Potassium-modulated δ-MnO2 as robust catalysts for formaldehyde oxidation at room temperature

Applied Catalysis B: Environmental 260 (2020) 118210

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Potassium-modulated δ-MnO2 as robust catalysts for formaldehyde oxidation at room temperature ⁎

Jian Jia, , Xiaolong Lua, Cheng Chena, Miao Hea, Haibao Huanga,b, a b

T

⁎

School of Environmental Science and Engineering, Sun Yat-sen University, 132 East Waihuan Road, Guangzhou 510006, China Guangdong Indoor Air Pollution Control Engineering Research Center, Sun Yat-sen University, 132 East Waihuan Road, Guangzhou 510006, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Formaldehyde oxidation Poor crystalline δ-MnO2 Potassium Surface active oxygen species Catalytic oxidation mechanism

Engineering MnO2 with rich surface active oxygen species is critical to effectively eliminate formaldehyde (HCHO) under mild conditions. Herein, we introduced a facile redox method to fabricate a series of δ-MnO2 samples by varying the concentration of K+, which efficiently modulated the layer size, morphology, crystallinity, redox properties, and thus the surface active oxygen species of the obtained δ-MnO2. The medium potassium concentration led to the optimized MneO bond strength, the abundant surface active oxygen species, and the complete conversion of ca. 22 ppm HCHO at 30 °C under a weight hourly space velocity (WHSV) of 200,000 mL/(gcat h). Surface adsorbed oxygen species (e.g., O2− and O−) and surface hydroxyl groups, were suggested to oxidize HCHO into intermediates (i.e., DOM, formate, and carbonate species). Water was critical for further transforming the intermediates into CO2. A Langmuir-Hinshelwood (LH) mechanism was proposed involving in the whole oxidation process.

1. Introduction The ubiquitous existence of formaldehyde (HCHO) in the living or public places, is mainly from building materials, decorative materials, furniture, and consumables, poses serious adverse health effects on humans [1]. Long-term exposure to formaldehyde, even at low concentrations (< 0.5 ppm), causes severe irritations, central nervous system damage, respiratory disease and even carcinogenic effects [2,3]. In fact, formaldehyde has been classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC) [4]. Abatement of indoor formaldehyde in a “green way” under mild conditions is highly desirable because humans generally spend 80–90% of their time indoors. Various technologies, such as adsorption [5], plasma destruction [6], biological or botanical degradation [7], photocatalytic oxidation [8], and thermal catalytic oxidation have been developed for formaldehyde abatement [9–13]. Among these technologies, catalytic oxidation is the most promising strategy due to its high removal efficiency, energy-saving, and mild reaction conditions. Since Zhang et al.’ pioneering work on Pt/TiO2 for complete oxidation of formaldehyde at room temperature [14], noble metal catalysts (e.g., Pt, Pd, Au, Rh, and Ag) have been extensively studied [9,10,12]. Their scarcity and high cost, however, greatly restrict further applications. Accordingly, tremendous effort has been devoted to developing non-noble metal oxides ⁎

based catalysts, including MnO2, TiO2, CeO2, Co3O4, Fe2O3, ZrO2, V2O5 and mixed metal oxides [11], among which manganese-based oxides have attracted increasing attention owing to their relatively low price, high activity, and low toxicity [13]. Sekine et al. [15] first performed catalytic oxidation of HCHO over several metal oxides (Ag2O, PdO, MnO2, TiO2, CeO2, and Mn3O4) in a static reaction vessel, and MnO2 exhibited the best activity, converting 91% HCHO at 25 °C after 24 h. The crystal structure of MnO2 was confirmed to significantly affect the catalytic performance. Zhang et al. [16] demonstrated that MnO2 with different crystal structures (i.e., α, β, γ, and δ-MnO2) was able to oxidize HCHO and δ-MnO2 was the most active owing to its abundant surface lattice oxygen species and layered structure enhancing the adsorption and desorption of HCHO. To date, only a few studies on HCHO removal over MnO2 at room temperature have been reported, most of which involved δ-MnO2. Zhang and coworkers [17–19] have explored bulk or supported birnessite-type manganese oxide on granular activated carbon or polyester fiber for formaldehyde removal and reported a high removal efficiency (> 70%) of formaldehyde with a concentration from 0.3 to 10 ppm at 25 °C. We previously prepared δ-MnO2 supported on coconut shell activated carbon and achieved complete removal efficiency of formaldehyde (10 ppm) at room temperature under a gas hourly space velocity of 65,000 h−1 [20]. Despite these efforts, formaldehyde oxidation over MnO2 at room temperature remains a great challenge.

Corresponding authors at: School of Environmental Science and Engineering, Sun Yat-sen University, 132 East Waihuan Road, Guangzhou 510006, China. E-mail addresses: [email protected] (J. Ji), [email protected] (H. Huang).

https://doi.org/10.1016/j.apcatb.2019.118210 Received 24 June 2019; Received in revised form 16 September 2019; Accepted 17 September 2019 Available online 20 September 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.

