Facile synthesis of homogeneous hollow microsphere Cu–Mn based catalysts for catalytic oxidation of toluene

Chemosphere 247 (2020) 125812

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Facile synthesis of homogeneous hollow microsphere CueMn based catalysts for catalytic oxidation of toluene Zhe Xiao a, Jingsi Yang a, Rui Ren b, **, Jing Li b, Ning Wang c, Wei Chu a, b, * a

Institute of New Energy and Low Carbon Technology, Sichuan University, Chengdu, 610207, China School of Chemical Engineering, Sichuan University, Chengdu, 610065, China c Physical Sciences and Engineering Division King Abdullah University of Science and Technology Thuwal, 23955-6900, Saudi Arabia b

h i g h l i g h t s
Homogeneous hollow microsphere CueMn catalysts were successfully synthesized.
HR-2Mn1Cu possessed abundant mesopores and long-range crystalline disorder.
HR-2Mn1Cu showed superb stability and catalytic performance for toluene oxidation.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 September 2019 Received in revised form 8 December 2019 Accepted 31 December 2019 Available online 13 January 2020

There emerges an urgent stipulation towards the enhanced toluene catalytic combustion nanocatalysts for whittling down the footprint of toluene, a notorious air pollutant. Unfortunately, Few materials which are currently made accessible both present the high catalytic performance lower than 250  C and keep durable at elevated temperatures. Herein, we demonstrate an expeditious salt hydrolysis-driven redoxprecipitation protocol wherein Hþ donated by the hydrolysis of copper salt was used to initiate the regioselective reduction of KMnO4 by H2O2 under controlled redox kinetics in order to assemble the homogeneous mixed solid solution hollow microsphere CueMn-based structure. Manifold characterization technologies unveil that in this unique nanbomicrosphere the abundant microscaled pores are successfully created across CueMn bulks with fine-modulating the chemical properties. In sharp contrast with the compact counterparts without tailed porosity, the tuned crystallinity, accessed edge sites with the unsaturated coordination, fast redox chemistry, and boosted gaseous diffusion during reactions synergize to result in the signally good toluene oxidation, with the complete elimination activity at 252  C, T90 at 237  C, and prominent long-term durability under the stringent reaction atmospheres. Our current study ushers in an alternative and tractable arena to excogitate the porous oxide materials for multifarious catalysis implementations. © 2020 Published by Elsevier Ltd.

Handling Editor: Rongshu Zhu Keywords: Cu-Mn catalyst Catalytic combustion Redox precipitation Toluene removal VOCs abatement

1. Introduction Volatile organic compounds (VOCs) are a large category of low boiling point organic chemicals and are extensively utilized in industrial applications (Huang et al., 2014b). Nevertheless, the net anthropogenic emission from an increasing body of sources, such as industrial processes, transports, house-hold activities and so on,

* Corresponding author. Institute of New Energy and Low Carbon Technology, Sichuan University, Chengdu, 610207, China., ** Corresponding author. E-mail addresses: [email protected] (R. Ren), [email protected] (W. Chu). https://doi.org/10.1016/j.chemosphere.2020.125812 0045-6535/© 2020 Published by Elsevier Ltd.

poses a severe threat, particularly considering that VOCs constitute the primary air pollutants and thus conduce to the growing ecopenalties such as the ground-level ozone, photochemical smog, PM2.5, toxic air emissions, and undesirable human health effects of respiratory tract and lung damage (Wang et al., 2008; Huang et al., 2014a). Currently accessible, the emission-control materials/technology always spin their wheels towards diminishing levels of VOCs in exhaust currents (Leson and Winer, 1991; Zou et al., 2019). Elevated temperature burning likewise engenders the release of the highly hazardous NOx and CO. Thus, it is imperative to revolutionize the existing techniques for efficiently managing the VOC release at the relevantly benign concentrations. As a promising strategy, combustion of toluene fostered by the heterogeneous

