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Redox behaviour of La-Cr compounds formed in CrOX/La2O3 mixed oxides and CrOX/La2O3/ZrO2 catalysts
(2003)
The redox behaviour of chromium oxide in CrOx/La2O3, CrOx/La2O3/ZrO2 and CrOx/ZrO2 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 La2CrO6, LaCrO4 was shown to occur by calcination of CrOx/La2O3 and CrOx/La2O3/ZrO2 at 600 °C. With a Cr content of ca. 4 wt.%, it was revealed that, having been reduced to Cr3+-containing compounds, La2CrO6 and/or LaCrO4 in both systems may be restored by reoxidation at 600 °C. In the case of CrOx/La2O3/ZrO2 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.
CrOx/La2O3 mixed oxides, prepared by impregnating La2O3 with appropriate aqueous solutions of (NH4)2CrO4 and calcining at 600 °C for 4 h, have been investigated by means of XRD, TPR, XPS, DRIFTS, and Raman spectroscopy (RS). The formation of the compounds La2CrO6, La(OH)CrO4 and LaCrO4 under these conditions was evidenced. Strong peaks at 864, 884, 913, and 921 cm-1, as well as weak peaks at 136, 180, 354, 370, and 388 cm-1 in the RS spectrum of CrOx/La2O3 have been assigned to La2CrO6.