Platinum washcoat

In addition to platinum and related metals, the principal active component ia the multiflmctioaal systems is cerium oxide. Each catalytic coaverter coataias 50—100 g of finely divided ceria dispersed within the washcoat. Elucidatioa of the detailed behavior of cerium is difficult and compHcated by the presence of other additives, eg, lanthanum oxide, that perform related functions. Ceria acts as a stabilizer for the high surface area alumina, as a promoter of the water gas shift reaction, as an oxygen storage component, and as an enhancer of the NO reduction capability of rhodium. 

Catalyst Deactivation. Catalyst deactivation (45) by halogen degradation is a very difficult problem particularly for platinum (PGM) catalysts, which make up about 75% of the catalysts used for VOC destmction (10). The problem may weU He with the catalyst carrier or washcoat. Alumina, for example, a common washcoat, can react with a chlorinated hydrocarbon in a gas stream to form aluminum chloride which can then interact with the metal. Fluid-bed reactors have been used to offset catalyst deactivation but these are large and cosdy (45). 

Platinum serves as the catalyst for the oxidation of CO and hydrocarbons. It is relatively insensitive to contamination by lead or sulfur. At high temperatures it is not known to dissolve in the washcoat, but sintering into larger particles may lead to a substantial loss of platinum surface area with dramatic consequences for the overall oxidation activity. 

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. 

Complex oxides of the perovskite structure containing rare earths like lanthanum have proved effective for oxidation of CO and hydrocarbons and for the decomposition of nitrogen oxides. These catalysts are cheaper alternatives than noble metals like platinum and rhodium which are used in automotive catalytic converters. The most effective catalysts are systems of the type Lai vSrvM03, where M = cobalt, manganese, iron, chromium, copper. Further, perovskites used as active phases in catalytic converters have to be stabilized on the rare earth containing washcoat layers. This then leads to an increase in rare earth content of a catalytic converter unit by factors up to ten compared to the three way catalyst. 

An example is washcoating of platinum and silica using a sol containing both a silica and a platinum precursor [58,59]. After calcination a catalyst containing platinum on silica-washcoated monolith is obtained. Since the platinum precursor is homogeneously mixed with silica, encapsulation of platinum may occur, which reduces the effectiveness of platinum as compared to that of a two-step preparation. In this respect, the method may be more suitable for less expensive active phases. 

Fig. 5.7. The three-way catalyst consists of platinum and rhodium (or palladium) metal particles on a porous oxidic washcoat, applied on a ceramic monolith.

It is difficult to separate the influence of the precious metals from the influence of the washcoat composition. Each of the precious metals platinum, palladium and rhodium, which have a specific task, interact with each other during the use and the aging of the catalyst, so that their influence on the catalyst performance is in most cases not additive. Figure 68 compares the performance of a fully formulated Pt/Rh catalyst to the performance of catalysts having either Pt or Rh alone in the same loadings and on the same washcoat as the fully formulated catalyst. 

Figure 68. The function of platinum and rhodium on a fully formulated three-way catalyst washcoat in the conversion of CO, HC and NO, (monolith catalyst with 62 cells cm" three-way formulation, aged on an engine bench for 20 h engine bench test at a space velocity of 60000NIC h exhaust gas temperature 673K exhaust gas composition lambda 0.999, dynamic frequency I Hz amplitude 1 A/F). Reprinted with permission from ref. [34], (C 1991 Society of Automotive Engineers, Inc.

Figure 69. Effect of platinum, palladium and rhodium at an equimolar loading on the temperature needed to reach 50% conversion of butene and butane, as a function of the exhaust gas oxygen current (monolith catalyst with 62 cells cm y-Al203 washcoat fresh precious metal loading 8.8 mmol 1" model gas light-off test at a space velocity of 60000N11 h model gas composition is stoichiometric at 1.0 vol % O2). Reprinted with permission from ref [30], (g) 1994 Society of Automotive Engineers, Inc.

Table 20. Sintering behavior of platinum, palladium and rhodium as a function of the aging atmosphere (washcoat La203-doped AI2O3, precious metal content 0.14wt.%). Reprinted from refs. [50, 51] with kind permission of Elsevier Science.

For the third type of lean bum engines, special catalysts are developed, which have the ability to convert NO to NO2 and to store the NO2 under the lean burn operation conditions. The stored NO2 is reduced to N2 once the engine is operated under stoichiometric and rich conditions. The catalyst proposed in the patent literature for this application contains precious metals such as platinum, on three-way washcoats modified so as to increase their capacity to store and release NO2. 

