2765
ADSOI~PTIOE OF OXYOEI’; O N SILVER
The Adsorption of Oxygen on Silver
by A. W. Czanderna U n i o n Carbide Corporation, Chemicals Diuision, South Charleston, West Virginia (Receii:ed dlurch 1.9, ILK.$)
The rato of adsorption and desorption of oxygen on silver powder has h e n iiieasured from -77 to 351” a t an oxygen pressurc of 10 torr employing a vacuum ultraniicro~~alatict.The ineasuretnents were made on ti silvcr surface on which both the total uptake arid the rate of adsorption were reprodueiMe a t any temperature and pressurc. Soininal activation energies of 3 , 8, and 22 kcal./inole which were tneasured are believed to correspond to dissociative adsorption, riiolcciilar adsorption, arid surface niobility of oxygen adatotiis, respectively. Data on thcriiiotlcsorption, vacuum desorption, and activation encrgy are corribincd to propose relativr surface coverages of the various adsorption types.
Introduction silver, like inany metals, undergoes both physical and chemical adsorption of gases. Nunierous studies of the physical adsorption of gases on silver have been made.’-3 Even mow investigations have been rvported on the rate of chemisorption of oxygen on silver. l--8 The best chernisorption data show that4--8 initially the rate of adsorption is very fast until the surface is covered or partly covered and then the rate decreases rapidly to saturation. Typical ranges of the activation energy reported for oxygen sorption on silver are 12-17,2 17-13,286 22,4 and 27-29 kcal./ t ~ ~ o l eThe . ~ wide ranges in values probably result froni calculations based on the adsorption a t diffcrcnt surface coverages on different saniples and a t different pressures. There are numerous literature references concerned with other aspects of the interaction between oxygen and silver. Those which appear to have a bearing on the interpretation of the data presented below will be introduced when they are pertinent. Howcver, in all cases of prior study of the adsorption of oxygen on silver, knowledge of surface coverages, reproducibility of chemisorption data, etc., have been somewhat incomplete. For the experimental work, a vacuum beam ultramicrobalance was chosen to study the adsorption of oxygen on silver. This instrument permits the ambient pressure and tmiperaturc to he varied over wide limits making it possible to nxasure froni the niasr changes that occur: (a) the saturation uptake as a function of pressure and temperature, (b) the rate of
adsorption and desorption, (c) the therniodcsorptiori of adsorbed species, (d) the surface area, arid ( e ) longtertii niass changes in the sample. It is possible to determine the heat of adsorption9 froni (a) and the activation energy of adsorption’” froiii (b). The desorption data of (b) cannot bc usrd for thc activation criergy of desorption if powders or films are cniployed.ll However, the desorption data of (b) and the data of (c), (d), and (e) provide important qualitative and historical information which are essential for assurance of reliability and reproducibility of the data. Although a comprehcnsivc study of oxygrn on silver has been conipleted, only data rrlating to the rate of adsorption and desorption will be presented in this paper.
Experimental J’lass changes were determined with a pivot-type beam ultramicrobalance. A detailed dcscription of the (1) IT. €1. Armbruster, J . Am. Chem. Soc., 64, 2545 (1942). (2) A . F.Deriton, ibid., 56, 255 (1934).
N. N. Knvtaradze, Chem. Abstr.. 17, 325b (195i). (4) \V. W. Smeltmr, Cun. J . Chem., 3 4 , 1040 (1950). (5) IC. W.It. Steacie, l’roc. Roy. SOC.(London), A112, 542 (1926). (3)
(6) 13. Taylor, 2 . p h y s i k . C‘hem. 1 3 o d t m h i n F ~ s t l i a n d 475 , (1931). (7) 31. I . Tcmkin and N. V. Kulkova, Z h k l . A k a d . . Y a ~ kS S S R , 105, 1021 (1955). ( 8 ) .J. N. Wilson, ct al., l’roc. Intern. (’onor. Surfmr. A c t i d ? / , B n d , London, 2 , 299 (1957). ((3) B. .\I. W. Trnpnell, “Cheniisorpt,ion.” Hiitterworths & Co.. London, 1955. g. 42. (10) B. N . UT. Trnpnell, i b i d . , p . 40. (11) R. AT. 1%’.Trapnell, ibid.. p. 79.
