Poisoning auto catalysts

Auto emissions have been closely monitored, and strict controls have been put Into place to minimize the amount of unbumed hydrocarbons released into the atmosphere. The Clean Air Act of 1990 was passed to help reduce hydrocarbon emissions from automobiles. The catalytic converter was developed to help react the unburned hydrocarbon and produce a less dangerous emission of carbon dioxide and water. (As a side benefit, lead had to be eliminated from gasoline because it poisoned the catalyst and made the catal3dic converter useless. The big campaign to get the lead out removed a major source of the deadly heavy metal from the environment.)... 

Since lead can poison auto exhaust catalysts, automobiles equipped with catalytic exhaust-control devices require lead-free gasoline, which has become the standard motor fuel. Sulfur in gasoline is also detrimental to catalyst performance, so sulfur levels in gasoline are kept very low. 

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. 

SSIMS has been applied to study of the chemical poisoning and thermal deactiviation of the catalyst that takes place with increasing distance travelled by the car. Figure 4.10 shows the positive and negative SIMS spectra obtained from a fresh fully formulated auto-exhaust system. 

Catalyst deactivation in hydrodemetallisation (HDM) is caused by the interaction of the metal deposits with the original active phase ( active site poisoning ) and the loss of pore volume due to the obstruction of catalyst pores i pore plugging) (1). However, metal deposits also have an auto-catalytic effect on the hydrodemetallisation reaction, thus active site generation may occur in low active phase loaded or bare support catalyst systems. 

Sulfur trioxide is probably a worse pollutant than sulfur dioxide, because SO3 is the acid anhydride of strong, corrosive sulfuric acid. Sulfur trioxide reacts with water vapor in the air, as well as in auto exhausts, to form sulfuric acid droplets. This problem must be overcome if the current type of catalytic converter is to see continued use. These same catalysts also suffer from the problem of being poisoned —that is, made inactive—by lead. Leaded fuels contain tetraethyl lead, Pb(C2H5)4, and tetramethyl lead, Pb(CH3)4. Such fuels are not suitable for automohiles equipped with catalytic converters and are excluded hy U. S. law from use in such cars. 

Organic sulphur- and nitrogen-compounds in motor fuels are a source for acid rain and harmful to the environment. Moreover, they are poisonous to the auto exhaust catalysts. To meet new developments in EU regulations on the S-concentration, a commonly applied one-step hydrodesulfurization (HDS), using conventional catalysts, e.g. C0-M0/7-AI2O3, is insufficient. A second HDS step, viz. a deep HDS step, can be more economical to reduce the S-content to the currently allowed European level of 350 ppm. This level will be reduced further to 50 ppm in 2005 [1]. In the first HDS step, often the heavy organic sulfur-containing polyaromatics survived, such as dibenzothiophene (DBT) and (4-, and/or 6-) alkylated DBTs [2,3]. They are the most refractory. In crude oils, there are also aromatic N-compounds, which suppress the performance of the HDS catalysts. Hence, a model feed for representative HDS-activity measurements should contain characteristic S- and N- compounds for practical relevance. 

Depending on the purity of the gas feedstock, there may be pre-treatment of natural gas to remove impurities such as sulphur compounds that will poison catalysts used in subsequent processes. There are three common technologies in use to convert natural gas into syngas steam methane reforming (SMR), partial oxidation (POX) and auto-thermal reforming (ATR). 

The spatial distributions of catalytic metals and contaminant poisons in auto exhaust catalysts were delineated by electron probe line scans. Element concentrations were characterized by element sensitivities, i.e. in counts per second (cps). The electron probe microanalyses (EPM) were qualitative or semiqualitative in nature. Accurate correlation between element sensitivity and element concentration requires rather sophisticated instrument calibration. A quantitative evaluation of the EPM findings is beyond the scope of this paper. In general, it can be stated that element concentration is directly proportional to element sensitivity. Furthermore, the proportionality constant between element concentration and element sensitivity varies greatly from element to element. 

Shelef et ah (2) reported the following representative contaminant retention values for monolithic noble metal HC-CO oxidation catalysts lead, 15% phosphorus, 9% zinc, 3% and sulfur, 0.05%. Furthermore, for monolithic noble metal HC-CO catalysts which had been subjected to 30,000 miles of vehicle testing, the ratios of front to rear contaminant poison concentrations were lead, 7 phosphorus, 16 and zinc, 11. Because NO, and HC-CO catalysts are normally operated under different ambient conditions, i.e. net reducing vs. net oxidizing atmosphere, it is expected that the nature, distribution, and retention of contaminant poisons will differ for these two types of auto exhaust catalysts. 

Catal dic converters led to the elimination of lead from gasoline, because lead poisons the converter catalyst. Similarly, sulfur poisons catalysts that may be used on future vehicles. Hence, the reduction of sulfur in gasoline and diesel fuel to ultra-low levels is a key requirement of Auto Oil n. 

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