Physicochemical Properties of LaFeO3 Perovskite Prepared by Various Methods and Its Activity in the Oxidation of Hydrocarbons

Barbara Kucharczyk,Juliusz Winiarski,Katarzyna Adamska,Irena Szczygieł

doi:10.1021/acs.iecr.0c03035

Abstract

LaFeO3 perovskites, obtained through co-precipitation of carbonates with (NH4)2CO3, co-precipitation of hydroxides with NaOH, citrate method, and through solid-state reaction using metal nitrates as precursors, were deposited on a heat-resistant foil support. Their physicochemical properties were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry and thermogravimetric analysis (DSC/TGA), and specific surface area measurements. DSC/TGA showed that phase transitions took place in the LaFeO3 perovskite up to 800–840 °C. Pure LaFeO3 perovskite phase was obtained only when the citrate method was used. The following decreasing order of methane and hexane oxidation activity at 700 °C was determined for the catalysts with the LaFeO3 perovskite obtained by different methods: citric acid > ammonium carbonate ∼ metal nitrates > sodium hydroxide. LaFeO3 catalysts obtained at 700 °C in the presence of citric acid or ammonium carbonate can be used as three-way automotive exhaust catalysts or as catalysts in environmental protection.

Highlights

Since perovskites exhibit good catalytic properties and are cheaper and more thermally resistant than noble metals, they can be used as catalysts in the oxidation of hydrocarbons and CO and as three-way catalytic converters (TWCs).[1−13]Metal cations are stable in the ABO3 catalyst provided that their cationic radii amount to 12-coordinated A and 6coordinated B sites: rA > 0.90 Å and rB > 0.51 Å, respectively.[4]
The results indicate that the number of iron moles and the number of lanthanum moles in all of the tested samples are close to the stoichiometric values
The perovskites obtained using the various methods differ in their properties and methane and hexane oxidation activity

Summary

Introduction

Since perovskites exhibit good catalytic properties and are cheaper and more thermally resistant than noble metals, they can be used as catalysts in the oxidation of hydrocarbons and CO and as three-way catalytic converters (TWCs).[1−13]Metal cations are stable in the ABO3 catalyst provided that their cationic radii amount to 12-coordinated A and 6coordinated B sites: rA > 0.90 Å and rB > 0.51 Å, respectively.[4]. The activity of perovskites in the oxidation of methane is similar to that of the Pt/Al2O3 catalysts and decreases in the following order: LaCoO3 > LaMnO3, LaFeO3 > LaCrO3.6 In the case of AFeO3 perovskites, where A = La, Nd, or Sm, prepared using the citrate method, their methane oxidation activity can be ordered as follows: LaFeO3 > NdFeO3 > SmFeO3.4 Perovskites LaMnO3 and LaCoO3 have been found to be most active in the oxidation of methane, CO, and soot Their stability is low at high temperatures.[4,6,10] The LaFeO3 perovskite exhibits lower activity in the above processes, but its thermal resistance is higher, especially under the variable conditions prevailing during the purification of exhaust fumes.[10] Already in 1971, attempts were made to replace the noble metals in TWC with perovskite.

Methods

Results

Conclusion