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catalysts is able to fully capitalize upon the accessible energy of toluene under smaller temperatures, augmenting the system behavior, and regulating the fingerprint via markedly abating the needed temperatures (Li et al., 2009; Liotta, 2010; Kamal et al., 2016). Considering the excellent chemical stability of toluene, heterogeneous nanomaterials for toluene combustion have to perform highly catalytically effective under small oxidation temperatures (no more than 300  C), while these nanocatalysts for such adhibition must as well possess exceptional reactive and mechanical endurance when operating under demanding working circumstances like the elevated reaction/flame temperatures, pressures, concentrations, and the complex mixture of the gaseous contaminants and plentiful moisture (Huang et al., 2015). Among a wide spectra of competitive candidates that run the whole gamut of repertoires including earth reserves, economic expense, energy dissipation, and catalytic functions, the transition metal manganese oxides are touted as one of the best catalysts for low-temperature catalytic elimination of toluene, despite the fact that their reaction mechanism and catalytically reactive phases still remain largely elusive (Liang et al., 2008; Wang et al., 2012; Li et al., 2014; Huang et al., 2018). Disappointingly, the single manganese oxides are still skewed toward exacerbating due to forfeiture of reactivity site accessibility by crystal growth, to the chlorine (Cl) poisoning during the catalytic oxidation of chlorinated VOCs, and to transmutation amid various crystal phases under temperatures bigger than 500  C (Wu et al., 2010a; Dai et al., 2012). Recently, both theoretical and experimental investigations decipher that introduction of second metal components (such as Ce, Fe, Co, Cu, La and so on) as a modifier into the base manganese system to form binary metal oxide hybrids can promote catalytic efficiency of individual manganese oxide by stabilizing the structural properties and heightening the electrophilic oxygen species, but the pure alien metals impart the restricted thermal tolerability and catalytic performance (Todorova et al., 2010; Fei et al., 2012; Yang et al., 2015; Wang et al., 2018b). In particular, the distinctive Jahn-Teller effect of copper can strikingly decorate both the tunnel structures and valence state of manganese oxides to reinforce the metal-MnOx interaction along with the higher average oxidation number (AON) of both Mn and metal Cu ion, thereby stepping up availability of reactive surface oxygen, intensifying the lattice oxygen mobility/ spillover, frustrating the drastic topological morphological fluctuation, and improving the catalytic property (Halcrow, 2013; Tang et al., 2015; Li et al., 2017b). Other systems built on metal oxides have also been tremendously explored, but their catalytic efficiencies are commonly tough to gratify rigorous industrial requirement in terms of stability and activity, with a thorough toluene conversion achievable merely above 300  C (Huang et al., 2012; Ma et al., 2013; Yang et al., 2019). Hence, catalysts which are competent to concurrently consolidate behaviors of Mn-based materials and constrain deterioration at high temperatures will remarkably accelerate the realistic application of the toluene oxidation technology. Porous architectures epitomize the peculiar paradigms wherein these created pores can ramp up the edge sites, improve the mass diffusion, and provoke the different advanced physical properties, which finds extensive usage within the various domains involving filtration membranes, electronics, energy storage devices and catalysis (Parlett et al., 2013). For catalysis, pores can serve as tunnels for target species to achieve exceptional transport across material planes and ultimate access to internal surfaces, while the synergy in the composite system can further strengthen the interactions between the components (Xia et al., 2010; Zhang et al., 2018a). Apart from the scenario of boosted catalytic attributes, the auto-organization mesostructuring way can provide an

impactful instrument toward, to greatest extent, foiling the catalytic behavior decay from both metal crystal thermo-growth and phase change through well-known nano-confinement mechanisms. This effect especially looms large for elevated-temperature catalysis, as is the situation of toluene oxidation (Argyrakis et al., 2001). Herein, we show one-pot preparation of mesostructure-typed composite catalysts motivated by the central tenet of the retrofitted co-precipitation chemistry in which the metal salt precursors can be homogenously mixed or dispersed on the atomic scales to maximally probe catalytic functions. The industrially-suitable coprecipitation method have witnessed intensive use in the heterogeneous catalysts by virtues of the ease in mediating the synergies between components at the nanoscale and of reliable repro~ o et al., 2015). However, the ductivity (Tian et al., 2012; Castan routine co-precipitation tool normally undergoes the severe futility in the mesostructuring in the absence of potent pore producers of block copolymer/surfactant and in the phase/element homogenization due to sequential precipitation of metal precursors from their vast disparity of solubility product constants under the condition of classic co-precipitation reaction, to the great detriment of catalytic interaction (Arena et al., 2007). Instead of having recourse to the costly and cumbersome supramolecular chemistry, both the intricate mesostructuring and the highly uniform element dispersing can be easily achieved by a hydrolysis-initiated redox precipitation without the assistance of any structure-directing agent, pore-forming agent, and strong acid/alkali precipitants (Chen et al., 2017, 2018). Two entities, Mn and Cu, were precipitated synchronously, afterwards self-assembled and organized in highly diluted H2O2 solution into the close-packed hollow microspheres inter-attached together via strong metal-metal interplays. The same element diffusion/precipitation rates guarantees that the final products exist as homogeneous “solid solution” like (Wang et al., 2018a). During self-assembly, abundant small-sized mesopores were created across the whole grain. Transmission electron microscopy evidences that, even upon exposure to harsh thermal annealing at a temperature as high as 300  C for 4 h, this graded porous morphology can be topologically transformed into a solid architecture missing the substantial “long-range” crystalline order due to the highly dispersed manganese and copper ions. The unique architecture of the gradational composite catalyst results in the extraordinarily elevated and prolonged behavior for the catalytic combustion of toluene at low temperatures. This distinctive conformation seems to maintain the tenable textural, structural, and chemical properties: stabilize the active phase of the catalysts, provide an extremely large surface area, expose a great deal of the largely active surface atoms with low coordination number on edges, and augment the reversibility of surface/bulk redox cycles. In addition, the mesopores enable close interfacial contacts/affinities between reactants and catalysts, in favor of the rapid diffusion of reactants and products. 2. Experimental section The materials preparation and characterization as well as catalytic performance test are depicted in detail in the Supporting Information.

