“…Following the nomenclature of published hydrogen TPD results, we identify the new peak as the β 3 state. The desorption temperature for this new state is similar to several well-known subsurface hydrogen states on the Ni(100), Ni(111), and Cu(110) surfaces. In addition, this new form of hydrogen activates adsorbed cyclopropane as previously observed for the subsurface state on nickel.…”
Carbon-carbon bond activation in adsorbed cyclopropane is observed following exposure to gas-phase atomic hydrogen on the Pt(111) surface for temperatures as low as 120 K despite the fact that this low index platinum surface generally does not activate C-C bonds. Cyclopropane desorbs molecularly at 140 K. In the presence of coadsorbed hydrogen, no reaction with cyclopropane is observed. In contrast, gas-phase atomic hydrogen reacts with cyclopropane at 120 K to form a propyl intermediate. During subsequent heating, this intermediate is hydrogenated to form propane at 170, 190, and 220 K. As indicated by the lack of methane and ethane formation, no multiple C-C bond activation processes were observed. Reaction mechanisms are proposed for each new TPRS peak observed based upon the products observed, isotope experiments, and comparisons with the literature. The primary 170 K propane peak is caused by surface hydrogenation of an adsorbed propyl formed by C-C bond activation in adsorbed cyclopropane by gas-phase atomic hydrogen. The 190 K propane peak is caused by reaction between adsorbed cyclopropane and a new reactive form of adsorbed hydrogen. The propane peak at 220 K is dominant only for low gas-phase atomic hydrogen exposures and may be associated with a coverage-dependent reaction path. Taken together, these results clearly indicate that the reaction between gas-phase atomic hydrogen and cyclopropane is more complex on the Pt(111) surface than the analogous reaction on the Ni(100) surface where only one propane peak was reported. Prolonged exposures of the clean Pt(111) surface to gas-phase atomic hydrogen at 120 K resulted in an unusually lowtemperature state of hydrogen that desorbs below 220 K, causes carbon-carbon bond activation, and has not previously been reported.
“…Following the nomenclature of published hydrogen TPD results, we identify the new peak as the β 3 state. The desorption temperature for this new state is similar to several well-known subsurface hydrogen states on the Ni(100), Ni(111), and Cu(110) surfaces. In addition, this new form of hydrogen activates adsorbed cyclopropane as previously observed for the subsurface state on nickel.…”
Carbon-carbon bond activation in adsorbed cyclopropane is observed following exposure to gas-phase atomic hydrogen on the Pt(111) surface for temperatures as low as 120 K despite the fact that this low index platinum surface generally does not activate C-C bonds. Cyclopropane desorbs molecularly at 140 K. In the presence of coadsorbed hydrogen, no reaction with cyclopropane is observed. In contrast, gas-phase atomic hydrogen reacts with cyclopropane at 120 K to form a propyl intermediate. During subsequent heating, this intermediate is hydrogenated to form propane at 170, 190, and 220 K. As indicated by the lack of methane and ethane formation, no multiple C-C bond activation processes were observed. Reaction mechanisms are proposed for each new TPRS peak observed based upon the products observed, isotope experiments, and comparisons with the literature. The primary 170 K propane peak is caused by surface hydrogenation of an adsorbed propyl formed by C-C bond activation in adsorbed cyclopropane by gas-phase atomic hydrogen. The 190 K propane peak is caused by reaction between adsorbed cyclopropane and a new reactive form of adsorbed hydrogen. The propane peak at 220 K is dominant only for low gas-phase atomic hydrogen exposures and may be associated with a coverage-dependent reaction path. Taken together, these results clearly indicate that the reaction between gas-phase atomic hydrogen and cyclopropane is more complex on the Pt(111) surface than the analogous reaction on the Ni(100) surface where only one propane peak was reported. Prolonged exposures of the clean Pt(111) surface to gas-phase atomic hydrogen at 120 K resulted in an unusually lowtemperature state of hydrogen that desorbs below 220 K, causes carbon-carbon bond activation, and has not previously been reported.
“…For the CH 3 I + D case, a reaction cross section smaller by a factor of 1.4 provides a good fit to the data (see Table ). This simple site-blocking model adequately describes all the kinetic results without the need to invoke nonlocal site effects such as compressed overlayers or island formation. − In the case of extremely crowded initial conditions (such as saturation preadsorption of hydrogen), however, the assumed initial number of empty sites is inadequate to hold the methyl iodide dissociation products without including compression and/or displacement effects. 12 Comparisons between the TPD yields of α-methane and kinetic model predictions: (a) dependence of yield on hydrogen precoverage at relatively constant methyl iodide coverage ((0.14 ± 0.02) × 10 15 /cm 2 ); (b) dependence of yield on methyl iodide coverage at two values of hydrogen precoverage (○, 1.1 × 10 15 /cm 2 ; ·, 1.3 × 10 15 /cm 2 ).…”
We have studied the reactions between submonolayer coverage
amounts of methyl iodide and adsorbed
hydrogen on Ni(111) using high-resolution electron energy loss
spectroscopy (HEELS), temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and
low-energy ion scattering (LEIS).
Methane is the only hydrocarbon product during TPD to 500 K and
consists of original methyl groups plus
one coadsorbed hydrogen atom. Methane desorbs in a sharp peak at
about 150 K and in broader peaks at
230 and 260 K. The 150 K process does not occur for coadsorbed
CH3 and H but rather involves abstractions
of surface hydrogen atoms by nascent, unaccommodated methyl groups
formed during CH3−I bond scission.
The higher temperature bond formation processes involve
accommodated methyl groups.
“…Similarly we did not see any evidence for extra adsorption states, or surface damage, caused by ion exposure. 26 Chorkendor † et al 15 observed that the subsurface TPD peaks shifted to higher temperatures as the ion energy was increased above 1 keV. This reÑects the implantation depth of D into the bulk and the kinetics of di †usion back to the surface, with formation of trapping states becoming important at higher energies.…”
The recombination of surface and subsurface D atoms on Ni(111) has been studied using resonance-enhanced multiphoton ionisation (REMPI) to measure the internal state and translational energy distributions of the desorbing product. By detecting formed D 2 during temperature-programmed desorption we were able to examine the reaction between subsurface and surface D atoms, and the recombination of two D atoms chemisorbed on the surface. Translational energy distributions for formed by D 2 recombination of surface D are very sensitive to coverage. Desorption from a low coverage surface produced a translational energy release of 2.6 kT , but a thermal rotational distribution, reÑecting an entrance channel barrier to dissociative chemisorption on the clean Ni(111) surface. Sticking probabilities predicted from detailed balance are consistent with molecular beam adsorption measurements. Desorption from D coverages above 0.5 ML resulted in a sub-thermal energy release, desorption being mediated by a molecular precursor state with dissociation occurring via a non-activated, D 2 trappingÈdissociation channel. In contrast, the reaction of subsurface D produces translationally hot with a mean energy approaching 8 at 180 K. This is consistent D 2 , kT s with the energetics for direct recombination of a chemisorbed D atom with a metastable subsurface D atom, which overcomes an activation barrier to resurface of between 0.35 and 0.47 eV depending on D concentration. The energy release decreases at higher temperature, probably as a result of a reduction in the energy of resurfacing D as the subsurface D concentration drops. This low energy component is attributed to accommodation of resurfacing D which is unable to react directly, followed by slow thermal desorption via the high coverage, surface D recombination channel. No internal rotational or vibration excitation was found in formed by reaction of subsurface D. D 2
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
