J. Phys. Chem. 1992,96,7703-7708 (24) Erwin, J. W.; Ring, M. A.; ONeal, H.E. In?. J. Chem. Kine!. 1985, 17, 1067-1083. (25) Coltrin, M. E.; Kee, R. J.; Evans, G. H.J . Electrochem. SOC.1989, 136, 819-829. (26) Purnell, J. H.; Walsh, R. Proc. R. SOC.London 1966,293,478-488.
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(27) JANAF Thermochemical Tables 1978 Supplement J . Phys. Chem. Ref. Data 1978, 7, 793. (28) Walsh, R. Thermochemistry and Reactivity of Silylenes. In Silicon Chemistry; Corey, E. R., Corey, J. Y., Gaspar, P. P., Eds.; Ellis Horwood Limited: Chichester, UK, 1988.
Low-Energy Electron-Induced Chemistry of Ethylene on Clean and CI- and D-Covered Ag(111) X.-L. Zhou and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: January 18, 1992)
The chemistry, induced by 50-eV electrons, of ethylene (C2H4) on clean and C1- and D-covered Ag( 1 1 1) has been studied. In the absence of electron irradiation, C2H4 is weakly *-bonded and desorbs, with no thermal decomposition, below 170 K. Consistent with *-donation and *-polarization, one monolayer of ethylene lowers the work function by 0.38 eV. Evidence is presented that adsorbed C2H4, exposed to low doses of electrons, is selectively decomposed to adsorbed H and vinyl (C2H3). The latter leads exclusively to 1,3-butadiene(C4H6)in subsequent temperature-programmeddesorption (TPD). Besides C4H6, higher doses of electrons lead to acetylene (C2H2),precursors to butene and tiny amounts of ethane, but no C, or C3 products. In the presence of C1, the parent ethylene is stabilized and the amount of C4H6produced in TPD is enhanced. For ethylene and coadsorbed D, there is, in the absence of electron irradiation, no reaction. With electron irradiation, isotopically labeled dihydrogen and partially deuterated ethylene and ethane form, but there are no C,, C3,or C4 products. We propose that bond-specific decomposition, e.g., C-H bond cleavage, to form vinyl occurs when the weakly bound ethylene is ionized by low-energy electrons. That further C-H bond cleavage occurs with much lower cross section and that there is no C-C bond cleavage may result from relatively effective quenching of excited ionic states of the strongly chemisorbed primary product, C2H3.
Introduction As a part of our continuing investigation of photon’ and electron2+ driven processes at adsorbatemetal interfaces, we report in this paper on the chemistry, induced by 50-eV electrons, of ethylene adsorbed on clean and C1- and D-covered Ag( 11 1) at 100 K. While intrinsically interesting, nonthermal means of activating adsorbed species also offer opportunities to prepare and study intermediates, thought to be catalytically important. For example, heterogeneous hydrocarbon catalysis is a complex multiphase kinetics problem; many intuitively attractive surface intermediates have been proposed, but few of these hydrocarbon fragments have been cleanly synthesized and characterized structurally and kinetically. One elegant approach to adsorbed CH3 fragments involves the dissociation of CH4 during energetic collisions with Ni( 111),5 but generalizing this to species containing more than one carbon atom appears difficult. A second approach, of greater generality but less attractive because a coadsorbate is unavoidable, is the thermal dissociation of alkyl iodides: including vinyl iodide.’ A third approach involves the photodissociation of alkyl halides.