A. W. CZANDERNA
2766
construction, operation, calibration] and limitations of the pivot-type balance has been given.12 It was found helpful to modify the beam of the balance to facilitate automation. Details of this change will be published. l 3 Automation of the balance was based on the transducer method devised by Cochran’* and more details of the automatic operation are avai1able.l6 A schematic representation of the automatic operation is presented in Fig. 1.
I
I
I
Figure 1. Block diagram of the apparatus: 1, sample container; 2, balance beam; 3, piano wire probe; 4, transducer; 5, translator; 6 , amplifier; 7, servomotor; 8, servo speed changers; 9, Helipot 40 turn Model E; 10, compensation solenoid; 11, standard resistor; 12, bucking voltage; and 13, two-pen recorder.
With the automated balance employed in this study, the rate of oxygen adsorption could be followed after 20 sec., changes in mass of *O.l to +0.2 pg. could be detected in vacuo or in the presence of the ambient gas, respectively, and changes in total sample mass of the order of 1 pg. could be monitored. Vacuum in the system was produced with a twostage mercury diffusion pump and mechanical forepump. LIercury vapor from the diffusion pump was condensed in a trap cooled with liquid nitrogen. The limiting pressure obtained in the balance housing with this system was 1 x lo-’ torr. Gas pressures in the gas-handling section of the vacuum system were read on a mercury manometer, a two-stage mercury h9cLeod gauge, or a thermocouple gauge. A pressure of 5 X IO-’ torr could be obtained in the gas-handling system when mercury vapor was condensed in a Dry Ice trap. Stainless steel bellows -type valves with Teflon gaskets were used in the gas-handling system between the sample chamber of the microbalance housing and the cold traps. The .JoiLmal of Phyaical Chemistry
The gases used in this study were obtained from the following sources: oxygen was prepared by thermal decomposjtion of chemically pure potassium permanganate, dried with niagnesium perchlorate, and stored in bulbs; nitrogen was prepared by the thermal decomposition of sodium azide; reagent grade hydrogen and carbon monoxide were obtained in 1-1. glass break-seal flasks from the Air Reduction Co. The silver powder used was Fisher reagent grade, with a nominal purity of 99.97y0. l\lajor impurities included 0.02% sulfate and 0.005% chloride; minor metallic impurities were 0.002% lead, 0.001% copper, and O.O005% iron. The silver powder was loaded into a spherical sample bulb, approximately 1 cm. in diameter, containing a 2-3-mm. diameter hole a t the top and suspended from one side of the balance. A dummy bulb was suspended from the other side of the balance. Two hinged tube furnaces (HeviDuty Type 70) were mounted about the sample bulb and dummy bulb. The voltage input into each furnace and hence its temperature was regulated by powerstats. The latter obtained power from a constant voltage regulator. I n the absence of room temperature fluctuations, furnace temperatures were controlled to better than 1 The furnace temperatures were measured with calibrated ten-junction chromel-alumel thermocouples positioned between the furnace and balance leg, as near as possible to the sample and dummy bulbs. Prior to adsorption studies the actual sample temperature was determined as a function of the chromelalumel thermopile temperature reading with a thermocouple placed inside the vacuum system. Thereafter] the thermopile e.m.f. was measured and the sample temperature obtained from this calibration curve. Specialized techniques for determining the surface area from nitrogen adsorption at low temperatures have been described.16 The difficulty encountered in obtaining the rate of adsorption because of thermomolecular flow forces a t pressures of 0.001 to 20 torr has also been discussed.” Because of this difficulty,
*
~~
(12) A. W. Czanderna and J. A I . Honig, Anal. Chem., 2 9 , 1206 (1957). (13) A. W. Czanderna, proceedings of a conference held in Pittsburgh, Pa., May. 1964, to be published in “Vacuum Microbalance Terhniques.” Vol. I V , Plenum Press, Inc., New York, N. Y. (14) C. N. Cochran, “Vacuum Microbalance Techniques,” Vol. I , Plenum Press, Inc., New York. N. Y., 1961, p. 23. (15) A. W. Czanderna and H. Wieder. “Vacuum Microbalance Techniques,” Vol. 11. Plenum Press, Inc.. New York, N. Y., 1962, pp. 151, 152. (16) A. W. Czanderna and J. M. Honig, J . Phys. Chem., 63, 620 (1959). (17) A. W. Czanderna, “Vacuum Microbalance Techniques,” Vol. I , Plenum Press, Inc., New York, N. Y., 1961, p. 129.
