J . Phys. Chem. 1988, 92, 1708-1712
1708
angle), the classical turning point for the v = 7 state of BrC1(3&+) and within 0.2 A of the classical turning point for the fifth vibrational level. We represent the structure of the probable transition state in Figure 3 and an estimated reaction potential energy diagram in Figure 4. Subsequent formation of the Br-Cl u bond and withdrawal of electron density from the already weak halogen-oxygen bonds lead to formation of ground-state 02(3Z;) (with two singly occupied A* orbitals properly oriented at 90°) and highly excited triplet BrCl by spin conservation. The resultant BrC1(3110+)may possess enough vibrational energy, or gain the necessary energy by V-T energy to predissociate into halogen atoms mimicking channel 1b. However, rapid quenching by Br, inhibits this predissociation, leading to a reduction in halogen atom formation at high concentrations of Br,. Finally, we note that the branching ratio for formation of employed in our simulations is similar to the branching ratios for C12 and Br2 formation in the C10 and BrO dispro~~~). portionation reactions (43% and 16%, r e ~ p e c t i v e l y ~ Emission from vibrationally excited C1,(311,+) has been observed in the C10 + C10 reaction,34and the experimental activation barrier for formation of C12is consistent with the experimental endothermicity for formation of C12(3110,+)and 02(3Z:g).7 Both 3110,+ 'ZOB+ and 311!u 'Zo,+ emissions from Br2 have accompanied the production of large concentrations of Br0.33 It is likely that the BrO and C10 disproportionation reactions form halogen molecules by mechanisms similar to the one posed here.
-
-
Conclusions We have argued that the BrO C10 reaction can proceed by a third channel not recognized in current evaluations of rate constant data for stratospheric modeling.1° The formation of BrCI(3110+)helps to qualitatively explain observations from this laboratory which indicate that bromine and chlorine atoms from reaction 1 are not produced by simple halogen-oxygen bond fmion. Unfortunately, due to the difficulty of carrying out direct mea-
+
(35) Clyne, M. A. A.; McDermid, 1. S.J . Chem. SOC.,Faraday Trans. 2
1978, 74, 807.
(36) Clyne, M. A. A,; McDermid, I. S.Faraday Discuss.Chem. SOC.1979, 67, 316. (37) Clyne, M. A. A.; Zai, L. C. J . Chem. SOC.,Faraday Trans. 2 1982, 78, 1221.
surements of a labile species (one that predissociates, fluoresces, collisionally decomposes, is quenched, or possibly reacts rapidly), it will be some time before we have a quantitative measure of the extent of this new channel in conditions appropriate for use in stratospheric models. We note, for example, that attempts to measure (IC) in excess ozone by converting all chlorine and bromine atoms from ( l a ) and (1 b) back into C10 and BrO are difficult to interpret since BrC1(3110+)may itself react with ozone to form Br and C10 or BrO and C1, both exothermic processes. Similarly, measurements of BrCl yields in excess Br, may also be difficult to interpret because formation of ground-state BrCl can be attributed to both reaction of C1 with Br, and quenching of BrCl(,&+) by Brz. Whether similar complications affect the kinetics of other halogen monoxide reactions such as C10 + C10 and BrO + BrO remains to be seen. Concerning Antarctic ozone depletion, the formation of BrCl can shut down bromine-catalyzed destruction of ozone rapidly after sunset by averting BrO into an inactive form, and this complicates attempts to assess bromine chemistry based on OClO observations.I2 Further complications arise when BrCl is produced in an excited state. Because kinetics experiments rarely mimic true atmospheric conditions, to assess the importance of this channel for ozone destruction and production of OClO the behavior of BrCl(3110+)must be well-understood. Quenching rates, fluorescence lifetimes, predissociative lifetimes, and collisional-dissociation rates must be accurately known; or, alternatively, the rate constants for all three channels must be measured under conditions that match or can be extrapolated confidently to stratospheric conditions. The most severe discrepancies will be between observations in low-pressure experiments and true chemical behavior over the Antarctic, where temperatures are low (180 K), pressures are relatively high (50-100 Torr), and the C10 and BrO reaction may be of importance. Acknowledgment. We thank Randall Fried1 and Stanley Sander for numerous invaluable conversations and are deeply grateful to William Brune and Jim Schwab for their continued support and guidance. We also thank Bill DeRoo for development of a new kinetics simulation program. This work was made possible by funding from the National Science Foundation, Grant ATM 8601 126. Registry No. BrO, 15656-19-6; CIO, 14989-30-1.
