First Principles Calculations of the Adsorption Properties of CO and NO on the Defective TiO2(110) Surface

First-principles calculations based on spin-polarized density functional theory (DFT) and the generalized gradient approximation (GGA) have been used to study the adsorption of CO and NO molecules on the rutile (TiO2) (110) surface in the presence of oxygen vacancy sites. The calculations employ slab geometry and periodic boundary conditions with full relaxation of all atomic positions. We have identified several possible adsorption configurations at both Ti five-coordinated (Ti(5f)) and four-coordinated (Ti(4f)) sites, finding that adsorption binding energies are dependent on the defect density. Among these configurations the most stable have been found at the Ti(4f) sites in the case of the surface with missing bridging-oxygen rows. In this case both CO and NO molecules can bind either on-top of Ti(4f) atoms or in vertical or tilted bridge configurations to neighbor Ti(4f) sites. The highest binding energies we have determined are 36 kcal/mol for CO and 87.15 kcal/mol for NO, respectively, and correspond to tilted bridge molecular configurations along the [001] direction. The large increase of the binding energies on the defective surface relative to the full oxidized surface indicates that the adsorption on vacancy defect sites takes place through a predominantly chemisorption mechanism. Additional calculations performed for N2O and N2O2 molecules indicate that on the defective surface the adsorption at Ti(4f) is also preferred with maximum adsorption energies of 51.2 and 126.7 kcal/mol.