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It is generally accepted that surface oxygen species (O2−, O−, or surface hydroxyl groups) or surface lattice oxygen directly involved in the activation of HCHO and its subsequent conversion into CO2 and H2O [13]. Surface oxygen species are preferentially located on the defects of MnO2, such as surface oxygen vacancies, which act as electron donors to activate molecular oxygen to form electron deficient adsorbed species (O2− and O−). These species also react with H2O to form surface hydroxyl groups (O2−, O− + H2O → −OH), as proposed to enhance the adsorption of HCHO or assist the oxidation of HCHO into CO2 and H2O [21]. Wang et al. [22] indicated that the pits on the surface of birnessite MnO2 served as defects in the activation of O2 and H2O into surface active oxygen species, hence resulting in high catalytic activity. Amorphous or weak crystalline transition metal oxides have been reported to have abundant lattice defects or oxygen vacancies, and excess surface oxygen species serve as active sites to efficiently catalyze various reactions, such as low-concentration NO removal [23], water oxidation [24], and CO oxidation [25]. Consequently, it is highly desirable to synthesize poorly crystallized δ-MnO2 for the catalytic oxidation of formaldehyde at room temperature. Potassium is usually used as a promoter to facilitate the HCHO oxygen reaction. For noble metal-based catalysts (e.g., Pt, Pd, and Ag), potassium can induce the formation of negatively charged metals, which enhance the adsorption of HCHO and facilitate the adsorption and activation of chemisorbed oxygen [26–28]. Potassium in MnO2 acted as a great electron donor, making the adsorption of O2 and H2O energetically favorable and promoting oxygen activation by creating hybrid d-sp orbitals [29,30]. Moreover, potassium can enhance the lattice oxygen activity by lowering the activation energy to release oxygen to form oxygen vacancies on MnO2 [31]. Notably, the formation of manganese oxides with layered frameworks generally proceeds through the so-called template reaction. δ-MnO2 has a layered structure constructed by 2D edge-sharing MnO6 octahedra, and water molecules and potassium ions insert into the layer’s interspace. Inspired by this, changing the concentration of K+ is expected to modify the microstructure of δ-MnO2, such as the crystallinity, for efficient catalytic activity. In this work, the poorly crystallized δ-MnO2 with different morphologies was prepared using a facile redox method by varying the amount of K+. The samples were used for catalytic oxidation of formaldehyde at room temperature. The δ-MnO2 samples were characterized by multiple techniques, such as N2 physisorption, XRD, SEM, TEM, XPS, H2-TPR, and O2-TPD. The structure-activity relationship of the various δ-MnO2 samples was correlated. Both the effects of potassium and water on the catalytic performance were also investigated to gain fundamental insights into the superior activity of the δ-MnO2based catalysts. The possible mechanism of formaldehyde oxidation was also proposed by using in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments.

were denoted as δ-MnO2-0, δ-MnO2-1, δ-MnO2-2, and δ-MnO2-5, respectively. Then, 1 g dried sample was added to 200 mL 5% HNO3 under continuous stirring for 12 h, respectively, in order to remove the potassium of δ-MnO2. Afterward, the mixture was filtered, washed with distilled water, and dried at 80 °C overnight. The obtained δ-MnO2 with potassium removal were denoted as δ-MnO2-xR (x = 0, 1, 2, and 5), corresponding to the δ-MnO2-x (x = 0, 1, 2, and 5) samples.

2. Experimental

2.4. Catalytic testing

2.1. Catalyst preparation

Catalytic oxidation of HCHO was performed in a fix-bed continuous flow reactor with an inner diameter of 8 mm. A gas mixture, including 22 ± 1 ppm HCHO and humidified air (1 L/min, which corresponded to a WHSV of 200,000 mL/(gcat h)), was fed for the reaction at room temperature (30 °C). Gaseous HCHO was generated by flowing dry air through paraformaldehyde in a round-bottomed flask at 40 °C in a water bath. HCHO and CO2 were analyzed using a Multi-Gas Analyzer (GASERA ONE, Gasear, Finland), and the formaldehyde removal efficiency was calculated as follows:

2.2. Catalyst characterization The morphology of the samples was characterized by field emission scanning electron microscopy (FE-SEM, ZEISS GeminiSEM 500, Germany). High-resolution transmission electron microscopy (HRTEM) imaging was carried out on a JEOL JEM-2100 F at an accelerating voltage of 200 kV. The texture properties, including the specific surface area and pore volume, were estimated using N2 adsorption and desorption isotherms collected at −196 °C using a Tristar II 3020 M (Micromeritics, USA) system. All the samples were pretreated by degassing at 200 °C for 6 h. The amount of potassium in the as-prepared δMnO2 was determined by inductively coupled plasma-optical emission spectrophotometry (ICP-OES, 730 Series, Agilent Technologies, USA). X-ray diffraction (XRD) patterns of different samples were recorded on a powder X-ray diffractometer (Smartlab-3KW, Rigaku Ltd., Japan) using Cu Kα radiation (λ = 0.15405 nm). Raman spectra were recorded with a Horiba HR800 Confocal Raman spectrometer (Jobin-Yvon, Inc., France) with 532 nm excitation. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo-ESCALAB 250XI (Thermo Fisher Scientific, USA). The binding energy values were all calibrated based on the hydrocarbon contamination using the C1s peak at 284.8 eV. H2 temperature-programmed reduction (H2-TPR) and O2 temperatureprogrammed desorption (O2-TPD) were performed on an AutoChem 2920 (Micromeritics, USA). Fourier transform infrared spectra (FTIR) spectra were collected using a Nicolet iS10 FTIR spectrophotometer (Thermo Fisher Scientific, USA) in the wavenumber range of 1000–4000 cm−1.