2767
ADSORPTIONOF OXYGEN ON SILVER
the oxygen adsorption and sample reduction were carried out a t 10 torr to maximize the accuracy of the rate data.
Results The purpose of the initial experiments was to establish a method of preparation of a silver surface that retained the highest possible surface area, that did not change in surface area, and possessed reproducible “chemisorption behavior” as a function of repeated adsorption reduction cycling. Both reproducibility of the rate of chemisorption and of the saturation uptake a t the same temperature and pressure are included in the term “chemisorption behavior’’ and both must be the same if the sample surface reaches a stable configuration and is reproducibly cleaned by the reduction treatment. This requirement of the surface led to an exhaustive study of rearrangements that occur during adsorption-reduction cycling. Since the results of this study are to be published at a future date, some of the pertinent findings will be reported here. Reproducible “chemisorption behavior” was obtained by cyclically outgassing, reducing, outgassing, and oxygenating as in Fig. 2. When the reducing gas was not hydrogen, one hydrogen treatment was necessary to remove adsorbed chloride ions. The first oxygen treatment removes sulfur as SOz; the chloride departs as HC1.18 Failure to employ a single hydrogen treatment prior to reduction with some other gas simply yields a surface that has reproducible “chemisorption behavior” a t a lower level of mass uptake per unit mass of silver. This is consistent with recent findings.l9 Progressively larger amounts, of the order of 0.1% of the sample mass, were lost during outgassing a series of silver samples a t progressively higher tempera-
tures. A perfectly reproducible “chemisorption behavior” based on mass uptake per unit mass of silver a t a given temperature and pressure could be achieved on different size aliquots of the silver samples when hydrogen, carbon monoxide, and other reducing gases were employed on the different samples. The results presented below were obtained on a 0.515-g. aliquot of silver powder that was treated cyclically with carbon monoxide and oxygen a t a pressure of 10 t.orr and a temperature of 360’ to obtain reproducible “chemisorption behavior” a t any arbitrary temperature below the reducing temperature. Before reaching constant mass, this sample lost 474 pg. by outgassing and 90 pg. by reduction in 16 complete cycles. The final surface area was 0.09 m.2/g. While the data presented below are explicitly for this sample, the author wishes to emphasize that these data have been reproduced for many other samples and that only one representative set is being presented to avoid cluttering the literature. The Temperature Dependence of Oxygen Adsorption. The rate of oxygen adsorption on a reduced silver surface was measured from -77 to 350’ a t 10 torr. Typical adsorption data are shown in Fig. 3. As can be seen, the initial rate of adsorption was very rapid a t all temperatures. The rate ultimately decreased until it was too slow to be measured. Above 0 = 0.65,20 the rate was observed to decrease markedly with decreasing temperature to about 137”. A t lower temperatures, the rate a t coverages above 6 = 0.65 became constant or decreased slightly with decreasing temperatures.