* A fourth approach, and the one used here, relies on controlled fluxes of low-energy electrons to drive bond-specific nonthermal adsorbate chemistry. In this paper we provide firm evidence for the production of an important C2intermediate, vinyl (-CH=CHJ? and H by controlled low doses of 50-eV electrons onto r-bonded C2H4 bound to Ag(ll1) at 100 K. H2and residual parent molecules are desorbed by annealing to 200 K,leaving only the vinyl fragments. Ag is an important and interesting catalytic metal. For example, the reaction of ethylene and oxygen to form ethylene oxide on Ag catalysts is well-known.’OJ1While adsorption of CzH4on and C1- and Ooovered Ag(1 10)lohas been studied, little attention has been paid to Ag( 11l),l4 Our earlier studies of hydrocarbon fragments (C,H,)3*4J5-17 indicate that Ag(ll1) is relatively unique among transition-metal surfaces; C-C and C-H bond formation from surface C a mfragments and H atoms have much lower activation energies than C-C and C-H bond dissociation. As one interesting example, the dissociation of benzene
on Ag(l1 l), induced by electrons (150 eV)? leads to surface phenyl and hydrogen, and during sukquent TPD these recombine to form biphenyl and dihydrogen. The results of this paper demonstrate that bond-specific electron-induced dissociation may be a general phenomena that deserves considerable study. Experimental Section We used a UHV chamberi8 equipped with a double-pass cylindrical mirror electron energy analyzer with a coaxial electron gun for Auger electron spectroscopy (AES), a differentially pumped He discharge lamp producing vacuum ultraviolet light (used here for work function change (A@) measurements), a quadrupole mass spectrometer (QMS)for temperature-programmed desorption (TPD), and an ion gun for surface cleaning. The chamber was ion-pumped and had an auxiliary titanium sublimation pump and a 170 L/s turbomolecular pump. The base pressure was 5 X Torr. The clean,19as verified by AES, Ag( 11 1) crystal (-0.8 cmz surface area) was cooled to 100 K with liquid nitrogen. The temperature, ramped at 2.5 K/s for TPD, was measured with a chromelalumel thermocouple spot-welded to a Ta loop that was pressed into a hole drilled in the edge of the crystal. A@ was determined from the low kinetic energy threshold of secondary emission of the He(1) UPS spectra. Ethylene (99.575, Linde) was dosed, with the Ag( 111) surface at 100 K,through a multichannel array doser positioned about 7 mm from the sample. To prepare for dosing, the crystal was turned away from the doser and the C2H4pressure was increased, using a leak valve, to give AP = 5 X Torr at the vacuum system ion gauge. To initiate the dose, the substrate was then turned to face the doser. While very reproducible, the exposures in langmuirs are not known. Surface C1 was prepared by electron-induced dissociation of monolayer CH3Cl followed by flashing the surface to 600 K to desorb undecomposed CH$l and C, fragments. All the CI fragments desorb as C,H, hydrocarbons, leaving only C1 on the surface.17 Surface D was prepared by exposing the surface at 100
0022-3654/92/2096-7703%03.00/00 1992 American Chemical Society
7704
Zhou and White
The Journal of Physical Chemistry, Vol. 96, No. 19, 1992
185
I
L
I
I
K
1 Sat'n C ~ H 4 / A g ( l l 1 )
i
if1'
Dosing time (s)
0
J 1
2
3
4
5
6
7
Electron Fluence e-) Figure 2. Work function change, measured at 100 K, versus electron fluence for a saturation coverage of C2H4 on Ag(ll1).
100
,
so . . . . . . . . . . . . . . . . . . . . . . . . 125
150
175
200
225
250
I
2.5 .
.
,
275
Temperature (K) Figure 1. TPD spectra of C2H4 from Ag(ll1) exposed to different amounts of C2H4 at 100 K. The dotted curve is for a 600 s dose of C2H4 on C1-covered Ag( 11 1).
