Engineering oxygen vacancies in Au/MnO₂ catalysts for complete formaldehyde removal at near-freezing temperatures

Abstract

Formaldehyde (HCHO) is a major indoor air pollutant that poses serious risks to human health, making its efficient removal a critical environmental concern. Catalytic oxidation at room and sub-ambient temperatures has attracted significant attention due to its potential to completely decompose HCHO into harmless CO₂ and H₂O. However, practical implementation remains challenging because of low reaction activation energy and limited catalyst performance at reduced temperatures. In this study, Au-loaded manganese oxide nanowire catalysts (x% Au/MnO₂-NWs) were synthesized using a colloidal deposition strategy to achieve efficient HCHO removal under ambient and sub-ambient conditions. The optimized 1% Au/MnO₂-NWs catalyst achieved complete conversion of 280 ppm HCHO at 30 °C and, remarkably, fully oxidized 20 ppm HCHO even at 0 °C, demonstrating outstanding low-temperature activity and practical potential. Comprehensive characterization studies including H₂-TPR, EPR, Raman spectroscopy, and in situ DRIFTS revealed that Au nanoparticles induced abundant oxygen vacancies, which acted as active sites for HCHO adsorption and promoted O₂ activation. The synergistic interaction between Au and MnO₂ significantly enhanced low-temperature catalytic performance, providing mechanistic insights and a solid foundation for the rational design of highly efficient catalysts for indoor formaldehyde removal.

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New concepts

This study presents a novel strategy for low-temperature formaldehyde (HCHO) elimination by developing Au-loaded manganese oxide nanowire catalysts (x% Au/MnO₂-NWs). A key innovation is their ability to achieve complete HCHO removal even at sub-ambient temperatures (0 °C) in a flow system, addressing a major challenge in indoor air purification. The process of loading Au nanoparticles can simultaneously introduce oxygen vacancies into the manganese oxide support, which synergistically enhances HCHO adsorption and O₂ activation. Moreover, the nanowire structure of MnO₂ enables efficient electron transfer and improved stability under mild conditions. Detailed spectroscopic analyses (H₂-TPR, EPR, Raman, and in situ DRIFTS) reveal the dual role of Au-MnO₂ interactions and nanowire architecture in generating reactive oxygen species and sustaining high catalytic activity. These findings provide mechanistic insights and a practical pathway for designing robust, environmentally friendly catalysts for efficient air purification at ambient and sub-ambient temperatures.

1. Introduction

With the advancement of modern society, the amount of time people spend indoors has steadily increased. Formaldehyde (HCHO), a major indoor pollutant originating primarily from building materials, decorative items, and furniture, poses serious health risks.^1–4 Prolonged exposure, even at ppm-level concentrations, can cause severe respiratory irritation, eye discomfort, headaches, and other adverse effects. Therefore, the efficient removal of HCHO at room temperature or below remains a critical challenge.

Various strategies have been developed for HCHO removal, including physisorption, photocatalysis, plasma decomposition, and thermal catalysis.^5–9 Among them, thermal catalysis has attracted particular attention because it can efficiently convert toxic HCHO into harmless H₂O and CO₂ at low temperatures. Numerous heterogeneous catalysts have been investigated for this reaction, which are generally categorized as either supported metal-based catalysts or metal oxides.^10–13 Notably, supported metal catalysts such as Pt-, Pd-, Ag-, and Au-based systems typically demonstrate superior HCHO removal performance at low temperatures.^8,14–19

Traditionally, Au was considered an “inert” metal until Haruta et al. demonstrated that Au nanoparticles (NPs) supported on metal oxides exhibit extraordinary activity toward CO oxidation at low temperatures.^20–22 Over the past decades, Au NPs dispersed on various metal oxides have been extensively studied, and their applications in HCHO oxidation have also been explored.^{14,17,23–27} However, the effective removal of HCHO with Au-based catalysts usually requires relatively high reaction temperatures. Extensive studies have shown that the particle size of Au NPs strongly influences catalytic performance, with particles smaller than 5 nm generally being considered highly active for low-temperature CO oxidation.^28,29

