5962
J. Phys. Chem. 1992,96, 5962-5965
to PtC14z-which react further with adsorbed OH- to form Pt(OH),. PtC162-(a) + 2e-cb PtC142-(a) + 20H-(a)
hv
PtC142-(a) + 2Cl-
CdS
pH = 13
Pt(OH),l
+ 4C1-
(3) (5)
(iii) Photoreduction of PtCb2- on high-temperature air treated CdS. As mentioned earlier, the CdO formed on the CdS surface when treated in high-temperature air can further react with the moisture in air to form Cd(OH),. Cd(OH)2, depending on the acidity of solutions, dissociates into two different kinds of complexes. in acidic solution: Cd(OH),(s) in basic solution:
-
+ H+(a)
+ OH-(a)-
Cd(OH)+(a) + H 2 0
(6)
+ H20
(7) Since XPS spectra reveal that the photoreduced product of PtCl& on high-temperature air-treated CdS is Pt(OH)2 (Figure 6b,c) in either acidic solution or basic solution. Therefore Cd(OH),(s)
PtCl,,-(a) PtC14,-(a)
+ 2e-cb
+ 20H-(a)
HCd(0H)-(a)
hv
PtC142-+ 2Cl-
Cd(OH)+
Pt(OH),i
+ 4C1-
(3) (8)
respectively.
4. Conclusions PtS or Pt(OH), is formed after photoreduction of PtClS2-on a CdS surface due to the different initial properties of CdS and
to the solutions of different pH values. PtS is not decomposable in UHV at 600 OC, but can be converted to Ptoin air a t 500 O C . Pt(OH)2 can be dehydrated in the UHV chamber at 600 OC and the dehydrated product is PtO.
Acknowledgment. We thank the Natural Science Foundation of Gansu Province for financial support of this work. We are also indebted to Prof. Hanqing Wang and Zhicheng Jiang for valuable discussions and Chengyun Wu for carrying out the DTA determination. Re&@ NO. PtQ2-, 16871-54-8; CdS, 1306-23-6; PtS, 12038-20-9; Pt(OH),, 12135-23-8;CdO, 1306-19-0; Cd(OH),, 21041-95-2. References and Notes (1) Cameron, R. E.;Bocorsly, A. B. Inorg. Chem. 1986, 25, 2910. (2) Shagisultanova, G. A. Koord. Khim. 1981, 7 , 1527. (3) Fadnis, A. G.; Kemp, T. J. J. Chem. Soc., Dalron Trans. 1989, 1237. (4) Kraeutler. B.;Bard, A. J. J. Am. Chem. Soc. 1978,100,4317. ( 5 ) Koudelka, M.; Sanchez, J.; Augustynski, J. J. Phys. Chem. 1982.86, 4211. (6) Dimitrijevic,N. M.; Li, Shuben; Gritzel, M. J . Am. Chem. Sa.1984, 106,6565. (7) Mills,A,; Williams, G. J. Chem.Soc., Faraday Trans. 1 1989,85,503. (8) Buhler, Meier, K.;Reber, J. F. J . Phys. Chem. 1984, 88, 3261. (9) Zhensheng, Jin; Qinglin, Li; Liangbo, Feng; Zhengshi, Chen J. Mol. Cafa. 1989, 50, 315. (10) Jaffrezic-Renanlt, N.; Pichat, P.; Foissy, A.; Mercier, R. J . Phys. Chem. 1986, 90,2733. (1 1) Hammond, J. S.;Winograd, N. J . Elecrroanal.Chem. 1977, 78,55. (12) Allen, G. C.; Tucker, P. M. J. Eleciroanal. Chem. Interfacial Elecrrochem. 1974, 50, 335. (13) Wang, T.; Vazguez, A.; Kato, A.; Schmidt, L. D. J . Catal. 1982. 78, 306. (14) Kim. K. S.;Winograd, N.; Dairs, R. E. J. Am. Chem. Soc. 1971,93, 6296. (15) Zhensheng, Jin; Qinglin, Li; Changjuan, Xi; Zhicheng, Jiang; Zhengshi, Chen Appl. Surf. Sei. 1988, 32,218. (16) Gmelins Handbuch der Anorgamschen Chcmie;System-NR.33, Cd Erganzungsband, p 608.
