Effect of MnO 2 Crystalline Structure on the Catalytic Oxidation of Formaldehyde

Manganese oxides prove to be a promising catalyst for formaldehyde (HCHO) elimination in catalytic oxidation. In this study, MnO2 with different crystalline structure (α-MnO2, β-MnO2, γ-MnO2, and δ-MnO2) were synthesized by hydrothermal method to investigate their catalytic performances towards the abatement of formaldehyde. The prepared catalysts were characterized and analyzed by the X-ray diffraction (XRD), hydrogen-temperature programmed reduction (H2-TPR), BET specific surface area, X-ray photoelectron spectroscopy (XPS), and ammonia-temperature programmed desorption (NH3-TPD). In addition, the apparent activation energy was also calculated by using Arrhenius plots. Among the above four prepared catalysts, the γ-MnO2 has the best destruction and removal efficiency (DRE), which was approaching to 100% for HCHO at 155°C. The catalytic activity of γ-MnO2 is associated with abundant mesopores, higher reducibility of surface oxygen species, and more oxygen vacancies as compared to other types of crystalline MnO2.


INTRODUCTION
Formaldehyde (HCHO), one of typical Volatile organic compounds (VOCs), is considered to be the major pollutant in indoor air (Wang and Li, 2010;Wu et al., 2015a;Zeng and Bai, 2016).However, HCHO has been widely used in the industrial production, such as plastic, rubber, resin, adhesives and plywood.Ii is easily emitted into the air from the production and consumption from the relevant products, resulting in many healthy and environmental issues.In addition, the indoor formaldehyde is mainly derived from the burning of cigarettes, as well as a variety of decorative materials (Kim, 2009;Tang et al., 2009;Wu et al., 2015b) .According to the reports, people who engaged in formaldehyde related occupations are more likely to have cancer, and the incidence of eye and respiratory irritation symptoms will be significantly increased as well (Collins et al., 2001;Silbergeld and Patrick, 2005).
At present, the removal methods of formaldehyde in the air mainly include adsorption, photocatalytic oxidation, thermal catalytic oxidation and so on (Spivey, 1987;Atkinson and Arey, 2003a;b;Zhou et al., 2011;Ewlad-Ahmed et al., 2012;Yu et al., 2015;Bai et al., 2016;Hosseini et al., 2016;Liang et al., 2016;Liu et al., 2016;Yu et al., 2016).High specific surface materials, such as activated carbon and molecular sieve are common used in adsorption technology to remove formaldehyde (Kosuge et al., 2007;Ma et al., 2011;Chen et al., 2014), but the difficulty of regeneration on absorbent have restricted the applicant in high concentration of HCHO.At room temperature, HCHO can be decomposed through the activity of hydroxyl radicals and superoxide radicals by using TiO 2 nanoparticles as photocatalyst.In photocatalytic oxidation, the requirement of UV excitation light source reduces the application of visible light in the environment.Moreover, the deactivation of catalyst is another restrictive factor for photocatalytic oxidation.Due to the advantage of its high removal efficiency, low cost, no secondary pollution, the thermal catalytic oxidation has been extensively used in the HCHO abatement.The catalyst can be classified to the supported noble metal catalyst (Au/ZrO 2 , Ag/Al 2 O 3 , Pd/TiO 2 and Pt/Fe 2 O 3 ) and transition metal oxide (CeO 2 , MnO x and Co 3 O 4 ) (Zhou et al., 2011;Chen et al., 2014Chen et al., , 2015;;Bai et al., 2016;Hosseini et al., 2016).Among them, manganese oxide catalysts showed excellent performance, even better than supported noble metal catalysts in the low temperature environment.
Among transition metal oxide materials, MnO 2 has an extremely rich structure and morphology.There are α-MnO 2 , β-MnO 2 , γ-MnO 2 and other forms, which has the same structure unit: MnO 6 octahedra, the oxygen atom is located on the top of the eight surface body angle, while the manganese atom is in the center of the eight surface body.The valence shell electron configuration of the metal manganese is 3d 5 4s 2 , which means five electrons in the D orbit are in the unsaturated state, and this will result in the electronic gain-and-loss easier to occur, as well as strong oxidation reduction performance.In the process of catalysis, the gain and loss of the electron is the key for the formation of active intermediates (Wang et al., 2015).
In this study, four different types of MnO 2 crystal structures were prepared and then, their properties, performance and reaction mechanism of the formaldehyde catalytic oxidation were investigated and discussed.