I nfrared Evidence for Ethylidyne Formation on Alumina-Supported Nickel Mark P. Lapinski and John G. Ekerdt* Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712 (Received: April 21, 1987; In Final Form: February 8, 1988)
Ethylene was dosed onto hydrogen-covered and hydrogen-free Ni/A1,03 at 228 and 248 K. The major infrared absorbance bands were observed at 2870, 1340, and 1125 cm-' and correspond respectively to the v,,,,(CH,), b,,(CH3), and v(C-C) modes of ethylidyne. This assignment was aided by a comparison to EELS and IR spectra for group VI11 metals and by observing parallel attenuation of peak intensities as a function of increasing temperature. Surface hydrogen and/or carbon-hydrogen moieties may have contributed to the formation and/or stabilization of ethylidyne on the Ni/A120, surfaces.
Introduction Ethylidyne (CCH,) formation has been observed with EELS following the chemisorption of ethylene on the fcc (1 1 1) single crystal surfaces of R,'Rh? and Pd,3on the 5 X 20 reconstructed surface of Pt(lOO)> and on the hcp (001) surface of R U . ~ .All ~ these surfaces are close-packed and contain 3-fold sites where *Author to whom correspondence should be addressed.
0022-3654/88/2092-1708$01.50/0
ethylidyne has been shown to exist by LEED studies on Pt( 11 1 ) and Rh(l1 Recently, ethYlidYne has been observed on (1) Steininger, H.; Ibach, H.; Lehwald, S.Surf. Sci. 1982, 117, 685. (2) Koel, B. E.; Bent, B. E.; Somorjai, G. A. Surf. Sci. 1984, 146, 211. (3) Kesmodel, L. L.; Gates, J. A. Surf. Sei. 1981, 111, L747. (4) Hatzikos, G. H.; Masel, R. I. Surf. Sci. 1987, 285, 479. (5) Barteau, M. A,; Broughton, J. Q.; Menzel, D. Appl. Surf. Sci. 1984, 19, 92.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 7. 1988 1709
Rh( 100) when the ethylene decomposition product, CCH, was present at greater than 0.5-monolayer coverage or when CO was preadsorbed at 0.5-monolayer coverage.'O Ethylidyne was proposed to bond at a 4-fold hollow site on this Rh surface. Ethylidyne has also been observed by using IR spectroscopy over Pd/A1203(ref 11 and 12), Rh/AI2O3, Ru/AI2O3, and PtjAl,O, (ref 12), and Pd/Si02 and Pt/Si02 (ref 13), and over Pt/A120, using N M R (ref 14). Table I summarizes the above ethylidyne observations. Ethylidyne has not been detected on Ni surfaces, specifically the Ni( 11 1 ) surface which contains 3-fold triangular sites. Ethylene adsorption on Ni( 111) led to adsorbed acetylene rather than ethylidyne. For the adsorption of ethylene at room temperature, Bertolini and Rousseau16 have proposed a C2H2residue on Ni( 11 1) with two u bonds and one a bond to three Ni atoms. Lehwald and Ibach" have proposed di-a-bonded ethylene (C2H4) at 150 K on Ni(ll1) and at temperatures above 230 K, the species dehydrogenated to coadsorbed acetylene and hydrogen. Ethylidyne has also not been detected on other Ni single-crystal surfaces such as N i [ 5 ( l l l ) ~ ( i l o ) ]Ni(l10),'8and ,~~ Ni(100).'9-2' Table I1 shows the observed species and peak assignments for C2H4adsorption onto clean Ni single-crystal surfaces. The purpose of this Letter is to present IR evidence for ethylidyne on Ni/AI2O3and to offer a tentative explanation for its observation.
Methods Transmission infrared spectra were obtained at a resolution of 2 cm-' with a Digilab FTS-15/90 FTIR equipped with a MCT detector. The IR cell and flow system have been described previousIy.22 The supported Ni preparation procedures were similar to those reported earlier22except that the pretreatment consisted of 4 h in H e (99.995%) at 673 K followed by H2 (99.999%) reduction for 8 h at 673 K. Alumina (Degussa C, 100 m2/g) was contacted with Ni(N0J2.6H20 by using a wet impregnation technique. The particle size distribution for this surface was determined by using a Siemens Elmiskop I transmission electron microscope. The surface-averaged Ni crystallite diameter was calculated to be 104
0
5 E
A.
