Nanocrystalline MnO₂ on an activated carbon fiber for catalytic formaldehyde removal

Abstract

Three different types of nanocrystalline MnO₂, namely, α-MnO₂, γ-MnO₂ and δ-MnO₂, were successfully synthesized via a co-precipitation method, which has the advantages of easy preparation, low cost, size uniformity and excellent crystallinity. It also avoids operating at high temperatures and pressures. The nanocrystalline MnO₂ were then tested for formaldehyde catalytic oxidation at 25 °C. The results suggested that δ-MnO₂ had the highest catalytic activity. Hence, δ-MnO₂ was synthesized using the same method to modify an activated carbon fiber (ACF) substrate. Further tests showed that the as-prepared MnO₂/ACF samples can significantly improve the removal of formaldehyde at room temperature. The MnO₂ content in MnO₂/ACF had great influence on the breakthrough time and we found that the MnO₂ content did not best perform at greater magnitudes but were optimal at 16.12 wt%. The formation method developed in this study may become a promising technique to improve the catalytic activity in formaldehyde removal.

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Introduction

Among the various volatile organic compounds (VOCs), formaldehyde continues to be a major pollutant, especially in an indoor environment. Formaldehyde is emitted from construction and decoration materials, and can cause physical and mental illness at elevated concentrations (WHO 2006). What is worse, exposure to a formaldehyde environment will result in nasopharyngeal cancer and other serious diseases in humans.^1–3 Thus, the removal of formaldehyde from an indoor environment is of utmost importance.

Many methods have been used for formaldehyde removal, among which physical adsorption,^2,4 catalytic oxidation^5,6 and plasma decomposition^7,8 are most commonly used. However, these methods have certain limitations. For the physical adsorption approach, which is the most widely used in commercial air-cleaner, its short saturated absorption time and difficult treatment of the abandoned adsorbent make it less effective. For the catalytic oxidation approach, many chemical catalysts only work in severe conditions, e.g., TiO₂ becomes less effective without UV light and may generate harmful by-products during the catalytic process,⁹ most of the metal catalysts have to work at high temperature and the exorbitant prices of noble metal catalysts made it impractical to be popularized.^10–12 Plasma technology shows poor performance at low concentrations and can also easily generate harmful by-products.³ Based on these studies, the combination of adsorption and catalytic oxidation should be an improved method to remove indoor formaldehyde at low concentrations.¹³

Activated carbon fiber (ACF) is regarded as the most promising adsorbent for formaldehyde removal due to its abundant micropores, large surface area, low pressure drop and excellent adsorption capacity.^4,14,15 The adsorption capacity of ACF for formaldehyde is not ideal at ambient temperature because of the weak interactions between the polar formaldehyde and the hydrophobic surface of porous carbon.^16,17 In order to enhance the adsorption capacity of ACF, some studies have utilized surface modification to promote its adsorption properties, e.g., introducing a functional substance such as p-aminobenzoic acid,¹⁷ CuO¹⁸ and TiO₂.¹⁹ Other transition metal oxide catalysts have also been proven to possess high catalytic activity for the complete of HCHO oxidation, such as CeO₂,^11,20–22 Co₃O₄ (ref. 23–25) and MnO₂.^13,26–28 While most of the previous research has focused on the removal of high-concentrations of formaldehyde (>100 ppm), in an indoor environment, however, the concentration of formaldehyde is generally low. Therefore, an investigation on the catalyst capacities for formaldehyde at low concentration levels has practical implications.

Recently, a multitude of research has concerned the catalytic removal of formaldehyde at ambient temperature.^{6,13,27,29–35} Table S1† summarizes the previously reported catalytic materials, concentration level, temperature, test methods and their activities during the catalytic oxidation of formaldehyde at ambient temperature over the related catalysts. To our knowledge, no research has focused on formaldehyde removal using MnO₂-based ACF substrates.

Herein, three kinds of nanocrystalline MnO₂ with different phase structures (α-MnO₂, γ-MnO₂ and δ-MnO₂) were synthesized via a co-precipitation method. MnO₂ was selected because no harmful by-products are generated during the formaldehyde oxidation process.³⁶ Catalytic tests at room temperature revealed that δ-MnO₂ had the best activity. Based on this result, ACF was modified with δ-MnO₂ and the obtained samples showed the significant removal of formaldehyde.

