C. E. Bamberger, J. Braunstein, and D. M. Richardson' Oak Ridge National Laboratory Oak R~dge.Tennessee 37830
Thermochemical Production of Hydrogen from Water
If mechanical and electrical energy consumption can be kept at a minimum, thermochemical cycles hold promise for producing hydrogen at higher efficiencies than electrolysis. temperature between source and sink and Tsi.k is the temHydrogen . . is becoming a n increasingly important element perature of the sink] is unlikely to exceed 50% with current ( I ) , not only because of its present uses, t ~ u also t because of technology. The only chemical reaction one can realistically its enormous potential for replacing natural gas and prrroleconsider is that of coal with water, and a t least in the United urn. H o w ~ v e r . ~ h ~ d r ois~not e n a primary sour& cdene&. The States it looks like a very likely candidate for producing Hz, present importance of hydrogen is underscored by the fact provided the mining of coal is increased accordingly and that a verv.Iaree " fraction of the US and worldwide uroduction means are developed for removing polluting impurities from of hydrogen is used in synthesizing ammonia, which in turn either the coal or the H7. is used as a fertilizer or to oroduce other fertilizers. Its enviIn this paper we wouid like to discuss the possible advansioned uses as an energy vector in a not-so-distant future are tages of decomposing water hv means of thermuchemical cvverv aopealina because its combustion oroduct is mainlv H70. elis; these consist ofseries ofchemical reactions, and t h i s shoild cause little effect on the environmeni. F&: a t high and low temperatures, in which the products of a rethermore. calculations indicate that for lone distance enerev action a t one temperature become the reagents of another transmis&on i t will he cheaper t o pump hidrogen throuih reaction a t a different temperature, and the net effect is the pipelines than to transmit electricitv throuah conductors. consumption of heat and water. If mechanical and electrical Another important aspect of hydrogen use isthat it is a very energy consumption can he kept a t a minimum, thermopromising candidate for energy storage, mainly of electrical chemical cycles hold the promise for producing hydrogen a t energy hut also of solar thermal energy. Three additional higher efficiencies than electrolysis. potential uses are worth mentioning: the gasification and liquefaction of coal, the reduction of iron ore for steel manuThermodynamic Principlesof ThermochemicalCycles facture, and as fuel for internal comhustion engines in cars or The efficiencies of thermochemical cycles are limited by the for iet engines in sub and suoersonic airolanes. Present and fullire what has heen termed - ~ ~ d ~ classical ~ , # laws ~ ~of t h e r m d ) namlrs as indicated in pluneeriny ~ ~ ~ ~onedoes ~ not necessarilv ~ ~ have : h,suhwribe " h ~ studies , hy ~Prof. Funk ~ of~the I!ni\,ersnv ~ ~ ( t i Kentucky (;!r.'l'hr processes in a thermochemical cycle occur repeatedly in a to all as&cts of the '.Hydrwgen EconwnS';to realize that the system with absorption of heat from a high temperature demaud for hvdroren udl rlse 111 the tuture. For one thme. ". the source and rejection of heat to a low temperature sink. The supply of naiural gas, the present major source for hydrbgen, thermodynamics of the cycle can he analyzed in terms of seis being exhausted: thus in the near future we have to face the quences of processes a t constant temperatures and constant compoinded problem of producing more hydrogen from other uressures. The aourouriate thermodvnamic functions under sources of supply and with other sources of enerev. Fortuthese conditions &e t i e Gihhs free energy changes, AG, which nately, there-is water as a virtually inexhaustible supply mav he written in terms of the e n t h a l ~ vchange. AH. and the source, hut its stability toward decomposition is quite high, eniropy change, AS, of each reaction-if the cjcie requiring a considerable expenditure of a primary source of energy. Primary sources of energy to he considered are: nuclear, solar, and coal, and they could he used to decompose water by, for example, direct thermolysis, electrolysis, T h e Gihhs free energy change a t constant temperature and chemical reaction, or thermochemical cycles. The first method pressure is the maximum amount of useful work (e.g., a s requires too high temperatures, in excess of 2500 K, and has electrical energy or as chemical conversion) which can he the most