Thermochemical production of hydrogen
BACKGROUND OF THE INVENTION 
     The invention described herein was made in the course of, or under, Energy Research and Development Administration Contract No. W-7405-ENG-48 with University of California. 
    
    
     This invention relates to a thermochemical reaction cycle for the generation of hydrogen from water, more particularly, to a thermochemical reaction cycle utilizing, as an intermediate, a metalloidal element selected from the arsenic-antimony-bismuth and the selenium-tellurium subgroups of the periodic system. 
     The potential use of hydrogen as a non-polluting energy source is well known. However, for hydrogen ever to achieve large scale use, there must be large scale production. The main problem is finding thermodynamically efficient methods for producing cheap hydrogen. The most obvious method, electrolysis of water, is relatively inefficient when one considers that the method is primarily dependent upon and limited by the efficiency of generating the electrical energy for the electrolysis. Direct thermal cracking of gaseous water using nuclear heat has been virtually discarded for thermodynamic reasons. No appreciable yields are possible under 2500°C, and present conventional reactors are not able to provide such temperatures during normal operation. 
     Multistep processes for the production of hydrogen from water have been suggested. If suitable reagents for a sequence can be found, it is possible to minimize the work requirement and chemically produce hydrogen and oxygen from water without electrolysis. The difficulty is finding a suitable set of reagents. 
     One reaction sequence, based on calcium, bromine and mercury compounds, has recently been proposed for the production of hydrogen and oxygen from water at temperatures under 730°C. (See &#34;Hydrogen Fuel Use Calls for New Source&#34;, Chemical and Engineering News, July 3, 1972, pp. 16-18.) The process consumes none of the materials required for the sequence; the net reaction is simply decomposition of water. However, one of the major problems associated with this reaction sequence is the formation of such compounds as HBr, the extremely corrosive effects of which at high temperatures and pressures are well known. 
     Another reaction sequence which utilizes, as an intermediate, a metalloidal element from Group V or Group VI of the periodic system and the compounds thereof is described in the copending application of Oscar H. Krikorian, Ser. No. 481,263, filed June 20, 1974, for &#34;Thermochemical Production of Hydrogen&#34; now U.S. Pat. No. 3,928,549 issued Dec. 23, 1975. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a reaction cycle for the thermochemical production of hydrogen from water based on the use of a metalloidal element and compounds thereof as intermediates. 
     Certain elements of the periodic system, particularly from Groups V and VI, possess the characteristics of both metals and non-metals. Such elements, termed semi-metals or metalloids, are described in &#34;The Metalloids&#34;, by Eugene G. Rochow, D. C. Heath and Co., 1966. (A list of those elements which can be considered to be metalloids is given on page 8 of the cited reference.) The terms &#34;metalloid&#34; and &#34;metalloidal element&#34; are used herein to refer to such elements. The present invention utilizes the metalloidal elements from Group V and Group VI, namely the elements of the arsenic-antimony-bismuth subgroup of Group V and of the selenium-tellurium subgroup of Group VI. 
     The present cycle is based on the reaction of a metal oxide with a metalloid and sulfur dioxide to produce the corresponding metal sulfate and an intermetallic compound of the corresponding metal and metalloid. It has been found that the presence of sulfur dioxide in the reaction of the metal oxide with the metalloid enables the reaction to proceed at a lower temperature with a more thermodynamically favorable outcome. The intermetallic product is then hydrolyzed with sulfuric acid to produce the corresponding metal sulfate and a hydrogen-metalloid compound which can readily be thermally decomposed to yield hydrogen and regenerate the metalloid. The product metal sulfate is then thermally decomposed to regenerate sulfur dioxide and the metal oxide and to produce oxygen and sulfur trioxide. The sulfur trioxide is then reacted with water to regenerate sulfuric acid. Thus, the net reaction is the decomposition of water into hydrogen and oxygen. 
     It is, therefore, among the objects of this invention to provide a thermochemical reaction cycle for the generation of hydrogen from water. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Broadly, the process of the present invention can be represented by the following sequence of reactions wherein M represents a suitable metal and Z represents a metalloidal element as hereinbefore described: 
     
         2MO + Z + SO.sub.2 → MZ + MSO.sub.4                 ( 1) 
    
     
         mz + h.sub.2 so.sub.4 → mso.sub.4 + h.sub.2 z       (2) 
    
     
         2mso.sub.4 →  2mo + so.sub.2 + so.sub.3 +  1/20.sub.2 ( 3) 
    
     
         h.sub.2 z → z + h.sub.2                             ( 4) 
    
     
         h.sub.2 o + so.sub.3 → h.sub.2 so.sub.4             ( 5) 
    
     the net reaction of the cycle is the decomposition of water into hydrogen and oxygen. 
     The preferred metalloid is selected from the selenium-tellurium subgroup of Group VI, most preferably selenium. The most suitable metal oxides are those which yield metal sulfates which decompose below 1000 K and which yields metal selenides which decompose in dilute sulfuric acid. The preferred metal oxides are the oxides of nickel, cobalt, iron, manganese, cadmium and zinc. 
    
