The chemistry of simple alkyl species on Pt(111) generated by hyperthermal collisions

Abstract

Kinetics and dynamics of the trappingmediated dissociative chemisorption of oxygen on Ru (001) The identification of the primary dissociation products of hyperthermal collisions of methane and ethane with a Pt͑111͒ surface at ϳ150 K and the study of their subsequent thermal evolution has been afforded by the use of reflection-absorption infrared spectroscopy, temperature programmed desorption, and Auger electron spectroscopy. In both cases, the primary dissociation products have been identified as an adsorbed alkyl moiety ͑methyl, CH 3 , and ethyl, C 2 H 5 , respectively͒ and an adsorbed hydrogen atom. The thermal treatment of both alkyl adlayers ultimately results in the formation of the ethylidyne ͑wCCH 3 ͒ moiety at temperatures between 300 and 400 K. At ϳ500 K, this species itself thermally decomposes to yield adsorbed carbon atoms and gaseous hydrogen. While this behavior may not be unexpected for the C 2 ethyl fragment, the formation of the C 2 ethylidyne fragment from the C 1 methyl fragment can only be explained through the occurrence of carbon-carbon coupling reactions. Such reactions have only recently been observed on the Pt͑111͒ surface. The C-C coupling reactions associated with the formation of ethylidyne from methyl show considerable dependence upon the surface coverage of the methyl fragment and are accompanied by the evolution of both gaseous hydrogen and methane. The latter results from recombinative desorption of methyl moieties with adsorbed hydrogen atoms.

Methane Dissociative Adsorption on the Pt(111) Surface over the 300−500 K Temperature and 1−10 Torr Pressure Ranges

“…This peak is assigned to the symmetric C−H methyl stretch. This assignment is consistent with those from previous vibrational spectroscopic studies of methyl groups deposited on the Pt(111) surface by various methods. ,− On the basis of the position of this vibrational peak and the absence of a peak at 2950 cm -1 because of the asymmetric C−H methyl stretch, the methyl groups are adsorbed on atop sites with C 3 v symmetry. , Increasing the reaction temperature leads to a change in the SFG spectrum, as seen in the figure. When the reaction temperature was increased to 400 K, the peak increased in intensity and the line width narrowed; however, the peak position remained at 2880 cm -1 .…”

“…The activation of carbon−hydrogen bonds in methane for conversion to hydrogen or to other hydrocarbons is one of the fundamental problems of catalytic surface chemistry. − Classical surface science studies of C−H dissociation in methane on Pt(111) under ultrahigh vacuum (UHV) conditions have been limited because of the low (<70 K) desorption temperature of the methane monolayer from this single-crystal surface. − Because of this limitation, the dissociative adsorption of methane on Pt(111), at temperatures above the monolayer desorption temperature, has been investigated previously using mainly molecular beam methods. − In some instances, various surface-sensitive techniques have been used to identify adsorbed surface intermediates. , On Pt(111), methyl groups were identified as products from the reaction of a hyperthermal molecular beam of methane at a surface temperature of 150 K using reflection absorption infrared spectroscopy (RAIRS) . For a surface temperature of 400 K, ethylidyne was proposed to form, on the basis of changes in the vibrational spectrum for the adsorbed surface species.…”

Sulfate-Enhanced Catalytic Destruction of 1,1,1-Trichlorethane over Pt(111)

“…It may be anticipated that such dissociation of the carbon−carbon bond following TCA dechlorination would liberate surface methyl fragments, consistent with the fingerprint low binding energy C 1s peak seen in Figure . Further support for low-temperature methyl formation is provided by the appearance of a strong C 2 H 6 desorption at 320 K, attributable to recombinative desorption of two surface bound methyl fragments. , Alternative routes to ethane, such as ethylidyne hydrogenation, can be discounted since our fast XP spectra show no evidence for a surface ethylidyne intermediate, and in any event ethylidyne is stable with respect to hydrogenation under ultrahigh vacuum . The 250 and 300 K water desorptions also suggest the direct interaction (and competitive dehydrogenation) of the methyl fragment of TCA with the Pt(111) surface at subambient temperatures.…”

“…A second set of coincident desorption peaks, arising from the combustion of residual surface carbon by the remaining adsorbed sulfoxy groups and hydrogenation of residual surface atomic chlorine and oxygen, occur at 400 K. Methyl dehydrogenation over clean Pt is believed to proceed in a stepwise fashion via a metastable methylene intermediate. , The second CO 2 desorption state may therefore be attributed to the oxidation of atomic carbon produced from methylene dehydrogenation. Surface hydrogen liberated in such a dehydrogenation step would account for simultaneous HCl and H 2 O desorptionthe latter oxygen extracted during the reduction of SO x to irreversibly adsorbed atomic sulfur, as shown in Figure .…”

Production of Synthesis Gas by Direct Catalytic Oxidation of Methane on Pt{110} (1 × 2) Using Supersonic Molecular Beams

“…Steps 1 and 2 describe methane and oxygen dissociative adsorption At surface temperatures above ∼550 K, methyl, formed from the dissociative adsorption of methane, rapidly dehydrogenates to C a and H a . , Thus, all reactions occur on the surface between carbon, oxygen, and hydrogen adatoms.…”