No abstract

On Pt(111) at 110 K, 1‐iodopropane, C3H7I, adsorbs molecularly, but for doses below 1.7 × 1014 molecules cm−2, only H2 and I appear in thermal desorption. C–I bond cleavage occurs between 160 and 220 K, forming adsorbed n‐propyl, C(a)H2CH2CH3, and atomic iodine, based on temperature‐programmed desorption (TPD), high‐resolution electron energy loss spectroscopy (HREELS), and X‐ray photoelectron spectroscopy (XPS). n‐Propyl undergoes β‐hydride elimination forming propylene, with desorption peaks at 185 and 240 K. At 240 K, hydrogenation to propane also occurs. Some di‐σ bonded propylene, C(a)H2C(a)HCH3, remains at 240 K and it rearranges to propylidyne near 300 K. Atomic H, bound to Pt, recombines and desorbs at ca. 260 K. Further desorption of H2 is limited by C–H bond breaking and occurs over a broad temperature range with local maxima at ca. 280, 320, and 420 K, typical of propylidyne fragments on Pt. Atomic iodine desorbs in a broad feature at 825 K.

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, <1016 cm-2, the chemistry involved is describable in terms of C−C bond breaking to open the ring and form metallacyclic species, some of which promptly lose H to form allylic (C3H5) moieties. In subsequent thermally activated reaction-limited processes, propylene, methane, ethylene, and hydrogen desorb. Only carbon remains above 800 K. Activation with 6.4 eV (193 nm) photons is also effective, but the cross section for loss of c-C3H6 is nearly 4 orders of magnitude lower than that for electron irradiation.

HREELS and XPS indicate negligible dissociation of ClC3H6I during adsorption at 100 K. During TPD, no ClC3H6I desorbs for coverages below 0.4 ML. For higher, but not multilayer coverages, parent ClC3H6I desorption occurs in two peaks, 200 and 230 K. After even larger doses, unsaturable multilayer desorption occurs at 170 K. HREELS indicates that most C−I bonds dissociate by 205 K. The following reaction paths are proposed on the basis of TPD and HREELS results. When the C−I bond breaks, 3-chloropropyl fragments, C(a)H2CH2CH2Cl, are formed and these either lose HCl to form η3- or η-allyl or lose a β-hydrogen to form 3-chloro-di-σ-propylene. Some η3-allyl groups hydrogenate to either propylene, some of which desorbs at 240 K, or n-propyl, some of which hydrogenates to release propane at 250 K. Other η3-allyl groups isomerize to η1-allyl. At 250 K, 3-chloro-di-σ-propylene eliminates chlorine as HCl and also releases H atoms that hydrogenate neighboring C3 fragments. The η1-allyl fragment either hydrogenates and desorbs as propylene at 325 K or isomerizes to propylidyne. Propyl and di-σ-propylene moieties rearrange to form propylidyne or release propylene at 325 K. Interestingly, there is some benzene desorbing at 375 K. To account for it, a diene metallacycle is suggested. Atomic iodine desorbs at 825 K. Comparisons of the thermal chemistry of ClC3H6I on Ag(111) and Ni(100) are made as are comparisons of ClC3H6I with other C3 adsorbates on Pt(111).

The low temperature thermal chemistry of CF3I on Ag (111) presents an example of competing reaction pathways; molecular desorption vs desorption of radical CF3. Temperature programmed desorption and angle resolved temperature programmed desorption, complemented with Auger electron spectroscopy and low energy electron diffraction, were used to discern the mechanism of the CF3 radical desorption channel. CF3 desorption is limited to the first monolayer of CF3I; 0.75 ML CF3I is the coverage used for angular dependence measurements. At 90 K most of the CF3I adsorbs molecularly to the metal, but also present under these conditions are dissociative adsorption and thermal decomposition channels limited to C–I bond cleavage. The decomposition product, CF3, desorbs as a radical at high temperatures (∼320 K) with the I remaining on the surface until 850 K. At submonolayer CF3I coverages, thermal activation produces a low temperature (100–150 K) radical desorption channel. Results indicate that low temperature CF3 thermal desorption occurs via dissociative electron attachment to molecular CF3I, yielding radical CF3 and adsorbed iodine.