K to atomic D produced by decomposition of D2 ( 5 X 10" Torr) on the filament of the QMS. Electrons from the QMS filament were used to irradiate the adsorbates, and to initiate electron-driven chemistry, the sample was turned to a line-of-sight position. The electron energy was 50 eV with a peak width at half-maximum of 1.5 eV (measured using the CMA with a pass energy of 25 eV). An estimate of the electron flux (-2.5 X 1013 electronsls) was obtained by measuring the electron current from the crystal to ground. As discussed in earlier this estimate has been corrected for collection of electrons on other parts of the manipulator but does not account for electron scattering from the crystal. Thus, the electron fluences (EF) reported here are lower limits, and we use them only in a relative sense and do not calculate cross sections. The temperature rise during electron irradiation was less than 1 K. Multiplexed TPD spectra were taken after electron irradiation, and to minimize the influence of electrons during TPD, the sample was turned away from the line-of-sight position. To unambiguously identify desorption products, careful attention was given to cracking patterns and the temperature profiles of various ions. Results Thermal Properties of C 2 H 4 on Ag(ll1). In the absence of electron irradiation, the TPD spectra of C2H4, Figure 1, show no thermal decomposition and only one saturable desorption state of C2H,; multilayer adsorption does not occur at 100 K and low pressure (-1O-Io Torr). At low exposure, the C2H4 peak temperature is 142 K. It broadens and shifts downward with increasing coverage, due to adsorbate-adsorbate interactions, and peaks at 128 K for a saturation exposure. Consistent with work on other Ag surfaces,12we estimate that the coverage at saturation is about 0.5 molecules of C2H4 for each surface atom of Ag. These results, implying nondissociative molecular adsorption and lateral interactions near saturation, are in good agreement with other work on Ag(1 10).10*12~'3 On most transition-metal surfaces, C2H4 is more strongly bound (di-a bonded) and adsorption involves rehybridization of the C atoms from sp2 to sp3.20Assuming that rehybridization requires participation of metal d-orbitals, the
-
absence of di-a-bonded C2H4 on Ag is readily understood; the d-band of Ag lies well below the Fermi level and does not interact strongly with C2H4 molecular orbitals. At C2H4 saturation, the Ag( 111) surface work function decreases by 0.38 eV (Figure 2), compared with a decrease of 0.69 eV on Ag(l10).12 Although C2H4does not have a permanent dipole moment, polarization of the charge within adsorbed C2H4 molecules will result in a surface dipole with the positive end pointing outward, causing the work function to decrease. Small amounts of charge transfer from C2H4 to Ag may also contribute. When C1 is coadsorbed (dotted curve of Figure l), C2H4 is still molecularly held but is stabilized (desorbing at 185 K). Similar stabilization occurs on Ag(1 1O).Io A simple explanation is that C1, by withdrawing electron density from Ag?' promotes charge donation from C2H4 to Ag and stronger adsorbate-substrate bonding. The donor (C2H4)acceptor(Cl) interaction can stabilize both adsorbates.22 EleCtron-Inducd Chemistry of Adsorbed q€b. Having briefly described the adsorption and desorption of C2H4 on clean and C1-covered Ag( 11l), we now turn to electron-induced chemistry. Unless otherwise noted, the surfaces were saturated with C2H4 at 100 K before electron irradiation. 1. Clean Ag(ll1). Figure 2 shows the work function change (A#) as a function of electron fluence (EF). It increases rapidly in the initial stages of electron irradiation and levels off at A@ = +0.07 eV for EF > 4.5 X 10I6e-. This variation indicates that electron irradiation leads to the production of electron acceptors and suggests, as confirmed below, that C-H bonds are broken and strong C-Ag bonds are formed. Interestingly, there is some structure in the curve of Figure 3, e.g., a change in slope around 1 X 10l6e- and a short plateau between 6 and 8 X 1015e-. While not investigated in detail, this structure is consistent with changes in the distribution of TPD products noted below. For an electron exposure where A# varies slowly (4.5 X 10l6 e-), the subsequent multiplexed TPD spectra (Figure 3) indicate a number of products (identified by the thermal profiles of various ions and the known cracking patterns of candidate species). The TPD products and the temperature ranges over which they desorb are summarized in Scheme I. Residual parent desorbs with peaks at 132 and 200 K. At about 155 K, there is a peak for C2H2 (difference between total signal at m l e = 26 and contributions from fragmentation of higher masses). The m / e = 2 (H2) signal begins in this region and peaks at 200 K. There is also a small peak at 200 K for C2H6 ( m / e = 30). The peak at -210 K is attributed to butene (C4H,) on the basis of signals at m / e 56,41, and 39, which all follow the same temperature pattern and the expected fragmentation pattern of butene. In Figure 3, the spectrum with peaks at 195, 265, and 320 K is attributed to 1,3-butadiene(C4H6);this assignment is confirmed by the 39, 50, and 53 amu signals (not shown) which follow the same thermal
The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7705
Chemistry of Ethylene
SCHEME I: Schematic of Electron-Induced Decomposition and Subsequent TPD for Large Doses of Electrons (>1 X 10l6e-) on CWAg(111)
~ 1 8 K8
........
