Manganese-Based Catalysts for Indoor Volatile Organic Compounds Degradation with Low Energy Consumption and High Efficiency

With the development of industrialization, the emission of volatile organic compounds (VOCs) to atmosphere causes serious environmental problems and the treatment of VOCs needs to consume a lot of energy. Moreover, indoor VOCs are seriously harmful to human health. Thus, there is an urgent requirement for the development of indoor VOCs treatment technologies. Catalytic degradation of VOCs, as a low energy consumption, high efficiency, and easy to achieve manner, has been widely studied in related fields. As a kind of transition metal catalyst, manganese-based catalysts have attracted a lot of attention in the catalytic degradation of VOCs because of their unique advantages including high efficiency, low cost, and excellent stability. This paper reviews the state-of-the-art progress of manganese-based catalysts for VOCs catalytic degradation. We introduce the thermocatalytic, photocatalytic and photo-thermocatalytic degradation of VOCs on manganese-based catalysts in this paper. The optimization of manganese-based catalysts by means of structural design, decorating modification and defect engineering is discussed.


Introduction
Energy and environmental problems are the two major problems in today's world; in particular, air pollution is becoming more and more serious [1][2][3][4][5]. VOCs are a kind of 1 3 organic compounds with melting point lower than room temperature and boiling point between 50 and 260 °C [3][4][5][6][7][8][9][10][11]. VOCs come from a wide range of sources, including coatings, organic chemicals, petrochemical, pharmaceutical and catering industries [3,5,[12][13][14][15][16]. VOCs are also important components of indoor air pollutants [16,17]. Therefore, improving indoor air quality by controlling VOCs pollution is conducive to improving living environment and human health [18][19][20][21]. There are three main ways to control indoor VOCs, including source pollution control, ventilation dilution and purification treatment [22][23][24][25][26][27][28]. Reducing the source pollution is an effective approach; however, the use of VOCs is necessary and inevitable [29]. Ventilation dilution, i.e., outdoor air replaces indoor air in a given space, is the easiest way to reduce the concentration of VOCs. However, the efficiency of this method is very low. Besides, the benefits of ventilation dilution depend on the relative concentration of pollution between indoor and outdoor air; sometimes, it may introduce new outdoor pollutants [23]. Purification treatment of VOCs is an ideal way to deal with indoor VOCs pollution, which transforms indoor VOCs into bound states or other harmless substances [25][26][27][28]. However, purification treatment of VOCs needs to consume a lot of energy. How to achieve the effect of pollutant degradation and reducing energy consumption through effective design is a subject worthy of in-depth research.
Based on whether the treated products contain VOCs, the purification treatment of VOCs can be divided into two types, i.e., recycling treatment and destruction treatment [30]. Recycling treatment of VOCs separates VOCs and clean air by passing the polluted air through a special device. The purification function is realized; however, a secondary treatment is needed to degrade the VOCs [31-34]. In contrast, the destruction treatment directly removes VOCs by decomposing them into non-toxic gases using catalytic/noncatalytic methods [30]. Among them, catalytic oxidation method possesses the advantages of low energy consumption, high efficiency and no secondary pollution [35][36][37].
Till now, a large number of catalysts including noble metals and transition metal oxides have been developed for catalytic oxidation of VOCs. Among them, manganesebased catalysts have attracted tremendous attention due to their excellent catalytic performance, unique structure, large specific surface area, excellent adsorption ability and low cost [36,[38][39][40][41]. Lots of progresses have been made using manganese-based catalysts for the degradation of VOCs [28,35,[42][43][44]. This review summarizes the state-of-the-art progress on the design, synthesis and application of manganesebased catalysts for VOCs degradation. A brief introduction of the VOCs degradation mechanisms is first described, followed by the discussions on the thermocatalysis, photocatalysis and photo-thermocatalysis of VOCs degradation.
Finally, we provide some personal insights into the challenges and future research directions in this field.

Mechanisms for VOCs Degradation
The mechanism for VOCs degradation is complicated due to the large number of different types of VOCs and catalysts. Four mechanisms for VOCs oxidation have been proposed, i.e., Langmuir-Hinshelwood (L-H) mechanism, Eley-Rideal (E-R) mechanism, simple power law (P-L) mechanism and Mars-van Krevelen (MVK) mechanism [45-48]. Among them, MVK mechanism can be used to explain most of the catalytic degradation reactions [47,49,50]. In this section, we will introduce different types of mechanisms with particular focusing on the MVK mechanism.

