Recent progresses in the synthesis of MnO₂ nanowire and its application in environmental catalysis

Abstract

Nanostructured MnO₂ with various morphologies exhibits excellent performance in environmental catalysis owing to its large specific surface area, low density, and adjustable chemical properties. The one-dimensional MnO₂ nanowire has been proved to be the dominant morphology among various nanostructures, such as nanorods, nanofibers, nanoflowers, etc. The syntheses and applications of MnO₂-based nanowires also have become a research hotspot in environmental catalytic materials over the last two decades. With the continuous deepening of the research, the control of morphology and crystal facet exposure in the synthesis of MnO₂ nanowire materials have gradually matured, and the catalytic performance also has been greatly improved. Differences in the crystalline phase structure, preferably exposed crystal facets, and even the length of the MnO₂ nanowires will evidently affect the final catalytic performances. Besides, the modifications by doping or loading will also significantly affect their catalytic performances. This review carefully summarizes the synthesis strategies of MnO₂ nanowires developed in recent years as well as the influences of the phase structure, crystal facet, morphology, dopant, and loading amount on the catalytic performance. Besides, the cutting-edge applications of MnO₂ nanowires in the field of environmental catalysis, such as CO oxidation, the removal of VOCs, denitrification, etc., have been also summarized. The application of MnO₂ nanowire in environmental catalysis is still in the early exploratory stage. The gigantic gap between theoretical investigation and industrial application is still a great challenge. Compared with noble metal based traditional environmental catalytic materials, the lower cost of MnO₂ has injected new momentum and promising potential into this research field.

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1. Introduction

The worldwide emissions of atmospheric pollutants, such as CO and volatile organic compounds (VOCs), have greatly increased year by year with the rapid development of the global economy.^1,2 For example, CO is generally regarded as a flammable, explosive, and toxic air pollutant, which is extremely harmful to human health and the living environment.^3–6 Besides, it is also a weak direct greenhouse gas and has an important indirect effect on global warming.^7–10 Generally, CO is mainly derived from the incomplete combustion of fuels.^11,12 As for the VOCs, they commonly include formaldehyde, benzene, toluene, xylene, etc.² They can enter into the human body through breathing and skin permeation, causing discomfort to the body and even inducing cancer. Besides, VOCs are also considered as important precursors of high concentrations of PM_2.5 and O₃ in the atmosphere. Therefore, it is greatly necessary to develop efficient technologies to control the emissions of CO, VOCs, and other atmospheric pollutants. The current technologies mainly include adsorption, separation, biodegradation, catalytic combustion, plasma coordinated catalysis, photocatalysis, etc. Among these strategies, the catalytic oxidation or combustion has attracted increasing attention due to the advantages of high purification efficiency, low reaction temperature, and low cost. Therefore, designing and developing the catalysts with advanced performances are the key solutions to the catalytic oxidation of these air pollutants.

Noble metals (Au,^13,14 Pt,^15–17 Pd,¹⁸ and Rh¹⁹) supported on particular metal oxides (CeO₂, Ce_xZr_1−xO₂ and Fe₂O₃) with excellent oxygen storage capacities usually perform high catalytic activity toward CO oxidation. However, they are extremely restricted in wide and large-scale application due to the high price and scarcity. Thus, it is very important to design and explore the low-cost catalysts with identical catalytic performance to the noble metals. It was reported that various transition metal oxides (Co₃O₄,^20,21 CuO,²² MnO₂,²³ and CeO₂ (ref. 24)) performed excellent catalytic activity for CO oxidation in the past decades. Among them, MnO₂ has been extensively investigated due to various advantages, such as low price, environmental friendliness, and relatively high activity.^25,26 With the continuous studies on MnO₂ based materials, it has been found that MnO₂ based catalysts usually perform good oxidation activities for the VOCs, such as formaldehyde, toluene, benzene, dimethyl ether, etc. Besides, the catalytic oxidation ability of MnO₂ can be greatly improved through crystal facet exposure engineering and regulating the loading methods.

Compared with the traditional bulk MnO₂, the nanostructured MnO₂ materials usually perform better physiochemical characteristics, such as the higher specific area, lower density, and more adjustable chemical nature. Furthermore, various studies have shown that MnO₂ with nanowire morphology usually perform high surface areas and strong metal–support interactions.^27–31 Therefore, the nanowire is the dominant morphology among the MnO₂-based nanostructured materials. Compared with MnO₂ nanorods, MnO₂ nanowires commonly possesses much more adsorbed surface oxygen, greater reducibility, higher specific area, and lower Mn–O bond strength, accounting for the better catalytic performances in the catalytic combustion of dimethyl ether²⁷ and plasma catalytic oxidation of toluene.²⁸ Saputra et al.²⁹ found that MnO₂ nanowire displayed much higher activity than the MnO₂ nanorod and nanofiber counterparts in activating oxygen ketones to degrade phenol. In addition to this, compared with rod-shaped and tubular MnO₂, the MnO₂ nanowire as the support behaved stronger interaction with Ag and displayed the much higher reactivity of toluene oxidation.³⁰ The α-MnO₂ nanowire also displayed the best catalytic activity toward gaseous benzene oxidation among the MnO₂-based nanomaterials with different morphologies (e.g., wire and rod-like α-MnO₂, β-MnO₂ microrods) even after decorating Co₃O₄ nanoparticles.³¹

In recent years, more and more advanced and important materials have been developed for the applications of MnO₂-based nanomaterials in the removal of environmental pollutants,^32,33 biosensing,³⁴ theranostic,³⁵ especially in the field of electrochemistry.^36–38 For example, Li et al.^37,38 synthesized α-MnO₂ nanowires with outstanding circulation stability, demonstrated rate performance, and high specific capacitance for supercapacitor electrode. Therefore, the previous reviews of MnO₂-based nanomaterials were mainly focused on their applications in electrochemistry.^39–43 However, there were few reviews paid close attention to their environmental applications. Yang et al.⁴⁴ systematically summarized the environmental applications of MnO₂-based materials. But they did not specially highlight the unique advantages of MnO₂-based nanowire materials, which have been considered as the basic assembly unit of two-dimensional or three-dimensional structure complex/heterostructure materials of MnO₂ (ref. 45) and normally have higher catalytic activity compared with MnO₂-based nanomaterials in other morphologies.^27–30 Meanwhile, the previous discussions about MnO₂-based nanowire materials were very scattered and their applications were mainly focused on the topic in the field of the electrochemistry. There has been few comprehensive reviews targeting the applications of MnO₂-based nanowire materials in the field of environmental catalysis. A systematic overview of frontier scientific research on the modulation of MnO₂-based nanowire materials for environmental catalysis application is urgent for the rational design and fabrication of high-efficiency MnO₂-based nanowire materials. Therefore, this review comprehensively summarizes the synthesis methods of MnO₂-based nanowires, such as hydrothermal, reflux, sol–gel, template method, etc., and analyses the effects of various influencing factors, such as the crystalline phase structure, crystal facet, morphology, doping, and loading, on the performances of catalytic oxidation of CO, VOCs, and other environmental pollutants. In addition, the latest research progresses of MnO₂-based nanowire materials in the field of environmental catalysis are also covered in this review. With the in-depth research on MnO₂-based nanowire materials, their environmental applications have been greatly improved. However, there is still big gap between the theoretical study and practical application. Therefore, it is of great necessity to summarize the recent progresses for better understanding the real surface reaction mechanism, which will promise the future environmental application in the fields of catalytic oxidation, combustion, purification, and other related research fields.

2. Synthesis of MnO₂-based nanowires

There are various preparation methods of MnO₂ nanomaterials, such as the liquid phase precipitation method,^46,47 reflux method,⁴⁸ electrodeposition method,⁴⁹ hydrothermal method,^50–53 sol–gel method,^54,55 template method,⁵⁶ thermal decomposition method,⁵⁷ and so on. As a result, the phase structures and chemical characteristics of MnO₂ nanomaterials fabricated by different methods will be completely different. Similarly, the synthesis of MnO₂-based nanowires also mainly employs the hydrothermal method, reflux method, sol–gel, and template method. Herein, the synthesis method, reaction condition, morphology parameters, and structural properties of MnO₂-based nanowires have been summarized in Table 1.

