Structure–performance relationships of MnO₂ nanocatalyst for the low-temperature SCR removal of NO_X under ammonia

Abstract

To investigate the corresponding relationship between catalytic efficiency and structure, MnO₂ nanomaterials (nanospheres, nanosheets, nanorods) have been prepared successfully, and were thoroughly characterized by SEM and TEM. Furthermore, the selective catalytic reduction (SCR) performance of NO_X under ammonia was used as an indicative reaction. Among the MnO₂ nanomaterials with different morphologies, it was found that their SCR activities showed an interesting variation tendency: nanospheres > nanosheets > nanorods of MnO₂. The NO conversion ratio of the MnO₂ nanospheres could reach 100% from 200 to 350 °C. Moreover, in order to study the probable mechanism for the best removal efficiency of the nanospheres, XRD, H₂-TPR, NH₃-TPD, BET, XPS and in situ DRIFTS were performed in detail. It is found that surface chemisorbed oxygen, specific surface area, reducibility and acid sites have great influence on the NO removal efficiency in the SCR reaction. In addition, how several process parameters affect the NO_X removal efficiency was carried out, such as time, H₂O and SO₂.

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1. Introduction

Emission of nitrogen oxides (NO_X) from the industrial combustion of coal and fossil fuels has been one of the major causes of environmental pollution. NO_X contributes to the formation of photo-chemical smog and acid rain, and ozone layer depletion, and have drawn wide concern due to their high toxicity to human health.^1–6 Many approaches have been developed to remove NO_X, among which SCR degradation of NO_X with NH₃ has been proven to be an effective technology.

As typical commercial catalysts, vanadium-based catalysts exhibit higher catalytic activity and selectivity in the SCR degradation of NO_X with NH₃. However, some inherent drawbacks still exist, such as the poor low temperature SCR activity, the toxicity of VO_X, and the narrow operating temperature window.^7–9 Therefore, it is necessary to develop several new types of catalysts that contain various metal oxides (Mn, Fe, Ni, Cu, Ce oxides) on different commercial supports. For example, Liu¹⁰ found that the addition of Ce not only enhanced the activity and alkali resistance of V₂O₅/TiO₂ but also reduced its doped vanadium content. Chen¹¹ found that novel Fe–Mn mixed-oxide catalysts showed the highest removal performance and 100% selectivity of N₂. Among the many transition metal oxides, manganese oxides have attracted much attention due to their high catalytic activity, good redox properties, relatively low toxicities, good sintering resistance and low cost. As we all know, MnO₂ has a redox cycle process between Mn²⁺ and Mn⁴⁺, providing oxygen mobility in the oxide lattice. However, Mn⁴⁺ ions are expected to exhibit higher activity compared to Mn³⁺ and Mn²⁺ in catalytic reactions.^12,13 Manganese oxide-based catalysts (MnO_X) exhibited a relatively high activity for the SCR reaction, such as MnO_X/CeO₂,^14,15 MnO_X/CuO,^16,17 MnO_X/ZrO₂–CeO₂,¹⁸ MnO_X/TiO₂,^19,20 MnO₂/Fe₂O₃,^11,21 MnO₂/NiCo₂O₄,²² and so on.

As we all know, SCR degradation of NO_X with NH₃ can be affected by many factors, for instance, the oxidation state of the transition metal element, the shape, the crystallinity and the specific surface.²³ However, few studies have focused on the corresponding relation between catalytic efficiency and morphology or dimensions. For example, Bai²⁴ et al. studied MnO₂ catalyst with different dimensions. They found that 3D-MnO₂ obtained the best catalytic properties, which is due to abundant surface adsorbed oxygen species, better reducibility and more Mn⁴⁺ ions. Zhao²³ found that Co₃O₄ nanorods showed much higher efficiency in SCR degradation of NO_X with NH₃ than Co₃O₄ nanoparticles, which is due to more Co³⁺ adsorption on the Co₃O₄ nanorods. Gao et al.¹⁸ studied the morphology-dependent properties of MnO_X/ZrO₂–CeO₂ nanostructures for NO_X reduction with NH₃. They found that the MnO_X/ZrO₂–CeO₂ nanorods achieved significantly higher NO_X removal efficiency than the nanocubes and nanopolyhedra, which is attributed to more Mn⁴⁺ species, adsorbed surface oxygen and oxygen vacancies.

In this paper, pure MnO₂ nanocrystals with different morphologies, including bulk particles, nanorods, nanosheets and nanospheres were synthesized successfully and their catalytic activities in the NH₃-SCR reaction were investigated, indicating that the MnO₂ nanospheres exhibited the highest NO conversion ratio in the NH₃-SCR reaction. Furthermore, its reaction mechanism was proposed using the analysis of SEM, TEM, BET, XRD, NH₃-TPD, H₂-TPR, XPS and in situ DRIFTS.

2. Experimental

2.1. Preparation of materials

All of the chemicals were analytical grade without further purification. Manganese(II) chloride tetrahydrate (MnCl₂·4H₂O) and urea were purchased from Sigma-Aldrich. Manganese sulfate (MnSO₄) and ammonium persulfate (NH₄)₂S₂O₈ were purchased from Aladdin Industrial Inc. PEG 400, potassium permanganate (KMnO₄) and sodium dodecyl benzene sulfonate (SDBS) were purchased from Jiangtian Chemical Technology Co. Ltd. (Tianjin, China).

