Nanoparticulate hydroxyapatite and calcium-based CO2 sorbents

This thesis is focused on the development of synthesis and characterisation protocols for two different nanoparticulate materials; hydroxyapatite (HA), a biomaterial well recognised as chemically akin to human bone, and CaO, a material often used for the sequestration of CO2 at elevated temperatures. For the analysis of these materials various bulk and particle level characterisation techniques have been employed, which are complemented by the versatile analytical methods available in the transmission electron microscope (TEM).

The first chapter of results reveal that a hydrothermal synthesis route achieved phase-pure nanoparticulate HA with Ca/P atomic ratios close to the stoichiometric target (1.67). Impure HA nanopowders were produced by a sol-gel synthesis route with analysis by X-ray diffraction (XRD) revealing secondary phases of calcium phosphates, CaCO3 and CaO.

The Ca/P ratios of each powder were determined at the particle level using TEM with energy dispersive X-ray spectroscopy (TEM-EDX), having first established a threshold electron fluence below which significant electron-beam-induced alteration of the composition of HA does not occur. Results showed a greater variability of particle composition from the sol-gel preparation route compared to the hydrothermal route. This technique provides results in reasonable agreement to bulk Ca/P ratio analysis carried out by X-ray fluorescence (XRF) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).

The second component of the thesis relates to the production of nanoparticulate CaO powder sorbents for the sequestration of CO2 gas. The CaO nanopowders were produced by the thermal decomposition of calcium acetate hydrate (CaAc); this process was analysed by thermogravimetric analysis (TGA) and by in-situ hot-stage XRD. The CO2 uptake capability of the CaO powder sorbents was analysed by TGA following the reaction:
CaO + CO2 ↔ CaCO3

Results showed a molar conversion ratio, χ (of CaO to CaCO3) of 0.92, after 15 minutes of carbonation with structural analysis by SEM and TEM showing consistent growth and densification of rounded CaCO3 crystals upon carbonation. Multiple cycles of carbonation and decarbonation were then carried out by TGA to investigate sorbent regenerability. A 0.32 decrease in χ was found after 9 cycles which is attributed to the sintering (reduction in surface area) of the sorbent with progressive decarbonations at 800 °C. Structural analysis of decarbonated samples extracted from the TGA, by XRD, SEM and TEM, highlighted the issue of sorbent hydration upon storage, sample preparation and analysis.

A TEM based technique has been developed for the structural analysis of multicycle CO2 capture using an ex-situ environmental cell (E-cell). This technique allows for multicycle capture to be carried out and then analysed in the TEM with minimal exposure to the atmosphere, therefore providing a closer microstructural match to what occurs in the TGA. Results showed that slow, low-vacuum decarbonation (in the E-cell) creates a densified ‘skeleton’ of CaO, consistent with the drop in capture capacity observed by TGA.

Finally, modifications of CaO sorbents using spacer materials has been carried out with the aim of declining the decay in sorbent performance during multiple cycles of carbonation and decarbonation in the TGA. Promising results were found using CaO sorbents modified a commercial YSZ powder and also with CaZrO3/ZrO2.