extensive unresolved materials and seuaration produced from a process occurring spontaneously (3). T A S problems. Electrolysis is well established, hut its eificiency is equal to the thermal energy absorbed if the reaction is carof conversion of thermal enerev into hvdroeen . .. is not as hieh ,.,. AH ried out isothermally under reversible conditions, Q as one would like (54On.4, and prospects (or xreatly imprmed is equal to the quantity of thermal energy absorbed if the reefficiency are not high. This is because even it' the cftlciencv action occurs isothermally without the generation of useful of the electrolytic process could be brought close to 100%, thk work. i.e., comuletelv irreversihlv" tQ;,.I. . ..... Fur the decom~osition Carnot cycle limited efficiency of conversion of thermal t o of water t o hidrogen and oxygen a t 1atm pressure a t 25%, electrical energy (ATIT.i,k) [where AT is the difference in the free enerav change is the negative of the standard free energy of formation of water from the elements, 238 Jlmole, Work sponsored by the Division of Basic Energy Sciences, Depnrtment of Energy under contract with the Union Carbide Corpothe positive sign indicating that energy would have to he ration. supplied to the system to accomplish the decomposition. The While this manuscript was being drafted D. M. Richardson passed enthalpy change, 285 Jlmole, is the negative of the heat genaway unexpectedly. With him we lost a friend and the scientific erated by the irreversible comhustion of hydrogen and oxygen community an honest, serious, and capable researcher. He will be to water. T h e difference, T A S = 47 .J/mule, is the heat that sadly missed. would have to he supplied, together with electrical energy, for At intermediateternoeraturesthere would be low nartial nresaures the reversible isothermal electrolytic decomposition of water. Althoueh the free enerev of water decom~ositiondecreases with increasing temperature, it requires, as has been noted energy to accomplish the separation of these gases and their eompression to useful pressures. above, impractically high temperatures, in excess of 2500 K.2
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Volume 55. Number 9, September 1978
1 561
Thequestion arises: is it possihle, thermodynamically, toaccomplish the thermal decomposition of water under less severe conditions by carrying out the reaction in a stepwise manner? For a sequence of steps allat a given temperature the answer is clearly no, by application of the first law of thermodynamics (e.g., through Hess' and Kirchoffs' Laws). If, however, the decomnosition is accomolished hv means of a seauence of reactions a t different temperatures, the answer may differ. The useful work or electrical energy required will be the sum of the free energies of the individual steps. Consider the simplest case of two reactions ( 1 ) and (2). whose combination is the decomposition of water; carried out a t two different temperatures, T I and T2
U represents hydrogen (or oxygen), while Z represents oxyg e n ( hydrogen), ~~ and M and V are intermediates or comhinations of intermediates. For example M might be a metal and V a metal oxide (or hydride). The sum of the two reactions is the decomposition of water, hut the sum of the free energies differs from that of the free energy of water a t any one temperature because of different temperature dependencies of the free energies AGAand AGm of reactions (I) and (2). The temperature dependence of the reaction free enemies is eiven Neby the entropychanges of the reactions, ASA anld glecting in this analysis the temperature dependence of the enthalpy and entropy of the two reaction steps, which is determined by the heat c a ~ a c i t ydifferences of products and reactants, and which is-probably much smaller than the temperature dependence of the free energies, the work required for water decomposition may be written in terms of the free energy of water decomposition at the two temperatures and the entropy of either reaction (I) or (2). By adding and subtracting T ~ A S to B the equation for I: W given above, and rearranging, we obtain
AS^^
Similarly, adding and subtracting T z A S yields ~
ZW = AGH,(T*) + (T2- TI)ASA I t may he seen from these equations that the net work (or e l e d r k l energy) input could, in special cases, be zero without violence to thermodynamic principles. In order for this to occur the thermodyn&ic funkion&f reactants and products would have to satisfy the relations obtained by setting I: W to zero.
