Combined In Situ Seiras and NMR Investigations of Ru@Pt Electrocatalysts for Methanol Oxidation Reaction

In this presentation, we will discuss a combined in situ SEIRAS (surface enhanced IR reflection absorption spectroscopy) and NMR (nuclear magnetic resonance) study of Ru core–Pt shell (Ru@Pt) electrocatalysts for methanol oxidation reaction (MOR) [1. 2]. The Ru@Pt electrocatalysts were synthesized with commercial Ru black (~ 3 nm, Johnson-Matthey) as starting material. Ethylene glycol (EG) was used as both reaction medium and reducing agent in the synthesis [3]. First, the Ru black was reduced at 140 °C. Then a given volume of H₂PtCl₆·6H₂O solution was added to the EG suspension of the pre-reduced Ru black and Pt was deposited onto Ru black at 120 °C.

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 [4], 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 [5] 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 [6] and on Ru deposited on Pt NPs [7], 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. 

Acknowledgments

The authors thank financial support from DOE (DE-FG02-07ER15895) and NSF (CHE-0923910).

References

[1]  Atienza, D. O.; Allison, T. C.; Tong, Y. Y. J. J. Phys. Chem. C 2012, 116, 26480−26486.

[2] Chen, D.; Hofstead-Duffy, A.; Park, I.; Atienza, D. O.; Susut, C.; Sun, S.-G.; Tong, Y. Y. J. J. Phys. Chem. C 2011, 115, 8735–8743.

[3] Du, B.; Rabb, S. A.; Zangmeister, C.; Tong, Y. Y. Phys. Chem. Chem. Phys 2009, 11, 8231–8239.

[4] Tong; Rice, C.; Godbout, N.; Wieckowski, A.; Oldfield, E. J Am Chem Soc 1999, 121, 2996–3003.

[5]  Korringa, J. Phys. 1950, XVI, 601.

[6] Wang, P. K.; Ansermet, J. P.; Rudaz, S. L.; Wang, Z.; Shore, S.; Slichter, C. P.; Sinfelt, J. H. Science 1986, 234, 35..

[7]  Tong, Y. Y.; Kim, H. S.; Babu, P. K.; Waszczuk, P.; Wieckowski, A.; Oldfield, E. J. Am. Chem. Soc. 2001, 124, 468.