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

The effect of H(2)O vapor on the oxygen isotope exchange between (18)O(2) and surface lattice oxygen (16)O* on a Pt(4+)/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 Pt(4+)/MgO catalyst with isolated Pt(4+) 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 H(2)O vapor increased the oxygen exchange rate by about a factor of ten. Both the single oxygen exchange, (18)O(2) + (16)O* --> (18)O (16)O + (18)O*, and the multiple oxygen exchange, (18)O(2) + 2 (16)O* --> (16)O(2) + 2 (18)O*, were promoted by H(2)O. The multiple oxygen exchange reaction proceeded only in the presence of H(2)O. The H(2)(18)O molecules readily dissociate to form hydroxyl groups on Pt(4+)/MgO and exchange with surface lattice (16)O* atoms to form H(2)(16)O 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 Pt(4+)/MgO, where about four lattice oxygen atoms per Pt(4+) ion can be exchanged with gaseous oxygen. The H(2)O-promoted oxygen isotope exchange may proceed on surface defects on Pt(4+)/MgO.