The Thermal Dissociation of Water The Forgotten Literature H. H. G. Jellinek Clarkson University. Potsdam. NY 13676 Thermal and uhotochemical decomuosition of water have herome o i g r e a t ' i m p o r a n during recent years for the production of alternate fuels such us hvdroaen. Solar enerav is envisaged as the energy source for thesedecomposition&ocesses. Recent literature on this topic is large. At the end of the last and a t the beginning of this century, appreciable research was undertaken on thermal gas dissociation equilibria including thermal decomposition of water; this work appears to have been forgotten as far as the literature on solar energy is concerned. Before 1900, there were schools under Henri Etienne Sainte Claire DeVille (1818-81) in France and August Wilhelm von Hofmann (181E-92) (EnglandIGermany), and there was also c lone investigator, Sir William Robert Grove, (1811-96). At the turn of the century Hermann Walther Nernst (1864-1941) and his graduate students, including Irving Langmuir (1881-1957), were working in Germany. On Nernst's suggestion many of the researches were concerned with gas equilibria including thermally catalyzed water decomposition; there were persons working with Nernst who became well known later in life. The dissociation equilibrium of water at not-too-high temueraturcs where dissociation into H atoms or OH radicalsdoes not play a role can be represented by (I) 2H20 (g) + 2H, (g) + 0, (g) + 56.93 kcal/(mol of H,O) The mass-action law with respect to pressure is,
Figure 1. W. R. Grove's exwriment.
1, 1899 (21.) Catalytic (Pt) decomposition of water was investigated by the well-known chemist Hofmann (Fig. 2). His work on water decomposition (3)represents only a very small part of his total research effort. He decomposed water by electric
Here P i s the total gas pressure; the other pressures are the respective partial pressures; a is the degree of dissociation, given by,
Further,
Early Experiments on Thermal Dlsoclatlon of Water Grove appears to have been the first who decomposed water using platinum as a catalyst (1847 (2)). A loop of P t wire (see Fig. 1) was sealed into the closed end of what essentially was a test tube. I t was inverted and dipped with its oven end into a pool of water in an inclined vosition. The Pt wire wns heated to incandescence with a spirit lamp, steam was e \ d v r d and enveloped the Pt loop, and H,O was decomposed to a small extent A small gas l;"hhle was left; this could be detonated, yielding HzO. Grove was horn in Swansea, Wales, on July 1, 1811. He became a barrister (lawyer) but because of poor health retired devoting himself to scientific experiments. Best known are his "Grove cells" (electric cells using Hq and 02,i.e., fuel cells). He was elected a F.R.S., returned to law, becoming a judge, and was knighted in 1872. (He died in London, August
m Figure 2 August Wllhelrn Hofrnann (1818.1892) llved in London
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Figure 3. H e m Saints-Claire DeViiie (1818-1881). sparks between P t electrodes and also on the surface of a red-to-white-hot P t spiral heated by batteries. A glass tuhe passing into a narrower one was closed by a cork carrying a P t wire loop (diameter 0.06 cm, length 16 cm). Steam was introduced through the narrow tube, dipping into water, and decomposed a t the hot spiral. The product gases were collected and exploded in an explosion pipet (eudiometer) yielding water. Hofmann was horn in the university town of Giessen, April 8, 1818, and died in Berlin May 2, 1892. Hearing lectures by Justus von Liehig, he was impressed and became Liehig's assistant. Hofmann was predominantly an organic chemist who was responsible to a large extent for the synthetic dve industrv (aniline derivatives). His PhD thesis dealr wiih der~vati;wuf indigo ichlmina;ed anilines. 