AU5994580A - Process for producing hydrogen from water using light energy and aqueous compositions - Google Patents

Description

PROCESS FOR PRODUCING HYDROGEN FROM WATER USING LIGHT ENERGY AND AQUEOUS COMPOSITIONS

BACKGROUND OF THE INVENTION

The present invention provides a process for produc¬ ing hydrogen gas from aqueous compositions using light as the energy source and useful compositions.

Hydrogen gas is used in industrial processes. But its market potential would be much larger for use as a fuel and also as a chemical raw material if it were avail¬ able in large quantities at an economical cost. At the present time, processes for obtaining hydrogen from hydro¬ carbons and/or producer gas or by the electrolysis of water are generally not economical. Electrolytic pro¬ cesses for separating water into hydrogen and oxygen using electricity as the energy source have been and continue to be investigated. To a lesser extent, the use of light energy to separate hydrogen from water has been investi¬ gated. Much of this work has been summarized in the "Current State of Knowledge of Photochemical Formation of Fuel", Report of Workshop Held at Bost University's Osgood Hill Conference Center, North Andover, Mass. , on September 23 and 24, 1974, sponsored by the National Science Foundation, Research Applied to National Needs Under Grant No. SE/AER/72-03597 to Boston University, NSF-RA-N-75-094, and in Gabriel Stein's review entitled "Chemical Storage of Solar Energy and Photochemical Fuel Formation", Israel Journal of Chemistry, Vol. 14, 1975, pp. 213-225 (25th IUPAC Congress, Jerusalem, July, 1975) , which includes a large discussion of the prior work in this field. The state of the art was reviewed in Mark S. Wrighton's Special Report entitled "Photochemistry" in the September 3, 1979 issue of Chemical & Engineering News. With the exception of the work described therein with europium, the reactions utilized ultraviolet light. U.S. Patents 4,045,315 and 4,105,516 disclose processes using europium salts and other salts, and U.S. Patent 4,169,030 discloses the use of a complex ion containing rhodium to produce hydrogen by the solar photolysis of water. The contents of (1) said NSF-RA-N-75-094 report, (2) said Stein and Wrighton's reviews, and (3) said patents are hereby incorporated by this reference. None of these processes has been of large-scale commercial utility because of process inefficiencies and/or uneco¬ nomical high cost.

Stein demonstrated that the yield of hydrogen gas increases when europium salts are photolyzed in water/ alcohol mixtures as solvent. It is postulated that part of the hydrogen originates from the water and part from the alcohol as -depicted in the following equations using methanol-as an example:

H⁺ + Eu⁺a⁺q. ■•hv_v) Eu⁺a⁺q⁺. + OH^" + H- (1)

2H. H₂ (2)

H. + CH₃OH > -CH₂OH + H₂ (3)

2 -CH₂OH HO-CH₂-CH₂-OH (4)

The alcohol is used as a sacrificial material and the

++ Eu , the starting cation, is not regenerated. The pro¬ duction of hydrogen is limited because the hydroxyl ions are not removed.

It is an object of the present invention to provide economical processes for producing hydrogen from water and compositions used therein. Hydrogen is produced form an aqueous composition, preferably a solution, comprising a photoexcitable reagent which absorbs strongly in the solar range at ground level and is excited to a higher state of oxida¬ tion and donates an electron which ultimately produces hydrogen gas from water. This photoexcitable electron donor reagent is hereinafter referred to as the "photo¬ excitable ion (D) " . The aqueous composition also con¬ tains a solvating agent, preferably an alcohol. In order to avoid buildup of reaction limiting materials in the cell, it also contains a chain reaction agent. The aqueous composition also preferably contains at least one spectral sensitizer which strongly promotes the reaction.

The aqueous composition is preferably a liquid composition which is most readily processed in ion- exchange resins as disclosed hereinafter. It is prefer¬ ably in the form of a solution, a colloid, a slurry, or a dispersion in water, or even in the form of an aqueous gel. Solids, preferably finely divided, may be present.

The excitable ion (D) may be a* cationic photo¬ excitable reagent which, when excited by absorption of light in the visible range at ground level, will donate an electron to the nearby solvated water. Suitable cationic agents include Fe ++, Cr++, Ni++, Co++, Ti +++,

+++ ++ + ++ ++ ++ Ce , V , Cu , Ru , Rh and Eu . Generally these cations are broadly included within the transition metal elements. Of the foregoing cations, the first four listed are preferred. Cobalt and copper are preferred when chloride ions are present; with iron and nickel being preferred when sulfate ions are present in the aqueous composition, largely based on ease of processing by ion-exchange to remove hydroxyl ions. Ions having higher oxidation potentials than those disclosed hereinbefore are not included in the afore- identified listing of excitable ions (D) because they might oxidize the resin of the ion exchange resins used in a preferred embodiment of the process discussed hereinafter. However, higher oxidizing excitable ions, such as Co , might be used if the ion exchange resin utilized is highly resistant to attack, for example, the sulfonated perfluorohydrocarbon ion exchange resins such as DuPont's Nafion, or in processes in which ion exchange resins are_. not required and/or used.

