Chemistry of Cyclopropane on Pt (111): Thermal, Electron, and Photon

J. Phys. Chem. B 1999, 103, 6791-6802

6791

Chemistry of Cyclopropane on Pt(111): Thermal, Electron, and Photon Activation T. B. Scoggins, H. Ihm, Y. M. Sun, and J. M. White* Center for Materials Chemistry, Department of Chemistry and Biochemistry, UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: April 26, 1999; In Final Form: June 15, 1999

The thermal, electron, and photon-induced chemistry of cyclopropane, c-C3H6, adsorbed on Pt(111) at 100 K has been studied. The thermal chemistry is simple. Adsorption is saturable, no multilayer accumulates, and thermal desorption exhibits only c-C3H6 with one peak at 144 K. High-resolution electron energy loss spectra results indicate that c-C3H6 adsorbs with the plane of the ring tilted away from the surface normal. The electron-induced chemistry is more complex. Irradiation with 50 eV electrons activates dissociation of adsorbed c-C3H6 at 100 K, and there are thermally activated reactions during subsequent temperature-programmed desorption. The total cross-section for destruction of c-C3H6 is 8.2 ((0.2) × 10-17 cm2. For low electron fluences, 1 atm), the dominant reactions are isomerization to propylene and hydrogenolysis (hydrocracking) to methane and ethane or ethylene. Both π-allyl and n-propyl species have been proposed as being responsible for propylene formation.26,34,35 UHV surface science studies of c-C3H6 on Ru,21 Ni,17 and Cu,20,36 but not Pt(111), have been reported. Results on Cu(111) and Cu(110)20,37 are related to the work reported here. On low-temperature (e100 K) Cu(110) and Cu(111), c-C3H6 adsorbs molecularly and there are no thermally activated reactions in subsequent thermal desorption. Low-energy (e15 eV) electron activation is effective and produces both C-H and C-C bond breaking to form adsorbed cyclopropyl and metallacyclobutane as evidenced in HREELS.

10.1021/jp991369h CCC: $18.00 © 1999 American Chemical Society Published on Web 07/20/1999

6792 J. Phys. Chem. B, Vol. 103, No. 32, 1999 In this paper, we report on the thermal, 193 nm photon, and 50 eV electron-induced chemistry of c-C3H6 on Pt(111) studied by temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and high-resolution electron energy loss spectroscopy (HREELS). With the Pt(111) substrate at 100 K, c-C3H6 is adsorbed molecularly and TPD leads to no new products. Photon and electron activations of adsorbed c-C3H6 are effective and lead to TPD products, some of which are analogous to those found when other C3 adsorbates are thermally activated. However, unlike the other C3 adsorbates, electronactivated c-C3H6 on Pt(111) forms intermediates that lead to some hydrogenolysis; in TPD after electron irradiation, methane and ethylene desorb in coincidence. While electron-induced dissociation of hydrocarbons on single-crystal metal surfaces is often dominated by C-H bond cleavage,38-41 electroninduced C-C bond breaking makes a major contribution for c-C3H6, perhaps because of the strain in the ring. The reaction paths followed during 193 nm photon irradiation remain unclear. 2. Experimental Section The experiments were carried out in standard ultrahigh vacuum instrumentation equipped with low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and high-resolution electron energy loss spectroscopy (HREELS) capabilities. The Pt(111) crystal was cleaned by one or both of two procedures: (1) sputtering with Ar+ ions for at least 5 min, and (2) oxidizing in 5 × 10-8 Torr of O2 at 800 K to remove surface carbon. Subsequent flashing to >1100 K removed residual argon and oxygen. LEED confirmed surface order, and XPS verified surface cleanliness. The temperature was monitored by a chromel-alumel thermocouple spot-welded to the back of the crystal. Over the course of the several months of experiments involved, the base Pt(111) temperature varied between 93 and 105 K. The TPD temperature ramp rates were typically 3 K s-1. HREELS measurements employed a primary beam of 2 or 3 eV and, typically, a resolution of 65 cm-1 full width at half-maximum (fwhm). C 1s X-ray photoelectron spectra were taken with Mg KR radiation and a hemispherical analyzer set at a pass energy of 80 eV and a step size of 0.05 eV. These spectra were fit using a Levenburg-Marquardt routine and a Shirley baseline. Generally, components were fit using fixed line width components, e.g., 1.8 eV fwhm for C 1s, and all binding energies are referenced to a Pt 4f7/2 binding energy of 70.9 eV.42 Absolute surface coverages (species per surface Pt or species cm-2) were determined from C 1s peak areas and a calibration using a reference Pt(111) surface saturated with CO at 300 K, for which the absolute coverage is 0.49 ( 0.02,43 i.e., one CO molecule per two surface platinum atoms. Cyclopropane (Aldrich) and cyclopropane-d6, 98.9 atom % D (CDN Isotopes), were used as received. Cyclopropane was dosed through a 10 ((2) µm pinhole connected to a directed tubular doser (3 mm ID). Assuming the pinhole diameter and assuming that every molecule entering the chamber collides with the substrate, an upper limit estimate for the dose rate (molecules s-1) was made by measuring the pressure behind the pinhole and using standard gas kinetic equations. For D2 coadsorption experiments, CP grade D2 (Linde) was passed through a liquid nitrogen trap and dosed by backfilling. Electron irradiation utilized either the mass spectrometer filament or a shrouded, thoriated tungsten filament. Both were biased at -50 eV with respect to the Pt(111). Electron currents were measured from sample-to-ground during irradiation. Since this method does not account for backscattered primary electrons

Scoggins et al.

Figure 1. C 1s XPS region of (a) clean Pt(111) and (b) monolayer c-C3H6.

and secondary electrons ejected into the vacuum, the reported cross sections are upper limits. Spatially uniform photon irradiation utilized ArF excimer laser pulses (30 Hz) of 193 nm photons that were incident along the surface normal and apertured so that a 2 mJ pulse-1 was incident on the substrate. 3. Results 3.1. Thermal Chemistry. The thermal chemistry of c-C3H6 dosed at 95 K is simple. Only c-C3H6 desorbs, indicating there is negligible thermal dissociation, and multilayers do not accumulate. C 1s XPS (Figure 1) of a surface saturated with c-C3H6 (defined as 1 monolayer, ML) is characterized by a single symmetric peak at 282.9 eV and fwhm of 1.8 eV. Only one chemical state is evident, consistent with weakly bound c-C3H6 in which the three carbons are indistinguishable. Using the C 1s calibration described in section 2, based on saturation CO (0.49 CO/Pt), the C 1s area in Figure 1 corresponds to 0.38 c-C3H6 per surface Pt atom or 5.7 × 1014 c-C3H6 cm-2. This is close to a rough estimate of 4.7 × 1014 cm-2 c-C3H6 calculated assuming adsorbed c-C3H6 occupies a cube of volume equal to that based on the density. After dosing c-C3H6 at 95 K, thermal desorption spectra (Figure 2) are characterized by a single parent peak at 144 K which saturates for a dose, estimated as described in section 2, of 1.15 × 1015 molecules cm-2 (inset). This saturation area is taken as the definition of 1 ML (monolayer). As on Ru(001),15,21 there is no evidence for multilayers or dissociation products. In particular, the 2 amu TPD trace indicated no H2 desorption (not shown). Since multilayers do not form at 95 K, the second peak at 112 K is attributed to desorption from heating leads. Finally, the TPD spectra of c-C3H6 and c-C3D6 are indistinguishable. The thermal desorption activation energy lies near 45 kJ mol-1 (0.47 eV ) 3800 cm-1) based on simple firstorder desorption kinetics. This energy is small compared with the C-C and C-H bond energies in c-C3H6 and slightly larger than the highest energy vibrational transitions (C-H stretching) in c-C3H6 (see Table 1). The linear relation between peak area and exposure (inset) indicates a constant sticking coefficient for all coverages within the first monolayer. Comparing the absolute coverage of 5.7 × 1014 c-C3H6 cm-2 reached at saturation with the estimated upper limit of 1.1 × 1015 c-C3H6 cm-2 dosed, the sticking coefficient is g0.5. HREELS after Dosing and Annealing. Figure 3 displays HREELS for 1 ML c-C3H6 adsorbed at 100 K (curve a), and annealed to 800 K and recooled to 100 K (curve b). The asdosed 1 ML spectrum (a) exhibits six modes (identified by vertical lines) that are assigned and compared to gas-phase and adsorbed phase data in Table 1. Confirming molecular adsorp-

