Reaction of NH3 with titania:: N-doping of the oxide and TiN formation

The thermal nitridation of titania powder (anatase and P25) by reaction with ammonia has been investigated by in-situ reaction studies and ex-situ characterizations using X-ray photoelectron spectroscopy (XPS), nearedge X-ray absorption fine structure (NEXAFS), and X-ray diffraction (XRD). At temperatures below 750 degrees C, the formation of TiN from the reaction of NH3 with the titania sample was negligible. Above 750 degrees C, in-situ reaction studies using XRD revealed a smooth phase transition from anatase or P25 to cubic TiN. On the basis of comprehensive characterizations, the crystalline structure of nitrogen-doped TiO2 is in general similar to that of TiO2. Incorporation of nitrogen into the interstitial sites of TiO2 anatase is supported by Rietveld refinement of XRD data. Interstitial nitrogen may form N-2-like species bound to either oxygen vacancies or the cavity-framework atoms with various degrees of bond strength. N 1s XPS and N K-edge NEXAFS spectra support the idea that nitrogen present in titania as atoms may combine to form N-2 molecules evolving into the gas phase upon heating to elevated temperatures, a behavior that agrees with the results of DFT calculations which show that nitrogen embedded in TiO2 is unstable energetically and spontaneously forms trapped or gaseous N-2.