18411. He became P r i w r Dozwr 31 the Lni\,ersitv of Ronn (1845). At this time, a committee had been forked in ~ n ~ l a n d chaired hv Prince Albert. who had been a student a t Bonn. considering the estul~lishmentof a (:trllege 01 (:hemismy in [.ondon. He and Queer Virroria visited Ronn at rl~eoccaiion o i I.. w n lkthnven's '75th anniversary. 'l'he I'rinre wished to riiir the apartment where he had lived as n srudrnt. 'This was now occupied by Hofmann, who had installed a private laboratory. The royal couple was enchanted by Hofmann's charming personality and his scientific demonstrations, and he was selected as Professor (1845) at the new college; he took two vears' leave of absence. hut actuallv in En. staved " gland 20 ;ears, having a brilliant career continuing his oraanic chemical research. Henrv Perkin (1838-1907: kniehti d 1906) was one of his co-wbrkers. perkin synthesized a comuound that is now known hv the name mauvein: with it, the synthetic dye industry was born. Hofmann was afavorite of society, became a F.R.S., and obtained mans distinctions and medals. At the height of his fame, he recei;ed an offer of a professorship in chemistry a t the University of Berlin (May, 1865), which he accepted. A laboratory was built according to his plans. The Deutsche Chemische Gesellschaft was founded by him, and he also started the Berichte in 1868. Hofmann and co-workers published 869 papers, 273 actually written by him. He was associated with the dye and 1030
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pharmaceutical industry. Once, when he was lecturing on benzene, he remarked, "Benzene has a characteristic odor; a lady told me it smells of cleaned gloves." The first three of his four wives died rather quickly; there were 11children. DeVille (4) (Fig. 3) developed sophisticated and ingenious apparatus for his work on the thermal decomposition of gases and also designed furnaces for high temperatures. He was, most likely, the first scientist who used flow systems, inventine the "hot-cold tuhe" method. ~ e ~ i lshowed ie that steam is partially decomposed at the melting point of P t (1750 'C) and also a t lower temperatures. A mixture of C02 and steam was passed through a redhot tuhe and was collected over strong potash. An explosive gaseous mixture was left behind. steam was also passed through a porous earthenware tuhe permeable to Hz enveloped by a gas-tight tube. In the space between the two tubes, COz flowed. Heated to 1100-1300 OC, the exiting Con contained H? and the other tuhe O*. Thus he demonstrated "the phenomenon of the spontaneois decomposition of water. A phenomenon which I [DeVille] propose to call dissociation." The success of DeVille's experiments was due to rapid coolingof the reaction mixtures (i.e., the rapid flow of cold water; the theory of flow systems was later evolved by Nernst) (5). Fiaure 4 shows one of DeVille's apparatus (I). A porcelain tube, P, is heated in a coal fire. A East stream of cold water flows through a thin-walled Ag tube, T. Any decomposition products are quickly quenched at the cold Ag wall, collected, and analyzed. Water is produced by detonating them. The apparatus was improved, showing ingenious features (see Fig. 5) (1). There is again a thin-walled Ag tuhe, T, throueh which cold water flows. Its rate can be reeulated hv the rap, R. Steam passed via \' to an inverred cullecrion tuhe wherr ir condenses while the permunenr gases H? and 0?are
Figure 4. DeViiie's original apparatus.
Figure 5. DeVille's
improved apparatus.
