Catalytic Reduction of NO by CO over Promoted Cu3Ce0.2Al0.8

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Catalytic Reduction of NO by CO Over Promoted CuCe Al Composite Oxides Derived From Hydrotalcite-Like Compounds Junning Qian, Xueyan Hou, Fan Wang, Qun Hu, Haoxuan Yuan, Lixia Teng, Ruonan Li, Zhangfa Tong, Lihui Dong, and Bin Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09041 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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The Journal of Physical Chemistry

Catalytic Reduction of NO by CO Over Promoted Cu3Ce0.2Al0.8

Composite

Oxides

Derived

From

Hydrotalcite-Like Compounds Junning Qiana, Xueyan Houa, Fan Wanga, Qun Hua, Haoxuan Yuana, Lixia Tenga, Ruonan Lia, Zhangfa Tonga, Lihui Dong*ab, Bin Li*ab a Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR. China. b Jiangsu Key Laboratory of Vehicle Emissions Control, Nanjing University, Nanjing 210093, PR. China.

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ABSTRACT: Catalytic reduction of NO by CO have been studied over a battery of Cu3CexAl(1-x) composite oxides catalysts prepared by co-precipitation method. The solids were further characterized by XRD, LRS, N2-physisorption (BET), H2-TPR, ICP-AES, XPS and in situ DRIFTS techniques. The assessment on the catalytic properties were conducted with the NO+CO model reaction. The influences of Cu and different ratios of Ce and Al on the catalytic performance have been investigated. When the ratio of Ce and Al was 1:4, this sample possessed the best catalytic properties, which was exactly derived from hydrotalcite-like compounds. The introduction of a few cerium species (Cu:Ce=15:1) in the structure improved the activity/selectivity towards SCR (Selective Catalytic Reduction) of NO by lowering the temperature of carbon monoxide oxidation.


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1. INTRODUCTION Economic development is accompanied by serious environmental pollution, especially for the air pollution. NOx that origins from coal-fired power plant and automobile exhaust is occupying the major air pollution component. It is one of the reasons that causes the formation of acid rain. besides it can also result in the greenhouse effect. Therefore, the development of low-cost and efficient catalysts for NOx abatement is vital in environmental catalysis. Though many studies had been conducted to find how NOx could be efficiently reduced or captured under lean condition, the reaction mechanism was still not very clear. To overcome this problem, many catalytic processes were investigated, such as NOx storage-reduction (NSR), selective catalytic reduction by NH31 or hydrocarbon (HC-SCR).2 Besides these reductive substances, CO is a primary reductant in some industrial processes.3 The reaction of NOx and CO is an important way to solve these kinds of air pollution, as it meets the concept of green chemistry. Especially, NOx and CO are also the pollution in exhaust. As a potential catalyst, CuO-CeO2-Al2O3 and its descendable system had been gradually studied in the selective catalytic reduction (SCR). Different parts of this catalyst possessed different roles. Copper-based oxides were found to be beneficial for the reduction of NOx.4,5 Ceria that had a good oxygen storage capacity was widely researched in catalysis.6 Al2O3 could promote the overall BET surface so that it was able to improve the catalytic ability in some ways.7,8 Lots of relevant researches on CuO/CeO2/Al2O3 system were mainly investigated by impregnation method.7-10 However, the results of their catalytic performance were not very good. On the whole, the minimum temperature that NO conversion reached 100% was still more than 250 °C. Different methods create different structures, usually the structure of material has an important effect on the properties. Therefore, it is meaningful to manufacture new structure with better properties using the same elements but in other ways. Hydrotalcite-like compounds, they are also named as layer double oxides. The hydrotalcite-like compounds can be respresented by the following formula:



{[M(II)1-xM(III)x(OH)2]x+[(An-)x/n·mH2O]x-,x=(0.2-0.34),

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M(II)

stands

for

any

divalent metal cation, M(III) is related to any trivalent metal cation, and An- stands for the anion}.11 The Hydrotalcite-like compounds (HTLCs) are often used as precursors of mixed metal oxides with their large surface areas, good thermal stability and high metal dispersion.12 Among many applications, they were known to be powerful for hydrocarbons oxidation13-15 and/or NOx reduction with hydrocarbons or ammonia.16,17 In addition, the precursor hydrotalcite materials and their derivatives are also catalytically active in dye decomposition, bio and oxidation. For example, Parida et al.18 investigated the effect of divalent metal ions on the catalytic activity with a series of Cu−Co/Cr ternary LDHs, they found that the layer structure could induce electron transfer and then prevent recombination of electrons and holes. Pavlovic et al.19 found that the existence of saturated heparin layer on the surface resulted in an enormous stabilization effect for Mg-Al-CO32--LDH-SOD-Hep materials in dispersion, such systems possessed huge potential applications in heterogeneous samples. Furthermore, Varga et al.20 prepared Mn-amino acid intercalated CaAl-LDHs, and they successfully achieved the aim of having the complexes exclusively among the layers of the LDHs. However, there were rare literature on the NOx reduction by CO with Hydrotalcite-like compounds. Hence, in order to prepare catalysts with good performance in NO+CO reaction, a series of Cu3CexAl(1-x) composite oxides catalysts were synthesized and characterized. XRD, LRS, N2-physisorption (BET), H2-TPR, ICP-AES, XPS, in situ DRIFTS were employed to characterize these catalysts. Subsequently, the catalytic performance was investigated under NO+ CO reaction. 2. EXPERRIMENTAL SECTION 2.1 Sample Preparation: A series of Cu3CexAl(1-x)-O (x=0, 0.1, 0.2, 0.3, 0.5, 0.7, 0.9, 1) catalysts were prepared through co-precipitation method. The certain number of Cu(NO3)2·3H2O, Al(NO3)3·9H2O and Ce(NO3)3·6H2O were dissolved into the water with the different molar ratios of Al and Ce (Al:Ce=1.0:0, 0.9:0.1, 0.8:0.2, 0.7:0.3, 0.5:0.5, 0.3:0.7, 0.1:0.9, 0:1.0). Then slowly added 150 mL (0.009 mol/L) sodium carbonate solution into the mixed solution. After the process above, added a certain amount of sodium hydroxide solution into the mixed solution and controlled


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the pH of the mixed solution system between 10 and 11. Behind that, stirred the solution and later let it precipitate at room temperature for 18 h. After the process of centrifugal, got the sediment dried at 80 °C for 20 h. Ground it into a fine powder and calcined at 800 °C for 4 h with air flow in the muffle furnace. Finally a series of black compound oxides were obtained. 2.2 Performance Evaluation: X-ray diffraction (XRD) patterns were gained on the D/MAX-RB X-ray diffractometer (Rigaku, Japan) with the Cu Kα (λ= 1.5418 Å) radiation. The X-ray tube worked under 40 kV and 40 mA. These samples were scanned over the 2θ range between 10° and 80° with the rate of 8°/min. Laser Raman spectra (LRS) was gathered on a Renishaw using Ar+ laser beam with the excitation wavelength of 532 nm. The structural characteristics of these samples gained from nitrogen adsorption on a Micrometrics TriStar II 3020 analyzer at 77 K were calculated by using the Brunauer–Emmet–Teller (BET) method for the specific surface area. In each analysis, about 0.1 g sample was degassed at 300 °C for 3 h in N2/He. The reductive performance was detected by the temperature programmed reduction process of FINESORB-3010 chemical adsorption apparatus. First of all, got about 50mg sample pretreated in the flow of N2 at 110 °C for 1 h and then cooled it to the room temperature. Turned on the stream of 7.03 vol% H2/Ar (10 mL/min) for half an hour. Behind that, the sample was heated at the rate of 10 °C/min from room temperature to 400 °C in N2 flow. ICP-AES results were gathered on a PerkinElmer Optima 5300 DV using the RF power of 1300 V. X-ray photoelectron spectroscopy (XPS) analysis was performed on the American Thermo ESCALAB 250Xi system, used the monochromatic Al Kα radiation (hv=1486.6 eV) operating with power of 150 W. The adventitious C1s peak at 284.6 eV was compensated for the existence of charging effect. In situ Fourier transform infrared (in situ DRIFTS) spectra was obtained on the Nicolet iS50 DRIFTS spectrometer, which was equipped with a MCT operating at a spectral resolution of 4 cm-1. The sample was pressed into an in situ chamber and flattened in the controlled environment room. In order to remove the impurities in the device, the sample was then pretreated with purified N2 at 300 °C for 1 h. Behind that procedure, the sample was treated with a controlled stream of CO-Ar ACS Paragon Plus Environment