700
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n
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Dose of electrons a t 100 K r o l l o w e d by h e a t l n g (TPD)
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e l e c t i o n dose
C4Hs
-500
.-3t
TPD Products
a 2400
.-P
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170
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v)
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I
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C4H 0
H2
heat 100
155
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I 200
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290 K 'A6
250
300
350 290
Temperature (K)
-
350 K C4H 6
Figure 3. TPD spectra of H2, C2H2,C2H4, C2H6, C4H6(1,3-butadiene), and C4Hs (butene) taken after exposing a C2H4-saturatedsurface to 4.5 X 10I6electrons (50 eV) a t 100 K. For C2H2and C2H4,the contributions from fragmentation of higher mass species have been subtracted. The dotted curve is for a submonolayer of 1-butene dosed on Ag(ll1).
profiles. We also monitored other masses corresponding to C14 hydrocarbons in TPD. They, however, were either undetectable or were fragments of the detected species in Figure 3. Thus, we conclude that after a dose of electrons sufficient to raise A4 to +0.07 eV, numerous hydrocarbons desorb in TPD-C2H2, C2H4, C2&, C& and C&. Interestingly, there are no C, or C3s p e s . Since dosed atomic H desorbs as H2 in the same temperature regime,23while H atoms derived from C,H, ( m / n < 5/6)fragments desorb above 600 K,4 we conclude, as indicated in Scheme I, that the H2observed here is due to recombination of H attached to Ag. The C2H2,which desorbs at 155 K, is most likely from surface C2H2molecules formed when two C-H bonds of C2H4 are dissociated by electrons. The 200 K C2H4 is certainly not *-bonded and can either be di-a-bonded C2H4 (the result of electron-induced activation of the C - C a-bond and subsequent formation of Ag-C bonds) or be from the hydrogenation of vinyl (C2H3) fragments. The desorption must be reaction limited, probably hydrogenation of di-u-bonded C2H4,since molecular C2H6is not retained on Ag( 111) at 100 K. The production of C4H6at 265 and 320 K is attributed to recombination of C2H3 fragments and that at 195 K to desorption C4H6formed during electron irradiation at 100 K (see evidence below). Turning to butene, we ascribe it to thermally activated fragment recombination reactions since directly dosed submonolayer 1-butene desorbs at 188 K (dashed curve of Figure 3), i.e., -20 K lower than any C4Hs desorbed after electron irradiation. As suggested by the structure of the A 4 curve (Figure 2), the distribution of TPD products is a strong function of electron exposure, particularly in the early stages. Figure 4 shows how the TPD peak areas vary with EF. C4H6grows in most rapidly and its intensity maximizes where A4 changes become slow. For larger EF, the c& signal drops, but relatively slowly. The butene and dihydrogen areas rise steadily. Acetylene desorption sets in near where the shoulder appears in the A4 curve. Its delayed onset suggests that the intermediate leading to it is, perhaps, not derived directly from a-bonded ethylene. Ethane desorption, not shown in Figure 4, is very weak, is detectable only for EF 22.25 X 10l6
Negllglble
101
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1
2
3
4
5
6
7
8
9
Electron fluence e') Figure 4. TPD peak areas of C4H6 (triangles), C4H8 (diamonds), H2 (squares), and C2H2(circles) versus electron fluence.