Langmuir-Hinshelwood (L-H) Mechanism
L-H mechanism takes surface reaction as the speed determining step. There are three steps in L-H mechanism, including the adsorption of reactants, catalytic reaction and the desorption of products [45]. Most of the surface reactions can be explained by L-H mechanism. Based on the L-H mechanism, Tarjomannejad et al. [45] carried out a kinetic study of the catalytic oxidation of toluene over LaMnO 3 catalyst. The results showed that LH-OS-ND (adsorption of reagents on the same type of sites and non-dissociative adsorption of oxygen) was the most likely mechanism to predict the experimental data. The correlation coefficient was R 2 = 0.9952. Hua et al. [51] proposed a new L-H dualsite mechanism to explain the experimental observations of the catalytic ozonation of toluene over MnO 2 /graphene via a steady-state kinetic study.

Eley-Rideal (E-R) Mechanism
Both E-R mechanism and L-H mechanism are based on the Langmuir model. The difference is that in E-R mechanism, only one of the reactants is adsorbed on the surface of catalyst, which then reacts with the non-adsorbed reactants [52]. Wang et al. [46] synthesized a series of spinel oxides ZnNi x Co 2-x O 4 (x = 0-0.8), and they found that the catalytic combustion behavior of methane on Ni-poor ZnNi x Co 2-x O 4 spinels could be fitted by E-R mechanism.
Power Law (P-L) Mechanism P-L mechanism is a dynamic distribution fitting method proposed by statistical physicists for scale-free phenomena in nature, which has been widely used in various fields [47]. Maghsoodi et al. [53] determined the apparent activation energy of the catalytic ozonation of toluene over MnO 2 /graphene using P-L mechanism.

Mars-van Krevelen (MVK) Mechanism
There are two steps in the MVK mechanism, i.e., lattice oxygen in the catalyst oxidizes VOCs, and then the catalyst adsorbs and converts gaseous oxygen into lattice oxygen [47][48][49][50]. Utsumi et al. [47] found out MVK model can well simulate the catalytic oxidation of acetyl acetate over La 1−x Ca x FeO 3 . But, without H 2 O and CO 2 , simple P-L mechanism can also fit the data. He et al. [49] studied the catalytic oxidation behavior and kinetics of benzene, toluene and ethyl acetate on Pd/ZSM-5 and found MVK model can express the reaction rate of all VOCs, while P-L model is only applicable to benzene. Du et al.
[50] pointed out the catalytic oxidation of toluene on low-Pt bimetal compounds (PtNi 3 -C and PtFe 3 -C) catalyst following the MVK mechanism, in which the active oxygen helps to remove toluene adsorbed on the catalyst (Fig. 1).
It should be noticed that not all catalytic reactions follow only one mechanism. Wang et al.
[54] studied the oxidation kinetics of benzene on ACo 2 O 4 (A = Cu, Ni and Mn) catalysts. The results indicate that the oxidation of benzene can be well fitted with both the MVK mechanism and L-H mechanism. The surface reaction of adsorbed MEK with oxygen was considered as the rate-determining step in the model.

Thermocatalytic Degradation of VOCs on Manganese-Based Catalysts
Thermocatalysis technology, developing from direct thermal degradation, is the most effective method in VOCs treatment [55]. For indoor air with low concentration of VOCs, the direct thermal degradation is difficult to achieve due to the high energy consumption. Besides, the thermal degradation also generates harmful by-products [56]. In contrast, thermocatalytic degradation is an ideal method for indoor VOCs degradation because of its better efficiency and lower energy consumption. To date, many catalysts including transition metal and noble metal-based catalysts have been used for thermocatalysis [57]. Although noble metal-based catalysts possess the advantages of high efficiency and low ignition temperature, the great cost and poor thermal stability limit their wide application [58]. Transition metal-based catalysts generally have the advantages of excellent stability and low cost; however, they suffer from low catalytic efficiency [36,[38][39][40][41]59]. Mn, a typical earth-abundant transition metal with diverse valence states, is widely used in catalytic reaction [60]. Mn-based catalysts have been widely studied in thermocatalytic degradation of VOCs; however, the catalytic activities still need to be improved [42,61]. In view of the existing problems, researchers have improved the activity and stability of Mn-based catalyst by means of structural design and decorating modification [36,38,39,42,[61][62][63][64].