The infrared experiments consisted of the following. Selfsupporting wafers were pressed at 50 000-60 000 kPa and were degassed in the IR cell under 5-mTorr vacuum for 30 min at 473 K followed by a 120-150-min H2 reduction at 643 K. The wafer was then cooled to the dosing temperature (228 or 248 K) under flowing H 2 to generate a hydrogen-covered surface. In some experiments the wafer was further treated with H e at 643 K for 30 min, after the H2 reduction, and then cooled to the dosing temperature in flowing He to create a hydrogen-free surface. The cell was evacuated for 1 min (5-mTorr vacuum) at the dosing temperature prior to adding ethylene. Ethylene (99.99%,
c
2
3
(6) Hills, M. M.; Parmeter, J. E.; Mullins, C. B.; Weinberg, W. H. J . Am. Chem. Soc. 1986, 108, 3554.
(7) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. Chem. Phys. Lett. 1978, 56, 267.
(8) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J . Chem. Phys. 1979, 70, 2180.
(9) Koestner, R. J.; Van Hove, M. A,; Somorjai, G. A. Surf. Sci. 1982, 121, 321.
(10) Bent, B. E. Ph.D. Thesis, University of California, Berkeley, 1986. (11) Beebe, T. P., Jr.; Albert, M. R.; Yates, J. T., Jr. J . C a r d 1985, 96, 1. (12) Beebe, T. P., Jr.; Yates, J. T., Jr. J . Phys. Chem. 1987, 91, 254. (13) Bandy, B. J.; Chesters, M. A.; James, D. I.; McDougall, G. S.; Pemble, M. E.; Sheppard, N. Philos. Trans. R . Soc. London, A 1986, 318, 141. (14) Wang, P.-K.; Slichter, C. P.; Sinfelt, J. H. J . Phys. Chem. 1985, 89, 3606. (15) Skinner, P.; Howard, M. W.; Oxton, I. A,; Kettle, S . F. A,; Powell, D. B.; Sheppard, N . J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1203. (16) Bertolini, J. C.; Rousseau, J. Surf. Sci. 1979, 83, 5 3 1 . (17) Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425. (18) Stroscio, J. A.; Bare, S. R.; Ho, W. Surf Sri. 1984, 148, 499. (19) Lehwald, S.; Ibach, H.; Steininger, H. Surf. Sri. 1982, 117, 342. (20) Zaera, F.; Hall, R. B. Surf. Sci. 1987, 180, 1. (21) Zaera, F.; Hall, R. B. J . Phys. Chem. 1987, 91, 4318. (22) Campione, T. J.; Ekerdt, J. G. J . Card. 1986, 102, 64.
.-E 9 .-
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1710 The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 TABLE 11: Specular EELS Studies of C2H,Dosed on Clean Nickel Surfaces C2H4dose, langmuirs structure formed surface temp, K 3 .O di-a-bonded C2H, Ni(ll1) 150Q
230 Ni(l11) Ni( 1 10)
298"
80"
acetylene and adsorbed H 6.0 3.0
220
acetylene di-a-bonded C2H4 acetylide (CCH) +di-a-bonded C2H4
Ni(100)
90"
satd
-
sp2 hybridized
C2H4
225
vinyl (-CH=CH2)
275 Ni[5(111)x(ilO)]
150"
240
acetylene 3.0
vinyl (-CH=CH2)
+di-u-bonded C2H4 sp3 hybridized type species
peak, cm-'
int
band assignt
ref
G H 2 ) (sym) 8(CH2) (scissors) u(CC) x(CH2) (wag) u(CH) u(CC)
17
V(CH) U(CC)
16
u ( C W (sym) 6(CH2) (scissors) x(CH2) (wag) u(CH) V(CC) x W 2 ) (wag)
18
S
u(CH2)
21
m
v(C=C) 6(CH2) (scissors) x(CH2) (wag) v(CHZ)(asym) u(CH2) ( s Y ~+) 4 C H ) 6(CH2) (scissors) A C H ) (rock) u(CH) V(CC)
2950 1440 1200 1100 2925 1220
s
2944 1218
S
2970 1435 1145 2990 1290 1175
vw w
2995 1560 1395 1100 3100 2940 1410 1335 2925 1340 3100 3000 1510 11201200 1420 2940 1460
s
m w vs
m m S
S
m w w
S
vs m S
w S
m S
m S
Matheson) was added by syringe to a pressure of 50.0 f 1.2 Torr and remained over the wafer for 15 min. The gas phase was sampled via syringe prior to the evacuation of the gas phase and was analyzed by gas chromatography. The effect of temperature was studied by slowly warming the cell under static cell vacuum.
