Combined In Situ Seiras and NMR Investigations of Ru@Pt Electrocatalysts for Methanol Oxidation Reaction
We have carried out in situ SEIRAS investigation of the Ru@Pt nanoparticles (NPs) of different Pt packing densities (PDs). We were able to identify the most active sites and surface water species involved in the carbon monoxide oxidation reaction (COR) and MOR on these NPs. We discovered that exposing the as- synthesized Ru@Pt NPs to air could turn them into largely surface-ruthenated NPs whose structure was rather stable under multiple potential cyclings between -0.2 and 0.7 V (vs Ag/ AgCl, 3 M) and reduction at -0.3 V but could be annealed by the COR. The SEIRAS data enabled the identification of the Ru-coordinated-to-Ru, Ru-coordinated-to-Pt, and Pt-islands- on-Ru-core sites on the COR-annealed Ru@Pt NPs among which the most active sites were the Pt-islands-on-Ru-core sites for the COR and MOR, as evidenced by an onset potential as low as -0.1 V for the COR. Among the three different surface water species, namely the water monomer, the weakly hydrogen-bonded water, and the strongly hydrogen-bonded water, the SEIRAS data pointed to the weakly hydrogen-bonded water as the dominant source that provided oxygen for the COR and MOR.
Point-by-point 195Pt NMR spectra of the as- received and EC-cleaned samples of Ru@Pt NPs in comparison with that of pure Pt/C NPs , all recorded in a supporting electrolyte (0.1 M HClO4) at 80 K. The vertical dashed line indicates the peak position (1.100 G/kHz, G stands for magnetic field). The as-received Ru@Pt NPs were freshly synthesized with a shelf life of only a few hours. With the protection of ethylene glycol, little surface oxidation would be expected in such a short time. Indeed, no significant surface oxidation was observed in the corresponding 195Pt NMR spectrum. The clean-surface peak now appeared at 1.1053 G/kHz, corresponding to a 4795 ppm negative shift with respect to the clean-surface peak position (1.1000 G/kHz) of the pure Pt NPs, revealing a strong electronic effect of Ru on Pt, which is opposite to that of Au. A Gaussian deconvolution of the surface peak gave a surface atoms fraction of ≈74%, indicating a dominant Ru core Pt shell structure. Results of T2 and T1 measurements at different spectral positions of the surface fall onto their respective single relaxation curves, indicating the same highly similar surface structural and electronic properties.
13C NMR spectra of 13CO on the Ru@Pt sample were also obtained. The peaks at ≈300 and ≈219 ppm can be assigned to CO on Pt and on Ru sites, respectively, based on literature values. The results of temperature-dependent T1 measurements show the pass-through-the-origin straight lines that are the hallmark of the Korringa relaxation behavior  which indicates that the adsorbed CO molecules on three different sites all acquired metallic characteristics through surface bonding. For CO on Ru sites, the shift (219 ppm) and Korringa constant T1T = 138 s·K (where T is the absolute temperature at which T1 is measured) are very close to those observed on pure Ru  and on Ru deposited on Pt NPs , which lends strong support to our peak assignment that is also consistent with the expected exposure of Ru core for a Pt PD of 40%. On the other hand, the Korringa constants measured at the same spectral position (333 ppm) were different for CO on surface Pt atoms of the Au@Pt and on those of the Ru@Pt
The above discussed combined in situ SEIRAS and NMR investigation enabled us to gain deeper insights into how Ru core influences the electronic properties of he Pt core by which highly active sites were identified.
The authors thank financial support from DOE (DE-FG02-07ER15895) and NSF (CHE-0923910).
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