J. Phys. Chem. 1903, 87, 3479-3462
at 0.13 eV and the other at 1.03 eV. The higher energy threshold at 1.03 eV has been ascribed32to ground-state SF, and was in agreement With the electron affinity of sF6 accepted a t the time, namely, 0.54 eV. The confusion32 in the discussion of thermochemical values for SFc and SF5-was due to the fact that the thermal energy was not considered as a contributing factor toward dissociative electron capture of SF6 to SF5-at an electron energy of 0 eV. In the double mass spectrometer experiment, ions are formed a t source pressures I-0.1 torr. I t is estimated that they undergo approximately 30 collisions within the source chamber before exiting. If they are initially formed in some excited state, the rate coefficient for collision deactivation must be IlO-" cm3/s in order for the excited species to survive. The transit time required for a projectile ion to reach the collision chamber is such that an excited species must have a lifetime of the order of 10 ps or longer to survive. Nevertheless, long-lived electronically excited metastable ions such as 02+(a4n,)have been observed3 by CID. An excited state was detected in 03-via CID, but it was unclear whether that was due to electronic or vibrational excitation, or to an isomeric We observe negligible or nonexistent collisional dissociation from the ground state. The excited state observed upon collisional dissociation may be vibrationally or electronically excited. If it is the latter, then it must be just -0.2 eV below the SF5-+ F limit. Stock et al.2shave suggested that SF, is preferentially formed unimolecularly over F- due to the topology of the potential surfaces for the electronically excited and ground electronic states of SF,-. It is now known16bthat the SF5-+ F pair is more stable thermodynamically than F- SF5,so that SF5formation can be explained within the framework of the QET.5s6J6bStreit2has suggested the additional electron-
+
(33) Tiernan, T. 0.;Marcotte, R. E. J. Chem. Phys. 1970,53,2107. (34) Wu,R.L. C.;Tieman, T. 0.; Lifshitz, C. Chem. Phys. Lett. 1977, 51, 211.
3479
ically excited SF actual value should then be higher, i.e., AHf0298(H02) 0.9 kcal/mol. Similar conclusions are arrived at from our recent study in the thermochemical analysis of the kinetics The results indicated that of HC1+ O2 and HBr 02.22 the reaction mechanisms proposed by Kochubei and Moin and those by Rosser and Wise were not applicable for the entire course of the reactions. However, the data obtained from Rosser and Wise and from our study of HI + O2 are in reasonable agreement with Howard's results.14J5 It is interesting to note that Howard's first value Afif0298(H02) = 2.5 f 0.6 kcal/mol from an equilibrium study of reaction 314was revised to mf02g8(H02) = 3.4 f 0.6 kcal/mol by his later study.15 Three values for ~ f 0 2 g 8 ( H 0can 2 ) be obtained from Lee and Howard's study. The first value is from the determination of the equilibrium constant shown previously; i.e., K = krb/ksb e 1 and m 0 4 b = 1.2 kcal/mol, which lead to AHHf0298(H02) = 3.4. It should be noted that this value was obtained on the basis of the assumption that reaction 5b was the only product channel. However, one cannot eliminate the possibility of formation of HC1 O2 which might form through the four-center role.22923Leu and Lin's measurements indicated that channel 5b proceeded from 68% to 81 % at 298 K. It would mean that the value 3.4 kcal/mol is uncertain due to the uncertainty in Leu and Lin's data. However, AHf0298(H02)can be evaluated from the temperature-dependent rate constant k4b. Lee and Howard15 obtained Elb = 1.0 f 0.1 kcal; if E..&= 0 is assumed for the reverse reaction (which is in general considered true), then m 0 4 b = E 4 b - E d b 1.0 kcal/mol and AHf0298(H02) = 3.6 kcal/mol is obtained. However, if the reverse reaction possesses a nonzero activation energy, E-4b= -RT = -0.6 kcal at 298 K, this will give m 0 4 b = 1.6 kcal/mol and iWf0298(H02) = 3.0 kcal/mol is obtained. The latter value would be the lower limit for AHHf0(H02)whereas the value 3.6 kcal/mol would be the upper bound in this case. All these three values are higher than the upper limit of Howard's first value ~ f 0 2 g 8 ( H 0=23.5 ) f 0.6 kcal/mol, but in excellent agreement with our estimation from the study of HI + 0 2 . In the early measurements from Foner and Hudson, they obtained the electron-impact HOz+appearance potential, AP(H02+)= 15.35 f 0.05 eV, from reaction 8 and the H 0 2
+
+
+
+ -
H202+ e-
H02++ H
+ 2e-
(8)
ionization potential, IP(H02) = 11.53 f 0.02 eV, from reaction 9. The difference, eq 8- eq 9, gives Solo = 88.3 H 0 2 e- H02+ + 2e(9) kcal/mol for reaction 10. AHHf0zs8(H02) can be calculated H202 H 0 2 + H (10)
3482
J. Phys. Chem. 1983, 87, 3482-3486
from the well-known i W f " 2 9 8 of H202and H. They obtained AHf"2gpjHO.J = 5.0 f 2 kcal/mol when the data were corrected to 25 "C. In their analysis Foner and Hudson assumed that the fragments at the observed threshold energy of the dissociative ionization process 9 separated with zero relative velocity and that the H02+ion was not vibronically excited, i.e., zero kinetic and internal energies. The most probable source of error in their measurement is that the fragments in process 9 may possess some excess kinetic and/or excitation energy; thus, their value would be most likely too high, i.e., AHHfOzss(HOz) C 5.0 kcal/mol. The value 1.6 kcal/mol from Khachatryan et al.18 is too low with a large uncertainty of f2.5 kcal/mol. The value of 0.4 from Wagman's calculation was derived from AH0224,25 and the value AHfo(H3+)which was obtained by quantum-mechanical calculation^.^^^^^ If the value of (26)M. E.Schwartz and L. J. Schaad, J.Chem. Phys., 47,5325(1967). (27)A. J. Duben and J. P. Lowe, J . Chem. Phys., 56, 2824 (1972).
