Single site of water-resistant asymmetric Bi–Ov–Mn for robust VOC ozonation at ambient temperature

Abstract

Manipulating the geometries and electronic structures of oxygen vacancies (Ovs) in oxides to increase their catalytic activity has been a critical focus of research, but the processes remain challenging, particularly because of the significant interference caused by ubiquitous water vapour. In this work, we employ a nanocrystal-to-crystal transformation methodology to integrate a single atom of bismuth (Bi) into MnO₂, resulting in the formation of Bi–Ov–Mn entities. This single site reduces the formation energy of ˙OOOH species, facilitating the formation of reactive oxygen species (ROS), particularly ˙OH, due to the nonuniform electron distribution in the presence of both ozone and water. Therefore, this unique asymmetric defective linkage provides excellent water vapour resistance (1.8 vol%), which significantly improves its performance in the removal of volatile organic compounds. In this study, a pioneering paradigm utilizing asymmetric active sites is introduced, which expands the potential of catalytic ozonation for VOC abatement.

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Introduction

Metal oxides, including MnO₂,^1,2 CeO₂,³ ZnO,⁴ TiO₂,⁵ and Co₃O₄,⁶ are becoming increasingly critical in catalytic chemistry for energy conversion and environmental protection practices. Oxygen vacancies (Ovs) are ubiquitous on these reducible oxides, providing a powerful force for driving surface chemistry and catalytic reactions.^7,8 For example, in the oxidation of organic compounds,⁹ Ovs govern redox ability and ultimately determine catalytic activity and stability.

Since Ovs are active sites in most oxidation reactions, regulating their structure and distribution is crucial. However, manipulating the chemical environment of the Ov structure, such as by constructing surface region asymmetric Ov sites, has proven to be more critical.^10,11 Asymmetric Ov sites, with asymmetric coordination of Ov and cations, have advantages over their symmetric counterparts because of their unique metastable oxygen-binding features, which enable superior oxygen exchange capabilities.¹⁰ Moreover, having an atomically dispersed metal stabilized on the surface of a substrate is an efficient strategy for creating asymmetric active sites, leading to the fabrication of highly efficient catalysts.^12,13 Conversely, water vapour is a ubiquitous component in most heterogeneous oxidation processes, such as water gas shift,¹⁴ ozone decomposition,¹⁵ and NO oxidation¹⁶ reactions, and it is one of the final products of the oxidation of organic compounds. Several studies have revealed that water vapour strongly interacts with Ov sites, occupying active sites and hindering their reactions.^5,17,18 Isolated asymmetric Ov sites with robust redox capacity and exceptional water vapour resistance are essential for the efficient oxidation of organic compounds. However, the precise design and creation of atomic-level asymmetric Ov sites have rarely been explored. Anchoring isolated metal cations on a pristine metal oxide might be a straightforward and feasible approach for creating surface region asymmetric Ov sites.

Bismuth (Bi), a low-valence element (trivalent oxidation state), has been effectively utilized as a low-cost and high-performance component in environmental catalysis.¹⁹ Moreover, Bi is frequently employed in aqueous phase oxidation reactions.^20,21 Owing to its combination of metallic and nonmetallic properties, Bi doping might offset the humidity-related limitations of heterogeneous gas–solid processes. Nevertheless, ensuring the uniform dispersion rather than clustering of Bi cations on the surface of a support is crucial for their effectiveness.

Typically, the use of α-MnO₂ as a support for Bi using conventional approaches results in Bi atoms being anchored onto the outer surface of α-MnO₂ (Scheme 1a), resulting in a limited ability to tune Ov structures. As a pioneering strategy, we (Scheme 1b) introduce a nanocrystal-to-crystal transformation (CCT) approach for anchoring Bi atoms onto MnO₂. This catalyst, termed α-MnO₂–Bi and abbreviated as MnO₂–Bi, easily creates isolated asymmetric Bi–Ov–Mn sites. The structural properties of MnO₂–Bi are characterized using techniques such as extended X-ray absorption fine structure (EXAFS), positron annihilation lifetime spectroscopy (PALS), in situ Raman spectroscopy and density functional theory (DFT) calculations. Asymmetric Bi–Ov–Mn sharply reduces the competitive adsorption of water vapour and simultaneously maintains the ability of O₃ to activate the formation of reactive oxygen species (ROS) such as ˙OH. This material, assisted by ozone, can efficiently degrade various inert volatile organic compounds (VOCs) at ambient temperatures under realistic humid conditions. This work provides valuable insights into the development of functional asymmetric entities that integrate oxygen vacancies and single atoms for environmental catalysis and can guide the design of a catalyst for the removal of VOC under real and complex conditions.