Table 1 The preparation method, raw materials, synthesis parameter, and the resultant structural properties of the MnO₂-based nanowires

MnO2 nanowire	Preparation method	Raw materials	Synthesis temperature (°C)	Synthesis time (h)	Length	Diameter (nm)	BET (m^2 g^−1)	Reference
α-MnO2	Hydrothermal method	MnSO4·H2O, KMnO4	150	12	2.5 μm	20–40	61.0	82
	Hydrothermal method	KMnO4, NH4Cl	200	48	Several μm	100–200	53.1	61
	Hydrothermal method	MnSO4·H2O, (NH4)2S2O8	140	12	0.4–5 μm	5–20	148.0	29
	Hydrothermal method	δ-MnO2, H2SO4	100	24	250–500 nm	30–60	93.1	27
	Hydrothermal method	KMnO4, CH3COOH	140	12	6–10 μm	30–60	39.9	83
	Reflux method	δ-MnO2 nanosheets	80–90	90–120	>10 μm	20–30	53.2	58
	Sol–gel template method	AAO template, Mn (CH3COO)2, citric acid	400	12	500–700 nm	70	—	54
β-MnO2	Hydrothermal method	MnSO4·H2O, (NH4)2S2O8	140	12	1.5–2.5 μm	60–100	55.7	84
	Hydrothermal method	Mn (NO3)2, O3	200	8	2–5 μm	6–12	73.5	58
γ-MnO2	Hydrothermal method	MnSO4·H2O, (NH4)2S2O8	90	24	—	—	—	85
δ-MnO2	Hydrothermal method	MnSO4·H2O, KMnO4	240	24	—	—	—
α-MnO2-100	Hydrothermal method	MnSO4·H2O, (NH4)2S2O8, (NH4)2SO4, KNO3	120	20	Several μm	15–20	87.9	86
α-MnO2-110	Hydrothermal method	KMnO4, (NH4)2SO4	180	24	—	45–50	119.5
	Hydrothermal method	KMnO4, (NH4)2C2O4·H2O	180	24	100–500 nm	10–50	—	87
α-MnO2-210	Hydrothermal method	MnCO3, HNO3, (NH4)2S2O8, H2SO4	140	2	100–500 nm	10–50	—
α-MnO2-310	Hydrothermal method	KMnO4, (NH4)2SO4	180	24	100–500 nm	10–50	—
	Hydrothermal method	KMnO4, (NH4)2C2O4	180	24	Several μm	15–20	135.3	86

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2.1 Hydrothermal synthesis method

The hydrothermal synthesis of MnO₂-based nanowires mainly involves oxidation–reduction reaction of MnO₄⁻ and/or Mn²⁺ or the phase transition of MnO₂ precursor. The hydrothermal method usually involves several key parameters, including the reagent amount, reaction time, hydrothermal temperature, pH, and so on. Therefore, the MnO₂-based nanowires with different phase structures can be synthesized in a precisely controllable way by tuning these key parameters in the hydrothermal process.

DeGuzman et al.⁵⁰ pointed out that pH and temperature could affect the final phase structure of MnO₂ in the synthesis of fibrous α-MnO₂ nanomaterials and α-MnO₂ would transit into β-MnO₂ when the temperature was kept above 120 °C. However, Wang et al.⁵¹ found that the effects of pH and temperature were not significant. In their study, 1D MnO₂ nanomaterials with different phase structures could be obtained in a wide temperature range (100–180 °C) just by varying the molar ratio of raw reactants (KMnO₄ and MnSO₄) without adjusting the pH (∼2.5) of the system. Besides, the type of the raw reactants greatly determines the hydrothermal reaction temperature of preparing the α-MnO₂ nanowire. For example, the optimal hydrothermal temperature was located between 120 °C and 160 °C when using KMnO₄ and MnSO₄·H₂O as raw materials;^58–60 whereas, the optimal hydrothermal temperature was 200 °C when using KMnO₄ and NH₄Cl as the precursors.^61–63 The effect of pH on the crystalline phase structure of the final product was also largely derived from the ion concentration as the tunnels of different MnO₂ crystal forms have different sizes and need different amounts of cations to stabilize.⁶⁴ Zhang et al.⁵³ showed that the phase structure of MnO₂ and the aspect ratio of the final product were greatly depended on the concentration of ions in the reaction system, especially K⁺. Because the high concentration of K⁺ could be used as the effective template, which was more conducive to the synthesis of α-MnO₂ nanowire.⁵³ Wang et al.⁵¹ believed that the layer structure of δ-MnO₂ tended to curl under the high temperature and pressure, which was considered as the determinant step for the formation of 1D nanostructures of MnO₂. Zhang et al.⁵³ also indicated that prolongating hydrothermal time could drive the self-assembly of α-MnO₂ into bundles and then transform into β-MnO₂. Therefore, the possible synthesis route of MnO₂-based nanostructures was described in Fig. 1. First, the MnO_x units appear in the solution and the δ-MnO₂ with the layer structure is then formed through a series of condensation reactions. δ-MnO₂ tends to curl under high temperature and high pressure. When K⁺ concentration is high, α-MnO₂ can be formed; otherwise, β-MnO₂ can be formed. With the prolonging of hydrothermal time, the [2 × 2] tunnel structure of α-MnO₂ directly collapses into the [1 × 1] tunnel structure of β-MnO₂.⁵³ Wei et al.⁵² studied the hydrothermal synthesis of MnO₂-based nanowires through the phase transition of granular MnO₂ precursor. They synthesized α-MnO₂ and β-MnO₂ single crystal nanowires by the hydrothermal treatment of γ-MnO₂ at different temperatures. This method not employing catalysts or templates could be easily reproduced. Therefore, the hydrothermal synthesis of different crystal types of MnO₂ nanowires with potential application prospects in large-scale could be realized.

Fig. 1 Schematic illustration of the self-assembly of α-MnO₂ nanowires into bundles and their subsequent phase transformation to β-MnO₂ microcrystals under prolonged hydrothermal conditions. Reproduced by Zhang et al.⁵³ with permission from Elsevier.

Generally, the hydrothermal method of preparing MnO₂-based nanowire has various advantages, such as high product purity, excellent dispersion, good crystallinity, uniform morphology, etc. Besides, the preparation conditions are simple and the fabrication time is short. Therefore, the hydrothermal method has been widely used for the synthesis of MnO₂-based nanowires. However, the hydrothermal method is usually limited by the capacity of the reaction system, so it is not suitable for the large-scale industrial production. Furthermore, the tightness of the reaction system will bring safety hazards.⁶⁵

2.2 Reflux method

The reflux is a kind of wet chemical synthesis method that does not require high-temperature calcination, and directly synthesizes nanomaterials with uniform particle size and high activity. The reflux method is suitable for the large-scale synthesis of MnO₂ nanowires owing to various advantages, such as simple operation and mild reaction conditions. Kijima et al.⁶⁶ synthesized MnO₂ with three different phase structures (α-, β-, and γ-) by the acid digestion of Mn₂O₃ under reflux conditions. The type of polymorph of MnO₂ products was strongly dependent on the reaction temperature, type of acid used, and the concentration. When H₂SO₄ was used as the acid, the α-MnO₂ was inclined to form at higher H₂SO₄ concentrations and lower reaction temperatures. In contrast, the β-MnO₂ was obtained at lower H₂SO₄ concentrations and higher reaction temperatures. The γ-MnO₂ were formed at intermediate conditions. When HNO₃ was used as the acid, only the β-MnO₂ and γ-MnO₂ could be formed. The α-MnO₂ could not be obtained in the HNO₃ solution. Compared with γ-MnO₂, the β-MnO₂ formation usually demanded higher reaction temperatures (>80 °C) and higher concentrations of HNO₃ (>10 mol L⁻¹).⁶⁶ Chen et al.⁶⁷ synthesized transition metal doped α-MnO₂ nanofibers by the reflux method. Compared to the pristine MnO₂, metal cation doped MnO₂ materials had slightly higher surface areas and pore volumes, especially when the dopant content was relatively high. Jin et al.⁶⁸ synthesized γ-MnO₂ homogeneous nanofiber with the length of ∼1 μm and the diameter of 50 nm by refluxing and used it as the catalyst for catalysing the atmospheric oxidation of toluene with molecular oxygen. Sinha et al.⁶⁵ prepared δ-MnO₂ nanosheets by the reflux method and further deepened the morphological evolution and shape transformation into ultra-long α-MnO₂ nanowires with extremely high purity by reflux. The length of the nanowire was observed to exceed 10 μm. The detailed preparation process and SEM results are shown in the Fig. 2. The reflux method also has the advantage of low chemical cost and avoids using autoclaves under harsh hydrothermal reaction conditions. As a result, the high-quality α-MnO₂ nanowires could be generated without using catalysts and stencils. The reflux method provides a promising preparation method of α-MnO₂ nanowires in large scale.⁶⁵ May et al.⁶⁹ synthesized α-MnO₂-based nanowires and further studied the effect of doping methods on low-temperature CO oxidation of CuO/α-MnO₂ catalysts based on the nitric acid reflux method of Uematsu et al.⁷⁰

Fig. 2 Preparation of gram level α-MnO₂ nanowires from aqueous KMnO₄/ethyl acetate biphasic system and their different stages (a−f) in digital/FESEM images. Reproduced by Sinha et al.⁶⁵ with permission from ACS.

