Fe(II), (c) to have a valence band potential lower than (as absolute value) the Redox level of O.sub.2 /H.sub.2 O, and (d) to be stable under the reaction conditions. Illustrative of suitable photocatalysts are WO.sub.3, TiO.sub.2, SrTiO.sub.3, Ta.sub.2 O.sub.5, In.sub.2 O.sub.5, SiC and FeTiO.sub.x where x is a number of 3-3.5.

It is preferred that a mixture of two or more different photocatalysts be used for reasons of effective and efficient utilization of various wavelengths encompassing UV and visible regions. The photocatalyst is generally used in the form of particles to ensure a large surface area. Preferably, the particle diameter of the photocatalyst ranges from 1-2,000 nm, more preferably 2-200 nm. To improve light exposing efficiency, it is also preferred that the photocatalyst be supported on carrier particles such as silica and zirconia. For the purpose of accelerating charge separation or reducing over voltage, the photocatalyst may be preferably composited with an additive such as RuO.sub.2, IrO.sub.2 or NiO in an amount of 0.01-20% by weight, preferably 0.1-5% by weight.

The kind of anions, pH and concentration of Fe.sup.3+ of the aqueous solution have an influence upon the catalytic reaction of the above energy accumulating step. These conditions are suitably determined in view of the succeeding electrolysis step which is a reversible reaction between Fe.sup.3+ and Fe.sup.2+.

The energy accumulation step may be carried out in an outdoor pool using sunlight as a light source. Of course, the energy accumulation step may be performed using an artificial light source. In any case, it is recommended to perform the energy accumulation step under conditions which ensure a high light irradiation efficiency and a high catalyst-water contact efficiency.

The aqueous solution in which at least a part of the ferric ions have been converted into ferrous ions is then subjected to electrolysis to form a hydrogen gas.

The electrolysis is preferably performed using a a cell separated by a diaphragm, preferably a cation exchange membrane, into anode and cathode chambers. The cathode at which hydrogen is generated is preferably a material having a small hydrogen over voltage, such as a carbon electrode having a small amount of Pt or Ni supported thereof. The anode at which ferrous ions are oxidized to ferric ions may be the same material as that of the cathode or may be a carbon electrode without metals.

Because the light energy has been accumulated as chemical energy (ferrous ions), the electrolysis can be carried out with a lower electrolysis bath voltage (e.g. 1 V or less) than that used in the conventional electrolysis (about 2 V). Thus, as an electric energy source, an ordinary solar battery may be used. To improve electrolysis efficiency, various measures can be adopted such as to stir the bath near the anode, to increase the bath temperature, to continuously feed the ferrous ion containing aqueous solution, to use a current collecting material, to decrease the electrode current density and to reduce the distance between the electrodes.

The following examples will further illustrate the present invention.

In a reactor equipped inside with a high pressure mercury lamp (400 V), a mixture containing 400 ml of water, 2 mmol of ferric sulfate and 1 g of WO.sub.3 was charged. The space above the mixture in the reactor was replaced with argon and then evacuated so that the pressure in the reactor was maintained at about 35 torr. The mixture was then irradiated with the lamp with stirring, while measuring the amount of oxygen generated by gas chromatography. The average oxygen producing rate MA between 2 and 4 hours from the commencement of the irradiation was 40 .mu.mol/hour, the cumulative amount MT of the oxygen produced up to 4 hours after the commencement of the irradiation was 270 .mu.mol, and the ratio R of the amount of Fe.sup.2+ to that of Fe.sup.3+ at 4 hours after the commencement of the irradiation was 1.2:1. The irradiation was continued for 20 hours. Then, the mixture in the reactor was immediately filtered and the filtrate was placed in one of two chambers separated by an ion exchange membrane. The same amount of pure water was placed in the other chamber. An amount of sulfuric acid was then added to each chamber so that the SO.sub.4.sup.2- concentration in the solution in each chamber was 0.5 mol/liter. A platinum wire was disposed in each chamber as an electrode. As a reference electrode, Ag/AgCl (NaCl) was used to measure the anode potential under electrolysis. The electrical potential between the anode and cathode was increased at a swept rate of 0.02 V/minute using a potentiostat. As a result, a current began flowing at an anode potential of 1.0 V(Ag/AgCl) with the simultaneous production of hydrogen gas.

A mixture containing 400 ml of water and 2 mmol of ferric sulfate was placed in one of two chambers separated by an ion exchange membrane. The same amount of pure water was placed in the other chamber. An amount of sulfuric acid was then added to each chamber so that the SO.sub.4.sup.2- concentration in the solution in each chamber was 0.5 mol/liter. Electrolysis was then carried out in the same manner as that in Example 1. A current began flowing at an anode potential of 1.5 V(Ag/AgCl(NaCl)) with the simultaneous production of hydrogen gas. A current-potential relationship of the electrolysis is shown by the solid line in the FIGURE.

In a reactor equipped inside with a high pressure mercury lamp (400 V), a mixture containing 400 ml of water, 2 mmol of ferric sulfate, 1 g of WO.sub.3 and an amount of sulfuric acid providing a SO.sub.4.sup.2- concentration of 0.5 mol/liter was charged. The mixture was then irradiated with the lamp in the same manner as that in Example 1. The average oxygen producing rate MA between 2 and 4 hours from the commencement of the irradiation was 40 .mu.mol/hour, the cumulative amount MT of the oxygen produced up to 4 hours after the commencement of the irradiation was 260 .mu.mol, and the ratio R of the amount of Fe.sup.2+ to that of Fe.sup.3+ at 4 hours after the commencement of the irradiation was 1.1:1. The irradiation was continued for 20 hours. Then, the mixture in the reactor was immediately filtered and the filtrate was placed in one of two chambers separated by an ion exchange membrane. The same amount of water having a SO.sub.4.sup.2- concentration of 0.5 mol/liter was placed in the other chamber. Electrolysis was then carried out in the same manner as that in Example 1. A current began flowing at an anode potential of 1.0 V(Ag/AgCl) with the simultaneous production of hydrogen gas.

