Dec., 1962
ADSORPTION OF GASESON ACTKVATED Iito~(II1)OXIDE
2397
the 2.1% weight loss due to heating from 290 to a model for the catalytic activity can be developed 520'. Also, the effects of irradiation and thermal based on Fe+3 impurity ions as the irradiation inactivation on the activation energy were quite dif- duced active sites. This model was shown to fit ferent. While thermal activation lowered the qualitatively the experimental data. activation energy a significant amount, irradiation (3) Results of ultraviolet irradiation have shown either raised t9he activation energy or left it un- that the bulk of the activity change must be atchanged. The mechanism of activation by ir- tributed to changes in the electronic structure of the radiation therefore could not be the same as that crystal rather than to changes in the number of of thermal activation. lattice imperfections. Conclusions.---( 1) Under proper conditions Acknowledgment.-The authors wish to exboth neutron and ultraviolet radiation have a press their appreciation to the Nat*ional Aeromarked effect on the catalytic activity of MgO. nautics and Space Administration for a grant to The radiation effects depend on the extent of the carry out this research. They also wish to previous degassing of the surface. thank Dr. W. Fulkerson and Mr. C. L. Hearn for ( 2 ) By comparing radiation effects on catalytic assistance in the experimental portion of this properties with those effects on the solid properties, study.
INFRARED AND VOLUMETRIC DATA ON THE ADSORYTIOS OF AMMONIA, WATER, AND OTHER GASES ON A4CTIVATEDIROK (111)OXIDE1 BY GEORGE BLYHOLDER AKD EDWIN A. RICHARDSOY Department of ChPwtastry, Cnaversity of Arkansas, Fa yetteazllr, Arknnsas Recezzed March 16, 1962
Infrared spectra in the 2 to 13 p range have been obtained of "3, H20, HzS, EtOH, and Et20 chemisorbed on a-Fez03. N2,OS,Ho, CO, Sot, CI?,C2Hs,and CtHa do not chemisorb a t 25" on a-Fe203. HtO chemisorbs by dissociating to form a OHion and a proton which reacts with a surface O + ion to produce another OH- ion. NH, upon chemisorption occupies the same surface sites as HzO and does not dissociate. Ion-dipole interaction between NH, and Fe+3ions together with hydrogen bonding furnishes the heat of chemisorption which was measured by equilibrium pressure data from 200 to 350" to be 11.5 kcal. per mole. NH4+formation occurred only whm physically adsorbed H?O and NH, were present and not when only NH3and surface hydroxyl groups from chemisorbed water were present. The infrared bands of physically and chemically adsorbed NH3showed no evidence of rotational structure superimposed on the vibrational bands. The adsorption of the other gases IS discussed briefly.
Introduction
,4 major improvement in the tools available for adsorption studies has occurred in the past few years. The application of infrared techniques has been reported by several investigator^.^-^ The utility of infrared methods in studies involving structural assignments for surface species is apparent Mapes and Eischens3 obtained the infrared spectra of S H 3adsorbed on a silica-alumina cracking catalyst. On the basis of the spectra indicating that most of the chemisorbed NH3 still had a NH3 structure as opposed to a NH4+structure, they concluded that the catalyst behaved largely as a Lewis acid. However, their spectra indicated some NH4+ was present which they attributed to the fact that they could never get rid of all hydroxyl groups on the surface. While the above system is sufficient to indicate the type of acidity involved in t'he catalyst, it is not well suited to a detailed st,udy of the structure of the (1) (a) This n-ork was supported in part b y a grant from t h e Smerican Oil Company and in part by a p a n t from t h e Monsanto Chemical Company. Acknowledgment is made to the donors of the Petroleum Research Fund, administered b y the America1 Chemical Society, for partial support of this research. (b) -4bstracted in part from a thesis presented b y E. A . Richardson t o t h e University of Arkansas in partial fulfillment of the rr:iuirwients for t h e degree of Doctor of Pliilosophy, 1962. (2) R. P. Eisctiens a n d JT. 1.Pliskin. ".ldvanees in Catalysis." Academic Press, Inc., Sh;pu.York, ps'. Y . .1958. (3) .J. E. N a p e s a n d R. P. Eischens. .I. P h m Chem., 68, 1039 (1954). (4) E. \I. ISyriogand RI. E. ~ ' a i l s ~ o r t M l ~i n. i n g E n y . . 5, 531 (1986).
