Langmuir 1985,1, 764-766
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Letters Thermal Desorption Studies of Methyl-Substituted Benzenes on Ni(ll1) and Ni(100) Surfaces D. G. Klarup, E. L. Muetterties, and A. M. Stacy* Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, California 94720 Received June 24, 1985 The thermal decomposition of partially deuterated 0-,m-,and p-xylene, (CD3)&H4, has been studied on Ni(ll1) and Ni(100) surfaces under ultrahigh vacuum conditions. The desorption of D2,HD, and H2 was monitored as a function of temperature. D2from the methyl C-D bonds desorbed at a lower temperature than H2 from the aromatic C-H bonds. This suggests that the aliphatic C-D bonds are dissociated at a lower temperature than the aromatic C-H bonds. However, it is interesting to note that the desorption energies of HD are the same as those of H2 and Dz. The best example of this regiospecific bond dissociation was of p-xylene on Ni(100) since the peaks were separated well and very little HD was produced. The decomposition results suggest that the open structure of the Ni(100) surface facilitates the abstraction of the methyl hydrogens while the Ni(ll1) surface facilitates the dissociation of the aromatic C-H bonds. Small changes in the thermal desorption spectra are observed for the various isomers. Although many hydrocarbons decompose on transition-metal surface^,^-^ little is known about the mechanisms by which these decompositions occur. By choosing molecules labeled with deuterium at specific sites, the temperature at which these C-D bonds dissociate relative to the other C-H bonds in the molecule can be determined. Friend and Muetterties' have shown that when toluene is decomposed thermally on nickel surfaces, the decomposition occurs by a stepwise mechanism. For C6D&D3 on Ni(lll), two D2thermal desorption maxima occur at -130 and -190 OC. For the analogous decomposition of cgH5CD3and C6D5CH3only a single D2desorption curve is observed at 130 and 185 "C, respectively. These experiments suggest that aliphatic C-H bonds dissociate at a lower temperature than aromatic C-H bonds. To extend these studies, we have investigated the interactions of partially deuterated 0-,m-, and p-xylene, (CD3)&H4, with both Ni(ll1) and Ni(100) surfaces under ultrahigh vacuum conditions. The desorption of HD was monitored in addition to D2 and H2 to check the regiospecific nature of the C-H bond activation. Experiments were performed in a Varian ultrahigh vacuum chamber with a base pressure of torr. The nickel crystals were cut from a single-crystal rod of 99.999% purity (Materials Research Corp.). Procedures for cutting and polishing the crystals are described elsewhere.' These nickel crystals were cleaned under vacuum by a combination of ion-sputtering and chemical techniques. The main contaminants were sulfur which was removed by bombardment with 500-eV Ar+ ions and carbon which was removed by treatment with -lo-' torr
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(1) Friend, C. M.; Muetterties, E. L. J.Am. Chem. SOC. 1981,103,773. 1982,104,2534. (2) Tsai, M A ; Muetterties, E. L. J.Am. Chem. SOC. (3) Tsai. M.-C.: Muetterties. E. L. J. Phvs. Chem. 1982. 86. 5067. (4j Tsai; M.-C.I Stein, J.; Fkend, C. M.; Muetterties, E.'L. Am. Chem. SOC. 1982,104, 3533. (5) Gentle, T. M.; Muetterties, E. L. J.Phys. Chem. 1983, 87, 2469.
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of O2 at 650 "C. When the surface was free of contaminants, the crystal was annealed for 60 s at 650 "C. The surface composition was monitored with a four-grid, retarding-field Auger electron spectrometer and low-energy electron diffraction was used to verify the crystallographic orientation. The chemicals used in this study were partially deuterated 0-, m-, and p-xylene, (CD3)2C6H4,and perdeuterio-p-xylene obtained from Merck and Co. The general procedure was to clean the nickel crystal and cool it to -125 OC, and then dose the surface with 1-10 langmuirs of the hydrocarbon. The temperature was ramped at -25 O C / s and the gases that desorbed were monitored with a quadrupole mass spectrometer (Uthe Technology International). Only H2, HD, and Dzdesorbed from the surface; xylene, benzene, and methane were not observed. On Ni(100), the carbon that remained on the surface after the thermal desorption experiments was removed using the O2treatment described above. In contrast, often no carbon remained on the surface of Ni(lll), in particular, for exposures of -5 langmuirs or less; the carbon obtained from the decomposition of the hydrocarbon evidently diffuses into the bulk of the crystal. The thermal desorption spectra of Dz from the decomposition of perdeuterio-p-xylene on Ni( 111)and Ni(100) as a function of the amount of p-xylene chemisorbed is shown in Figure 1. These spectra are much sharper than the corresponding thermal desorption spectra of H2 adsorbed on nickel surfaces as measured by Christmann and co-workers.6 At high coverage, two distinct D2 maxima are observed on Ni(ll1) at 110 and 219 "C and on Ni(100) at 75 and 290 "C. A t low coverage, these peaks are not resolved as well and, for Ni(lll), only one maximum at 150 "C is observed. For Ni(100),while the low-temperature peak is constant with coverage,the high-temperature peak (6) Christmann, K.; Schober, 0.; Ertl, G.; Neumann, M. J. Chem. Phys. 1974,60,4528.