(10% volume CO) or/with NO-Ar (5% volume NO) at a rate of 10.0 mL·min-1 for 40 min. Then the DRIFTS spectra was gained from 50 °C to 300 °C by deducting the corresponding background reference spectrum. 2.3 Catalytic Activity Measurements: Reaction NO+CO was aimed to gain the information of catalytic performance. The reaction was implemented with a stream of a certain composition 5% NO, 10% CO and 85% He, and the corresponding GHSV (Gas Hourly Space Velocity) was 12 000 h−1. So as to remove the impurities on the surface, around 50 mg (40–60 mesh) sample was piled into a quartz tube and treated with the stream of N2 at 110 °C for 1 h. After that, cooled it down to the room temperature before passing into the mixed gas. Then the reaction was carried out from 150 °C to 300 °C. When the reaction was completed, recorded the data of NO conversion, N2 selectivity and N2 yield. 3. RESULTS AND DISCUSSION 3.1 Structural Characteristics (XRD and LRS): The XRD patterns of precursors are shown in Fig. 1a. It can be seen that only the precursors of Cu3Al-O and Cu3Ce0.2Al0.8-O present a typical layered structure of intercalated carbonate anions.21 They show some different crystallographic planes of (003), (006), (012), (015) and (018), which are the characteristic planes of Cu-Al LDHs. The result can be well matched with the literature.22 This means that only the ratio of Ce and Al is proper can the HT-like structure form. Fig. 1b displays a series of XRD of Cu3CexAl(1-x)-O (x=0, 0.1, 0.2, 0.3, 0.5, 0.7, 0.9, 1) samples. Pure CuO and CeO2 are also presented in Fig. 1b to compare with those samples. All of the diffraction peaks in the picture can be retrieved and they belong to the mono-clinic crystal structure CuO (PDF-ICDD80-1916) and cubic fluorite-type CeO2 (PDF-ICDD34-0394).23 The typical diffraction peaks at 32.5°, 35.5°, 38.7°, and 38.9° match the crystallographic planes of (110), (-111), (111) and (-202), accordingly. The diffractogram patterns of CeO2 at 28.6°, 33.1°, 47.5°, 56.4° correspond to the (111), (200), (220) and (311) crystallographic planes. Moreover, except the sample Cu3Ce-O, the others calcined at 800 °C all emerge the CuAl2O4 spinel phase (PDF-ICDD33-0448), which shows two main peaks at 31.3°, 36.9°. No other impurity


