The effect of hydrogen reduction of <i>α</i>-MnO<sub>2</sub> on formaldehyde oxidation: The roles of oxygen vacancies

The effect of hydrogen reduction of α-MnO₂ on formaldehyde oxidation: The roles of oxygen vacancies

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Liping Zhou^a^,^b, Chunying Wang^b^,^c, Yaobin Li^b^,^c^,^*, Xiaofeng Liu^b, Hua Deng^b, Wenpo Shan^a^,^b^,^c^,^*, Hong He^b^,^d

Chinese Chemical Letters | 2023, 34(3) : 107605

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Chinese Chemical Letters | 2023, 34(3): 107605

• Communication •

The effect of hydrogen reduction of α-MnO₂ on formaldehyde oxidation: The roles of oxygen vacancies

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Liping Zhou^a^,^b, Chunying Wang^b^,^c, Yaobin Li^b^,^c^,^*, Xiaofeng Liu^b, Hua Deng^b, Wenpo Shan^a^,^b^,^c^,^*, Hong He^b^,^d

Affiliations

^a College of Resources and Environmental Sciences, Fujian Agriculture and Forestry University, Fuzhou 350002, China

^b Center for Excellence in Regional Atmospheric Environment, Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China

^c Zhejiang Key Laboratory of Urban Environmental Processes and Pollution Control, Ningbo Urban Environment Observation and Research Station, Institute of Urban Environment, Chinese Academy of Sciences, Ningbo 315800, China

^d State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

Published: 2023-03-15 doi: 10.1016/j.cclet.2022.06.028

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Abstract

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A series of α-MnO₂ catalysts with various Mn valence states were treated by hydrogen reduction for different periods of time. Their catalytic capacity for formaldehyde (HCHO) oxidation was evaluated. The results indicated that hydrogen reduction dramatically improves the catalytic performance of α-MnO₂ in HCHO oxidation. The α-MnO₂ sample reduced by hydrogen for 2 h possessed superior activity and could completely oxidize 150 ppm HCHO to CO₂ and H₂O at 70 ℃. Multiple characterization results illustrated that hydrogen reduction contributed to the production of more oxygen vacancies. The oxygen vacancies on the catalyst surface enhanced the adsorption, activation and mobility of O₂ molecules, and thereby enhanced HCHO catalytic oxidation. This study provides novel insight into the design of outstanding MnO_x catalysts for HCHO oxidation at low temperature.

Key words

Hydrogen reduction / α-MnO₂ / Formaldehyde oxidation / Oxygen vacancies

Cite this Article

Liping Zhou, Chunying Wang, Yaobin Li, Xiaofeng Liu, Hua Deng, Wenpo Shan, Hong He. The effect of hydrogen reduction of α-MnO₂ on formaldehyde oxidation: The roles of oxygen vacancies[J]. Chinese Chemical Letters, 2023 , 34 (3) : 107605 - . DOI: 10.1016/j.cclet.2022.06.028

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HCHO is a serious threat to human health in indoor environments, and its main sources are wood furniture and artificial boards [1]. Chronic exposure to HCHO causes a number of diseases such as skin allergies, asthma, acute poisoning, neurasthenia and even a variety of cancers [2]. Therefore, finding an effective way to control indoor formaldehyde is an urgent issue. Several HCHO purification technologies have been developed in recent years, including adsorption [3], photo-catalytic oxidation [4], plasma decomposition with a catalyst [5], and catalytic oxidation [6-8]. Among these technologies, the catalytic oxidation method is an efficient and feasible approach for indoor HCHO elimination without secondary pollution [9].

