On the mechanism of sodium migration in transition aluminas with calcination

Abstract

We have characterized unwashed agglomerate Bayer gibbsite, a major precursor for industrial-grade and specialty aluminas, and the transition aluminas/corundum formed on calcination to follow the migration of sodium with phase evolution. Sodium is found initially to be intercalated, likely in the OH-rich planes in hydroxide precursors, with Na 1s XPS spectra demonstrating much becomes incorporated into the alumina structure with calcination. 23Na-NanoSIMS, XRF, and XPS measurements reveal sodium then migrates from within the crystallites to accumulate in cracks, interfaces or defects and, eventually, at the grain exterior during calcination without significant losses. Changes in pore structure with acid leaching and subsequent re-calcination suggests sodium occupies surface Al-O-Na environments analogous to surface hydroxide, and that near-surface bulk sites act as a reservoir to repopulate lost surface sodium. These bulk sites are argued to be the alumina cation vacancies, the distribution and occupancy of which is still debated despite implications for bulk and surface chemistry properties, a contention strongly supported by XPS and Na K-edge NEXAFS spectra. This indicates that, while surface segregation is thermodynamically favoured, bulk incorporation of sodium occurs for kinetic reasons. We tentatively suggest that bulk hydrogen may similarly be present in activated transition aluminas due to analogous kinetic considerations.