The effect of oxygen vacancies on the binding interactions of NH3 with rutile TiO2(110)-1 × 1.

A series of NH(3) temperature-programmed desorption (TPD) spectra were taken after dosing NH(3) at 70 K on rutile TiO(2)(110)-1 × 1 surfaces with oxygen vacancy (V(O)) concentrations of ~0% (p-TiO(2)) and 5% (r-TiO(2)), respectively, to study the effect of V(O)s on the desorption energy of NH(3) as a function of coverage, θ. Our results show that in the zero coverage limit, the desorption energy of NH(3) on r-TiO(2) is 115 kJ mol(-1), which is 10 kJ mol(-1) less than that on p-TiO(2). The desorption energy from the Ti(4+) sites decreases with increasing θ due to repulsive NH(3)-NH(3) interactions and approaches ~55 kJ mol(-1) upon the saturation of Ti(4+) sites (θ = 1 monolayer, ML) on both p- and r-TiO(2). The absolute monolayer saturation coverage is determined to be about 10% smaller on r-TiO(2) than that on p-TiO(2). Additionally, the trailing edges of the NH(3) TPD spectra on the hydroxylated TiO(2)(110) (h-TiO(2)) appear to be the same as that on r-TiO(2) while those on oxidized TiO(2)(110) (o-TiO(2)) shift to higher temperatures. We present a detailed analysis of the results and reconcile the observed differences based on the repulsive adsorbate-adsorbate dipole interactions between neighboring NH(3) molecules and the surface charge associated with the presence of V(O)s.