Usually, thermal catalytic catalysts include supported noble metal and metal oxide catalysts. Although noble-metal-based catalysts such as Pt- [10-12], Pd- [13-16], Au- [17-19] and Ir-based [20, 21] catalysts exhibit excellent catalytic performance at ambient temperature, their commercial applications are limited by their high cost. On the contrary, metal oxide catalysts such as MnO_x-SnO₂ [22], MnO_x-Co₃O₄-CeO₂ [23], Ag/CeO₂ [24], Co₃O₄ [25], Ce-MnO₂ [26], and Cu/MnO₂ [27] have attracted more and more attention due to their low price and promising performance. In 2001, Sekine and Nishimura found that MnO₂ possessed the best HCHO removal efficiency by comparing various transition metal oxides including CeO₂, CuO, TiO₂, Fe₂O₃, Mn₃O₄ and so on [28]. The past two decades have witnessed the development of manganese dioxide catalysts for the oxidation of HCHO. Manganese dioxide possesses variable valence states and unique physicochemical properties, resulting in superior HCHO catalytic activity compared with other transition metal oxides. Recently, the performance of manganese dioxide in HCHO oxidation has been investigated from many aspects, including crystal structure [29], morphology [30], and crystal facets [31]. Zhang et al. found that δ- and α-MnO₂ displayed better HCHO catalyst performance than β- and γ-MnO₂ because δ- and α-MnO₂ possessed unique tunnel structures and abundant surface lattice oxygen species, which enhanced the HCHO oxidation activity [32]. Chen et al. reported that α-MnO₂ possessed a wealth of surface lattice oxygen species, which play a major role in HCHO oxidation [33]. Rong et al. also found that the {310} facets of α-MnO₂ promoted the formation of oxygen vacancies, which led to visibly promoted activity for HCHO elimination [34]. α-MnO₂, consisting of a tunnel structure with size 0.46 × 0.46 nm, exposes more MnO₆ edges, which is conducive to the formation of surface oxygen vacancies [35]. Hence, α-MnO₂ is a promising catalyst in the field of indoor HCHO purification. Great efforts have been made to develop methods for improving the catalytic activity of manganese oxides at lower temperatures, including noble metal doping [36, 37], composite material manufacturing [31] and defect adjustment [38]. According to previous works, surface oxygen vacancy formation can enhance the formation of surface-active oxygen species and then improve the performance of MnO₂ catalysts for HCHO oxidation [26, 38, 39].

Therefore, in this work, α-MnO₂ catalysts were reduced by hydrogen for different periods of time to control the concentration of oxygen vacancies and then tested for their performance in the catalytic oxidation of HCHO at low temperatures. A dramatic difference in the catalytic activity was clearly observed between the four kinds of α-MnO₂. The as-obtained catalyst α-MnO₂-2h showed the best catalytic activity among the four catalysts, achieving 100% HCHO conversion at 70 ℃ with a weight hourly space velocity (WHSV) of 60,000 mL g⁻¹ h⁻¹ (Table S1 in Supporting information). The catalysts were carefully characterized by various methods. Based on the results, factors affecting the catalytic activity were elucidated.

The catalytic performance of α-MnO₂-0h, α-MnO₂-1h, α-MnO₂-2h and α-MnO₂-3h in HCHO oxidation was tested, and the results are shown in Fig. 1a. The HCHO conversion of the catalysts at the initial temperature of 30 ℃ were very low, (less than 20%). The HCHO conversion ratio of α-MnO₂-0h (30% at 100 ℃) was obviously lower than the other three catalysts, indicating that hydrogen pre-reduction of the α-MnO₂ catalyst can effectively enhance its catalytic activity of HCHO oxidation. The temperatures for complete HCHO oxidation over α-MnO₂-1h, α-MnO₂-2h and α-MnO₂-3h catalysts were 90, 70 and 70 ℃, respectively. Meanwhile, as seen in Fig. 1b, there were no other by-products besides CO₂. Fig. S1 (Supporting information) shows a comparison of α-MnO₂-2h catalyst activities in RH = 0 and RH = 35%. Obviously, the catalytic activity decreased after adding water. T_100% of α-MnO₂-2h increased from 70 ℃ to 90 ℃. Among these catalysts, α-MnO₂-2h showed the best performance of HCHO oxidation, thus the stability of α-MnO₂-2h was further evaluated at 70 ℃ (Fig. S2 in Supporting information). The result showed excellent catalytic stability for α-MnO₂-2h, maintaining 99% of HCHO conversion for the tested 20 h.