In addition to particle size, the nature of the support also plays a critical role in determining catalytic activity. Transition metal-based catalysts (e.g., Fe,³⁰ Co,³¹ Ti,³² and V³³) have been extensively studied for formaldehyde oxidation due to their natural abundance and low cost. Among these, MnO₂ is considered one of the most promising candidates for HCHO removal, owing to its diverse crystal structures, tunable morphologies, environmental friendliness, and low cost.^12,34–38 Furthermore, extensive studies have shown that MnO₂ not only provides excellent activity and stability as a support³⁹ but also actively participates in the catalytic process,⁴⁰ thereby facilitating reaction kinetics and enhancing overall catalytic efficiency. For example, Dong et al.⁴¹ investigated the catalytic activity of a series of metal oxide catalysts synthesized at 120 °C and found that MnO₂ exhibited the highest formaldehyde conversion, reaching up to 80%. Dai et al.⁴² reported that δ-MnO₂ with abundant manganese defects efficiently oxidized formaldehyde to carbon dioxide and water at room temperature. In our previously work, colloidally deposited Au NPs (2–3 nm) on manganese oxides demonstrated remarkable activity at −80 °C, achieving 50% CO conversion.⁴³ These results highlight manganese oxides as highly effective supports for Au nanoparticles. Accordingly, the combination of uniformly dispersed Au nanoparticles with MnO₂ supports is highly desirable for efficient HCHO oxidation at ambient or even sub-ambient temperatures.

Herein, we synthesized a series of Au NPs supported on MnO₂ nanowires (x% Au/MnO₂-NWs) catalysts via a colloidal deposition method. The as-prepared catalysts exhibited a uniform distribution of Au NPs with an average particle size of ∼2.0 nm on the MnO₂ supports. Notably, the 1% Au/MnO₂-NWs catalyst demonstrated excellent low-temperature HCHO removal performance, achieving 100% conversion of 280 ppm HCHO at 30 °C with outstanding durability. It also completely converted 20 ppm HCHO at 0 °C, maintaining stable operation for up to 25 h. This performance is particularly significant for HCHO removal in near-freezing (0–10 °C) environments, such as cold-climate spaces, refrigerated facilities, and passive houses, where conventional catalysts often fail and preheating for room-temperature catalysts consumes additional energy. Comprehensive characterization studies, including H₂-TPR, EPR, Raman spectroscopy, and in situ DRIFTs, revealed that Au NPs not only generated additional oxygen vacancies serving as active adsorption sites for HCHO but also promoted O₂ adsorption and activation. This synergistic effect between Au and MnO₂ significantly enhanced low-temperature performance, providing valuable insights for the rational design of highly efficient catalysts for HCHO oxidation under mild conditions.

2. Experimental section

2.1. Materials

KMnO₄ and MnSO₄·H₂O were obtained from Sinopharm Chemical Reagent Co. Mn(NO₃)₂·4H₂O was purchased from Macklin, and NaBH₄ from Aladdin. Polyvinyl alcohol (PVA, M_w ≈ 1000) was supplied by Sigma-Aldrich. HAuCl₄·4H₂O was obtained from Shanghai Chemical Reagent Co. All chemicals and reagents were of analytical grade and used without further purification. Millipore water is made in the laboratory using an Eco-Q15 water purifier.

2.2. Synthesis

2.2.1. Synthesis of MnO₂ nanowires (MnO₂-NWs). MnO₂-NWs were prepared via a hydrothermal method according to our previously established procedures.⁴⁴ Typically, 2.25 g of MnSO₄·H₂O and 1.40 g of KMnO₄ were dissolved in 160 mL of distilled water and stirred vigorously at room temperature until a homogeneous solution was obtained. Afterward, the obtained solution was transferred into a 200 mL Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at 160 °C for 12 h. The resulting MnO₂-NWs were collected by filtration, thoroughly washed with distilled water and ethanol, and dried at 50 °C overnight.

2.2.2. Synthesis of x% Au/MnO₂-NWs. x% Au/MnO₂-NWs were synthesized using a colloidal deposition method.⁴⁵ In a typical procedure, PVA was added to an aqueous HAuCl₄ solution (100 mg L⁻¹, Au/PVA = 1.5/1 mg mg⁻¹) under vigorous stirring at room temperature and in the dark. The mixture was stirred for 10 min, after which a freshly prepared aqueous NaBH₄ (0.1 mol L⁻¹, Au/NaBH₄ = 1/5 mol mol⁻¹) was rapidly injected. The MnO₂ supports were subsequently introduced into the colloidal gold solution and stirred vigorously for 3–8 h. Finally, the obtained x% Au/MnO₂-NWs were collected by centrifugation, washed repeatedly with Millipore water, and dried at 50 °C.

