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(4) G. B. Pariiskii and V. B. Kazanskii, Kinet. Katai., 5, 96 (1964). (5) A. Abou-Kais, J. C. Vedrine, J. Massardier, G. Dalmai-Imelik, and B. Irnelik, C . R. Acad. Sci.. Ser. C , 272, 883 (1971). (6) J. C. Vedrine, J. S. Hyde, and D. S.Leniart, J. Phys. Chem., 76, 2087 (19721. (7) A. Abou-Kais, T h k e doctoral, L'UniversitB de Lyon, 1970. (8) G. C. Bond, "Catalysis by Metals", Academic Press, London, 1962. (9) J. L. Hall and R. T. Schumacher, Phys. Rev., 127, 1892 (1962). (IO) B. Welber, J . Chem. Phys., 43, 3015 (1965). (11) R. G. Bessent, W. Hayes, J. W. Hodby, and P. H. S. Smith, Proc. R. SOC., London, Ser. A , 309, 69 (1969). (12) D. M. Gruen, B. Siskind, and R. 8. Wright, J. Chem. Phys., 65,363 (1 976). (13) P. A. Sermon and G. C. Bond, Catai. Rev., 8, 211 (1973). (14) Performed commercially by the Baron Consulting Co., Connecticut. (15) L. E. Iton, Ph.D. Thesis, Princeton University, 1976. (16) T. H. Vanderspurt, Ph.D. Thesis, Princeton University, 1972. (17) (a) F. W. Jones, R o c . R. SOC.,London, Ser. A , 166, 16 (1938); (b) H. P. Klug and L. E. Alexander, "X-ray Diffraction Procedures", Wiley-Interscience, New York, N.Y., 1974. (18) B. Welber, Phys. Rev. A , 136, 1408 (1964). (19) E. R. Bernsteinand G. M. Dobbs, Rev. Sci. Instrum., 44, 1314(1973). (20) G. J. Tramrnell, H. Zeldes, and R. Livingston, Phys. Rev., 110, 630
(1958). (21) A. M. Portis, Phys. Rev., 91, 1071 (1953). (22) T. G. Castner, Jr., Phys. Rev., 115, 1506 (1959). (23) E. L. Cochran, F. J. Adrian, and V. A. Bowers, J . Chem. Phys., 34, 1161 (1969). (24) L. E. Iton and J. Turkevich, J . Phys. Chem., 81, 435 (1977). (25) A. Abou-Kais, J. C. Vedrlne, J. M. Massardier, and G. Dalmai-Imelik, J . Chem. SOC.,Faraday Trans. 7 , 70, 1039 (1974). (26) E. D.Sprague and D. Schulte-Frohlinde, J. Phys. Chem., 77, 1222 (1973). (27) T. E. Gunter, J . Chem. Phys., 46, 3818 (1967). (28) H. C. Box, E. E. Budzinski, K. T. Lilga, and H. G. Freund, J . Chem. Phys., 53, 1059 (1970). (29) J. C . Vedrine, J. Massardier, and A. Abou-Kak, Can. J . Chem., 54,
1678 (1976). (30) K. Kayama, J. Chem. Phys., 39, 1507 (1963). (31) (a) H. E. Radford, Phys. Rev., 122, 114 (1961);(b) ibid., 126, 1035 (1962). (32) (a) J. C. Vedrine, T h h e doctoral, Universitl de Lyon, 1968. (b) J.