2.3. In-situ DRIFTS tests In-situ DRIFTS spectra were recorded using an FTIR spectrophotometer (Nicolet iS10, Thermo Fisher Scientific, USA) with an in-situ cell. All samples were pre-purged with N2 for 1 h at room temperature before switching to formaldehyde with a concentration of 50 ppm and a flow rate of 30 mL/min. All spectra were acquired with a resolution of 8 cm−1 for 16 scans, over a wavenumber range between 650 and 4000 cm−1.

δ-MnO2 were synthesized by a facile redox precipitation method using KMnO4 (Guangzhou Chemical Reagent Factory, China), MnSO4 (Tianjin Damao Chemical Reagent Co., Ltd, China), and KCl (Sinopharm Chemical Reagent Co., Ltd., China) as follows: 2MnO4− + 3 Mn2+ + 2H2O → 5MnO2 + 4H+ Typically, the designed amount of KMnO4 and MnSO4 with a stoichiometric ratio of 2:3, and KCl with various amounts (0, 0.01, 0.02, and 0.05 mol), were sequentially added to 100 mL deionized water under magnetic stirring at 60 °C for 1 h in a water bath. The obtained brown precipitates were filtered and washed with deionized water for several times, and then dried at 80 °C for 24 h. The as-prepared samples

CHCHO = ([HCHO]in – [HCHO]out)/[HCHO]in * 100% where CHCHO, [HCHO]in, and [HCHO]out represent the formaldehyde removal efficiency and the inlet and outlet HCHO concentrations, respectively. 2

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explained by that more MnO6 octahedral units were stacked along the c-axis with extended growth along the a–b plane. K+ was generally assumed to act as a template to synthesize MnO2 with various structures, and more K+ or H2O molecules are required to stabilize the layer structures of δ-MnO2 [36,37]. The excessive addition of K+ further strengthens the crystallinity and increases the layer size of the samples. The XRD patterns of the samples indicate that the facile redox method employed here was favorable for the formation of δ-MnO2, and the concentration of K+ strongly affects the crystal phases and crystallinity of the manganese oxides. The high K+ concentration can improve the crystallinity and form large-sized layers of δ-MnO2. To gain further insights into the structural differences among the δMnO2 samples, Raman spectroscopy was used. Fig. 1b shows that δMnO2 displays a characteristic spectrum in the range from 400 to 800 cm−1 with three dominant bands: a prominent Raman feature between 550 and 570 cm−1 corresponding to an in-plane MneO compressing and stretching vibration along the octahedral layers in δ-MnO2 and two other bands at ∼500 and ∼630–650 cm−1 associated with out-of-plane MneO vibrations perpendicular to the layers [38]. The bands at ∼480 cm−1 were attributed to an A1g-like stretching mode of the MnO6 octahedral [39]. The weak peaks for δ-MnO2-0 (628.5 cm−1), δ-MnO2-1 (631.1 cm−1) and δ-MnO2-2 (634.8 cm−1) confirm the presence of short-range order in the samples, further demonstrating their poorly crystalline nature [39]. The relatively intense peak for δ-MnO2-5 reveals that this sample was relatively highly crystallized, which was in good agreement with the XRD results. In addition, the peaks at approximately 635 cm−1 display a slight blue shift from δ-MnO2-0 to δMnO2-5 (641.2 cm−1). The MneO force constant (k) can be determined by the following equation [40]:

ν=

1 2πc

k μ

where ν is the wavenumber (cm−1), c is light velocity, and μ is the effective mass of the bond. The calculated force constant was in this order δ-MnO2-5 (299.7 N/m) > δ-MnO2-2 (293.8 N/m) > δ-MnO2-1 (290.4 N/m) > δ-MnO2-0 (288.0 N/m). Because the force constant is related to the average Mn-bond length by the equation k = 17/r3, the excessive addition of K+ thus leads to the strengthening of the MneO bonds in δ-MnO2-5, thus lowering the lattice oxygen activity. To investigate the difference in the microstructure of the as-prepared samples, SEM images were collected and are shown in Fig. 2. From the panoramic morphology, δ-MnO2-0 (Fig. 2a), δ-MnO2-1 (Fig. 2c), δ-MnO2-2 (Fig. 2e), and δ-MnO2-5 (Fig. 2g) are composed of irregularly shaped microparticles, small microspheres (200–1000 nm), large microspheres (1.5–2.0 μm), and wool brush shapes, respectively. The corresponding magnified images of all the samples (Fig. 2b, d, f, and h) clearly display a hierarchical structure, which is assembled from intersected ultrathin nanosheets via strong interparticle forces and the high surface energy of small nuclei [41]. Increasing the amount of KCl from 0.1 to 0.5 mol during the synthesis process led to an increase in the layer size, which further induced the morphological differences. In the initial stage of δ-MnO2 growth, primary nanopetals preferentially undergo edge-to-edge oriented attachment and grow sufficiently large along the a–b plane [42]. Since the hexagonal birnessite layers are rich in negative charges owing to the presence of vacancy sites and Mn(III) in MnO6 octahedra, hydrated K+ or H+ in solution adsorb onto the vacancy sites and Mn(III) to balance the negative charges [43]. Once K+ is adsorbed, a nanopetal will attract another one with negatively charged surface vacancy sites via Coulombic interactions and hydrogen bonding. More K+ allows larger nanopetals to interact with another one and induced the formation of largely assembled microspheres (δ-MnO2-1 and δ-MnO2-2). Nevertheless, nanopetals that are too large have difficulty forming microspheres and develop a wool brush shape (δ-MnO2-5). TEM images in Fig. 3a and b further confirm the hierarchical

Fig. 1. (a) XRD patterns and (b) Raman spectra of the various δ-MnO2 samples. Standard patterns of δ-MnO2 (JCPDS 18-0802) and α-MnO2 (JCPDS 44-0141) are presented in (a).

