Structure – Activity Relationship Study of Mn / Fe Ratio Effects on Mn − Fe − Ce − O x / γ-Al 2 O 3 Nanocatalyst for NO Oxidation and Fast SCR Reaction

A series of Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts were synthesized with different Mn/Fe ratios for the catalytic oxidation of NO into NO2 and the catalytic elimination of NOx via fast selective catalytic reduction (SCR) reaction. The effects of Mn/Fe ratio on the physicochemical properties of the samples were analyzed by means of various techniques including N2 adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H2-temperature-programmed reduction (TPR), NH3-temperature-programmed desorption (TPD) and NO-TPD, meanwhile, their catalytic performance was also evaluated and compared. Multiple characterizations revealed that the catalytic performance was highly dependent on the phase composition. The Mn15Fe15−Ce/Al sample with the Mn/Fe molar ratio of 1.0 presented the optimal structure characteristic among all tested samples, with the largest surface area, increased active components distributions, the reduced crystallinity and diminished particle sizes. In the meantime, the ratios of Mn4+/Mnn+, Fe2+/Fen+ and Ce3+/Cen+ in Mn15Fe15−Ce/Al samples were improved, which could enhance the redox capacity and increase the quantity of chemisorbed oxygen and oxygen vacancy, thus facilitating NO oxidation into NO2 and eventually promoting the fast SCR reaction. In accord with the structure results, the Mn15Fe15−Ce/Al sample exhibited the highest NO oxidation rate of 64.2% at 350 ◦C and the broadest temperature window of 75–350 ◦C with the NOx conversion >90%. Based on the structure–activity relationship discussion, the catalytic mechanism over the Mn−Fe−Ce ternary components supported by γ-Al2O3 were proposed. Overall, it was believed that the optimization of Mn/Fe ratio in Mn−Fe−Ce/Al nanocatalyst was an extremely effective method to improve the structure–activity relationships for NO pre-oxidation and the fast SCR reaction.


Introduction
Nitrogen oxides (NO x ) are one of the strong contributing factors of air pollutants, which result in acid rain, global warming and ozone depletion via photochemical redox [1].The selective catalytic reduction (SCR) of NO x by NH 3 or urea over high efficient catalysts is the most economical and effective technology to eliminate the pollution of NO x [2,3].The main SCR reaction involves the typical standard SCR reaction (1), the fast SCR reaction (2) and the NO 2 SCR reaction (3) [4,5]: (1) The catalytic mechanism of the main SCR reaction has been systematically researched during the recent years, and the reaction processes of fast SCR have also been proposed in detail [4].Koebel et al. [6] revealed the fast SCR reaction rate was decupled larger than that of standard SCR under 200 • C.However, the typically components of NO x were NO 2 at ~5% and NO at ~95% [2], and thus the oxidation of NO into NO 2 previous to the main SCR progress is noteworthy so as to increase the molar ratio of NO 2 /NO to 1.0 for facilitating fast SCR reaction artificially.
In the SCR reaction progress, the optimized catalyst takes an important role to the NO x removal with NH 3 .The V 2 O 5 /TiO 2 catalyst, promoted with WO 3 or MoO 3 usually, is the most widely used commercial catalyst.The disadvantage of V 2 O 5 −WO 3 (MoO 3 )/TiO 2 is the strict temperature window limit [7].These vanadium-based catalysts are not efficient enough to eliminate NO x as the catalytic temperature below 250 • C. In recent years, the catalysts suitable for the low temperature SCR have become required, and are appropriately installed in a downstream electrostatic precipitator and desulfurizer [8].Many research groups dedicated to utilizing high efficient active elements and structure supports to optimize low-temperature SCR catalysts with super activities, excellent stabilities and wide-ranging temperature windows.
During the past few years, a large amount of low-temperature SCR catalysts making up of transition metal oxides on multifarious supports have been investigated.The Mn−Fe−O x based catalysts, in particular, exhibited remarkable catalytic activities, such as Mn−Fe [9], Mn−Fe/TiO 2 [10] and V 2 O 5 −Mn−Fe attapulgite [3].Meanwhile, it was found that Mn−Fe−O x based catalysts could induce the oxidation reaction of NO into NO 2 during the NH 3 -SCR progress [11,12].On this basis, some researchers have recently been absorbed oxidizing NO into NO 2 , due to the more high-efficiency reaction of NO 2 with NH 3 than NO [13].Zhang et al. [13] proved the NO oxidation into NO 2 or bidentate nitrite took place on the surface of FeMnO x /TiO 2 via the adsorbed oxygen.Fang et al. [14] revealed the high density of lattice oxygen in the noncrystalline Mn−Fe−O x played a leading and main role in NO oxidation.But Mn−Fe−O x based catalysts usually needed other active elements to improve their catalytic selectivity and SO 2 resistance [15].Among various promoter, ceria performed extremely selectivity and SO 2 resistance for the low-temperature SCR in Mn−Fe−O x based catalysts [16].At the same time, ceria as alkaline material with excellent redox abilities to adsorb and desorb active oxygen, also exhibited promoting effects on NO oxidization into NO 2 and NO 2 absorption into nitrites or nitrates [17].However, the effects of Fe/Mn ratio on the activity of Mn−Fe−Ce−O x catalyst for NO oxidation and the fast SCR reaction have not been intensive researched, especially the combined effects of γ-Al 2 O 3 as carriers.The γ-Al 2 O 3 is an outstanding support material for NO oxidation due to the Brönsted acid sites on its surface besides its prominent surface area [18], remarkable mechanical strength [19], great thermal stability [1] and low production cost [20].
In this research, we systematically manufactured a series of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts with a Mn atomic ratio (Fe balance) from 0% to 100%.Their catalytic abilities of NO oxidization and the fast SCR were researched to understand the Mn/Fe ratio effects on the reactions.The physicochemical characteristics of the nanocatalyst samples were analyzed by N 2 adsorption, scanning electron microscopy (SEM) mapping, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H 2 -temperature-programmed reduction (H 2 -TPR), NH 3 -temperature-programmed desorption (NH 3 -TPD) and NO-TPD, in order to reveal the structure-activity relationship.The mechanism of Mn/Fe ratio influence on Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts was also discussed.The purpose of this work was to clarify the nature of Mn−Fe−Ce based materials for catalytic performance and to explore the possibility of manufacturing nanocatalysts with outstanding capabilities in both the NO oxidization into NO 2 and the fast SCR reaction progress.