2.3 Sol–gel method

As for the sol–gel method, it employs the metal alkoxides or inorganic salts as precursors to achieve gelation through continuous hydrolysis and polycondensation processes. Then, the gel experiences the heat, and other subsequent treatments to obtain the final target product. The sol–gel synthesis of nanostructured MnO₂ commonly includes various combinations of hydrated metal cations, such as manganese acetate (MnAc₂) and citric acid,⁷¹ MnAc₂ and n-propyl alcohol,⁵⁴ tetrabutylammonium permanganate and methanol,⁷² and KMnO₄ and fumaric acid (C₄H₄O₄).^73,74 The structures of the obtained MnO₂ are strongly depended on the molar ratio between the KMnO₄ and the organic reducing agents.⁷⁵ For example, the 3 : 1 reactant ratio of KMnO₄ to glucose favoured the cryptomelane (2 × 2 tunnel) synthesis, while the 5 : 1 ratio generated the layered birnessite-type material.⁷² Hastuti et al.⁷⁴ synthesized irregular spherical α-MnO₂ particles by the sol–gel method and discovered that transition metal (Fe³⁺, Cu²⁺) doping did not affect the phase structure but only changed the lattice parameters and morphology of the final product. Hashem et al.⁷⁶ prepared the α-MnO₂ materials by redox reaction of KMnO₄ and ascorbic acid using the sol–gel method. The morphology of the final sol–gel samples were long needles with uniform diameter of ∼15 nm and average length of ∼0.8 μm, and further aggregated to urchin-like balls. Hashemzadeh et al.⁷¹ prepared γ-MnO₂ polymorph using the manganese acetate and citric acid as the raw reagents in the sol–gel way. The obtained γ-MnO₂ polymorph was composed of particles with surface covered with needle-like nanorods. The researches have demonstrated that the sol–gel method could prepare 1D nanostructure MnO₂ nanowire with different morphologies. Ghorbani et al.⁷⁷ successfully synthesized MnO₂ nanowires with 45 nm in diameter using simple precursors, such as KMnO₄ and NaNO₃, by sol–gel method. They found that the optimal annealing temperature for achieving crystalline structure and nanowire morphology was 600 °C. Chin et al.⁷⁸ successfully prepared MnO₂ nanowires via the self-assembly of MnO₂ nanoparticles using the sol–gel method with KMnO₄ and Mn(ClO₄)₂ as the precursors. The microstructure and morphology of sol–gel derived MnO₂ nanowire were affected by the synthesis conditions and post synthesis heat treatment. Acidic pH value (pH 5.7) and calcination temperature at 300 °C were effective on achieving nanowire morphology and high surface area. Liu et al.⁷⁹ synthesized wire-like ion sieve MnO₂·0.5H₂O with about 50–200 nm in diameter and 0.5–2 μm in length using the sol–gel method. The ion sieve MnO₂·0.5H₂O nanowire had excellent adsorption performance for lithium extraction in seawater system and had a good application prospect. Furthermore, the combination of sol–gel and template has been considered as a better way to synthesize highly-ordered MnO₂-based nanowires than the sol–gel itself.^54,55 As shown in Fig. 3, Tang et al.⁵⁵ used CTAB as the surfactant to obtain highly dispersed ultrafine MnO₂ nanowires with the diameter of only 7 nm by the template directed sol–gel template method.

Fig. 3 (a) SEM image, (b) XRD pattern, (c) and (d) TEM images of MnO₂ nanowire prepared by sol–gel approach. Reproduced by Tang et al.⁵⁵ with permission from Elsevier.

2.4 Template method

The template method has been considered as a common method for preparing nanomaterials by employing various structure directing agents or templates in recent years. Nanomaterials with ordered structures are generated through the guidance of the template in the form of self-assembly by using the organic molecules as template agents. For example, Wang et al.⁵⁴ successfully prepared α-MnO₂ nanowire with the diameter of about 70 nm and the length between 500 nm and 700 nm using the sol–gel template method. The length and diameter of the nanowires greatly depended on the pore size and thickness of the employed AAO template. Gu et al.⁵⁶ used mesoporous silica SBA-15 as the hard template to prepare highly ordered array of mesoporous β-MnO₂ nanowires by the nano-casting method. Fig. 4 shows the schematic representation of the preparation of MnO₂ by using the SBA-15 as the template.⁸⁰

Fig. 4 Schematic representation of the preparation of MnO₂ by SBA-15 template. Reproduced by Zhang et al.⁸⁰ with permission from Elsevier.

The morphology of materials synthesized by the template method can be tunable by the choice of the template. However, the use of templates obviously increases the production cost. Therefore, reducing the costs of the templates is still an important concerned issue. Wang et al.⁸¹ fabricated hierarchical MnO₂ microfibers by using cotton as the template and KMnO₄ as the precursor. In comparison with other templates, biological materials are “green” and easy to obtain, and the morphology of cotton fiber is more uniform than other kinds of plant fibers. This green, sustainable, and low-cost biotemplating method can be extended to the synthesis of MnO₂-based nanowires and other metal oxide materials in the view of economical concern.

3. The factors affecting the catalytic performances of MnO₂ nanowires

3.1 Phase structure

The [MnO₆] octahedral unit is the basic building block for crystalline-phase MnO₂ materials. As shown in Fig. 5, when [MnO₆] octahedral units are linked in different ways, the MnO₂ can form a variety of polymorphs, such as α-, β-, γ- and δ-MnO₂.^88–90 Studies have found that the phase structure can significantly affect the catalytic activity of MnO₂, and both α-MnO₂ and δ-MnO₂ are the dominant phase structures of MnO₂-based catalytic nanomaterials.^91–94

Fig. 5 Ball-stick and polyhedron representations of (a) β-, (b) α-, (c) γ- and (d) δ-MnO₂. Reproduced by Li et al.⁹¹ with permission from Elsevier.

Specifically, α-MnO₂ occupies more space than β-, γ-, and δ-MnO₂ due to the double tunnel structure of [2 × 2] and [1 × 1]. As a result, it obtains more adsorption sites for the reactants (such as CO and VOCs) than other phases. At the same time, the existence of surface open pore structure can also increase the surface area of α-MnO₂ nanowires, thereby providing abundant oxygen adsorption sites for the reactants.⁹² In addition, α-MnO₂ also has the weakest Mn–O bond strength among these crystalline phase structures, which could provide the lattice and absorbed oxygen during the oxidation reaction. Therefore, α-MnO₂ performs much better activity than other phase structures in catalytic reactions, such as CO oxidation,²⁶ O₃ catalytic degradation,⁹³ plasma oxidation of toluene,²⁸ and degradation of phenol by activating oxyketone.²⁹

δ-MnO₂ is composed of [MnO₆] octahedrons shared by the double layer edges. The special layer structure makes it easy for toluene molecule to absorb on the surface of δ-MnO₂ and thereby behaves excellent activity in the catalytic oxidation of toluene.⁹¹ As the comparison, the α-MnO₂ performs weaker toluene oxidation activity due to the lower content of Mn⁴⁺ and lattice oxygen.⁹¹ Besides, δ-MnO₂ has also been proved to be the best crystalline phase structure for low-temperature catalytic oxidation of formaldehyde.⁹⁴

β-MnO₂ with [1 × 1] tunnels have the most stable redox ability among these four phase structures. Besides, the surface area of β-MnO₂ nanowire materials is usually the smallest among the four phase structures, which will greatly hinder the adsorption and activation of the reactants and oxygen. As a result, it usually performs the lowest activity in the catalytic oxidation of CO and VOCs. The [2 × 1] and [1 × 1] tunnel structures of γ-MnO₂ have relatively mediocre catalytic activity in the oxidation of CO and VOCs.

3.2 Crystal facet

Studies have shown that the catalytic activities toward specific substances can be significantly improved by synthesizing nanomaterials with a high proportion of exposure specific crystal facets.^95–97 Recently, more and more studies have paid attention to the influence of specific crystal facets on their catalytic effects, such as TiO₂,⁹⁵ Fe₂O₃,⁹⁶ CeO₂ (ref. 97) and Co₃O₄,²⁰ because they are closely related with the final catalytic performances. Meanwhile, the great progresses have been made in the application of specific crystal facet of MnO₂-based nanowires to the catalytic oxidation of VOCs, such as formaldehyde,⁸⁶ methylmercury⁹³ and toluene,⁸⁷ etc.

Theoretically, the reactivity and activity of crystal faces are proportional to their surface energy. Therefore, the crystal facets with higher surface energies are usually more reactive in heterogeneous reactions.⁹⁸ Many studies have been carried out on the α-MnO₂ nanowires because α-MnO₂ has highest activity among the four phase structures of MnO₂-based nanowires. The characteristic crystal facets of α-MnO₂ generally include (310), (100), (110), and (210). Among them, (310) crystal facet possesses the highest surface energy, which is beneficial to the formation of oxygen vacancy. Thereby, the adsorption and activation of O₂, H₂O (ref. 86) and O₃,⁹³ etc., could be promoted. As a result, (310) crystal facet performs obviously better activities on the oxidation of formaldehyde and the ozonation of methylmercury than (100) and (110) counterparts (see Fig. 6). Rong et al.⁸⁶ reported that the 310-α-MnO₂ could completely convert 100 ppm formaldehyde to CO₂ at 60 °C, which was even better than some silver-based catalysts.^99,100 As shown in Fig. 7, the (310) facets with high surface energy could not only facilitate adsorption/activation of O₂ and H₂O but also be beneficial to the generation of oxygen vacancies, which resulted in significant enhancement of HCHO oxidation activity. He et al.⁹³ reported that the 310-α-MnO₂ could completely remove 70 ppm of methyl mercaptan within 20 minutes at room temperature. However, crystal planes with higher surface energy usually disappear in most crystals with the fast growth and the thermodynamically stable crystal planes would preferentially dominate the surface to minimize the total surface energy of the crystal.⁸⁶ Meanwhile, Huang et al.⁸⁷ found that (210) crystal plane with the lower surface activation energy (24.75 kJ mol⁻¹) showed better catalysis activity and stability for toluene than (110) and (310) crystal plane. The lower surface activation energy accelerated the activation and oxidation of toluene. The alternating arrangement of Mn–O atoms in the electronic structure also speeds up the oxidation and desorption of intermediate products, thus exhibiting excellent activity.