2.3. Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku SmartLab X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å) operated at 40 kV and 50 mA, with a scan step of 2.5° min⁻¹. Nitrogen adsorption–desorption measurements were performed on a TriStar II 3020 analyzer at the temperature of −196 °C. All the catalysts were degassed at 150 °C for 12 h under vacuum before the sorption measurements. The morphology and microstructure of the catalysts were characterized via scanning electron microscopy (SEM, Zeiss Merlin Compact) and transmission electron microscopy (TEM, JEOL-F200). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Fisher Scientific ESCALAB250Xi instrument to determine surface elemental composition and oxidation states, with binding energies calibrated against the C 1s signal at 284.8 eV. Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker EMXplus spectrometer. Raman spectra were collected on a Renishaw Raman system (Model 1000, 514.5 nm). Inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 730) was used to quantify the actual Au loadings.

Hydrogen temperature-programmed reduction (H₂-TPR) was carried out using an iChem-700 instrument equipped with a thermal conductivity detector (TCD). Typically, 50 mg of catalyst was pretreated in high-purity N₂ at 300 °C for 30 min to remove surface adsorbates, followed by cooling to room temperature. The flow was then switched to 5 vol% H₂/Ar (50 mL min⁻¹), and reduction profiles were recorded from room temperature to 700 °C at a constant heating rate of 10 °C min⁻¹.

Oxygen temperature-programmed desorption (O₂-TPD) was performed on an iChem-700 equipped with a TCD detector. In a typical test, 50 mg of catalyst was pretreated in N₂ at 300 °C for 30 min and cooled to 50 °C. The sample was then exposed to 5 vol% O₂/He (30 mL min⁻¹) until saturation, followed by purging for 1 h to remove physiosorbed O₂. Desorption profiles were recorded from room temperature to 800 °C under Heat 10 °C min⁻¹.

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed on a Thermo Fisher Nicolet iS10 spectrometer equipped with a liquid-nitrogen-cooled MCT detector. The catalyst was first pretreated in a flow of 20 vol % O₂ and 80 vol% N₂ at 300 °C for 30 min, and then cooled to room temperature under inert gas. Subsequently, 100 ppm HCHO in synthetic air was introduced at a flow rate of 100 mL min⁻¹, and in situ DRIFTS spectra were recorded.

2.4. Catalytic activity of HCHO oxidation

Catalytic activity tests were performed in a fixed-bed quartz reactor (inner diameter: 6 mm). Gaseous HCHO was generated by passing a mixture of 20 vol% O₂ and 80 vol% N₂ through a formalin solution (37 wt% in H₂O) maintained in a 30 °C water bath. The reactant stream consisted of 280 ppm HCHO and 20 vol% O₂, with N₂ as the balance gas, at a relative humidity (RH) of 35%. A thermocouple placed in the catalyst bed was used to measure reaction temperature. Typically, 50 mg of catalyst (35–65 mesh) was loaded into the reactor, pretreated in situ at 300 °C for 30 min in 20 vol% O₂/80 vol% N₂, and then cooled to the target temperature. The reactant gas was subsequently introduced at a total flow rate of 60 mL min⁻¹, corresponding to a weight hourly space velocity (WHSV) of 72 000 mL g_cat⁻¹ h⁻¹. Formaldehyde concentration was continuously monitored using an online multigas analyzer (Bruker Omega 5). The HCHO conversion was calculated using eqn (1):where [HCHO]_in and [HCHO]_out are the inlet and outlet concentrations, respectively.

3. Results and discussion

3.1. Structural features and morphologies of catalysts

A series of x% Au/MnO₂ nanowire catalysts with varying Au loadings were synthesized via a colloidal deposition method. Their crystal structures and microstructural parameters were systematically characterized via X-ray diffraction (XRD). As shown in Fig. 1A, the diffraction peaks closely match those of α-MnO₂ (PDF2 No. 44-0141), confirming that all samples retain the α-phase crystal structure.⁴⁶ Notably, the crystalline phase and structure of the MnO₂-NWs support remained unchanged after the anchoring of Au nanoparticles (NPs). No characteristic diffraction peaks of Au were detected, likely due to the low Au loading and/or the high dispersion of Au species within the MnO₂-NWs.⁴⁷

Fig. 1 (A) TEM image; (B) particle size distribution of Au NPs; (C and D) HR-TEM images of the 1% Au/MnO₂-NWs catalyst.