surface complex ; the silica-alumina cracking catalyst is essentially opaque beyond 7.5 p. Since two of the fundamentals of ammonia complexed with metals5 occur beyond 7 p , it is of interest to obtain the spectra on a metal oxide in which this range is accessible. It has been found that a-FezOanot only meets the criterion of sufficient infrared transmission but it also can be completely dehydrated. A number of volumetric studies have been carried out involving the adsorption of various gases on iron(II1) oxide. Isotherms of NH3 on A1203, Fe2O3, and Cr2O3 in the 10 to 700 mm. pressure range have been described.6 Cremer and Gruner' observed no adsorption of S H 3 beyond a simple monolayer a t 20' on Fep03. A Freundlich isotherm was obtained a t high coverages and the first portion of "3 allowed to contact the activated cr-F'e203was taken up non-reversibly. S o measure of this quantity was reported. These investigators evidently were unaware of the difficulty in removing the surface hydroxyl groups, and hence chemisorption was noted to take place only in minor amounts since OH groups probably were covering most of the active chemisorption sites on the adsorbent. (3) J. P. Faust a n d .T. 1'. Quagliano, J . . l m . Chsiik. Soc., 76, 5346 ( 195.2).
(6) Von N. Xiktin, Z. anorg. allgem. C k m . , 156, 338 (1926). (7) E. Crrwer and R . Gruner, %. php-ik. Chern.. 196, 319 (1931).
GEORGE BLYHOLDER AND EDWIN A. RICHARDSON
2598
It therefore was necessary to redetermine the isotherms of Cremer and Gruner using activated (OH-free) Fe203. In this manner the ratio of the number of moles of NH3 required to complete the chemisorbed layer (or fill all chemisorption sites) to the number of moles of NH3required to complete the monolayer of chemisorbed and physically adsorbed NH3 can be determined for a given weight of activated Fe2O3. The principal purpose of this article is to describe the adsorption of ammonia, water, and other gases on iron(II1) oxide and to ascertain the structure of the surface complexes. Another article in preparation deals with the absorption spectrum of the solid Fez03. Experimental Apparatus.-A Perkin-Elmer Model 21 infrared recording spectrometer was employed. To expand the low transmission range of the instrument, the reference beam was partially blocked off with screen wires. The sample disk was mounted in a modified gas cell as described by Blyholder and Neff.8 The effective range of the system was 2 to 13 cc.
A conventional vacuum apparatus capable of maintaining 0.001 p pressure was employed in degassing the disk. Materials.-Certified reagent grade iron(II1) oxide obtained from Fisher Scientific Company wm em loyed without further treatment. Particle sizes were in tge 5 p range and the material had a surface area of about 15 m.2/g. The far infrared spectrum of this material is essentially that of a-FezOIQand so the material is considered to be a-FezOl. Anhydrous ammonia obtained from the Matheson Company was used without further purification. Commercial grade welding oxygen obtained from the National Cylinder Gen. Division of Chemetron Corp., Chicago, Illinois, was used after passing through a 2-ft. column of anhydrous magnesium perchlorate to remove any moisture. The vapor from distilled water, degassed at room temperature, was employed. Commercial tank CO was used after heating to 350' to decompose iron carbonyl and passage over activated charcoal a t liquid air temperatures to remove any other impurities. Commercial tank SO1 obtained from the Matheson Company was employed without further purification. Hydrogen sulfide was used after degassing and distillation. Commercial tank gas was employed after assing through a 2-ft. column of anhydrous magnesium percilorate to remove moisture. Absolute ethyl alcohol and ethyl ether were used after distillation and degassing. Preparation of Disks.-About 0.3 g. of m-Fe2O8was transferred t o a I in. diameter die and pressed at 8000 p.s.i. The die was heated before pressing in an oven at 140' t o prevent sticking of the disk to the die face. By careful handling, the disk could be removed from the die without breaking and transferred to the cell. Activation of a-FelOs Disks.-The disks prepared above contained considerable water and thus were not active in chemisorption. The activation process consisted of degassing the disk a t room temperature for several hours or until a vacuum in the order of 0.05 p could be maintained. This disk then wm heated to 375" in an atmosphere of On and maintained for several hours. This oxidized any impurities (such as magnetite) which might cause subsequent reduction. The cell then was evacuated and the temperature raised t o 475' for the final activation step, which was com lete after about 16 hr. when the absorption spectrum of &e disk no longer indicated the presence of hydroxyl groups. Chemisorption.-A controlled amount (usually 100 p ) of adsorbate was allowed to contact the disk. If the pressure in the system (about 300 ml. volume) dropped immediately to less than 1 p pressure, the adsorbate was assumed t o be chemisorbed. The adsorption spectrum thus D. Neff. J . Pht~s.Ciiem., 66, 1464 (1962). Private communication, Shell Oil Company, Houston, Texas.
