Aging three way catalysts

Birgersson, H., Boutonnet, M., Klingstedt, F. et al. (2006) An investigation of a new regeneration method of commercial aged three-way catalysts, Appl. Catal. B Environ., 65, 93. 

Figure 59. Influence of space velocity on gas temperature needed to reach 50% and 70% conversion of CO, HC and NOjt over a fresh and an engine aged three-way catalyst (monolith catalyst with 62 cells cm, three-way formulation with Pt 1.42gl-, Rh 0.28gl->, engine bench light-off test at lambda 1.02 for CO and HC, and at lambda 0.986 for NO engine bench aging during 200 h).

Figure 63. Influence of monolith radius at fixed catalyst volume on the conversion of CO, HC and NO.v reached over fresh and engine aged three-way catalysts at fixed exhaust gas temperature, space velocity and exhaust gas composition (monolith catalyst with 62cellscm" engine bench test space velocity 60000 N11 h exhaust gas composition lambda 0.995 dynamic frequency 1 Hz amplitude 1A/F).

Figure 67. Influence of the washcoat formulation on the conversion of CO, HC and NO , reached over various engine aged three-way catalysts as a function of the exhaust gas lambda value (monolith catalyst with 62cells cm three-way formulation with Pt 1.16gC, Rh 0.23gl" engine bench test A/F scan at a space velocity of 60000 Nl 1 h exhaust gas temperature 723 K dynamic frequency 1 Hz amplitude 1 A/F engine bench high temperature aging cycle for lOOh). Sample A baseline formulation, samples B-D increasing amounts of stabilizers.

Fig. 4.1. Illustration of the structure gap for an automotive three-way catalysts (TWO) depicted by a schematic drawing of a monolithic, supported catalyst. The lower right image shows a TEM picture of aged automotive catalyst 12-nm Pt on AI2O3. Note, the presence of small Pt single crystals. Well-defined model catalysts such as those made by electron-beam lithography (EBL lower middle image) are used to mimic real catalyst and bridge the gap between single crystal studies (bottom left) and real-life catalysts...

Figure 68. The function of platinum and rhodium on a fully formulated three-way catalyst washcoat in the conversion of CO, HC and NO, (monolith catalyst with 62 cells cm" three-way formulation, aged on an engine bench for 20 h engine bench test at a space velocity of 60000NIC h exhaust gas temperature 673K exhaust gas composition lambda 0.999, dynamic frequency I Hz amplitude 1 A/F). Reprinted with permission from ref. [34], (C 1991 Society of Automotive Engineers, Inc.

Figure 75. Secondary ion mass spectra (SIMS) showing the extent of the solid state reactions between alumina and ceria in a three-way catalyst washcoat (a) as a function of the type of precious metal and (b) as a function of the total precious metal loading (model catalyst on monoliths with 62 cells cm Pt, Pd or Rh loading 1.76 g 1 various total loadings at fixed Pt Rh ratio of 5 1 g/g model washcoat with 70 wt % alumina and 30 wt % ceria, after aging under air at 973 K).

Figure 86. Stability of the internal surface area of a three-way catalyst washcoat as a function of temperature and aging duration in an air atmosphere. Reprinted with permission from ref [34], CO 1991 Society of Automotive Engineers, Inc.

Table 21. Order of magnitude of the poisoning effects during the aging of three-way catalysts.

Figure 90. XRF impulse rate of Pb, Ca, P, Zn and S, relative to A1, as a function of the depth in the washcoat layer for a vehicle-aged monolithic three-way catalyst used in the Federal Republic of Germany in 1985-1989.

Figure 92. Conversion of CO, HC and NOv in three different phases of the European vehicle test cycle, reached over a three-way catalyst in the fresh state and after high temperature engine bench aging (phase 1 first city driving cycle phase 2 second, third and fourth city driving cycle phase 3 extra-urban driving cycle).

The ceria surface area of a commercial Pt-Rh three-way catalyst was determined after laboratory hydrothermal aging at 1173-1373 K and after 200 h on engine bench. It was measured by X-ray diffraction (XRD) line broadening analysis and by a method based on the exploitation of the hydrogen temperature programmed reduction (TPR) profiles. In this case, the hydrogen uptakes below about 900 K include the ceria surface reduction and that of the oxidized noble metals. They are analyzed and discussed, assuming two possiblities for the metals oxidation state. 

The effect of the ageing procedure upon the activity of a three way catalyst working under transient conditions... 

The question of ageing for the three-way catalyst is a real problem. The literature reports the importance of the mode of preparation [1], and of the ageing conditions [2], but few studies have been carried out in alternating flow conditions at high temperatures. Authors have only studied the effects of such fluctuations using simplified gas mixtures (O2/H2) or very low frequencies of oscillation between the two gas compositions [3,4,5]. 

Effect of ageing on the redox behavior of Ce in three-way catalysts... 

Oxidizing conditions have been observed to damage three-way catalysts at lower temperatures than reducing conditions. A platinum-rhodium three-way catalyst (base metal additives present but not identified) aged on an engine dynamometer was deactivated more readily (at lower temperature) during a brief exposure to lean air-fuel ratios than to rich air-fuel ratios [13]. Activity loss as measured at 600 F at stoichiometry was appreciable following only 20 minutes exposure to lean exhaust at 1600 F [13]. 

GIXD results are summarized in Table 3 which also indicates the phases detected for an aged Pd/Al203 - D (PdO and d-Al203) and an aged three-way Pt-Rh/Al203 catalyst (Pt° and d-Al203). 

A chemisorption teclmique developed by Koinai et al., based on CO methanation, was successfrilly used to analyze noble metal dispersions of both fresh and vehicle-aged Pt/Rli and Pd/Rli commercial automotive three-way catalysts. The teclmique is relatively rapid (< 2 hours), extremely sensitive, and largely free from complications due to adsorption of CO on non-noble metal components of the washcoat (support, promoters, stabilizers, etc.). Particle sizes of the vehicle-aged catalysts, calculated by applying the spherical particle assumption to the dispersions measured by the CO methanation method, agreed well with particle sizes calculated from x-ray diffraction line-broadening data. These results indicate that the CO methanation teclmique can be applied routinely to obtain fast and accurate measurements of noble metal surface areas in automotive catalysts retrieved from tlie field, even tliose with metal dispersions ca. 2% or less. 

Frequently exposed to very high temperatures, the catalysts used for depollution of motor vehicle exhaust gases are deactivated mainly by structural and textural evolution processes. This paper describes how the catalytic activity of a typical three-way catalyst (platinum-rhodium on a wash-coated cordierite monolith) was determined for the removal of hydrocarbons of various types before and after high-temperature treatments. Electron microscopy (CTEM and STEM) was used to determine metal particle size in fresh and aged catalysts. 

The influence of simultaneous thermal and chemical cycling on commercial three-way catalysts has been examined after ageing in a specifically designed automated laboratory bench. For all cycles tested, reproducing repeated fiiel cutoff procedures between two temperatures (850°C-850°C, cycle 1 850°C-950°C, cycle 2 850°C-1050°C, cycle 3), X-ray diffraction evidences the formation of platinum/rhodium alloys only when the atmosphere cycle comprises a reducing step. Evaluation of the rhodium concentration in alloyed phases suggests that some rhodium remains unalloyed in catalysts. 

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