    
     The process of the present invention is illustrated by the following sequence of reactions in which zinc oxide is the representative metal oxide and selenium is the representative metalloid. 
     EXAMPLE I 
     
         ______________________________________                                    
                      ΔH°.sub.298 (kJ/mol)                   
______________________________________                                    
2ZnO + Se + SO.sub.2 → ZnSe + ZnSO.sub.4                           
                        -149     (6)                                      
 (˜773 K and 4 atm SO.sub.2)                                        
ZnSe + H.sub.2 SO.sub.4 (aq) → ZnSO.sub.4 + H.sub.2 Se             
                        +102     (7)                                      
 (1M H.sub.2 SO.sub.4, &lt; 373 K)                                           
2ZnSO.sub.4 → 2ZnO + SO.sub.2 + SO.sub.3 + 1/20.sub.2              
                        +569     (8)                                      
 (˜ 1000 K)                                                         
H.sub.2 Se → Se + H.sub.2                                          
                        -30      (9)                                      
 (˜473 K)                                                           
H.sub.2 O + SO.sub.3 → H.sub.2 SO.sub.4 (aq)                       
                        -206     (10)                                     
H.sub.2 O → H.sub.2 + 1/20.sub.2                                   
                        +286                                              
______________________________________                                    
 
    
     The reaction products obtained in the above cycle are readily separable on the basis of solubility, volatility, or diffusion rates. ZnSe is substantially insoluble in water whereas ZnSO 4  is water soluble; thus, ZnSe is easily separated from ZnSO 4 . Hydrogen can be separated from selenium on the basis of differences in volatility, hydrogen being considerably more volatile than selenium at reaction temperature. SO 2 , SO 3  and O 2  can be separated by gas chromatographic techniques familiar to those skilled in the art. 
     The heat efficiency for the above cycle, estimated as the ratio of water decomposition enthalpy (286 kJ/mol) to the enthalpy of the high temperature endothermic step, the zinc sulfate decomposition (569 kJ/mol), is 50%. 
     The following side reaction may occur at low temperatures, particularly at temperatures below about K: K: 
     
         4znO + 4SO.sub.2 → ZnS + 3ZnSO.sub.4                (11) 
    
     in order to avoid the above reaction, it is preferred to conduct reaction (6) at a temperature of about 773 K or above. If reaction (11) does occur to some extent, the products can be reintroduced in the cycle in the following manner: 
     
         ZnS + H.sub.2 SO.sub.4 → ZnSO.sub.4 + H.sub.2 S     (12) 
    
     
         h.sub.2 s + 3/2  o.sub.2 → h.sub.2 o + so.sub.2     (13) 
    
     alternatively, ZnS can be treated with O 2  to yield ZnO and SO 2 . Thus, a competing side reaction can feasibly be integrated into the overall cycle. 
     Although ZnSe is shown only as an intermediate in the cycle, the compound is useful in its own right as a photoconductor. Thus, reaction (6) can afford an inexpensive and convenient method for preparing ZnSe. 
     EXAMPLE II 
     The following reaction sequence illustrates the use of cadmium oxide as the representative metal oxide. 
     The reaction of CdO with Se and SO 2  proceeds analogously to reaction (6) above: 
     
         2CdO + Se + SO.sub.2 → CdSe + CdSO.sub.4            (14) 
    
     cdSe hydrolyzes in 12M HCl, but not in H 2  SO 4 . Therefore, the hydrolysis of CdSe to form CdSO 4  is carried out by way of an intermediate hydrolysis step with HCl as follows: 
     
         CdSe + 2H.sup.+ + 3Cl.sup.- → CdCl.sub.3.sup..sup.- 1 + H.sub.2 Se (15) 
    
     The CdCl 3   -   1  solution from reaction (15) is subsequently treated with H 2  SO 4  to regenerate the HCl and produce CdSO 4  : 
     
         cdCl.sub.3.sup..sup.- 1 + H.sub.2 SO.sub.4 →  2H.sup.+ + 3Cl.sup.- + CdSO.sub.4                                                (16) 
    
     the remainder of the cycle is analogous to reactions (8), (9), and (10) above: 
     
         2CdSO.sub.4 →  2CdO + SO.sub.2 + SO.sub.3 +  1/20.sub.2 (17) 
    
     
         h.sub.2 se → Se + H.sub.2                           (18) 
    
     
         h.sub.2 o + so.sub.3 → h.sub.2 so.sub.4             (19) 
    
     the net reaction is the decomposition of water into hydrogen and oxygen. The intermediate, cadmium selenide, is also useful as a photoconductor. 
     A metal oxide-SO 2  reduction can also be applied to a methane-methanol cycle for the generation of hydrogen from water as described in the copending application of Robert M. Dreyfuss and Robert G. Hickman for &#34;Methane-Methanol Cycle for the Thermochemical Production of Hydrogen&#34;, Ser. No. 590,748 filed concurrently herewith. This cycle can be represented by the following sequence of reactions wherein M is a suitable metal: 
     
         CH.sub.4 + H.sub.2 O → CO + 3H.sub.2                               
                          (20)                                            
CO + 2H.sub.2 → CH.sub.3 OH                                        
                          (21)                                            
CH.sub.3 OH + SO.sub.2 + MO → MSO.sub.4 + CH.sub.4                 
                          (22)                                            
MSO.sub.4 → MO + SO.sub.2 + 1/20.sub.2                             
                          (23)                                            
H.sub.2 O → H.sub.2 + 1/20.sub.2                                   
 
    
     Closed process cycles, such as those of the present invention, are decidedly advantageous from an economic and environmental standpoint; by-products of the reaction are recycled for reuse so that raw materials are not wasted, and no waste materials are produced to pollute the environment. Other advantages of the present closed cycle process include low operational temperatures (below about 1000 K) and avoidance of corrosive substances. 
     Although the invention has been described in detail with respect to specific examples, it will be appreciated that various changes and modifications can be made by those skilled in the art within the scope of the invention as expressed in the following claims.