Vanadyl Porphyrin: Efficient Spin Probe to Study the Alumina Surface of Supported Catalysts

Abstract

The EPR method and vanadyl tetraphenylporphyrin complex (VOTPP) as a spin probe were employed to study the transformation of the γ-Al₂O₃ surface during temperature treatment. It has been shown that VOTPP complex is stable at least up to a temperature of 450 °C and \({P}_{{\text{O}}_{2}}\) ≲ 3 Pa. Four distinct forms of vanadyl complex were identified on the γ-Al₂O₃ surface, depending on the annealing temperature. The intensity ratio between these forms, exhibiting significantly different spin-Hamiltonian parameters, depend on the degree of surface hydration, which in turn determines the ratio between sites with different acidity on the γ-Al₂O₃ surface. It has been shown that VOTPP complex demonstrate reversible transitions between different adsorption forms on the alumina surface during dehydration/hydration cycles. The observed phenomenon can be explained by the coordination vanadyl spin probe along the axial axis to the surface centers of different polarity/acidity.