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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-induced emissions following dissociative Brz adsorption 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 torr. Throughout this paper all pressure of 2 X
* 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 dissociative Br2adsorption (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
Langmuir, Vol. 1, No. 6, 1985 767
Letters
....
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Figure 1. Adsorption of CBr4on Fe(ll0) at room temperature. The increase of the Br(4p) orbital photoemission features is shown
as a function of CBr4exposure. The light incidence angle was 45" while the vector potential A is parallel to the (110) direction. The photoelectrons were collected normal to the surface with the incident photon energy of 21 eV. CBr,/Fe (110) A , ~
..
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normal emission hv = 21 eV
* .
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I I
I
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Figure 2. Light incidence angle dependence. Angle-resolved
photoemission spectra for 15" and 45O light incidence angles. The component vector potential A in the surface plane is parallel to the (110) direction of the clean Fe(ll0) substrate. The photon energy is 21 eV and the photoelectrons were collected normal to the surface. Both energy distribution curves were taken following 2 langmuirs CBr, exposure on Fe(ll0) at 300 K. on Fe(ll0) at 300K is attributable to the Br(4p) orbitals.' Bromine-induced photoemission features have also been
60 40 20 Binding Energy [eV]
EF
Figure 3. Adsorption of Br, on Fe(ll0) at room temperature. The increase of the Br(4p)orbital emissionsis shown as a function of Br2exposure. The light incidence angle was 45" while the vector potential A is parallel to the (110) direction. The photoelectrons were collected normal to the surface with the incident photon energy of 21 eV.
observed at about 5-6 eV binding energy, for dissociative bromine adsorption on Fe(100),14W (100),15Ni(100)16and Pd(lll)," and Cu(100).18 Hence we postulate that CBr, adsorbs dissociatively on Fe(ll0) a t 300 K. With the light incidence angle of about 15" the major component of the vector potential A is parallel with the surface (s-polarized light). In this geometry, one expects, from dipole selection rules, the principal contribution from the adsorbate to the energy distribution photoemission curve taken at normal emission to arise from the C(2px,) and Br(4px,) orbitals following dissociative CBr, adsorption. The photoemission signal from carbon following dissociative halocarbon adsorption is typically quite small! For dissociative CBr, adsorption on Fe(ll0) we would also expect that photoemission features attributable to carbon are smaller than those that arise from bromine since typically the photoionization cross-section of carbon is quite small, particularly with respect to bromine. In addition, there is a four to one proportion of bromine to carbon on the surface. Furthermore, with dissociative CBr, adsorption on Fe(100) carbon was observed to adsorb in a binding site beneath the bromine overlayer4 and this would lead to a further reduction in photoemission signal from carbon. Thus the 4.5 f 0.2 eV binding energy feature observed with s-polarized light may be attributable to the Br(4p,,) orbitals. The light incidence angle of 45O would have equal components of A parallel to the surface and normal to the surface. In this light incidence orientation both the Br(4pJ and Br(4px,) orbitals should be observed with normal (14)Dowben, P. A.;Grunze, M.; Tomaneck, D. Phys. Scr. 1983,T4, 106. (15)Bhattacharya, A. K.;Broughton, J. Q.;Perry, D. L. J. Chem. Soc., Faraday Trans. 1 1979,75, 850. (16)Dowben, P. A.;Sakisaka, Y.; Rhodin, T. N. J. Vac. Sci. Technol. (17)Lloyd, D. R.; Netzer, F. P. Surf.Sci. 1983,131,139. (18)Richardson, N.V.;Sass, J. K.Surf. Sci. 1981,103,496.