0 1985 American Chemical Society
Letters
Langmuir, Vol. 1, No. 6, 1985 765 ,
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(C03)2CSH4 on Ni (100)
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Figure 1. Thermal decomposition of perdeuteriep-xylene to yield gaseous D2 from (A) Ni(ll1) and (B)Ni(100). The p-xylene exposures are given in langmuirs as well as the temperature of the desorption peak.
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Figure 3. Thermal decomposition of partially deuterated xylenes on Ni(100)to yield gaseous D2,HD, and H2from (A) o-(CD3)&H4,
(B)m-(CDs)2C6H4,and (C) p-(cD&C6H4. The approximate exposure was 4 langmuire for 0-xylene and 8 langmuirs for m- and p-xylene, saturation coverage.
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Figure 2. Thermal decomposition of partially deuterated xylenes on Ni(ll1) to yield gaseous D2,HD,and H2 from (A) o-(CD&C&,
(B)m-(CD3)2C6H4, and (C) The approximate exposure was 8 langmuirs in each case, saturation coverage. shifts to higher temperature as the coverage is increased. These spectra show that (1) the decomposition of p-xylene occurs in a stepwise fashion with two distinct dissociation energies for the C-D bonds in the molecule and (2) the decomposition is sensitive to the surface geometry. By partially labeling p-xylene with deuterium a t the methyl carbons only, it is possible to distinguish between the dissociation of aliphatic and aromatic C-H bonds. Figures 2C and 3C show the thermal desorption of D2,HD, and H2 from p-(CD3)2C6H4 on Ni(ll1) and Ni(100), respectively. H2 is found to desorb at higher temperature relative to D2on both surfaces. However, it is interesting to note that the desorption energies of HD are the same as H2and Dp We conclude that most of the aliphatic C-D bonds are dissociated at lower temperature than the aromatic C-H bonds. The ratio of the energies needed to
dissociate the aromatic C-H bonds relative to the aliphatic C-H bonds in xylene on Ni(ll1) estimated from a ratio of peak desorption temperatures is 1.3:l.O. This ratio compares favorably with the relative AH of dissociation measured in the gas phase, 112 kcal/mol for C6H5-H and 85 kcal/mol for CJ)5CH2-H, which is also a ratio of 1.31.0. The decomposition of P - ( C D ~ ) ~ Cdepends ~ H ~ on the surface geometry. D2 desorption occurs at a lower temperature on Ni(100) than on Ni(ll1) and the reverse is true for the H2 desorption. While the Ni(ll1) surface is close packed-with hexagonal geometry, the Ni(100) surface is more open with 4-fold symmetry. The decomposition results suggest that the open structures of the Ni(100) surface facilitates the abstraction of the methyl hydrogens. On the other hand, the hexagonal geometry of the Ni(ll1) surface facilitates the dissociation of aromatic C-H bonds from the hexagonal benzene ring. The decompositions of partially deuterated 0- and mxylenes are shown also in Figures 2 and 3. While the o-xylene is similar to p-xylene, m-xylene has an extra H2 peak at -330 "C on Ni(ll1) and an extra D, peak at -170 OC on Ni(100). These extra peaks suggest that the interaction of m-xylene with Ni(ll1) is such that not all the aromatic C-H bonds are equivalent and therefore these dissociate at different temperatures. On Ni(100), not all of the aliphatic C-D bonds are equivalent once the molecule is chemisorbed on the surface. In conclusion, xylene decomposes on nickel surfaces in a stepwise fashion similar to toluene, with aliphatic C-H bonds dissociating a t a lower temperature than aromatic C-H bonds.' The decomposition depends on the geometry of the surface; since dissociation of aromatic and aliphatic C-H bonds occurs at well-separated energies for p-xylene on Ni(100), it may be possible to activate the aliphatic C-H bonds selectively to produce a modified product. Furthermore, while carbon remains on the surface of Ni(100) after the thermal decomposition reactions, often no carbon remains on Ni(ll1). This implies that for catalytic ap(7) Hirota, K.;Ueda, T.Bull. Chem. SOC.Jpn. 1962, 35,228.
Langmuir 1985,1,766-768
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plications, Ni(ll1) surfaces will not be poisoned by graphitic overlayers. Acknowledgment. This research was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Depart-
ment of Energy under Contract DE-AC03-76SF00098. This paper is dedicated in fond memory of E. L. Muetterties for all his support and friendship. Registry No. Ni, 7440-02-0; o-xylene, 95-47-6; m-xylene, 108-38-3; p-xylene, 106-42-3; hydrogen, 1333-74-0.