Synthesis of FuHy Dehydrated Fully Zn2+-ExchangedZeolite Y and Its Crystal Structure Determined by Pulsed-Neutron Dlffractlon Petie B. Peapples-Montgomery a d Karl Sew Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822-2275 (Received: December 23, 1991; In Final Form: April 6, 1992)
Fully dehydrated, fully Zn2+-exchangedzeolite Y has been synthwized by the reduction of all H+ ions in H-Y by zinc vapor. This solvent-free ion-exchangereaction goes to completion at 420 OC with about 0.2 Torr of Zno to give Zn27,sSi137Alss0384 (a,, = 24.4688 (3) A). The crystal structure was determined in the cubic space group F&m by pulsed-neutron powder-diffraction methods at 10 K and was refined to Rp = 0.0268 and Rw = 0.0368. Two different Znz+positions were found in the structure. The Zn(1) position is located on a threefold axis in the sodalite unit adjacent to a single 6-ring (site II'), 2.183 (12) A from three nearest framework oxygens. The Zn(2) position is also on a threefold axis in the sodalite unit, but is adjacent to a double 6-ring (site I'), 2.228 (15) A from three nearest framework oxygens. It must be true, on the basis of refined fractional occupancy parameters and to avoid 3.1-A Zn( 1)-Zn(2) distances, that about half of the sodalite cavities contain about four Zn(1) ions in the four tetrahedrally arranged single 6-rings and that the other half contain about three Zn(2) ions in three of the four tetrahedrally arranged double 6-rings. Based on this crystal structure, it is proposed that Zn(2) is initially preferred Zn2+can bond to 0(3), the most electronegative oxygen) but that at higher loadings the increased number of short 3.90 intrasodalite Zn(2)-Zn(2) distances causes Zn( 1) to be the preferred site.
L
Introduction Most dipositive cations cannot be completely exchanged into zeolites by aqueous methods. The only dipositive cations reported to have exchanged completely into zeolite Y by aqueous methods at 25 OC are Mg2+and Ca2+,and at 80 O C , S?+.' The maximum extent of exchange of the ions Co2+,Ni2+, Cu2+,and Zn2+ at 25 OC is 80, 70,86, and 94.5%, respectively.2 For zeolite X,Ca2+ fully exchanges at 25 OC,' Zn2+ a t 45 0C,2 and Sr2+and Ba2+ 0022-3654/92/2096-5962S03 .OO/O
a t 80 O C . I Complete aqueous exchange of dipositive cations into zeolite A has been achieved for Ca2+,35S P p Ba2+PZn2+,7Cd2+,8 and Pb2+.9 Zeolite A generally accepts extra molecules (over ion exchange) of salts of Cd2+or Pb2+, or of Cd(OH)2or Pb(OH)2, from aqueous solutions of those ions. The complete ion exchange of metal ions into zeolites has been accomplished by solvent-free redox methods. Na-A,Io K-A," and Ca-AI2 successfully underwent complete ion-exchange with 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5963
Fully Dehydrated Fully Zn2+-ExchangedZeolite Y
I
I
I
0 0 00
I
+
++
I
+
0 0
-
w o
-
9 9
z
0 0
LOO 0;t
U
8I O
-
\ O CnN
tz 3 0 00
~
~
~
~
~ I
~
~
~
~
~
~
~
~IIIIIIIIIIII ~ ~
IIIIII ~ ~ II I I I ~I I 11~I 11 II
II I 11 I
I11 II
II I I I I I
I1 1 I I
I 11 I I I
0
0
nl I
I
I
I
I
1.5
2.0 0 D-SPACING, A Figure 1. Observed, calculated, and difference profiles for fully exchanged Zn-Y. 1.0
Cs vapor,'O as did Na-AI3 and Ag,Ca-A14 with Rb vapor. Cso also reacted to completion with all of the Na+ ions in Na-X.15 Finally, Ag-A was partially ion-exchanged with Pb2+by reaction with liquid lead.I6 additional In some previous redox ion-exchange atoms were sorbed to form cationic alkali-metal clusters (or retained in the case of Ag14J6to form neutral Ag, clusters). For example, clusters of ( C S ~ )formed ~+ in C S ~ ~ - A - ~ / the ~ C product S, of the reaction of Cso with Na-A." Also, (Rb6)4+clusters were found in zeolite A after the reaction of Na-A with RbO at various ~0nditions.l~ This work was done to learn whether the solvent-free redox method could be used to exchange dipositive cations into a zeolite. A possible product, fully Zn2+-exchangedzeolite Y, had not been reported previously. Beyond that, it was recognized that additional atoms of Zn might be retained by zeolite Y to give cationic Zn clusters whose structure could be determined crystallographically. Zno gas was expected to react completely with H+ in zeolite Y. The aqueous &'O value, generally a good indicator for intrazeolitic reaction, is large and positive, +0.76 V. In addition, any H,(g) which might be produced during synthesis would be dynamically removed. It is interesting to note that for zeolites A and X the reverse reaction,17 Zn-A