e-, and increases slowly with EF. While we do not show the EF and coverage-dependent details here, some products desorb in multiple peaks reflecting multiple reaction pathways which we do not undertake to identify here. For example, 1,3-butadiene desorbs in a single peak (285 K) for low EF, two peaks (265and 320 K) appear for EF > 1.2 X 10l6 e-, and a third peak (-195 K) appears for EF > 4.5 X 10l6e-. Butene becomes detectable for EF 1 3 X loL5electrons, and two peaks (183 and 220 K) are resolved. These merge into a single broad peak (-210 K) for EF 2 4.5 X 10l6e-. Significantly,there is never any dihydrogen desorption above 300 K and, after TPD, we find no AES evidence for C. Because there is interference between C and Ag AES signals, the latter indicates only that C accumulation is not more than 5% of a monolayer. The former is very strong statement that hydrocarbon fragments containing
Zhou and White 'C2H4/CVAg(lll)
+ 4 . 5 ~ 1 0 ~e.6
900 -
C ~ H q l D I A g ( l l 1 )+ 1 . 2 ~ 1 e' 0~~
110 K I
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H2
0
I . _ . . I . . . . I . . . . I . . . . ~ . . . . I . . . ,
150
200
250
300
150
350
200
Figure 5. TPD spectra (solid curves) of HI. C2H4,C1H6, and C4H8taken after exposing a C2H4-saturated, CI-covered Ag( 111) surface at 100 K to 4.5 X 10l6 electrons (50 eV). The C1 coverage is -0.3 ML (Cl(181)/Ag(256) = 0.28).21 The TPD spectra from CI-free Ag(ll1) (dotted curves) are shown for comparison. The dashed curve for C4H6, in the uppermost group of curves, is from C2H,I/Ag( 11 1)
H lose no C-H bonds above 300 K. We conclude that the C, if any, left after TPD must be formed during the electron irradiation. 2. (X"lAg(ll1). We now turn to ethylene decompition on C1-covered Ag( 111). Before electron irradiation, the surface C1(181)/Ag(256) AES ratio was 0.28 (-0.3 ML24). This dec r d to 0.25 after irradiation with 4.5 X loL6e-, probably due to slow electron stimulated desorption of surface C1. TPD after EF = 4.5 X loi6e- (Figure 5 ) shows C4Ha,C4Hs,H2,and C2H4. The profiles of these peaks differ significantly from those obtained in the absence of C1 (dotted curves). Compared to the C1-free case, C4H6desorbs in a narrower temperature range and with a peak temperature of 270 K, close to the low-temperature peak on the C1-free surface (-265 K);the yield is nearly 3 times that from the C1-free surface. For comparison, we also show C4H6 formed from C2H3 fragments derived from thermal dissociation of C2H31on Ag( 111) (dashed curve). Its desorption in a single relatively sharp peak at -253 K suggests that the C4H6observed here has a similar origin, Le., thermally activated recombination of C2H3 fragments. For C a s , the yield is lower and the desorption peak temperature (-220 K)slightly higher on Cl/Ag( 111) than on Ag( 11 1). Undecomposed C2H4 desorbs at 135 and 170 K; these peaks are assigned to C2H4 unperturbed and stabilized by C1, respectively. Again, there are no CI or C3products but, unlike the C1-free case, there is no C2H2or C2H6 desorbed here. Compared to the C1-free surface, the H2 demrption area is 40% lower and the peak temperature is 10 K lower. 3. DCovered Ag( 111). On D-covered Ag( 111). 40% of sat~ r a t i o nthe , ~ ~post-EID TPD results are quite different (Figures 6 and 7). When no C2H4 is dosed (dotted C U I V ~ ) ,D2is dominant and there is some HD but negligible Hz in TPD. All the isotopes desorb at 180 K. There is no thermal reaction between cadsorbed D and CzH4; the TPD spectra for both are the same as for each alone on Ag( 111) and there are no other products, not even isotope exchange. Electron irradiation, e.g., 1.2 X l o r 6e- for solid curves
-
-
250
350
300
Temperature (K)
Temperature (9)
Figure 6. TPD spectra of D2, HD, and H2 taken immediately after atomic D exposure (dotted curves) and after exposing CzH4-saturated D-covered A g ( l l 1 ) to 1.2 X 1OI6 electrons (50 eV) at 100 K (solid curves). The D coverage is -40% of ~ a t u r a t i o n . ' ~
C2HqlD/Ag(lll) +1.211016 e'
132 K
I
500
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.-,400 cY
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.-9 3 0 0 *
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.
.
.
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.
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.
.
.
J
.
.
.
.
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Temperature (K) Figure 7. TPD spectra of m / e = 27 and 29-33 taken after exposing C,H,-saturated D-covered Ag(ll1) to 1.2 X I O t 6 electrons (50 eV) at 100 K. The D coverage is -40% of ~aturation.'~
in Figure 6, leads to peak shifts (200 K),lower D2 peak area (-65%), less HD, and somewhat more H2.
The Journal of Physical Chemistry, Vol. 96, No.19, 1992 7707
Chemistry of Ethylene
SCHEME II: Reaction Scheme and TPD Products for 1 ML of CzH4on Clean and 0.4 ML of D-Covered Ag(ll1) at 100 K, before and after Exposure to Low Dose (7.5 X lOI4 e-) 50-eV Electrons e dose 500
*IO0 . ,400 c) cn
T e m p e r a t u r e Programmed Desorption
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. ....,.: . . .. .........