2. Results and discussion 2.1. Fabrication of CuMnOx with molecular-scale homogeneity At present, increasing attention has been paid to the preparation of catalysts with maximum dispersion of the active phases within the multicomponent systems, which is markedly conducive to elevating the catalytic performance. The co-precipitation method, a

Z. Xiao et al. / Chemosphere 247 (2020) 125812

common way to prepare mixed metal oxide catalysts, is difficult to modulate the generation of the uniform mutual-dispersion of element in the hybrid metal oxides, resulting in poor catalytic efficiency (Chen et al., 2018). As delineated in Scheme 1, the precipitation of mixed Cu and Mn hydroxides comes true under the condition of aqueous ammonia solution. The precipitation kinetics of each cation substantially differs because of the huge discrepancy in the solubility product constant (Kqsp ([Cu2þ][OH]2 ¼ 2.2  1020 and Kqsp ([Mn2þ][OH]2 ¼ 1.9  1013), thereby making the Cu2þ ions be more easily precipitated than Mn2þ. This prominent difference first induces the preferential self-conglomeration of Cu(OH)2 previous to its exceptionally homogenous distribution within the in situ precipitated MnO2 matrix, and finally forms an undesired structure of a copper-enrich oxide core surrounded by the Mn oxide after calcination. As confirmed latter, this mixture of “monophase” precipitate particles will fail both in enhancing the reducibility of the “poorly dispersed” manganese ions with higher average oxidation numbers (AON) and in the reversibility of the surface redox cycle, two fundamental benchmarks for the catalyst activity and stability in the catalytic combustion of toluene. In order to maximize the dispersibility of Cu and Mn ions and raise their atomic utilization efficiency, a suitable synthetic route must be explored to achieve simultaneous reactions of Mn2þ and Cu2þ towards constructing a molecularly dispersed copper-manganese binary oxide with the proper “tuning” of the structural and electronic features of the active phase, as shown in Scheme 1. Fortunately, we find that, by means of accurately devising the redox reactions of the MnO4 with the Cu2þ-Mn2þ ions in the neutral solution, a salt-triggered hydrolysis redox precipitation mechanism can unleash the possibility of synchronizing the precipitation reactions of MnO4, Cu2þ, and Mn2þ ions into MnO2 and CuO to synthesize CueMneO nanohybrids with a “molecular mixing” of the oxide species. Typically, under acidic conditions, KMnO4 is spontaneously reduced to MnO2 by H2O2, as the following Eq. (5). In this synthetic route, a stable acidic environment can be ensured by Hþ produced by hydrolysis of copper salts. Since the proportion of Hþ consumed by redox reaction is equal to that produced by hydrolysis of salt, it will lead to the continuous hydrolysis of copper salts towards the formation of Hþ until complete precipitation, as

3

shown in Eq. (6), which signifies that MnO2 and Cu(OH)2 will be precipitated synchronously in the reaction process to attain a mixture of the oxide components at an atomic level. The new synthetic pathway is shown in Eq. (7), which vividly illustrates the feasibility of initiating the spontaneous redox-precipitation reactions among MnVII, CuII and MnII precursors in the neutral aqueous solution towards efficiently architecting the molecularly dispersed CuMnOx composite nanocatalysts with the obviously enhanced textural and structural properties. It is noteworthy that the redox process driven by salt hydrolysis does not require additional strong acid and alkali to maintain the stable pH, and that the textural modulation does not need to resorting to any costly/ cumbersome supramolecular chemistry, which is different from the previously reported homogeneous system by redox reaction coprecipitation and soft chemistry (Wang et al., 2018a, 2018b). þ 2MnO 4 ðaqÞ þ 3H2 O2 ðlÞ þ 2H ðaqÞ / 2MnO2 ðsÞ þ 3O2 ðgÞ

þ 4H2 O ðlÞ (5) Cu2þ ðaqÞ þ 2H2 O ðlÞ # CuðOHÞ2 ðsÞ þ 2Hþ ðaqÞ

(6)

Cu2þ ðaqÞ þ 2MnO 4 ðaqÞ þ 3H2 O2 ðlÞ / 2MnO2 ðsÞ þ CuðOHÞ2 ðsÞ þ 3O2 ðgÞ (7)

2.2. Phase and structural properties of samples ICP-OES and XPS were used to analyze the distribution of metal elements in the bulk and surface of each sample, as shown in Table 1. The contents of metal elements in MnO2 and CuO are close to the eigenvalues (Mn/MnO2 ¼ 63.2 wt% and Cu/CuO ¼ 79.9 wt%, separately). For HR samples, the bulk Mn/Cu atomic ratio of 2.27 determined by ICP-OES is close not only to the nominal value of 2.0, and but also to the surface layer atomic ratio (2.18) measured by XPS analysis, evidencing the high atom utilization efficiency during the synthetic and the highly homogenous dispersions. Conversely, the surface Mn/Cu atomic ratio of 4.95 is much larger than the bulk and nominal composition in the CP samples, which confirms that the different precipitation rates of Mn2þ and Cu2þ indeed leaded to the preferential accumulation of Cu in the core and the enrichment of Mn on the superficial layer. This unique element distribution trait can be further substantiated by the results of high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) and EDS. As shown in Fig. 1(b), the basic overlap of different elements mapping indicates that Mn and Cu are uniformly distributed in the materials prepared by HR method, which is consistent with the results of XPS and ICP-OES, and confirms the

Table 1 Physical properties of all samples. catalysts

HR-2Mn1Cu CP-2Mn1Cu MnO2 CuO a b

Scheme 1. Illustration of two pathways to synthesize CueMn oxide.

c

ICP-OES (wt.%)

Mn/Cu molar ratio

Mn

Cu

ICP-OES

XPS

36.6 39.7 60.6 e

18.6 21.9 e 79.8

2.27 2.16 e e

2.18 4.95 e e

Surface area calculated by the BET method. BJH Desorption cumulative volume of pores. Average pore diameter.