3. Results and discussion 3.1. Crystal structure and morphology The XRD patterns of the as-prepared samples in Fig. 1a feature three obvious diffraction peaks at 12.3, 37.0, and 66.2°, which can be indexed as the (002), (006), and (119) planes, respectively, corresponding to the hexagonal δ-MnO2/birnessite phase (JCPDS 18-0802) [32]. The broad and diffused diffraction peaks indicate the poor crystallinity of δ-MnO2, probably due to the low crystallization temperature and nucleation rate that precludes long-range ordering of the MnO6 octahedra [33]. When no additional K+ ions were added for δ-MnO2-0, the sample shows more intense peaks at 42.1, 56.0 and 66.2°, and a less intense peak at 12.3° relative to the peaks of δ-MnO2-1 and δ-MnO2-2. This result indicates the possible presence of another crystal phase in δ-MnO2-0, i.e., α-MnO2 (JCPDS 44-0141), which has also been detected using a similar preparation method at low temperature [34]. The weak (001) and (006) diffraction peaks at 12.3 and 37.0° in this sample possibly result from the small-sized and randomly stacked MnO2 sheets, and few stacked interlayers along the c-axis [35]. When the addition of KCl increased from 0.01 to 0.05 mol, the diffraction peaks of (001) and (006), which indexed to δ-MnO2, gradually strengthened. This can be 3

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Fig. 2. SEM images of (a) δ-MnO2-0, (c) δ-MnO2-1, (e) δ-MnO2-2, and (g) δ-MnO2-5. The corresponding high magnification images of (b) δ-MnO2-0, (d) δ-MnO2-1, (f) δ-MnO2-2, and (h) δ-MnO2-5.

structure of δ-MnO2-0, which consists of small, dense nanoplates with a particle size of ca. 20 nm. HRTEM image in Fig. 3c indicates that several interplanar spacings are observed for these nanoplates, i.e., 0.71 and 0.28 nm, which are assigned to the (001) and (001) planes of δ-MnO2 and of α-MnO2, respectively [44,45]. This result further demonstrates the formation of a mix-phase in δ-MnO2-0, which is consistent with the XRD and SEM results. Fig. 3d, g, and j clearly indicate that the size of the nanosheets, in turn, become larger, possibly due to the different sized template derived from K+ and water molecules. The corresponding enlarged images (i.e., Fig. 3e, h, and k) also demonstrate the change in the particle size of the nanosheets/layer. HRTEM images for δ-MnO2-1 (Fig. 3f), δ-MnO2-2 (Fig. 3i), and δ-MnO2-5 (Fig. 3l) show an interlayer spacing of 0.71 nm, which matches the interlayer spacing of

the (001) planes of birnessite-type MnO2 and indicates the main crystal phase of δ-MnO2. The layered structure becomes more obvious as the K+ concentration increases, which is favorable for the growth of layered MnO6 octahedra. This result agrees well with the enhanced peak intensity of (001) in the XRD patterns.

3.2. Catalytic oxidation of HCHO The as-obtained MnO2 was used for the catalytic oxidation of formaldehyde at room temperature (30 °C), and the results are shown in Fig. 4a. All catalysts exhibited superior activity for formaldehyde removal with efficiencies more than 95% at the initial stage (first 130 min). As the reaction proceeded, δ-MnO2-1 and δ-MnO2-2 4

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Fig. 3. TEM images of (a) δ-MnO2-0, (d) δ-MnO2-1, (g) δ-MnO2-2, and (j) δ-MnO2-5. Enlarged view of (b) δ-MnO2-0, (e) δ-MnO2-1, (h) δ-MnO2-2, and (k) δ-MnO2-5. HRTEM images of (c) δ-MnO2-0, (f) δ-MnO2-1, (i) δ-MnO2-2, and (l) δ-MnO2-5.

stage and then gradually declined to 9.2, 7.4, 8.0, and 7.6 ppm as the reaction proceeded for 300 min. The poorly crystallized δ-MnO2 (i.e., δMnO2-1 and δ-MnO2-2) is energetically favorable for HCHO and oxygen activation and improve the catalytic activity due to the weak MneO bond while δ-MnO2-5 exhibited the lowest activity due to the lowest lattice oxygen activity. Although δ-MnO2-0 has a mixed phase, it shows the best stability in CO2 formation. It is suggested that the location of

maintained a high catalytic activity of ca. 96% for formaldehyde removal efficiency, while δ-MnO2-0 and δ-MnO2-5 started to decline at ca. 220 min and 130 min to reach 86% and 59%, respectively. Fig. 4b shows the corresponding CO2 generated from formaldehyde oxidation throughout the reaction over different δ-MnO2 catalysts. The generated CO2 over δ-MnO2-0, δ-MnO2-1, δ-MnO2-2, and δ-MnO2-5 was approximately 10.7, 9.8, 10.2, and 10.0 ppm, respectively, in the initial 5