Scheme 1 Schematic diagram of catalyst preparation: (a) Bi/α-MnO₂ and (b) MnO₂–Bi.

Results and discussion

Textural and physicochemical properties of the catalysts

The synthesis of MnO₂–xBi is based on a CCT approach, with the Bi loading controlled by varying the concentration of Bi(NO₃)₃. Inductively Coupled Plasma (ICP) analysis of the Bi contents in MnO₂–xBi reveals that the measured values closely match the calculated theoretical weight fractions (Table S1). The XRD patterns in Fig. S1a show that MnO₂–1Bi, MnO₂–5Bi, and MnO₂–10Bi align closely with the standard α-MnO₂ structure (PDF #44-0141), indicating that Bi cations are uniformly distributed among Mn atoms without forming crystalline Bi oxides when the Bi loading level is below 10 wt%.²² However, at 50 wt% Bi, either Bi₂Mn₄O₁₀ or Bi₂O₃ is detected in MnO₂–50Bi.²³ The broadening and slight redshift of the characteristic Raman peaks²⁴ of α-MnO₂ with increasing Bi content, as illustrated in Fig. S1b, suggest the formation of asymmetric Bi–O–Mn or Bi–Ov–Mn sites.

The morphologies of pristine MnO₂, MnO₂–xBi and Bi/α-MnO₂ are illustrated in Fig. S2 and 1a–c. The pristine MnO₂ sample has a nanowire structure, while stacked nanorods emerge and increase with increasing Bi content. Compared with the clear and regular lattice fringes of the (2 0 0) surface plane of MnO₂, some distorted lattice fringes are observed in MnO₂–xBi, with (2 0 0) and (2 1 1) serving as the exposed planes. No discernible clusters are detected in the samples via high-angle annular dark-field scanning TEM (HAADF-STEM). STEM-coupled EDS elemental mapping, as depicted in Fig. S3, confirms the uniform distribution of Bi in MnO₂. Aberration-corrected transmission electron microscopy (STEM) images (Fig. 1a–c) reveal that in the MnO₂–5Bi catalyst, Bi is atomically dispersed both on the framework (yellow dots, empty tunnels) and lattice fringes (cyan dots) of α-MnO₂. Conversely, the Bi/α-MnO₂ sample shows Bi atoms dispersed on the framework but not within the lattice fringes. The pore structures and specific surface areas (SSAs) are determined by N₂ adsorption/desorption measurements (Fig. S4 and Table S1). The pore structures are basically maintained after various levels of Bi incorporation, although the SSA values decrease with increasing Bi loading.

Fig. 1 Characterization results for the pristine MnO₂ and MnO₂–xBi catalysts. (a–c) Representative aberration-corrected-STEM images of pristine MnO₂, MnO₂–5Bi and Bi/α-MnO₂ catalysts. (d and e) Wavelet transform images of the Mn K-edge and Bi K-edge for the MnO₂–5Bi catalysts. (f and g) EXAFS and corresponding fitted spectra of the Mn K-edge and Bi K-edge for pristine MnO₂ and MnO₂–xBi catalysts. (h) ESR spectra. (i) Positron lifetimes and corresponding intensities.