Experimental

Materials

KMnO₄ and MnSO₄ (analytical pure reagent) were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification. The pitch-based ACF was obtained from Osaka Gas Chemicals Co., Ltd. ACF was first washed three times with deionized water to remove ash and dried at 130 °C for 12 h in vacuum before use.

Synthesis of α-MnO₂, γ-MnO₂ and δ-MnO₂

α-MnO₂, γ-MnO₂ and δ-MnO₂ were prepared via a co-precipitation method. The redox reaction is listed below:^37,38

2KMnO₄ + 3MnSO₄ + 2H₂O → 5MnO₂ + 2H₂SO₄ + K₂SO₄ (1)

To synthesize α-MnO₂, 3.16 g of KMnO₄ and 4.53 g of MnSO₄ were dissolved in 50 mL of deionized water, respectively and stirred at 90 °C. Then, the KMnO₄ and MnSO₄ solutions were added dropwise into 50 mL of deionized water, respectively. The obtained solution was then stirred and reacted at 90 °C for 2 h. The products were washed, centrifuged and dried at 80 °C for 12 h. For γ-MnO₂, 3.16 g of KMnO₄ and 18.12 g of MnSO₄ were synthesized at 80 °C for 2 h. For δ-MnO₂, 3.16 g of KMnO₄ and 2.26 g of MnSO₄ were synthesized at 80 °C for 2 h.

Preparation of δ-MnO₂ modified ACF

Nanocrystalline MnO₂ was incorporated into ACF via a co-precipitation method. Briefly, ACF was first immersed in an MnSO₄ solution and then a KMnO₄ solution was added dropwise and with vigorous stirring at 80 °C for 2 h. Subsequently, the ACF was taken out and heated to 80 °C for 12 h and finally dried at 120 °C for 5 h in vacuum. By altering the concentrations of the KMnO₄ and MnSO₄ solutions, different MnO₂ contents in the MnO₂/ACF can be obtained.

Characterization

Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4800 field emission scanning electron microscope. X-ray diffraction (XRD) patterns were measured on a D/max-II B, (SHIMADZU, Japan) using Cu Kα radiation (λ = 0.1542 nm) operating at 40 kV and 30 mA. The α-MnO₂, γ-MnO₂, δ-MnO_2, ACF and MnO₂/ACF were scanned from 5° to 90° at a scan rate of 5° min⁻¹. The δ-MnO₂ content of each MnO₂/ACF sample was determined according to ASTM D2866-11. The ash content in the individual ACF (n = 5) was first measured using the following methods. A crucible was placed in a furnace at 650 °C for 1 h and cooled down to room temperature to record its weight (W₀). Next, the ACF was added into the crucible and its weight was calculated (W₁). Then, the crucible was transferred into a preheated muffle furnace at 650 °C for 3 h. Finally, the crucible was cooled to room temperature in a desiccator and its weight was recorded as W₂. The ash content was calculated using eqn (2):

Then, the MnO₂/ACF was added into the crucible and the total weight was calculated (W₃). The crucible was placed into a muffle furnace at 650 °C for 3 h. Finally, the crucible was placed in a desiccator for conditioning and its weight was determined (W₄). The weight of ash in MnO₂/ACF was defined as W₅. The δ-MnO₂ content of each MnO₂/ACF was derived as follows:

W₅ = (W₃ − W₄) × ash% + (W₃ − W₄) × ash%² (3)

