Carbon-supported Pt–Sn electrocatalysts for the anodic oxidation of H 2, CO, and H 2/CO mixtures. : Part II: The structure–activity relationship

Abstract

The catalytic activity of three different carbon-supported Pt–Sn catalysts for the anodic oxidation of hydrogen, carbon monoxide, and H 2/CO mixtures is correlated with the bimetallic microstructure established in Part I [V. Radmilovic, T.J. Richardson, S.J. Chen, P.N. Ross, Jr., J. Catal. 232 (2005) 199–209] of this study. These catalysts differ primarily by having differing amounts of Pt, Pt 3Sn, PtSn, and SnO 2 phase nanoparticles distributed on the carbon support. Further surface chemical characterization of the Pt 3Sn nanoparticles is provided by comparison of in situ vibrational spectra of CO adsorbed on the nanoparticles with CO adsorbed on Pt 3Sn( h k l ) surfaces [V. Stamenkovic, M. Arenz, B.B. Blizanac, K.J.J. Mayrhofer, P.N. Ross, N.M. Markovic, Surf. Sci. 576 (2005) 145–157; V.R. Stamenkovic, M. Arenz, C.A, Lucas, M.E. Gallagher, P.N. Ross, N.M. Markovic, J. Am. Chem. Soc. 125 (2003) 2736–2745]. Qualitative measures of CO oxidation activity were also obtained from the FTIR spectra for the electrode potential at which CO 2 is first detected (appearance potential). Quantitative kinetic measurements using the carbon-supported catalysts were obtained with the thin-film rotating disk electrode (TF-RDE) method. The CO ad stripping voltammetry clearly indicated that the 500N sample exhibits the lowest degree of alloying, showing a superposition of Pt 3Sn and Pt features, whereas the catalyst consisting entirely of stoichiometric Pt 3Sn cubic phase (E270 sample) exhibits no features characteristic of pure Pt. None of the three carbon-supported Pt–Sn catalysts had the splitting of the C O stretching band of characteristic of CO ad on Pt 3Sn(111), the surface with the highest specific activity (turnover number) for CO and H 2/CO oxidation. All three catalysts had very high levels of activity for H 2 oxidation. The mass activity for CO and H 2/CO oxidation was proportional to the amount of Pt 3Sn phase in the catalyst. There is no clear evidence that either the PtSn phase or Pt/SnO 2 clusters contribute any significant activity for H 2/CO or CO oxidation in these catalysts.