An Evaluation of the Ethanol Oxidation Activity of Ternary Pt-Rh-SnO2 catalysts Prepared from the Vapor Phase

Ethanol holds promise as a non-toxic, transportable, energy dense (8kWh/kg), and fuel that, unlike hydrogen, is amenable to use in the existing fuel infrastructure. However, slow oxidation kinetics and incomplete CO₂ formation, indicating unbroken C-C bonds at practical potentials, limit usage in a fuel cell. Incomplete formation of CO₂, a complete 12 electron transfer, leads to the formation of adsorbed intermediates such as CO, acetaldehyde, and acetic acid. These intermediates can poison the catalyst surface leading to a loss in cell efficiency.  The use of Pt/Rh/Sn ternary catalysts has proven promising owing to bi-functional, electronic, and synergistic effects between the constituents. The role of Rh is to cleave the C-C bond of ethanol, SnO₂provides the OH species to oxidize intermediates (freeing the Pt and Rd sites), and Pt is for ethanol dehydrogenative adsorption [1]. Typical synthesis methods include polyol [2], Bönneman [3], co-impregnation-reduction [4], or cation-adsorption-reduction-galvanic displacement [5] techniques.

 Previous flame-based deposition of catalysts for ethanol oxidation have focused on Pt-Sn combinations and found that 10 wt.% Sn showed the best onset potential (~0.3V vs RHE) and largest oxidation peaks in 0.5 M H₂SO₄and 1 M ethanol at 1 mV/s [6].

 Reactive spray deposition technology (RSDT) has been developed by Maric et al. to produce nanoparticles in vapor phase for catalysts comprised of Pt [7,8], Ir_xPt_1-xO_2-y, and Ir_xRu_1-xO_2-y [9]. In this work we extend recent studies on Pd-Ru and Pd cores made by the RSDT process, with subsequent Pt monolayer attachment by galvanic displacement, to the ternary Pt/Rh/Sn system. Elemental ratios of 3:1:3 and 3:2:3 are examined for their performance toward ethanol oxidation.  Figure 1 shows the nodular morphology of Pt/Rh/Sn (3:2:3) as grown on a gas diffusion layer along with the XEDS elemental mapping. Strategies for electrode activation using potential cycling in HClO₄, ethanol and CO stripping are discussed. Figure 2 is a plot of the CV after various pre-treatment approaches. The performance toward ethanol oxidation at room temperature and 60^oC will be discussed in respect to chemical composition. A representative linear sweep voltammogram is shown in Figure 3.  Infrared reflection-absorption spectroscopy (IRRAS) is explored to detail the EOR mechanism. Microscopy studies of the structure and chemical composition are presented.

References

[1] M. Li, W.P. Zhou, N. Marinkovic, K. Sasaki, and R. Adzic, Electrochim. Acta 104 (2013).

[2] M. Li, A. Kowal, K. Sasaki, N. Marinkovic, D. Su, E. Korach, P. Liu, and R.Adzic, Electrochim. Acta 55(2010).

[3] C. Lamy, S. Rousseau, E. Belgsir, C. Coutanceau, J. Leger, Electrochim. Acta 49 (2004).

[4] F. Vigier, C. Coutanceau, A. Perrard, E. Belgsir, C. Lamy, J. Appl. Electrochem. 34 (2004).

[5] A. Kowal, M. Li, M. Shao, K. Sasaki, M. Vukmirovic, J. Zhang, N. Marinkovic, P. Liu, A. Frenkel, R. Adzic, Nature Materials 8 (2009).

[6] K. Fatih, R. Neagu, V. Alzate, R. Maric, W. Haijiang, ECS Trans. 25(1) (2009).

[7] J.M. Roller, R. Neagu, F. Orfino, R. Maric, J. Mater. Sci. 47 (2012).

[8] J.M. Roller, J. Arellano-Jiménez, H. Yu, R. Jain, C.B. Carter, R. Maric, Electrochim. Acta 107 (2013).

[9] J.M. Roller, J. Arellano-Jiménez,R. Jain,R. Jain, H. Yi,  C.B. Carter and R. Maric, J. Electrochem. Soc. 160(6) (2013).