Surface reactivity and spectroscopy of alkaline-earth-oxide powders. Part 3.—1H/2H and 18O/16O exchange on specpure CaO

Abstract

Observations on the activity of calcium oxide powder for heterophase hydrogen-isotope exchange processes of the type: ²H₂(g)+¹HO^–(s)⇌²H¹H(g)+²HO^–(s) and/or ²H₂(g)+ 2 ¹HO^–(s)⇌¹H₂(g)+ 2 ²HO^–(s) after outgassing in vacuo at progressively higher temperatures, T_og, provide a sensitive monitor for variations in the number of surface hydroxyls capable of H/D exchange using 573 or 673 K as the fixed reaction temperature, T_rxn.

Observations on activity for heterophase oxygen-isotope exchange with T_rxn= 623 K reveal a strong similarity in dependence upon T_og to that for heterophase hydrogen exchange. This similarity indicates that ¹⁸O₂(g)+¹⁶OH^–(s)⇌¹⁶O¹⁸O(g)+¹⁸OH^–(s) rather than ¹⁸O₂(g)+¹⁶O^2–(s)⇌¹⁶O¹⁸O(g)+¹⁸O^2–(s) is the dominant pathway to heterophase oxygen-isotope exchange at T_rxn= 623 K after T_og in the range 573–973 K. Information on the extent to which surface O^2–(s) or OH^–(s) contributes to ¹⁶O/¹⁸O exchange at T_rxn= 698 or 773 K is sought from a sequence of measurements at each T_rxnafter T_og in the range 623–1123 K.

Delayed onset of the homophase oxygen-isotope equilibration process ¹⁸O₂(g)+¹⁶O₂(g)⇌2¹⁶O¹⁸O(g) until after the CaO powders had experienced T_og 973 K is observed and attributed to a need for most surface hydroxyls to be removed before defect sites required for this exchange become available.