Gasoline catalyst poisoning

Another limitation on the use of catalytic oxidation is the susceptibility of the catalysts to various deactivators or poisons, although according to the vendor the phase out of the use of volatile lead alkyls as antiknock agents in U.S. gasoline, catalyst poisoning is today rarely encountered. 

Catalyst Poisons. Synthesis gas prepared by the partial combustion of sweet natural gas can be charged to the reactors directly without purification. However, synthesis gas containing more than 0.1 grain of sulfur per 100 cubic feet must be purified before use over fluidized iron catalysts. Other catalyst poisons are known, such as chlorine (14), but they are not likely to be encountered in the natural gas to gasoline process. 

One of the most significant cases of catalyst poisoning occurred at the gasoline pump. For many years lead was used as an antiknock component in gasoline. While effective as an octane enhancer, it also poisoned the catalytic afterbimier, which reduced NO, CO, and hydrocarbons in the exhaust. Consequently, lead had to be removed from gasoline. 

One of the most widely known examples of catalyst poisoning is taken from the automobile industry. Though tetra-ethyl lead has been removed from essentially all gasoline in North America, the ban on leaded gasoline is not worldwide, and leaded and unleaded gasoline is available in many countries. Catalytic converters, which contain precious metals like platinum, palladium, and rhodium, are used to both reduce NO c and oxidize CO and unburned hydrocarbons. Lead irreversibly destroys the catalytic ability of the converter. Concentrations of lead in leaded gasoline are nominally 150mg/L. 

Typical catalyst poisons are lead and phosphorus. Lead is present at very low levels in unleaded gasoline. Typical lead levels are 0.003 g/gal although 0.05 g/gal is the maximum allowed lead level in unleaded fuel. Lead is not believed to be a major catalyst poison at the 0.003 g/gal level. On the other hand, use of leaded fuel will poison three-way catalysts, and catalyst activity is not fully recovered upon changing back to unleaded fuel. Figure 8... 

The main catalyst site poison for many years was tetraethyllead [78-00-2]y C H ven after use of unleaded gasoline. Not only is lead a catalyst poison, but automotive source lead is also a health hazard (66). The source of this lead came from manufacture and distribution of leaded and unleaded gasoline in common transport equipment and storage facilities (67). In the eady 1990s, so Htde leaded gasoline was being distributed that Pb contamination was approaching zero (<0.26 ppm/L). 

The main catalyst site poison for many years was tetraethyllead [78-00-2] even after use of unleaded gasoline. Not only is lead a catalyst... 

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

Beginning with the 1975 U.S. automobiles, catalytic converters were added to nearly all models to meet the more restrictive emission standards. Since the lead used in gasoline is a poison to the catalyst used in the converter, a scheduled introduction of unleaded gasoline was also required. The U.S. petroleum industry simultaneously introduced unleaded gasoline into the marketplace. 

By 1999, General Motors, Daimler-Clirysler, Toyota, and Nissan all had demonstration fuel cell vehicles operating on niethanol, with plans to start introducing vehicles into the market by 2005. Auto makers have shown a preference for methanol over gasoline primarily because of the likelihood of the sulfur content in gasoline poisoning some of the catalysts used in the fuel cell. 

Sulfur in gasoline contributes to the SO air quality problem and deactivates the catalyst in the catalytic converter. Emissions from a poisoned converter contain higher levels of VOC, NO, and CO. As stated earlier, VCR2 and NOj are catalyzed by sunlight to form smog. 

For the U.S., the new EPA rules will limit sulfur in gasoline to 30 ppm, phased between 2004 and 2006. The automobile industry has made a strong case for lower sulfur because of its effect on the catalytic converter. The converter has the same catalyst as the refinery reformer and it is poisoned just as easily by sulfur. 

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