186
F. H. HEALRY, J. M. FETSRO AND A. C. Z:~TTJ,EMOYIZR
Vol. A7
iodine, the iodine aioms could be expected to cover theory of adsorption in preferred positions. Part an area of 17.67 A.2 each. On the other hand, of this discrepancy may be attribute’d to experiiodine may be adsorbed as diatomic molecules in mental errors, but it should be pointed out that which case the arrangement may be that depicted even with adsorption measurements of perfect in the lower left of Fig. 2. In either case the accuracy the calculated areas should show some molecular area is 35.33 8.,2in agreement with the scatter from the ideal value of 100 due to edge effects. Perfect agreement with the theory could experimental data. be expected only if the magnesium oxide crystal For the chemisorptios of carbon dioxide the faces were of infinite extent and without impermolecular area of 44.16 A.2 might be selected from fection. Actually, since the cry$allite faces of Table I11 since it gives a relative area of 99.2. This value cannot reasonably be selected on the material used were only about 80 A. on a side, the basis of the known dimensions of the carbon dioxide edges and corners might well produce a measurable distortion of the results. Thus the figures in the molecule, however. A special mechanism has been postulated for the last column of Table V indicate as good an agreechemisorption of carbon dioxide on magnesium ment with the theory as can be expected. oxide which gives a molecular area of 46.71 Conclusions and is also included in Table V where it shows fair The data presented here are not extensive enough agreement with the data. This mechanism is based on the formation of a carbonate ion involving to establish conclusively that adsorption on magone of the oxide ions in the magnesium oxide nesium oxide surfaces does take place in preferred crystal face and with the other two oxygens of the positions. They are explained much more satiscarbonate free to rotate. It is further based on the factorily on this basis, however, than they are by chemisorption being essentially irreversible under any other approach suggested to date. Thus the experimental conditions so that once a molecule there is considerable evidence that adsorption on is chemisorbed it stays in position. Thus, once a magnesium oxide surfaces takes place in preferred molecule has been chemisorbed, all positions where positions. How widespread this phenomenon may be is the adsorption of another molecule would produce interference of rotation are effectively blocked. difficult to determine since the surface geometry of The application of the concepts of probability to m0s.t solids is muoh too complex to be handled by the approach used’here. Also the doubly charged this gives the molecular area mentioned above. The molecular areas tabulated by Livingston8 ion structure of magnesium oxide may make it have also been applied to these data. For the nearly unique in this respect. Acknowledgment.-The authors wish to acknowcases where he has values, the agreement is considerably better than that for liquid close-packing ledge the support of the Westvaco Chemical Divibut not as good as for adsorption in preferred sion of the Food Machinery and Chemical Corporapositions. This result might be expected since tion in the early phases of this work and of the these values are not based on any theoretical Lehigh University Institute of Research in the conconcept but are averages of relative experimental cluding phases. They also wish to acknowledge the results reported in the IiteEature. assistance of William E. Kelly, Richard J. MikovIt will be noted that the figures in the last column sky, and the General Aniline and Film Corporation of Table V do not show perfect agreement with the in obtaining some of the adsorption data.
PHYSICAL ADSORPTION ON CADMIUM OXIDE1 BY F. H. HEALEY, J. M. FETSKO AND A. C. ZETTLEMOYER William H . Chandler Chemistry Laboratory, Lehigh University, Bethlehem, Penna. Received March 67, 1062
Cubic crystallites of cadmium oxide were prepared by burning spectroscopically pure cadmium vapor. Adsorption isotherms were obtained with nitrogen, oxygen, carbon monoxide and argon at - 195 and - 183’. A comparison of calculated V , values showed no evidence of the adsorption in preferred position observed by Walker and Zettlemoyer in the case of cubic magnesium oxide. An explanation of the observed difference in the adsorptive properties of ,cadmium and magnesium oxides is offered on the basis of cation polarizabilities.
The state of gases physically adsorbed on solids is usually assumed to be non-localized at the temperatures ordinarily used.2n3 Due to the lack of constraint on the two-dimensional motion of the adsorbed molecules, the use of molecular areas calculated from liquid densities has often been found satisfactory. However, in the case of a
strongly inhomogeneous surface field, it would not be surprising to find that the motion of the adsorbed molecules was restricted and that an approach was made toward localized adsorption. Evidence pointing toward the latter type of adsorption has been found by Orr4 on CsI and by Walker and Zettlemoyer6 on MgO. Both of these sub-
(1) Presented before the Colloid Division of the American Chemical Society, Houston, Texas, in March, 1950. (2) 8. Brunauer, “The Adsorption of Gases and Vapors,” Prinaeton University Prens, Princeton, N. J., 1048, pp. 448-461. (8) T,L, Hill, J . Chrm. Phua., 14, 441 (l946),
(4) W. J. C. Orr, Trans. Farada?, ~ o c . 36,1247 , (1939). (6) W. C. Walker and A. C. Zettlemoyer, presented before the Division of Colloid Chemistry at the 113th Meeting of the American Cbemiorl looiety.
