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

Rees, Gregory J. and Orr, Simon T. and Barrett, Laurence O. and Fisher, Janet M. and Houghton, Jennifer and Spikes, Geoffrey H. and Theobald, Brian R. C. and Thompsett, David and Smith, Mark E. and Hanna, John V. (2013) Characterisation of platinum-based fuel cell catalyst materials using 195Pt wideline solid state NMR. Physical Chemistry Chemical Physics, 15 (40). pp. 17195-17207. ISSN 1463-9076

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 Na2PtCl6(aq)) which has a very different local chemical environment.