Rhodium monoliths

The rhodium monolith showed a lower hydrogen selectivity of 50% at the same temperature and residence time. For the FeCrAlloy monolith impregnated with nickel, full conversion was achieved at 900 °C, but at a lower hydrogen yield [55],... 

Mayer et al. [43] compared their results generated at a micro structured monolith (see Section 2.4.3) with literature data [137]. The degree of conversion and the hydrogen selectivity of the rhodium monolith outperformed both metal-coated foam monoliths, Pt-Rh gauzes and extruded monoliths. This was partially attributed to the higher activity of the rhodium monolith, but also to the lower cross-sectional channel area of the metallic monolith, which reduced mass transfer limitations, and to the improved heat conductivity. 

New developments in the field of ceramic foam monoliths could also potentially provide new catalytic process technology for the conversion of methane into synthesis gas. For example, workers at Minnesota University [9] have achieved high synthesis gas yields at both high temperatures and space velocities using rhodium supported on a ceramic foam. 

GP 8[ [R 7[ Syngas generation with commercial Pt-Rh gauzes, metal-coated foam monoliths and extruded monoliths has been reported. For similar process pressure, process temperature, and reaction mixture composition, methane conversions are considerably lower in the conventional reactors (CH4/O2 2.0 22 vol.-% methane, 11 vol.-% oxygen, 66 vol.-% inert species 0.14—0.155 MPa 1100 °C) [3]. They amount to about 60%, whereas 90% was reached with the rhodium micro reactor. A much higher H2 selectivity is reached in the micro reactor the CO selectivity was comparable. The micro channels outlet temperatures dropped on increasing the amount of inert gas. 

This is explained by a possible higher activity of pure rhodium than supported metal catalysts. However, two other reasons are also taken into account to explain the superior performance of the micro reactor boundary-layer mass transfer limitations, which exist for the laboratory-scale monoliths with larger internal dimensions, are less significant for the micro reactor with order-of-magnitude smaller dimensions, and the use of the thermally highly conductive rhodium as construction material facilitates heat transfer from the oxidation to the reforming zone. 

Another important highly selective and stable hydroformylation sol gel catalyst is made of silica-supported rhodium covalently bound to supported Xantphos family of ligands.36 By incorporating monoliths of the sol-gel doped material into the paddles of an autoclave stirrer, the catalyst (Rotacat) can be used in a continuous liquid flow process. A single sample of this catalyst was used for a variety of different hydroformylation reactions under widely varying conditions over a period of more than a year, still retaining its selective 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. 

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. 

Metallic monoliths made of both rhodium ([HCR 1]) and FeCrAlloy (72.6% Fe, 22% Cr and 4.8% Al ([HCR 3]) carrying micro channels of 120 pm x 130 pm cross-section at various length (5 and 20 mm) were applied. The monoliths were prepared of micro structured foils by electron beam welding. After bonding, the FeCrAlloy was oxidized in air at 1 000 °C for 4 h to form an a-alumina layer, which was verified by XRD. Its thickness was determined as < 10 pm by SEM/EDX. The alumina layer was impregnated with rhodium chloride and alternatively with a nickel salt solution. The catalyst loading with nickel (30 mg) was much higher than that with rhodium (1 mg) (see Table 2.4). The amount of rhodium on the catalyst surface was determined as 3% by XPS. 

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.

A development in catalyst support systems in which half of the 5-10% rhodium-platinum alloy gauzes were replaced by nonnoble metal supports or by ordinary metal catalysts gave cost economies without adversely affecting operating efficiency [47]. More recently, ammonia oxidation in a two-bed system (Pt gauzes followed by monolithic oxide layers) gave nearly the same ammonia conversion while reducing platinum losses by 50% [48]. 

Until 1995, most of the monolithic three-way catalysts contained platinum and rhodium, in mass ratios of about 5-20 1 Pt Rh. The total precious metal loading is typically 0.9-2.2gU catalyst volume. These are only typical values, as the amount of precious metals and the mass ratio of platinum to rhodium depends on the specific application of the catalyst and is governed by factors such as the composition of engine-out emissions, the emissions targets to be reached, the catalyst operating conditions and the properties of the fuels used. 

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.

A ceramic (cordierite) monolith-type three-way automotive exhaust catalyst was used for laboratory-simulated sintering and activity tests and was characterized at Aliieo-Signa1 Materials Research Center. The catalyst contained platinum and rhodium at a weight ratio of 5 to and a total noble metal loading of approximately 1.1 weight percent based on the weight of the... 

The present investigation was conducted to identify and determine the degree of Rh-base metal oxide interaction, using unsupported rhodium oxides and bulk aluminum and rare earth metal rhodates. Catalytic activities were determined using monolithic catalysts containing various bulk rhodium species exposed to a simulated stoichiometric auto exhaust composition. The activities were correlated with information obtained from CO chemisorption measurements, temperature-programmed reduction,... 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

The data sets from this initial engine work shows that the thrifted rhodium coating at this loading reduces the NOx eluent from the palladium only monolith by ca 50%. Work is underway to optimise the rhodium coating and the data will be published elsewhere. 

The catalyst was prepared on a cordierite monolith having 62 cells per cm2, xhe support was coated with a promoted alumina-ceria (6 % Ce02) wash-coat and impregnated with 1.06 g/1 of platinum plus rhodium, with a Pt/Rli mass ratio of 5. After impregnation, the catalyst was calcined for 2 hours at 500°C. Activity measurement... 

Frequently exposed to very high temperatures, the catalysts used for depollution of motor vehicle exhaust gases are deactivated mainly by structural and textural evolution processes. This paper describes how the catalytic activity of a typical three-way catalyst (platinum-rhodium on a wash-coated cordierite monolith) was determined for the removal of hydrocarbons of various types before and after high-temperature treatments. Electron microscopy (CTEM and STEM) was used to determine metal particle size in fresh and aged catalysts. 

Since 1975 catalysts have been fitted to vehicles in the USA to control emissions, initially of HC and CO (oxidation catalysts), and latterly also of NOx (three way catalysts). The mode of operation of these catalyst systems in the USA and Japan is now well characterised (1). The catalysts typically comprise the precious metals platinum, palladium and rhodium, either singly or in combination, together with base metal promoters or stabilisers, supported on alumina pellets or alumina coated ceramic monoliths. Catalysts for the US market are designed to withstand 50,000 miles of road use and must be operated in conjunction with lead free fuel since they are poisoned by lead. 

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