Decomposition of 1,3-butadiene on Ru(001): Evidence for a (CH)(4) metallacycle

The adsorption and thermal decomposition of 1,3-butadiene has been studied on Ru(001). Starting with a condensed multilayer at 80 K, the decomposition of I,3-butadiene was monitored as a function of temperature using temperat ure programmed desorption (TPD) and high-resolution electron energy loss sp ectroscopy (HREELS). Annealing the multilayer-covered surface to 800 K resu lts in the decomposition of some of the adsorbed 1,3-butadiene, leaving a s urface carbon coverage corresponding to 0.67+/-0.03 carbon adatoms per surf ace ruthenium atom, as determined by titration with oxygen. As the butadien e decomposes between 230 and 700 K, hydrogen desorbs in an amount correspon ding to 1.01+/-0.05 adatoms per surface ruthenium atom. A HREEL spectrum ta ken after annealing to 260 K for 10 s provides spectroscopic evidence for t he presence of a (CH), metallacycle decomposition product. This assignment is consistent with and supported by the determination of a surface stoichio metry of CH1.04+/-0.11 using TPD spectra. The formation of the catalyticall y important (CH)(4) metallacycle has been predicted theoretically, but this work represents the first experimental evidence for the formation of this intermediate from the decomposition of 1,3-butadiene. (C) 2000 American Vac uum Society. [S0734-2101(00)15404-1].