The preparation methods of the three precursors were as follows: the β-MnO₂ nanorods²⁵ were synthesized by a hydrothermal method. In a typical process, 0.2 g KMnO₄ and 4 mL PEG (400) were added into a 100 mL Teflon-sealed autoclave. Then 80 mL deionized water was added into the autoclave and kept under rapid magnetic agitation at room temperature for 10 min. The Teflon-sealed autoclave was heated to 160 °C for 5 h.

The synthesis process of γ-MnO₂ nanosheets²⁶ was similar to the above steps. 1.3 g SDBS and 0.4 g MnCl₂·4H₂O were added into 45 mL deionized water, then 0.2 g urea was dropped into this homogeneous solution under stirring. After continuous stirring for 30 min, the mixture was transferred into a Teflon-sealed autoclave and maintained at 110 °C for 12 h.

In a similar process, 1.5 g MnSO₄ and 23 g (NH₄)₂S₂O₈ were prepared to synthesize γ-MnO₂ nanospheres,²⁷ which were dissolved in 50 mL deionized water and maintained at 110 °C for 10 h in a Teflon-sealed autoclave.

Then the obtained three precursors were filtered and washed with deionized water and absolute ethanol several times, respectively. After drying at 60 °C in an oven, the resulting powders were finally calcined at 400 °C for 5 h in air.

As a comparison, the MnO₂ bulk nanoparticles were prepared directly by calcining Mn(NO₃)₂·4H₂O (97%) at 400 °C for 5 h.

2.2. Characterization

MnO₂ nanomaterials with different morphologies were investigated by field-emission scanning electron microscopy (FESEM) (JSM-6700F), and the size and morphology of the samples were observed by transmission electron microscopy (TEM) using a JEOL Model JEM-1200EX instrument with an accelerating voltage of 200 kV. The X-ray diffraction (XRD) measurements of the MnO₂ materials were carried out on an X-ray diffractometer (Rigaku D/Max 2200 PC) over a 2θ range between 10° and 80° with the electric current being fixed at 40 mA and the voltage being operated at 40 kV, with a graphite monochrometer and Cu Kα radiation (λ = 0.15418 nm) at room temperature. Temperature-programmed reduction by hydrogen (H₂-TPR) was conducted on a chemisorption analyzer (Micromeritics Autochem 2920 II instrument) containing a thermal conductivity detector (TCD). The experimental pretreatment was as follows. MnO₂ material was outgassed at 100 °C for 1 h in flowing Ar with about 30–40 mg different catalysts. These samples were heated up to 700 °C under a 10% H₂/Ar gas flow (50 mL min⁻¹) at a rate of 10 °C min⁻¹ after cooling to room temperature. Temperature-programmed desorption by ammonia (NH₃-TPD) was performed on a chemisorption analyzer (Quantachrome Autosorb iQ-C-TCD-MS). Approximately 100 mg MnO₂ samples were saturated with highly purified NH₃ at 50 °C for 1 h and then flushed at 50 °C for 1 h to remove physically adsorbed NH₃ after degassing at 300 °C for 2 h in flowing N₂. In the end, these samples were heated from 200 °C to 700 °C at a heating rate of 10 °C min⁻¹. The X-ray photoelectron spectroscopy (XPS) of the three different MnO₂ samples was carried out on a spectrometer (Thermal ESCALAB 250, UK) with Al Kα radiation, and the binding energies were calibrated by using C 1s (C 1s = 284.6 eV). The specific surface areas and the pore size distributions of the three different shapes of MnO₂ nanocrystals were observed respectively by the Brunauer–Emmett–Teller (BET) method, a cylindrical pore model (BJH method) and the adsorption branches. The in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) was carried out on a spectrometer (Thermo Nicolet iS50 FTIR). Prior to each test, the MnO₂ catalyst was disposed at 350 °C in flowing argon for 60 min and then cooled to 150 °C. The sample spectra were recorded using the KBr pellet method by collecting 32 scans at a resolution of 4 cm⁻¹ in the range of 400–4000 cm⁻¹.

2.3. Catalytic performance tests

The catalytic activity tests for the three different MnO₂ samples were conducted in a fixed-bed quartz reactor (inner diameter of 10 mm) with 500 mg of samples measuring 40–60 mesh and ranging from 50–400 °C under atmospheric pressure. The SCR reaction activity measurement by ammonia was as follows: 500 mg catalyst was put in the reactor. The total flow rate of the mixed gases (500 ppm NH₃, 500 ppm NO, 5% H₂O (when used), 3% O₂, all balanced by nitrogen) was 200 mL min⁻¹, amounting to a gas hourly space velocity (GHSV) of 2.8 × 10⁴ h⁻¹. The concentrations of O₂, NO, NO_X and NO₂ were continually monitored by an FTIR spectrometer (GASMET FTIR DX4000, Finland). All the concentrations of the outlet gases were recorded when the SCR process reached a steady-state after half an hour at each temperature. To further count the activity of the three different shaped MnO₂ samples, the NO conversion was obtained from the following equation, where NO_out is the concentration of the outlet NO and NO_in is the concentration of the inlet NO.

NO oxidation to NO₂ and N₂ selectivity were obtained by using the following equations, where NO_X is the sum of the NO and NO₂ concentrations.

