MSCE  Vol.5 No.12 , December 2017
Effects of CuO-CeO2 Addition on Structure and Catalytic Properties of Three Way Catalysts
ABSTRACT
The noble metals (Pt, Pd, Rh) supported on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate were investigated as catalytic performance of Three Way Catalysts (TWC) under simulated automotive exhaust feed gas. The structural, morphological features and catalytic activity were observed by X-ray diffractometry (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS) and GC-TCD (Varian CP-4900). The catalytic performance of noble metals (Pt, Rh, Pd) supported on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate was be compared with noble metals (Pt, Rh, Pd) supported on Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and only γ-Al2O3 washcoat/FeCrAl substrate at various stoichiometric ratio of oxygen. The results showed that the addition of Cu-Ce mixed oxides improved CO oxidation reaction at lower temperature during stable lambda of 1, the highest CO conversion of 99% is observed for the noble metals (Pt, Pd, Rh) support on Cu-Ce with γ-Al2O3 washcoat/FeCrAl substrate. The results also showed that, the addition of Cu-Ce mixed oxides promoted released oxygen, thus it improved strongly CO and C3H8 conversion at lean oxygen stoichiometric operation.

1. Introduction

Three way catalysts (TWCs) are capable of simultaneous converting CO, hydrocarbon (HC) and nitrogen oxides (NOx) into harmless CO2, H2O and N2. In TWCs, some noble metals, such as Pt, Rh and Pd act as the active components. Oxygen storage capacity (OSC) is one of the crucial factors for the performance of TWCs, and the higher OSC promotes the better dynamic performance of catalysts in converting CO, HC and NOx under conditions from rich to lean A/F (λ-value) in automotive. CeO2-ZrO2 solid solution is well-known as an excellent supporter for OSC [1] . CeO2 exhibits oxygen storage/release behavior by the redox reaction of Ce ions between Ce3+ and Ce4+ [2] , and the introduction of ZrO2 into CeO2 improves the reduction temperature of CeO2 through structural modification of CeO2 [3] . Among many studies on CeO2 based materials such as CeO2-Al2O3 [4] , CeO2-SiO2 [5] , CeO2-La2O3 [3] [6] [7] , CeO2-TbOx [8] , and CeO2-PrOx [9] have been reported to improve OSC and increase the thermal stability.

As legislation becomes tighter, it is necessary to improve the efficiency of TWCs at lower temperatures and under oxygen-rich atmospheres. The copper/copper oxides as CuMO (M = Al, Fe, Mn, Ga) [10] have been found with its oxygen storage/release behavior at lower temperatures. Recently, it was found by authors that CuO-CeO2 prepared by mechanical milling shows excellent OSC at lower temperatures [11] , and it showed higher value of total OSC due to the valence change between Ce4+/Ce3+ and Cu2+/Cu+/Cu. So it is necessary to know the effect of noble metals supported on Cu-Ce mixed oxides with alumina washcoat. Many reports based on the thermal stability and catalytic performance of noble metals coated on CeO2-ZrO2 or coated on γ-Al2O3 which supported for traditional TWCs was shown in previously reported [12] [13] [14] . H. He et al. reported the performance and redox properties of Pd, Pt, Rh loaded Ce0.6Zr0.35Y0.05O2 [12] . The aim of this study is to investigate catalytic performance of noble metals supported on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate. The catalytic performance of noble metals (Pt, Rh, Pd) supported on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate will be compared with noble metals (Pt, Rh, Pd) supported on Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate.

2. Experimental Section

2.1. Catalysts Preparation

Powdery CuO, CeO2 (Kojundo Chemical) and γ-Al2O3, ZrO2 (Nilaco Corporation) were used as starting materials. The powder mixture of CuO-CeO2 (50 mol% of CuO content) and CeO2-ZrO2 (20 mol% of ZrO2 content) were milled by using a high-energy vibratory ball milling up to 18 hours (dry milling).

