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 (H-2, CO, O-2) 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 rim, 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 Ti-IV 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 W 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 Ti-III. The zeolite matrix ensures good stability of the highly dispersed Ti-IV oxide species. The response time of hosted Ti-IV oxide species for the optical registration of the presence of H-2 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 Ti-IV 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.