SBET a (m2/g)

Pv b (cm3/g)

APD c (nm)

193.3 34.3 104.7 14.4

0.904 0.242 0.470 0.103

17 29.2 18.3 25.3

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Fig. 1. TEM images of (a, c) and (b) mapping result of HR-2Mn1Cu sample, TEM images of (d) CP-2Mn1Cu, (e) MnO2 and (f) CuO catalysts.

great feasibility of synthesizing the homogeneously dispersed binary compounds via a selective ‘‘cogeneration’’ of the MnO2 and CuO phases by redox interactions among proper precursors. Conversely, copper-rich phase deposition can be clearly observed from the image of HAADF-STEM mapping for CP sample (Fig. S1), implying the formation of ‘‘monophasic’’ solid particles because of the different precipitation kinetics of each Mn and Cu cation. The discrepancy in phase composition and element distribution must inextricably correlates with the phase structural characteristics. Thus, the X-ray diffraction is used to characterize the phase structure of all samples, presented in Fig. 2(a). In general, irrespective of the synthetic preparation and element composition, a poorly crystallized material was achieved exhibiting strong and anisotropic line broadening for all specimens. To be precise, pure MnO2 showed a few weak signals at 2q values of 28.6 , 37.3 , 42.8 , 56.6 and 72.1, corresponding to the crystal planes of (110), (101), (111), (211) and (301) of the standard mode a-MnO2 (pyrolusite, PDF # 72e1984), respectively. It has been affirmed that the abatement of intensity of some diffraction peak in MnO2 is caused by poor crystallinity (Gu et al., 2015). For the single Cu oxide, all peaks deal with the monoclinic CuO phase (PDF # 89e2530) and no other signals exist. For the binary oxide materials, the XRD diffractograms evidently revealed that CP-2Mn1Cu have all peaks of each singlephase oxide, with the manganese species being mainly present as Mn3O4 (PDF # 80e0382). This simple combination of the typical diffraction lines of individual oxides and their comparatively high intensity unambiguously unravels that CP sample practically consist of the highly crystallized and large-sized manganese and copper oxides with the less lattice defects, further denoting the inadequacy of the traditional coprecipitation route in getting an effective mixing of the oxide components and the incipient crystallization of manganese and copper oxides (Ye et al., 2018). It is worth noting that these peaks are slightly shifted, especially at the peak of 35.9 (2q) corresponding to CuO (111) or MnO2 (100), which

indicates the variation of metal-oxygen bond length due to the formation of CueMneO solid solution. However, in sharp contrast with XRD pattern of the CP sample, HR-2Mn1Cu is distinguished by the mere atypical broad and smooth diffraction bands in the range of 33 e39.2 , without other identifiable diffraction signals in the whole 2q range, signifying that an amorphous architecture of MneCu solid solution lacking a significant “long-range” crystalline order is formed (Wakihara et al., 2006; Wang et al., 2018b). Considering the same calcination conditions, such peculiar XRD patterns mirror a “disordered” arrangement, consequent not primarily on the “soft” thermal annealing but on the nature/extent of the ion-ion interaction during the synthesis. Combined with synthetic mechanisms, the results confirms that controlling the precipitation rates of metal ion via our novel redox-precipitation can serve as the robust tool for tuning the incipient structural rearrangement of a very intimate sticking of MnOx and CuOx molecules, which can offer huge physical barriers to oppress the growth of ‘‘large’’ crystalline domains and thus results in the long-disordered binary metal oxide with both high spatial anisotropy and “attaching” of MnOx and CuO species at a (quasi) molecular level. The aforementioned conclusions are further supported by transmission electron microscope (TEM) in Fig. 1(c). A sporadic presence of small crystalline domains, randomly dispersed into a prevalently “amorphous” matrix, renders Mn or Cu poorly evident at high magnification. It seems extremely hard to seek out the abundant clear lattice fringes in image of HR-2Mn1Cu compared with other three samples with the good crystalline phase Fig. 1(d-f). Besides, the average spacing of scarce lattice fringes in image of HR2Mn1Cu (0.269 nm) is wider than those of MnO2 (101) and CuO (111) lattice planes. These unusual phenomena indicate that Cu ions are successfully incorporated into the crystal lattice of MnO2, thus inhibiting the crystallization of pure MnO2 and forming an amorphous CueMn catalyst. The structure of non-crystalline phase promotes the occurrence of the lattice defects and the exposure of

Z. Xiao et al. / Chemosphere 247 (2020) 125812

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Fig. 2. (a) XRD patterns of as-synthesized samples. (b) N2 adsorption-desorption isotherms. (c) pore-size distribution. (d) H2-TPR profiles of all catalysts.