In order to parallelly compare the activities of the three different MnO₂ samples, we performed turnover frequency (TOF) experiments. The total flow rate of the mixed gases (500 ppm NH₃, 500 ppm NO, 3% O₂, all balanced by nitrogen) was 3000 mL min⁻¹, which is equivalent to a gas hourly space velocity (GHSV) of 4.2 × 10⁵ h⁻¹. By assuming that the reaction components were free of diffusion limitations, TOF values can be calculated according to the following equation:

where F_NO is the concentration of inlet NO (L min⁻¹), X_NO is the NO conversion, W is the catalyst weight (g), and n_Mn⁴⁺ is the mole number of Mn⁴⁺ ions calculated by H₂-TPR.

3. Results and discussion

3.1. Structural properties

The SEM images of the obtained MnO₂ nanostructures are shown in Fig. 1. A large number of MnO₂ nanorods are presented in Fig. 1a, where the nanorods are shown to have a smooth surface and typical rod-like shape. These high-purity MnO₂ nanorods are about 100 nm in diameter and their lengths are in the range of 2–10 μm. Meanwhile, from the SEM image in Fig. 1b, the product is presented as a uniform hexagonal nanosheet with a side length of 1.2 μm and a thickness of approximately 200 nm. The surfaces of these uniform hexagonal nanosheets are smooth. Fig. 1c shows urchin-like nanospheres which are composed of numerous randomly oriented nanorods, and the diameters are about 4 μm. These nanorods are 400–800 nm in length and 100–150 nm in width.

Fig. 1 SEM and HRTEM images of MnO₂ nanorods (a, d), MnO₂ nanosheets (b, e) and MnO₂ nanospheres (c, f).

The detailed morphology structures of the MnO₂ nanorods, nanosheets and nanospheres are further analyzed by HRTEM in Fig. 1. As shown in Fig. 1d, it can be seen that the MnO₂ nanorods are well crystallized and the interplanar distances are 3.098 nm and 2.35 nm, corresponding to the (110) and (101) crystal planes, respectively. A pretty morphology is displayed in Fig. 1e. The interplanar distance of the MnO₂ nanosheets is 2.25 nm, which corresponds to the (022) crystal plane. In Fig. 1f, the MnO₂ nanospheres are composed of numerous randomly oriented nanorods, and the interplanar distance is 3.398 nm, corresponding to the (101) crystal plane.

Fig. 2 shows the XRD patterns of the MnO₂ materials with different morphologies. The diffraction peaks of MnO₂ bulk nanoparticles can be well identified as β-MnO₂ (JCPDS 24-0735).²⁸ The MnO₂ nanorods can be determined as tetragonal-phase β-MnO₂ (JCPDS 24-0735), which are produced by the pyrolysis of MnOOH nanorods, as illustrated in Fig. 2. The lattice constants are calculated, and a is 4.38 Å and c is 2.88 Å, which are very consistent with the reported values (a = 4.40 Å and c = 2.87 Å).²⁹ The intensity ratios of (110) and the other reflections are different, a result of the preferential growth of the materials. The mechanism has been proved using many one-dimensional nanocrystals.³⁰ The MnO₂ hexagonal nanosheets could be identified as γ-MnO₂ (JCPDS 30-0820), which is from the calcination of MnCO₃ nanosheets and can be easily indexed to the lattice constants (a is 2.80 Å, b is 2.80 Å, c is 4.45 Å). The XRD patterns of MnO₂ nanospheres show that the calcined products were γ-MnO₂ (JCPDS 30-0820). From the full width at half maximum and intensity of the peaks, the diffraction peaks of the MnO₂ nanospheres are higher than that of the MnO₂ nanosheets, suggesting that the MnO₂ nanospheres obtained high crystallinity.

Fig. 2 XRD patterns of the MnO₂ nanomaterials with different morphologies, such as nanorods, nanosheets, nanospheres and bulk particles.

The N₂ adsorption–desorption isotherms and the pore size distribution, and the specific surface area and pore volume of the MnO₂ bulk nanoparticles, MnO₂ nanorods, MnO₂ nanosheets and MnO₂ nanospheres are shown in Fig. 3 and Table 1, respectively. In Fig. 3a, it can be seen that the MnO₂ nanospheres and nanosheets all have a typical type IV isotherm with type-H3 hysteresis loops according to the definition of IUPAC. The hysteresis loops indicate the presence of mesopores.^31,32 However, both the MnO₂ bulk nanoparticles and MnO₂ nanorods did not show the typical IV-type isotherm, indicating that the MnO₂ bulk nanoparticles and MnO₂ nanorods have no mesoporous channels. It can be further observed over the N₂ adsorption–desorption isotherms that the turning point of the nitrogen-adsorbed volume for MnO₂ nanorods and nanospheres is 0.9, and the N₂ adsorbed volume further increases sharply under higher relative pressures, which is ascribed to the large mesopores (>10 nm) in the samples.³³ As illustrated in Fig. 3b, the MnO₂ nanospheres show a wide peak at 40–60 nm, which might correspond to the staggered arrangement of numerous randomly oriented MnO₂ nanorods. The consequence is consistent with the results of SEM and TEM analysis. The MnO₂ nanosheets exhibit a sharp peak at around 8 nm, which might result from the packing of the nanosheets. The pore distribution of MnO₂ bulk nanoparticles and MnO₂ nanorods shows that a few large mesopores exist. As shown in Table 1, the specific areas of the MnO₂ bulk nanoparticles and MnO₂ nanorods are lower than the MnO₂ nanosheets and nanospheres, which is in good conformity with the results of NO removal efficiency in the SCR reaction. Furthermore, it can be seen that the specific surface area of the MnO₂ nanosheets is almost equal to the nanospheres at about 16 m² g⁻¹. However, the pore volume of MnO₂ nanospheres (0.14 cm³ g⁻¹) is significantly higher than that of the nanosheets (0.07 cm³ g⁻¹), which is due to them being composed of numerous randomly oriented nanorods of urchin-like MnO₂ nanospheres.²⁴ The similar specific surface area and large differences in the total pore volume of the MnO₂ nanosheets and nanospheres are due to the pore diameters of the nanosheets being less than for the nanospheres.³⁴ It is reported that the larger specific surface area and porous structures can offer more active sites and increase the adsorption of reactants in the SCR reaction.^35,36 The specific surface area and pore volume of the MnO₂ nanospheres are higher than the MnO₂ nanosheets and nanorods, which is beneficial for the exposure of more active sites and adsorption of the reactants. From what has been discussed above, we may draw the conclusion that the MnO₂ nanospheres obtained the highest catalytic activity in the SCR reaction and it might be due to the large specific surface area and pore volume.³⁷ This is consistent with the above discussion.

