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

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

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