The morphologies and microstructures of x% Au/MnO₂-NWs were examined using SEM and TEM. As depicted in Fig. S2A, the SEM image of MnO₂-NWs synthesized by hydrothermal treatment exhibits a uniform nanowire morphology with diameters between 40 and 50 nm and lengths on the micrometer scale. After introduction of Au NPs, the SEM image of 1% Au/MnO₂-NWs (Fig. S2B) shows that the nanowire morphology is preserved, indicating the high structural stability of the supports. TEM and HR-TEM were further employed to analyse the size and distribution of Au NPs on 1% Au/MnO₂-NWs (Fig. 1). The TEM image (Fig. 1A) reveals highly dispersed small Au NPs anchored on the surface of MnO₂ nanowires. The average Au particle size was 2.0 nm (Fig. 1B), which was nearly identical across all x% Au/MnO₂-NWs samples (x = 0.1, 0.4, 0.7, and 2) (Fig. S3). Size distributions of Au NPs, calculated from approximately 100 particles, are summarized in Table S1. This finding is consistent with the absence of Au peaks in high-sensitivity XRD.^48,49 The Au NPs, with particle sizes below 5 nm, are consistent with our previous reports,⁵⁰ confirming the reproducibility of the colloidal deposition method. High-resolution TEM (HR-TEM) images (Fig. 1C and D) further demonstrate the crystalline nature of both MnO₂ nanowires and Au NPs. Lattice fringes of 0.70 nm and 0.24 nm were clearly observed, corresponding to the (110) facet of MnO₂ nanowires and the (111) plane of metallic Au, respectively.

Moreover, the specific surface areas of the samples were determined by nitrogen sorption measurements. As shown in Fig. S4, the isotherms of 1% Au/MnO₂-NWs and MnO₂-NWs exhibit nearly identical profiles. The corresponding specific surface areas (Table S1) are 29 and 27 m² g⁻¹, respectively. Considering the high density of MnO₂ (5.02 g cm⁻³) and the low amount of microporosity,^51,52 these measured values are relatively high. The surface area of 1% Au/MnO₂-NWs shows only a slight decrease compared with the parent MnO₂-NWs, indicating that the structural framework was largely preserved after Au NP deposition. Therefore, it is reasonable to attribute the catalytic enhancement in HCHO oxidation to the introduction of Au species. The Au loading in 1% Au/MnO₂-NWs, determined via ICP-OES, was 1.11 wt% (Table S1), which agrees well with the theoretical value.

3.2. Catalytic activity of HCHO oxidation

Gold nanoparticles smaller than 5 nm supported on metal oxides are widely regarded as highly active catalysts for CO oxidation at low temperatures.^53–56 Accordingly, the 1% Au/MnO₂ nanowire catalyst, featuring gold nanoparticles with an average size of 2.0 nm, is expected to exhibit comparable efficiency in low-temperature oxidation reactions such as formaldehyde (HCHO) oxidation. Its catalytic performance was evaluated in a gas mixture containing 280 ppm HCHO, 20 vol% O₂, and N₂ as the balance gas, under 35% relative humidity and various temperatures (Fig. 2A and Fig. S5, S6). The observed activity differences are primarily attributed to the interaction between the Au nanoparticles and the MnO₂ nanowires. Compared to previously reported catalysts (Table S2), the current system demonstrates outstanding performance. The optimized 1% Au/MnO₂-NWs catalyst achieved complete HCHO conversion at 30 °C (WHSV = 72 000 mL g_cat⁻¹ h⁻¹), whereas MnO₂-NWs alone required 180 °C to reach 100% conversion. These results indicate that Au nanoparticle deposition substantially enhances the intrinsic activity of MnO₂-NWs, likely by promoting O₂ adsorption and activation. Additional evaluations under varying space velocities further confirmed the superior performance of the 1% Au/MnO₂-NWs catalyst (Fig. S7). Notably, complete HCHO conversion was achieved at 60 °C even at a WHSV as high as 144 000 mL g_cat⁻¹ h⁻¹.

