ArticleMechanochemical redox-based synthesis of highly porous CoxMn1-xOy catalysts for total oxidation
Graphical Abstract
A mechanochemical redox reaction (MnO4− + 3Co2+ +5OH− = MnO2 + 3CoOOH + H2O) was introduced to obtain a CoxMn1–xOy catalyst with a high specific surface area. The synthesized CoxMn1–xOy catalyst exhibited good performance in the catalytic combustion reaction of VOCs.
Introduction
The catalytic combustion of hydrocarbons is of great importance in the control of volatile organic compounds (VOCs) and automobile exhaust gases [1, 2, 3, 4, 5, 6]. At present, most commercial catalysts are noble-metal-based materials with low combustion temperatures and long catalyst lifetimes [1, 7, 8, 9, 10, 11]. In contrast, transition metal oxide catalysts are inexpensive and based on earth-abundant elements, but their activities and stabilities are somewhat low [4, 12, 13, 14, 15], obstructing their path toward commercialization. In particular, cobalt-manganese oxides (CoxMn1–xOy) with multi-redox cycles are promising candidates in the catalytic oxidation of hydrocarbons [16, 17, 18, 19]. Several synthetic methods have been developed for these catalysts. Among the parameters that determine the catalytic performance of combustion catalysts, the porosity and surface active oxygen species appear to be most critical [11, 20, 21].
The traditional strategies for CoxMn1–xOy synthesis are the co-precipitation (CP) method, sol-gel (SG) method [22] and redox method [23], all of which are solution-based processes (SBPs). From the viewpoint of process intensification, there is still some room to modify the SBP. For example, the scope of metal precursors in SBPs is limited to liquid or soluble solid materials. Several operation units, such as the dissolution of metal precursors and liquid-liquid mixing, are required. Meanwhile, good dispersion of cobalt and manganese species is a key factor in CoxMn1–xOy catalysts, since active oxygen species are expected to form at the CoxMn1–xOy interfaces. Because of the different equilibrium constants for the Co2+ (1.6 × 10−15) and Mn2+ (1.9 × 10−13) ions involved, local separation of cobalt and manganese oxides is often observed when the CP method is used. Thus, the development of efficient methods for the synthesis of CoxMn1–xOy catalysts is of great interest.
Recently, mechanochemistry has been revisited; mechanochemical methods could enable the synthesis of porous catalytic materials (e.g., zeolites, ordered mesoporous polymers and carbons, porous metal oxides, metal-organic frameworks, MgAl layered double hydroxide), and are attractive due to their solvent-free nature. Herein, we demonstrate a mechanochemical redox process (MRP) for the facile synthesis of a CoxMn1–xOy catalyst via ball milling. The solution-based redox reaction involving the oxidation of a cobalt (II) salt with potassium permanganate (KMnO4) is a well-known method for the synthesis of CoxMn1–xOy catalysts [23, 24]. However, in solution processes, the oxidative ability of KMnO4 depends on its concentration, which may affect the formation of high valence species and high porosity within the CoxMn1–xOy. Unexpectedly, the CoxMn1–xOy catalyst prepared via MRP exhibited a surface area of 479 m2 g−1, which was higher than that of control samples prepared via CP (34 m2 g−1), SG (72 m2 g−1), and liquid redox (131 m2 g−1) methods. The physical effects of the mechanical action continually reduced the particle size during redox, leading to the formation of interstitial porosity.
In the aerobic combustion of propylene, propylene was completely oxidized at 200 °C using the CoxMn1–xOy prepared by MRP; this temperature was much lower than those of the control CoxMn1–xOy catalysts fabricated via CP (450 °C) and SG (400 °C). In addition, the CoxMn1–xOy obtained by MRP also showed good resistance to water vapor (4.2%, >65 h) and sulfur dioxide (20–100 ppm SO2). At the same time, this catalyst can be extended to other substrates, such as carbon monoxide and methane, to give low combustion temperatures (T90 = 120 °C for CO, T90 = 150 °C for ethanol, T90 = 225 °C for acetone, T90 = 250 °C for toluene, T90 = 540 °C for CH4). Furthermore, this mechanochemical redox process is scalable, and the kilogram–scale preparation is already complete, making it an attentive alternative to the traditional methods.
Section snippets
Preparation of catalysts
Typical procedure for the synthesis of CoxMn1–xOy by MRP. CoxMn1–xOy was synthesized by MRP based on the redox reaction of cobalt chloride, sodium hydroxide, and potassium permanganate without any templates. In a typical process, 2.8600 g of CoCl2·6H2O and 0.8000 g of NaOH were added to a stainless-steel reactor (25 mL) along with six stainless steel balls (two with a diameter of 1.0 cm, and four with a diameter 0.6 cm). The reactor was placed in a high-speed ball milling apparatus (Focucy
Catalyst synthesis and characterization
The chemistry in the MRP mainly followed the redox equation below: MnO4− + 3Co2+ +5OH− = MnO2 +3CoOOH + H2O
The synthesis of CoxMn1–xOy by the mechanochemical redox process was conducted in two steps, as shown in Scheme 1. First, cobalt chloride and sodium hydroxide were added to the ball-milling apparatus, and the solid mixture was ball-milled for 30 min, resulting in the formation of cobalt ions in a basic environment. Next, the oxidant KMnO4 was added to the reactor, and the
Conclusions
In summary, we used a mechanochemical redox process to prepare the catalyst CoxMn1–xOy. XRD, XPS, HRTEM, ICP-AES, and TGA results proved that CoxMn1–xOy existed as MnO2 and CoOOH with a Co:Mn molar ratio of 2.97:1, which was close to the theoretical value (3:1). The obtained CoxMn1–xOy catalyst had a specific surface area of 479 m2 g−1, which was higher than that of the control catalysts prepared by the co-precipitation method (34 m2 g−1), sol-gel method (72 m2 g−1), and solution redox process
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Published 5 December 2020
This work was supported by the National Natural Science Foundation of China (21776174), the Open Foundation of the State Key Laboratory of Ocean Engineering (Shanghai Jiao Tong University of China) (1809), Shanghai Jiao Tong University Scientific and Technological Innovation Funds (2019QYB06), China Shipbuilding Industry Corporation (CSIC) and Zhejiang XinAn Chemical Industry Corporation.