I t is conceivable that reaction sequences might exist with entropies such that, a t the proper t&nperaturei, no electrical work would be required to accomplish the splitting of water. Since the numerators are positive, satisfying the above relations requires that the high temperature (T2) process have a positive entropy change and the low temperature (TI) process a negative entropy change. This suggests that one should seek high temperature steps in which many bonds are broken, liquid or solid reactants are converted to gaseous products, or significant structural demadation occurs. The reverse would be sought for the low temperature steps. For chemists, the first impulse is to seek reactions occurring readily with high yield. In a thermochemical cycle, however, i t should be clear that very stable compounds are generally undesirable, since every bond formed in one reaction will require comparable energy to break in a subsequent reaction. As a general screening criterion, processes with positive or negative standard free energies up to about 40 kJImole may he used. Of course one generally does not work a t conditions corresponding to the standard states; the yield of an endoergic reaction can he en562 / Journal of Chemical Education
hanced by removing continuously one or more components. Free enerkes more negative than 40 kJ1mole. however. sueeest too high ;stability f& a product, and its subsequentde&v position is likely to be energeticdly expensive in closing a cycle. This is quire evident in reaction ( 6 ) in the cycle described helow, the decomposition of FerO., requiring a - . - auite high temperature. In the literature, thermochemical hydrogen cycles are often, hut entirely erroneously, designated as catalytic in nature. IC may be seen that the ahove feasibility criteria are h ~ s e don equilibria and not on rates, and can he considered, in effect, an application of the law of mass action. However, the rates are certainly important considerations in finding practical cycles, and catalysis of the individual reactions is frequently a necessity. The thermodynamic analysis has been extended by Funk et al. (21, to include multi-step reaction sequences of more than two steps at more than two temperatu;es, the effect of heat capacities, ie., the temperarure dependence of AH and AS, and the work terms associated with the separation of mixtures of reaction products. But the key criteria of feasible processes remain the production of entropy a t high temperatures and the consumption of entropy a t low temperatures. In thelight of theaboveconsiderations, an examination of m a w two-step processes such as the reactions of water to form either an oxid'and hydrogen, or a hydride and oxygen, followed by thermal decom~ositionof the oxide or hvdride. Funk (2) found no feasible two-step process and conclided that one was unlikely. Abraham and Schreiner ( 4 ) have made the stronger statement that a feasible two-step process is impossible because the required entropy change of the high temperature reaction step, taking the upper and lower temperatures as lOOOK and 298K, is twice the entropy change for decomposition of water into its elements, and hence too large for real substances. They concluded that the minimum number of reaction steps is four (two heat absorption steps a t high temperature and two heat rejecting steps at low temperature). In actual cases, however, particularly where complex compounds are formed and where changes of the state of aggrecation occur simultaneouslv. it mav be difficult to define without ambiguity the number of steps or processes (see, e.g. reaction (7) below). As an evidence of chemical ingenuity the literature discloses a large variety of cycles, in excess of 200, which are formed by widely differing numbers of reactions (anywhere between two and more than six). But a large fraction of such published cycles is unworkable ( 5 , 6 )mainly hecause when they were conceived no free energies of reaction had been estimated. Some of these turned out to be too positive for any practical application. Equations (1)and (2) give the conditions for a two stepcycle for water decomposition with no input of electrical energy (ZAG = 0). Consideration has been given also to so-called hybrid cycles, in which a sequence of chemical reactions at differing temperatures is closed by means of a photolytic step or an electrolytic step with a lower decomposition voltage (by virtue of a lower free energy of decomposition) than that of etc. Such electrolyticsteps pure water, e.g. HBr (7),HzSOa (8), for incorporation into a cycle must be carefully chosen to avoid inefficiencies. Since the maximum efficiency for conversion of thermal energy to electrical energy is currently less than 50%,thecost in thermal energy would he twice that equivalent to the electrical enerev ... reauired . for the electrolvsis. Obv~ouslythe above app11e.i to any chem~calcompound, indenendentlv of the rlrmet~tsthat form it. and in theorv onlv the periodic table limits the selection of possible candihates. In practice, however, economics, i.e., the overall cost of hydrogen produced, rules strongly the future of its production by thermochemical processes. Thus, in order to attain low capital (inventory)costs oirhemi~nkit is advisable toconsid~tr only those formed by elements which arr at present of low cost or are sufficiently abundant to he of low cost if a strong demand arises in the future. This reduces significantly the
numher of elements in the periodic table that can he used. This numher is further reduced hv the reauired multinle valence of the elements that have t u art, dlrertly or indirectly, as reductmts and o~ldantsof water ( 5 . 6 ) Another asnect that affects the cost of the product hydrogen is the invesiment in construction materials for the chemical reactor(s) and heat exchangers. Most of the chemicals involved in thermochemical cycles are, by nature, quite corrosive, especially a t the high temperatures required for some of the reactions (9). Thus the containment for some reactions may require the use of exotic materials whose cost will add to the cost of the product. The tolerance to even a very small degree of corrosion has t o be ascertained very carefully and demonstrated experimentally, because corrosion products may build up and either inactivate catalysts or produce undesired products that have to he discarded. This affects the efficiency of the process as well as the chemicals and materials that have to he reolaced. In order to illustrate some of the concepts discussed above the followine examole of a cvcle exoerimeutallv demonstrated in the labor&ry (i0) is presented.