The nitrogen adsorption–desorption isotherms obtained for ACF and MnO₂/ACF were measured on an ASAP 2020 (Micromeritics Instrument, USA) at 77 K. The surface area of α-MnO₂, γ-MnO₂, δ-MnO₂, ACF and MnO₂/ACF was obtained using the Brunauer–Emmett–Teller method.^2,39 The pore size distribution of α-MnO₂, γ-MnO₂, δ-MnO₂, ACF and MnO₂/ACF was calculated using non-localized density functional theory (NLDFT).⁴⁰ X-ray photoelectron spectroscopy (XPS) was performed on a PHI5700 ESCA system equipped with aluminum anode (Al Kα = 1486.6 eV radiation) at a pressure of 2 × 10⁷ Torr. All the binding energies were calibrated using contaminated carbon (C 1s = 284.6 eV). The number of binding energy peaks was determined using a deconvolution process. Hydrogen temperature-programmed reduction (H₂-TPR) measurements were performed on a chemisorption analyzer (AutoChem II 2920), the H₂ consumption and the mass signal was detected using thermal conductivity (TCD) measurements. δ-MnO₂, ACF and MnO₂/ACF (ca. 50 mg) were introduced in the U-type quartz microreactor with a flow of 5% H₂/Ar at a flow rate of 50 mL min⁻¹. The samples were heated from 60 °C to 800 °C at a heating rate of 10 °C min⁻¹.

Adsorption and catalytic activity tests

The room temperature adsorption and catalytic activity measurements for α-MnO₂, γ-MnO₂, δ-MnO₂, ACF and MnO₂/ACF were performed in a fixed-bed quartz reactor (length = 500 mm, diameter = 10 mm) at 25 °C. Gas formaldehyde was generated by flowing pure air (20 ± 2% RH) through an S-4000 gas mixing system (Environics, USA) and the total gas flow rate was 200 mL min⁻¹, as monitored by a mass flow control system. The concentration of formaldehyde was kept at 15 ppm. For the three different MnO₂ samples, 100 mg of sample was used in each test, corresponding to a gas hourly space velocity (GHSV) of 120 000 mL (g_cat h)⁻¹. While 200 mg of the MnO₂/ACF sample with a packing length of 20 mm was kept in a quartz reactor for each test and GHSV was 60 000 mL (g_cat h)⁻¹. HCHO, CO₂, CO and water vapor were analyzed online using a photoacoustic IR multigas monitor (INNOVA AirTech Instruments Model 1412i). Before each test, the upstream concentration was first measured for at least 4 h to ensure the HCHO concentration was constant. During the test, the downstream gas was also collected hourly and subsequently analyzed using a GC 9800 gas chromatograph equipped with TCD and FID detectors. No other carbon containing compounds except CO₂ in the products were detected using the catalysts studied. The HCHO conversion was determined using the following equation:where [HCHO]_in (ppm) is the inlet concentration of HCHO before passing the catalyst and [HCHO]_out (ppm) is the output concentration of HCHO after passing the catalyst. The above parameters for HCHO conversion were used to calculate the removal capacity of the MnO₂/ACF samples.

The activity tests were also performed in the static state to clarify the mechanism of HCHO removal. The method used for the static activity tests was adapted from previous reports.^29,31 A 0.5 L stainless steel reactor was covered by a polytetrafluoroethylene layer on its inner wall. The MnO₂/ACF sample was first placed on the bottom of a quartz Petri dish. After placing the Petri dish into the reactor, 300 ppm of HCHO was emitted into the reactor using an S-4000 gas mixing system (Environics, USA). After the concentration of HCHO was stabilized at 150 ppm, the cover of the sample dish was removed to start the adsorption and catalytic reaction of HCHO. HCHO, CO₂, CO and water vapor were measured respectively during test at 25 °C. The yield of CO₂ (ΔCO₂) and the concentration variation of HCHO were calculated to analyze the removal ratio.

Results and discussion

Morphology characterization

Fig. 1 depicted the procedure of MnO₂/ACF synthesis. Nanocrystalline MnO₂ was incorporated in ACF via a co-precipitation method. Fig. 2a–f shows the SEM images of α-MnO₂, γ-MnO₂, δ-MnO₂, ACF, 0.03-MnO₂/ACF and 0.05-MnO₂/ACF. The morphology of each MnO₂ sample was determined by their different crystal phases. α-MnO₂ showed a needle-like morphology with diameters of 50 nm and lengths of 300–900 nm. γ-MnO₂ presented an irregular nanoparticle structure, while δ-MnO₂ displayed a flower-like nanostructure without clear interparticle boundaries with an average diameter of 200 nm. As shown in Fig. 2d, the surface of ACF was smooth before being loaded with MnO₂. After loading, the surface of ACF became coarser (see Fig. 2e and f). When the concentration of MnSO₄ reached 0.05 mol L⁻¹, the entire fiber surface was covered with MnO₂, suggesting the maximum MnSO₄ feeding capacity was 0.03 mol L⁻¹.

