Enhanced performance of a Pt-based three-way catalyst using a double-solvent method

Abstract

A new Pt three-way catalyst supported on CeO₂–ZrO₂–La₂O₃–Al₂O₃ via a double-solvent method was developed. For the conversion of a simulated compressed natural gas (CNG) vehicle exhaust, the catalyst showed a significant enhancement of the activity, which decreased the 100% conversion temperature (T₁₀₀) for CH₄ by about 46 °C compared with the catalyst prepared by conventional impregnation method.

---

In recent years, compressed natural gas (CNG) vehicles have attracted great attention due to their high fuel-efficiency and low exhaust emission. However, conversion of nitrogen oxide and unburned CH₄ in CNG exhaust emission is still very challenging.^1–4 To meet increasingly more stringent legislations (EURO 6, LEVII, and J-SULEV), there is a strong desire for development of catalysts with very high activity and durability for after-treatment of CNG vehicle exhaust emission.^5,6 Recently, a number of catalysts supported on ordered porous materials such as SBA-15, nanotube and metal–organic framework (MOF) were prepared by a so-called double-solvent technique, and the catalysts showed enhanced activity in cyclohexanol oxidation and Fischer–Tropsch synthesis (FTS).^7–11 However, the supports may suffer from either significant pore blockage or poor hydrothermal stability,¹² which would lead to reduced catalytic performance. Al₂O₃-based composite supports may be candidates for addressing the problems as they normally have a 3D channel structure and superior hydrothermal stability.^13–16 Nevertheless, few papers using the double-solvent method to support catalyst on these materials have been reported so far.

Herein, we report the use of the double-solvent method to support Pt on a CeO₂–ZrO₂–La₂O₃–Al₂O₃ (CZLA) composite, and a new Pt-based TWC was developed. The catalyst exhibited a superior catalytic performance in the elimination of a simulated CNG vehicle exhaust, compared with the catalyst prepared by conventional incipient wet impregnation method. The double-solvent technique can significantly decrease the 100% conversion temperature (T₁₀₀) for CH₄ by about 46 °C and the 100% N₂-selectivity temperature from 427 °C to 380 °C.

The synthesis of the CeO₂–ZrO₂–La₂O₃–Al₂O₃ material with a C/Z/A molar ratio of 1 : 1 : 2 and CZA/L mass ratio of 95 : 5 was carried out according to our previous work.¹⁷ The resulting CZLA powder was calcined at 900 °C for 3 h. The catalysts were synthesized using two methods. The first one was a typical double-solvent technique.^7,8,18,19 Briefly, 10.000 g of CZLA powder was firstly suspended in 500 ml of dry n-hexane and the mixture was sonicated for 30 min until it became homogeneous. Then, a desired amount of Pt(NO₃)₂ precursor was dissolved in distilled water, whose quantity is corresponded to the pore volume of support determined by N₂ adsorption. Finally, the hydrophilic Pt(NO₃)₂ solution was added dropwise to the hydrophobic n-hexane over a period of 20 min with constantly vigorous stirring. The resulting solution was continuously stirred for 3 h (300 rpm). The powder was recovered by careful filtration and dried in air at room temperature. The synthesized catalysts were further dried at 120 °C for 12 h, and then calcined at 550 °C for 3 h. The resulted catalyst was referred to as Pt-ds. For comparison, the second catalyst was prepared by the conventional incipient wet impregnation method and was denoted as Pt-im. The catalyst powders were ball milled to get homogeneous slurry. Then, the resulting slurry was coated onto cylindrical cordierite monoliths (Corning, USA, 400 cells per in.², 2.5 cm³), then dried at 120 °C overnight and calcined at 550 °C in air for 3 h to obtain the monolithic catalysts.²⁰ Both catalysts had the same Pt loading amount, with a content of 1.8 wt% relative to the amount of CZLA powder. The loading of washcoat was kept at about 160 g L⁻¹ with a relative error of ±2%.

