Interaction of cyclobutane with the Ru(001) surface: Low-temperature molecular adsorption and dissociative chemisorption at elevated surface temperature
Cj. Hagedorn et al., Interaction of cyclobutane with the Ru(001) surface: Low-temperature molecular adsorption and dissociative chemisorption at elevated surface temperature, J CHEM PHYS, 110(3), 1999, pp. 1745-1753
We have studied the interaction of cyclobutane with the hexagonally close-p
acked Ru(001) surface. High-resolution electron energy loss spectroscopy (H
REELS) has been used to identify the vibrational modes of both c-C4H8 and c
-C4D8 adsorbed at 90 K as a function of cyclobutane exposure. We have obser
ved a vibrational mode not observed in the gas phase at 2600 cm(-1) (2140 c
m(-1)) which is attributed to the strong interaction of the cyclobutane C-H
(C-D) bonds with the ruthenium surface. Two different adsorption geometrie
s for cyclobutane on Ru(001) have been proposed based on the dipolar activi
ty of this softened C-H mode. We have also measured the trapping-mediated d
issociative chemisorption of both c-C4H8 and c-C4D8 at surface temperatures
between 190 and 1200 K. The measured activation energies with respect to t
he bottom of the physically adsorbed well for c-C4H8 and c-C4D8 are 10 090
+/- 180 and 10 180 +/- 190 cal/ mol, respectively. The trapping- mediated c
hemisorption of cyclobutane is believed to occur via C-C bond cleavage, as
judged by the absence of a kinetic isotope effect. The measured ratios of t
he preexponential factors for desorption relative to reaction of 21 +/- 2 a
nd 47 +/- 64 for c-C4H8 and c-C4D8 respectively, are in the expected range
considering the greater entropy gain associated with the transition state f
or desorption relative to the transition state for C-C bond cleavage. (C) 1
999 American Institute of Physics. [S0021-9606(99)70103-0].