Studies of Gas Adsorption on ZnO Using ESR, FTIR Spectroscopy, and MHE (Microwave Hall Effect) Measurements

This paper describes the application of a new technique-Microwave Hall Effect (MHE) measurements-to measure electron mobilities and to determine the effect of adsorption on electron densities of powders. Conduction electron densities calculated from microwave measurements of both mobilities and conductivities, as well as ESR spectroscopy and chemisorption measurements, have been applied to characterize high-surface-area ZnO (up to 30 m2/g) samples before and after exposure to O2, CO2, CO, and H2. Evacuation at 673 K removed lattice oxygen to produce paramagnetic lattice vacancies, (V+0)− and an approximately equal number of conduction electrons, e−. Adsorption of O2 decreased both species and created an O−2 species, but a larger quantity of O2 also adsorbed and appeared to reoxidize the surface and remove neutral V0 sites. Reversible CO adsorption occurred which had little effect on (V+0)− and e−; however, CO adsorption on an oxygen-precovered surface reestablished (V+0)− and e− species and decreased the O−2 species. After this step, very weak carbonate bands were observed using DRIFTS. CO2 adsorption on ZnO after evacuation at 673 K decreased the e− and (V+0)− concentrations equally, while much smaller decreases of each species occurred after CO2 adsorption on an oxygen-precovered surface, although O−2 species were unaffected. This showed that CO2 does not react with adsorbed O−2 species, but can interact with both (V+0)− and conduction electrons at sites not covered by molecular oxygen.