The valence state of the cations specified as the respective excitable ion (.D) are apparently the speci¬ fied valence state at which the ion acts to donate an electron described hereinbefore based upon theoretical considerations. Because of the extreme activity of the contents of the cells during operation, it cannot be stated with certainty as to the exact valence state of an ion when hydrogen is being produced. In certain cases the ion may be added to the cell in the form of a compound in which the ion exists at a different valence state than that specified hereinbefore. Thus, Example 2 illustrates -the production of hydrogen from a cell in which ruthenium trichloride or rhodium trichloride is added as the excitable ion (D) . It is assumed that the iiss rreedduucceedd ttoo tthhee dd±i~valent state under the conditions existing in the cell.

Stein's article in the Israel Journal of Chemistry identified hereinbefore postualtes that the hydrogen cell may produce hydrogen in accordance with the postulated reactions depicted in the following equations.

OMPI +++

Fe ++ aq hv (Fe⁺⁺) ^•aq- >Fe aq + OH^" aq + H* (5)

Fe⁺⁺ aq + H" ->(FeH)⁺⁺ aq- (6)

+++

(FeH) ++ aq + H -^ e aq + H. (7)

^♦indicates photoexcited state

Other possible (or alternate) reaction mechanisms are also disclosed therein. The foregoing equations do not illustrate the function of the spectral sensitizer(s) which is preferably also present in the cell.

The chain reaction agent forms free radicals and functions to (1) promote the formation of hydrogen, (2) reduce the oxidized ion (D) , (3) prevent the forma¬ tion of hydrogen from the solvating agent, and (4) protects the spectral sensitizers from becoming reduced. They form chain reaction promoting free radicals as a result of reaction with hydrogen radicals and regenerate the photoexcitable ion. They are postulated to function as depicted in the following reactions, using a mercap- tan as the illustrative chain reaction forming agent:

H* + RSH H₂ 4- RS- (8)

2RS- > • RSSR (9)

RSSR - hv ■ 2RS • (10)

H- + RSSR- — RSH + RS< (11)

++■+• 2RS- + M - - M⁺⁺ + RSSR (12)

M ,+++ +++ is for example, Fe

I found that the useful chain reaction agents include the sulfur-containing chain transfer agents used in polymerization reactions to regulate.the molecu¬ lar weight of polymers. Such agents are described in William Austin Pryon, Mechanics of Sulfur Reactions , McGraw-Hill, New York, NY (1962) , particularly pages 50-90; Sulfur in Organic and Inorganic Chemistry, Alexander Senning, Vol. I, Vol. II and Vol. Ill, Marcel Dekkar, New York, N.Y. ; and Radicals, Nonhebel, Teddar and Walton, Cambridge University Press (1979) which are incorporated by reference.

The organic chain reaction agents have the struc¬ ture RZnH or RZmR wherein n is at least one and m is at least 2, and preferably not larger than about 8-10, e.g. polysulfides; more preferably, n is 1 and is 2, i.e., RZH and RZZR. Z is boron and also the atoms in the third and higher periods (rows) of the Periodic Table which can accommodate more than eight outer valence electrons by using its d orbitals. Z is prefer¬ ably boron or an atom of the III, IV, V or VI Groups of the Periodic Table and of the third, fourth or fifth rows thereof and is more preferably a member of Group IVA, VA and VIA. The particularly preferred Z atoms are boron, sulfur, phosphorus, selenium, tellurium, and - tin. The R group is an organic_^ group as described here¬ inafter, which in conjunction with the metal atom forms a moiety which can form a free radical, e.g., when in the form of RZZR by splitting under' the influence of light, and/or when in the form of RZH forms free radi¬ cals (RZ') as the result of reaction with hydrogen radicals and two RZ- radicals can form RZZR.