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Fig. 5. N2 adsorption-desorption isotherms measured for various δ-MnO2 samples, and the inset shows the corresponding pore size distribution curves.

leading to the smallest specific surface area. Although δ-MnO2-1 and δMnO2-2 both consist of microspheres, the nanosheets in δ-MnO2-1 are much smaller, resulting in the higher specific surface area. Since flowerlike δ-MnO2-2 is mainly composed of smaller nanosheets than δ-MnO25 [47], the stacked nanosheets in the “flower” structure will occupy many exposed sites and lead to a lower specific surface area than the brush structure. The specific surface area of the four catalysts followed this order: δ-MnO2-1 > δ-MnO2-5 > δ-MnO2-0 ≈ δ-MnO2-2. It is previously proposed that the layered structure δ-MnO2 may lead to the intensive adsorption ability of HCHO by enhancing the diffusion in the interlayer space [16]. The inconspicuous layered structure in δ-MnO2-0 could contribute to the reduced activity for HCHO oxidation. However, the number of exposed sites may not explain the difference in catalytic behaviors of other samples. Fig. 6 shows the XPS spectra of the samples. The XPS survey spectrum in Fig. 6a reveals that the various δ-MnO2 samples consist of the elements Mn, K, and O. The O 1s core-level spectrum (Fig. 6b) illustrates that the observed values of the binding energies, i.e., 529.8–530.1, 531.4–531.6, and 532.6–533.1 eV can be assigned to the lattice oxygen (Olatt, MneOeMn), surface oxygen species (Osurf, e.g., MneOeH or defect-oxide), and the surface adsorbed H2O (OH2O, HeOeH), which are in good agreement with the reported values for MnO2 [46,48]. The amount of lattice oxygen in δ-MnO2-0 (73.4%) is higher than that in δ-MnO2-1 (61.1%) but lower than that in δ-MnO2-2 (74.2%) and δ-MnO2-5 (77.1%). These results confirmed the crystallinity increased with the K+ concentration except for δ-MnO2-0, possibly due to the presence of the mixed phase. The surface oxygen species, however, were expected to show a contrary trend, i.e., δ-MnO2-1 (30.8%) > δ-MnO2-0 (16.1%) > δ-MnO2-2 (16.0%) > δ-MnO2-5 (11.7%). The average oxidation state (AOS) of the surface Mn in δMnO2 can be calculated using the equation AOS = 8.956 – 1.126ΔE, in which ΔE stands for the splitting width of Mn 3s [49]. The corresponding AOS values were determined to be 3.73, 3.31, 3.40, and 3.63 for δ-MnO2-0, δ-MnO2-1, δ-MnO2-2, and δ-MnO2-5, respectively (Fig. 6c). A lower AOS indicates a greater fraction of Mn3+ in δ-MnO2. This result was further confirmed by the Mn 2p3/2 spectra, which were fitted into two peaks (Fig. 6d), i.e., at approximately ∼642.5 eV and ∼643.5 eV, which can be indexed to Mn3+ and Mn4+ species, respectively. The fractions of Mn3+ and Mn4+ in these samples are listed in Table 1. δ-MnO2-1 and δ-MnO2-2 have a large proportion of Mn3+ and a relatively high fraction of Osurf. It is generally accepted that surface Mn3+ facilitates the formation of surface oxygen vacancies [50], which allows easy activation of oxygen to surface active species

Fig. 4. (a) HCHO removal efficiency as a function of time over various δ-MnO2 samples, and (b) the corresponding formed CO2. Reaction conditions: 22 ppm of HCHO, 50% RH, 0.3 g cat., air, WHSV 200, 000 mL/(gcat h), 30 °C.

K+ strongly affected the activity of HCHO oxidation and the localized K+ is more active than the isolated K+. It is proposed that δ-MnO2-0 may contain more localized K+ thus showed the most stable CO2 formation [30]. For comparison, referenced δ-MnO2 samples with relatively high crystallinity were also evaluated for HCHO oxidation and only form 4.0 and 6.4 ppm CO2 (Fig. S1). This result indicates that poorly crystallized δ-MnO2 are favorable for HCHO oxidation and CO2 formation. 3.3. Mechanistic insights into the catalytic performance 3.3.1. Catalyst characterization In order to gain more insight into the catalytic performance of the different δ-MnO2 samples, systematic characterizations were conducted. Fig. 5 shows the N2 adsorption-desorption isotherms and the corresponding pore size distribution of the samples. The N2 adsorption/ desorption isotherms reveal that the BET surface areas of δ-MnO2-0, δMnO2-1, δ-MnO2-2, and δ-MnO2-5 is 70.7, 205.2, 71.8, and 96.1 m2/g, respectively. The well-defined hysteresis loop is typical of a classical type IV isotherm with a typical H3-type hysteresis loop, indicating the presence of slit-shaped mesopores [46] in these samples. The difference in the adsorbed volume, as well as the hysteresis loop, is mainly related to the microstructure of the samples. δ-MnO2-0 depicted a relatively high crystallinity and was composed of small crystallized nanoplates, 6

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Fig. 6. XPS spectra of various δ-MnO2: (a) XPS survey spectrum, (b) O 1s, (c) Mn 3s, and (d) Mn 2p XPS spectra.