Brunauer-Emmett-Teller (BET) Measurements
In order to compare the change of physical properties of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts with different Mn/Fe ratios, the test data of BET specific surface areas, pore volumes and pore diameters were summarized in Table 1.The specific surface area and pore volume of Fe30−Ce/Al sample (Mn(x)Fe(y)−Ce/Al for short, x and y represented the mass percentage of Mn and Fe in the nanocatalysts, respectively) were originally 58.2 m 2 /g and 0.33 cm 3 /g, respectively.As the Mn/Fe molar ratio reached 0.5, the specific surface area rose to 77.4 m 2 /g and the pore volume rose to 0.56 cm 3 /g.The maximum specific surface area of 122.7 m 2 /g and the maximum pore volume of 0.73 cm 3 /g were both achieved over Mn15Fe15−Ce/Al with the Mn/Fe molar ratio of 1.0, which was possible due to manganese addition-enhancing active components better dispersing on the nano-Al 2 O 3 support [18,21].The manganese addition into Fe−Ce−O x /γ-Al 2 O 3 promoted mesoporosity formation and at the same time suppressed the macroporosity formation, which resulted in remarkable improvements in pore volumes [12].Further increasing the Mn/Fe molar ratio, both the specific surface area and the pore volume started declining notably.The typical micrographs of SEM, TEM and Mappings of the as-prepared Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts with different Mn/Fe ratios were presented in Figure 1.In the SEM images shown in Figure 1a-c, the Mn10Fe20−Ce/Al sample particles were irregular.There were lots of stacking particles distributing on the catalyst surface with abundant pores collapsing produced by the fast evaporation of crystal water [22].While the Mn15Fe15−Ce/Al sample shaped unbroken mesoporosity mesh structure on the catalyst surface.The detailed morphology structures of Mn15Fe15−Ce/Al nanoparticle were further analyzed by TEM, as revealed in Figure 1d.The Mn15Fe15−Ce/Al sample exhibited fine uniform elliptic particles with a narrow size distribution and without hard aggregate.As the Mn/Fe molar ratio rising to 2.0, the Mn20Fe10−Ce/Al sample exhibited spinel microstructure with the nanoparticle size increasing notably.According to the element mappings of the Mn15Fe15−Ce/Al sample as displayed in Figure 1e-j, it was obvious that the corresponding components of manganese, iron and cerium species were highly dispersive on the catalyst surface without regional accumulations or crystallizations.

X-Ray Diffraction (XRD) Analysis
The XRD results of Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts with different Mn/Fe ratios are exhibited in Figure 2. It was obvious that the diffraction peaks in XRD patterns matched γ-Al2O3 with several strong and distinguished peaks at 32.3°, 37.0°, 45.3°, and 67.0° (International Centre for Diffraction Data (ICDD) PDF card # 79-1558) [23].Although the peaks for the structure of Al2O3 support remained complete with different Mn/Fe molar ratios, the diffraction angles of corresponding peaks shifted towards lower angles at varying degrees.Among all tested catalysts, the Mn15Fe15−Ce/Al sample obtained the lowest diffraction angles for each peak, which manifested the potential interaction among MnOx, FeOx, CeOx and Al2O3.For the curve of the Mn10Fe20−Ce/Al sample, the diffraction peaks coinciding with Fe2O3 at 2θ = 23.7°,42.3°, 53.3°, and 63.0°, in accordance with (012), (020), ( 116) and (300) crystallographic plane reflections, respectively (ICDD PDF card # 88-2359) [24].While the diffraction peaks of Fe3O4 or FeO were not found in the Mn10Fe20−Ce/Al sample.Comparing the curves of the Mn15Fe15−Ce/Al and Mn20Fe10−Ce/Al samples with that of Mn10Fe20−Ce/Al sample, it could be found that the diffraction peaks of Fe2O3 were lowered as the Mn/Fe molar ratio increased, which probably indicated the addition of manganese into Fe−Ce−Ox/γ-Al2O3 decreased the crystallization of Fe2O3.As for the Mn20Fe10−Ce/Al sample with Mn/Fe molar ratio of 2.0, the diffraction peaks coinciding with MnOx were very complex.The characteristic peaks at 37.5°, 42.5°, 53.8°, and 64.6° could be ascribed to MnO2 (ICDD PDF card # 89-5171), the peaks at 32.3° and 55.1° attributed to Mn2O3 (ICDD PDF card # 24-0508), and the peaks at 33.5°, 36.4° and 59.7° assigned to Mn3O4 (ICDD PDF card # 89-4837) [25].At the same time, the peaks matched Fe2O3 remarkably weakened and no peaks of other FeOx appeared.
In the curve of Mn15Fe15−Ce/Al sample, there were no evident distinct diffraction peaks of FeOx or MnOx were detected.These indicated the appropriate Mn/Fe molar ratio not only facilitated the dispersion of FeOx entirely, but also enhanced the dispersion of MnOx completely on the catalyst surface.As a result, the coexistence of iron oxides and manganese oxides increased the active species distributions, minished the particle size and reduced the crystallinity.The smaller size particles of active element species were beneficial to the fast SCR reaction [26], but the larger oxide clusters promoted NH3 oxidation and restrained fast SCR efficiency [27].So, the better distribution of FeOx and MnOx on the nanocatalyst surface had a significant impact on the fast SCR reaction.Furthermore, no evident peaks for crystalline of CeO2 or Ce2O3 were observed with Mn/Fe molar ratio of 0.5~2.0 in this work, which suggested the CeOx might be highly dispersed on the surface of Mn−Fe−Ce/Al or the crystal diameters of CeOx were too small (<5 nm) to be distinguished.

X-Ray Diffraction (XRD) Analysis
The XRD results of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts with different Mn/Fe ratios are exhibited in Figure 2. It was obvious that the diffraction peaks in XRD patterns matched γ-Al 2 O 3 with several strong and distinguished peaks at 32.3 • , 37.0 • , 45.3 • , and 67.0 • (International Centre for Diffraction Data (ICDD) PDF card # 79-1558) [23].Although the peaks for the structure of Al 2 O 3 support remained complete with different Mn/Fe molar ratios, the diffraction angles of corresponding peaks shifted towards lower angles at varying degrees.Among all tested catalysts, the Mn15Fe15−Ce/Al sample obtained the lowest diffraction angles for each peak, which manifested the potential interaction among MnO x , FeO x , CeO x and Al 2 O 3 .For the curve of the Mn10Fe20−Ce/Al sample, the diffraction peaks coinciding with Fe 2 O 3 at 2θ = 23.7 • , 42.3 • , 53.3 • , and 63.0 • , in accordance with (012), (020), (116) and (300) crystallographic plane reflections, respectively (ICDD PDF card # 88-2359) [24].While the diffraction peaks of Fe 3 O 4 or FeO were not found in the Mn10Fe20−Ce/Al sample.Comparing the curves of the Mn15Fe15−Ce/Al and Mn20Fe10−Ce/Al samples with that of Mn10Fe20−Ce/Al sample, it could be found that the diffraction peaks of Fe 2 O 3 were lowered as the Mn/Fe molar ratio increased, which probably indicated the addition of manganese into Fe−Ce−O x /γ-Al 2 O 3 decreased the crystallization of Fe 2 O 3 .As for the Mn20Fe10−Ce/Al sample with Mn/Fe molar ratio of 2.0, the diffraction peaks coinciding with MnO x were very complex.The characteristic peaks at 37.5 • , 42.5 • , 53.8 • , and 64.6 • could be ascribed to MnO 2 (ICDD PDF card # 89-5171), the peaks at 32.3 • and 55.1 • attributed to Mn 2 O 3 (ICDD PDF card # 24-0508), and the peaks at 33.5 • , 36.4 • and 59.7 • assigned to Mn 3 O 4 (ICDD PDF card # 89-4837) [25].At the same time, the peaks matched Fe 2 O 3 remarkably weakened and no peaks of other FeO x appeared.
In the curve of Mn15Fe15−Ce/Al sample, there were no evident distinct diffraction peaks of FeO x or MnO x were detected.These indicated the appropriate Mn/Fe molar ratio not only facilitated the dispersion of FeO x entirely, but also enhanced the dispersion of MnO x completely on the catalyst surface.As a result, the coexistence of iron oxides and manganese oxides increased the active species distributions, minished the particle size and reduced the crystallinity.The smaller size particles of active element species were beneficial to the fast SCR reaction [26], but the larger oxide clusters promoted NH 3 oxidation and restrained fast SCR efficiency [27].So, the better distribution of FeO x and MnO x on the nanocatalyst surface had a significant impact on the fast SCR reaction.Furthermore, no evident peaks for crystalline of CeO 2 or Ce 2 O 3 were observed with Mn/Fe molar ratio of 0.5~2.0 in this work, which suggested the CeO x might be highly dispersed on the surface of Mn−Fe−Ce/Al or the crystal diameters of CeO x were too small (<5 nm) to be distinguished.

