Adatoms-induced local bond contraction, quantum trap depression, and charge polarization at Pt and Rh surfaces

The extremely high catalytic efficiency of undercoordinated noble metal adatoms is indeed fascinating, but its chemical and electronic origin remains yet puzzling. Incorporating the BOLS correlation theory [Sun, C. Q. Prog. Solid State Chem. 2007, 35, 1] into the high-resolution XPS measurements [Baraldi, A.; et al. New J. Phys. 2007, 9, 143; Bianchettin, L.; et al. J. Chem. Phys. 2008, 128, 114706] has affirmed the BOLS expectations that the broken bonds induce local strain and quantum trapping in addition to polarization of the otherwise conductive half-filled s-shell charge by the tightly- and densely-trapped inner electrons of the adatoms. Both the trapped and polarized states would be detectable from the density-of-states evolution of the valence and the core bands. The trapped states have been discovered at the bottom edges of Pt(5d ¹⁰6s⁰) 4f_7/2 and Rh 3d_5/2 bands, and the polarized states only present at the upper edge of Rh(4d⁸⁵s ¹) 3d_5/2. It is suggested that the quantum trapping increases the electroaffinity and the polarization does oppositely. Therefore, the Rh adatom serves as a donor and the Pt adatom as an acceptor in the process of catalytic reaction.