WATER-PROMOTED OXYGEN-ISOTOPE EXCHANGE ON A PT4+ MGO CATALYST/

The effect of H2O vapor on the oxygen isotope exchange between O-18(2) and surface lattice oxygen O-16 on a Pt4+/MgO catalyst that was prep ared by using Pt(acac)(2) as a precursor has been systematically inves tigated in the temperature range 423-573 K for the first time. The oxy gen isotope exchange on the Pt4+/MgO catalyst with isolated Pt4+ ions embedded at the top-layer lattice of MgO proceeded, whereas the oxygen isotope exchange on a conventional Pt-0/MgO catalyst with metallic Pt particles (1.9 nm in size) did not proceed under similar conditions. It was found that the presence of H2O vapor increased the oxygen excha nge rate by about a factor of ten. Both the single oxygen exchange, O- 18(2) + O-16 --> O-18 O-16 + O-18*, and the multiple oxygen exchange, O-18(2) + 2 O-16 --> O-16(2) + 2 O-18*, were promoted by H2O. The mu ltiple oxygen exchange reaction proceeded only in the presence of H2O. The (H2O)-O-18 molecules readily dissociate to form hydroxyl groups o n Pt4+/MgO and exchange with surface lattice O-16 atoms to form (H2O) -O-16 molecules. In the presence of water vapor, surface lattice oxyge n atoms and hydroxyl groups may be active oxygen species for oxygen ex change with gaseous oxygen. The adsorbed water (hydroxyl groups) activ ates the surface lattice oxygen atoms of Pt4+/MgO, where about four la ttice oxygen atoms per Pt4+ ion can be exchanged with gaseous oxygen. The H2O-promoted oxygen isotope exchange may proceed on surface defect s on Pt4+/MgO.