Alloying Effects on N−O Stretching Frequency: A Density Functional Theory Study of the Adsorption of NO on Pd3Mn (100) and (111) Surfaces

Abstract

We present total energy and N−O stretching frequency calculations for the low-coverage adsorption of NO on palladium−manganese Pd3Mn (100) and (111) surfaces, on the basis of density-functional theory periodic calculations. A complete description of all the different adsorption sites and corresponding N−O vibrations is given and a theoretical interpretation of the experimental IR spectra is proposed. On both Pd3Mn (100) and (111) surfaces, the highly coordinated vertical adsorption sites are always energetically favored. The atop adsorption on the surface manganese atom is also a stable site. On Pd3Mn (100), a new horizontal dibridge site is reported. The adsorption on these palladium−manganese alloy surfaces is weaker than the adsorption on the pure corresponding palladium surfaces. The anharmonic N−O stretching frequencies on the Pd3Mn surfaces are shifted by 60−100 cm-1 toward the lower frequencies by comparison with the pure palladium surfaces. The weakening of the adsorption strength and the global shift for the N−O frequencies has been correlated with the presence of the surface manganese atoms, which play a predominant role for the electronic interactions between the magnetic NO molecule and the alloy periodic surface. An interpretation of the alloying effect on the strength of the N−O bond and the NO adsorption is proposed on the basis of a qualitative Mulliken population analysis. The empty states on the surface manganese atoms are responsible for an increased electron-transfer toward NO, and hence of the smaller vibrational frequency on the alloy compared to pure Pd. Indeed these empty states interact with the π*NO and push it below the Fermi level, resulting in a transfer from the “surface electron reservoir” toward the π*NO molecular orbital.