X-Ray Photoelectron Spectroscopy (XPS) Analysis
In fast SCR reaction, the components of catalyst surface and the oxidation states of active species had significant effects on the catalytic activity [5].For the purpose of exploring the atomic chemical valences and revealing the element concentrations on the catalyst surface, XPS analysis of Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts was undertaken.The XPS spectra of Mn 2p, Fe 2p, Ce 3d, and O 1s in the nanocatalysts were exhibited in Figure 3.The valence states of each element were confirmed numerically by Gaussian fitting, respectively.According to these fittings, the relevant binding energy and the respective atomic concentration of elements in diverse valence states on Mn−Fe−Ce−Ox/γ-Al2O3 surface are shown in Table 2.

X-Ray Photoelectron Spectroscopy (XPS) Analysis
In fast SCR reaction, the components of catalyst surface and the oxidation states of active species had significant effects on the catalytic activity [5].For the purpose of exploring the atomic chemical valences and revealing the element concentrations on the catalyst surface, XPS analysis of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts was undertaken.The XPS spectra of Mn 2p, Fe 2p, Ce 3d, and O 1s in the nanocatalysts were exhibited in Figure 3.The valence states of each element were confirmed numerically by Gaussian fitting, respectively.According to these fittings, the relevant binding energy and the respective atomic concentration of elements in diverse valence states on Mn−Fe−Ce−O x /γ-Al 2 O 3 surface are shown in Table 2.

X-Ray Photoelectron Spectroscopy (XPS) Analysis
In fast SCR reaction, the components of catalyst surface and the oxidation states of active species had significant effects on the catalytic activity [5].For the purpose of exploring the atomic chemical valences and revealing the element concentrations on the catalyst surface, XPS analysis of Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts was undertaken.The XPS spectra of Mn 2p, Fe 2p, Ce 3d, and O 1s in the nanocatalysts were exhibited in Figure 3.The valence states of each element were confirmed numerically by Gaussian fitting, respectively.According to these fittings, the relevant binding energy and the respective atomic concentration of elements in diverse valence states on Mn−Fe−Ce−Ox/γ-Al2O3 surface are shown in Table 2.It can be seen from Figure 3a that the XPS spectra of Mn 2p in the investigated nanocatalyst samples were matched with two constituents, attributed to Mn 2p 3/2 (peak at around 642 eV) and Mn 2p 1/2 (peak at about 653 eV) existing in MnO x simultaneously [28].The dissymmetric peak of Mn 2p 3/2 further confirmed the complex co-existence of manganese species in various states.The spectra of Mn 2p 3/2 could be split into three peaks via the peak-fitting deconvolution.As reported in previous studies [22], the first peak at around 641.0 ± 0.3 eV associated with MnO, the second one at 642.2 ± 0.3 eV is consistent with Mn 2 O 3 , and the third one at 644.2 ± 0.3 eV was assigned to MnO 2 , respectively.It was apparent that the three valence states of MnO x were difficult to separate within the binding energy difference value of 3.3 eV.The surface atomic compositions and relative intensities of Mn n+ on the catalyst surface were calculated on account of the area covered under the separated peaks, as summarized in Table 2.With the Mn/Fe molar ratio increasing from 0.5 to 1.0, the Mn 2+ concentration on the sample surface reduced obviously from 33.9% to 6.8%; by contrast, the concentration of Mn 4+ rose significantly from 29.9% to 53.8% which became larger than that of Mn 3+ .From the above results, it was proposed that the principal phase of manganese species in the Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts changed from Mn 2 O 3 to MnO 2 .This was also possibly ascribed to the major amount of Mn mainly existing as +4 state covering dispersedly on the catalyst surface.Keeping on increasing the Mn/Fe molar ratio to 2.0, the percentage of Mn 4+ in Mn n+ began to decline, at the same time the Mn 3+ /Mn n+ ratio increased remarkably.The NO catalytic activity of pure manganese oxides had been ranked as MnO 2 > Mn 2 O 3 > Mn 3 O 4 [25].Furthermore, there were studies revealed that the increased concentration of Mn 4+ on the catalyst surface was beneficial to SCR reactions [29].Consequently, it could be expected that the Mn15Fe15−Ce/Al nanocatalyst would provide a superior fast SCR activity compared to Mn10Fe20−Ce/Al and Mn20Fe10−Ce/Al samples.
The Fe 2p XPS spectra of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts are displayed in Figure 3b with two distinctive peaks presented for Fe 2p 3/2 (709.1-712.0eV) and Fe 2p 1/2 (723.8-725.0eV).The broad peak centered at 711.0 eV included two overlapped peaks, the first one at about 709.4 ± 0.4eV was related to Fe 2+ and the second one at around 711.8 ± 0.5eV was ascribed to Fe 3+ .Meanwhile, a satellite peak of Fe 3+ in Fe 2 O 3 appeared at 718.4 eV [10].The overlapping peaks manifested the coexistence of iron in +2 and +3 states on the catalyst surface, which were quantified in Table 2.With the Mn/Fe molar ratio increasing from 0.5 to 1.0, the percentage of Fe 3+ in Fe n+ decreased, while the Fe 2+ concentration was improved.These could be attributed to the synergistic effects taking place in the redox equilibrium between Fe and Mn: Fe 3+ + Mn 3+ ↔ Fe 2+ + Mn 4+ [30].
Figure 3c showed the Ce 3d spectra results for Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts.The Ce 3d curve contained u and v multiplets conforming to the spin orbit split 3d 5/2 and 3d 3/2 core holes, which were further split into eight peaks on the basis of binding energy, labeled as u 0 , u , u , u and v 0 , v , v , v , respectively [31].The two peaks labeled u and v were attributed to the 3d 10 4f 1 initial electronic state of Ce 3+ , and the other six bands corresponded to the 3d 10 4f 0 state of Ce 4+ [32].These indicated the species of Ce 3+ and Ce 4+ coexisted on the surface of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts.As shown in Table 2, it was obvious that the Ce 3+ /Ce n+ ratio of Mn15Fe15−Ce/Al sample was 33.8%, as calculated by Equation (4) [33], which was greater than that of the other two catalyst samples.Therefore, the Mn/Fe molar ratio of 1.0 might be positive to the conversion from Ce 4+ to Ce 3+ , which resulted in an improvement of Ce 3+ species on its surface.
The generated species of Ce 3+ were significant inducements for the formation of electric charge balance, the generation of unsaturated chemical bonds, the improvement of surface oxygen vacancies, and the development of more chemisorbed oxygen, which would be advantageous to the activate reactants adsorption or intermediate species transformations [5].As the temperature was under 200 • C, the main catalytic reactions followed the Eley-Rideal mechanism.NH 3 absorbed on active sites to generate NH 2 and −OH with oxygen.Subsequently, the achieved NH 2 further reacted with NO forming intermediate NH 2 NO, and finally the intermediate species decomposed into N 2 and H 2 O [34].Thus, it was proposed that the quantity improvement of oxygen vacancies on the catalyst was conducive to facilitating the intermediate formation and enhancing catalytic performance in the fast SCR reaction.In the crystal lattice of Mn15Fe15−Ce/Al sample, the negative charge transferred from manganese to cerium intensifying the interaction between Mn and Ce: Mn 2+ + Ce 4+ ↔ Mn 3+ + Ce 3+ [16,35], Mn 3+ + Ce 4+ ↔ Mn 4+ + Ce 3+ [36,37].The course of oxygen storage and release on the Ce 3+ /Ce 4+ redox couple was brought forward: Ce 2 O 3 + 1/2O 2 → 2CeO 2 and 2CeO 2 → Ce 2 O 3 + O* (adsorbed) [36].Overall, it could be easier for the catalysts with more Ce 3+ species on the surface to develop surface oxygen vacancies, which was beneficial to the oxygen adsorption to achieve chemisorbed oxygen [10].
The XPS spectra of O 1s for Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts are illustrated in Figure 3d.According to the curve-fitting results, the O 1s spectra was split into two peaks: the first peak labeled by O α was ascribed to lattice oxygen appearing at binding energy of 529.5-530.3eV, the second peak denoted as O β was attributed to chemisorbed oxygen on the catalyst surface at the binding energy of 531-532 eV.Compared with the XPS spectra of the Mn10Fe20−Ce/Al sample, both binding energies of O α and O β in the Mn15Fe15−Ce/Al and Mn20Fe10−Ce/Al samples shifted towards lower values, which might be caused by the existence of more Mn 4+ species [9].Furthermore, the surface concentration of chemisorbed oxygen of the Mn15Fe15−Ce/Al sample reached 37.4% as shown in Table 2, which was much larger than that of the Mn10Fe20−Ce/Al and Mn20Fe10−Ce/Al samples.The chemisorbed oxygen with high mobility was regard as the most active oxygen species in the SCR process, facilitating NO oxidation to NO 2 and promoting the fast SCR reaction in the gas mixture:

H 2 -Temperature-Programmed Reduction (H 2 -TPR) Analysis
The reduction behavior of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts were analyzed by H 2 -TPR and the curves were matched by Gaussian functions as shown in Figure 4.For each of the three nanocatalyst samples, there were five main hydrogen consumption peaks locating in the range from 100 to 900 • C. Since the support of alumina had no obvious H 2 consumption peaks in the temperature range [38], all the peaks exhibited in Figure 4 could be associated with the reduction of different species of MnO x , FeO x and CeO x .For Mn10Fe20−Ce/Al sample, the initial wider reduction peak (P1) from 160 • C to 440 • C was assigned to the high oxidation state of manganese ion transformation from MnO 2 reducing to Mn 2 O 3 [15].Considering the existence of Fe 2 O 3 crystallization had been confirmed by XRD analysis, the second prominent reduction peak (P2) centered at 366 • C could be mainly attributed to Mn 2 O 3 species combining with the majority of Fe 2 O 3 to Fe 3 O 4 , where iron species were settled in easily reducible sites in the form of isolated ions, oligomeric clusters or nanoparticles [39].According to a previous report [40], the Mn 2 O 3 reduction consists of two processes, Mn 2 O 3 to Mn 3 O 4 and Mn 2 O 3 to MnO.The reduction from Mn 2 O 3 to Mn 3 O 4 happened more easily on the initial amorphous Mn 2 O 3 , and the conversion from Mn 2 O 3 to MnO occurred more easily at the higher temperature range.Therefore, the third reduction peak (P3) centered at 434 • C was due to the reduction process of Mn 2 O 3 to MnO and the reduction of residual Fe 2 O 3 to Fe 3 O 4 .Meanwhile, the fourth reduction peak (P4) at around 643 • C and the fifth one (P5) at approximately 761 • C were potentially associated with the reduction process of Fe 3 O 4 to FeO and FeO to Fe 0 in isolated ions, respectively [41].potentially associated with the reduction process of Fe3O4 to FeO and FeO to Fe 0 in isolated ions, respectively [41].In comparison, the redox characteristic of Mn15Fe15−Ce/Al sample was significantly different from that of Mn10Fe20−Ce/Al sample.The H2-TPR curve of Mn15Fe15−Ce/Al sample exhibited as a disequilibrium bimodal pattern, and the two main reduction peaks were at around 279 °C and 353 °C, respectively.It was rather remarkable that the MnO2 reduction peak and the Fe2O3 reduction peak varied drastically with the Mn/Fe molar ratio increasing from 0.5 to 1.0.As shown in Figure 4c, the increasing Mn/Fe molar ratio resulted in the significant promotion of reducibility at a low temperature range.The reduction peak P1 was much stronger and broader with the hydrogen consumption maximum obtained at a lower temperature region.Comparing the H2-TPR curves of Mn15Fe15−Ce/Al sample with that of Mn10Fe20−Ce/Al sample, it was noteworthy that all the reduction peaks of MnOx and FeOx shifted towards the lower temperature regions, signifying enhanced catalytic activities at lower temperature.Similar results were achieved by Wang et al. [12] that confirmed the Mn-Fe mixtures leading to lower temperature for Fe3O4 reducing to FeO and FeO converting to Fe 0 at the same time.However, as the Mn/Fe molar ratio increased to 2.0 in the Mn20Fe10−Ce/Al sample, the reduction peaks involving MnOx and FeOx (P1, P2 and P3) moved to higher temperature section slightly, while the reduction peaks only containing FeOx (P4 and P5) still shifted towards lower temperature region.In consideration of the crystallization of MnO2, Mn2O3 and Mn3O4 observed in the XRD results as shown in Figure 2, it was supposed that the actual Mn/Fe molar In comparison, the redox characteristic of Mn15Fe15−Ce/Al sample was significantly different from that of Mn10Fe20−Ce/Al sample.The H 2 -TPR curve of Mn15Fe15−Ce/Al sample exhibited as a disequilibrium bimodal pattern, and the two main reduction peaks were at around 279 • C and 353 • C, respectively.It was rather remarkable that the MnO 2 reduction peak and the Fe 2 O 3 reduction peak varied drastically with the Mn/Fe molar ratio increasing from 0.5 to 1.0.As shown in Figure 4c, the increasing Mn/Fe molar ratio resulted in the significant promotion of reducibility at a low temperature range.The reduction peak P1 was much stronger and broader with the hydrogen consumption maximum obtained at a lower temperature region.Comparing the H 2 -TPR curves of Mn15Fe15−Ce/Al sample with that of Mn10Fe20−Ce/Al sample, it was noteworthy that all the reduction peaks of MnO x and FeO x shifted towards the lower temperature regions, signifying enhanced catalytic activities at lower temperature.Similar results were achieved by Wang et al. [12] that confirmed the Mn-Fe mixtures leading to lower temperature for Fe 3 O 4 reducing to FeO and FeO converting to Fe 0 at the same time.However, as the Mn/Fe molar ratio increased to 2.0 in the Mn20Fe10−Ce/Al sample, the reduction peaks involving MnO x and FeO x (P1, P2 and P3) moved to higher temperature section slightly, while the reduction peaks only containing FeO x (P4 and P5) still shifted towards lower temperature region.In consideration of the crystallization of MnO 2 , Mn 2 O 3 and Mn 3 O 4 observed in the XRD results as shown in Figure 2, it was supposed that the actual Mn/Fe molar ratio in the MnFeO x nano particle was reduced, and the dominant reduction peak P1 was mainly caused by the reduction of crystallographic MnO 2 [11,30].
