Growth of Ultra-Thin PtxPb1-x Electrocatalysts By Surface Limited Redox Replacement and Study of Their Adsorption Properties

The development of inexpensive, active and stable Pt catalysts relevant to fuel cell applications remains a very active area of research. The most promising approaches to improve the performance of Pt catalysts while reducing the total noble metal content include Pt thin-film overlayers¹ and Pt-X alloy skin and skeleton structures that comprise a Pt-rich surface supported on a Pt-X alloy². The confinement of the dimensions to a few atomic layers, coupled with the neighbourhood of Pt to another metal, causes electronic and geometrical effects that substantially alter the catalytic performance and adsorption binding energies. Furthermore, this work is motivated by promising results concerning the catalytic activity of Pt_xPb_1-x alloys for formic acid oxidation³ and the suppression of CO poisoning⁴. Thus, in this work we have explored the effect of controlling film thickness and Pb content on adsorption and catalytic activity.

One of the most successful methods to grow thin film overlayers is known as Surface Limited Redox Replacement (SLRR)⁵. In this method, a monolayer of underpotentially deposited (UPD) metal is galvanically displaced by Pt. Recently, it was demonstrated that both the UPD step and the redox replacement by more noble ions can be performed in a single electrochemical cell, assisted by Pb UPD⁶. Recent work has shown the application of Pb UPD to the formation of Pt₄Cu overlayers⁷. However, the application of this method to Pt alloy thin films has not been thoroughly explored.

In this work, we explored the SLRR-based deposition of Pt_xPb_1-x alloys in a single solution containing 0.1 M NaClO₄, 1 mM Pb(ClO₄)₂, 1 mM HClO₄ and 0.5 mM K₂PtCl₄. The deposition protocol included (1) Pb UPD on a Au substrate by application of a 1 s pulse at -0.85V vs MSE and (2) a redox replacement reaction under open circuit conditions. The OCP was monitored and interrupted at selected values in the potential region for Pb UPD on Pt. In this way, the Pb content in each layer was controlled. Films of 10 ML in thickness of different composition were grown by repeated deposition cycles.

The variation in surface composition with upper limit potential and thickness has been verified by XPS measurements. The coverage of H UPD has been shown to decrease dramatically with Pb content, as shown in Figure 1A. In fact, the estimated Pt active areas obtained from the H UPD charge are much lower than the ones measured by XPS on the same alloys. This observation can possibly be explained by a screening effect of Pb causing an inhibition of H-adsorption. A similar effect has been observed in other Pt bimetallic systems^8,9.

In this work, we have also examined the CO stripping behaviour on alloys. CO stripping was performed by polarising the electrode at -0.6 V in CO saturated 0.5 M H₂SO₄solution. Linear sweeps in the CO free solution are presented in Figure 1B.

A negative shift of the onset potential for CO stripping and change in peak shape as the Pb content increases can be observed. These changes can be ascribed to an electronic effect from Pb, or a bifunctional effect affecting the onset of OH adsorption¹⁰. The charges obtained by integration of the stripping peaks are in quantitative agreement with those obtained by H UPD, indicating the coverage obtained by CO follows the same trend.

References

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²              V. R. Stamenkovic, B. S. Mun et al.,  J. Am. Chem. Soc. 128, 8813 (2006).

³              S.-M. Hwang, J. E. Bonevich et al.,  J. Electrochem. Soc. 158, B1019 (2011).

⁴              N. de-los-Santos-Álvarez, L.R. Alden et al.,  J. Electroanal. Chem. 626, 14 (2009).

⁵              S. R. Brankovic, J. X. Wang et al.,  Surf. Sci. 474, L173 (2001).

⁶              M. Fayette, Y. Liu et al.,  Langmuir 27, 5650 (2011).

⁷              L. Bromberg, M. Fayette et al.,  Electrocatalysis 4, 24 (2013).

⁸              Dennis F. van der Vliet, Chao Wang et al.,  Angew. Chem. Int. Ed. 51, 3139 (2012).

⁹              H. Schulenburg, J. Durst et al.,  J. Electroanal. Chem. 642, 52 (2010).

¹⁰             H. A. Gasteiger, N. M. Markovic et al.,  J. Phys. Chem. 98, 617 (1994).