A comparative study of OH radical desorption in the H 2 + O 2 reaction on Pt, Pd, Rh, Ir, and Ni

Abstract

OH radicals emitted from Pt, Pd, Rh, Ir, and Ni during the reaction between H 2 and O 2 have been studied by laser-induced fluorescence. The OH emission yields from the different catalysts were determined as a function of total pressure (5 × 10 −3−0.2 Torr), temperature of the catalyst (950–1300 K), and H 2 O 2 partial pressure ratio. When varying the total pressure at a constant mixing ratio of the reactants, a maximum intensity of OH yield is obtained at 0.7 ± 0.1 Torr for all catalysts. The OH production is very sensitive to the mixing ratio of H 2 and O 2, and shows a maximum at a specific ratio, which varies for the different catalysts. The maximum in the OH production does not coincide with the maximum in the H 2O production. The absolute intensity of desorbed OH radicals varies strongly with the different catalysts in the order Pt > Pd > Rh > Ir > Ni. When the H 2 gas supply is closed, OH radicals are still detected, indicating that the reaction H 2O ads + O ads → 2OH ads contributes to the observed OH intensity. The apparent desorption energy of OH shows a weak dependence on total pressure but a strong dependence on the mixing ratio.