| Popis: |
The redox behaviour of chromium oxide in CrO x /La 2 O 3 , CrO x /La 2 O 3 /ZrO 2 and CrO x /ZrO 2 mixed oxide systems has been investigated by temperature-programmed reduction (TPR), photoelectron spectroscopy (XPS) and Raman spectroscopy (RS). The formation of La-Cr compounds such as La 2 CrO 6 , LaCrO 4 was shown to occur by calcination of CrO x /La 2 O 3 and CrO x /La 2 O 3 /ZrO 2 at 600 °C. With a Cr content of ca. 4 wt.%, it was revealed that, having been reduced to Cr 3+ -containing compounds, La 2 CrO 6 and/or LaCrO 4 in both systems may be restored by reoxidation at 600 °C. In the case of CrO x /La 2 O 3 /ZrO 2 catalysts, it is suggested that these compounds were real surface species anchored as an overlayer on zirconia. Such a structure may stabilise, on the one hand, the tetragonal modification of zirconia and, on the other hand, highly dispersed state of Cr species in calcined as well as in reduced catalysts. |
Full Text from ScienceDirect