The surface chemistry of metallurgical aluminas

We have studied the evolution of surface basicity as a function of calcination temperature in a range of transition alumina‐rich analogues of metallurgical grade aluminas produced by calcination of Bayer gibbsite. Specific basicity, the number of basic sites per unit surface area, was determined by thermometric back‐titration and found to increase with calcination temperature up to ~700°C to 800°C, decreasing dramatically thereafter. The population of tetrahedrally coordinated Al3+, which exhibits a qualitatively similar evolution, was determined by Al K‐edge near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy but found not to correlate with basicity changes. Interestingly, a shift to higher and then lower binding energy of Al 2p photoelectrons by X‐ray photoelectron spectroscopy and changes in the intensity of O K‐edge NEXAFS spectra do appear to correlate with surface basicity. Exploiting the differing surface sensitivities of NEXAFS spectra collected in partial electron and total fluorescence yield modes, we find O K‐edge spectra intensities, Al 2p X‐ray photoelectron spectrometer binding energies, and surface basicity all reflect the reorganisation of internal surfaces rather than changes in AlO4:AlO6 occupation. Copyright © 2017 John Wiley & Sons, Ltd.