,Jan.. 196%
~’OLIJhIlC CIIANGE OK AIIXING I N BINARY
LIQUIDALKALINITRATES
103
librium cxxprcssion and this is solved for (e-), we find where in* is the cffcctive mass of thc electron, fi is I’lanck’s conslalit, 7’ is the absolute tcmperaturr and k is Boltzmann’s constant. Rrportcd ~.a!iic.s of m* range from 0.1 to 0 . 5 . 5 This leads to a v:~luc> of &d betwcn 0.1 and 0.2 C.V. \'slues based on conductivitirs of powders are roughly 0.1 C.V. or Type 13 adsorption: 1 ~ ~ 8 5hence, ; thc values of K and ;Ird a r not ~ unreasonable. Finally, it should be noted that the ahovc picture offws an explanation for the small c4’fTtict of type u-hcrcin ’ivc have made usc of thc relation KN,i = I3 adsorption on T y p ~A adsorptioii. Type A (00-)2. adsorption is an examplc of deplctiw cheniisorpSincc. (ti-) and ( c y - ) arc presumably determined tion and as such is governed by the nnniber of by 1he integrated intensity of the signal, the derived free electroiis and the. boundary layrr potential. expression for type A chemisorption, which con- According to the derived equations type B adsorptains no adjustable parameters, completely de- tion affects the number of free cxlectronsfarlws than scribes the data in Fig. 3. The solid line in Fig. 3 type A adsorption; hence, the dccrcase of type A is calculated on this basis. The above expression adsorption by the prescwce of pr+adsorbcd oxygen for type B adsorption specifics the data in Fig. 4 should he far less if the pre-adsorbed oxygcn is i n trrms of thc adjustable parameter ( X d ) ; the put on a t 400” than when it is put on a t 25”. solid line in Fig. 4 is that calculated for Ard = Acknowledgment.-We :ire grateful t o Mrs. Olga 9 x 1017 electrons/g. Shaffer who ran some of the spectra reported herein. The viilue of K dctermincd by the above valiie Acknowledgment also is made to the donors of the of .Vd is 3.2 x IO1; cm.-3. It is possible to check Petroleum Research Fund, administered by the the validity of this valiic by computing the value American Chcmical Socicty, for support of this rcof the donor ionization E,i c n c ~ g yfrom thc formula5 search.
THE
nxrm
CI-IALVGE ON MIXING IN sonm BISARY LIQUID ALIcfu,I KITRATES“ 131- 13. I;. POWIXS,
t
~
I,. K A T Z AND ’ ~ 0. J. r(LEI>P.k .
Institute foi th? S t i i d i j of .Iletnls and thp Department of ChPrnishi, ?‘he TJnzoei s t t i / of Chicago, Chicago 3Y, Illinozs Rezezied July IS, 1961 ‘I’hci voli~nic~ cilitlngc on mixing has bwn mumiircd i n liquid mixtures of sodium nit>ratewith lithium, pot:tssium, rubidiiim md cwiuin riitratw It is foritid that the ex(~essvolunics are all positive. To a firsti:~pl)roxirti:itioiithey mny be rrprcscnted 1 ) t,hc ~ ~mpiri(*aI rcla1.ion AVC = i - 2 . 2 X 10: X ( l - X ) [ ( d ,-,&)/(di f &)I4 cc./rnole. Hcre X is the molt? fracation in tht: mixtiire, n-hilo ti, :tnd d, i ~ r ethc intcrionic. dist:tncc:r char:tcter~sticof the two pure salts.
Introduction In a rcccnt conimunicalioriZ the aiiihors have
Unfortunately, our attempts to study the othc.r systems which have lithium nitrate as a common component so far have hwn inconclnsive due to the thermal instability of this salt.
