Coverage Effects on the Kinetics of Benzene Formation from Acetylene on Pd(111): A Laser-Induced Thermal Desorption/Fourier Transform Mass Spectrometry Investigation

Reported are investigations regarding the coverage dependence of the kinetic factors for acetylene cyclotrimerization on Pd(111). Laser-induced thermal desorption/Fourier transform mass spectrometry is used to monitor the rate of the surface reaction prior to conventional desorption. Surface benzene formation is rate limited by the addition of acetylene to C 4 H 4, in contrast to gas-phase benzene production which is rate limited by subsequent benzene desorption. Increasing initial acetylene exposures lead to a substantial increase in both the activation energy and preexponential factor for benzene formation. For initial acetylene exposures from 1.0 to 2.5 langmuir, the activation energy varies from 24.4 ± 3.9 to 43.5 ± 3.9 kJ/mol (95% confidence). Fitting to pseudo-first-order kinetics, the preexponential factors vary from 10 3.1±0.1 to 10 9.8±0.1 s -1 . A change in diffusion rate and steric blocking can both be ruled out as significant contributors to the coverage effect, since both of these processes would lead to the opposite trend in preexponential factors. The change in kinetic factors is most likely a consequence of an increased barrier to C−Pd bond breaking due to more tightly bound species at higher acetylene coverage. Supporting evidence for this is cited from the literature.