Fig. 1a shows the N₂ adsorption–desorption isotherms of the samples. All samples exhibit type IV isotherms with H2 hysteresis loop according to the IUPAC classification.²¹ This type isotherm is characteristic properties of an irregular mesoporous material.¹⁵ The BJH method was employed to calculate the pore size distributions (PSD) of the support and catalysts (inset in Fig. 1a). The support and catalysts have almost the same pore size distributions within the range of 2–20 nm. The specific surface areas of Pt-ds and Pt-im decreased by 18.1% and 22.7%, respectively, compared with that of the support (Table 1). Fig. 1b shows the powder X-ray diffraction (XRD) patterns of the Pt containing catalysts, which are very similar to those of the supports. The main diffraction peaks of all samples are consistent with the characteristic peaks of tetragonal Ce_0.5Zr_0.5O₂ (PDF-ICDD38-1436).²² There are no separated Al₂O₃ or La₂O₃ peak, which implies that Al³⁺ or La³⁺ may enter into the Ce_0.5Zr_0.5O₂ solid solution. Moreover, no diffraction peaks corresponding to Pt species are detected, which may be attributed to low Pt content or high dispersion of Pt species. It is concluded that the main textural property and the crystal phase structure of the support are retained upon Pt-loading via both methods.

Fig. 1 (a) The nitrogen adsorption–desorption isotherms and pore-size distribution (inset figure) of the samples; (b) the XRD patterns of the support and catalysts.

Table 1 Specific surface areas, pore volumes and mean pore radii of the samples

Sample	SBET (m^2 g^−1)	V (ml g^−1)	R (nm)
Support	110.9	0.36	6.5
Pt-im	85.1	0.29	6.9
Pt-ds	90.0	0.29	6.5

---

Table 2 shows the Pt dispersion, active metal surface area²³ (MSA) and average Pt particle size on the catalyst surface determined by CO chemisorption. Compared to the conventional incipient wet impregnation method, the double-solvent technique can significantly increase Pt dispersion from 21.2% to 35.5% and active MSA by about 60%. Moreover, the calculated size of Pt-ds is much smaller than that of Pt-im. Thus, enhanced activity for catalytic removal of pollutants in vehicle exhaust emissions may be obtained due to large quantities of highly dispersed Pt, the size effect and structure sensitivity of Pt nanoparticles.^24,25

The morphology and microstructure of the as synthesized catalysts were analysed by Transmission electron microscopy (TEM) experiments (Fig. 2). The presence of Pt nanoparticles is confirmed by d-spacing measurements, and Pt nanoparticles are crystalline with the spacing of 0.27 nm and 0.24 nm corresponding to Pt(1 1 0) and Pt(1 1 1), respectively.^7,27 It demonstrates that the smaller Pt nanoparticles (about 2–3 nm) of the Pt-ds catalyst are more homogeneously dispersed than those bigger ones (about 5–6 nm) of the Pt-im catalyst, which is consistent with the CO-chemisorption results (Table 2).

Fig. 2 The TEM images of Pt-im (a and c) and Pt-ds (b and d).

Table 2 Physical characteristics of Pt-im and Pt-ds determined by CO chemisorption

Catalyst	Dispersion (%)	Active MSA (m^2 g^−1)	dPt^a (nm)
a Average Pt particle size, calculated based on that dPt (nm) = 1.1/dispersion.^26,27
Pt-im	21.2	0.95	5.2
Pt-ds	35.5	1.60	3.1