Turkevich et al. C. Vedrine, G. Dalmai, C. Naccache, and B. Imelik, J. Chim. Phys.,
65,2129 (1968). (33) A. Abou-Kais, J. C. Vedrine, and J. Massardier, J. Chem. SOC., Faraday Trans. 7 , 71, 1697 (1975). (34) P. H. Kasai, J. Chem. Phys., 43, 3322 (1965). (35) J. A. Rabo, C. L. Angell, P. H. Kasai, and V. H. Schomaker, Discuss. Faraday SOC.,41,328 (1966). (36) (a) W. Bontinck, Physica, 24, 650 (1958);(b) H. Bill and R. Lacroix in "Proceedings of the Coiloque Ampsre XIV", R. Blinc, Ed., North Holland, Amsterdam, 1967. (37) P. B. Barraclough and P. G. Hall, J . Chem. SOC.,faraday Trans. 7 , 71, 2266 (1975). (38) R. G. Bessent, W. Hayes, and J. W. Hodby, Proc. R. SOC.,London, Ser. A , 297, 376 (1967). (39) M. B. D.Bloom, R. S.Eachus, and M. C. R. Symons, J. Chem. SOC. A , 833 (1971). (40)L. Kevan in "Progress in Solid State Chemistry", Vol. 2,H. Reiss, Ed., Pergarnon Press, Oxford, 1965. (41) J. K. Thomas in "Advances in Radiation Chemistry", Vol. 1, M. Burton and J. L. Magee, Ed., Wiley-Interscience, New York, N.Y., 1969. (42)A. H. Samuel and J. L. Magee, J . Chem. Phys., 21, 1080 (1953). (43)R. L. Platzman, NASNRC Publ., 305, 34 (1953). (44) T. Henriksen, Radiat. Res., 23, 63 (1964). (45)(a)D. W. Feldman, J. G. Castle, Jr., and J. Murphy, Phys. Rev. A , 138, 1208 (1964);(b) P. G. Klemens, Phys. Rev., 125, 1795 (1962). (46) F. J. Adrian, J . Chem. Phys., 32, 972 (1960). (47)H. Zeldes and R. Livingston, Phys. Rev., 96, 1702 (1954). (48) V. B. Kazanskii, G. B. Pariiskii, I.V. Aleksandrov, and G. M. Zhidomirov, Sov. Phys. Solid State, 5, 473 (1963). (49) N. Papp and K. P. Lee, J . Magn. Reson., 19, 245 (1975). (50) E. N. Fortson, D. Kleppner, and N. F. Ramsey, Phys. Rev. Lett., 13,
22 (1964). (51) P. G. H. Sandars, Proc. Phys. SOC.,92, 857 (1967). (52) C. Schwartz, Ann. Phys. (N.Y.), 6, 156 (1959). (53) J. A. Rabo and L. Poutsma, Adv. Chem. Ser., No. 102, 284 (1971). 1541 Published in ref 35. (55) C. L. Gardnar, E. J. Casey, and C. W. M. Grant, J. Phys. Chem., 74, 3273 (1970). (56)D. N. Stamires, USCFTI AD Reo.. 644983 119661. (57) R. B. Levy and M. Boudart, J . Catai., 32, 304 (1974). (58) P. A. Sermon and 0. C. Bond, J . Chem. SOC.,Faraday Trans. 1 ,
72, 730 (1976).
Hydrogenation of Hexene over Platinum on Alumina Vs. Platinum in a Na-Y Zeolite R. S. Miner, Jr.,+ K. G. Ione,'S. Namba,+ and J. Turkevicht" Chemistry Department, Princeton university, Princeton, New Jersey 08540 and the Institute of Catalysis, Siberian Division, USSR Academy of Sciences (Received September 27, 7977) Publication costs assisted by Princeton University
In order to study the efficacy of zeolites as supports, several platinum H-Y zeolites were prepared by ion exchanging an H-Y zeolite with Pt(NH3)&12 and reducing these products with hydrazine hydrate (A, B, C). Another preparation was made by adsorbing 32-A platinum 901 on the zeolite crystallites (D). These catalysts were studied for hydrogenation and isomerization of hexene-1, ethylene hydrogenation, hydrogen chemisorption, and poison titration. They were compared with monodisperse Pt (32 A diameter) on alumina. A marked difference was found between the behavior of hexene-1 with the platinum-in-zeolite and with the platinum-on-alumina.
