ACTIVE MAGXESIA.
iG3
111
(5) POOLE, H. J.: Trans. Faraday SOC.22, 82 (1926). (6) PRYCE-JONES, J.: J. Oil and Col. Chem. Ass. 19, 295 (1936). O., ASD GOLDBERG, H.: Anal. Chem. 19, 123 (1947). (7) SANDVIR, (8) WILLIAMSON, R. V . : Ind. Eng. Chem. 21, 1108 (1929).
ACTIVE MAGSESIA.
111
X-RAY versus NITROGEN ADSORPTION SURFACE AREAS A. C. ZETTLEMOYER
AND
W. C. WALKER
W m . H . Chandler Chemzstry Laboratory, Lehigh Unaversity, Bethlehem, Pennsyluania Received December 10, 1946
The method of Emmett and Brunauer (2) for determining surface areas by nitrogen adsorption a t the temperature of liquid nitrogen is u-ell established. The method of determining particle sizes, and consequently surface areas, by the x-ray method has also received extensive development (4)since it was originated by Schemer (6). There has been, honever, no direct comparison between the two methods. The most extensive application of the. x-ray method to the determination of surface areas has been carried out by Hofmann and Wilm (3) on various activated carbons. S o gas-adsorption values were obtained on these samples, but for similar materials other investigators have reported areas of the same order of magnitude as was obtained by the x-ray method. In most cases, however, it seems likely that the latter method would indicate higher areas than the former, because aggregates formed by the particles and too small pores would prevent the nitrogen molecules from reaching all crystal faces. In the course of a series of studies on the nature of active magnesia, x-ray diffraction patterns vere obtained on a series of samples n hich had been carefully examined previously by the nitrogen-adsorption technique (9) and for fluoride adsorption (10). From the broad diffraction bands in the ponder diagrams, the half-intensity breadths were estimated, and from these the particle sizes were calculated according to the theoretical equation of Sherrer ( G ) for cubic crystals. A direct comparison between surface areas as determined by x-ray and by nitrogen adsorption thus became available. Warren (8) and Birks and Friedman (1) showed that x-ray line broadening could be used in determining the size of magnesium oxide particles. Birks and Friedman concluded that particle sizes above 100 A. agreed t o =k 10 per cent with those estimated from electron micrographs; below 100 A. these investigators report an accuracy of the measurements on the electron micrographs of the order of f 25 per cent, so that a comparison with the particle sizes as determined by x-ray measurements was difficult to make in this range. The particle sizes of the samples examined here are mostly below 100 A., and in
764
A . C . ZETTLEYOYER AKD TT. C. W.ILKER
no case was it possible to determine ultimate particle size or shape from electron micrographs. lL4TERIALS
The active magnesias were prepared from the hydroxide obtained from sea water ( 7 ) . Magnesia 26G1, although not an active grade, was again included here for comparison. The production conditions and chemical analyses have been summarized elsewhere (9). In table 1 the iodine adsorption numbers, which provide the usual commercial activity test, are presented in the second column. -4new sample, Magnesia AS, was added to the series. A preparation of this sample is t o be described as part of another study to be reported elsewhere. With this additional sample, as indicated in the third column, the areas as determined by nitrogen adsorption extend from 0.8 to 296 square meters per gram. TABLE 1 Characteristics of magnesias I C1ADI:
'
IODINE ATUBER
I
I AS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X P . ,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2642 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2652-S . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2652 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2661.. . . . . . . . . . . . . . . . . . . . . . . . . .
1
I
~
1
202 210 150 130 146 76 4
I 1 1
~
~
,
1
AREA ~
Kitrngen
~
X-ray
q w , e nirltri per gram
296 230 154 125 146 71 0.8
1
1 , '
1
304 270 234 205 189 135 43
i ~
CELL C O I S I A Y T
A.
4.211
~
'
1 I
1
4.213 4.206 4.204 4.201
METHODS
Back-reflection x-ray patterns of the various magnesia samples were taken using a General Electric XRD unit, Type 1. Each sample was sifted onto a greased piece of cardboard in such a manner that a fine layer of the povdered sample adhered to the cardboard without falling off when placed in a vertical position. This sample was mounted in a General Electric rotating specimen holder and was placed a t a distance of 60.52 mm. from the film, using a calibrated pointer. The specimen and film were rotated in order to give smooth diffraction bands for the larger grained samples and t o obtain a more representative diffraction band. Eastman duplitized non-screen x-ray film was used for registering the diffraction bands. Each pattern was developed for 5 min. in Eastman x-ray developer and fixed for 10 min. in Eastman x-ray fixer. After washing for 30 min., the films were sponged off and dried for 8 hr. before microphotometer measurements mere made. This latter precaution \vas taken to minimize errors due to film shrinkage. To measure intensities across the diffraction bands a Soci6t6 Genevoise
microphotometer having a suitably controlled light source and photvcell \\-as used. The equation for the particle size D in terms of the width B at half-maximum of the intensity curve is:
D
=
0.94-
x
B cos 0
