Honeycomb, porous alumina

In a related approach, these same porous alumina membranes serve as a mask through which O2 plasmas are used to etch underlying carbon films. This etching process produces honeycomb carbon structures that are positive replicas of the alumina-membrane mask. This process has successfully produced honeycomb structures of both diamond and graphitic carbon, with pore sizes in the carbon replica around 70 nm. 

Zeolite is a crystalline, porous aluminosilicate mineral with a unique interconnecting lattice structure. This lattice structure is arranged to form a honeycomb framework of consistent diameter interconnecting channels and pores. Negatively charged alumina and neutrally charged silica tetrahedral building blocks are stacked to produce the open three-dimensional honeycomb framework. 

For example, continuous multilayered ZSM-5 films were grown on cordierite modules.[59] Similar films were generated on as-prepared and acid-treated honeycomb substrates.[60] The latter treatments led to silica-rich surface layers their composition affected the Si/Al ratios of the zeolites crystallized on the cordierite. Thin, defect-free Mid-type films were also made on porous alpha-alumina and yttria-doped zirconia substrates using tclrapropylammonium hydroxide (TPAOH) as a structure-directing agent.[61]... 

A second catalyst support type is commonly called a monolith which is a thin walled multi channeled honeycomb. The ceramic walls between the channels are the base support surfaces for the catalyst. (FIGURE 3) Although they are porous, they are not the direct surface for the precious metal. An intermediate alumina coating called "washcoat provides an ultra high surface for the catalyst sights (FIGURE 4 is an illustration of the washcoat precious metal relationship). The catalyzed monolith is likewise assembled into the metal container using a compressible interface material. 

The physical form of the support has to be chosen with a view to the type of reactor in which its use is intended. Silica and alumina are available as coarse granules or fine powders, and may be formed into various shapes with the aid of a binder (stearic acid, graphite) they can then be used in fixed bed reactors. For fluidised beds, or for use in liquid media, fine powders are required. Ceramic monoliths having structures resembling a honeycomb are used where (as in vehicle exhaust treatment) very high space velocities have to be used, but they are made of a non-porous material (a-alumina, muUite) and have to have a thin wash-coat of high area alumina applied, so that the metal can be firmly affixed. 

To maintain catalytic performance during long-term vehicle operation, porosity of the cordierite material is important to fix the catalyst layer on the cell wall of the honeycomb substrates. Figure 13.1.7 shows the washcoat layer, which contains y-alumina composite carrier and precious metal catalyst, on the porous cell wall surface. This photograph shows a 6mil/400cpi standard cell structure catalyst made of 35% porosity cordierite. 

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