. I
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220 K I
C4H6 from CqH(j/Ag(lll) ( ~ 0 . 3 ) ....................................................................
.........
g
i
13i
W
D i hydrogen
3300
. I
Butadiene
............................................................................................................................
E
Y
Zoadsorbed D i
. I
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......... J?............:. ............................................................................................... I
9
Difference between C2H4 spectra
104
di-Ethylene
p 0--c
H 2 (XI)
Temperature (K) Figure 8. TPD spectra (solid curves) of Hz, C2H4 and C4H6(1,3-butadiene) taken after exposing 1 ML of CzH4 (defined as saturation coverage) on Ag( 1 1 1) to 7.5 X lOI4 electrons. The temperature ramping rate was 2.5 K/s, and the dosing temperature was 100 K. The CzH4 curve is the difference between the C2H4spectra taken before and after electron irradiation. The dotted curve is the TPD spectrum after dosing molecular 1,3-butadiene.
any thermal reactions; only D2and C2H,were found in TPD. The same coverages dosed with 7.5 X 1014e- gave products, dominated by C2H3D,peaking at 200 K and accompanied by small amounts of fully hydrogenated ethylene. Residual ethylene desorbed at 128 K, but C1, C3, and C4 species were not detected. As indicated above, it is important to keep the fluence low if the only C-containing product is vinyl; TPD after doses up to 6 X 1015 electrons is still dominated by 1,3-butadiene, i.e., vinyl recombination, but small amounts of butene and acetylene desorb too.
Only C2hydrocarbon products are detected (Figure 7), in strong contrast to what we find in the absence of preadsorbed D, where C4species dominate. Reflecting the incorporationof D, C2signals are detected for m/e up to 32 (C2H4D2)but no higher. We do not attribute m/e = 32 to C2D4 because the other fragments, particularly their temperature profiles, are all consistent with ethane desorption. Furthermore, formation of C2D4 would require a relatively high probability for complete hydrogen isotope exchange. While unlikely in any case, it is particularly unlikely on Ag(ll1) where the activation of C-H bonds is difficult and generally not observed at low temperatures. Thus, part of the intensity at 200 K in Figure 6 can be assigned to ethanes, i.e., C2H4D2,C2H5D,and C2H6. However, since the m/e = 29 intensity is much higher (Le., 2X) than m/e = 30, C2H3D (ethylene-dl) must make a major contribution. Consistent with the fact that no butadiene is detected, we conclude that hydrogenation of C2H3to C2H3D,m r s at a much lower temperature than does the recombination of C2H3. Thus, ethylene-dl becomes dominant in the presence of high surface D atom concentrations and butadiene formation is completely suppressed. 4. Production of Vinyl. We now focus on the products formed after a low dose of electrons, e.g., 7.5 X 1014e- (Figure 8). The only molecules desorbing in TPD were residual parent (128 K), H2 (155 K), and 1,3-butadiene (C4H6,285 K); careful searches for methane, ethane, C3's and other C i s revealed no detectable intensities ( 4 % of the detected dihydrogen and 1,3-butadiene). The C2H4 trace in Figure 8 is a difference spectrum, before and after electron irradiation and thus measures the loss of C2H4. After TPD to 400 K, AES and A@ measurements indicated a clean surface. When 1,3-butadiene itself was dosed and followed by TPD, all multilayer and monolayer desorption occurred below 250 K (dotted curve in Figure 7). When, as described above, the surface was dosed to 50% of saturation with D and then saturated with ethylene but not dosed with e-, there was no evidence for
Discussion Scheme I1 s u m m a k and interprets, in four horizontal panels, the observations made when the electron dose is omitted or is small. As discussed above, adsorbed ethylene is ?r-bonded on Ag( 111) and desorbs molecularly at 128 K (topmost panel). The thermal inactivity of Ag( 111) is indicated by the absence of both hydrogenation and isotope exchange when ethylene and atomic deuterium are coadsorbed (third panel). Incident 50-eV electrons readily activate adsorbed ethylene to form vinyl (second and fourth panels)-this is the key observation. The electron-driven C-H bond breaking most likely occurs via an ionization mechanism, i.e., the incident electrons ionize surface species. Related to this point, we tried, with no success, to drive this chemistry with low-energy electrons (
