Reactions of ammonia on stoichiometric and reduced TiO2(001) single crystal surfaces

The reaction of NH3 on the surface of the {011}-faceted structure of the TiO2(001) single crystal is studied and compared to that on the O-defected surface. Temperature-programmed desorption (TPD) conducted after NH3 adsorption at 300 K shows only molecular desorption at 340 K. Modeling of TPD signals as a function of surface coverage indicated that the activation energy, E-d, and pre-exponential factor, v(eff), decrease with increasing coverage. Near zero surface coverage, E-d was found to be equal to 92 kJ/mol and v(eff) to be close to 10(13)/s. Both parameters decreased to similar to52 kJ/mol and similar to10(7)/s at saturation coverage. The decrease is due to a repulsive interaction of adsorbed NH3 molecules on the surface. Computing of the TPD results show that saturation is obtained at 1/2 monolayer coverage (referred to Ti atoms). Both the amount and shape of NH3 peak change on the reduced (Ar+-sputtered) surfaces. The desorption peak at 340 K is considerably attenuated on mildly reduced surfaces (TiOsimilar to1.9) and has totally disappeared on the heavily reduced surfaces (TiO1.6-1.7), where the main desorption peak is found at 440 K. This 440-K desorption is most likely due to NHx + H recombination resulting from ammonia dissociation upon adsorption on Ti atoms in low oxidation states.