Deactivation Correlations of Pd/Rh Three-way Catalysts Designed for Euro IV Emission Limits

Effect of Ageing Atmosphere, Temperature and Time

Ulla Lassi

Department of Process and Environmental Engineering, University of Oulu

Abstract

The aim of this thesis is the knowledge of the most relevant deactivation mechanisms of Pd/Rh three-way catalysts under different ageing conditions, the deactivation correlation of laboratory scale ageing and engine bench/vehicle ageings, and the evaluation of the deactivation correlation. In the literature review, the phenomena involved in the three-way catalyst operation and its deactivation are considered. In the experimental section, ageing-induced phenomena in the catalyst are studied and deactivation correlations between laboratory scale and engine bench/vehicle ageings are presented, based on the results of several surface characterization techniques. The effects of ageing atmosphere and temperature, and time are considered in particular.

Fresh and aged catalysts used in this study were metallic monoliths designed for Euro IV emission limits. Thermal ageings were carried out in the reductive, oxidative and inert atmospheres in the temperature range of 800°C to 1200°C, and in the presence of water vapour (hydrothermal ageing). The engine ageing was carried out in the exhaust gas stream of a V8 engine during a 40 hour period. The ageing procedure composed of rich and stoichiometric air-to-fuel ratios carried out consecutively. The vehicle ageing was accomplished under real driving conditions (100 000 kilometres).

According to the results, deactivation of a Pd/Rh monolith is a combination of several ageing phenomena. The most important deactivation mechanisms are the sintering of active phase, the collapse in surface area and ageing-induced solid-solid phase transitions in the bulk washcoat. Furthermore, poisoning is a relevant deactivation mechanism of the vehicle-aged catalyst. High ageing temperature, gas phase composition and exposure time are essential variables to the deactivation of a Pd/Rh three-way catalyst.

This thesis presents an approach to discover the deactivation correlation between the laboratory scale ageing and under the vehicle"s operation in an engine bench or on-road. Based on the characterization results, the accelerated laboratory scale air ageing does not correspond to the ageing-induced changes in the catalyst under the vehicle"s operation. Therefore, there is a need for a modified ageing cycle and according to the results, a deactivation correlation between the laboratory scale ageing and the engine bench ageing can be presented as a function of ageing temperature and atmosphere, and time. Instead, after the vehicle operation, the deactivation correlation cannot be presented based solely on the studied variables because, after 100 000 kilometres of driving, the role of poisoning should be taken into account in the ageing cycle.

The results of this thesis can be utilized and applied in the development of laboratory scale ageing cycles, which corresponds closely to the ageing-induced changes in the catalyst during the vehicle operation. This enables a rather fast testing of the catalyst"s performance and reduces the cost during the manufacturing of catalysts.

