Alumina-washcoated

If the same quantity of active ingredient is concentrated in an outside shell of thickness 0.015 cm, one obtains y> = 2.27. This would yield an effectiveness factor of 0.431 in a slab geometry, and the apparent kinetic constant has risen to 99.2 sec-1. If the active ingredient is further concentrated in a shell of 0.0025 cm, one obtains y> = 0.38, an effectiveness factor of 0.957, and an apparent kinetic constant of 220 sec-1. These calculations are comparable to the data given in Fig. 15. This analysis applies just as well to the monolith, where the highly porous alumina washcoat should not be thicker than 0.001 in. 

Figure 3.1 Cross section of micro channels coated with alumina washcoat exhibiting U- or V-shapes [6 ...

Table 3.9. shows the results of barometric hydrogen chemisorption on Cu/AlaOj, Ni/A Oj, Pt/Al203 and a Ni on alumina-washcoated monolith. 

Lenz and Aicher reported the experimental results obtained with an autothermal reformer fed with desulfurized kerosene employing a metallic monolith coated with alumina washcoat supporting precious metal catalysts (Pt and Rh) [78]. The experiments were performed at steam-to-carbon ratios S/C = 1.5-2.5 and... 

The cordierite extruded monoliths, having 400 square cellsAn, were similar to those used in automobile catalytic converters. However, instead of using an alumina washcoat as in the catalytic converter, these catalyst supports were loaded directly with 12 to 14 wt.% Pt in the same manner as the foam monoliths. Because these extruded monoliths consist of several straight, parallel channels, the flow in these monoliths is laminar (with entrance effects) at the flow rates studied. 

R 20] [P 3] In Figure 4.49 some experimental results are shown. The y-alumina washcoat impregnated with Cu/ZnO delivers a hydrogen yield of 78% at 98% selectivity. 

Aluminas are used in various catalytic applications, a-, y-, and -aluminas are all used as support materials, the first one in applications where low surface areas are desired, as in partial oxidation reactions. The latter two, and especially y-alumina, in applications where high surface areas and high thermal and mechanical stability are required. One of the most prominent applications of y-alumina as support is the catalytic converter for pollution control, where an alumina washcoat covers a monolithic support. The washcoat is impregnated with the catalytically active noble metals. Another major application area of high-surface aluminas as support is in the petrochemical industry in hydrotreating plants. Alumina-supported catalysts with Co, Ni, and/or Mo are used for this purpose. Also, all noble metals are available as supported catalysts based on aluminas. Such catalysts are used for hydrogenation reactions or sometimes oxidation reactions. If high... 

FIGURE 25 Image of alumina washcoat on aluminum foam. Left closeup SEM left, aluminum middle, anodized layer (aluminum oxide, about 5 pm thick) right, washcoat (20 pm thick). Right image piece of solid foam from which part of the washcoat had been removed (broken off), thus showing the washcoat layer. 

X 10 Alumina-washcoat steel-alloy support impregnated with nickel... 

The application of monolith catalysts to a variety of commercial synthesis processes has been investigated because of the potentially smaller size and lower pressure drop through the chemical reactors. One of the earliest of these investigations was for methanation, the chemical reaction between carbon monoxide and hydrogen to produce methane selectively. In a detailed study [14] a comparative evaluation involved the use of nickel catalyst on (1) spherical alumina pellets (0.32-cm diameter), (2) alumina washcoated (10-20% by weight) cordierite monoliths with 31- and 46-cells (square)/cm density, (3) an alumina... 

Kim et al. [40] made an attempt to oxidize phenol in water solutions using a monolith reactor. Alumina-washcoated cordierite monoliths (62 celLs/cm ) impregnated with copper... 

Alumina washcoating is taken as an example to illustrate the washcoating of an alumina layer onto a monolith surface. 

Figure 1 shows the block scheme of washcoating. A dry monolith is dipped in an Al-sol. Afterwards, it is drained or blown with air to remove the remaining sol. After drying and calcination at appropriate temperatures, the alumina washcoating is completed. 

Carbon coating can be achieved using pyrolysis of hydrocarbons at elevated temperatures [69]. Figure 2 shows a device used for carbon coating via hydrocarbon pyrolysis. In the example described here, an alumina-washcoated monolith is covered with carbon by pyrolysis of cyclohexene. A gas mixture of cyclohexene in nitrogen is passing the reactor at a certain flow rate. The monolith block to be coated is placed in the middle of the heated tubular reactor. The reaction takes place at 873-973 K, and the amount of carbon deposited can be controlled by the temperature and the time on stream. Up to 3-10 wt% carbon can be homogeneously coated onto the monolith in this way. It appears that the surface area of the carbon-coated alumina-washcoated cordierite monolith is of... 

Our preliminary results of nickel on carbon-coated monolithic catalysts show that in a hydrogenation reaction it is five times more active than the corresponding nickel on alumina-washcoated monolithic catalyst without carbon coating. 

Monoliths made of metal foils can also be used as substrates in combustion catalysts [19, 20]. The metal is generally an iron- or nickel-based steel containing small amounts of aluminum. The aluminum diffuses to the surface on heating and oxidizes to form an adherent alumina layer. This alumina layer gives the alloy high oxidation resistance and is essentially self-healing as it arises from diffusion from the bulk material. It also provides good adhesion for the alumina washcoat. 

Figure 10.12. AI solid-state NMR chemical shift data from a fresh Pd-only catalyst and from inlet, middle, and outlet sections of a high-mileage taxi-cab aged catalyst of the same formulation. Note the strong feature at 40 ppm in the vehicle-aged catalyst corresponding to hydrated aluminum phosphate. The broad features near 5 and 60 ppm are associated with octahedrally and tetrahedrally coordinated aluminum cations in the alumina washcoat. [30]...

A method for coating microchannel walls with layers as thick as 25 pm was developed by Stefanescu et al. [181]. The microreactor was built from FeCrAl (Aluchrom ). The metal surface was first chemically treated in several steps and afterward annealed at 1200 °C for 1 h to trigger the segregation of aluminum and the formation of an alumina layer on the metallic surface. An alumina washcoat was subsequently deposited from a slurry onto the microstructure and characterized by various physical methods. The authors varied the properties such as viscosity, particle size, and pH of the slurry. Acrylic acid, a component used as dispersant and binder, was found to be particularly important for the adhesion of the alumina layer. 

Cordierite monoliths were coated with an alumina washcoat and stabilised at 550°C. Some of the samples were then immersed in either an aqueous solution of cerous or cobalt nitrate, dried and calcined in air at 550°C at which the metal nitrates decomposed into their oxides [11]. The samples were weighed and the procedure was repeated until 40 mg of the metal oxide had been deposited onto the alumina washcoated monolith samples. Pt and Pd were applied by direct impregnation using aqueous solutions of HaPtCla and PdCb followed by diying and calcination in air at 550°C [8]. The Pt and Pd loadings (2.0 and 1.09 mg, respectively) of the catalysts were equal on molar basis. The nominal composition of the eight catalysts prepared are listed in Table 1. 

Flow reactor studies were performed using monolith samples with an alumina washcoat, a storage compound (BaO) and noble metals (Pt, Rh). In order to elucidate the importance of the various ingredients, samples were also prepared without storage compoimd or noble metals, respectively. For the FTIR studies, similar powder catalysts were pressed into thin discs. 

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