Fig. 1 A schematic of the synthesis of MnO₂/ACF.

Fig. 2 SEM images of (a) α-MnO₂, (b) γ-MnO₂, (c) δ-MnO₂, (d) ACF, (e) 0.03-MnO₂/ACF and (f) 0.05-MnO₂/ACF.

XRD analysis

XRD patterns were measured to identify the crystallographic structures of α-MnO₂, γ-MnO₂, δ-MnO₂, ACF and MnO₂/ACF (see Fig. 3).

Fig. 3 XRD patterns of α-MnO₂, γ-MnO₂, δ-MnO₂, ACF and MnO₂/ACF.

As shown in Fig. 3, the typical peaks of the different MnO₂ samples can be clearly identified as α-MnO₂ (JCPDS 44-0141), γ-MnO₂ (JCPDS 14-0644) and δ-MnO₂ (JCPDS 80-1098). The ACF pattern displays two broad diffraction peaks at 25° and 43°, respectively, both of them are the characteristics peaks of disordered carbon.^41,42 When compared to the pattern obtained for pure ACF, the MnO₂/ACF sample showed more diffraction peaks at 12.5°, 37.1° and 66.0°, which were in accordance to the characteristics peaks of δ-MnO₂, suggesting that the δ-MnO₂ has been successfully loaded on the surface of ACF.

Content of δ-MnO₂

The initial ash content in the pure ACF sample was ∼6.72%. The content of δ-MnO₂ in the MnO₂/ACF samples was calculated using eqn (5) and the results were ∼5.62%, 11.74%, 16.12%, 22.42% and 29.33%, depending on the initial concentration of MnSO₄ solution (0.01 mol L⁻¹, 0.02 mol L⁻¹, 0.03 mol L⁻¹, 0.04 mol L⁻¹, 0.05 mol L⁻¹, respectively). The obtained MnO₂/ACF samples with different δ-MnO₂ contents were labeled as #X MnO₂/ACF (X represents the concentration of MnSO₄ solution used). Detailed information is shown in Table S2.†

Specific surface area and porosity

Table 1 lists the specific surface area (S_BET), average pore size (D_p) and total pore volume (V_p) of α-MnO₂, γ-MnO₂ and δ-MnO₂. δ-MnO₂ exhibited the highest S_BET, D_p and V_p values. The nitrogen adsorption isotherms at 77 K obtained for ACF, 0.03-MnO₂/ACF and 0.04-MnO₂/ACF are presented in Fig. 4a. The curve for ACF exhibited a type I isotherm, according to the IUPAC classification,⁴³ which indicated that the sample was microporous. In the presence of MnO₂, a hysteresis loop emerged in the adsorption–desorption isotherms, which was attributed to capillary condensation in the mesopores of MnO₂. The relevant BET surface areas of ACF and MnO₂/ACF are shown in Fig. 4a. The results showed that pure ACF had the highest value of 961.20 m² g⁻¹, whereas 0.03-MnO₂/ACF and 0.04-MnO₂/ACF showed values of 679.12 m² g⁻¹ and 574.92 m² g⁻¹, respectively. The results showed that as more nanocrystalline MnO₂ was loaded on ACF and a slight decrease in the surface area occurred. This is because the nanocrystalline MnO₂ on the surface of ACF blocked the pores, leading to a decrease in the adsorption amount.⁴² Fig. 4b shows the pore size distribution of ACF, 0.03-MnO₂/ACF and 0.04-MnO₂/ACF. Besides the micropores, the mesoporous values were also observed in the patterns of 0.03-MnO₂/ACF and 0.04-MnO₂/ACF, which were identical to the results of the adsorption–desorption isotherms. The results also indicated that the pore size increased with an increase in the MnO₂ loading amount.

