Comprehensive Comparison between Nanocatalysts of Mn−Co/TiO₂ and Mn−Fe/TiO₂ for NO Catalytic Conversion: An Insight from Nanostructure, Performance, Kinetics, and Thermodynamics

Abstract

The nanocatalysts of Mn−Co/TiO₂ and Mn−Fe/TiO₂ were synthesized by hydrothermal method and comprehensively compared from nanostructures, catalytic performance, kinetics, and thermodynamics. The physicochemical properties of the nanocatalysts were analyzed by N₂ adsorption, transmission electron microscope (TEM), X-ray diffraction (XRD), H₂-temperature-programmed reduction (TPR), NH₃-temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). Based on the multiple characterizations performed on Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts, it can be confirmed that the catalytic properties were decidedly dependent on the phase compositions of the nanocatalysts. The Mn−Co/TiO₂ sample presented superior structure characteristics than Mn−Fe/TiO₂, with the increased surface area, the promoted active components distribution, the diminished crystallinity, and the reduced nanoparticle size. Meanwhile, the Mn⁴⁺/Mnⁿ⁺ ratios in the Mn−Co/TiO₂ nanocatalyst were higher than Mn−Fe/TiO₂, which further confirmed the better oxidation ability and the larger amount of Lewis acid sites and Bronsted acid sites on the sample surface. Compared to Mn−Fe/TiO₂ nanocatalyst, Mn−Co/TiO₂ nanocatalyst displayed the preferable catalytic property with higher catalytic activity and stronger selectivity in the temperature range of 75–250 °C. The results of mechanism and kinetic study showed that both Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism reactions contributed to selective catalytic reduction of NO with NH₃ (NH₃-SCR) over Mn−Fe/TiO₂ and Mn−Co/TiO₂ nanocatalysts. In this test condition, the NO conversion rate of Mn−Co/TiO₂ nanocatalyst was always higher than that of Mn−Fe/TiO₂. Furthermore, comparing the reaction between doping transition metal oxides and NH₃, the order of temperature−Gibbs free energy under the same reaction temperature is as follows: Co₃O₄ < CoO < Fe₂O₃ < Fe₃O₄, which was exactly consistent with nanostructure characterization and NH₃-SCR performance. Meanwhile, the activity difference of MnO_x exhibited in reducibility properties and Ellingham Diagrams manifested the promotion effects of cobalt and iron dopings. Generally, it might offer a theoretical method to select superior doping metal oxides for NO conversion by comprehensive comparing the catalytic performance with the insight from nanostructure, catalytic performance, reaction kinetics, and thermodynamics.

1. Introduction

Nitrogen oxides (NO_x) generated from fossil fuels are regarded as the primary reason for acid rain, ozone depletion, photochemical smog, and greenhouse effects [1]. Selective catalytic reduction of NO_x with NH₃ or urea as a reductant (NH₃-SCR) is proposed to be the most efficient method for eliminating NO_x from stationary source and mobile source [2]. In recent decades, the commercial catalyst of V₂O₅-WO₃(MoO₃)/TiO₂ used for NH₃-SCR process exhibited an excellent catalytic property in the typical standard SCR reaction within the temperature range of 300–400 °C [3,4].

$${4\mathrm{NO}+4\mathrm{NH}}_3{+\mathrm{O}}_2\to {4\mathrm{N}}_2{+6\mathrm{H}}_2\mathrm{O}$$

(1)

Nowadays, for the purpose of reducing the inhibiting effects of ash and SO₂ over the SCR catalysts, there is an intense interest in developing catalysts for NO reduction at lower temperature [5]. However, the V₂O₅−WO₃(MoO₃)/TiO₂ catalysts demand a strict temperature window, which limits their arrangement flexibility. The vanadium-based catalysts cannot reach satisfactory efficiency of eliminating NO_x as the reaction temperature lower than 250 °C. Hence, the catalysts appropriate to low temperature SCR are strongly desired, which could be placed at the downstream of electrostatic precipitator and desulfurizer [6]. A great deal of catalysts comprised of different transition metal oxides on various supports were analyzed to improve the catalytic ability for low-temperature deNO_x. The typical transition metals, such as Mn [7], Co [8], Fe [9], Ni [10], Zn [11], and Cr [9], were well-reported to display satisfactory properties at the low temperature. Among the various transition metal elements applied in the catalysts for NO_x reduction, manganese oxides display superior activity especially at the low temperature, which can be attributed to the various types of labile oxygen and high mobility of valence states [12]. Meanwhile, it was found that cobalt and iron species can combine with manganese to produce mixed nanoparticle-oxides and exhibit high SCR activity and excellent N₂ selectivity with a wide temperature window from 100—300 °C [13,14]. These mixed nanoparticle-oxides contain abundant oxygen vacancies on the catalyst surface, forming strong interaction bands at atomic scale, such as Mn-O-Fe [15,16] and Mn-O-Co [17]. Meanwhile, the active metal species of CoO_x and FeO_x are also regarded as the typical promoters for NO_x conversion, which serve as core catalyst components of active metal oxides, supplying surface oxygen to accelerate NO_x elimination [17,18].

Another crucial field of the investigation on NH₃-SCR process is the reaction mechanism on the active sites with a better understanding of the surface chemistry and developed kinetic models to evaluate the reactions. It was widely accepted that the main reaction of low temperature SCR complies with both the Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism, which is further depended on the reaction temperature and the catalyst components [19,20]. According to the Eley-Rideal mechanism, the reaction occurred between NO and NH₃ (adsorbed) to generate an activated transition state and further decomposed into H₂O and N₂ [20]. Based on the Langmuir-Hinshelwood mechanism, the reaction happened between adsorbed NO and adsorbed NH₃ on the adjacent sites to form H₂O and N₂ [21]. Meanwhile, it was widely accepted that the first step in SCR was an oxidative abstraction of the hydrogen from adsorbed ammonia [22]. Therefore, oxidative NH₃ played an important part in the mechanism of NH₃-SCR, which could be evaluated by thermodynamics in details [7]. However, the comprehensive comparison of NO elimination over the nanocatalysts of MnCoO_x and MnFeO_x on TiO₂ support, especially with an insight from structure, performance, kinetics, and thermodynamics, has not been explored clearly.