---

In order to evaluate the reducibility of the samples, H₂-temperature programmed reduction (TPR) experiments were carried out (Fig. 3a). For the support, the reduction peak (about 508 °C) can be ascribed to the simultaneous reduction of the surface and bulk oxygen owing to the high oxygen mobility in CeO₂–ZrO₂ solid solution. The TPR profiles of Pt-im and Pt-ds catalysts are significantly different from that of the support. For Pt-im catalyst, the reduction peak with a high intensity appears at 147 °C and a shoulder appears at 256 °C, which can be associated with the reduction of PtO_x species and the interaction between PtO_x species and the support, respectively.²⁸ Additionally, the reduction peak maxima at ca. 404 °C can also be attributed to the reduction of surface CeO₂.²⁹ For Pt-ds catalyst, the main peak located at 133 °C can be ascribed to the reduction of PtO_x species and the shoulder peak at 223 °C can be assigned to the reduction of surface CeO₂ having interaction with Pt. Note that the reduction peaks of Pt-ds shift remarkably to lower temperatures, which may be due to the fact that Pt-ds has smaller Pt particles, better dispersion and stronger interaction between Pt and the support. Thus, Pt-ds can provide better reducibility than Pt-im.

Fig. 3 (a) H₂-TPR profiles of the samples; (b) OSC values of the support and catalysts obtained by the O₂ pulses at different temperatures.

Fig. 3b displays the oxygen storage capacity (OSC) amount of the samples at 200, 300, 400, 500, and 600 °C, respectively. For all the samples, the OSC values increase with increment of temperature. However, compared with the support, the catalysts display better OSC performance because of the more oxygen vacancies generated by the Pt species.³⁰ The generation may follow a route of the reduction of support by spilt H atoms from Pt to the support or a route of the electron transfer from the conduction band of support to the Pt phase.^30–33 Interestingly, Pt-ds exhibits higher OSC value than that of Pt-im at the same temperature, which could be due to the fact that Pt-ds have the larger number of oxygen vacancies and the stronger interaction between Pt species and the support.^26,30 Thus, Pt-ds may display higher activity in TWC application. However, it is important to note that the transient rate of labile oxygen supply of support to the Pt surface can also affect the TWC performance of the Pt catalysts.^33–35

The chemical state of Pt and the surface composition of the catalysts were studied via the X-ray photoelectron spectroscopy (XPS) technique (Fig. 4). The method of curve fitting for the XPS Pt 4f spectra is according to the reported method.³⁶ It is found that Platinum nanoparticles of both catalysts exist mainly in an oxide form after being calcined in air³⁷ and the relative abundance (Table 3) of PtO₂ (45.1%) on Pt-ds is higher than that on Pt-im (39.6%). The result indicates that the double-solvent method is favorable to oxidize Pt species, which could be due to the smaller platinum particles for Pt-ds that can be oxidized more easily in ambient air.³⁸ Similarly, Pt-ds with smaller PtO_x particles should also be reduced more easily under the flow of a reducing gas,³⁷ which is confirmed by the results of H₂-TPR. Furthermore, for Pt-ds, the shift of the XPS Pt²⁺ 4f_7/2 spectrum (0.3 eV) relative to the reference line³⁶ is higher than that of Pt-im (0.0 eV), which can be due to the smaller particles of Pt-ds with stronger metal–support interaction (SMSI).³⁶ Therefore, the results show that the double-solvent method can enhance the reducibility of Pt and the interaction between Pt and the support which are beneficial for the improvement of the catalytic activity for Pt-ds.

Fig. 4 XPS (Pt 4f) spectra of Pt-im and Pt-ds.

Table 3 Relative abundance of different Pt species and obtained XPS peak positions from XPS analyses over catalysts

Catalyst	Pt species (%)			Pt^2+ 4f7/2 (eV)	Pt^4+ 4f7/2 (eV)
	Pt^0	PtO	PtO2
Pt-im	0	60.4	39.6	72.4	74.2
Pt-ds	0	54.9	45.1	72.1	74.3

