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 sieve (ETS-10), confirm that the peculiar optical properties of the solid are associated with the presence of -OTiOTiO- linear chains behaving as quantum wires. The optical [in the UV-Vis (ultraviolet-range)] and the magnetic [(ESR) 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 total (Ti/Na similar to 4) restoration of the chain stoichiometry and to the predominant formation of sodium oxide. The formation of a minor fraction of superoxide negative ions whose magnetic properties are revealed by ESR spectroscopy is also observed. Total restoration of the optical properties of the original samples is always obtained when the oxygen adsorption is made at 473 K. The sample keeps its structural integrity during the reduction and successive oxidation process. (C) 2000 American Institute of Physics. [S0021-9606(00)70506-X].