Fig. 3 N₂ adsorption–desorption isotherms (a) and pore diameter distribution (b) of the MnO₂ nanomaterials with different morphologies.

Table 1 Specific area, pore volume and pore diameter distribution of the MnO₂ samples

Materials	Specific area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
MnO2 bulk nanoparticles	5.6	0.01	3.83
MnO2 nanorods	9.6	0.02	24.78
MnO2 nanosheets	16.0	0.07	12.58
MnO2 nanospheres	15.7	0.14	68.74

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The chemical states of the elements in the near-surface region were investigated by XPS. The survey spectra of three different samples (Fig. 4a) show the existence of Mn and O, with C from the reference. Moreover, the atom molar rate of Mn and O is about 1 : 2 in the survey spectra, demonstrating the formation of MnO₂. As illustrated in Fig. 4b, two XPS binding energies of Mn 2p_1/2 and Mn 2p_3/2 at 654.1 eV and 642.4 eV were observed for MnO_X. It can be seen that the binding energy separation is 11.7 eV between the Mn 2p_1/2 and Mn 2p_3/2 states.³⁸ As shown in Fig. 4b, Mn 2p_1/2 can be divided into two characteristic peaks which can be ascribed to Mn⁴⁺ and Mn³⁺ ions displayed with binding energies of 654.8 eV and 654.1 eV, respectively. Moreover, Mn 2p_3/2 can be divided into two characteristic peaks which can be ascribed to Mn⁴⁺ and Mn³⁺ ions displayed with binding energies of 642.9 eV and 641.8 eV, respectively. The relative atomic percentages of Mn³⁺ and Mn⁴⁺ over the surface of the MnO₂ catalysts are shown in Table 2, respectively. We find that the MnO₂ nanospheres display the highest Mn⁴⁺/Mnⁿ⁺ molar ratios. As we all know, higher oxidation state manganese species are more active for the SCR reaction over manganese-based catalysts. It is well known that Mn⁴⁺ ions play a major role in low-temperature SCR reactions, which could promote the oxidation of nitric oxide to nitrogen dioxide.^24,39 Therefore a high Mn⁴⁺/Mnⁿ⁺ ratio will be conducive to the low-temperature SCR reaction. The Mn⁴⁺ content in the MnO₂ nanospheres (55.6%) was much higher than that in the MnO₂ nanosheets (49.1%) and nanorods (37.2%), which is consistent with the results of SCR activity.

Fig. 4 XPS spectra of the MnO₂ nanorods, MnO₂ nanosheets and MnO₂ nanospheres.

Table 2 XPS results of the MnO₂ samples

	Surface Atomic Concentration (%)
MnO2 materials	2p		Oα (531.2)	Oβ (529.5)
	Mn^4+/Mn^n+	Mn^3+/Mn^n+
Nanorods	37.2	62.8	36.73	63.27
Nanosheets	49.2	50.8	47.90	52.10
Nanospheres	55.6	44.4	43.72	56.28

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The O 1s XPS spectra of the three samples in Fig. 4c were deconvoluted using a curve-fitting procedure, and contain two kinds of distinguished surface oxygen species. The lower binding energy peak at 529.0–530.0 eV might be assigned to the surface lattice oxygen (O_β), such as defective oxides or surface oxygen ions bonded to manganese in a coordinatively saturated environment, and the higher binding energy peak at 531.0–532.0 eV could belong to the surface chemisorbed oxygen (O_α).³⁹ Generally, the surface chemisorbed oxygen (O_α) is more reactive than the surface lattice oxygen (O_β), which is due to the higher mobility of O_α. Hence a high O_α/(O_α + O_β) ratio will be conducive to the SCR reaction.⁴⁰ The O_α ratio between the MnO₂ nanospheres (43.72%) and nanosheets (47.9%) has no significant difference. The O_α rate of the MnO₂ nanospheres and nanosheets were much higher than that of the MnO₂ nanorods (36.73%), which corresponds to the lowest NO conversion efficiency of the MnO₂ nanorods in the SCR reaction.