Introduction The use of zeolites as supports for precious metal catalysts has been of interest for some time since these supports offer the possibility of obtaining either small clusters of metal, or metal atomically dispersed and may endow the catalyst with greater stability. Furthermore zeolite catalysts with uniform pore size permit selectivity of reacting molecules by allowing small molecules (e.g., straight chain) to enter and rejecting large diameter molecules (branched chain). Rabo, Pickert, Stamires, and Boylel reported that Na-Y zeolite, either 85% deca+PrincetonUniversity. *USSR Academy of Sciences. 0022-3654/78/2082-02 14501.OO/O
tionated or ion exchanged with Ca2+,supporting 0.5 wt 70 of Pt or Pd, is an excellent isomerization catalyst for normal C5-C6 paraffins a t 350-375 "C in the presence of 450 psig of hydrogen. In a subsequent publication, Rabo, Shomaker, and Pickert2reported that the activity of Ca-Y zeolite (containing 0.5 wt % Pt, preheated to 350 "C and reduced with hydrogen a t that temperature) for n-hexane isomerization was not decreased by traces of thiophene in the reactant stream if the platinum had been introduced as the cation, Pt(NH3)42+.If, however, the platinum is introduced as the anion, (PtCl,P, rapid deactivation occurs. Hydrogen chemisorption on the catalyst prepared by cation exchange gave a H-to-Pt ratio of 2 a t 100 and 200 "C. There was considerable desorption of hydrogen
0 1978 American
Chemical Society
Hydrogenation of Hexene on Pt-Supported Zeolites
a t intermediate temperatures. The high value of the ratio suggests atomic dispersion of the platinum with the formation of HPtH species. The H-to-Pt ratio for the catalyst prepared by anion exchange was 1 at both temperatures, showing a larger particle size. Boudart3 first reported that "titration of platinum surface of the catalyst SK-200 reduced as prescribed by Rabo et al. .., shows by contrast that the dispersion of the metal is rather poor and does not exceed 0.34". In a more detailed examination of platinum-zeolite systems, Dalla Betta and Boudart4 prepared a catalyst by exchanging a Ca(NH3)4-Y zeolite with the Pt(NH3):+ complex and showed that if, prior to reduction at 400 "C, the sample is subjected to oxygen treatment a t 350 "C, an H-to-Pt ratio of 1.19,0.99, or 1.15 is obtained. Using an ingenious set of experiments, Dalla Betta and Boudart suggest that the platinum is produced in clusters of six or less atoms. The OH concentration in the zeolite is measured by the infrared absorbance at 3640 cm-l. The material is then treated with deuterium gas and the drop in absorbance is determined and ascribed to formation of OD bonds. These are postulated to be located only next to the platinum atoms and that the mobility of the H or D atoms on the zeolite is negligible. Knowing the amount of platinum, the geometry of the zeolite and the number of H atoms replaced by D, Dalla Betta and Boudart conclude that the platinum exists either as clusters of atoms or is atomically dispersed. The group at the Institute of Catalysis in Lyon, France5 extended in a significant way the investigations of the platinum-in-zeolite system which they prepared by ion exchanging the Na-Y zeolite with an ammoniacal solution of PtClz to give two samples with differing platinum-tosodium ratios of 1:1.7 and 1:12. They established that the temperature of oxygen activation was critical in determining the dispersion. If the catalyst was activated at 300 "C, and then reduced also at 300 "C, the H-to-Pt ratio was 1 (we found 0.85) while if it was activated at 600 "C and reduced a t 300 "C, the dispersion dropped to 0.25 (we found 0.16). Gallezot et al. ascribe the difference to Pt being in the supercages at 300 "C where it is available to adsorb hydrogen. In the sample activated at 600 "C most of the platinum is in the sodalite cages, unavailable to hydrogen. X-ray diffraction results are offered as support for this point of view. Crystal powder diffraction used for determining crystal structure is limited to determining the presence of platinum in sodalite cages. Using this technique, platinum was shown to be present in the sodalite cages in the 600 "C sample and absent in the 300 "C sample. While small angle x-ray scattering was incisive for measuring sintering processes, it did not differentiate clearly between the product with platinum clusters in the supercages and platinum subdivided atomically in sodalite cages. Thus platinum catalysts supported o n or i n zeolites are the focus of interest in many centers of catalytic research. It was considered desirable to carry out research in this area in cooperation between the Institute of Catalysis of the Siberian Branch of the USSR Academy of Sciences and the catalytic group of the Chemistry Department of Princeton University. The objectives of the "investigations" which we are reporting were the following: (1) Study of the differences in behavior of platinum particles adsorbed on the surface of zeolite crystallites and platinum clusters inside the pores of zeolites. (2) Preparation of a platinum-zeolite catalyst by chemical reduction at low temperature of a zeolite
The Journal of Physical Chemistry, Vol. 82, No. 2, 1978 215
which had been ion exchanged with a platinum complex ion. (3) Determination of its activity for hydrogenation and isomerization of hexene-1 and comparison with that of 32-A Pt on alumina. (4) Determination of the activity of the Pt-zeolite catalyst prepared by chemical reduction at low temperature (20 and 65 "C) for the hydrogenation of ethylene and again comparing it with that of Pt-on-alumina.