Fig. 6 TEM and HRTEM images of (a–c) α-MnO₂-310, (e–g) α-MnO₂-110, and (i–k) α-MnO₂-100. Atomic structure of the (d) {310}, (h) {110}, and (l) {100} surface of α-MnO₂. Small (red) spheres are oxygen and large (purple) are manganese. Reproduced by Rong et al.⁸⁶ with permission from ACS.

Fig. 7 α-MnO₂ nanowires with (100), (110) and (310) facets for HCHO oxidation. Reproduced by Rong et al.⁸⁶ with permission from ACS.

All in all, the specific VOCs oxidation reaction mechanism on the MnO₂-based nanowires is the key to improving the oxidation ability of MnO₂-based nanowire catalyst to VOCs through crystal facet engineering. It is also greatly necessary to strengthen the research on the mechanism of catalytic oxidation of VOCs because of the specific advantages of each crystal facet of MnO₂-based nanowires.

3.3 Morphology

As well known, the physiochemical properties of MnO₂ with different morphologies are often different.^101,102 The common one-dimensional structure of MnO₂ includes nanorods, nanotubes and nanowires. Among these morphologies, the MnO₂ nanowires occupies an important place. Because one-dimensional nanowires can be used as the basic assembly unit of two-dimensional or three-dimensional structure materials.⁴⁵ Furthermore, the composite materials with novel structures and properties can be facilely prepared on the basis of them.

One of the basic parameters greatly affecting the performance of nanowires is the length or aspect ratio. For example, the photocatalytic performance of W₁₈O₄₉ nanowires is greatly affected by its aspect ratio;¹⁰³ the dielectric properties of BaTiO₃ nanowires also have close relationship with its length.¹⁰⁴ Therefore, the performance of the nanowire can be tuned by controlling the length or aspect ratio. Liao et al.¹⁰⁵ investigated the influence of the length of α-MnO₂ nanowires on the catalytic combustion performance of dimethyl ether. The results revealed that the length of the nanowires would affect the redox property of α-MnO₂ and the final catalytic combustion performance of dimethyl ether. Among them, α-MnO₂ nanowires with medium length (4–8 μm) possessed the best catalytic performance with ignition temperature at 167 °C and the complete combustion temperature at 240 °C. The 20 h life test demonstrated that the sample behaved good stability without deactivation.¹⁰⁵ Li et al.⁹¹ prepared three kinds of δ-MnO₂ with different crystallinities and morphologies. The complete toluene conversion was achieved at 250–275 °C for δ-MnO₂ and the toluene conversion efficiency of δ-MnO₂ would be promoted with the decrease of crystallinity degree and particle size. Although the effect of the crystallinity and morphology were not as significant as the phase structure, they indeed greatly affected the activity of MnO₂ in the catalytic oxidation of toluene.

3.4 Metal loading and doping

The metal loading and doping are two common used modification methods when preparing catalyst. Both of them could significantly improve the catalytic activity of the MnO₂-based nanowire. The loading method could highly and uniformly disperse metal or metal oxide on the surface of catalytic support and the catalysts with higher activities could be achieved due to the formed strong metal-support interactions.¹⁰⁶ The doping method is to replace a small portion of the second metal cations in the host metal oxide with different cations can greatly change the chemical bonding on the surface of the host metal oxide, thereby improving its catalytic performance.¹⁰⁷ The active site in such catalytic system could be either the oxygen atoms near the dopant or the dopant itself. As a result, the activity of MnO₂-based nanowire can be further improved by dispersing more active ingredients on its surface.

The studies related to the metal modification of MnO₂ usually starts with the loading of noble metals, such as Au and Ag. For example, Gu et al.⁵⁶ loaded Au nanoparticles on α-, β-MnO₂ nanowires and developed Au/MnO₂ catalyst with extremely high activity and stability for CO oxidation at low temperature. The Au/α-MnO₂ nanowire catalyst showed the CO conversion of around 37% at −90 °C. As the temperature increased, the CO conversion increased rapidly to 90% below −60 °C, and slowly reached 100% at around −34 °C. The low-temperature (−85 °C) CO oxidation rate over this catalyst (2.8 mol_CO h⁻¹ g_Au⁻¹) was almost 30 times higher than that of the Au/Mn₂O₃.¹⁰⁸ Qin et al.¹⁰⁹ investigated the toluene oxidation activity over the α-MnO₂ nanowires supported catalysts with different Ag loading amounts, which greatly affected the toluene oxidation performance. As could be seen in Fig. 8, 4% Ag/MnO₂ catalyst performed the highest activity of the toluene oxidation and the lowest apparent activation energy (45.5 KJ mol⁻¹) among Ag/MnO₂ catalysts. They found that 4% loading of Ag could greatly facilitate the adsorption of toluene molecules and optimally increase the activation capacity of MnO₂ lattice oxygen between the interface of Ag and MnO₂, thereby increasing the reaction rate and promoting deep oxidation between toluene and lattice oxygen.¹⁰⁹ Apart from the loading of noble metals, Sadeghinia et al.⁸⁴ reported that the catalytic activity of CO oxidation could be enhanced by loading the α-MnO₂ nanowires with different contents of CuO. The 30% CuO/α-MnO₂ catalyst could achieve the complete CO conversion at about 100 °C, which was nearly 100 °C lower than pristine α-MnO₂ nanowire. May et al.⁶⁹ further confirmed that the loading of CuO by the deposition precipitation (DP) method would make the one-dimensional nanostructure of MnO₂ with more excellent CO catalytic oxidation activity than the incipient wetness impregnation. Specifically, the most active 5CuO/MnO₂ could achieve 80% CO conversion rate at 30 °C.

Fig. 8 (a) Catalytic performance of the catalysts for toluene oxidation; (b) Arrhenius plot of the reaction rate of the catalysts. Reproduced by Qin et al.¹⁰⁹ with permission from Elsevier.

In view of the excellent performance of CuO loaded MnO₂-based nanowire catalysts,⁸⁴ the researches gradually focused on the transition metals doped catalysts because of the low cost and abundant availability. Lambert et al.¹¹⁰ reported that the catalytic activities of α-MnO₂ nanowires doped with Cu and Ni were significantly better than that of the pristine α-MnO₂ nanowires. Gao et al.⁶¹ investigated the effects of doping Fe, Co, Ni, and Cu transition metals on the catalytic performance CO oxidation of α-MnO₂ nanowires. As shown in Fig. 9, the appropriate amount of transition metal doping would not apparently change the morphology, phase structure, and specific surface area of MnO₂ nanowires. As could be observed in Fig. 10, the CO oxidation activities of the transition metal doped α-MnO₂ nanowires were also significantly increased. Among them, Cu doped α-MnO₂ nanowires showed the best CO oxidation activity. The density functional theory calculations further revealed that doping Cu could more easily form oxygen vacancies in MnO₂ than doping Fe, Co, and Ni, which was the determinant of the CO oxidation reaction rate. Therefore, the Cu-doped nanowires performed the higher CO oxidation catalytic activity than those for noble-metal-free catalysts, such as Cu-substituted Co₃O₄,¹¹¹ Cu-doped CeO₂,^112,113 CuO/Al₂O₃.¹¹⁴ In addition, Jampaiah et al.⁶² also studied the MnO₂ nanowires doped with two transition metals. Their results showed that the Cu and Co doped MnO₂ nanowires could greatly improve the soot oxidation activity and its T₅₀ of soot conversion was as low as 279 °C. The advantage of two transition metals doped MnO₂ nanowires was that it enhanced the strong synergistic interaction between the transition metals. However, this study had shown that there was an upper limit for the amount of surface metal doping of MnO₂ nanowires. Because the original nanowire morphology would be damaged when the surface metal doping amount reached 20%.⁶²

Fig. 9 SEM images of (a) MnO₂, (b) Fe_0.1MnO_x, (c) Co_0.1MnO_x, (d) Ni_0.1MnO_x, and (e) Cu_0.1MnO_x nanowires and (f) N₂ adsorption–desorption isotherms. Reproduced by Gao et al.⁶¹ with permission from Elsevier.

Fig. 10 CO oxidation activity tests of the MnO₂ and Fe-, Co-, Ni-, and Cu-doped MnO₂ catalysts. Reproduced by Gao et al.⁶¹ with permission from Elsevier.

