Extraction of phosphorus and metal contaminants from aged commercial three-way catalysts by using weak organic acid solutions
The developed procedure consists of the use of weak organic acid solutions (citric, oxalic acid) at a low temperature (T=50ºC) to remove phosphorous and other metal contaminants (e.g., Pb, Fe, Cu, Ni, Cr) from a commercial TWC. The potential application of the described process is to permit the extraction of metal poisons from aged three-way catalytic converters in an easy and cheap way, and also from similar commercial catalysts the deactivation of which is also due to metal deposition.
This result contains the findings and conclusions of the economic and environmental assessment for the regeneration / reactivation procedure of used catalysts, in the framework of the Work Package 4 of the REGEN-CATS project. The objectives are twofolds: - to provide a State of the Art of the economic and environmental aspects of the TWC; - to highlight the key economic and market issues. Indeed, it is essential to define which conditions should be fulfilled by the regeneration procedure (in terms of emissions reduction performance and logistic constraints) in order to maximise its market "acceptability". Two contrasted scenarios have been envisaged and discussed: - "Re-use of the regenerated TWC on an other car" - "Several uses of the regenerated TWC on the same car" Both scenarios are based on the principle that a TWC is designed to comply with the emissions regulations during the whole life of a car (" fit and forget" principle).
The following have been identified as the main causes of the deactivation: - Sintering of the cerium oxide containing phases - Loss of noble metals - Sintering of the noble metals - Loss of oxygen storage capacity - Accumulation of poison (phosphate, lead, calcium, zinc, iron, etc..) The microactivity test of different used catalysts showed that elimination of NO and hydrocarbons are seriously impaired in used catalyst and the extent to which this occurs is greater in the front section of the monoliths although the rear section of the monoliths are also deactivated but to a lesser extent. Although CO oxidation performance is diminished as a function of catalysts age, it is not affected to the same extent as the NO reduction and hydrocarbon oxidation reactions. The consequence of these finding will impact on the future design of 3 way catalysts: the result show which of the active sites are most affected by the different causes of deactivation (contaminants or poison accumulation, loss and deterioration of the different active sites of the washcoat, etc). This knowledge should contribute to a better design of the TWC by catalysts manufacturers to increase and to guarantee longer periods of operation. The barriers for future use of this result will came from the development of new clean fuels for cars. For instance the use of the H2 fuel cell will not require such a catalyst for the elimination of toxic exhaust pipe emissions.
The thermal treatment of TWC has shown a reactivation in the catalytic activity during the development of the studies. The thermal treatment has removed the organic poison from the TWC providing an increment in the life cycle of the TWC. The application of the process could be used with many other catalytic system, which involve organic matter: The deposits of this organic matter will deactivate the catalytic surface but this could be recovered by applying the thermal treatment. This procedure has proven to be successful using the methods described: - Oxygen treatment at 500 to 700ºC. - Chlorine (0.5-5%)/Oxygen treatment at 300ºC. - Chlorine (<1%)/Oxygen treatment at 500ºC. - Hydrochloric acid pre treatment at 300-500ºC.