Langmuir 1985, 1 , 768-770
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emission. We can therefore assign the 5.8 f 0.3 eV binding energy photoemission feature to the Br(4pz). With dissociative Br, adsorption on Fe(110) the binding energy of the B r ( 4 ~ , , ~is) notably coverage-dependent ranging from 5.4 f 0.2 eV at low coverage to 4.3 f 0.1 eV at high coverage when the vector potential A of the incident light is parallel to the (100) direction.' The Br(4pJ orbital is not observed to have a pronounced binding energy coverage dependence and varies in value between 5.7 and 6.1 eV, at most, with dissociative Br, adsorption on Fe(l10).7 No pronounced coverage dependence of the Br(4pZ)and Br(4p,,) orbitals was observed with dissociative CBr, adsorption (Figure 1). The absence of marked coverage-dependant binding energies of the bromine orbitals with dissociative CBr4 adsorption may also be a result of different sequence of overlayer structures, as compared to dissociative Br2 adsorption, as the bromine coverage increases. The carbon may also influence bonding and surface adsorption sites of the bromine overlayer resulting in some differences in the photoemission spectra for dissociative CBr, and Br, overlayers. The binding energies of the Br(4pJ and Br(4pxJ)orbitals at 5.8 f 0.3 and 4.5 f 0.2 eV, respectively, observed following dissociative CBr, adsorption do, however, resemble the binding energies of the Br(4p2)and Br(4pJ orbitals at 6.0 f 0.2 and 4.3 f 0.1 eV, respectively, observed following saturation of the bromine ov'erlayer with dissociative Br2 adsorption on Fe(l10h7 The small difference in binding energy of the Br(4p,,) orbital at saturation of the dissociative CBr, overlayer as compared to the dissociative Br, overlayer is likely to be a result of differences in saturation coverage. Dissociative CBr, adsorption has been
observed to end at a somewhat smaller bromine saturation coverage on Fe(100) than that for dissociative Br, adsorption., For the free bromine atom the bromine 4p orbitals are degenerate. Since the Br(4pJ orbital has a greater binding energy than the Br(4pJ orbitals at i=' (normal emission), this suggests that following dissociative CBr4 adsorption, the bromine is bound to the surface via the Br(4pZ)orbital while the B r ( 4 ~ ~orbitals , ~ ) are nonbonding. A similar bonding behavior has also been observed with bromine adsorption on Pd(lll).17 For bromine on Cu(lOO)'* and bromine on Ni(100),16 bonding occurs via the B r ( 4 ~ ~ , ~ ) orbitals while the Br(4p,) is nonbonding. The broad half-width of the bromine emissions for bromine bound to the Fe(ll0) surface is probably attributable to lifetime broadening. Similar lifetime broadening of chemisorbed bromine photoemission features has also been observed for bromine on Ni(100).16 In conclusion, we find that CBr, adsorption on Fe(ll0) at 300 K is completely dissociative. A bromine overlayer is formed with the bromine bonding to the surface via the Br(4p2)orbital.
Acknowledgment. We thank the staff of the Synchroton Radiation Center of the University of Wisconsin, Madison for their excellent support. The Tantalus I storage ring is supported by NSF Grant DMR 77-21888. This research is supported by Syracuse University and the Research Corporation, as well as the University of Cornel1 Material Science Center (NSF-DMR 82-17227 and NSFDMR 83-13742). Registry No. CBr4, 558-13-4;Fe, 7439-89-6.
Transient-Kinetic Study of Catalysts Aging: Methanation over Raney Nickel Y. Soong and P. Biloen* Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 Received March 25, 1985 Abrupt switches in the isotopic composition of the reactants have been utilized to abstract transient-kinetic information at, essentially, steady state. In the context of methanation over Raney nickel this method demonstrates that aging is a process which originates in the catalyst itself rather than in its adlayer. We report on the use of non-steady-state kinetic methods for the study of one of the less tractable problems in heterogeneous catalysis: catalyst deactivation. A Raney nickel catalyst, when exposed to hydrogen-lean syngas, initially declines at a rate of approximately 0.2% /h. After a period of approximately 120 h, the residual decline rate is only approximately 0.03% /h. On the basis of the existing methanation literature'" and without further evidence we tentatively conclude that the catalyst's adlayer slowly deteriorates. Less reactive surface species are being slowly produced and removed, with a steady-state level of these surface-blocking species being attained after some 120 h. Below we show this tentative conclusion to be incorrect: aging originates in the catalyst itself rather than in the adlayer.
* To whom correspondence should be addressed.
We tested the "adlayer deterioration" hypothesis by following the line out with transient-kinetic experiments. Thereto we switched repeatedly the feed from 12CO/H2 to 13CO/Hzand vice versa. This leads to a transient-kinetic phenomenon: a decay of the 12CH4production, compensated by the concurrent increase of the W H , production. In what follows we call F13(t) and Fl2(t) isotopic transients, in which
(1)McCarty, J. G.; Wise, H. (2) (3) (4) (5)
J. Cotal. 1979, 57, 406. Wentrcek, P. R.; Wood, B. J.; Wise, H. J . Catal. 1976, 43, 363. Araki, 3. M.; Ponec, V. J. Catal. 1976, 44, 439. Biloen, P.; Helle, J. N.; Sachtler, W. M. H. J. Catal. 1979,58,95. Bonzel, H. P.; Krebs, H. J. Surf. Sci. 1980, 91, 499.
0 1985 American Chemical Society 0743-7463/S5/2401-076~~0~.50/0
