Water-promoted oxygen isotope exchange on a Pt4+/MgO catalyst

Abstract

The effect of H2O vapor on the oxygen isotope exchange between 18O2 and surface lattice oxygen 16O* on a Pt4+/MgO catalyst that was prepared by using Pt(acac)2 as a precursor has been systematically investigated in the temperature range 423–573 K for the first time. The oxygen 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 P/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 exchange rate by about a factor of ten. Both the single oxygen exchange, 18O2 + 16O* → 18O16O + 18O*, and the multiple oxygen exchange, 18O2 + 216O* → 16O2 + 218O*, were promoted by H2O. The multiple oxygen exchange reaction proceeded only in the presence of H2O. The H218O molecules readily dissociate to form hydroxyl groups on Pt4+/MgO and exchange with surface lattice 16O* atoms to form H216O molecules. In the presence of water vapor, surface lattice oxygen atoms and hydroxyl groups may be active oxygen species for oxygen exchange with gaseous oxygen. The adsorbed water (hydroxyl groups) activates the surface lattice oxygen atoms of Pt4+/MgO, where about four lattice oxygen atoms per Pt4+ ion can be exchanged with gaseous oxygen. The H2O-promoted oxygen isotope exchange may proceed on surface defects on Pt4+/MgO.