Structural Design of Manganese-Based Catalysts
A lot of efforts have been put into the structural design of manganese-based catalysts in order to improve their efficiency toward the thermocatalytic degradation of VOCs. Yang et al. [38] reported the preparation of α-, β-, γ-and δ-MnO 2 catalysts and evaluated their catalytic performance for toluene degradation. The results showed that the order of degradation efficiency was δ-MnO 2 > α-MnO 2 > γ-Mn O 2 > β-MnO 2 , which was consistent with the order of oxygen adsorption capacity and low-temperature reducibility ( Fig. 2a, b). Li et al. [65] also prepared MnO 2 with different phase structures and reported their catalytic oxidation activities of toluene (Fig. 2c, d).
[42] prepared palygorskite-supported Mn oxides, which catalyzed the oxidation of formaldehyde in a complete and efficient way. De Luna et al. [67] prepared octahedral molecular sieve type manganese oxide (K-OMS 2) catalysts with outstanding low-temperature activity, which achieved 100% toluene conversion efficiency and 98% benzene conversion efficiency at 523 K and 565 K, respectively. Nguyen Dinh et al. [68] reported the synthesis of MnCO 3-carbon composites and 3D porous ε-MnO 2 microcubes (PEMD) with great porosity, strong reducibility, high lattice oxygen reactivity and large Mn 4+ fraction, significantly improving the catalytic performances of toluene complete oxidation. Piumetti et al.

Decorating Modification of Manganese-Based Catalysts
Decorating modification is another effective method to improve thermocatalytic activity. Ali et al. [59] prepared a series of triple-oxide (CeO 2 , ZrO 2 and TiO 2 ) as support and then used (i) Mn impregnation or/and (ii) Au deposition-precipitation to obtain the corresponding catalyst (Mn-only catalysts, Au-only catalysts and Au-Mn-containing catalysts). In their report, propane was used to evaluate the catalytic performance. The results showed that a strong metal-metal interactions between Au and Mn significantly increased the catalytic activity of the Au-Mn/TOS catalyst compared with the sole metal Au/TOS and Mn/TOS catalysts (Fig. 4a) [59]. Zhang et al. [69] decorated different amounts of single-atom Pt into MnO 2 , among which, the activity of 0.1% Pt/MnO 2 sample showed the best performance. The conversion rate of toluene reached 100% at 80 °C, and more than 80% of toluene conversion was achieved at the gas hourly space velocity (GHSV) of 60 L/gh at room temperature (28 °C) ( Fig. 4b) [69].
Perovskite-type catalysts (ABO 3 ) have the characteristics of great thermal stability, unusual valence states of the transition metal ions in their structures, tunable redox properties and excellent low-temperature activity. Moreover, cations at A and B sites can be decorated by other ions, which produces more defects sites and oxygen vacancies [25]. Li and his collaborators [70] studied the catalytic oxidation of hexane by La 0.8 Ce 0.2 MnO 3 /mesoporous ZSM-5. The results showed that Ni doping at Mn sites in La 0.8 Ce 0.2 MnO 3 could promote the low-temperature catalytic activity due to the enhancing electronic transfer among the La, Ce, Ni, Mn and mesoporous ZSM-5, as well as the promoting migration and distribution of the surface oxygen species. Maghsoodi et al. [53] proved that excess manganese in LaMn 1+x O 3+δ  b Temperature-dependent toluene conversion by MnO 2 and Pt-deposited MnO 2 catalysts. Reprinted with permission from Ref. [69]. Copyright 2019 Elsevier perovskite also had a promoting effect on the oxidation of trichloroethylene (TCE) in air. Tarjomannejad et al. [45] showed that Fe-containing perovskite catalyst had a better activity than Cu-containing perovskite catalyst. Liu et al.
In summary, the thermocatalytic degradation of VOCs on different manganese-based catalysts is listed in Table 1.

