Electronic alterations caused by ruthenium in Pt-Ru alloy nanoparticles as revealed by electrochemical NMR

We have carried out a series of Pt-195 and C-13 NMR spectroscopic and electrochemical experiments on commercial Pt-Ru alloy nanoparticles and compared the results with those on Pt-black samples having similar particle sizes. The Pt NMR spectrum of the alloy nanoparticles consists of a single Gaussian peak, completely different from the broad "multi-Gaussian" NMR spectra, which are generally observed for carbon-supported Pt catalysts. Spin-echo decay measurements show that the intrinsic spin-spin relaxation time (T-2) is much larger in the alloy compared to Pt-black. A "slow-beat" is observed in the spin-echo decay curve of the alloy, implying that the NMR frequencies of spin-spin coupled Pt nuclei in the alloy nanoparticles are quite similar, unlike the situation found with Pt-black. These Pt-195 NMR results strongly suggest that there is a surface enrichment of Pt atoms in the Pt-Ru alloy nanoparticles. The CO-stripping cyclic voltammograrn (CV) of the Pt-Ru alloy nanoparticles is broader than that observed with platinum black and is shifted toward lower potential. The two-peak structure observed previously for the CO-stripping CV behavior of Pt-black containing spontaneously deposited Ru (Tong et al. J. Am. Chem. Soc. 2002, 124, 468-473) is absent in the alloy sample. The C-13 NMR spectrum of CO adsorbed on the Pt-Ru alloy consists of a single peak, exhibiting only a small Knight shift. An analysis of the C-13 spin-lattice relaxation results indicates that Ru addition causes a reduction in the Fermi level local density of states of the clean metal surface atoms and the 2pi* orbital of adsorbed CO. These NMR results suggest that alloying with Ru reduces the total density of states (DOS) at the Pt sites, in accord with conclusions drawn previously from synchrotron X-ray absorption studies of Pt-Ru electrocatalysts. This electronic alteration could be the basis for the ligand field contribution to the "Ru enhancement".