The Effect of Citric Acid Concentration on the Properties of LaMnO3 as a Catalyst for Hydrocarbon Oxidation

Zakaria Sihaib,Fabrizio Puleo,Sonia Gil,Valeria La Parola,Anne Giroir-Fendler,Giuseppe Pantaleo,José Luis Valverde,Leonarda Francesca Liotta

doi:10.3390/catal9030226

Abstract

LaMnO3 (LM) catalysts with a molar ratio of citric acid (CA) to metal (La3+ + Mn2+) nitrates ranging from 0.5 to 2 (LM0.5 to LM2) were synthesized by the citrate sol–gel method with the aim of studying the effect of the citric acid ratio on the physicochemical properties and the catalytic performance in hydrocarbon oxidation. Structural and morphological properties of these catalysts were characterized by X-ray diffraction (XRD) and specific surface area (N2 adsorption) measurements, while the chemical composition was determined by inductively coupled plasma atomic emission spectroscopy (ICP-OES). In the selected samples, additional characterizations were carried out by thermogravimetric and differential thermal analysis (TGA/DTA), Fourier Transform Infrared Spectroscopy (FT-IR), temperature-programmed reduction by hydrogen (H2-TPR), and X-ray photoelectron spectroscopy (XPS). The results showed that the amount of citric acid used significantly influenced the TGA/DTA profile of gels along with the physicochemical properties of the catalysts. The XRD patterns are consistent with the perovskite formation as the main phase. The segregation of a small amount of Mn3O4, detected for molar ratios ranging between 0.5 and 1.5, suggested the formation of a slightly nonstoichiometric LaMn1−xO3 phase with a relatively high content of Mn4+. The catalytic performance was evaluated in the total oxidation of two selected hydrocarbons, toluene and propene, which represent typical volatile organic compounds (VOCs). Typically, three consecutive catalytic cycles were performed in order to reach steady-state performance in toluene and propene oxidation. Moreover, the stability of the catalysts under reaction conditions was investigated through 24-h experiments at 17% of toluene conversion. The catalysts LM1.2, LM1.3, and LM1.5 showed the best catalytic performance in both hydrocarbon oxidations, well comparing with the Pd/Al2O3 used as a reference.

Highlights

Since the early investigations elucidating the peculiar activity of LaMnO3 -based perovskites in CO oxidation and the important role played by oxygen mobility [17], many further studies have been carried out to elucidate the relationships between the solid-state properties and their catalytic activity
The LaMnO3 samples prepared after calcination at 750 ◦ C were characterized by X-ray diffraction (XRD)
The constant BE of the La3d5/2 and Mn2p3/2 levels suggests, according to the results reported in the literature [42], that the chemical bonding for LaMnO3 was not affected by different synthetic procedures

Summary

Introduction

Catalysts based on lanthanum manganites (LaMnO3 ) with a perovskite structure have attracted wide interest in the last few decades due to their potential application in electrocatalysis and solidCatalysts 2019, 9, 226; doi:10.3390/catal9030226 www.mdpi.com/journal/catalystsCatalysts 2019, 9, 226 oxide fuel cells [1,2,3], as semiconductors thanks to their giant magnetoresistance properties [4], and in catalysis, especially for NOx removal [5,6,7] and methane, CO, and VOCs oxidation [8,9,10,11,12,13,14,15,16]. Catalysts based on lanthanum manganites (LaMnO3 ) with a perovskite structure have attracted wide interest in the last few decades due to their potential application in electrocatalysis and solid. Since the early investigations elucidating the peculiar activity of LaMnO3 -based perovskites in CO oxidation and the important role played by oxygen mobility [17], many further studies have been carried out to elucidate the relationships between the solid-state properties and their catalytic activity. A strong correlation between oxygen mobility and the rate of N2 O decomposition was found in La1−x Srx MnO3 samples prepared by the Pechini route [20]. The highest values of the content of fast-exchangeable oxygen and oxygen diffusion coefficient were found for a multiphase sample containing layered perovskite, providing a strong correlation between oxygen mobility and catalytic activity

Methods

Results

Conclusion