Characterization and Catalytic Activity of Mn-Co/TiO2 Catalysts for NO Oxidation to NO2 at Low Temperature

Qiu, Lu; Wang, Yun; Pang, Dandan; Ouyang, Feng; Zhang, Changliang; Cao, Gang

doi:10.3390/catal6010009

Open AccessArticle

Characterization and Catalytic Activity of Mn-Co/TiO₂ Catalysts for NO Oxidation to NO₂ at Low Temperature

by

Lu Qiu

,

Yun Wang

,

Dandan Pang

,

Feng Ouyang

^*,

Changliang Zhang

and

Gang Cao

Environmental Science and Engineering Research Center, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China

^*

Author to whom correspondence should be addressed.

Catalysts 2016, 6(1), 9; https://doi.org/10.3390/catal6010009

Submission received: 7 November 2015 / Revised: 27 December 2015 / Accepted: 5 January 2016 / Published: 11 January 2016

(This article belongs to the Special Issue Surface Chemistry and Catalysis)

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Abstract

:

A series of Mn-Co/TiO₂ catalysts were prepared by wet impregnation method and evaluated for the oxidation of NO to NO₂. The effects of Co amounts and calcination temperature on NO oxidation were investigated in detail. The catalytic oxidation ability in the temperature range of 403–473 K was obviously improved by doping cobalt into Mn/TiO₂. These samples were characterized by nitrogen adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM) and hydrogen temperature programmed reduction (H₂-TPR). The results indicated that the formation of dispersed Co₃O₄·CoMnO₃ mixed oxides through synergistic interaction between Mn-O and Co-O was directly responsible for the enhanced activities towards NO oxidation at low temperatures. Doping of Co enhanced Mn⁴⁺ formation and increased chemical adsorbed oxygen amounts, which also accelerated NO oxidation.

Keywords:

NO Oxidation; NO₂; mixed oxides; manganese; cobalt; interaction

Graphical Abstract

1. Introduction

Nitrogen oxides (NO_x) in the exhaust from stationary and mobile sources are toxic to human’s health and have brought environmental problems, such as photochemical smog, acid rain and ozone depletion. NO_x storage-reduction (NSR) and selective catalytic reduction (SCR) are considered to be promising technologies to achieve high NO_x reduction efficiency. It is known that most nitrogen oxides from exhaust emissions exist in the form of NO (>90%). NO oxidation is considered to be a key step for NO_x reduction on both the NSR and SCR reactions. In the process of NSR, NO is first oxidized to NO₂ and then stored on the basic components of catalysts as nitrates [1]. For SCR, NO₂ is favored for NO_x conversion according to the so-called Fast SCR reaction, which is thought to be faster by one order of magnitude than the Standard SCR reaction under oxidizing conditions [2]. The light-off temperature in the NO oxidation to NO₂ is significant for the low-temperature SCR of NO_x reduction. Therefore the research into NO oxidation catalysts is important for the NO_x removal and various catalysts have been researched, including the supported noble metals [3,4,5,6] and other metal oxides [7,8,9].

Among these catalysts, there has been some attention focused on cobalt oxides [10] and manganese oxides [11]. It was reported that cobalt oxides possess better oxidation ability [12,13,14]. Co²⁺/Co³⁺ oxidation states were found to interconvert readily in oxidizing and reducing conditions [15]. The presence of Co₃O₄ means high activity for oxidation reactions [10,16]. In addition, Mn-based catalysts have been proven to be highly active for the low-temperature SCR reaction, which would avoid the disadvantages associated with the commercial high-temperature catalysts. Several manganese based catalysts, such as the Mn/TiO₂ [17,18] and the Mn catalysts doped with other metal (Fe, Co, Ni, Cu, Ce, etc.) oxides [19,20,21], have been reported for the low-temperature SCR reaction. As reported in the literature [22,23], Mn oxides not only acted as highly efficient SCR catalysts, but also showed certain catalytic activity for NO oxidation. The Mn-Co/TiO₂ catalysts have been evaluated for the low temperature SCR reaction [19]. However, the activities of Mn-Co/TiO₂ for NO oxidation to NO₂ have been reported rarely.

