A Band Dispersion Mechanism for Pt Alloy Compositional Tuning of Linear Bound CO Stretching Frequencies

• Published in: The journal of physical chemistry. B 2005-02, Vol.109 (5), p.1839-1848
• Main Authors: Dimakis, Nicholas, Iddir, Hakim, Díaz-Morales, Robert R, Liu, Renxuan, Bunker, Grant, Chung, Eun-Hyuk, Smotkin, Eugene S
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp046332y

The C−O stretching frequency (νCO) of atop CO/Pt in PtRu alloys is compositionally tuned in proportion to the Pt mole percent. The application of a Blyholder−Bagus type mechanism (i.e., increased back-donation from the metal d-band to the hybridized 2π* CO molecular orbitals (MOs)) to compositional tuning has been paradoxical because (1) a Pt−C bond contraction, expected with increased back-donation as the Pt mole percent is reduced, is not observed (i.e., calculated Pt−C bond is either elongated or insensitive to alloying and the binding energies of CO/Pt decrease with alloying) and (2) the lowering d-band center and increased d-band vacancies upon alloying (suggesting less back-donation to the higher energy metal hybridized 2π* CO MOs) must be reconciled with the alloy-induced red shift of the νCO. A library of spin-optimized Pt and Pt alloy clusters was the basis of density functional theory (DFT) calculations of CO binding energies, νCO values, shifts, and broadening of 5σ/2π* CO MO upon hybridization with the alloy orbitals and a DFT derived Mulliken electron population analysis. The DFT results, combined with FEFF8 local density of states (LDOS) calculations, validate a 5σ donation−2π* back-donation mechanism, reconciling the direction of alloy compositional tuning with the lowering of the d-band center and increased vacancies. Although the d-band center decreases in energy with alloying, an asymmetric increase in the dispersion of the d-band is accompanied by an upshift of the metal cluster HOMO level. Concomitantly, the hybridization and renormalization of the CO 5σ/2π* states results in a broadening of the 5σ/2π* manifold with additional lower energy states closer to the upshifted (with respect to the pure Pt cluster) HOMO of the alloy cluster. The dispersion toward higher energies of the alloy d-density of states results in more 5σ/2π* CO filled states (i.e., enhanced 2π*-back-donation). Finally, Mulliken and FEFF8 electron population analysis shows that the increase of the average d-band vacancies upon alloying and additional 2π* back-donation are not mutually exclusive. The d-electron density of the CO-adsorbed Pt atom increases with alloying while the average d-electron density throughout the cluster is reduced. The localized electron density is manifested as an electrostatic wall effect, preventing the Pt−C bond contractions expected with increased back-donation to the 2π* CO MOs.