3.5 Interface effect

The interface effect derived from nanostructures plays an important role in enhancing the catalytic activity. It is widely believed that multi-component metal oxides with coupling interfaces often exhibit better catalytic activity than single-component metal oxides.^115–119 For example, as regards the α-MnO₂@Co₃O₄ catalyst, the heterogeneous interface constructed by MnO₂ nanowires and Co₃O₄ nanoparticles could not only enhance the redox pair of Mn⁴⁺/Mn³⁺ and Co²⁺/Co³⁺ but also strengthen the adsorption of oxygen on the surface and accelerate the fluidity of oxygen. As a result, the catalytic activities of α-MnO₂@Co₃O₄ toward the oxidation of benzene³¹ and toluene¹²⁰ were much better than that of pristine MnO₂ nanowires. The α-MnO₂ nanowire@Co₃O₄ could reduce the T₉₀ of benzene oxidation to 247 °C (see Fig. 11) and the T₉₀ of toluene conversion to 229 °C, which were 100 °C and 47 °C lower than that of the pure α-MnO₂ nanowire, respectively.^31,120 Deng et al.¹²¹ successfully prepared the MnO₂ nanowire@NiO nanosheet core–shell hybrid nanostructure by uniformly decorating NiO nanosheets on α-MnO₂ nanowires. The obtained catalyst could achieve the complete oxidation of benzene at 320 °C, which was 60 °C lower than the pure MnO₂ nanowire. Ma et al.¹²² prepared MnO₂@TiO₂ by immobilizing crystalline anatase TiO₂ on the surface of α-MnO₂ nanowire. The as-prepared MnO₂@TiO₂ exhibited much higher maximum adsorption capacity (105.3 mg g⁻¹) of the radioactive element U(VI) than the pure MnO₂ (13.3 mg g⁻¹), illustrating that the performance of the surface-modified MnO₂ was greatly improved.

Fig. 11 Benzene oxidation activity tests of the 1D MnO₂@Co₃O₄ and 1D MnO₂ catalysts. Reproduced by Tang et al.³¹ with permission from Elsevier.

Homojunctions are junctions layer made of the same semiconductor materials.⁴⁴ It has been revealed that the construction of homojunctions could enhance the adsorbing and catalytic performance of the MnO₂ nanomaterials.^123,124 Zhou et al.¹²³ constructed α-MnO₂@δ-MnO₂ homojunction by loading layered δ-MnO₂ nanoflakes on the framework of α-MnO₂ nanotubes. The α-MnO₂@δ-MnO₂ homojunction integrated the exposed surfaces of the α-MnO₂ (100) surface and δ-MnO₂ (001) surface at the same time and displayed outstanding catalytic performance toward the oxidation of formaldehyde. The α-MnO₂ (100) surface was conducive to the adsorption and activation of oxygen molecules and the δ-MnO₂ (001) surface was favourable for the desorption of H₂O. The synergistic effect of the two surfaces significantly enhanced the catalytic activity for formaldehyde oxidation at room temperature. Huang et al.¹²⁴ synthesized the special two-phase structure MnO₂ catalyst simultaneously with α-MnO₂ and β-MnO₂ (α-MnO₂: β-MnO₂ = 1 : 1). As a result, toluene could be completely oxidized over the α@β-MnO₂ two-phase catalyst at about 205 °C, which was lower than the pure α-MnO₂ (220 °C) and β-MnO₂ (260 °C). Compared with the single-phase MnO₂, the α@β-MnO₂ with two-phase structure generated more abundant active interface and oxygen vacancy, thus promising enhanced catalytic performances.

4. Application in catalytic oxidation

4.1 CO oxidation

CO is one of the most common atmospheric pollutants harmful to the health of human beings. Its capacity of binding to hemoglobin is about 210 times higher than O₂.¹²⁵ Therefore, the combination of CO and hemoglobin severely hinders the delivery of blood oxygen, which will cause the supply shortage of blood oxygen and further threaten the life safety. Given these drawbacks, the low-temperature catalytic oxidation of CO has been considered as one of the most important environmental catalysis processes. As shown in Fig. 12, the proposed reaction mechanism of CO oxidation on manganese oxide catalyst commonly abides by Langmuir–Hinshelwood reaction mechanism (L–H mechanism) at low temperature (<90 °C) and Mars-van Krevelen reaction mechanism (MvK mechanism) at high temperatures (T > 190 °C).¹²⁶ When the temperature is low (50–90 °C), gaseous O₂ are captured by oxygen vacancies and the active oxygen species (O₂⁻ or O₂²⁻) could be formed. Subsequently, the adsorbed CO (CO–Mn³⁺) tent reacts with active oxygen species to generate bidentate carbonate species, and then decompose into CO₂. Finally, the dissociated O species could be restrained by oxygen vacancies and react with the adsorbed CO to form CO₂ which follows the L–H mechanism. When the temperature is high (T > 190 °C), the surface lattice oxygen species can directly react with adsorbed CO to generate CO₂ and oxygen vacancies due to the activation of surface lattice oxygen species. Subsequently, the oxygen vacancy was refilled by gaseous O₂ and the redox cycle is completed by following the MvK mechanism. The T₁₀, T₅₀, T₉₀, T₁₀₀ and BET of MnO₂-based nanowire materials on the catalytic oxidation of CO and some VOCs have been summarized in Table 2.

Fig. 12 Proposed reaction mechanism of CO oxidation on metal oxide. Reproduced by Tian et al.¹²⁶ with permission from Elsevier.

Table 2 T₁₀, T₅₀, T₉₀, and T₁₀₀ of MnO₂-based nanowire catalysts toward different catalytic substances

MnO2 based nanowire material	BET (m^2 g^−1)	Catalytic substance	T10 (°C)	T50 (°C)	T90 (°C)	T100 (°C)	Reference
α-MnO2	31.3	CO	—	∼180	∼210	260	84
20% CuO/α-MnO2	22.0		—	∼75	∼100	150
Au/α-MnO2	40.0		—	−83	−60	−34	56
α-MnO2	53.1		∼180	234	∼275	∼300	61
Fe0.1MnOx	57.2		∼170	212	∼260	∼275
Co0.1MnOx	42.5		∼110	133	∼160	∼180
Ni0.1MnOx	43.0		∼140	178	∼220	∼260
Cu0.1MnOx	55.7		50	80	—	100
α-MnO2	80.6		∼80	∼110	126	∼155	127
β-MnO2	55.7		∼95	∼150	169	∼185
Ag/α-MnO2	64.2		∼45	∼75	90	∼120
Ag/β-MnO2	40.0		∼35	∼65	80	∼115
α-MnO2	51.1	Soot	492	547	602	—	62
5Co–MnO2	50.2		436	513	567	—
10Co–MnO2	51.7		420	475	546	—
20Co–MnO2	50.7		479	528	586	—
5Ni5Co–MnO2	58.3		379	445	513	—
5Cu5Co–MnO2	59.4		368	431	485	—
α-MnO2	52.7	Benzene	243	301	345	∼375	31
α-MnO2@Co3O4	58.5		205	227	247	∼265
α-MnO2	52.7		243	301	∼350	380	121
α-MnO2@NiO	88.9		233	273	∼300	320
α-MnO2	—	Toluene	∼186	∼210	∼220	∼230	109
4% Ag/α-MnO2	—		—	∼195	∼215	∼220
α-MnO2	39.9		214	∼230	237	∼245	83
α-MnO2-110	—		—	142	170	200	87
α-MnO2-210	—		—	88	109	140
α-MnO2-310	—		—	130	170	200
α-MnO2	60.6		∼240	264	276	∼285	120
α-MnO2@Co3O4	54.9		∼205	220	229	∼235
α-MnO2	93.1	Dimethyl ether	200	∼230	274	∼290	27
α-MnO2(1 ∼ 3 μm)	—		176	—	—	250	105
α-MnO2(4 ∼ 8 μm)	—		167	—	—	240
α-MnO2(6–10 μm)	—		170	—	—	250
α-MnO2-100	87.9	Formaldehyde	∼80	125	∼145	150	86
α-MnO2-110	119.5		∼50	100	∼125	130
α-MnO2-310	135.3		—	35	∼55	65

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Previous studies found that the crystalline phase structure could significantly affect the CO catalytic activity of MnO₂.^26,92,127 Liang et al.²⁶ synthesized four different crystalline MnO₂ nano-oxides. They found that the order of activity for catalytic oxidation of CO was α ≈ δ > γ > β-MnO₂. Because the tunnel structure of [2 × 2] and [1 × 1] of α-MnO₂ could occupy more space to obtain more CO adsorption sites; whereas, other crystalline MnO₂ only involved β-MnO₂ with [1 × 1] tunnel, γ-MnO₂ with [2 × 1] and [1 × 1] tunnels, and δ-MnO₂ composed of [MnO₆] octahedrons shared by the double-layer edges. Another reason accounting for the enhanced catalytic CO oxidation performance of α-MnO₂ was that the elongated Mn–O bond length stemming from the distorted [MnO₆] units could benefit the cracking of the Mn–O bond, promoting CO oxidation.^26,127 As a result, α-MnO₂ could easily react with CO and thus further improved its catalytic performance.

In addition, various studies have been conducted to improve the catalytic activity of MnO₂-based nanowires for CO oxidation. In order to achieve this goal, effective strategies including element doping, surface engineering and combination with other active substances have been extensively conducted. Gao et al.⁶¹ found that the hydrothermal doping of Cu would significantly increase CO oxidation activity (TOF = 9.1 × 10⁻³ s⁻¹ at 70 °C) and water resistance of α-MnO₂ nanowires. As for γ-MnO₂, Zn-doped MnO₂ could achieve the 90% CO conversion at 160 °C and perform the best CO oxidation activity among Cu, Zn, Mg, Co, Ni, Ca, Al, Fe, and La-doped γ-MnO₂.¹²⁸ Besides, the surface engineering is considered as another effective method to improve the catalytic activity of MnO₂. Li et al.⁹² synthesized α-MnO₂ with a large number of surface open cells through acid treatment. The open cells were rich in adsorption sites for O₂, thus enhancing the catalytic activity of MnO₂ for CO oxidation. Moreover, combining Au,⁵⁶ Ag,¹²⁷ CuO,^69,84 etc. with MnO₂ nanomaterials can effectively improve CO oxidation efficiency. Xu et al.¹²⁷ investigated the CO oxidation activity over the α-MnO₂ nanowires supported Ag based catalysts and the T₉₀ of Ag/α-MnO₂ below 100 °C could be achieved. Although many studies have been carried out to enhance the catalytic activity of MnO₂ for CO oxidation and the complete CO conversion could be achieved at −34 °C,⁵⁶ the current research hotspot is still focused on the loading or doping noble/transition metals. The further explorations should be concentrated on the surface engineering or the rare earth metals modification.