The redox ability of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts could be determined by the starting reduction peak temperature [42], which followed the order of Mn15Fe15−Ce/Al > Mn20Fe10−Ce/Al > Mn10Fe20−Ce/Al.This indicated that the suitable Mn/Fe molar ratio enhanced the redox activities of MnO x and FeO x , and was further confirmed by the total H 2 consumption as shown in Table 3.As the Mn/Fe molar ratio increased from 0.5 to 2.0, the total H 2 consumption rose from 4.93 mmol/g to 6.11 mmol/g, and then declined to 5.97 mmol/g.On account of the redox property being an important factor affecting the fast SCR reaction, it was reasonable that the Mn15Fe15−Ce/Al sample exhibited higher NO oxidation rate and fast SCR reaction activity than the Mn20Fe10−Ce/Al and Mn10Fe20−Ce/Al samples.Meanwhile, it could be noticed that there were no distinguishable reduction peaks corresponds to cerium species.According to previous research [43], the typical reduction peaks at around 450 • C and 740 • C could be associated with the surface Ce 4+ converting to Ce 3+ and the bulk Ce 4+ transforming to Ce 3+ , respectively.In this work, the reduction peaks of cerium species were very weak due to the low-quality content of cerium.At the same time, the characteristic reduction peaks of cerium were overlapped with the reduction process of Mn 2 O 3 to MnO and Fe 2 O 3 to Fe 3 O 4 at 380~500 • C (P3), and covered by the FeO to Fe 0 reduction at 700~800 • C (P5). Figure 5 provides a graphical representation of the reduction process, in which each active component is embodied qualitatively.
Overall, on the basis of the reduction process analyzed above, the optimal Mn/Fe molar ratio for Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts was 1.0 with increased H 2 consumption and lower reduction peak temperature.This indicated that the reinforced reducibility in Mn15Fe15−Ce/Al sample was potentially due to the strong interaction between MnO x and FeO x with the best material proportion.ratio in the MnFeOx nano particle was reduced, and the dominant reduction peak P1 was mainly caused by the reduction of crystallographic MnO2 [11,30].The redox ability of Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts could be determined by the starting reduction peak temperature [42], which followed the order of Mn15Fe15−Ce/Al > Mn20Fe10−Ce/Al > Mn10Fe20−Ce/Al.This indicated that the suitable Mn/Fe molar ratio enhanced the redox activities of MnOx and FeOx, and was further confirmed by the total H2 consumption as shown in Table 3.As the Mn/Fe molar ratio increased from 0.5 to 2.0, the total H2 consumption rose from 4.93 mmol/g to 6.11 mmol/g, and then declined to 5.97 mmol/g.On account of the redox property being an important factor affecting the fast SCR reaction, it was reasonable that the Mn15Fe15−Ce/Al sample exhibited higher NO oxidation rate and fast SCR reaction activity than the Mn20Fe10−Ce/Al and Mn10Fe20−Ce/Al samples.Meanwhile, it could be noticed that there were no distinguishable reduction peaks corresponds to cerium species.According to previous research [43], the typical reduction peaks at around 450 °C and 740 °C could be associated with the surface Ce 4+ converting to Ce 3+ and the bulk Ce 4+ transforming to Ce 3+ , respectively.In this work, the reduction peaks of cerium species were very weak due to the low-quality content of cerium.At the same time, the characteristic reduction peaks of cerium were overlapped with the reduction process of Mn2O3 to MnO and Fe2O3 to Fe3O4 at 380~500 °C (P3), and covered by the FeO to Fe 0 reduction at 700~800 °C (P5).Figure 5 provides a graphical representation of the reduction process, in which each active component is embodied qualitatively.
Overall, on the basis of the reduction process analyzed above, the optimal Mn/Fe molar ratio for Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts was 1.0 with increased H2 consumption and lower reduction peak temperature.This indicated that the reinforced reducibility in Mn15Fe15−Ce/Al sample was potentially due to the strong interaction between MnOx and FeOx with the best material proportion.