+ H2
-
H-A
+ Zn
has been observed to occur in flowing hydrogen at 500 "C. Experimental Section Zeolite Y powder of composition Na55Si137A1550384 was kindly provided by Royal Shell Laboratories, Amsterdam (KSLA). A 10-g sample was NH,+-exchanged by contact with 1.0 M NH4C2H302(5-fold excess) at 93 "C. The solution was renewed five times, approximately daily. The slurry was then suctionfiltered, rinsed with 10 mL of deionized water, and allowed to dry at room temperature. A flame test for sodium on the resulting colorless powder was negative, indicating that ion exchange was complete. A 4.0-g sample of NH4-Y was placed in a Pyrex tube over a 10-fold excess of mossy zinc and was evacuated at ca. Torr and 25 OC for 26 h to remove absorbed water.18 The temperature was then raised to 100 "C for 24 h. It was raised further to 420 OC, driving off ammonia to give the hydrogen form of zeolite Y. At this temperature (zinc vapor pressure ca. 0.2 Torr) Zn(g) was allowed to react with H-Ye The powder became pale cream in color during the first two days, and no further changes were observed during the next five days. After cooling to 25 "C, the
2.5
I I
3.0
TABLE I: Neutron Powder Diffraction Data and Structure Refinement for Zn-Y chemical composition space group ao, A
Zn27.5SiI
37A1550384
FdJm
24.46883 (27) 1.50 no. of data points 4565 no. of reflections 1251 no. of parameters 58 0.0268 R, = C l Z o - ~ c l / C t J 0.0368 R,, = (C[w(Z0 - Zc)2]/C[~Z~])'/2
Pcalcd,
g
tube containing the powder sample and residual Zn was sealed under vacuum. For the neutron diffraction experiment, the sample was transferred in a helium-filled glovebag to a vanadium can fitted with an indium seal. Diffraction data were gathered at 10 K for about 13 h on the GPPD (general purpose powder diffractometer) instrument of the IPNS (Intense Pulsed Neutron Source) at Argonne National Laboratory. Data from the f148" banks of detectors were analyzed by Rietveld refinement techniques.19 Crystallographic calculations were done using GSAS.20 Table I provides a summary of the crystallographic data and data collection parameters. Structure Determination. Full-matrix least-squares refinement was initiated in the space group Fd3m using the atomic parameters of the zeolite framework ((Si,Al), O( l), 0(2), 0(3), and O(4)) of dehydrated Na-Y. Since the Si04 and A104 tetrahedra are indistinguishable in this space group, only the average species, (&,AI) is considered in this work. Initial anisotropic refinement of only the framework converged with R, = 0.0378 and R,, = 0.0523 (see Table I). An ensuing difference Fourier function revealed several peaks, the four largest of which were refined by least squares. This refinement led to the location of 15.4 Zn(1) ions at x = y = z = 0.1952 and 11.8 Zn(2) ions at x = y = z = 0.0686; the other two peaks did not refine to give significant occupancy. The value of 0.023 A2 for Zn was chosen so that the reasonable U,,, total number of Zn2+ions per unit cell would be about 27.5, to balance the framework charge. This corresponds to about 2 Zn(1) and 1.5 Zn(2) ions per sodalite unit. Inclusion of these positions in least-squares refinement with U, fmed resulted in convergence with R, = 0.0268 and R,, = 0.0368. Further Fourier functions did not reveal any new atomic positions. Atomic coordinates are presented in Table I1 and bond distances and angles in Table 111. Figure 1 shows the agreement between observed and calculated peak profile int ensities.