Preparation of Catalysts
The catalysts were prepared by hydrothermal method.In a typical synthesis of the α-MnO 2 catalysts, 1.05 g MnSO 4 •H 2 O and 2.50g KMnO 4 were mixed in the distilled water (160 mL) and adding 4mL concentrated HNO 3 , then magnetically stirred about 1 h to form a homogeneous mixture.After stirring, the mixture was transferred into a Teflon reactor (200 mL) and heated at 100°C for 24 h.Similarly, the δ-MnO 2 were obtained from the reaction of KMnO 4 (3.0 g) and MnSO 4 .H 2 O (0.55 g) at 240°C for 24 h.For the preparation of the β-MnO 2 , (NH 4 ) 2 S 2 O 8 (4.56 g) and MnSO 4 •H 2 O (3.38 g) were well mixed and hydrothermally treated at 140°C for 12 h.The products of the above reaction were collected and centrifugal washed for 3-5 times, then dried at 100°C overnight, the obtained solid sample was calcinated at 400°C for 2 h, then ground to 40-60 mesh standby (Yuan et al., 2003;Jin et al., 2009;).For γ-MnO 2 , 0.06 mol L -1 KMnO 4 solution and 0.08 mol L -1 MnSO 4 solution (the mixed ratio is 5:6, respectively) were mixed and transferred into 1 L beaker under vigorous stirring for 30 min.Then the mixed solution was further aged for 30 min.After that, the precipitation was collected by filtration using buchner funnel, and washed by deionized water until there is no SO 4 2-in filtrate (test with 0.2 mol L -1 barium chloride solution).The precipitation is dried at 80°C, crushing to obtain γ-MnO 2 (Jin et al., 2009).

Characterization
XRD analysis was performed using a D/max-RB type X ray diffraction instrument, with a Cu radiation source, at a scanning speed of 4° min -1 , 50 kV tube voltage, and 30 mA tube current, the scanning range is 5°-70°.The specific area (BET) analyses of the sample were investigated using a Quantachrome NOVA 3000e analyzer at -196°C.The pore size distribution was calculated by the desorption branch of the N 2 adsorption-desorption isotherm using the BJH method.Before the N 2 physisorption, the catalysts were degassed at 300°C for 5 h.H 2 -TPR test was carried out in a micro reactor from room temperature up to 600°C under 5% H 2 /Ar mixture atmosphere over 50 mg catalyst, gas velocity is 30 mL min -1 and heating rate is 10 °C min -1 .For NH 3 -TPD analysis, 100 mg catalyst was taken in a tubular fixed bed flow reactor of quartz (i.d.= 6 mm).heated to 300°C and then swept with Ar for 1 h.After natural cooling to 100°C, the catalyst was purged with 0.05% NH 3 /Ar for about 100 min.Until the adsorption ended, NH 3 desorption was carried out from 100°C to 450°C at a heating rate of 10 °C min -1 in Ar atmosphere, and then the temperature was maintained at 450°C for 1h to ensure complete desorption of samples.A mass spectrometer (Hiden QIC-20) was used as a gas detector in the outlet gas.

Catalytic Activity Test
The quartz tube of 6 mm id is used as a fixed bed reactor while experimental temperature ranges from 100°C to 350°C.60 mg catalyst placed in the reactor is heated by a tubular furnace at a heating rate of 10 °C min -1 .The feed gas composition was 1400 ppm HCHO balanced by the air.The high concentration of HCHO used in this study was to simulate the normal condition of industrial formaldehyde.Gaseous HCHO was produced by flowing dry air over the paraformaldehyde at 38°C.Before the air flow through the reactor, all dry air was purged by sodium hydroxide pellet to clean out residual CO 2 and H 2 O.The gas velocity was 100 mL min -1 .The removal rate of HCHO was calculated by the yield of CO 2 .
where C * CO2 is the CO 2 concentration in the outlet gas when HCHO oxidized completely and C CO2 is that at different temperatures.The concentration of CO 2 in the effluent was recorded by a Jena Analytik multi N/C 2100 TOC analyzer (Liu et al., 2013).
Specific surface area is used to characterize the adsorption properties and activity of porous materials.Table 1 shows the specific surface area (BET), total pore volume (Vp) and average pore size (Dp) of the catalyst samples.It turns out that there are big differences between the specific surface area of MnO 2 catalysts with diverse crystal type, and the average pore size variation law is α-MnO 2 > β-MnO 2 > δ-MnO 2 > γ-MnO 2 , while the change law of the total pore volume is [MnO 6 ] octahedra is thought as fundamental building unit in manganese oxides, which share two opposite edges to form a single octahedral chain, and two single chains further share two other adjacent edges from each octahedron to build a double chain (Meng et al., 2014).Manganese oxides have diverse structures, depending on the connectivity between the [MnO 6 ] units via sharing corners or edges.The most common one is layered material δ-MnO 2 , which has MnO 6 -shared edges in each layer with cations such as Li + , Na + , and K + and other alkaline metals and water molecules occupying the space between the layers.One dimensional MnO 2 has tunnel structure, which would change along with the tunnel sizes.For instance, α-MnO 2 (2 × 2), β-MnO 2 (1 × 1), γ-MnO 2 (1 × 1 and 1 × 2) all belong to MnO 2 with tunnel structure (Huang et al., 2010; Meng et al., 2014).