Table 1 The physical parameters of the three samples of nanocrystalline MnO₂

Samples	Surface areas SBET (m^2 g^−1)	Pore diameter Dp (nm)	Pore volume Vp (cm^3 g^−1)
α-MnO2	70.11	14.48	0.13
γ-MnO2	74.84	15.70	0.24
δ-MnO2	87.50	19.85	0.28

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Fig. 4 (a) Nitrogen adsorption/desorption isotherms and (b) the corresponding pore size distributions for ACF, 0.03-MnO₂/ACF and 0.04-MnO₂/ACF.

Activity test

The HCHO oxidation activities of the three kinds of MnO₂ catalysts are shown in Fig. 5. The catalytic activities of δ-MnO₂ decreased dramatically during the first 5 h and the rate of HCHO conversion maintained at ∼25% after 40 h. The other catalysts, α-MnO₂ and γ-MnO₂ showed lower catalytic activities than that of δ-MnO₂. The HCHO conversion using α-MnO₂ turned to 20% after 40 h. Because the pattern of α-MnO₂ manifested a similar trait to δ-MnO₂, the α-MnO₂ catalyst had an oxidation activity similar to δ-MnO₂. γ-MnO₂ displayed the lowest catalytic activity. A low conversion rate was found at the beginning of the test, after which it dropped continuously and then stabilized at 15%. Previous studies^28,44 have found that when compared with other properties (such as the specific surface area, degree of crystallinity, reducibility and oxidation state of manganese), the channel structures of MnO₂ are the primary factor that determines the catalytic activity. δ-MnO₂ normally forms a 2D layer structure, while α-MnO₂ and γ-MnO₂ obtained 1D channels in their structures. In detail, for α-MnO₂, it is comprised of [2 × 2] and [1 × 1] channels and γ-MnO₂ contains both [1 × 1] and [1 × 2] channels.⁴⁵ Since the effective diameter of the [2 × 2] channel was more suitable for HCHO diffusion and adsorption of HCHO molecules than the [1 × 1] channel during the reaction, the α-MnO₂ had a higher catalytic activity during HCHO oxidation when compared to γ-MnO₂. For δ-MnO₂, the interlayer structure will facilitate the diffusion and absorption of HCHO molecules to the active sites more than the [2 × 2] channel, so the δ-MnO₂ showed the best catalytic activity. Therefore, δ-MnO₂ was selected in this article to modify ACF in order to enhance the oxidation of HCHO.

Fig. 5 (a) HCHO catalytic performance with time on stream over the α-MnO₂, γ-MnO₂ and δ-MnO₂ catalysts. Reaction conditions: 25 °C, 15 ppm HCHO, 20 ± 2% RH, GHSV = 120 000 mL (g_cat h)⁻¹. (b) The breakthrough curves obtained for HCHO over ACF and MnO₂/ACF. Reaction conditions: 25 °C, 15 ppm HCHO, 20 ± 2% RH, GHSV = 60 000 mL (g_cat h)⁻¹.

According to Pei's study, a sorbent bed with a higher than 80% breakthrough can be considered ineffective in practice for indoor air cleaning.⁴⁶ Hence, the 80% breakthrough point was chosen to identify the abilities of ACF and MnO₂/ACF in the catalytic oxidation of HCHO. As shown in Fig. 5b, pure ACF had a fast breakthrough time of less than 60 min and the 80% breakthrough time was nearly 100 min. In comparison, both of the breakthrough times of 0.01-MnO₂/ACF were prolonged to 50 min and 120 min, respectively. A greater MnO₂ content could also lead to better catalytic activities, which could be proved by the results of 0.02-MnO₂/ACF and 0.03-MnO₂/ACF. However, when the MnO₂ content increased to more than 22.42% (critical point), the HCHO oxidation efficiency declined, suggesting that overloading MnO₂ might cause agglomeration of the nanoparticles (see Fig. 2f), which blocks the pores of the ACF substrate. Therefore, the optimal content of MnO₂ was set at 16.12%, in which, the 80% breakthrough time and formaldehyde removal capacities at 80% breakthrough point were 550 min and 0.74%, respectively (see Fig. S1†).

