Regular Article
Efficient catalytic degradation of toluene at a readily prepared Mn-Cu catalyst: Catalytic performance and reaction pathway

https://doi.org/10.1016/j.jcis.2021.01.096Get rights and content

Abstract

Fabricating of economical transitional metal oxide-based materials with satisfied low-temperature catalytic performance and application perspective is still a challenge in deep degradation of VOCs. Here, Mn-Cu bimetallic oxides were facilely prepared by one-step hydrothermal-redox method, which displayed much higher catalytic activity in toluene oxidation than those synthesized by hydrolysis-driven redox-precipitation or co-precipitation approach. It is shown that the lattice defect and oxygen vacancy concentration over prepared materials can be tuned by controlling Cu/Mn molar ratio. Amongst, spinel structured MnCu0.5 exhibited the highest catalytic activity, superior durability and water resistance in toluene total oxidation owing to abundant surface adsorbed oxygen species, excellent low-temperature reducibility, and high amounts of Cu+ and Mn3+. In detail, the reaction rate of MnCu0.5 was over 9.0 times higher than that of MnCu0.75, MnCu0.75-P and MnCu0.75-H2O2 at relative low temperature of 210 °C. The cyclic redox process with easier oxygen species mobility played a key role in the catalytic oxidation of toluene. Typical reaction intermediates as benzyl alcohol, benzaldehyde, benzene, phenol, and benzoquinone could be detected by PTR-MS, which further decomposed to acetone, ethanol, ketone, acetic acid, methanol, formaldehyde and acetaldehyde species by ring opening before total mineralization.

Introduction

Volatile organic compounds (VOCs) mainly derived from human activities and industrial processes have negatively impact on the human and environmental health [1], [2], [3]. A number of techniques (e.g., biotechnology, photocatalysis, thermal combustion, adsorption, condensation and catalytic oxidation) are utilized to remove VOCs, among which catalytic oxidation is one of the most efficient and promising technique due to its low-energy consumption, high efficiency and controllable reaction yield [4], [5], [6], [7]. Until now, transition metal oxides and noble metal-based catalysts have been usually developed for catalytic oxidation of VOC. Due to the susceptibly poisoning and high cost of noble metal-based materials [8], transition metal (e.g., Cu, Cr, Mn, V, Co, and Ni) oxides have been considered as the alternative for VOCs elimination because of their superior reducibility, anti-poisoning ability and relatively low cost [9]. Among them, MnOx catalysts as one category of the most active ones [8] can be further modified by different cations such as Cu2+, Co2+, Cr3+, Ni2+ and Ce4+ to form mixed oxide, which improve the redox ability and oxygen storage capacity [7], [8].

The intrinsic activity of mixed oxide materials including perovskite-based, spinel-based, cordierite-based, hydrotalcite-based and hexaaliminate-based catalysts also plays a crucial role in determining their catalytic performance [5]. Amongst, spinel-based catalysts have advantages of low cost and excellent catalytic stability [5], [10]. Mn-Cu bimetallic oxides as one category of spinel-based catalysts exhibited excellent catalytic performance and played a key role in various catalytic reactions such as total oxidation of VOCs, catalytic oxidation of CO [11], water gas shift reaction [12] and low temperature reduction of NO with NH3 [13]. The type, distribution and charge of spinel-type materials determined the catalytic activity. There are two spinel types including stoichiometric and non-stoichiometric types, and CuxMn3-xO4 as one of the non-stoichiometric spinel is rather complex and has different kinds of cations [14]. The different x values in CuxMn3-xO4 exhibit different structural and electrical properties. Cu1.5Mn1.5O4 as one of such a CuxMn3-xO4 material displays a distinguished feature of having a flexible valence. The abundant Mn4+ and Cu+ cations of Cu1.5Mn1.5O4 material are stabilized in the octahedral and tetrahedral sites without needing help from stabilizing agent, which was beneficial for catalytic stability and performance. Some previous studies reported that Cu1.5Mn1.5O4 based materials displayed superior catalytic performance for VOC degradation. Morales et al. [15] found that the excellent activity of Mn-Cu mixed oxide for ethanol total oxidation was attributed to Cu1.5Mn1.5O4 phase with manganese oxides owing to its enough oxygen vacancies and structure defects. Kim and co-workers [16] proposed that the existence of Cu1.5Mn1.5O4 phase in the Mn-Cu bimetallic catalysts significantly enhanced the catalytic activity for propane total oxidation, which was much higher than that of CuO. Behar et al. [10] claimed that the Cu1.5Mn1.5O4 nanoparticles exhibited the best catalytic performance for toluene combustion.

