J O U R N A L OF THE AMERICAN CHEMICAL SOCIETY (Registered in U. S P a t e n t Office)
TOLLME i9
(a
Copyright, l(257, b y t h e American Chemical Society)
JULY 25, 1 9 5 i
NUMBER 14
PHYSICAL AND INORGANIC CHEMISTRY [CUSTRIBUTION S o . 92 FROM
Palladium Hydride.
THE
CRYOGESIC LABORATORY OF THE COLLEGE OF CHEMISTRY ASD PHYSICS,THE PENSSYLVANI.4 S T A T E USIVERSITY]
I. The Thermodynamic Properties of Pd,H between 273 and 345OK. BY DONALD 11. SACE AND J. C;. ASTON RECEIVED FEBRUARY 13, 1957
T h e 30' absorption isotherm for hydrugeri dissolved in a highly active palladium black has been measured and fouiid t o agree very closely with an equilibrium desorption isotherm obtained previously by Gillespie and Hall. Partial molar heats of desorption for the system pa lladium-hydrogen have been calculated from t h e isotherms. Partial molar heats of absorption and desorption of hydrogen from palladium hydride also have been measured in a n adiabatic calorimeter. These values are used t o calculate the free energy and the entropy change associated with the formation of P d l H from t h e elements at 30". Agreement between the calorimetric heats and those calculated from the temperature coeficient of the equilibrium pressure establishes a reasonable criterion of equilibrium for t h e system.
Introduction Palladium is a rapid exothermic absorber of large volumes of hydrogen. -1lthough this phenomenon has been the subject of many previous investigation^,^^^ no entirely satisfactory explanation of the nature of the bonding force has been established. The phase equilibria of the hydrogenpalladium system have been extensively investigated for different forms of palladium (wire, foil, sponge and black). Hysteresis effects have been quite evident in all the isotherms, especially those for temperatures below 160". By using a special heat treatment technique, Gillespie and Hall4 obtained, a t temperatures as low as 0 and 30°, apparently true equilibrium isotherms which are located between the usually different absorption and desorption curves. The equilibrium isotherm is interpreted as indicating two solid solutions; one is a hydrogen-poor solution (a-phase), the other is a hydrogen-rich solution (Rphase) of approximate composition P&H. For both a - and @-phasesX-ray analysis identifies the palladium atom array as being face-centered cubic, (1) Aided in p a r t by contract Nljonr-2tiY, 'I. 0 . I11 of t h e Office of Naval Research. Reproduction in whole or in p a r t is permitted f o r any purposes of t h e United States goi.einment. ( 2 ) A complete bihli,igraphy of t h e literature u p to 1Y47 is given Iry D. P. S m i t h in the hook "Hydrugen in U e t a l s , " University o f Chicayo Press, Chicago, I l l , 1948. ( 3 ) (a) P. Wright, Pl'O O ) ; (13) K . E . Xrxhert, P h r s K w r . . 86, 7-t.5 (1952). ( 1 , I, 1. C;illv\i)ic xnrl I C . P. H x l l , T I I I S JorrKvnr., 48, I 2 O i i l ! j ? f i ) .
the a-phase having a paraQieter of 3. the R a parameter of 4.020 A.5 The data presented in this paper were compiled for the purpose of calculating the entropy for the system PdaH a t near room temperature. Such a value is needed in order to determine the absolute entropy of Pd2H a t 0°K. from a third law study. I t is best that this be determined on the sample used for the third law study. Therefore, a series of equilibrium pressure and heat of absorption measurements have been made on a o u y activc palladium black between 2i:3 and 345°K. Materials.-The palladium used is L: special grade of palladium black obtained from Baker and Company, Kewark, S e w Jersey. This extremely active palladium is made for their own research purposes, so the method o f preparation is not available. The purity is giveii as S9.9',.,. Although the density of bulk palladiuiii is 18.0 g./cc.. tlie packet1 density of this black powder is only 0.6 g./cc. One saniplc, weighing 31.277 g . , was sealed into ;I gold calorimeter and used for d l of the experimental work. The sample w a s never heated over 100D because tlie material supposedly sinters a t higher temperatures. An?- possible oxidc o f palladium was reduced initiallJ- by adding Ii!drogen to thc saniple and then degassing a t 98'. Following this reduction, t h e sample w a s exposed only to very pur? lirliuni, nitrogen and hydrogen. High purity hl-drogen made by electrolysis and purified by liquefying and then compressing into a high pressure cylinder was used throughout. All portions of the hydrogen allowed to react with the palladium were first passed througli :L trap packed with acti\-:ited ch:irci,al aiid iinnierscd i i i :I liquid nitrugen-liath.