In this study, the nanocatalysts of Mn−Co/TiO₂ and Mn−Fe/TiO₂ were synthesized by hydrothermal method. The physicochemical properties of the obtained samples were researched by SEM, BET, XRD, H2-TPR, NH3-TPD, and XPS. In the meantime, the catalytic performance comparation of Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts was investigated using kinetic and thermodynamic analysis. The purpose of this work was mean to explore a comprehensive perspective for optimizing multimetals SCR catalysts, especially for the Mn-based bimetals nanocatalysts.

2. Results and Discussions

2.1. Physicochemical Properties of the Nanocatalysts

2.1.1. TEM Analysis

The topographic characteristics and the porous structures of the Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts were collected by TEM test. As shown in Figure 1a, the most part of Mn−Co/TiO₂ nanocatalyst was formed with uniform elliptic nanoparticles with glabrous surfaces and well-proportioned size distribution and without apparent agglomeration, although there was a spot of tightly aggregated TiO₂ nanoparticles interfused into the smaller regular particles. Therefore, a distinct and unbroken mesh structure of micropore was form in Mn−Co/TiO₂ sample. However, according to the TEM images of Mn−Fe/TiO₂ nanocatalyst from Figure 1b, a noticeable augment in the particle size was observed. The nanoparticles were irregular, lots of which stacked on the nanocatalyst surface. According to Figure 1(c)(l), captured at a larger scope, it could be found that all active elements contained in Mn−Co/TiO₂ nanocatalyst were well dispersed without obvious irregular stacked particles. On the surface of Mn−Fe/TiO₂ nanocatalyst, the element of manganese appeared slight regional accumulation, which might be caused by the abundant micropore structure collapsing [23].

2.1.2. BET Analysis

The structure parameters of Mn−Co/TiO₂ and Mn−Co/TiO₂ nanocatalysts, such as the specific surface areas, total pore volumes, and average pore diameters, were investigated by nitrogen-adsorption-desorption, and the test data was summarized in

Table 1. Physical properties of Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts.

Samples	Specific Surface Aarea (m2·g−1)	Total Pore Volume (cm3·g−1)	Average Pore Diameter (nm)
Mn−Co/TiO2	189.9	0.531	33.06
Mn−Fe/TiO2	104.6	0.424	54.85

. Mn−Co/TiO₂ nanocatalyst attained the larger specific surface areas than Mn−Fe/TiO₂, which was possible due to the improved dispersion of MnCoO_x species on the nanocatalyst surface. In the meantime, the average pore diameters increased obviously, from 33.06 nm in the Mn−Co/TiO₂ sample to 54.85 nm in the Mn−Fe/TiO₂ sample. Therefore, the MnCoO_x species were more likely to promote nanocatalyst to form micropores compared with MnFeO_x species [24,25]. Nevertheless, the difference of total pore volumes between Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts was not conspicuous. The total pore volumes reduced slightly from 0.531 cm³·g⁻¹ in Mn−Co/TiO₂ sample to 0.424 cm³·g^-1 in Mn−Fe/TiO₂ sample, which was probable due to the mesoporosity generation, which blocked the micropore formation, resulting in a small decrease of total pore volume. Considering the comprehensive test results, the Mn−Co/TiO₂ nanocatalyst exhibited superior physical properties than the Mn−Fe/TiO₂ sample with higher specific surface area, larger total pore volume, and smaller average pore diameter, which were contributed to abundant micropores structures.

2.1.3. Components Analysis

The XRD patterns of Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts were displayed in Figure 2. The diffraction peaks for TiO₂ support contained in the nanocatalysts were preserved entirely, with the strong and distinguished reflections appearing at 2θ = 25.3°, 37.8°, 48.0°, 53.9°, 62.7°, 68.8°, 70.3°, 75.1°, and 82.7°. These accorded with the XRD pattern of anatase TiO₂ (ICDD PDF card # 71-1166) [26], with the diffraction angles of the matching peaks shifted at tiny degrees. For Mn−Co/TiO₂ nanocatalyst, each corresponding peak of anatase TiO₂ appeared at the lower diffraction angle, which indicated the interaction between MnCoO_x and anatase TiO₂ was stronger than that of MnFeO_x and anatase TiO₂. Meanwhile, the diffraction peaks of anatase TiO₂ in the Mn−Co/TiO₂ sample were broader and weaker than that in Mn−Fe/TiO₂ sample, which demonstrated the crystalline of anatase TiO₂ reduced with MnCoO_x loading.

The diffraction peaks corresponding to MnO_x contained in Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts were very complex due to the transformation among the incomplete crystallization of manganese oxides, such as MnO₂, Mn₂O₃, Mn₃O₄, and MnO. The characterization reflections for MnO_x in the Mn−Co/TiO₂ sample were obviously weaker than that in Mn−Fe/TiO₂ sample, which manifested the active species in the Mn−Co/TiO₂ nanocatalyst, which were superior dispersed on the nanocatalyst surface or the active species incorporated into TiO₂ lattice better [27]. The diffraction peaks fitted MnO₂ (ICDD PDF card # 82-2169) precisely at about 2θ values of 22.10°, 35.19°, 36.96°, 38.72°, 47.86°, and 57.166°, which were coinciding with the crystallographic plane reflections of (110), (310), (201), (111), (311), and (420), respectively [12]. Meanwhile, both Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts exhibited the diffraction peaks corresponding to Mn₂O₃ and Mn₃O₄, apparently. The Mn₂O₃ (ICDD PDF card # 78-0390) was well-matched with intensive distinct signals at 2θ = 23.08°, 26.72°, 32.87°, and 56.89°, attributed to the crystallographic plane reflections of (211), (220), (222), and (433), respectively. The Mn₃O₄ (ICDD PDF card # 75-0765) was fitted to characteristic peaks at 36.28°, 40.67°, 41.80°, 57.73° and 64.17°, ascribed to the crystallographic plane reflections of (112), (130), (131), (115), and (063), respectively [23].

