Oxygen-18 isotope exchange and equilibration processes over preoxidised and promoted ceria. Part I. 15N18O(g) or (18O2+16O2(g))

Changes effected in the isotopic composition of monolayer equivalent amounts of (NO)-N-15-O-18 Or equimolar (O-18(2) + O-16(2)) probe gases by contact with pure or rhodia-promoted CeO2 and ZrO2 are reported and analysed. A prerequisite for efficient and selective promotion of the Re-type homophase exchange process, O-18(2(g)) + O-16(2(g)) --> 2(18)O(16)O((g)) at 295 K over preoxidised samples was dispersal thereon of greater than or equal to 2 wt% rhodia originated from chloride-free precursors. Temperatures for onset of heterophase isotope exchange over preoxidised CeO2 or ZrO2 under Tramp were 790 or 873 K. However, dispersion of 0.5, 2.0 or 4.0 at.% rhodia thereon resulted in onset of O-18(2(g)) + 2(16)O((s))(2-) --> O-16(2(g)) + 2(18)O((s))(2-) heterophase exchange at progressively lower temperatures. Nature of the sites/mechanisms via which dispersed rhodia promotes those R-0 or R-2 changes in the dioxygen probe gases are considered. Also presented are results from analagous experiments in which (NO)-N-15-O-18 replaced dioxygen as probe gas over preoxidised CeO2 or RhOx//CeO2. Observations under Tramp demonstrated the onset at ca. 450 K of a decrease in (NO)-N-15-O-18 mirrored by onset of an increase in (NO)-N-15-O-16. Continuation of both processes at similar rates brought about an equimolar ((NO)-N-15-O-16 + (NO)-N-15-O-18) mixture at 700 K. The 1:1 stoichiometry of that conversion would be consistent with either of the R-1-type heterophase oxygen isotope exchange processes (NO(ads)-N-15-O-18) + O-16((s))(2-) --> (NO(ads)-N-15-O-16) + O-18((s))(2-); or (NO(ads)-N-15-O-18) + (OH(s)-O-16)(-) --> (NO(ads)-N-15-O-16) + (OH(s)-O-18)(-) over preoxidised CeO2 or 0.5% RhOx//CeO2. Modifications of their activity and selectivity in (NO)-N-15-O-18 conversions following reductions in H-2 at 423 or 823 K are described and interpreted. (C) 1999 Elsevier Science B.V. All rights reserved.