The organic chain reaction agents include those having, e.g., the formula RSH, and RSSR. Organic R groups include aliphatic (including cycloaliphatic) or aromatic or heterocyclic group, preferably containing up to 16 carbon atoms, although the number of carbon

*_* atoms can be 20 or more so long as the agent has at least limited solubility in the aqueous solution of the cell. The cyclic rings may include the atom Z in the ring. When R is an aromatic group, it is preferably a monocyclic hydrocarbon group such as benzyl. It is pre¬ ferred that the chain reaction agent should be soluble in the aqueous composition. Those in which R is such an aliphatic group containing between C~ and C,_fi are more preferably between C-, and C,₄ and which are liquids at ambient pressure are preferred. The use of an agent which is a gas would require that the cell be under some pressure. The chain reaction agents having the formula RSH, R(SH)₂, and RSSH are generally preferred with those having the formula RSH present being the more highly preferred because they are cheaper. As illustrated, it is postulated that the RZ H, e.g. RSH, compounds and the RZ R, e.g. RSSR, compounds are both present at different stages and either may be the starting agent. It is preferred that when using organic agents, the agent in the aqueous composition form RZ R, e.g. RSSR, which undergoes spontaneous decomposition in the pres¬ ence of light to form the RZ • radicals, e.g. RS* radi¬ cals. When the agent is in the RZ H form, it forms free RZ • radicals when it reacts with hydrogen radicals.

The preferred organics are also those having low molecular weight, e.g., up to about 200 when Z is B, P, S, and higher, e.g. up to about 800 with higher molecular weight Z atoms. The presently specifically preferred organic chain reaction agents are those illustrated in the examples, and the following: l-oxo-4,5-dithiacycloheptane; 6,8-thioctic acid; benzothiazolyl disulfide; tetramethylthiuram disulfide Cand the corresponding ethyl) ; p-ethoxyphenyl disulfide; o-, m- and p-tolyl disulfide; 2,6-dimethylphenyl di- sulfide; ethyl thioglycolate; thiophenol; benzyl er- captan; ethane dithiol; and trimethylene disulfide. The inorganic chain reaction agents have struc¬ tures which in their essence (as they relate to their function in the present invention) are analogous to the structures of the organic chain reaction agents. The inorganics have the general structures (Y-X—-H)~ and (Y-X_m-Y) The minus symbol may be a single minus value or a multiple minus value dependent upon the valence of the X atom. The above-noted structures are anions which may be associated with any cation which does not adversely effect the function of the aqueous composition to produce hydrogen. Potassium, sodium and ammonium are preferred. X is sulfur, selenium, tellurium or phosphous. Y is oxygen, sulfur, selenium or tellurium. When there are multiple Y groups which is the usual instance, some of the Y groups may be replaced by hydrogen or an organic group. These inorganics either lose a hydrogen atom or split under the reaction conditions to form free radicals as illus¬ trated in the following equations.

O ~ ^" 0 il II

0= S— H + H- " H. II o=s- o o

It is believed that the above-depicted radicals reduce the oxidized metal excitable cation to its reduced valence state, and also may couple in a manner analogous to that depicted hereinbefore with respect to RS- radicals. The preferred inorganic chain reaction agents.are the sodium, potassium, and ammonium

« salts of the following anions: hydrogen sulfite, thio- sulfite, thiosulfate, dithionite, dithionate, hydrogen selenite, and hydrogen tellurites. Those which are

OM odorless and water soluble are particularly preferred for use as the sole chain reaction agent or as one of a number of chain reaction agents used in combination.

The borides which have at least one hydrogen atom bonded to boron exhibit the highest activity or effec¬ tiveness as the chain reaction agent in that when a boride is the only chain reaction agent in an aqueous composition of the present invention, the production of hydrogen is larger than with the same aqueous compo¬ sition containing one of the other chain reaction agents. The borides are compounds having the structure (A₂B) -H- wherein A is a metal atom and particularly the transition metals and noble metals, such as cobalt, nickel, copper, iron, titanium, vanadium, zirconium, manganese, tin, platinum, rhodium, ruthenium, palladium, osmium, and iridium have proven effective as the metal atom A, with cobalt, nickel, iron and tin being pre¬ ferred based upon yield and cost considerations.

The borides are presently produced by reaction of the metal (A) halide in the anhydrous form with dry sodium borohydride by mixing them in the desired stoichiometric amount. The borides have also been formed in situ by adding sodium borohydride to an aqueous composition of the present invention which contains a salt of one of the metals (A) , and hydrogen was subsequently produced by exposing the aqueous compo¬ sition to light.

It is believed that the boride functions as a chain reaction agent by losing one of its hydrogen atoms in the aqueous composition when the reaction occurs, probably by reaction with a hydrogen radical, to form a radical as depicted in the following equation

^{A2^B)5^H3 ^{+ H* (A}2^B)5^H2^{* + H}2 The boride radical depicted above functions to reduce the photoexcitable cation from its oxidized state to its reduced state and may also couple, in the manner described hereinbefore the for RS* radicals. It is preferred to use a boride chain reaction agent, preferably together with one of the other chain reaction agents, in particular, a mercaptan and/or disulfide.