Catalytic Oxidation for Formaldehyde
The catalytic activity of MnO 2 with different crystal structures over formaldehyde is shown in Fig. 2. In Fig. 2(a), all the MnO 2 can oxidize formaldehyde into CO 2 completely at 300°C, but different crystal forms show different performances.Complete conversions of HCHO over γ-MnO 2 and δ-MnO 2 were achieved at about 160°C, while that of α-MnO 2 and β-MnO 2 were both close to 300°C.To further explore the differences in activity of the γ-MnO 2 and δ-MnO 2 , the top 2 activity catalysts, we have refined the temperature range of the two reactions in Fig. 2(b).Taking T 90 as a starting point, every 5°C set as a temperature point, staying 20 min at each point to achieve completely decompose of formaldehyde, until the end of reaction.

Reducibility of Catalysts
Fig. 3 represents the profiles of H 2 -TPR, which are used to evaluate the reducibility of the four samples.The final reduction products of MnO 2 could be MnO with Mn 2 O 3 and Mn 3 O 4 as intermediates (Jia et al., 2016).As for α-MnO 2 , two reduction peaks were observed at 280 and 328°C.The reduction peaks of δ-MnO 2 were similar to that of α-MnO 2 at 299 and 324°C, respectively.According to the literature, the lower temperature peak might be associated with the reduction of MnO 2 to Mn 3 O 4 , whereas the higher reduction peak should be attributed to the reduction of Mn 3 O 4 to MnO (Liang et al., 2008).The TPR profiles of β-MnO 2 and γ-MnO 2 were different from that of the α-and δ-MnO 2 .Two reduction peaks were showed at 342 and 360°C for β-MnO 2 , but the distribution of this two peaks was unclear, which might be related to the reduction of MnO 2 to MnO with Mn 2 O 3 and Mn 3 O 4 as intermediate (Xiang and Xie, 2001).There are three reduction peaks in γ-MnO 2 observed at 239, 263, and 343°C, respectively.However, the low  The reducibility of catalyst improves along with the decrease of reduction temperature (Jia et al., 2016).From the results of TPR, it is observed that γ-MnO 2 has the optimal reduction performance since it shows the reduction peak at the lowest temperature.Thus, the order of reduction is γ-MnO 2 >α-MnO 2 >δ-MnO 2 >β-MnO 2 .This indicates that except β-MnO 2 , there may exist relatively abundant loosely coupled surface oxygen species in other three catalysts (Jia et al., 2016).Among the four samples, γ-MnO 2 is proven to possess the most mobile oxygen species.

NH 3 -TPD
NH 3 -TPD was used to identify the acidity of the catalysts.The quantities of acidic sites were estimated by integration of the area under the NH 3 -TPD curve (Jia et al., 2016) and the results are summarized in Table 3 and Fig. 4. The position of the peak can reveal the binding energy between acid sites and NH 3 molecule.Though α-MnO 2 contains the largest amount of acid sites, γ-MnO 2 contains much less acid sites than α-MnO 2 , the desorption peaks at high temperature area of γ-MnO 2 appeared at 420 ºC which is higher than that of  α-MnO 2 .For β-and δ-MnO 2 three desorption peaks were observed, which were slightly lower than those of γ-MnO 2 .
The binding energy of acid sites and NH 3 molecule will increase along with the temperature rise, indicating that γ-MnO 2 might possess the strongest acid sites of all.Besides, the exposed crystals of metal compound catalysts have significant effects on the catalytic activities.The crystal planes differ not only from the surface atom densities but also in the potential chemical reactions, hence resulting in the different surface energies.

