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

The chemisorption properties of NO on the oxidized TiO2(110) surface have b een investigated using both experimental and theoretical methods. The resul ts of temperature-programmed desorption measurements indicate that for NO e xposures less than 1.1 x 10(14) molecules/cm(2) NO adsorbs weakly and desor bs at similar to 127 K. The thermal desorption kinetics are almost independ ent of the coverage of adsorbed NO molecules. The experimental activation e nergy 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. T he weak interaction between the NO molecule and the TiO2(110) surface has b een also revealed from first-principles calculations based on density funct ional theory and the pseudopotential method in which NO molecules are adsor bed at the in-plane Ti cation sites. These calculations employ slab geometr y and periodic boundary conditions with full relaxation of all atomic posit ions. 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 confi guration. At half coverage the adsorption energies of 10.52 and 5.75 kcal/m ol have been determined for Ti-NO and Ti-ON binding configurations, respect ively, in good agreement with the experimental results. At full coverage th e adsorption energies were found to decrease by about 1.50-1.75 kcal/mol re lative to the half-coverage case. The lack of large chemical effects indica tes that the adsorption takes place through a predominantly physisorption m echanism. Besides the independent adsorption configurations of NO molecules , in the case of full coverage the formation of the N2O2 species was also o bserved theoretically. Among several different N2O2 isomers analyzed, the m ost 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 theoret ically investigated different adsorption configurations of N2O and NO2 spec ies on the TiO2(110) surface. These studies indicate that for N2O the most favorable adsorption configuration corresponds to a vertical Ti-N-N-O orien tation 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 determine d theoretically.