NH 3 -Temperature-Programmed Desorption (NH 3 -TPD) and NO-TPD Analysis
Except for the satisfactory redox behavior, it was another pivotal factor that the acid sites on the catalyst surface were sufficient to enhance the catalytic activities in the SCR reaction [9,17].For the purpose of intensive investigation of the complicated relationship between the surface acidity and the activities, NH 3 -TPD and NO-TPD experiments were performed on Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts and the corresponding curves according to data analysis were showed in Figures 6 and 7 Except for the satisfactory redox behavior, it was another pivotal factor that the acid sites on the catalyst surface were sufficient to enhance the catalytic activities in the SCR reaction [9,17].For the purpose of intensive investigation of the complicated relationship between the surface acidity and the activities, NH3-TPD and NO-TPD experiments were performed on Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts and the corresponding curves according to data analysis were showed in Figures 6 and  7, respectively.The NH3-TPD patterns for these catalysts were ascribed to three desorption peaks of chemisorbed NH3 in the temperature range from 120 to 700 °C.For the Mn10Fe20−Ce/Al sample, the first weak peak (P1) was ranged at around 213 °C, which was attributed to NH3 desorption from the weak acid sites.The second stronger peak (P2) positioned at about 383 °C approximately was ascribed to the medium-strong acid sites.The third dominating desorption peak (P3) at around 547 °C signified the distribution of strong acid sites, which exhibited a large amount of strong acid sites matching to the desorption of strongly bound ammonia on the potential Lewis acid sites [44].Compared with the NH3-TPD pattern of the Mn10Fe20−Ce/Al sample, the Mn15Fe15−Ce/Al sample displayed superior acidity property at the medium-and low-temperature regions, where all the three desorption peaks shifted towards lower temperatures and converted to much wider spans.The enhancements of medium-strong acid sites and weak acid sites were positive to ammonia adsorption, which was proposed that NH3 probably adsorbed on both Lewis acid sites and Brönsted acid sites on the catalyst surface [12].As the Mn/Fe molar ratio increased to 2.0 in the Mn20Fe10−Ce/Al sample, the NH3 desorption peaks moved further to the lower temperature region, and meanwhile the two strong peaks of medium-strong acid sites and weak acid sites became overlapped from 150 °C to 450 °C.The desorption peak of strong acid sites apparently reduced, which would be the reason for the decrease in fast SCR activity of Mn20Fe10−Ce/Al sample.For the purpose of accurately analyzing the total surface acidity, the desorption peaks of the Mn10Fe20−Ce/Al, Mn15Fe15−Ce/Al and Mn20Fe10−Ce/Al samples were quantitatively analyzed and are summarized in Table 4.The total NH3 consumption grew from 1.24 mmol/g to 1.61 mmol/g with the Mn/Fe molar ratio increasing from 0.5 to 1.0, which further confirmed the promotion effects of the Mn/Fe molar ratio on the surface acidity of nanocatalyst samples.These could be explained by the creation of more Brönsted acid sites on the catalyst surface [16].It was noteworthy that the Mn20Fe10−Ce/Al sample exhibited the best acidity properties at the low temperature, 0.56 mmol/g at 210 °C, which was beneficial to ammonia adsorption and advantageous to the low-temperature SCR reaction.However, it was found that the blocking effects of NH3 applied especially to the fast SCR reaction at low temperatures [45].The inhibiting effects were probably due to a competition of NH3 with NO for adsorption and activation onto the metal oxidizing centers on the catalyst surface, and a disadvantageous electronic interaction between the adsorbed NH3 and the metal sites [46].As such, the acidity property on the nanocatalyst surface was closely associated with the redox behavior.In order to obtain the optimal catalyst, it was desirable to seek an appropriate equilibrium among the surface components, the oxidation states of metal species and the acidity property.For the purpose of accurately analyzing the total surface acidity, the desorption peaks of the Mn10Fe20−Ce/Al, Mn15Fe15−Ce/Al and Mn20Fe10−Ce/Al samples were quantitatively analyzed and are summarized in Table 4.The total NH 3 consumption grew from 1.24 mmol/g to 1.61 mmol/g with the Mn/Fe molar ratio increasing from 0.5 to 1.0, which further confirmed the promotion effects of the Mn/Fe molar ratio on the surface acidity of nanocatalyst samples.These could be explained by the creation of more Brönsted acid sites on the catalyst surface [16].It was noteworthy that the Mn20Fe10−Ce/Al sample exhibited the best acidity properties at the low temperature, 0.56 mmol/g at 210 • C, which was beneficial to ammonia adsorption and advantageous to the low-temperature SCR reaction.However, it was found that the blocking effects of NH 3 applied especially to the fast SCR reaction at low temperatures [45].The inhibiting effects were probably due to a competition of NH 3 with NO for adsorption and activation onto the metal oxidizing centers on the catalyst surface, and a disadvantageous electronic interaction between the adsorbed NH 3 and the metal sites [46].As such, the acidity property on the nanocatalyst surface was closely associated with the redox behavior.In order to obtain the optimal catalyst, it was desirable to seek an appropriate equilibrium among the surface components, the oxidation states of metal species and the acidity property.
It was obvious that the NO-TPD patterns of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts as shown in Figure 7 are quite similar to the NH 3 -TPD patterns discussed above.With temperature rising from 120 to 750 • C, each of the three catalyst samples exhibited three desorption peaks of chemisorbed NO.The dominating desorption peak of Mn10Fe20−Ce/Al sample was at about 597 • C attributed to the strong Lewis acid sites.While the main desorption peaks of Mn15Fe15−Ce/Al and Mn20Fe10−Ce/Al samples were settled at around 420~430 • C, which were ascribed to the overlapped peaks of Lewis acid sites and Brönsted acid sites.With the Mn/Fe molar ratio increasing from 0.5 to 2.0, the NO desorption at low temperature apparently augmented from 0.32 mmol/g to 0.59 mmol/g, while NO desorption in the high-temperature region declined from 0.70 mmol/g to 0.39 mmol/g, which was supposed to be conducive to fast SCR at low temperature [17].