The N₂ adsorption-desorption isotherms of the four catalysts are shown in Fig. S3 (Supporting information). The four catalysts showed typical Ⅳ adsorption isotherms according to the classification criteria proposed by IUPAC, indicating that all of the catalysts were mesoporous materials. Meanwhile, as shown in Table S2 (Supporting information), the specific surface area of the catalyst decreased after hydrogen reduction, indicating that hydrogen reduction may lead to the agglomeration of α-MnO₂ particles.

SEM and TEM images of the MnO₂ samples are shown in Fig. S4 (Supporting information). With the increase of hydrogen reduction time, the samples converted from a dispersed sheet structure to solid particles gradually, indicating that the sample particles agglomerated gradually during hydrogen reduction, which was consistent with the results of N₂ adsorption-desorption.

The XRD patterns of the catalysts are shown in Fig. 2. Some peaks were observed for all of the samples at 12.8 (110), 18.1 (200), 28.8 (310), 37.5 (211), 39.0 (330), 60.3 (521), which were assigned to α-MnO₂ (JCPDS No. 44-0141) [36, 40]. After prereduction treatment, the XRD patterns of the samples became broader and weaker, indicating the decline of crystallinity. When the sample was reduced for 3 h in hydrogen, diffraction peaks located at 32.4 (103) and 58.5 (321) appeared, which were assigned to Mn₃O₄ (JCPDS No. 80–0382) [30]. It suggested that MnO₂ was partially reduced to Mn₃O₄ after 3 h hydrogen reduction.

The electronic states of manganese on the surface of as-obtained samples were measured by XPS, and the results are show in Fig. 3. The average oxidation state (AOS) of Mn was calculated by following formula [41]: AOS = 8.956 − 1.126 ∆E_S, where ∆E_S represents the binding energy difference of Mn 3s (Fig. 3a). The AOS of Mn on α-MnO₂-0h, α-MnO₂-1h, α-MnO₂-2h and α-MnO₂-3h catalysts were 3.7, 3.5, 3.4 and 3.04, respectively, as shown in Table S2 (Supporting information). The results indicated that the longer the reduction time, the smaller the AOS of Mn, which revealed that MnO_x was reduced gradually. According to the results of Mn 2p XPS (Fig. S5 in Supporting information), there were three peaks at 640.4, 641.6 and 643.7 eV for all of the samples, which were attributed to Mn²⁺, Mn³⁺ and Mn⁴⁺ species [42, 43], respectively. The proportions of Mn³⁺/Mn_Total were calculated, and the results are listed in Table S2 (Supporting information). It is worth noting that the sample of α-MnO₂-2h exhibited the largest Mn³⁺/Mn_Total ratio. As we know, oxygen vacancies will be produced when Mn³⁺ exists in pure manganese dioxide in order to maintain charge balance [44]. That is, α-MnO₂-2h possessed the largest content of oxygen vacancies on account of the largest content of Mn³⁺. The existence of abundant oxygen vacancies on α-MnO₂-2h catalyst can enhance the adsorption and activation of molecular oxygen and water and then improve the mobility of surface oxygen species [13, 38, 43, 45], which may be the main reason for the excellent activity of the α-MnO₂-2h catalyst for HCHO oxidation. However, when the reduction time further extended to 3 h, Mn²⁺/Mn_Total species increased sharply, which was due to the formation of Mn₃O₄. Mn₃O₄ species appeared on α-MnO₂-3h catalyst, which may affect the performance of α-MnO₂-3h catalyst for HCHO oxidation [46]. As shown in Fig. S6 (Supporting information), compared with Mn₂O₃ and Mn₃O₄, the α-MnO₂-2h catalyst possessed the best performance of HCHO oxidation.