The service life is a critical factor in practical applications for adsorbents and catalysts. The removal amount of HCHO using MnO₂/ACF at room temperature was tested (n = 5) and the results are shown in Fig. 6. After 5 cycles, the removal capacity showed no significant difference when compared with the first cycle, while the yield of CO₂ presented a large variation. Besides, it's encouraging to see that the MnO₂/ACF sample can be reactivated by being dried at 130 °C for 4 h in vacuum. Based on the results obtained for the HCHO oxidation rate and CO₂ generation rate, the reactivated MnO₂/ACF was able to maintain its original function indicating the HCHO molecules absorbed by active carbon fiber and MnO₂ can be partially degassed and removed at high temperature under vacuum. The HCHO removal activity of MnO₂/ACF was a multi-step process (see Fig. 8). Based on previously reported results,^13,47–49 the mechanism of the catalytic oxidation of HCHO using a catalyst is described as an adsorption–degradation–desorption process, which can also be used here to explain the oxidation process of HCHO using the MnO₂/ACF sample. As shown in Fig. 7, the concentration of HCHO decreased sharply in the first 60 min, after which the decreasing speed became slow. In comparison, the CO₂ concentration kept increasing linearly, regardless of the reaction time. This trend indicates that the removal mechanism of HCHO can be attributed to the absorption of ACF during the first 60 min and the subsequent oxidization of MnO₂ till 540 min. Initially, most of the HCHO molecules were absorbed by active carbon and only a tiny proportion of HCHO was oxidized by MnO₂, emitting a small amount of CO₂. During the second stage, HCHO was oxidized by MnO₂ thus generating lots of CO₂. Finally, the HCHO removal reached a stationary state (stage III).

Fig. 6 The variation of HCHO and CO₂ concentration with reaction time for 0.03-MnO₂/ACF in the static recycling tests.

Fig. 7 The variation of HCHO and CO₂ concentration with reaction time for 0.03-MnO₂/ACF in the static test.

Fig. 8 The proposed mechanism for the catalytic oxidation of HCHO over MnO₂/ACF.

In general, the ACF substrate and nanocrystalline MnO₂ act synergistically during the HCHO removal process. The MnO₂ provided catalytically active sites and promoted the effective degradation of HCHO. The ACF can provide fields for those active sites and also protect the MnO₂ from potential flue gas poisons. When compared with Table S1,† the MnO₂ and MnO₂/ACF samples are competitive in the HCHO oxidation at room temperature. Although the catalytic activity for HCHO oxidation of the MnO₂/ACF material was less effective than some noble metal catalysts, it has a great advantage in catalytic HCHO removal at room temperature.

XPS analysis

The surface electronic state of MnO₂/ACF was investigated by XPS analysis and the results are shown in Fig. 9 (“Before” represents the as-prepared sample while “After” represents the sample that has been used in HCHO oxidation). After deconvolution of the Mn 2p spectrum at 2p_3/2, three peaks emerged. The peaks at 642.6 eV and 641.5 eV indicate the presence of Mn⁴⁺ and Mn³⁺, respectively, while another peak at 644.0 eV was assigned as a satellite peak.^50–52 When compared with the spectra of the as-prepared sample, the spectrum of after-test sample became broader and both peaks for Mn⁴⁺ and Mn³⁺ were shifted by 0.5 eV to a lower binding energy. This indicates that there was a significant change in the chemical state of manganese before and after the reaction. MnO₂/ACF acts as a catalyst in the oxidation process of HCHO. Table 2 further revealed the molar ratio of Mn⁴⁺/Mn³⁺ on the surface of the as-prepared and after-test samples, and the value decreased from 1.54 to 1.13. Zhang et al.⁶ recommended that more surface Mn⁴⁺ ions may provide more oxygen vacancies for an oxide material, which is positive for the adsorption, activation and migration of oxygen in the gas phase. For this reason, Mn⁴⁺ plays an important role in HCHO oxidation and the redox cycle for Mn⁴⁺/Mn³⁺ also occurs in the HCHO oxidation process. Deconvolution of the O 1s XPS spectrum is displayed in Fig. 9b. The spectrum of the as-prepared sample consisted of two main peaks at 530.0 and 531.4 eV. The former peak was attributed to the lattice oxygen (O²⁻) denoted as O_latt, and the latter peak was assigned to surface adsorbed oxygen (such as OH) denoted as O_ads. From Table 2, it was clear that the surface molar ratio of O_latt/O_ads for the as-prepared and after-test samples was 1.71 and 0.55, respectively, suggesting that when the HCHO oxidation progress was complete, the surface molar ratios of O_latt/O_ads declined drastically. Another peak at a binding energy of 533.0 eV was caused by the presence of adsorbed water molecules on the surface of ACF and MnO₂, which also appeared in the spectrum of the after-test sample. Room temperature oxidation of HCHO follows the Mars and Van Krevelen mechanism as mentioned in previous studies, according to which the formaldehyde is finally oxidized to H₂O and CO₂.⁴⁵ Therefore, it can be deduced that during the reaction, the formaldehyde was oxidized by the lattice oxygen of MnO₂ with the consumption of lattice oxygen and molecular oxygen generated in the gas phase. These results proposed that abundant lattice oxygen will enhance the activity of the MnO₂ catalysts for the HCHO oxidation.