1
0
0
Figure 2. Typical adsorption-reduction cycles: 1, outgassing; 2 and 2’, oxygen adsorption; 3, oxygen desorption; 4, chemical reduction; and 5, thermodesorption. T h e rate and amount of oxygen adsorption ( 2 and 2’) could be reproduced for many months aa long as the surface stabilization temperature was not appreciably exceeded.
.IO0
1 PO0 1tYlMI
I
1
300
400
Figure 3. T h e adsorption of oxygen on reduced silver powder a t various temperatures. (18) A. W. Smith, unpublished results. (19) R. G. hieisenheirner and R. N. Wilson, J . Catalysis, 1, 151 (1962).
Volumr 88. ,Virmbrr IO
October, 1,964
A. W. CZANDERNA
2768
The apparent activation energy for adsorption was obtained from Arrhenius plots a t constant oxygen coverage 6 of the data shown in Fig. 3. A typical plot for 6 = 0.77 is shown in Fig. 4. Plots similar to the one shown in Fig. 4 were obtained for 8-values from 0.23 to 1.07. The activation energies depend primarily on the temperature of oxygen adsorption and only slightly on the coverage as shown in Fig. 5. Three distinct ranges of activation energies were obtained. The activation energy range of 22-24 kcal./mole a t temperatures above 150' and 6 greater than 0.5 is in agreement with values most frequently quoted in the literat~re.~ The lower activation energy ranges of 0-10 and 0-4 kcal./mole reveal quantitative changes in the oxygen adsorption that occur a t about 100' and a t about 25'. These changcs, which are different from any previous findings, will be discussed in more detail below. The physical adsorption of oxygen on top of chemisorbed oxygen probably accounts for the decrease in the activation energies toward zero at increased coverage. This can be seen more explicitly in Fig. 6 from the adsorption of oxygen on silver at room temperature as a function of pressure. The cover-
*
-
30
O
t
4
t*100-I5O TO 280
T. 150-180
&pooo
0
0
-
n 00
IO 25-85 TO 100-150
a
.3
.4
.s
.8
e
a
; I
,a
1.0
Figure 5 . Activation energies for adsorption of oxygen on reduced silver and temperature ranges in which they are obtained.
h
0
t
t
t
w
100
im
i LOO
t(MlN)
Figure 6. Oxygen adsorption on reduced silver at various oxygen pressures. ~
(20) In this report 8 , the coverage of oxygen, is defined as the mass of oxygen adsorbed a t any temperature divided by the mass of one monolayer of nitrogen adsorbed a t 78OK. This convention was chosen in preference to mole ratios since the oxygen adsorption is not
*'I h El
-01
I
1
I
I
I
103
Figure 4. Arrhenius plot for 0
x
=
The Journal of Physical Chemistry
3.4 X c o l /MOL
I
I/T
0.77 (ref. 20).
1
entirely molecular. Since it is generally believed that the adsorption is dissociative, the calculation of the true 8 must assume a crystal face or set of faces, a mechanism for adsorption, and a ratio of molecular and atomic oxygen on the surface. Hence, B as presented is actually $ ( e ) in terms of the usual adsorption nomenclature where $ can be related to 0 by proper selection of the variables mentioned above. Since the relationship between $(e) and B actually may he close to unity, the term 0 will be used throughout this manuscript. Finally, 0 = 0 is an arbitrary reference point of a reproducible surface e'. While the value of contamination which is in actuality e = 0 e is unknown, 8' must be a constant since constant sample mass is ultimately achieved.