The present work aims to investigate the catalysts of Mn-Co composite oxides loaded on TiO₂ for NO oxidation at low temperatures. The catalysts with different Co contents and calcined at different temperatures were investigated to determine the interactions between cobalt oxides and manganese oxides. Special attention was paid to the Mn-O-Co mixed oxides which can change the properties of the catalysts and enhance the oxidation ability. The Mn-Co/TiO₂ catalysts were characterized by means of N₂ adsorption-desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and hydrogen temperature-programmed reduction (H₂-TPR).

2. Results and Discussion

2.1. Activity of NO Oxidation to NO₂

Figure 1 shows the oxidation efficiency of NO to NO₂ over a series of Mn-Co/TiO₂ catalysts with different cobalt contents and a constant loading of 10 wt. % manganese. These catalysts were calcined at 773 K. As a reference, the activities of 10Co/TiO₂, 10Mn/TiO₂ and 20Mn/TiO₂ were also tested. As shown in Figure 1, the NO conversion efficiencies of 10Mn/TiO₂ and 20Mn/TiO₂ were higher than that of 10Co/TiO₂ below 513 K. Dramatic increase in the NO oxidation ratio within the temperature range of 403–473 K was observed upon addition of cobalt oxides to Mn/TiO₂. However, the NO conversion increased slowly with increasing Co content above 473 K. The 10Mn-10Co/TiO₂ exhibited a maximum oxidation efficiency of 61% at 543 K, which was nearly equal to that of 10Co/TiO₂. The activity results implied that strong interactions might exist between Co and Mn oxides, which could promote the NO oxidation to NO₂ at low temperatures.

Figure 1. NO oxidation efficiency of Mn-Co/TiO₂ catalysts with different Co contents. Reaction conditions: [NO] = 400 ppm, [O₂] = 5%, Ar balance, GHSV = 42,000 h⁻¹.

Figure 2 shows the oxidation efficiency of NO to NO₂ over a series of 10Mn-5Co/TiO₂ catalysts calcined at various temperatures. It can be seen that the catalytic activity increased with the calcination temperature increase from 573 to 773 K, and then decreased. The catalyst Mn-Co/TiO₂ (773 K) had the highest oxidation efficiency at 543 K among these samples. However, the Mn-Co/TiO₂ calcined at 573 and 673 K exhibited excellent catalytic activities in the temperature range of 403–483 K.

Figure 2. NO oxidation efficiency of 10Mn-5Co/TiO₂ catalysts calcined at different temperatures. Reaction conditions: [NO] = 400 ppm, [O₂] = 5%, Ar balance, GHSV = 42,000 h⁻¹.

2.2. Nitrogen Adsorption-Desorption Characterization

The specific surface areas and pore volumes for Mn-Co/TiO₂ catalysts were determined by nitrogen adsorption-desorption method. The results are listed in Table 1. A consistently decreasing trend of surface areas with increasing calcination temperature was noted for 10Mn-5Co/TiO₂ samples, and a little decrease with the increased Co contents was also showed.

Table 1. BET surface areas and average pore diameters of Mn-Co/TiO₂ catalysts with various Co contents and calcined at different temperatures.

**Table 1.** BET surface areas and average pore diameters of Mn-Co/TiO₂ catalysts with various Co contents and calcined at different temperatures.
Catalyst	S_BET (cm²·g⁻¹)	Average Pore Diameter (nm)
10Mn/TiO₂ (773 K)	48.35	8.67
10Mn-5Co/TiO₂ (573 K)	50.57	7.21
10Mn-5Co/TiO₂ (673 K)	50.38	6.86
10Mn-5Co/TiO₂ (773 K)	45.79	8.52
10Mn-5Co/TiO₂ (873 K)	30.51	8.38
10Mn-5Co/TiO₂ (973 K)	7.01	6.27
10Mn-10Co/TiO₂ (773 K)	42.13	8.02

2.3. XRD Characterization

Figure 3 shows the X-ray powder diffraction patterns of the Mn-Co/TiO₂ catalysts calcined at 773 K, with different Co contents ranged from 0 to 10 wt. %. It can be seen that the relative strong patterns showed diffraction peaks corresponding to anatase TiO₂ phase (PDF#21-1272) and rutile phase (PDF#21-1276), while anatase was the main form in these catalysts. In the pattern of 10Co/TiO₂ (Figure 3a), several weak diffraction peaks were detected at 2θ values of 18.8°, 31.2°, 44.7°, 59.3° and 65.2°, corresponding to Co₃O₄ phase (PDF#43-1003). In the pattern of 10Mn/TiO₂ (Figure 3b), these weak peaks at 2θ values of 37.3° and 42.7° can be attributed to MnO₂ (PDF#50-0866). However, the crystalline MnO₂ was poor since its corresponding diffraction peaks were very weak.