inner atoms, and enlarges the surface area of catalysts. In short, the results manifest that the doping of transition metal oxides by HR process has a remarkable effect on the phase structure of manganese oxide. The overall microstructural characteristics of as-prepared CueMn catalysts were studied by N2 adsorption-desorption isotherms and pore size distribution (PSD). As shown in Fig. 2(b), four samples exhibited similar type-IV isotherms with an H3 hysteresis loop, indicating the presence of mesopores. In the case of the redox-precipitated sample, the marked mesostructuring was induced through the strong interplay between Cu and Mn precursor, thus displaying a more obvious hysteresis loop in the P/P0 range of 0.5e1.0 and relatively high N2 adsorption volume. Contrarily, the pore configuration of CuO and CP-2Mn1Cu is just associated with the irregular compiling of larged-sized CuO and MnOx nanoparticles, so their isotherms showed both a relatively low N2 adsorption volume and a hysteresis loop at a relatively higher P/P0 value. As shown in Fig. 2(c), it can be clearly seen that redoxprecipitated HR-2Mn1Cu forms a highly porous structure with a random particle size distribution in the range of 3e90 nm and a maximum at 16 nm. The smaller pores are the intrinsic pore of materials, the larger ones from 15 nm to 30 nm are derived from the corrosion of the surface of nanoparticles by hydrogen peroxide, as confirmed by TEM images (Fig. S2), and the other ones at the range between 30 nm and 90 nm may ensue from the accumulation of tiny irregular nanoparticles. This multi-model pore size

distribution clearly elucidates a hierarchical and loose structure, as also seen by SEM images (Fig. S3(a)). For a quantitative assessment, the corresponding textural results are summarized in Table 1. The values of SBET and pore volume of HR-2Mn1Cu were (193.3m2/g; 0.904cm3/g) and MnO2 (140.7m2/g; 0.407cm3/g) assembled by the HR reaction are much higher than those of CuO (14.4m2/g; 0.103cm3/g) and CP-2Mn1Cu (34.3m2/g; 0.242cm3/g) prepared by conventional precipitation method. This clearly evidence that the “redox-precipitated” sample is characterized by a much larger surface exposure than other co-precipitated samples. The higher specific surface area and higher pore volume are mainly attributed to the large amount of oxygen released during the redox reaction, which suppresses the agglomeration of small particles and enlarges the accumulated pore volume. On the contrary, CuO and CP2Mn1Cu prepared by co-precipitation exhibit compact aggregation structure formed by small particles, resulting in the lower SBET. Generally speaking, higher SBET can provide more active sites, and the pore-rich structure can efficiently reduce the adsorption and diffusion resistance of the reactant molecules, which is a positive factor for the catalytic oxidation of toluene (Kim and Shim, 2010; Bai et al., 2012). A critical index on the feasibility of using this material in catalytic oxidation of toluene is to identify the extent of redox capacity and the ability for oxygen maneuverability. Shown in Fig. 2 (d) is the normalized results of temperature programmed reduction of all samples by H2. The CuO sample prepared by conventional

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precipitation shows only an asymmetric peak of H2 consumption at 350  C indicating its inferior oxidizing ability compared with that of other three samples. There emerged two obvious peaks for the pure MnO2: a very intense one at 284  C and a slightly weaker one at 379  C, which were affiliated with the reduction of MnO2 to Mn3O4 and Mn3O4 to MnO, respectively (Stobbe et al., 1999; Tang et al., 2014; Wang et al., 2019). The lower reduction temperatures well developed in multicomponent oxides compared to single metal oxides shows a strong synergy between manganese and copper. In the meantime, introducing copper ions (Cu2þ and Cu1þ) will enhance the content of high valent manganese ions (Mn4þ or Mn3þ) under the action of charge balance, which can also boost the low temperature reduction of catalyst (Tang et al., 2015). Specifically, HR-2Mn1Cu and CP-2Mn1Cu evince the first peak of hydrogen consumption at 232  C and 243  C, respectively, which is much lower than the first peak temperature of other single-phase catalysts, implying that lattice oxygen in them is less bound and more active. This low temperature reduction peak is associated with the reduction of exiguous CuO granules to Cu or the reduction of amorphous MnO2 existing in HR-2Mn1Cu to Mn3O4. The high temperature peak is attributed to the stepwise reduction of MneCu oxides containing the lattice oxygen species of Cu and oxygen of MnOx in the mixed valence (Tang et al., 2006). Generally, a nanomaterial with equal mass consisting of smallsized nanograin has a higher system surface energy resulting in fewer coordination numbers of metal and oxygen, which could expose more surfaces to H2 and lead to better low temperature reducibility (Deng et al., 2016a). Because the crystallites of CuO/ MnOx are smaller and well dispersed on matrix surfaces in line with the XRD and SEM results, the reduction peak temperature of HR2Mn1Cu is lower than that of CP-2Mn1Cu and the spacing is about 12  C. Previous studies have shown that the lower reduction temperature and the greater hydrogen consumption is in virtue of the higher oxygen fluidity and abundant oxygen overflow, which has favorable impacts on the catalytic oxidation (Stobbe et al., 1999; Fan et al., 2018). The order of low temperature reductive ascension is as follows: CuO＜MnO2＜CP-2Mn1Cu＜HR-2Mn1Cu, while hydrogen consumption based on peak area calculation increases as below: CuO＜CP-2Mn1Cu＜MnO2＜HR-2Mn1Cu. All token together, the largest hydrogen consumption and the lowest low temperature reducibility seen on HR-2Mn1Cu clearly suggest the boosted reducibility and surface affinity of the MnCuOx system towards gas-phase oxygen, and these two factors codetermine its the best catalytic ability for the oxidation of VOCs. Raman spectroscopy was applied to expound the bulk and surface texture information because it was sensitive to vibration of both the M  O stretching and lattice defects. Fig. 3 showed the Raman spectra of all the samples. As observed for pure MnO2 sample, only three visible Raman bands centered around 182, 289 and 634 cm1 appeared, which was fairly similar to that of reported for alpha-MnO2 type materials (Polverejan et al., 2004; Gao et al., 2008; Cheng et al., 2014). More concretely, the bands below 500 cm1 were associated with octahedron stacking models, but no spectral signal was detected near 380 cm1, indicating the longrange disordered accumulation of octahedrons, mainly due to the low polarization or the intersection of certain vibration structures (Gao et al., 2008). The appearance of typical band at 634 cm1 related to the Mn4þ-O stretching in octahedron implies the existence of MnO2 (Cheng et al., 2014; Chen et al., 2016). For HR2Mn1Cu, the introduction of Cu causes the most intense peak to slightly negatively shift from 634 cm1e639 cm1, indicating that the lattice defects occur due to the doping of copper ions into the MnO2 lattice by partially supplanting Mn ions (Zhang et al., 2007). Simultaneously, the dominant peak (639 cm1) is broader than that of pure MnO2 (634 cm1), while other peaks decay out of the

