J. Phys. Chem. 1992, 96, 1409-1417 with i, j = 1, 2 and rl = x, r2 = y . Now, the principal axes of the ellipse, ux, by, may be fixed in order to verify that Jd2r' r'/,Mhpe(F';uz,uy)= A,,
645)
where with Mhpc(~ux,u,,) we explicitly show the dependence of the Mayer function on the principal axes. This criterion takes into account both the macroscopic symmetry (through the angular
1409
distribution function) and the molecular geometry and it fixes both the orientation of the principal axes and their magnitude. In summary, in this Appendix we have provided a nonlocal density functional model for grafted hard rods which needs as input the excess free energy and direct correlation function for HD, as well as the Mayer function of the grafted rods. We believe that such a model can be used to introduce positional correlations in the model for Langmuir monolayers described in the main text.
Adsorption and Thermal Decomposition of Benzene on Ni( 110) Studied by Chemical, Spectroscopic, and Computational Methods D.R. Huntley,* S . L.Jordan,' Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6201
and F. A. Grimm Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996- 1600 (Received: July 25, 1991; In Final Form: September 20, 1991)
The chemisorption and reactions of benzene on Ni( 110) have been studied by temperature-programmed desorption (TPD) including isotopic labeling, X-ray photoelectron spectroscopy(XPS), high-resolution electron energy loss spectroscopy(HREELS), and low-energy electron diffraction (LEED) as a function of coverage and adsorption temperature between 100 and 300 K. At saturation, 70430% of the benzene is irreversibly chemisorbed, and C-H bond scission commences at 320 K. For high exposures, molecular desorption competeswith decomposition. A ~ ( 4 x 2LEED ) pattern is observed at saturation coverage of chemisorbed benzene (0.2 monolayer by XPS). HREEL spectroscopy indicates that the benzene ring lies parallel to the surface. Semiempirical molecular orbital calculations have been made and predict the most likely adsorption site for benzene chemisorption to be the atop site at a height of about 1.75 A or the short bridge site at 1.90 A. Upon annealing above 300 K, the benzene decomposes, evolving H2and forming a surface carbide. Additionally, a species forms which ultimately desorbs as benzene at 460 K but also undergoes H-D exchange with benzene-& An unambiguous identification of this fragment has not been made, but the vibrational spectroscopy and isotopic exchange data are consistent with the assignment of a phenyl or benzyne group. The major effects of coadsorbed sulfur and oxygen are to inhibit dissociation and to weaken the interaction between the benzene and the surface.
Adsorption and reactions of benzene on clean, partially oxidized and partially sulfided Ni( 110) were examined with two major goals. The first was to help understand the interactions of the aromatic molecule with the Ni( 110) surface, as an extension to a study of the interactions of benzenethiol with nickel.' The second was to compare the reactivity of benzene on the corrugated Ni( 110) face with previous studies of benzene on the closepacked Ni( 111) face2s3and with the Ni( 100) Benzene chemisorption on metal surfaces has been extensively studied on Ni( 111),2*3*611 Ni( 100),"6*8-11Ni( 110),'Jo and other metal ~ u r f a c e s , ' ~usually - ~ ~ on overlayers adsorbed near 300 K. Fewer studies have been made following low-temperature ads ~ r p t i o n . ~ ~In* general, ' ~ - ~ ~ chemisorbed benzene is thought to be x bonded, with the molecular plane parallel to the surface. The only proposed exception to that model was for Pd(1 lo), where the benzene molecule apparently lies tilted about 10-20° into the troughs.I2 This possibility also exists for Ni(ll0) since it is isostructural with Pd(ll0). Previous studies of the decomposition of benzene on Ni( and Ni( 111)2have indicated some interesting differences. In both casts, the only desorption products observed by heating a chemisorbed overlayer were hydrogen and benzene. On Ni(100), H2 (actually D2since these studies concentrated on perdeuterobenzene) was evolved prior to molecular benzene. The hydrogen peak temperature was 472 K with a low-temperature tail, while the benzene peak desorption temperature was 483 K. In contrast, To whom correspondence should be addressed. Great Lakes Colleges Association Science Semester participant.
Steinriick et al. have made similar studies of benzene on Ni( 111) and find that benzene desorption precedes hydrogen desorption.2 (1) Huntley, D. R. Submitted for publication in J. Phys. Chem. (2) Steinriick, H. P.; Huber, W.; Pache, T.; Menzel, D. Surf. Sei. 1989, 218, 293. (3) Huber, W.; Steinrrlck, H. P.; Pache, T.; Menzel, D. Surf. Sci. 1989, 217, 103.
(4) Blass, P. M.; Akhter, S.; White, J. M. Surf. Sci. 1987, 191, 406. (5) Myers, A. K.; Benziger, J. B. Lungmuir 1987, 3, 414. (6) Jobic, H.; Tardy, B.; Bertolini, J. C.; J. Electron Spectrosc. Relat. Phenom. 1986, 38, 55. (7) Bertolini, J. C.; Massardier,J.; Tardy, B.; J . Chim. Phys. 1981,78,939. (8) Bertolini, J. C.; Dalmai-Imelik, G.; Rousseau, J. Surf. Sci. 1977,67, 478. (9) Bertolini, J. C.; Rousseau, J. Surf. Sci. 1979,89, 467. (10) Friend, C. M.; Muetterties, E. L. J. Am. Chem. Soc. 1981, 103,773. (11) Meyers, A. K.; Shoofs, G. R.; Benziger, J. B. J . Phys. Chem. 1987, 91. 2230. (12) Netzer, F. P.; Rangelov, G.; Rosina, G.; Saalfeld, H. B.; Neumann, M.; Lloyd, D. R. Phys. Rev. E 1988, 37, 10399. (13) Polta, J. A.; Thiel, P. A. J. Am. Chem. Soc. 1986, 108, 7560. (14) Jakob, P.; Menzel, D. Surf. Sci. 1988, 201, 503. (15) Jakob, P.; Menzel, D. Surf. Sci. 1989, 220, 70. (16) Liu, A. C.; Friend, C. M. J . Chem. Phys. 1988,89,4396. (17) Grassian, V. H.; Muetterties, E. L. J . Phys. Chem. 1987, 91, 389. (18) Waddill, G. D.; Kesmodel, L. L. Phys. Reu. E 1985, 31, 4940. (19) Ohtani, H.; Bent, B. E.; Mate, C. M.; van Hove, M. A,; Somorjai, G. A. Appl. Surf. Sci. 1988,33134, 254. (20) Koel, B. E.; Crowell, J. E.; Mate, C. M.; Somorjai, G. A. J . Phys. Chem. 1984,88, 1988. (21) Koel, B. E.; Crowell, J. E.; Bent, B. E.; Mate, C. M.; Somorjai, G. A. J . Phys. Chem. 1986, 90, 2949. (22) Mate, C. M.; Somorjai, G. A. Surf. Sci. 1985, 160, 542. (23) Surman, M.; Bare, S . R.; Hofmann, P.; King, D. A. Surf. Sci. 1987, 179, 243.