XAFS results are obtained to analyse the local atomic structures and defect sites (Fig. 1d–g, S5 and Table S2). The curve-fitting results of the Mn K-edge EXAFS of MnO₂ (Fig. 1f and Table S2) reveal the presence of reduced Mn–O coordination numbers, suggesting the production of oxygen vacancies (Ovs) near the Mn sites. Fig. 1g and Table S2 show that the Bi–O coordination numbers increase with increasing Bi content. Bi anchoring results in the formation of a Mn–Bi shell, and the peak intensity increases with increasing Bi content. At low Bi-doping levels (<10 wt%), Bi atoms remain isolated and are separated by Mn, forming Bi–O–Mn. However, the Fourier transform peak of the MnO₂–50Bi sample suggests the coexistence of a Bi–Bi shell (Fig. S5b), indicating clustering at relatively high Bi loadings.

The chemical elements present and their electronic states in the MnO₂–Bi samples are investigated using X-ray photoelectron spectroscopy (XPS; Fig. S6), which reveals that the metal species are present only in oxidized states (Mn⁴⁺ at 643.5 eV, Mn³⁺ at 642.0 eV, and Bi³⁺ at 164.1 eV and 158.9 eV) rather than in metallic states.^25,26 The electron spin resonance (ESR) spectra in Fig. 1h indicate that the Ov signal at a g value of 2.003 increases with increasing Bi content, suggesting the creation of additional Ovs. The positron lifetimes (τ) and occupancies (I) shown in Fig. 1i and Table S1 indicate that there are three distinct Ov environments:²⁷ isolated Ovs within the lattice (τ₁, 130.7–207.2 ps) and Ov clusters (τ₂, 335.4–374.3 ps), with τ₃ (743–1804 ps) representing positron lifetimes in the pore structure. The abundance of Ov clusters, as evidenced by the high I₂ value (69.2–87.4%), underscores their dominance among oxygen vacancies, a feature with reduced τ₂ values (from 360 to 335 ps) that is further amplified by appropriate Bi anchoring. This enhancement creates Bi–Ov–Mn sites with higher electron density compared to pristine MnO₂, highlighting the effective modulation of the electronic structure by Bi.

In short, the XRD, Raman and electron microscopy results suggest that at low levels of Bi anchoring, Bi cations are atomically distributed by substituting for Mn in the α-MnO₂ lattice, while excess Bi leads to the formation of Bi oxides. The ESR results reveal that the Ov content increases with increasing Bi content, and the PALS results further verify that the loading of isolated Bi atoms facilitates the formation of asymmetric Bi–Ov–Mn clusters.

Enhanced ozone activation and moisture resistance abilities due to Bi doping

Through ESR analysis, we detect ROS in the MnO₂ or MnO₂–xBi/ozone/water three-phase systems (Fig. 2a–d). An increased Bi content promotes ¹O₂ and ˙OH generation while reducing ˙O₂⁻ formation. This result suggests that symmetric structures favour ˙O₂⁻ production, whereas asymmetric structures are more effective for ¹O₂ and ˙OH production. DFT calculations are conducted to reveal the Ov structures and to propose an O₃ activation mechanism (see Text S3 for details). The optimized Ov structures are presented in Fig. S7. Compared with bare α-MnO₂, Bi loading decreases the energy required for Ov formation,²⁸ aligning with the ESR results shown in Fig. 1h. The variations in the adsorption energy and structure of O₃, including bond lengths and angles, are detailed in Fig. S8 and Table S3, respectively. Compared with a pristine Bi-anchored (2 0 0) slab, the Ov species increases O₃ activation, with substantial deformation of the O₃ molecule upon adsorption on the oxygen dimer vacancy cluster. This result is further confirmed by the evident electron transfer between the adsorbate and the asymmetric dimer Ov, as shown in Fig. S9. By combining these results with our previous findings,²⁸ we can deduce that oxygen dimer vacancies, whether symmetric or asymmetric, serve as efficient active sites for ozone activation.

Fig. 2 Chemical properties of the pristine MnO₂ and MnO₂–xBi catalysts. (a–d) ESR spectra of the ROS when the pristine MnO₂ and MnO₂–xBi catalysts are exposed to O₃ in the dark. (e) Static adsorption isotherms for water. (f) Optimized adsorption structures for water on pristine MnO₂ and MnO₂–Bi with dimer vacancies.