Experimental Section Catalyst Preparation. Platinum(II) A m m i n e H-Y Zeolite. A 10-g charge of H-Y zeolite was agitated for 1-1.5 h at room temperature with 50 mg of Pt as Pt(NH3)4C12 dissolved in 50 mL of distilled water. The reaction mixture was filtered. The residue was washed free from chloride ion and air dried at 38 "C under a low intensity tungsten lamp. The platinum content of the product after reduction was found by analysis to be 0.6 wt 7'0. The material was light grey in color (prepared at Novosibirsk). Preparation A . A 10-g charge of Pt(I1) ammine H-Y zeolite was refluxed for about 5 h with 10 mL (-0.23 mol) of 100% hydrazine hydrate dissolved in 40 mL of methanol. The material assumed a light tan color. The reaction mixture, after cooling, was filtered, washed with methanol, and dried overnight in a desiccator in a stream of helium. During the drying process the product turned grey (prepared at Novosibirsk). Preparation B. A 5-g charge of Pt(I1) ammine H-Y zeolite was mixed thoroughly in a test tube with 10 mL (0.23 mol) of 100% hydrazine hydrate and the test tube allowed to stand at room temperature with occasional agitation for 4.5 h. It was then filtered on a Buchner funnel and the filter paper plus residue dried in a desiccator under a stream of helium. No color change was observed on drying overnight. Exposure to air produced the evolution of a heavy white vapor. The drying was continue&for another 24 h in helium (prepared at Novosibirsk). Preparation C. A 10-g charge of H-Y zeolite containing 8 wt 7'0 of Na-Y zeolite was agitated at room temperature for 1.5 h with 139 mL of very dilute NaOH solution (pH 4-5) containing 0.18 g of La3+as lanthanum chloride. The solid was filtered off, washed chloride-free with distilled water, and air dried overnight. It was then impregnated with Pt(NH3)4C12(see above) and treated with hydrazine hydrate in methyl alcohol a t reflux, using the same procedure as for preparation A (prepared at Novosibirsk). Preparation D. A 2.23-g charge of H-Y zeolite was dried and then agitated intermittently for 2 h in 210 mL of colloidal Pt (11.1 mg of 32-A monodisperse particles) prepared by citrate reduction of platinic chloride.6 The solids were filtered off, washed free of chloride ion, and air dried overnight (prepared at Novosibirsk). Preparation E. H-Y zeolite prepared by overnight evacuation at 400 "C of NH4-Y zeolite was exchanged with an aqueous solution of Pt(NH3),C12to give 0.6 wt 7'0 Pt. After the exchange the zeolite was filtered off, washed free of chloride ions, and then allowed to react with a large excess of 85% N2H4-H20 in water. The product was filtered, washed well with methanol, and dried overnight at room temperature in a stream of nitrogen (prepared at Princeton University). Preparation F was monodisperse colloidal platinum (32-A diameter) dispersed on platelike alumina (0.48% Pt) prepared and used as a standard at Princeton.6 The gases and all reagents were research grade materials. Apparatus and Procedure. The apparatus for the study of the hydrogenation and isomerization of hexene-1 is given
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Turkevich et al.
CATALYST TESTING APPARATUS
(Continuous Flow Scheme 1
Flow
Room temp.
