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Reactivation of an Aged Commercial Three-Way Catalyst by Oxalic and Citric Acid Washing
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
2006 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 40, 2030-2036 p.Article in journal (Refereed) Published
Abstract [en]

The efficiency of dilute oxalic and citric acid solutions on improving the oxygen storage capacity (OSC) and catalytic activity of a severely aged (83 000 km) commercial three-way catalyst (TWC) has been investigated. Washing procedures applied after optimization of experimental parameters, namely, temperature, flow-rate, and concentration of acid solution, led to significant improvements of OSC and catalytic activity (based on dynamometer test measurements) of the aged TWC. The latter was made possible due to the removal of significant amounts of various contaminants accumulated on the catalyst surface (e.g., P, S, Pb, Ca, Zn, Si, Fe, Cu, and Ni) during driving conditions, as revealed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) and X-ray Photoelectron Spectroscopy (XPS) analyses. For the first time, it is demonstrated that dilute oxalic acid solution significantly improves the catalytic activity of an aged commercial TWC toward CO, CxHy, and NOx conversions under real exhaust gas conditions (dynamometer tests) by two to eight times in the 250-450 degrees C range and the OSC quantity by up to 50%. Oxalic acid appears to be more efficient than citric acid in removing specifically P- and S-containing compounds from the catalyst surface, whereas citric acid in removing Pb- and Zn-containing compounds, thus uncovering surface active catalytic sites.

Place, publisher, year, edition, pages
2006. Vol. 40, 2030-2036 p.
Keyword [en]
oxygen storage, release properties, calcination temperature, exhaust catalysts, chemical-analysis, metal-oxides, supported rh, deactivation, phosphorus, oxidation
National Category
Engineering and Technology
URN: urn:nbn:se:kth:diva-5635DOI: 10.1021/es052310tISI: 000236213900046ScopusID: 2-s2.0-33645220203OAI: diva2:10067
QC 20100812Available from: 2006-04-27 Created: 2006-04-27 Last updated: 2011-11-08Bibliographically 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
Trita-KET, ISSN 1104-3466 ; 227
automotive catalyst, TWC, aging, sintering, Pd/Rh, Pt/Rh, regeneration, oxy-chlorination, dispersion, acid-leaching, mathematical modeling, reaction kinetics
National Category
Chemical Engineering
urn:nbn:se:kth:diva-3931 (URN)91-7178-343-1 (ISBN)
Public defence
2006-05-12, Sal F3, Lindstedtsvägen 26, Stockholm, 10:00
QC 20100812Available from: 2006-04-27 Created: 2006-04-27 Last updated: 2010-08-12Bibliographically approved

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