The solvents are materials which solvate the photo^¬ excitable reagent in the aqueous solution of the cell. They preferably also absorb energy in the visible and near infrared, i.e., from 4000A to 9000A, portion of the solar light spectrum which is the energy source for the production of hydrogen. Solar light is that por¬ tion of the spectrum of radiant energy having wave¬ lengths from 3000A to 12,000A.

The alcohols, including polyols, which are at least partially water miscible and preferably completely miscible in water, and which also solvate the photo¬ excitable reagent are the preferred solvents. These alcohols include the mono-, di- and tri- hydroxyl alcohols; the aliphatic mono- alcohols containing up to 20 carbon atoms are generally preferred. The pre¬ ferred alcohols are those containing from about 1 to 6 carbon atoms. Those alcohols which are resistant to oxidation are particularly preferred, for example the tertiary alcohols. The presently preferred alcohols are illustrated in the examples. The polyols, such as polyethylene glycol and polypropylene glycol which meet the aforenoted criteria, may also be used.

The solvent may importantly function to form a solvated electron which has been disclosed to have an absorption peak at from 6000A to 8000A. The chemistry of solvated electrons is disclosed in Leon M. Dorfman's, Chapter 4, and Max S. Matheson, Chapter 5, of "The Solvated Electron" , edited by Edwin J. Hart, Advances In Chemistry Series, American Chemical Society (1965) , which is hereby incorporated by this. reference.

«

The spectral sensitizers increase the production of gas from the cells. It is postulated that they absorb radiant energy and transfer it to the photoexcitable reagent thereby promoting the production of hydrogen. The spectral sensitizers are generally the same mate¬ rials which have been disclosed to be spectral sensi¬ tizers in the photographic process. Spectral sensi¬ tizers in the photographic process and dyes useful as spectral sensitizers are disclosed in the following which are incorporated by 'this reference.

Charles .Edward Mees, "The Theory Of The Photo¬ graphic Process", Edited by T.R. Jones, Third Edition (1966) , Maxmillian, New York, N.Y.

John R. Thirtle, CHEMTECH, January 1979, pages 25-35.

"The Cyanine Dyes and Related Compounds" , by Frances M. Ha er, Interscience Publishers, a division of John Wiley & Sons, New York, N.Y. (1964) .

"The Chemistry of Synthetic Dyes" by K. Venkataraman, Academic Press, New York, N.Y. , Vol. I, Vol. II, Vol. Ill, Vol. IV, Vol. V, Vol. VI, Vol. VII (1952 to 1974).

"The Chemistry of Synthetic Dyes and Pigments" , Edited by H.A. Lubs, American Chemical Society, Monograph Series, Robert E. Krieger Publishing Company, Huntington, N.Y.

Fieser & Fieser's Organic Chemistry (1944), pages 827-889, Reinhold, New York, N.Y.

F.C. Pennington, H.H. Strain, W.A. Snec, and J.J. Katz, Chlorophyll article, J. A er- Chem. Soc, Vol. 86, No. 7, pages 1418-1426 (1964).

The spectral sensitizers are preferably selected in conjunction with the photoexcitable reagents to be used in the system, and preferably have absorption characteristics matched to those of the photoexcitable reagents. In many -instances it is desirable to provide a combination of spectral sensitizers for best results. For use with the presently preferred system in which Co⁺⁺ or Fe⁺⁺ is the photoexcitable ion (D) , the presently preferred sensitizers are phenol sulpho- phthalein, methylene blue, 3,3'-diethyl-thiodicarbo- cyanine iodide, 3,3'-diethylthiotricarbocyanine iodide, and chlorophyll a. Other preferred sensitizers are disclosed in the examples and include the illustrative combinations of sensitizers. The spectral sensitizers are preferably present in and at least partially sol¬ uble in the aqueous compositions. They may, however, be immobilized, e.g. on silica gel in a two phase system in the aqueous composition, or applied outside of the light transparent upper surface of the cell, e.g. as a coating or film, or as a membrane or liquid layer sandwiched between two panes of glass.

Generally the preferred spectral sensitizers are selected from the following classes of materials: cyanines; phthalocyanines; triphenylmethanes; methylene* blue; thionine; thiazenes; porphyrin dyes, e.g. chloro¬ phyll; phenolphthaleins; phenolsulfophthaleins; vat dyes, e.g. indigo, indanthrene; and azo dyes, e.g. aniline blue.

The spectral sensitizers are effective even when used in very small concentrations. Successful results have been obtained with cells containing. 0.000001 g per ml of cell solution. The maximum practical amount is determined by the solubility of the spectral sensi- tizer in the aqueous composition and the economics since they are relatively expensive materials. They are presently preferably used in amounts between about 0.0001 and 0.001 g per ml of aqueous composition.

