Mechanism and cross sections for HD and CH4−xDx (x=1–4) formation in D(g)+CH3/Cu(111) reaction at 100 K

Abstract

We have investigated the kinetics of the gas-phase D atom reaction with methyl (CH3) adsorbed on Cu(111) at 100 K, in which the product molecules desorbing from the surface were directly monitored with a mass spectrometer. While adsorbed D atom combines with CH3(ad) to form CH3D at temperatures ⩾ 300 K, the incident D(g) atom readily abstracts CH3(ad) to form CH3D at 100 K. In addition, the D(g) atom also abstracts a H atom from CH3(ad) to form HD and CH2(ad) which is subsequently converted to CH2D(ad) by addition of an incident D(g) atom. CH2D(ad) thus formed in turn undergoes the same reaction sequence to lead to desorption of CH2D2, CHD3, and CD4. A kinetic analysis was made to extract the reaction cross sections for the elementary reactions involved: σCH3=2.9±0.2, σH=0.5±0.05, and σa⩾22 Å2 for the CH3(ad) abstraction, H abstraction, and D atom addition reactions, respectively. These are essentially barrierless reactions, showing almost no temperature dependence. σCH3 varied with the beam incidence angle θi as cos0.63 θi and σH as cos0.85 θi. The mechanism of each elementary reaction was discussed in terms of a direct Eley–Rideal (ER) and primary hot atom (p-HA)—trapped but not accommodated incident atom—mechanisms by considering the cross section, energetics, and reaction dynamics. Finally, we concluded that (1) the H abstraction from CH3(ad) occurs exclusively by an ER mechanism, (2) the CH3(ad) abstraction predominantly occurs by an ER mechanism with some contribution by a p-HA mechanism, and (3) the D(g)-addition to CH2(ad) exclusively by a p-HA mechanism.