A. C. ZETTLERSOYER, YVSG-FANG Yrr
~ N J. D
J. CHESSICK
Vol. 59
ADSORPTION STUDIES ON METALS. IV. THE PHYSICAL ADSORPTION O F AKGOAT ON OXIDE-COATED AND REDUCED NICKEL BY -4.c. ZETTLEMOTER,Yt:NG-F.kXG YU A N D J. J. CHESSICK Contribution fi,oni the Sui:faceChemistry Labolato?y, Lehiyh University, Bethlehem, Pa. Received Oclobev 88, 1064
Isotherms for the adsorption of argon on oxide-coated nickel powder and on this sample after each of several reductions with dry hydrogen a t 350" were determined at - 195 to - 183". Equilibrium functions, isosteric heats of adsorption and integral molar entropy values were calculated. These data show a marked difference in the surface nature of the unreduced and reduced samples. However, the adsorption behavior of argon on the reduced samples was similar above monolayer coverage. I n this region of adsorption the only effect of reductions at high temperature was to cause an increase in particle size due to sintering without markedly changing the surface nature of the reduced metal powder. The equilibrium function and isosteric heat data indicate that both the unreduced and reduced samples were heterogeneous. Adsorption sites of higher energy as well as a wider distribution of sites characterized the unreduced samples. Stronger cooperative effects leading to an increase in heat values and a subsequent maximum monolayer coverage was found for argon adsorption on the reduced sample. On the basis of experimental and calculated integral entropy values, a model of localized adsorption on a heterogeneous surface without adsorbate interactions explains the adsor tion of argon on the unreduced and reduced sample. This model is probably not valid beyond 0 = 0.5 for the unreduceg sample and beyond the relatively low value of e = 0.2 for the reduced sample; above these e values lateral interaction set in. At very low coverages on both samples the heterogeneous nature of the surfaces was evident from the low molar entropy valnes SS and small configurational entropy coGtribution. when first prepared greater than 09%, and a particle size Introduction from 2 to 25 f i . The sample was passed through a Gas adsorption techniques were employed in this range 400 mesh sieve before use to remove large particles. Laboratory t o study the surface heterogeneity of High purity tank argon and helium were used. Thc molybdenum metal.' A thick oxide film was argon was further purified by passage t,hrough fine coppel' heated to 500" and dried with magnesium perchlorate. formed on the molybdenum surface after exposure gauze Helium was purified by passage through a charcoal trap to moist air during storage. Reduction of this immersed in liquid nitrogen. polymoleculnr oxide film by high temperature Tank hydrogen, 99.8% pure, was passed slowly through a treatment with dry hydrogen increased the surface Baker Deoxo Unit, containing a palladium catalyst, and then through a drying train consisting of two magnesium perroughness or physical surface heterogeneity. After chlorate driers, a phosphorus pentoside tube and a cold successive heatings in the presence of hydrogen a trap immersed in liquid nitrogen. Previous work in this lower limit of surface area \vas reached. For a Laboratory showed that water diffused into the reduction further understandiiig of metallic surface proper- system if rubber connections were used. Therefore, the reduction system was of all-glass construction with Tygon t,ies,this work has been esteiided to nickel. tubing connections. Through the developmentmof adsorptioii thermoAn Orr tjype5 apparatus \vas used. Apiezon B oil which dynamics and its application to various adsorption had a very low vapor pressure and did not dissolve measursystems in the last decade, it is now nrell established able amounts of the gases studied wvns used in the manomPressures as low as 0.005 em. were measured with that thermodynamic fuiictioiis are \very useful for etem. oil. However, the height of the manometer restricted characterizing the properties of adsorbed films. t,liis yessure measurements to about 5 em. To increase the However, because of the complexity of the different range of the manometer, n back pressui'e was applied to the existing forces and the wide differences in surfaces, vacuum side of the manometer for pressures greater than 5 em. In this way, adsorption pressures as high as 20 cm. it is helpful to investigate more than one function were obtained. hefore reaching final conclusions. A sharp initial I t was shown previously6 that samples with an area as low drop of the isosteric heat with increasing surface as 0.5 m.2 could be studied with precision with this appacoverage is generally attributed to the surface ratus. Nevertheless, in t.his investigation sufficiently large heterogeneity. Following Hill's2 statistical de- samples were taken so that the total area ranged from 3 to Trelopment, the entropy of the adsorbed phase 7 m.z. Pretreatment of Samples.-The unreduced, polycrystalcould be evaluated, which serves as a parameter for line samples were first washed with absolute alcohol to defining a surface. Furthermore, Morrison and remove any organic film. They were then degassed a t Drain3 were able to calculate the configurational room temperature for a t least 12 hours to an ultimate presbelow 10-5 mm. before adsorption measurements were and non-configurational entropy values of systems sure made. consisting of a heterogeneous surface. Recently, Reductions of the surface oxide which had formed on the Graham4 has introduced the equilibrium constant nickel powders on exposure to moist air during storage,' were carried out a t 350". The hydrogen after thorough of a localized adsorption system for characterizing a drying was passed slowly through the samples a t atmospheric surface. Very little work has been done with the pressure. The reduction times are listed in Table I. Two last method. I n the present work the conclusions nickel samples which differed slightly i n nrea hut were obfrom this free energy fuiiction are compared to tained from the same batch were studied. The first sample was subjected to four reduct,ions. After each reduction the those of isosteric heats and entropies. sample was degassed a t 25" to 10-5 mm. for 6 hours, t)hen nrgon adsorption isotherms were determined a t - 195'. Experimental Materials and Apparatus.-The nickel sample furnished by the International Nickel Company had a reported purit#y (1) F. H. Haaley. J. J. Clirsniok and A. C. Zettlenroyor, THIB JOURNAL, 57, 178 (1053). (2) T. L. Hill, J . Chern. Phys., 17, 520 (1940). F a r a d a y Soc.. 48, 310 (3) J . Rf. Drain and J. A. Rlorrison, TIYZIIS. 11952). (4) D. Graham, THIS J O U R N A L87, , 005 (1053).
