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 <0.1% of paramagnetic [Ti3+& UARR; VO: & UARR;Ti3+] reduced edge sites and >99.9% of reduced spin-paired [Ti3+& UARR;& DARR; 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.

Automated summary

A model is proposed to determine the size of acicular octadecahedral microcrystallites of rutile titanium dioxide powder. Reduction, ESR, electrical conductivity and controlled re-oxidation techniques were used to study reduced microcrystallites. The model suggests the presence of paramagnetic and spin-paired sites on the external crystal faces and polygonal bulk crystallographic shear structures. These shear structures are oxidized at different rates on the microcrystallite faces by anionic vacancy transfer.