High catalytic activity for formaldehyde oxidation of an interconnected network structure composed of δ-MnO₂ nanosheets and γ-MnOOH nanowires

Formaldehyde (HCHO) is a typical indoor air pollutant that is mainly released from furnishings and building materials [1‐3]. Long-term exposure to HCHO can induce adverse health consequences, such as allergic rhinitis, asthma, respiratory system diseases, hepatitis, pneumonia, nasopharyngeal cancer, and leukemia [4‐7]. In previous studies, several approaches for HCHO oxidation have been developed, such as biological degradation, photocatalytic oxidation, adsorption, and thermal catalytic oxidation. It has been proved that room-temperature catalytic oxidation is one of the most promising and cost-effective methods for high-efficiency conversion of HCHO into harmless species (H₂O, CO₂) over a metal oxide catalyst [8‐10].

Noble metal catalysts, mainly containing Pt, Au, Pd, and Ag as the active component supported on various metal oxides (TiO₂, Al₂O₃, ZrO₂, and Fe₂O₃), show high performance for HCHO oxidation at ambient temperature [11‐14]. However, their high cost, poor thermal stabilities, and limited resources have prevented a wide application of noble metal catalysts. It was found that transition metal oxide catalysts had enormous potential for HCHO oxidation at room temperature. In particular, the transition metal oxide catalysts such as MnO_x, Co₃O₄, and CeO₂ show good thermal stability and are less expensive and more abundant than noble metal catalysts [15‐18].

Among the transition metal oxide catalysts, manganese oxides have been widely used in catalysis because of their high activity, nontoxicity, low cost, polybasic structures, and morphologies [10‐22]. The structures of manganese oxide, including α-MnO₂, β-MnO₂, γ-MnO₂, and δ-MnO₂, play an important role in catalytic capacity [23‐27]. Zhang et al. [25] found that δ-MnO₂ had the highest activity because of its special 2D layer tunnel structure that contained the most active oxygen species and the highest amount of lattice oxygen species on the catalyst surface. Birnessite (δ-MnO₂) [23, 25] is a layered manganese dioxide constructed by an edge-sharing [MnO₆] octahedra framework with a certain number of cations (Li⁺, Na⁺, Ca²⁺) and water molecules located between the manganese oxide layers for charge balance. Wang et al. [17] found that the water content in the birnessite was essential for HCHO oxidation—that was, better activity was obtained for materials with higher water content. They also found that the HCHO oxidation efficiency was influenced by the different types of interlayer cation (K⁺, Mg²⁺, Ca²⁺, Fe³⁺) [18, 19]. However, the agglomeration of the nanosheets in δ-MnO₂ can reduce the exposed active sites. In the past few years, MnO₂ has usually been supported on carbon materials (graphene hydrogel, carbon foam, carbon nanotubes, and carbon fibers) to solve the problem of agglomeration [28, 29]. However, these carbon materials are inert components in catalysts and may cover the active sites for catalytic reactions. To solve this problem, Rong et al. [30, 31] developed an organic and template-free approach to prepare 3D MnO₂ by freeze-drying aqueous solutions of the MnO₂ nanosheets and nanowires. Although improved catalytic performance was obtained, this method involved multiple steps and was cumbersome.

In this report, a one-step hydrothermal method for the fabrication of a MnO₂-based catalyst (M-MnO₂) composed of δ-MnO₂ nanosheets anchored on a nanowire-shaped γ-MnOOH framework is described. It exhibits high performance for HCHO oxidation at room temperature.

Figure 1 shows the X-ray diffraction (XRD) patterns of the as-synthesized samples. As 2.81 mmol (NH₄)₂C₂O₄ was added, all peaks of the sample could be indexed to the birnessite (JCPDS No. 80-1098); thereafter, it was named “δ-MnO₂”. With the increase of (NH₄)₂C₂O₄ (5.98 mmol), the final product was a mixed phase (named “M-MnO₂”) of δ-MnO₂ and manganite γ-MnOOH (JCPDS No. 88-0649). After more reductants (11.26 mmol) were added, the birnessite phase disappeared, and there was only manganite; therefore, the sample was named “γ-MnOOH” [32, 33]. The wide peak widths of birnessite in δ-MnO₂ and M-MnO₂ indicate poor crystallinity, which may be caused by the damage to the ordered structure.