. r.
PHYSICAL ADSORPTION ON CADMIUM OXIDE
Feb., 3 953
stances were cubic crystals of high purity so t h t the adsorbent surface was well defined. To elucidate further the conditions under which preferred position adsorption may occur, additional measurements on other well defined surfaces are needed. In the present study cadmium oxide was chosen because it also belonged to the cubic system and could be prepared in a state of high purity.
187
2.c
,,~
J.W.M. PLOTS G A S E S ON C A D M I U M OXIDE A T -ISS°C.
$1;
Nr4.5
0.E
Experimental The cadmium oxide cubic crystals were' 0.1 prepared by burning spectroscopically pure cadmium after vaporizing it into a stream of prepurified nitrogen from a porcelain boat in a 1.5-inch diameter quartz tube at 700800'. The mouth of the quartz tube was closed with Sauereisen cement so that only a small (ca. 2 mm.) hole remained. Thk Fig. 1.-Adsorption of nitrogen, carbon monoxide and oxygen on cadmium cadmium oxide was formed when the metal vapor passed into the.air. The oxide dust oxide a t -195", plotted after the method of Joyner, Weinberger and Montwas collected on a buchner funnel connected gomery. to a vacuum pump and inverted above the mouth of the quartz tube. More rapid streams of nitrogen plot was somewhat in doubt. For this reason, the produced finer particle sizes; the largest surface area ob- data were also plotted according t o the method of tained by this method was about 5 square meters per gram. Electron photomicrographsa revealed that the cadmium ox- Joyner, Weinberger and Montgomery, l 1 in order to extend the linear region up to a relative pressure ide crystals so prepared were practically entirely cubic. The absence of cadmium nitride was demonstrated by of 0.6. The extended BET plots were quite linear testing with the sensitive Nessler reaction. The nitride is and consequently no ambiguity remained in choosformed only with active nitrogen and gives a very strong ing the best straight lines. Typical plots are Nessler test as demonstrated by Strutt.7 Degassing the cadmium oxide proved to be a difficult shown in Fig. 1. problem. A temperature of 350" under a high vacuum was The value of n, which according to the BET found to cause partial decomposition to metallic cadmium. theory represents the average maximum number of Subsequently, the degassing temperature was reduced to 100' before there was certainty that decomposition had not adsorbed layers, was found t o fall between the occurred. Electron diffraction patterns8 were taken of the limits of 4.5 and 5.0 for all adsorbates; the average surface of the oxide particles after degassing at 100'. The value was 4.6. Lack of any hysteresis in the nitrocadmium oxide crystals gave excellent patterns and no trace gen isotherms and approximate agreement in surof cadmium lines was found. The standard degassing conditions used before each run were a temperature of 100' and face areas calculated from adsorption data and a pressure of 6 ' 0 1 mm. for 12 hours. Adsorption measure- from electron micrography was interpreted to ments were rerun several times to he1 to assure cleanliness mean that the cadmium oxide was non-porous. of the surface according to the methoxof Harned.g Finite values of n for non-porous adsorbents has The adsorption apparatus was of the conventional volualso been reported by other investigators. 11,12 metric Brunauer-Emmett-Teller design previously deThe V m values calculated from both the BET and scribed.lO High purity tank nitrogen, argon, oxygen and helium were used. Nitrogen and argon were further purified JWM plots are given in Table I for the four adby passage through h e copper gauze heated to 500' and sorbates: nitrogen, oxygen, carbon monoxide and dried with. a mixture of drierite and soda lime. Oxygen In most was dried with drierite-soda lime mixture. The helium argon, measured a t -195 and -183". used in dead space determinations was purified by passing cases the two methods gave values of Vm which it slowly through a charcoal trap immersed in liquid nitro- agreed within 2%. The average Vm's and calgen. culated surface areas, based on the JWM results, Carbon monoxide was prepared by decomposition of Baker Reagent Grade formic acid with concentrated sulfuric TABLE I acid. The gas was purified by passage through a glass wool trap, soda lime, and finally through a trap immersed in ADSORPTION OF GASESON CADMIUM OXIDE liquid oxygen. Liq. On temp. - 183' Liq. Nn temp. - 195'
Results and Discussion The isotherms for all of the adsorbates were of the usual type 11; however, in several instances a slight dent in the curve was observed at a relative pressure of about 0.2. Because of this dent the best straight line to draw for the corresponding BET
(6) Electron photomicrographs were made b y Miss M. H. Polley, Godfrey L. Cabot Co., Boston, Mass. (7) R. J. Strutt, Proc. Rou. SOC.(London), ASS, 539 (1913). (8) Electron diffraction etudies were made by Dr. R. G. Pioard, Radio Corporation of America, Camden, New Jersey. (9) H. S. Harned, J . Am. Chem. Soc., 43, 372 (1920). (10) A. C. Zettlemoyer and W. C, Walker, I n d . Bnu. Chsm., 89, 69 (1947).
