Preadsorbed Oxygen Atoms Affect the Product Distribution and Kinetics of Acetylene Cyclization to Benzene on Pd(111): A Laser-Induced Thermal Desorption/Fourier Transform Mass Spectrometry Study

摘要

The study presented here focuses on determining the role of oxygen as a modifier on Pd(111) and its effects on the cyctization of acetylene to benzene. Laser-induced thermal desorption/Fourier transform mass spectrometry (LITD/FTMS) is used as a sensitive tool for measuring the in situ kinetics of benzene formation from acetylene on O/Pd(111). Low exposure of acetylene on O/Pd(111) leads to the anticipated formation of benzene and 1,3-butadiene. Though there is no evidence of furan formation on the surface, oxidation products, such as CO and H↓(2)0, are observed. An enhancement in the yield of benzene has been observed with increasing oxygen preexposure. Our evidence suggests that this enhancement is caused by oxygen-island compression of acetylene molecules into bare patches of Pd, which effectively increases the local coverage of acetylene in those regions. Isothermal kinetic studies of 1.1 langmuirs of acetylene on a 50% saturated layer of O on Pd(111) (from a 0.25 langmuir exposure of O↓(2) at 250 K) yield an Ea of 37.8 ±3 kJ/mol using initial rates (and 36.2 ±3 kJ/mol using a pseudo-first-order model). Both the activation energy and preexponential factor from a 50% saturation coverage of oxygen on Pd(111) correspond to the values expected for twice the acetylene exposure on a clean surface. The apparent contradiction between increased benzene yields and activation barrier for the O/Pd system can be rationalized by the compensation effect where a more tightly bound reactant can lead to a greater entropy of activation.