Comparing the XRD patterns of the Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts, it could be observed that the diffraction peaks of both Mn₂O₃ and Mn₃O₄ were visibly weakened in the Mn−Co/TiO₂ sample. In the meantime, the diffraction peaks of anatase TiO₂ in the Mn−Co/TiO₂ sample were also notably lower than that in Mn−Fe/TiO₂ sample. It was believed that the doping of cobalt into MnO_x obtained superior enhancement than iron on reducing the crystallization of MnO_x and TiO₂ simultaneously. Moreover, in the XRD patterns of the Mn−Co/TiO₂ nanocatalyst, there were no distinguished diffraction peaks for CoO_x, which manifested the doping of cobalt not only improved MnO_x dispersion, but also enhanced CoO_x dispersing on the nanocatalyst surface completely. The similar results were received over Mn−Fe/TiO₂ sample. Based on the phase characteristics comparation between Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts, it was believed that the Mn−Co/TiO₂ sample exhibited more excellent properties with smaller crystallinities of chemical compounds and better distribution of the active species, which were conducive to catalytic reactions.

2.1.4. Reducibility Properties

The oxidation states of the active compounds contained in nanocatalysts of Mn−Co/TiO₂ and Mn−Fe/TiO₂ were tested by H₂-TPR. The H₂ consumption curve was fitted by reduction peak separation with Gaussian function, as shown in Figure 3. The details of H₂ consumptions and reduction temperatures for each peak were listed in

Table 2. H₂-TPR quantitative analysis of Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts.

Samples	Temperature (°C) / H2 Consumption (mmol·g−1)
	Peak 1	Peak 2	Peak 3	Peak 4	Total
Mn−Co/TiO2	218/1.14	327/0.21	418/2.12	517/0.96	- /4.43
Mn−Fe/TiO2	276/0.83	387/0.41	436/0.54	501/1.59	- /3.37

. In Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts, the anatase TiO₂ support did not bring remarkable reduction peaks within the test temperature range. Hence, all H₂ consumption could be attributed to the reduction process of MnO_x, CoO_x, and FeO_x. It was proposed that the typical reduction process of MnO_x contained in the nanocatalysts of Mn−Co/TiO₂ and Mn−Fe/TiO₂ complied with the following order: MnO₂ → Mn₂O₃ (Mn₃O₄) →MnO [28]. Mn−Co/TiO₂ nanocatalyst exhibited four reduction peaks with temperature rising from 50—850 °C, as shown in Figure 3b. The first reduction peak centered, at about 218 °C, was attributed to the high oxidation state of manganese ion converting from MnO₂ to Mn₂O₃ [29]. The dominant reduction peak (Peak 3) was caused by two sequential processes of Mn₂O₃ reducing to Mn₃O₄ and Mn₂O₃ reducing to MnO, as reported in previous literatures [12,28]. The reduction process of Mn₂O₃ to Mn₃O₄ was more liable to happen over the original amorphous Mn₂O₃ [30], while reduction process of Mn₂O₃ to MnO preferred to occur at relatively higher temperature [31]. For the Mn−Co/TiO₂ sample, the typical reduction process of cobalt oxides commonly displayed two separated peaks, the one of Co₂O₃ reducing to CoO appeared at around 327 °C, presenting as an asymmetrical reduction peak (Peak 2), the other one of CoO reducing to Co⁰ occurred at about 418 °C and overlapped with MnO_x reduction peaks in whole or partly (Peak 3) [8,32]. Therefore, the fourth medium reduction peak in the Mn−Co/TiO₂ nanocatalyst was related to the reduction processes of Mn₃O₄ to MnO.

In comparison, the peak intensions and the reduction temperatures of the Mn−Fe/TiO₂ nanocatalyst were evidently different from that of the Mn−Co/TiO₂ sample. The peak of MnO₂ reducing to Mn₂O₃ weakened remarkably and shifted to higher temperature (276 °C). There were wide joint peaks (Peak 2 and Peak 3) at temperature of 330–530 °C, which were ascribed to the coinstantaneous reduction processes of Mn₂O₃ to Mn₃O₄ and Fe₂O₃ to Fe₃O₄. It was proposed that the majority of Fe₂O₃ was reduced at around 380 °C, which located at effortlessly reducible sites in the form of oligomeric clusters, nanoparticles or isolated ions [33]. After the majority of Fe₂O₃ reduction, there was a small quantity of residual Fe₂O₃ reducing to Fe₃O₄ at the higher temperature [34]. The dominating peak (Peak 4), at around 501 °C, was assigned to the overlapped peaks of Mn₃O₄ to MnO and Fe₃O₄ to FeO. Figure 4 provided a graphical representation of the reduction process, in which each active component embodied qualitatively [1,35]. In these two kinds of nanocatalysts, Mn−Co/TiO₂ sample displayed the higher low-temperature reducibility and exhibited a noticeable medium temperature reduction peak at the same time, which manifested the higher oxidation states of manganese ion (Mn⁴⁺ and Mn³⁺) constituted the dominating phase [31].

According to Table 2, the total H₂ consumption of Mn−Co/TiO₂ nanocatalyst was 4.43 mmol·g⁻¹ much larger than that of Mn−Fe/TiO₂ sample. Meanwhile, the starting reduction peak temperature of Mn−Co/TiO₂ nanocatalyst was at 218 °C lower than that of Mn−Fe/TiO₂, which was regarded as an important factor influencing the reducibility. It was believed that the catalyst performed superior catalytic ability at low temperatures as the reduction peaks arising in lower temperature regions [1].

2.1.5. Ammonia Adsorption Properties

The acid capacity of the Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts was measured by NH₃-TPD test, which was another crucial factor effecting the catalyst performance in SCR process [36]. The results were exhibited in Figure 5 and

Table 3. Quantitative analysis of NH₃-TPD profiles.