Effect of Mn/Fe Ratio on fast SCR Activity
The fast SCR activity of the prepared Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts with diverse Mn/Fe molar ratios at different reaction temperatures were illustrated in Figure 9.It was found that the Mn/Fe molar ratio had remarkable effects on the catalytic performances of Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts.Among all analyzed catalyst samples, the Mn15Fe15−Ce/Al sample exhibited the

Effect of Mn/Fe Ratio on fast SCR Activity
The fast SCR activity of the prepared Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts with diverse Mn/Fe molar ratios at different reaction temperatures were illustrated in Figure 9.It was found that the Mn/Fe molar ratio had remarkable effects on the catalytic performances of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts.Among all analyzed catalyst samples, the Mn15Fe15−Ce/Al sample exhibited the highest fast SCR activity in the whole tested temperature window, with NO x conversion above 90% at 75-350 • C. As illustrated in Figure 9a, the Fe30−Ce/Al sample showed the lowest catalytic activity in NO x conversion with the temperature rising from 25 • C to 200 • C. The addition of Mn into Fe−Ce/Al sample enhanced the fast SCR activity obviously.The appropriate amount of Mn doping enhanced the catalytic activity, while the excessive Mn doping caused a negative impact.When the Mn/Fe molar ratio grew to 0.5, the NO x conversion of Mn10Fe20−Ce/Al sample achieved approximately 90.3% at 150 • C. When the Mn/Fe molar ratio was larger than 1.0, the NO x conversion of Mn20Fe10−Ce/Al and Mn30−Ce/Al samples began to decline slightly.However, as the temperature changed within 250-350 • C, the variation tendency of NO x conversion with the Mn/Fe molar ratios increasing were entirely different.In the high temperature range, the Mn30−Ce/Al sample showed the lowest catalytic activity in NO x conversion, and the Mn20Fe10−Ce/Al sample had as high a catalytic activity as Mn15Fe15−Ce/Al.The reduction of NO x conversion was caused by the non-selective oxidation of NH 3 at high temperatures, which was probably due to the redox property enhancement produced by manganese addition [47].Moreover, the NO conversion performance of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts was similar to that of NO x conversion according to Figure 9b.
The NO 2 conversion of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts could be divided into two phases as shown in Figure 9c.As the temperature below 200 • C, the NO 2 conversion of all tested catalysts improved with the temperature increasing.In this temperature range, the NO 2 conversion of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts raised initially and then reduced with the Mn/Fe molar ratio increasing.The highest NO 2 conversion was obtained over the Mn15Fe15−Ce/Al sample with the Mn/Fe molar ratio of 1.0.As the temperature above 200 • C, the NO 2 conversion of all tested catalysts stabilized at approximately 99%.Based on the Figure 9, the fast SCR activity of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts could be ranked Fe30−Ce/Al < Mn10Fe20−Ce/Al < Mn30−Ce/Al < Mn20Fe10−Ce/Al < Mn15Fe15−Ce/Al.Generally, the variation tendency of the specific surface area and the pore volume with the Mn/Fe molar ratio increasing coincided with that of fast SCR activity.Thereby, it could be believed that the fast SCR activity of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts was bound up with the specific surface area as reported in prior studies [30,47].
From Figure 9d, it is obvious that the variation tendency of NH 3 conversion has a close resemblance to that of NO 2 conversion over Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts.In the range of 25-200 • C, the NH 3 conversion was slightly higher than the NO x conversion for each Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalyst at the same temperature.Meanwhile, the NH 3 conversion reached above 99% for all tested catalysts within the temperature range of 200-350 • C. According to the results exhibited in Figure 9e, it can be basically confirmed that Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts achieved satisfying N 2 selectivities.In the test temperature region, the Mn30−Ce/Al sample presented excellent N 2 selectivity that remained stable above 97.5%.The lowest N 2 selectivity was 88.3% obtained at 350 • C over the Mn15Fe15−Ce/Al sample.In order to better understand the enhancing effect of fast SCR on NO conversion than that of standard SCR, a contrast test was carried out under NO without NO 2 as shown in Figure 9f.Comparing the curves of Figure 9b,f, it could be confirmed that the participation of NO 2 in deNO x reations boosted the NO conversion at relatively low temperature and broadened the active temperature window.Therefore, it was regarded as an effective method to improve NO x conversion at low temperature by oxidizing NO to NO 2 and accelerating the fast SCR reaction.
selectivity that remained stable above 97.5%.The lowest N2 selectivity was 88.3% obtained at 350 °C over the Mn15Fe15−Ce/Al sample.In order to better understand the enhancing effect of fast SCR on NO conversion than that of standard SCR, a contrast test was carried out under NO without NO2 as shown in Figure 9f.Comparing the curves of Figure 9b,f, it could be confirmed that the participation of NO2 in deNOx reations boosted the NO conversion at relatively low temperature and broadened the active temperature window.Therefore, it was regarded as an effective method to improve NOx conversion at low temperature by oxidizing NO to NO2 and accelerating the fast SCR reaction.

Reaction Mechanism Analysis
Based on the experiment data of physicochemical characteristics and catalytic performances of the tested catalyst samples exhibited above, the Mn/Fe molar ratio in Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts influenced the oxidation and reduction characteristics of active chemisorbed sites notably, and changed the redox activities of active components significantly.

Reaction Mechanism Analysis
Based on the experiment data of physicochemical characteristics and catalytic performances of the tested catalyst samples exhibited above, the Mn/Fe molar ratio in Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts influenced the oxidation and reduction characteristics of active chemisorbed sites notably, and changed the redox activities of active components significantly.
The synergistic effect among manganese, iron and cerium increased the percent of Mn 4+ /Mn n+ , Fe 3+ /Fe n+ and Ce 3+ /Ce n+ , formed more lattice oxygen and plenty of oxygen vacancy on the catalyst surface [16].Among the three tested catalyst samples in this study, the Mn15Fe15−Ce/Al sample with the Mn/Fe molar ratio of 1.0 presented the highest probability of nitrate intermediate formation via the oxidization of NO into NO 2 , which was regarded as a pivotal catalytic step for accelerating the fast SCR reaction [47].The fast SCR process mainly comprised four catalytic reactions [5]. 3 interacted with NO to produce NO − 2 and NO 2 .Among these catalytic reactions, the NO transformation to NO 2 was considered as the crucial step in the reaction mechanism of fast SCR through the nitrate process.It had been revealed that MnO 2 was abundant in active oxygen and conducive to NO oxidation to NO 2 [37].Mn 4+ had the highest catalytic activity for NO elimination in consideration of the promotion effect on the reaction of NO oxidation to NO 2 , which accelerated the fast SCR reaction remarkably [30].Meanwhile, the MnO 2 phase presented better a catalytic property than Mn 2 O 3 due to its lattice structure defect [50].Therefore, Mn 4+ achieved the strongest redox ability comparing to the other valence states in MnO x -based catalysts [28].The increase of the Mn 4+ /Mn n+ ratio in the Mn15Fe15−Ce/Al sample indicated the species transformation from Mn 3+ and Mn 2+ to Mn 4+ , and the chemical circumstance variation among the Mn10Fe20−Ce/Al, Mn15Fe15−Ce/Al and Mn20Fe10−Ce/Al samples.This variation was owing to the powerful interaction among manganese, iron and cerium with different Mn/Fe molar ratios, which had been fully proved by the results exhibited above.The increase of Mn 4+ contained in the Mn15Fe15−Ce/Al sample was advantageous to NH 3 adsorption and beneficial to NO catalytic oxidation.The Mn 4+ =O in the catalyst could react with NH + 4 and NO to form Mn 3+ −OH and release N 2 and H 2 O.The formed Mn 3+ −OH could interact with NO − 3 to turn back to Mn 4+ =O and generate NO 2 and H 2 O.As a result, the highest Mn 4+ /Mn n+ ratio made a large contribution to intensify the fast SCR reaction [30].Therefore, the Mn15Fe15−Ce/Al sample presented superior catalytic ability than the Mn10Fe20−Ce/Al and Mn20Fe10−Ce/Al samples.
Furthermore, it is proposed that the redox couples of Fe 3+ /Fe 2+ and Ce 4+ /Ce 3+ contained in Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts strengthened the redox cycle of Mn 4+ /Mn 2+ and improve the amount of active oxygen, which would further promote the catalytic reactions [9,35].The percentage of Fe 2+ /Fe n+ and Ce 3+ /Ce n+ improved with the Mn/Fe molar ratio of 1.0, probably attributed to the strong electron transfer between Fe 2+ ↔ Mn 4+ [50], Ce 3+ ↔ Mn 3+ [35] and Ce 3+ ↔ Mn 4+ [37].Hence, it is proposed that the appropriate Mn/Fe molar ratio could develop the intimate electronic interaction among manganese, iron and cerium.Comprehensive considering the XPS, H 2 -TPR, NH 3 -TPD and NO-TPD analysis in this study, the Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalyst with the Mn/Fe molar ratio of 1.0 achieved the optimal physicochemical property in accordance with the catalytic performance.The possible redox catalytic pathway of fast SCR reaction over Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts is exhibited in Figure 10.

