HYDROXYL RADICAL DESORPTION FROM POLYCRYSTALLINE PALLADIUM - EVIDENCE FOR A SURFACE PHASE-TRANSITION

The desorption of hydroxyl radicals, as detected by laser-induced fluorescence, during the reaction of H-2 and O2 over polycrystalline palladium has been studied as a function of temperature and O2/H-2 ratio. The apparent activation energy for OH desorption shows a discontinuity at almost-equal-to 1210-1220 K for O2/H-2 ratios > 4. This abrupt change in E(a) is taken as evidence that a phase transition, possibly involving the decomposition of a two-dimensional PdO phase, has occurred in the surface region.