Development of solid state NMR to understand materials involved in catalytic technology used in fuel cells
Rees, Gregory J.
The utility of the little used Field Sweep Fourier transform (FSFT) method is demonstrated for recording wideline nuclear magnetic resonance (NMR) of 195Pt resonances for various sized platinum nanoparticles, as well as platinum-tin bimetallics used in fuel cell catalysts, and various other related platinum (Pt3X; X = Al, Sc, Nb, Ti, Hf and Zr) alloys. The lineshapes observed from PtSn for both 195Pt and 119Sn suggest that it is more ordered than other closely related intermetallics, which might be expected from other measurements (e.g. XRD linewidths). From these reconstructed spectra the mean number of platinum atoms in the 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 platinum 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 spectra from spin I = 1 2 nuclei shows the advantages of the automated FSFT technique over the spin echo height/integration approach that dominates the literature. A study of small 13 atom platinum clusters, with variable particle size dispersion for which there is no experimental characterisation in the literature, provides evidence for an isotropic chemical shift of these platinum nanoparticles and provides a better basis for determining the Knight shift when compared to referencing against the primary IUPAC standard which has a different local structure.\ud \ud Rare earth apatite oxide ion conductors are novel candidates for electrolytes in solid oxide fuel cells. It has been shown that La8Y2Ge6O27 is an excellent oxide conductor at lower temperatures when compared to the market leader yttrium stabilised zirconia (YSZ). To understand the mechanism of its conduction 17O-labelled water was allowed to conduct through the sample and 17O solid state NMR was employed to comment on this pathway in a series of germanium and silicon substituted apatites. The linear channels running through the centre of the structure were believed to contain vacancies and as with perovskites it was commonly believed these allowed hopping of the oxygen to enable the apatite to conduct. It was shown that a limited amount of the 17O-oxygens made it to the channel and almost all of the label was located in the tetrahedra. This suggested that the mechanism of conduction was via the tetrahedral backbone. Molecular dynamics studies on these systems confirmed this SN2 mechanism of conduction as the excess oxygen hopped onto the tetrahedral site to form a five coordinate bridging oxygen which then forced a neighbouring oxygen to hop onto another tetrahedra.\ud \ud A comparison of analytical techniques used to characterise hydrogen bonding in benzoic acid and its corresponding group IA hemibenzoates indicates the need to draw upon multiple methods to fully understand the nature of the bond. The X-ray diffraction (XRD) data cannot confirm precisely the position of the hydrogen in the complex and hence cannot comment on the nature of the bond. Traditionally the angle at the central bonded proton and the oxygen-oxygen bond distance are used to comment on the strength of the hydrogen bonding, the results present here show the limitations of these analysis methods. Due to the oxygen-proton-oxygen bond angle variations commenting on the oxygen-oxygen length and correlating it to the hydrogen bonding is not feasible. There is heavy literature present on correlating the 1H isotropic shifts to the hydrogen bond strength, here we show a step wise change in hydrogen bonding from benzoic acid and lithium hemibenzoate down the periodic table to potassium, rubidium and cesium hemibenzoate. We show that the anisotropic tensor, 22, is pointed along the carbonyl bond and changes with the hydrogen bonding strength. However this method of characterising the bonding interaction gives a linear correlation from benzoic acid to cesium hemibenzoate. The 17O MAS of the carbonyl groups show an in ated quadrupole coupling constant when compared to the hydroxyls. There is a correlation between the anisotropic 13C 22 parameter and the quadrupole coupling (CQ), as the 22 decreases the CQ seems to give an overall increase. These oxygen results have been confirmed by multiple field double rotation results. All the crystallographic and solid state NMR data present is tied together by density functional theory calculations which show varying degrees of agreement with the achieved results.
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