r r p o r t d soin(’ now information oil the volume chang:~on mixing in liquid mixtures of sodium nitratc.-potassium nitrate. Rased on 21 siicccssful Experimental and Errors expcrimenIs at &j0 and 425” we found that the The equipment risrd in tlir prment work is a modified EXCPSS .\-oliirric>ii n this systrm arv positiw and, and iniprovcid vrrsion of L L I ~apparatus rwent ly developed within our precision, independent of temperature. by on(^ of the authors for dctmniiiation of thc volumc chiingc I he mxximum oc(’i~rsnear the 50-50 compoqitioii on mixing in liquid :illoy SI stcms.3 The principal fraturrs will he readily undrrstood from the schematic^ diiigmin givrn a i d is +0.07 0.02 cc./mole. Fig. 1 of ref. 3. T h r follo\~ingmodifirations werr ~ i s d c . In the pr-sent communication we give a more in Thc ball and socltrt joint (on the a ~ i CC s in the earlier dciailcd rc~porton our study of the volume change version) has been replncrd by a sttlnditrd taper joint. This on mixing i n binary alk;ili iiitratc sysl c>ms which permits rocking of the V-tiilw ahoiit t h r standard taper joint. open cnd. manome1c.r to 1 1 : ~sodium ~ iiitratt. as a common romponent. it drterniiiiation of t Iir Our results indicate that systems w.liic-hdo not have sodium nitrate or lithium nitrate as one of thc comThc coiirsr of n typical cxpriirnciit is as follows: The poriciit s wi!l havv voliimc. changes on mixing which purr salts are mrllcd and cast into sticks of R diameter arc’ too small to bc determined by our tec.hiiiqnr. slightly smallrr than Ihe insidr di:bmrtcr of the Pvrcx UI ,
(1) (a) R ork supported by the Ofice of Ni~valRrwnrch a t the Criivcrsity of Cliirnao tinder Contract No S o r i 2121 ( l l ) , (h) Ameripan Cheinical S o r i c t y P ~ t r o l r u n lRerearrti Fund Prrdoctoral Eellow. (2) J. L. 1Catz. B. F. Powersand 0 J. Klrpps. J . C h e m . I ’ h y s , 35, 705 ( l 0 t l l ) .
tube. The two salts arc wciglird (total amount about I/? the gram molwular w i g h t ) and plitccti in the two “lrgs” of the crll. These then are scaled off and the cell inimcrsrd in the salt-bath. ______-
(3) 0 J. R l ~ p p J~ .Phys Cliem , 64, 1512 IIOe(1).
B. F. POWERS, J. L. KATZAND 0. J. KLFPPA
104
Vol. 66
TABLE I BINARYLIQUID NITRATESYSTlillS INVOLVING SODIUM NITRATE
EXCESSs’OLUMES
IN
Temp., OC.
Inert
350
Ar
425
Ar
(Na-Li)PiOa
310
Ar
(Xa-Rb)NQ8
340
Ar
System
(Na-K)X08
gas
J
J
/’ 1
Mole fraction, XKsxoa
0.3-0.7
AVE/X(I - X ) , cc./mole 0.26 & 0.08 (mean
of 9 expt.) 0 . 3 - 0 . 7 0.28 5Z 0.08 (mean of 12 expt.) 0.51 0.26 ’1 .51 ’25 0.26 5Z 0.02 .50 e28 .51 .24 0.80 0.61 1 .50 .50 .49 0.52 .52 .51 0.66 .62 1.45 .58 1’41 > 1.37i0.12 1.49 .46 -38 1.21 1.16, .35 0.51 0.51
}
1
He
425 [(d,-d2)/(d,fd21]4x
Ar
lo5.
Fig. 1.-Dependence of the excess volume (AVE/X(l X))on the parameter ( d l &)/(dl d2). Open symbols, argon atmosphere; closed symbols, helium atmosphere.