Fig. 3. Raman spectra of all catalysts.

detection limit, manifesting the long-range disordered accumulation of octahedrons. The missing of long-range crystalline order commonly results from the size dependence between nanoparticles and the crystallinity of samples, which can be explained by the heterogeneous strain of particle size dispersion. The analytical results are consistent with the XRD and TEM measurements, where HR-2Mn1Cu has the smallest particle size and the worst crystallinity. In general, the non-uniform strain and phonon confinement hold accountable for the broad and asymmetric character of the band when the grain size is reduced (Wu et al., 2010b). In short, the broadening and weakening of the Raman peaks indicate the long-range disordered accumulation in the structure, mainly due to the lattice defects and poor crystallization (Cheng et al., 2014; Chen et al., 2016). Furthermore, the Raman peaks of CuO cannot be found in the as-prepared HR-2Mn1Cu meaning that Cu ions were in a highly dispersed state and Cu oxide particles were not aggregated in the sample. This result is also accordant with the XRD discussion mentioned above. Hence, the formation of homogeneous MneCu intermixed oxide is further verified. Noteworthy, the CP-2Mn1Cu has a distinct peak at 261 cm1, which is very close to the obvious peak of pure CuO. This is due to the presence of phase-separated CuO oxides in CP2Mn1Cu, even if the XRD signal cannot be detected. All in all, Raman data evidence that the “redox-precipitated” sample has a much more homogeneous chemical composition and metal grain size distribution as well as a more disordered long-range order than the conventionally coprecipitated one. 2.3. Catalytic performance The catalytic performance of toluene combustion was tested at different temperatures from 150 to 280  C with a total flow rate of 50 mL/min, and the corresponding WHSV ¼ 30,000 mL/(g h), pure air as the feed gas. The catalytic activity was evaluated by the reaction temperature T50 (indicating the temperature at the time of conversion of 50% toluene) and T90 (indicating the temperature at the time of conversion of 90% toluene) listed in Table 2. It can be clearly seen that the catalytic activity of our synthesized catalysts for toluene oxidations strongly depends upon the preparation conditions and element composition.

Z. Xiao et al. / Chemosphere 247 (2020) 125812 Table 2 Combined results of surface elemental compositions, and toluene combustion activity. catalysts

Olatt/Oads

Mn3þ/Mn4þ (2p3/2)

HR-2Mn1Cu CP-2Mn1Cu MnO2 CuO

2.78 1.69 2.09 1.30

2.83 0.55 0.94 e

Catalytic activity ( C) T50

T90

228 245 233 257

237 259 244 273

Ea (kJ$mol1)