Chemistry of Cyclopropane on Pt(111)

J. Phys. Chem. B, Vol. 103, No. 32, 1999 6793

TABLE 1: Vibrational Frequencies and Assignments for c-C3H6 Pt(111) this work HREELS

gas phase

Ru(001)16 HREELS

840 (-26)a 1015 (-14) 1180 (-8) 1430 (-8) 2955-3030 (-72)

866 1029 1188 1438 3025-3103

570 880 1010 1200 1480 2980-3080

a

Cu(111)20 HREELS 839 1032 1177 1451 3064

Ni(111)17 RAIRSb

Cu(100)36 RAIRS

assignment21

1024 1181 1421 3014, 3020, 3072, 3093

855 1020 1180 1424 2986, 3005, 3069

M-CCC ring deformation (symmetric) ring deformation (asymmetric) ring breathing CH2 scissor C-H stretch

Difference: (Pt energy - gas-phase energy). b RAIRS: Reflection-absorption infrared spectroscopy.

Figure 3. HREELS of 1 ML c-C3H6 adsorbed on Pt(111) at 100 K, before (a) and after (b) heating to 800 K and recooling. Electron beam energy was 3.0 eV. Dashed lines estimate decay of elastic peak.

Figure 2. TPD spectra (42 amu traces) of c-C3H6 adsorbed on Pt(111) at 100 K for the following exposures (× 1015) molecules/cm2: (a) 0.5, (b) 0.75, (c) 1.0, (d) 1.15, (e) 1.5. The heating rate was 3 K s-1. The inset shows the integrated TPD area versus exposure. Saturation, indicated by the arrow, defines monolayer (ML) coverage.

tion on Pt(111), there are distinctive and relatively narrow ring deformation and ring breathing modes at 840, 1015, and 1180 cm-1. Methylene scissoring occurs at 1430 cm-1, and there are losses from symmetric and asymmetric C-H stretching at 2955 and 3030 cm-1. Compared to the gas phase, c-C3H6 on all the metals exhibits small downward mode-dependent shifts of the transition energies.17,19,21 On Pt(111), the shifts (parentheses in Table 1) range from a negligible 8 cm-1 for ring breathing to a more substantial 70 cm-1 for the C-H stretches. Thus, while the coupling of c-C3H6 to Pt(111) is relatively weak, it is interesting that the C-H stretching modes weaken much more than any of the others. The band intensity pattern of Figure 3 matches observations on Ru(001),21 but not on Cu(111),17,19 where the C-H stretching mode is relatively much stronger and the ring breathing mode is much weaker. Thus, while TPD confirms that c-C3H6 is adsorbed weakly on the four metal substrates listed in Table 1, differences in vibrational characteristics point to detectable differences in adsorbate-substrate electronic coupling and/or orientation. After heating to 800 K and recooling (curve 3b), the HREEL spectrum is reduced to negligible intensity throughout. There may be some intensity in the 800-1200 cm-1 region attributable to tiny amounts of readsorbed c-C3H6. Consistent with the

Figure 4. HREELS intensities elastic peak and six vibrational modes of 1 ML cyclopropane on Pt(111) as a function of off-specular angle. Electron beam energy was 3.0 eV.

absence of thermal dissociation of adsorbed c-C3H6, XPS after such an experiment indicates negligible amounts of carbon (not shown). Off-Specular HREELS. Integrated HREELS intensities as a function of the detection angle are shown in Figure 4. The elastic peak decays by a factor of 25 in passing from on-specular to 8° off-specular. The modes at 840 and 1015 cm-1 decrease roughly by a factor of 10, while the remaining modes decrease less than a factor of 5. We conclude that all six modes involve a mixture of impact and dipole scattering and that the modes at

6794 J. Phys. Chem. B, Vol. 103, No. 32, 1999

Figure 5. TPD after irradiation of 1 ML c-C3D6 with 1.6 × 1016 e cm-2 of 50 eV electrons. Products are D2 (4 amu), methane (20 amu), ethylene (32 amu at 260 K), propylene (48 amu at 200, 250, and 310 K), and cyclopropane (48, 32 amu at 126 K). The heating rate was 3 K/s.

840 and 1015 cm-1 involve larger contributions from dipole scattering. As generally found, the C-H stretching regions involve significant impact scattering. As noted in the discussion section, the results are consistent with the c-C3H6 plane tilted away from the surface plane. We conclude from these TPD, HREELS, and XPS observations that there is negligible thermally activated chemistry of adsorbed c-C3H6 and that the coupling of c-C3H6 to Pt is best described in terms of polarization forces, i.e., physisorption, that perturb the vibrational modes of c-C3H6 to varying degrees. 3.2. Electron-Induced Chemistry. Irradiation of saturated monolayer coverages of c-C3D6 or c-C3H6, Figures 5 and 6, with 50 eV electrons clearly activates surface chemistry at 100 K. After irradiation with ca. 1016 e- cm-2, dihydrogen, propylene, methane, and ethylene appear in TPD. The c-C3D6 TPD results are somewhat easier to interpret because carbon monoxide (28 amu) and dihydrogen (2 amu) in the background do not interfere with C2D4 (32 amu) and D2 (4 amu). For electron-irradiated c-C3D6, Figure 5, there are large, easily identified D2 peaks at 315 and 420 K. The signal remains relatively strong above 430 K, and full dehydrogenation is indicated by the loss of D2 intensity near 750 K. The D2 desorption exhibits a leading edge at 270 K ascribed to recombination of D(a). Thus, there is some C-D bond breaking below 270 K, but it is not extensive. We suppose that isomerization reactions and hydrogen transfer reactions, mediated by Pt, occur to keep most of the D bound to C. Overall, the D2 TPD profile resembles that found during thermal activation of propylene.4 Thermally activated propylene forms di-σ-bonded propylene that converts to propylidyne. In turn, propylidyne dehydrogenates, a process signaled by characteristic multipeaked H2 TPD. Evidently, propylidyne makes a major contribution to the thermal chemistry of electron-irradiated c-C3H6.

Scoggins et al.

Figure 6. Product distribution observed following electron irradiation of 1 ML c-C3H6 with 3.3 × 1016 e cm-2. Products are H2 (2 amu), methane (16 amu), ethylene (27 amu, 260 K), propylene (42, 41, and 27 amu at 185, 208, and 310 K), cyclopropane (42, 41, and 27 amu at 126 K). The heating rate was 3 K/s.

