Insight into the catalytic performance and reaction routes for toluene total oxidation over facilely prepared Mn-Cu bimetallic oxide catalysts

Highlights

• Mn-Cu bimetallic catalysts were facilely prepared by one-step hydrothermal-redox method.

• Hydrothermal-redox was superior to other preparation methods for Mn-Cu catalysts.

• MnCu spinel type catalyst exhibited excellent low-temperature catalytic performance.

• PTR-MS results revealed the reaction routes of toluene oxidation over MnCu catalyst.

Abstract

Developing facile preparation method to obtain the satisfied low-temperature catalytic performance of transitional metal oxide-based materials is still a challenge in deep degradation of VOCs. Here, a series of Mn-Cu bimetallic oxide catalysts were prepared by one-step hydrothermal-redox method for catalytic total oxidation of toluene. The CH₃COOH concentration, Cu/Mn molar ratio and calcination temperature greatly affected the crystal structure, micromorphology and catalytic performance. Amongst, MnCu spinel structured catalyst exhibited excellent low-temperature catalytic activity, superior durability and water resistance in toluene total oxidation owing to its abundant surface adsorbed oxygen species, higher amount of Cu⁺ and Mn³⁺ and excellent low-temperature reducibility. The reaction rate of MnCu was 7.0 times higher than that of MnCu_0.5 at 210 °C. The cyclic redox process with enough oxygen vacancy played a vital role in toluene oxidation. The deep oxidation of benzene was the key step in the toluene oxidation. Proton transfer reaction-mass spectrometry (PTR-MS) results revealed the reaction intermediates including benzaldehyde, benzene and phenol, which further decomposed to acetone, ethanol, acetic acid, ketone and acetaldehyde by ring opening before total mineralization. Therefore, PTR-MS provided a facile method to investigate the reaction mechanism of toluene oxidation.

Introduction

With the rapid development of urbanization and industrialization, the emissions of volatile organic compounds (VOCs) are increasing from industrial processes and human activities [1], [2]. Many different types of VOCs can directly cause harm to environmental and human health because they are smelly, poisonous and carcinogenic. Several techniques including thermal combustion, photocatalysis, biotechnology, adsorption, condensation and catalytic oxidation are utilized to remove VOCs [3], [4], [5]. Amongst, catalytic oxidation was one of the most promising techniques due to its low-energy consumption, high efficiency and less harmful reaction by-products [6]. However, the catalysts still have some unsolved issues such as complex preparation and high cost. Thus, developing facile method to improve catalytic activity of catalyst in VOCs oxidation has been attracted more attention.

Due to the high cost and susceptibly poisoning of noble metal-based materials [7], transition metal oxides have been considered as the alternative for VOCs elimination because of their anti-poisoning ability, superior reducibility and relatively low cost [8]. However, the catalytic activity of a single transition-metal oxide such as Fe₂O₃, Co₃O₄, MnO₂ and CuO is still relatively poor [3]. Therefore, developing mixed metal oxides was a better and promising strategy for VOCs combustion owing to complementary advantages of different metals in catalyst. Indeed, the introduction of Cu, Co and Fe into Mn-based materials has been widely used in the catalytic oxidation of VOCs, which enhanced the catalytic activity [9], [10], [11], [12]. Among them, Mn-Cu oxides have attracted significant attention for their applications in various catalytic reactions such as catalytic oxidation of CO [13], total oxidation of VOCs [7], [14], [15], water gas shift reaction [16] and low temperature reduction of NO with NH₃ [17].

The catalytic performance of Mn-Cu oxides closely depended on the preparation method [18]. In order to improve the textural property and intimate interaction, Mn-Cu oxides have been intensively investigated by various preparation methods including impregnation [8], co-precipitation [18], combustion [10], [19], [20], hydrothermal [7], redox [13], solid-state reaction [21], sol-gel [22] methods. Amongst, the combustion, sol-gel, hydrothermal and redox methods have been applied to optimize the densities of catalytic active sites. For instance, Li et al. [10] proposed that the highly active layered CuO-δ-MnO₂ hybrid composites was synthesized by self-propagated flaming technique using KMnO₄ and Cu(CH₃COO)₂ as precursors. Papavasiliou et al. [19], [20] reported that a series of Cu-Mn spinel oxides synthesized by the urea-nitrate combustion method exhibited high catalytic activity. It should be pointed that the safety of combustion and flaming methods needs to be strictly controlled, which limited their wide application. Behar et al. [22] found that Mn-Cu catalysts with different Cu/Mn molar ratios were synthesized by sol–gel method using the ionotropic alginate gels as precursors, which provided better nanoscale oxide dispersion. However, some of sol/gel materials could be as hazardous wastes with high cost after preparation, which was harmful to environment if they were not well treated. Recently, hollow microsphere structure Mn-Cu catalyst with high BET surface area was prepared by hydrolysis-driven redox-precipitation method [1]. Although the Mn-Cu catalyst displayed higher catalytic activity than that of catalyst prepared by co-precipitation method, the catalytic activity was still lower than that of other catalysts prepared by hydrothermal and hydrothermal-redox methods [1], [7], [13].

In previous study, Luo et al. [7] proposed that copper-modified manganese oxides prepared by hydrothermal method exhibited preferable catalytic performance. However, the precursor must be aged for one day at a certain pH value before hydrothermal reaction at 210 °C for two days. Wang and co-authors [13] reported that the redox reaction of layered Mn-Cu oxide precursor was firstly carried out at a certain pH value and temperature for 2 h before hydrothermal reaction. Therefore, the hydrothermal and hydrothermal-redox methods reported by above references were still complex for synthesis of Mn-Cu catalysts. In order to enhance the performances of catalysts with using a facile preparation method, a series of Mn-Cu catalysts were synthesized by one-step hydrothermal-redox method with or without CH₃COOH. The affecting factors of CH₃COOH concentration, Cu/Mn molar ratio, calcination temperature, space velocity and water vapor on the catalytic performances were illustrated comprehensively. With the help of several characterization analyses, the MnCu catalyst exhibited the best catalytic activity, durability and excellent water resistance for toluene oxidation. Moreover, the reaction routes of toluene oxidation were studied by proton transfer reaction-mass spectrometry (PTR-MS), providing a facile approach to investigate the reaction mechanism.