XPS Analysis
To further confirm the oxidation state of the surface elements of the catalyst, the X-ray photoelectron spectrum analysis was carried out with the peaks associated with Mn 3s and O1s.Fig. 5(a) reveals energy spectrum of Mn 3s region.The average oxidation state (AOS) of Mn was estimated based on the following formula: AOS = 8.956 -1.126ΔE S , where ΔE S is the binding energy difference between the doublet Mn 3s peaks (Yang et al., 2013).ΔE S value is the basis for judging the oxidation state of the catalyst, and the ΔE S value of MnO, Mn 2 O 3 and MnO 2 are 6.0, 5.5 and 4.7 eV, respectively (Liu et al., 2016).It shows that the range of ΔE S value over four kinds of manganese oxides is 4.80-5.2eV,which indicates Mn 4+ is not the only one valence for Mn element.The ΔE S value of α-MnO 2 is 5.128, while the ΔE S value of β-MnO 2 , γ-MnO 2 and δ-MnO 2 are 4. 81, 4.83 and 4.80, respectively.This data demonstrates the valence of α-MnO 2 is in favor of +3 oxidation state, and the valence of other three types of catalysts tends to +4 oxidation state.On the basis of results, AOS decreases in the order of δ-MnO 2 (3.55) > β-MnO 2 (3.53) > γ-MnO 2 (3.52) > α-MnO 2 (3.18), which suggests that the part of Mn 4+ in δ-MnO 2 is the highest.It was reported that high Mn 4+ concentration is advantageous to the catalytic combustion of hydrocarbons (Machocki et  al., 2004; Wu et al., 2013).Comparison of Mn 4+ ion concentration of catalysts, β-MnO 2 should have the best catalytic activity of all, but this does not match the experimental results.Obviously, γ-MnO 2 possesses better catalytic activity, and this might be attributed to its high specific surface area, which was conducive to the adsorption and diffusion of formaldehyde gas.
The O1s spectrum is often used to determine the types of surface oxygen species on the oxide.There are three obvious peaks in Fig. 5(b), which correspond to surface oxygen (O α ), lattice oxygen (O β ), and surface adsorbed water molecules (O α' ), respectively.For all of samples, the binding energy of 529.7-530.1 eV is attributed to lattice oxygen (like O 2-); the binding energy of 531.3-531.8eV is attributed to surface oxygen (like O 2 2-, O -, OH -, CO 3 2-); And the binding energy of 532.4-533.2eV is attributed to surface adsorbed water molecules (Dai et al., 2011).It can be seen from Table 4 that γ-MnO 2 has the highest proportion of O α /O β , demonstrating γ-MnO 2 contains the most abundant surface adsorbed oxygen (O -, OH -).This result suggests γ-MnO 2 owns the highest density of oxygen vacancies, because oxygen molecules are usually absorbed at the oxygen vacancies of an oxide material (Wang et al., 2012).
Generally speaking, the higher oxidation state of manganese oxide, the more favorable to the catalytic oxidation process.But this is contrary to the results of Fig. 1, which shows that the oxidation state is not the only factor to determine the catalytic activity of manganese oxide.Combining with previous characterization results, γ-MnO 2 has large specific surface area, richest surface oxygen vacancies, open pore structure and abundant surface oxygen species with loose combination.All these characteristics provide more active sites for the formaldehyde gas, as well as accelerate the diffusion of O 2 and enhance the surface oxygen flow.For β-MnO 2 , its lowest specific surface area and average pore diameter will restrict the internal diffusion rate of pollutants enormously, along with lacking acid sites, thus led to its weak catalytic capability.For α-MnO 2 , the lowest Mn 4+ concentration might be responsible for its worse oxidation activity.As for δ-MnO 2 , despite its highest AOS and specific surface area, it cannot provide enough active sites, as well as the weak oxygen vacancies, consequently reducing its catalytic activity.

Calculation of E a Value
Fig. 6 shows the Arrhenius plots for formaldehyde decomposition over the four MnO 2 catalysts.According to the slope of the curve, the apparent activation energy of oxidation of formaldehyde can be evaluated and summarized in Table 3.The apparent activation energy is 0.60 kJ mol -1 on δ-MnO 2 , 0.66 kJ mol -1 on γ-MnO 2 , 1.74 kJ mol -1 on β-MnO 2 and 0.79 kJ mol -1 on α-MnO 2 .Among these, the apparent activation energy of δ-MnO 2 and γ-MnO 2 are nearly the same, which suggests that the surface of these two samples are easier to be activated at low temperature.Although the apparent activation energy of γ-MnO 2 is slightly higher than that of δ-MnO 2 , γ-MnO 2 still has better catalytic activity for its abundant surface oxygen vacancies, which eliminates the tiny gap between the E a value of these two catalysts.As we can see, the catalytic activity is not consistent with the changing trend of the activation energy as usual.Shi et al.  (2012) had found the same phenomenon as we did, in which the best catalyst owned the highest E a value among the three synthetic samples.