For the Mn-Co/TiO₂ samples with different Co contents, several weak peaks appeared at 18.4°, 30.8°, 33.4°, 44.1°, 58.9° and 65.5° (Figure 3d–g). The intensities of these peaks were enhanced slightly by the increase of Co contents. Compared with the pattern of 10Co/TiO₂, these peaks had a slight shift toward lower or higher 2θ value. These peaks at 30.8°, 33.4°, 58.9° and 65.5° can be assigned to CoMnO₃, while the peaks at 18.4° and 44.1° were ascribed to CoMn₂O₄. These peaks were also weak, inferring that most of the binary metal oxides existed in highly dispersed state. However, the intensities of these peaks were enhanced when the content of Mn increased to 20 wt. % and the Co content was 10 wt. % (Figure 3g). The existence of CoMnO₃ and CoMn₂O₄ was also demonstrated by Meng et al. [24] when Co oxides and Mn oxides were mixed without the support TiO₂. It can be deduced that CoMnO₃ and CoMn₂O₄ were two types of Mn-Co composite oxides formed on surface of TiO₂, in which Mn ions existed mainly in valence states of Mn⁴⁺ and Mn³⁺, respectively.

Figure 3. XRD patterns of the catalysts Mn-Co/TiO₂ (773 K) with different Co contents: (a) 10Co/TiO₂; (b) 10Mn/TiO₂; (c) 10Mn-2Co/TiO₂; (d) 10Mn-5Co/TiO₂; (e) 10Mn-8Co/TiO₂; (f) 10Mn-10Co/TiO₂; and (g) 20Mn-10Co/TiO₂. (A = anatase phase of TiO₂, R = rutile phase of TiO₂).

Figure 4 shows the XRD patterns of 10Mn-5Co/TiO₂ catalysts calcined at various temperatures. It can be seen that the intensities of rutile TiO₂ peaks were enhanced with calcination temperature increasing while the intensities of anatase peaks were decreased, indicating the transformation of anatase to rutile. The samples calcined at 573 and 673 K were low crystallized with the Mn-O-Co mixed oxides (Figure 4a,b). The weak crystal structure of CoMnO₃ and CoMn₂O₄ were only detected until the calcination temperature reached 773 K, represented by the peaks at 18.4°, 30.8°, 33.4°, 44.1° and 65.5°. These peaks decreased or disappeared above 873 K. Evidently, NO oxidation efficiency is related to the dispersion of CoMnO₃ and the crystal types of TiO₂.

As the calcination temperature increased to 873 and 973 K, the peaks at 18.4°, 30.8° and 33.4° shifted slightly toward lower 2θ value. We considered that was due to the transformation of Mn⁴⁺ to Mn³⁺ and Mn³⁺ to Mn²⁺ in the process of sintering. The peak at 32.9° can be attributed to CoMn₂O₄ phase and the peaks appeared at 18.1°, 29.2° and 60.6° can be attributed to a more complex (Co,Mn)(Co,Mn)₂O₄ phase, which had a higher crystallinity.

Figure 4. XRD patterns of the 10Mn-5Co/TiO₂ catalysts calcined at various temperatures: (a) 573 K; (b) 673 K; (c) 773 K; (d) 873 K; and (e) 973 K. (A = anatase phase of TiO₂, R = rutile phase of TiO₂).

2.4. TEM Characterization

Figure 5 shows the TEM images of 10Mn-5Co/TiO₂ calcined at 673 and 773 K. For Mn-Co/TiO₂ (673 K) in Figure 5A, the lattice fringes a and b were determined to be 0.36 and 0.45 nm, matched CoMnO₃ (012) (standard value is 0.362 nm) and (003) (standard value is 0.457 nm) crystal faces, respectively. For Mn-Co/TiO₂ (773 K) in Figure 5B, the lattice fringe c was determined to be 0.48 nm, corresponding to CoMn₂O₄ (standard value is 0.485 nm). The lattice fringes d and e were determined to be 0.35 and 0.52 nm, corresponding to anatase (101) of TiO₂ and MnO₂ (200) (standard value is 0.515 nm) crystal faces, respectively. The lattice fringes f and g were determined to be 0.36 and 0.41 nm, matched CoMnO₃ (012) and (101) (standard value is 0.408 nm) crystal faces, respectively. These results demonstrated the existence of CoMnO₃ and CoMn₂O₄ phases in Mn-Co/TiO₂ samples.

