Probing O(³P) Reactivity with Chemisorbed Hydrocarbons: Insights from Experiment and Theory

Heterogeneous oxidation of hydrocarbons via low-pressure nonthermal plasma has traditionally focused on nonvolatile compounds since volatile or semivolatile hydrocarbons can partition into the plasma, reducing product selectivity. However, adsorption of volatile compounds can prevent the hydrocarbon from vaporizing and reaching the plasma-phase, allowing reactions to take place between free radicals generated via nonthermal plasmas and the volatile or semivolatile hydrocarbon. In this work, we present a state-of-the-art chamber for the heterogeneous reactions between adsorbed hydrocarbons and ground-state oxygen (O(³ P)) generated via nonthermal plasma. The chamber enabled a hydrocarbon monolayer on an alumina thin film to be exposed to a plasma plume generated with a radio frequency (RF) generator. In situ vibrational spectroscopy of the alumina-coated surface was used to investigate the relative kinetics of two model hydrocarbons, 1-hexene and cyclohexane, chemisorbed onto alumina. Finally, the functionalization of the chemisorbed hydrocarbon on the alumina powder via nonthermal plasma was investigated in situ in order to determine the conditions for an effective oxidation. Our results show a novel and effective method for the reaction with adsorbed volatile compounds with O(³P). For the adsorbed compounds, the reaction of adsorbed cyclohexane is twice as fast as adsorbed 1-hexene, which represents a significant change with respect to gaseous phase rates, where 1-hexene reaction with O(³P) is approximately 37 times faster than cyclohexane.