Fig. 3b shows the XPS spectra of O 1s. The O 1s spectra can be deconvoluted into three peaks to further access the information on intrinsic quality of oxygen species. The peak at the range of 529.3–530 eV can be assigned to lattice oxygen (marked as O_latt); the peak at binding energy 530.7–531.2 eV is assigned to the surface-adsorbed oxygen species (marked as O_ads), and the peak at the higher binding energy of 532.2–533.7 eV can be assigned to the surface-adsorbed molecular water (marked as O_wat) [25]. It is clearly shown that lattice oxygen and surface-adsorbed oxygen account for a large part of oxygen species on the catalyst surface. Table S2 displays the O_ads/O_total ratio of four as-obtained samples. The O_ads/O_total ratio of the samples followed the order: α-MnO₂-2h > α-MnO₂-3h > α-MnO₂-1h > α-MnO₂-0h, in consistent with the order of activity. This result showed that α-MnO₂-2h possesses the most abundant surface-adsorbed oxygen. Because α-MnO₂-2h possessed sufficient oxygen vacancies, their existence led to oxygen atoms near oxygen vacancies becoming unsaturated, which enhanced the migration of lattice oxygen and acted as surface active oxygen species [47-49].

Raman and ESR tests were carried out to further illustrate the formation of oxygen vacancies on the catalysts. The Raman spectra of the samples are shown in Fig. 4a. The Raman band at near 650 cm⁻¹ can be assigned to the symmetric stretching vibration Mn—O bonds in the MnO₆ octahedron [41, 50, 51]. The Mn-O bond force constant (k) could be calculated by Hooke's law as follows (Eq. 1) [52]:

(1)

where ω is the Raman shift (cm⁻¹), c is light velocity, and μ is effective mass. According to the formula, there is a positive correlation between ω and the Mn-O bond force constant (k). As shown by the spectra, the band at 656.3 cm⁻¹ for α-MnO₂-0h shifts to 650 cm⁻¹ (α-MnO₂-1h), 643.8 cm⁻¹ (α-MnO₂-2h) and 650 cm⁻¹ (α-MnO₂-3h), respectively, after hydrogen reduction for different times. The peak-shift phenomenon indicated the change in the Mn-O bond force among the catalysts. The Mn-O bonds in α-MnO₂-2h had the weakest force among the catalysts, which contributed to the production of oxygen vacancies and finally favored HCHO catalytic oxidation [34]. Fig. 4b shows the ESR spectra of the four catalysts. The signal at g ≈ 2.004 was observed for all of the samples, which was ascribed to oxygen vacancies [13, 53]. The signal intensities of oxygen vacancies followed the order of α-MnO₂-2h > α-MnO₂-3h > α-MnO₂-1h > α-MnO₂-0h, which was consistent with the results of activity test. It shown that hydrogen reduction weakened the Mn-O bond and increased the content of oxygen vacancies on the catalysts. The presence of abundant oxygen vacancies in α-MnO₂-2h facilitated the adsorption, activation, and migration of O₂ molecules and further improved the catalytic activity [43, 54].

The activated oxygen capacities of the samples were further evaluated by H₂-TPR, and the results are shown in Fig. S7 (Supporting information). As we know, the peak located in the region below 200 ℃ can be assigned to the surface-activated oxygen species [33]. It is worth noting that the reduction peak of α-MnO₂-2h moved to lower temperature in comparison with other samples. In other words, it is more likely to adsorb oxygen molecules from the air to form surface-adsorbed oxygen [55]. The temperature of the reduction peak for α-MnO₂-2h was the lowest. It indicated that α-MnO₂-2h has superior catalytic capacity, which can facilitate the adsorption and activation of molecular oxygen to form active oxygen species, in consistent with the result of O 1s XPS. Therefore, the α-MnO₂-2h catalyst showed the best catalytic performance for HCHO oxidation. The O₂-TPD was tested to further enrich the discussion of oxygen activation, which was shown in Fig. S8 (Supporting information). The higher temperature peak at 700 ℃ is attributed to the conversion of bulk lattice oxygen [50]. The desorption peaks of α-MnO₂-0h, α-MnO₂-1h and α-MnO₂-2h remained basically unchanged. However, the desorption peak intensity of α-MnO₂-3h is obviously weaker than those of the other samples, indicating the consumption of bulk lattice oxygen in α-MnO₂-3h during H₂ reduction. This is a good explanation for the presence of the Mn²⁺ species in α-MnO₂-3h.