55.74 74.07 62.65 64.95

The catalytic activity of binary oxides (HR-2Mn1Cu and CP2Mn1Cu) and pure MnO2 containing Mn species are more superior to that of CuO, inferring that Mn is the main active center of toluene oxidation in this reaction. In general, bimetallic oxides have much higher activity for toluene catalytic combustion than singlephase oxides due to the strong interaction between metals (Li et al., 2017a). However, in our case, incredibly, the catalytic performance of MnO2 for toluene oxidation is much higher than that of CP2Mn1Cu, and the T50 and T90 of toluene conversion are 233  C and 244  C respectively, which are about 15  C lower than that of CP-2Mn1Cu. This anomalous behavior unquestionably reflects that no strong synergy between Cu an Mn species in CP-2Mn1Cu is enacted. Meanwhile, HR-2Mn1Cu shows the lowest toluene conversion temperature with T50 and T90 of 228  C and 237  C, respectively (Fig. 4(a)), and the toluene conversion at 240  C is much higher than that of other three catalysts (Fig. S4). It is generally accepted that the intrinsic properties of metal oxides such as structure, morphology and redox properties determine their catalytic activity, and these factors can ordinarily be improved by some means, such as optimized preparation method (Deng et al., 2016b). According to the previous discussion on structural characterization, the optimum activity of the HR sample is due to the uniform introduction of copper ions by the hydrolysis-driven redox co-precipitation to form a long-rang disordered mesostructure. With the formation of surface hollow structure and accumulative pores caused by H2O2 corrosion, the specific surface area of HR sample is increased further, thereby augmenting the accessibility of surface edge sites and inner atoms. Meanwhile, the strong interaction between metals ensures the low temperature redox and higher oxygen storage capacity, which are validated by the investigation of XRD, SEM, TEM, H2-TPR etc. Moreover, the effect of WHSV on the catalytic performance of HR-2Mn1Cu catalyst is shown in Fig. 4(b). As expected, the catalytic

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efficiency decreases with the increase of WHSV values. Specifically, when WHSV is elevated from 30,000 to 120,000 mL/(g h), the corresponding conversion rates (T90) are 237, 255, and 268  C. Even at an extremely high WHSV of 120,000 mL/(g h), HR-2Mn1Cu can completely decompose toluene at 290  C. In other words, the catalyst assembled by the HR method exhibits good tolerability to high WHSV, which is a crucial factor in actual use. To further elucidate the intrinsic source for high-effective catalysis, the reaction kinetics of four catalysts were determined. Fig. S5 shows the Arrhenius diagram of toluene oxidation when the toluene conversion is less than 20% (Liu et al., 2012; Tang et al., 2015). From the slope of Arrhenius curve, we can calculate apparent activation energy (Okumura et al. 2003) of toluene oxidation over these catalysts, as displayed in Table 2, and the sample with lower Ea value corresponds to the preferable intrinsic catalytic activity (Liu et al., 2012). It can be observed that the Ea value of catalysts prepared by redox precipitation method is lower than that of catalyst derived by traditional precipitation method, while the uniform copper-manganese catalyst (HR-2Mn1Cu) with a hollow sphere structure has the lowest Ea value (55.74 kJ/mol) among the four samples (55.74e74.07 kJ/mol) suggesting that HR sample is most effective in toluene catalytic combustion to achieve the optimum catalytic capability. This finding confirms that toluene combustion is carried out more easily over the redox-precipitated sample and has the close correlation with the structure of the catalyst. 2.4. Surface elemental analysis O1s spectrum could be used to analyze the surface oxygen species and deconvoluted into several sections as shown in Fig. 5(a). All samples prepared by different methods have two peaks at 531.4e531.3 eV and 529.8e529.7 eV, corresponding to the adsorbed oxygen (simplified to Oads) such as O2, O and OH groups and the surface lattice oxygen (simplified to Olatt) like O2- 2, respectively (Santos et al., 2010; Hu et al., 2018). And another signal at high binding energies (533.5e533.3 eV) can be attributed to adsorbed molecular H2O, which only appears in HR-2Mn1Cu and MnO2 prepared by redox precipitation (Zhang et al., 2018b). Referring to Mars-van Krevelen mechanism, the lattice oxygen, considered as the foremost active oxygen, plays a vital part in catalytic oxidation of toluene (Sun et al., 2015). When the content of accessible surface lattice oxygen is insufficient, toluene is physically adsorbed in the oxygen vacancies or the catalyst surface. Thus, a higher molar ratio of Olatt/Oads is decisive to their catalytic

Fig. 4. Relationship between toluene conversion efficiency and temperature over the CuMn oxide, CuO, and MnOx catalysts (1000 ppm toluene, WHSV ¼ 30,000 mL/(g h)), (b) Effect of WHSV on catalytic efficiency over the HR-2Mn1Cu catalyst (1000 ppm toluene, WHSV ¼ 30,000e120,000 mL/(g h)). Toluene conversion vs reaction time over HR-2Mn1Cu.

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Fig. 5. XPS profiles of all catalysts (Li et al.) O  1s orbital, and (b) Mn  2p3=2 orbital.

applications, which is thought to be propitious to the MVK mechanism for VOCs oxidation. As is listed in Table 2, HR-2Mn1Cu and MnO2 (Olatt/Oads ¼ 2.78 and 2.09, respectively) evidently teem with surface active oxygen species compared with CP-2Mn1Cu and CuO (Olatt/Oads ¼ 1.69 and 1.30, respectively). Such high proportion of active oxygen specie will exert a salutary effect upon building the exceptional reversibility of the surface redox cycle during the toluene combustion. The different valence of manganese ions, including Mn2þ, Mn3þ, and Mn4þ, play a momentous role in the catalytic oxidation of manganese oxides. Fig. 5(b) manifests the peak-splitting results of Mn2p spectrum, in which the peak around 642.0 eV is attributed to Mn3þ and the peak at 643.1 eV is assigned to Mn4þ (Sun et al., 2015; Morales et al., 2017; Chen et al., 2018; Zhang et al., 2018b). Another unique peak at 640.5 eV is detected in CP-2Mn1Cu which could belong to surface Mn2þ ions. This is because Mn3O4 is the main crystalline phase in CP-2Mn1Cu, which has been authenticated by XRD (Chen et al., 2017; Li et al., 2017a). Referring to the homologous literature, we could know that the presence of high proportions of oxygen defects and oxygen vacancies in the crystal may lead to the formation of relatively large amounts of Mn3þ, so a high proportion of Mn3þ/Mn4þ could cause better redox ability and superior catalytic capability on toluene (Chen et al., 2017; Huang et al., 2018;