0022-3654/92/2096-1409$03.00/00 1992 American Chemical Society
1410 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
The benzene desorbs in two peaks, one at 294 K (at high coverage) and another at 440 K, while the hydrogen desorption peaks at 450 K with a high-temperature tail. These data suggest that the chemisorption energy is very comparable to the C-H(D) bond activation energy on nickel surfaces and that structural differences can strongly affect the competition between desorption and decomposition. Despite the differences in this chemistry, vibrational spectra from both surfaces following benzene adsorption at 300 K adsorption are virtually i d e n t i ~ a l . ~In, ~both cases, the most intense feature was a peak near 745 cm-I, which is due to the dipole-activeout-of-plane C-H bending mode (vll in the Wilson26 notation). An early HREELS study of benzene on Ni(ll0) adsorbed at room temperature was also carried out, and the frequency of this mode was found to be somewhat lower, 700 cm-l.' Semiempirical molecular orbital calculations of the relative chemisorption energies of benzene on Ni(100)531'and Ni(l1 1)27 have been made previously. The conclusions were that the most stable site for benzene chemisorption on both surfaces was the highest coordination site, i.e. the 3-fold hollow site on Ni( 111) and the 4-fold hollow site on Ni(100). A comparison of the chemisorption energies on Ni( 111) and Ni( 100) suggested that the benzene was considerably more strongly bound on Ni( 100) than on the close-packed surface." In this study, the adsorption and reaction of benzene on clean, partially oxidized and partially sulfided Ni( 110) were studied using a variety of spectroscopic techniques, including high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS),temperature-programmed-desorptionmass spectrometry (TPD) (including isotopic labeling studies), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). In addition, semiempirical molecular orbital calculations of benzene adsorbed in various sites on the Ni( 110) face were performed to predict likely adsorption sites and compared to the previous studies on the other low-index surfaces.
Experimental Section The experiments were conducted in a stainless steel ultrahigh vacuum chamber which operated with a base pressure of about 7 X 10-I' Torr. This system was equipped to do high-resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed-desorptionmass spectrometry, Auger electron spectroscopy, and low-energy electron diffraction. A high-purity nickel crystal was cut to within 0.75O of the (1 10) face and mechanically polished. Prior to initial insertion into the vacuum chamber, the crystal was heated in 1 atm of H2 gas at 673 K for 12 h. The sample was cleaned in vacuum by a combination of neon ion sputtering and annealing at 1000 K. Sputtering was required to remove sulfur, but both carbon and oxygen could be removed by annealing at 1000 K for 60 s. The surface cleanliness was ascertained by AES. The temperature of the sample could be controlled from 100 to over 1000 K. Cooling the sample to 100 K after a 1-min anneal at 1000 K required approximately 8 min. The benzene and benzened6was stored over type 4A molecular sieves and subjected daily to freezepumpthaw cycles until pure, as determined by mass spectrometry. Exposures to these vapors were accomplished via a directed doser, and the units (Torres) refer to the pressure behind an 8-rm aperture multiplied by the time of the dose. Other gases were dosed by back-filling the chamber to a fixed pressure measured by an uncorrected ion gauge, and exposures are reported in langmuir units (1 langmuir = lo4 Torr SI.
The oxidized surfaces were prepared by exposing the clean Ni(l10) surface to O2at 300 K followed by annealing at 600 K for 60 s. The sulfided surfaces were prepared by exposing the (24) Surman, M.; Bare, S. R.; Hofmann, P.; King, D. A. Surf. Sci. 1983,
Huntley et al. crystal to H2S at 110 K and annealing at 800 K. The temperature-programmed-desorption data were obtained with a linear heating rate of 5.0 K/s. This ramp was feedback controlled and was both very accurate and highly linear. The sample was positioned with a direct line of sight to the mass spectrometer ionizer, at a distance of about 2 cm. The sample was biased at -70 eV to eliminate the possibility of electronstimulated decomposition, which had been observed earlier on Ag(l1 1)25and in the benzenethiol/Ni(llO) system.' However, separate experiments were done to probe electron-induced chemistry on the Ni( 110) surface, and no effects were observed. X-ray photoelectron data were obtained using a Mg anode and a double-pass cylindrical-mirror analyzer. The analyzer was operated at pass energies of either 25 or 50 eV, resulting in analyzer resolutions of 0.5 or 1.0 eV, respectively. The binding energies were referenced to Ni core levels obtained from the clean surface. A two-point calibration was done using the Ni 2p and 3p core levels at 852.8 and 65.6 eV?* respectively, to correct for a slight nonlinearity in the binding energy scale. HREEL spectra were obtained with a primary beam energy of about 4 eV. The count rates in the elastic peak varied considerably, depending on the order and nature of the overlayer. Typical spectral resolution as measured by the elastic scattering peak full width at half-maximum was 11 meV. Auger spectra were obtained with a primary beam energy of 3 keV and with a modulation voltage of 3 V, except the high-resolution spectra obtained for line-shape information. There the modulation voltage was reduced to 1 VFp. When HREEL, Auger, or photoemission spectra were obtained as a function of annealing temperature, the crystal temperature was ramped at 10 K/s to the indicated temperature. The samples were annealed for 60 s and cooled to below 150 K prior to data collection.
