Platinum monoliths

S.E. Voltz and D. Liederman, "Thermal Deactivation of a Platinum Monolithic Carbon Monoxide/Hydrocarbon Oxidation Catalyst", Ind. Eng. Chem. Prod. Res. Dev.. 1974,1314). 243-250. 

Alvarez, E., Blanco, J., Otero de Becerra, J., Olivares, J., Salvador, L. 2002. Platinum monolithic catalysts for SO2 abatement in Dust-Free Flue Gas from combustion units. Latin American Applied Research. 32,123-129. 

T he successful use of platinum monolithic oxidation catalysts to control automobile emissions over many thousands of miles requires an intimate understanding of the many factors which contribute to catalyst degradation. Contamination of the active catalyst by lead and phosphorus compounds present in fuel and lubricating oil is a major factor in catalyst deterioration. 

Salomons S, Votsmeier M, Hayes R, Drochner A, Vogel H, GieshofJ CO andH2 oxidation on a platinum monolith diesel oxidation catalyst, Catal Today 117 491—497, 2006. 

Since NO production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore, many new designs incorporate DENOX units that employ catalytic methods to reduce the NO limit. Platinum-containing monolithic catalysts are used (36). Each catalyst performs optimally for a specific temperature range, and most of them work properly around 400°C. 

Starting with a ceramic and depositing an aluminum oxide coating. The aluminum oxide makes the ceramic, which is fairly smooth, have a number of bumps. On those bumps a noble metal catalyst, such as platinum, palladium, or rubidium, is deposited. The active site, wherever the noble metal is deposited, is where the conversion will actually take place. An alternate to the ceramic substrate is a metallic substrate. In this process, the aluminum oxide is deposited on the metallic substrate to give the wavy contour. The precious metal is then deposited onto the aluminum oxide. Both forms of catalyst are called monoliths. 

Serious research in catalytic reduction of automotive exhaust was begun in 1949 by Eugene Houdry, who developed mufflers for fork lift trucks used in confined spaces such as mines and warehouses (18). One of the supports used was the monolith—porcelain rods covered with films of alumina, on which platinum was deposited. California enacted laws in 1959 and 1960 on air quality and motor vehicle emission standards, which would be operative when at least two devices were developed that could meet the requirements. This gave the impetus for a greater effort in automotive catalysis research (19). Catalyst developments and fleet tests involved the partnership of catalyst manufacturers and muffler manufacturers. Three of these teams were certified by the California Motor Vehicle Pollution Control Board in 1964-65 American Cyanamid and Walker, W. R. Grace and Norris-Thermador, and Universal Oil Products and Arvin. At the same time, Detroit announced that engine modifications by lean carburation and secondary air injection enabled them to meet the California standard without the use of catalysts. This then delayed the use of catalysts in automobiles. 

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 ... 

The fit of these equations to the data is very good, as seen in Fig. 18. These equations are valid to very small values of CO concentrations, where the reaction becomes first order with respect to CO. In a mixture of CO with oxygen, there should be a maximum in reaction rate when the CO concentration is at 0.2%, as shown in Fig. 19. When the oxidation of olefins and aromatics over a platinum loaded monolith is over 99% complete, the conversion of higher paraffins may be around 90% and the conversion of the intractable methane is only 10%. 

Thermal radiation becomes important at higher temperatures, especially above 2000°F, when thermal destruction of the monolith substrate probably takes place. Thermal radiation intensities are proportional to the emissivity of the surface multiplied by the absolute temperature raised to the fourth power. The thermal emissivity of the monolith may be close to 1.0 due to the blackened surfaces from deposition of platinum. Each point of the channel is completely visible from any other point of the channel. The... 

Many elements of a mathematical model of the catalytic converter are available in the classical chemical reactor engineering literature. There are also many novel features in the automotive catalytic converter that need further analysis or even new formulations the transient analysis of catalytic beds, the shallow pellet bed, the monolith and the stacked and rolled screens, the negative order kinetics of CO oxidation over platinum,... 

GP 10] [R 18]The best HCN yield of 31% at a p-gauze platinum catalyst (70 ml h methane 70 ml h ammonia 500 ml h air 1 bar 963 °C) is much better than the performance of monoliths (Figure 3.49) having similar laminar flow conditions [2]. A coiled strip and a straight-channel monolith have yields of 4 and 16%, respectively. The micro-reactor performance is not much below the best yield gained in a monolith operated under turbulent-flow conditions (38%). 

The difference in reactor performance is due to the difference in hydraulic diameters of the reaction channels, i.e. related to varying mass-transfer limitations. The micro channels of the p-gauze platinum catalyst amount to 70 pm, whereas the monoliths have channel/pore diameters of 500-1200 pm. 

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. 

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

Catalytic materials can be physically supported on either pelleted or monolithic substrates. In the case of the pelleted catalyst, the support is an activated alumina. A typical monolithic catalyst is composed of a channeled ceramic (cordierilc) support having, for example. 300 to 400 square channels per square inch on which an activated alumina layer is applied. The active agents (platinum, palladium, rhodium, etc.) arc then highly dispersed on the alumina. 

The upstream monolith (containing platinum) catalyzes oxidation of hydrocarbons and CO to CO2 and NO to NO2, which is highly reactive... 

When used as monolithic, porous catalytic membranes, the platinum group metals provide higher mechanical stability and heat conductivity than conventional supported metal catalysts. The cost, however, can be an issue. An economically feasible solution to... 

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