The aqueous compositions preferably contain above about 0.01% and more preferably between about 1% and 20% by weight based on the total weight of the aqueous compo¬ sition of the photoexcitable ion reagent. Although higher concentrations of the photoexcitable ion would be operative in the production of gas, they present difficulties in the recycling system wherein they are removed in the ion exchange operation discussed here¬ inafter.

The solvent content of the aqueous composition may vary widely, for example when methanol is a solvent, between 0.01% and 90% by volume of the total liquid composition. A similar range of other solvents which are totally miscible can be used. It is preferred that the solvents should not be present in excess of its solubility in the system. It is preferred that with solvents that are miscible in the range of 5-50% of the total liquid, the solvent concentration should be within this range. For methanol, the preferred ratio of alcohol to water is 1:1 to 1:2 by volume.

When halide ions are present in the aqueous solu¬ tion, for example tap water or brine, the solvent is preferably a mixture of methanol and t-butylalcohol.

The chain reaction agents are preferably present in the aqueous composition in amounts between about 0.1% and 30% by weight and preferably in an amount between about 1 and 20%. The chain reaction agents are prefer¬ ably present in a molar ratio to excitable ion (chain reaction agent:excitable ion) of between about 1:30 and 10:1 with the range of about 1:15 and 1:1 being broadly preferred, and the range of 1:10 and 1:2 being more preferred, with the range of between 1:8 and 1:4 being most preferred.

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O The pH of the hydrogen cell, which will operate when alkaline so long as it is not sufficiently alkaline to precipitate all of the ions, preferably is operated at a pH below 7. Broadly the pH range of about 1-7 is preferred with operation between about 2 and 6.5 being ^" particularly preferred.

The processes of the present invention also pro¬ vide heat in that the hydrogen cell functions as solar receptors of heat. Additionally the production of the gas results in concomitant heating indicating exothermic reactions. This physical heat may be utilized by use of heat exhcange apparatus in the system. If the system is constructed with ^"pressure resistant equipment, the cells may be operated at temperatures above the boiling point of the solvent-aqueous system permitting operation at substantially elevated temperatures and the production of substantial amounts of heat energy which may be utilized.

The surface of the hydrogen cell through which light is applied to the aqueous composition is prefer¬ ably a glass that passes the maximum amounts of radiant energy. This is usually a glass which transmits the broadest range of wavelengths. Suitable materials include Pyre glass (borosilicate glass) , fused silica, sapphire (A1₂0₃) , Vycor (high silica glass) and clear plastic which transmits light including partial or sub¬ stantial transmission of the ultraviolet portion of the radiation. The remaining interior surfaces of the cells, i.e. those through which the light is not applied by transmission, are preferably lined with, o -r if transparent, backed with a reflective coating or film which reflects the light back into the cell.

" uREAf During operation of the hydrogen cell, the hydroxyl ions which are formed concomitant with the hydrogen should be removed and additional hydrogen ions are fed to the cell to permit long service operation. This can be accomplished by removing a portion of the aqueous composition from the cell and adding an excess of chloride ions based on the cation reagent concentra¬ tion. The positively charged cations are converted to negatively charged anion complexes as follows using Co⁺⁺ as an example of the excitable ion. CoCl₂ + excess Cl" » (CoCl_χ) negatively charged

Sodium chloride is the preferred source of chloride ions. The aqueous composition containing the anion complex is passed through an anion exchange resin such as Amberlite IRA 400 (Cl form) sold by Rohm & Haas. The anion complex is selectively adsorbed on the - resin. The effluent is enriched with hydroxyl ions which are produced as a coproduct in the hydrogen cell. A mixture of distilled water and alcohol is then passed through the columns to regenerate_# the ion exchange resin to its OH form. The effluent which contains Co and which is enriched with hydrogen ions is fed to the hydrogen cell. Sodium chloride solution is passed through the ion exchange column (OH form) to convert it to. its Cl form for reuse.

Ion exchange membranes which may be utilized are disclosed in Diffusion and Membranes Technology, pages 200-206, Reinhold (.1962); and Encyclopedia of Polymer Science and Technology, Vol. 8, pages 620-638, Wiley (1968) , which are incorporated by this reference. Based on presently available technology, it is more con¬ venient to use anion exchange resins for removing reagent ions in their anion form and to produce effluents

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_ OMPI enriched with hydrogen ions to achieve pH control in the hydrogen cell.

Those cations which form anion complexes can be separated using anion exchange resins. For example, this includes Co which forms (CoCl- when the chloride ion concentration is above 6 M (molar) and Fe which forms Fe(Cl ) when the chloride ion con- centration is above 10 M. In aqueous sulfate solu- tions, Cr ++ and Ni++ can also be used because they form anion sulfate complexes when a large molar excess of sulfate ions is present.