Gas
Vm
BET
Nz
0.671 .667
0 2
.701
CO A
.714 .719 .726 .663 .660
c
29 53 24 22 50 45 25 27
JWM
Vm
0.668 .652 .719 .726 .703 .699 .702 .662
BET Vm
c
JWM
Vm
0.580 34 0.604 .615 .599 36 .701 .684 18 .703 .656 17 ,662 ,630 48 .660 .638 ,51 .722 .716 17 .741 .691 20
(11) L. G. Joyner, E. B. Weinberger and C. W. Montgomery, J . Am. Chsm. Soc., 67, 2182 (1946). (12) F,E. Bnrtell nnd 0.Q. Dadd, THIBJOURNAL, 64, 114 (1960).
188
F. H. HEALEY, J. M. FETSKO AND A. C. ZETTLEMOYER
VOl. 57
'are given in Table I1 together with their values relative to the average of the eight determinations taken as 100. The molecular areas shown in Table I1 were calculated by the usual method on the basis of liquid close-packing.
packing intermediate between that of the solid and liquid. Even a t - 183", the use of liquid densities gave a calculated surface area 7y0above the average value. Again the choice of such packing is suspect for the polar molecule. It is of interest here that the c values determined for CO were noticeably TABLE I1 larger than for any of the other adsorbates. Temp., The V , values for all adsorbates except argon Relative Molecular Surface Relative Gas OC. Vm Vm area area area were smaller a t the higher temperature. This Nt -195 96 16.2 2.88 102 0.660 behavior is customary when adsorption is in close- 183 89 17.1 2.81 100 .610 packed array, and again gives no evidence for pre2.66 95 02 -195 .723 106 13.7 ferred position adsorption. - 183 2.66 95 14.1 .702 103 As pointed out earlier-, localized adsorption has 16.2 3.06 109 CO -195 .701 102 been reported for magnesium oxide, but the present - 183 .66l 97 1 6 . 9 3 . 0 0 107 investigation'gives no evidence of localized adsorp14.0 2.56 91 A -195 .682 100 tion on cadmium oxide. Since both oxides are 14.5 2.85 101 -183 .732 107 cubic crystals of the NaCl type, an explanation of the apparent difference in the state of the adsorbed Since the purpose of this research was to deter- film must lie in the difference in cation properties. mine whether or not adsorption in preferred posiWey1la,l4has discussed the effect of ion polariztion took place on cadmium oxide, it is of interest ability on the strength of the surface force field. to examine the values for the relative Vm)s given He pointed out that the more polarizable an ion in in Table 11. The average V , for the eight results a surface was the better the electronic structure of was 0.682 with an average deviation of =k4.5%. that ion could adjust itself to the unbalanced force The maximum deviation from the average was 11% field. This internal ionic adjustment would be in for the V , of nitrogen at - 183 '. such a direction as to lower the forces emanating This variation in V , would not be expected if from the surface. adsorption in preferred position took place. Rather Since the Cd++ ion is considerably more polarizit would be expected that all V , values would be able than the Mg ++ ion, the above' reasoning would the same. Thg accepted value for a0 of cadmium lead to the expectation that cadmium oxide would oxide is 4.689 A. whick would give a unit cell area have a much weaker net surface field and lower in the 100 face of 22.0 A.2. Since this is a consider- energy barriers between adsorption sites than ably larger value than the cross-sectional area of magnesium oxide. In this connection it is interestany of the four molecules, each site could easily ing to note that the BET c values given in Table I accommodate one molecule. Judging from the are lower than is usually obtained for these sdsorbreproducibility of the individual isotherms, pre- ates on inorganic adsorbents. WeylI4 has also discussed the instability of the ferred position adsorption should lead to a maximum average deviation in V , of l.8yO.Thus the oxides of cations having weak potential fields. The average deviation of 4.5% found experimentally ease with which cadmium oxide decomposed was is an indication that preferred position adsorption noted in the early degassing experiments. Magdoes not occur on cadmium oxide for the gases nesium oxide, on the other hand, is exceedingly stable even a t very high temperatures. Thus, the measured. The assumption that the adsorbed, material is in relatively greater instability of cadmium oxide and close-packed, liquid-like array aiso does not lead to the absence of localized adsorption can both be concordant results. The surface areas, actual and explained on the basis of the weak potential field relative, calculated from liquid densities are shown of the Cd++ ion as compared to the strong field in the last two columns of Table 11. Again, the from the small Mg++ ion. deviat,ion of *4.8% is outside the maximum limit Acknowledgment.-The support of this work by set by the reproducibility of individual isotherms. the American Philosophical Society, Grant NO. It should be noted that for CO at - 195" the use of 1016 of the Penrose Fund, is gratefully acknowlthe liquid density results in a surface area 9% above edged. average. It would not be unexpected that the (13) W. A. Weyl, Trans.". Y. Aead. Sei., Series 11, 12, 245 (1950) partial orientation of the carbon monoxide dipole (14) W. A. Weyl, O N R Technical Report No. 39, Contract NBonron the surface, a t low temperatures, might lead to a 269, Taak Order 8, NR-032-265.
.
d