Regarding isotope effects, the first major D2 peak is not shifted with respect to its H2 counterpart (Figure 6) but the most intense D2 peak, 420 K, is shifted downward by 10 K. From these results, we conclude that C-H(D) bond coordinate changes are significant in forming the transition state leading to the 420 K, but not the 315 K, desorption of D2. In addition to these peaks, the broad D2 desorption at higher temperatures (Figure 5) has local maxima at 480, 590, and 675 K. Analogous peaks are not well-resolved in the H2 spectra of Figure 6 but both dihydrogen spectra are attributable to propylidyne dehydrogenation.4 As already mentioned, the thermal chemistry of electronirradiated c-C3H6 differs from the thermal chemistry of other C3 adsorbates on Pt(111)1-4 hydrogenolysis products are observed in TPD after electron irradiation of c-C3D6. There is a large 32 amu peak at 260 K ascribed to C2D4 and, in coincidence, a 20 amu peak ascribed to CD4. Fragmentation pattern analysis rules out other hydrocarbons, i.e., propane, propylene, and ethane. Both the C2D4 and CD4 signals are limited by formation reactions, not by adsorbate desorption kinetics.44 Since the C2D4 and CD4 TPD peaks are nearly symmetric and track each other, and since the temperature region over which they desorb is relatively narrow (fwhm ≈ 30 K), a single C3 precursor species, derived from c-C3D6, is proposed. Commonly discussed intermediates derived from other C3 adsorbates, e.g., η3-allyl, η1-allyl, n-propyl, and propylidyne, lead to no C1 or C2 species in TPD. Thus, another candidate is proposed, namely, metallacyclobutane (see discussion below). The absence of a 42 amu peak at 260 K in Figure 6 rules out propylene and cyclopropane desorption, and the absence of a 30 amu peak (not shown) rules out C2H6. Methane and ethylene (based on 27 amu) peaks appear at 260 K, the same peak temperature as for electron-irradiated c-C3D6. The absence of an isotope effect on the desorption peak temperature indicates that C-H(D) bond alterations are not energetically important in forming the transition state that controls the evolution of these two products. We propose that the rate is determined by C-C bond breaking within metallacyclobutane to form adsorbed methylene and π-bonded ethylene.



Figure 8. The distribution of products as a function of initial coverage of c-C3D6 for a fixed fluence (1.6 × 10+16 cm-2) of 50 eV electrons. Figure 7. Integrated TPD area for indicated products versus electron fluence. the initial coverage was 1 ML c-C3H6. Curve for CH4+ is offset for clearer viewing. The dashed lines define the zero baselines. Inset: Semilog plot from which cross section for loss of parent is determined.

For deuterated cyclopropane, the 48 amu signal profiles (Figure 5) contain contributions from two isomers of C3D6s cyclopropane and propylene. After irradiation, the residual cyclopropane TPD peak appears at 126 K, just as for c-C3H6 in Figure 6, compared to 144 K before irradiation. In the presence of dissociation products, cyclopropane c-C3D6 is destabilized by interactions with strongly bound hydrocarbon fragments produced during irradiation. The 48 amu peaks at 200, 250, and 310 K are all attributable, based on fragmentation pattern analysis, to fully deuterated propylene. All the propylene desorption is reaction limited since physisorbed propylene desorbs at lower temperatures (∼125 K).45,13 Multiple propylene desorption peaks suggest multiple intermediates, and comparison of Figures 5 and 6 provides important guidance in terms of isotope effects. For c-C3H6 irradiation, the residual TPD peak is at 126 K, unshifted compared to c-C3D6. However, the 200 and 250 K propylened6 peaks shift downward for propylene-H6 (185 and 210 K), indicating the participation of C-H(D) bond rearrangement in the transition states associated with these propylene desorptions. The higher temperature peak is attributed to hydrogenation of η3-allyl, and the lower temperature peak to β-hydride elimination of n-propyl species (see discussion). Interestingly, the 310 K propylene peak shows no detectable isotope shift, indicating that C-H(D) rearrangement is not energetically crucial at the transition state leading to this propylene desorption. This peak is attributed to the reductive elimination of η1-allyl (propenyl), i.e., C-H bond formation is involved but the C-H coordinates are apparently not critical. Starting with 1 ML c-C3H6, Figure 7 plots, as a function of electron fluence, the decay of parent and growth of product TPD peak areas (the CH4+ curve monitoring methane desorption is shifted down for easier viewing). The parent decay is replotted (inset) in semilog form from which the cross section for loss of

c-C3H6 is (8.2 ( 0.2) × 10-17 cm2. While this is an upper limit, it indicates that c-C3H6 is quite vulnerable to electron-induced decomposition. In a similar electron irradiation of monolayer CH4 on Pt(111), the analogous cross section is of the same order, ca. 10-16 cm2.44 From an inspection of Figure 7, the following points are made regarding products. There is a sharp onset in the appearance of H2 but delays in the appearance of methane, ethylene, and propylene (about 5 × 1015 e- cm-2). We suppose that the Pt sites occupied by the weakly held c-C3H6 are readily available for the more strongly bound products that form during electron irradiation, e.g., metallacycles. The resulting, even more weakly held, c-C3H6 then desorbs at 126 K. At low fluences where few products form, thermal activation during TPD after electron irradiation leads to intermediates which, as for TPD of directly dosed propylene, dehydrogenate without releasing any hydrocarbons to the gas phase. For longer irradiations (>5 × 1015 e- cm-2), site occupancy begins to limit the formation of these intermediates. As a result, other thermally activated paths involving hydrocarbon desorption become competitive. Thus, most of the hydrogen added incrementally (in the form of c-C3H6) is carried off by these hydrocarbons, and the H2 peak area increases much more slowly than at the outset. Over the range covered by Figure 7, the methane, ethylene, and propylene yields all increase steadily above the onset. Furthermore, the yield ratios among these three products are constant. The H2 yield, on the other hand, becomes constant for fluences above 1.5 × 1016 e- cm-2. Parenthetically, the hydrocarbon yields also reach limiting values for electron fluences exceeding 6 × 1016 e- cm-2 (not shown). Figure 8 shows how, at a fixed fluence of 1.6 × 1016 ecm-2, the TPD peak areas vary with the initial c-C3D6 coverage in ML units. D2 is by far the dominant product, and its area is scaled down by a factor of 10 in Figure 8. It is the only product below 0.25 ML, and its dominance becomes steadily less as the initial coverage of c-C3D6 increases above 0.25 ML, where C2D4, CD4, and C3D6 begin to appear. When the initial c-C3D6 coverage exceeds 0.35 ML, the D2 yield becomes constant while


Figure 9. The left panel shows the C 1s X-ray photoelectron spectra of (a) 1 ML c-C3H6 on Pt(111), irradiated with (b) 0.26, (c) 0.66, (d) 2.6, (e) 5.3 × 1016 e- cm-2. The right panel shows the spectrum in (e) annealed to (f) 150, (g) 200, (h) 350, and (i) 600 K (and recooled for XPS). Numbers on each curve are total peak areas with high BE component (labeled CO) removed and normalized to (a).

