O –2 formation on magnesium oxide powders containing preadsorbed hydrogen

Abstract

Temperature-programmed desorption (t.p.d.) and e.s.r. spectroscopy have been used to study the mechanism of O^–₂ formation following the admission of O₂ onto the surface of MgO powders which had previously been outgassed at high temperatures (mainly at 1123 K) and then exposed to hydrogen at 308 K. Four types of preadsorbed hydrogen have been previously identified and oxygen has been found to interact with all of them. For a given amount of preadsorbed hydrogen twice as much as oxygen can be adsorbed (as O^–₂). The presence of at least four varieties of O^–₂ has been identified by e.s.r. spectra and five desorption peaks of O₂ have been found in the t.p.d. spectra following O^–₂ formation. However, no specific relation has been found between the types of preadsorbed hydrogen and the O^–₂ species resulting from subsequent oxygen adsorption.

O^–₂ formation can be explained on the basis of the presence of heterolytically dissociated hydrogen; a first O^–₂ species is formed by electron transfer between H^– and O₂ and a resultant H atom forms a second O^–₂. This mechanism is also consistent with energy considerations.

Thermal decomposition of O^–₂in vacuo proceeds in three steps: (i) O^–₂ is transformed into O^2–₂ with the simultaneous desorption of O₂, forming four t.p.d. peaks in the temperature range 350–790 K; (ii) by further heating O^2–₂ is decomposed into O^2– and O₂, forming a fifth t.p.d. peak at 790–1000 K; (iii) preadsorbed hydrogen is desorbed as H₂O above 900 K, with the O atom thought to originate from O^–₂ from material-balance considerations.