Band edges positions prediction of the of Ag nanocluster-decorated titania surfaces and their relationship to NO and NO2 interaction from first-principles calculations

Abstract

Metal nanoclusters deposited on oxides have been widely used in photocatalysis playing an important role in the design of model catalysts with applications in heterogeneous catalysis. In particular, we are interested in the potential activity of these cluster-supported systems for the removal of nitrogen oxides either by possible catalytic reduction and/or by their adsorption. In this work, using first-principles methods, we evaluate the main characteristics of Ag_n (n = 1-4) nanoclusters isolated and deposited on anatase TiO₂(101) and rutile TiO₂(110) surfaces. Our results indicate that they are preferably adsorbed on rutile surface. The different formation energy at each surface can be explained using a Bader charge analysis. Particularly for Ag₄ the lowest formation energy is obtained for tetrahedral geometry while the isolated Ag₄ geometry is planar. Small silver deposits placed superficially on titania surfaces modify its electronic structures and improve the conduction band edges positions for possible NO reduction. Band edges positions with respect to the vacuum potential have been studied. The comparison of the conduction band minimum with the reduction potentials of NO/N₂O and N₂O/N₂ shows that they are higher, being Ag₃ on rutile and Ag₁, Ag₂ and Ag_4P on anatase better for NO reduction. To complete the analysis, the calculation of work function, energy gap, ionization energy and electron affinity are relevant since they allow the location of semiconductor band edges at point of zero charge. Finally, the adsorption of nitrogen oxides is studied where the NO₂ adsorption is favored over NO.

Keywords: Band edges; DFT; NOx; Silver nanoclusters; TiO(2).