Oxide ceramics, monolith

Monolithic non-oxide ceramics Monolithic oxide ceramics Non-oxide CMC Oxide CMC... 

The most widely used exhaust control device consists of a ceramic monolith with a thin-waHed open honeycomb stmcture. The accessible surface of this monolith system is iacreased by applyiag a separate coatiag, a wash coat, of a high surface area material such as gamma-alumiaa with the catalyticaHy active species impregaated iato this washcoat. The catalyst aeeds to oxidize hydrocarboas, coavert CO to CO2, and reduce NO. The whole system forms a catalytic converter that, suitably encased, is placed between the engine and the muffler/silencer unit. 

The ceramic oxide carrier is bonded to the monolith by both chemical and physical means. The bonding differs for a ceramic monolith and a metallic monolith. Attrition is a physical loss of the carrier from the monolith from the surface shear effects caused by the exhaust gas, a sudden start-up or shutdown causing a thermal shock as a result of different coefficients of thermal expansion at the boundary between the carrier and the monolith, physical vibration of the cataly2ed honeycomb, or abrasion from particulates in the exhaust air (21) (see Fig. 6d). 

A ceramic monolith catalyst support, cordierite, consisting of silica, alumina and magnesium oxide. The purpose of this is to provide support, strength and stability over a wide temperature range. 

There are a number of examples of tube waU reactors, the most important being the automotive catalytic converter (ACC), which was described in the previous section. These reactors are made by coating an extruded ceramic monolith with noble metals supported on a thin wash coat of y-alumina. This reactor is used to oxidize hydrocarbons and CO to CO2 and H2O and also reduce NO to N2. The rates of these reactions are very fast after warmup, and the effectiveness factor within the porous wash coat is therefore very smaU. The reactions are also eternal mass transfer limited within the monohth after warmup. We wUl consider three limiting cases of this reactor, surface reaction limiting, external mass transfer limiting, and wash coat diffusion limiting. In each case we wiU assume a first-order irreversible reaction. 

The modem catalytic converter installed on most automobiles is a washcoat consisting of precious metal oxides, supported on a ceramic monolith. After passage of the... 

Weiland et al observed that a small amount of Pt metal present in the Rh-based catalyst could significantly improve the catalyst activity for ATR of gasoline range fuels. They claimed that the role of Pt is to enhance oxidation activity, whereas Rh provides high SR activity. The Rh-Pt/alumina catalyst used in the study was supported on monolithic honeycombs and had a Rh to Pt ratio of 3-10 by weight. The geometry (metal monolith, ceramic monolith, or ceramic foam) of the support did not affect the product composition. ... 

The oxidation of gaseous ethanol (first-order reaction) was studied in a spinning basket reactor with Pt/Al203 on a ceramic monolith as catalyst. The inlet concentration of ethanol... 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

Due to its high photocatalytic activity towards the complete mineralisation of VOCs [7,8] titania in its anatase form is normally used. Using ceramic monoliths with high titania content (50%) the total oxidation of chlorinated organic compounds at low temperature has been demonstrated [9]. However, since the photons from natural light may only penetrate a few microns into the catalyst surface the use of a wash-coating technique, where only a thin active film of titania is applied to the ceramic or metallic support can be considered as an ideal technique to produce maintenance free photocatalytic reactors. 

Most of the current converters consist of a flow-through ceramic monolith with its channel walls covered with a high-surface-area 7-AI2O3 layer (the washcoat) which contains the active catalyst particles. The monolith is composed of cordicrite, a mineral with the composition 2MgO 2AI2O3 5Si02. The chemical composition of a modern TWC is quite complex. In addition to alumina, the washcoat contains up to 30 wt% base metal oxide additives, added for many purposes. The most common additives are ceria and lanthana in many formulations BaO and Zr02 are used, and in some converters NiO is present. The major active constituents of the washcoat are the noble metis Pt, Pd, and Rh (typically 1-3 g). Most of the TWC systems in use today are still based on Pt and Rh in a ratio of about 10 1. 