Results and Discussion ChemiPorptionof Benzeneon Ni(ll0). Exposure of the Ni(ll0) single crystal to benzene resulted in strong chemisorption for temperatures below 300 K. The coverages were estimated using the intensity of the carbon 1s core level photoemission and were calibrated by comparing the integrated intensity with the intensity of the carbon peak from benzenethiol. The carbon coverage for benzenethiol could be calculated from the molecular formula, C6H5SH,since the sulfur calibration had been previously well e~tablished.~~ The saturation coverage of carbon following benzene adsorption at 200 or 300 K or by annealing the multilayer exposures to 200 K was estimated to be 1.12 monolayers (ML) (0.2 ML of benzene). LEED studies following a saturation exposure at 200 K, a temperature chosen to avoid physisorbed benzene, revealed a ~ ( 4 x 2 pattern, ) similar to observations on Pd( 11O).I2 Annealing the surface to 300 K had no discernable effect on the LEED pattern in either symmetry or quality, as determined visually. However, annealing to 400 K resulted in a high background and loss of the fractional-order beams. Further annealing to 500-600 K resulted in the formation of a ~ ( 4 x 5 pattern, ) but annealing to higher temperatures caused a loss of all fractional-order spots and a gradual sharpening of the p(lX1) pattern. Since other studies22have indicated that CO plays a role in benzene ordering, CO thermal desorption was monitored during these LEED experiments and indicated only trace amounts (10.03 ML), which had presumably come from ambient adsorption, were present. The predicted coverage for a ~ ( 4 x 2 LEED ) pattern is 0.25 ML, which is consistent with the XPS coverage determination of 0.2 ML of benzene at saturation. A comparison of studies of lowtemperature benzene adsorption on other metals indicates that the saturation coverages of benzene generally are limited by the size of the benzene molecule.2 The unit cells of the ordered overlayers are generally in the range of 37-45 A2,which is com-
126, 349.
(25) Zhou, X. L.; Castro, M. E.; White, J. M. Surf. Sci. 1990, 238, 215. (26) Wilson, E. B., Jr. Phys. Rev. 1934, 45, 706. (27) Anderson, A. B.; McDevitt, M. R.; Urbach, F. L. Surf. Sci. 1984, 146, 80.
(28) Powell, C.J.; Erikson, N. E.;Jach, T. J . Vac. Sci. Techno/. 1982,20, 625. (29) Huntley, D. R. Surf. Sci. 1990, 240, 13.
Benzene Adsorption and Decomposition on Ni( 110)
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1411 TABLE I: SlPter Atomic Orbital Panmeters'
Bsnzene/Ni(l 10)
30000
atom H C
Ted-300 K
h
u
VI
>5 20000
i
I II I
Ni
I
845
a
0
1000
2000
orbital 1s 2s 2p 4s 3P 3d
VSIP6/eV 12.600 19.000 10.260 8.635 4.990 11.000
CI
1.0 1.0 1.0 1.0 1.0 0.5683
c2
1.200 1.685 1.618 1.800 1.500 5.750
0 0 0 0 0 0.6290
t-2
0 0 0 0 0 2.0000
Parameters taken from ref 27. Valence-state ionization potential.
3000
Energy Loss (CH' )
F i i 1. High-resolutionenergy loss spectrum under specular scattering conditions of benzene adsorbed on Ni(ll0) at 300 K. The surface was saturated, resulting in a coverage of -0.2 ML of benzene.
parable to the twedhnensionalvan der Waals area of the benzene molecule. The observed ~ ( 4 x 2 on ) the Ni( 110) unit cell corresponds to a unit cell area of 70.1 A2, but since there are two benzene molecules per unit cell, the effective unit cell area per benzene molecule is 35 A*, in good agreement with the hypothesis that benzene lateral interactions control the saturation coverage. On the Ni( 110) surface the benzene molecules are very tightly packed (vide infra), which may explain the difference in the XPS coverage determination and the ideal coverage based on the LEED. The vibrational spectrum obtained for a saturation exposure of benzene on Ni(ll0) at 300 K is shown in Figure 1. As in previous studies on Ni surfaces,@ two major losses are observed at 740 and 875 cm-'and assigned to the C-H out-of-plane bending mode (vI1in the Wilson notation) and a C< ring stretching mode (Y'). These assignments are based on the normal-coordinate analysis of Jobic et aL6 The intensity of the 740- and 875-cm-' modes and the absence of a substantial C-H stretching mode near 3000 cm-' indicate that the adsorption is associative and that the molecule lies essentially parallel to the surface. The similarity of the vibrational spectra between Ni( 11 l), Ni(100), and Ni(ll0) indicate either that the HREELS experiment is insensitive to the symmetry of the adsorption sites or that the molecule is adsorbed in structurally similar sites on all three surfaces. The small number of vibrational modes in the specular energy loss spectrum argues that the symmetry of the molecule (e6") determines the spectrum more than the symmetry of the adsorption site. The HREELS spectra are consistent with the presence of a single adsorption site on each surface, although that site has not been determined. The assignment of the 875-cm-' loss as the vI mode suggests substantial perturbation of the ring since the frequency in the gas-phase spectrum is 992 cm-I. Jobic et al. have interpreted similar data for Ni( 111) and Ni( 100) as indicating a reduction in bond order from 1.5 to approximately 1.0.6 Modeling the Chemisorption. A series of molecular orbital calculations was done using the atomic superposition and electron delocalization molecular orbital (ASED-MO) method developed by A n d e r ~ o n ~with ~ . ~ 'the goal of determining the most stable adsorption sites on Ni( 110). This method is based on the extended Hockel method but includes pairwise repulsive terms based on the Hellman-Feynman theorem.29 The inclusion of the repulsive terms gives a total energy and therefore allows structural modeling. The orbital parameters used in these calculations are shown in Table I. The choice of parameters profoundly affects the absolute numbers calculated, and we have simply chosen to use those of Andersoq2' although they seriously overestimate the adsorption energies. A consistent set of calculations should, however, reveal relevant trends. An important note is that the adsorption energies calculated by this method are strongly dependent on the size of ~
~~
~
(30) Anderson, A. B. J. Chem. Phys. 1975, 62, 1187. (31) Anderson, A. B.; Grimes, R. W.; Hong, S.Y . J . Phys. Chem. 1987, 91, 4245.
nnnnnnn
Figure 2. Results of the semiempiricalmolecular orbital calculations. (a) Sites modeled by ASED calculations. The benzene molecules are drawn to scale with the lines representing the bonding distances in the molecule. (b) A diagram of a structure consistent with the ~ ( 4 x 2 LEED ) pattern and the ASED calculations. The hexagons represent the outer extent of the covalent radii of the hydrogen atoms. The surface is quite tightly packed, and lateral interactions at high coverage may alter this structure.
the metal cluster (especially for clusters of less than about 20 metal atoms), so care should be exercised when absolute energies between clusters of different sizes are compared. In this study, all surfaces (Ni(1 lo), Ni(l1 l), and Ni(100)) were modeled as 30-atom clusters to minimize size effects. The structure of the Ni(ll0) "surface" was fixed and assumed to be an unreconstructed bulk termination. For most of the calculations, the structure of the benzene molecule was also fmed, but a few calculations were done with hydrogen atoms bent away from the surface. A more detailed description of the calculations will be presented elsewhere.32 In order to model the differences in adsorption energies on the various crystallographic sites shown in Figure 2a,certain structural information was required, including the height of the benzene molecule above the surface. Since experimental information is not available for benzene on Ni surfaces, a structural optimization was done by adjusting the height of the benzene molecule above the surface on a small cluster (13 atoms, 9 first layer and 4 second layer). The height with the lowest energy was chosen and used (32) Huntley, D.