The Fe-Cr (17 - 21 wt%)-Al (2 - 4 wt%) alloy sheet (Nilaco Corporation) was used as the substrate. In order to achieve good adhesiveness between the substrate and the washcoat layer, the substrate was first immersed in an HCl solution (69 wt%) for 2 - 3 min to increase the roughness, followed by immersion in an HNO3 solution (68 wt%) at 80˚C for 5 min to clean the superficial oxide. The substrate was then pre-oxidized at 900˚C for 10 h to produce a fine precipitation layer of alumina on the substrate, which is known to exhibit a good contact with a γ-Al2O3 washcoat layer [15] . Finally, the treated substrate was rinsed with acetone. Dip-coating method was employed to deposit a γ-Al2O3 washcoat/FeCrAl substrate as well as to deposit a Cu-Ce and Ce-Zr mixed oxides layer on γ-Al2O3 washcoat/ FeCrAl substrate.

The γ-Al2O3 slurry was prepared by mixing 23 mass% of the Al(NO3)3 binder solution (Wako), 23 mass% of γ-Al2O3 powder, and 54 mass% of distilled water, followed by vigorous stirring (600 rpm) for 8h at room temperature to make slurry solution. After the dip-coating process, the Al2O3 washcoat layer was dried at room temperature for 30 min and heated at 250˚C for 2 h, followed by sintering at 650˚C for 2.5 h [16] , where the atmospheres were ambient.

In the Cu-Ce and Ce-Zr mixed oxides deposition, the CuO-CeO2 slurry and CeO2-ZrO2 slurry were prepared by mixing 23 mass% of the Al(NO3)3 binder solution, 23 mass% of milled CuO-CeO2 powder and CeO2-ZrO2 powder and 54 mass% of distilled water, followed by high-energy ball milling (wet milling) for 30 h. The dip-coating process, drying and sintering procedure was repeated as γ-Al2O3 deposition.

The noble metals (Pt, Pd, Rh) supported on Cu-Ce mixed oxides, Ce-Zr mixed oxides with γ-Al2O3 washcoat on FeCrAl substrate and γ-Al2O3 washcoat on FeCrAl substrate were prepared by being impregnated with the mixed solution of Pt(NO3)2, Pd(NO3)2 and Rh(NO3)3 (Wako), the mole ratio of Pt:Pd:Rh = 1:14:1 (total 3.7 gram/L). The total loading amount of noble metal was wt. 4% and kept at the same weight level for Cu-Ce, Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate respectively.

2.2. Surface Characterization

The structure and morphology of catalysts samples were analyzed by X-ray diffractometry (XRD) using Cu-Kα radiation (RINGAKU RINT-2100CMT) and by scanning electron microscope (SEM, TEOL JSM-5800). The surface area was estimated by N2 adsorption method (Brunauer-emmett-Teller). X-ray photoelectron spectroscopy (XPS) was carried out on a JEOL XPS using monochromatic Mg Kα radiation.

2.3. Catalytic Activity Measurements (CO, C3H8, NO)

The simulant exhaust gas containing O2, CO (1.5%), H2 (0.5%), CO2 (12%), C3H8 (0.1%), NO (0.05%) and N2 (balance) was prepared, the λ value as oxidants/reductions factor was defined to be λ = (2O2 + NO)/(CO + H2 + 10C3H8). The λ was adjusted by controlling the concentration of oxygen, CO, C3H8 and NO conversion was analyzed by GC-TCD (Varian CP-4900), the samples were put in a reaction tube (i.d = 8 mm) made from quartz. The catalytic performances were carried out at various oxygen stoichiometric operations. At stationary stoichiometric operation (λ = 1, oxygen concentration of 1.5%), the catalytic performance was measured at the reaction temperature of 30˚C - 540˚C, the heating rate of 3˚C/min and the gas flow of 20 ml/min. At lean/rich oxygen stoichiometric operation (oxygen concentration was changed from 2.1% to 0.42% respectively, λ value in range of 1.4 - 0.3), the catalytic performances were measured at the temperature of 500˚C.