collected. The latter were led into the final container where they were analyzed. The ingenious part consisted of a small hole at the underside of the Ag tuhe around which a flame plays. The gases of this flame, which have suffered in part thermal decomnosition are sucked into the tuhe and taken with the stream. The temperature is very high in the immediate neiehhorhood around the hole. Water cannot flow out of the hoL: rather, thr tlamepises are sucked into the tuhe. Thus L)r\'illecould show that gases areavtuallydecon~posed . thermally. DeVille was horn a t St. Thomas, Virgin Islands, March 11, 1818, and died in Boulogne on July 1, 1881 (4). His father was the French Consul there and a ship owner. All of DeVille's education took place in France. He obtained an MD in 1843 but was attracted by L. J. T h h a r d ' s lectures and decided to studv chemistrv. obtaining his PhD verv quickly. He was appdinted ~rofe$sorof ~Cemistryand ~ e a of n t h e newly established facultv a t the Universitv of Besancon (1845-57). By that time he had a good reputation as being an independent and tireless worker having published important papers. His appointment as Professor a t the ficole Normal Superieur (1851-80) followed and also a titular professorship a t the Sorhonne (1853) substituting for J. B. A. Dumas' lectures. His early work was concerned with organic chemistry. Equipment and a library for research were nonexistant. DeVille, however, established research laboratories and a flourishine school. Later. he occuoied himself with work in ~ h v s i cal a i d inorganic cl;emistry:~e discovered N205 (1849);his method produced Al in large quantities; he crystallized silicon, studied the metallurgy of Pt, and became the authority in the field of high temperatures including thermal decomposition of gases. ~e worked togethe; with Friedrich Woehler (1800-82) on boron and titanium compounds. M. Faraday (1791-1869) and DeVille visited each other repeat-
Figure 7. Nernst's research gmup, Berlin 1906. Fmm ken, top: L. Lowenstein, A. Eucken. Sirk. DeOsa. Vogei won Falkenstein. A. Magnus, M. Pier. F. Horak, F. Weicert. G. Bullbock. H. Jahn. W.van Wartenberg. H. Schlessinger. G. Felk. H. Schluter, 0 . Brill.
Figure 8. Nernst and colleagues, university of Berlin. 1906. From Leftto right: G. Buchbock. A. Magnus, A. Eucken. F. Horak, U. Jellinek, ?,?.?,0. Brill. ?. H. Schluter. Sirk, H. Jahn, W. Nernst.
edly. Vapor densities were determined a t high temperatures in the 1000-1200 'C range with his co-worker L. Troost (1825-1911). In addition, diffusion of gases in metals at high temperatures was investigated. His work on dissociation is collected in "Lecons Sur La Dissociation" (4). Hermann wilther Nernst and his graduate students (Fies. 6-8) researched thermal gas dissociation eauilihria "eri intensely around the year I900 (1, 5). All this work oriainated from Nernst's desire of ascertainina whether (entropy) S 0 or (free energy) F 0 at 0 K; this involved the determination of the integration constant J in the GibhsHelmholtz equation,
-
-
M=H-TAS
Figure 6. Hermann Wallher Nernst (1864-1941).
Here A F is the free enerev. A S the entrow. .".and AH the heat of reaction; a, 8, and y are constants; and T is the absolute temoerature. If J = 0 a t or near 0 K. then AF = AH and 7'A.q = 0: Fur crystalline wlids this was found generally to he the case. Eventuallv, Nernst'i heat theorem emerred from these and uther investigations. Nernst and his r~,-workc~s studied many gaseous dissoriation equilibria. Some examples are: 2 H?O.' 2 H, + Ol 16). Volume 63 Number 12
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+ +
+
0 p 4 HCl+ 2 Clp 2 Hz0 (Deacon Process) (7), 2 COz * 2 3 Hp (9), 2 NO + N2 + 0 2 (10). CO O2 (a),2 NH3 +N2 The early experiments of water decomposition were simple, hut as work in this field continued quantitativemethods were evolved. Avariety of experimental and other methods were used: 1) flow, 2) explosion, 3) heated catalyst, 4) semipermeable walls, 5) calculation of an equilihrium from other equilibria, 6) heat conduction, 7) effusion (not used by Nernst), and 8) emf determinations (not discussed here). Nernst and T. Wulf (11) used DeVille's flow method. Steam was passed through an electrically heated quartz capillary. Some Hp and O2 (Knallgas) was detected hut its amount changed with the steam flow rate. H. von Wartenberg (1886-1960) (6), working for his PhD deeree. - . develoned the flow method of Nernst's suggestion. His first experimental arrangement was quite simpli(6). An Ae" tuhe. 10 cm lona- and 1-2 cm in diameter, was used. Ag capillaries were welded at either end. The exit capillary was water cooled. The wide tuhe was heated to incandescence. However, satisfactory results could not he ohtained (red-hot silver absorbs 0 2 and upsets the water equilihrium). Next, a P t capillary was tested (6), hut Hp permeated through its wall and this attempt was abandoned. Quartz tuhes of -2-crn diameter and 12-cm length were tested next (6); to either end