( 8 ) 0 . Blyholder a n d L. (Y)
Vol. 66
could be obtained before and after each such addition and changes caused by the adsorbate readily detected. Any material which remained on the disk after evacuation for 1 hr. at 25' was taken as chemisorbed. Isotherms and Surface Area.-Isotherms for NHI a t 25' were obtained in the usual manner by adding incrementa of NHHand measuring the e uilibrium pressure. The total surface area for activated a%esOa was calculated from this isotherm. The physically adsorbed ammonia then was desorbed by evacuating the system at room temperature overnight and another isotherm obtained. This I essentially the method of Cremer and Gruner? and yields the amount of material on a physically adsorbed monolayer from which the area involved can be calculated. Subtracting this from the value for the total gives the area involved in chemisorption. This subsequently will be referred to as method 1 for determining the amount of chemisorption. It was found possible to measure the amount of material adsorbed before a measurable equilibrium pressure persisted in the system and this was taken to represent chemisorption. The surface area involved was calculated directly. Th? will be referred to as method 2. The amount of ammonia physically adsorbed at low pressurea is negligible compared to the chemisorbed monolayer. These methods give somewhat different amounta of chemisorption. To obtain isotherms at several hundred degrees centigrade where the chemisorbed NHa is in equilibrium with the gas phase, a small bulb was filled with about 10 g. of FelOo and sealed to the manifold of the vacuum system with a 6 in. capillary tube. A furnace was placed around the bulb. The volume of the bulb is negligible compared to the volume of the manifold so that the amount of gas desorbed by heating a sample in the bulb can be determined from the pressure in the manifold.
Results A small known amount of each adsorbate was added to an adsorption cell containing an activated disk of Fez03. Since each chemisorbed gas gave easily detected infrared bands, the occurrence of chemisorption could be verified by ascertaining that the intensities of these bands were constant upon prolonged evacuation of the cell. From these measurements HzO, "3, HzS, EtOH, and Et20 were found to chemisorb at 25' on activated cyFeZO3. Similarly N2, 0 2 , Hz, GO, S02, Clz, C2H6, and C2H4were found not to chemisorb. For two powder samples tested (Fisher Fez03) and one disk sample, the total surface area of a monolayer including chemisorbed and physically adsorbed material was found to be 12.1, 15.9, and 14.1 m.2/g., respectively. This indicates that the disk pressing procedure does not seriously diminish the surface area and the differences found are well within the precision expected of the method. Adsorption isotherms were obtained for pressures from less than 1 to 650 mm. Since these appear just like Cremer'~,~ they are not reproduced here. The isotherms leveled out past about 400 mm., so the amount adsorbed at 450 mm. pressure was assumed to represent a monolayer. The aTea covered per NH3 molecule was taken as 12.9 A2. It should be pointed out that these calculations are based upon the assumption that XH3 does not physically adsorb on top of the chemisorbed ammonia. This appears valid since multilayers of physically adsorbed NH3 do not form under these conditions' and a chemisorbed NH3 molecule is assumed to present a surface for interaction with other Tu",molecules just like a physically adsorbed NH, molecule. The results of using the two methods of determining the surface area for chemical adsorption of
ADSORPTIOSO F CASES
Dec., 1963
S H , on a-Fe203in both powder and disk form a t 20' compared to the total area determined, as described in the preceding paragraph, are given in Table I. TABLE I TO TOTAL A U S O I t B h I > x&
TFlE ]