A cation exchange resins (H form) can also be used to remove positively charged cations from the hydrogen cell. But acidic solutions must be used to regenerate the cation exchange resins (H form) . Anion exchange resins are preferred because when in the anion complex form they can be regenerated using water.

Because of the variety of different components which may be included in the aqueous compositions as disclosed herein, a wide variety of- separating (includ- ign ion exchange) techniques will be applicable, with different and specific techniques preferred for specific aqueous compositions.

The following examples illustrate various aspects of the invention and/or materials used therein. All parts and percentages are by weight unless volumetric units are indicated. EXAMPLES

In Examples 1-7, the light-activated photochemical reactions were carried out in a 250 ml. Pyrex Erlenmeyer flask (7.1 square inches at the bottom) which was equipped with a water-cooled reflux condenser and ther¬ mometer. A U-type exit tube attached to the top of the reflux condenser was inserted under water into a graduated buret filled to the top with water to collect gas formed in the Erlenmeyer flask. A photolamp (General Electric sunlamp, 250 watts, 110-125 volts; or a General Electric ENH Quartzline lamp, C-C-8 fila¬ ment having a color temperature of 3,250°K) was used. The energy distribution of the Quartzline lamp approxi-

9 mates the radiation distribution of sunlight. The lamp was positioned 3 inches below the bottom of said flask to provide approximately 30 suns of delivered radiation. The external sides and upper part of the Erlenmeyer flask were covered with a wrinkled aluminum reflector which, in turn, was covered with a sheet of urethane foam insulating material.

In the following examples, water, the active ion, solvent(s) , chain reaction agent(s) -and/or spectral εensitizer(s) were added to the Erlenmeyer flask and dissolved by gentle swirling. No de-aeration of the aqueous solution was carried out. Usually, after a brief induction period of 5-15 seconds after the photo¬ lamp was turned on, gas evolution started. The rate of gas evolution and the total volume of gas collected were recorded as the flask was shaken gently in static experiments for a total reaction time of 15 minutes. The temperature was controlled by the reflux condenser. In similar experiments without the condenser, the temp¬ erature increased from the starting 25°C. to about 77-85°C. with the solvent (methanol) distilling off. The static experiments were largely used to screen various materials used as reagents and to determine relative performance of different reagent combinations and conditions. The amount of hydrogen produced in Examples 1-7 was determined mass spectrometrically.

EXAMPLE 1

This example illustrated the use of different solvents and solvent combinations. Each hydrogen cell contained 0.2 g of Fe(NH₄) ₂(S0₄) ₂* 6H₂0, 10 ml. distilled water, 0.05 g 1-dodecane thiol, and 0.0015 g methylene blue. The results are set forth below

Solvent Ml. Total Volume Hydrogen (cc) methanol 5 4.2 methanol 0.5

3 8

1-propanol 4.5 J methanol 0.5? tert. butyl alcohol 4.5 j 4.0

A combination of methanol (0.5 ml) and ethylene glycol (4.5 ml) was used in an early experiment with the same aqueous composition to produce hydrogen.

EXAMPLE 2

Various cations were tested as the photo active ion (D) . 0.2 of the cation reagent was dissolved in an aqueous composition containing 10 ml distilled water, 10 ml methanol, 0.05 g 1-dodecane thiol, and 0.0015 g methylene blue. The results are set forth

Cation Reagent Total Volume Hydrogen (cc)

RuCl₃ 4.6

Cu₂Cl₂ 4.4

CoCl₂ 4.0

Fe( H₄)₂(S0₄)₂-6H₂0 4.2

CrCl₂ 3.8

NiCl₂ 3.8

EU(S0₄)₂ 3.2

RhCl₃ 3.2 vso_Λ 3.2

EXAMPLE 3

This example illustrates the use of various spectral sensitizers when used alone or in combina¬ tion. Each hydrogen cell contained- 0.2 g Fe(NH₄)₂(S0₄)₂'6H₂0, 10 ml distilled water, 5 ml methanol, 0.05 g 1-dodecane thiol, and a total of 0.0015 g of the specified one spectral sensitizer or combination of spectral sensitizers listed in the following table.

