Palladium washcoat

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. 

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 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.

The mass transfer model presented in Equation (30) was applied by Tsoligkas et al. [117] to interpret experimental results characterizing the hydrogenation of 4-nitrobenzoic acid to 4-aminobenzoic acid. The reaction was conducted in a capillary with a circular cross-section and a washcoat incorporating an alumina-supported palladium catalyst. 

Palladium was deposited onto three different supports by incipient-wetness impregnation, as described elsewhere [16]. The supports, supplied by Condea GmbH, possess the following chemical composition AI2O3, Ba-Al203 and La-ALOs. The concentration of Ba and La, respectively, was 3 wt% for both supports. A Pd(N03)2 solution (Alfa Aesar, 8.41 wt%) was used as the metal precursor. The concentration of the solution was calculated to obtain a washcoat with a metal loading of 2.5 wt %. 

The morphology and size of the palladium particles on the alumina washcoat were studied using a transmission electron microscope (TEM - JEOL 2000 FX) operated at 200 kV. The microscope was equipped with an EDS (energy dispersive x-ray spectroscopy) detector (LINK AN 10000). The samples were prepared by applying a few droplets of a dispersion of a finely ground catalyst in ethanol onto a hollow carbon film supported by a copper grid. 

The stronger deactivation of the catalyst supported on modified alumina is apparently due to the presence of Ba and La. As PdAS, PdBS and PdLS exhibit the same size of palladium particles at the surface of the washcoat, the nature of the support is responsible for the difference observed in combustion activity. Nevertheless, this is observed only in the presence of SO2 in the gas stream. 

Palladium and platinum was added to the washcoat, corresponding to 2.5 wt-%. This was carried out with a conventional incipient wetness technique, using platinum and palladium nitrates. After addition of the precious metals the catalysts was once again calcined at 1000 °C for 4 h. 

For the CH4 combustion all of the catalysts except the palladium based ones showed a similar activity, i.e. igniting the CH4 around 700°C. Palladium based catalyst showed a difference depending on the washcoat material and the ignition occurs in the following order Pd-YAG>Pd-MAS>Pd-LMA. This is probably due to the difference in surface area between the washcoats and shows the importance of a sintering resistant washcoat material. 

Fig. 2. Poisoning of the catalysts, LMA- (top left), MAS- (top right) and F/lG-catalysts (bottom). The difference in temperatures for 10% (TIO), 50% (T50) and 90% (T90) conversion for the different fuel components between the fresh and the poisoned catalysts palladium (stripy bars), platinum (white bars) and washcoat only (black bars).

These catalytic converters contain a high surface area, a honeycombed ceramic or stainless steel core that is coated with silica and alumina, called a washcoat. Precious metal catalysts, such as platinum, palladium, and rhodium, are added as a suspension to the washcoat. As the hot gases pass through the catalytic converter, they are converted by the catalysts to the reduced or oxidized products. 

A similar approach was adopted on single micro structured metal foils covered in a metal housing for catalyst screening during CPO of propane [49]. Platinum, palladium and rhodium on a y-alumina washcoat catalysts were used, with the best... 

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