Dedication

To my Family

Table of Contents
Acknowledgements
List of symbols and abbreviations
1. Introduction
1.1. Background
1.2. Aims of the work
1.3. Scope and outline of the work
2. Three-way catalysis
2.1. General
2.2. Structure of a three-way catalyst
2.3. Phenomena in the three-way catalyst operation
2.3.1. Catalytic reactions and chemical kinetics
2.3.2. Heat and mass transfer phenomena
2.3.3. Oxygen storage capacity
2.3.4. Cold start and catalyst fast light-off
3. Catalyst deactivation – Ageing
3.1. Overview
3.2. Catalyst deactivation mechanisms
3.2.1. Deactivation by thermal degradation and sintering
3.2.2. Deactivation by poisoning
3.2.3. Other relevant mechanisms of deactivation
3.3. Accelerated catalyst ageing
3.4. Vehicle test cycles
4. Experimental
4.1. Catalysts
4.2. Ageing procedures
4.3. Catalyst characterization techniques
4.3.1. Scanning Electron Microscopy
4.3.2. Physisorption analyses
4.3.3. Chemisorption analyses
4.3.4. Temperature-programmed techniques
4.3.5. X-ray diffraction
4.3.6. X-ray Photoelectron Spectroscopy
4.3.7. Activity measurements
4.3.8. Chemical analyses
5. Ageing-induced changes in the catalyst
5.1. Collapse in surface area and pore structure
5.2. Loss in active surface area and sintering of metal particles
5.3. Ageing-induced changes in the desorption behaviour of NO
5.3.1. Interpretation of NO-TPD spectra
5.3.2. Effect of ageings on the desorption of NO
5.3.3. Modelling of desorption curves
5.4. Solid-solid phase transitions in the bulk material
5.5. Changes in the chemical states of active metals
5.6. Loss of catalytic activity
5.7. Poisoning
5.8. Volatilization and encapsulation of metal particles
5.9. Other aspects on deactivation
6. Deactivation correlations
6.1. Integration of results
6.2. Deactivation correlations
6.3. Evaluation and utilization of the results
6.4. Requirements for further research
7. Summary and conclusions
References
List of Tables
1. Typical concentrations of the exhaust gas constituents of gasoline-fuelled engines. Air-to-fuel ratio contributes significantly to these concentrations (Taylor 1993).
2. European emission limits (g/km) for gasoline-fuelled passenger cars and light commercial vehicles (Directive 98/69/EC).
3. The overall reactions in the catalytic converter (Lox & Engler 1997).
4. A summary of the deactivation mechanisms of three-way catalysts (Carol et al. 1989, Koltsakis & Stamatelos 1997, Sideris 1998).
5. Effect of ageing atmosphere on the sintering behaviour (particle size) of Pt, Pd and Rh (washcoat La2O3-doped Al2O3, precious metal content 0.14 wt-%) (Shinjoh et al. 1991).
6. Vapour pressures (torr) of Pt, Pd and Rh at 800°C in air (Shinjoh et al. 1991).
7. Ageing procedures.
8. The experimental procedure for the H2-chemisorption measurements at the University of Oulu (Anon 1992).
9. Composition of the test gas mixture for the activity measurements.
10. Catalyst characterization results for thermally aged catalysts. Thermal ageing was carried out in the oxidative ageing atmosphere (air) for 3 and 24 hours.
11. Catalyst characterization results for thermally aged catalysts. Thermal ageing was carried out in the reductive ageing atmosphere (5% H2 and N2 balance) for 3 and 24 hours.
12. Catalyst characterization results for hydrothermally aged catalysts. Hydrothermal ageing was carried out either in the reductive (5 vol-% H2, 10 vol-% H2O and N2 balance) or in the oxidative (air and 10 vol-% H2O) atmospheres for 3 hours.
13. Catalyst characterization by H2-chemisorption for fresh, engine-aged, vehicle-aged and thermally aged catalysts.
14. Peak temperatures for fresh, engine-aged and vehicle-aged catalysts.
15. Relative peak heights for fresh, engine-aged and vehicle-aged catalysts.
16. Comparison of CO light-off temperatures (°C) after 3 hours of reductive, oxidative and inert ageings, in lean reaction conditions.
17. Comparison of CO light-off temperatures (°C) after 3, 24 and 42 hours of ageing in the reductive ageing atmosphere.
18. CO and NO light-off temperatures (T50 values) after hydrothermal ageings in the oxidative and reductive ageing atmospheres. Ageing time was 3 hours.
19. The comparison of dry and wet ageing atmospheres in lean and rich reaction conditions after 3 hours of ageing at 1200°C.
20. Contaminant levels of the fresh, engine-aged and vehicle-aged catalysts in axial and radial directions.
