200
Vol. 67
NOTES
fragment by hydrogen abstraction. Since the yield of 4-carbon compounds was reduced to zero in the presence of iodine, reactions to form these are more likely radiation induced or thermal. Acknowledgment.-The authors gratefully appreciate the assistance of Dr. A. Bureau and the synchrotron staff, particularly Mr. J. R. McConnell, who operated the synchrotron for the many irradiations, and nlr. D. Clark, who performed Fricke dosimetry in the synchrotron. T H E STATE OF PLATINUM IN RE-FORMING CATALYSTS BY MARVINF. L. JOHNSON AND CARLD. KEITH Sinelair Research, Inc., Harvey, Illinozs Received August 0, 1066
Alumina with a small amount of platinum is widely used as a re-forming catalyst-to catalyze reactions of dehydrocyclization, dehydrogenation, isomerization, and hydrocracking of hydrocarbons. Such catalysts normally are prepared by impregnating alumina with a soluble platinum compound, followed by calcination in air a t an elevated temperature. The chemical state and degree of dispersion of the platinum in platinum-alumina have been the subject of several recent papers. Since the catalysts are employed under reducing conditions, one presumes that platinum has been converted to the metallic state. Indeed, proposed mechanisms for the various reforming reactions1 presuppose the existence of metallic platinum as the dehydrogenation component of these dual-function catalysts. Hydrogen consumption measurements in these Laboratories and by Mills, et C L E . , ~ indicate nearly complete reduction of Pt+4to Pto in a few minutes in hydrogen, a t temperatures as low as 245’. As suggested by Kluksdahl and Houston,3 the darkening and dehydrogenation activity which appear upon reduction with hydrogen suggest the production of platinum metal. Furthermore, it has been observed in these Laboratories that hydrogen reduction a t an elevated temperature is necessary to cause platinum-alumina to catalyze HrD2 exchange; rate constants of the order of 20-30 min.-l at 195.5’ and 518 mm. are observed for various preparations containing 0.6% Pt. The above constitutes evidence for the existence of platinum metal upon treatment of platinum-alumina with hydrogen at elevated temperatures. Several workers have employed adsorption techniques to show that this platinum exists in a high degree of dispersion. Hughes, et ~ 1 . ~ used 4 carbon monoxide adsorption, while 0thers6-~employed hydrogen adsorption to draw this conclusion. Hughes, et u L , ~showed, in addition, that hydrogen reduction must be carried out above a t least 200° to obtain adsorption by platinum metal; they showed further that a minimum extent of plati-
-
(1) F. G. Ciapetta, R. M. Dobres, and R. W. Baker, “Catalysis,” Vol. 6, ed. by P. H. Emmett, Reinhold Publ. Corp., New York, N. Y.,1958. (2) G. A . Mills, S. Weller, and E. B. Cornelius, “Second International Congress on Catalysis,” Vol. TI, Paris. 1960, Paper 113. (3) H. E. Kluksdahl and R. J. Houston, J . Phys. Chsm., 66, 1469 (1961). (4) T. R. Hughes, R. J. Houston, and R. P. Sieg, Preprints Pet. Div., ACS, April, 1959. ( 5 ) L. Spenadel and M. Boudart, J. P h w Chem., 64, 204 (1960). ( 6 ) S. F. Adler and J. J. Keavney, ibid., 64, 208 (1960). (7) €1. L. Gruber, &id., 66, 48 (1962).
