INTERACTION OF CO2 AND CO WITH NON-POLAR ZINC-OXIDE SURFACES

The interaction of CO2 and CO with ZnO (101&#x0304;0) was studied by means of changes in the electrical conductivity Δσ and surface potential Δφ{symbol}s, thermal desorption, AES, EELS and LEED. For CO2, completely reversible adsorption and desorption at 298 ≤ T ≤ 473 K is found. Initial sticking coefficients are close to unity (S0 = 0.6 at 300 K). Chemisorption equilibria are described by Freundlich type isotherms with isosteric heats of adsorption between 140 and 70 kJ/mol for 10-3 < θ < 10-2. Decreasing Δσ and increasing Δφ{symbol}s on CO2 exposure is tentatively discussed on the basis of a charge transfer model in terms of small fractions of ionized molecules or partial negative charges δ and in terms of dipole moments μad of the adsorption complex. Small values δ ≈ 3.5 × 10-3 attributed to the chemisorption complex indicate predominant covalent bonding. Extremely large μad values (μad > 10 D for θ < 10-2) can not simply be explained by localized surface complexes. Chemisorption induced changes in the surface atom reconstruction are suggested to explain μad values and corresponding pronounced variations in LEED I/U curves. For CO, irreversible interaction is found even at room temperature leading to simultaneous increase of Δσ and Δφ{symbol}s. Subsequent TDS yields CO2. The effects are interpreted in terms of formation of chemisorbed CO2 and oxygen vacancies at the surface. © 1979.