Monolith poisoning

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to 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.

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. 

A sophisticated quantitative analysis of experimental data was performed by Voltz et al. (96). Their experiment was performed over commercially available platinum catalysts on pellets and monoliths, with temperatures and gaseous compositions simulating exhaust gases. They found that carbon monoxide, propylene, and nitric oxide all exhibit strong poisoning effects on all kinetic rates. Their data can be fitted by equations of the form ... 

Lead aerosol in the air is poisonous to breathe, especially for young children. Many people called for the abolition of lead in gasoline. In the 1970s, the photochemical smog in California was attributed to unburned hydrocarbons and carbon monoxide from automobile tailpipes, and the best solution was the catalytic converter which works with finely divided platinum particles deposited on alumina monoliths. When leaded gasoline is used, these platinum atoms would be quickly covered by a barrage of lead aerosols. This finally led to the abolishment of TEL as a gasoline additive. 

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... 

Stabilizers can be introduced into the pellets or the washcoats with the intention of slowing down the thermally induced decrease in the surface area of the porous structure itself, or of the active component. Both, the active materials and the stabilizers, are put sometimes only on the outer layers of the pellets or monoliths, while, in other cases they penetrate the porous structures completely. Such preferential distributions have very specific aims, the utilization of the active materials and their protection from poisoning being the most important ones. There exists a vast body of patent literature on such designs. 

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. 

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... 

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). 

Since typical monolith catalysts have a thin coating of catalytic ingredients on the channel walls, they can be susceptible to poisoning. 

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. 

S.H. Oh and J.C. Cavendish, Design aspects of poison-resistant automobile monolithic catalysts, lEC Prod. Res. Dev. 22 509 (1983). 

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. 

The distribution of the elements in the direction across the monolith was analyzed by EPMA line analysis to identify the poisonous elements, because the change in the selectivity of NO reduction as shown in Fig. 2 is not thought... 

The fraction of the poisoning elements retained by the catalyst is distributed nonuniformily over the length of the catalyst and the depth of the washcoat layer. Highest concentrations occur at the entrance to the monolith (Figs. 90 and 91). Each of the poisoning elements interact in a different way with the washcoat con-... 

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. 

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