The morphology and microstructure of these three samples were observed using SEM and TEM, as shown in Fig. 2. The morphology of γ-MnOOH nanowires can be observed clearly in Figs. 2a, d, and g, and the nanowires are randomly intertwined. The particles reveal a nanorod like morphology with a width of less than 50 nm and a length of approximately 1 μm, and the lattice fringes of 0.48 nm can be assigned to the (100) planes. The particles shown in Figs. 2b, e, and h are δ-MnO₂. Figure 2b shows a flowerlike structure comprising nanoparticles, and the size of the nanoparticles observed in Fig. 2h is approximately 5–10 nm. The distances between the adjacent lattice fringes are 0.24 nm and 0.35 nm, which are related to the (−111) and (002) planes, respectively. However, the particles of the flowerlike structure in Fig. 2b show severe agglomeration. As shown in Figs. 2c, f, and i, M-MnO₂ shows a special interconnected network structure with the flowerlike δ-MnO₂ anchored on the γ-MnOOH nanowires. The γ-MnOOH as a framework support plays an essential role in the network structure, and δ-MnO₂ is dispersed on the framework. The distances between the adjacent lattice fringes of M-MnO₂ were 0.24 nm and 0.48 nm for δ-MnO₂ and γ-MnOOH, respectively. It was concluded that the sample with this special morphology was a mixture of δ-MnO₂ and γ-MnOOH. This is also supported by the XRD results. The morphology of the as-synthesized products can be regulated by modulating the ratio of (NH₄)₂C₂O₄/KMnO₄ during the hydrothermal process. With the increase of the ratio of (NH₄)₂C₂O₄/KMnO₄, the low-valent manganese compound γ-MnOOH appeared. The δ-MnO₂ nanosheets tended to anchor on a nanowire-shaped γ-MnOOH, which increased the specific surface area and the pore volume compared with the pure δ-MnO₂, as shown in Table 1.

For the case in which the main catalyst is δ-MnO₂, XPS measurements are performed to analyze the surface chemical compositions and the valence state of γ-MnOOH, δ-MnO₂, and M-MnO₂, and the results are shown in Figs. 3a and b. The Mn 2p_3/2 peak can be deconvoluted into two peaks, located at 641.1–641.2 eV and 642.4–642.7 eV that correspond to the Mn³⁺ and Mn⁴⁺ species, respectively. The Mn⁴⁺/Mn³⁺ ratios of γ-MnOOH (0.85), δ-MnO₂ (1.71), and M-MnO₂ (1.18) are calculated from the XPS data by their peak area ratios and are summarized in Table 1. The Mn⁴⁺/Mn³⁺ ratios were in the order δ-MnO₂ > M-MnO₂ > γ-MnOOH.

In addition, as shown in Fig. 3b, the O 1s spectra of the three different samples were deconvoluted into two peaks located at 529.9 eV and 531.4 eV that were associated with lattice oxygen (O_latt) and surface adsorbed oxygen (O_ads) species, respectively. The O_ads/O_latt ratios of the three samples are calculated from the XPS data using their peak area ratios and are presented in Table 1. The O_ads/O_latt ratio of M-MnO₂ was 0.62, while that of δ-MnO₂ was 0.53 and that of γ-MnOOH was 0.46. The O_ads/O_latt ratios followed the order M-MnO₂ > δ-MnO₂ > γ-MnOOH. The M-MnO₂ surface has the most-abundant surface adsorbed oxygen, which is mainly present in the form of active oxygen species, such as ·O₂⁻ and ·O⁻ [34]. Previous studies have shown that the catalytic oxidation activity at low temperatures is closely related to the content of surface oxygen species, and most of the O_ads is generated from the adsorption and activation of adsorbed oxygen at oxygen vacancies [31]. The highest O_ads/O_latt ratios are ascribed to the high-specific-surface area and can expose more active sites, which can be deduced from the SEM images and higher-surface-area data.

H₂-TPR experiments were performed to investigate the reducibility of the three samples. Figure 3c shows the H₂-TPR profiles of γ-MnOOH, δ-MnO₂, and M-MnO₂ catalysts with increasing temperature that can be deconvoluted into four peaks, named peaks 1, 2, 3, and 4. Among these, the peak area ratio of peaks 2, 3, and 4 is approximately 3:1:2 for all samples. This can be associated with the sequential reduction of MnO₂ to Mn₂O₃, Mn₂O₃ to Mn₃O₄, and Mn₃O₄ to MnO [28]. However, the first reduction peaks for γ-MnOOH, δ-MnO₂, and M-MnO₂ are located at 240 °C, 228 °C, and 197 °C, respectively. M-MnO₂ showed the lowest initial reduction temperature among all of the samples, suggesting that its surface oxygen species showed the highest reducibility and activity. It is clear that the highest oxidation potential is in the order of M-MnO₂ > δ-MnO₂ > γ-MnOOH, which is consistent with the results for the catalytic activity of HCHO oxidation by these samples.

