KOTES
1736
T'ol. 66
NOTES A STUDY OF *4DSORF'TIOK OF TTARIOUS GASES AT 300'K. BY J. TUUL Amerzcan Cuanamzd Company, Stamford, Connectzcut
Received February 6, 1966
While developing a method for a quick estimation of surface areas of catalyst materials by the use of an air pycnometer, the writer attempted to find some basic facts about the observed air adsorption. The method' referred to consisted of measuring the amount of air adsorbed on the sample at room temperature when the pressure of air surrounding the sample was increased from one to two atmospheres. Under specified conditions it was possible to correlate this adsorption with the surface area of the adsorbent. The investigations to be described here were carried out with the following three materials, all in the form of l/l,j in. extrudates: (1) A1203, ( 2 ) 90% Alsoa 10% MOOS,and (3) 82% ,41203 1.3% 3100~ 3% COO. By carrying out measurements with nitrogen and oxygen separately, it had been found' that the amount of oxygen adsorbed was on the average close to 90% of the amount of nitrogen adsorbed under identical conditions. I n order to illuminate further the problem of room temperature adsorption on inorganic oxides, similar measurements were carried out with carbon monoxide as pycnometer gas. CO adsorption was found to exceed that of air by about 50%. Otherwise, it mas similar to the adsorption of air, being reversible and complete in less than 10 see. Like the adsorption of air, the adsorption of CO could be increased by 100% or more by suitable heat treatments of hydrated samples. However, the ratio of CO adsorption to air adsorption on the same sample was always approximately 1-5. Ilhen COa was used as pycnometer gas, it was found to behave in a rather different manner. Its adsorption was about an order of magnitude larger than that of air, and much slower in reaching completion so that its progress could be conreniently followed. The process could best be approximated by a logarithmic function of time, indicating Elovich-type kinetics. T h e n plotting the amount of GOz adsorbed vs. the logarithm of time. two portions of the curve could be approximated by a straight line. The first of these was in the range between 1.5 see. and 3 min., and the second in the region above 5 min. About one half of the COn taken up at 1 atm. pressure could be desorbed by 5-min. pumping with a mechanical pump, whereas the remainder, or at least most of it. could be desorbed by repeated evacuations alternated with exposures to atmospheric pressure of air. Some samples Rere exposed t o COZ without air
+
+ +
(1) 6, Tuul and W. R, Innes, Ana2 C h e n , 94, 818 (1'2621,
having been evacuated from them. As can be seen from Fig. 1, the adsorption of COZproceeded well on these samples and reached about the same proportions as on samples from which air had been evacuated prior to exposure to COz. The seemingly larger amount of adsorption on the unevacuated sample is not real but indicative of the limitations of the method. Thus, the equilibrium between gaseous C 0 2and the oxide surface, partly covered with adsorbed C02 molecules, is independent of a relatively small concentration of nitrogen or oxygen. It could be assumed that C01 and air are adsorbed on different surface sites. However, it also is possible that so many sites are available for the adsorption of COz that a small fraction of sites covered by preadsorbed oxygen or nitrogen molecules would not make an observable difference with our method. Less COz is adsorbed on hydrated than on dehydrated samples, but this effect is only about half as pronounced as in the case of the previously mentioned gases. As a consequence, the ratio of C 0 2 to air adsorption is about 7 on dehydrated samples, but as high as 1 5 on hydrated samples. An explanation was sought for the differences in adsorption of the various gases. It is generally accepted that the following kinds of forces are operative in physical adsorption : (1) dispersion forces, ( 2 ) repulsive forces, (3) electrostatic interactions with induced dipoles, and (4) electrostatic interactions with permanent electric moments. As far as the adsorption of nitrogen and oxygen is concerned, Drain2 has suggested that the difference in their quadrupole moments might account for the difference in their heats of adsorption on ionic crystals. An inspection of Table I reveals that there is a positive correlation between the quadrupole moments and the relative adsorption of Nz, 0 2 , CO, and COz. It generally is assumed3 that adsorption on dielectric surfaces is determined mainly by van der Waals and dipolar forces. When comparing the data for GO and COz in Table I, it would seem that the quadrupole moment plays a more important role than the dipole moment. However, the overriding factor may be the polarizability. The principal polarizabilities of the molecules under study also are given in Table I . The relative adsorption coefficients for the four gases correlate with bl as well as bz and b3, and also with the arithmetic mean of the polarizabilities. In an attempt to determine which of the three polarizabilities, if any, is determining for the amount of adsorption, measurements were carried out with ammonia for which hl is smaller, but bz and b3 are larger than for any of the previously used gases. Ammonia adsorption was found to be similar to that of CC,, but much larger. TThen ammonia was included, the relative adsorptioll (2) LAE. Dram, TransAFaraday Soc., 49,836 (1953). (3) -4. Ca Zettlemaynri Chem. 8 e u . i 69, Q3t (19iSQ)a
NOTES
Sept., 1962
1737
TABLE I RELATIVEADSORPTION, DIPOLEMOMENTS,QUADRUPOLE MOMENTB, AND POLARIZABILITIES OB GASESSTWDIED Quadrupole moment
Cas
Nz
Relative adsorption coefficient
Dipole moment, Debyes
1.00 0 2 0.87 co 1 5 COZ 7--15 CzHn 9--12 NHI 25-200 a 61, 62, and 6 8 are tion ellipsoid.
