Full Length ArticleInsight into the catalytic performance and reaction routes for toluene total oxidation over facilely prepared Mn-Cu bimetallic oxide catalysts
Graphical abstract
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.
Section snippets
Catalyst synthesis
In a typical synthesis, KMnO4 (3.0 mmol) and (CH3COO)2Cu·H2O (3.0 mmol) were dissolved in different molar concentrations of CH3COOH solutions (30 mL), which was transferred to a Teflon-lined stainless steel autoclave (50 mL) after stirring for about 30 min, and it was heated at 140 °C for 12 h. After cooling to room temperature, the products were dried at 80 °C overnight after washing by ethanol and water for several times, and then they were calcined in air at 450 °C for 3 h. The obtained
Crystal structure and TG-DTG
The XRD patterns are shown in Fig. 1. The peaks of α-MnO2 material (Fig. 1a) at Bragg angle 2θ = 12.8, 18.0, 25.6, 28.7, 36.5, 37.4, 41.9, 49.7, 56.1, 60.1, 69.4 corresponded to (1 1 0), (2 0 0), (2 2 0), (3 1 0), (4 0 0), (2 1 1), (3 0 1), (4 1 1), (6 0 0), (5 2 1), (5 4 1) reflections of α-MnO2, respectively (PDF 44-0141) [23]. It indicated that the high purity of the α-MnO2 catalyst was successfully synthesized. The peaks of CuO catalyst (Fig. 1a) at Bragg angle 2θ = 32.5, 35.5, 38.7, 48.7, 53.5, 58.3, 61.5,
Conclusions
Mn-Cu bimetallic oxide catalysts for toluene oxidation were successfully prepared by one-step hydrothermal-redox method. The catalytic activities of catalysts increased with the decreasing of CH3COOH concentration. Effect of calcination temperature for MnCu precursor was investigated by TG-DTG, BET and FE-SEM, which indicated that the calcination temperature of 450 °C led to more active site. Both the catalytic activity and reaction rate decreased as MnCu > MnCu1.5 > MnCu-0.1 > MnCu0.5
CRediT authorship contribution statement
Jian-Rong Li: Conceptualization, Investigation, Writing - original draft, Funding acquisition. Wan-Peng Zhang: Investigation. Chang Li: Investigation. Hang Xiao: Formal analysis, Validation. Chi He: Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (51708535, 21922606, 21876139) and Ningbo Science and Technology Project of China (2017C50004). The authors also appreciate the editor and reviewers for their professional work and valuable comments.
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