Samples	Temperature (°C) / NH3 composition (mmol·g−1)
	Peak 1	Peak 2	Peak 3	Peak 4	Total
Mn−Co/TiO2	276/0.11	402/0.49	587/0.52	653/0.32	--/1.44
Mn−Fe/TiO2	209/0.14	398/0.24	585/0.39	649/0.20	--/0.97

, respectively. The NH₃-TPD curves for the Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts were ascribed to four desorption peaks of chemisorbed NH₃ with the temperature rising from 150 °C to 750 °C. For Mn−Fe/TiO₂ nanocatalyst, the first NH₃ desorption peak occurred at around 209 °C, attributed to NH₃ desorption from weak acid sites, which was too feeble to bound NH₃ steadily in the gas mixture during SCR process [36]. Peak 2 and peak 3 were combined from 398 °C to 585 °C, which were attributed to the medium strong acid sites on the nanocatalyst surface. Peak 4, at around 649 °C, was assigned to the strong acid sites, which were regarded as abundant Lewis acid sites adsorbing a great deal of strongly bound NH₃ [37]. In comparation, the NH₃ desorption results of the Mn−Co/TiO₂ nanocatalyst exhibited better acidity capacity at medium and high temperatures. However, there was an undesired temperature shift to higher temperature occurred in the meantime. For Mn−Co/TiO₂ sample, the desorption temperature of weak acid sites, medium strong acid sites and strong acid sites were 276 °C (Peak 1), 402 °C (Peak 2), 587 °C (Peak 3), and 653 °C (Peak 4), respectively. It was obvious that the medium strong acid sites and the strong acid sites were enriched in the Mn−Co/TiO₂ sample, which were positive to form more abundant Brønsted acid sites and Lewis acid sites promoting NH₃ adsorption on the nanocatalyst surface [38,39].

The quantitative comparation of the total acid capacity between the Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts was performed and summarized in Table 3. The Mn−Co/TiO₂ sample achieved larger total NH₃ desorption of 1.44 mmol·g⁻¹ than Mn−Fe/TiO₂, which further confirmed the superior enhancement of MnCoO_x on the surface acidity than MnFeO_x. Although the Mn−Fe/TiO₂ sample generated weak acid sites at the lower temperature, the NH₃ desorption on the weak acid sites was only 0.14 mmol·g⁻¹, which was too little to offer enough NH₃ for SCR process. However, it was reported that NH₃ could restrain the adsorption and activation of NO onto the active sites on catalyst surface via the undesirable electron transfer between the adsorbed NH₃ and the metal sites [40]. Consequently, the excessive adsorbed NH₃ could inhibit SCR reactions. Therefore, the surface acidity properties of the Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts were bound up with their reducibility properties.

2.1.6. Oxidation States of Active Species

In order to better understand the oxidation states and the atomic concentrations of active species on the surface of Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts, the XPS spectra of Ti 2p, Mn 2p, Co 2p, Fe 2p, and O 1 s were tested and numerically analyzed by Gaussian fitting, respectively, as shown in Figure 6. The binding energy and atomic concentration of each element was summarized in

Table 4. Surface atomic compositions of the catalysts determined by XPS.

Samples	Binding Energy (eV) / Atomic Composition (%)
	Mn			Fe		Co		O
	Mn2+	Mn3+	Mn4+	Fe2+	Fe3+	Co2+	Co3+	Oα	Oβ
Mn−Co/TiO2	641.2/ 13.8	642.6/ 39.4	644.1/ 46.8	-/-	-/-	779.6 39.3	782.1/ 60.7	531.4/ 33.7	530.3/ 66.3
Mn−Fe/TiO2	641.4/ 21.3	642.8/ 38.5	644.6/ 40.2	709.6/ 43.4	711.7/ 56.6	-/-	-/-	531.6/ 28.2	530.3/ 71.8

.

As shown in Figure 8a, the XPS spectrum of Ti 2p was consisted of two characteristic peaks, attributed to Ti 2p_1/2 at around 464.3 eV and Ti 2p_3/2 at about 458.7 eV, respectively [41]. It was obvious that Ti⁴⁺ ion was stabilized on the catalyst surface and appeared as the dominating state in the nanocatalysts of Mn−Co/TiO₂ and Mn−Fe/TiO₂. The XPS spectra of Mn 2p was composed of Mn 2p_1/2 peak at about 653 eV and Mn 2p_3/2 peak at around 642 eV [42], as exhibited in Figure 6b. The Mn 2p_3/2 peak could be further split into three peaks for Mn²⁺ at 641.2 ± 0.3 eV, Mn³⁺ at 642.6 ± 0.2 eV, and Mn⁴⁺ at 644.1 ± 0.5 eV, respectively, according to the multi-peaks Gaussian fitting results [43]. It is evidently difficult to distinguish the three valence states of manganese in MnO_x with the binding energy difference value less than 3.7 eV. For accurate comparison of the atomic concentration of Mnⁿ⁺ on the surface of Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts, a quantitative analysis was performed according to the integral area under every divided peak, as exhibited in Table 4. It was widely accepted that the catalytic ability of MnO_x ranked as: MnO₂ > Mn₂O₃ > Mn₃O₄ [12,22,30]. The abundant MnO₂ generated on the nanocatalyst surface was beneficial to SCR reactions [5]. As listed in Table 4, the major amount of manganese in both Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts was Mn⁴⁺. Meanwhile, the Mn⁴⁺/Mnⁿ⁺ atomic composition was 46.8 % in Mn−Co/TiO₂ sample higher than that of 40.2 % in Mn−Fe/TiO₂ sample, generating more lattice oxygen and plenty of oxygen vacancy on the catalyst surface [1], which was regarded as the main reason for higher reducibility properties of Mn−Co/TiO₂ sample as discussed above.

In Co 2p spectrum of Mn−Co/TiO₂ nanocatalyst, the two individual peaks were attributed to Co 2p_1/2 at about 796.5 eV and Co 2p_3/2 at around 780.6 eV, respectively. Both of these two main peaks had satellite peaks at 803.1 eV and 786.8 eV, correspondingly, as shown in Figure 6c. The gentler and broader satellite peaks appeared at relatively higher binding energy, which were introduced by the metal-to-ligand electron transfer or the shakeup process of cobalt in its high spin state. However, this shakeup process was only observed with the high spin state of Co²⁺ ion, but did not appear with the diamagnetic low-spin Co³⁺ ion [13]. The intense peak of Co 2p_3/2 was composed of two overlapped peaks, one attributed to Co³⁺ seated at about 780.0 eV and the other ascribed Co²⁺ and located at around 781.6 eV. These two distinguishing peaks indicated the co-occurrence of cobalt in +2 and +3 valence states on the Mn−Co/TiO₂ nanocatalyst surface. There was a dynamic equilibrium sustained on the nanocatalyst surface with the electron transfer between Mn and Co ions during the catalytic oxidation process, expressed as Co³⁺ + Mn³⁺ $\boxed{\leftrightarrow }$ Co²⁺ + Mn⁴⁺ [44]. Moreover, Co³⁺ species were exhibited as the major atomic concentration of 60.7%, which were at the comparatively higher valence state of Coⁿ⁺ generating vast anionic defects, forming abundant surface oxygen and facilitating adsorption and oxidation during SCR process [45].