\\-here 1 is the n-ave length of the x-rsdiation and @ is tlic Uragg angle. B must be corrected for the instrument width, that is, the Ti-idth of the half-maximum of the intensity ciir~-efor very large particles. For this purpose the intensity curve of a sample of JIagriesia 2GG1 heated for 48 hr. at 1000°C. \vas mcnwred. Thiscurveshowed the t\\.opeaks of the S i Iiaaland the S i Iicu,? radi:rtion. Thc average of the ividths at the tn.0 half-maxima was 0.03.552t5a n d thiz \V:LS apjilietl :is a correction. 11'. in the usual form: B? = - bl' (2) From the d u e of D , thc s-ray surface area can be calculated. This theoretical surface area takes into account each face of each cubic crystallite. If the particle size D is divided by the cell constant for magnesium oside. 1.20, the length I in terms oi unit cells is obtained. The s-ray area, it may lie shown, ciin he calciilaied from the eqriation: X-ray : ~ r m= 3860, I
(3)
The apparatus and p r u e o d ~ i for r ~ deter~niningthe areas by nitrogen adsorption h a w been described prcviously (9). Sitrogen \vas adsorbed at liquid-nitrogen temperatures on the evacu:ited samples. Some minor difficulty was encountered in calculating the areas from the adsorption data. The ai'eas tabulated in tlicl third column of table 1 are the so-called BET :ireas obtained from the RET plots. The BET plots, however. \\-ere not linear, a i is i~suallyfound, but possessed a slight curratuic concavc to the pressure nsi.5. Except for Magnesia -is, the nitrogen areas fall in the same order as the iodine numbers. Some of thc pores arailablc to nitrogen molecules in thc skiniplc were apparently too sinall to accommodate the larger iodine molecules. The shapes of the nit rogen-adsorption isotherms \\-ere explained by hypothesizing a structui,c consisting of plates of checlrer\rork of cubic holes and ci,ystnllites. The perforated plates ]!-ere later found on electron micrographs RLSCLTS A S D DISCUSSIOS
The particle sizcr of the active grades as deterniined from the s-ray line I)i,oatlenings yarietl f r o m 120 for Magnesia 2641 t o 53.4 -1.for Magnesia ;1S. IIagnesia 2661. an inacti1.e grade. had a particle size of 374 A. as calculated from equation 3 are tabulated in the fourth column of table 1. Tliese theoretical areas, representing the surface.; available if none of the particles \\.ere touching each other. are all larger than the nitrogen areas. In figure 1 the nitmgpn areas are plotted against the x-ray areas. The
766
A . C. ZETTLEMOYER AND W. C. WALKER
dotted line represents the situation that would exist if both methods were to yield equal surface areas. A smooth curve fits the data surprisingly well. As the theoretical surface area increases, the fraction available to the nitrogen
X
AREA S Q , M./G.
FIQ.1. Nitrogen versus x-ray areas
FIG.2. Per cent deviation zersus x-ray areas
molecules increases, until for Magnesia AS the nitrogen area very nearly coincides with the x-ray area. The per cent deviations of the nitrogen area N from the x-ray area X ,
-
X
N1OO, were calculated from the smooth curve in figure 1. For x-ray
areas above 100 sq.m. per gram, these values, when plotted versus the x-ray areas, produce the straight line in figure 2. The equation of this line is:
N1OO X
=
-0.267X
+ 87.3
(4)
ACTIVE MAGKES1.4.
76i
I11
Small deviations from the smooth curve of figure 1 yield large deriations from the linear plot in figure 2. From these results, then, it may be concluded that for active magnesia the x-ray areas and the nitrogen areas agree closely only a t the highest areas yet attained. Except for the inactive grade, the x-ray area nerer exceeds the nitrogen area at any level by more than a factor of tn-o. The cell constants for the various grades as calculated from the distances between the maxima in the intensity curves are presented in the fifth column of table 1. While these results are not completely consistent, owing to difficulties in estimating the precise positions of the masima in the smaller-grained samples, the cell constants are generally larger for the samples with the larger areas. In the catalytic decomposition of ethyl alcohol over magnesium oxide, Rubinshtein showed that the cell constant was a critical factor ( 5 ) . SV1lMIART
X-ray diffraction patterns n-ere obtained on a series of active magnesias prepared from the hydroxide. The particle sizes, and from these the surface areas, n-ere estimated from the broad diffraction bands. The surface areas as determined by s-ray measurements were compared with the surface areas which had been determined previously by nitrogen adsorption. The x-ray areas and the nitrogen areas were found to agree closely at the highest level of 300 sq.m. per gram and to deviate in a regular manner as the particle size decreased. The cell constants ryere found to be generally larger for the samples with the larger areas. REFERESCES (1) BIRKS,L. S., A N D FRIEDYLX, 13.: J. Applied Phys. 1 7 , 6 8 i (1946). R , J. .hi. Chem. Soc. 56, 35 (1934); 59, 310 (193;). (2) EXXETT,P. H . , AND B R C - S . ~ ~ Es.: U.AND W I L ~ ID.: , Z. physik. Chem. B16, 401 (1932); Z. Elektrochem. 42, (3) HOFDIANN, 504 (1936). (4) LAUE,11.v o s : Z.Iirist. 64, 115 (1926). BRILL,R . : Z. Iirist. 66,38i (19251. BRILL? R . , A N D PELZER, 13.: Z. lirist. 72, 398 (1929); 74, 147 (1930). JOSES,F. W,:Proc. Roy. Soc. (London) A166, 16 (1938). (5) RUBISSHTEIX, A . &I.: Bull. acad. sci. U.R.S.S. 1943, 427-34. (6) SCHERRER, P.: Sachr. Ges. Wiss. Gottingen 1918,98. M. T.: Am. Inst. Mining Met. Engrs. 146, 22 (1942); U.S. patents 2,219,728 (7) SEATON, and 2,219,726. (8) WARREN,B. E.: J. Applied Phys. 12, 375 (1941). (9) ZETTLEMOYER, A . C., A N D TALKER, W.C.: Ind. Eng. Chem. 39, 69-74 (1947). (10) ZETTLEMOYER, A . c.,ZETTLEDIOYER, E . A , , A N D ~ ~ A L K E R , c.:To be published.
w.