of a tuhe, a quartz capillary was fused. The wide Dart was heated electrically in a nickel furnace tuhe into an asbestos box filled with magnesia. A thermocouple and cooling of the exit tuhe were provided. When a mixiure of Hz and O2 was passed through the assembly a t 900 OC, a large percentage of this gas mixture did not combine to water. Apparently, the velocity of recombination is quite slow a t this temperature. A catalyst (i.e., small particles of Pt) was introduced, hut at the points where P t touched the quartz wall, deep holes developed which were covered with a white coating (silicic acid) subliming towards the exit capillary. Next the rube's inside wall was piatinieed; ho\vwer. the Pt laver wnsdestroved within 15 min. porcelain vesselb and tuhes w&e then tried. Glazed porcelain was not available at that time and tuhes had to he selected for their gas impermeability. The reaction vessel, a sphere of about 13 cm3 and 0.3-cm wall thickness had, on opposite sides, one capillary each sealed t o it (diameter 0.05 em, length 20 cm). The sphere was inserted into a clay tube, wound with P t wire. and heated electricallv. Fair results were ohtained. The techniaue and aDDaratus were further improved. The method of semipermeable membranes was used. L. L6wenstein (12). followinr! Nernst's suggestion, had developed it. His apparatus consisted of a P t ;&el (8 cm long, diameter 1.2 cm) which had a long P t capillary welded to it (length 12 cm, diameter 0.05 cm). I t extended 6 cm beyond the P t furnace tuhe. The manometer liquid was oil. The P t vessel was evacuated; steam was passedthrough the furnace tube. 0.01% of Hz in the gas mixture gave 1 mm pressure; 0.001%
+
Hz could still be estimated. The P t vessel appeared to have some small pinholes as some steam permeated into the vessel. Introduction of P205 before analysis eliminated this source of error. Another fault consisted in Hz diffusing through the P t furnace tuhe. This was eliminated by an unglazed hut almost gas-tight porcelain tuhe inserted into the furnace tuhe. I t was somewhat longer than the latter; clearance between the tuhes was -0.2 cm. The ends were sealed gas tight. Steam was also passed through this clearance: i t was eenerated in a flask ~ r o v i d e dwith P t wire elec" trodes. The generation of a very tiny amount of Hp and Op by a verv weak current resulted in even boiling. The exitina steam condrnsrd and product gases were analized. ~ ~ u i l i h l rium inside and outside the I't \.essel was reached in ahout 10 min a t about 1700 K, results were as follows, K 100 a ( i . ~ . .% decomposition df H*0)
1705 0.102
1783 0.183
1863 0.354
1968 0.518
Von Wartenberg based his further research (6) on the method olsemiprrmt:ahle memhmnes. Higher temperatures than Liiwrnstein's were desired 1>2000 "CJ. Nernst had used I't and also Ir previously for the determination of molecular weights (5)a t very high temperatures and also for detecting dissociation. The vessel was made of Ir (Fig. 9). At high temperatures, Hz permeates through Ir as well as through P t walls. An Ir tuhe (1) 25 cm long and 1.5 cm in diameter served as a furnace tuhe; 1-mm-thick P t discs (2) were welded to this furnace tuhe and were extended by 2-cm-long P t tuhes. Brass tubes (3) were hard-soldered to these P t tubes. The large hrass tuhe was 7 cm long. Durina an experiment coolingwater reached the boiling point. he othe; end was closed by a hrass disc (4) carrying an inlet tuhe for steam. The wide Ir furnace tuhe was located in an asbestos box filled with magnesia. The Ir vessel passed over into a long narrow Ir tuhe (6) to which a 3-em-long P t tuhe was welded. A Cu tube was soldered to the P t tuhe which was provided with a small water cooler and was extended by a glass tube having a 1-m-long T piece (diameter 0.2 cm) serving as open manometer. The other branch of the T ~ i e c eled to the pumps. The manomrtcr could he read withu telescope to 1 X 10-'rm He. " Steam (5)exited at the other side ( 7 ) ;product gases were collected in an explosion pipet while steam condensed. Temperatures were measured by a "Wanner" pyrometer. The bottom of the Ir vessel was blackened with a mixture of MnO and MgO. Also, blackened diaphragms were placed into the furnace tuhe, so that the bottom of the Ir vessel became virtuallv a black-bodv radiator. Decrease in temperature was never observed while introducing steam. The Ir vessel was evacuated, the furnace tuhe heated and the temperature of the Ir vessel was determined. Steam was introduced. After about 5 min, the pressure inside and outside the Ir vessel became almost constant. The degree of dissociation was calculated on the basis of the equilihrium written as,
thus, PH. = P H ~ O(decomposed)
= Pop). The partial pressure of H Zwas measured ('/~PH~ The analysis was carried out with an amount of steam exiting during 1 min. All values were normalized to STP. Preliminary results were as follows, K I00 a (i.e.. % H20 decomposed) Figure 9. van Wartenberg's experiment.
