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.