In Fe 2p spectra of the Mn−Fe/TiO₂ nanocatalyst, as shown in Figure 6d, the two main peaks were ascribed to Fe 2p_1/2 at about 724 eV and Fe 2p_3/2 at around 710 eV. There was a satellite peak at 718.3 eV assigned to Fe³⁺ in Fe₂O₃ [46]. The XPS spectra of Fe 2p_3/2 was composed of two characteristic peaks, ascribed to Fe³⁺ at around 711.6 eV and Fe²⁺ at about 709.6 eV, which meant these two kinds of Feⁿ⁺ coexisted on the surface of the Mn−Fe/TiO₂ nanocatalyst. It was proposed that the promotion effect between manganese and iron was attributed to the electron transfer in the redox reaction: Mn³⁺ + Fe³⁺ $\boxed{\leftrightarrow }$ Mn⁴⁺ + Fe²⁺ [47].

The O 1 s spectrum of Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts were compared in Figure 6e, which was composed of the chemisorbed oxygen peak at 531.2–531.6 eV (O_α) and the lattice oxygen peak at 530.2–530.3 eV (O_β). According to the curve-fitting results, the binding energy of O_α shifted to higher values from 531.4 eV in Mn−Co/TiO₂ sample to 531.6 eV in Mn−Fe/TiO₂ sample. Meanwhile, the similar changes happened to the binding energy of O_β. It was believed that the chemisorbed oxygen (O_α) was the most active oxygen species owing to its high mobility [48]. On the surface of Mn−Co/TiO₂ nanocatalyst, the atomic concentration of O_α reached 33.7% much higher than that of Mn−Fe/TiO₂ sample, which was regarded as another reason for its superior redox ability.

2.2. Catalytic Performance

Figure 7 exhibited the NO conversion and catalytic selectivity over Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts. It can be seen that the NO conversion improved conspicuously with the temperature increasing from 25 °C to 250 °C. For both Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts, the satisfactory conversion (>90%) was obtained above 150 °C. Meanwhile, the Mn−Ce/TiO₂ nanocatalyst exhibited higher catalytic activity than the Mn−Fe/TiO₂ sample within the temperature range of 50–175 °C. Furthermore, the catalytic selectivity of Mn−Ce/TiO₂ nanocatalyst was higher than that of Mn−Fe/TiO₂ within 75–250 °C. Therefore, both the catalytic activity and the catalytic selectivity over Mn−Co/TiO₂ were remarkably improved compared to Mn−Fe/TiO₂, potentially due to the interaction between manganese and cobalt stronger than that between manganese and iron, which enhanced the generation of Brønsted acid sites and Lewis acid sites on the nanocatalyst surface, resulting in promoting Langmuir–Hinshelwood mechanism reactions and Eley-Rideal mechanism reactions, simultaneously [49].

2.3. Reaction Kinetic Study

It was proposed that both the Langmuir–Hinshelwood mechanism and the Eley–Rideal mechanism contributed to the SCR reactions over nanostructured Fe-Mn oxides [36]. According to Langmuir-Hinshelwood mechanism, the SCR reactions over Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatsalysts could be approximately described as:

$${\mathrm{M}}^{\mathrm{n}+}=\mathrm{O}+\mathrm{NO}\left(\mathrm{ad}\right)\to {\mathrm{M}}^{(\mathrm{n}-1)+}-\mathrm{O}-\mathrm{NO}$$

(2)

$${\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{O}-\mathrm{NO}+\mathrm{NH}}_3\left(\mathrm{ad}\right)\to {\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{O}-\mathrm{NO}-\mathrm{NH}}_3\to {\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{OH}+\mathrm{N}}_2{+\mathrm{H}}_2\mathrm{O}$$

(3)

$${\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{O}-\mathrm{NO}+\mathrm{O}}_2\to {\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{O}-\mathrm{NO}}_2$$

(4)

$${\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{O}-\mathrm{NO}+\mathrm{NH}}_3\left(\mathrm{ad}\right)\to {\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{O}-\mathrm{NO}}_2{-\mathrm{NH}}_3\to {\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{OH}+\mathrm{N}}_2{\mathrm{O}+\mathrm{H}}_2\mathrm{O}$$

(5)

$${\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{OH}+\mathrm{O}}_2\to {\mathrm{M}}^{\mathrm{n}+}{=\mathrm{O}+\mathrm{H}}_2\mathrm{O}$$

(6)

The adsorbed NO(ad) was oxidized by Mⁿ⁺ (Mn⁴⁺, Mn³⁺, Co³⁺, Fe³⁺) on the nanocatalyst surface to form ${\mathrm{NO}}_2^{-}$ (Reaction 2). The ${\mathrm{NO}}_2^{-}$ reacted with adsorbed NH₃(ad) to generate NH₄NO₂ and decomposed to N₂ and H₂O, subsequently (Reaction 2). In the meantime, an undesired oxidization reaction occurred on ${\mathrm{NO}}_2^{-}$ which was oxidized to ${\mathrm{NO}}_3^{-}$ under O₂ (Reaction 4). The formed ${\mathrm{NO}}_3^{-}$ reacted with adsorbed NH₃(ad) to generate NH₄NO₃ and further decomposed to N₂O and H₂O (Reaction 5). Finally, the active species ${\mathrm{M}}^{(\mathrm{n}-1)+}-\mathrm{OH}$ was oxidized by O₂ to regenerate ${\mathrm{M}}^{\mathrm{n}+}=\mathrm{O}$ (Reaction 6).

According to the Eley-Rideal mechanism, the SCR reactions over Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatsalysts could be described as:

$${\mathrm{M}}^{\mathrm{n}+}{=\mathrm{O}+\mathrm{NH}}_3\left(\mathrm{ad}\right)\to {\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{OH}+\mathrm{NH}}_2\left(\mathrm{ad}\right)$$

(7)

$${\mathrm{NH}}_2\left(\mathrm{ad}\right)+\mathrm{NO}\to {\mathrm{N}}_2{+\mathrm{H}}_2\mathrm{O}$$

(8)

The adsorbed NH₃(ad) on the nanocatalyst surface was activated by Mⁿ⁺ to generate NH₂ (Reaction 7). Then, the NH₂ reacted with NO to form N₂ and H₂O (Reaction 8).