21. The effect of poisoning of engine-aged and vehicle-aged catalysts on the BET surface area, oxygen storage capacities (measured at adsorption temperature of 750°C) and catalytic activities for the removal of CO and NO (in lean conditions).
List of Figures
1. Purification system of exhaust gases of gasoline engines. Purification system includes an electrically controlled air-to-fuel management system (cf. Holmgren 1998).
2. The conversion efficiency (%) of a three-way catalyst as a function of A/F-ratio. The lambda window, an A/F-ratio of 14.6 corresponds to stoichiometric operation, λ =1 (cf. Holmgren 1998).
3. The structure of a monolithic exhaust gas catalyst.
4. Close-up view of a metallic monolithic support (cf. Rahkamaa-Tolonen 2001).
5. Conversion as a function of temperature: rate controlling regimes.
6. Automotive emission control system showing the pre- and main catalytic converters.
7. Deactivation mechanisms: A) Coke formation, B) Poisoning, C) Sintering of the active metal particles, and D) Sintering and solid-solid phase transitions of the washcoat and encapsulation of active metal particles (cf. Suhonen 2002).
8. Phase transitions and surface areas of Al2O3 (boehmite) as a function of temperature.
9. Thermodynamic equilibrium calculations of volatilization of A) Pd and B) Rh in a 5 % O2/N2 atmosphere (lean) (Turpeinen & Maunula 1993).
10. The European Driving Cycle (EC2000): The speed of a vehicle as a function of time.
11. FTP-75 test driving cycle established by the EPA.
12. A) A metallic monolith and B) the fresh catalyst on the left and the engine-aged catalyst on the right.
13. A SEM image of an engine-aged catalyst.
14. The experimental set-up of the ageing furnace.
15. Test samples from the different zones of the pre-catalyst.
16. Experimental set-up for NO-TPD measurements in a vacuum.
17. Experimental set-up for the activity measurements at the University of Oulu.
18. Activity measurement system equipped with the GASMETTM gas analyzer (University of Oulu).
19. Effect of ageing temperature on the BET surface areas of catalysts. The comparison after 3 hours of ageing in the reductive (▪), inert (○), and oxidative (●) atmospheres.
20. The comparison of surface areas in different ageing atmospheres after 3 hours of ageing.
21. Effect of ageing time on the BET surface areas of catalysts at ageing temperatures of 900°C, 1000°C and 1100°C. The comparison after 3, 24 and 42 hours of ageing in the reductive ageing atmosphere (%5 H2/N2).
22. The effect of ageing temperature on the sintering rate at 800°C, 900°C, 1000°C, 1100°C and 1200°C; A) after the reductive ageing and B) after the oxidative ageing.
23. Adsorption (○) and desorption (●) isotherms measured for the engine-aged catalyst.
24. Pore size distributions for fresh, engine-aged and vehicle-aged catalysts; A) micropores up to 2 nm (differential HK volume) and B) mesopores from 2 nm up to 100 nm (differential BJH volume).
25. The effect of ageing atmosphere, temperature and time on the precious metal dispersions of catalysts (based on H2-chemisorption). Comparison between reductive (3 hours (●) or 24 hours (○)) and oxidative (3 hours (▪) or 24 hours (□)) ageing atmospheres.
26. Effect of ageing atmosphere, temperature and time on the sintering behaviour of the aged catalysts. The normalised active metal surface area D/D0 as a function of ageing time (hours).
27. NO-TPD curve for a fresh catalyst measured under the carrier gas flow at 1 atm and in a vacuum at 10-7 atm.
28. NO-TPD curves for the fresh catalyst, measured in a vacuum at a linear heating rate of 30°C/min; following the adsorption of 5% NO/Ar at room temperature for 10 minutes.
29. NO-TPD curves for the fresh catalyst, and for separate washcoats of the catalyst; measured in a vacuum at a linear heating rate of 30°C/min, following the adsorption of 5% NO/Ar at room temperature for 10 minutes.
30>. NO-TPD curves for fresh and thermally aged catalysts; following the adsorption of 5% NO/Ar at room temperature for 10 minutes; Fresh catalyst (A); H2/900°C/3h-aged (B); H2/900°C/24h-aged (C); H2/1100°C/24h-aged (D); air/900°C/3h-aged (E), and air/1100°C/24h-aged (F) catalysts.
31. NO-TPD curves for fresh and hydrothermally aged catalysts; following the adsorption of 5% NO/Ar at room temperature for 10 minutes; Fresh catalyst (A); H2O-H2/800°C/3h-aged (B); H2O-H2/1000°C/3h-aged (C); H2O-H2/1200°C/3h-aged (D); H2O-air/800°C/3h-aged (E), and H2O-air/1200°C/3h-aged (F) catalysts.