n u n area, as measured by carbon monoxide adsorption, is necessary to obtain activity for re-forming methylcyclopentane. Similarly, Mills, et u L , ~observed that a loss of catalytic effectiveness paralleled the growth of platinum metal particles as observed by X-ray diffraction. Platinum surface area thus is an important factor in re-forming, A different point of view has been proposed by McHenry, et aL18who postulated the active component of these catalysts to consist of a platinum-alumina complex. This conclusion was based on their finding that dehydrocyclization activity was dependent on the amount of complex present, defined as that portion of the platinum which was soluble with the alumina in aqueous hydrofluoric acid or acetylacetone. The present paper presents evidence to show that this apparent discrepancy can be resolved if one considers the “complex” to exist in the oxidized state and to be responsible for high platinum metal dispersion upon subsequent reduction. Experimental Platinum solubilities of Table I were measured by the method described by McHenry, et aZ.8 Table I1 results were obtained by digestion in 1:4 H&Ok a t 60’ for several days; the solutions and the residues each were analyzed. Chemisorptions of carbon monoxide were performed after reduction for 12 hr. at 482” in flowing purified hydrogen, and evacuation for 9 hr. at 300°, at about 5 X 10-8 mm.; this temperature of evacuation was chosen to minimize adsorption by alumina t o about 0.05 cc. (STP)/g. After cooling to room temperature in a few mm. of helium and evacuation of the helium, a measured quantity of carbon monoxide is admitted to the catalyst. One hour later, automatic Toepler pumping is started to return unadsorbed gas to the calibrated system. This pumpTABLE I EFFECT OF OXIDATION ON SOLUBLE PLATINUM yo soluble Sample
Pt”
History
A Used catalyst B A 3 hr., 440°, one atm. C A 3 hr., 480°, one atm. D A 3 hr., 505’) one atm. E C 20 hr., 505’, 10 atm. Solubilityin HF; 0.6% totalPt.
+
+ + +
air air air air
CO ohemisorption STP/g.)
(00.
0.23 .27 .32 .34 -48
0.14 .20 * 22 .24 .33
TABLE I1 EFFECTOF OXIDATIVE CONDITIONS ON SOLUBLE PLATINUM Temp., OC.
Partial pressure of Oe, atm.
% solubfe platinum”
HE-Dn exchange rateb
..
427 0.015 0.27 427 0.11 .427“ 22 427 1.o .452 30 482 0.21 .493 30 1.0 .509 46 482 510 0.015 .433 39 510 0.21 .508 50 1.o .52 45 510 538 0.21 .504 57 538 1.o .53 47 18 582 0.015 .367 582 0.21 .378 43 .533 44 582 1 .o 650 1.o .374 33 “Solubility in HzS04; 0.55% total Pt. bSee text. OThis sample wm used for the other treatments. (8) K. W. McHenry, R. J. Bertolacini, H. M. Brennan, J. L. Wilson, and H. 8. Seelig, “Second International Congress on Catalysis,” Vol. 11, Paris, 1960, Paper 117.
NOTES
Jan., 1963 ing is continued for 6-12 hr.; no further carbon monoxide is recovered after 6-8 hr. The chemisorption is defined as the difference between gas admitted and gas recovered. The H2-D2 exchange rates also are measured after reduction for 12 hr. a t 482'. The catalyst is cooled to liquid nitrogen teniperature in hydrogen, after which a mixture of 25% D2 and 75% HZis passed over the catalyst a t a constant rate and a t a constant pressure of 518 mm.; the effluent is analyzed for HD by maiw spectrometry. From several consecutive runs a t varying flow rates, the exchange rate constant is attained as the slope OF a linear plot of -In (1 X/X,) us. l / S V , where X is the conversion to HD, X , the equilibrium conversion, and S'v is expressed as cc. (STP)/g./min.
