ADSORPTION S T U D I E S
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ilDSORPTIOS STCDIES TOLUEKE, BESZENE,ETHYL IODIDE, HYDROGEX SCLFIDE,WATERVAPOR, CARBON DISULFIDE, AND
P H Y S I C a L ADSORPTIOX O F NITROGEN,
P E K T A S E OX V A R I O U S P O R O U S 9 N D N O N - P O R O U S S O L I D S
B. L. HARRIS
AND
P. H. ER.Z;ZIE'I?I"
Chemical Engineering Department, The Johns Hopkins University, Baltimore, .Varyland Received September 6,1948 INTRODCCTIOX
In 1938 a general kinetic theory of multimolecular physical adsorption of gases by solids was developed by Brunauer, Emmett, and Teller (4), which gave for adsorption on plane surfaces an equation of the form
P
-
V(p0 - p )
1
c - l p
- v,c + lr,c
(1)
where V is the volume of gas (S.T.P.) adsorbed a t pressure p a t a temperature a t which the vapor pressure of the adsorbate is po. V,,, is the volume of gas required t o form a single layer on the solid adsorbent and C is a constant equal t o a1
bl: e ( B I - E L ) I R T
azbl
E1 being the average heat of adsorption in the first layer of adsorbed gas, EL the heat of liquefaction of the adsorbate, and albp/a2bla group of kinetic constants which has usually been assumed t o have a value of approximately unity but actually (5, 6) may deviate considerably from unity. In testing the general applicability of equation 1 to various adsorbates and adsorbents, it has seemed worthwhile t o measure the adsorption of a variety of adsorbates on both porous and non-porous adsorbents. The present paper contains a number of such data not heretofore reported. EXPERIMESTAL
Low-temperature apparatus The adsorption determinations m-ere carried out in one of two types of appara.us. The one used for l o w r temperature runs (nitrogen at - 195"C., hydrogen julfide a t -78"C., ethyl iodide, carbon disulfide, and pentane a t OOC.) was essenially the same as that described in many previous publications (8, 10, 11). The lowest temperature n-as obtained by means of a bath of commercial liquid titrogen and was measured by an oxygen vapor-pressure thermometer. The ,ath a t -78°C. was prepared from dry ice and acetone, and was stirred in a arrow-necked Dewar flask by tank carbon dioxide. This temperature was ieasured by a copper-constantan thermocouple checked against a carbon diox1 Present address : Gulf Research and Development Company's Alultiple Fellowships, [ellon Institute, Pittsburgh, Pennsylvania.
812
B. L. HARRIS
AND P. H. EMMETT
ide vapor-pressure thermometer. The 20°C. bath was a water bath maintained constant to 0.1”C. by means of an automatically regulated gas-heated circulation coil. For use with organic vapors, one such apparatus \vas modified by addition of a storage chamber for the liquid adsorbate and an ampoule breaker as described later. All stopcocks which came in contact with the vapor were lubricated with Glydag, a, colloidal dispersion of graphite in glycerol, a small quantity of which
r I I
I I I I
I I I I I
I
I
FIG.1. Adsorption apparatus used at higher temperatures. 31,manometer; C, mercury cutoff; B, calibrated buret; S,sample; U , ampoule breaker system (before breaking ampoule); Y and S,upper and loner zero settings for manometer; L, entrance tube for helium and nitrogen, sealed off before breaking ampoule. Connections t o vacuum and atmosphere were through the large three-xvay stopcock on the manometer.
was effectively freed of 11-ater by outga4ng under full vacuum at room temperature for 24 hr. This lubricant was found to hold a fairly good vacuum for a
few days, but the stopcocks \\-ere re-lubricated every day and fresh adsorbate was introduced into the apparatus. Niliiio.-teinper.ati~~eapparatus
For n-ork \\-ith organic v::pors at i 5 ” C . a special apparatus was constructed (figure 1). I t consisted of a manometer (31) connected by a special mercury
ADSORPTIOS STUDIES
813
cutoff C (designed to prevent mercury from being carried into sample bulb S with the first rush of gas) to the adsorbent sample hulb and to a calibrated bulbtype buret (B). The buret was connected by a simple cutoff t o the adsorbate chamber (P). The entire apparatus nith the exception of the top of the manometer and the adsorbate chamber was immersed in a \rater thermostat controlled at '75°C. t o 10.013"C. by means of a tubular forced-draft circulatorheater controlled by a mercury thermoregulator through an electronic relay system. The entire apparatus n-as visible through a removable +-in. plate-glass front on the thermostat. Removal of the glass front allon-ed lon--temperature runs to be made n-ithout removing the adsorbent from the apparatus. The position of the mercury confining liquid in the buret and manometer n a s controlled by varying the pressure in the reservoirs by the pressure and vacuum connections thereto. The apparatus n-as operated essentially as folloir s: The sample 11-asthoroughly outgassed at 100-400°C.,or if iron, it was first reduced by hydrogen as later described, the hydrogen entering the sample tube S through a tube at the bottom which lras sealed off after reduction. -1fter the outgassing, a low-temperature nitrogen run v-as made in order to determine the surface area of the adsorbent. In this run the '.dead spaces" from the top of the h r e t to the mark Y on cutoff C, and from mark ITto mark S,including the interstices and pores of the sample, were calibrated by helium in the U ~ U Rmanner. ~ Both helium and nitrogen irere introduced t o the apparatus