32. NO-TPD curves for fresh catalyst (A); engine-aged (B); vehicle-aged (C); H2/900°C/3h-aged (D), and air/900°C/3h-aged (E) catalysts; following the adsorption of 5%NO/Ar at room temperature for 10 minutes.
33. O2-TPD curves for the fresh catalyst (A); air/550°C/3h-aged (B); H2/800°C/3h-aged (C); H2/900°C/24h-aged (D); air/900°C/3h-aged (E), and engine-aged (F) catalysts; following the adsorption of 5%NO/Ar at room temperature. O2-TPD multiplied by 5 compared to the desorption rate of NO presented earlier in Fig. 28.
34. Fitted NO-TPD curves for A) fresh, B) engine-aged and C) vehicle-aged catalysts (Lassi et al. 2002d).
35. Crystallization of the catalysts after reductive and oxidative ageings; XRD diffractograms for A) air/800°C/24h-aged; B) air/950°C/3h-aged; C) H2/800°C/3h-aged, and D) H2/900°C/3h-aged catalysts. a = CeO2, b = Zr-rich mixed oxide.
36. Formation of CeAlO3 in different ageing atmospheres; XRD diffractograms for A) H2/1000°C/24h-aged; B) inert/1050°C/24h-aged, and C) air/1200°C/24h-aged catalysts. a = CeO2, a* = Ce-rich mixed oxide , b = Zr-rich mixed oxide, c = CeAlO3, d = Ce-Zr mixed oxides (30 mol-% < Ce < 70 mol-%) formed in the ageings, e = α-Al2O3.
37. XRD diffractograms for A) engine-aged; B) vehicle-aged, and C) H2/1200°C/24h-aged catalysts. a = CeO2, a* = Ce-rich mixed oxide, b = Zr-rich mixed oxide, c = CeAlO3, d = Ce-Zr mixed oxides (30 mol-% < Ce < 70 mol-%) formed in the ageings, e = α-Al2O3, f = CeAl11O18.
38. Effect of water vapour on the phase transitions after the ageings. XRD diffractograms for A) air/1000°C/3h-aged; B) H2O-air/1000°C/3h-aged; C) H2/1000°C/3h-aged, and D) H2O-H2/1000°C/3h-aged catalysts. a = CeO2, b = Zr-rich mixed oxide, c = CeAlO3.
39. Rh 3d XPS peaks measured both before (solid line) and after (dashed line) reduction of the samples in situ in 400 mbar of static H2 at 300°C for 30 minutes. A) fresh; B) H2/900°C/3h-aged; C) H2/1000°C/3h-aged; D) H2/1100°C/3h-aged; E) air/1000°C/3h-aged; F) engine-aged, and G) vehicle-aged catalysts.
40. Rh 3d XPS peaks measured both before (solid line) and after (dashed line) reduction of the samples in situ in 400 mbar of static H2 at 300°C for 30 minutes. A) fresh; B) H2/900°C/24h-aged; C) H2/1000°C/24h-aged; D) engine-aged; E) vehicle-aged, and F) air/1000°C/24h-aged catalysts.
41. Pd 3d XPS peaks together with the partially overlapping Zr 3p peaks measured from the catalyst’s surface. A) fresh; B) air/1000°C/3h-aged; C) H2/900°C/3h-aged; D) vehicle-aged; E) engine-aged; F) H2/1000°C/3h-aged, and G) H2/1100°C/3h-aged catalysts.
42. Catalytic activities in lean conditions: determined as the light-off temperatures of A) CO and B) NO after 3 and 24 hours of oxidative and reductive ageings.
43. Catalytic activities in rich conditions: determined as the light-off temperatures of A) CO and B) NO after 3 and 24 hours of oxidative and reductive ageings.
44. Comparison of the light-off curves of CO after the reductive and oxidative ageings: A) H2/800°C/3h-aged; B) air/800°C/3h-aged; C) H2/1000°C/3h-aged; D) H2/1200°C/3h-aged; E) air/1000°C/3h-aged, and F) air/1200°C/3h-aged; lean reaction conditions.
45. Light-off curves of CO after hydrothermal and thermal ageings for A) H2/800°C/3h-aged; B) H2O-air/800°C/3h-aged; C) air/1000°C/3h-aged; D) H2O-H2/1000°C/3h-aged; E) H2/1000°C/3h-aged; F) H2/1200°C/3h-aged; G) H2O-air/1000°C/3h-aged; H) air/1200°C/3h-aged, and I) H2O-air/1200°C/3h-aged catalysts.
46. Light-off curves of CO for A) the fresh catalyst; B) H2/800°C/3h-aged; C) H2/1000°C/3h-aged; D) engine-aged; E) H2-H2O/1200°C/3h-aged; F) air/1200°C/3h-aged, and G) vehicle-aged catalysts; lean reaction conditions.
47. Visual analysis of the vehicle-aged catalyst (this thesis): A) metallic pre-converter, B) enhanced view of the metallic monolith, C) poisoned channel walls of the monolith, and D) a back-scattered electron image taken at the inlet of the vehicle-aged catalyst. The figures A, B and C are adapted from Härkönen et al. (2001).
48. Elemental analysis (X-ray maps) data taken at the inlet of the vehicle-aged catalyst (magnification 1000x); A) back-scattered electron image showing the metal foil, washcoat and contaminant overlayer; EDS elemental maps of B) Al, C) O, D) Ca, E) P, and F) Cr.
49. A top view of a SEM back-scattering image (scale: 30 µm) taken at the inlet of the vehicle-aged catalyst.
50. SEM back-scattering images (scale: 30 µm) of A) fresh and B) air-aged (1200°C) catalysts.
51. Ageing-induced structural and chemical changes in the catalyst.
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