Additionally, we investigate water vapour adsorption on pristine MnO₂ and MnO₂–Bi catalysts via static water vapour adsorption. Fig. 2e shows that the water adsorption amount decreases with increasing Bi loading, indicating that the asymmetric entities (Bi–Ov–Mn) significantly reduce water vapour adsorption. DFT calculations further verify the impact of asymmetric Bi–Ov–Mn sites on water vapour adsorption. As shown in Fig. 2f, Bi doping significantly reduces the water affinity at the vacancy sites due to the decreased adsorption energy, regardless of water coverage. This finding aligns with the static water adsorption results.

Interface evolution of asymmetric Bi–Ov–Mn linkages during ozonation

The structural changes in the catalysts and the dynamics of catalytic ozonation are analysed using in situ Raman spectroscopy, as depicted in Fig. S10 and 3a, b. Fig. 3a shows that MnO₂–5Bi has the highest peak intensities at 839 cm⁻¹ (O₂²⁻) and 1676 cm⁻¹ (adsorbed O₃),^29–31 indicating that ozone activation is most effective with this catalyst. MnO₂–50Bi exhibits poor ozone activation due to its inadequate redox capability, as shown in Fig. S11. Under humid conditions (Fig. 3b), pristine MnO₂ produces fewer ROS as water occupies the active sites. In contrast, MnO₂–5Bi maintains peak intensity, indicating that the process of Bi anchoring increases ozone adsorption and activation, even in the presence of water.

Fig. 3 In situ Raman spectra and DFT calculation results. (a) In situ Raman spectra of pristine MnO₂ and MnO₂–xBi in ozone gas. (b) In situ Raman spectra of pristine MnO₂ and MnO₂–5Bi with the addition of water. (c) Theoretical models of ˙OOOH on MnO₂ and MnO₂–5Bi catalysts. (d) Potential energy surface scanning for ˙OOOH along the middle O–O bond.

Above, we confirm that the asymmetric oxygen vacancy structure formed by Bi addition effectively enhances O₃ activation and reduces water adsorption, thereby benefiting the catalytic ozonation of VOCs. To elucidate the benefits of asymmetric Bi–Ov–Mn in generating water resistance, DFT calculations are conducted to study the interactions between O₃ and H₂O molecules. The DFT results in Fig. 3c indicate that Bi anchoring reduces the formation energy of ˙OOOH. A potential energy scan is performed for the ˙OOOH species along the linear axis of the middle O–O bond (Fig. 3d). Near Bi–Ov–Mn, the energetically favourable formation of active ROS occurs, thereby promoting ˙OH formation. Consequently, the asymmetric Bi–Ov–Mn moiety is expected to function as an active site because of its ability to activate both water and ozone.

To investigate the electronic mechanism, the electron transfer and density of states (DOS) associated with the activation of O₃ are analysed. Unlike O₃ in a vacuum layer, the O₃ molecules undergo dissociative adsorption on both pristine MnO₂ and MnO₂–Bi (Fig. S12), further verifying their ability to activate O₃. To better understand the nature of symmetric Mn–Ov–Mn and asymmetric Bi–Ov–Mn sites, the interactions of H₂O and O₃ with pristine MnO₂, MnO₂–Bi and Bi/α-MnO₂ (200) surfaces are investigated (Fig. 4 and S13). Unlike H₂O in a vacuum layer, H₂O molecules undergo similar dissociative adsorption on the surfaces of the pristine MnO₂, MnO₂–Bi and Bi/α-MnO₂ catalyst surfaces (Fig. S13). O₃ exhibits significantly different electron delivery (Bader charge) patterns across the above three surfaces in the presence of water vapour. For the pristine MnO₂ and Bi/α-MnO₂ catalysts, which feature symmetric Mn–Ov–Mn entities, the electron accumulation in both terminal oxygen atoms of adsorbed O₃ is identical, as shown in Fig. 4b and d, indicating symmetric electronic redistribution. In contrast, the electronic transfer to the terminal oxygen atoms of O₃ on asymmetric Bi–Ov–Mn (MnO₂–Bi catalyst) is asymmetric. Owing to the unique metal and nonmetal characteristics of Bi, the electron donation to the nearest terminal oxygen atom is less than that to the other terminal oxygen atoms (+0.24 vs. +0.59e), as illustrated in Fig. 4c. Moreover, the DOS pattern of MnO₂–Bi exhibits a significantly greater density of unpaired electron spins than those of the MnO₂ and Bi/α-MnO₂ catalysts. This observation aligns with the experimental results (Fig. 2d), which show that the asymmetric Bi–Ov–Mn structure promotes the generation of ROS.