4.2 Catalytic removal of VOCs

The volatile organic compounds (VOCs) typically refer to organic compounds with boiling points between 50 °C and 260 °C at atmospheric pressure (101.325 kPa) according to the World Health Organization.¹²⁹ Besides, most VOCs emitted into the atmosphere can cause the formation of secondary pollutants, such as tropospheric ozone, peroxyacetyl nitrate, and secondary organic aerosols.^130–132 Various means have been used to remove the VOCs, such as the physical adsorption,¹³³ catalytic oxidation,^134,135 photocatalytic degradation,^136,137 plasma catalytic oxidation,¹³⁸ and biological decomposition.¹³⁹ Among these strategies, the catalytic oxidation has been considered as one of the most effective ways owing to various advantages, such as high efficiency, less secondary pollutants, and low reaction temperature. As for the catalysts of VOCs oxidation, compared with precious metal catalysts, the transition metal oxides catalysts are considered to be more promising candidates owing to their low costs and excellent catalytic performances.¹⁴⁰ MnO₂ based nanomaterials are widely investigated as the effective catalysts and/or supports in the fields of pollutant removal, energy storage, and water treatment due to the excellent oxygen storage/release capacities, adsorption performances, and redox performances.¹⁴¹

4.2.1 Benzene. Benzene is extensively used in the fields of basic chemical and petrochemical industries. However, it is also harmful to human health at high risk. It is classified as the human carcinogen causing leukemia by the WHO.¹²⁹ Up to now, the supported noble metal based catalysts, such as Pt, Pd, Au, Ag based catalysts,^142–145 are well known as the active catalysts for the oxidation of the benzene at low temperature. However, the development of the low-cost catalysts for the oxidation of benzene is in greatly urgent demand and the MnO₂ based catalysts are promising candidates due to facile availability.

Hu et al.¹⁴⁶ found that the order of catalytic activities on benzene combustion over MnO₂ with different phase structures was γ-MnO₂ > β-MnO₂ > α-MnO₂ > δ-MnO₂. The surface adsorbed oxygen species concentration and low temperature O₂ desorption of these manganese oxides were in good agreement with the sequence of their catalytic performance on benzene combustion. Besides, the strategies such as metal doping and loading have been investigated to improve the catalytic activities on benzene combustion over MnO₂. Liu et al.¹⁴⁷ found that the Ce³⁺ and Cu²⁺ doped of MnO₂ nanosheet could significantly improve the activity of MnO₂ for benzene decomposition. For the pristine MnO₂, benzene was hardly decomposed below 200 °C and only ∼40% conversion was achieved at 400 °C. Whereas, as for the Ce–MnO₂ and Cu–MnO₂ catalysts, the benzene started to decompose around 100 °C and nearly all benzene could be removed around 250 °C. Moreover, the Cu–MnO₂ catalyst also exhibited high resistance to high-temperature deactivation and water resistance during continuous long-term testing. Recently, Ma et al.¹⁴⁸ synthesized the tungsten-doped MnO₂ (W–MnO₂) with excellent performance of the photo-assisted thermal catalytic oxidation/removal of benzene. They found that doping W could not only generate abundant oxygen vacancies in MnO₂, but also produce strong covalent interactions with adjacent O atoms through W–O bridge bonds, thereby promoting O₂ polarization and electron transference.

As for the metal loading, Ye et al.¹⁴⁹ prepared nanosized α-MnO₂-supported silver catalysts for benzene oxidation. They found that the 5Ag/MnO₂ catalyst performed the best activity of the benzene oxidation. Tang et al.³¹ modified one-dimensional MnO₂-based catalysts (wire-like, rod-like α-MnO₂ and β-MnO₂ microrods) by the Co₃O₄ nanoparticles (see Fig. 13) and investigated them as the catalysts for benzene oxidation. They found that the modified MnO₂@Co₃O₄ composite exhibited better catalytic performance than the pristine MnO₂. The specific conversion efficiencies of the modified material to benzene abided by the order of α-MnO₂ nanowires > α-MnO₂ nanorods > β-MnO₂ nanorods. The reason for this could be attributed to the enhanced low-temperature reducibility and abundant active surface adsorption of oxygen derived from the heterogeneous hybrid interface.

Fig. 13 Schematic process for the preparation of 1D MnO₂@Co₃O₄ hetero-nanostructures. Reproduced by Tang et al.³¹ with permission from Elsevier.

4.2.2 Toluene. Toluene could directly threaten the human health and safety because of its carcinogenicity and teratogenicity.^150,151 Therefore, the effective removal method of toluene should be explored in great urgency considering the harmful effects of toluene on the human health and the strict environmental regulations. Cheng et al.⁸³ found that α-MnO₂ nanowires performed much higher catalytic activity than the commercial MnO₂ counterpart toward toluene combustion. The reason for this could be attributed to the enhanced surface oxygen mobility and surface area of α-MnO₂ nanowires, which were significantly better than the commercial MnO₂. The possible oxidation reaction mechanism of toluene over α-MnO₂ catalyst was summarized in Fig. 14. Firstly, toluene adsorbed on α-MnO₂ catalyst surface. The adsorbed toluene would be subsequently oxidized into benzyl alcohol by surface lattice oxygen. Meanwhile, the resultant oxygen vacancies could be supplemented by gas-phase O₂, and active oxygen species (O₂/O₂⁻/O₂²⁻/O⁻/O²⁻) would be formed.⁸⁶ The benzyl alcohol would be oxidized by active oxygen species into the benzaldehyde, and then further oxidized into the benzoic acid. Finally, the benzene ring would be destroyed by active oxygen species. After ring-opening, the unsaturated long chain enol and alkyne could be ultimately transformed into the CO₂ and H₂O quickly.⁸⁷

Fig. 14 Proposed mechanism of toluene degradation pathway on α-MnO₂. Reproduced by Huang et al.⁸⁷ with permission from Elsevier.

It has been reported that the catalytic performances of the toluene oxidation over the MnO₂ nanomaterials were strongly related to the crystalline phase structures.^91,124,152 Li et al.⁹¹ prepared MnO₂ with different crystalline phases (β-, α-, γ-, and δ-). The results showed that catalytic toluene oxidation activities followed the sequence of δ > γ > α > β-MnO₂. The morphology of the MnO₂ nanostructure also greatly influences the catalytic performance of the toluene oxidation. The previous study showed that the MnO₂ nanowire showed the most excellent toluene combustion catalytic activity (T₁₀₀ = 220 °C) after loading silver among the MnO₂-based one-dimensional nanostructures (nanowire, nanorod, and nanotube).³⁰

Besides, the metal loading and doping can also greatly enhance the catalytic performance in toluene oxidation removal.^153,154 Dong et al.¹⁵³ found that the introducing Na⁺ and Ca²⁺ into MnO₂ promised the higher catalytic activity for toluene oxidation than pristine MnO₂. Specifically, the Ca–MnO₂ catalyst exhibited the best catalytic activity (T₉₀ = 215 °C) and possessed good water resistance. The outstanding catalytic performance of Ca–MnO₂ was closely related to the abundant acidic sites, high oxygen vacancy concentration, and the weak CO₂ adsorption capacity. Min et al.¹⁵⁴ found that transition metal (Ni and Co) doped α-MnO₂ and β-MnO₂ exhibited better catalytic performances in toluene oxidation removal owing to their lower crystallinities, more defects, larger specific surface areas, more oxygen vacancies, and better low-temperature redox ability than the pristine α-MnO₂ and β-MnO₂.

The catalytic performance of the toluene oxidation was also influenced by the interface between different metal oxides. Ren et al.¹²⁰ fabricated ZIF-derived Co₃O₄ grown in situ on 1D α-MnO₂ materials (as displayed in Fig. 15) and used it as the catalyst for toluene oxidation. They found that the α-MnO₂@Co₃O₄ could reduce the T₉₀ of toluene conversion to 229 °C, which was 47 °C and 28 °C lower than those of the pure α-MnO₂ nanowire and Co₃O₄-b, respectively. The main reason for the enhanced catalytic activity could be attributed to the synergistic effect produced by the coupling interface constructed between α-MnO₂ and Co₃O₄. The specific reaction route of toluene oxidation on α-MnO₂@Co₃O₄ catalyst could be illustrated as follows: toluene → benzoate species → alkanes containing oxygen functional group → CO₂ and H₂O. In addition, the α-MnO₂@Co₃O₄ catalyst also exhibited excellent stability and water resistance during the toluene oxidation.¹²⁰ Zhang et al.¹⁵⁵ found that the fluorine (F)-doped titanium dioxide (TiO₂) coupled with a small number of α-MnO₂ nanowires could effectively improve the photocatalytic degradation effect of toluene. Specifically, the toluene degradation efficiency before coupling was about 90%, and the toluene degradation efficiency after coupling was increased to 96%.