-
+
To remove dissolved gases from the two molten salts the cell is maintained under a floor pump vacuum overnight. It then is filled with an inert gas (argon, helium) and connected by means of the 3-way stopcock (A) to the octoilfilled 0.1 cc. constant pressure gas “buret.” After a constant volume has been achieved in the cell-capillary -buret system the two salts are mixed by gently rocking the cell for about 5 min. One or two additional 1 min. mixing periods were used to check for completeness of mixing. After mixing, a new constant volume is established in the system, and the excess volume is calculated from the volume increment. For further details on procedure and calculation the reader is referred to earlier work.8 Most of our experiments were carried out with argon as the inert gas, although helium was used in some cases. The lithium, sodium and potassium nitrates were Mallinckrodt Analytical Reagents, and were used without further purification:‘ The rubidium and cesium nitrates were purchased as 99.9% pure or better” from the Millmaster Chemical Corporation, and were recrystallized once from distilled water before casting. In order to avoid possible thermal decomposition of the lower melting salt we adjusted the operating temperature of each series of experiments to a value slightly higher than the melting point of the higher melting salt (310” for lithium nitrate-sodium nitrate, 340” for sodium nitrate-rubidium nitrate and 425’ for sodium nitrate-cesium nitrate). The experimental results recorded in Table I adequately illustrate the magnitude of the random error in our experiments. However, they of course give no hint as to possible systematic errors. It is believed that the only potentially significant systematic error would arise from the possible difference in the solubility of the inert gas in the pure salts, on the one hand, and in the salt mixture on the other. Very few data are available for the solubility of rare gases in molten salts and their mixtures. Protsenko and Bergman4 report a solubility of xenon in the sodium nitratepotassium nitrate eutectic of about 10-7 mole Xe/cc. solvent in the temperature range 260-450’ a t 1 atmosphere. In observations on fluoride melts Grimes, Smith and Watsonb found the solubility of argon to be 2-3 times that of xenon, while that of helium is about 13 times that of xenon. If we assume that these ratios apply also in the nitrate melts, we estimate a maximum total solubility of about 0.8 (4) P. I. Protsenko and A. G. Bergman, J . Gen. Chem. U.S:S.R., 20, 1365 (1950). (5) W. R. Grimes, N. V. Smith and G. M. Watson, J. Phus. Chem., 62, 862 (1958).
He
cc./mole of argon and about 3 cc./mole of helium in our fused salt mixtures. In the course of the present investigation we have been able to confirm that the solubilities are of this order of magnitude. On the other hand, we have been unable to find solubility data for the considered inert gases in the pure fused salts, and to make numerical estimates of differential solubilities. However, since the actual solubility of helium in our salt melts presumably is about 4 times that of argon, its differential solubility also might be expected to be much larger than that of argon. Fortunately, our experimental results for sodium nitrate-rubidium nitrate and sodium nitratecesium nitrate, where we used both argon and helium as inert gases, give no indication of a large difference in differential solubility. Therefore, we conclude that our reported results are not modified significantly by differential solubility of the inert gas.
Results and Discussion Our experimental data for all the four binary systems covered in the present study are presented in Table I. It will be noted that the excess volumes in all cases are positive, the values of AVE/X(l X ) ranging from about +0.26 cc./mole in sodium nitrate-lithium nitrate and sodium nitrate-potassium nitrate to t 0 . 8 in sodium nitrate-rubidium nitrate and 1.5 in sodium nitrate-cesium nitrate. Thus, it is apparent that the excess volumes increase quite markedly with increasing difference in size between the two participating cations. It is recalled here that in a recent study of the heats of mixing in the binary alkali nitrates, Kleppa and Hersh6 found that these systems are all exothermic, and that the enthalpies of mixing to a good first approximation obey the semiempirical relation
+
AH^ = -140X(1
- X ) [ ( d l - d z ) / ( d ~f &)I2
kcal./mole
I n this expression dl and dz are the interionic (6) 0. J. Klepps and L. 9. Hersh, J . Chem. Phys., 34, 351 (1961).
distances characteristic of the two pure salts. Thc niimcrical factor (140 kcal./molc) is of the ordrr of magnitudc of tlie lnt,ticc energies of the salts. The simplicity of this equation has aroused consider:il)le interest. I t bcgs the question whether equally simple relations may apply for the oiher cxccss thcrmodpnamic functions, and notably for thc cxccss volumes considered in thc present work. In ordcr to obtain an a n s m r to this question TW h a w plotted our excess volumc data against diffcrent powcrs of the parametcr (d, - d2),/(dl+d2). It‘igurc 1 shows that wc obtain a reasonably good straight line that passcs through the origin in a ploi, against the fourth power of this parameter. This s i q e s t s for the excess volumc an empirical rc.Iation ol thc type AITi =
For
t h c k
+V’x(l - A’)[(dr - d z ) / ( d i -k
&)I4
considcred alkali nitrate mixtures, the
value of the numerical constant V’ is about 22,000 cc./mole. It, was shown by Longuet-Higgins,’ in his thcory of conformal solutions, that any first-order solution thcory will predict the s m e sign for all the excess thermodynamic functions (APE, AHE, ASE and A P ) . Thus, our volume and enthalpy data for the considcred alkali nitrate systems demonstrate that a satisfactory theory for them mixtures must bc sccond ordcr or higher. An attcmpt has been made recently to account for the observed enthalpies of mixing by mcans of sccond-order conformal solution tlieory.* However, no similar attack has becn madc as yet on the problem of the excess volumes. (7) 11. C. I,onguet-€Tingins, P r o r . Rov. Soc. (London), A205, 247 (1951). (8) 13. Reiss, J. L. Katz and 0. J. Kleppa, J. Chem. I’hus. (in press).