---

The conversions of CH₄ (a), NO (b) and CO (c), and selectivity of N₂ (d) over Pt-im and Pt-ds are presented in Fig. 5. The conversions of CH₄ and CO for both catalysts increase with temperature. For NO, however, it mainly reacts with CO at low temperature and with CH₄ at high temperature, and there is a balance between the two reactions.³⁹ Pt-ds shows 100% CH₄ conversion (T₁₀₀) at 396 °C, while for Pt-im, the value increases to 442 °C. Moreover, the ΔT (ΔT = T₁₀₀ − T₅₀) value of CH₄ for Pt-ds (28 °C) is smaller than that for Pt-im (40 °C), suggesting that CH₄ over Pt-ds can more easily reach complete conversion after light-off. Interestingly, for the conversion activity of NO and CO, the performance of Pt-ds is also better than that of Pt-im. It is worth noting that a similar behavior is observed for the selectivity towards N₂. Pt-ds has a superior N₂-selectivity, which is up to 100% at 380 °C. However, for Pt-im, the other N-containing product species (NO₂, N₂O and NH₃) can be formed (Fig. S1†) and a relatively poor N₂-selectivity (60%) is obtained.

Fig. 5 Conversion curves of CH₄ (a), NO (b) and CO (c), and selectivity of N₂ (d) over Pt-im and Pt-ds.

In summary, two catalysts, Pt-im and Pt-ds, were synthesized using the conventional incipient wet impregnation method and the double-solvent technique, respectively. The results of nitrogen adsorption–desorption, XRD, CO-chemisorption, TEM, H₂-TPR, OSC and XPS indicate that Pt-ds has higher dispersion and better reducibility of Pt species and stronger interaction between these species and the support. Thus, Pt-ds exhibits a significant enhancement of the activity in the elimination of NO, CO and CH₄ from a simulated CNG vehicle exhaust, compared with Pt-im. It is found that all three pollutants can be converted completely over Pt-ds and the N₂-selectivity can be up to 100% at a low temperature (about 400 °C). Our work may provide a facile but efficient method to develop highly efficient catalysts for the conversion of the vehicle exhaust. We note that a future suitable TWC for CNG exhausts emissions would be Pd-based catalyst due to its significantly lower cost compared to Pt-based one. Further work on developing the Pd-based TWC catalyst via double-solvent method is underway in our lab.

Acknowledgements

This work is sponsored by the National Hi-tech Research and Development Program of China (863 Program, 2015AA034603 and 2013AA065304), and Sichuan Province science and technology support program (2014SZ0143 and 2015GZ0054).

