Poison-Resistant Automotive Catalysts

Poison-resistant Catalyst for Automotive Emission Control... 

Noble metal catalysts are highly active for the oxidation of carbon monoxide and therefore widely used in the control of automobile emissions. Numerous recent studies on noble metal-based three-way catalysts have revealed characteristics of good thermal stability and poison resistance(l). Incorporation of rare earth oxides as an additive in automotive catalysts has improved the dispersion and stability of precious metals present in the catalyst as active components(2). Monolith-supported noble-metal catalysts have also been developed(3). However, the disadvantages of noble metal catalysts such as relative scarcity, high cost and requirement of strict air/fuel ratio in three-way function have prompted attention to be focused on the development of non-noble metal alternatives. 

The catalytic species of current automotive catalysts are balanced mixtures of precious metals and promoters selected, as discussed previously, on the basis of application. Precious metals are favoured due to high catalytic activity and selectivity, particularly at low temperatures (as experienced with cold start tests). Additionally their supported dispersions are relatively stable at high temperatures and exhibit good resistance to poisoning. 

One notable exception has been the development of the catalytic exhaust system for automobiles, one of the most intense catalyst development efforts ever undertaken. An automotive catalyst normally consists of Pt/Pd and some Rh on a ceramic support. Catalytic exhaust control systems function under severe and rapidly changing conditions and must be active for several reactions that reduce automotive emissions—CO oxidation, hydrocarbon oxidation, and reduction (this is the so-called three-way catalyst). Typical operating conditions are temperatures of 400 to 600 C (or much greater under certain conditions) and 150,000 hr space velocity. Numerous reviews of the development and performance of these catalysts are available, and these catalysts are of interest because they are frequently used for control of VOC-emissions, particularly in conjunction with open flame preheaters. Unfortunately, these catalysts are not designed to resist poisoning by many VOC-type compounds, particularly those containing chlorine and sulfur. 

The parameters that affect the degradation of supported platinum and palladium automotive exhaust catalysts are investigated. The study includes the effects of temperature, poison concentration, and hed volume on the lifetime of the catalyst. Thermal damage primarily affects noble metal surface area. Measurements of specific metal area and catalytic activity reveal that supported palladium is more thermally stable than platinum. On the other hand, platinum is more resistant to poisoning than palladium. Electron microprobe examinations of poisoned catalyst pellets reveal that the contaminants accumulate almost exclusively near the skin of the pellet as lead sulfate and lead phosphate. It is possible to regenerate these poisoned catalysts by redistributing the contaminants throughout the pellet. 

All NOx-containing combustion gases also contain a significant amount of water vapor (2-15%), therefore resistance to water poisoning is important for practical application of a catalyst. Iwamoto, et al and Li and Hall (9) reported that the catalytic activity of Cu-ZSM-5 for NO conversion to N2 was decreased in the presence of 2% water vapor, but it could be recovered after removal of water vapor. However, Kharas and coworkers (10) found that Cu-ZSM-5 was severely deactivated under a typical automotive fuel lean exhaust gas (10% H2O, GHSV = 127,(X)0 h" ) over the temperature range of 600 to SOO C. The authors attributed the catalytic deactivation of Cu-ZSM-5 to the formation of CuO and the disruption of zeolitic crystallinity and porosity. 

Monolithic catalytic converters are now of wide use on automotives, however their efficiency and their resistance to thermal shocks, poisons, misfiring, etc., must be improved. Beside new catalyst formulations and experimental tests, modeling and understanding of the main physical and chemical processes can help to suggest design improvements. 

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