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Title: Solid state nuclear magnetic resonance on quadrupolar nuclei in disordered catalysis based materials
Author: Hooper, Thomas J. N.
ISNI:       0000 0004 7227 457X
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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The behaviour of a catalyst is intrinsically linked to its structure and, therefore, accurate structural refinements are desired to tune their overall function. Higher functional demands on catalysts systems, require more complex disordered materials which are inherently difficult to characterise with conventional analytical techniques. Solid state NMR is an excellent probe of local order and, hence, is utilised in this thesis for the structural determination of several catalytic related materials. The first direct105 Pd solid state NMR measurements of diamagnetic (K2PdCl6, (NH4)2PdCl6 and K2PdBr6) complexes is reported, thereby introducing an effective 105 Pd chemical shift ranges with respect to the newly proposed 105 Pd chemical shift reference (0.33 M H2PdCl6(aq)). The enormous 105 Pd quadrupolar moment, makes the interaction very sensitive to small structural distortions as demonstrated by the measurable quadrupolar parameters for the three complexes, despite the high symmetry octahedral Pd coordination. The detected deviation from a cubic symmetry, was corroborated by XRD PDF analysis. The 105 Pd quadrupolar parameters are shown to be more sensitive to minute disorder than conventional XRD and other quadrupolar nuclei NMR. Ambient temperature 105 Pd NMR observations of Pd metal determined the Knight shift as K = −3.205 ± 0.006 %, where variations in the 105 Pd Knight shift allowed for detection of defects in the cubic metal structure and for differentiation of Pd nanoparticle sizes. The developed 105 Pd NMR methodology was then applied in a multi-technique structural investigation of doped Pd catalysts, that confirmed the interstitial location of the dopants. The use of the newly developed structure-generation software, supercell, in combination with GIPAW-DFT calculations and solid state NMR, is shown to be a thorough tool for structural determination of disordered materials. The methodology is applied to two phases of the aluminosilicate mullite (3:2 mullite and 2:1 mullite), and provides complete assignment of the 17O,27 Al and 29 Si NMR spectra. The distribution of AlO4/SiO4 sites in the mullite structure is shown to be random, proving the presence of SiO4 moieties in the tritetrahedral (T3O) environments. The observation of said moieties directly contradicts Loewenstein’s avoidance principle. Additionally, a quasi-tetrahedral site with an additional long bond ((Al/Si)O4+1) is discovered and a vacancy adjacent three-coordinated Al site (AlO3[]) is proposed. The findings from this investigation are then applied to the 27 Al MQMAS study of boron doped mullites, providing additional evidence for the AlO3[]motif. A thorough 11 B and 27 Al solid state NMR investigation was undertaken on three series of aluminium borate phases (A9B2, A2B and metastable Al6-xBxO9 (where 1 ≤ x ≤ 3)) with varying Al/B ratios. A solid solution of AlO4 and BO4 tetrahedra was discovered in all three phases (to varying extents), justifying the conflicting compositional/structural reports present in the literature. Differences in the crystallinity of commercially available, sol-gel synthesized, and solid state synthesised A9B2 samples were documented. The solid state NMR study of the disordered phases, A2B and metastable Al6-xBxO9, utilised multiple fields and MQMAS measurements to constrain the simulation of the 1D spectra, which allowed for complete assignment of the structure and corrected previous erroneous reports. An AlO4+1 site, analogous to the discovered mullite environment, is found in both disordered phases.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; QD Chemistry