Catalysts' Preparation
The Mn−Fe−Ce/Al nanocatalyst samples were prepared by the co-precipitation method.

Catalysts' Preparation
The Mn−Fe−Ce/Al nanocatalyst samples were prepared by the co-precipitation method.Mn(NO 3 ) 2 (analytical pure 50%, Sinopharm, Shanghai, China), Fe(NO 3 ) 3 •9H 2 O (analytical pure 99.9%, Sinopharm, Shanghai, China), and Ce(NO 3 ) 3 •6H 2 O (analytical pure 99.9%, Nanjing-reagent, Nanjing China) were introduced as the precursors of MnO x , FeO x and CeO x , respectively.The nano γ-Al 2 O 3 (ultra pure 99.99%, Aladdin, Seattle, Washington, USA) was used as the carrier for the active species.The precursors were all dissolved into deionized water followed by the addition of ammonia solution till pH rose to 8.Then, the nano γ-Al 2 O 3 powder was put into the aqueous solution with continuous agitation to form a homogeneous gel.Subsequently, the gel was dried in N 2 at 150 • C for h and calcinated at 450 • C for 4 h.Lastly, the generated Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts were triturated and filtered with 40-60 mesh, at which sizes the diffusion effect on the catalytic performance was less than 0.5%.Hence, it could be regarded as no diffusional control with the catalytic activity calculations in this study.Meanwhile, the same size catalyst particles were used as the object for the characterization tests.In this contribution, a series of Mn−Fe−Ce−O x /γ-Al 2 O 3 samples with different Mn/Fe ratios were prepared and all samples were made up of (Mn+Fe) 30 wt% and Ce 5 wt%.The molar ratios of Mn:Fe were calculated on the basis of the mass ratios as displayed in Table 5.

Catalysts' Characterization
The Maxon Tristar II 3020 micropore-size analyzer (Micromeritics, Norcross, GA, USA) was introduced to test the N 2 adsorption isotherms of the nanocatalysts at −196 • C. The nanocatalysts were vacuum degassed at 350 • C for 10 h and then the surface areas and the pore-size distributions were measured.The specific surface areas were determined according to the BET plot linear portion.The pore-size distributions were calculated according to the desorption branch with the Barrett-Joyner-Halenda (BJH) formula.The XRD profiles were achieved by a Bruker D8 advance analyzer (Bruker, Billerica, MA, USA) with Mo K α radiation.The diffraction intensity was tested from 10 • to 90 • with point counting time of 1s and step of 0.02 • .International Center for Diffraction Data (ICDD) was used to distinguish the element phases in XRD patterns by comparing characteristic peaks.The surface image data on the nanocatalysts was captured by a FEI Quanta 400 FEG scanning electron microscope (FEI, Hillsboro, OR, USA), and the advanced microstructural and compositional information of the nanocatalysts was obtained by a high-resolution transmission electron microscope JEOL JEM-2010 (JEOL, Tokyo, Japan).The Micromeritics Autochem II 2920 chemical adsorption instrument (Micromeritics, Norcross, GA, USA) was employed to complete the H 2 -TPR, NH 3 -TPD and NO-TPD tests.In the H 2 -TPR experiment, 50 mg nanocatalyst was pretreatment in He at 400 • C for 1 h, then was cooled to ambient temperature in the gas mixture of H 2 and He at 30 mL/min.The H 2 consumptions were tested within 50-850 • C at the heating rate of 10 • C/min.The experimental procedure of the NH 3 -TPD and NO-TPD tests were quite similar to H 2 -TPR test, with NH 3 and NO replacing H 2 .XPS were analyzed via a Thermo ESCALAB 250XI under the pass energy of 46.95 eV, Al K α radiation at 1486.6 eV, X-ray source at 150 W and binding energy precision within ± 0.3 eV.C 1s line at 284.6 eV was introduced as a reference.

Figure 5 .
Figure 5. Graphical representation of the reduction process of Mn−Fe−Ce−Ox/γ-Al2O3 nanocatalysts with the reduction agent of H2 from 100 to 900 °C.

Figure 5 .
Figure 5. Graphical representation of the reduction process of Mn−Fe−Ce−O x /γ-Al 2 O 3 nanocatalysts with the reduction agent of H 2 from 100 to 900 • C.
, respectively.The NH 3 -TPD patterns for these catalysts were ascribed to three desorption peaks of chemisorbed NH 3 in the temperature range from 120 to 700 • C. For the Mn10Fe20−Ce/Al sample, the first weak peak (P1) was ranged at around 213 • C, which was attributed to NH 3 desorption from the weak acid sites.The second stronger peak (P2) positioned at about 383 • C approximately was ascribed to the medium-strong acid sites.The third dominating desorption peak (P3) at around 547 • C signified the distribution of strong acid sites, which exhibited a large amount of strong acid sites matching to the desorption of strongly bound ammonia on the potential Lewis acid sites [44].Compared with the NH 3 -TPD pattern of the Mn10Fe20−Ce/Al sample, the Mn15Fe15−Ce/Al sample displayed superior acidity property at the medium-and low-temperature regions, where all the three desorption peaks shifted towards lower temperatures and converted to much wider spans.The enhancements of medium-strong acid sites and weak acid sites were positive to ammonia adsorption, which was proposed that NH 3 probably adsorbed on both Lewis acid sites and Brönsted acid sites on the catalyst surface [12].As the Mn/Fe molar ratio increased to 2.0 in the Mn20Fe10−Ce/Al sample, the NH 3 desorption peaks moved further to the lower temperature region, and meanwhile the two strong peaks of medium-strong acid sites and weak acid sites became overlapped from 150 • C to 450 • C. The desorption peak of strong acid sites apparently reduced, which would be the reason for the decrease in fast SCR activity of Mn20Fe10−Ce/Al sample.Catalysts 2018, 8, x FOR PEER REVIEW 11 of 22
Firstly, the disproportionation of NO 2 formed NO − 2 and NO − 3 on the catalyst [48,49].Secondly, the formed NO − 2 reacted with NH + 4 composing the intermediate NH 4 NO 2 on the catalyst surface, and then NH 4 NO 2 decomposed rapidly into N 2 and H 2 O. Thirdly, the formed NO − 3 reacted with NH + 4 composing the intermediate NH 4 NO 3 and further reacted with NO to produce NO 2 , N 2 and H 2 O.At the same time, some formed NO −

Figure 10 .
Figure 10.The possible redox catalytic pathway of the fast SCR reaction over Mn−Fe−Ce−O x / γ-Al 2 O 3 nanocatalysts.

Table 1 .
Physical properties of the nanocatalysts with different Mn/Fe ratios.

Table 2 .
Surface atomic compositions of the catalysts determined by XPS.