Reactions involving electron transfer at semiconductor surfaces. Part 10.—Oxygen isotope equilibration on non-illuminated zinc oxides

Abstract

Rapid isotopic equilibration, ¹⁶O₂+¹⁸O₂⇌ 2¹⁶O¹⁸O, was observed upon contacting an equimolar mixture of (¹⁶O₂+¹⁸O₂) at 295 or 77 K with oxygen-deficient surfaces of pure and doped zinc oxides from which light was excluded at all stages. Kinetic expressions for opposing second-order reactions accurately described variations in mole fraction of ¹⁶O₂, ¹⁶O¹⁸O and ¹⁸O₂ in the gas phase during the approach to full isotopic equilibration at 295 or 77 K. Rate constants thereby derived for this R₀-type exchange did not correlate with reported concentrations of conduction-band electrons for the zinc oxides, indicating that the rate-determining process for exchange was not collective-electron type charge transfer at the oxygen-deficient surfaces in the absence of illumination. Surfaces could be rendered inactive by extensive preoxidation in ¹⁶O₂ at 650 K in the dark, but heating for 2 h periods in the dark under continuous evacuation restored activity to a progressively increasing extent at temperatures of 400–650 K. Preadsorption of H₂O, H₂ or (CH₃)₂CHOH at 295 K strongly inhibited activity. An explanation of the observed results is developed based on: (i) dissociative chemisorption at e^–|Zn_cus sites on the oxygen-deficient surfaces; (ii) formation of triatomic oxygen surface intermediates; (iii) equilibration by reversible second-order reactions of O₂ with these triatomic oxygen intermediates.