Characterisation of platinum-based fuel cell catalyst materials using 195Pt wideline solid state NMR

Abstract

This study demonstrates the utility of the novel Field Sweep Fourier Transform (FSFT) method for acquiring wideline ¹⁹⁵Pt NMR data from various sized Pt nanoparticles, Pt–Sn intermetallics/bimetallics used to catalyse oxidative processes in fuel cell applications, and various other related Pt₃X alloys (X = Al, Sc, Nb, Ti, Hf and Zr) which can facilitate oxygen reduction catalysis. The ¹⁹⁵Pt and ¹¹⁹Sn NMR lineshapes measured from the PtSn intermetallic and Pt₃Sn bimetallic systems suggest that these are more ordered than other closely related bimetallic alloys; this observation is supported by other characterisation techniques such as XRD. From these reconstructed spectra the mean number of atoms in a Pt nanoparticle can be accurately determined, along with detailed information regarding the number of atoms present effectively in each layer from the surface. This can be compared with theoretical predictions of the number of Pt atoms in these various layers for cubo-octahedral nanoparticles, thereby providing an estimate of the particle size. A comparison of the common NMR techniques used to acquire wideline data from the I = 1/2 ¹⁹⁵Pt nucleus illustrates the advantages of the automated FSFT technique over the Spin Echo Height Spectroscopy (SEHS) (or Spin Echo Integration Spectroscopy (SEIS)) approach that dominates the literature in this area of study. This work also presents the first ¹⁹⁵Pt NMR characterisation of novel small Pt₁₃ nanoclusters which are diamagnetic and thus devoid of metallic character. This unique system provides a direct measure of an isotropic chemical shift for these Pt nanoparticles and affords a better basis for determining the actual Knight shift when compared to referencing against the primary IUPAC shift standard (1.2 M Na₂PtCl_6(aq)) which has a very different local chemical environment.