(23) Morrow, B. A.; Sheppard, N. Proc. R. SOC.London, A 1969,311,391,
17
-m
S
6(CH2) (scissors) not assigned
W
Dosing temperature. The subsequent temperatures are those to which the surfaces were heated.
Results Gas chromatography revealed that ethane (gas) was produced from both the H-covered and H-free surfaces, at both dosing temperatures, after the 15-min dosing period. Only trace amounts of methane were detected. Infrared spectra following the adsorption of ethylene at 228 K are shown for H-covered and H-free Ni/A1203in Figure 1, parts A and B, respectively. The absorbance intensities decreased less than 4% within a 15-20-min period under cell vacuum at 228 K. Figure 1 shows three major absorbance bands at 2870, 1340, and 1125 cm-I. Similar spectral features were observed at 248 K (not shown). Weak to medium intensity absorbance bands were observed in some of the 228 and 248 K spectra at 2960,2932,2800, 1420, 1240, and 1165 cm-I. The absorbance bands in the C-H stretching region (3100-2700 cm-I) resemble those obtained by Morrow and Sheppard at 195 K for ethylene adsorption after 1 min on Ni/Si02.23 The above IR bands were not observed in experiments that were done on pure alumina wafers. The effect of hydrogen coverage can be seen by comparing parts A and B of Figure 1. The intensity ratios for the 1125/1340 absorbances varied slightly with values of 0.57 for the H-covered surface and 0.47 for the H--free surface. A much larger difference can be seen for the 2870/ 1340 ratio with values of 1.4 and 0.44 for the H-covered and H-free surfaces, respectively. The behavior Of the I R bands was monitored with increasing temperature. In general, for both Ni/AhO3 surfaces, the 2870and 1340-cm-' bands shifted little with temperature. The
u(CH2) (asym) [de]* V(CH2) (sym) u(C=C) 6(CH)
[de] = deconvoluted.
2870
228 K on Ni/Alumina A. H-Covered 8 . H-Free 1340
W
0
z
4
m
U 0
cn
m 4
u 2900 2700
3100
1500
1300
1100
WAVENUMBER (cm-1)
Figure 1. Infrared spectra of ethylene adsorbed at 228 K Ont0'9 wt % Ni/A120p.The dosing pressure of ethylene was 50.0 f 1.2 Torr, and the dosing time was 15 min. The IR spectra were recorded after a 1-min evacuation of the gas phase. (The absorbances associated with alumina have been subtracted out.)
1 125-cm-l band shifted to lower wavenumbers with increasing temperature; it was centered at 1120 cm-1 between 245 and 260 K and at approximately 1115 cm-' at temperatures greater than 270 K. Figure 2 shows the normalized peak heights of the bands at 2870, 1340, and 1125 cm-' versus temperature. The 1340- and the 1 125-cm-1 bands displayed parallel attenuation for both surfaces as the temperature was increased from 228 K to at least 270 K, suggesting that these absorbances belong to the same
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5
e
0
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 1711 1.2
r
-
I
H-Covered NilAlumina
-
0.6
225
235
245
255
265
275
285
TEMPERATURE (K) H-Free Ni/Alumina
W
I Y
a
W
e 0
w c!
0
U
2
0.0
'
225
1340cm-1
I
I
235
245
255
265
275
285
TEMPERATURE (K) Figure 2. Temperature behavior of absorbance bands at 2870, 1340, and 1125 cm-' for H-covered and H-free Ni/AI2O3. All absorbances were normalized to the peak height observed following the I-min evacuation
of gas-phase ethylene. species. The 2870-cm-I band was also attenuated for bothsurfaces but did not parallel the behavior of the 1340- and 1125-cm-' bands. Two new strong bands at 2879 and 2890 cm-I grew in with increasing temperature and may have overlapped with and contributed to the absorbance peak intensity of the 2870-cm-' band displayed in Figure 2.