The second sample was studied pi,imai.ily to substantiate the result,s found with the first sample as well as to obtain some additional data for the adsoiyt8ionof argon a t -188 and - 195'. After the desired adsorption measurements were made, t,he samples were again subjected to hydrogen treatment, at 350'. Since the reduced nickel was never ( 5 ) \V. .J. c'. O r r . P ~ c n.o ] / . Soe. ( L o n d o n ) , l'ldA, 349 (1939) (13) .I J . Ctiesnick, P h . D . Thesis, Leliigh University, 1952.
July, 1955
AADSORPTION O F
ARGONON
OXIDE-COBTED A N D
exposed to any gas except non-reactive helium nnd argon aftw t,he first reduction, the second and subsequent8hydrogen treatments were not strictly reductions. Rather they were conducted to allow a study of the change of the surface properties of tthe metal as sintering. progressed; they are labeled reductions merely as a convenience. No effort WZLSmade t,o remove sorbed hydrogen from the reduced samples by degassing a t elevated temperatures, since the primary objective of this investigation was to characterize two different surfaces-the oxide-coated and that produced by reduction-by gas adsorption met,hods. However, to determine the effect of this sorbed hydrogen on the reduced samples on the adsorption of argon, a separate sample was reduced under the usual conditions and degassed at 350' for 3 hours. Large amounts of hydrogen were evolved. Isotherms reduced t o unit surface area for this sample and other samples which were veduced but not degassed a t high temperature were found to coincide exactly above monolayer coverage indicating t,hat in this region the argon adsorption was independent of any hydrogen presorbed. Beeck? also found that the adsorption of krypton on nickel films wns independent of hydrogen presorbed in his sample Procedure .-One of the often unrecognized difficultiee in adsorption studies a t low temperatures is the slow attninmerit of equilibrium a t small ndsorption pressures. Spurious rates or heat curves have been shown to result when insufficient time has been allowed for equilibrium.638 To accelerate t,hernial equilibrium in the low pressure region, it ~ n found s expedient to precool the sample in the presence of helium a t the adsorption temperature for 2 or 3 hours before adsorption measurements were made. The system was then degassed for 30 minutes a t the adsorption temperature and argon adsorption mensureintwts begun. As n result of I)i,ecooling, equilibr.iuni nt pressures I J ~ I O W0.1 mm. \vas utt:tined within 15 minutes; however, at least 1 hr. was allo.wed for equilibrium to be reached in the low pressure region.
Results and Discussion Adsorption Isotherms.-Argon adsorption isotherms were measured a t - 195" on unreduced, oxide-coat.ed nickel and on this sample after each of four successive reductions at) 350". The shapes of t,he isothernis indicated a marked difference in the behavior of argon adsorbed on the unreduced and reduced powders. Contrariwise, isotherms for argon adsorbed on the reduced nickel samples were found to be very nearly parallel in the region above monolayer coverage although tliey showed a decrease in the amount adsorbed after each reduct'ion. This decrease, which exponentially approached a limiting value, resulted from a definite increase in particle size of the metal powder due t o sintering at t#he reduction temperature. Confirmation of these initial results was obtained when similar studies mere conducted on a new sample of lower initial area but obtained from the same batch of nickel pon.der. Furthermore, the temperature dependency for adsorption on this second sample was determined a t -183 and -195" and isosteric heats of adsorption were calculated. Some tentative conclusions concerning the natures of the nickel surfaces before and after reductioii were possible From a knowledge of the prehistory of the nickel and a visual inspection of these isot,herms. First, since the metal was polycryst,alline, it was reasonable to assume that the surfaces were heterogeneous. Further, it was coiicluded that the distributions of adsorption energy sites on the oxide-coated and reduced metal were quite different. Finally, it appeared that (7) 0. Beeck, "Advances in Catalysis," VoI. 11, Academic Press, Inc., New York, N. Y.,1950, p. 160. (8) F. C. Toninkins, T r a n s . F a r a d a ~Soc., 34, I4GY (1938).