To investigate the reducibility of the MnO₂ samples with different morphologies in the SCR reaction, including bulk nanoparticles, nanorods, nanosheets and nanospheres, the H₂-TPR spectra of the MnO₂ catalysts are shown in Fig. 5a. It can be seen that the three reduction peaks of the MnO₂ bulk nanoparticles ranging from 350 to 600 °C are attributed to the reduction of MnO₂ to Mn₂O₃, Mn₂O₃ to Mn₃O₄ and Mn₃O₄ to MnO, respectively.^34,41 Moreover, the TPR spectra of the MnO₂ nanorods, nanosheets and nanospheres all show two overlapping strong reduction peaks. The first reduction peak may be the conversion of MnO₂ to Mn₂O₃, and the second reduction peak may be attributed to the reduction of Mn₂O₃ to MnO. These features correspond to the reduction of MnO₂ with low crystallinity.⁴¹ However, some differences in the four spectra can be seen. The first reduction peak of MnO₂ nanospheres appeared at lower temperature than the two other MnO₂ catalysts, which might imply that the MnO₂ nanospheres have better low temperature reducibility than the two other MnO₂ catalysts. Furthermore, the reduction peak region of the MnO₂ nanospheres is also wider than those of the two other MnO₂ catalysts, which might imply that the MnO₂ nanospheres have a wider operating temperature window. All the features correspond to the SCR activity results. The reduction peaks 1 and 2 of the MnO₂ nanorods correspond to H₂ consumptions of 8.35 and 3.58 mmol g⁻¹, respectively. The reduction peaks 1 and 2 of the MnO₂ nanosheets correspond to H₂ consumptions of 6.01 and 4.55 mmol g⁻¹, respectively. Meanwhile, we find that the reduction peaks 1 and 2 of MnO₂ nanospheres correspond to H₂ consumptions of 7.22 and 4.01 mmol g⁻¹ and are located at 277 °C and 422 °C, respectively. From the above we can come to the conclusion that the total hydrogen consumption of the samples approaches the theoretical hydrogen consumption (11.49 mmol g⁻¹). The MnO₂ samples show the first reduction step of MnO₂ to Mn₂O₃ (peak 1), which corresponds to the reduction of Mn⁴⁺ to Mn³⁺.⁴² Therefore, we can calculate the Mn⁴⁺ total active sites of the MnO₂ nanospheres, nanosheets and nanorods to be 0.835, 0.601 and 0.702 mmol, respectively. The H₂ consumption of the reduction peaks of the MnO₂ nanosheets are less than that of the MnO₂ nanospheres, and the MnO₂ nanospheres obtained a higher NO conversion efficiency than the MnO₂ nanosheets (Table 3).

Fig. 5 H₂-TPR (a) and NH₃-TPD (b) patterns of the MnO₂ bulk nanoparticles, MnO₂ nanorods, MnO₂ nanosheets and MnO₂ nanospheres.

Table 3 H₂-TPR results of the MnO₂ samples

Samples	Temperature (°C)		H2 consumption (mmol g^−1)		Mn^4+ total active sites (mmol)
	Peak 1	Peak 2	Peak 1	Peak 2
Nanorods	299	409	8.05	3.58	0.805
Nanosheets	309	412	6.01	4.55	0.601
Nanospheres	277	422	7.22	4.01	0.722

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Temperature-programmed desorption by ammonia (NH₃-TPD) experiments were performed in order to investigate the surface acidity of the catalyst. As we all know, ammonia storage capacity plays an important role in the SCR reaction system. Thus, it is necessary to research the adsorption–desorption behavior of the catalyst by NH₃.⁴³ Based on analysis in previous reports, the thermal stability of the NH₄⁺ associated with the Brønsted acid sites was lower than the NH₃ associated with the Lewis acid sites. The low-temperature desorption peak below 350 °C corresponds to the NH₄⁺ desorbed from Brønsted acid sites and the high-temperature desorption peak above 400 °C corresponds to NH₃ desorbed from Lewis acid sites.⁴⁴ As shown in Fig. 5b, two obvious desorption peaks were observed for the MnO₂ nanospheres. The lower temperature desorption peak of the MnO₂ nanospheres at 310 °C is associated with NH₄⁺ adsorbed on the Brønsted acid sites and the higher temperature desorption peak at 550 °C is associated with NH₃ adsorbed on the Lewis acid sites. Compared with the MnO₂ nanospheres, no Brønsted acid sites were seen for the MnO₂ bulk nanoparticles, MnO₂ nanorods and MnO₂ nanosheets. Moreover, the intensity of the higher temperature desorption peak of the MnO₂ nanospheres is also higher than the three other samples. As we all know, Lewis acid sites exert a major influence in SCR systems, which may be a very significant reason for the SCR reaction activity of the MnO₂ nanospheres being higher than those of the MnO₂ bulk nanoparticles, MnO₂ nanorods and MnO₂ nanosheets. All the features correspond to the SCR activity results.