Fig. 2 (A) HCHO conversion over x% Au/MnO₂-NWs (x = 0–1) catalysts at different temperatures. Reaction conditions: 280 ppm HCHO + 20 vol% O₂ + 80 vol% N₂, RH = 35%, WHSV = 72 000 mL g_cat⁻¹ h⁻¹; (B) Arrhenius plots and apparent activation energies for HCHO oxidation over freshly synthesized catalysts. Reaction conditions: 280 ppm HCHO + 20 vol% O₂ + 80 vol% N₂, RH = 35%, WHSV = 1 200 000 mL g_cat⁻¹ h⁻¹; (C) durability test of the 1% Au/MnO₂-NWs catalyst under different WHSV values at 30 °C. Reaction conditions: 280 ppm HCHO + 20 vol% O₂ + 80 vol% N₂, RH = 35%.

It is well established that the preparation method significantly influences the catalytic performance of metal-based catalysts in HCHO oxidation.⁵⁷ To further validate this effect, a comparison sample, DP-1% Au/MnO₂-NWs, was prepared via the deposition–precipitation (DP) method. As shown in Fig. S8A, its XRD pattern displays distinct diffraction peaks at 2θ = 38.2° and 44.4°, characteristic of metallic Au. In addition, TEM images (Fig. S8B) show the formation of relatively large Au nanoparticles. These results confirm that the DP method produces larger Au particles, which in turn leads to inferior HCHO oxidation activity relative to the colloidally synthesized 1% Au/MnO₂-NWs.

The Arrhenius plots for HCHO oxidation over various catalysts are presented in Fig. 2B. The apparent activation energy (E_a) decreases markedly upon the introduction of Au NPs. Among the catalysts, 1% Au/MnO₂-NWs exhibits the lowest E_a value, following the trend: 1% Au/MnO₂-NWs < 0.1% Au/MnO₂-NWs < DP-1% Au/MnO₂-NWs < MnO₂-NWs. Remarkably, the E_a value for 1% Au/MnO₂-NWs is lower than that of DP-1% Au/MnO₂-NWs, demonstrating that the colloidal deposition method is more effective in preparing highly active Au-based catalysts for HCHO oxidation.

Catalyst durability under harsh reaction conditions is a critical factor for practical applications. To evaluate stability, the 1% Au/MnO₂-NWs catalyst was subjected to long-term tests under different WHSV conditions, as shown in Fig. 2C. At around room temperature (30 °C) and a space velocity of 72 000 mL g_cat⁻¹ h⁻¹, the catalyst achieved 100% HCHO conversion without detectable deactivation after 30 h. To further probe its durability, the space velocity was increased to 144 000 mL g_cat⁻¹ h⁻¹ to maximize the utilization of active sites. Under these conditions, approximately 60% HCHO conversion was achieved within the first hour, and the catalytic activity showed only a slight decline after 30 h, confirming excellent long-term stability. Importantly, indoor HCHO concentrations are generally much lower than those used in laboratory evaluations. Thus, catalysts with high activity and stability at low concentrations are highly desirable. To this end, the 1% Au/MnO₂-NWs catalyst was evaluated at an inlet concentration of 20 ppm HCHO (Fig. 3A), where complete conversion was achieved even at 0 °C. Subsequent durability tests conducted at 0 °C (Fig. 3B) showed no detectable HCHO for up to 25 h, demonstrating that the 1% Au/MnO₂-NWs catalyst maintains outstanding stability under ultra-low-temperature operating conditions.

Fig. 3 (A) HCHO conversion over the 1% Au/MnO₂-NWs catalyst at different temperatures; (B) durability test of the 1% Au/MnO₂-NWs catalyst at 0 °C. Reaction conditions: 20 ppm HCHO +20 vol% O₂ + 80 vol% N₂, WHSV = 72 000 mL g_cat⁻¹ h⁻¹.