AHHfO(H3+)is off by 3 kcal/mol (which is more than likely), this would bring the value up to AHf0298(H02) = 3.4 f 2 kcal/mol, which is in good agreement with most data. A mean value AHHf0%&H02)= 3.5 kcal/mol is obtained by considering most of the data except those from Kochubei and Moin (which was ruled out) and from Khachatryan et al. (which has a large uncertainty). An upper limit of 4.5 is adopted from Foner and Hudson's measurements and Heneghan and Benson's measurements. A lower limit of 3.0 is assigned since three s t ~ d i e s ' ~ Jshow ~9~~ that the value of AHf0(H02)has to be greater than 3.0 kcal mol. Therefore, we recommend AHf02g8(H02)= 3.5G.50 kcal/mol as the best value. Acknowledgment. This work has been supported by the
US.Army Research Office for scientific research under grant No. DAAG29-82-K-0043and the National Science Foundation under grant No. CHE-79-26623. Registry No. Hydroperoxyl radical, 3170-83-0.
Kinetics of H2 Desorption from Silica-Supported Rhodium Arthur A. Chlnt and Alexls T. Bell' Department of Chemical Englneering, University of California, Berkeley, Callfornia 94720 (Received: October 7, 1982)
Temperature programmed desorption spectroscopy has been used to characterize the kinetics of Hzdesorption from a Rh/Si02catalyst. Desorption is observed to occur from two states, designated p1and p2. By simulation of the experimentally observed spectra, it is determined that the preexponential factor for desorption from both states is 3.8 x cm2/s. The activation energy for desorption from the p1state is 14 kcal/mol and that for the Pz state is 21 kcal/mol.
Introduction Temperature programmed desorption (TPD) spectroscopy is a powerful method for characterizing the binding states of molecules adsorbed on a catalyst Theoretical methods for determining desorption kinetics and rate parameters have been established and used extensively to interpret TPD spectra obtained from unsupported metals under ultrahigh vacuum conditions. Similar methods can also be applied to interpret TPD spectra obtained from supported metal catalysts studied under conditions in which desorption occurs into a flowing carrier gas.8 Recent theoretical workgJOhas shown that readsorption of the desorbing gas must be taken into account in the analysis of such experiments. Furthermore, experimental conditions must be selected carefully to avoid mass transfer effects which can give rise to modifications in peak position and shape. In the present paper we report on a study of the kinetics of H2 desorption from a silicasupported Rh catalyst. TPD spectra were obtained by rapid desorption into a stream of flowing helium. The distribution of binding states and the rate parameters for desorption from these states were determined by simulation of the observed TPD spectra. Studies by various authors1'-" have shown that H2 adsorbs dissociatively on both supported and unsupported Present address: Mobil Research and Development Corp., Research Department, Paulsboro, N J 08066.
Rh. Measurements of the initial sticking coefficient for H2 have been made on both polycrystalline and singlecrystal Rh Previous studies of H2 desorption have been restricted to unsupported Rh.l1-l5 The TPD spectra obtained in these investigations suggest that H2 desorbs primarily from a single state. Moreover, spectra obtained for smooth and stepped surfaces do not differ ~ignificantly.'~J~J~ In those cases where the TPD spectra (1) Redhead, P. A. Vacuum 1962,12,203.
(2)Cvetanovic, R.J.;Amenomiya, Y. Ado. Catal. Related Subj. 1967, 17, 103. (3)Cvetanovic, R. J.; Amenomiya, Y. Catal. Rev. 1972,6,21. (4)Petermann, L.A. In "Adsorption-DesorptionPhenomena";Ricca, F. Ed.; Academic Press: New York, 1972. (5)Schmidt, L.D.Catal. Reu. 1974,9, 115. ( 6 ) Smutek, M.; Cemy, S.; Buzek, F. Ado. Catal. Related Subj. 1975, 24,343. (7)Madix, R. J. Catal. Reo. 1977,15, 293. (8) Falconer, J. L.; Schwartz, J. A. Catal. Reo. Sci. Eng. 1983,25,141. (9) Herz, R. K.; Kiela, J. B.; Marin, S. P. J. Catal. 1982,73,66. (10)Gorte, R. J. J . Catal. 1982,75, 164. (11) Mimeault, V. J.; Hansen, R. S. J . Chem. Phys. 1966,45, 2240. (12)Castner, D.G.; Sexton, B. A.; Somorjai, G. A. Surface Sci. 1978, 71,519. (13)Castner, D.G.; Somorjai, G. A. Surface Sci. 1979,83,60. (14)Yates, Jr.,J. T.;Thiel, P. A.; Weinberg, W. H. Surface Sci. 1979, 84,427. (15)Gorodetakii, V. V.; Nieuwenhuys, B. E.; Sachtler, W. M.H.; Boreskov, G. K.Surface Sci. 1981,108,225. (16)Edwards, S. M.; Gasser, R. P. H.; Green, D. P.; Hawkins, D. S.; Stevens, A. H. Surface Sci. 1978, 72,213. (17)Zakumbaeva, G. D.;Omashev, K. G.Kinet. Katal. 1977,18,450.
0022-3654/83/2087-3482$01.50/00 1983 American Chemical Society