R.;Grimm, F. A. To be published.
Huntley et al.
1412 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 a t
TABLE II: Calculated Distances and E d e s
sitea atop ( A l ) short bridge ( S l ) atop rotated (A2) short-bridge rotated (S2) short-bridge tilted (S2T) hollow ( H l ) atop tilted (A2T) hollow rotated (H2) long-bridge rotated (L2) long bridge ( L l )
height/A 1.75 1.90 1.80 2.00 2.00 1.90 1.90 2.00 1.90 2.05
a, Desorption State
chemisorption enerav/eV 13-atom 30-atom cluster cluster 4.336 5.375 4.229 5.256 4.952 4.061 4.805 3.908 4.814 3.838 4.194 3.713 4.599 3.772 4.302 3.264 4.263 3.164 4.122 3.138
r;
D
P
I
t
6.0
6.5
7.0
7.5
I
“Code in parentheses refers to the sites shown graphically in Figure 2b.
in subsequent modeling. The surface-benzene distance chosen for each site is given in Table 11. Typical distances for the height of the benzene above the surface determined by the calculation are 1.75 A ( N i x distance of 2.24 A) for the atop site, which is comparable to the Ni-arene distance of 1.693 A in ($-mesitylene)bis(pentafluorophenyl)nickel(II)33 and to the Cr-benzene distance of 1.61 A in ~ h r o m o c e n e . ~In~ the hollow site where the repulsive terms are much larger, a distance of 2.00 A ( N i x distance of 2.14 and 2.67 A, both in the first layer) was determined. The height above the atop site is considerably shorter than that calculated by Andersonz7for benzene on Ni(ll1) (2.1 A) but is plausible on the more open Ni( 110) surface, which is better able to accommodate the benzene molecule on the atop site due to a smaller repulsive term. The results of this structural modeling are quite reasonable and provide a measure of the validity of this method. The results for all sites shown in Figure 2a are summarized in Table I1 for both the 13-atom cluster and a 30-atom cluster (3 layers with 12 atoms in each of the first and the third layers and 6 in the second layer). According to this calculation, the most stable sites on the Ni( 110) surface are the atop site (Al) and the short-bridge site (Sl). The energies of these two sites differ by only 0.12 eV, which is not a signifcant difference. The least stable sites are the rotated hollow site (H2), and the two long bridge sites (L1 and L2). These results are in contrast to the earlier calculations by AndersonZ7and Benziger5J1on the Ni( 111) and Ni(100) surfaces (which we verified by our calculations), where, in both cases,the highest coordination sites were found to be most stable. However, this difference is quite reasonable. On Ni(1 lo), the benzene molecule can approach the atop site much more closely due to the decreased repulsive energies, which result from the more open structure. As a result of the shorter bond distances on Ni(1 lo), the one-electron HDckel energy is much more favorable on the atop site than on the other surfaces. Two different tilted geometries were also tested, since Netzer et al.Iz suggest a tilted geometry for benzene on Pd(ll0). These sites were obtained by tilting the benzene molecule into the trough by 15O for the atop (A2) and short-bridge (S2) sites. This did not result in increased stabilization, and the calculations still predict the flat geometry to be more stable. Calculations were also done on the two atop sites with the hydrogen atoms bent away from the surface 10’ relative to the C atom plane. This resulted in a slight destabilization. The structure shown in Figure 2b corresponding to the ~ ( 4 x 2 ) LEED pattern is based on these calculations. However, the calculations are most valid in the low-coverage limit and do not account for lateral interactions in the benzene overlayer. For the tightly packed overlayer shown in Figure 2b, lateral interactions are likely to be important and may alter the structure to relieve crowding (see note added in proon. (33) Radonovich, L. J.; Koch, F. J.; Albright, T.A. Inorg. Chem. 1980, 19, 3373. (34) Keuler, E.; Jellinek, F. J . Organome?. Chem. 1966, 5 , 490.
100
150
200
250
300
350
Temperature (K)
Figure 3. Benzene (mass 78) thermal desorption profiles for the initial stages of multilayer formation. The inset shows a leading-edge analysis, which yielded an activation energy for the desorption of the a3state of 9.9 kcal/mol.
PhyS”of Benzene on Ni( 110). Following large exposures at 100 K, the desorption of physisorbed benzene is observed in three states at 163, 147, and 160 K, which populate in that order with exposure, as shown in Figure 3, and are designated as a l , az,and a3. Similar to what was observed on Ni( 11l)z, the az peak disappeared at exposures sufficient to populate the ajstate. This is in contrast to studies on Ni(100)4 where the a2 state persisted for high exposures. The a3state is associated with the desorption of bulk benzene; the peak does not seem to saturate, and an analysis of the leading edge of the a3state indicates an activation energy for desorption of 9.9 kcal/mol, which is in good agreement with the heat of sublimation of benzene (10.5 kcal/ molj5). The al and azdesorption states are associated with the first two layers of physisorbed benzene.I5 The three a states observed in the desorption of physisorbed benzene from Ni(ll0) are similar to those observed on RU(OOO~),~~-’~ Mo(1 10),l6 Ni(l1 l),z and Ni(100)4 surfaces. In all cases, the states are populated in the same order, and the desorption temperatures are similar. An early study on Ru(OOO~)~~ suggested that the a states correspond to benzene weakly chemisorbed into gaps in the strongly bound benzene overlayer. In light of the tight packing of the benzene molecules in the chemisorbed overlayer on Ni(ll0) and other surfaces, this does not seem likely. More recently, Jakob and MenzelI5 have published a detailed analysis based primarily on HREELS of the physisorption on Ru(0001) and conclude that the a1state is the second layer of benzene and is essentially parallel to the surface, the azstate is a metastable layer in which the molecular plane is far from parallel to the surface, and the a3state corresponds to the bulk crystalline benzene structure where the molecules are tilted in a herring bone configuration. Similar HREELS data on Ni( 110) obtained in this study (not shown) confirm these conclusions. The only difference in the present study is that substantial intensity is seen in the C-H stretching mode even for the al state, suggesting that the molecules are not parallel to the surface on Ni(ll0). Thermal Decomposition of Benzene on Ni(110). Molecular hydrogen and benzene are the only molecules observed in the temperature-programmed reaction profiles. The exposure dependence of the hydrogen temperature-programmed-desorption profiles is shown in Figure 4. At low exposure, only hydrogen is observed. Its desorption profile indicates three features at 310, 375, and 435 K. As the coverage is increased, the 375 K peak becomes the prominent feature near the first layer saturation (35) Handbook of Chemistry and Physics, 56th ed.; CRC Press: Boca Raton, FL, 1975-1976, p C716.