+
AI)SOI~ITIOS OF OXYCESON SILVER
2789
agc?or aniount of oxygen adsorbed is nearly independent, of pressure below 0.1 torr, but increases sharply at highrr pressures. T h e pressur(:-indepcrideiit coverage o c c w s up to 6 = 0.S. which is the same covcragc where the activation cwrgy falls toward zero. It can be readily swn froni Icig. 5 how difTercvit irivcstigators obtain activation eiicigics ranging froin 11 to 27 kcal./ iiiolc as indicated earlier. The oxygc~i adsorption isobar is shown in I(ig. 7 . tcinpcmtures above 160'. a dccwasc in thc saturation amount of adsorption is apparcwt. This iiitlicates the ratc of desorption is bcconiiiig niorc iniporiant than the adsorption rate at high c o \ - e i ' a g ~ ~.It . ttwipc~raturesbelow loOo, the iatC of atisoi ption is too slo\v to produce the niaxiniuin covcIi.agc1 pcrniitt,cd within thc time allowed for the cxpcvhmt. ?'he Idate oj Iksorptzon uizd Themodesorplion of O.c,ygcn f ) ~ i iis z~l i w . I t was fouiid that most of the oxygc~iadsorbed at :1504 vould ultiinately be rtnioxwl simply by outgassing a t the sanic trinpcrature. After adsorhing oxygen at a s ~ r i c sof lowfr tenipcraturcs (Fig. 3), thv systcni was evacuated. T h e imss loss nieasurcd during rvacuation at each tcmpcrature is plotted in I'ig. 8. The rat(. of dcsorption below 160' and 0 = 1 0 is ric~gligiblr,but abovv '200" it is quite rapid until low covcragcs are roached. T h e apparent activatloll ( ~ n ( ~ t g for y dcsorptiori was found to be about 25 kcal i i i o l c for 0 of 0.3 to 0.7. Ilowcver. as was pointed out hforc., an activation clnergy calculated from the rat0 of desorption f roni powders cannot be exact. 'I'hv t r w a c t ivat ion t.nergy for desorption niust be grcatctr than 25 kcal , iiiolc. IYhcn the rate of niass loss on outgsssitig at the adsorption temperature rc.acbhc~la ticy$igiblc or zoro rate, t h e furnace tcniperat u w \vas iiirwasd a t a newly constant rate to ahout
I
I
IS
LO
9 -
9B
10
f
9
r 5
i
5
0
o o 26)
IW
300
Figure 8. Desorption of oxygen from silver at various temperatures.
350'. This method wa5 fouiid particiilarly useful for adsorption temperatures below 140', where the rat(. of desorption became negligible cvcri at high covcrages. h typical heating and inass loss curve is shown in Fig. 9. ,4 curve similar to this was obtained after each subsequent oxygen adsorption. T h e t)reak, A , in 1Jig. 0 appcarcd in all theriiiodcsorption curves between I50 and 173' after adsorption at 130' or less and was independent of the heating rate. The. drastic change in rate, 13, occurred in all desorption and thcriuodesorption curves. Welow 140", tlirw ~ ' e i ' ealways five distinct fractionb of oxygen reiiiovcd fioni the silver surface. These were: (1) a sinall fraction reinoved quickly a t the adsorption tcmperaturc. by cvacuation alone; ( 2 ) a fraction which could ht. renioved qukkly by heating to 130- 175" or by punipirig fn:. s e l ~ ~ a l days a t lower teiiiperatuiw; (3) a fiactiori which could bc rcwovcd quickly by hrating t o 260 2530" o r by
40.3
IIYI")
Figuro 7 . Oxygcii adsorpt.ion isotmr on reduced silver (P,,,?= 10 torr).
*oQ
iwm
Figure 0.