3. Results and Discussion

3.1. Textural, Structural and Morphological Characterizations

Table 1 shows the surface areas of the noble metals (Pt, Pd, Rh) support on Cu-Ce, Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate. The surface area of 142 m2/g is demonstrated for Pt, Pd, Rh/γ-Al2O3/FeCrAl substrate, surface area trends decrease due to the deposition of Cu-Ce and Ce-Zr mixed oxides on γ-Al2O3 washcoat, the lower surface area with the CuO content is probably due to the formation of the soft Cu phase, which may cause agglomeration of powders.

Figure 1 shows XRD patterns of the noble metals (Pt, Pd, Rh) support on Cu-Ce, Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and the noble metals (Pt, Pd, Rh) supported on γ-Al2O3 washcoat/FeCrAl substrate. The peaks of FeCrNi, γ-Al2O3 and α-Al2O3 are observed after pre-oxidized FeCrAl substrate at 900˚C for 10 h (Figure 1(a)) [15] , γ-Al2O3 peaks become sharper and the intensity of FeCrAl decrease slightly when γ-Al2O3 is deposited on FeCrAl substrate (Figure 1(b)). The reflection peak intensities of the CeO2, CuO, ZrO2 phase occurred after the CeO2-CuO and ZrO2-CeO2 slurry is deposited on γ-Al2O3/FeCrAl substrate (Figure 1(c) and Figure 1(d)), no phase change of CuO, ZrO2 and CeO2 are observed. The XRD also shows that, no peaks of noble metals (Pt, Pd, Rh) or noble metal oxides are observed due to tiny loading of the noble metal (Pt, Pd, Rh) support on Cu-Ce, Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl and γ-Al2O3 washcoat/FeCrAl substrate.

Figure 2 shows the morphology Pt, Pd, Rh/CuO-CeO2/γ-Al2O3/FeCrAl substrate. A cross-sectional SEM image of the layered catalyst is shown in Figure 2(a), the particles appear often agglomerated, the fine particles of agglomerates (of size between 5 and 10 μm) are spread on the γ-Al2O3 washcoat surface, those particles are interlocked with each other and the tighter the packing by bond mechanical or interfacial nature, the wachcoat thickness is about 20 μm, the Figure 2(b) shows surface of CuO-CeO2/γ-Al2O3/FeCrAl substrate, the higher magnification reveals a compact CuO-CeO2 microstructure of aggregated nano-particles.

Table 1. The surface areas of the noble metals (Pt, Pd, Rh) support on Cu-Ce, Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate.

Figure 1. XRD patterns of (a) FeCrAl after pre-oxidation at 900˚C for 10 h; (b) Pt, Pd, Rh/g-Al2O3 washcoat/FeCrAl; (c) Pt, Pd, Rh/CeO2-CuO/γ-Al2O3 washcoat/FeCrAl; (d) Pt, Pd, Rh/ZrO2-CeO2/γ-Al2O3 washcoat/FeCrAl.

(a) (b)

Figure 2. SEM photographs of Pt, Pd, Rh/CuO-CeO2/γ-Al2O3/FeCrAl substrate (a) interface; (b) surface.

3.2. Chemical State Analysis of Pt, Pd, Rh/CuO-CeO2/γ-Al2O3/FeCrAl Substrate

The noble metals (Pt, Pd, Rh) support on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl (sintered at 650˚C for 2.5 h) is used for the chemical state analysis of noble metals. The Figure 3(a) shows Pt 4f photoelectron spectra of sample, the doublet at 71.1 and 74.2 eV are observed which may be attributed to Pt0 sites. The Figure 3(b) shows Pd 3d photoelectron spectra, the doublet at 336.1 eV and 338.1.1 eV can be attributed for Pd2+ and Pd4+. The Figure 3(c) shows Rh 3d for sample, the Rh 3p5/2 peaks are observed at 306.0 eV and 308.3 eV can be attributed for Rh0 and Rh3+ respectively. The results show that the chemical state of noble support on Cu-Ce mixed oxides with γ-Al2O3 washcoat/ FeCrAl. Xiaodong Wu et al. [12] or S.Suhonen et al. [17] reported that those