Example 1 was repeated in the same manner as described above except that ferric chloride was substituted for ferric sulfate and that hydrochloric acid was substituted for sulfuric acid. The average oxygen producing rate MA between 2 and 4 hours from the commencement of the irradiation was 20 .mu.mol/hour, the cumulative amount MT of the oxygen produced up to 4 hours after the commencement of the irradiation was 250 .mu.mol, and the ratio R of the amount of Fe.sup.2+ to that of Fe.sup.3+ at 4 hours after the commencement of the irradiation was 1.0:1. In the electrolysis, a current began flowing at an anode potential of 0.9 V(Ag/AgCl) with the simultaneous production of a hydrogen gas.

A mixture containing 400 ml of water and 2 mmol of ferric chloride placed in one of two chambers separated by an ion exchange membrane. The same amount of pure water was placed in the other chamber. An amount of hydrochloric acid was then added to each chamber so that the Cl.sup.- concentration in the solution in each chamber was 0.5 mol/liter.

Electrolysis was then carried out in the same manner as that in Example 1. A current began flowing at an anode potential of 1.3 V(Ag/AgCl(NaCl)) with the simultaneous production of hydrogen gas.

Example 1 was repeated in the same manner as described above except that TiO.sub.2 (rutile) was substituted for WO.sub.3. The average oxygen producing rate MA between 2 and 4 hours from the commencement of the irradiation was 20 .mu.mol/hour, the cumulative amount MT of the oxygen produced up to 4 hours after the commencement of the irradiation was 260 .mu.mol, and the ratio R of the amount of Fe.sup.2+ to that of Fe.sup.3+ at 4 hours after the commencement of the irradiation was 1.1:1. In the electrolysis, a current began flowing at an anode potential of 1.0 V(Ag/AgCl) with the simultaneous production of a hydrogen gas.

Particles of WO.sub.3 were impregnated with an aqueous solution containing RuCl.sub.3 and then calcined at 500.degree. C. for 2 hours in air to produce four kinds of WO.sub.3 --RuO.sub.2 Composites (I)-(IV) having RuO.sub.2 contents of 0.3, 1, 3 and 5% by weight, respectively.

Example 1 was then repeated in the same manner as described except that each of WO.sub.3 --RuO.sub.2 Composites (I)-(IV) was substituted for WO.sub.3. The average oxygen producing rate MA between 2 and 4 hours from the commencement of the irradiation was 120 .mu.mol/hour in the case of Composite (I), 190 .mu.mol/hour in the case of Composite (II), 180 .mu.mol/hour in the case of Composite (III) and 120 .mu.mol/hour in the case of Composite (IV). The cumulative amount MT of the oxygen produced up to 4 hours after the commencement of the irradiation was 380 .mu.mol in the case of Composite (I), 410 .mu.mol in the case of Composite (II), 440 .mu.mol in the case of Composite (III) and 400 .mu.mol in the case of Composite (IV). The ratio R of the amount of Fe.sup.2+ to that of Fe.sup.3+ at 4 hours after the commencement of the irradiation was 3.2:1 in the case of Composite (I), 4.6:1 in the case of Composite (II), 7.3:1 in the case of Composite (III) and 4.0:1 in the case of Composite (IV). In the case of Composite (III), a current was found to begin flowing at an anode potential of 0.85 V(Ag/AgCl(NaCl)) with the simultaneous production of hydrogen gas. A current-potential relationship of the electrolysis is shown by the dotted line in the FIGURE.

Particles of WO.sub.3 were impregnated with an aqueous solution containing RuCl.sub.3 and then calcined for 2 hours in air at three different temperatures of 300.degree. C., 400.degree. C. and 600.degree. C. to produce three kinds of WO.sub.3 --RuO! Composites (V)-(VII), respectively, each having a RuO.sub.2 content of 3% by weight.

Example 5 was then repeated in the same manner as previously described using Composites (V)-(VII). The average oxygen producing rate MA between 2 and 4 hours from the commencement of the irradiation was 90 .mu.mol/hour in the case of Composite (V), 90 .mu.mol/hour in the case of Composite (VI) and 80 .mu.mol/hour in the case of Composite (VII). The cumulative amount MT of the oxygen produced up to 4 hours after the commencement of the irradiation was 340 .mu.mol in the case of Composite (V), 380 .mu.mol in the case of Composite (VI) and 320 .mu.mol in the case of Composite (VII). The ratio R of the amount of Fe.sup.2+ to that of Fe.sup.3+ at 4 hours after the commencement of the irradiation was 2.1:1 in the case of Composite (V), 3.2:1 in the case of Composite (VI) and 1.8:1 in the case of Composite (VII).

Particles of TiO.sub.2 were impregnated with an aqueous solution containing ammonium tungstate and then calcined in air at 500.degree. C. to produce a WO.sub.3 --TiO.sub.2 Composite having a WO.sub.3 content of 10% by weight.

Example 1 was then repeated in the same manner as described except that the WO.sub.3 --TiO.sub.2 Composite was substituted for WO.sub.3. The average oxygen producing rate MA between 2 and 4 hours from the commencement of the irradiation was 70 .mu.mol/hour, the cumulative amount MT of the oxygen produced up to 4 hours after the commencement of the irradiation was 400 .mu.mol, and the ratio R of the amount of Fe.sup.2+ to that of Fe.sup.3+ at 4 hours after the commencement of the irradiation was 4.0:1.

The above results are also summarized in Table below.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all the changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.