Figure 103. The amount of soluble organic fraction (SOF) adsorbed on an aged diesel oxidation catalyst, as a function of the washcoat formulation (monolith catalyst with 62cellscm", dedicated diesel washcoat formulations with platinum at a loading of l.76gl diesel engine bench aging for 50 h diesel fuel containing 0.15 wt. % sulfur). Reprinted with permission from ref. (68], C 1991 Society of Automotive Engineers, Inc.

Figure 1 5. Conversion of carbon monoxide, gaseous hydrocarbons and sulfur dioxide reached over a diesel catalyst with and without measures to suppress the formation of sulfates, as a function of the exhaust gas temperature (monolith catalyst with 62 cells cm dedicated diesel washcoat formulations with platinum at a loading of 1.76 g I" diesel engine test bench light-off test at a space velocity of 120000 N1 F h diesel engine bench aging procedure for 100 h at a catalyst inlet temperature of 773 K).

Figure 109. Conversion of carbon monoxide and gaseous hydrocarbons reached over a diesel oxidation catalyst at various settings of the exhaust gas temperature, for a model gas composition with and without SO2 (monolith catalyst with 62cells cm dedicated diesel washcoat formulations with platinum loading of 1.76gl" in the fresh state model gas light-off test at a space velocity of 50000Nir h model gas simulates the exhaust gas composition of an IDI passenger car diesel engine at medium load and speed).

Figure 111. Emission of aldehydes, acrolein and various polynuclear aromatic hydrocarbons of two passenger cars equipped with an IDI/NA and with a DI/NA diesel engine, once without and once with a diesel oxidation catalyst, in the US-FTP 75 vehicle test cycle (monolith catalyst with 62 cells cm dedicated diesel washcoat formulation with a platinum loading of 1.76 g 1 in the fresh state vehicle dynamometer tests according to the US-FTP 75 vehicle test procedure, with passenger cars equipped with a DI/NA and with an IDI/NA diesel engine of displacement 2.0 1). Reprinted with permission from ref [70], 1990 Society of Automotive Engineers, Inc.

Porous and thermally stable washcoating layer on mechanically strong support is an important component in both oxidative and three-way catalysts used for car exhaust gas cleaning. The washcoat provides a high and stable surface area for dispersion of the active component of the catalysts consisting of platinum and /or paladium. Usually for the preparation of this layer aluminas modified by La, Ce, Zr, Si etc. are used [1-3]. As it was shown in [4-6] the properties of modified aluminas depend on the method of introduction of the additives In this work we present the results on the preparation and study of model alumina systems modified by La, Ce and Zr as well as of monolith supports washcoated by optimal compositions of alumina and additives. 

The trapping component was formulated into a washcoat and supported on a ceramic monolith with 400 cells per square inch (cpsi). The trap material was chosen for NOx adsorption, regenerability, thermal stability and rate of adsorption/desorption. Platinum is incorporated within the trap to oxidize the NO and the injected hydrocarbon. The lean NOx catalyst was Pt (60 gft- ) deposited on y-Al203 on a 400 cpsi cordierite monolith. 

Figure 7. Estimated cost and platinum content of a WGS monolith washcoated with PEceria catalysts.

The typical deactivation curves plotted in Figure 8 depict the poisoning of Johnson Matthey platinum metal oxidation catalyst by lead and phosphorus. A section of the spent catalyst was subjected to electron probe microanalysis (Figure 9) the lead and phosphorus density photographs were obtained by monitoring back reflected x-rays. Phosphorus accumulated on the periphery of the washcoat at the gas-solid interface, whilst lead appeared to be more evenly distributed throughout the... 

The phosphorus deactivation curve is typical type C, and, according to the Wheeler model, this is associated with selective poisoning of pore mouths. Phosphorus distribution on the poisoned catalyst is near the gas-solid interface, i.e. at pore mouths, which confirms the Wheeler model of pore mouth poisoning for type C deactivation curves. Thus we may propose that in the fast oxidative reactions with which we are dealing, transport processes within pores will control the effectiveness of the catalyst. Active sites at the gas-solid interface will be controlled by relatively fast bulk diffusional processes, whereas active sites within pores of 20-100 A present in the washcoat aluminas on which the platinum is deposited will be controlled by the slower Knudsen diffusion process. Thus phosphorus poisoning of active sites at pore mouths will result in a serious loss in catalyst activity since reactant molecules must diffuse deeper into the pore structure by the slower Knudsen mass transport process to find progressively fewer active sites. 

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