Stoichiometric and sodium-doped titanium silicate molecular sieve containing atomically defined -OTiOTiO- chains: Quantum ab initio calculations, spectroscopic properties, and reactivity

Ab initio calculations on a linear -OTiOTiO- chain embedded in an envelope of (SiO4) tetrahedra, mimicking the structure of Na2TiSi5O13 molecular siev e (ETS-10), confirm that the peculiar optical properties of the solid are a ssociated with the presence of -OTiOTiO- linear chains behaving as quantum wires. The optical [in the UV-Vis (ultraviolet-range)] and the magnetic [(E SR) electron spin resonance] properties of these chains can be modified by adsorbing Na vapors. The sodium atoms diffusing into the channels undergo a ionization process with formation of Na+ (localized in the main channels) and Ti3+ (in the -OTiOTiO- chain, which so becomes a nonstoichiometric wire ) characterized by Ti/Na ratios in the 2-4 range. Successive adsorption of oxygen at room temperature leads to the partial (Ti/Na similar to 2) or tot al (Ti/Na similar to 4) restoration of the chain stoichiometry and to the p redominant formation of sodium oxide. The formation of a minor fraction of superoxide negative ions whose magnetic properties are revealed by ESR spec troscopy is also observed. Total restoration of the optical properties of t he original samples is always obtained when the oxygen adsorption is made a t 473 K. The sample keeps its structural integrity during the reduction and successive oxidation process. (C) 2000 American Institute of Physics. [S00 21-9606(00)70506-X].