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–|Zncus 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.