Jan., 1962
CHEMISOR~PTION OF OXYGEN AND ELECTRON SPINRESOXANCE OF ZINC OXIDE
Heats of Vaporization.-The heat of vaporization of 2,2-diethoxypropane (7.61 kcal./mole) was determined from the variation of its vapor pressure with temperature, using a Smith-;CIenzies1O isoteniscope thermoregulated to =k 0.01". The values are shown in Table VI. -4ssuming that entropies of vaporization of the two similar ketals are equal, the heat of vaporization of 2,2-dimethoxypropane is 7.03 kcal./mole. The error introduced in this calculation by the above assumption is likely to be very small.11 The Additivity Group Theory and Calculated Heats of Formation.-From each of the heats of formation of the two ketals a d the available group combinations we obtain the average value of the new combination [C-(O),(C),]-2 [C-(0) (H)g] (g) = 7.15 =k 0.75 kcal. Thus the calculated heats of formation (--109.1 and -127.4 kcal./mole for 2,2-dimethoxypropane (1) and 2,2-diethoxypropane (l), respectively) are within 0.8 kcal. (0.6%) of the experimental heats. The theory is based on data (LO) Reference 7, p. 436. (11) G. PJ. Lewis and M. Randall, "Thermodynamics," 2nd Edition, McGraw-Hill Book Co., Xew York, N. Y., 1961, p. 519.
99
with uncertainties of comparable absolute magnitude. Calculations for other ketals are useful, particularly in the absence of experimental heats. Table VI1 includes heats of formation based on the average value of the above group combination. The calculated heat of 2-methoxy-2-butoxypropane (1) mas checked by measurements on an unpurified sample yielding a value of -132.2 kcal./ mole. The difference is 2.3%) and it is beliered that in this case the calculated heat is more reliable. The group property theory in conjunction with relatively few experimental measurements predicts quite reliable values of thermodynamic properties of many compounds. The theory is valuable when experimental measurements are not feasible. 2,2Dihydroxypropane of Table VI1 probably would offer serious calorimetric difficulties, since it is unstable. Acknowledgment.-We wish to express our gratitude to Mr. Lloyd Peak for carrying out some of the measurements, to Dr. Sidney W. Benson for fruitful discussion, and to the Research Corporation for financial support. -
THE ISFEUESCE OF CHEMISORPTIOS OF OXYGEN ON THE ELECTRON SPPS RESONAKCE OF ZISC OXIDE BY R. J. KOKES Department of Chemistry, The Johns Hopkins University, Baltimore 18, Maryland Reeeioed J u l y $6, 1061
Amounts of oxygen chemisorption have been determined on preparations of zinc oxide prepared by ( a ) decomposition of zinc oxalate, ( b ) decomposition of zinc carbonate and (e) oxidation of metallic zinc. The results indicate the existence of two types of oxygen chemisorption, one appearing a t room temperature and the other appearing a t 200" or above. Zinc oxide, doped or as is, has been shown to have an electron spin resonance signal that reflects the concentration of free electrons. The effect of low and high temperature oxygen adsorption on this signal has been examined as a function of coverage. Interpretation of the results leads to the tentative conclusion that the low temperature form of chemisorption occurs as 0 ions formed by reaction with conduction electrons, and the high temperature form occurs as O= stabilized by migration of donors to the surface.
Introduction Many of the chemical and physical properties of zinc oxide are profoundly influenced by the chemisorption of oxygen. When zinc oxide is used for photocatdysis,l such as the production of hydrogen peroxide from water and oxygen, the presence of chemisorbed oxygen is an obvious factor. When used as a hydrogenation c a t a l y ~ zinc t ~ ~0xid.e ~ must be pretreated a t elevated temperatures either i n vacuo or in hydrogen to remove oxygen, a catalytic poison. Other properties such as conductivityJ4photoconductivity and lumiiiescence,j and sintering ratoe6are influenced to some degree by oxygen chemisorption. But in spite of the fact that oxygen chemisorpt'ion plays such an important role in determining the properties of zinc oxide, (1) G. M. Sohwab, Advances in Catalysis, 9, 229 (1957). (2) J. F. Woodman and 13. S. Taylor, J . Am. Chem. Soc., 62, 1393 (1940). (3) E. Mol.inari and G. Parravano, ibid., 78, 5233 (1953). (4) D. J . M. Bevan and J. S. Snderson, Discussions Faraday Soc. 8 , 238 (1960). ( 5 ) G. Heiland, E. Mollwo and F. Stookmann, Solid State Phys., 8 , 191 (1959). (6) T. J. Grray, J. Am. Ceramic Soc., 37, 534 (1954).