Iksorption
:tiid
t h c r ~ t ~ o t l c s o r ~ ~oft i o r ~
oxygen from silver after adsorptiorl
:tt
65'.
pumping a t 160-200’ for 24 hr. or more; (4)a fraction which could be removed by pumping a t 350’ for several hours; and (5) a fraction which required chemical reduction for its removal. The five fractions obtained on desorption after adsorption a t various temperatures are plotted as 6 us. the temperature of adsorption in Fig. 10. Above 150-170’, regions 1, 2, and 3 merge since differentiation between them is merely a rate effect. The ease with which oxygen can be removed from the surface is qualitatively associated with the strength of the bond formed between the oxygen and silver. Thus, in the progression from regions 1 to 5, the relative strength of the oxygen adsorbed is: (1) very weakly adsorbed oxygen, probably physically adsorbed ; (2) weakly chemisorbed oxygen, possibly as charged molecules, Ozs-; (3) chemisorbed oxygen, possibly as charged oxygen atoms, O b - ; (4) strongly chemisorbed oxygen which may be the same form as in fraction 3 but either placed far apart on the surface so that desorption as 02 molecules is limited by the rate a t which oxygen atoms can diffuse together, or partially submerged into the bulk silver lattice so the desorption rate is limited by the rate of their diffusion to the surface; (5) very tightly chemisorbed oxygen, probably on relatively high-indexplane adsorption sites. Of particular interest in Fig. 10 is the increase in coverage of chemisorbed oxygen and the decrease in the amount of weakly chemisorbed material from 100 to 140’ a t 6 of 0.5-1.0. The significance of the regions becomes more evident if the results obtained for the values of the activation energies are plotted as in Fig. 11. The numbers within this figure specify activation energies a t various 6 and T in kcal./mole. A variation in E with 6 is indicated by the arrows between the numbers. The shaded regions indicate the 6 and T
1.0
e
0 5
I
a
I
I
100 200 AOSORPTIOH T E M P E R I T W E ‘C
Figure LO. Composite of deqorption and therrnodevorption dflta
I
3QP
J
I
e 0
I
I
0
100
200
T *C
Figure 11. Comparison of activation energies of adsorption with thermodeaorption and desorption “breaks.”
1.0
-
8
CUAROED ATOMS
- -- - --------0
-2’
Km
200
Figure 12. Species of oxygen probably adsorbed on silver a t various coverages and temperatures.
where experimentally determined activation energies of adsorption have been obtained thus far. The broken lines define areas where different forms of oxygen exist as previously discussed. As can be seen, the regions where the activation energies are 3 and 22 kcal./ mole coincide with the region where chemisorbed oxygen of type 3 and 4 occurs. The activation energy of about 8 kcal./mole coincides with the region where weakly chemisorbed oxygen (type 2) occurs. If the qualitative assignment of oxygen specified present from thermodesorption and desorption data is correct, then the species present a t various coveragcs would be as shown in Fig. 12.
Discussion I n considering the forms of oxygen adsorbed on silver, the over-all adsorption mechanism might! include all of the following steps.
277 1
r\l)SOltPTIOS OF OXYGEN O N S I L V E R
O&) -+ O?(adsj -+ (h.-(ads) + 2 0 6 (ads) + 20*-’(ads)
\
/
026-‘(ads)
(1)
Since it is not yt.t possible to distinguish 0z6-from OZ6*, the mechanism to be coilsidered can be simplified to o2(g) -+ ()?(ads) -+ ( h - ( a d s ) --+ 206-(ads) (2)
where the charges arc assigned merely to indicate that the adsorbed species probably exist as ions, though of uiispecificd charge. I t is known that sonie form of oxygen molecules and atonis must coexist on the surface frorn oxygen exchange ~ o r k ,and ~ , that ~ ~ oxygen molecules are adsorbed and subsequently dissociate. Thus, frorn this and cq. 2 , it appears that only two different adsorption sites are requirfd, i e . , for charged oxygw atonis and charged oxygen molecules. The primary site for atom adsorption is envisioned to be the tetrahedral hole in an array of three silver atoms having face centered cubic packing. When ncighboring clusters of this type are not available, oxygen rnust adsorb as a molecule, possibly by