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

Abstract

The study presented here focuses on determining the role of oxygen as a modifier on Pd(111) and its effects on the cyclization 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 H2O, 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 O2 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.