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LAWRENCE SPENADEL AND MICHEL BOUDART
mechanisms must exist and marker motion is absent, it is concluded that the sucrose and water clusters interdiffuse through a coordinated isosteric process.
T’ol. 64
Acknowledgment.-The authors wish to express their sincere appreciation to Dr. Scott E. Wood for helpful suggestions.
DISPERSION OF PLATINUM ON SUPPORTED CATALYSTS BY LAWRENCE SPENADEL AND MICHEL BOUDART Esso Research and Engineering Company, Linden, New Jersey Received June 89, 1969
Hydrogen chemisorption at saturation can be used to determine the specific surface area and an average crystal size for platinum highly dispersed on an alumina support in a range of dispersion not accessible to measurements by X-ray line broadening. For a fresh catalyst containing 0.6 wt. % platinum, the ratio of platinum atoms to adsorbed hydrogen atoms is almost equal to unity. This shows that the metal is very highly dispersed on the support. Platinum crystal growth is brought about by a heat treatment a t elevated temperatures. Good agreement is obtained between platinum particle size calculated from chemisorption and from X-ray line broadening in a crystal size range where the latter method becomes applicable.
Introduction platinum is smaller than about 50 A. Since X-ray Cheniisorption has been used in the past to de- measurements are not appIicable in this region, it termine the surface area of catalyst.’ The method was decided to study the dispersion of platinum requires that the adsorbate form a chemisorbed on an alumina support by means of hydrogen monolayer and that there exist a simple relation chemisorption. between the number of molecules adsorbed a t Experimental saturation and the number of surface atoms. A volumetric glass system of standard design’ was used Thus, hydrogen forms an almost complete monothe adsorption measurements. The apparatus included layer on a nickel evaporated film with each ad- aforMcLeod gauge, a mercury manometer read with the help of sorbed hydrogen atom corresponding to one surface a cathetometer, sample and gas storage bulbs, and an oil metal atom.2 diffusionpump used in conjunction with a mechanical pump. The gases used were helium for dead space determinations, These special requirements have prevented for catalyst reduction and adsorption, and nitrogen chemisorption from becoming a general method of hydrogen argon for BET surface area determinations. Commersurface area determination. Nevertheless, its prin- and cial cylinder gases were purified as noted: hydrogen by ciple has considerable merit in particular situations. diffusion through a palladium thimble; helium by passing I€,for example, a metal is dispersed on an oxide i t through a train composed of activated charcoal a t liquid nitrogen temperature, a drying column and a liquid nitrogen support in small quantities and in a particle size cold range that cannot be detected by physical meas- trap.trap; argon by passage through a liquid nitrogen cold urements, the problem of determining the disThe adsorbents or catalysts used were 7-alumina, 7persion of the supported metal may be solved in alumina supported platinum catalysts and platinum black. principle by selective chemisorption. This has The supported catalysts were reforming catalysts, prepared described in the literature.8 The catalysts were made by been done by Boreskov and Karnaukhov3 who aa impregnation of the supports with aqueous chloroplatinic studied the preferential chemisorption of hydrogen acid and then drying at 550” for one hour. The chloride by platinum supported on silica gel. In this in- and platinum are present in near stoichiometric amounts. vestigation, the average particle size of platinum The surface areas of the alumina supports are given in crystallites calculated from the metal specific Table I. The platinum black used (>98w0 platinum) was obtained from Baker and Co. surface area was never less than 40 8. Platinum crystal size was determined from the breadth On the other hand, the dispersion of platinum of the (311) X-ray diffraction line, recorded on a Norelco on acidic bases (acidic aluminas or silica-alu- X-ray spectrometer. The standard equation of Patterson9 minas) has not been reported until r e ~ e n t l y . ~ - with ~ Warren’s correctionslo was used in the calculation of Unpublished data obtained in this Laboratory X-ray crystal size. measurement of a hydrogen adsorption isotherm, indicate that catalysts containing less than 1 wt. theBefore adsorbent or catalyst was subjected to a standard preyo platinum on 7-alumina are characterized by a treatment. First the sample was outgassed zn Vacuo a t 500” w r y high dispersion of the metal. In fact, the for a few hours. Then it was kept under about one atcrystal size of the platinum is, in most cases, too mosphere of hydrogen a t 500” for 30 minutes in order to the surface adequately. Finally it was outgassed a t small to be determined by X-ray line broadening. reduce 500” overnight (16 hours). No difference in adsorption isoIt can be assumed that the crystal size of the therm was detectable if the final outgassing time was in(1) A. F. Benton, J . A m . Chem. Soc., 48, 1850 (1926). (2) 0. Beeck, ”Advances in Catalysis,” Vol. 11, Academic Press, Inc., New York, N . Y . , 1950, p. 151. (3) G . K . Boreskov and A. P. Karnaukhov, Zhur. Fia. Khim., 2 6 , 1814 (1952). (4) L. Spenadel and M. Boudart, paper presented at Boston A.C.S. meeting, 1959. ( 5 ) J. J. Keavney and S. F. Adler, paper presented at Boston A C.S. meeting, 1959. (6) T. R. Hughes, R. J. Houston and R. P. Sieg, paper presented at Boston .4.C.S. meeting, 1959.
creased to 72 hours. I n this work, outgassing was considered completed when a pressure of less than mm. could be maintained for more than 15 minutes in the system cut off from the pumps a t the temperature of the experiment. (7) G. 2. Joyner, “Scientific Glass Blowing,” Instruments Publishing Co., Pittsburgh, Pa., 1949, p. 257. (8) F. G. Ciapetta and C. J. Plank, “Catalysis,” Vol. I, Reinhold Publ. CorD., New York, N. Y.,1954, p. 315. (9) A. C. Patterson, Phgr. Rev., 49, 884 (1936). (10)”B. E. Warren, J . Amer. Ceramzc Soc., 21, 49 (1936).
DISPERSIOX OF
Feb., 1960
205
P L A T I N U M O X S C P P O R T E D CATALYSTS
TABLEI DATAON PLATIXUM DISPERSION
m.Z/g.
Adsorpt. a t 250° 24 cm. in H atoms moles Hz X 105/g. ads. per Pt Sample Support atom in sample
Pt black Pt black3 0.60 wt. 70 Pt on A1203 (fresh) 0 . GO wt. yo Pt on A1203 (heat treated 2 hr., 650') 4.46 wt. % Pt on A1103 (heat treated 24 hr.,
0.55 0.17 195
4.98 1.62 19.8
4.G5
135
5.50
3.40
.14
650') 3.10 wt. % Pt on 750')
115
8.80
3.45
.017
95
3.50
0.85
.033
BET S A ,
A1203
..
..
0.0019 ,00063 .03
5.4. of P t ,
P t crystal size from adsorpt.,
A.
rn.z/g.
0,54 0.17 273
P t crystal size from X-ray line broadening,
A.
..
4,300 13,700
