Studies on chromia/zirconia catalysts. 2. ESR of chromium species

The characterization of CrO{sub x}/ZrO{sub 2} samples (Cr content 0.05 to 6 wt%) by means of ESR spectroscopy is reported. On samples heated in O{sub 2} at increasing temperatures up to 1173 K, the presence of Cr(V) is detected by ESR. Its concentration (Cr(V) ions nm{sup {minus}2}) is found to increase with temperature, remaining about constant above 773 K. Experiments with the {sup 53}Cr isotope allow assignment of the species to a surface mononuclear chromyl-complex in a square pyramidal configuration. At higher temperatures (generally at T {ge} 973 K, depending also on textural features of the ZrO{sub 2} support and Cr content) the ESR signals of (1) a chromia-like phase and (2) a-Cr{sub 2}O{sub 3} are observed in addition to Cr(V). In the more concentrated samples and after extensive reduction only, and ESR signal from Cr(III) is observed, and assigned to weakly interacting Cr(III) ions exposed on the surface of ZrO{sub 2}. If reduced samples are treated with H{sub 2}O at increasing temperatures up to 1073 K, the selective oxidation of Cr(II) to CR(III)-{beta} species is observed. Species Cr(III)-{beta} and Cr(III)-{beta}{prime} differ from each other by cluster size only, as indicated by their different redox behavior. Reoxidation with O{sub 2} at room temperature only minimallly restores the {gamma}-signal, and hardly affects the {delta}-signal. Full reversibility is achieved upon heating in O{sub 2} at 773 K. ESR results and average oxidation numbers from redox cycles allow the identification of two distinct redox couples on the ZrO{sub 2} surface: Cr(III)/Cr(V) and Cr(II)/Cr(VI). The stabilization effect of the ZrO{sub 2} matrix on the various chromium species is discussed.