Mechanistic aspects of the reduction of rutile titanium dioxide and its Re-oxidation. Development and destruction of crystallographic shear structures

A model is presented giving the mean dimensions of acicular octadecahedral microcrystallites of a rutile titanium dioxide powder. Reduction at 823 K, in conjunction with ESR, electrical conductivity and controlled re-oxidation has enabled the model to be applied to reduced microcrystallites. At 300 K they contain 99.9% of reduced spin-paired [Ti3+↑↓ Ti3+ VO:] sites. These sites are situated on the external crystal faces and on polygonal bulk crystallographic shear (CS) structures inclined to the microcrystal four-fold symmetry axis. CS structures are quantum-sized [Ti4O7VO:] environments which broaden the paramagnetic signals at 78 K. Temperature programmed reduction in H2(g) reveals atomic hydrogen as a precursor to CS structure formation via a lattice template formed on microcrystallite faces. Shear structures are oxidised on their polygonal perimeters at differing rates on the respective microcrystallite faces by anionic vacancy transfer from sub-surface regions. [Display omitted] •A mean acicular octadecahedral model of microcrystalline rutile is presented.•Vacuum reduction at 823 K results in oxygen loss from low coordination anion sites.•Paramagnetism from Ti3+ centres critically depends upon post-reduction procedures.•Bulk CS structures form through a template in the reduced surface shell.•CS reoxidation occurs at differing rates according to their perimeter sizes.