Formation and desorption of hydroxyl radicals during Pt-catalyzed oxidation

Abstract

Heterogeneously catalyzed reactions often involve reactive radical intermediates that are produced on the surface and subsequently desorb into the gas phase. In this work, laser-induced fluorescence (LIF) has been used to measure OH radical desorption from a catalytically active polycrystalline platinum wire during the oxidation of methane and hydrogen. The temperature range of the wire was 900 K to 1300 K, and reactant partial pressures were kept between 1 and 20 mtorr to minimze any gasphase reactions. Under these conditions the surface reactions and energetics involving the hydroxyl radical can be studied for Pt-catalyzed oxidation by observing the LIF intensity when changing reaction conditions. The concentration of gas-phase OH radicals depends not only on the concentration of surface hydroxyl radicals but also on the apparent activation energy for OH desorption. This activation energy was found to be inversely related to oxygen coverage, varying from ~33 kcal/mol under oxygen-rich conditions to ~55 kcal/mol under oxygen-depleted conditions. The results indicate that the breaking of the Pt-OH is the rate limiting step for the appearance of OH radicals in the gas phase.