O₂-TPD experiments were performed to investigate the activity of the surface adsorbed oxygen in different samples. Several O₂ desorption peaks were observed from in the 40–500 °C, as shown in Fig. 3d. The first desorption peak was assigned to the surface adsorbed oxygen (·O₂⁻/·O⁻), while the rest of the peaks were related to the release of lattice oxygen [35, 36]. The first surface adsorbed oxygen desorption occurred at temperatures of 113 °C, 100 °C, and 72 °C for γ-MnOOH, δ-MnO₂, and M-MnO₂, respectively. M-MnO₂ showed the lowest surface adsorbed oxygen desorption temperature, indicating that the surface adsorbed oxygen of M-MnO₂ exhibited the highest mobility and activity. Furthermore, the special interconnected network structure of M-MnO₂ favors mass transfer and exposes more active sites.

The specific surface area and pore volume of γ-MnOOH, δ-MnO₂, and M-MnO₂ are shown in Table 1. The specific surface area and the pore volume of γ-MnOOH were higher than those of the other two species, and those of δ-MnO₂ were the lowest. This was because of the high dispersion of γ-MnOOH nanowires and the severe agglomeration of δ-MnO₂ nanosheets. M-MnO₂ has a special structure composed of δ-MnO₂ nanoparticles anchored on nanowire-shaped γ-MnOOH, which avoids the severe agglomeration of δ-MnO₂.

The catalytic activities of γ-MnOOH, δ-MnO₂, and M-MnO₂ for HCHO removal at room temperature are compared in Fig. 4. For the static experiment, shown in Fig. 4a, the concentration of HCHO was decreased from 180 to 142, 80 and 34 mg/kg within 1 h by γ-MnOOH, δ-MnO₂, and M-MnO₂, respectively. In the static catalytic activity test, M-MnO₂ showed significantly higher HCHO removal ability than the other samples, and the catalytic efficiency of γ-MnOOH was quite low. The test demonstrated that δ-MnO₂ was the main active component for HCHO oxidation in the M-MnO₂ catalyst.

The dynamic experiment results further illustrate the catalytic activities of the different samples, as shown in Fig. 4b. The catalytic efficiencies of γ-MnOOH, δ-MnO₂, and M-MnO₂ were 4%, 36%, and 53% at room temperature, respectively. Meanwhile, T₁₀₀ refers to the temperature of complete HCHO conversion. The T₁₀₀ of γ-MnOOH, δ-MnO₂, and M-MnO₂ were 160 °C, 120 °C, and 90 °C, respectively. This shows that the γ-MnOOH catalyst needs a tremendous amount of energy (160 °C) to achieve complete HCHO conversion. The three catalysts’ variation tendency in Fig. 4b is similar to that of the static experiments in Fig. 4a. The order of the catalytic efficiency is M-MnO₂ > δ-MnO₂ > γ-MnOOH.

The catalytic stability and HCHO removal efficiency are important for the practical applications of the catalysts [34]. Therefore, the catalytic HCHO removal efficiencies of the three samples were tested in a series of relatively long-term experiments, as shown in Fig. 4c. The catalytic efficiency of M-MnO₂ was greater than 50% and remained stable during the entire 10 h test. Meanwhile, the HCHO removal efficiency of pure δ-MnO₂ decreased sharply from 36% to 27% within 10 h, while that of pure γ-MnOOH was initially 6% and then rapidly decreased to zero. These results further indicate that the catalytic efficiency of M-MnO₂ is higher than that of the other two catalysts and can be maintained for a long time at room temperature.

The apparent activation energies (E_a) of the three samples are given in Fig. 4d and were calculated according to the Arrhenius plots of the catalysts for HCHO oxidation. The E_a values for γ-MnOOH, δ-MnO₂, and M-MnO₂ were 34.1, 18.2 and 15.1 kJ/mol, respectively. It is clear that HCHO is oxidized more easily by M-MnO₂ because it has a lower E_a value than γ-MnOOH and δ-MnO₂.

Thermogravimetry (TG) analysis was used for evaluating the weight loss and qualitative analysis for detachable species in different samples with increasing temperature (see Fig. 5). Mass spectral analysis was used to confirm the detachable species according to mass charge ratio (m/z). It was found that the m/z of all detachable species was close to 18, which was consistent with the water molecule. The content of the largest surface adsorbed water loss (less than 100 °C) was 9.21% for M-MnO₂, followed by δ-MnO₂ and γ-MnOOH, which were only 3.68% and 2.71%, respectively. According to the literature, the surface adsorbed water can provide a rich surface hydroxyl group, which promotes the efficiency of HCHO oxidation at room temperature [17].