3.3. Surface physicochemical analysis

The XPS spectra of 1% Au/MnO₂-NWs and MnO₂-NWs are shown in Fig. 4A and B to investigate their surface chemical states. As presented in Fig. 4A, the Mn 2p_3/2 spectra exhibit peaks at 641.9–642.3 eV and 643.0–643.7 eV, which can be assigned to Mn³⁺ and Mn⁴⁺ species, respectively.^46,58,59 A decrease in Mn⁴⁺ valence leads to the generation of oxygen vacancies to maintain charge balance. Therefore, the Mn³⁺/Mn⁴⁺ ratio is commonly regarded as an indicator of oxygen vacancies.⁵⁹ As listed in Table S3, the Mn³⁺/Mn⁴⁺ ratio follows the order 1% Au/MnO₂-NWs > MnO₂-NWs, suggesting that 1% Au/MnO₂-NWs contains a higher density of oxygen vacancies. Fig. 4B shows the O 1s spectra of the two catalysts, which were deconvoluted into three peaks at 529.0–530.2, 530.7–531.6, and 532.6–533.4 eV. These peaks are attributed to the lattice oxygen (O_I),^60,61 surface-adsorbed active oxygen (O_II, e.g., O⁻, O²⁻, and OH groups),⁶² and surface-adsorbed water (O_III), respectively.⁶³ Based on previous reports,³⁵ the O_II/O_I ratio is considered to be an essential parameter for evaluating the abundance of reactive sites in oxidation reactions. As shown in Table S3, this ratio decreases in the order 1% Au/MnO₂-NWs > MnO₂-NWs. Since O_II species are more reactive and strongly interact with reactants,⁶⁴ the enriched O_II species and abundant oxygen vacancies in 1% Au/MnO₂-NWs account for its superior catalytic performance, consistent with the above conclusions.⁵⁹ The Au 4f spectrum of 1% Au/MnO₂-NWs (Fig. S9) exhibits an Au 4f_7/2 peak at 84.0 eV, assigned to metallic Au,^57,65 which agrees with previous reports on Au catalysts prepared via colloidal deposition.^45,66

Fig. 4 (A) Mn 2p XPS spectra; (B) O 1s XPS spectra; (C) EPR spectra, and (D) Raman spectra of 1% Au/MnO₂-NWs and MnO₂-NWs.

To further evaluate the oxygen vacancy content of the catalysts, electron paramagnetic resonance (EPR) and Raman spectroscopy were performed. The EPR spectra of the catalysts (Fig. 4C) display a characteristic signal at g = 2.003, indicative of paramagnetic oxygen vacancies. As reported in the literature, variations in EPR signal intensity correlate with differences in oxygen defect concentration among catalysts.⁵⁹ The 1% Au/MnO₂-NWs catalyst exhibits a significantly stronger signal, suggesting a higher oxygen vacancy concentration, which likely contributes to its superior catalytic activity. The Raman spectra (Fig. 4D) provide further evidence. For MnO₂-NWs, the peak at 641 cm⁻¹ corresponds to the symmetric υ₂ (Mn–O) stretching of MnO₆ octahedra,⁶⁷ whereas the peak at 575 cm⁻¹ is attributed to υ₃ (Mn–O) stretching, and the weak band at 385 cm⁻¹ corresponds to framework vibrations.⁶⁷ For 1% Au/MnO₂-NWs, the main Mn–O peak shifts from 641 to 636 cm⁻¹, accompanied by notable peak broadening. This shift and increased half-peak width signify the formation of additional oxygen vacancies.⁶⁷ Collectively, these Raman results confirm that Au nanoparticle deposition introduces abundant oxygen vacancies into MnO₂ nanowires, consistent with the XPS and EPR analyses.

The reducibility of the catalysts was evaluated by H₂-TPR, as depicted in Fig. 5A. For 1% Au/MnO₂-NWs, a reduction peak at 162 °C can be attributed to the consumption of surface oxygen and hydroxyl species, while the broad reduction peak at 281 °C can be assigned to the reduction of MnO₂ to Mn₂O₃. In addition, the distinct reduction peak at 324 °C is associated with the reduction of Mn₂O₃ to Mn₃O₄, and the higher-temperature peak at 376 °C can be ascribed to the further reduction of Mn₃O₄ to MnO.³⁵ Notably, the reduction temperatures of the catalysts are significantly lowered after the introduction of Au nanoparticles. This shift to lower reduction temperatures indicates enhanced reducibility and improved mobility of oxygen species.³⁵ Consequently, the superior oxygen mobility of 1% Au/MnO₂-NWs facilitates the generation of active oxygen species, thereby contributing to excellent catalytic activity in HCHO oxidation.⁵⁸

Fig. 5 (A) H₂-TPR profiles and (B) O₂-TPD profiles of 1% Au/MnO₂-NWs and MnO₂-NWs.

The oxygen activation ability of the catalysts was further investigated by O₂-temperature-programmed desorption (O₂-TPD), as displayed in Fig. 5B. The desorption peak below 400 °C corresponds to adsorbed surface oxygen species.⁵⁹ The lower desorption temperature observed for 1% Au/MnO₂-NWs demonstrates improved low-temperature oxygen mobility, indicating that it possesses better reactivity at low temperatures.