However, in addition to the main Eley-Rideal mechanism reactions, undesirable side reactions occurred on the active species of ${\mathrm{M}}^{\mathrm{n}+}=\mathrm{O}$.

$${\mathrm{M}}^{\mathrm{n}+}{=\mathrm{O}+\mathrm{NH}}_2\left(\mathrm{ad}\right)\to {\mathrm{M}}^{(\mathrm{n}-1)+}-\mathrm{OH}+\mathrm{NH}\left(\mathrm{ad}\right)$$

(9)

$${\mathrm{M}}^{\mathrm{n}+}=\mathrm{O}+\mathrm{NH}\left(\mathrm{ad}\right)+\mathrm{NO}\to {\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{OH}+\mathrm{N}}_2\mathrm{O}$$

(10)

$${\mathrm{M}}^{\mathrm{n}+}{=\mathrm{O}+\mathrm{NH}\left(\mathrm{ad}\right)+\mathrm{O}}_2\to {\mathrm{M}}^{(\mathrm{n}-1)+}-\mathrm{OH}+\mathrm{NO}$$

(11)

The activated NH₂(ad) formed in Reaction 7 was further oxidized to NH(ad) over ${\mathrm{M}}^{\mathrm{n}+}=\mathrm{O}$. Then NH(ad) reacted with NO to form N₂O or react with O₂ to generate NO.

According to Reactions 3 and 8, the kinetic equations of N₂ formation rates via the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism could be described as:

$$\begin{gathered} k_{\mathrm{scr}}=\frac{{\mathrm{d}[\mathrm{N}}_2]}{\mathrm{dt}}= {\frac{\mathrm{d}\left[{\mathrm{N}}_2\right]}{\mathrm{d}t}|}_{\mathrm{LH}}+{\frac{\mathrm{d}\left[{\mathrm{N}}_2\right]}{\mathrm{d}\mathrm{t}}|}_{\mathrm{ER}}= -{\frac{\mathrm{d}\left[{\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{O}-\mathrm{NO}-\mathrm{NH}}_3\right]}{\mathrm{d}\mathrm{t}}|}_{\mathrm{LH}}-{\frac{\mathrm{d}\left[\mathrm{N}{\mathrm{H}}_2\left(\mathrm{a}\mathrm{d}\right)\right]}{\mathrm{d}\mathrm{t}}|}_{\mathrm{ER}} \\ {=k}_3\left[{\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{O}-\mathrm{NO}-\mathrm{NH}}_3\right]{+k}_8\left[{\mathrm{NH}}_2\left(\mathrm{ad}\right)\right]\left[\mathrm{NO}\right] \end{gathered}$$

(12)

where k₃ and k₈ were the kinetic constants of Reactions 3 and 8,$\left[{\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{O}-\mathrm{NO}-\mathrm{NH}}_3\right]$, $\left[{\mathrm{NH}}_2\left(\mathrm{ad}\right)\right]$, and $\left[\mathrm{NO}\right]$ were the concentrations of NH₄NO₂, NH₂(ad), and NO, respectively. When the reactions above reached stable conditions, the concentrations of NH₄NO₂ and NH₂(ad) on the surface of Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts could be regarded as constants without connecting with the gaseous concentrations of NH₃ and NO [50]. Therefore, the SCR reaction rate was in a linear relationship with NO concentration as exhibited in Equation (13), the slope and the intercept could be obtained from the linear regression in Figure 8, as listed in

Table 5. Reaction kinetic constants of NO reduction.

Sample	Temperature (°C)	kSCR-ER/106	kSCR-LH	R2	kSCRa
Mn−Co/TiO2	100	0.029	5.75	0.993	23.5
	150	0.046	13.25	0.994	41.1
	200	0.064	20.50	0.988	58.2
	250	0.075	25.01	0.984	68.4
Mn−Fe/TiO2	100	0.018	2.35	0.989	13.5
	150	0.027	8.25	0.990	24.4
	200	0.033	16.01	0.991	36.0
	250	0.062	14.50	0.998	52.1

^a [NO] = [NH₃] = 600 ppm.

.

$$k_{\mathrm{scr}}{=k}_{\mathrm{SCR}-\mathrm{LH}}{+k}_{\mathrm{SCR}-\mathrm{ER}}\left[\mathrm{NO}\right]$$

(13)

$$k_{\mathrm{SCR}-\mathrm{LH}}{=k}_3\left[{\mathrm{M}}^{(\mathrm{n}-1)+}{-\mathrm{O}-\mathrm{NO}-\mathrm{NH}}_3\right]$$

(14)

$$k_{\mathrm{SCR}-\mathrm{ER}}{=k}_8\left[{\mathrm{NH}}_2\left(\mathrm{ad}\right)\right]$$

(15)

2.4. Thermodynamic Calculation

The standard thermodynamic characteristics of the chemical individual species of M_xO_y and M in crystalline states, as well as NH₃, N₂ and H₂O in gaseous states, were listed in

Table 6. Standard thermodynamic properties of M_xO_y [7,53,54,55].

	Cp J mol−1 K	△fH0 kJ mol−1	S0 J mol−1 K	△fG0 kJ mol−1
Mn	26.3	0.0	32.0	-
MnO	45.4	−385.2	59.7	−362.9
Mn3O4	139.7	−1387.8	155.6	-1283.2
Mn2O3	107.7	−959.0	110.5	−881.1
MnO2	54.1	−520.0	53.1	−465.1
Co	24.8	0.0	30.0	-
CoO	55.2	−237.9	53.0	−214.2
Co3O4	123.4	−891.0	102.5	−774.0
Fe	25.1	0.0	27.3	-
FeO	-	−272.0	-	-
Fe3O4	143.4	−1118.4	146.4	−1015.4
Fe2O3	103.9	−824.2	87.4	−742.2
NH3	-	45.9	192.8	-
N2	29.1	0.0	191.6	-
H2O (g)	33.6	−241.8	188.8	−228.6

. The oxidative capacities of M_xO_y were evident according to the reliable standard thermodynamic parameters, which was a convincing support for investigating the roles of M_xO_y in NH₃-SCR process. The Gibbs free energies for the reactions between M_xO_y species and ammonia were calculated for estimating their oxidative capacities according to Equation (16), displayed in

Table 7. Gibbs free energy for the reaction between M_xO_y and NH₃ under different temperatures.