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1413
Benzene Adsorption and Decomposition on Ni( 1 10)
I
I
benzene desomtion
! I
I\
242 K 349 K
466 K
200 K
0.0 100
,300K
200
300
400
500
600
IO
Temperature (K)
Figure 6. Benzene (mass 78) desorption profiles for equivalent (saturation at 200 or 300 K) exposures of benzene as a function of adsorption temperature. This allowed the full development of the three peaks due to chemisorbed benzene with no interference from the multilayer peak.
100 200 300 400 500 600 700 800
Temperature (K)
Figure 4. Hydrogen (mass 2) thermal desorption profiles as a function of benzene exposure at 100 K. For the lowest exposures, the peak observed at 320 K is due primarily to ambient H2 adsorption. 1.4
1
-" E
-
c
L
E
0.7
e c
x 100
200
400 500 Temperature (K)
300
600
700
Figure 5. A comparison of the desorption profiles of H2and C6H6(solid lines) with D2 and C6D6(dashed lines). This figure demonstrates that the evolution of hydrogen (deuterium) occurs in a reaction-limited step where C-H (C-D) bond scission is rate limiting and that benzene desorption and decomposition are competitiveprocesses above 300 K. The adsorption temperature was 200 K, and the initial coverage was -0.2 ML.
coverage. Similar experiments done with benzene-d, indicated that most of the 3 10 K peak was due primarily to impurity hydrogen adsorbed from the ambient. The higher temperature hydrogen desorption features are due to the decomposition of the benzene ring. The presence of two peaks and broad peak shapes suggests that the decomposition occurs via several hydrocarbon intermediates, each of which decomposes to produce hydrogen. A comparison of the deuterium desorption from C6D6and the hydrogen desorption from c,& indicates that C-H bond scission occurs at a lower temperature than C-D bond scission, as indicated by the peak desorption temperatures of 375 K for H2and 395 K for D2, as shown in Figure 5. Since the H2 or D2 is desorbed in a reaction-limited step, the desorption rates are a measure of the rates of bond scission. This shift in the desorption profile is a manifestation of a primary kinetic isotope effect, where the rate-limiting step involves the cleavage of the C-H(D) bond. As can be seen in Figure 5, the molecular desorption is halted by the ollset of C-H bond scission, i.e. the molecular desorption decreases as the hydrogen desorption begins. The identical effect is seen in the perdeuterobenzene data,indicating that, for high coverages, desorption and decomposition occur competitively above 300 K. Molecular benzene is the only other desorption product and is observed only near the first layer saturation coverage. Three
desorption stam of chemisorbed benzene are observed at 242,349, and 466 K. The relative intensities of the 242 and 349 K peaks varied somewhat for nominally equivalent exposures. Figure 6 shows benzene desorption profiles following saturation exposures at 100,200, and 300 K and subsequent cooling to 100 K. The higher temperature adsorption experiments allowed full development of the three desorption peaks with no interference from the multilayer adsorption. The low-temperature desorption state, at 242 K, which is populated at the highest coverage, may result from destabilizing lateral interactions in the crowded overlayer. The hydrogen desorption profiles were unchanged as a function of exposure temperature in the 100-300 K range. The extent of associative adsorption or H-D exchange was probed by two types of isotopic labeling experiments. In the first, the surface was exposed to deuterium followed by benzene. There was no evidence for incorporation of deuterium into the desorbed benzene. The hydrogen desorption profile was identical to that observed from benzene desorption from a clean surface. The molecular deuterium desorption profile was shifted to significantly lower temperature when benzene was coadsorbed than in the absence of benzene. HD was also observed and had a desorption profile similar to that of D2, although much weaker. It is assumed that the hydrogen which was incorporated into the HD was due to adsorption of H2from the ambient prior to deuterium exposure since no deuterium was incorporated into the benzene. Deuterium desorption was complete by 380 K, and, therefore, the highest temperature desorption state of benzene (466 K) was not probed by this method; however, the lower temperature desorption states (at 242 K and 349 K) were shown to be associative. In order to determine the nature of the high-temperature benzene desorption state, isotope scrambling was examined between coadsorbed benzene and benzene-d,. The sample was exposed to a mixture (data shown in Figure 7 for an approximately 2:3 ratio) of benzene and benzene-d,. In agreement with the D2/benzene coadsorption experiments, no isotopic mixing was evident in the two low-temperaturedesorptionstates, and the peak intensities of the mass 78 and 84 peaks mirrored the dosing gas composition as expected. However, as depicted in Figure 7, the desorption profiles indicated an enhancement of benzene-d, in the high-temperature desorption state as well as clear evidence for isotopic scrambling, since a peak in the mass 83 profile was observed with nearly the intensity of the mass 84 peak. The H2 and D2 profiles were similar to those obtained from the reactions of benzene and benzene-d, adsorbed separately on the clean surface. The H2desorption preceded the D2desorption and was more intense. The benzene desorption peaks at 242 and 349 K can clearly be assigned to associatively and reversibly adsorbed benzene. The
Huntley et al.
1414 The Journal of Physical Chemistry, Vol. 96, No, 3, 1992 TABLE IIk Vibrational Frequencies (cm-I) and Peak Aapi-tS'' species r(CH) (vi11 gas phaseb 675 multilayer/Ni(l 10) 692 300 K anneal/Ni( 110) 737 400 K anneal/Ni( 110) 748 300 K saturation/Ni( 110) 743 300 K saturation/Ni( 1 700 300 K saturation/Ni( 1 1 l)d 745 750 300 K saturation/Ni(
4CH)
(VI)
r(CH)
992 1009 a75 a79 845 845 a45
(4
'
4 C H ) (VZ)
b19)
1479 1477
1178
1460
3062 3065 3080 3085
1110 1110 1120
1420 1420 1430
3020 3020 3025
'Mode numbers are in the Wilson notation, ref 26. bReference 6. CReference7. dReference 9. I
4CC)
1177 1160
60000
k\
Benzene/Ni(l 10; Ta,=lOO K I
.