+ ZHzOk) + HLR)
(3)
AG = -102.2 kJ. AH = -17.2 kJ and AS = 106.9 JIK
AG = -66.2 kJ, AH = -198.5 kJ and AS = -444.8 JIK
have large entropy increases, while the low temperature reaction has a larce entronv decrease. The main reason for the &.ropyincreaie in thehigh temperature reactions is the liberation of eases.. Hv- and 01 -.. a disorderine -.orocess. The low temperature reaction has a large entropy decrease coniributed in Dart hv the renlacement of a more disordered crvstal N~F~O dy?a moreordered crystal Fe(OHI3. Thermochemical cvcles have been generated in manv institutions by means of computer progr&s which includea set of thermodvnamic data and a screening criterion arbitrarily set by the brogrammer, such as maximum temperature df reaction, numher of reactions, free energy of reaction, etc. Such searches for cycles, although capable of assembling a large numher of cycles (-300) with only very few elements (i.e. four), are seriously limited by the lack of accuracy and the scarcity of some thermodynamic data. Furthermore, these programs do not deal with the kinetics of reactions although there are no inherent limitations, other than lack of data. Kinetics is important from an economic point of view because i t affects directly the cost of hydrogen produced, i.e., a slower reaction reauires a lareer inventorv of chemicals and a larger supply of heat. ~ l t h o u g hc a ~ c u l a t ~ofnfree energies of re&tion, either hv comnuter or bv hand. indicates whether a r e action mightpoceed as written it isalways advisable to test the reaction exoerimentally in order to uncover some effects orerlvoked in ihe ralculnt& which may jeopardize the use of that reaction in a cycle. This can be illu.;tmted by considering the following example 6FdOH)ds)
AG = -710.7 kJ, AH = 307.1 kJ and AS = 1272.2 JIK 16WK
3Fe20a(s)--,2Fe30As) + %O&)
(6)
AG = 18.5 kJ,AH = 233.9 kJ and AS = 134.6 J K
The reaction of maanetite (FenOd with sodium hydroxide was performed in hi& fired dumina orcuppw cruc;t,les in a silica enclosure under k~wingnrgnn aicarrier yas.The cvulved hydrogen was meajured by thermal conductivity. The hydrolysis of Nave02 was r f t c t e d with boiling water and the FetOHh formed was senarated b\, filtration. The thermal decomposition of hematite (Fe203)contained in a platinum boat was oerformed at about 1400K under areon flow and the oxygen evolved was measured by means of a Eeckman oxygen analyzer, model 741. All the crystalline solids were identified by powder X-ray diffraction. In order to demonstrate that unreacted FenOl could he separated from Fe(0Hh a magnetic separation ofthe two solidswas tested. A portionof N ~ F from a purposely not completed reaction was loaded into a 50-ml huret and connectedto water flowing upwards through the stopcock; the exiting suspension was collected on a glass frit filter and continuously filtered by suction. Two or three small horseshoe laboratory magnets held to the buret with rubber bands prevented the Fe304 from migrating upwards. This simple device demonstrated that the separation is easy to accomolish. This cycle has the advantage of being simple, using elements which are abundant and of low price, and although the alkali hydroxides are very corrwive, the relatively low temperatures make the corrosion oroblems tractable. T h e main difficultv associated with this cycle is the high tempernture rrquired tin the evolution of oxvgen (reactiun r61j. 'i'hermorhemiral calculations ( 1 1 ) indicate that l atm of oxygen is in equilibrium with the two solid iron oxides at 1735K. This temnerature is at present too high for nuclear reactors of the HTGR type (High Temperature Gas Cooled Reactor). I t is achievable in solar furnaces; however, the industrial scale application of such equipment may he barred presently by the costs of collecting and concentrating solar energy. The example given, although it involves four reactions a t three temneratures. rather than two. illustrates aualitativelv some of the princi$es discussed. ~hermodynam.icdata (117 indicate that the intermediate and high temperature reactions ~
~
ZK 2Fe30ds) + 9HzO(g) + lIzOz(g)
(7)
where AG = -783 kJ. Such a large negative free energy indicates that reaction (7) should oroceed snontaneouslv: however it does not. The reason lithat ( 7 )is in thrt compoied of tworeartions, 15, one with a lnrae A C and (61m e with a oosllive AG. The latter - negative thus is a major controlling factor of the extent of reaction of 17). Because of lack of adequate information another case, not always contemplated in calculations, which is frequently uncovered by experimental demonstration is the formation of compounds, namely double oxides, resulting from reactions with container materials. Such effects and others suooort .. our contention that it is imperative to demonstrate all the reactions and separation processes involved in thermochemical cycles; otherwise the successful development of thermochemical oroduction of hvdroeen mav remain an interestina exercise on paper.