Substance	△fG /kJ
	298 K	323 K	373 K	423 K	473 K	523 K
MnO2	−46.25	−49.09	−54.77	−60.45	−66.12	−71.80
Mn3O4	42.00	39.27	33.81	28.34	22.88	17.42
MnO	84.06	81.65	76.83	72.01	67.19	62.37
Co3O4 a	−85.71	−88.74	−94.79	−100.84	−106.89	−112.94
CoO	−64.64	−67.17	−72.22	−77.28	−82.34	−87.39
Fe2O3	−31.40	−34.23	−39.89	−45.55	−51.21	−56.87
Fe3O4	−24.99	−27.69	−33.09	−38.49	−43.89	−49.29

^a Co₃O₄ was regard as the compound of Co₂O₃ and CoO

. In order to discuss the reaction mechanism conveniently, simplifying assumptions were introduced as follows. Only the kind of main oxidation state was considered in the thermodynamic calculation, although there were several kinds of oxidation states for each M_xO_y [51]. Meanwhile, it was assumed that every M_xO_y was reduced into metal, although the metallic oxide in the high-oxidation state was reduced step-by-step in the actual process. It had been proved that the entropy of metals would vary drastically as the metal physical states transforming [52]. Furthermore, the transformation could bring about the changes of the straight slope of the temperature–Gibbs free energy curve which was named Ellingham diagrams. The Ellingham diagrams were a particular graphical form of the principle that the thermodynamic feasibility of a reaction depends on the Gibbs free energy change (Δ_fG⁰), which could be calculated using Equation (16), where Δ_fH⁰ was the enthalpy change and ΔS⁰ was the entropy change under standard state.

$${\Delta }_{\mathrm{f}}{\mathrm{G}}^0={\Delta }_{\mathrm{f}}{\mathrm{H}}^0-\mathrm{T}\Delta {\mathrm{S}}^0$$

(16)

The Ellingham diagram plotted Δ_fG for each oxidation reaction as a function of temperature. In order to compare different reactions, all Δ_fG values referred to the same quantity of oxygen, chosen as one mole O (1/2 mol O₂) [53], as shown in Equation (17)

$$\frac{1}{\mathrm{y}}{\mathrm{M}}_{\mathrm{x}}{\mathrm{O}}_{\mathrm{y}}+\frac{2}{3}{\mathrm{NH}}_3\to \frac{\mathrm{x}}{\mathrm{y}}\mathrm{M}+\frac{1}{3}{\mathrm{N}}_2{+\mathrm{H}}_2\mathrm{O}$$

(17)

According to Equations (16) and (17), the redox reactions that happened between M_xO_y and NH₃ were discussed under thermodynamic calculation, assuming that the enthalpy, entropy, and Gibbs free energy remained unchanged with temperature. The Δ_fG in Table 7 for each kind of M_xO_y was calculated in order to estimate its oxidative capacities and gain semiquantitative conclusions. Meanwhile, this research just focused on the catalytic performance and reaction mechanism of cobalt and iron doped in the Mn/TiO₂ nanocatalysts, so that only one M_xO_y was different at a time. The negative Δ_fG values for Co₃O₄ (the compound of Co₂O₃ and CoO) and Fe₂O₃ indicated M_xO_y was effective for the oxidative abstraction of the hydrogen from adsorbed ammonia [7].

The Ellingham diagrams for the Gibbs free energy of reactions between M_xO_y and NH₃ with the reaction temperature rising was displayed in Figure 9. As the △_fG was negative (△_fG < 0), the reaction took place more easily. On the contrary, the reaction would happen at higher temperature or would not happen if the △_fG was positive (△_fG > 0). Under the same reaction temperature, the order of temperature−Gibbs free energy for the reaction between M_xO_y and NH₃ is as follows: Co₃O₄ (compound of Co₂O₃ and CoO) < CoO < Fe₂O₃ < Fe₃O₄ (only considering the doping transition metals). These results were exactly consistent with the NH₃-SCR performance and almost coincided with the data of the nanostructure characterization, as discussed above, which demonstrated the simplifying assumptions used in the thermodynamic calculations was reasonable. However, the activity of MnO_x exhibited in Ellingham Diagrams was not in well accord with the reducibility properties, which might be on account of positive kinetics or simplifying assumptions during the thermodynamic calculations [51,53]. It manifested that MnO_x, as the dominate phase in nonacatalysts of Mn−Co/TiO₂ and Mn−Fe/TiO₂, was activated by the doping of cobalt or iron. A large number of researches had been carried out focusing on optimizing catalysts via doping transition metals in order to enhance NO catalytic conversion remarkably. Based on the thermodynamic study, Ellingham diagrams would provide a novel insight for developing catalysts doped proper metal oxides with higher NH₃-SCR activity.

3. Materials and Methods

3.1. Catalysts Preparation

The noncatalysts of Mn−Co/TiO₂ and Mn−Fe/TiO₂ were prepared by hydrothermal method. Co(CH₃COO)₂·4H₂O (analytical pure 99.9%, Kermel, Tianjin, China) and Fe(NO₃)₃·9H₂O (analytical pure 99.9%, Sinopharm, Shanghai, China) were used as the precursors of CoO_x and FeO_x, respectively. The aqueous solution of Mn(NO₃)₂ (analytical pure 50.0%, Sinopharm, Shanghai, China) was used as the precursors of MnO_x. The tetrabutyl titanate was introduced as the precursors of anatase TiO₂ to support the bimetals oxides. The precursors were added into deionized water at room temperature. One-hundred fifty mL glycol was added into the above mixture with magnetic stirring continuously to regulate the reaction rate of hydrothermal reaction. Then the homogeneous solution was heat at 180 °C for 8 h in a Teflon-lined stainless steel autoclave. After that, tetrabutyl titanate was added into the solution and reheated in the autoclave at 180 °C for 3 h. The precipitate was obtained by reduplicative centrifugation and wash. In the end, the nanocatalysts were dried at 150 °C for 12 h and calcined in air at 500 °C for 4 h. The nanocatalyst was denoted as Mn−Co/TiO₂ and Mn−Fe/TiO₂ with the molar ratios of Mn:M:Ti = 2:1:7.

