STUDIES ON CHROMIA ZIRCONIA CATALYSTS .2. ESR OF CHROMIUM SPECIES

The characterization of CrOx/ZrO2 samples (Cr content 0.05 to 6 wt%) by means of ESR spectroscopy is reported. On samples heated in O2 at increasing temperatures up to 1173 K, the presence of Cr(V) (γ-signal, g{norm of matrix} = 1.960 and g{norm of matrix} = 1.979) is detected by ESR. Its concentration (Cr(V) ions nm-2) is found to increase with temperature, remaining about constant above 773 K. Experiments with the 53Cr isotope allow assignment of the species to a surface mononuclear chromyl-complex in a square pyramidal configuration. At higher temperatures (generally at T ≥ 973 K, depending also on textural features of the ZrO2 support and Cr content) the ESR signals of (i) a chromia-like phase (β′-signal, g = 1.98 and ΔHpp = 1500-1800 G) and (ii) α-Cr2O3 (g = 1.98, ΔHpp = 480-500 G, spectra recorded at T ≥ 308 K) are observed in addition to Cr(V). The particle size of the β- is too small (≤7 nm) to show strong antiferromagnetic interactions. On samples reduced with CO, the γ-signal sharply decreases with increasing temperature of the reduction, and disappears at 623 K. In the more concentrated samples and after extensive reduction only, an ESR signal from Cr(III) is observed (δ-species, σ ≈ 2.2 with a broad maximum at g in the range 3.8 to 5.0), and assigned to weakly interacting Cr(III) ions exposed on the surface of ZrO2. If reduced samples are treated with H2O at increasing temperatures up to 1073 K, the selective oxidation of Cr(II) to Cr(III)β species (g = 1.98, ΔHpp = 1500-1600 G) is observed. Species Cr(III)-β and Cr(IIl)-β′ differ from each other by cluster size only, as indicated by their different redox behavior. Reoxidation with O2 at room temperature only minimally restores the γ-signal, and hardly affects the 8-signal. Full reversibility is achieved upon heating in 0, at 773 K. ESR results and average oxidation numbers from redox cycles allow the identification of two distinct redox couples on the ZrO2 surface: Cr(III)/Cr(V) and Cr(II)/Cr(VI). The stabilization effect of the ZrO2 matrix on the various chromium species is discussed. © 1991.