MnO2: adsorbed surface oxygen species (< 500 °C), surface or subsurface lattice oxygen (500 to 600 °C), and bulk lattice oxygen (> 600 °C) [52,53]. The adsorbed surface oxygen species of the δ-MnO2 samples follows this order: δ-MnO2-1 (39.2%) > δ-MnO2-0 (36.5%) > δMnO2-2 (35.3%) > δ-MnO2-4 (28.1%), which is in good agreement with the O1 s spectra. Surface oxygen species favorably adsorbed on the oxygen vacancies of metal oxides. This implies that both δ-MnO2-1 and δ-MnO2-2, which have abundant of oxygen vacancies, superiorly adsorb and activate oxygen, enhancing the catalytic performance [13]. The intensity of the peaks from 700 to 900 °C gradually increases, mainly from the increased crystallinity of the layer structure. These results were also confirmed by SEM, TEM, and H2-TPR; i.e., the high redox property and more surface active oxygen species contributed to the high removal efficiency of HCHO.

Table 1 The surface atomic ratio estimated by XPS analysis. Sample

Mn3+ (%)

Mn4+ (%)

Olatt (%)

Osurf (%)

OH2O (%)

AOS

δ-MnO2-0 δ-MnO2-1 δ-MnO2-2 δ-MnO2-5

27.0 68.4 59.3 38.9

73.0 31.6 40.7 61.1

73.7 60.9 74.3 77.1

16.1 30.8 16.0 11.7

10.2 8.3 9.7 11.2

3.71 3.31 3.40 3.63

(e.g., O2− and O−) and enhances the oxidation ability [22]. This can explain the high activity of the catalysts for HCHO oxidation. The oxygen mobility of transition metal oxides greatly impacts their catalytic activity toward VOCs oxidation. H2-TPR experiments were performed to investigate the reducibility of δ-MnO2 and the results are shown in Fig. 7a. The H2-TPR profiles of all the δ-MnO2 samples show peaks at approximately 250 °C, which could be assigned to the reduction of easily reducible surface adsorbed oxygen species (e.g., O2− and O−) [51]. Peaks from 270 to 500 °C could be assigned to the stepwise reduction of MnO2 to Mn2O3/Mn3O4, and finally to MnO [34–36]. In this reduction stage, δ-MnO2-1 and δ-MnO2-2 display the lower reduction temperatures, i.e., 300 and 284 °C, respectively, indicating higher redox properties. An additional peak appears between 600 and 700 °C. This peak could be the reduction of the large-sized layer of δMnO2. The results of the O2-TPD are displayed in Fig. 7b. The peaks in the O2-TPD spectra can be assigned to the three types of oxygen in

3.3.2. Role of potassium in the catalytic oxidation of HCHO Previous research has demonstrated that potassium in birnessite is an important factor for formaldehyde oxidation [13,54]. In this work, the role of potassium in different MnO2 samples was also investigated. Fig. 8a shows formaldehyde oxidation over δ-MnO2-R with removable potassium ions. Clearly, all catalysts were deactivated rapidly as the reaction proceeded, and the deactivation rate followed this order: δMnO2-2R > δ-MnO2-1R ≈ δ-MnO2-5R > δ-MnO2-0R. Unexpectedly, all catalysts hardly converted formaldehyde to CO2 (Fig. 8b). The K 2p XPS spectra in Fig. 8c show that the intensity of the peak increased with 7

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wavenumbers, only the broad negative peaks centered at 3586 cm-1 appears, which are attributed to the stretching vibration ν(OH) of surface hydroxyl species [58,59]. In considering the presence of interlayer water in δ-MnO2 according to the TG analysis (weight loss in the range from 180 to 300 °C [60], Fig. S3) and the absence of peaks (at approximately 3440 cm−1 [61]) for interlayer water in Fig. S2, the interlayer water possibly not involve in the oxidation of HCHO. The surface hydroxyl species herein mainly have been presented on the surface of the samples. Consumption of surface hydroxyl species led to the negative peaks at 3586 cm−1, implying a –OH-assisted formaldehyde adsorption process [62]. The peak at 1649 cm−1 is contributed from the bending-stretching vibration δ(OH) of surface adsorbed water [18], which formed from the complete oxidation of formaldehyde. Compared with other MnO2 samples, δ-MnO2-0 formed the lowest amount of formate species, which may be ascribed to the different active sites in the mixed MnO2 sample. Despite the rapid formation of formate species by δ-MnO2-1 and δ-MnO2-5, the generated water was accumulated during the reaction. δ-MnO2-1 has the largest specific surface area and thus adsorbs relatively more water on the surface. δ-MnO2-5 contains the most potassium among the δ-MnO2 samples, which possibly prohibits the desorption of formed water and thus leads to the catalytic deactivation [63]. In view of this, δ-MnO2-2, which contained a medium potassium concentration, was the optimal catalyst for HCHO oxidation. The removal of potassium from δ-MnO2 (δ-MnO2-R) resulted in a great reduction in formaldehyde oxidation and CO2 formation. In-situ DRIFTS spectra over various δ-MnO2-R samples are presented in Fig. S4. Compared with δ-MnO2, the IR peak at 3586 cm−1 was not observed whereas the peaks at 1006, 1045, and 1139 cm−1 appear. These peaks are attributed to the surface adsorbed formaldehyde [64,65], indicating the slow conversion of formaldehyde over δ-MnO2-R samples. But even then δ-MnO2-R can convert formaldehyde to formate (1352 and 1583 cm−1) and carbonate (1328 and 1431 cm−1) species [22,30,56–58]. The intermediates may occupy the active sites and result in the catalyst deactivation. The absence of negative peaks at approximately 3586 cm−1 indicated that the absence of surface hydroxyl groups with a lower concentration, leading to the decreased adsorption of HCHO. It is reported that K+, as one kind of cations, maintain the layered structure of δ-MnO2 [36,37]. The presence of interlayer K+ facilitates the dissociation of O2 and H2O via charge transfer from K to O [30]. The formed active oxygen species will enhance the oxidation of HCHO. However, excess of K+ showed detrimental effects on CO2 desorption and O2 desorption [63]. Moreover, K+ enhance the lattice oxygen activity by lowering the activation energy to release oxygen to form oxygen vacancies on MnO2 [31], which can further activate oxygen to form electrophilic oxygen species, such as O2− and O−. These species further react with water to form surface hydroxyl groups (O2−, O− + H2O → 2-OH), which is critical converting the intermediates to generate CO2 [21,55].

