Reduction kinetics of zeolite-hosted mono- and polynuclear titanium oxide species followed by UV/Vis diffuse reflectance spectroscopy: Influence of location and coordination

The kinetics of the reversible reduction of faujasite-hosted mononuclear titanium oxide species prepared by reaction of dehydrated zeolites with TiCl4 at different temperatures are (i) evaluated recording the intensity of an absorption band of Ti(III) species at 16200 cm(-1) by UV/vis diffuse reflection spectroscopy and (ii) compared to those of faujasite-hosted polynuclear titanium oxide species, titanosilicates, and bulk anatase. The reduction kinetics of mononuclear Ti(IV)O-x species could be fitted assuming the existence of three types of Ti sites, one being located at the surface of the zeolite NaY crystals and two inside the void structure. The distributions of Ti, i.e., the ratios Ti-surface/Ti-bulk, resulting from this fit agree with those obtained from photoelectron spectra. The reduction rates for different Ti(IV)O, species increase with increasing excitation energy of the O-->Ti charge transfer, i.e., increasing deviation from octahedral symmetry, and decrease with increasing nuclearity of titanium oxide clusters. The response time of the mononuclear Ti(IV)O, species toward reduction to Ti(III) is shortened by a factor of 60 in comparison to bulk TiO2. Titanium tetrahedrally coordinated in framework positions in TS-1 cannot be reduced.