3.2. Catalysts Characterization

The advanced microstructural image data and the surface element contents of the nanocatalysts was achieved by a high resolution transmission electron microscope JEOL JEM-2010 (Japan electronics corporation, Tokyo, Japan) combined with EDS. The Maxon Tristar II 3020 micropore-size analyzer (Maxon, Chicago, IL, USA) was used for testing N₂ adsorption isotherms of the prepared nanocatalysts at −196 °C. The surface areas and the pore-size distributions of the nanocatalysts were measured after the nanocatalysts degassing in vacuum at 350 °C for 10 h. BET plot linear portion was used to determine the nanocatalysts specific surface areas, and the desorption branch with Barrett–Joyner–Halenda (BJH) formula was introduced to calculate the pore-size distributions. The XRD data was captured by a Bruker D8 advance analyzer (Bruker, Frankfurt, Germany) with Mo K_α radiation, diffraction intensity from 10° to 90°, point counting time of 1 s and point counting step of 0.02°. The element phases contained in the nanocatalysts were distinguished by comparing characteristic peaks presented in the XRD patterns with the International Center for Diffraction Data (ICDD). H₂-TPR and NH₃-TPD tests were performed with a Micromeritics Autochem II 2920 chemical adsorption instrument (Micromeritics, Houston, TX, USA). During H₂-TPR experiment, nanocatalysts were pretreatment in He at 400 °C for 1 h, and then cooled to environment temperature in H₂ and He gas mixture at 30 mL/min. The test temperature range of H₂ consumptions was from 50 °C to 850 °C with the heating rate of 10 °C/min. The operating process of NH₃-TPD test was similar to that of H₂-TPR test with NH₃ replacing H₂. XPS analysis was performed on a Thermo ESCALAB 250XI (Thermo Fisher, Boston, MA, USA) with pass energy 46.95 eV, Al K_α radiation 1486.6 eV, X-ray source 150 W, and binding energy precision ±0.3 eV. The C 1 s line at 284.6 eV was measured as a reference.

3.3. Catalytic Performance Tests

The catalytic properties of the Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts were tested on a fixed-bed quartz tube reactor including a tube furnace and a temperature control unit, as shown in Figure 10. The mass of the catalyst was 250 mg with the particle diameter of 40–60 mesh. During NO conversion test, 5 mL nanocatalyst with the particle diameter of 40–60 mesh was used under the total flow rate of 2000 mL/min, corresponding to the gas hourly space velocity (GHSV) of 24,000 h⁻¹. The feed gas contained 300 ppm NO, 300 ppm NH₃, 5% O₂, and N₂ as balance gas. During reaction kinetic study, the reaction conditions were NO 150–600 ppm, NH₃ 600 ppm, O₂ 5%, catalyst mass of 200 mg, total flow rate at 4000 mL/min, and GHSV at 1 200 000 cm³·g⁻¹·h⁻¹. The concentrations of NO, N₂O, and NH₃ in the outlet were continually monitored using German MRU MGA-5 analyzer (MRU, Berlin, Germany) joint with an external special detector for N₂O and NH₃. The NO conversion was calculated according to Equation (18). The N₂ selectivity was calculated by the concentrations of N₂O and NO, as show in Equation (19). Each experiment was repeated three times to confirm the results accuracy.

$$\mathrm{NO}\mathrm{conversion}=\left(\frac{{\left[\mathrm{NO}\right]}_{\mathrm{in}}-{\left[\mathrm{NO}\right]}_{\mathrm{out}}}{{\left[\mathrm{NO}\right]}_{\mathrm{in}}}\right)\times 100\%$$

(18)

$${\mathrm{N}}_2\mathrm{selectivity}=1-\frac{2{\left[{\mathrm{N}}_2\mathrm{O}\right]}_{\mathrm{out}}}{{\left[\mathrm{NO}\right]}_{\mathrm{in}}-{\left[\mathrm{NO}\right]}_{\mathrm{out}}}\times 100\%$$

(19)

4. Conclusions

The multiple characterizations performed on the Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts indicated that the Mn−Co/TiO₂ sample obtained the superior structure characteristics than Mn−Fe/TiO₂ with the surface area increased by 85.3 m²·g⁻¹, the nanoparticle size reduced by 21.79 nm, the active components distribution improved, and the crystallinity decreased. Meanwhile, these results further confirmed the catalytic property was highly dependent on the phase compositions of the catalysts. The ratios of Mn⁴⁺/Mnⁿ⁺ in the Mn−Co/TiO₂ sample was 46.8% higher than that of 40.2% in the Mn−Fe/TiO₂ sample, which demonstrated the better oxidation ability and the larger amount of Bronsted acid sites and Lewis acid sites on the sample surface. Both Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts exhibited the satisfactory conversion (>90%) above 150 °C. When compared to the Mn−Fe/TiO₂ nanocatalyst, the Mn−Ce/TiO₂ sample displayed the preferable catalytic property with higher catalytic activity and stronger selectivity in the temperature range of 75—250 °C. In the meantime, the mechanism and kinetic study showed that Langmuir-Hinshelwood mechanism and Eley-Rideal mechanism reactions contributed to NH₃-SCR catalysis, simultaneously. Under the same test conditions, the NO conversion rate of Mn−Co/TiO₂ nanocatalyst was always higher than that of Mn−Fe/TiO₂ sample. Furthermore, Ellingham Diagrams of Mn−Co/TiO₂ and Mn−Fe/TiO₂ nanocatalysts were in accordance with their catalytic performances, based on the Gibbs free energy calculation of the reactions between ammonia and each kind of active metal oxide. Generally, Mn−Co/TiO₂ nanocatalyst exhibited the better catalytic properties than Mn−Fe/TiO₂ sample according to the comprehensive comparison in this research. The comparison with the insight from structure, performance, kinetics, and thermodynamics might supply a theoretical method to select superior metal oxides for NH₃-SCR.