Fig. 7. (a) H2-TPR profile and (b) O2-TPD spectra of various δ-MnO2 samples.

the potassium content in this order: δ-MnO2-5 > δ-MnO2-1 > δMnO2-2 > δ-MnO2-0, which was further confirmed by ICP-OES analysis (Fig. 8d). The δ-MnO2-0 sample has the least amount of potassium (0.67%) in comparison with the other catalysts, i.e., 0.99%, 1.08%, and 1.0% for δ-MnO2-1, δ-MnO2-2, and δ-MnO2-5, respectively. In view of δ-MnO2-2R having the highest activity, the remained potassium or the microstructure of δ-MnO2 is possibly very important in formaldehyde oxidation. To identify the role of potassium in the catalytic performance, in-situ DRIFTS measurements were conducted on δ-MnO2 and δ-MnO2-R samples. As shown in Fig. S2, several peaks at approximately 1316, 1340, 1343, 1374, 1397, 1436, 1453, 1558, 1649 and 3586 cm−1 were detected over various δ-MnO2 samples. No obvious peaks for adsorbed formaldehyde were observed during the reaction while weak peaks at 1397 and 1453 cm−1 attributed to the ω(CH2) and δ(CH2) vibrations of dioxymethylene (DOM) intensified [20,22,55], indicating the fast transformation of adsorbed formaldehyde to DOM. The peaks at 1316 and 1436 cm−1 can be attributed to the adsorbed surface carbonate species [22,56,57]. The peaks at 1340, 1374, and 1558 cm−1 are attributed to the symmetric stretching and asymmetric stretching of COO (νs(COO) and νas(COO)) from formate species [18,22,30,56,58]. As the reaction proceeded, both the formate and carbonate species accumulated and formate species were the dominant intermediates. At high

3.3.3. Role of water in the catalytic oxidation of HCHO Formaldehyde oxidation over δ-MnO2-2 under different relative humidities was tested. As shown in Fig. 9a and b, a lower relative humidity, i.e., RH = 10% and 25%, was beneficial for the conversion of HCHO to intermediates rather than generated CO2. As the RH increased to 50% and 75%, the removal efficiencies remained almost 100%, and only a slight decrease was observed for 75% after 250 min reaction. The CO2 formation under both conditions significantly increased to ca. 8.0 ppm, which may be due to the more water in the reaction converting the intermediates to CO2. Unexpectedly, further increasing the RH to 90% and 100% leads to rapid catalyst deactivation at 150 and 130 min, respectively, and simultaneously lowers the formation of CO2 to 5.3 and 4.7 ppm, respectively. This result implies that too much water shows adverse effects on the reaction, mainly due to the competitive adsorption with HCHO on the catalyst surface, occupation of the active sites, and deactivation of the catalyst. To confirm the positive 8

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Fig. 8. (a) HCHO removal efficiency as a function of time over various δ-MnO2 samples with potassium removal and (b) the corresponding formed CO2, (c) K 2p XPS spectra of various δ-MnO2, and (d) HCHO removal efficiency over δ-MnO2 and δ-MnO2-R at 180 min with different potassium contents which were measured by ICP-OES. Reaction conditions: 22 ppm of HCHO, 50% RH, air, WHSV 200, 000 mL/(gcat h), 30 °C.

role of water, HCHO oxidation over δ-MnO2-2 under dry air and varying RH levels was conducted. Fig. 9c shows that under dry air, the catalyst maintained nearly 100% removal efficiency of HCHO for ca. 168 min while gradually deactivating to 47.8% removal efficiency of HCHO after 500 min reaction. Notably, only less than 1 ppm CO2 was generated during the whole reaction process. In contrast, under a varying RH, i.e., in dry air and air with 50% RH in Fig. 9d, the removal efficiency of HCHO was maintained at nearly 100% throughout the reaction process. CO2, however, was only generated under an RH of 50% (ca. 9 ppm) and hardly formed when RH was switched to 0. The

presence of water was suggested to react with the surface oxygen species to form surface hydroxyl groups, which assist the adsorption of formaldehyde and also can react with the intermediates, such as formate, to produce CO2 and H2O. In addition, it has been reported that water also interacts with the surface carbonate species to form CO2 [17]. 3.3.4. Reaction mechanism of HCHO oxidation over δ-MnO2 Formaldehyde oxidation over manganese oxides generally involves the oxidation reactions between initial reactant (HCHO) and various Fig. 9. (a) HCHO removal efficiency and (b) CO2 formation as a function of time over δMnO2-2 in different relative humidities (RH = 10%, 25%, 50%, 90%, and 100%), and HCHO removal efficiency and CO2 formation over δ-MnO2-2 in. (c) dry air (RH = 0 and (d) varying relative humidity. Reaction conditions: 22 ppm of HCHO, 0.3 g cat., air, WHSV 200, 000 mL/(gcat h), 30 °C.

