Multilayer-Induced Reaction of Cyclobutane on Ir(111):  Identification of Reaction Products and Quantification of Reaction Kinetics^†

We have studied the low temperature adsorption of cyclobutane on the hexagonally close-packed (hcp) Ir(111) surface using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Monolayer and submonolayer cyclobutane coverages desorb essentially completely in TPD experiments, whereas in the presence of a condensed cyclobutane multilayer, significant reaction of first-layer cyclobutane is observed. Based on the HREELS and TPD measurements, a C4H8 metallacycle is believed to be the predominate reaction intermediate that is formed in the initial reaction step during multilayer-induced reaction (MIR) of cyclobutane on Lr(lll). The TPD spectra suggest that MIR of cyclobutane on Lr(lll) results in three different reaction products: (1) surface carbon and hydrogen adatoms formed via complete decomposition of cyclobutane, (2) desorption of n-butane, and (3) desorption of I-butene, Moreover, the extent of MIR has been quantified and is used in conjunction with a kinetic model in order to quantify the kinetics of the MIR of cyclobutane on this surface. The MIR kinetics are well described by a rate expression which is first order in the coverage of unreacted cyclobutane and the rate coefficient of which can be expressed in Polanyi-Wigner form with a preexponential factor of k(r)((0)) = 6.4 x 10(13) s(-1) and an activation energy of E-r= 8 500 +/- 700 cal/mol. This value represents a significant decrease in the activation barrier fur reaction of cyclobutane on Ir(lll) compared to the value of 10 270 cal/mol recently measured for the trapping-mediated dissociative chemisorption of cyclobutane on this same surface in the submonolayer regime at low temperature (300-400 K). This activation barrier reduction for MIR of cyclobutane on Ir(111) is qualitatively similar to that observed recently for the MIR of cyclobutane on Ru(001). The potential implications of these results toward providing a better understanding of catalytic reactions that occur at the solid-liquid interface are discussed.