Evaluation of NO Oxidation Properties over a Mn-Ce/γ-Al2O3 Catalyst

With the purpose of studying the effect of diesel oxidation catalyst (DOC) on the NO oxidation activity, a series of xMn10Ce/γAl 2 O 3 (x = 4, 6, 8, and 10) catalysts were synthesized by acid-aided sol-gel method.The physicochemical properties of the catalysts were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and Transmission Electron Microscope (TEM). Result showed that the crystalline size of MnOx and CeO2 ranges from 5 nm to 30 nm and manganese existed mainly in the catalysts in the form of manganese dioxide. Moreover, NO oxidation experiments were carried out to evaluate the activity of the catalysts; according to the results, 6Mn10Ce/γ-Al 2 O 3 catalyst showed the supreme NO oxidation activity with a NO to NO 2 conversion rate of 83.5% at 300C. Compared to 500 ppm NO inlet concentration, the NO conversion was higher than that of 750 and 1000 ppm NO over 6Mn10Ce/γ-Al 2 O 3 catalyst in the temperature range of 150–300C.


Introduction
Diesel engines are widely used as commercial vehicle power due to the higher thermal efficiency, durability, and fuel economy compared to gasoline engines.However, with increasingly stringent vehicle exhaust gas emission regulations and the attention to health, the NO x (NO, NO 2 ) emission from diesel engines that caused many serious environmental problems, such as acid rain and photochemical smog, has met severe challenge [1,2].
Recently, catalysis technique adopted to oxidize NO to NO 2 has been attracting enormous attention due to its role in several catalytic processes, such as NO x reduced by SCR with hydrocarbons and soot combustion in the atmosphere of NO x /O 2 [3,4].The diesel oxidation catalyst (DOC) can not only oxidize CO, HC, and PM, but also facilitate the oxidation of NO to NO 2 , which is beneficial for NO x removal by the downstream SCR or NSR units [5,6]; for urea SCR, a NO 2 : NO ratio of 1 : 1 is the most effective.In the NSR process, NO needs to be oxidized to NO 2 before being absorbed by storage components during the lean phase.However, according to the literature [7], the amount of NO 2 in diesel engine exhaust is usually less than 10% in the total NO x .Hence, to improve the NO 2 /NO x ration is of great necessity for the sake of improving the posttreatment efficiency.
Many metal mixed catalysts, such as CeO 2 /ZrO 2 [8] and MnO x -TiO 2 [9], can effectively oxidize NO to NO 2 .The MnO x -CeO 2 mixed oxides have been reported to have much higher catalytic activities compared to the individual CeO 2 and MnO x , for the reason that the strong interaction between MnO x and CeO 2 can form solid solution [10,11].Li and coworkers reported that Mn-Ce oxides were active for NO to NO 2 oxidation with about 80% NO conversion at 150 ∘ C over Mn-Co-Ce(20)-400 catalyst [12].According to the literatures [13][14][15][16], CeO 2 has been widely studied for its oxygen storage and redox properties, among which the most important property is that ceria can store and release oxygen via the redox shift between Ce 4+ and Ce 3+ under oxidizing and reducing conditions.
In this work, in order to study the NO oxidation performance by DOC, Mn-Ce/-Al 2 O 3 mixed oxides catalyst was synthesized by acid-aided sol-gel method.X-ray diffraction (XRD), scanning electron microscope (SEM), and Transmission Electron Microscope (TEM) were carried out to investigate the physicochemical properties of the catalyst structure, and NO oxidation experiments were carried out to evaluate the activities of the catalysts.