Figure 3. Photoelectron spectra of (a) Pt 4f; (b) Pd 3d and (c) Rh 3d support on CuO-CeO2/γ-Al2O3/FeCrAl substrate.

chemical states depend strongly on heated treatment condition, the noble metals oxide are observed after sintering of 650˚C for 2.5 h, the reason for this may be due to the incomplete decomposition of noble metals salt.

3.3. Catalytic Performance of Samples

Figure 4 shows CO, C3H8, NO conversion of the noble metals (Pt, Pd, Rh) support on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl as a function of temperature at λ = 1. The reactions appear at the temperatures of 180˚C, with the increase of temperature in range from 30˚C to 540˚C, the CO, C3H8 and NO conversion increase rapidly, the highest CO conversion of 99% is observed while C3H8 and NO conversion are 62% and 84.3% respectively. Figure 5 shows comparative CO, C3H8, NO conversion of the noble metals (Pt, Pd, Rh) support on Cu-Ce, Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate due to the increase of reaction temperature in range from 30˚C to 540˚C and at λ = 1, the highest CO conversion performance is observed for the noble metals (Pt, Pd, Rh) support on Cu-Ce with γ-Al2O3 washcoat/FeCrAl substrate, the result also shows that the CO oxidation reaction of the noble metals (Pt, Pd, Rh) support on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate appears at temperature of 150˚C which is lower than the noble metals (Pt, Pd, Rh) support on Zr-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate (Figure 5(a)). It is reasonable to consider that the additive Cu promotes CO oxidation reaction, Meng-Fei Luo et al. [18] and G. Avgouropoulos et al. [19] are also reported that the additive Cu promotes CO oxidation reaction at low temperatures. The same trend of C3H8 and NO conversions are observed for the noble metals (Pt, Pd, Rh) support on Cu-Ce, Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate (Figure 5(b) and Figure 5(c)).

Figure 6 shows comparative CO, C3H8, NO conversion at 500˚C of the noble metals (Pt, Pd, Rh) support on Cu-Ce, Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate at rich/lean

Figure 4. CO, C3H8, NO conversion of the noble metals (Pt, Pd, Rh) support on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl at λ = 1.

(a) (b) (c)

Figure 5. Comparative (a) CO; (b) C3H8; (c) NO conversion of the noble metals (Pt, Pd, Rh) support on Cu-Ce, Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate at λ = 1.

(a) (b) (c)

Figure 6. Comparative (a) CO; (b) C3H8; (c) NO conversion at 500˚C of the noble metals (Pt, Pd, Rh) support on Cu-Ce, Ce-Zr mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate at rich/lean oxygen stoichometric.

oxygen stoichometric. At lean oxygen stoichometric, CO and C3H8 conversion performance of the noble metals (Pt, Pd, Rh) support on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate decrease from 99.1% to 33.3% and from 55.1% to 16.2% respectively during the decrease of λ from 1 to 0.3 (Figure 6(a) and Figure 6(b)), where flat NO conversion of 55.7% is also observed (Figure 6(c)). Compared with CO and C3H8 of the noble metals (Pt, Pd, Rh) support on Zr-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate, those of the noble metals (Pt, Pd, Rh) support on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate is higher those of noble metals (Pt, Pd, Rh) support on Zr-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate (from 96.3% to 20% and from 53.9% to 13% respectively) and the γ-Al2O3 system (from 95.7% to 15.1% and from 53.3% to 10.3%). It can be suggested that the high OSC activities play an important role in the oxidation of CO and C3H8.

