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

Abstract

A series of NH₃ temperature-programmed desorption (TPD) spectra were taken after dosing NH₃ at 70 K on rutile TiO₂(110)-1 × 1 surfaces with oxygen vacancy (V_O) concentrations of ∼0% (p-TiO₂) and 5% (r-TiO₂), respectively, to study the effect of V_Os on the desorption energy of NH₃ as a function of coverage, θ. Our results show that in the zero coverage limit, the desorption energy of NH₃ on r-TiO₂ is 115 kJ mol⁻¹, which is 10 kJ mol⁻¹ less than that on p-TiO₂. The desorption energy from the Ti⁴⁺ sites decreases with increasing θ due to repulsive NH₃–NH₃ interactions and approaches ∼55 kJ mol⁻¹ upon the saturation of Ti⁴⁺ sites (θ = 1 monolayer, ML) on both p- and r-TiO₂. The absolute monolayer saturation coverage is determined to be about 10% smaller on r-TiO₂ than that on p-TiO₂. Additionally, the trailing edges of the NH₃ TPD spectra on the hydroxylated TiO₂(110) (h-TiO₂) appear to be the same as that on r-TiO₂ while those on oxidized TiO₂(110) (o-TiO₂) 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 NH₃ molecules and the surface charge associated with the presence of V_Os.