Discussion A . Peak Assignments. Table 11 shows the species observed on Ni single crystals, and these species may be expected over Ni supported on alumina. The species are molecularly adsorbed ethylene ( -sp2 hybridized), vinyl (-CH=CH2), di-u-bonded ethylene, acetylene, and acetylide (CCH). A comparison of bands between the Ni/A1203 IR spectra and the EEL spectra must be consistent with the surface selection rule. If molecularly adsorbed ethylene is *-bonded with the H-C-H plane oriented parallel to the Ni/A1,03 surface, then the C H 2 wag at 1100 cm-I would be the only IR band expected in our spectra. If molecularly adsorbed ethylene is H-bonded or a-bonded with the H-C-H plane not parallel to the Ni/A1203 surface, then the CH2 stretches and bends should be observed in IR. Molecularly adsorbed ethylene can be discounted as the species responsible for the strong IR absorbances in both cases. In the first case, the IR bands at 1125 and 1340 cm-I acted in parallel and no band is expected at 1340 cm-I for ethylene a-bonded parallel to the surface. In the second case, no strong band at 2995 cm-' (CH, stretch of a sp2 hybridized species) was observed in the IR spectra. Vinyl species on Ni(100) and Ni[S(111)X(i10)] have medium to strong bands above 3000 cm-I (CH2 stretches). No strong bands at 3000-3100 cm-' were observed in our IR spectra, thus eliminating the vinyl species as a major IR contributor. Di-o-bonded C2H4 on Ni( 111) and Ni(l10) displays a strong peak at 1 100-1 175 cm-' (CH, wag) and a weak to medium peak at about 1430 cm-' (CH, scissors). Since the band a t 1125 cm-' in our spectra behaved in parallel with the 1340-cm-I band and no band is expected for di-a-bonded ethylene at 1340 cm-', the di-u-bonded species can be discounted as a major IR contributor. Acetylene-type species are widely observed on Ni single crystals by 230 K with a C H stretch at 2925-3000 cm-l.
If acetylene-type species were present on Ni/A1,0,, then the C H stretch shifted down at least 55 cm-' to 2870 cm-'. Acetylene-type species cannot be associated with the pair of bands at 1125 and 1340 cm-'. Two possibilities are suggested to account for the three strong absorbance bands observed in Figure 1. The first is a combination of acetylene-type species and a second species which caused the bands at 1125 and 1340 cm-', and possibly contributed to the absorbance at 2870 cm-l. The second possibility is a single species, one which has not been observed over Ni single-crystal surfaces under UHV conditions. A comparison of C2H4 adsorption on various group VI11 metals (Table I) reveals that the absorbance bands a t 2870, 1340, and 1125 cm-' are well matched to the vsym(CH3),6,,,(CH3), and v(C-C) modes of ethylidyne, respectively. (Asymmetric modes of ethylidyne are not expected because the surface selection rule24,2sshould be in operation for our Ni/Al,O,.) The medium-intensity peak at 2800-2830 cm-I in the Ni/A120, spectra is consistent with an ethylidyne overtone, 26asym(CH3).13*15 The infrared evidence presented here clearly supports the presence of ethylidyne. The possibility that an acetylene-type species is a major surface species and contributes to the 2870-cm-' band cannot be proven with the present IR data. The weak bands in the Ni/A1203spectra will not be assigned here. B. Adsorption Site. From the LEED studies of ethylidyne on P t ( l l 1 ) and Rh(l1 1),7-9ethylidyne is believed to bond at 3-fold triangular sites. Beebe and Yates26have used ethylidyne to determine the percentage of Pd( l l l ) sites on Pd/A120, by assuming that ethylidyne only formed above 3-fold hollow sites. Recently, Bentlo has observed ethylidyne on Rh( IOO), but another adsorbate such as CO or CCH was required at a coverage of 0.5 monolayer in order for ethylidyne to form. The ethylidyne was proposed to bond in a 4-fold site on the Rh( 100) surface. Large metal particles of oxide-supported Ni are thought to contain a significant amount of (1 11) and (100) plane^.