Titanium Oxide Species in Molecular Sieves:  Materials for the Optical Sensing of Reductive Gas Atmospheres

The reversible redox behavior of intrazeolitic titanium dioxide clusters in reducing and oxidizing atmospheres (H2, CO, O2) is studied by in situ diffuse reflectance UV/vis spectroscopy (UV/vis DRS). Titanium oxide species, mononuclearly dispersed or as clusters with a particle size up to 2 nm, have been stabilized in the pores of molecular sieves by chemical vapor deposition (cvd) or ion exchange from aqueous solutions and subsequent calcination. The structure of different TiIV oxide species and their location with respect to the zeolite matrix, that is, inside the pore system or on the external surface, has been deduced from (i) the blue shifts of absorption edges in diffuse reflectance UV/vis spectra, (ii) the interpretation of XANES and EXAFS spectra giving coordination numbers and bond lengths, (iii) XPS measurements, and (iv) the reduction behavior to TiIII. The zeolite matrix ensures good stability of the highly dispersed TiIV oxide species. The response time of hosted TiIV oxide species for the optical registration of the presence of H2 in surrounding gas atmospheres decreases with their decreasing cluster size and is lowest for mononuclear species with a highly distorted oxygen coordination sphere. In comparison to bulk TiO2 the response times of TiIV oxide/zeolite composites is shortened by a factor of about 10 and are comparable to those values found for solid-state electrolyte sensors. Also, changes in the ratio of CO:air mixtures can easily be monitored.