the C2D4, CD4, and C3D6 yields each increase approximately linearly. The D2 thermal profiles (not shown), i.e., two major peaks and a broad high-temperature desorption, are all shaped like those of Figure 5. These observations support the model described in conjunction with the discussion of Figure 7. Below a critical initial coverage (0.25 of saturation), all the products of electronirradiated c-C3D6 find sites that lead to complete dehydrogenation. Above this initial coverage, site competition allows other thermally activated paths and reaction intermediates to compete. Figure 9 shows the C 1s XPS region for (a) unirradiated monolayer c-C3H6, (b-e) increasing electron fluence, and (fi) annealing after (e) from 150 to 600 K. Each of the spectra represents a separate experiment, i.e., clean, dose, irradiate, anneal, and gather spectrum. Repeating Figure 2, spectrum a is the monolayer fit with one peak at 282.9 eV and fwhm of 1.8 eV. With electron irradiation, the C 1s signal broadens on the high binding energy (BE) side, spectra b and c, and arbitrarily fitting to peaks with 1.8 eV fwhm leads to the two-component decomposition shown (dashed curves). The number associated with each spectrum is the total C 1s area. For curve d, a clear high BE shoulder (>285 eV) is evident. This is ascribed to background CO adsorption based on the high BE, ca. 285.6 eV,42 and increased total C 1s area (with this component included, the total C 1s area in (e) is about 1.2 of that in (a)). Its continued contribution with annealing, even well above the CO desorption temperature, is attributed to variable background adsorption during the lengthy XPS data gathering time (g1 h) and to increasing CO background through a day of experiments. For lengthy experiments where Pt sites become available, the accumulation of CO is confirmed in HREELS data presented below. Omitting the area assigned to CO, the total C 1s areas, normalized to spectrum a, are constant for curves a-e. We conclude that irradiating a monolayer of c-C3H6 with up to 5.3 × 1016 e- cm-2 of 50 eV electrons does not desorb detectable quantities of C-containing species. The broadening is consistent with alteration of the adsorbed c-C3H6 but diagnosis of the species formed is impossible.

Scoggins et al. Following the same fitting procedure, the right-hand column in Figure 9 reflects the effects of annealing after electron irradiation. Comparing curves e and f of Figure 9 shows that negligible amounts of carbon are removed by annealing to 150 K after irradiation. This is consistent with TPD; extrapolation of TPD data from Figure 7 predicts that irradiation with 5.3 × 1016 e- cm-2 would leave no more than 2% of the initial amount of c-C3H6. According to Figures 5 and 6, heating to 200 K desorbs propylene, and the XPS carbon signal (CO removed) drops from 1.0 to 0.9, curve g. Further decay to 0.75 occurs upon heating to 600 K which, according to TPD, desorbs all hydrocarbon products but leaves some C-H bonds. The C 1s decay is associated mainly with the lower BE (282.8 eV) portion of the signal. It decays by 60% between 100 and 600 K and is accompanied by a modest (∼15%) increase in the higher BE (283.9 eV) portion of the signal. Even when all the hydrocarbons have desorbed (T > 350 K), there remains significant intensity at 283.9 eV, underscoring the point that CHx with much of the C linked strongly to Pt makes contributions to this BE region. While the XPS data cannot provide further detail, the following key points are evident: (1) ejection of C-containing species during electron irradiation is minimal; (2) when electron irradiation is sufficient to chemically alter all of a saturated monolayer of c-C3H6, about 75% of the C associated with electron irradiation products is retained after heating to 600 K, implying that 25% desorbs as hydrocarbons during TPD; (3) electron irradiation, sufficient to alter all the adsorbed c-C3H6, does not bring about strong changes in the C 1s BE profile, only broadening toward high BE is noted. In this context, we note that the fitting to two C 1s peaks is arbitrary and does not demonstrate two distinct chemical environments for the carbon atoms. This general picture does not change as hydrogen is removed by heating after electron irradiation. We conclude that C 1s XPS, while providing powerful conclusions regarding retention of C-containing species, does not provide compelling evidence regarding the bonding changes that accompany electron irradiation and subsequent annealing. HREELS. HREELS taken before and after electron irradiation of 1 ML c-C3H6, Figure 10, reflects changes that accompany the conversion into products. Compared to Figure 3, the resolution was significantly better (65 vs 90 cm-1 fwhm for the elastic peak). After irradiation with 0.68 × 1016 e- cm-2 that, based on TPD, alters about two-thirds of the initial monolayer, the HREEL spectrum, curve b, differs measurably from molecularly adsorbed c-C3H6 (curve a). Compared to curve a, the losses after irradiation are characterized by (1) broadened peaks in the ring breathing and deformation mode regions (800-1200 cm-1), (2) a weakened ring deformation mode (840 cm-1), (3) a stronger methylene scissors mode (1420 cm-1), (4) stronger C-H stretching (2800-3000 cm-1) with an intensity distribution that is shifted to lower energy, and (5) the appearance of chemisorbed hydrocarbon-Pt modes at 420 cm-1. Thus, unlike XPS, HREELS provides compelling evidence for chemical alteration of adsorbed c-C3H6 when it is irradiated with 50 eV electrons. For a 4-fold greater dose of 50 eV electrons (2.7 × 1016 ecm-2), no more than 10% of the initial parent remains (TPD). The corresponding HREELS, Figure 10c, when compared to the lower dose, curve b, is characterized by (1) 2× overall intensity increase (scaling by 150× versus 300×), (2) further weakening of the 840 cm-1 ring deformation mode, reflecting alteration of nearly all the initial monolayer c-C3H6, (3) more intense (4.8×) and narrower (190 cm-1 fwhm) C-H stretching modes peaking at slightly lower loss energy (2910 cm-1), and


Figure 10. HREELS of 1 ML of c-C3H6 on Pt(111) (a) as dosed, and irradiated with (b) 0.68 and (c) 2.7 × 1016 e- cm-2. HREELS primary beam energy was 3.0 eV.

either overlapping or becoming part of the 2825 cm-1 band, and (4) a more intense (4.3×) CH2 scissors mode (1420 cm-1). The broadening of the peaks in the 800-1200 cm-1 region can arise from several sources including (1) loss of local order, (2) heterogeneous restructuring of the remaining c-C3H6, and (3) formation of products with vibrational bands in the same loss region, e.g., cyclopropyl, allyl groups, and metallacycle moieties. The latter have been reported on copper substrates.20 The downward energy shift associated with the C-H stretching region is consistent with alterations that increase the sp3 hybrid character of the bonding at the carbon atoms, e.g., increased C-Pt coupling and larger C-C-C bond angle. The alterations occurring before heating that are evident from HREELS indicate that, like c-C3H6, the products contain methylene, CH2, groups that exhibit enhanced vibrational excitation cross sections for scissoring and stretching. Since the products cannot increase the total number of methylene groups, the increased intensity requires one or both of the following plausible changes: (1) the transition dipoles associated with the scissoring and stretching modes of CH2 move toward the surface normal where image forces amplify them, compared to adsorbed c-C3H6; (2) the structural changes involve charge redistributions that enhance the transition dipole matrix elements. We propose that one important electron-activated process opens the ring to form metallacycles. This process preserves the number of methylene groups while altering the charge distribution and likely altering the orientation of the three carbons with respect to the plane of the substrate. As outlined in more detail below, we propose that, at 100 K, metallacyclobutane is stable, whereas metallacyclopentane is transient and rearranges to η3-allyl (H2C(a)-C(a)H-C(a)H2) by losing one H atom. While forming η3allyl reduces the number of methylene groups, this could be more than compensated for by changes in geometry and electronic coupling. C-H bond cleavage to form cyclopropyl groups may also occur, but we have no unambiguous evidence of their presence. Plausibly, if they are formed, prompt isomerization to η3-allyl could occur.


