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),
1738
neodymium, and samarium formate all accord with the formula La(HC00)a.0.2H20. The results are in good agreement with an earlier reportq4 The fractional water molecule could be removed only a t 300' with simultaneous decomposition of the formate. TABLE
1
PARTIAL POWDER DIFFRACTIOP;DATA FOR FORXATES Cehlcl Hex.
T'ol. 66
NOTES
hkl Rhomb.
(HCqO)a d, A.
Pr(HCO0)a d , b.
5.272 3.727 3.066 2.664 2.169 2.012 1.991 1.882 1.860 1.776
5.304 3.720 3.065 2.649 2.159 2.018 1.983 1.871 1.860 1.769
(a) (h) LANTHANON (e)
NdSm(HCO0)s (HCQ0)s d,
formates here investigated are isomorphous with gadolinium formate.5 Gadolinium formate has a trigona1,unit cell and its space group is R3m. The positions in this space group are filled in the gadolinium formate as
A.
Gd atom in 000 3 C atoms in xxz, xzx, xxx 3 01 atoms in xxx, xxx, xxx 3 0, atoms in xxx, xzx, zxx
x
5
0.43 0.19 0.33
0.85 0.81 0.58
d, A .
The oxygen atoms are placed in two non-equivalent sets of threefold positions, the O1 atoms forming a ring about each metal atom, whereas the O2 atoms form a somewhat closer ring around each threefold axis, equidistant between two metal atoms. I n order to prove the isomorphism between gadolinium formate and the formates investigated in this work, the calculated and the observed intensity values of neodymium formate were compared. Intensity measurements were made only for this compound, because the intensities of the reflections of the different compounds are similar. Intenslty X-Ray Results.-Powdered samples of the form- measurements were made with a diffractometer ates were examined by the X-ray diffraction using filtered CuK radiation. Intensity data were method, using a General Electric diffractometer and recorded on a strip chart while scanning the reflecfiltered CuK radiation. Only samples that were tions a t (26) per minute, the areas of the bands proven to have a stoichiometric composition were then were measured with a planimeter. The caltaken; none of the lanthanum formate prepara- culations were made by taking the same atomic tions came up to this requirement. parameters as determined for gadolinium formate. The powder patterns of the formates may be Absorption and temperature factors mere omitted indexed on the basis of a hexagonal cell. Further in these calculations. examination of the hexagonal pattern showed that In Table I11 observed and calculated intensities only lines of h-1-k = 3n are present, indicating a rhombohedral lattice. Powder data w e listed in for the first six reflections of neodymium formate Table I. The agreement b e t w e p observed and are listed. The agreement between the observed calculated d values was kO.015 A. The 012, 202, and calculated values leaves little doubt that these 122, 312, and 232 lines (hexagonal indices) of formates have the same structure as gadolinium cerium and praseodymium formates overlapped on formate. While in Pabst's work with gadolinium the diffractometer pattern a t the 410, 321, 330, formate low intensity values were obtained for the 241, and 520 lines. They could be observed only by ioi, 100, 112, i i i reflections, in our case this distaking a powder picture with a high resolution crepancy was observed only for the 100 reflection. focussing Guinier camera. TABLE I11 Unit cell dimensions of these compounds are ORSERVED AND CALCULATED INTEXSITIES FOR NEODKMIW recorded in Table 11.
110 ioi 101 100 300, 021 i i $ i i i 220, 211 202,20i 131 212 410, 401 3 i 2 , 3 i i 012 110 321 302 202 200 330 302
,
5.241 3.720 3.055 2.649 2.159 2.000 1.979 1.868 1.857 1.766
5,272 3.674 3.035 2.627 2.139 1.987 1.063 l.853 1.836 1.753
FORMATE
TABLE I1 LATTICECONSTAXTS O F LAXTH4NOX FORMATES Formates of
Cerium( 111) Praseodymium(II1) Neodymium(II1) Samarium(II1) C:adolinium(III)" a See ref. 5 .
Hexagonal cell c , A. a , A.
10.67 10.63 10.61 10.53 10.44
4.08 4.07 4.06 4.02 3 98
Rhombohedral cell
a,
A.
6.31
6.28 6 27 6 22 6.17
aki
Ioba
ioi
356 30
357 90
6.4
71
100
62 41
100
IOSlC
a
115'36' 115'36' 115'36' 115'36' 115'30'
Table I1 shows the regular decrease in unit ce! dimensions in the order of diminishing crystal radii of the trivalent rare earth ions. This corresponds with the analytical results which indicated that the metal ions in these salts are all in the trivalent state. Thus, there is ample evidence that the lanthanon (4) B. Sahoo, 8, Panda, and D. Patnqik, J . Indian Chenh. SOC.,87, 894 (1980).
212 312 3ii
1
103
Acknowledgments.-The authors wish to thank Mr. Z. Ka.lman of the Department of Physics for help in carrying out the X-ray work, and Mrs. M. Goldstein of the Department] of Organic Chemistry for analyzing the samples. (5) A . Pabst, J . Chsm. Phys., 11, 145 (1943).