only a few studies have been made in which the amount of oxygen chemisorption has been determined.'-lO The nature of this chemisorption still is not clear. In this present work the effect of oxygen adsorption on the electronic state of the zinc oxide has been studied by the examination of the electron spin resonance spectra. Experimental Catalysts.---ZnO-I was the 8.P-500 pigment manufactured by the New Jersey Zinc Company by burning zinc in air. Representative analysis showed that the concentration of paramagnetic ions in this sample was less than 1 p.p.m.; the concentration of aluminum, known to function as a donor, mas found to be less than 10 p.p.m. The surface area of this catalyst repeatedly degassed a t 550" was 3 m.2/ g.11
(7) E. R. S. Winter, Aduances in Catalysis, 10, 196 (19%). ( 8 ) S. R. Morrison, zbmd., T , 25Q (1956). (9) T. I. Barry and F. 8. Stone, Pror. Roy. SOC. (London), 3356, 124 (1960). (10) T. I. Barry, Paper KO.70, International Conference on Catalysis, Paris, 1960. (11) It ha3 been noted by Xesavulu and Taylor [ J . Phys. Chem , 64, 1124 (1960)l that ZnO-I evacuated a t 360' for 26 hr. retained large amounts of carbon dioxide and RTater. They claim that as a result there is no chemieorption of hydrogen between 30 and 360O. We have
R. J. KOKES
100
;i 14
ol 0
I
I
200
400
t
I
I
600
800
1
"C.
Fig. 1.-Isobar for 2.5 g. ZnO-111. The sarriple was calciricd in air 16 hr. a t 1000". For thc data indicated by the opcn cirrles ( P = 50 p ) the sample was degassed 16 hr. a t 1000°; for thc data indicatcd by the closed cirt4cs ( P = 20 p ) the sample was degassed 16 hr. at 800".
- - - - . - h / t d x H
'dt i
---3
Vig. 2.-Spectra of doped arid undoped ZnO-I: A, ZnO-I (0.5 mole yo Ga) x 1 scale; B, ZnO-I undoped X 1 scale; C, ZnO-I undoped X 13 scale; 13, ZnO-I (0.5 mole % Li) X 13 sralo. The ordinate, d x j d H , is a recorder trace of the derivative of the absorption expressed in arbitrary unite. T h e s w c ~ pamplitude was 1.5 gauss. ZitO-I1 was prepard by the decomposition of zinc oxalate in air a t 500" followed by c:tlcina,tion at 500" for 16 hr. %no-I11 was prepared by the hrating of zinc carbonate in 0sygc.n at 500 . Doped samples with lithia, alutninn or gallia were prepared by slurrying 2 g. of zinc oxide with 4 cc. of the approprinte nitrate solution of the ion being added. The resulting slurry was evaporatpd to dryness and calcined. In every batrh prepared, a blank was run in which zinc oxide was slurried with distilled aster, evaporated to
Vol. tiG
dryness, and calcined a t the same time ati the doped eatalysts. Adsorption Experiments.-Adsorption experiments were earrird out in u volumetric system equipped with mercury cut-offs and designed for studies a t low pressures. Thc sample container was made of fused silica to permit high temperature studies. Studies of itdsorption as a function of temperature were carried out by admitting a measured amount of oxygen to the sample a t or below room temperature and measuring the amount adsorbed after one hour a t cach of a serirs of higher temperatures. Spectra studies mere rarried out on weighed samples of catalysts (about 0.2 g.) in 2 mm. i.d. fused silica tubes that were sealed to the sample container used in adsorption studies. After evacuation or adsorption, these tubes were sealed off and the spectra werc detcrmincd. In all studies of oxygen adsorption the pressure when the tubes were scaled off was less than 2 p . Electron Spin Resonance Measurements.-All Spectra were obtained with the Varian-4500 spectrometer at a frequency of 9200-9525 i l k . This instrument yields a recorder trace of the derivative of the adsorption curve. The center of the signal and the half-width (peak-to-peak distance on the recorder trace) were determined by field measurements with a proton probe. At the power levels used, saturation effects were absent. The number of spins/g. sample was determincd by comparing the integrated intensity of the signal from zinc oxide to that from a known amount of di henylpicrylhydrazyl diluted 1000:1 with ZnO-I. Initiali)y, this comparison was made by double integration of the recorder tiace; it later was established that the same results were obtaincd by assuming that the integrated intensity was proportional to the height X (peak-to-peak distance).$ Unless otherwise specified all spectra were obtained a t 24'.