Photocatalytic Degradation of VOCs on Manganese-Based Catalysts
Since Fujishima and Honda [71] first reported the photolysis of water over TiO 2 , lots of efforts have been devoted to developing photocatalysts for various photocatalytic reactions. In fact, the research on photocatalytic degradation  of VOCs also began with using TiO 2 as catalyst, and then extended to other catalysts, including manganese-based catalysts [43, 72,73]. Frank and Bard [74,75] studied the reduction of CN − in water, which was the first application of TiO 2 in environmental purification. Kraeutler and Bard [76] proposed the first organic photocatalysis reaction (CH 3 COOH → CH 4 + CO 2 ). Pruden and Ollis [72] carried out a semiconductor sensitized reaction for the oxidative mineralization of organic pollutants (the degradation of trichloroethylene in water), which introduced photocatalysis into the field of VOCs catalytic degradation for the first time. However, TiO 2 has a wide band gap energy (3.2 eV for anatase) and can only utilize the ultraviolet light (about 3%-5% of the solar energy), which greatly limits its practical application [77,78]. Therefore, it has become an important research topic to develop UV-Vis photocatalyst with narrow band gap. MnO 2 has become an attractive candidate for photocatalytic reaction in wastewater and waste gas treatment due to its low cost, small band gap and non-toxic characteristics [43, [78][79][80].

Mechanism of Photocatalytic Degradation of VOCs
Generally, there are three key steps in a semiconductor photocatalytic reaction. First, photo-induced electrons (e − ) and holes (h + ) are generated when the photocatalyst is excited by photons with energy greater than or equal to the band gap energy of photocatalyst (Eg) [81]. Second, the photoexcited carriers separate and migrate to the surface of photocatalyst.
Third, e − with strong reducibility reacts with O 2 to form O 2 − , while h + with strong oxidation combines with H 2 O or OH − adsorbing on the photocatalyst surface to form ·OH.
These two substances further react with VOCs and convert them into CO 2 and H 2 O (Fig. 6) [81].
Although Fig. 6 is the mechanism summarized by scientists based on TiO 2 , it is still applicable to other semiconductor catalysts, including manganese-based photocatalyst.

Optimization of Manganese-Based Photocatalyst
As a kind of traditional thermocatalysts, manganese-based catalysts have also been used as photocatalysts in photocatalytic reactions. Chhabra et al. [80] prepared manganese dioxide (MnO 2 ) nanorods over reduced graphene oxide (RGO) nanocomposites, which exhibited efficient photocatalytic activity for the removal of a colored dye (neutral red) from water. This demonstrated that manganese-based catalysts could be used in photocatalytic degradation of VOCs. Other researchers also studied the synthesis of series manganese oxides and evaluated their photocatalytic performance [82][83][84]. The mixed valence state of Mn and the presence of active oxygen (·OH and ·O 2 − ) were found to play important roles in promoting the photodegradation of VOCs.
Manganese-based photocatalysts have also been used in the dye removal, which is a combination of adsorption, oxidation and photocatalysis. The catalytic mechanism is similar to the photocatalytic degradation of VOCs. Fang et al. [10] prepared Mg-doped OMS-2 nanorod catalysts using a hydrothermal redox reaction, and the catalytic degradation of benzene increased significantly under full solar spectrum (UV-Vis-IR) irradiation from a Xe lamp. The main reasons for the degradation of outstanding benzene were the substitution of a small amount of Mg cation for Mn 4+ in OMS-2. Chan et al. [85] prepared β-MnO 2 nanotubes and found the photocatalytic degradation Fig. 6 Main processes occurring on a semiconductor particle: electron-hole pair generation, charge transfer, electron-hole pair recombination at the surface or in the bulk, and electron and hole-induced chemistry at the surface. Reprinted with permission from Ref. [81]. Copyright 2019 WILEY-VCH efficiency of RhB dye reached 90.3% after 120 min. Zhang et al. [86] compared the photocatalytic degradation efficiency of phenol by several manganese oxides (acidic birnessite (BIR-H), alkaline birnessite (BIR-OH), cryptomelane (CRY) and todorokite (TOD)). After 12 h of UV-Vis irradiation, the total organic carbon (TOC) removal rate of CRY, BIR-H, TOD and BIR-OH reached 62.1%, 43.1%, 25.4% and 22.5%, respectively. Compared to the reactions in the dark condition, UV-Vis exposure improved the TOC removal rates by 55.8%, 31.9%, 23.4% and 17.9%.
In summary, the photocatalytic degradation of VOCs on different manganese-based catalysts is listed in Table 2.

