title: Reactions of ammonia on stoichiometric and reduced TiO(2)(001) single crystal surfaces.
abstract: The reaction of NH(3) on the surface of the 011-faceted structure of the TiO(2)(001) single crystal is studied and compared to that on the O-defected surface. Temperature-programmed desorption (TPD) conducted after NH(3) 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 approximately 52 kJ/mol and approximately 10(7) /s at saturation coverage. The decrease is due to a repulsive interaction of adsorbed NH(3) 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 NH(3) peak change on the reduced (Ar(+)-sputtered) surfaces. The desorption peak at 340 K is considerably attenuated on mildly reduced surfaces (TiO( approximately )(1.9)) and has totally disappeared on the heavily reduced surfaces (TiO(1.6)(-)(1.7)), where the main desorption peak is found at 440 K. This 440-K desorption is most likely due to NH(x) + H recombination resulting from ammonia dissociation upon adsorption on Ti atoms in low oxidation states.
authors.affiliation: Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand.
authors.forename: J N
authors.initials: JN
authors.lastname: Wilson
authors.forename: H
authors.initials: H
authors.lastname: Idriss
mesh_terms: D000641:Ammonia / Q000737:chemistry*; D003460:Crystallization; D010084:Oxidation-Reduction; D010316:Particle Size; D013237:Stereoisomerism; D013499:Surface Properties; D013696:Temperature; D013997:Time Factors; D014025:Titanium / Q000737:chemistry*
chemical_list: C009495:titanium dioxide; D000641:Ammonia; D014025:Titanium