Figure 11. HREELS of 1 ML c-C3H6 irradiated with 3.5 × 1016 ecm-2, flashed to the indicated temperature, and recooled. HREELS primary beam energy was 2.0 eV.

Two sets of HREELS annealing experiments were done in an effort to characterize the species produced thermally. In one set, Figure 11, the focus was on identifying the species produced by electron irradiation. This was approached by annealing to remove the residual parent following an electron irradiation that reduces the c-C3H6 TPD signal to 5% of its initial value (3.5 × 1016 e- cm-2). After annealing to 130 K to remove residual parent, curve a of Figure 11 is comparable to curve c of Figure 10; the peak positions are the same within experimental uncertainty, and the intensity distribution differs only in the sense that the methylene scissoring and C-H stretching peaks are narrower and more intense in Figure 11. Heating to 175 K, just prior to the desorption of some propylene, and to 185 K, into the middle of the first propylene desorption region, provides spectra b and c of Figure 11. Curves a and b differ only slightly as follows: (1) absolute intensities are stronger in (b); (2) loss peaks are generally slightly narrower in (b); (3) there is, in (b), a slight increase of intensity on the high-energy side (>3000 cm-1) of the C-H stretching region. To account for the unshifted but more intense loss peaks, we propose that the products produced by electron irradiation remain at 175 K but gain some order during annealing between 130 and 175 K. Heating to 185 K desorbs some propylene, but there are only modest changes in the HREEL spectrum c of Figure 11, compared to (b), as follows: (1) reduced intensity of all bands; (2) increased relative intensity in the 925 cm-1 region; (3) a shift of the C-H intensity distribution to slightly lower energies forming a more symmetrical peak. As indicated in Figure 12, the HREEL spectra for electronirradiated c-C3H6 annealed to 185 K, curve a, is similar to that for η3-allyl formed by the thermal decomposition of allyl bromide, curve b.1 In each of the spectra, there are seven identifiable peaks with loss energies that are comparable (vertical dashed lines). While the individual peak widths and the intensity distribution in the 700-1600 cm-1 region are equivalent, there


Figure 12. HREELS comparing: (a) 1 ML c-C3H6 irradiated with 3.5 × 1016 e- cm-2 and flashed to 185 K and (b) η3-allyl formed thermally by dosing submonolayer allyl bromide at 100 K and heating to 250 K.

Figure 13. HREELS of 1 ML c-C3H6 irradiated with 6.3 × 1016 ecm-2, flashed to the indicated temperature, and recooled. HREELS primary beam energy was 3.0 eV.

is more relative intensity in the C-H (2950 cm-1) and Pt-C (400 cm-1) stretching for curve b. We conclude that η3-allyl is one significant product of the electron irradiation of c-C3H6. However, since no methane or ethylene desorbs in the allyl bromide experiments, another species must also be present, but not dominant, in the electron-irradiated c-C3H6, and metallacyclobutane is proposed. In another set of HREELS experiments, Figure 13, the emphasis was on changes brought on by annealing to temperatures where the hydrocarbons desorb, removing, as noted above, 25% of the initial carbon. The electron dose was 6.3 × 1016 e- cm-2, which converts nearly all the initial monolayer cyclopropane coverage into adsorbed products. Before annealing, the spectrum matched closely that of Figure 11c, except that the line widths were somewhat broader, probably indicating more disorder in the absence of any annealing. Heating to 220 K (curve a of Figure 13) promotes some ordering and desorbs

Scoggins et al.

Figure 14. TPD of 1 ML c-C3H6 dosed on D-covered Pt (111) and irradiated with 50 eV electrons.

some propylene, but none of the ethylene or methane that is produced in the full TPD spectrum. Compared to Figure 11c, there are, in Figure 13a, (1) stronger HREELS peaks at 760 and 925 cm-1, (2) significantly enhanced relative intensity in the 1420 and 2950 cm-1 regions, and (3) intensity attributable to CO stretching (1850 cm-1). The latter is consistent with XPS observations and indicates that, as the result of propylene desorption, some bridging sites become available on the Pt surface and are filled by background CO. Heating to 290 K, curve b, desorbs all of the hydrocarbons leaving 75% of the initial (before irradiation) carbon. Compared to HREELS data taken at lower temperatures, there are now easily recognized differences. The C-H stretching region narrows and the intensity distribution shifts lower (2870 cm-1). The methylene scissors mode intensity at 1422 cm-1 decays strongly (2 ×), the intensity between 1000 and 1300 cm-1 becomes very weak, and the intensity increases at 925 cm-1 and at 400 cm-1. The background CO uptake is stronger and there is an emerging band at 2035 cm-1, assigned to CO bound to Pt in an atop configuration. Given the H2 TPD, which is indicative of propylidyne formation, the HREELS of Figure 13b is expected to contain significant contributions to its precursors, e.g., η3-allyl, and assuming hydrogen is available, it hydrogenates to di-σ-bonded propylene. While there are many similarities with the published spectra for propylene annealed to 300 K,45 there are also significant differences; in particular, there is no significant intensity at 760 cm-1 for annealed propylene. This is taken as reflecting, in Figure 13b, the presence of multiple CxHy species, including η3-allyl on the basis of Figure 12a. This underscores the complexity of this C3 system. Finally, upon heating to 330 K which, in TPD, is just above the first H2 TPD peak, the HREELS 1400 cm-1 region intensifies and broadens, reflecting emerging modes above and below 1420 cm-1, the 1170 cm-1 mode vanishes, the mode at 760 cm-1 decays, a band above 800 cm-1 becomes evident, and there is evidence for both bridging and atop background CO bonding to Pt sites. This spectrum is consistent with propylidyne and coadsorbed CO. Additional insight is gained by irradiating a mixture of c-C3H6(a) and D(a) prepared at 100 K by dosing D2, to form submonolayer adsorbed D, followed by saturation with c-C3H6. From post-irradiation TPD (Figure 14), the following significant observations are made. (1) The first propylene peak appears at 180 K, within 5 K of that in Figure 6, and, significantly, contains no more than one D. (2) The 208 and 310 K propylene peaks