Fig. 9 The XPS spectra of the MnO₂/ACF samples: (a) Mn 2p and (b) O 1s.

Table 2 The physical parameters of the three samples of nanocrystalline MnO₂

Samples	Binding energy (eV)		Molecular ratio	Binding energy (eV)		Molecular ratio
	Mn^4+	Mn^3+	Mn^4+/Mn^3+	Olatt	Oads	Olatt/Oads
As-prepared	624.6	641.5	1.54	531.0	531.4	1.71
After-test	642.1	641.0	1.13	531.0	531.6	0.55

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Reducibility of the catalyst

H₂-TPR measurements were performed to investigate the reducibility of the related samples. Fig. 10 shows the TPR profiles obtained for δ-MnO₂, ACF and MnO₂/ACF. For the δ-MnO₂ catalyst, two reduction peaks were observed at 288 °C and 336 °C. It is obvious that the ratio of the lower temperature peak to the higher temperature peak was about 1, which can lead to the conclusion that the main product of the δ-MnO₂ catalyst could be MnO. The reduction route may be started with MnO₂ to Mn₂O_3, and then to MnO.^6,53 However, pure ACF displayed only one reduction peak at 625 °C, which can be attributed to the decomposition of functional groups, such as oxygenic groups in the ACF sample. When nanocrystalline δ-MnO₂ was introduced into the ACF, the reduction behaviour differed from that observed on ACF. The TPR profiles of the MnO₂/ACF sample exhibited three reduction peaks, two of which shifted to lower temperature at 275 °C and 368 °C, while the other one shifted to 505 °C and showed a higher peak intensity. The results illustrated that the decomposition of the functional groups in ACF was enhanced due to the existence of nanocrystalline MnO₂.⁵⁴ In addition, since there existed an intense interaction between the ACF and δ-MnO₂, MnO₂/ACF was more easily decomposed. To a certain extent, such decomposition can reflect the oxygen mobility in the samples. Since both δ-MnO₂ and ACF were involved with adequate mobile oxygen during the reaction, active oxygen will be motivated and enhanced during the reduction of formaldehyde.

Fig. 10 H₂-TPR profiles obtained for the δ-MnO₂, ACF and MnO₂/ACF samples.

Conclusions

α-MnO₂, γ-MnO₂ and δ-MnO₂ were successfully prepared via a co-precipitation method. The formaldehyde catalytic oxidation results proved that all the nanocrystalline MnO₂ samples had catalytic activity at room temperature and the δ-MnO₂ sample displayed the highest activity. The influence of MnO₂ content in MnO₂/ACF on the breakthrough time was also studied and the optimal MnO₂ content was found. The results suggest that the ACF substrate modified with nanocrystalline MnO₂ may be potentially used for formaldehyde removal in an indoor environment.

Acknowledgements

This work was supported by the Shanghai Municipal Science and Technology Commission (No. 14520502800), “Chen Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (No. 14CG34) and Fundamental Research Funds for the Central Universities (No. 2232014d3-15).

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Footnote

† Electronic supplementary information (ESI) available: Summary of the catalytic oxidation for formaldehyde removal at ambient temperature in recent literature in Table S1 and amounts of δ-MnO₂ attached in MnO₂/ACF in Table S2. The formaldehyde removal amount was calculated. See DOI: 10.1039/c6ra15463h

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