Notes and references

1. P. Gélin and M. Primet, Appl. Catal., B, 2002, 39, 1–37.
2. M. Hussain, F. A. Deorsola, N. Russo, D. Fino and R. Pirone, Fuel, 2015, 149, 2–7.
3. D. Fino, N. Russo, G. Saracco and V. Specchia, Prog. Solid State Chem., 2007, 35, 501–511.
4. N. Russo, P. Palmisano and D. Fino, Chem. Eng. J., 2009, 154, 137–141.
5. A. Winkler, P. Dimopoulos, R. Hauert, C. Bach and M. Aguirre, Appl. Catal., B, 2008, 84, 162–169.
6. F. Klingstedt, H. Karhu, A. K. Neyestanaki, L. E. Lindfors, T. Salmi and J. Väyrynen, J. Catal., 2002, 206, 248–262.
7. A. Aijaz, A. Karkamkar, Y. J. Choi, N. Tsumori, E. Ronnebro, T. Autrey, H. Shioyama and Q. Xu, J. Am. Chem. Soc., 2012, 134, 13926–13929.
8. M. Imperor-Clerc, D. Bazin, M.-D. Appay, P. Beaunier and A. Davidson, Chem. Mater., 2004, 16, 1813–1821.
9. J. Taghavimoghaddam, G. P. Knowles and A. L. Chaffee, J. Mol. Catal. A: Chem., 2012, 358, 79–88.
10. H. C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 2012, 112, 673–674.
11. J. van der Meer, I. Bardez-Giboire, C. Mercier, B. Revel, A. Davidson and R. Denoyel, J. Phys. Chem. C, 2010, 114, 3507–3515.
12. J. Taghavimoghaddam, G. P. Knowles and A. L. Chaffee, J. Mol. Catal. A: Chem., 2013, 377, 115–122.
13. X. Yao, C. Tang, F. Gao and L. Dong, Catal. Sci. Technol., 2014, 4, 2814.
14. M. H. Yao, R. J. Baird, F. W. Kunz and T. E. Hoost, J. Catal., 1997, 166, 67–74.
15. J. Wang, J. Wen and M. Shen, J. Phys. Chem. C, 2008, 112, 5113–5122.
16. N. S. Priya, C. Somayaji and S. Kanagaraj, J. Nanopart. Res., 2014, 16, 2214.
17. X. Zhang, E. Long, Y. Li, J. Guo, L. Zhang, M. Gong, M. Wang and Y. Chen, J. Nat. Gas Chem., 2009, 18, 139–144.
18. I. Lopes, N. El Hassan, H. Guerba, G. Wallez and A. Davidson, Chem. Mater., 2006, 18, 5826–5828.
19. Q.-L. Zhu, J. Li and Q. Xu, J. Am. Chem. Soc., 2013, 135, 10210–10213.
20. L. Lan, S. Chen, Y. Cao, S. Wang, Q. Wu, Y. Zhou, M. Huang, M. Gong and Y. Chen, J. Mol. Catal. A: Chem., 2015, 410, 100–109.
21. K. S. W. SING, Pure Appl. Chem., 1985, 57, 603.
22. G. Vlaic, R. Di Monte, P. Fornasiero, E. Fonda, J. Kašpar and M. Graziani, J. Catal., 1999, 182, 378–389.
23. S. B. Kang, S. J. Han, S. B. Nam, I.-S. Nam, B. K. Cho, C. H. Kim and S. H. Oh, Top. Catal., 2013, 56, 298–305.
24. J. Garciacortes, J. Catal., 2003, 218, 111–122.
25. S. Vajda, M. J. Pellin, J. P. Greeley, C. L. Marshall, L. A. Curtiss, G. A. Ballentine, J. W. Elam, S. Catillon-Mucherie, P. C. Redfern, F. Mehmood and P. Zapol, Nat. Mater., 2009, 8, 213–216.
26. J. Fan, X. Wu, X. Wu, Q. Liang, R. Ran and D. Weng, Appl. Catal., B, 2008, 81, 38–48.
27. J. Fan, X. Wu, L. Yang and D. Weng, Catal. Today, 2007, 126, 303–312.
28. Y. Zheng, Y. Zheng, Y. Xiao, G. Cai and K. Wei, Catal. Commun., 2013, 39, 1–4.
29. L. Liu, Y. Cao, W. Sun, Z. Yao, B. Liu, F. Gao and L. Dong, Catal. Today, 2011, 175, 48–54.
30. C. Bozo, N. Guilhaume and J.-M. Herrmann, J. Catal., 2001, 203, 393–406.
31. M.-F. Luo and X.-M. Zheng, Appl. Catal., A, 1999, 189, 15–21.
32. N. W. Hurst, S. J. Gentry, A. Jones and B. D. McNicol, Catal. Rev., 2007, 24, 233–309.
33. C. N. Costa, S. Y. Christou, G. Georgiou and A. M. Efstathiou, J. Catal., 2003, 219, 259–272.
34. A. M. Efstathiou and S. Y. Christou, Catal. Sci. Series, in Catalysis by Ceria and Related Materials, ed. A. Trovarelli and P. Fornasiero, Imperial College Press, London, 2nd edn, 2013, vol. 12, ch. 3, pp. 139–222.
35. P. S. Lambrou, C. N. Costa, S. Y. Christou and A. M. Efstathiou, Appl. Catal., B, 2004, 54, 237–250.
36. L. Olsson, J. Catal., 2002, 210, 340–353.
37. Z. Yang, N. Zhang, Y. Cao, M. Gong, M. Zhao and Y. Chen, Catal. Sci. Technol., 2014, 4, 3032.
38. E. Fridell, A. Amberntsson, L. Olsson, A. Grant and M. Skoglundh, Top. Catal., 2004, 30–31, 143–146.
39. M. Salaün, A. Kouakou, S. Da Costa and P. Da Costa, Appl. Catal., B, 2009, 88, 386–397.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06285g

---