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Journal of Chemical Education
1882 1.18
1984 1.77
For the final experimental arrangement (Fig. 10 (6)), von Wartenberg returned to the flow method. The vessel was of
similar shape as before but made of glazed porcelain. At its ends, the vessel was extended by narrow porcelain tubes having diameters of 0.6 cm. One had a T piece attached leading to a flask for boiling HzO. The latter was provided with P t electrodes for producing very small amounts of Hq and 0 2 (Knollgas) ensuring even boiling. The wide tuhe passed over into a capillary (diameter 0.05 cm) at the other side increasina the steam flow rate a ~ ~ r e c i a b l i.e.. v . the equilibrium was "frozen in". The exiting (Hz ? oq)'was analyzed in an explosion pipet. Three hundred amDeres and 1.5 v ac were needed for heating the furnace t u i e which consisted of P t (0.02-cm wall thickness). I t was located in an asbestos box filled with magnesia. A thermocouple was provided. Results were ohtained as follows (6)
The next investigation is of great significance for today's attempts of thermal, catalytic Hz production using solar radiation as heat source. Langmuir (13) (Fig. 11)-who became famous in later life and received the Nobel Prize in 1932-worked on his PhD thesis in Nernst's lahoratorv: Nernst suggrsted the topic. -On thr Partial ~ecomhinatir;l; of Dissuci3ted Gases Ihrina Coolinr" t 1905). Lanamuir undertook, also on Nernst's kggesti& 'research on the dissociation of water a t a heated P t wire. This work can he considered as the heginning of Langmuir's subsequent extensive research on gases decomposing on hot wires at the G. E. Laboratories leading eventually to the perfection of the incandescent electric lamp. The title of the study was "The Dissociation of Water Vapor and Carbon Dioxide a t High Temperatures" (13). Langmuir made use of the method of the heated catalyst (13). A P t wire was stretched along the axis of a hard glass tuhe and was heated electrically to white incandescence. Steam was introduced, quickly decomposing at the surface of the hot P t wire and estahlishinr the water eauilihrium: the products %,ereat once removed from the wire su that the equilibrium was preserved. 'I'hat I his was the case was shown by experiment. T h e glass tuhe (Fig. 12) had an internal diameter of 0.9 cm. The voltage between P and P' was measured very accurately as the temperature was ohtained from the resistance as a function of temperature. This was the most troublesome part leading to small inaccuracies. The wire consisted of highly purified P t having a diameter of 0.006 cm. The lower end of the class tube fitted the neck of a flask and was provided with a ~g seal. Steam was produced in this flask. The upper end of the glass tube had aT piece, T, whose tip could be dipped into water. The product gases Hz
Figure
11. I. Langmuir in 1920.
and 0% bubbled into a water-filled inverted tube. Steam condensed in the water ~ o o lAnalysis . was carried out in an rrpl