,
748
0
5
. I
C
30000 v
2 C
al
-
-Id
C
lLo 1009
200
400 600 800 Temperature (K)
1180 1477
1000
Ngure 7. Desorption profiles of C6H6 (mass 79, C6HSD (mass 79), C$SH (mass 83),and C6D6 (mass 84) following an exposure of the clean Ni(l10) surface to a 2:3 mixture of benzene and benzene-d6 at 200 K.
high-temperature benzene desorption peak onset is well above the peak in the hydrogen desorption, suggesting that a species is formed which does not undergo C-H bond scission as readily as the other chemisorbed benzene. This species could be a more strongly bound benzene molecule or perhaps a phenyl or benzyne fragment. Benzyne species have been proposed previously on Mo(1 10).l6 The isotopic mixing between C6D6 and C6H6 in the high-temperature state provide evidence for a partially dissociated fragment. The experiments only conclusively demonstrated the formation of benzene-d5. This might seem to suggest a phenyl fragment as the likely species, but that assignment is ambiguous. Due to enhanced rates of C-H bond scission compared to C-D bond scission in the range of 300-400 K, by 400 K, the surface should be enhanced in fragments formed from the deuterated benzene. The higher coverage of deuterium on the surface (compared to hydrogen) would preferentially lead to formation of the highly deuterated benzene. Thus, while isotopic mixing is evidence for a dissociated fragment in this temperature range, it is not conclusive in determining the stoichiometry of the fragment. Since isotope exchange could occur in the absence of a stable partially dissociated fragment (by either elimination/additionor addition/elimination mechanisms), vibrational spectra were o b tained as a function of annealing temperature to investigate the presence of a phenyl or benzyne species near 400 K. Shown in the lower curve in Figure 8 is the spectrum following an exposure sufficient to produce multilayer benzene. This spectrum is extremely similar to gas-phase benzene, as expected. The peaks and their assignments are given in Table 111. The most intense peak in the HREEL spectrum is the out-of-plane C-H bending mode, uIl. Annealing this surface to 200 or 300 K results in the loss of the physisorbed benzene and leaves a saturation coverage of the chemisorbed benzene. The spectrum of this phase is shown in the middle curve of Figure 8. The most intense peaks are at 740 and 875 cm-'and are assigned to the CH aut-of-plane bending mode as in the physisorbed benzene and to the uI mode.6 The
0
SO65
100
0
1000
2000
3000
4000
Energy Loss (cm-')
Figure 8. High-resolution electron energy loss spectra recorded under specular conditions for multilayer benzene adsorbed at 100 K, as a function of annealing temperature. The crystal was annealed for 60 s at each temperature. The peak with the asterisk in the 300 K spectrum is assigned to CO, which had adsorbed from the ambient.
symmetric stretching mode at about 3000 cm-' is very weak. These features and the absence of any ring modes suggest that the benzene molecule is lying essentially parallel to the surface, as observed following room-temperature adsorption. Annealing the surface to 400 K results in significant changes, as depicted in the upper curve of Figure 8. New modes appear at 1180 and 1460 cm-', there is now a pronounced peak at 3085 cm-l, and the mode at 740 cm-' has shifted somewhat to 750 cm-'. The modes at 1180 and 1460 cm-' are assigned to ring mod-, as indicated in Table 111. The 3085-cm-I peak is assigned to the C-H stretching mode and its intensity under specular scattering conditions suggests that the ring is no longer parallel to the surface. These data are consistent with the formation of stable phenyl or benzyne fragments on the surface. The loss features at 750 and 3085 cm-l could also be due to C-H groups on the surface, but the simultaneous presence of the ring modes leads to the identification of phenyl or benzyne species. Annealing the surface further to 600 K results in a loss of all hydrocarbon species. The mochanism of the rehydrogenation of the aryl fragment can be considered in more detail. A comparison of Figures 4 and 6 (or 7) indicate that both benzene and molecular hydrogen desorb in the 400-500 K temperature range. This suggests that at these temperatures the rate of hydrogen desorption and the rate of aryl rehydrogenation/benzene desorption must be comparable. Since hydrogen and benzene desorption are very fast above 350 K,we conclude that the ratelimiting step for hydrogen desorption and hydrogenation of the aryl fragment must be hydrogen production via hydrocarbon decomposition. Some product of the partial decomposition of benzene continues to decompose, producing surface hydrogen. The surface hydrogen can then react with either other surface hydrogen atoms and desorb as H2 or can react with the aryl fragments on the surface to form benzene. The fact that
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1415 3 , 900 K
285.1
-
203.7
700 K 2 500 K
2 c Y
-C
200 K
L
2
5.582’
0-
U
1
0
0
220
290 200 286 284 202 280 278 276
Binding Energy (eV)
Figure 9. Carbon 1s core level X-ray photoelectron spectra following multilayer exposure at 100 K as a function of annealing temperature. The crystal was annealed for 60 s at each temperature indicated. A integral background4’was subtracted from each spectrum.
