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 hexago nally close-packed (hcp) Ir(111) surface using temperature-programmed desor ption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Monolayer and submonolayer cyclobutane coverages desorb essentially comple tely in TPD experiments, whereas in the presence of a condensed cyclobutane multilayer, significant reaction of first-layer cyclobutane is observed. B ased on the HREELS and TPD measurements, a C4H8 metallacycle is believed to be the predominate reaction intermediate that is formed in the initial rea ction step during multilayer-induced reaction (MIR) of cyclobutane on Lr(ll l). The TPD spectra suggest that MIR of cyclobutane on Lr(lll) results in t hree different reaction products: (1) surface carbon and hydrogen adatoms f ormed via complete decomposition of cyclobutane, (2) desorption of n-butane , and (3) desorption of I-butene, Moreover, the extent of MIR has been quan tified 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 a re well described by a rate expression which is first order in the coverage of unreacted cyclobutane and the rate coefficient of which can be expresse d 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 v alue represents a significant decrease in the activation barrier fur reacti on of cyclobutane on Ir(lll) compared to the value of 10 270 cal/mol recent ly measured for the trapping-mediated dissociative chemisorption of cyclobu tane on this same surface in the submonolayer regime at low temperature (30 0-400 K). This activation barrier reduction for MIR of cyclobutane on Ir(11 1) is qualitatively similar to that observed recently for the MIR of cyclob utane on Ru(001). The potential implications of these results toward provid ing a better understanding of catalytic reactions that occur at the solid-l iquid interface are discussed.