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

Abstract

The C-O stretching frequency (nu(CO)) 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 2pi 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 2pi CO MOs) must be reconciled with the alloy-induced red shift of the nu(CO). A library of spin-optimized Pt and Pt alloy clusters was the basis of density functional theory (DFT) calculations of CO binding energies, nu(CO) values, shifts, and broadening of 5sigma/2pi 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 5sigma donation-2pi 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 5sigma/2pi states results in a broadening of the 5sigma/2pi 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 5sigma/2pi CO filled states (i.e., enhanced 2pi-back-donation). Finally, Mulliken and FEFF8 electron population analysis shows that the increase of the average d-band vacancies upon alloying and additional 2pi 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 2pi CO MOs.

Symmetry-based IR group modes as dynamic probes of Nafion ion exchange site structure

“…(1) DFT calculated peaks tend 9 to be at higher frequency than the observed bands. [43] For example, the observed 969 cm -1 bands corre-10 sponds to a 983* cm -1 band. Thus, an observed Nafion-Li C 3v, LF , calculated to be at 940* cm -1 , would 11 5 940* cm -1 eigenvector animation snapshot available in Fig.…”

Theoretical and experimental infrared spectra of hydrated and dehydrated Nafion

“…While the calculated peak at 1405 cm −1 is fortuitously close to the observed peak at 1414 cm −1 , the calculated peak at 786 cm −1 differs from the observed 910 cm −1 by 13%. DFT tends to overestimate vibrational frequencies by only a few percent: Scaling factors are applied to calculated peak frequencies . However the effect of scaling factors may be small in comparison to the effects of choosing a polymer chemical repeat unit as the basis for a DFT model.…”

Effect of Particle Size and Adsorbates on the L3, L2 and L1 X-ray Absorption Near Edge Structure of Supported Pt Nanoparticles

“…The presence of adsorbates such as hydrogen or CO on metals has also been observed to shift the XANES edge location and intensity [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Hydrogen has been found consistently to broaden the L 3 edge and shift it to higher energy [27].…”