hydrogen desorbs over a wide range of relatively high temperatures and especially that it desorbs simultaneously with the high-temperature benzene suggest that surface hydrogen is continuously being produced and is available for hydrogenation of the aryl fragment. The thermal decomposition of benzene on the Ni( 110) surface was also probed by X-ray photoelectron spectroscopy and Auger electron spectroscopy. Several different states of carbon were identified, as indicated in Figures 9 and 10. Carbon 1s core level photoemission spectra of a multilayer benzene exposure are shown as a function of annealing temperature in Figure 9. The binding energy of physisorbed benzene was found to be 285.5 eV, in good agreement with other studies on physisorbed or condensed bmzene.’6.M Chemisorbed benzene, produced either by annealing the multilayer to 200 K or by a lower exposure has a binding energy of 284.5 eV. Annealing the surface above 200 K results in a decrease in the total integrated intensity and annealing above 300 K mults in a shift to lower binding energy, which is caused by the development of a carbidic state with a binding energy of 283.7 eV. At about 700 K another state of carbon is observed, with a binding energy of 285.1 eV, which is the predominant state present at 800 K. Further annealing to higher temperature causes a continued decrease in total intensity until by 900 K no carbon is evident on the surface. Since no carbonaceous species desorbed above 500 K, it is assumed that the carbon is adsorbed into the bulk, and the high-temperature state of carbon is then proposed to be subsurface carbon. The assignment of the peaks in the photoemission spectra was confinned by a study of the Auger line shapes, as shown in Figure 10. There is very little difference in the Auger line shape between the spectra of the multilayer (100 K) and the chemisorbed benzene (200 K)despite the 1.0-eV binding energy shift in the carbon 1s core level spectrum. The binding energy shift in the XPS is then interpreted as a relaxation effect and not as an initial-state chemical shift. Annealing the surface to 400 or 500 K results in a drastic change to a carbidic line shapeI3’which is consistent with the XPS bindq energy shift to 283.7 eV. Annealing to 700 K d t e d in another change in the line shape, which is associated with the formation of the subsurface carbon which has a binding energy of 285.1 eV. Annealing the surface to 900 K resulted in (36) Gelius, U.; HedCn, P. F.; Hedman, J.; Lindberg, B. J.; Manne, R.; Nordberg, R.; Nordling, C.; Siegbahn, K.Phys. Scr. 1970, 2, 70. (37) Haas, T. W.;Grant, J. T.; Dooley, G. J., 111. J . Appl. Phys. 1972, 43, 1853.
100 K
240
260 280 300 Kinetic Energy (eV)
320
Figure 10. Carbon Auger spectra for multilayer benzene adsorbed on clean Ni( 110) at 100 K as a function of annealing temperature. The crystal was annealed for 60 s at each temperature. Very little difference is observed between the physisorbed (100 K) and chemisorbed benzene (200 K), but substantial differences are observed as the benzene decomposes to form a carbide ( 5 0 0 K) and subsurface carbon (700 K).
a strong attenuation of the carbon Auger signal as the subsurface carbon was adsorbed into the bulk. A comparison of the thermal chemistry of benzene on Ni( 11l)? Ni( 100): and Ni( 110) suggests that decomposition is more facile on the Ni(l10) surface than the others. This can be easily seen by comparing the hydrogen desorption profiles following benzene adsorption. In all cases, the hydrogen desorption is reaction limited. On Ni( 111) and Ni( loo), the peak desorption temperatures are at about 450 and 470 K, respectively, while on Ni( 110) the peak desorption temperature is substantially lower at 375 K. In addition, decomposition is more extensive on Ni( 110) than Ni(ll1). The selectivity at saturation coverage was estimated based on the H2 desorption yields, and it was determined that 70-8096 of the chemisorbed benzene decomposes. This is a much higher fraction than on Ni( 11l), where only 39% decomposes.2 It has been suggested that C-H bond scission does not occur on terrace sites on Ni(ll1) but does occur on step sites.’* This is consistent with the slower rates of C-H bond scission on Ni( 111) compared with Ni(1 lo), which as a corrugated surface may resemble a stepped surface. Ni( 110) appears to be somewhat unique in the formation of the phenyl or benzyne species and its subsequent rehydrogenation to form benzene. The benzyne intermediate was proposed on Mo(1 10),l6 but no evidence for rehydrogenation was observed. Isotopic exchange reactions on Pt(1 have been done and demonstrate complete scrambling of the hydrogen and deuterium, but no evidence was seen for a-bonded species such as phenyl or benzyne groups. Rerctiollso f R e ” e 00 Sutfur- and Oxygen-Modified Surfaces. Because of the interest in poisoning of metal surfaces by heteroatoms, desorption studies of benzene on partially sulfided and oxidized surfaces were done. Both sulfur and oxygen passivate the surface with respect to benzene decomposition. Temperature-programmed-desorption data are shown in Figures 11 and 12 for the presulfided and preoxidized surfaces. In all desorption profiles, the benzene exposure was constant and resulted in a coverage of about 0.15 ML of benzene. As suggested by the data in Figures 11 and 12, the presence of submonolayers of sulfur or oxygen reduce the hydrogen desorption and increase the benzene desorption yields. No H20, phenol, or benzenethiol were observed in the temperatureprogrammed-desorptionprofiles. The benzene (38)
Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425.
Huntley et al.
1416 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
Benzene/S/Ni( 1 10) I
I
I
I
I
I
I
I
I
I
I
I
1
benzene
I
I
H2 1.o
I n v) v)
0.5
N
-.v)
CQ
3 Q
\
-
0.24
I
1
1
1
0.24 0.13 -h 0.00 I
I
I
l
1
0.58 I
I
I
I
I
0.0
I
100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 900
Temperature (K) Figure 11. Temperature-programmed-desorption profiles for hydrogen (mass 2 ) and benzene (mass 78) following equivalent exposures of benzene as a function of sulfur coverage. The coverage of benzene was -0.15 ML. Note that the desorption curves are presented in reverse order with respect to sulfur coverage for H2and benzene.
Benzene/O/Ni( 1 10) I
1
I
1
1
1
I
I
1
I
I
I
l
l
1 0.8
0
I n
c
.-I/)0
v)
0.4
03 I\
Ni-04
0 I
1
1
1
1
1
I
I
I
I
I
I
v)
0.0
I
100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 900
Temperature (K) Figure 12. Temperature-programmed-desorption profiles for hydrogen (mass 2) and benzene (mass 78) following equivalent exposures of benzene as ) pattern, 0.33 a function of oxygen coverage. The coverage of benzene was -0.15 ML, and the oxygen exposures were 0.22 langmuir ( ~ ( 3 x 1 LEED ) pattern, 0.5 ML), 3.0 langmuirs ( ~ ( 3 x 1 LEED ) pattern, 0.67 ML), and 30 langmuirs ('oxide"). Note that the ML), 0.8 langmuir ( ~ ( 2 x 1 LEED desorption curves are presented in reverse order with respect to oxygen coverage for H2and benzene.