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carbonate species. HCHO is first adsorbed on the surface of δ-MnO2 via the assistance of a surface hydroxyl group. Then the adsorbed HCHO interacts with surface O2− and O− to generate DOM, which transforms to formate species with the surface active oxygen species (e.g., O2− and O−). The formed formate species are then converted to CO2 and H2O with the surface hydroxyl group, which is generated by water and surface oxygen species (O2−, O− + H2O → 2-OH) [21,63]. Because the formates were the main intermediates, the conversion of formates to CO2 and H2O is likely the rate-determining step in the whole process. With the continuous feeding of oxygen, the consumed O2− and O− will then be replenished by the activation of oxygen on the surface vacancies, such as oxygen vacancies, for further oxidation of formate species. 4. Conclusions

Fig. 10. In-situ DRIFTS spectra over δ-MnO2-2 in a flow of air + HCHO for 120 min, followed by N2 purging for 120 min, air purging for 120 min, and then by air + H2O purging for 120 min at room temperature.

In summary, we reported a facile redox method to prepare δ-MnO2 by varying the concentration of potassium. A medium concentration of potassium was favorable for the formation of δ-MnO2 with poor crystallinity, weakened MneO bonds, high redox properties, and more surface oxygen species. This type of δ-MnO2 was highly effective for HCHO oxidation at room temperature, i.e., completely converting 22 ppm HCHO at 30 °C under a WHSV of 200,000 mL/(gcat h). The presence of potassium in δ-MnO2 was critical to generate surface oxygen vacancies and surface oxygen species (O2−, O−, and –OH) and thus efficiently convert HCHO to intermediates. Surface hydroxyl groups were dominant in transforming the formed intermediates into CO2 and H2O. Moreover, LH mechanisms are mainly involved in the reaction between the prepared δ-MnO2 and HCHO to maintain the removal efficiency of HCHO. This work provides a facile method to synthesize poorly crystallized δ-MnO2 for effective HCHO oxidation, and identify the active sites for the evolution of the reactant and the intermediates, which will allow the rational design of δ-MnO2 for HCHO oxidation with long-term activity and high stability

active surface oxygen species, including surface oxygen species (e.g., O2− and O−), surface hydroxyl groups, surface lattice oxygen, and even bulk lattice oxygen. In this study, we carried out in-situ DRIFTS experiments by stepwise introducing air + HCHO, N2, air, and air + H2O for HCHO oxidation over δ-MnO2-2 at room temperature. Fig. 10 presents the steady state of the spectra in each stage. After exposure to a flow of air + HCHO for 120 min, formate (1347, 1387, 1560, and 2835 cm−1) [18,22,30,56,58] and DOM (1387 and 1445 cm−1) [20,22,55] continuously accumulated as the reaction proceeded while the surface adsorbed water (1668 cm−1) [18] and surface hydroxyl groups (3600 cm−1) [58,60] were consumed. When the gas was switched to pure nitrogen, almost all peak intensity is basically unchanged after 120 min feeding. Then changing the gas to air maintains the intensity of the peaks even after 120 min. This result indicates that oxygen without water cannot convert the formed intermediates. When the sample was subsequently switched to air + H2O for 120 min, peaks at 1298, 1347, 1387, 1445, 1560, and 2835 cm−1 all weakened, indicating the conversion of intermediates by water addition. The increased intensity of the peaks at 1668 and 3600 cm−1 indicates the surface adsorbed water and formed surface hydroxyl groups, respectively. The peak at 3640 cm−1 was suggested to be another type of surface OeH. It is noted that even water was introduced, the intermediates were also accumulated on the surface of the sample. These species strongly affected the stability of the catalyst. We have performed HCHO oxidation over δ-MnO2-2 for 1500 min, the catalyst gradually deactivated especially after 400 min (Fig. S5). Further research to enhance the stability of δ-MnO2-based catalysts is also expected. According to the aforementioned results, we proposed a LangmuirHinshelwood reaction mechanism of HCHO oxidation over δ-MnO2 (Scheme 1). Since formate species are the dominant intermediates, the reaction mechanism only involved formate species and excluded

Declaration of Competing Interest 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. Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (NSFC) (No. 21506054 and No. 21677179), Natural Science Foundation of China (NSFC) and the Research Grants Council (RGC) of Hong Kong Joint Research Scheme (No. 51561165015 or No. N_HKU718/15), Science and Technology Planning Project of Guangdong Province (No. 2015B020236004 and No. 2017B050504001).

Scheme 1. Schematic diagram of the proposed pathway for catalytic oxidation of formaldehyde over δ-MnO2. 10

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Appendix A. Supplementary data

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