Figure 7 shows the amount of oxygen released from oxygen storage materials to support oxygen for the oxidation reaction of CO and C3H8, during the

Figure 7. Oxygen release capacity at 500˚C of the noble metals (Pt, Pd, Rh) support on (a) Cu-Ce and (b) Zr-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate at lean oxygen stoichiometric.

decrease of λ values from 1 to 0.3. The amount of released oxygen is calculated based on γ-Al2O3 washcoat which results depict no release of oxygen. The result shows that amount of released oxygen the noble metals (Pt, Pd, Rh) support on Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate is much higher those of the noble metals (Pt, Pd, Rh) support on Zr-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate. It has been reported [11] that a ceria-copper oxide compound (CuO-CeO2) prepared by high energy mechanical milling to significantly promote the OSC during the valence change of Cu2+/Cu and Ce4+/Ce3+. It is also quite interesting that the amount of released oxygen from oxygen storage materials is found during the decrease of λ from 1 to 0.3. It is reasonable to believe that the released oxygen may improve CO and C3H8 conversion at lean oxygen condition. Thus, it can improve the efficiency of TWCs.

Under an engine real working conditions, the value of λ oscillates at around 1 with a frequency of about 1 Hz [1] . Hence, the efficient conversion of TWCs decreases during the oscillation of λ. The dynamic of released oxygen from oxygen storage materials is therefore an important parameter in the improvement of the TWCs conversion efficiency. The dynamics of released oxygen was estimated for values of λ from 1 to 0.9. Result shows that, the dynamics of released oxygen of the noble metals (Pt, Pd, Rh) support on (a) Cu-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate (6.7 μmol∙mg−1∙s−1) is much higher than those of the noble metals (Pt, Pd, Rh) support on Zr-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate (1.1 μmol∙mg−1∙s−1).

Figure 6 also shows CO, C3H8 and NO conversion at rich oxygen stoichiometric (λ in range 1 - 1.4). The results show that CO and C3H8 conversion are flat (Figure 6(a) and Figure 6(b)) while NO conversion decreases from 55.9% to 38.9% (Figure 6) caused by the decrease of reduction agent. Compared the noble metals (Pt, Pd, Rh) support on Cu-Ce, Zr-Ce mixed oxides with γ-Al2O3 washcoat/FeCrAl substrate and γ-Al2O3 washcoat/FeCrAl substrate, it shows similar CO, C3H8 and NO conversions during λ varies from 1 to 1.4.

4. Conclusions

Effects of CuO-CeO2 additions on structure and catalytic Properties of Three Way Catalysts were investigated. The samples were characterized by means of XRD, SEM, Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS) and GC-TCD.

The catalytic performance of noble metals (Pt, Rh, Pd) supported on Cu-Ce mixed oxides with alumina washcoat was compared with noble metals (Pt, Rh, Pd) supported on Ce-Zr mixed oxides with alumina washcoat and γ-Al2O3 washcoat at various stoichiometric ratio of oxygen. The results showed that the addition of Cu-Ce mixed oxides improved CO oxidation reaction at lower temperature during stable lambda of 1, the highest CO conversion of 99% is observed for the noble metals (Pt, Pd, Rh) support on Cu-Ce with γ-Al2O3 washcoat/FeCrAl substrate. The results also showed that, the addition of Cu-Ce mixed oxides promoted released oxygen, thus it improved strongly CO and C3H8 conversion at lean oxygen stoichiometric operation.

Acknowledgements

This research has been supported by The Ministry of Education and Training, Vietnam; Laboratory of internal combustion engine at Hanoi University of Science and Technology, Global Center of Excellence (GCOE) Program, Japan.

Cite this paper
The Luong, N. , Duy Tien, N. , Yamasue, E. , Okumura, H. and N. Ishihara, K. (2017) Effects of CuO-CeO2 Addition on Structure and Catalytic Properties of Three Way Catalysts. Journal of Materials Science and Chemical Engineering, 5, 28-39. doi: 10.4236/msce.2017.512003.
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