Apparatus.-The container for the pdladiuni black compressioit. The equation for making thew corrvctir~ii~ sample is a closed gold cylinder fitted a t the top with a gold to the measured values is inlet tube arid having a n internal volume o f 55.6 cc. Cotinectioii between the g.)ltl calorimeter atid a glass xtcuum system is made through a platinutti-rliotliuiii tube which has an inside diameter (if 0.0292 in. anti which is gild where C,,, is the measured lieat c:tpacity :intl AtL i y tile inwelded inside the calorimeter inlet tube. The calorinieter crease in tlie number of moles of gas in the dead space. contains a 2.5 in. long re-entrant well which is gold weltleil After each tneasurement of heat evolved LIL- ' t giveti in the center of the ch-linder bottom. The re-entrant well quantity of gas, the calorimeter was cimled to the iiiitial is designed for insertion of :L platinuni resistaiicc therrniinihcat capacity I I I C ; L . ; ~ ~ ~ I I ~ ~ I I ~ ' : temperature and one or eter. Tvere made betwecti the in and final tenilier,itures of the \-olumes of gas tire iiieasureil i i i a c,tlibratetl &liter Pyres heat o f absorption iiieasu Kilt. The hc:tt I J f al~si~rpticiti I)ulb set up inside a SO-galliiii insulated stirred \VatKr-b;ith was calculated by a series of reiterations of eq. 1 a n t l 2 which is kept a t room temperature. Mercury friitri a stor- since (I ;itid C,, :tppe:ir in both equations. age tank immersed in the water-bath can be raised into lir The Absorption Isotherm.-A :jO" isotherin for lowered from the %liter bulb in order to traiisfer the gas to the absorption of hydrogen in palladium black W:LS tlie calorimeter. The system is patterned after one described bv Giauque a n d Jolinstoii.6 The temperature of the determined in the isothermal apparatus. The water is -measured on ;I inercur>- tlierrnoinrter \\-hich has isotherm is shown as the solid curve in Fig. 1 heen calibrated against a platiuuni resistance tliertiiometcr. where i t is compared with the ,30" isotherin obtained The pressure of the g a i is measured w i n g :L 13 tntn. bore. copper cased, mercury inaniiineter. The mercury levels by Gillespie and Hall.4 Since the Gillespie and m d meniscus heights were referred to :i staiiiless steel, Hall isotherm is a desorption isotherm brought to calibrated meter bar by means of a Gaertner Scientific equilibrium by the use of a i3iiO" heat treatnient Corporation h1901 cathetoinetcr. prior to each pressure iiieasurement, the siiriilarThe calorimeter vessel was iinniersed in tlie inner bath of a ity between these isotherms indicates t h a t true double constant temperature b:itti for the isothermal presiurc~-cotii~~ositici~i study and the pressiire-tei~~per:it~~rc equilibrium has also been reached in the absarption studies a t fixed ctmiposition. The inner b:itli consists of a isothertn of this paper. Equilibrium, as revealed 1Ol)O-cc. i n d a t e d Pyrex beaker equipped with :L Traiisite by the constant pressure portion of the isotheriii, lid and pri~vitledwith :Lstirrer, calihrated platinum resisthas never reportedly been obtained for the a b : L I I C ~ therinornrtcr, :in unc;ilibr:Lted platinuni resistancis tlierrnoineter, a r i d L: six-\v:itt light bulb. The uncaliliratctl sorption of hydrogen in palladium below 1 (io3 resistance thcrrnonieter w a s used to control the temper:iture without it heat treatment. The reasoii for the by means of a K'heatstone bridge arrangement in which miy lack of hysteresis in the present in\-estig;ttioti is unbalance of the bridge resulting from :i temperature change in the bath of 0.001' or more is detected by :I tl-pc undoubtedly inherent in the extreniely finely tlivitiecl state of the palladium black used. Pressure I< galvanometer. A light b e ; m reflected froin the galvunometer mirror activates :L photocell relay circuit which equilibrium for the isotherm mas obtained in two turns (1x1 or o f f thc six-nxtt bulb. (;lycerol i m s iised as the to six hvurs after addition of a h>-drogen incremediurn in hotli inner a n d outer liath5. The iiutrr h i t 1 1 ment. 