Monoliths alumina pellets

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... 

Two basic catalyst structures were used, distinguished by the configuration of the catalyst support. The two support types are alumina pellets and alumina coated ceramic monoliths (Figure 2). The pellets are approximately 1/8th inch in diameter and are composed of thermally stable transitional alumina. The monoliths are made of a ceramic material such as cordierite (2Mg,2Al203,5Si02). 

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. 

The raw materials needed to supply about ten million new automobiles a year do not impose a difficult problem except in the case of the noble metals. Present technology indicates that each car may need up to ten pounds of pellets, two pounds of monoliths, or two pounds of metal alloys. The refractory oxide support materials are usually a mixture of silica, alumina, magnesia, lithium oxide, and zirconium oxide. Fifty thousand tons of such materials a year do not raise serious problems (47). The base metal oxides requirement per car may be 0.1 to 1 lb per car, or up to five thousand tons a year. The current U.S. annual consumption of copper, manganese, and chromium is above a million tons per year, and the consumption of nickel and tungsten above a hundred thousand tons per year. The only important metals used at the low rate of five thousand tons per year are cobalt, vanadium, and the rare earths. 

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. 

In the earliest catalysts, two basic support configurations were used. The first was thermally stable alumina in the form of cylindrical pellets or spheres, typically 3 mm in diameter, that had for several decades been used in the chemical processes industry, such as petroleum refining. The second type of catalyst support was the so-called monolith, made from metal, as described in this chapter, or a ceramic material such as cordierite (2MgO 2Al203 5Si02) the subject of the previous chapter. The monolith has strong thin... 

Figure 8 Steam reforming of hexane at flow rates of 2 0 and 0 64 Ib/hr of water and hexane, respectively Axial bed-temperature and composition profiles for a metal monolith (250 cells/in consisting of Kanthal support/7-Al203 washcoat/NiO catalyst, and a packed bed of Girdler G-9(X pellets (j in. X in ) of alumina impregnated with nickel. (From Ref. 9.)...

Summary data for different conversion/temperature conditions are provided in Tables 5-7. Rate constants were calculated from these data, and it was determined that although the operations at 140 kPa were influenced by mass transfer, this was not the rate-limiting step however, the reaction was mass transfer limited at 1000 kPa. The higher carbon monoxide conversion values and methane production observed for the monolith-supported nickel compared to pellets were explained to be due to the provision by the monoliths of smaller pore diffusion resistance and higher mass transfer rates at higher temperatures, primarily a result of shorter diffusion paths in thin alumina coatings on the monolith walls. 

Two types of catalytic converters are currently being used for meeting the passenger car emission standards in the U.S. three-way converters and dualbed converters. Both converters contain three-way catalysts, but with the dual-bed converter the three-way catalyst is followed by an air injection/ oxidation catalyst system. As for the earlier oxidation catalysts two forms of catalyst support are used pellets (thermally stable transitional alumina) and monoliths (cordierite honeycombs coated with a thin alumina washcoat). Figure 7 shows four catalytic converters currently being used by General Motors. 

Nearly identical kinetic results are obtained with pellet or monolith samples, i.e., there is practically no difference in this regard between the outer layer of a pellet and the coating of a monolith, if y-alumina and activation are the same. 

Transition aluminas help solving the problem of automotive pollution control. Their intrinsic advantages chemical inertia, suitable porosity are found in the forms of pellets or washcoated monoliths, as well as one important drawback thermal aging. This very important feature was studied,... 

Characteristics of the alumina common to both pellets and monolithic substrates In each case, alumina must provide a suitable surface to precious metals. This requires both chemical purity for the nature of the surfaces and thermal stability of surface area. 

Thus, recent automotive catalyst development tends toward depositing platinum in a thin washcoat of alumina on low-surface-area, ceramic monoliths or in egg shell layers on the exterior of pellets. 

The catalysts developed during the fifties and sixties were prepared on pellet substrates, displaying sphere or more irregular grain shape, and made of porous transition alumina this alumina is generally manufactured by a dedicated preparation procedure to resist thermal shocks. The development of car exhaust catalysts leads to the use of monoliths as catalyst supports [3,4] and cellular ceramics are today easily commercially available. 

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