In situ structural changes during toluene complete oxidation on supported EuCoO₃ monitored with ¹⁵¹Eu Mössbauer spectroscopy

Abstract

Ceria–zirconia supported (10 and 20 wt.% EuCoO₃) versus bulk EuCoO₃ perovskite were prepared via citrate decomposition at 700 °C and tested for toluene complete oxidation. S_BET, XRD, XPS, H₂-TPD and in situ ¹⁵¹Eu Mössbauer investigations were performed. Electron delocalization processes around the Eu cations are induced by the reaction, as proved via the evolution of the Mössbauer parameters. All characterization data indicate the formation of a well-dispersed EuCoO₃ at the surface of Ce_0.9Zr_0.1O₂ for the 10 wt.% loading and larger crystallite formation for 20 wt.% loading. In terms of intrinsic activity for toluene combustion, supported catalysts were more active than bulk EuCoO₃. All structural changes during operation time are reversible.

Introduction

Catalytic total oxidation of various volatile organic compounds (VOC) became an interesting and stringent solution for environment protection [1]. For such purposes noble metals are mainly used but they lack thermal stability, they can be easily poisoned/deactivated and are extremely expensive [2], [3]. On the other hand, mixed oxides are a suitable alternative in view of thermal stability, lower price and tunable by composition catalytic activity. Perovskite structure – ABO₃ (A—usually lanthanide, dodecahedral coordinated, B—transitional metal for catalytic purposes, octahedral coordinated) – may accommodate almost all the elements of the Periodic Table. This compositional flexibility induces interesting and useful properties and the capabilities of such materials were thoroughly studied from electrical, magnetic, optical and chemical point of view [4], [5], [6], [7], [8], [9], [10]. But only a few base-formulations are known for their activity in total oxidation reactions, namely those based on Fe, Co, Ni and Mn at B-site with La (partially substituted by other lanthanide or alkaline-earth metals) at A-site.

However, the major drawback of perovskite large-scale use for catalytic purposes is the low surface area and increased tendency to sinter at high temperatures. Deposition or embedding on adequate refractory supports is a suitable route for solving these problems [3], [11], [12], [13], [14]. The most common mesoporous supports such as alumina and silica were intensively studied. Even if stabilized with inert elements toward perovskite, components still yield catalytically inactive compounds such as spinels. Ceria-based materials and zirconia were reported as good carriers, leading to no interaction with the most common cations of perovskites [15], [16], [17]. In many occasions, they were found to provide an adequate surface area such as the case of cerium-based materials having good oxygen storage-and-release properties [18], [19]. At the same time, some studies on Co-based perovskites showed that in a reductive atmosphere Co can be reduced to Co⁰. This is an interesting way of preparing highly dispersed Co on lanthanide oxide active for synthesis gas production [20], [21]. However, under an oxidant atmosphere, the perovskite structure is reformed [21], [22].

The aim of this paper was to investigate the behavior of a Eu–Co-perovskite in total oxidation of toluene in in situ conditions. The literature is scarce reporting such studies for perovskites. Because they are black materials, it is hard to follow their behavior by using spectral techniques. In addition, the changes in the intrinsic properties of these materials are rather small when exposed to temperature, and consequently require highly sensitive techniques. One technique suitable for such a study is Mössbauer spectroscopy but it lacks the applicability for most of the interesting cations. Fortunately, ¹⁵¹Eu is a Mössbauer atom. The idea of Eu replacement for La in Co-based perovskites is not new but the literature reported only a partial replacement. These studies indicated a beneficial effect for methane combustion [23], [24]. The presence of europium in the perovskite formulation decreases the Goldschmidt tolerance factor (t = (r_A + r_O)/[2^1/2(r_B + r_O)], considering the oxide described by formula ABO₃) and increases the reducibility of B-site cation [5], [21]. Other reports dealt with fundamental studies or with comparative photocatalytic behavior of different lanthanide cations at A-site [25], [26] but not with application of Eu-cobaltites for VOC total oxidation. EuFeO₃ was one of the most active catalysts for photocatalytic destruction of different organic dyes [26].