A XAFS Investigation of Pt Monolayer on Au(111) Obtained from Displacement Reaction

Abstract

The urgent need of our modern society for the shift to clean energy from fossil energy resources has driven many research efforts into proton exchange membrane fuel cells (PEMFCs), which emit no greenhouse gases during operation and thus the applications to the automobile is promising. However, several issues concerning the electrocatalysts, including the cost and durability, have greatly inhibited the practical use of PEMFCs. To ultimately increase the surface area of mainly Pt-based fuel cell catalyst, and thus reduce the cost, 2D metal monolayer catalyst has attracted many research interests1. Surface-limited redox replacement reaction (SLRR)2, as a convenient, low cost way to achieve monolayer deposition of noble metal such as Pt, Ru, has attracted extensive attentions and lead to its wide applications3 , 4. However, there were only limited numbers of studies regarding the fundamental mechanism of the replacement reaction. For example, there is still controversy regarding the chemical state of the monolayer Pt obtained from displacement reaction5 , 6. The correct understanding of the stoichiometry and Pt monolayer structure is of fundamental importance to better design the electrocatalysis. In this work, the Pt monolayer on well-defined Au(111) synthesized from SLRR of Cu UPD was studied by in situ PTRF-XAFS (polarization-dependent total reflect fluorescence X-ray absorption fine structure), which is especially powerful in characterizing the surface structure and electronic state of monolayer adsorbate on atomically flat surface even in electrochemical condition7. Measurements were conducted in two polarizations, s- and p-, whose surface orientations are parallel and perpendicular to the polarization direction of the incident X-ray, respectively. The in situ XANES (X-ray absorption near edge structure) spectra taken at 0.3 V (vs. sat. Ag/AgCl) presented a larger white line intensity compared to the spectrum of Pt foil as shown in Fig. 1(a). The Pt L edge intensity is a direct reflection of the level of unfilled d state, i.e, the Pt monolayer was not completely reduced Pt0 even at this potential which was at the overpotential region for Pt deposition from [PtCl6] 2-. Furthermore, by analyzing the EXAFS (extended X-ray absorption fine structure) spectra, we found that the Pt-Cl instead of Pt-Pt oscillation in s-polarization (Fig. 1(b)). This result diverts from the previous understanding that Pt4+ in [PtCl6]2-was reduced and Pt-Cl bond breaks when replacing Cu UPD layer. Further XPS measurement also provided the evidence that the Pt monolayer had the Pt-Cl complex structure and the reduction kinetics was very slow, meaning the Pt complex monolayer is stable on the Au(111) surface. Figure 1. a) Pt L3-edge XANES intensity comparison between the Pt monolayer at E=0.3 V (s- and p-polarization) with standard reference samples with Pt oxidation state 0, +2, and +4 (Pt foil, K2PtCl4 and H2PtCl6·6H2O, respectively). b) Pt L3-edge EXAFS comparison of s- and p- polarization of Pt monolayer at 0.3 V with kweight = 3. Reference: (1) Uosaki, K.; Ye, S.; Naohara, H.; Oda, Y.; Haba, T.; Kondo, T. J. Phys. Chem. B 1997, 101, 7566. (2) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Surf. Sci. 2001, 474, L173. (3) Wang, J. X.; Inada, H.; Wu, L. J.; Zhu, Y. M.; Choi, Y. M.; Liu, P.; Zhou, W. P.; Adzic, R. R. J. Am. Chem. Soc. 2009, 131, 17298. (4) Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Science 2007, 315, 220. (5) Gokcen, D.; Bae, S. E.; Brankovic, S. R. J. Electrochem. Soc. 2010, 157, D582. (6) Cheng, S.; Rettew, R. E.; Sauerbrey, M.; Alamgir, F. M. ACS Appl. Mater. Inter. 2011, 3, 3948. (7) Asakura, K. In Catalysis; Spivey, J. J., Gupta, M., Eds.; RSC Publishing: London, 2012; Vol. 24, p 281. Figure 1