Fig. 15 Illustration of the fabrication of α-MnO₂@Co₃O₄ samples. Reproduced by Ren et al.¹²⁰ with permission from Elsevier.

4.2.3 Phenol. Phenol is deemed as one of the most indispensable organic compounds widely used in various fields. However, it is also an aromatic environment pollutant and widely exits in the industrial wastewater, such as petroleum refinery, wood preservation, and coking.¹⁵⁶ Phenol is a highly toxic substance, which has been listed as one of the most important organic pollutants with carcinogenic, mutagenic, and teratogenic hazards.¹⁵⁷ Although MnO₂-based nanowires have been widely used in environmental catalysis, yet there are few studies investigating the application of MnO₂-based nanowire materials in water treatment. Dong et al.¹⁵⁸ found out that the β-MnO₂ nanowires possessed outstanding catalytic performance for phenol degradation. Saputra et al.¹⁵⁹ found that the α-MnO₂ materials exhibited different activities in activating peroxymonosulfate to produce sulfate radicals for phenol degradation because of the variety in the phase structure and morphology. The results demonstrated that α-MnO₂ nanowire presented the highest activity with stable performance among the investigated nanomaterials. Then, they investigated the effect of phase structure of MnO₂-based nanomaterials on the activities in activating oxone to generate sulfate radicals for phenol degradation. As can be seen in Fig. 16, the wire-like α-MnO₂ still exhibited higher activity than the rod-like β-MnO₂ and fiber-like γ-MnO₂ because of the large specific surface area and unique tunnel structure. Besides, the β-MnO₂ exhibited the lowest activity due to single tunnel and stable oxygen reducibility. Meanwhile, Peng et al.¹⁶⁰ also illustrated that α-MnO₂ nanowires performed higher activity than α-MnO₂ nanotubes in the catalytic phenol degradation. The probable reason for this was that α-MnO₂ nanowire possessed much greater surface area and lower negative surface charge density than the α-MnO₂ nanotube, further accounting for the higher catalytic activities. Liang et al.¹⁶¹ synthesized the mesoporous Co₃O₄/α-MnO₂ catalyst exhibiting greatly high activity in peroxymonosulfate activation for phenol degradation with 100% conversion in 20 min. The stability tests also showed that the mesoporous Co₃O₄/MnO₂ catalyst presented excellent stability of phenol degradation in several runs without evident deactivation.

Fig. 16 Phenol degradation of MnO₂-based nanomaterials with different phase structures. Reproduced by Saputra et al.²⁹ with permission from ACS.

4.2.4 Formaldehyde. Formaldehyde (HCHO) has been extensively considered as one of the most harmful indoor VOCs closely related to the human health. As well known, most of the indoor decoration materials will release formaldehyde. Besides, the incomplete combustion of fuel and tobacco will also generate formaldehyde. The formaldehyde is not only a carcinogen but also greatly harmful to the eyes, nose, respiratory system, nervous system, and cardiovascular system.^162–164 Therefore, it is greatly urgent and necessary to effectively reduce the indoor HCHO to improve air quality and reduce public health risks. The catalytic oxidation of HCHO has been considered as a facile, green, and sustainable route. The noble metal catalysts commonly used for HCHO oxidation mainly contain platinum.^165–167 Therefore, their widely industrial and commercial applications are severely restricted because of the high cost. Sekine et al.¹⁶⁸ found that MnO₂ could react with HCHO and release CO₂ even at room temperature. This indicated that MnO₂ possessed great potential applications as the efficient catalysts in the catalytic oxidation of HCHO and could also be used as catalyst supports. Wang et al.¹⁶⁹ proposed a possible HCHO oxidation mechanism over the partially crystallized mesoporous MnO_x catalyst under dry condition based on the MvK mechanism. As shown in the Fig. 17, firstly, the free oxygen molecules can be captured by oxygen vacancies on the surface of the catalyst. Then, the captured oxygen molecules are stripped and further decomposed into active oxygen species (i.e., adsorbed oxygen: O_ads); Secondly, Mn³⁺ is oxidized into Mn⁴⁺ by O_ads and meanwhile one electron is released to O_ads to produce O⁻.^170,171 Thirdly, the adsorbed HCHO on the surface of manganese oxide is oxidized into the HCOOH intermediate by the O⁻. During this process, an electron will transfer from O⁻ to manganese cations and reduce Mn⁴⁺ to Mn³⁺ at the same time. Subsequently, the HCOOH intermediate further decomposes into H⁺ and HCOO⁻.^168,172 The HCOO⁻ subsequently reacts with another O_ads to produce HCO₃⁻ and HCO₃⁻ is finally degraded into H₂O and CO₂.

Fig. 17 The possible mechanism of catalytic oxidation of HCHO to CO₂ on partially crystallized mesoporous MnO_x under dry condition. Reproduced by Guan et al.¹⁷³ with permission from Elsevier.

Li et al.¹⁷⁴ reported that the sphere-like mesoporous MnO₂ exhibited good catalytic activity and reusability for the formaldehyde gas oxidation and the formaldehyde purification efficiency could reach 99% after reacting at room temperature for 2 hours. Zhang et al.⁹⁴ studied the catalytic activity of formaldehyde oxidation over the MnO₂ nanomaterials with different phase structures (α-, β-, γ- and δ-). It was found that the δ-MnO₂ catalyst performed the best activity among these catalysts and could achieve the complete conversion of the HCHO at 80 °C, which was much lower than the α, β, and γ-MnO₂ at 125 °C, 200 °C, 150 °C, respectively. Besides, previous researches revealed that the Au-doped¹⁷⁵ and Ag-loading¹⁷⁶ α-MnO₂ nanorods exhibited superior catalytic performances of HCHO oxidation. The 100% conversion of HCHO could be achieved at 75 °C and 80 °C over the Au-doped and Ag-loading α-MnO₂ nanorods, respectively. Rong et al.⁸⁶ prepared (310) facet exposed α-MnO₂ nanowire catalyst through crystal facet engineering with superior HCHO oxidation activity. It could completely convert HCHO into CO₂ at 60 °C and achieve 86% conversion rate of 0.53 mg m⁻³ HCHO at room temperature. He et al.¹⁷⁷ enhanced the catalytic oxidation of HCHO at ambient temperature over the α-MnO₂ with four well-defined morphologies (nanowire, nanotube, nanorod, and nanoflower) with the help of the mechanical vibration. Among them, α-MnO₂ nanowires exhibited the best catalytic activity with 100% conversion of 125 ppm HCHO at ambient temperature owing to the high oxygen vacancy concentration.

4.2.5 Methyl mercaptan. Methyl mercaptan (CH₃SH) is a highly toxic and corrosive sulfur-containing VOC (S-VOCs). It is widely released from sewage treatment and sanitary landfill.¹⁷⁸ The exposure of CH₃SH at few ppm-level can make the human feel unpleasant and dozens of ppm-level can cause significant poisoning.^179,180 In these years, the catalytic removal of CH₃SH has been extensively studied. Catalytic combustion technology is considered as one of the most efficient technologies to convert S-VOCs into less toxic products (CO₂, SO₄²⁻, etc). But the temperature of catalytic combustion technology is usually higher than 300 °C, which consumes high energy and leads to the sintering of catalysts.^181–185 Currently, catalytic ozonation has been recognized as a promising approach owing to its advantages of high efficiency and complete oxidation of S-VOCs under mild conditions.^186,187 In most studies, supported noble metals (Au, Ag, Pt and Pd, etc.) and low-cost metal oxides like NiO, Fe₂O₃/Fe₃O₄, MnO_x, and Co₃O₄ have been developed as efficient catalysts for the catalytic ozonation of S-VOCs.¹⁸⁷ MnO₂ is the most widely studied due to its plentiful valence states, versatile structures, high stability, and low cost.