Peapples-Montgomery and Seff
5964 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 TABLE II: Poaritional. F r r c t i o ~ Occuwae~. l a d Thermal Parrmeters for Z b Y atom Si, AI O(1)
O(2) O(3) O(4) Zn(1) Zn(2)
Wyckoff position xo Y 2 frac VI, or Uiw* U2, 192(i) -0.05055 (19) 0.03449 (25) 0.12487 (23) 1.0 37 (28) 21 (19) 1.o 257 (26) 257 (26) 96(h) -0.10032 (21) 0.10032 (21) 0.0 371 (27) 371 (27) 96(g) 0.00078 (23) 0.00078 (23) 0.14911 (29) 1.0 305 (26) 305 (26) 96(g) 0.17672 (21) 0.17672 (21) -0.02217 (36) 1.0 347 (29) 347 (29) 96(g) 0.17253 (25) 0.17253 (25) 0.32326 (27) 1.0 32(e) 0.1952 (5) 0.1952 (5) 0.1952 ( 5 ) 0.482 (21) 23W 32(e) 0.0686 (6) 0.0686 (6) 0.0686 (6) 0.369 (20) 23W
u 3 3 UI 2 3 239 (35) 13 (19) -59 (28) 408 (45) -146 (30) -289 (24) 307 (40) 207 (35) -14 (28) 692 (55) 169 (31) -132 (26) 201 (36) 521 (31) -165 (26)
u 2 3
-137 -289 -14 -132 -165
(21) (24) (28) (26) (26)
‘Origin at 1. *The anisotropic temperature factor = exp((-2~/o2)(h2UII+ k2U2, + PUj3 + 2hkU12+ 2h/Ul, + 2k/U2,)). Values given are Xlo4. The thermal ellipsoids of (Si,Al), 0(1), and O(4) are nonpositive definite. (This thermal parameter was fixed in least-squares refinement.
TABLE III: Selected Interatomic Distances (A) and Aaglea (deg) for ZFY (Si,AI)-0(1) (Si,AI)-0(2) (Si,AI)-0(3) (Si,AI)-0(4)
1.599 (6) 1.615 (6) 1.725 (7) 1.666 (6)
Zn(l)-0(2) Zn(1)-O(4)
2.183 (12) 3.229 (10)
Zn(2)-0(3) Zn(2)-0(2)
2.228 (15) 3.064 (10)
O(l)-(Si,Al)-0(2) O(l)-(Si,Al)-0(3) O(l)-(Si,Al)-0(4) 0(2)-(Si,A1)-0(3) O(2)-(Si,AI)-O( 4) 0(3)-(Si,A1)-0(4)
117.4 (5) 108.8 (5) 110.0 (5) 103.7 (5) 108.9 (5) 107.4 (5)
(Si,Al)-O(l)-(Si,AI) (Si,A1)-0(2)-(Si,AI) (Si,AI)-0(3)-(Si,A1) (Si,A1)-0(4)-(Si,AI) 0(2)-Zn(l)-0(2) O(3)-Zn(2)-0(3)
148.3 (6) 131.2 (6) 130.0 (6) 140.6 (6) 105.1 (6) 95.7 (7)
Figure 2. A stereoview of the Zn( 1)-containing sodalite cavity in fully exchanged Zn-Y. Atoms are drawn as spheres of arbitrary size.
