Alloying effects on N-O stretching frequency: A density functional theory study of the adsorption of NO on Pd3Mn (100) and (111) surfaces

We present total energy and N-O stretching frequency calculations for the l ow-coverage adsorption of NO on palladium-manganese Pd3Mn (100) and (111) s urfaces, on the basis of density-functional theory periodic calculations. A complete description of all the different adsorption sites and correspondi ng N-O vibrations is given and a theoretical interpretation of the experime ntal IR spectra is proposed. On both Pd3Mn (100) and (111) surfaces, the hi ghly 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 o n the pure corresponding palladium surfaces. The anharmonic N-O stretching frequencies on the Pd3Mn surfaces are shifted by 60-100 cm(-1) toward the l ower frequencies by comparison with the pure palladium surfaces. The weaken ing of the adsorption strength and the global shift for the N-O frequencies has been correlated with the presence of the surface manganese atoms, whic h play a predominant role for the electronic interactions between the magne tic NO molecule and the alloy periodic surface. An interpretation of the al loying effect on the strength of the N-O bond and the NO adsorption is prop osed on the basis of a qualitative Mulliken population analysis. The empty states on the surface manganese atoms are responsible for an increased elec tron-transfer toward NO, and hence of the smaller vibrational frequency on the alloy compared to pure Pd. Indeed these empty states interact with the pi*(NO) and push it below the Fermi level, resulting in a transfer from the "surface electron reservoir" toward the pi*(NO) molecular orbital.