Meter
Flgure 1. Continual flow catalyst evaluation apparatus
in Figure 1 (Novosibirsk). A 10-20-mg charge of catalyst was diluted 5-20-fold, charged to the reactor whose diameter was 7 mm, and heated at 145-150 "C for 1.5 h in a stream of hydrogen. A helium stream was then used to vaporize the hexene and it was combined with the hydrogen stream. The maximum hydrogen flow rate was 10-15 L/h. The olefin concentration was in the range of 2-5 mmol/L of hydrogen-helium mixture. The contact s/cm. Stopcock 2 permitted time was about 6 X sampling of the initial gas stream for determination of olefin concentration or allowing it to pass through the catalyst. Stopcock 3 permitted either venting the reaction products or filling a measured-volume sampling loop with the gaseous mixture for analysis. A magnetic pump facilitated the flow of gases. The order of gases coming out of the vapor column chromatograph was (1)a trace of pentene-1 present as an impurity in hexene-1, (2) hexane, product of hydrogenation, (3) hexene-1, unreacted material, (4)hexene-3, principal isomerization product, (5) hexene-2, (6) trace of pentane from hydrogenation of pentene impurity. The reaction rate constant K was calculated as follows
K=
mL/s A X flow rate X mass of catalyst (g) (product olefin concn)'
'*
where A is the difference in olefin concentration in mmol/L before and after the reaction. The square root of product olefin concentration is based on the assumption that the reaction rate is 0.5 order in olefin. The apparatus for hydrogen titration and poisoning titration has been discussed in a previous publication from the Princeton Laboratories.'
Results and Discussion The hexene hydrogenation and isomerization results are presented in Table I and Figures 2 and 3. The different preparations of platinum supported on or dispersed in zeolites all had about the same activity with the exceptioh of La-H-Y catalyst (C). The 32-A Pt sol supported on
TABLE I: Catalytic Activity for Hydrogenation and Isomerization of Hexene-1 Sample T, C
%hydro- % isomerigenation zation
165 190 210 236 270 200 162
12.9 30.7 59.1 92.8 98.7 51.4 14.0
34.5 29.7 8.3 7.2 -0
145 17 3 2 26 259
4.2 6.7 53.5 79.7
7.3 20.9
160 175 225 253
15 32 86 85
145 169 185 220 245 160
8.9 21.9 24.5 36.8 30.7 17.7
22.2 27.0 25.0 32.9 13.0
146 167 215 251 176 155 120
39.5 51.7 62.1
0 0
62.1
56.8 43.5 17.2
K(hydrogenation 0.04 2 0.116 0.295 1.103 -2.98 0.265 0.05 0.0255 0.0418 0.465 1.083 0.053 0.237 0.750 1.185
0 0 0 0 0
0.028 0.085 0.102 0.159 0.126 0.067 0.286 0.422 0.568 0.567 0.4 87 0.332 0.109
H-Y zeolite (D) also had about the same activity at and below 182 "C. The stability of all of these catalysts except D was excellent in the temperature range 120-275 "C, the same activity being observed a t the lowest temperature before and after runs at the highest temperature. Catalyst D, which was made by adsorption of 32-A monodisperse Pt sol onto the zeolite crystals, showed an irreversible loss of activity which must be associated with sintering of the
The Journal of Physical Chemistry, Vol. 82, No. 2, 1978 217
Hydrogenation of Hexene on Pt-Supported Zeolites
t
O.'r
Pt on Alumina -0 5 -
A
Y
4)
- 1.0-
2
-1,oL
t -2.0
1
283 253
I
'
227
203
182
162
-2'o
T "C Figure 2. Catalytic activity for hydrogenation of Pt-zeolite samples. log of reaction constant K plotted against reciprocal of temperature: (X) A, hydrazine reduction at -65 OC; (0)B, hydrazine reduction at -22 O C ; (e)C, hydrazine reduction at -65 OC (material contained La'); (A)D,zeolite with colloidal Pt. 1OOr
70
A
c
/
10
120
150
200
250
275
T "C
Figure 3. Comparison of activity for hydrogenation and isomerization of Pt-zeolite and Pt-alumina catalysts.