Experimental
2.1.Catalyst Preparation.A series of Mn10Ce/-Al 2 O 3 ( = 4, 6, 8, and 10, wt%) catalysts were prepared by acidaided sol-gel method as described in the literature [17].First, Ce(NO 3 ) 3 ⋅6H 2 O, C 4 H 6 MnO 4 ⋅4H 2 O, and -Al 2 O 3 were weighed by different Mn/Ce molar ratio and dissolved in deionized water, respectively, to obtain solution, into which citric acid was added, the amount of which was twice the sum of Ce 3+ and Mn 2+ total molar.Then polyethylene glycol, the quality of which is 10% of the citric acid, was introduced and magnet-stirred at 80 ∘ C until transparent gel was obtained.Then it was dried at 110 ∘ C for 24 h and calcined in air for 5 h at 500 ∘ C. Then the powder was ball-milled to obtain the required 20-40-mesh powder.

Catalyst
Characterization.X-ray diffraction (XRD) of the as-prepared sample was measured on a Bruker D8 ADVANCE X-ray diffractometer with a Ni-filtered Cu K ( = 0.154068 nm) radiation source at 40 kV and 40 mA.Powder XRD patterns were recorded at 0.02 ∘ interval in the range of 20 ∘ ≤ 2 ≤ 80 ∘ with a scanning velocity of 7 ∘ min −1 .The crystalline size was calculated by Scherrer's formula  = K/( cos ).Transmission Electron Microscope (TEM) measurement was done by using a Philips Tecnai 12 microscope, with a 120 kV accelerating voltage.Before testing, the catalyst powder was ground and dispersed ultrasonically in anhydrous ethanol and then dropped onto a carbon-coated Cu grid of 200 mesh.Scanning electron microscope (SEM) testing was performed on JSM-7001F microscope made by JEOL.Accelerating voltage was 0.5-30 kV, magnification 10-800 K, and resolution 1.2 nm (30 kV)/3.0nm (1 kV).

Catalyst Evaluation.
The oxidation of NO to NO 2 activity was measured at the temperatures in the range of 150-450 ∘ C. The catalysts were placed in a fixed-bed quartz mircoreactor with an inside diameter of 10 mm and plugged and sandwiched between two silica wool layers to prevent the sample from being blown away.The gases used in the tests were 500 ppm NO, 10% O 2 , and N 2 in balance with a total flow rate of 280 mL/min, giving a space velocity of 56,000 h −1 .The outlet NO and NO 2 concentrations were detected by Thermo Scientific Model 42i-HL NO x Analyzer.Before each NO oxidation experiment, the catalyst sample was heated to 450 ∘ C in N 2 atmosphere for 30 min, in order to clean up the residual gases adsorbed in the catalyst surface, and then cooled to the required measurement temperatures.200), (220), and (311) planes of cubic fluorite CeO 2 (JCPDS: PDF43-1002) [18,19].Moreover, the diffraction peaks of CeO 2 shifted to lower degrees for about 0.5 ∘ , the reason of which is that a part of Ce 4+ ions is replaced by Mn 4+ and Mn 3+ to form solid solution [20].According to Scherrer's formula, the crystalline size of CeO 2 (111) is 26 nm.Other two diffraction peaks at 2 = 28.823∘ and 56.197 ∘ (which are overlapped with the peaks of CeO 2 ) are identified as the phase of MnO 2 (JCPDS: PDF 65-7467).In addition, when  = 4, the diffraction peaks of Mn 2 O 3 are not detected by XRD, which is in accordance with the small amount of manganese oxides, while the intensity increased with  (when  ≥ 6).

SEM and EDS
Analysis.SEM analysis of as-prepared catalysts was used to elucidate the surface morphology and distribution of the different components present.Figure 2(a) shows the micrograph of 6Mn10Ce/-Al 2 O 3 catalyst.As can be seen, the catalyst presents a morphology in the form of crisscrossed nanorods, which is -Al 2 O 3 , on which there are some particles of 10-30 nm (red circles) deposited that are CeO 2 nanoparticles; the result is in accordance with the XRD (the size of CeO 2 particle is 26 nm).In addition, some agglomerate particles with different sizes (80-300 nm) also deposited on -Al 2 O 3 , which can be the aggregates of MnO x and CeO 2 (black circles).

