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While other aspects o f microreactor design can be very different from conventional reactor design, some o f the established methods o f incorporating catalysts have proven useful. Fixed bed microreactors have been made by flowing particles into the reaction chamber (Schmidt, et al, 1997, Losey, et al, 1999). Reactants and catalyst particles can enter either through the same or different inlets, while pillars with spacing smaller than the diameter o f the particles are required at the outlet o f the reaction channel to act as a trap for the particles. Pressure drop can be an issue due to the small particle size. Another relatively easy, albeit expensive, catalyst incorporation method is to machine plates from the catalytic metal itself (Mayer, et al, 1999). The microstructured layers are then sealed by diffusion bonding.

When high catalytic surface area is required, various techniques can be employed to create porous surfaces. A method (for conducting materials) is anodisation, which is performed by immersing the substrate into a suitable acidic solution and passing an electric current through it. Varying the current density and time gives rise to a range o f surface morphologies and porous layer thicknesses. Surface area increase factors o f 100 are readily achievable. Normal impregnation or immobilisation methods can then be used for catalyst deposition (W ie^meier and Honicke, 1996b, Drott, et al 1997, Fichtner, et al, 1999, Honicke, 1999, Wunsch, et al, 2000). Kursawe, et al (2000) used sputtering after anodisation to deposit the catalyst. Sol-gels based on alumina, silica, titanium have been used to make porous catalyst supports as well. The use o f stabilising chelating agents to improve adhesion has been recommended. Thermal annealing at different temperatures gives rise to different surface areas, with crack-free highly porous layers which are mechanically stable. Following that, the surface is

ready to be impregnated with the catalyst (Fichtner, et al, 1999, Kursawe, et al, 2000). Zeolites (silicalite, ZSM-5, TS-1) with controlled crystal orientation have been grown by hydrothermal synthesis on silicon and stainless steel microstructures producing uniform thickness layers (Wan, et al, 2001, Rebrov, et al 2001). By prior seeding o f the microchannel area, selective zeolite growth can take place only within the confines o f the microchannel.

Anodisation and surface coating methods mentioned above result in regular, porous catalyst layers with controlled thicknesses. Anodised structures are formed from the substrate itself, thus adhesion is usually not an issue, while sol-gel structures can have their adhesion modified by the addition o f various additives. Porous silica can also be formed within the whole micro channel by passing potassium silicate solution mixed with formamide. The catalyst can then be immobilised by impregnation (Christensen, et al, 1998, Greenway and McCreedy, 1999).

Alternatively, aerosols o f metal salt can be applied to the microreactor surface and evaporated at the point o f contact to give rise to high surface area catalysts. This method was developed after it was determined that drying an aqueous or organic solution to deposit the catalyst precursor results in the catalyst coating the comers o f the channel due to surface tension effects. Platinum, silver and rhodium were deposited in this manner (Franz, et al, 1999b). Preformed catalyst particles can be immobilised on the reactor walls by first dispersing them in suitable liquids. Pfeifer, et al (1999) applied nanoparticles o f palladium or copper catalyst mixed with ZnO promoter dispersed into a polymer (hydroxyethylcellulose) to a microreactor. After drying, and sintering a BET surface area o f up to 9.3 m^/g was achieved, with a

catalytic layer thickness o f 20 pm. Alternatively, catalytic particles can also be incorporated on to a layer o f PDMS coated microchannel and then baked to give an immobilised high surface area catalyst (Wilson and McCreedy, 2000).

Chemical vapour deposition (CVD) can also be used to deposit catalysts or carriers. Activated carbon catalyst has been deposited by polymer CVD followed by carbonisation, or using a sooting flame (Franz, et al, 1999b). Atmospheric pressure CVD was used to deposit a porous ceramic coating (alumina) with a surface enhancement factor o f 100 and thickness o f 100 pm (Fichtner, 1999).

If high catalytic surface area is not required, less conventional methods from the microfabrication avenue can be used, especially sputtering processes. Either the shadow mask or lift-off technique is used to define the target area. In the former, a sheet (usually metallic) containing the deposition pattern is placed against the substrate. In the latter, conventional photolithography is used to pattern a layer o f photoresist on the substrate. Once the mask is in place, the catalyst material is sputtered on to the substrate. In the lift-off process, the photoresist is then removed, bringing with it the excess deposited catalyst (Srinivasan, et al, 1997, Madou, 1997). It should be noted that these thin films have been reported to degrade when subjected to high temperatures (Firebaugh, et al, 1998).

If the substrate is electrically conducting, electroplating can be used to deposit the catalyst from suitable catalyst precursor containing solutions (Franz, 1999b). Another method that has been used recently is micro-contact printing. A master is replicated using a PDMS stamp. The metallic catalyst is then deposited on to the stamp.

Meanwhile, the microreactor is covered with a suitable ligand using a self-assembly process to give rise to an ordered structure. The stamp is then contacted with the reactor and treated to transfer the catalyst. Suhmicron particles can also be deposited with a stamping process (Xia and Whitesides, 1998, Braun, et al, 1997).