Adsorption of NO on the TiO₂(110) Surface:  An Experimental and Theoretical Study

Abstract

The chemisorption properties of NO on the oxidized TiO 2 (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 × 10 14 molecules/cm 2 NO adsorbs weakly and desorbs at ∼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 TiO 2 (110) surface is 8.4 kcal/mol in the limit of zero coverage. Above a critical NO surface exposure of 5.5 × 10 14 molecules/cm 2, partial conversion of NO to N 2 O is observed yielding N 2 O desorption processes at ∼169 and ∼250 K. The weak interaction between the NO molecule and the TiO 2 (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 TiO 2 (110) surface is tilted. There 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 N 2 O 2 species was also observed theoretically. Among several different N 2 O 2 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 N 2 O and NO 2 species on the TiO 2 (110) surface. These studies indicate that for N 2 O 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 NO 2 molecule, a small binding energy of 2.11 kcal/mol was determined theoretically.