^^-^^ Since ethylidyne has not been observed on Ni single crystals, neither this species nor a possible surface site can be proposed by a direct comparison to these studies. However, a likely site is the closepacked, 3-fold sites on Ni( 111) based on a comparison to the other (1 11) fcc s u r f a c e ~ . ~ - Other ~ , ~ - ~sites cannot be discounted. For example, the 4-fold site may be possible as in the case of Rh( 100)'O because other carbon-hydrogen species are suggested by the weak peaks in Figure 1 on the Ni/A1203 surfaces. C. Effect of Hydrogen and Carbon-Hydrogen Moieties. Two factors may be responsible for the observation of ethylidyne: adsorbed hydrogen may be allowing a hydrogenation pathway to ethylidyne and/or some carbon-hydrogen moieties may affect decomposition reactions during ethylene adsorption. One proposed mechanism for ethylidyne formation over group VI11 metals proceeds by dehydrogenation at one carbon of di-ubonded ethylene to form vinyl (CHCH2) and vinylidene (CCH,) followed by rehydrogenation at the other carbon rcsulting in ethylidyne (mechanism A).30931 A second proposed mechanism proceeds by H insertion into a-bonded C2H4to form ethyl (CH,CH,) followed by two dehydrogenation steps at the a-carbon resulting in ethylidene (CHCH,) and then ethylidyne (mechanism B).'O Evidence for the ethyl species on Pt(l11) and R h ( l l 1 ) consisted of (1) ethylene in the presence of surface hydrogen could be hydrogenated to ethane (gas) at the ethylidyne formation t e m p e r a t ~ r e ' and ~ . ~ (2) ~ preadsorption of D,, followed by ethylene, incorporated more D into the ethylidyne methyl group than adding D2 after ethylene, suggesting that some hydrogenation of surface (24) Pearce, H. A,; Sheppard, N. Surf.Sci. 1976, 59, 205. (25) Greenler, R. G.; Snider, D. R.; Witt, D.; Sorbello, R. S. Surf.Sci. 1982, 118, 415. (26) Beebe, T. P., Jr.; Yates, J. T., Jr. Surf.Sci. 1986, 173, L606. (27) Van Hardeveld, R.; Van Montfoort, A. Surf.Sci. 1966, 4 , 396. (28) Van Hardeveld, R.; Hartog, F. Surf Sci. 1969, 15, 189. (29) Desai, P. H.; Richardson, 5. T.J . Catal. 1986, 98, 392. (30) Kang, D. B.; Anderson, A. B. Surf.Sci. 1985, 155, 639. (31) Stuve, E. M.; Madix, R. J. J . Phys. Chem. 1985, 89, 105. (32) Godbey, D.; Zaera, F.; Yeates, R.; Somorjai, G.A. Surf.Sci. 1986, 167, 150.
J . Phys. Chem. 1988, 92. 1712-1715
1712
species must occur in the formation of ethylidyne.I0 One difference between the Ni/AI2O3 surfaces and the single-crystal surfaces was the presence of adsorbed H during ethylene adsorption. The EELS studies of ethylene adsorption on Ni single crystals (Table 11) were conducted over surfaces that were initially free of H; one exception is C2D, on Ni(100).33 We believe that adsorbed H was present on both Ni/AI2O3surfaces during C2H, adsorption. Some ethylene dehydrogenation is proposed to occur during initial adsorption on the H-free surface. Three facts support this: ( 1) dehydrogenated species (acetylene-type) are observed on Ni( I 1 I), Ni( 1 lo), and Ni( 100) by 230 K;’’.**.*’(2) dehydrogenation and C-C scission reactions were observed to occur at lower temperatures over the stepped surface, Ni[5( 11l)X(TlO)], as compared to N i ( l 1 l ) ” (supported Ni should contain steps, kinks, etc.); and (3) self-hydrogenation of ethylene occurred on the H-free surface producing ethane (gas) at 228 and 248 K. Dehydrogenation reactions would make hydrogen available, through surface diffusion, to the entire Ni surface. The observation of ethane (gas) at the ethylidyne formation temperature for both H-free and H-covered Ni/A1203 suggests that adsorbed H on the surface during C2H, dosing allows a hydrogenation pathway to ethane (gas) and possibly to the ethylidyne species on our Ni/ A1203. Mechanism B is suggested but mechanism A, as well as other mechanisms, cannot be ruled out by this study. The second factor responsible for the observation of ethylidyne may be that adsorbed carbon-hydrogen species block decomposition pathways and thereby enable ethylidyne to form, or stabilize it once formed, in a manner similar to the carbon species over Rh(lOO).10 The Ni/A1203 surfaces were exposed to 50 Torr of ethylene for 15 min (ca. loL0langmuirs), which may lead to a (33) Zaera, F.; Godbey, D.; Somorjai, J. A. Presented at the 192nd National Meeting of the American Chemical Society, Anaheim, CA, Sept 1986. Adsorbing hydrogen prior to C2D4on Ni(100) at 173 K did not produce ethylidyne. This is expected because ethylidyne has only been observed on a non-close-packed surface, Rh(100),I0 when other carbon species such as CO or CCH were present.