The coherent neutron scattering lengths are Si, 4.15; Al, 3.45; 0, 5.81; and Zn, 5.68 fm. Conventional background scattering was fitted with a 12-parameter analytical function, and three parameters were allowed to vary to fit a 7-parameter peak-shape function. Data were gathered for the d-spcing range 0.685-2.872 A. Additional experimental and refinement details may be found in Table I.
Results and Discussion The reaction of H-Y with Zn(g) was quantitative. Complete solvent-free ion exchange was achieved, and no additional atoms of ZnO were sorbed to give cationic clusters. With Vi, fixed at 0.023 A2 for Zn2+, 15.4 (7) Zn2+ ions per unit cell were found at Zn( 1) and 11.8 (6) were found at Zn(2). The total, 27.2 (9) Zn2+ions, is approximately the number needed (27.5) to balance the -55 charge per unit cell of the zeolite framework. The Zn(1) position is on a threefold axis in the sodalite unit, 0.87 A from the plane of the single 6-ring, at site II’.21 The Zn(2) position is also on a threefold axis inside the sodalite unit, but opposite a double 6-ring at site 1’, and 1.15 A from the nearer 6-ring plane. (The respective 6-ring planes are taken to be those of the three oxygens nearest each Zn2+ion.) The mean Zn-0 distance, 2.21 (2) A, is longer than the sum of the ionic 0.74 1.32 = 2.06 A. It is also longer than that found for 3-coordinate 6-ring Zn2+ions (1.99 (1) A) in zeolite A,26consistent with the lower negative charge of the zeolite Y framework. In addition, the oxygen positions found crystallographically in Zn-Y are averages over Zn2+containingand empty brings, so the Zn-0 bond lengths found may be artificially long. Other work” suggeats that sites I’ and 11’ should not be occupied by divalent cations unless water is present in the zeolite. However, those authors allow that the small size of Zn2+,rather than coordination by water, may permit Zn2+to occupy sites I’ and 11’. T h m is no reason to believe than an experimental error occurred which allowed the sample to become hydrated. The 1.9 Zn(1) and 1.5 Zn(2) ions per sodalite unit must arrange themselves so that some sodalite units contain only Zn( 1) ions and others only Zn(2). Otherwise a sodalite unit with more than one of each cation would have one or more very short (3.14 (2) A) Zn(1)-Zn(2) (Zn2+to Zn2+)distances. Therefore, about half of the sodalite units contain about 3.8 Zn(1) ions (Zn(1)-Zn(1) = 4.86 A; see Figure 2). and the other half about 3.0 Zn(2) ions (Zn(2)-Zn(2) = 3.90 A; see Figure 3). Perhaps the Zn(1) sodalite units are full (contain 4.0 Zn2+ions arranged tetrahe-
+
Figure 3. A stereoview of the Zn(2)containing sodalite cavity in fully exchanged Zn-Y. Atoms are drawn as spheres of arbitrary size.