Chemistry of Cyclopropane on Pt(111) of Figure 6 are absent. (3) There is a new propane peak at 240 K incorporating at least three D atoms. (4) Ethylene and methane desorb at 225 K, compared to 260 K without D(a). (5) No more than two D’s are evident in methane desorption. (6) Because of mass interferences, the ethylene isotopic composition could not be determined. The dominance of C3H6 in the first propylene peak indicates that the intermediate leading to its formation does not involve surface-bound hydrogen and that isotope exchange reactions are slow up to 180 K. By comparison with the surface chemistry of 1-iodopropane, the first propylene peak in Figures 5 and 6 is attributed to a β-hydride reaction of n-propyl, C3H7. If so, the formation of this fragment does not involve surface bound H or D. A plausible proposal is its formation during electron irradiation by a disproportionation reaction between electronactivated c-C3H6 and a neighboring c-C3H6 to form η3-allyl, C3H5, and n-propyl, C3H7. The absence of the other two propylene desorptions when D is present indicates either that the proposed η3- and, perhaps, η1-allyl precursors are not formed during electron irradiation or, more likely, that they are formed but thermally activated hydrogenation is competitive above 180 K. This view is consistent with hydrogenation, rather than isomerization, leading to the desorption of propane, peaking at 240 K and containing up to three D atoms. Assuming coadsorbed D becomes active at 180 K is also consistent with the formation of methane and ethylene peaking at 225 K instead of 260 K. Once D becomes activated, we propose that it irreversibly hydrogenates the bridging methylene group in metallacyclobutane to form a transient CH2-CH2D-CH2 intermediate which, in the presence of additional D, simultaneously forms CH2D2 and CH2CH2 both of which desorb. Since, in the absence of coadsorbed D, methane and ethylene desorptions move 35 K higher, we conclude that sufficient concentrations of thermally activated H atoms are not formed either during electron irradiation or, until the temperature reaches roughly 260 K, in subsequent TPD. This is taken as an indication that large amounts of adsorbed H are not formed during electron irradiation and, further, that the rate of C-H bond breaking in the adsorbed species formed by electron irradiation is slow below 260 K. 3.3. Photon Activation of c-c-C3H6. In a separate set of experiments, monolayer c-C3H6 was irradiated with 193 nm photons. Subsequent TPD as a function of total photon fluence indicated very little loss of parent molecules and, except for H2, undetectable product formation. In semilogarithmic form (not shown), the decay of the c-C3H6 TPD peak at 144 K gives a cross section of 1.4 ((0.9) × 10-20 cm2, nearly 4 orders of magnitude smaller than that for 50 eV electron irradiation. 4. Discussion 4.1. Thermal Properties of Adsorbed c-C3H6. Taken together, the TPD, XPS, and HREELS results for monolayer c-C3H6 are consistent with nondissociative adsorption into a weakly bound state in which the plane of the C3 ring is tilted with respect to the surface normal. The C 1s spectrum lacks any evidence for dissociation since there is only a single maximum with a fwhm of 1.8 eV. HREELS data, Figure 3a, confirms the weak molecular adsorption; there is overall agreement (Table 1) of the full set of measured vibrational frequencies with those found in the gas phase and other cases where c-C3H6 adsorbs molecularly. Finally, there is no H2 and only a single c-C3H6 TPD peak, the latter positioned at 144 K. On the basis of simple first-order TPD analysis, the desorption activation energy for removing c-C3H6 from Pt(111) is 45 ( 5 kJ mol-1, independent of coverage over the first monolayer.

J. Phys. Chem. B, Vol. 103, No. 32, 1999 6799 To assess the molecular orientation, we note that off-specular HREELS data (Figure 4) indicates dipole and impact contributions to all six modes.46 Both specular and off-specular HREELS results for c-C3H6 on Ru(001)21,16 have the same characteristics, and the authors conclude that the adsorption symmetry is Cs(σν). The resulting adsorption geometry has two C atoms in contact with the substrate (C-C bond parallel to the substrate) and the c-C3H6 ring tilted with respect to the surface normal. This geometry is consistent with our data and is taken as the structure for monolayer c-C3H6 adsorbed on Pt(111). The adsorption energy changes associated with coverage increases, and accompanying orientation changes, if any, are undetectable in TPD, Figure 1, implying that the effective adsorption energy per adsorbate species does not change measurably. In this situation, alignment effects are weak and, particularly at submonolayer coverage, many orientations of the C3 plane lie within a thermally accessible distribution of ca. (5 kJ mol-1. Near monolayer coverage, packing effects will narrow the range of configurations. It is of interest to compare the thermal behavior of adsorbed c-C3H6 with C3H6 (propylene) and C3H8 (propane). There are only strong C-C σ-bonds in propane, and it is the least reactive; propylene contains a C-C π bond which generally increases reactivity, and c-C3H6 possesses C-C bonds with intermediate π character and reactivity. On Pt(111), these trends are followed. Propane adsorbs and desorbs reversibly, the monolayer desorbing at 120 K.44 c-C3H6 adsorbs reversibly, but the monolayer desorbs at higher temperature (144 K) than propane. Propylene, on the other hand, chemisorbs in di-σ-bonded form,4 and unless the coverage is very high, does not desorb in TPD. Rather, it dehydrogenates, beginning near 275 K.4 Multilayer propylene desorbs at 125 K and, under crowded conditions where dissociation is inhibited, di-σ propylene desorbs at 255-270 K.45 4.2. Electron-Induced Chemistry of Adsorbed c-C3H6. The experimental results described here provide compelling evidence for electron-induced chemistry of monolayer c-C3H6. HREELS clearly detects changes brought on by 50 eV electron irradiation, especially suppression of modes associated with the C3 rings 840, 1015, and 1140 cm-1. On the basis of C 1s XPS, this electron-induced chemistry involves no ejection of C-containing species but is directly evidenced by the appearance of TPD products. The question of ejection of atomic hydrogen during electron irradiation remains unanswered. For discussion purposes, we focus on monolayer coverages, where each c-C3H6 is surrounded initially by other c-C3H6’s. Irradiation with 50 eV electrons, which activates this adsorbatemetal system, can do so by several means; we follow a simple adsorbate ionization model, which is intuitively reasonable and adequately accounts for the results. The gas-phase ionization potential of c-C3H6 is 10 eV and the measured gas-phase fragmentation pattern between 50 and 70 eV is, in the form (mass, relative intensity), c-C3H6+ (42 amu, 0.94), c-C3H5+ (41 amu, 1.00), and C2H3+ (27 amu, 0.48). In the presence of Pt(111), these patterns will change if, as expected, the time scale for fragmentation competes with that for reneutralization. Intuitively, since more nuclear rearrangement is involved, the time required to form C2H3+ is likely longer than that needed to form c-C3H5+, and both can be quenched by charge transfer from the substrate. Although not explicitly considered here, the scattering of 50 eV electrons in the near-surface region will produce secondaries that can, themselves, activate chemistry through attachment or ejection of electrons. Assuming impact ionization to form C3H6+ is the major activation step and that neutralization is competitive with

