Abstract:
Alumina (Al2O3) from a range of sources have been investigated as potential catalysts for
the destruction of fluorocarbon gases produced in the aluminium smelting process.
Alumina is the feed used in aluminium production and contains Lewis acid sites on its
surface. Under calcined aluminas, such as the transitional γ-Al2O3, contain higher surface
areas and more Lewis acid sites on the surface, compared to the fully calcined α-Al2O3. At
elevated temperatures (>400 ℃), the surface hydroxyls are released which creates
coordinatively unsaturated sites (CUS) on the surface.
The surface of potential catalysts were then analysed to quantify the acid sites on the surface
of the catalyst (as catalysis is believed to be underpinned by Lews acidity). Surface analysis
methods were deployed to determine the speciation on the surface of the catalyst (XPS)
and semi-quantitatively determined the proportion between strong and weak acid sites
(NEXAFS). Surface analysis methods were deployed to determine the phases present in
the catalyst (XRD). Quantitative acid site calculations used thermogravimetric analysis
(TGA) and back-titration methods.
Potential catalysts were then tested to decompose SF6 using a custom-made rig. SF6 was
used as a proxy to PFC’s as both produce a key decomposition product (HF) and are
completely fluorinated (which leads to both being inert and potent as greenhouse gases).
Therefore, the performance of the alumina-based catalyst in the decomposition of SF6
provided insight into how well the catalyst would perform in the presence of PFC’s. Using
these methods, alumina derived from an industrial, commercial Bayer-gibbsite (ABG) was
found to decompose PFC’s at a high % conversion. Above a threshold temperature of
500 ℃, an increase in reaction temperature produced higher catalyst performance (up to
700 ℃). Alumina derived from laboratory-grade gibbsites, prepared by calcination of
laboratory grade reagents, were found to exhibit higher SF6 % conversion than ABG
samples.
The experiments undertaken demonstrate that alumina catalysts are a viable method of PFC
destruction in conditions that allow the surface to be dehydroxylated. It is possible that
these conditions are already replicated in certain regions of an aluminium reduction cell
which bodes well for future in-situ application.