Fig. 4 Electron density difference and DOS analysis of O₃ for O₃ + H₂O in the vacuum layer (a), on MnO₂ (b), on MnO₂–Bi (c), and on Bi/α-MnO₂ (d). The cyan and green colours indicate a decrease and increase in the electron density, respectively.

Subsequent investigations aim to thoroughly explore the catalytic ozonation activity and reaction mechanism of pristine MnO₂ and MnO₂–Bi catalysts to verify the impact of asymmetric Ov units on VOC ozonation under humid conditions.

Catalytic ozonation performance

The CCT method outperforms traditional preparation methods in controlling the generation of Ovs in MnO₂ (Fig. S14 and S15). Thus, the catalytic activity under various conditions is examined carefully. While pristine MnO₂ synthesized using the CCT strategy exhibits superior catalytic activity under dry conditions, its performance significantly decreases under humid conditions (Fig. S16). This limitation is effectively mitigated by the incorporation of foreign atoms. The introduction of promoters such as Co, La and particularly Bi into the MnO₂ lattice markedly enhances both toluene conversion and CO₂ yield in the presence of water vapour (Fig. S17), confirming the general applicability of the CCT strategy for doping various metals. Furthermore, by optimizing the Bi concentration, we further improve catalytic ozonation, achieving superior performance under humid conditions (Fig. 5b and S18). MnO₂–5Bi presents the highest CO₂ yield of 0.65 μmol g⁻¹ s⁻¹ at less than 0.5 vol% moisture. In contrast to MnO₂–5Bi, Bi/α-MnO₂ with the same Bi loading level is produced by doping Bi onto the outer surface of MnO₂ following the same CCT approach. The catalytic performance is shown in Fig. 5a. As expected, the Bi/α-MnO₂ catalyst exhibits notably inferior catalytic activity compared with that of MnO₂–5Bi. The substitution of Bi for Mn within the α-MnO₂ lattice particularly facilitates the formation of asymmetric Bi–Ov–Mn structures. In contrast, the interaction between Bi and the outer surface of MnO₂ is too weak to form the single-site Bi–Ov–Mn entity; instead, it leads to the formation of Mn–Ov–Mn entities, as illustrated in Fig. 1b, c and 4c, d. In summary, this phenomenon underscores the efficacy of the single-site Bi–Ov–Mn entity resulting from the substitution of lattice Mn with Bi.

Fig. 5 Catalytic ozonation performance. (a) Catalytic ozonation of toluene over various catalysts under humid conditions at 190 min (toluene: 100 ppm, ozone: 1600 ppm, 120 L h⁻¹ g⁻¹, reaction temperature: 30 °C, water: 1.1 vol%). (b) Catalytic ozonation efficiency of toluene over MnO₂–xBi catalysts at 300 min (toluene: 100 ppm, ozone: 1400 ppm, WHSV: 120 L h⁻¹ g⁻¹, reaction temperature: 30 °C, water: 1.1 vol%). (c) Long-term activity of MnO₂–5Bi for the catalytic ozonation of toluene (toluene: 100 ppm, ozone: 2000 ppm, WHSV: 60 L h⁻¹ g⁻¹, reaction temperature: 30 °C, water: 1.8 vol%; if used, RH = 42%). (d) Catalytic ozonation of mixed VOCs (33.3 ppm AC + 33.3 ppm EA + 33.3 ppm toluene, ozone: 1600 ppm, WHSV: 120 L h⁻¹ g⁻¹, reaction temperature: 30 °C, water: 0.6 vol%).