6800 J. Phys. Chem. B, Vol. 103, No. 32, 1999 fragmentation, we develop a model which accounts for the observed products and allows, but does not require, ejection of H, (ionic or neutral) during irradiation. After formation, and on a picosecond time scale, the center of mass of c-C3H6+ will move toward the Pt(111) substrate under the influence of image forces. At the same time, C-H and C-C bond lengths will increase in response to the removal of an electron from a bonding orbital. In a molecular orbital description, these motions bring the frontier molecular orbitals of c-C3H6+ into overlap with filled and empty Pt orbitals energetically positioned near the Pt Fermi level. The excellent catalytic activity of Pt is associated with such interactions; it is a matter of getting a CxHy species (neutral or ionized) close enough, a distance dependent on the species, to allow mixing of filled and empty orbitals on both partners, i.e., mixing LUMO and HOMO of c-C3H6+ with orbitals just above and just below the Fermi level of Pt. In such a description, this mixing provides new orbitals in which electron density is shared between c-C3H6+ and Pt, i.e., bondbreaking and bond-forming chemistry. In gas-phase electron impact ionization, C-H cleavage to form c-C3H5 and H+ or c-C3H5+ and H occurs with high probability. On Pt(111), the analogous process would form adsorbed C3H5 and either adsorbed or gas-phase H or H+. For c-C3H6 aligned in the tilted geometry described above, ejection of H or H+ from the CH2 group furthest from the Pt is plausible. To this point, neutralization has not been considered. As c-C3H6+ moves closer to the Pt(111), the neutralization probability will increase. Reneutralization at a given configuration of the c-C3H6+-Pt(111) system will position the resulting neutral fragment at a variety of locations on one of the accessible potential energy surfaces. Assuming the electronic ground state is formed, the c-C3H6 will be on the repulsive portion of the c-C3H6-Pt potential, and the internal configuration of c-C3H6 will not be equilibrated with the substrate because energy has accumulated in vibrational modes during the ion’s lifetime. This excitation energy can be used to meet the activation energy requirements of subsequent chemical bond rearrangements. The extent of vibrational excitation will vary from mode-to-mode and, after neutralization, will migrate among various modes. Specific reactions are promoted by energy accumulation in critical coordinates involved in forming the transition state for that process. These chemical changes are assisted by being close to Pt(111) where orbital mixing is occurring, analogous to the mixing described above for c-C3H6+. For vibrationally excited c-C3H6, catalyzed by Pt(111), both C-C bond breaking (ring opening) and C-H bond breaking are expected in competition with relaxation. On the basis of the known chemical activity of Pt(111) for breaking C-C bonds, coupled with the relatively high thermodynamic potential of c-C3H6, i.e., relatively weak C-C bonds and a strained C3 ring, we anticipate that ring opening will be a relatively low activation energy path and will be aided by energy accumulating in the ring breathing and bending modes. However, the inertia for this motion will be larger than that for H atom motion, and since time scale considerations are central, accumulating energy in C-H modes during the lifetime of c-C3H6+ will generally occur more rapidly than in the ring modes. Thus, even though the C-H bonds are stronger than the C-C bonds, breaking C-H might be favored dynamically. However, on Pt, unlike Cu, there is no evidence for stabilization of cyclopropyl groups at 100 K; the absence of ring modes at or near 840, 1015, and 1140 cm-1 in the post-irradiation HREELS (Figure 11a) makes this clear. If cyclopropyl groups form, they might rearrange to form η3-allyl, a rearrangement

Scoggins et al. known in liquid-phase reactions,47 thereby accounting for the η3 character of post-irradiation HREEL spectra. The H atom liberated during cyclopropyl formation could also be transferred to a neighboring c-C3H6 to form an adsorbed n-propyl group. For the tilted configuration of adsorbed c-C3H6, two types of C-C bonds are present, one parallel to the surface, and two pointed away from the surface, cleavage of each will likely lead to different adsorbed products. Cleaving C-C parallel to the surface minimizes the atomic motion that must occur to form two C-Pt bonds, i.e., form nascent metallacycles, in concert with C-C rupture. An attractive scenario involves placing activated c-C3H6 over a single Pt atom and rearranging (rehybridizing) the electron density around each of the carbons in concert with overlap with Pt orbitals. At the same time rotation of the CH2 groups around an axis through the uppermost C atom and parallel to the plane of the carbon atoms is initiated. Changing the C-C distance may not be necessary to reach the transition state to metallacyclobutane; that is, the ring opening may be activated simply because c-C3H6 (ionic or neutralized) is located close enough to the Pt surface to access the attractive chemical forces that break a C-C bond and form two Pt-C bonds. The C-C distances could relax (extend) later on the portion of the potential energy surface leading from the transition state to the products. Alternatively, if c-C3H6 is located across two Pt atoms, the ring cleavage could lead to metallacyclopentane. In the second case, where C-C bonds that point away from the surface are broken, several processes may occur. Direct metallacycle formation, while plausible, is less probable than for the first case since concerted formation of two C-Pt bonds is less likely. More likely, the CH2 group nearest the surface forms a C-Pt bond. The resulting very labile adsorbed radical species, Pt-CH2CH2CH2, can rearrange along several paths, consistent with no desorption: (1) form a metallacycle; (2) undergo a hydrogen shift reaction to form di-σ-bound propylene; (3) lose an H atom to the Pt surface to form η1 allyl (Pt-CH2CHd CH2); (4) lose an H atom to form η3-allyl, C(a)H2C(a)HC(a)H2. To some extent, the path followed will depend on the availability of sites in the region around the nascent radical. The paths to η1 allyl minimize the site requirements and may become more important when little c-C3H6 remains, i.e., when weakly held Pt sites become very scarce. However, there is only circumstantial evidence here for η1 formation. Evidence for its formation has been reported in a study of allyl bromide on Pt(111).1 Studies of 1-chloro-3-iodo propane adsorbed on Pt(111) indicate that metallacyclopentane is not stable when formed thermally, slightly above 200 K, by cleavage of the C-Cl bond in Pt-CH2CH2CH2Cl, the latter formed below 200 K by C-I bond rupture.3 The presumed metallacyclopentane undergoes immediate β-hydride elimination to η3-allyl, and we presume, even though the temperature is lower, the same occurs when formed by electron irradiation of c-C3H6 on Pt(111). While compelling direct evidence for stabilization of metallacyclobutane is absent, several considerations are consistent with it being formed by electron irradiation of c-C3H6 on Pt(111). (1) On Cu, a stretching mode at ca. 2839 cm-1 was diagnostic for metallacyclobutane and a mode emerges in this region after electron irradiation on Pt (Figure 10b). (2) Metallocyclobutane complexes typically rearrange by C-C bond cleavage, and a somewhat analogous process could account for the desorption of ethylene and methane observed in our TPD. (3) There is evidence on other metals for the participation of metallacyclobutane in the formation of methane and ethylene from c-C3H6. We now briefly expand on each of these points.

Chemistry of Cyclopropane on Pt(111) The HREELS evidence for metallacyclobutane is weak because of insufficient resolution to separate the numerous vibrational modes that likely contribute to the C-H stretching and ring modes. Ring modes from cyclopropane, cyclopropyl fragments, and C3 metallacycles do not vary greatly; on Cu, the asymmetric ring deformation at 1034 cm-1 shows a shift to 967 cm-1, the symmetric ring deformation shifts from 872 to 838 cm-1, and the ring breathing mode shifts from 1179 to 1177 cm-1.37,20 In our case, the intensity that remains in the regions expected for metallacyclobutane ring modes, e.g., 700-1300 cm-1, is ambiguous, since other intermediates, e.g., η3-allyl, have numerous modes that contribute to this region. While there is some intensity at 780 cm-1, which may have shifted from the strong 840 cm-1 band of c-C3H6, it is clear that an intact metallacycle fragment is not the dominant product of electron irradiation. Literature on organometallic complexes provides precedent for the formation of metallacylobutane and its relation to methane and ethylene. The 500 nm photolysis of unligated nickel(II) cyclobutane results in a methylidenylnickel ethylene complex, i.e., a Ni atom with one CH2 group and one π-bonded ethylene moiety attached.48 The major products observed in the reaction of Co+,49-51 Ni+,50 and Cu+ 50 with cyclopropane are MCH2+ and gaseous ethylene. In all cases, there is also a low probability reaction path to C3H5+ and MH.49-51 As noted several times above, electron-irradiated cyclopropane is distinct among the C3 adsorbates we have studied on Pt(111); from among C3H7I, ClC3H6I, C3H5Br, and C3H6, it alone exhibits CH4 and C2H4 in TPD. For the others, only C3 hydrocarbons, C3H6 and C3H8, desorb. Among the systems we have studied, the intermediate leading to the simultaneous desorption of CH4 and C2H4 apparently contributes significantly only to the chemistry of electron-irradiated c-C3H6. We attribute this to the decomposition of metallacyclobutane, even though the details of the C-C bond-breaking process may not be the same as for the organometallic complexes. Finally, there is evidence for metallacyclobutane formation on other metals. It has been proposed in the formation of methane and ethylene/ethane from reactions of cyclopropane on Ir(111) 23 and supported Ni catalysts.33 A metallacycle was also proposed on Ru(0001) and Ru(1120),31 but the authors do not specify whether they support bonding to one or two Ru atoms. While metallacyclopentane was proposed, only propane was observed for reactions of cyclopropane with H2 on Pt thin films,14 on supported Pt catalysts, and on stepped Pt(111).30 Thus, electron activation of cyclopropane on Pt(111) opens a new pathway for C3 reactions, and based on the above evidence, we propose that metallacyclobutane, formed during electron irradiation, is responsible for methane and ethylene desorption. The formation of CH4 and C2H4 requires synthesis, by electron irradiation, of one or more intermediates that can supply H for hydrogenation of CH2 to form CH4. Considering that the H2 TPD signal is roughly 10 times that of these hydrocarbons, the small rise of CH4 after the H2 TPD appears to saturate (Figure 7), creates no serious stoichiometric problem. Scheme 1 summarizes the proposed electron-induced reactions of 1 ML of cyclopropane adsorbed on Pt(111). During electron exposure, we propose that metallacyclopentane, cyclopropyl, and metallacyclobutane are formed and that the latter is stabilized at 100 K, while the former two species form η3allyl at 100 K. During subsequent heating, thermally activated reactions of these species lead to new surface species and desorbing products. As for other C3 adsorbates on Pt(111), the