TEM Analysis.
TEM is executed to ascertain the morphology of the catalyst.Figure 3 shows the morphology of 6Mn10Ce/-Al 2 O 3 catalyst.Obviously, the main structure of the catalyst is strip-shaped with nanosize, which is similar to the SEM analysis results.The small near-spherical particles with the size of about 5∼30 nm (marked by the red circles), homogeneously dispersed on the surface of the catalyst, could be regarded as CeO 2 or MnO x nanoparticles.Besides, the darker zone on the TEM pattern indicates a certain degree of aggregation between CeO 2 and MnO x .

Effect of Temperature on NO Conversion.
Results on NO conversion as a function of temperature over Mn10Ce/-Al 2 O 3 ( = 4, 6, 8, and 10) catalysts are given in Figure 4.It is obvious that, in the range of 150 ∘ C to 300 ∘ C, NO conversion of all the catalysts increased with temperature, but when temperature is above 300 ∘ C, NO conversion decreased with temperature except for 4Mn10Ce/-Al 2 O 3 catalyst (maximum NO conversion at 350 ∘ C).The NO conversion rate decreased mainly due to the accelerated thermal decomposition of NO 2 under the influence of high temperature.In the whole range of temperature, the steady-state NO conversion goes  catalyst obtained the best NO oxidation activity when the Mn/Ce ratio is 6/10.

Effect of NO Concentration on NO Conversion.
As described above, the 6Mn10Ce/-Al 2 O 3 catalyst showed the best activity on NO to NO 2 conversion of 83.5% at 300 ∘ C. In order to further study the effect of inlet NO concentrations on the NO to NO 2 oxidation activity, the experiment, in the atmosphere of 500, 750, and 1000 ppm NO in 10% O 2 with N 2 as balance, respectively, was carried out.The NO conversion rate was calculated when the outlet NO concentration stabilized for 400 s.Results on NO conversion as a function of temperature over 6Mn10Ce/-Al 2 O 3 catalyst are given in Figure 5.The result indicates that, in the range of 150-300 ∘ C, when inlet NO was 500 ppm, NO conversion rate was 32.9%-83.5%.In addition, with inlet NO concentration increasing, the NO conversion deceased by around 3% (NO = 750 ppm) and 5% (NO = 1000 ppm), respectively.Moreover, the NO conversion was almost maintained the same, whatever the inlet NO concentration in the range of 350-450 ∘ C was.However, the catalysts are sensitive to other pollution gases, such as SO 2 , CO, and H 2 O, which was still needed for further research.

Conclusions
In the present work, a series of Mn10Ce/-Al 2 O 3 ( = 4, 6, 8, and 10) catalysts were prepared by acid-aided sol-gel method and found to be well crystallized and dispersed, CeO 2 and MnO x with the crystalline size of 5-30 nm.Compared with other catalysts, 6Mn10Ce/-Al 2 O 3 catalyst showed higher activity for NO oxidation to NO 2 over the temperature range of 150-450 ∘ C, up to 83.5% at 300 ∘ C.
In addition, NO oxidation to NO 2 was further studied under different NO concentrations over 6Mn10Ce/-Al 2 O 3 catalyst.Generally, NO conversion augmented rapidly with temperature elevating in the range of 150-300 ∘ C for all different NO conditions.On the contrary, NO conversion decreased with temperature above 300 ∘ C. NO conversion reached maximum of 83.5%, 80.9%, and 76% for 500 ppm, 750 ppm, and 1000 ppm NO concentration, respectively, at 300 ∘ C. We also found that there was almost no effect of NO concentration on NO conversion above 350 ∘ C.

Figure 2 (
b) shows the surface elements of the sample; the main elements are Al, Mn, Ce, O, and C, which are from the white box area in Figure 2(a), among which the presence of C element can be attributed to the decomposition of citric acid and C 4 H 6 MnO 4 ⋅4H 2 O at high temperature during calcination.