To explain why the M-MnO₂ catalyst shows excellent catalytic activity and to observe the reaction process and mechanism, in situ DRIFTS spectra of the three samples exposed in the 80 mg/kg HCHO/N₂ + 20% O₂/N₂ for HCHO oxidation at room temperature were obtained (see Fig. 6). There was no obvious material change on the surface of γ-MnOOH, which indicated that the catalytic reaction was weak. The material change on the surface of δ-MnO₂ and M-MnO₂ was obvious, which also confirmed that δ-MnO₂ was the main catalytic agent, and the catalyst activity of γ-MnOOH could be ignored at room temperature. The band at 1 465 cm⁻¹ represents the dioxymethylene (DOM) species (δ(CH₂)), suggesting that HCHO first absorbed on the samples to form DOM because the carbonyl carbon in HCHO is electrophilic and tends to adsorb on the nucleophilic surface oxygen atom. The weak intensity of the DOM band indicates that DOM can be rapidly transformed into formate species with surface oxygen. Moreover, the band at 1 465 cm⁻¹ is much lower in the M-MnO₂ sample, suggesting that the DOM on M-MnO₂ can be decomposed faster than that on a pure δ-MnO₂ sample.

Formate species are the well-known intermediates during the catalytic oxidation of HCHO and are detected in the experiments. Four intense bands located at 2 914 (ν(CH)), 1 564 (ν_as(COO⁻)), 1 415(δ(CH)), and 1 340 (ν_s(COO⁻)) cm⁻¹ can be ascribed to the formate species [30, 31]. The intensity of the formate species bands on δ-MnO₂ and M-MnO₂ increased with the HCHO exposure time, suggesting that formate species accumulated gradually on the catalyst surface. Meanwhile, the pure δ-MnO₂ sample showed a much higher relative intensity of the formate species than M-MnO₂. According to the previous studies [28, 29], the catalytic performance of the catalysts decreased, with the formate species occupying the active sites of the catalysts. This can explain the gradual decrease in the catalytic performance of δ-MnO₂ over time. The decomposition of formate species is the governing process for HCHO oxidation at room temperature, and the formate species can be transformed to carbonate species by the surface hydroxyl groups [17]. The intensity of the negative and large band at 3533 cm⁻¹ (ν(OH)) decreased gradually with the exposure time, indicating that the hydroxyl groups in the structure were continuously consumed during the reaction. Moreover, the carbonate species, located at 1 685 (ν_as(CO₃)) cm⁻¹ and 1 201 (ν_s(CO₃)) cm⁻¹, were only found on M-MnO₂, as shown in Fig. 6c. These indicate that the decomposition efficiency of the formate species on M-MnO₂ is higher than on pure δ-MnO₂. It has been reported that hydroxyl groups play a key role in MnO₂-based catalysts and the surface hydroxyl groups can be regenerated by the reaction between surface active oxygen (·O⁻/·O₂⁻) and surface adsorbed water through the reaction (·O⁻/·O₂⁻ + H₂O → 2-OH). The higher content of ·O⁻/·O₂⁻ and surface adsorbed water on M-MnO₂ inevitably leads to a higher catalyst efficiency.

Based on the previous analysis, the mechanism of HCHO oxidation over M-MnO₂ at room temperature can be described as shown in Fig. 7. Normally, HCHO is oxidized by active oxygen (·O⁻/·O₂⁻) into DOM and formate species after adsorbing on the catalyst surface, while the formate species tend to deposit on the catalyst surface, which requires more energy or surface hydroxyl groups to decompose. However, M-MnO₂ is composed of δ-MnO₂ nanosheets dispersed on γ-MnOOH nanowires, and this structure can be beneficial for the surface active oxygen compared with δ-MnO₂. According to the above-described experiments, the enhanced performance results from the special interconnected network structure, the dispersion and specific surface area of M-MnO₂ are clear. This special morphology can provide more surface active oxygen species, more surface adsorbed water, and thus, higher surface hydroxyl groups. Therefore, the formate species can be rapidly transformed to carbonate species and then decomposed to CO₂ and H₂O.

A special interconnected network structure, M-MnO₂, composed of δ-MnO₂ nanosheets and γ-MnOOH nanowires for HCHO oxidation was prepared by a facile one-step hydrothermal method. The obtained M-MnO₂ exhibited high catalytic activity, which reached 50% for 100 mg/kg HCHO oxidation for 10 h under a gas hourly space velocity (GHSV = 60 000 mL/(g_cat·h) higher than that of pure δ-MnO₂ nanosheets. It was found that the interconnected network structure provided sufficient surface active oxygen species and surface adsorbed water, resulting in excellent catalytic activity and stability for HCHO oxidation at room temperature. Moreover, the synthetic method is simple and can be easily implemented for large-scale industrial application.