Results Adsorption.-Figurc 1 shows an isobarI2 obtained for ZnO-I11 calcincd and dcgasscd a t high temperatures. Results for ZnO-I1 and ZnO-111 were similar insofar as they showed a minimum amount of adsorption a t 200-300" and the absence of any maximum a t the higher temperature. This behavior was observed for catalysts degassed a t tcmperatures as low as 500". Other observations that have a bearing on the nature of this chemisorption are the following. (a) The reproducibility of adsorption for cvacuated virgin catalyst samples was fairly good, but if thc catalyst was exposcd to osygcn for a long time a t 550", the oxygen adsorption was found to be reduced even after cvacuation for 16 hr. a t 550 O . (b) Adsorption bctn-een -100 and $100" was esscntially complete in 30 min.; at 400" an initial rapid adsorption was followed by a slowcr adsorption that continued even aftcr several days. (c) The presence of pre-adsorbed osygcn put on at 400" has little effect on the oxygen adsorption a t 25". For example, a sample of degassed ZnO-I adsorbed 0.9 pl.,/g. at room temperature. Adsorption of 2.5 pl. of osygcn/g. a t 400" reduced subscqucnt adsorption at 23" to 0.7 pl./g. Extrapolation of these and othcr data indicate that i t would take 15-20 kl./g. of osygcn adsorption a t 400" to blockout, adsorption (-1 pl./g.) a t 2 5 O . Resonance Spectra of Various Catalysts.Table I summariecs the rcsults obtaincd for various catalyst prcparations. All spectra were ob-
found that after evaciintion at 55O0 for 16 hr. ZnO-I does n d w r b
1ivdrolrr.n in amounts comparable t o thoar observed by Kesavulu and Taylor for zinc oxide from zinc oxalatr. (We found -0.01 cm.*/ni.' at 1 miit. coiiipared to their findina oi 0.01 i ~ i n . * j i n . ' nt An mm.) Thus. it mould srrin that the residunl carbon dioxide nnd antcr is not a [)roblent under our condXons of pretreatment.
(12) I t is apparent from the method of measuremrnt that the data do not truly represent isobars innamuch a8 the preasure vaned 801116" what. Run8 wherein the prrauurr variation was negligible showed that the shape of these curves was little inlluenccd by changes in pressure.
,Jnn., 1902
~ I I E M T S O R P T I O NOF OXYGEN AND I;'LECTRON SPIX
tained a t room temperature and the signal observed occurred a t g = 1.06. The reproducibility of tho runs with ZnO-I degassed a t 550" is indicated by the first three rows of Table I. If the catalyst is examined as is or briefly degassed a t room temperature (fourth row) no signal is observed, but after prolonged evacuation a t room tcmperature 8 faint signal is delcctable. The results obtained for the blank in the doping experiments show that slurrying with distilled water has little effect on the signal; hence, changes duc to doping are the rcsult of thc added ion. For purposes of illustration the spectra obtained with iindoped and doped ZnO-I are shown in Fig. 2. T h e horizontal scale is the same in all cases; the half-width for curve C is 7.1 gauss. TABLEI ELECTRON SPIN RESONAKCE SPECTRA OF VARIOUSCATALYSTS'
Sample
Pretreatment b
ZnO-I
Degassed 550' DcAgassed 550' Degassed 550" Ihgassed (5 min.) 25' Flurried with distilled water; dried and degassed 550' 1)oped 0.5 mole % Li deg:tsscd a t 550" Doped 0.5 mole 76 Ga degassed
%no-I
ZnO-I ZiiO-1 ZnO-I ZnO-I
%no-I
Spins/g.
x lo-"
2 3 2.2 2.4 : 10'6//g.)