(Q X
lo'*),
cm.2
Polarizabilitiesa ba and bi X 1O*s, br X 1026, 8.S.U.
8.S.U.
24.3 14.3 0 0.27 24.3 11.9 0 .09 26.0 16.2 0.1 .34 41.0 19.3 0 .65 56.1 35.9 0 .48 24.2 21.9 1.46 .28 the principal semi-axes of the polariza-
coefficient could be correlated only with b? and ba. However, the large dipole moment of the ammonia molecule might be responsible, a t least in part, for the large amount of adsorption. Finally, measurements with ethylene proved that no single lproperty of the adsorbate molecule determines the extent of physical adsorption a t room temperature, but rather a combination of the properties considered. The adsorption of ethylene was found to be approximately as large as GOz adsorption, but not as large as its polarizability would indicate. I n addition, ethylene adsorption was fast, almost as fast as that of 0 2 , N2, and CO. By the use of ethylene instead of air, the sensitivity of the previously described method of evaluating surface areas1 could be increased considerably. The extent of surface coverage occurring with the various gases in the pycnometer measurements has been estimated. Using the effective crosssectional area of 16.2 A.2 for nitrogen, and applying Henry's law, a total coverage of 3-401, was found for nitrogen a t 2 atm. on freshly calcined samples (the exact figure depends on the type of material). Similarly, assuming a n effective cross-sectional area of 14.1 A.2for the oxygen molecule, a coverage of 2-3% was calculated for oxygen under the same conditions. Using approximate cross-sectional areas for the other four molecules, calculated from the densities of these gases in the solid state, the following coverages were estimated at 2 atin. pressure: 5-7% for CO, 30-40% for CO?, 40-5091, for C,H,, snd 50-60% for NHB. These estimates may be too low, since the effective cross-sectional areas a t room temperature may be larger than the ones assumed in these calculatioiis. Severtheless, the quoted figures give an indication of the large differences in the adsorption of the various gases under identical conditions of adsorbent, pressure, and temperature. Measurements with known mixtures of these gases would be of interest, as they might give an answer to the question of whether different gases are adsorbed on different surface sites. The influence of preadsorbed water can be correlated with the affinity of the respective gases for water. The solubility of the gas in water may be taken as a manifestation of this affinity. The solubilities of X2, 0 2 , and GO in water are very low. The adsorption of these gaws on samples which @on-
-1
L-
I-_-1-2--
I
0 10 20 30 40190200210220230240250260280 Time, min. Fig. l.-d4dsorption of carbon dioxide on yalumina under various conditions: sample weight, 5.83 g.; surface area, 200 m.2/g.; curve I, air not evacuated from sample; curve 11, air evacuated from sample.
tain about 10% water is less than 40% of the adsorption on the same samples immediately after calcination. The influence of preadsorbed water on the adsorption of ethylene and COS is similar but smaller, since these two gases have considerably higher solubilities in mater than the three above mentioned ones. The solubility of ammonia in water is about 100 times larger than that of Kz, 0 2 , or CO. This fact is reflected in the singular behavior of ammonia inasmuch as it is adsorbed to a larger extent on hydrated than on dehydrated samples. I n Table I the adsorption of the other gases is related to the adsorption of nitrogen. A range of values of the relative adsorption coefficient has been assigned to C02, C2H4,and XH3. The lowest value refers to freshly calcined samples, and the highest to samples hydrated by several months of atmospheric exposure.
THE PREPARATION OF SOME RARE EARTH FORMATES AND THEIR CRYSTAL STRUCTURES BY
1.
h[AYCR,
hf. STEINBERG, F. FEIGENBLATT, GLASNER~
AND
Ad.
Department of Inorganic and Analytical Chemistru, The Hebrew Unaverszty, Jerusalem, lsrael Recezved March 10, 1969
I n the course of the study of the thermal decomposition of lanthanon salts, a series of formates was prepared.2 These were crystallized from formic acid soIutions of the oxides or nitrates, washed with water, and then with ethanol or acetone. The composition of the salts was determined3 by the following procedures: (a) calcination of the formates to the corresponding sesquioxides (in the case of cerium, to the dioxide); (b) titration of the formate ion with potassium permanganate2 ; ( c ) determination of carbon and hydrogen by microanalysis; (d) absorption spectra in the infrared region (2-16 p ) . From the above analyses it was concluded that our preparations of cerium(III), praseodymium, (1) On sabbatical leave a t the Department of Chemistry, Princeton University, Princeton, N. J, ( 2 ) F. Feigenblatt, iM.So. Thesis, 1961. (3) A. Cilaaner, M. Steinberg, F. Feigenblstt, and W. Bodenheimer, BuZZ. Rea. GsunciZlrraeZ, l o & 8 (IQ61),