As we all know, Mn⁴⁺ ions are most likely the active sites of NO oxidation to NO₂ because the oxidation process often occurs from redox cycling of high and low valence state cations.^45,46 Therefore, the TOF was defined as the number of NO_X converted per Mn⁴⁺ ion. The catalytic activity can be seen from TOF analysis (Fig. 6a), and different TOF values of the three samples at six different temperature points was obtained, indicating that the TOF values increase with increasing temperature. Moreover, the TOF value of the MnO₂ nanospheres is 0.07 h⁻¹ at 100 °C, and 0.15 h⁻¹ at 140 °C. Furthermore, it can be seen that the TOF values varied with the different morphology. The MnO₂ nanospheres obtained the highest TOF values from 100 to 200 °C over the MnO₂ nanosheets and MnO₂ nanorods. Therefore, it can be concluded that abundant Mn⁴⁺ ions in the MnO₂ nanospheres promoted their catalytic performance in the SCR reaction.^45,46 The Arrhenius plots of NO oxidation of the three different samples are shown in Fig. 6b. Furthermore, the reaction activation energy can be calculated based on the slope of the plot. It can be seen that the activation energy of the MnO₂ nanospheres (20.5 kJ mol⁻¹) is lower than the MnO₂ nanosheets (30.7 kJ mol⁻¹) and nanorods (44.5 kJ mol⁻¹), indicating that surface adsorbed oxygen species are easily activated and favour enhanced SCR performance, which also corresponds to the SCR results.

Fig. 6 TOF values (a) and Arrhenius plots of the NO oxidation rates (b) of the MnO₂ nanorods, MnO₂ nanosheets and MnO₂ nanospheres.

3.2. NH₃-SCR activity

Stability tests and H₂O resistance studies of the three different catalysts in the SCR system were performed, and the results are shown in Fig. 7. Fig. 7a showed the SCR by ammonia performance of the MnO₂ with different structures and the traditional V₂O₅–WO₃/TiO₂ catalyst. It is seen that the NO conversion of all of the catalysts increases at first and finally decreases as the temperature is elevated. However, some inherent differences in the catalyst performance are discovered. It can be seen that the MnO₂ nanospheres showed about 100% NO_X conversion at 200–300 °C and obtained over 90% NO_X conversion at 150–350 °C with a wide operating temperature window. The MnO₂ nanosheets obtained an almost 100% conversion efficiency only at 200 °C, with a narrow operating temperature window. The MnO₂ nanorods were also observed to show very low activity and a best NO conversion of only 80% at 300 °C in the investigated temperature window in the SCR by NH₃. In contrast, the MnO₂ bulk nanoparticles obtained the lowest activity in the investigated temperature window, and the traditional V₂O₅–WO₃/TiO₂ catalyst displays poor low temperature SCR activity. Comparing all of the above catalysts, MnO₂ nanosphere catalysts display a wider operating temperature window and the highest conversion efficiency over the other catalysts. As shown in Fig. 7b, the NO conversion of the three different catalysts showed little change during a 72 h continuous running duration, indicating that all these catalysts have excellent stability. Compared to the samples before the SCR reaction, there is almost no change of the morphology, surface area and crystallinity of the reacted samples, which was proved by TEM, SEM, XRD and BET (Fig. S1–S3†). Therefore, after the reaction the three MnO₂ materials still maintain the original structure, which proves that the three MnO₂ morphologies are still stable after the SCR reaction.

Fig. 7 NO conversion (a), stability test (b), H₂O resistance (c), effect of SO₂ (d) and co-effect of SO₂ and H₂O (e) on NO conversion for the MnO₂ nanorods, nanosheets and nanospheres at the optimum temperature. Reaction conditions: 500 ppm NO, 500 ppm NH₃, 3% O₂, 5% H₂O (when used), 5% SO₂ (when used) and N₂ balance gas.

Fig. 7c indicates the effects of H₂O on NH₃-SCR activity. After adding water, it can be seen that the NO conversion efficiency of the MnO₂ nanospheres decreased to 93% and then remained at 97% for the next 18 h. The NO conversion efficiency returned to 100% quickly after the removal of H₂O. Moreover, the NO conversion efficiency of the MnO₂ nanorods and nanosheets decreased quickly after the introduction of H₂O. However the NO conversion efficiency returned quickly after cutting off the H₂O. All these features indicate the recoverable activity of the samples in the NH₃-SCR system. Hence, it can be trusted that the inhibition effect of H₂O was possibly attributed to the competitive adsorption of H₂O and NH₃ onto the catalyst surface.⁴⁷ Meanwhile, Fig. 7d shows the effects of SO₂ on the NH₃-SCR activity. The NO conversion of the MnO₂ nanospheres decreased to 87% with the introduction of SO₂ and was maintained at 87%. After removing the SO₂ gas, the NO conversion returned to 95%, suggesting that the MnO₂ nanospheres have a good SO₂ resistance in the NH₃-SCR reaction. The NO conversion of the MnO₂ nanorods and nanosheets decreased rapidly after the introduction of SO₂. On removing the SO₂ gas, the NO conversion of the MnO₂ nanosheet does not fully recover, suggesting that manganese sulfate formed on the catalyst surface.⁴⁸ Compared with the MnO₂ nanorods and nanosheets, the MnO₂ nanospheres obtained the highest SO₂ resistance in the NH₃-SCR reaction. Meanwhile, the co-effect of SO₂ and H₂O was also studied. When SO₂ and H₂O were introduced into the feed gases at the same time, the NO_X conversion ratios of the three MnO₂ samples all decreased, but the NO_X conversion of the MnO₂ nanospheres was still 70%. The above results suggest that the MnO₂ nanospheres had good SO₂/H₂O durability. Additionally, N₂ selectivity tests are shown in Fig. S5.† As can be seen, the N₂ selectivity decreases and N₂O concentration increases for all the catalyst with the elevating temperature. Moreover, the MnO₂ samples can obtain higher N₂ selectivity at low temperature (<200 °C). However, the N₂ selectivity became lower in the high temperature SCR reaction.