Figure 5. TEM images of the catalysts: (A) 10Mn-5Co/TiO₂ (673 K); and (B) 10Mn-5Co/TiO₂ (773 K). (a. CoMnO₃ (012); b. CoMnO₃ (003); c. CoMn₂O₄ (111); d. anatase (101); e. MnO₂ (200); f. CoMnO₃ (012); g. CoMnO₃ (101)).

2.5. XPS Characterization

The surface oxidation states and atomic concentrations of 10Mn/TiO₂ (773 K) and 10Mn-5Co/TiO₂ calcined at 673, 773 and 973 K were determined by XPS analysis. The photoelectron profiles of Mn 2p and O 1s are shown in Figure 6. The atomic concentrations of Mn, Co and O are listed in Table 2. To evaluate Mn valence states, a peak-fitting deconvolution was performed for these profiles fitted with a mixing of Gaussian and Lorentzian peaks after removal of a Shirley background.

Figure 6. XPS spectra for (A) Mn 2p and (B) O 1s of the catalysts: (a) 10Mn/TiO₂ (773 K); (b) 10Mn-5Co/TiO₂ (673 K); (c) 10Mn-5Co/TiO₂ (773 K); and (d) 10Mn-5Co/TiO₂ (973 K).

In Figure 6A, two peaks were detected at 641.2–642.1 eV and 652.8–653.7 eV, belonging to Mn 2p_3/2 and Mn 2p_1/2, respectively. The spectra of Mn 2p_3/2 could be separated into three peaks. The peaks corresponding to the higher, middle and lower binding energy were attributed to Mn⁴⁺, Mn³⁺ and Mn²⁺, respectively [25,26]. Their relative atomic ratios in the surface layer are listed in Table 2. The increase of Mn⁴⁺ ratio after Co doping indicated the transformation of Mn²⁺ to Mn³⁺ and Mn³⁺ to Mn⁴⁺ by the addition of cobalt oxides. The reduction was resulted from the strong interaction between Mn and Co oxides. In addition, as the calcination temperature increased, the relative ratio of Mn⁴⁺ decreased gradually while the relative ratio of Mn²⁺ increased. The main valence state of manganese in Mn-Co/TiO₂ (673 K) was Mn⁴⁺, whereas Mn²⁺ was the dominant valence state in the catalyst calcined at 973 K. It can be concluded that the Mn⁴⁺ oxidation state was responsible for the activity of NO oxidation at low temperatures.

Figure 6B shows the O 1s XPS spectra for these catalysts. The O 1s spectra can be separated into two peaks at around 528.6–529.2 eV and 530.5 eV. The higher binding energy one with less intensity was ascribed to adsorbed oxygen or surface hydroxyl species [27,28], referred to as O_α, whereas the lower binding energy one was due to lattice oxygen O²⁻ [28,29], denoted as O_β. The value of binding energy of O_β decreased from 529.2 to 528.6 eV when the calcination temperature increased from 773 to 973 K. The binding energies of 529.2 and 528.6 eV were attributed to the lattice oxygen of anatase TiO₂ and rutile TiO₂, respectively. The ratios of O_α:O_β in Table 2 showed an increase in chemical adsorbed oxygen by doping of Co oxides. The reason is that as a nonstoichiometric compound-like, Co oxides can adsorb and exchange oxygen easily in several surface layers [15,30], and then promote oxygen adsorption. In addition, it was found out that the relative ratio of O_α reduced from 54% to 17% when calcination temperature increased from 673 K to 973 K, indicating the inhibition for adsorption of oxygen after high-temperature calcination. The XPS results indicated that the decline of NO oxidation efficiency was related to the decrease of chemical adsorbed oxygen. Wu et al. [31] have suggested that chemisorbed oxygen was helpful to the oxidation of NO to NO₂.

Table 2. Surface atomic concentrations and ratios for the catalysts determined by XPS spectra.