PERTUItBATIONS OF THE NICKEL METAL I< X-RAY ABSORPTION EDGE DUE TO SNALL CRYSTAL SIZE ,4XD IlYDROGES CHEAfISOllPTION BY P. 11. LEWIS Tcxaco Research Center, Beacon, New I’arlc Receztrd Julv SI,1961
‘The X-ray absorption edge of small nickrl crystals (ca 30 A.) has been found to have small pcrturbutions from that of bulk nic-ltel. Thc pertiirbation has tlilTerctit chractciristics from those obscrwd whcn gases arc adsort)cd ’Yhe small crvsr,al pwtnrbationc, have h n tentatively associated with the posr-ession of atom-like energy statcu. ‘ l h effert of chemisorbed hydrogen on the electron-empty energy levels of the small cr) st:d nicbel IS q u d i t ativcly the same as that of c~liemisorbrd osygen, but quantitatively about half ns small. It is suggestcd that thc changes in cnutalyst absorption edge spectrum due to gas chelmsorption are better correlutcd u ith the gas molcculcs spicading surface ut o m apart rrtthrr thaii n ith an increase in potential ficld about each stom.
Introduction since they have no clectrorls in them. It is with Although catalytic reactions on the sixfaces of these last bands that this paper will tic concerned. small metal crystals have bcen studicd extensively, The X-rav spectral methods measure the density the knowlcdgc of many fundamental aspects is of cncrgy levels, the extent of electron filling, and still incompletc. Little is known about horn the their angular momentum quantum niimber. The first object of this paper is to show the diffcrenergy lei& of these crystals tire affected by their crystal sizc, their contact with support niai crials or ences bctween the I< X-ray absorption edge of with adsorbed gases. X-Itav spectroscopy can be very small nickel crystals and that of bulk metal. used to study those energy levels (closely grouped The second object is to compare the catalyst nickel to form bands) that arc affected when the metal X-ray absorption edge after the chemisorption of crystals are small enough (ca. 50 A. or smaller). hydrogen with that after the chemisorption of Then the atoms on the surface form a sufficiently oxygen. The similarities found in this comparison large fraction of the total so that perturbation ef- tend to lead to a unified picture of t,hc effect of fects arc iiot diluted bcyond observation by h r g o chemisorbed gas on a metal. Since the mathematical methods used to study numbers of unaffected interior atoms. The bands that arc affected are in an encrgy rcgion roughly 40 the X-ray absorption edge were riot conventional, C.V.wide and, for nickel, 8 k.e.v. above the Bohr a description of thcse preccdcs thc experimental IC-level. I h r g y levels in the lower 10 e.v. part resid t s. Use has been made of X-ray absorption edge contain valence electrons For nickel metal thcse valcnc(J electrons occupy t hc 3d and the lower part spectra to mcnsure solid state characteristics of of the I s band. Sincc thcse bands contain elcc- catalysts. The valence stsatc of siipported transifrons, they should be studied using the profiles of tion metal oxides2 has been determined. A study X-my emission 1int.s. The energy bands in thc has been made of the absorption spcctnim 10-100 The problems uppcr 30 e.v. section: the higher enerqy part of the e.v. above the cdge dis~ontinuity.~ nickel 4s hand and the entire 4p band, can be connect cd with the obscrvation of small changes studied using met,al absorption edge’ measurements in the X-ray absorption spectrum of metals due to chemisorbed gas have been shown to be surmount(1) An absorption edge occitrs wlirn the X-rays have just solfirient enrrgy t o raise a n inner electron to the empty levels. .4 discontinuous change in abaorntion cw(tioient as a function of X-ray a a v c length characterizes tlie edge.
(2) (a) 11. P. Hanson and W.0. hlilligan, J. Phv8. Cham., 60,1144 (1958): (b) R. 0. Keelinq, Jr., J . Chem. Phua , 91,279(1959). (3) R. A. Van Nordntrand, Aduanceszn Catalums, 12, 1 4 0 (1960).