Wang et al., 2018a, 2018b). The atomic ratio of surface Mn3þ/Mn4þ can be calculated by fitting data and listed in Table 2. HR-2Mn1Cu has the highest Mn3þ/Mn4þ ratio of 2.81, which is three times as much as MnO2, while that of MnO2 is 0.94. However, CP-2Mn1Cu assembled by the traditional co-precipitation method gave the lowest ratio of Mn3þ/Mn4þ (0.55), representative of the absence of Mn3þ, hinting to the worst catalytic activity. Combined with the foregoing XRD, BET, TEM, and XPS data, there must be a strong interaction between Cu and Mn ions during the redox-precipitation process, which could bring forth a pronounced electron transfer between Cu and Mn species. As a result of the mismatch of ion radii and charge between Cu and Mn ions, inclusion of Cu ions into the MnO2 crystal lattice to substitute for the Mn ions must lead to the lattice distortion, ion vacancies and charge imbalance. Thus, Mn3þ/ Mn4þ ratio must mount up to maintain a charge balance. This possibility can occur when a solid-solution is formed or the “sticking” of CuOx and MnOx molecules takes place at the molecular scales, as is the case for the redox-precipitated binary metal oxide. 2.5. Catalytic stability The assessment of catalyst stability under reaction conditions is one of the main mediums to judge its merits and demerits. The

Fig. 6. Toluene conversion vs reaction time over HR-2Mn1Cu (Li et al.) at 240  C for 48 h continuous test and (b) under four consecutive cycles. Reaction conditions: toluene concentration ¼ 1000 ppm, and WHSV ¼ 30,000 mL/(g h).

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durability of typical HR-2Mn1Cu at 240  C was evaluated. The evaluation temperature is lower than the temperature at which toluene is completely converted, thus, the possible deactivation could be intuitively shown. Fig. 6(a) depicts the trend of catalyst performance over time. It can be seen that the activity of HR catalyst is basically stable in 48 h consecutive test. Then, the catalytic recyclability of the HR sample was run (Fig. 6(b)). The results of four consecutive reuse experiments on fresh HR-2Mn1Cu show that the performance is practically unanimous in the first three operation cycles and slightly increases in the fourth operation. This phenomenon can be attributed to the gradual activation process of the catalyst. The XRD diffraction patterns of the samples before and after the reaction are consistent (Fig. S6), indicating that the structure of the catalyst remains unchanged during the reaction. 3. Conclusion We infer via highlighting that a binary metal oxide composite possessing elaborate nanostructuring can be derived with the finetuned physiochemical properties (which can noticeably favor the low-temperature catalytic combustion of toluene) by a wieldy and eco-benign salt hydrolysis-driven redox-precipitation approach. Such merits signify that this novel redox-precipitation avenue is likewise able to yield the metastable mixed phases and homogenous hollow microspheres teeming with the rich microscaled pores throughout bulks without resorting to both any expensive porogen (like P123, F127, and CTAB) and any additional strong acid/alkali, largely extending the purview of available porous material families. Benefiting from this capability, we envisage that this binary metal oxide hybrid with the hierarchical morphostructure might have great avail in the pursuit of untraditionally porous/compositional codependent catalytic phases and functions. As a potential demonstration, convergent characterizations substantiate that the uniform hollow CuMn microspheres without obvious phase segregation display textural connectivity/hierchary and single crystalline nature. High mesoporosity of hollow CuMn microspheres impart great accessible surface areas and microenvironments, the chemical and steric confinement effects of which remain poorly fathomed out for toluene combustions. Extraordinary ability to induce the highly dispersed multicomponent oxide mesostructure due to the rapid element diffusion and uniform precipitation rates of metal precursors during the hydrolysisdriving redox co-precipitation afford the versatile platform for discovering the defect chemistry, diffusion physics, and adhibitions in novel sorts of hetero-nanostructures. For instance, reminiscent of the latest findings of anomalous nano-confinements of the ordered mesopororous, more exotic correlated componential/steric catalysis are yet to be studied in mesopores across the singlecrystalline mixed oxides. The universality in terms of the controlled reaction kinetics of a wide variety of metal salt precursors during the hydrolysis-driving-redox precipitation possibly allows for the direct inclusion of strongly correlated properties (particularly the gain in densities of reactive edge sites, mass diffusion, oxygen storage capacity, oxygen vacancies and redox traits) within common metal oxide materials, laying the preliminary foundation towards producing a new kind of multifunctional porous materials in the membranes, catalysis, and energy storage devices. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21872098). The authors greatly appreciate the technical support of Institute of New Energy and Low Carbon Technology.

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