**Table 2.** Surface atomic concentrations and ratios for the catalysts determined by XPS spectra.
Catalyst	Mn⁴⁺:Mn³⁺:Mn²⁺	O_α:O_β	Mn at. %	Co at. %
10Mn/TiO₂ (773 K)	34:36:30	30:70	6.48	-
10Mn-5Co/TiO₂ (673 K)	43:39:17	54:46	5.47	2.32
10Mn-5Co/TiO₂ (773 K)	40:37:23	32:68	6.19	3.73
10Mn-5Co/TiO₂ (973 K)	14:36:50	17:83	6.73	1.54

2.6. H₂-TPR Characterization

Figure 7 presents the H₂-TPR profiles of the Mn-Co/TiO₂ catalysts with various Co contents and calcined at 773 K. The reduction profile of 5Co/TiO₂ was also provided to identify the peaks of Co oxides (Figure 7a). The profile of 5Co/TiO₂ was characterized by two reduction peaks at 693 and 824 K, due to reduction of Co₃O₄ to CoO and CoO to Co°, respectively [32]. In terms of 10Mn/TiO₂ (Figure 7b), two separate reduction peaks were observed. According to the literature [33], the lower temperature one at 670 K (T₁) could be ascribed to the reduction of MnO₂ to Mn₂O₃. The higher temperature peak could be separated to two peaks at 763 and 796 K, corresponding to the reduction of Mn₂O₃ to Mn₃O₄ and Mn₃O₄ to MnO, respectively [33]. The prominent peak at 670 K indicated that MnO₂ was present in Mn/TiO₂.

For the Mn-Co/TiO₂ catalysts with various Co contents (Figure 7c–f), it can be seen that after the addition of Co oxides into Mn/TiO₂, the profiles of the first and second peaks were similar to the reduction peaks of Mn/TiO₂. These two peaks became more and more narrow and their intensities increased with increasing Co content. Meanwhile, the summits shifted toward low temperature with the increase of Co content. As can be seen, after 10 wt. % Co was added into the catalyst, the first peak at 625 K was lower for about 45 K than the first reduction peak of Mn/TiO₂, and lower for about 68 K than that of Co/TiO₂. Consequently, it can be deduced that an interaction existed between Mn and Co oxides, leading to the down-shift of the reduction temperature and related to the higher NO oxidation efficiency at low temperatures for Mn-Co/TiO₂. Combining with the XPS results, the interaction between Mn and Co ions can be represented as reaction (1):

Mn³⁺ + Co³⁺→Mn⁴⁺ + Co²⁺

(1)

Figure 7. H₂-TPR profiles of the catalysts Mn-Co/TiO₂ (773 K) with various Co contents: (a) 5Co/TiO₂; (b) 10Mn/TiO₂; (c) 10Mn-2Co/TiO₂; (d) 10Mn-5Co/TiO₂; (e) 10Mn-8Co/TiO₂; and (f) 10Mn-10Co/TiO₂.

It is known that the Co³⁺ probably corresponds to [Co²⁺Co³⁺₂O₄] with a normal spinel structure [30]. Based on XRD and TEM results, CoMnO₃ is the main physical phase on Mn-Co/TiO₂ (773 K). From the perspective of standard electrode potential, the standard electrode potential of Co³⁺→Co²⁺ is higher than that of Mn⁴⁺→Mn³⁺ [34], and the lower the standard electrode potential, the more easily the coordinated oxygen is obtained. Apparently, if the oxygen vacancy exists in the mixed oxide of Co₃O₄ and CoMnO₃, the oxygen ion will preferentially coordinate to Mn⁴⁺ ion rather than Co³⁺ ion, which will lead to oxygen vacancy of Co₃O₄. Meanwhile, Co₃O₄ adsorbs oxygen easily on Co-contained surface and exchange with the oxygen on several surface atom layers to form Co₃O_4+y^.CoMnO₃ [15,30]. These oxygen ions are highly active and reduced easily, leading to the shift of the reduction peak toward lower temperatures compared with the Co/TiO₂. Consequently, Co doping into Mn/TiO₂ can lower the reduction temperature. When the y is small, we write Co₃O_4+y^.CoMnO₃ as Co₃O₄^.CoMnO₃. Therefore, the first peak of Mn-Co/TiO₂ could be ascribed to the reduction of the compound Co₃O₄^.CoMnO₃. The second and third reduction peak temperatures were also lower than those of Mn/TiO₂ in the similar way. Combining with XRD and XPS results, the second peak could be ascribed to the reduction of the composite oxide CoMn₂O₄ and the third peak could be ascribed to the reduction that the final product was CoO^·MnO. These two peaks trended to combination with increasing Co content, probably due to the strong oxygen adsorption ability of Co oxides. Consequently, the Mn³⁺ ions were easy to be directly reduced to Mn²⁺. The fourth peak was due to the reduction of Co²⁺ to Co°. It shifted toward higher temperature after co-doping of Mn oxides. The Mn³⁺ ions are much reducible than the Co²⁺ ions and request lower activation energy for reduction. Accordingly, the Co²⁺ can be reduced only until the Mn³⁺ ions were reduced completely.