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OM

, $ ~ j Code Total Volume No. Spectral Sensitizer Hydrogen (cc)

1 chrysodine y 2.8

2 phenosulfonphthalein 4.2

3 crystal violet 4.0

4 indigo 4.0

5 aniline blue 3.0

6 methylene blue 4.2

7 phthalocyanine 2.4

8 copper phthalocyanine 3.0

9 manganese phthalocyanine 3.0

10 3,3'-diethylthiadicarbo- 2.1 cyanine iodide

11 cryptocyanine 3.7

12 3,3'-diethylthiatricarbo- 4.1 cyanine iodide

13 chlorophyll a * 4.2

14 equal parts by weight of Nos. 4.4

6, 10, 12- and 13 (total weight: 0.0015 g)

15 equal parts by weight of Nos. 4.2 2, 6, 10, 12 and 13 (total weight: 0.0015 g)

*ρrepared using procedure disclosed in Ann. N.Y. Acad. Sci. , 84, 617 (1960)

EXAMPLE 4

This example compares the results using 0.05 g of various chain reaction forming agents in aqueous compositions containing^' 0.2 g Fe(NH₄) ₂ (S0₄) ₂* 6H₂0, 10 ml distilled water, 5 ml methanol, and 0.0015 g methylene blue. The amount of hydrogen produced is reported below

Chain Reaction Forming Agent Total Hydrogen (cc) none 1.8 sodium hydrogen sulfite 4.1 sodium dithionate 4.2 diphenyl disulfide 4.0 di-tert. butyl disulfide 4.2

1-dodecane thiol 4.2 n-butyl mercaptan 4.4

The following chain reaction agents were used in earlier experiments in the same aqueous compo¬ sition to produce hydrogen. Di-tert.-dodecyldisul- fide, tert.-dodecylmercaptan, tert.-butylmercaptan, bis-(dimethyl thiocarbamoyl) disulfide, di-thioctic acid, thiophenol, dimethyl diselenide, diphenyl diselenide, diphenyl ditelluride, hexa-n-butylditin, trimethyl tin hydride, sodium thiosulfate, sodium dihydrogen phosphite, and sodium hydrogen selenite.

EXAMPLE 5

This example illustrates the operation of the process in sunlight using a concentrating solar collector manufactured by Jacobs-Del Solar Systems, Inc., Oxnard, Calif. This equipment consists of a silver-mirrored glass concentrating trough 8 feet long with a 2-foot aperture. When mounted east-west and equipped with sun tracker, it delivers 10 to 12 suns to its central pipe, a pyrex glass tube 8 feet long.

The aqueous composition consisted of 3.34 g CoCl₂-6H₂0, 0.84 g Na₂S₂0_g, and 0.025 g of the spectral sensitizer Code No. 15 dissolved in a mixture of 83 ml of methanol and 167 ml of distilled water. This compo¬ sition was pumped into the pipe reactor at the rate of 10 to 15 ml per minute so as to maintain a surface area of aqueous composition of 64.5 square inches inside the reactor pipe. During the photochemical reaction, the direct beam sunlight was 80 to 80 milli¬ watts per square centimeter. After a 22 minute running time, 66.5 cc of hydrogen was produced. The hydrogen was confirmed by gas chromatography using Linde molecular sieve 5A as absorbent.

EXAMPLE 6

This example illustrates how the process can be operated continuously. It demonstrates the processes for removing hydroxyl ions by ion exchange and illus¬ trates how hydrogen ions are fed back into the hydrogen cell using water as the source of hydrogen ions.

200 g of sodium chloride was added to 721 ml of spent solutions obtained from these trials described in Example 5. The combined solution (pH of 6.2) after photolysis was passed through an ion exhcange column having a bed volume of 2.0 inches diameter and 60 inches high of IR Amberite 400 (Cl form) . The solu¬ tion was passed through at the rate of 10 ml per minute. The effluent (pH of 9.2) was colorless, thus Co was absent. 620 ml of solvent (two-thirds dis¬ tilled water and one-third methanol by volume) was passed through the column. The effluent was pink indicating the presence of Co and had a pH of 6.0. This effluent was used as feedstock for the produc¬ tion of hydrogen using the Jacobs-Del parabolic collector described in Example 5. The ionexchange column now (OH form) was converted to the Cl form by passing through it 400 ml of a 10% solution of sodium chloride dissolved in the solvent (two-thirds distilled water and one-third methanol) .

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OM EXAMPLE 7

This Example compares the results using various borides as the chain reaction agent in aqueous compo¬ sitions containing 10 ml. distilled water, 5 ml. methanol, 0.1 g sodium dithionate, 0.0015 g methylene blue, and 0.2 g CoCl₂ as the excitable ion. The borides were prepared by reacting a dry mix of 0.1 g NaBH₄ with 0.2 g anhydrous metal halide listed in the following table. A dry mixture of the sodium borohydride and the anhydrous metal chloride is slowly added to the aqueous composition. lυt x vυxuωc - — 1