Reaction Mechanism of Formaldehyde Catalytic Oxidation
In previous study, Mars-Van Krevelen (MVK) mechanism has been explained for the catalytic oxidation of VOCs over metal oxides (Wu et al., 2011; Huang et al., 2015).
The reaction procedure consists of two redox steps: firstly, the catalyst in the gas phase is oxidized by oxygen to form the surface oxygen species, including surface lattice oxygen; Then, VOCs molecules are oxidized by surface oxygen and reduction of the oxidized catalysts by hydrocarbon compounds.Especially for HCHO, its oxidation over metal oxide includes the reaction between HCHO and surface oxygen to form the intermediate formate (Eqs.( 2) and ( 3)) , and the oxidation of formate to H 2 O and CO 2 by surface oxygen at different sites, accompanied with the catalyst reduced (Eqs.( 4) and ( 5)) (Sekine, 2002).where (g) and (a) refers to the gaseous and absorbed species, respectively.Thus, the catalytic oxidation of formaldehyde depends on many factors.In addition to the physical and chemical properties of the catalyst itself, the removal efficiency of formaldehyde is also closely related to the reduction and surface oxygen vacancies content of the catalysts.

Effect of H 2 O and SO 2 over γ-MnO 2 Catalyst
Since H 2 O is an integral part of the composition of the air, and SO 2 has become an assignable factor for the release of some chemical companies after desulfuration.It is necessary to discuss the influence of H 2 O and SO 2 on the catalytic activity.In order to simulate the content of H 2 O and SO 2 in air, we refer to the relevant knowledge and determine the concentration of H 2 O and SO 2 .As shown in Fig. 7   stays steady at first, then rapidly falls to 65% and only recover to 80% after removing the H 2 O.These results show that the inhibiting effect of H 2 O on the catalytic activity is incomplete reversible.When 100 ppm of SO 2 is added, the CO 2 generation of γ-MnO 2 decreases from 100% to about 20%, and can not recover at all when SO 2 is off.This might be owing to the formation of sulfate ion under the presence of SO 2 and H 2 O.When SO 2 and formaldehyde were led into the reactor, sulfate ion formed and adsorbed on the catalyst's surface, which will compete with pollutants for active sites and result in irreversible inactivation (Ao et al., 2004a, b).The co-existence of SO 2 and H 2 O in feed gas causes a rapid decline at the beginning of the reaction, and then decreases slowly from 34% to 12% and remain stable.

CONCLUSIONS
Four different crystal forms of MnO 2 nanoparticles were prepared by hydrothermal method and were tested for the destruction and removal (DRE) of formaldehyde.The physicochemical properties of the catalysts were characterized by the XRD, BET, H 2 -TPR, NH 3 -TPD, and XPS techniques.Under the condition of formaldehyde concentration = 1400 ppm, and SV = 100000 mL g -1 h -1 , the active temperature window of γ-MnO 2 was 155°C, while the same conversions on α-MnO 2 , β-MnO 2 and δ-MnO 2 were obtained at 275, 300 and 165°C, respectively.The apparent activation energies of the four forms of MnO 2 were in the range of 0.6-1.8kJ mol -1 .According to the experimental result, the synthesized MnO 2 possessed almost 100% Mn 4+ ion enriching on the surface.It can be concluded that the surface concentration of Mn 4+ ion is significant to its high catalytic activity for the catalytic oxidation of formaldehyde.The TPR and TPD results suggest γ-MnO 2 possess large specific surface area, strong binding sites and loosely combined surface oxygen, which will be propitious to the adsorption and diffusion of the reactants.Besides, the proportion of O α /O β for γ-MnO 2 is 0.53, which is apparently higher than those of the other three catalysts.Indicating γ-MnO 2 owns the most abundant oxygen vacancies, which is ascribed as the most critical factor for the catalytic oxidation of formaldehyde in this novel.

Fig. 2 .
Fig. 2. The formation rates of CO 2 over four crystal forms of MnO 2 catalyst (a) and the top 2 activity catalysts (b).

Table 1 .
BET results of MnO 2 catalysts with different crystal phases.

Table 2 .
Catalytic activity of MnO 2 for formaldehyde oxidation.

Table 3 .
The apparent activation energy of the catalytic oxidation of formaldehyde and the amounts of desorbed NH 3 over four crystal forms of MnO 2 .
, when 1% H 2 O is added, the CO 2 generation of γ-MnO 2

533.05 531.80 530.10 Binding energy/eV
After shutting off SO 2 and H 2 O, the CO 2 generation increases only 10%.As simultaneous existence of SO 2 and H 2 O, competitive adsorption of SO 2 and H 2 O causes the deactivation of catalyst and shortens its life.