Characteristics of chromium oxides supported on TiO2 and Al2O3 for the decomposition of perchloroethylene

Abstract

The effects of supports including TiO2 and Al2O3 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 I 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 Al2O3 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 alpha-Cr2O3 on the surface of Al2O3. CrOx/TiO2 catalyst formed polymeric chromium oxide species on its surface in the wide range of Cr content of the catalyst from I to 17.4 wt%. The high degree of polymerization of chromate species on the surface of TiO2 containing the strong redox ability is contrasted with the state of the chromate species on Al2O3. The rate of PCE oxidation was enhanced as the Cr loading increased for both catalysts. CrOx/TiO2 catalyst revealed a relatively higher reaction rate than CrOx/Al2O3 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 CrOx/TiO2 catalyst is attributed mainly to the formation of the polychromate species containing a stronger redox ability on the surface of TiO2. This clearly suggests that the metal-oxygen-support interaction (MOSI) is critical in the present catalytic reaction system. (C) 2003 Elsevier Inc. All rights reserved.