Rhodium 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. 

Alumina, present in the gamma modification, is the most suitable high surface area support for noble metals. The y-Al203 in washcoats typically has a surface area of 150-175 m g However, at high temperatures y-alumina transforms into the alpha phase, and stabilization to prevent this is essential. Another concern is the diffusion of rhodium into alumina, which calls for the application of diffusion barriers. 

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. 

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.

Zirconium oxides are the preferred supports for the precious metal component rhodium. The cerium oxide and/or the zirconium oxide are added to the washcoat either as preformed oxides or as oxide precursors, such as their respective carbonates or nitrates - the oxides are then formed in situ during washcoat drying and calcination. 

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.

Within the current TWC catalyst washcoats, rhodium is susceptible to deleterious interactions with various components during a prolonged lean high temperature excursion. To elucidate the potentially detrimental rhodium compounds formed under such circumstances, unsupported rhodium oxides, rare earth metal rhodates, and aluminum rhodate are characterized and measured for catalytic activity. The intrinsic activities at 673K of NO, CO and CjHg conversions over various unsupported rhodium oxides species are basically structure insensitive. However, the intrinsic activities at the same temperature of both the rare earth metal rhodates and aluminum rhodate appear to be sensitive to their structure. The interaction between rhodium and the rare earths especially cerium, is found to be much stronger than that between rhodium and aluminum. 

When the NO addition was done after the preadsorption of CO on the catalyst surface, absorption bands of linearly adsorbed NO on Pt and Rh were observed. The corresponding bands are seen at 1890 (Rh-NO ) and 1667 cm-1 (Pt-NO or Rh-NO"). The absorption band at 1786 cm" is due to the rhodium nitrosyl band (Rh-NO ) or platinum nitrosyl band (Pt-NO). Strong absorption bands at around 2258 cm" indicate the presence of isocyanate intermediates (Pt-NCO and Rh-NCO) which appeared after the evacuation. If NO was introduced into the chamber before CO the isocyanate complex was formed earlier. NO and CO were also adsorbed strongly on the washcoat giving absorption bands below 1600... 

The support is a ceramic honeycomb widi a washcoat of AI2O3 that contains the rhodium metal catalyst (0.2 %). Indications of die mechanism of the reaction are provided by literature data on analogous reactions ... 

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 wider variety of alcohols [18] and alkenes [17] were investigated under CPO conditions on platinum- and rhodium-coated monoliths with 5 wt.% of catalytically active species. The results obtained with monoliths having the same dimensions as for methanol CPO revealed that a small surface area of active species can lead to higher alkene selectivity by homogeneous reactions. Platinum was also judged to be unable to dissociate higher alkenes completely. Thus, an increase in catalytically active surface area by means of a washcoat or a reduction in the channel size can lead to a higher H2 yield. 

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... 

The flat stagnation disk was coated with a Rh/Al203 catalyst, where rhodium particles were distributed in a porous AI2O3 washcoat. Appropriate amounts of aqueous solution of rhodium(III) nitrate (Umicore) (9 wt%... 

---