desorption temperature is lowered substantially relative to the clean surface for both oxygen- and sulfur-covered surfaces. On sulfided surfaces, the desorption profile indicates that the benzene is only very weakly adsorbed on the surface. The benzene desorption from the oxidized surfaces is at higher temperature and broader than on the sulfided surfaces and may indicate a stronger interaction with the surface. The benzene desorption from the sulfided surface can be interpreted on the basis of sulfur islanding. Even at relatively low coverages, sulfur tends to form islands on Ni( 1lo).% The hydrogen desorption shown in Figure 11 comes from decomposition of
benzene on clean regions of the surface, which can exist below 0.5 ML of sulfur. The desorption of benzene observed above 400 K is similar to that attributed to hydrogenation of an aryl fragment on the clean surface. The low-temperature benzene desorption is essentially due to benzene weakly physisorbed on the sulfided regions of the surface. The qualitative features of the benzene adsorption and reactions on partially oxidized Ni(l10) are similar to those on the sulfided surface. The details however are somewhat different and can be discussed in relation to structural features on the oxidized surfaces. Oxygen induces various reconstructions of the Ni( 110) surface
J. Phys. Chem. 1992,96, 1417-1423 which tend to leave open troughs of exposed nickel atoms.39 Since nickel sites are still available, the benzene can be more strongly bound to the partially oxidized surfaces than the sulfided surfaca, which explains the higher benzene desorption temperature for the oxidized surfaces. The passivation effect of sulfur and oxygen may be due to simple siteblockingeffects where the presence of the coadsorbed adatoms limit the availability of adjacent sites for decomposition. An alternative explanation is that the passivation may result from subtle changes in the A bonding (and possibly r back-bonding) to the surfaces in the presence of surface sulfur or oxygen. S-ry
The major conclusions which emerge from this study are that benzene chemisorption on Ni( 110) results in molecular adsorption below 300 K. The adsorbed benzene lies essentially parallel to the surface. Semiempiricalmolecular orbital calculationsindicate that the most likely adsorption site is the atop site at a height of 1.75 A or a short-bridge site at a height of 1.90 A. At saturation coverage, 0.2-0.25 ML, a ~ ( 4 x 2 LEED ) pattern was observed. Decomposition is competitive with molecular desorption above 320 K and, at saturation, decomposition accounts for about 7040% of the chemisorbed benzene. Carbon-hydrogen bond scission commences at about 320 K, resulting in reaction-limited hydrogen desorption. The formation of a stable partially dissociated fragment such as a phenyl or benzene species near 400 K was based on spectroscopic and isotopic mixing data. This fragment undergoes rehydrogenation to form benzene at 460 K. In addition to this fragment, benzene decomposition also results (39) Engel, T.; Rider, K.
H.; Batra, I. P. SurJ Sci. 1984, 148,
321.
1417
in the formation of a surface carbide. Interestingly, heating the surface to 700 K results in the formation of a subsurface carbon, which is distinguishable from the surface carbide by its carbon 1s core level binding energy and its Auger line shape. The coadsorption of oxygen or sulfur inhibits decomposition and weakens the bonding between benzene and the nickel surface. Note Added in Proof. Very recent angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) studies by Huber et a1.40 have shown that at low coverage the benzene molecule is oriented as predicted by the ASED calculations (shown in Figure 2b). Their data did not permit exact identification of the site along the [ IT01 direction but did distinguish the azimuthal orientation, which agreed with our modeling studies. Furthermore, as suggested by the high packing density of benzene on Ni( 110) at high coverages, lateral interactions cause a reorientation of the molecules, which was detected by ARUPS. Acknowledgment. This research was sponsered by the Division of Chemical Sciences, Office of Basic Energy Sciences, U. S. Department of Energy, under Contract DE-AC05-840R2 1400 with the Martin Marietta Energy Systems, Inc. We acknowledge the late David Onwood for his work on the ASED programs, Audrey Companion for making the programs available to us, and Steven Overbury and Phillip Britt for critically reading the manuscript. Registry No. Ni, 7440-02-0; H2,1333-74-0; 02, 7782-44-7; S,770434-9; benzene, 7 1-43-2. (40) Huber,W.; Weinelt, M.; Zebisch, P.; Steinrtick, 1991, 253, 72. (41) Shirley, D. A. Phys. Rev. B 1972, 5, 4709.
H.-P. Surf. Sci.
Interaction and Catalytic Decomposltlon of 1,I, 1-Trichloroethane on High Surface Area Alumina. An Infrared Spectroscopic Study Todd H. BaUjnger and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: August 2, 1991; In Final Form: September 25, 1991)
Transmission infrared spectroscopy has been used to study the catalytic decomposition of l,l,l-trichloroethane, CH3CC13, on high surface area alumina, A1203,in the temperature range 300-600 K. At 300 K CH3CC13is reversibly adsorbed on the isolated surface hydroxyl groups of M203via hydrogen bonding. At T 2 400 K an &HC1 elimination occurs, forming CH2=CC12(g). A small amount of surface carboxylate was also formed by means of a minor reaction pathway. It was found that Lewis acid (A13+)surface sites were involved in causing the primary reaction. This was shown by the observation that irreversible adsorption of pyridine on the A13+sites efficiently quenched the surface reaction. Conversely, surface A l 4 H groups are not involved in the CH3CC13decomposition.
I. Introduction Contamination of ground water and soil by various chlorinated hydrocarbons is a major environmental concern. The lifetimes in the environment of such compounds and the end decomposition products which are formed are two important factors in environmental control. Thus, studies have focused on the biotic1>and abiotic3" transformations, along with incinerati~n,~ of compounds (1) Vogel, T. M.; McCarty, P. L.Environ. Sci. Technol. 1987, 21, 1208. (2) Bouwer, E. J.; McCarty, P. L.Appl. Enuiron. Microbiol. 1983, 45, 1286. (3) Pearson, C. R.; Mdhnnell, G. Proc. R.Soc. London, B 1975,189,305. (4) Haag, W. R.;Mill, T. Enuiron.Sci. Technol. 1988, 22, 658. ( 5 ) Graham, J. L.;Hall, D. L.;Dellinger, B. Enuiron. Sci. Technol. 1986,
such as l,l,l-trichloroethane. As shown in other studies, the half-life of CH3CC13may be rather long; for example, the half-life for the hydrolysis of CH3CC13in Ocean water at 25 'C is 1 year.6 A desirable method of environmental cleanup could involve the adsorption and/or reaction of undesirable substances on naturally occurring mineral surfaces in the ground such as alumina, A120!. A 1 2 0 3 is a good model material for research purposes because it contains a variety of chemical sites on its surface which can participate in chemical processes. These sites result because A 1 2 0 3 has a defect spinel structure where A13+ can be in either the octahedral or tetrahedral holes of the oxygen lattice. Various transitional aluminas,' as classified by the ratio of octahedral and (6) Jeffers,
20, 703.
0022-3654/92/2096-1417$03.00/0
P. M.; Wolfe, N. L. Science 1989, 246, 1638.
0 1992 American Chemical Society