IO 0
I NI-
0
5 0
D
c
v) v)
00 00
I 5 0
I
I
10 0
15.0
00
1-11 / g Fig. 4.-Effect of oxygen adsorption a t 400" on electron spins/g., Run K28; spin resonance a t room temperature: 0, e, spins/g., Run K27 heat treated at 400'; A, half-width of adsorption signal. Dotted line corresponds to a decrease in the signal of one spin/adsorbed oxygen atom. Solid line for integrated intensity plot is calculated on the basis indicated in the discussion. (& m'as assumed to be 9 >: 1017/g.; this corresponds to K , = 3.2 X 101D/cm.3.)
tures. He assumed that the low temperature variety (henceforth called type A) n-as adsorbed 0- ions and that the high temperature variety (type 3)was adsorbed 0- ions. Barry and Stone.9 in opposition to Morrison, postulate that type A adsorption occurs as 0-ions and type B adsorption occurs as 0- ions that may be stabilized by the migration of excess zinc to the surface under the influence of the boundary layer p0tentia1.l~ The data in Fig. 1 confirm the existence of type A and B chemisorption. The fact that no maximum in the isobar occurs at high temperatures is consistent with the supposition of Barry and Stone that migration of excess zinc, an activated process, accompanies type B adsorption. The slow adsorption above 400" that persisted for several days and the partial irreversibility of the adsorption also are consistent with a process involving bulk diffusion. The results of adsorplion experiments substantiate the suggestions of Barry and Stone, but little can be said about the charge of these adsorbed species without measurements of the change in the (13) K. Hauffe, Advances zn Cotal~sms.7 , 213 (1965).
Vol. 66
electronic state of the solid. The electron spin resonance signal has been shown to reflect changes in the number of free electrons. On the basis of the data in Table I, there can be little doubt that, the signal observed for zinc oxide a t g = 1.96 is due to un-ionized donors and/or conduction electrons. (Similar observations have been reported for other semiconductors.l 4 - I 6 ) One cannot rule out the possibility that unionized donors contribute to the signal, but the lack of splitting due to the nuclear spin of gallium and aluminum for the doped samples together with the increase in the signal during ultraviolet irradiation of undoped samples are consistent with the interpretation that the signal is due to conduction electrons. On the above basis the data in Fig. 3 and Fig. 4 can be interpreted in a straightforward fashion. In undoped zinc oxide conduction electrons are assumed to be produced by the follom-ing equilibrium
where (e-), (Zni) and (Zni+) are the concentrations of conduction electrons, interstitial zinc, and interstitial zinc ions, respectively. n The assumed mechanism of oxygen adsorption can be represented as
+ +
Type A: '/20? e- -+ 0,Type B: 0,Zn,+ --+ Zn,++ 08
where 0,- represents an adsorbed ion and Zn,++ Os= represents an adsorbed O= on stabilized by the migration of an interstitial zinc ion to the surface. For the degassed catalyst
Where (eo-) is the electron concentration in the absence of adsorbed oxygen. Provided the ioiiization of Zni is only partially complete a t 25", we can write (-Td)
= (Zn,) + (Zn,+)
0
(Zn,)
and hence, (eo-)2 = K(1j'd). With the same approximation we can indicate the effect of adsorption on the species (Zni), (Znif) and (e-) as Tvw A adsomtion:
Type B adsorption: ( h i ) = ( N d ) - (Zni++Ob) - (e-) or (Zni) = ( N d ) - (Zni-+O,=) (Zni+) = (e-)
In the above, the amounts of adsorption are expressed as adsorbed ions per unit volume of catalyst. When these values are substituted in t'he equi(14) 6. Feher, P h y s . Rev., 114, 1219 (195g). (15) G. Feher, D. K. Wilson and E. -4.Gere, Phgs, Rev. Letters, 8, 25 (1959), (16) J. Lambe a n d C. Kikuchi, J. Phys. Chem. Solids. 8 , 492 (1959). (17) Recently, it has been suggested t h a t the conductivity of undoped zinc oxide may be due t o donors other than interstitial zinc. [D. G. Thomas, ibid., 3, 229 (1957).] Clearly, the above discussion mould be applicable m e n if the donors were oxygen vacancies.
,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 at 400” than when it is put on at 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 cnc~gyfrom 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).