-
Results and Discussion A commercial catalyst consisting of 0.6% Pt om alumina, partially deactivated by use and subsequent regenerations, was treated with dry air at various temperatures. The changes in carbon monoxide chemisorption and platinum solubility are shown in Table I. Sample A, the regenerated catalyst, was found to have a small amount of platinum metal, barely detectable by X-ray diffraction; this plus the low carbon monoxide chemisorption value indicates that some growth of platinum crystallites has occurred. Correspondingly, less than half of the platinum was soluble, compared to nearly 100% for the fresh catalyst. As the severity of oxidative treatment was increased, the extent of platinum solubility increased. At the same time, the platinum surface area upon Subsequent reduction incrertsed. Thus, the formation of platinum-, alumina complex results in greater platinum surface area, as measured by carbon monoxide chemisorption. The existence of a platinum-alumina complex is, therefore, desirable, not per se, but because it leads to a high dispersion of the platinum upon subsequent reduction. Consider the reaction
Pt
+ Oa J_ PtOz
PtOz.alumina
It is clear that in the presence of an oxygen partial pressure, there will be a tendency for the oxide to form, and that the oxidized state will be stabilized by complex formation with alumina. Therefore, an increase in oxidation severity increases the extent of complex formation. At the same time, since the particles of platinum are dispersed by the complex formation, subsequent reduction produces more highly dispersed platinum metal. There are limits to the improvements one can attain by increasing oxidation severity. As platinum-alumina is treated in a given oxygen partial pressure at successively higher temperatures, a temperature will be reached at which the oxygen pressure equals the decomposition pressure. Above this temperature, there will be decomposition to form platinum metal. The critical tempera1,ure wiIl increase as oxygen partial pressure increases. If the reasonable assumption be made that mobility is greatest in the oxidized state, the treatment at a temperature such that the decomposition pressure exceeds the ambient pressure will result not only in the formation of platinum metal, but in an increase in crystallite size. Herrmann, et uL19 have reported a decrease in the amount of soluble platinum on heating in air a t 593 and 625'. The results given in Table I1 illustrate this hypoth(9) R. A. IIcrrmann, S. F. Adler, M. S. Goldstem, and R. R.1. DeBaun, J . Phus. Chem., 6 6 , 2189 (1961).
201
esis. As oxygen partial pressure is increased with temperature held constant, the amount of platinumalumina complex increases. As in Table I, this increase is accompanied by a subsequent increase in platinum dispersion upon reduction, as measured by the rate of H2-D2 exchange. Also, as temperature is increased at constant oxygen pressure, the extent of complex formation passes through a maximum, as predicted. This critical temperature, at which the oxygen partial pressure equals the decomposition pressure, is near 510' at 0.21 atm. and near 580' a t 1.O atm. It is concluded, therefore, that in the oxidized state a platinum-alumina complex does exist, but that it is converted to metal on treatment with hydrogen. Furthermore, the degree of dispersion of this metal increases with an increase in the fraction of platinum soluble in the oxidized state. Acknowledgment.-The authors wish to acknowledge the assistance of J. S. Melik in determining rates of H2-D2 exchange.
THE KINETICS OF THE OXIDATION OF HYDROGEN PEROXIDE BY CERIUM(1V) BYG. CZAPSIU,B. H. J. BIELSKI, AND N. SUTIN Chsmistry Department, Bronkhaven National Laboratory, Upron, Long Island, New York neC&ved M a y B8, 1063
Baxendale has reported that the oxidation of hydrogen peroxide by cerium(1V) is complete within a few seconds at room temperature, even at micromolar concentrations of the two reactants, and has proposed that the reaction proceeds in two one-electron steps.2 The kinetics of the hydrogen peroxide-induced cerium(II1)-cerium(1V) exchange in 0.8 N sulfuric acid solutions have been studied by Sigler and Masters.3 Following Baxendale, they have proposed that the hydrogen peroxide-cerium(1V) reaction proceeds via the two-stage process Ce(1V)
+ HzOz
kl Ce(1II)
+ HO2 + Hf
(1)
k-1 k2
Ce(1V) 3- HOz +Ce(II1)
+ O2+ H+
(2) with Ic-llkz = 0.129 f 0.013 a t 0'. According to Baer and Stein, on the other hand, the reverse of reaction 1 does not occur to any significant extent.4 The conclusions of Baer and Stein are based on studies of the stoichiometry of the hydrogen peroxide-cerium(IV) system. The formation of perhydroxyl radicals as intermediates in the hydrogen peroxide-cerium(IV) reaction, as required by the above reaction scheme, recently has been confirmed by means of electron spin resonance spectroscopy.K16 (1) Research performed under the auspices of the U. S. Atomic Energy Commission. (2) J. H. Baxendale, J. Chem. SOC.(London), Spec. Publ. No. 1 , 40 (1954). (3) P. B. Sigler and B. J. Masters, J . A n . Chem. SOC.,79, 6353 (1957). (4) S. Baer and G. Stein, J . Chem. Soc., 3176 (1953). (5) E. Srtito and B. H. J. Bielski, J. Am. Chem. Soc., 83, 4467 (1961). (6) B. €1. J. Bielski and E. Balto, J. Phys. Chem., 66,2266 (1962).
