Highly Active Transition Metal-Promoted CuCeMgAlO Mixed Oxide Catalysts Obtained from Multicationic LDH Precursors for the Total Oxidation of Methane

Hussein Mahdi S Al-Aani,Ioana Fechete,Ionel Popescu,Ioan-Cezar Marcu,Mihaela Badea,Mihaela M Trandafir,Cătălin Negrilă,Mihaela Florea,Lucia N Leonat

doi:10.3390/catal10060613

Abstract

To improve the catalytic performance of an active layered double hydroxide (LDH)-derived CuCeMgAlO mixed oxide catalyst in the total oxidation of methane, it was promoted with different transition-metal cations. Thus, two series of multicationic mixed oxides were prepared by the thermal decomposition at 750 °C of their corresponding LDH precursors synthesized by coprecipitation at constant pH of 10 under ambient atmosphere. The first series of catalysts consisted of four M(3)CuCeMgAlO mixed oxides containing 3 at.% M (M = Mn, Fe, Co, Ni), 15 at.% Cu, 10 at.% Ce (at.% with respect to cations), and with Mg/Al atomic ratio fixed to 3. The second series consisted of four Co(x)CuCeMgAlO mixed oxides with x = 1, 3, 6, and 9 at.% Co, while keeping constant the Cu and Ce contents and the Mg/Al atomic ratio. All the mixed oxides were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) coupled with X-ray energy dispersion analysis (EDX), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption-desorption at −196 °C, temperature-programmed reduction under hydrogen (H2-TPR), and diffuse reflectance UV-VIS spectroscopy (DR UV-VIS), while thermogravimetric and differential thermal analyses (TG-DTG-DTA) together with XRD were used for the LDH precursors. The catalysts were evaluated in the total oxidation of methane, a test reaction for volatile organic compounds (VOC) abatement. Their catalytic performance was explained in correlation with their physicochemical properties and was compared with that of a reference Pd/Al2O3 catalyst. Among the mixed oxides studied, Co(3)CuCeMgAlO was found to be the most active catalyst, with a temperature corresponding to 50% methane conversion (T50) of 438 °C, which was only 19 °C higher than that of a reference Pd/Al2O3 catalyst. On the other hand, this T50 value was ca. 25 °C lower than that observed for the unpromoted CuCeMgAlO system, accounting for the improved performance of the Co-promoted catalyst, which also showed a good stability on stream.

Highlights

Volatile organic compounds (VOC) are known as major contributors to air pollution, directly affecting the health of human beings through their toxic nature [1]
The M-free CuCeMgAl layered double hydroxide (LDH) precursor consisted of a mixture of both LDH
The absence of diffraction lines corresponding to cerium- or transition metal-containing additional phases can be noted, suggesting that these cations were well dispersed in the M(3)CuCeMgAl LDH precursor samples, as already observed in CuCeMgAl LDH

Summary

Introduction

Volatile organic compounds (VOC) are known as major contributors to air pollution, directly affecting the health of human beings through their toxic nature [1]. These compounds contribute to the depletion of beneficial stratospheric ozone, the formation of harmful tropospheric ozone, and to the increasing of photochemical smog [1,2]. Much effort has been made to find new highly active oxide-based catalytic materials with the aim of replacing the noble metal catalysts for VOC destruction, the main findings being the object of several review papers and book chapters [1,5,6,7,8]

Methods

Results

Conclusion