n-bonding. The multiple activation energies measured could result if there are, in addition, immobile and niobile forms of the cl1arg.d atonis or in the clusters of silver atoms containing the charged atom. It is suspected from the data that there is a transition from iriiniobility to mobility at about 100’. The adsorption below 100’ could proceed by molecular adsorption followed by imrncdiate dissociation into charged atoms which are irnniobilc. This process has a negligible activation energy ( 3 kcal./mole) and continues as long as adjacent sites are available to accept the two charged oxygen atoms froni each dissociated molecule. The random nature of thc adsorption process prevents the surface from being completely covered by charged atoms because some silver atoms will become isolated in clusters of insufficient size and thereby become unable to dissociate oxygen molecules. Additional adsorption must then consist of charged oxygen n?olecules. This would corrrspond to a more highly activated process (8 kcal./inole). Above 100’ the adsorbed charged oxygen atoms become mobile on the surface. 3Iovernent of the charged oxygen atoms would then allow formation of additional primary adsorption sites which facilitate additional molecular dissociation until the iiiaxiiiiuin number of charged atoms that can be held on the surface is reached. The highest activation encrgy (22 -24 kcal.:’inc)le) is associated then with the movement of charged oxygen atoms on the surface to
form additional primary adsorption sites. Coverage greater than the maxiilium charged atom concentration again would consist of weakly chemisorbed charged molecules- again possibly n-bonded and/or physically adsorbed oxygen n~olccules.
Conclusions Activation energics of 3, 8, and 22 kcal./mole have been determined indicating that the nature of the oxygen adsorption on silver is more complex than previously reported. It is also shown that nearly all activation energy data obtained by other workers are obtainable if the proper teniperaturc and surface coverage are chosen. A primary adsorption site consisting of an oxygen adatoni in a tetrahedral hole has been proposed and that mobility or immobility of this adatom or site conibiried with dissociative oxygen adsorption can be used to explain the existcncc of threc activation energies.
Acknowledgment. The author is grateful for the mass spectrometric riicasurenicnts carried out by Dr. A. W. Smith. TIC also appreciates the numerous stimulating discussions of the results of this study with Dean HenFy Eyring and Drs. F. G. Young, A. W. Smith, T. H. George, and P. 0. Schissel. He is gratcful for the extra careful manner in which AIiss L. I. Forrest carried out the measurements. Finally, the author appreciates prepublication information released to him by hIr. C. S . Cochran.
Discussion M .I,. W 1 i 1 1 (13ell ~ Telephor~e,Murmy Hill). I)o your results irnply t h a t silver does not oxidize in oxygeii a t teniperrttures u p to :300°? Carl silver be oxidized at any tenrper:rture, under these conditions? A. W. CZANI)EHXA. These results show t h a t a t oxygen gressures of 10 torr or less and a t ternperatures below 350°, silver adsorbs :tbout one rnonolayer of oxygen. There w : no ~ evidence froin the rnass d a t a in this study t h a t incorporation of osygeii above o r below the silver srirfac:e occurred eit,her by c:ornpounti formation or by formation of a solid solut>ion. ‘l‘here is no evidencv? frorn this study that silver ctm be oxidized :it t,hese os>’gen pressures at any ten-iperttture. At higher teriiper:tt,ures, it is most likely that a solid solution of oxygen ntorns in silver will be forrned. ‘l’he results here do not rontradicat the recwit, results of Menzel (15. Merizel and C:. Rlenzel-Kopp, Preprints, Intern:ttional Conference on the l’hysics arid Cheniist,ry of Solid Surfaws, I3rown I;niversity, June 21--28, 1064, p. LL-I), who slioweti that AglO rrystallites can t)c fornied at, oxygen pressures of :35 atin. and 2JOo, heciiuse of the therinodynail~ic.sof the reac,t.iori . 4 g ( ~ ) Odg) A&(Y).
+
e
(21) L. YY.Xlargolis, It?. Aknd. .Vauk SSSR. Otd. Khim. S a u k , 2 1 , 225 (1959).
Volume 68,.L’irmher 10 Octoher, IRK!+