J. Phys. Chem. B, Vol. 103, No. 32, 1999 6801 SCHEME 1

major reaction pathway of these intermediates, accounting for about 75% of the carbon associated with all the above intermediates, is complete dehydrogenation during TPD leaving adsorbed carbon at temperatures above 600 K. Propylidyne, formed by thermal activation, dominates the species present at 300 K that lead to this carbon. The remaining 25% of the carbon desorbs as propylene, methane, and ethylene. We propose that C-H bonds cleave rapidly at above 250 K, providing activated surface H atoms that hydrogenate the bridging methylene in metallacyclobutane to methane. Simultaneously, the remaining methylene groups, which are bound to a single Pt, recombine and desorb as ethylene. This scheme is consistent with the results found in the presence of coadsorbed atomic deuterium. There are three propylene peaks. The first (185 K) is attributed to β-hydride elimination of n-propyl, C3H7, on the basis of work on n-propyl iodide.2 The presence of adsorbed n-propyl necessitates hydrogenation of one of the C3H6 species formed by electron irradiation. As noted above, a disproportionation reaction of electron-excited c-C3H6 with neighboring c-C3H6 to form n-propyl and η3-allyl could be responsible. In this regard, n-propyl has been invoked to describe the isomerization of cyclopropane to propylene over transition metal catalysts.35 The second and third propylene peaks are accounted for, by analogy with a study of allyl bromide on Pt(111),1 in terms of hydrogenation of allyl intermediates, η3-allyl at 208 K and η1allyl at 310 K. Some η1-allyl rearranges to η3-allyl near 310 K and any η3-allyl surviving to this temperature rearranges to propylidyne. Forming propylidyne is a major contributor to the thermal chemistry. In other work, electron irradiation of cyclopropane on Cu(111) and Cu(110) shows that dissociative electron attachment induces both C-C and C-H bond cleavage.37,20 A C-H mode at ca. 2839 cm-1 was diagnostic for metallacyclobutane.20 Compared to Pt(111), Cu is much less active for C-C and C-H bond scission in adsorbed hydrocarbons. In fact, while allyl bromide dissociates thermally on Cu(100) at 100 K, the major subsequent thermally activated process is linking allyl fragments to form 1,5- hexadiene,36 not dehydrogenation as on Pt(111).

6802 J. Phys. Chem. B, Vol. 103, No. 32, 1999 On Cu(111), electron activation of c-C3H6 leads to C-H bond cleavage to form a stable c-C3H5 (cyclopropyl).37 There is no evidence for isomerization to allyl at 100 K on Cu, indicating its much lower chemical activity compared to Pt. Photochemistry. Compared to electron irradiation, the cross section for 193 nm photon driven loss of c-C3H6 is negligible (10-20 vs 10-16 cm2). Another comparison is of interest. For 193 nm irradiation, the cross section for loss of CH4 on Pt(111) is 10 times larger52 than that for photolysis of c-C3H6. The latter was attributed to a direct photon absorption by CH4 molecules that are weakly held but, nonetheless, slightly distorted (C3V vs Td local symmetry). This proposed structural change has been verified by reflection-absorption infrared measurements.53 Gas-phase cyclopropane exhibits electronic transitions in the deep ultraviolet; the optical absorption cross section extends to wavelengths as long as 160 nm, compared to 144 nm for CH4.54 Moreover, as confirmed here, the symmetry of c-C3H6 generally is lowered when absorbed on metals.16,21 Despite these factors, which might favor direct photon absorption by c-C3H6, as compared to CH4, the cross section for rearrangement of c-C3H6 is an order of magnitude lower. Speculatively, this could imply that the excited state of adsorbed c-C3H6 is not dissociative or that the time scale for bond breaking is not competitive with relaxation. If so, then the critical distinction between the photondriven and electron-driven processes might be the formation of ions in the latter with attending strong attractive forces that move the c-C3H6+ moiety close enough to the Pt surface so that, after neutralization, c-C3H6 is located on a chemisorption potential. 5. Summary and Conclusions Monolayer c-C3H6 adsorbs without dissociation on Pt(111) at 100 K and desorbs molecularly at 144 K. Nonthermal chemistry is activated by either 50 eV electron or 6.4 eV photon irradiation. A model involving C-C bond breaking to form metallacyclobutane, and through transient metallacyclopentane, η3-allyl, accounts for the observed TPD, XPS, and HREELS measurements. When 1 ML c-C3H6 is irradiated at 100 K until nearly all the c-C3H6 is reacted, only 25% of the total carbon desorbs as hydrocarbons in subsequent TPD. The other 75% is completely dehydrogenated during TPD and remains adsorbed after TPD. The accompanying H2 TPD profile tracks that expected for propylidyne formation and dehydrogenation. The desorbing hydrocarbon products include three peaks of propylene, attributed to β-hydride reaction of n-propyl groups and to hydrogenation of allyl groups, and CH4 and C2H4 attributed to the hydrogen-assisted decomposition of metallacyclobutane. The desorption of CH4 and C2H4 distinguishes electron-irradiated c-C3H6 from other C3 adsorbates. Acknowledgment. This work was supported in part by the U. S. Department of Energy, Office of Basic Energy Sciences, and by the Robert A. Welch Foundation. T.B.S. would like to thank E. D. Pylant and K. H. Junker for their helpful discussions. References and Notes (1) Scoggins, T. B.; White, J. M. J. Phys. Chem. 1997, 101, 795867. (2) Scoggins, T. B.; Ihm, H.; White, J. M. Israel J. Chem. 1998, 38, 353-363.

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Chemistry of Cyclopropane on Pt (111): Thermal, Electron, and Photon

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