different surface environment for adsorbing ethylene than the low doses under the UHV conditions reported in Table 11. Carbonhydrogen species other than ethylidyne are suggested by the weak peaks in Figure 1 on both the H-covered and H-free Ni/Al,O, surfaces. D. Implications to Hydrogenation Reactions. In regard to the mechanism of catalytic ethylene hydrogenation, Godbey et ai.32have presented evidence on Pt( 11 1) that ethylidyne, and perhaps ethylidene, is directly involved in ethylene hydrogenation at room temperature. Beebe and Yates have carried out an in situ IR study of ethylene hydrogenation over Pd/Alz03 at 300 K and suggested that ethylidyne was a spectator species.34 Ethylene hydrogenation studies that might reveal the mechanism over our Ni/Alz03 surfaces have not been performed. The data in Figure 2 reveal that ethylidyne is unstable and unlikely to saturate the surface of Ni at 300 K as it appears to over Pd and Pt. Therefore, ethylene hydrogenation by transfer of H via ethylidyne/ethylidene3* is not expected to be a dominant path over Ni. In summary, ethylidyne has been observed on Ni/A1,03 at ethylene dosing temperatures of 228 and 248 K. This species has not been observed on Ni single-crystal surfaces; it is the first observation of ethylidyne on Ni. Surface hydrogen and/or carbon-hydrogen moieties may have contributed to the formation and/or stabilization of ethylidyne in our experiments. Further studies are in progress to help explain ethylidyne formation over Xi. Acknowledgment. This work was supported by the National Science Foundation under Grant CBT-8319494. The FTIR was funded by the Department of Defense Instrumentation Grant DAAG-29-83-0097. We also thank F. Zaera, A. Campion, and J. M. White for helpful comments. Registry No. Ethylene, 74-85-1; ethylidyne, 67624-57-1 (34) Beebe, T.P., Jr.; Yates, J. T., Jr. J . Am. Chem. SOC.1986, 108, 663.
Ab Initio Calculations on Localized Electrons in Alcoholic Matrices: Hydrogen-Bond Defect Model H. Tachikawa, M. Ogasawara,* Faculty of Engineering, Hokkaido University, Sapporo 060,Japan
M. Lindgren, and A. Lund Department of Physics and Measurement Technology, Linkoping University, S-581 83 Linkoping, Sweden (Received: November 30, 1987)
The validity of a new hydrogen-bond defect model of localized electrons in irradiated crystalline alkanediols has theoretically been examined by using ab initio molecular orbital calculations. A molecular cluster together with an excess electron was treated as a supermolecule, and the optimum configuration was searched for by calculation. Since the cluster of octanediol or hexanediol was too large, a simplified system composed of methanol or ethanol molecules and an excess electron was chosen. The calculationswere made on the different configurations of the constituent molecules. The results suggested that the localization of the excess electrons in the middle of the cluster was disturbed by the existence of hydrogen bonding, but the sites composed of a hydroxy group and a methyl (methylene) group were able to stabilize the excess electrons. The computational results together with the experimental evidence obtained previously led to the conclusion that the excess electrons were localized at the defects of the hydrogen-bonded network in a form of a cluster anion in alcoholic matrices. Introduction In 1979 Box and co-workers presented convincing evidence by means of ENDOR spectroscopy that the excess electrons could be stably trap@ in carbohydrate and other polyhydroxy crystals.’
Soon afterward optical data by means of pulse radiolysis were obtained.’ In 1981 Samskog et al. found for the first time a broad, transient absorption attributable to localized electrons (elK-) in pulse-irradiated single crystals of 1,6-hexanediol at room tem-
(1) (a) Budzinski, E. E.; Potter, W. A,; Potienko, G.; Box, H. C. J . Chem. Phys. 1979,70,5040. (b) Box, H. C.; Budzinski, E. E.; Freund, H. G.: Potter, W. R . J . Chem. P h j r . 1979, 70. 1320.
(2) (a) Buxton, G. V.; Salmon, G. A. Chem. Phys. Lerf. 1980, 73, 304. (b) Samskog, P.-0.; Lund, A,; Nilsson, G.; Symons, M. C.R . J . Chem. Phys. 1980, 73, 4862.
0022-3654/88/2092-1712$01.50/0
0 1988 American Chemical Society