3.4. In situ DRIFTS

In situ DRIFTS spectra were obtained to investigate the reaction mechanism of HCHO oxidation over these catalysts at room temperature and 50% relative humidity (RH) (Fig. 6). When the 1% Au/MnO₂-NWs catalyst was exposed to HCHO in humidified air for 40 min, characteristic bands corresponding to intermediates such as dioxymethylene (DOM, 1477 cm⁻¹), formate (1593 cm⁻¹), and carbonate species (1430 cm⁻¹) were observed.^16,24,68,69 Two negative bands at 3740 and 3845 cm⁻¹ were assigned to surface hydroxyl groups, and their peak intensity varies with increasing reaction time, suggesting that intermediate formation is partially related to the consumption of surface hydroxyl groups.⁸ By contrast, for the MnO₂-NWs, the intensities of the negative bands at 3619 and 3728 cm⁻¹ were much weaker than those of 1% Au/MnO₂-NWs. Moreover, characteristic absorption peaks of molecularly absorbed HCHO (1216 and 1720 cm⁻¹) were observed on MnO₂-NWs.^52,68,70 This indicates that absorbed HCHO species are difficult to cleave on the surface of MnO₂-NWs, where only the DOM intermediate adsorption band at 1071 cm⁻¹ is detected. In contrast, for the 1% Au/MnO₂-NWs catalyst, three major intermediate bands, rather than signals corresponding to molecular HCHO, were clearly observed, confirming that Au deposition significantly promotes HCHO activation.

Fig. 6 In situ DRIFTS spectra of (A) 1% Au/MnO₂-NWs and (B) MnO₂-NWs under a flow of air + HCHO + H₂O (RH = 50%).

Reactive oxygen species (ROS) are essential for HCHO oxidation, as they drive the conversion of DOM intermediates into CO₂ and H₂O.⁷¹ On MnO₂-NWs, the O₂ molecules are adsorbed at the oxygen vacancies and dissociate to form active oxygen species.⁷⁰ In contrast, on 1% Au/MnO₂-NWs, O₂ can adsorb on Au nanoparticles, dissociate into active oxygen species, and participate directly in HCHO oxidation. Based on these findings, a possible reaction mechanism is proposed. Oxygen vacancies act as adsorption sites for HCHO, since water molecules preferentially dissociate at these sites to form hydroxyl groups, which promote HCHO adsorption.^72–75 Meanwhile, Au NPs serve as active centres to facilitate O₂ dissociation. Specifically, HCHO molecules in the reaction stream are initially adsorbed on surface –OH groups via hydrogen bonding. Then, molecularly absorbed HCHO is subsequently oxidized to DOM intermediates by activated oxygen species. Subsequently, DOM undergo further oxidation by –OH groups and/or activated oxygen species to form formate species, which rapidly converted into carbonate species, and ultimately decomposes into CO₂ and H₂O.^76,77

4. Conclusions

In summary, a series of Au nanoparticles supported MnO₂ nanowire catalysts (x% Au/MnO₂-NWs) were successfully synthesized via a colloidal deposition strategy. The obtained catalysts featured uniformly dispersed Au nanoparticles with an average size of ∼2.0 nm and were applied in the low-temperature catalytic oxidation of HCHO. Notably, the 1% Au/MnO₂-NWs catalyst achieved complete HCHO conversion (280 ppm at 30 °C) at a WHSV of 72 000 mL g_cat⁻¹ h⁻¹, while maintaining excellent stability for up to 30 h. Moreover, complete conversion of 20 ppm HCHO was realized even at 0 °C, with stable performance sustained for 25 h. This outstanding catalytic activity is attributed to the synergistic effect between oxygen vacancies and Au nanoparticles: oxygen vacancies enhance HCHO adsorption and dissociation, whereas Au nanoparticles facilitate O₂ activation, enabling efficient low-temperature oxidation. These findings offer valuable guidance for the rational design of catalysts for high-performance, low-temperature HCHO removal.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be made available on request.

The data supporting this article have been included as part of the Supplementary Information. Supplementary information: further experimental details, Fig. S1–S9, Tables S1–S4, and additional references. See DOI: https://doi.org/10.1039/d5mh01869b.

Acknowledgements

The work was supported by the National Key R&D Program of China (2023YFA1508001 and 2018YFE0201703), the National Natural Science Foundation of China (22572059), the “1000-Youth Talents Plan” and the Natural Science Foundation of Guangdong Province (2025A1515012924).

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Footnote

† Zhenghuan Yin and Yajun He contributed equally to this work.

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