Identification of the Most Active Sites and Surface Water Species: A Comparative Study of CO and Methanol Oxidation Reactions on Core−Shell M@Pt (M = Ru, Au) Nanoparticles by in Situ IR Spectroscopy

This paper reports a detailed electrochemical in situ surface-enhanced infrared reflection absorption spectroscopic (SEIRAS) investigation of two different core−shell, Ru@Pt and Au@Pt/C, metal nanoparticles (NPs). We were able to identify the most active sites and surface water species involved in the carbon monoxide oxidation reaction (COR) and methanol oxidation reaction (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. For the Au@Pt/C NPs, the SEIRAS data showed a much higher onset potential (0.45 V) for the COR that accounted for their much lower activity observed as compared to that of the Ru@Pt in terms of COR and MOR. 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. Furthermore, the SEIRAS data showed that the surface water structure was very different in reaction media, i.e., with preadsorbed CO or methanol, from that in pure supporting electrolyte, an observation that strongly cautions the practice of using the latter to identify the active water species.