NH3 and O2 Interaction with Tetrahedral Ti3+ Ions Isomorphously Substituted in the Framework of TiAlPO-5. A Combined Pulse EPR, Pulse ENDOR, UV-Vis and FT-IR Study

Continuous Wave (CW), pulse Electron Paramagnetic Resonance (EPR) and pulse Electron Nuclear Double Resonance (ENDOR) spectroscopies, in conjunction with UV-Vis and Infra Red (IR) spectroscopies are used to investigate the chemical reactivity of tetrahedrally coordinated Ti3+ ions isomorphously substituted in the framework of AlPO-5 towards NH3 and O2. The coordination of ammonia to Ti3+ centres is followed in detail by complementary vibrational and electron magnetic resonance techniques. In particular HYSCORE spectra allow identifying the coordination of two ammonia molecules to Ti3+ centres resolving the full hyperfine and quadrupole 14N coupling tensors. The reactivity of the reduced TiAlPO sample towards molecular oxygen is detailed by means of CW-EPR and pulse ENDOR spectroscopy. 17O2 is employed, allowing to establish the formation of a “side-on” O2--Ti4+ electrostatic complex. Pulse ENDOR spectra provide detailed information on the local environment of the formed superoxide radical anion which acts as a paramagnetic probe, providing evidence for Ti-O-Ti oligomeric species