Nano-deposits of Ni and Co, supported on porous oxide materials, serve as heterogeneous catalysts within Johnson-Matthey plc. in the steam reforming and Fischer-Tropsch processes, with the size, shape and dispersity of the metal crystallites linked to the catalytic profile. Here we study the magnetic properties of nickel systems synthesised on the nano-scale, with the aim of developing an industrially viable technique by which the diameter of the nickel species can be evaluated. A series of nickel nanoparticles, synthesised via the thermal decomposition of Ni(acac)2, are studied as a model for the catalytic systems. The nanoparticles were studied via magnetometry and microscopy to identify the super-paramagnetic and nuclear volume of the particles, respectively. The magnetisation studies demonstrate that the widely used Langevin function based method of particle sizing does not reflect the total nuclear volume, and a surface correction term is introduced based on the low temperature, high applied field magnetisation. To demonstrate the applicability of the proposed analysis, the study of a series of industrially-viable precipitation catalysts are reported. The catalysts are studied via x-ray diffraction (XRD) and gas adsorption to establish comparable values of crystallite diameter. The values of crystallite diameter determined from the magnetic analysis are demonstrated to be consistent with the range of sizes determined from the XRD and gas adsorption studies, with additional sensitivity to the polydispersity of the crystallites. During the study of the precipitation catalysts, the magnetic volume was demonstrated to be reduced from the nuclear volume. This behaviour was also confirmed via small angle neutron scattering experiments, which demonstrated a magnetic scattering volume reduced from the nuclear by = 1 nm, on the order previously reported for nano-ferromagnetic materials. Through these studies we have established the methods for determining the surface correction term to magnetic granulometry studies. We have demonstrated that the corrected values are in agreement with the nuclear volumes determined via TEM, gas adsorption and XRD and that our proposed technique for the study of catalyst crystallites requires a short time scale, is insensitive to the catalytic support and is sensitive to the distribution of crystallite diameters.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:632879 |
| Date | January 2014 |
| Creators | Cook, Robert M. |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/63943/ |
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