Autothermal reforming is a teim adopted for the process in which a mixture of air and steam serves as the oxidant in the conversion of hydrocarbon fuels to a hydrogen-rich product. This process has also been reported to become more efficient as a result of the use of monolithic catalyst beds [11]. An example of this has been the demonstration of a modified version of the fuel-rich partial oxidation process in which noble metal catalysts were used in place of nickel on ceramic monoliths [2j. In earlier reports where packed catalyst beds were used, the concept to control carbon formation, which was predicted by thermodynamic equilibrium at low air-to-fuel ratios, was demonstrated by introducing steam, in addition to air, as an oxidant. 

A number of methods can be used to prepare monoliths. Extrusion is widely used for the manufacture of ceramic monoliths, whereas corrugation is used mostly for those of metallic monoliths. As to the techniques used to coat a thin layer of oxide(s) on a monolith, the sol-gel method is used the most. Other methods, e.g., surface oxidation, slurry dip-coating, or CVD and CVI techniques, can also be used. 

Ahluwalia, R.K., Zhang, Q., Chmielewski, D.J., Lauzze, K.C., and Inbody, M.A. Performance of CO preferential oxidation reactor with noble-metal catalyst coated on ceramic monolith for onboard fuel processing applications. Catalysis Today, 2005, 99, 271. 

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.

Metal monoliths have a number of advantages compared to ceramics. They are more robust than ceramic monoliths and have excellent resistance to thermal shock, provided that the metal is not oxidized at high temperatures. The foil sheet that is used to manufacture the walls of the metal monolith is much thinner than the walls in a ceramic monolith (typically 0.05 mm in the metal vesus 0.25 mm in the ceramic). The thinner walls give a larger surface to volume ratio which produces a smaller pressure drop for a given mass transfer limited performance. 

Figure 1. Sketch of reactor configuration used for catalytic oxidation on monolith reactors at millisecond contact times. Gases slightly above atmospheric pressure flow at high velocities through porous ceramic monolifiis coated with Rh or Pt.

The increasing volume of air pollutants such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) has become a serious global environmental problem. Currently the three-way catalytic converters, which contain Rh, Pt and Pd noble metals supported on a ceramic monolith, are used for the simultaneously control of all the above pollutants from... 

The trapping component was formulated into a washcoat and supported on a ceramic monolith with 400 cells per square inch (cpsi). The trap material was chosen for NOx adsorption, regenerability, thermal stability and rate of adsorption/desorption. Platinum is incorporated within the trap to oxidize the NO and the injected hydrocarbon. The lean NOx catalyst was Pt (60 gft- ) deposited on y-Al203 on a 400 cpsi cordierite monolith. 

Four of these solid solutions, supported on ceramic monolithic supports, were selected for further study with simulated automobile exhausts. These systems were chosen because they demonstrated good nitric oxide reduction activity and were among the best candidates for oxidation activity. The four systems consisted of CuFe204 and CuMn204 class I spinels in solid solution with NiAl204 and ZnAl204 class II spinels. Surface area and porosity of the monolithic support and of the support... 

System Preheating. - The thermal efficiency of a catalytic oxidation system may be enhanced by preheating the feed gas in air prior to catalytic combustion. Tichenor and Palazzolo [5] have determined the relative contribution of the pre-heater to the overall efficiency. A mixture of iso-propanol, methyl ethyl ketone, ethyl acetate, benzene and n-hexane was combusted at a space velocity of 50,000h" in the temperature range 300-450°C over a bi-metallic Pt-Pd catalyst supported on a ceramic monolith. The results are shown in figure 1. 

At DLR, three different classes of ceramic materials have been developed for the use in energy applications Oxide/oxide and non oxide Ceramic Matrix Composites (CMC) as well as monolithic SiSiC materials. 

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