Energetics and mechanism for hydroxyl radical production from the Pt-catalyzed decomposition of water

The catalytic decomposition of H₂O over a polycrystalline platinum surface has been studied in a low pressure flow system. Hydroxyl radicals were detected in gaseous decomposition products above about 820 K by laser-induced fluorescence at 300 nm. The activation energy for the HO desorption process, HO* → HO(g)+*, was determined to be 30 ± 1 kcal/mol, which is in excellent agreement with the values reported previously for the Pt-catalyzed oxidation of H₂ by O₂ and N₂O. The effects of added H₂, D₂ and O₂ on HO formation were also investigated and the observed data can be satisfactorily accounted for by a mechanism which involves chemically adsorbed H, O and HO species. A similar but brief experiment was carried out for the catalytic decomposition of NH₃ over the same catalyst. However, no NH or NH₂ radicals were detected in gaseous decomposition products up to 1100 K.