Poison in monolithic catalysts

Acres et al. (22) have speculated on the modes of phosphorus and lead poisoning in monolithic catalysts, based on data obtained in simulated aging. Conversion vs. time-of-exposure curves for catalysts poisoned by either lead or phosphorus show quite different shapes, which the authors attribute to pore-mouth poisoning for phosphorus, and uniform poisoning... 

Fig. 6. Catalyst inhibition mechanisms where ( ) are active catalyst sites the catalyst carrier and the catalytic support (a) masking of catalyst (b) poisoning of catalyst (c) thermal aging of catalyst and (d) attrition of ceramic oxide metal substrate monolith system, which causes the loss of active catalytic material resulting in less catalyst in the reactor unit and eventual loss in performance.

The amount of catalyst used per vehicle depends on engine size, catalyst location, desired efficiency, and several other considerations. Since it is necessary to relate the amount of catalyst to that of the poison which may come in contact with it, we indicate that on a typical U. S. eight-cylinder vehicle, made in 1977, two monolithic catalysts, each weighing about 1 kg, are employed. The weight of a pelleted catalyst on a similar vehicle is of the order of 3 kg. Furthermore, the pelleted catalysts... 

Shelef, M., Dalla Betta, R. A., Larson, J. A., Otto, K., and Yao, H. C., Poisoning of Monolithic Noble Metal Oxidation Catalysts in Automotive Exhaust Environment, Am. Inst. Chem. Eng., New Orleans Meet. (1973). 

The monolith catalysts are the least tested in pilot scale, however they have the advantage that they offer good mechanical strength and have high catalytic activity. On the other hand their cost is considerably higher and they arc more prone to poisoning and deactivation than dolomite and related catalysts. Because of their cost, the most important operational variable is the life of the catalyst. 

It is assumed that gases within a channel of the monolith are well mixed in the direction perpendicular to the wall and plug flow axially. Buzanowski and Yang [49] have presented an analytical solution for the overall NO conversion as an explicit function of space velocity and other parameters for a monolithic catalyst under isothermal conditions. Three monoliths were prepared, one of which was poisoned with K2O. 

Corrosiveness. Sulfur in the fuel facilitates corrosion. The combustion of sulfur containing fuels results in the formation of corrosive gases that attack the engine, the exhaust as well as the environment. Sulfur also adversely affects the alkyl lead octane response. Furthermore, it adversely affects the exhaust catalysts. However, the monolithic catalysts usually recover when the sulfur content of the fuel is reduced. In this case, sulfur is considered to be an inhibitor rather than a catalyst poison. The copper strip corrosion test and the sulfur content specification are used to ensure fuel quality. The copper strip test measures the active sulfur whereas the sulfur content reports the total sulfur present. 

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. 

Certain factors are analyzed to determine their effects on automotive catalyst activity. At operating gas velocities, spherical catalysts were more active than monolithic catalysts at comparable catalyst volumes and metals loadings. Palladium was the most active catalyst metal. Platinum in a mixed platinum palladium catalyst stabilizes against the effects of lead poisoning. An optimum activity particulate catalyst would contain about 0.05 wt % total metals on a gamma-alumina base with a platinum content of 0.03-0.04 wt % and a palladium content of 0.01-0.02 wt %. A somewhat thick shell of metals located near the outer surface of the particle provides better catalyst activity than a shell type distribution of metals. 

Since 1975 catalysts have been fitted to vehicles in the USA to control emissions, initially of HC and CO (oxidation catalysts), and latterly also of NOx (three way catalysts). The mode of operation of these catalyst systems in the USA and Japan is now well characterised (1). The catalysts typically comprise the precious metals platinum, palladium and rhodium, either singly or in combination, together with base metal promoters or stabilisers, supported on alumina pellets or alumina coated ceramic monoliths. Catalysts for the US market are designed to withstand 50,000 miles of road use and must be operated in conjunction with lead free fuel since they are poisoned by lead. 

Almost all catalytic converters have to contend with the decay or poisoning of the catalyst In some form and the catalytic monolith Is no exception. Indeed this Is notorious In the automotive application where the catalytic converter must survive 50,000 miles of operation and still perform adequately. Although we shall use the kinetics of carbon monoxide oxidation over a platinum catalyst as an obvious and Important example, our main objective Is to develop a model which can handle any catalyst decay question and to point out the differences In two types of poisoning. Thus our study comes within the third main division of the subject as laid out by Butt (1 ) In 1972 not the mechanism or rate determination but the effect of deactivation on the operation of the reactor. 

Xu et al ° studied the poisoning effect of phosphorus in both model and monolithic catalysts aged on bench-engine dynamometers and vehicles. They found that the direct reaction of ceria with P2O5 present in the exhaust gas or with other P compounds... 

Base Metal Catalyst - An alternate to a noble metal catalyst is a base metal catalyst. A base metal catalyst can be deposited on a monolithic substrate or is available as a pellet. These pellets are normally extruded and hence are 100% catalyst rather than deposition on a substrate. A benefit of base metal extruded catalyst is that if any poisons are present in the process stream, a deposition of the poisons on the surface of the catalyst occurs. Depending on the type of contaminant, it can frequently be washed away with water. When it is washed, abraded, or atritted, the outer surface is removed and subsequently a new catalyst surface is exposed. Hence, the catalyst can be regenerated. Noble metal catalyst can also be regenerated but the process is more expensive. A noble metal catalyst, depending on the operation, will typically last 30,000 hours. As a rule of thumb, a single shift operation of 40 hours a week, 50 weeks a year results in a total of 2,000 hours per year. Hence, the catalyst might have a 15 year life expectancy. From a cost factor, a typical rule of thumb is that a catalyst might be 10%-15% of the overall capital cost of the equipment. 

The distribution of contaminants within the porous layer again has to be considered separately for monolithic and pelleted catalysts. Gradients of the contaminant concentration in both cases can be very steep or relatively flat. Some inferences on the poison-carrying species can be deduced from such gradients. 

Awareness of the importance of air quality has triggered several home appliances of catalysts, whereby monoliths play a role. Table 6 gives several applications in the consumer sector. The limited lifetime of such applications overcomes a disadvantage of monolithic systems, in that they contain usually a relatively small amoimt of catalyst, and as a consequence, the buffer capacity against poisoning is less than that of conventional packed-bed reactors. In industrial operation, either robust catalysts or pure feeds are needed, but for home appliances, these criteria may be less of an issue. Similarly, for the use of N2O in hospitals, ambulances, etc., catalyst poisoning is less important. N2O decomposition can be achieved easily, and the use of a monolith structure is highly attractive for mobile applications. 

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