through tube 1, prior to breaking the ampoule containing the more condensable a d m h t e . a\fter the nitrogen run the sample TI as again outgassed, the remornble thermoqtat front reassembled, and the thermostat filled uith water and heated to i j " C . , the mercury level in the cutoff lieing held above marl< I-, thiii isolating the sample. \Then conditions Irere a i desired. tuhe L \\.a ealed off, the adsorbate tube U \\.as cooled u i t h dry ice, and the ampoule hroken by pulling the catch electronagneticnlly, allon-ing the ampoule to fall on it. fine tip. The tube from L to lie thermostat n a s heated by a wire 11inding t o prevent condensation therein. l'he amount of gas allon-ed into the buret could tie controlled by controlling the emperature at U. From here on the manipulation v a s cimilar to that emiloyed on the other apparatus. Increments of gas were added with the cutoff iercury level at Y and the amount \\as determined by noting the pressure and olume (huret reading). The gas v-ab then pasqed into the sample by lowering i e mercury in the cutoff to mark S. The pressure v a s re-read as coon as the :stem had equilibrated. Successive readings I\ ere macle by raiqing the mercury L buret B. More gas could be added to the system by returning the mercury vel to mark Y and dropping the mercury level in the buret system, firat making reading at T to determine the amount of gas remaining in the dead space. esorption runs Trere made by slightly modifying the above procedure. Adsorbents
The porous glass used was a sample of acid-leached high-silica glass obtained lm the Corning Glass Corporation, and used in work reported previously (13). was sieved t o 100-200 mesh and \vas evacuated at 140°C. prior to runs.
814
B. L. HARRIS AND P. H. EMMETT
Two types of iron synthetic ammonia catalysts were studied, both of which had been previously studied by Emmett and Brunauer. Iron 973 was an unpromoted catalyst containing 0.13 per cent alumina as impurity. It was prepared for use by crushing the oxide to 8-14 mesh, reducing it in purified hydrogen for 48 hr. a t 400°C. a t a space velocity of 5000, and evacuating it for 1 hr. a t the same temperature. Iron 652 was an 8-14 mesh sample of a doubly promoted catalyst containing 0.44 per cent potassium oxide and 1.89 per cent alumina. It mas reduced for 48 hr. a t 5000 space velocity and a t 430"C., and was then evacuated for 1 hr. a t the same temperature. Approximately 10 cc. of each of these catalysts was used for a particular run. When changing gases in the reservoir or otherwise manipulating the apparatus a t atmospheric pressure, the sample was filled with prepurified nitrogen dried by passage through phosphorus pentoxide. Whenever the sample was exposed to air, it was re-reduced for a period of 12-24 hr., depending upon the extent of the exposure. The hydrogen used was tank hydrogen purified by passage over copper a t 350"C., thence through a dry ice trap, and finally through phosphorus pentoxide. The Pyrex glass was prepared by crushing laboratory Pyrex tubing, wet grinding it for 30 hr. in a ball mill with steel balls, treating it with dilute sulfuric acid to remove iron, and washing it on filter paper in a Biichner funnel. It was evacuated a t a temperature of 140°C. for all determinations. The silver was prepared by the method of Benton and Elgin (2) by precipitating, washing, and drying the hydroxide, and reducing it by hydrogen a t temperatures that slowly increased from 25" to 100°C. in the first 12 hr. and remained a t 100°C. for 12 hr. additional. Glass spheres were made from Pyrex glass by the method of Bloomquist and Clark (3). A sample was isolated by fractional settling in water; it mas in the size range 3-5 microns as judged by settling rate and by microscopic observation. About 13 g. was used for a run; the sample was evacuated a t 140°C. for 1 hr.
Gases Helium was purified by passage over outgassed charcoal a t -195°C. Prepurified water-pumped nitrogen was dried by passing it slowly over phosphorus pentoxide. All adsorbates which were liquids a t room temperature were of C.P. grade; they n-ere further purified and freed of dissolved air by being condensed in a trap a t -195OC., and were evacuated and slon-ly distilled under vacuum into ampoules which were sealed for later use. The vacuum pumps were closely connected to the distillation system during the entire process and the first and last thirds of each distillation n-ere discarded. Hydrogen sulfide Tras treated similarly but was stored in the gas phase rather than in ampoules. RESULTS
Glass spheres The glass spheres were run successively with all of the various adsorbates ai temperatures close to or belov their respective boiling points. Equilibrium wa: attained rapidly with all adsorbates except water. I n the run with \rate
815
ADSORPTIOS STUDIES
vapor the pressure in the apparatus never became constant. The results on water vapor were therefore discarded. After the sample was thoroughly outgassed at 140OC. (following the water vapor run), it was used for a second run with nitrogen. The surface area as judged by nitrogen was found to have increased from 19.0 to 24.4 sq. m. per gram, owing presumably to etching of the glass surface by water vapor in the manner noted by previous workers (14, 17). The adsorption results are plotted as isotherms in figure 2; the values of Vm,C, and surface areas as calculated from the various adsorbates are sum-
p/