In order to improve the textural properties and Cu-Mn interaction, the Cu1.5Mn1.5O4 based catalysts have been intensively investigated for the various preparation methods including impregnation [9], co-precipitation [17], combustion [18], [19], [20], hydrothermal [8], redox [11], solid-state reaction [21] and sol–gel [10] methods. Among them, the hydrothermal or redox method was often applied to optimize the densities of active sites, which can improve the structure and catalytic activity. For example, Luo and co-authors [8] reported that copper modified manganese oxide catalysts were synthesized by hydrothermal method with 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, et al. proposed that the redox reaction of layered copper manganese oxide precursor was firstly carried out at pH of 8.5 and 80 °C for 2 h before hydrothermal reaction. As a result, the hydrothermal and hydrothermal-redox methods reported by above references were still complex for synthesis of Mn-Cu catalysts [11]. In order to enhance the catalytic active of catalysts under a facile method, a series of spinel type Mn-Cu based catalysts were prepared by one-step hydrothermal-redox method, which was compared with the materials synthesized by the co-precipitation and hydrolysis-driven redox-precipitation methods. The affecting factors of Cu/Mn ratio, space velocity and water vapor on the catalytic performances were illustrated comprehensively. The structures, morphologies, surface properties and catalytic performance of prepared catalysts were also extensively studied. MnCu0.5 catalyst exhibited the best catalytic activity with long-term stability and excellent water resistance for toluene oxidation, exhibiting great potential in the field of VOCs elimination. Moreover, the role of lattice oxygen and the reaction pathway of toluene oxidation were studied detailedly by proton transfer reaction-mass spectrometry (PTR-MS), providing a facial approach to investigate the reaction mechanism.

Section snippets

Preparation of catalysts by the co-precipitation method

0.05 mol of Mn(NO3)2 (50 wt% in H2O) and 0.029 mol of Cu(NO3)2·H2O were dissolved in 100 mL of water. Subsequently, NaOH solution (2 mol·L−1) was added into the mixture at a constant rate of 0.5 mL·min−1 to maintain pH of 9–10. The formed precipitate was stirred for 2 h, and then it was left to age for 6 h at room temperature. The precipitate was washed by water for several times until the pH of supernatant was≤ 7. The precipitate was dried at 80 °C overnight, and then calcinated at 450 °C for

Crystal structure

The XRD patterns of prepared catalysts are shown Fig. 1. 2θ angles of 32.5, 35.5, 38.7, 48.7, 53.5, 58.2, 61.5, 66.3, 68.1, 72.4, 74.8 corresponded to the (1 1 0), (−1 1 1), (1 1 1), (−2 0 2), (0 2 0), (2 0 2), (−1 1 3), (−3 1 1), (2 2 0), (3 1 1), (0 0 4) planes of CuO phase (ICDD 01-080-0076). The diffraction peaks of MnO2 catalyst at 2θ of 28.7, 37.4, 41.0, 42.9, 46.2, 56.7, 59.4, 67.1, 72.3 could be well identified as the (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 0), (2 1 1), (2 2 0), (3 1 0), (3 0 1) planes of the MnO2 phase

Conclusions

A series of Mn-Cu bimetallic oxide catalysts for toluene oxidation were fabricated by co-precipitation, hydrolysis-driven redox-precipitation and hydrothermal-redox methods, respectively. The Mn-Cu catalysts prepared by the one-step hydrothermal-redox exhibited higher catalytic active than others. The varied lattice defects and oxygen vacancy concentration can be tuned by controlling Cu/Mn molar ratios. Both the catalytic activity and reaction rate decreased as MnCu0.5 > MnCu0.25 > MnCu0.75

CRediT authorship contribution statement

Jian-Rong Li: Conceptualization, Data curation, Formal analysis, Funding acquisition, Project administration, Writing - original draft. Wan-Peng Zhang: Investigation, Methodology, Software. Chang Li: Investigation, Methodology, Software. Chi He: Writing - review & editing, Funding acquisition, Supervision.

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). The authors also appreciate the editor and reviewers for their professional work and valuable comments.

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