The Kinetics of Ethylidyne Formation from Ethylene on Pd(111)

Recent density functional theory (DFT) calculations of the activation energy for the conversion of ethylene to ethylidyne on Pd(111) (Moskaleva, L. V.; Chen, Z-X.; Aleksandrov, H. A.; Mohammed, A. B.; Sun, Q.; Rosch, N. J. Phys. Chem. C 2009, 113, 2515) predicted an activation barrier with respect to gas-phase ethylene of similar to 46 kJ/mol, substantially lower than the results of previous DFT calculations. Thus, the kinetics of formation of ethylidyne from ethylene on Pd(111) are measured as a function of sample temperature using reflection-absorption infrared spectroscopy to monitor the ethylidyne coverage as a function of time. The results yield an experimental value of the height of the activation barrier with respect to gas-phase ethylene of 49 +/- 5 kJ/mol, in good agreement with the results of the DFT calculations. The agreement between experiment and theory indicates that the proposed ethylidyne formation pathway involving an initial, rate-limiting ethylene dehydrogenation step to form vinyl species that finally form ethylidyne via an ethylidene intermediate is correct.