drally); the occupancy 3.8 (2) differs insignificantly from 4.0. The higher occupancy at the Zn(1) site would indicate that it is lower in energy than Zn(2). If this were true, it would follow that all Zn2+ ions should occupy Zn(1) sites: if they did, the shortest Zn2+-Zn2+approaches would increase from 3.90 to 4.86 A. To understand why this has not occurred, note that the O(3) oxygen appears to be the most electronegative,25*26 so the Zn(2) site should be lower in energy. For this reason, it is proposed that the Zn(2) position is preferred initially (in highly siliceous Y), but that the short intrasodalite Zn2+-Zn2+distances (Zn(2)-Zn(2) = 3.90 A) diminish the stability of this site at higher loadings (increasing A13+ and corresponding Zn2+content) with respect to Zn(1). This crystal structure shows that the Zn(2) site is favored in sodalite units with three Zn2+ ions, where there are only three 3.90-A &ions (each ion experiences two). However, in sodalite units with four Zn2+ions, where there would be six such repulsions (each ion would experience three), the Zn(2) site has become higher in energy, so the Zn( 1) sites are occupied. It follows that the maximum number of sodalite units would contain exactly three Zn2+ions at Zn(2) and that the remainder would be full with four Zn2+ions at Zn(1). Therefore, in a unit cell with 27.5 Zn2+ions, 4.5 sodalite units would be of the Zn(2) kind and 3.5 would be Zn(1). For a unit cell with no more than 48 A P ions, and therefore no more than 24 Zn2+ions, only Zn(2) positions (site 1’) would be occupied.
Acknowledgment. This work has benefited from the use of the Intense Pulsed Neutron Source at Argonne National Laboratory.
5965
J . Phys. Chem. 1992,915, 5965-5914 This facility is funded by the US.Department of Energy, BESMaterials Science, under Contract W-31-109-Eng-38. R. L. Hitterman and J. W.Richardson of the IPNS were of great assistance. Registry No. Zn,7440-66-6.
References and Notes ( I ) Gcdber, J.; Baker, M. D.; Ozin, G. A. J . Phys. Chem. 1989, 93, 1409-1421. (2) Macs, A.; Cremers, A. J . Chem. Soc., London, Faraday Trans. I 1975, 71, 265-277. (3) Firor, R. L.; Seff, K. J . Am. Chem. SOC.1978, 100, 3091-3096. (4) Pluth, J. J.; Smith, J. V. J . Am. Chem. Soc. 1983, 105, 1192-1195. (5) Mellum, M. D. M.S.Thesis, University of Hawaii, Honolulu, HI, 1983. (6) Kim, Y.; Subramanian, V.; Firor, R. L.; Seff, K. ACS Symp. Ser. 1980, 135, 137-153. (7) McCusker, L. B.; Seff, K. J . Phys. Chem. 1981,85,405-410. ( 8 ) McCusker, L. B.; Seff, K. J. Am. Chem. Soc. 1979,101,5235-5239. (9) Ronay, C.; Seff, K. J . Phys. Chem. 1985,89, 1965-1970.
(IO) Heo, N . H.;Seff, K. J . Chem. Soc., Chem. Commun. 1987, 1225-1226. (11) Heo, N . H.; Seff, K. ACS Symp. Ser. 1988, 368, 177-193. (12) Hw, N . H. Ph.D. Thesis, University of Hawaii, Honolulu, HI,1987. (13) Scong, H. S.;Kim, U. S.;Kim, Y.; Seff, K. J . Phys. Chem., submitted for publication. (14) Swng, H. S.;Kim, Y.; Seff, K. J . Phys. Chem. 1991,95,9919-9924. (15) Sun,T.; Heo, N. H.; Seff, K., unpublished work. (16) Stamovlasis, D.; Wilson, J. R.; Seff, K., unpublished work. (17) Yates, D. J. C. J . Phys. Chem. 1%5,69, 1676-1683. (18) McCusker, L. B.; Seff, K. J . Am. Chem. Soc. 1981,103, 3441-3446. (19) Rietveld, H. M. J . Appl. Crystallogr. 1969, 2, 65-71. (20) Larson, A. C.; Von Dreele, R. B. "Generalized Structure Analysis System"; Los Alamos National Laboratory, LAUR 86-748, 1986. (21) Smith, J. V. ACS Symp. Ser. 1971, 101, 171-200. (22) Demmev. E.: Olson. D. H. J . Phvs. Chem. 1970. 74. 305-308. (23) Hand&k ofChemistiy and Physks, 63rd ed.; Chemikl Rubber Co.: Cleveland, OH, 1982; p F-179. (24) McCusker, L. B.; Seff, K. J . Phys. Chem. 1981,85,405-410. (25) Mortier. W. J.: Pluth. J. J.: Smith. J. V. J . Catal. 1976.45. 367-369. (26) Jirak, Z.; Vratislav, S.; Bosacek, V. J. Phys. Chem. Solids 15%0,41, 1089-1 095.
