A LASER-INDUCED FLUORESCENCE STUDY OF OH DESORPTION FROM PT IN H2O/O2 AND H2O/H2 MIXTURES

The desorption of OH from high-temperature-annealed polycrystalline Pt in H2O/O2, H2O/H-2 (D2), and H2O/O2 + H-2 (D2) mixtures has been investigated using the laser-induced fluorescence (LIF) technique. The measurements were done in the pressure range 1-1000 mTorr and temperature interval 900-1300 K. The experiments yield new interesting information about water decomposition on Pt, unimolecularly and in the presence of O2 and/or H-2, and provide complementary results to earlier studies of water formation from H-2 + O2. There is a small but significant OH desorption even in pure H2O. In H2O/O2 mixtures the OH desorption rate has a maximum at 22-30% O2 content. The absolute desorption rate increases nearly linearly with total pressure. The maximum yield occurs at successively lower relative O2 concentration, the higher the pressure. In H2O/H-2 Mixtures the OH desorption is strongly suppressed due to reaction of OH with adsorbed hydrogen atoms. H2O/D2 MiXtures yield OD and OH desorption. The experimental results are well described by a previously developed kinetic model for the H-2 + 1/2O2 --> H2O reaction. Approximate analytical expressions are derived for the kinetics and compared with full numerical solutions. Interestingly the dominant path for H2O decomposition seems to be the unimolecular decomposition H2O(a) reversible H(a) + OH(a) rather than the bimolecular reaction H2O(a) + O(a) reversible 2OH(a) (a = adsorbed). The promoting effect of oxygen is kinetic, rather than mechanistic; i.e., the presence of O atoms moves the surface equilibrium to higher OH concentration, rather than providing a new reaction path via direct H2O(a) + O(a) interaction. An evaluation procedure is briefly outlined on how to extract the true OH desorption rate from the measured LIF intensities at pressures where collisional gas-phase quenching and pumping speed effects are important.