Interaction Of Cyclobutane With The Ru(001) Surface: Low-Temperature Molecular Adsorption And Dissociative Chemisorption At Elevated Surface Temperature

We have studied the interaction of cyclobutane with the hexagonally close-packed Ru(001) surface. High-resolution electron energy loss spectroscopy (HREELS) 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 observed a vibrational mode not observed in the gas phase at 2600 cm(-1) (2140 cm(-1)) which is attributed to the strong interaction of the cyclobutane C-H (C-D) bonds with the ruthenium surface. Two different adsorption geometries for cyclobutane on Ru(001) have been proposed based on the dipolar activity of this softened C-H mode. We have also measured the trapping-mediated dissociative chemisorption of both c-C4H8 and c-C4D8 at surface temperatures between 190 and 1200 K. The measured activation energies with respect to the 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 chemisorption 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 the preexponential factors for desorption relative to reaction of 21 +/- 2 and 47 +/- 64 for c-C4H8 and c-C4D8 respectively, are in the expected range considering the greater entropy gain associated with the transition state for desorption relative to the transition state for C-C bond cleavage. (C) 1999 American Institute of Physics. [S0021-9606(99)70103-0].