Characteristics of chromium oxides supported on TiO 2 and Al 2O 3 for the decomposition of perchloroethylene

Abstract

The effects of supports including TiO 2 and Al 2O 3 on the molecular structure and catalytic activity of chromium oxide for the complete oxidation of perchloroethylene (PCE) have been examined with respect to the content of Cr on the catalyst surface. The results from XRD, XPS, Raman, and TPR-TPO indicate that the state of the surface species of chromium oxide strongly depends on catalyst support and Cr loading. The amount of Cr(VI) in the high oxidation state on the surface of the catalyst increased as the content of Cr increased from 1 to 17 wt%, while the relative ratio of Cr(VI)/Cr(III) existing on the catalyst surface decreased with respect to the Cr loading, regardless of the supports examined in the present study. The molecular structure of Cr(VI) was also altered with respect to the content of Cr and the surface nature of support. Monochromate species appeared on the surface of Al 2O 3 at 1 wt% of Cr loading on the catalyst surface. However, further increase in the Cr contents up to 17 wt% formed di-, tri-, and tetrachromate species of Cr(VI) and crystalline α-Cr 2O 3 on the surface of Al 2O 3. CrO x /TiO 2 catalyst formed polymeric chromium oxide species on its surface in the wide range of Cr content of the catalyst from 1 to 17.4 wt%. The high degree of polymerization of chromate species on the surface of TiO 2 containing the strong redox ability is contrasted with the state of the chromate species on Al 2O 3. The rate of PCE oxidation was enhanced as the Cr loading increased for both catalysts. CrO x /TiO 2 catalyst revealed a relatively higher reaction rate than CrO x /Al 2O 3 at the given content of Cr and its surface density. TOF of the catalyst increased along with the degree of polymerization of the chromate species on the catalyst surface. The superior performance of CrO x /TiO 2 catalyst is attributed mainly to the formation of the polychromate species containing a stronger redox ability on the surface of TiO 2. This clearly suggests that the metal–oxygen–support interaction (MOSI) is critical in the present catalytic reaction system.