Adsorption of NO on the TiO2(110) surface:: An experimental and theoretical study

The chemisorption properties of NO on the oxidized TiO2(110) surface have been investigated using both experimental and theoretical methods. The results of temperature-programmed desorption measurements indicate that for NO exposures less than 1.1 x 10(14) molecules/cm(2) NO adsorbs weakly and desorbs at similar to 127 K. The thermal desorption kinetics are almost independent of the coverage of adsorbed NO molecules. The experimental activation energy for NO desorption from the nondefective TiO2(110) surface is 8.4 kcal/mol in the limit of zero coverage. Above a critical NO surface exposure of 5.5 x 10(14) molecules/cm(2), partial conversion of NO to N2O is observed yielding N2O desorption processes at similar to 169 and similar to 250 K. The weak interaction between the NO molecule and the TiO2(110) surface has been also revealed from first-principles calculations based on density functional theory and the pseudopotential method in which NO molecules are adsorbed at the in-plane Ti cation sites. These calculations employ slab geometry and periodic boundary conditions with full relaxation of all atomic positions. As shown by the full relaxation of the atomic system, the most stable configuration of the NO molecule on the TiO2(110) surface is tilted. Then is a clear preference for the Ti-NO orientation compared to the Ti-ON configuration. At half coverage the adsorption energies of 10.52 and 5.75 kcal/mol have been determined for Ti-NO and Ti-ON binding configurations, respectively, in good agreement with the experimental results. At full coverage the adsorption energies were found to decrease by about 1.50-1.75 kcal/mol relative to the half-coverage case. The lack of large chemical effects indicates that the adsorption takes place through a predominantly physisorption mechanism. Besides the independent adsorption configurations of NO molecules, in the case of full coverage the formation of the N2O2 species was also observed theoretically. Among several different N2O2 isomers analyzed, the most stable has a cis-ONNO configuration with a binding energy of 13.6 kcal/mol in the singlet state, In addition to the bonding of NO, we also theoretically investigated different adsorption configurations of N2O and NO2 species on the TiO2(110) surface. These studies indicate that for N2O the most favorable adsorption configuration corresponds to a vertical Ti-N-N-O orientation with a binding energy of 7.73 kcal/mol at half coverage. In the case of the NO2 molecule, a small binding energy of 2.11 kcal/mol was determined theoretically.