Decomposltlon of Cyclohexene on Pt(ll1): A BPTDS and HREELS Study F. C. Hem, A. L. Diaz, M. E. Bu~sell,~ M. B. Hugenschmidt, M. E. Domagala, and C. T. Campbell* Department of Chemistry, BG- 10, University of Washington, Seattle, Washington 981 95 (Received: January 2, 1992; In Final Form: March 2, 1992)
The interactions of cyclohexene with a Pt( 111) surface have been studied using a combination of bismuth postdosing thermal desorption mass spectroscopy (BPTDS) and high-resolution electron energy loss spectroscopy (HREELS). BPTDS is a technique which utilizes vapor-deposited Bi (at a surface temperature of 100 K) to passivate a previously prepared adlayer against intraadsorbatebond-breaking reactions. After such passivation, the surface is heated and adsorbed intermediates desorb intact for mass spectral identification, provided they have stable gas-phase analogues. Here we show that BPTDS is a useful technique for monitoring the coverages of adsorbed intermediates produced during the dehydrogenation of cyclohexene on Pt(ll1). At 95 K, cyclohexene adsorbs molecularly in a di-a fashion. By 200 K (E, z 13.7 kcal/mol) this species converts to another form of di-a, molecularly adsorbed cyclohexene,which itself converts to r-allyl c-C6H9, (plus adsorbed hydrogen) at 200-240 K (E, z 14.4 kcal/mol). At about the same temperature, part of the cyclohexene decomposes, producing small amounts of adsorbed benzene and cyclohexadiene. At about 340 K, the C-CgHg,, species converts to adsorbed benzene (E, = 20.8 kcal/mol) and the adsorbed hydrogen desorbs as H2 The C-CgHg, intermediateis identified in BPTDS by its deuteration to c-CsH9Dgas at 190 K when coadsorbed with deuterium. No significant H-D exchange occurs between coadsorbed deuterium and molecularly adsorbed hydrocarbons when probed by BPTDS.
-
-
I. Introduction The dehydrogenation of cyclohexeneto benzene on platinum surfaces has been studied in ultrahigh vacuum (UHV) by a number of researchers.'+ This reaction, its mechanism, and its kinetics are of fundamental importance since they are prototypical of the kind of surface chemistry occurring on platinum catalysts in hydrocarbon conversion.I0 No definite mechanism for the overall reaction has yet been determined. In previous UHV surface science studies of this reaction, a number of techniques have been used to characterize the species present on the surface and the reactions these species undergo. Some of these techniques include thermal desorption mass spectroscopy (TDS),I4 laser-induced thermal desorption-Fourier transform mass spectroscopy (LITD-FTMS),8.9 low-energy electron diffra~tion,~ X-ray photoelectron spectroscopy (XPS),' and work function measurements? It is known that cyclohexene, when adsorbed at -100 K on Pt( 11l), can both dehydrogenate and desorb molecularly when heated during TDS.' At very low coverages (0.1), where free Pt sites needed for hydrogen abstraction and 'Department of Chemistry, Western Washington University, Bellingham, WA98225.
dehydrogenation are not available. By -360 K, the cyclohexene has all either desorbed or dehydrogenated to produce adsorbed benzene. However, between 255 and 350 K, it is thought that other partially dehydrogenated species such as c-C6H9,,, exist on the surface.' The adsorbed benzene produced from cyclohexene partially desorbs at -500 K if it is produced at high enough coverages but dehydrogenates above 450 K to graphitic carbon if produced at low coverages (