Figure 8 shows the H₂-TPR profiles of 10Mn-5Co/TiO₂ catalysts calcined at various temperatures. The catalysts calcined at 573 and 673 K clearly showed similar reduction peaks. The T₁ and T₂ peaks were mainly due to the reduction of Co₃O₄^.CoMnO₃ and CoMn₂O₄, respectively. The intensities of the two peaks decreased gradually and the peaks broadened with the increase of calcination temperature. It indicated that the Mn-O-Co mixed oxides formation and the oxygen mobility of Mn-O-Co were affected by the calcination temperature. The TPR results were associated with the NO oxidation ability in Figure 2. For the Mn-Co/TiO₂ calcined at 573 and 673 K, the H₂ reduction peaks of T₁ corresponded to their NO oxidation peaks at 443 K. This also demonstrated that the formation of highly dispersed Co₃O₄^.CoMnO₃ through synergistic interaction between Mn-O and Co-O was responsible for the enhanced activities towards NO oxidation. The high calcination temperature leads to catalyst sintering, difficulty in oxygen mobility of Mn-O-Co, the reduction of Mn⁴⁺ to Mn³⁺ and Mn²⁺, and the transformation of CoMnO₃ to (Co,Mn)(Co,Mn)₂O₄. These factors are responsible for the decrease of NO oxidation efficiency.

Figure 8. H₂-TPR profiles of 10Mn-5Co/TiO₂ catalysts calcined at various temperatures: (a) 573 K; (b) 673 K; (c) 773 K; (d) 873 K; and (e) 973 K.

3. Experimental Section

3.1. Catalyst Preparation

The catalysts were prepared by wet impregnating method using TiO₂ P25 (99.5%, Evonic Degussa, Germany) as support materials. Mn(NO₃)₂ solution (50.0%, Damao, Tianjing, China) and Co(NO₃)₃·6H₂O (99.0%, Damao, Tianjing, China) were used as the precursors. The required amounts of precursors were dissolved into 5 mL deionized water and then 6.0 g of support was added into the solution. The mixed solution was stirred for 1 h and left at room temperature for 24 h. Subsequently, the samples were dried at 383 K for 12 h, followed by calcination in air for 3 h at 573–973 K. The manganese loadings were selected as 10 wt. %, and the cobalt contents were varied ranging from 0 to 10 wt. %. The catalysts were simply denoted as xMn-yCo/TiO₂ (T), where x and y represented the weight percentages of Mn and Co to TiO₂, respectively, and T represented the calcination temperature.

3.2. Catalyst Evaluation

The catalytic activity tests were carried out in a fixed bed quartz reactor (i.d. 8 mm) containing 0.15 g of the catalyst (60–100 mesh). The simulated gas mixture (contained 400 ppm NO, 5 vol. % O₂ and Ar balance) was fed to the reactor with a gas hourly space velocity (GHSV) of 42,000 h⁻¹. The reaction temperature was increased from 373 to 593 K at a heating rate of 1 K/min. The reactor temperature was controlled by a thermocouple and a PID-regulation system (CKW-2200, Bachy, Beijing, China). The outlet gas was monitored using an online nitrogen oxides analyzer (EC9841B, Ecotech, Ferntree Gully, Australia) to test the concentrations of NO, NO₂ and NO_x.

The percentage of NO oxidation to NO₂ was calculated as Equation (2):

[Conv.]_NO=([NO]_in − [NO]_out)/[NO]_in × 100%

(2)

3.3. Catalyst Characterization

Specific surface area and pore size distribution of the samples were measured using a BELSORP-mini II instrument (Ankersmid, Holland), through nitrogen adsorption at liquid nitrogen temperature (77 K) after degassing samples in vacuum at 573 K for 3 h.