Anhydrous Metal Halide (cc)-15 minutes

CoCl₂ 161

NiCl₂ 170

Cu₂Cl₂ 149

SnCl₂ 155

FeCl₂ 151 τicι₃ 141

CrCl₂ 133

ZrOCl₂ 119

H₂PtCl₆ 151

RhIII trichloride 170

RuIII trichloride 198

The above-noted results were obtained with the aqueous compositions which also contained the 0.1 g sodium dithionate which also acts as a chain reaction agent. The same experiment was repeated except that the sodium dithionate was omitted from the aqueous composition, using each of the first three of the above-noted metal halides, i.e. CoCl₂, NiCl₂,_# and Cu₂Cl₂, and in each case substantial hydrogen was pro¬ duced in an amount of about 80-85% of that reported in the preceding table. Example 5 establishes that sunlight is a practical and useful light source for carrying out the methods of the present invention. In particular instances, the aqueous compositions used in the hydrogen cell may be utilized by applying ultraviolet light instead of visible light. Since ultraviolet light is not trans¬ mitted by Pyrex glass, the cells would require having at least one ultraviolet light transmissive surface, such as specialized glass, e.g. borosiiicate Vycor glass, or an acrylic plastic which does not contain a UV stabilizer, or direct application of the ultra¬ violet radiation to the aqueous solution.

During long-time continuous operation of cells and when utilizing impure water sources, there may be an undesirable buildup of anions in the cells (even when using cation photoexcitable reagents) , eventually resulting in the formation of salts in sufficient concentration to exceed their solubility. Since aqueous solutions which may be colloidal are preferred, these excess-salts should be removed, for example, by filtration and/or centrifuging, in an operation positioned prior to-the ion exchange operation. The highly concentrated salt solutions obtained from such separation can be used in the salt splitting operation of strongly basic anionic exchange resins to regenerate them.

The aqueous compositions used in the hydrogen cell may be used as a source of hydrogen atoms. It is possible to utilize the reactive hydrogen atoms as reactants for a variety of reduction (or hydrogena- tion) reactions. Various changes and modifications may be made and features described in connection with any one of the. embodiments may be used with any of the others within the scope of the inventive concept, and in particular includes chain reaction agents which function in the manner described herein but may not fall within the definition of the formulae of the three specific types disclosed herein.

iϊU EΛ ^

Claims (8)

I CLAIM:

1. An aqueous composition wherein hydrogen is formed when light is applied thereto comprising (1) at least one photoexcitable cation which strongly absorbs light energy at ground level causing it to enter its excited state and donate an electron whereby the cation is oxidized, (2) at least one solvent which solvates said photoexcitable cation; and at least one reagent (3) or reagent (4) , said reagent (3) being at least one chain reaction agent which forms a free radical in said aqueous liquid composition when exposed to light or when it reacts with a hydrogen radical; and said reagent (4) being at least one spectral sensitizer.

2. The composition of Claim 1 which contains at least one of said chain reaction agents (3) and at least one of said spectral sensitizers (4) .

3. The composition of Claims 1 or 2 wherein said photoexcitable ion is at least one ion selected from the group consisting of Fe , Cr , Ni , Co⁺⁺, m Ti- +⁺⁺ , C r,e +++ , τ Vτ++ , C r,u + , * R^■*,u ++ , Rh and Eu - .

4. The process of any one of Claims 1, 2 or 3. wherein said chain reaction agent is a boride having the formula .(A₂B) ₅H-, wherein A is a transition metal or a noble metal and particularly cobalt, nickel, copper, iron, titanium, vanadium, zirconium, manganese, tin, platinum, rhodium, ruthenium, palladium, osmium or iridium.

5. The process of any one of Claims 1, 2 or 3 wherein said chain transfer agent is an anion having the formula (Y-Xn-H) wherein X is at least one element selected from the group consisting of sulfur, selenium, tellurium and phosphorus, and Y is at least one element selected from the group consisting of oxygen, sulfur, selenium and tellurium.

6. The process of any one of Claims 1, 2 or 3 wherein said chain reaction agent is selected from the group consisting of those having the formula

RZnH and RZmR wherein Z is an atom in the third or higher periods of the Periodic Table which can. accommodate more than eight outer valence electrons by using its d orbitals, R is an organic group, n is an integer which is at least 1, and m is an integer which is at least 2.

-βUREAtT

OMPI

7. The process for producing hydrogen from water comprising applying light to the aqueous composition of any of Claims 1 through 6 to evolve hydrogen and form hydroxyl ions in said aqueous composition; and applying means to prevent the hydroxyl ion concentra¬ tion from increasing to a level sufficient to substan¬ tially inhibit the evolution of said hydrogen.

8. The process of Claim 7 wherein said means to prevent the hydroxyl ion concentration from increasing comprises removing at least a portion of said aqueous composition from which hydrogen has been evolved and passing said portion of aqueous composition in contact with an ion exchange resin to absorb cations thereon, and to pass the hydroxyl ions, and then recovering said cations and returning them to said aqeuous composition together with additional water.

ITυR

OM