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

Abstract Changes effected in the isotopic composition of monolayer equivalent amounts of 15 N 18 O or equimolar ( 18 O 2 + 16 O 2 ) probe gases by contact with pure or rhodia-promoted CeO 2 and ZrO 2 are reported and analysed. A prerequisite for efficient and selective promotion of the R 0 -type homophase exchange process, 18 O 2(g) + 16 O 2(g) →2 18 O 16 O (g) at 295 K over preoxidised samples was dispersal thereon of ≥2 wt% rhodia originated from chloride-free precursors. Temperatures for onset of heterophase isotope exchange over preoxidised CeO 2 or ZrO 2 under T-ramp were 790 or 873 K. However, dispersion of 0.5, 2.0 or 4.0 at.% rhodia thereon resulted in onset of 18 O 2(g) +2 16 O 2− (s) → 16 O 2(g) +2 18 O 2− (s) 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 15 N 18 O replaced dioxygen as probe gas over preoxidised CeO 2 or RhO x //CeO 2 . Observations under T-ramp demonstrated the onset at ca. 450 K of a decrease in 15 N 18 O mirrored by onset of an increase in 15 N 16 O . Continuation of both processes at similar rates brought about an equimolar ( 15 N 16 O+ 15 N 18 O) 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 15 N 18 O (ads) + 16 O 2− (s) → 15 N 16 O (ads) + 18 O 2− (s) ; or 15 N 18 O (ads) + 16 OH − (s) → 15 N 16 O (ads) + 18 OH − (s) over preoxidised CeO 2 or 0.5% RhO x //CeO 2 . Modifications of their activity and selectivity in 15 N 18 O conversions following reductions in H 2 at 423 or 823 K are described and interpreted.