Dissociative CBr4 Adsorption on Fe(110) D. Mueller and T. N. Rhodin School of Applied and Engineering Physics and Cornell Material Science Center, Cornell University, Ithaca, New York 14853
B. Sturm and P. A. Dowben* Department of Physics, Syracuse University, Syracuse, New York 13210 Received J u n e 10,1985 CBr4 adsorption on clean Fe(ll0) at 300 K was studied by angle-resolved photoemission. The photoemission features resemble bromine-inducedemissions following dissociative Brzadsorption on Fe(110). Adsorption of CBr4 was found to be completely dissociative. The Br(4pJ orbital was identified at 5.8 f 0.3 eV and the Br(4p,,J orbital was identified at 4.5 f 0.2 eV. Halocarbon adsorption on iron surfaces has now been the subject of a number of studies.' The halogenated methanes cc14?3 CBr414and CFC1a5v6have all been found to dissociatively adsorb Fe(100) a t 300 K. For CBr4 the dissociative nature of the adsorption process on Fe(100) was inferred from coverage estimates made by using LEED and Auger electron spectroscopy (AES)., While no electron beam effects were observed, dissociation as a result of the incident LEED and AES electron beams cannot be completely excluded. The adsorption of molecular CFC12 on Fe(100) at 110 K indicated that adsorbed CFC1, molecular species were subject to fragmentation as a result of incident electrons. In this paper we report the observed dissociative adsorption of CBr, by photoemission and compare these results with dissociative Br2 adsorption. Both molecular' and dissociative8 Br, adsorption on Fe(ll0) have been investigated. Experiments were performed on a Fe(ll0) surface. The clean surface was prepared by Ar+ ion bombardment and annealing. The cleanliness was monitored by using AES and photoemission spectroscopy. The photoemission measurements were carried out at the University of Wisconsin, Madison, Synchroton Radiation Center. The radiation was dispersed by a 1-m vertically mounted SeyaNamioka monochrometer. Angle-resolved energy distribution curves were taken using a VG Scientific ADES-400 spectrometer with a total energy resolution of 0.2 eV and an angular acceptance of 4O. Throughout the experiments, the direction of the surface component of the polarization vector of A of the incident light was parallel to the (110) crystal direction, and the photoelectrons were collected normal to the sample surface. Data were normalized to the relative incident photon flux determined by the photoyield of a tungsten mesh diode. The chamber was pumped by a 400 L/s ion pump, a 150 L/s turbomolecular pump, and a titanium sublimation pump giving a base pressure of 2 X torr. Throughout this paper all
* Address correspondence to this author. 0743-7463/85/2401-0766$01.50/0
binding energies are reference to the Fermi level of the clean Fe(ll0) surface. With CBr, adsorption on Fe(ll0) at room temperature, a broad emission induced at 4-6 eV binding energy in the angle-resolved photoemission energy distribution curves is seen in Figure 1. This broad emission with a half-width in the region of 1.5 eV (full width at half-maximum) has a maximum at 4.5 f 0.2 eV binding energy with a light incidence angle of 15' off the surface normal. With light incidence angles of 45O off the surface normal two emissions can be observed in angle-resolved photoemission at 4.5 f 0.3 and 5.8 f 0.3 eV binding energies, as seen in Figure 2. The photoemission features induced by CBr4adsorption, for exposures less than 20 langmuirs, on Fe(ll0) at 300 K in no way resemble the expected features for an ironbromidegJOthat might be formed by halide formation. Neither can the observed features at 5-6 eV binding energy be considered to resemble the photoemission features attributable to free gaseous CBr,."-13 The general feature at 5-6 eV binding energy is similar to the emission induced by dissociative Br2adsorption on Fe(llO), as seen in Figure 3. This emission observed with dissociativeBr2adsorption (1) Grunze. M.: Dowben. P. A. ADDLSurf. Sci. 1982. 10. 109. (2) Jones, R. G. Thesis, Universi& of Cambridge, 1977. ' (3) Jones, R. G. Surf. Sci. 1979,88, 367. (4) Dowben, P. A.; Jones, R. G. Surf. Sci. 1979,89, 114. (5) Dowben, P. A.; Grunze, M. Ber. Bunsenges. Phys. Chem. 1981,85, 728.
(6) Dowben, P. A.; Grunze, M.; Jones, R. G.; Illenberger, E. Ber. Bunsenges. Phys. Chem. 1981,85, 734. (7) Mueller, D.; Sakisaka, Y.; Rhodin, T. N. J. Vac. Sci. Technol., A 1984, 2, 1018. (8) Dowben, P. A.; Mueller, D.; Rhodin, T. N.; Sakisaka, Y. Surf. Sci. 1985, 94. (9)Berkowitz, J.; Streets, G. D.; Garritz, A. J.Chem. Phys. 1979, 70, 1305 (10) Sakisaka, Y.; et al. J. Phys. SOC.Jpn. 1974, 36, 1372. (11) Green, J. C.; Green, M. L. H.; Joachim, J.; Orchard, H. F.; Turner, D. W., Philos. Trans. R. SOC.London, A 1970, A268,lll. (12) Potts, A. W.; Lempka, H. J.; Streets, D. G.; Price, W. C., Philos. Trans. R. SOC.London, A 1970,268,59. (13) Bassett, P. J.; Lloyd, D. R. Chem. Phys. Lett. 1969, 3, 22.
0 1985 American Chemical Society
