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Thermal gas treatment to regenerate spent automotive three-way exhaust gas catalysts (TWC)
KTH, Superseded Departments, Chemical Engineering and Technology.
KTH, Superseded Departments, Chemical Engineering and Technology.
KTH, Superseded Departments, Chemical Engineering and Technology.
2004 (English)In: Applied Catalysis B: Environmental, ISSN 0926-3373, E-ISSN 1873-3883, Vol. 54, 193-200 p.Article in journal (Refereed) Published
Abstract [en]

The effect of regeneration on the metal dispersion and catalyst activity of commercial automotive three-way catalysts (TWC) in oxygen, hydrogen and oxy-chlorine at elevated temperatures has been investigated. In addition, characterisation of the catalysts has been performed using XRD, TEM, TPR, CO chemisorption and BET surface area measurements. Activity tests on monolith fragments were conducted in a micro-reactor supplied with real exhaust gases. An increase in activity was observed with oxygen treatment on catalysts showing heavy sintering as well as a high noble metal loading. Hydrogen proved to be an ineffective treatment procedure, whereas the addition of chlorine to the oxygen stream resulted in an activity increase even on catalysts less sintered and with lower noble metal contents. A comparison of the XRD and BET profiles showed no or small changes in the washcoat structure compared to fresh catalysts after successful regeneration. The TEM, TPR and CO chemisorption measurements suggest a decrease in the size of the largest noble metal agglomerates on the catalyst surface, as well as some Pd and RhO restructuring. The oxy-chlorine regeneration procedure is shown to be the most efficient, both regarding the catalyst activity and the metal dispersion. An increase in the amount of noble metal particles with a size of 20-70 nm, coupled with a decrease in size of larger clusters up to 100 nm in size, was observed for this treatment.

Place, publisher, year, edition, pages
2004. Vol. 54, 193-200 p.
Keyword [en]
Automotive catalyst; Dispersion; Oxy-chlorine; Regeneration; Thermal gas treatment; TWC
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-5633DOI: 10.1016/j.apcatb.2004.09.001ISI: 000224948100007Scopus ID: 2-s2.0-6044269328OAI: oai:DiVA.org:kth-5633DiVA: diva2:10065
Note
QC 20100812 QC 20110928Available from: 2006-04-27 Created: 2006-04-27 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Development and Assessment of Regeneration Methods for Commercial Automotive Three-Way Catalysts
Open this publication in new window or tab >>Development and Assessment of Regeneration Methods for Commercial Automotive Three-Way Catalysts
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Car exhaust catalysts were introduced in the early 1980’s, to limit the release of pollutants such as hydrocarbons, carbon monoxide and nitrogen oxides. These catalysts contain noble metals such as palladium (Pd), platinum (Pt) and rhodium (Rh) and are able to simultaneously abate all three of the above-mentioned pollutants, hence the name three-way catalyst (TWC). The exposure to high temperatures (800-1000 °C) during operation and the presence of additives in gasoline and lubricants will, after a certain time, lower the activity of the TWC. High temperatures reduce the active area by causing the noble metals to agglomerate and sinter, whereas the additives alter the activity either by fouling the pores of the support material or by interacting with the metals.

The main objective of this work was to develop a method which allows for the removal of contaminants (additives) from the washcoat and enables the redispersion of the active sites (noble metals), in an effort to recover lost catalyst activity. For this purpose, regeneration experiments were carried out on a wide spectrum of different commercial car exhaust catalysts.

The influence of a thermal treatment in a controlled gas atmosphere, such as oxygen or hydrogen, and a redispersing agent, e.g. chlorine, on the activity of TWC was investigated by means of laboratory-scale activity measurements. Several complementary characterization methods such as SEM/TEM, XRD, BET, LA, XPS and TPR were employed to verify the effects of the regeneration treatments on the catalyst morphology (Papers I, II). The results show that partial regeneration of catalyst activity and noble metal dispersion was achieved after thermal treatment in an oxygen-chlorine rich atmosphere at temperatures below 500 °C. A wet-chemical regeneration treatment with dilute oxalic and citric acid solutions is evaluated in Paper III. These acidic solutions are able to dissolve and remove contaminants from the washcoat, thus partly restoring the catalyst activity.

An investigation of the effects of an oxy-chlorine thermal treatment for regeneration of a ‘full-scale’ commercial automotive three-way catalyst was carried out (Paper IV). Improved catalyst activity for a high mileage catalyst could be observed, with emissions lowered by approximately 30 to 40 vol.% over the EC2000 driving cycle.

The properties of fresh, aged and regenerated catalysts were then studied by means of labscale experiments, on a local as well as a global level using a mathematical model (Paper V). The model allows for comparison of the intrinsic properties of the active surface by deriving and tuning parameters of a fresh catalyst and verifying the activity of a regenerated or aged catalyst.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006
Series
Trita-KET, ISSN 1104-3466 ; 227
Keyword
automotive catalyst, TWC, aging, sintering, Pd/Rh, Pt/Rh, regeneration, oxy-chlorination, dispersion, acid-leaching, mathematical modeling, reaction kinetics
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-3931 (URN)91-7178-343-1 (ISBN)
Public defence
2006-05-12, Sal F3, Lindstedtsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100812Available from: 2006-04-27 Created: 2006-04-27 Last updated: 2010-08-12Bibliographically approved

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