The crystalline phases of the catalysts were determined by X-ray diffractometer (RIGAKU, D/Max 2500PC, Tokyo, Japan) in the 2θ angle range of 10–80° using Cu Kα radiation combined with nickel filter.

Detailed physical structural characteristics were observed with a transmission electron microscope (Tecnai G² F30, FEI, Hillsboro, OR, USA). Samples were prepared by ultrasonic dispersion in ethanol. The suspension was deposited on a Lacey-carbon film (Beijing, China), which was supported on a copper grid.

The surface oxidation states and atomic concentrations of samples were analyzed by a X-ray photoelectron spectrometer (ULVAC-PHI 1800, Tokyo, Japan) using Al Kα as a radiation source. The binding energy of the C 1s peak at 284.6 eV was taken for correcting the obtained spectra.

The H₂-Temperature programmed reduction experiments were carried out with 0.05 g catalysts under a total flow rate of 40 mL/min. Before the TPR measurements, the catalysts were pretreated in a flow of N₂ at 573 K for 1 h and subsequently cooled to 323 K. Then the TPR runs were carried out from 323 to 923 K with a flow of 5% H₂/N₂ at a heating rate of 10 K/min. The consumption of H₂ was continuously monitored using an online gas chromatograph (GC 6890, Qingdao, China) with a thermal conductivity detector (TCD).

4. Conclusions

A series of Mn-Co/TiO₂ catalysts were prepared by wet impregnation method and developed for the catalytic oxidation of NO to NO₂ below 593 K. The bimetallic catalysts Mn-Co/TiO₂ showed higher catalytic abilities than the Mn/TiO₂ and Co/TiO₂ within the temperature range of 403–473 K. The Mn-Co/TiO₂ calcined at 573 and 673 K showed excellent activities for NO oxidation at low temperatures. The correlations among the catalytic performances and the redox properties of the catalysts were investigated. It was found from the XRD, TEM and H₂-TPR results that the CoMnO₃ mixed oxides were formed, which led to the shift of the reduction peak toward low temperatures. The XPS results revealed the increase of chemical adsorbed oxygen and Mn⁴⁺ ratio by doping Co into Mn/TiO₂. Increasing calcination temperature led to the decrease of chemical adsorbed oxygen amounts and the reduction of Mn⁴⁺ to Mn³⁺ and Mn²⁺. In conclusion, the higher the amount of chemically adsorbed oxygen, the higher the amount of Mn⁴⁺ ions, and the formation of dispersed Co₃O₄^.CoMnO₃ mixed oxides were helpful to the oxidation of NO to NO₂ at low temperatures.

Acknowledgments

This project was financially supported by Foundation Science and Technology innovation Committee of Shenzhen, China (No. JCYJ20140417172417138 and No. ZDSYS20140508161622508).

Author Contributions

Feng Ouyang, Lu Qiu, and Yun Wang contributed to the experimental design. Lu Qiu, Yun Wang and Changliang Zhang contributed to all the experimental data collection. Lu Qiu wrote the first draft of the manuscript, which was then extensively improved by Feng Ouyang, Dandan Pang and Gang Cao.

Conflicts of Interest

The authors declare no conflict of interest.

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Qiu, L.; Wang, Y.; Pang, D.; Ouyang, F.; Zhang, C.; Cao, G. Characterization and Catalytic Activity of Mn-Co/TiO₂ Catalysts for NO Oxidation to NO₂ at Low Temperature. Catalysts 2016, 6, 9. https://doi.org/10.3390/catal6010009

AMA Style

Qiu L, Wang Y, Pang D, Ouyang F, Zhang C, Cao G. Characterization and Catalytic Activity of Mn-Co/TiO₂ Catalysts for NO Oxidation to NO₂ at Low Temperature. Catalysts. 2016; 6(1):9. https://doi.org/10.3390/catal6010009

Chicago/Turabian Style

Qiu, Lu, Yun Wang, Dandan Pang, Feng Ouyang, Changliang Zhang, and Gang Cao. 2016. "Characterization and Catalytic Activity of Mn-Co/TiO₂ Catalysts for NO Oxidation to NO₂ at Low Temperature" Catalysts 6, no. 1: 9. https://doi.org/10.3390/catal6010009

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Characterization and Catalytic Activity of Mn-Co/TiO₂ Catalysts for NO Oxidation to NO₂ at Low Temperature

Abstract

1. Introduction