~ O ~
Conclusions The importance of hydrogen in the developing energy economv. " . and the varietv of methods .o r o-~ o s e dfor its eeneration, suggest it as a useful topic for study in the chemistry curriculum. Electrolysis of water is already widely dealt with, and the application of thermodynamic principles in the search for alternative methods should be instructive. The thermochemical cycle presented here may not prove to be the most economical. hut it illustrates some of the orinciples with reactions that can he verified in the lahoratbry. Literature Cited (11 namherrer.C.%and Rrsunnein..l..~mrr. Scimtirt. 63. #4,4:38 119751. iP1 Funk. J. E.. in " P r ~ c p p d m g(4 ~ Hydroyen Energy Fundamentals." I E d ~ l e cNejaf Verirarlu, T I Miami Reach. Fls.. Mareh 1975.p. S2~:3. 131 Lowis. C.N.. and Randall. M.. revised by Pitzer. K. S.. and Brewer. I r . , " T h e r m o d ~ ~ namies."Znd Ed.. McCrsw-Hill Rcnh Cu.. New York. 1961. (41 Ahraham. R. M., and &hreine?. F.. Ind. E n 8 Chsm. Fund.. 13. SOSl19741. 1s) Bamher~er,C. E., and Richardson. D.M.. Cryomnics. 197 1April 19761. 16) Hnmbeqer,C. E..C r j o w u c s . 170 (March 19781. (71 Srhuekz.C.H.,in"Pn~c~edingsof lstWorldHydrogenEn~r~yConferenco."1Edilor: Nejnt Veriroglu.T.1. Vol. I , Miami Reach. Fla., March 1976, p. RA-49. 181 Warde, C. .I.. and Hrecher. I.. R.. in "Praeedingr nl 1st World Hydmgen E n e w Ccmference," 1Milor Nejar Vezirn~lu.T.1. Vnl. I. Miami Hench. Fla. Mareh 1976. p. 9A-51. 191 Ramhereer, C.E.. and DeVan, .I. H.. Mei. Tienr.. 9A,?Ol (Peh 1918). 11111 Ramberser,C. E.. Richsrdsun, D. M..snd lirimen. W. R., U.S.I3aUnL:3,929.979. Dec.
:10 1071 ., . .. .. (111 All the thermodynamic data is from Stull. D.R.. and Prophet. H.. "JanafThermochemical Tables." 2nd Rd.. NSRDS-NRS 37. 1971 with t h e e r c e p h n of Lhnt lor
Volume 55. Number 9, September 1978 / 563
N e e O * [Koehler. M. F.. Barany. R.,and Kelley, K. K., RI S i l l , Bureau of Mines. 19611 and for NaOH(d) [Nstl. Bureau of Standards, Circular SW. 19521.
General References N e t Vezimglu. T. (Editor). "Hydmgen Energy," Parts A and B, Plenum Pub. Cow.,
.".".
*wc
Nejat Vairaplu. T. (Editor). "1st World Hyd-n Energy Conferenex P d i n g a ? I3 VOW.. Schoal of Continuing Studie, Univ, of Miami, Coral Gables, Fla. 33124,1976. ERDA, "Hydrogen Fuels, A Bibliography," TIC-3356, 1976. Available fmm National Technical Information Service, U.S. Dept. ofCommerce, Springfield. VA 22161. University of Kentucky, "A T~chnoeconomicAnalysis of Large Scale Thermwhemieal Production of hydrogen." EPRl Report EM-287, Project 467,1976.
564 / Journal of Chemical Education