It is now widely accepted that the enhancement of NO to NO₂ over catalysts in the SCR reaction could significantly promote the catalytic activity, and can be attributed to the faster reaction: NO + NO₂ + 2NH₃ → 3H₂O + 2N₂.^49,50 Therefore, the experiments of NO to NO₂ in the absence of NH₃ on the catalyst are studied. As shown in Fig. 8, we can see that the oxidation of NO to NO₂ increases at first and finally decreases with the elevating temperature. Moreover, the oxidation of NO to NO₂ reaches the maximum conversion at 350 °C, and then decreases above 350 °C, which is attributed to the thermal equilibrium compositions. Meanwhile, the MnO₂ nanospheres obtained a higher activity of oxidation of NO to NO₂ than the MnO₂ nanosheets and nanorods. The formation of NH₄NO₂ from NO₂ and NH₃ in the SCR reaction has been reported.^50,51 Therefore, a high NO₂ concentration, or a high activity of oxidation of NO to NO₂, enhance the SCR activity.

Fig. 8 Oxidation of NO to NO₂ over the MnO₂ samples at different temperatures.

3.3. In situ DRIFTS

The acid site distribution and the gas molecule adsorption behavior on the surface were studied by in situ DRIFTS experiments. At first the catalysts were purged with N₂ for 1 h and cooled down to 50 °C. NH₃ was then introduced into the IR cell and spectra were recorded as a function of temperature. The in situ FTIR spectra of adsorbed NH₃ on the three different shaped MnO₂ nanomaterials at different temperatures are shown in Fig. 9. With the increase of temperature, the intensities of all the bands decreased. Several bands at 1387, 1300, 1143, 1106 and 993 cm⁻¹ were observed on the MnO₂ nanorods catalyst in Fig. 9a. The bands at 1300, 1143 and 1106 cm⁻¹ could be ascribed to coordinated NH₃ Lewis acid sites.^52–54 The weak band at 1387 cm⁻¹ may be due to the oxidization species of adsorbed ammonia species.^35,55 The band at 933 cm⁻¹ was assigned to gas or weakly adsorbed NH₃.³⁵ As shown in Fig. 9b, the NH₃ adsorption peaks of the MnO₂ nanosheets are at 1450, 1141, 1110 and 1082 cm⁻¹, of which the weak peak at 1450 cm⁻¹ can be assigned to the NH₄⁺ bound to the Brønsted acid sites, and the adsorption peaks at 1141, 1110 and 1082 cm⁻¹ can be ascribed to the NH₃ bound to the Lewis acid sites.^56,57 Several bands at 1642, 1395, 1137 and 1113 cm⁻¹ can be found for the MnO₂ nanospheres in Fig. 9c. The adsorption peaks at 1642 and 1395 cm⁻¹ were ascribed to the NH₄⁺ bound to the Brønsted acid sites. Two bands at 1113 and 1137 cm⁻¹ were also found, which could be ascribed to coordinated NH₃ Lewis acid sites. It is evident that the band intensity of Lewis acid sites over the MnO₂ nanospheres is higher than that over the MnO₂ nanorods and nanosheets. All the characteristics corresponded to the above analysis in NH₃-TPD.

Fig. 9 In situ DRIFTS of (a) NH₃ adsorption on the MnO₂ nanorods; (b) NH₃ adsorption on the MnO₂ nanosheets; (c) NH₃ adsorption on the MnO₂ nanospheres.

The in situ DRIFTS of NO + O₂ adsorption over the three different shaped MnO₂ catalysts at different temperatures are shown in Fig. 10. First of all the catalysts were purged with N₂ for 1 h and cooled down to 50 °C. NO + O₂ was then introduced into the IR cell, and spectra were recorded as a function of temperature. As shown in Fig. 10a, the band at 1379 cm⁻¹ could be ascribed to the adsorbed NO₃⁻, which is due to the disproportionation of chemisorbed NO₂ on the sample surface.⁵⁸ The bands at 1559, 1312 and 1209 cm⁻¹ be assigned to bridged nitrate, bidentate nitrates and bridging nitrites, respectively.⁵⁷ As shown in Fig. 10b, three adsorption bands were observed over the MnO₂ nanosheets catalyst. Besides the bands assigned to monodentate nitrate (1499 cm⁻¹), trans-N₂O₂²⁻ (1461 cm⁻¹) and M–NO₂ nitro compounds (1375 cm⁻¹) were also observed.^{53–55,59–61} Several bands at 1568, 1541, 1499, 1385, and 1215 cm⁻¹ were observed on the MnO₂ nanospheres catalyst shown in Fig. 10c. The bands at 1541 cm⁻¹ and 1499 cm⁻¹ could be attributed to the monodentate nitrate species and the bands at 1215 cm⁻¹ could be assigned to the disproportionation of NO bridging nitrates.^54,55,62 The band at 1568 cm⁻¹ is in accordance with the literature and can be assigned to the bidentate nitrate.^52,63,64 The band at 1385 cm⁻¹ is close to the value in the previous study and could be attributed to monodentate nitrite.⁵⁷ Remarkably, the bands of the MnO₂ nanospheres are much more than those of the MnO₂ nanorods and nanosheets. The above in situ DRIFTS analyses show that the larger pore volume and hierarchical porous structure have enhanced the chemisorption and activation of reactant molecules on the MnO₂ nanospheres, resulting in the best NO conversion efficiency in NH₃-SCR.

