Temperature-programmed reduction study of chromium oxide supported on zirconia and lanthana-zirconia

Temperature-programmed reduction (TPR) has been applied to study the reduct ion behavior of chromium oxide supported on pure zirconia and on lanthana-c ontaining zirconia. The results revealed influences of support nature and c hromium loading on the state of the supported chromium oxide. With catalyst s containing lanthana and only low amounts of chromium, the decrease of the chromium oxidation state during TPR corresponded to the reduction of Cr6to Cr3+. With lanthana-free catalysts, lower changes of oxidation state hav e been found, hinting on thermal instability of higher valent chromium spec ies during thermal catalyst pretreatment. Chromium oxide supported on pure zirconia is reduced at temperatures between 340 and 370 degrees C and is no t quantitatively reoxidized by an oxygen treatment. Chromium oxide on lanth ana-zirconia is reduced at higher temperatures between 370 and 415 degrees C and is completely reoxidable at Cr loadings less than or equal to 4.0 wt. %. The results suggest that lanthana strengthens the interaction between th e zirconia surface and chromium species and, therefore, stabilizes a disper sed state of chromium species. (C) 2000 Elsevier Science B.V. All rights re served.