Manganese oxides,including MnO, MnO₂, Mn₂O₃, and Mn₃O₄,are important in a range of applications such as ion exchange, molecular adsorption,catalysis, electrochemical reactions, batteries, and magnetic processes,because of their structural flexibility and novel chemical and physicalproperties ^{[1, 2, 3, 4, 5]}. Three-dimensional (3D) orderedmesoporous MnO₂ has disordered polycrystalline walls and a unit cellparameter of 20.85 nm; it has a large number of exposed (110) crystal planes,with a lattice spacing of 0.311 nm, and more Mn⁴⁺ ions are presenton these than on other planes ^[6]. 3D-MnO₂ has been used as aneffective environmentally friendly material for ethanol, carbon monoxide,benzene, and toluene oxidation, and in electronic components ^{[6, 7, 8, 9]}. The catalytic oxidation of volatile organiccompounds (VOCs) over 3D-MnO₂ shows the potential usefulness of thisnovel material. It has excellent hydrophobicity and a strong affinity towardVOCs, and can selectively adsorb VOC molecules and oxidize VOCs to CO₂and H₂O even in the presence of water vapor ^{[10, 11]}.

The development ofeffective catalysts for complete oxidation of low concentrations of HCHO at lowtemperatures is still a challenge. In this study, an ordered mesoporous 3D-MnO₂material was synthesized using a nanocasting method, and its catalyticperformance in HCHO oxidation was evaluated. This work was motivated by thefact that HCHO is becoming a major indoor pollutant. It is emitted from widelyused construction and decorating materials, and long-term exposure to indoorair containing even a very low concentration of HCHO is potentially harmful tohuman health ^{[12, 13]}.

The 3D-MnO₂ was prepared using KIT-6mesoporous silica as a hard template ^{[11, 13]}. After KIT-6 sample wassynthesized, KIT-6 molecular sieves (4.0 g) were added to a solution of Mn(NO₃)₂·4H₂Oin ethanol (0.91 mol/L, 40 mL). The sample was evaporated to dryness at 80 °Cand calcined at 200 °C for 6 h. These casting and evaporating steps wererepeated. Finally, the material was calcined at 400 °C for 6 h. The KIT-6 hardtemplate was removed using a NaOH solution (2 mol/L). Centrifugal separationwas used to remove sodium silicate, and the samples were dried at 100 °C andcalcined at 400 °C to yield 3D-MnO₂. 3D-MnO₂ grain growthis related to the initial crystallite size of the KIT-6 template andcalcination temperature ^[2]; for example, 3D-Mn₂O₃ can beobtained by increasing the calcination temperature.

α-MnO₂ and β-MnO₂ nanorods were prepared using the method reported in theliterature ^[14]. MnSO₄·H₂O (0.008 mol) and an equalamount of (NH₄)₂S₂O₈ were added todistilled water at room temperature to form a homogeneous solution. Thesolution was transferred to a 40-mL Teflon-lined stainless-steel autoclave, andthe autoclave was sealed and maintained at 140 °C for 12 h. After the reactionwas complete, the solid product was filtered, washed with distilled water to removeany ions in the final product, and dried at 120 °C to yield β-MnO₂.α-MnO₂ was prepared by adding analytical grade (NH₄)₂SO₄(0.015 mol) to the reaction system.

Powder X-raydiffraction (XRD) was performed using a TTR3 X-ray diffractometer with a Cu K_αradiation source, at a scanning speed of 0.05°/min, 40-kV tube voltage, and40-mA tube current. The pore size distribution and N₂ adsorptiondesorptionisotherms of the sample were investigated using an Autosorb-1MP instrument.Transmission electron microscopy (TEM) was performed using a JEM-2011instrument at a voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) wasperformed using a PHI-5300 instrument at 300 W, with Mg K_αX-rays as the excitation source. HCHO oxidation tests were performed in afixed-bed quartz-tube reactor (ϕ10 mm) using 0.2 g of catalyst (40-60 mesh). HCHO gas was generated and injectedusing a N₂ bubbler in a low-temperature thermostatic bath at 0 °C; theN₂ was passed through a container filled with formalin (a HCHOaqueous solution (37%)). The total gas flow rate through the reactor was keptat 100 mL/min using mass-flow meters; the gas consisted of 400 ppm HCHO, 20% ofO₂, and balance N₂. The space velocity was 30000 mL g^-1 h^-1). Thereaction products were detected online using an Agilent 7890A gaschromatograph, with a thermal conductivity and mass-selective detectorconnected to Porapak-Q and HP-INNOWAX columns, respectively. No carbon productsother than CO₂ were detected.

Fig. 1(a) shows theN₂ adsorption-desorption isotherms. The isotherm has a hysteresisring and is a type IV adsorption isotherm; this indicates the presence ofmesopores ^{[9, 13]}. Fig. 1(b) shows the Barrett-Joyner-Halenda pore sizedistribution of 3D-MnO₂ calculated from the desorption isotherm. The3D-MnO₂ pore size distribution is 3.7-11.4 nm, therefore the material has a mesoporous structure. 3D-MnO₂has a high specific surface area (87 m²/g), which is consistent withprevious literature reports ^[6].