Photo-Thermocatalytic Degradation of VOCs on Manganese-Based Catalysts
Although compared with thermal degradation technology, the thermocatalytic degradation of VOCs has achieved a great reduction in energy consumption, it is still a high energy consumption process. As for the traditional photocatalytic degradation of VOCs, the utilization efficiency of solar energy is very low. Though some studies have extended the photoresponse of various photocatalysts from ultraviolet to visible light or even infrared region by decorating modification, the efficiency is still low [84,[87][88][89][90]. Photo-thermocatalytic oxidation, which combines the excellent catalytic efficiency and excellent durability of thermocatalytic oxidation with low energy consumption of photocatalytic oxidation, has become a very effective method for reducing air pollutants. Based on different reaction ways, photo-thermocatalysis can be divided into three categories: photo-assisted thermocatalysis, thermo-assisted photocatalysis and photo-thermal synergetic catalysis. There are very few reports on the use of manganese-based catalysts in thermo-assisted photocatalysis. In this part, we will focus on the use of manganese-based catalysts in photo-assisted thermocatalytic and photo-thermal synergetic catalytic degradation.

Photo-Assisted Thermocatalytic Degradation
Fundamentally, photo-assisted thermocatalysis is also a kind of thermocatalysis, but the energy primarily comes from light. The catalyst absorbs solar energy and converts solar energy into thermal energy, which increases the temperature of catalyst to the ignition temperature of VOCs and then drives the thermocatalytic reaction [91,92]. Therefore, to achieve high efficiency in photo-assisted thermocatalytic VOCs degradation, catalysts should not only possess good thermocatalytic performance, but also have excellent photo-thermal conversion efficiency. Yang et al. [91] prepared a hollow sphere ramsdellite MnO 2 (R-MnO 2 -HS), which exhibits efficient catalytic activity for the purification of benzene under the whole solar spectrum. Excellent photo-assisted thermocatalysis efficiency was also observed under visible-infrared and infrared radiation. The excellent catalytic activity of R-MnO 2 -HS was resulted from its great thermocatalytic activity and efficient photo-thermal conversion efficiency in the whole solar spectrum. The catalytic efficiency of manganese-based catalysts had been improved by ion doping. Hou et al. [92] showed strong absorption in the whole solar spectrum region through the Ce ion substituted cryptomelane-type octahedral molecular sieve (OMS-2) catalyst. The catalyst could effectively convert the absorbed solar energy to thermal energy, which significantly increased the temperature of the catalyst. Combined with the efficient photo-thermal conversion and excellent thermocatalytic activity, the efficient degradation of benzene, toluene, acetone and other VOCs was realized under the irradiation of whole solar spectrum, visible-infrared and infrared light. Moreover, the catalyst still maintained stable activity after 40 cycles, which indicated the excellent durability of the catalyst (Fig. 7) [92]. Interestingly, the catalyst had no photocatalytic activity through the traditional photocatalytic route at room temperature. Li et al. [93] synthesized CeO 2 / LaMnO 3 , which realized the efficient photo-thermocatalytic degradation of VOCs under infrared irradiation. Wang et al. [94] reported the synthesis of MnO 2 and graphene nanohybrid (MnO 2 -G). The as-synthesized nanohybrid showed better HCHO oxidation efficiency than pure MnO 2 or graphene  [86] under the whole solar spectrum. The excellent photo-thermal effect of graphene played an essential role in the catalytic reaction. In addition, the decorations of Mg, Fe, Cu and other non-noble metal ions also significantly improved the photo-thermocatalytic activity of manganese-based catalysts under UV-Vis-IR irradiation [95,96]. Defect engineering, which involves the manipulation of the types, concentrations and spatial distributions of defects, is considered to be one of the most effective strategies to regulate the electronic structure of materials and improve the catalytic performance [97]. Lee et al. [98] prepared mesoporous copper manganese oxides exhibiting better performances for the catalytic oxidation of benzene, achieving the best benzene conversion rate of 90% at around 219 °C, which is 23 °C lower than that for mesoporous manganese oxides. The reason was that the desorption of lattice oxygen caused by copper addition in the manganese oxide, increasing the concentration of oxygen vacancy defects (OVDs) on the catalyst surfaces. Mao et al. [83] also reported the synthesis of OMS-2 nanorod catalysts with different OVDs concentrations. The catalysts showed great activity and excellent durability toward the oxidation of benzene, toluene, acetone and other VOCs under the irradiation of full solar spectrum, visible-infrared and infrared light.
Noble metals, such as Au, Ag and Pt, exhibit unique surface plasmon resonance (SPR) effect, which absorb and scatter light strongly [91]. Therefore, to improve the light harvesting ability of the catalysts, and thus achieve better photo-thermocatalytic performance, metal nanoparticles with strong SPR effect are incorporated into manganesebased catalysts. Ali et al. [59] synthesized Au-Mn/tripleoxide catalyst which showed great catalytic activity in propane oxidation due to the strong synergism between Mn and Au. Ag/MnO x and Pt/MnO x also have been synthesized as efficient catalysts toward VOCs degradation under natural conditions [99,100].