Fig. 10 In situ DRIFTS of NO + O₂ adsorption on the MnO₂ nanorods (a), NO + O₂ adsorption on the MnO₂ nanosheets (b); NO + O₂ adsorption on the MnO₂ nanospheres (c).

3.4. Reaction mechanism analysis

Herein, γ-MnO₂ nanospheres, γ-MnO₂ nanosheets, and β-MnO₂ nanorods were found to display different catalytic effects. Furthermore, it is found that the γ-MnO₂ nanospheres and γ-MnO₂ nanosheets obtained significantly higher NO conversions than the β-MnO₂ nanorods and β-MnO₂ bulk nanoparticles. As described in the above sections, the XPS results show that γ-MnO₂ has more surface chemisorbed oxygen than β-MnO₂. According to the literature,^24,38 the surface chemisorbed oxygen on γ-MnO₂ participates easier in the reaction than β-MnO₂, because the surface chemisorbed oxygen on γ-MnO₂ is more active. All the above analysis implies that surface chemisorbed oxygen is the main factor in promoting the activation of NO and NH₃. Based on the above analysis and discussion of the XPS measurements, the MnO₂ nanorods have a relatively high Mn⁴⁺ ion content (61.59%) but the lowest surface chemisorbed oxygen (36.73%) on the surface, and the MnO₂ nanosheets have abundant surface chemisorbed oxygen (47.9%) but the lowest Mn⁴⁺ ion content (35.18%) on the surface. Generally, the MnO₂ nanospheres have the highest Mn⁴⁺ ion content (78.35%) and higher surface chemisorbed oxygen (43.72%). As we all know, Mn⁴⁺ ions and surface chemisorbed oxygen play important roles in the low-temperature SCR reaction. Therefore a high Mn⁴⁺/Mnⁿ⁺ ratio and O_α/(O_α + O_β) ratio will be conducive to the low-temperature SCR reaction. As shown in Fig. 11, the first oxidation process of the MnO₂ nanocatalysts is NO to NO₂, which is consistent with the reduction of Mn⁴⁺ to Mn²⁺. Furthermore, the TPD and in situ DRIFTS results show that the MnO₂ nanospheres have more acid sites on the outside surface than the MnO₂ nanosheets and MnO₂ nanorods. Meanwhile, the MnO₂ nanospheres show a higher catalytic effect than the MnO₂ nanosheets. The MnO₂ nanospheres have more exposed Mn⁴⁺ and acid sites due to their three dimensional structure. However, the MnO₂ nanosheets with a two dimensional structure are easy to pile up into pieces, which may result in fewer Mn⁴⁺ and acid sites on the outside surface. All the features correspond to the above results of in situ DRIFTS, TPD and XPS. Moreover, the MnO₂ nanospheres obtained higher TOF values from 100 to 200 °C than both the MnO₂ nanosheets and MnO₂ nanorods. Therefore, it can be concluded that the abundant Mn⁴⁺ ions in the MnO₂ nanospheres promote the catalytic performance in the SCR reaction.^45,46 Meanwhile, it can be found that activation energy of the MnO₂ nanospheres (20.5 kJ mol⁻¹) is lower than the MnO₂ nanosheets (30.7 kJ mol⁻¹) and nanorods (44.5 kJ mol⁻¹), indicating that surface adsorbed oxygen species are easily activated and in favour of SCR performance. This result is consistent with the SCR results. Therefore, surface chemisorbed oxygen, Mn⁴⁺ and acid sites on the outside surface play important roles in the SCR reaction.

Fig. 11 Reaction mechanism of NH₃-SCR on MnO₂ with different morphologies.

4. Conclusions

In this paper, MnO₂ nanomaterials with different morphologies were prepared by the redox hydrothermal method, such as nanorods, nanosheets, nanospheres and bulk particles. Moreover, their NO conversion efficiency was studied and compared through the SCR reaction. The γ-MnO₂ nanospheres showed significantly higher NO conversion and a significantly higher rate constant with respect to NO conversion than the bulk particles, MnO₂ nanorods and MnO₂ nanospheres in the NH₃-SCR of NO. The NO conversion efficiency of the MnO₂ nanospheres reached 100% over a wide temperature window from 200 to 350 °C. Moreover, the MnO₂ nanospheres show excellent stability, H₂O resistance and SO₂ tolerance. The high NO conversion efficiency of the MnO₂ nanospheres is due to their large specific area, a large number of surface chemisorbed oxygen species, more Mn⁴⁺ on the outside surface, higher reducibility and more acid sites, based on XPS, H₂-TPR, NH₃-TPD and in situ DRIFTS.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21377061, 81270041), Independent Innovation fund of Tianjin University (2015XRG0020), Key Laboratory of Colloid and Interface Chemistry (Shandong University, Ministry of Education) (201401), and by the Natural Science Foundation of Tianjin (Grant No. 15JCYBJC48400 and 15JCZDJC41200).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03108k

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