platinum particles on the surface of the zeolite. This observation may establish a criterion for distinguishing between particles on the zeolite crystals and platinum clusters within the cavities of the crystal. Thus if the platinum zeolite catalyst retains its activity up to 280 "C it must mean that the platinum is dispersed in cavities of the zeolite. If the platinum is present on the exterior of the crystal, it should sinter at about 180 "C. Another criterion for the location of the platinum in the interior of the zeolite rather than on the surface is the presence of catalytic activity for a gaseous reaction and its absence for a reaction in the liquid phase. Thus catalyst A has little, if any, activity for hydrogen peroxide decomposition, while it has good activity for ethylene hydrogenation. This undoubtedly is due to the fact that in the hydrogen peroxide decomposition the zeolite cages are filled with
\\
1
144 127
onfeolite
'
283
1.800
253 224
203
2.000
182 2,200
162
X A
144 127 2.400
TC '
Figure 4. Arrhenius plot for hydrogenation of hexene using Pt-in-zeolite and Pt-on-alumina.
water, hindering the diffusion of hydrogen peroxide molecules in and the oxygen gas out. In the case of gaseous ethylene hydrogenation this diffusion problem is not as marked. Platinum dispersed in zeolite has good isomerization activity, shifting the double bond from the first to the second and to the third carbon atoms. These reactions become less important as the reaction temperature is raised, when the competing hydrogenation process predominates. On the other hand, platinum-on-alumina shows no catalytic activity for double bond migration. In the hydrogenation of hexene, the platinum-on-alumina (F) shows at low temperatures a fivefold higher activity than Pt dispersed in zeolite (A). However, the hydrogenation activity of Pt-on-alumina to produce hexane levels off a t 62% above 175 "C,while the Pt-in-zeolite increases steadily up to 250 "C. The leveling-off of the activity of Pt-alumina is not due to the sintering of the Pt catalyst since, on lowering the temperature, the activity points fall on the same activity curve as that obtained on increasing the temperature. The Pt-in-zeolite catalyst gave a good Arrhenius plot with an activation energy of about 18 kcal/mol while the Pt-on-alumina catalyst gave a satisfactory Arrhenius plot only at low temperatures, with activation energy of 9.7 kcal/mol (Figure 4). The Benson-Boudart titration at 20 "C gave a 2.8% dispersion for the Pt-in-zeolite reduced at 150 " C and 5.1% dispersion for reduction at 300 "C after oxidation at 300 "C. After carrying out the hydrogen titration the catalyst was reduced at 400 "C and pumped off for 2 h at that temperature. Hydrogen chemisorption a t room temperature gave a dispersion of 1 2 % . The number of active centers for ethylene hydrogenation as determined by CS2 poisoning titration for material reduced at 150 "C corresponds to a dispersion of less than 4%. This is consistent with the data obtained from hydrogen titration. The activity of the Pt-in-zeolite catalyst for hydrogenation of ethylene at 0 O C using the pulse technique at 0 "C was the same as for Pt-on-alumina. Since the dispersion of Pton-alumina is six times greater than that of Pt-in-zeolite,
218
R. C. Garvie
The Journal of Physical Chemistry, Vol. 82, No. 2, 1978
this would indicate a sixfold greater turnover number for Pt-in-zeolite than for Pt-on-alumina. This high turnover number is undoubtedly associated with the greater adsorption of ethylene on the zeolite than on the alumina, a point of view supported by the observation that water vapor completely suppresses the hydrogenation of ethylene but does not affect the chemisorption of hydrogen.
Conclusion It, was found that platinum particles supported on the exterior of the zeolite crystals lose activity a t about 180 “C while those with platinum dispersed in zeolite crystals are stable to about 500 “C. Furthermore, the latter show much lower activity for liquid phase reactions such as hydrogen peroxide decomposition than for gaseous reactions such as hydrogenations. The reduction of a platinum complex dispersed in a zeolite at 25 or 65 “C with hydrazine hydrate produces a preparation with much lower dispersion than gas phase reduction. This is undoubtedly due’to diffusion control of the reactants in the liquid phase reduction using hydrazine hydrate. This reducing agent is on the outside of the zeolite crystal. The reduction takes place at the pore entrance, forming small platinum particles which further hinder the interaction of the platinous complex inside the pore, the hydrazine hydrate outside the crystal, and the platinum particles between them. The activity of Pt-in-zeolite for hydrogenation of hexene is fivefold smaller at lower temperature than that of Pton-alumina. However the Arrhenius plot is regular and at the higher temperatures (>225 “C), it is greater than that of Pt-on-alumina. The latter catalyst shows an unusual behavior with the conversion of hexene to hexane leveling off at about 60% from 170 to 252 “C. Pt-in-zeolite shows good double bond isomerization activity at the lower
temperatures while Pt-on-alumina has no isomerization activity (4). A low value (ca. 5%) for dispersion of Ptin-zeolite was obtained both by hydrogen chemisorption and by poison titration, using ethylene hydrogenation. The value was sixfold smaller than that obtained for Pt-onalumina. In spite of this the reactivity for ethylene hydrogenation using the pulse technique was the same. This is undoubtedly due to greater adsorption of ethylene in the zeolite as compared to the alumina and points out the danger of comparing by the pulse technique catalysts of radically different adsorptive power for the organic reactant.