Photo-Thermal Synergetic Catalytic Degradation
Photo-thermal synergetic catalysis integrates the advantages of both photocatalysis (TiO 2 ) and thermocatalysis (manganese-based catalysts) and shows better performance through synergistic effect. Ma et al. [44] prepared MnO x /TiO 2 nanocomposites and found that there was a synergistic effect This synergistic effect significantly improved the catalytic activity of MnO x /TiO 2 nanocomposites. Excellent catalytic activity and durability of MnO x /TiO 2 for the gas-phase oxidation of benzene under full solar and visible-infrared light irradiation were demonstrated. Ren et al. [61] prepared Ce-MnO x catalyst by coprecipitation method and then deposited TiO 2 and Pt, respectively, to obtain Pt-TiO 2 /Ce-MnO x catalyst. The kinetic constant of the photo-thermocatalytic degradation of benzene (k TP = 5.17 mg 1/2 (m 3/2 h)) was 7.72 times that of single photocatalysis (k P = 0.67 mg 1/2 (m 3/2 h)) and 2.32 times the sum of the kinetic constants of the photocatalysis and thermocatalysis (k T = 1.55 mg 1/2 (m 3/2 h)). This result indicated that the photocatalytic and thermocatalytic of benzene on Pt-TiO 2 /Ce-MnO x catalyst had remarkable synergistic effect (Fig. 8a) [61]. Lan et al. [101] prepared MnO x /TiO 2 nanocomposites with dominant facets (Fig. 8b). The catalytic realized the synergistic effect of photo-assisted thermocatalysis and photocatalysis, which effectively extended the catalytic benzene oxidation response from ultraviolet to the whole solar spectral region.
In summary, the photo-thermocatalytic degradation of VOCs on different manganese-based catalysts is listed in Table 3.

Prospects
Although significant progress has been made in the efficient catalytic degradation of VOCs with manganesebased catalysts, there are still some challenges and possible opportunities. Firstly, manganese oxide is a narrow band gap semiconductor, which has good photo-thermal conversion performance. There are only a few reports on the use of manganese-based catalysts for photocatalytic and photothermocatalytic reactions. More efforts should be devoted into this research field. Moreover, noble metals are often doped into manganese-based catalysts to improve their  catalytic performance. However, the high cost of noble metals strongly restricts their wide application. Therefore, it is necessary to develop catalysts doped with less amount of noble metals or earth-abundant elements. In addition, although a lot of researches on the catalytic degradation of VOC by manganese-based catalysts have been carried out, there is still a lack of effective understanding of the specific reaction pathway. Therefore, more experiments and theoretical simulations are needed to deeply understand the pathway of decomposition of VOCs by manganese-based catalysts, such as the combination of in situ characterization and computational chemistry.

Conclusions
This paper summarizes the state-of-the-art application of manganese-based catalysts in VOCs catalytic degradation. The optimization of manganese-based catalysts by means of structural design, decorating modification and defect engineering is introduced. Proper structure design can improve the specific surface area, increase the active sites and improve the catalytic performance. Certain amount of decorating with other metal ions, especially noble metals, can greatly improve the catalytic performance. Defect engineering is considered to be one of the most effective strategies to regulate the electronic structure of materials, which greatly improve the catalytic performance of the catalyst.  (9)