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peaks are found, it indicates the high purity of these samples. The characteristic peaks attributed to Al2O3 are absent, suggesting that Al2O3 species are highly dispersed in amorphous state.24 Comparing the pure CuO with Cu3Al-O on the crystalline plane of CuO (111) in Fig. 1(b), it is not difficult to find that Al3+ doesn’t enter into the CuO crystalline. The ionic radii of Cu2+ and Ce4+ are 0.072 nm and 0.102 nm, respectively. When Ce4+ partially occupies the place of Cu2+, the CuO lattice consequently expands. According to the Bragg formula 2d sin θ = nλ , the increasing plane interval distance naturally causes the diffraction peak to move to lower angle, which should be responsible for the change of CuO diffraction angle. The intensity of CuO peak turns to get down but wider than pure CuO with the increase of CeO2 amount. It should be ascribed to reason that the addition of CeO2 restrains the development of CuO crystallize.25 Moreover, the intensity of CeO2 peak gets enhanced and it also becomes sharper with the increase of Ce content. This result shows that the crystal phase of CeO2 increases continuously on the surface of the samples. Fig. 1c shows the Raman spectra of the samples. It can be seen that Raman peaks at 278, 342 and 624 cm-1 belong to CuO.26 The three Raman bands centered at 296, 345 and 629 cm-1 correspond to the vibration modes of crystalline CuO. Moreover, the Raman bands of Cu3Al-O sample are nearly close to those of pure CuO, which indicates that the function between Al2O3 and CuO is not strong. The Raman bands shift to higher wave-number direction and the peak intensity also becomes stronger with the increase of Ce content. In addition, the Raman bands of Cu3Ce-O shift to the higher

direction

than

that

of

other

samples.

Furthermore,

the

samples

[(Cu3CexAl(1-x)-O)(x≤0.5)] display broader Raman band and larger value of FWHM, which could be possibly connected with the formation of oxygen vacancy and defect on the surface structure.27,28 However, it reflects an opposite case for these samples [(Cu3CexAl(1-x)-O)(x>0.5)]. As seen in Fig. 1b, the precipitation of CeO2 keeps increasing with the increase of Ce content, so the oxygen vacancies on the surface of these samples are more covered by CeO2. It’s known that the band at 462 cm−1 belongs to the Raman vibrational F2g of CeO2 lattice, which is attributed to the vibrational mode of fluorite structure.29,30 This band is higher than that of pure ACS Paragon Plus Environment


CeO2,29 it should be related to result that Cu2+ enters into the lattice of CeO2 on the surface.31 A few internal oxygen vacancies also form with the interaction of Cu and Ce cations.32 Combining with XRD results, the changes of width and height for the bands reflect the existence of oxygen vacancies.

3.2 Textural Characterization (N2-Physisorption): The N2 desorption-adsorption isotherms are shown in Fig. 2. All samples belong to the classical IV-type isotherms with H3-type hysteresis loops, which means that they include the mesopore (2–50 nm) and narrow plate-like particles.33 It is obvious that a relatively higher desorption branch observed in high-pressure range (P/P0>0.8) appears in each sample. While the adsorption branches rises slowly in low-pressure range (P/P0≤0.8), it suggests that the dense mesopore exist in all samples. These densely distributed mesopores play an important role in the capacity of adsorption in high-pressure range. Similarly, the appearance of Hysteresis loops is relevant to capillary condensation effect. At the same time, the hysteresis loop becomes smaller with the increase of Ce content. As shown in Table 1, the pore volume generally presents a decreasing trend with the increase of Ce content, and the size of pore volume decreases from 0.208 cm3/g to

0.06 cm3/g. These changes are mainly due to the gradual accumulation of CeO2 in the pore.34 The increasing amount of CeO2 is in agreement with the XRD results. Furthermore, the BET surface generally starts to decrease with the decrease of Al content until the ratio of Ce and Al is 1, because the existence of Al2O3 in system can improve the BET surface.35 Interestingly, as the content of Ce further increases, the BET surface tends to increase. In large part, it is due to a decrease in the structural stability of Al2O3.34 The structural collapse degree is gradually increasing with a continuously incremental amount of CeO2 precipitation, so that the BET surface gets increased.

3.3 The Analysis of Reduction with H2-TPR: The curves of H2-TPR are presented in Fig. 3. It can be seen that there are three reduction peaks ( α, β, γ ) in the picture. The CeO2 presents two reduction peaks, one is a high-temperature peak (~850 °C) and the other one is a low-temperature peak (~500-600 °C).36-38 So the three peaks (

Catalytic Reduction of NO by CO over Promoted Cu3Ce0.2Al0.8

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