REDUCED NICKEL
589
while a new type of surface resulted after reduction of the oxide film, the effect on this iiew surface of sintering a t high temperature was to cause a decrease in area wit,hout drastically changing its site energy distril~ution,particularly a t higher coverages. Considerable evidence was obtained to sub. stantiate these findings as well as to yield more quantitative information on the nature of these tli o different surfaces. Adsorption Isotherms Reduced to Unit Surface Area.-Adsorption in the first layer, u hetlier chemical or physical, depends on two factors: the extent of the surface and the energy of interaction between surface and adsorbate. Brunauerg designated the former the "non-specific" factor, the latter the "specific" factor in adsorption. Thib delineation was, of course, an oversimplification since the interactions between adjacent adsorbed molecules can be an important additional factor. Holyever, if physical adsorption were completely "non-specific," then surface area alone would be controlling and isotherms for a gas adsorbed on all types of surfaces would coincide if volumes adsorbed per unit area were plotted against pressure. This situation would not esist, on the other hand, if surface "specificities" were also important. Isotherms of this nature were developed in order to differentiate between the adsorption behavior of argon on both t,he unreduced and reduced nickel samples. For this reason a measure of the specific areas of these samples TPRS necessary. The values for V, were obtained by the usual BET method and represented the STP volumes of gas required to cover the surface of one gram of sample with one adsorbed layer. The specific areas of the samples were calculated from these V,, values assuming that the adsorbed molecules had the same hexagonal close-packing as in the liquid state.I0 Another method, the "B" point method of obtaining Tinl and corresponding area values, was to use the volume adsorbed a t the beginning of the linear portion of the Type I1 isotherms. The results of this latter method were adopted here TABLE I .kDSORPTIO\
Rediiction no.
Unreduced
Time, hr.
,
..
1 2
2.0
3 4
2.5 3.0
Unreduced
...
1
2.5 2.0
i3.5
O F .L\RGON 0'4
I'm,
rnI./g.
Unreduced 1 2
"B" Point Area,
ni.*/g.
Snmple 1, -105" 0.1'35 0.73 .1G8 .63 ,110 .45 ,107 .40 ,100 .38 Sample 2 ,
2
NICKEL
BET
0.154 ,117 ,100
Sample 2, .., 0,152 2.5 ,122 2.0 ,104
Vm,
Area,
InI./g.
iii.*/g.
0.lGi ,155 ,113 .IO3 ,098
0.63 .58 .43 .39 .37
0.133 ,112 ,008
0.50
0.126 ,107 .095
0.49 .42 .37
- 195" 0.57 ,44
.37
.42
.37
- 183' 0.59 .4i .40
(9) S. Brunnuer. "The Adsorption of C l ~ s e and s Yaixxs," Princzt.on Univ. Press, Princeton, N. J., 1043. p. 324. (10) Ref. 4 , p. 287.
-4. e. ZETTLEMOTER,Y.4NG-FANG Y U
590
because of the better agreement in area values calculated from V m values a t -183 and -195” obtained by this “B” point method. This agreement is shown in the last column of Table I for sample 110. 2 . The isotherms measured for the adsorption of argon 011 unreduced nickel and on this same sample after each of four, successive. high temperature reductions are shown in Fig. 1. The adsorption values were reduced to unit area; i.e., V / V , , values were plotted against relative pressure. There was no doubt that the “specific” forces whether surface, adsorbate interaction or both were different for argon adsorbed 011 unreduced and reduced nickel. On the other hand, the coincidence of the four isotherms for the reduced samples, although they differed in area by as much as 36%, strongly suggested that oiily small changes in these ‘(specific” forces occurred as a result of sintering at the reduction temperature.
’4ND
J. J.
\.(A. 59
CHESSICK
The equilibrium functions for the adsorption of argon on the second sample of the unreduced nickel and for this same sample after each of two successive reductions are shown plotted in Fig. 2 as a function of e. The shape of the equilibrium function curve for argon adsorbed 011 the unreduced sample indicate adsorption on a heterogeneous surface. Initial adsorption took place 011 sites of high energy. Furthermore, these site energies decreased markedly with increasing 8. The equilibrium function was found to be nearly constant between 0 = 0.5 to 0 = 0.8, the criterion for adsorption on a uniform surface. HoJvever, a balance between cooperative adsorption and surface heterogeneity, itself, could be responsible for the constancy of K in this region,
\\
t
REDUCTION
I
”
n
&
0
UNREDUCED
0
REDUCTION I
x
REDUCTION
P
m
P
m
.,
0
I
0‘
0.02
a04
a06
0.00
0.10
RELATIVE PRESSURE
0.12
0.14
0.16
P/ Po.
Fig. l.-Adsorpt,ion isotherms of argon on nickel nt -195” reduced t o unit surface.
Free Energy Change of Adsorption.-Graham4 developed a simple distgributtionfunction which he claimed provides the basis for a sharp classification of physical :dsorption in terms of the surface properties of the adsorbent as well as adsorliate int,ernction. The equilibrium constant for the transfer of one mole of sdi;orlmte from the stJandarcl state as n liquid to the adsorlied phase can be written
e = ___ (1
- S)X
where e is the fraction of surface covered and X is the relative pressure of the gas in equilibrium with solid. The free energy change is simply given by the relationship -AF”
=
RT In I