In-situ DRIFTS was tested to explore the effect of H₂ reduction on reaction intermediates during HCHO oxidation process and to clarify the HCHO oxidation mechanism. As shown in Fig. S9 (Supporting information), 150 ppm of HCHO and O₂ were used to pass through the α-MnO₂ with the detection of the accumulation of surface reacted intermediates. From the figure we can see that the intermediates, formate species (1375, 1595 cm⁻¹ for ʋ(COO)) [13, 38] appeared in all the samples. However, when the O₂ was introduced into the system, the formate species decreased and the hydroxyl species (3677 cm⁻¹ for ʋ(OH)) consumed in α-MnO₂-2h evidently. That is, the intermediates conversion rate of α-MnO₂-2h was higher than those of the other samples. To sum up, H₂ reduction can improve the conversion of intermediates to CO₂ and H₂O on α-MnO₂-2h.

In summary, the strategy of hydrogen reduction was utilized to obtain α-MnO₂ catalysts with different concentrations of oxygen vacancies. The HCHO catalytic oxidation performances of the obtained catalysts followed the order: α-MnO₂-2h > α-MnO₂-3h > α-MnO₂-1h > α-MnO₂-0h, and the α-MnO₂-2h catalyst could completely oxidize HCHO to CO₂ and H₂O at 70 ℃. The characterizations indicated that the reduction of the catalyst further generates oxygen vacancies which could enhance the adsorption, activation and mobility of O₂ molecules, and thereby enhanced HCHO catalytic oxidation. However, the existence of Mn₃O₄ decreased the reaction activity. To sum up, the properties and density of oxygen vacancies played a crucial role in the catalytic degradation of HCHO. This work provides a reasonable explanation for this phenomenon and offers a new approach to the design of efficient low-price catalysts for HCHO removal.

Declaration of competing interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

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The work was supported by the Cultivating Project of Strategic Priority Research Program of Chinese Academy of Sciences (No. XDPB1902), the Science and Technology Planning Project of Xiamen City (No. 3502Z20191021), the Science and Technology Innovation "2025″ major program in Ningbo (No. 2022Z028) and Youth Innovation Promotion Association of Chinese Academy of Sciences (No. 2020310).

Supplementary materials

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.06.028.

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Year 2023 volume 34 Issue 3

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doi: 10.1016/j.cclet.2022.06.028

Receive Date：2022-02-15
Online Date：2025-11-21
Published：2023-03-15

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Received：2022-02-15
Revised：2022-04-13
Accepted：2022-06-11

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^a College of Resources and Environmental Sciences, Fujian Agriculture and Forestry University, Fuzhou 350002, China

^b Center for Excellence in Regional Atmospheric Environment, Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China

^d State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Fig. 1. (a) HCHO conversion and (b) CO₂ yield of α-MnO₂-0h, α-MnO₂-1h, α-MnO₂-2h and α-MnO₂-3h catalysts. Reaction conditions: HCHO concentration = 150 ppm, 20% O₂, He balance, and WHSV = 60,000 mL g⁻¹ h⁻¹.

Fig. 2. XRD patterns of α-MnO₂-0h, α-MnO₂-1h, α-MnO₂-2h and α-MnO₂-3h catalysts.

Fig. 3. XPS spectra of α-MnO₂-0h, α-MnO₂-1h, α-MnO₂-2h, and α-MnO₂-3h catalysts. (a) Mn 3s. (b) O 1s.

Fig. 4. (a) Raman spectra and (b) ESR spectra of α-MnO₂-0h, α-MnO₂-1h, α-MnO₂-2h and α-MnO₂-3h catalysts.

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