'Three of the experimental points of the n ~ i scontriillcd by means of i i itiercury tlieriniistat a t :t fcw isotheriii were obtained :Liter allowing the pressure tentlis of ;I degree lielii\r tlie teinpefi~tureI J f t h e iiiner Ixitli. Measured fluctuation.; of teinpcr:tturc i n tlic iilnvr h t l i to come to equiliLriuiii overnight, but the pwssurc' ;~rniiuntedto iO.Ol)2". increased only 0.02 iniii. after the secoiitl hour. Heats of a h i r p t i o t i a n d tlcsorption \wrc inca.;ured in the The pressure during the horizontal portion of the same gold calorimeter vessel after it had been incorlxirated isotherm varies only 1 0 . 2 0 r n m from ai1 average in ;in usenibly designed for adiabatic operation with high vacuum insulation. This adiabatic assetnhly is deicribctl of 17.;33 nim. This value is 1.4.5 inni. (1F = 51 in the follon-ing paper.' Temperatures arc measured with LL cal.) below the \ d u e obtained by Gillespic :uid strain-free platinum resistance thermometer, Pt-G-5, Hall. based on the International Temperature Scale. The calo*After sufficient hydrogen had been absorbed i l l rimeter was held adiabatic during the addition of gas and until equilihrium was established, or 111 m a n s casvs, the palladium to pass the second break in the iso~ i i i t i lpressure and temperature chattgei were small etlough therm, about 1 2 5 of the hydrogen was tlesorbetl. to allow extrapolation to a11 ;tpprosimate equilibrium. The pressure fell to a low \ d u e quite rapidly, a n t l The hrat rif absorpticm per rririle of hydrogen i.; calculated the calorimeter had to be heated to SO" in ortler to friiin the same eupresstoii usetl for heats of atlsorptiiiii, desorb such a large quantity of hytlrogen. This namely\
desorption curve, as shown in Fig. I , is identical with that obtained by Gillespie aiid Hall on tlesor1)tion of hydrogen where the 360" heat treatment was not previously applied. No increase iii pressure where A I ' is the temperature rise, 71 is the number of rnoles occurred over R period of four days after desorptioti of hydrogen absorbed, A i ' is the change in pressure during absorption antl V C a is ~ the dead space volume of the call)- of Iiydrogeri. Ry heating the system agaiii ant1 ritneter. The calorimeter was cooled between increttients of holding it a t SO3 for 24 hours, the p r e s s u r e alter hydrogen added so t h a t the mean temperature for e:tch rise the system w a s cooled to 30' differed frciiii t h a t was about 30'. The mean heat capacity of the systetii before before the second heat treatnlent by oiily 0. I O i i 1 1 i i . : i d aftcr ahsorption of hydrogen, C, was interrmlatetl ti) the itteiiti temperature fiir the temperature rise. The ~neasuretl Iieabsorption of hydrogen raises thc prcssure to : I heat capacities have been corrected for the heat of desorption, point where it coincides with the origiiial :ibscirl)\vhich is assumed to be equal in magnitude to tlie measured tiori isotheriii. .I second set of tlesorptioii m t l heat of absorption b u t negative in sign, and the heat o f reabsorption isotherms is shown in Fig. 1 . Isosteric Heats of Desorption.-The palladium (6) W. F. Giauque and H . L. Johnston, T r r r s J O C R X A L , 51, 2300 hydride a t a composition of about 0. (1929). ( 7 ) Donald 11.S a c e a n d J . 6. Aston, ibid., 79, 3023 (1957). hydrogen per atom of palladiunl was h (8) G. L. Kington and J. G . Aston, ibid.. 73, 1929 (1951). I n and pressures were measured at ;L se equation 1 a term, due t o temperature rise, has been neglected since peratures froin 4.5 to 7.5". The temperature w a $ s el]. I 1 this term is insignificant. This is t h e term ~ ? , K ( a 7 ' / 6 > r , )in controlled with an accuracy of 0.002' a n ( l
