Laser-induced luminescence of model Fe/Al2O3 and Cr/Al2O3 catalysts

The laser-induced luminescence of Cr3+ impurity ions in model Fe/Al2O3 and Cr/Al2O3 catalysts with different calcination temperatures was studied. It was found that an additional luminescence band at 770 nm appeared in the luminescence spectra of low-temperature samples as a result of the interaction of octahedrally coordinated Cr3+ ions with Fe3+ impurity ions. In the theta-Al2O3 phase with a concentration of Cr3+ ions higher than 0.1 wt %, the interaction of the Cr3+-Cr3+ ion pairs in the immediate surroundings resulted in the appearance of N-theta lines due to the splitting of R-theta lines. The differences of these lines from the N-alpha lines of alpha-Al2O3 were related to the individuality of the crystal lattice of the theta phase and the coordination of Cr3+ impurity ions in the immediate surroundings, which is different from that in the alpha phase. Based on the laser-induced luminescence spectroscopic data, it was found that regions with a local Cr3+ concentration higher than the average Cr3+ concentration in the bulk of a catalyst by one order of magnitude were formed in the alpha-Al2O3-Fe2O3 system with the bulk Fe and Cr concentrations of 2.5 and 0.04 wt %, respectively, which was calcined at 1220 degrees C, as a result of the diffusion of chromium and iron ions.