The effects of differences in weight hourly space velocity (WHSV), reaction temperature and initial ozone concentration on catalyst performance are presented in Fig. S19. The stability test of the MnO₂–5Bi catalyst with 1.8 vol% water vapour reveals that it maintains stable toluene conversion and high CO₂ selectivity (Fig. 5c and S20). Fig. 5d and S21 confirm the efficiency of MnO₂–5Bi in degrading various VOCs, including acetone (AC), ethyl acetate (EA), toluene and their mixture. Table S4 summarizes the catalytic ozonation performance of various catalysts from recent studies, highlighting the outstanding activity and stability of our catalysts in VOC elimination. It should be noted, however, that real waste gas streams contain complex components such as dust and acid gases. Therefore, investigating the catalyst's performance under actual industrial conditions represents a critical next step.

Taking toluene as an example, the catalytic oxidation mechanism is revealed by GC-MS analysis (Fig. S22). The intermediates are summarized in Table S5. The pristine MnO₂ catalyst primarily produces benzaldehyde, acetophenone, and benzoic acid, while MnO₂–5Bi generates acetic acid. Under humid conditions, pristine MnO₂ exhibits a reduced peak intensity of the byproduct, which is correlated with decreased activity because water accumulation blocks the active sites. Conversely, the main byproduct of MnO₂–5Bi shifts to amylene, and its activity remains stable, demonstrating its superior moisture resistance. Collectively, the identified intermediates outline a comprehensive degradation network wherein toxic aromatics are sequentially broken down into low-carbon acids and finally mineralized to CO₂ and H₂O, confirming the efficacy and environmental benefit of the process.

Conclusion

We synthesized a series of MnO₂–xBi catalysts via the CCT synthetic method, adjusting the Bi content to tune the lattice oxygen environment, resulting in the formation of an asymmetric Bi–O–Mn moiety. These units, particularly the MnO₂–5Bi variant, demonstrated exceptional redox behaviour and water resistance due to the metastable nature of the oxygen sites, increasing ozone activation even in the presence of water. DFT calculations confirmed that water molecules interacted more actively and released ˙OH more readily from these sites, particularly at oxygen vacancy dimers. Thus, the superior catalytic performance of MnO₂–5Bi was due partially to the dynamic occupation and exposure of active sites—specifically, the asymmetric Bi–Ov–Mn dimer—by water molecules. To our knowledge, this work is the first report on the fabrication of water-resistant Mn-based catalysts using the simple CCT synthetic method. The current strategy has far-reaching implications for designing and preparing numerous water-resistant metal oxidation catalysts and can serve as a potential forefront in VOC degradation technology, especially with regard to its use in practical applications.

Author contributions

Yuqin Lu designed and performed the experiments, analyzed the data and wrote the original draft of the paper. Huayang Zhang analyzed the data and contributed to modify the manuscript. Jianguo Ding and Tingting Pan performed the experiments. Wenjie Tian, Yunbo Yu, Changbin Zhang, Wenpo Shan and Shaobin Wang analyzed the data. Hua Deng conducted the theoretical calculations. Hua Deng, Hong He and Joseph S. Francisco supervised the project and contributed to modify the manuscript. All authors contributed to the preparation of the manuscript.

Conflicts of interest

The authors declare no competing financial interests.

Data availability

The data that support the findings of this study are available upon reasonable request from the corresponding author.

Supplementary information: catalyst preparation and activity, catalyst characterization and DFT calculations. See DOI: https://doi.org/10.1039/d5sc06166k.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (52270111, 52570134, 52500128, 52225004), Fujian Provincial Natural Science Foundation of China (2024J010042) and the Foundation of Institute of Urban Environment, the guiding project of seizing the commanding heights of “self-purifying city” (No. IUE-CERAE-202401). W. T. acknowledges the partial support from the Australian Research Council Discovery Early Career Researcher Award (ARC DECRA) (Project ID: DE220101074), and H. Z. and S. W. acknowledge the support from the Australian Research Council Discovery Project (DP230102406).

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Footnote

† These authors contributed equally to the work and are co-first authors.

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