The oxygen vacancies in MnO₂ are usually considered as the active sites for O₃ adsorption and catalysing ozonation.¹⁸⁸ Fig. 18 shows the adsorption of CH₃SH over the MnO₂ catalyst and the mechanism of catalytic ozone.⁹³ O₃ can be captured and decomposed by the structural defects on the MnO₂ surface, then transformed into activate oxygen species (O²⁻/O₂²⁻), and quickly evolved into ˙O₂⁻/˙OH/¹O₂ through a chain cycle reaction, which can directly participate in CH₃SH and adsorbed CH₃S⁻ oxidation. It is desirable to obtain highly active crystal facets with abundant oxygen vacancies through surface engineering to improve the catalytic activity. He et al.⁹³ prepared α-MnO₂ nanowire with different crystal facets exposed and successfully employed them to catalyse the ozonation of CH₃SH. The synthesized 310-MnO₂ catalyst performed higher activity than the 110-MnO₂ and 100-MnO₂. In addition, previous researches found that loading the metal active sites on MnO₂ nanomaterials could also enhance the ozonation catalysis of CH₃SH.^187,189 Xia et al.¹⁸⁷ prepared Ag deposited three-dimensional MnO₂ porous hollow microspheres (Ag/MnO₂ PHMSs). The optimal 0.3% Ag/MnO₂ PHMSs could completely degrade 70 ppm CH₃SH within 600 seconds, displaying much higher efficiency than that of the pristine MnO₂ PHMSs support (79%). Additionally, the catalyst retained long-term stability and could be facilely regenerated to its initial activity with ethanol and HCl. Yang et al.¹⁸⁹ synthesized urchin-like oxygen vacancy-rich (V_O) hollow-structured MnO₂ with the loading of CuO nanoparticles on the surface (CuO/V_O-MnO₂). The well-dispersed CuO on the surface of urchin-like hollow MnO₂ enhanced the chemisorption of CH₃SH, thus promoting the efficient decomposition of CH₃SH on the CuO/V_O-MnO₂. Among the as-prepared CuO/V_O-MnO₂ catalyst, the 5CuO/V_O-MnO₂ realized the optimum CH₃SH removal efficiency (98.5%) within 1200 s reaction under 25 °C, which was obviously higher than that of individual MnO₂ (70.7%) and CuO (57.0%).

Fig. 18 Mechanism of adsorption and catalytic ozonation toward CH₃SH by 310-α-MnO₂ catalysts. Reproduced by He et al.⁹³ with permission from ACS.

However, there are few researches about the wire-like MnO₂ for the ozonation of CH₃SH except for the study of He.⁹³ In view of the strong interaction between wire-like MnO₂ and noble/transition metals, more attentions will be attributed to the metal loading/doping of MnO₂ nanowire for the ozonation of CH₃SH in the future research.

4.3 Ozone

Ozone (O₃) is a light blue gas with special odor and strong oxidizing ability. O₃ usually can accomplish the complete oxidation of VOCs at room temperature. Therefore, the ozone is widely used as the oxidant in environmental treatment. However, when the amount of the pollutants, such as VOCs, are very large, the balance of ozone in the atmosphere will be broken, leading to the ozone pollution. Besides, the exposure to low-level ozone usually can cause serious effects on the human body's heart, lungs, respiratory tract, and cardiovascular system.^190–193 The catalysts used for the degradation of ozone can be divided into noble metals,^194,195 transition metal oxides^196–201 and noble transition compounds.²⁰² In the view of the cost and large-scale application, the studies on the catalytic decomposition of ozone have been mainly focused on transition metal oxides, especially MnO₂.

α-MnO₂ is a newly emerging material for catalytic decomposition of ozone. Jia et al.¹⁴¹ investigated the ozone removal activity over the MnO₂ with different phase structures (α, β, and γ). They found that α-MnO₂ performed the highest ozone removal efficiency. Zhu et al.²⁰³ developed a hierarchical structure consisted of graphene encapsulating α-MnO₂ nanofiber to address the challenges of the low stability and water inactivation during ozone decomposition. The optimized catalyst exhibited high ozone conversion efficiency of 80% and excellent stability over 100 h under a relative humidity (RH) of 20%. Even though the RH was as high as 50%, the ozone conversion also could reach 70%, well beyond the performance of α-MnO₂ nanofiber. Zhang et al.¹⁵⁵ found that introducing α-MnO₂ nanowires into the fluorine (F)-doped titanium dioxide (TiO₂) could not only improve the photocatalytic degradation of toluene but also remove almost the total O₃ in the system. As the comparison, only 15.8% of O₃ could be removed by the pristine F–TiO₂ catalyst without mixing with α-MnO₂. This indicated that α-MnO₂ nanowire was an excellent catalyst for O₃ decomposition. Previous study showed that the oxygen vacancies on MnO₂ nanomaterials were active sites for ozone adsorption and further decomposition,¹⁹⁹ which largely determined the catalytic degradation activity of MnO₂. Zhu et al.⁸² developed the α-MnO₂ nanowire with high K⁺ content in tunnel structure through the simple KOH solution post-treatment. As shown in Fig. 19, the substantial increase in K⁺ content reduced the formation energy of oxygen vacancies, which was further conducive to the formation of oxygen vacancies in the crystal lattice. As a result, the ozone removal performance was improved by doping K⁺. However, peroxide species (O₂²⁻) could accumulate on the surface of the catalyst and further evolve into relatively stable oxygen, resulting in the decrease in the ozone removal rate. Therefore, the accumulation of intermediate oxygen was a key factor limiting the high efficiency and sustainability of ozone decomposition. Liu et al.²⁰⁴ prepared cerium-modified todorokite-type manganese dioxides (Ce–MnO₂). The as-obtained catalyst showed greatly improved stability, which could keep ∼98% of removal efficiency for ∼115 ppm of O₃ within 5 h under a very high space velocity of 1200 L g⁻¹ h⁻¹ at room temperature. Besides, the Ce–MnO₂ catalyst also exhibited highly stable O₃ removal efficiency at temperature as low as 0 °C.

Fig. 19 Schematic of ozone catalytic decomposition on the K-rich MnO₂. Reproduced by Zhu et al.⁸² with permission from ACS.

5. Summary and prospect

MnO₂-based nanowires are becoming one of the most popular and promising catalysts in environmental applications due to their high catalytic performance, low cost and environmental friendliness. This review systematically summarizes the preparation methods of MnO₂-based nanowires, and the effects of phase structure, crystal facet, and morphology on their environmentally catalytic performance. In addition, this review also summarizes their applications in environmental pollution control, such as CO oxidation, removal of VOCs, and the decomposition of O₃.

(1) Hydrothermal is a flexible and diverse preparation method, which can facilely control the phase structure, morphology, and exposed crystal facets of the product by changing hydrothermal reaction conditions and has been widely used in laboratory research of MnO₂ nanowires. As for the reflux method, it has various advantages (e.g. simple operation procedure, mild reaction condition, and low cost) and is suitable for the large-scale industrial synthesis. However, it is difficult for the sol–gel method alone to obtain wire-like MnO₂-based nanomaterials. Therefore, it is often used in combination with template method to prepare highly ordered MnO₂ nanowires. However, the employment of the templates will increase the cost of the preparation.

(2) Crystal facet engineering is the research hotspot in the field of MnO₂ nanowires. Compared with the influences of phase structure and morphology, it has more significant impact on the catalytic activity of MnO₂ nanowires in environmental catalysis.

(3) Doping and/or loading the metals or their oxides can effectively improve the chemical bond strength, phase structure, and electronic structure of the MnO₂ nanowires. As a result, the catalytic performance will be improved by increasing the fluidity of oxygen.

Up to now, lots of MnO₂-based nanowire material modulation strategies have been developed and their performances in the fields of environmental catalysis have also been promoted. However, the environmental applications of MnO₂-based nanowire materials are still at the early stage with various challenges and have a long way to go before the practical applications, specifically in the following aspects.

(1) The complex crystal structures and the morphology diversity of MnO₂ result in challenging obstacles in obtaining pure phase structures and uniform morphologies of MnO₂ nanowires. Therefore, it is urgent to develop stable and efficient strategies to fabricate MnO₂ nanowires in single-crystalline solids with uniform morphology. Considering the remarkable improvement of the catalytic oxidation ability of MnO₂ nanowire by the crystal facet engineering, novel synthesis strategies for MnO₂ nanowire exposed a high proportion of specific crystal facet should be investigated in depth.

(2) The crystal facet engineering of MnO₂ nanowire is mostly focused on the catalysis of VOCs. It can be predicted that it will also have great potential application in catalytic CO oxidation and ozone degradation. However, there are few studies in these areas. Different crystal facets have different catalytic oxidation performances toward specific VOCs. Therefore, it is of great urgency and necessity to systematically investigate the special functions of each crystal facet of MnO₂, and finally verify the types of VOCs matched.

(3) Previous studies mainly focused on the noble metals (Au and Ag) and transition metals (Fe, Co, Ni, and Cu) loading or doping. However, few studies have focused on the modification of MnO₂ nanowire by loading or doping rare earth metals (La, Ce, Pr, and Sm). In view of the improvement of the O₃ removal efficiency of MnO₂ at low temperature by the cerium doping, the effect of catalytic degradation of other air pollutants by the rare earth metals modification should be explored in the future.

(4) Although MnO₂-based nanowire materials as the supports or the efficient catalysts are gradually emerging in the field of environmental catalysis, yet there is still giant gap between the laboratory results and actually industrial requirement in the aspects of the catalytic performance, long-term stability, and water resistance of removing CO and VOCs at relatively low temperature compared with traditional noble metal catalysts.

Author contributions

LX: conceived ideas, made comments, edited; YX: investigation, writing – original draft; GC, CW, NW: assisted YX in literature research and summary; LX, MC, XH: investigation, supervision, funding acquisition. All authors discussed the results and contributed to the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors sincerely acknowledge the financial support from National Natural Science Foundation of China (Grant No. 21503113, 21871144, 21577065, 21976094), the National Key Research and Development Project (Grant No. 2018YFC0213802), Environmental Protection Projects of Jiangsu Province (2017022), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX20_0309), and the Natural Science Foundation of Jiangsu Province (BK20190786). This study was also supported by Startup Foundation of Nanjing University of Information Science and Technology.

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Footnote

† These authors contributed equally to this work.

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