Reactivity of the surface paramagnetic species on polycrystalline ZnO and ZnO supported ruthenium with hydrogen and carbon monoxide: an electron paramagnetic resonance investigation
The ZnO paramagnetic centres, previously identified as interstitial Zn+ and ionized oxygen vacancies, V+0, have been checked for their reaction towards oxygen, hydrogen and carbon monoxide. Contact at room temperature reduces oxygen to O2– or O– which fix onto Zn+ centres, quenching their resonance lines. Hydrogen and carbon monoxide both remove the ionosorbed reduced oxygen from Zn+, inducing an apparent increase of Zn+ lines. Thus the variations in the intensity of these resonances parallel the changes in height of the Schottky barrier which are related to different amounts of oxygen ionosorbed at the surface. Contact at high temperatures (T > 473 K) reduces oxygen to diamagnetic lattice oxide, while reducing gases cause an increase of V+0 centres.
If ruthenium is supported on ZnO contact with H2 produces new V+0 centres even at room temperature while the production of new vacancies by CO is promoted by ruthenium only at high temperature. The maximum number of new donor centres is obtained under reducing gas atmosphere at ca. 673 K, on Ru-supported ZnO samples. The process can be followed by EPR spectroscopy.