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J . Phys. Chem. 1985,89, 2972-2973
pressure is increased to 3 kbar. As mentioned in the Introduction, the trapped state energy ET will also be affected by pressure (density) changes.j Fueki and Kimura’* have calculated this change for n-hexane based on a semicontinuum model with six hexane molecule in the first coordination layer symmetrically arranged around the electron. The trapped state energy is predicted to increase by 0.40 eV going from 1 bar to 3 kbar. Thus both quantities change and since Voincreases more than ET, an increase in the activation energy is predicted. Since E , is the difference between two large numbers there is some uncertainty in this quantity, but the calculated change in E, is in qualitative accord with the experimental results. For 2,2-DMB the electron mobility increases by 33% with pressure. The mobility at 1 bar is 12 cm2/(V s) and a small activation energy of 0.06 eV has been reported.19 The small (18) Kimura, T.; Fueki, K. private communication. (19) Dcdelet, J.-P.; Shinsaka, K.; Freeman, G. R. Can. J . Chem. 1976, 54, 744.
activation energy at 1 bar reflects that V, N ET and it is conceivable that the delicate balance of Voand ET changes in such a way as to cause p to increase with pressure. Whether pressure changes the mobility in hexane and 2,2-DMB as a result of changes in the activation energy or a result of changes in the extended state mobility, or both, is uncertain at present. Additional studies are underway to see if other hydrocarbons behave similarly. That is, to see if for low mobility liquids the mobility decreases and for higher mobility liquids the mobility increases with pressure. Temperature studies are also planned to determine the role of activation energy changes. Acknowledgment. We thank K. Walther, P. Roman, and C. Koehler for their help in the construction of the vacuum system, and T. Kimura and K. Fueki for their advice and semicontinuum calculations. This research was carried out at Brookhaven National Laboratory under contract DE-AC02-76CH00016 with the U S . Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.
MetaLSupport Effects on H2 and CO Heats of Adsorption on Ti0,-Supported Platinum M. A. Vannice,* L. C. Hasselbring, and B. Sen Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: February 25, 1985; In Final Form: April 3, 1985)
Isothermal, integral heats of adsorption of H2 and CO on supported Pt have been measured with a differential scanning calorimeter. For Pt dispersed on S O 2 , q-A1203,and Si02-A1203, AH(ad)values were 30 & 4 kcal/mol for H, and 28 f 4 kcal/mol for CO. For Pt/TiOZreduced at 473 K, similar, but slightly lower, values were obtained. However, after reduction at 773 K the markedly decreased chemisorption uptakes on TiOz-supportedPt also showed significantlylower heats of adsorption for both H2 and CO. These results provide direct evidence that adsorption capacity for CO and Hzcan be decreased by a change in the chemical nature of surface Pt atoms as well as by physical blocking of the Pt surface by migrating TiO, species.
Introduction We report here what we believe to be the first direct measurements of heats of adsorption on a Pt catalyst after it is in the “SMSI” state, which is obtained after a high-temperature (773 K) reduction in H2.’ Since the first reports of suppressed H2 and C O chemisorption on titania-supported group 8-10 metals’ and their enhanced ~.~ catalysts specific activity for CO h y d r ~ g e n a t i o n , metal/Ti02 have received much a t t e n t i ~ n .This ~ suppression of chemisorption was originally attributed to a strong metal-support interaction (known as SMSI) involving electron transfer from the support to the metal$ however, other explanations have since been proposed.4 The lower chemisorption capacity has been attributed to a weakened adsorbate-metal i n t e r a ~ t i o n ,and ~ it has subsequently been assumed that heats of adsorption of H2and CO are lower on Ti02-supported metals after any possible metalsupport intera~tion.~,’However, a decrease in chemisorption capacity can be due to either a decrease in the heat of adsorption or a decrease in the number of adsorption sites (or both). Therefore, to determine the contribution of the former explanation, we embarked on an experimental study to directly measure isothermal, integral heats of adsorption of CO and H2 on a family of Pt (1) S. J. Tauster, S. C. Fung, and R. L. Garten, J. Am. Chem. Soc., 100, 170 (1978). (2) M. A. Vannice and R. L. Garten, J. Cutal., 56, 236 (1979). (3) M. A. Vannice, J . Cutul., 74, 199 (1982). (4) “Studies in Surface Science and Catalysis, Vol. 11, B. Imelik, et al., Eds., Elsevier, New York, 1982. (5) S. J. Tauster, S.C. Fung, R.T. K.Baker, and J. A. Horsley, Science, 211, 1121 (1981). (6) B.-H. Chen and J. M. White, J . Phys. Chem., 86, 3534 (1982). (7) X.-2. Jiang, T. F. Hayden, and J. A. Dumesic, J. Cutal. 83, 168 (1983).
0022-3654/85/2089-2972$01.50/0
catalysts which had been characterized previously by X-ray diffraction, a variety of chemisorption techniques, and infrared spectroscopy.8 Experimental Section These Pt catalysts had been reduced by using standard pretreatments in a volumetric adsorption system providing a vacuum Pa (lod torr), and uptakes of C O and H2 had below 1.3 X been measured between 50 and 250 torr after correction for physical adsorption on the support.8 Small quantities (ca. 40-100 mg) of these catalysts were then placed in a specially modified Perkin-Elmer DSC-2C differential scanning ~ a l o r i m e t e r . ~An equal amount of the pure support was placed in the reference cavity to compensate for energy changes due to physical adsorption on the support. High-purity Ar (99.99%, MG Scientific) was further purifed by passage through an Oxytrap (Alltech Ass.) to reduce O2 concentrations below 1 ppm. Carbon Monoxide (99.99%, Matheson) was used after flowing through a heated molecular sieve trap. Hydrogen (