Fig. 1. N2 adsorption-desorption isotherms (a) and pore size distribution (b) of 3D-MnO2.

Fig. 2 shows the XRDpatterns of 3D-MnO₂. The low-angle diffraction peaks at 2θ =1.00° and 1.04° correspond to the (211) and (332) planes (Fig. 2(a)),indicating that 3D-MnO₂ has the 3D ordered mesoporous structure withcubic symmetry (ia3d) of its template. The wide-angle diffraction peaks at 2θ= 28.7°, 37.3°, 42.8°, 56.7°, 59.4°, 64.8°, and 72.3°, corresponding to the(110), (101), (111), (211), (220), (002), and (301) planes, respectively, areattributed to the crystalline phase of β-MnO₂ pyrolusite ^{[8, 9]}. Thisindicates that the 3D-MnO₂ has a rutile structure.

Fig. 2. Low‐angle (a) and wide‐angle (b) XRD patterns of 3D‐MnO2.

Fig. 3 shows TEMimages of 3D-MnO₂, and α-MnO₂ and β-MnO₂nanorods. The α-MnO₂ (Fig. 3(c)) and β-MnO₂ (Fig. 3(d))nanorods are both nonporous materials, and have one-dimensional single-crystalnanostructures of diameters about 10 and 100 nm, respectively. The α-MnO₂nanorod diameter is much smaller than that of the β-MnO₂ nanorods. Orderedmesoporous structures can be clearly observed in Fig. 3(a) and (b). The 3D-MnO₂has exposed (110) crystal planes with a lattice spacing of 0.311 nm. In theβ-MnO₂ unit cell, more Mn⁴⁺ ions are exposed on the (110)crystal planes than on the other planes ^{[15, 16]}. The different structures ofthe samples may result from the different amounts of Mn⁴⁺ exposed onthe (110) crystal facets.

Fig. 3. TEM images of (a, b) 3D-MnO2, and (c) α-MnO2 and (d) β-MnO2 nanorods.

Fig. 4 shows thatthe Mn 2p_2/3 XPS profiles have two components, at bindingenergy of 641.6 and 642.9 eV, corresponding to surface Mn³⁺ and Mn⁴⁺ions, respectively. The peak areas indicate that the surface Mn⁴⁺/Mn³⁺molar ratios for 3D-MnO₂, and α-MnO₂ and β-MnO₂nanorods are 3.9, 3.1, and 1.6, respectively. These results show that largernumbers of Mn⁴⁺ species are exposed on the 3D-MnO₂surface. This large number of Mn⁴⁺ ions provides sufficient activesites for HCHO oxidation, as previously reported ^[6].

Fig. 4. Mn 2p2/3 XPS profiles of 3D-MnO2, and α-MnO2 and β-MnO2 nanorods.

Fig. 5 shows theHCHO conversions over 3D-MnO₂, and α-MnO₂ and β-MnO₂nanorods. 3D-MnO₂ is a promising catalyst and has the best oxidationactivity, entirely converting HCHO to CO₂ and H₂O at 130°C. Complete conversions of HCHO over α-MnO₂ and β-MnO₂nanorods were achieved at 140 and 180 °C, respectively. The catalytic activityof 3D-MnO₂ is therefore better than that of α-MnO₂and β-MnO₂ nanorods, indicating that the catalyst morphology has asignificant effect on the catalytic activity. The specific mesoporousstructure, higher specific surface area, and larger number of surface Mn⁴⁺ions of 3D-MnO₂ are responsible for its good catalytic activity inHCHO oxidation. 3D-MnO₂ has a special pore channel structure andhigh surface area, which are beneficial to the adsorption and diffusion ofreactants and products, and the large number of Mn⁴⁺ ions exposed onthe (110) facets provide sufficient active sites for HCHO oxidation. Inaddition, the oxidation performance over 3D-MnO₂ is similar to thatpreviously reported for a 3D-Co₃O₄ catalyst ^[13], and isbetter than that of CeO₂ nanospheres and Ag-supported CeO₂bulk particles ^[17]. These results indicate that 3D-MnO₂ has goodpotential as an efficient catalyst for HCHO removal. Further investigations onthe optimum preparation and possible metal-doping of 3D-MnO₂ are inprogress.

Fig. 5. Catalytic activities in HCHO oxidation over (1) 3D-MnO2, and (2) α-MnO2 and (3) β-MnO2 catalysts.

Acknowledgements

This study wassupported by State Environmental Protection Key Laboratory of Sources andControl of Air Pollution Complex, and also supported by State Key Laboratory ofEnvironmental Criteria and Risk Assessment, Chinese Research Academy ofEnvironmental Sciences.