Acknowledgment. We acknowledge the hospitality and scientific support given to one of us (R.S.M., Jr.) at the Institute of Catalysis at Novosibirsk and particularly to express our gratitude to its Director, Academician G. K. Boreskov and also to V. Romanikov, whose catalytic test unit was used. We also acknowledge the support given to the US-USSR cooperative Program in Chemical Catalysis by the National Science Foundation, and two of us (S. Namba and J. Turkevich) acknowledge support from the U.S. Energy Research and Development Agency. References and Notes (1) J. A. Rabo, P. E. Picket?, D.N. Stamires, and J. E. Boyle, Proc. Int. Cong. Catal., 2nd, 2, 2055 (1961). (2) J. A. Rabo, V. Schomaker, and P. E. Picket?, Roc. Int. Cong. Catal., 3rd, 2, 1264 (1965). (3) M. Boudart, Adv. Catal., 20, 153 (1969). (4) R. A. Dalla Betta and M. Boudart, Proc. Int. Cong. Catal., 5th, 1, 1329 (1973). (5) P.Gallezot, A. Alarcon-Diaz, J. A. Dalmon, A. J. Renouprez, and B. Imelik, J . Catal., 39, 334 (1975). (6) K. Aika, L. L. Ban, I. Okura, S. Namba, and J. Turkevich, J. Res. Inst. Catal., Hokkaido Univ., 24,54-64 (1976). (7) L. Gonzalez-Tejuca, K. Aika, S. Namba, and J. Turkevich, J. Phys. Chem., 81, 1399 (1977).
Stabilization of the Tetragonal Structure in Zirconia Microcrystals R.
C. Garvle
CSIRO, Division of Tribophysics, Melbourne, Australia (Received March 4, 1977) Publication costs assisted by CSIRO
The occurrence of “metastable” tetragonal zirconia as a crystallite size effect is reviewed in the light of recently published experimental evidence. Evidence is presented to show this effect may be general and is a necessary consequence of a structural phase transformation associated with an endothermic heat effect during heating. Hydrostatic and nonhydrostatic stresses influence the microcrystal size-transformation temperature relationship profoundly, Consideration of the combined effects of these variables can account for all the experimental observations reported in the literature.
Introduction Zirconium dioxide is normally monoclinic at room temperature but undergoes a reversible martensitic phase transformation at about 1200 “C to a tetragonal structure;l the high temperature phase cannot be quenched. However it has been known for some time that the tetragonal structure exists at room temperature in microscrystals. Garvie2 earlier advanced the hypothesis that the tetragonal form had a lower surface free energy than the monoclinic, thereby accounting for the spontaneous occurrence of the former structure at a critical crystallite size at room temperature. Filipovich and Kalinina3 independently 0022-3654/78/2082-0218$01 .OO/O
showed in a more general way that the high temperature polymorph of a crystal could be stabilized at temperatures below its normal transformation temperature at some critical crystallite size if the high temperature polymorph had a reduced surface free energy with respect to the low temperature structure. This effect of crystallite size may be more widespread than realized. Takada and Kawai4 observed the existence of “metastable” cubic barium titanate in 10-nm crystals at room temperature. Normally the room temperature structure is tetragonal; this phase is ferroelectric with a Curie point of 120 “C, above which the cubic phase is
0 1978 American
Chemical Society
