Influence of La₂O₃ promoter on the structure of MnO_x/SiO₂ catalysts

Abstract

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses have been used to characterize the structure of a La₂O₃-promoted MnO_x/SiO₂ catalyst, before and after its utilization in the oxidative dehydrogenation of ethylbenzene (EB). MnO_x/SiO₂ and MnO_x/La₂O₃/SiO₂ catalysts were prepared by pore volume impregnation, using aqueous solutions of (i) La³⁺-nitrate at an atomic ratio of La/Si = 0.08, and (ii) Mn²⁺-nitrate at an atomic ratio of Mn/Si = 0.10, followed by drying and calcination at 500°C in air. XRD data show no diffraction patterns specific to MnO_x on the La₂O₃-promoted MnO_x/SiO₂ catalyst, after calcination. Thus, the presence of La₂O₃ apparently favors the dispersion of manganese oxides during calcination, presumably by forming mixed Mn-La oxides. On the fresh promoted and unpromoted catalysts, after calcination, XRD and XPS analyses indicated that Mn was present mostly as MnO₂ and Mn₂O₃. In the used catalyst, Mn from the unpromoted catalyst degenerated from Mn⁴⁺ to Mn²⁺, resulting in formation of Mn₃O₄ species, whereas in the case of La₂O₃-promoted catalyst Mn remained well dispersed as MnO₂ and Mn₂O₃. It appears that La₂O₃ precludes the formation of Mn₃O₄ during the EB dehydrogenation, conserving Mn structure and oxidation state.