Porous coating technique

PtSn/C electrocatalysts with R Sn atomic ratios of 50 50 and 90 10 were prepared by alcohol-reduction process, using ethylene glycol as solvent and reducing agent, and by borohydride reduction. The electrocatalysts were characterized by EDX, XRD and cyclic voltammetry. The electro-oxidation of ethanol was studied by cyclic voltammetry using the thin porous coating technique. The electrocatalysts performance depends greatly on preparation procedures and R Sn atomic ratios. 

Electrochemical studies of the electrocatalysts were carried out using the thin porous coating technique [12,13]. An amormt of 20 mg of the eletrocatalyst was added to a solution of 50 mL of water containing 3 drops of a 6% polytetrafluoroethylene (PTFE) suspension. The resulting mixture was treated in an ultrasound bath for 10 min, filtered and transferred to the cavity (0.30 mm deep and 0.36 cm area) of the working electrode. The quantity of electrocatalyst in the working electrode was determined with a precision of... 

A thin dense separating layer (in the range of 0.5-5 p thick) is then coated on this substructure. Different coating techniques are in use, but most commonly a solution of polymer in an appropriate solvent is spread onto the porous substructure. The solvent is evaporated, followed by further treatment to crosslink the polymer. 

The solution coating technique was used in the preparation of the cellulose triacetate membrane discussed above. A solution of cellulose triacetate in chloroform was deposited on the porous support and the solvent was then evaporated leaving a thin film on the porous support. Thin film polymerization was used to prepare a polyfuran membrane barrier layer on polysulfone. In this case, the monomer furfuryl alcohol is polymerized in situ by adjustment of pH and temperature. This membrane proved to be highly susceptible to oxidizing agents and is of limited value. By far the most valuable technique in the formation of membrane barrier layers is interfacial polycondensation. In this method, a polymer is formed on the porous support surface at the interface of organic and aqueous phases by reaction of specific molecules dissolved in each phase. It is by this method that a number of polyamides and polyurea membrane barrier layers have been formed on polysulfone. Elements containing these membranes are available commercially. 

At the current time, liquid precursor (solution/sol/slurry) coating techniques have been the most successful for the deposition of oxides, such as monazite (LaP04) and porous oxides (carbon-oxide mixtures) onto oxide fiber tows [178, 179, 191, 192]. Atypical fiber coater is shown in Figure 10a [193]. In the coating process, a lighter, immiscible liquid is floated on the surface of the coating precursor. The immiscible liquid is used to remove excess sol from the coated tow and it allows for the coating of individual filaments with... 

An alternative technique is to use zinc soaps (e.g., at 200°C) to impregnate porous coatings such as sintered iron parts (Kovarskii et al., 1988) which, it is said, gave results comparable to coated steel after one year in a marine environment. 

The geometric surface of the microchannels can be increased for performing catalytic reactions. Porous coatings are typically applied for this purpose. The porous layer can be catalytically active or serve as a support for a catalytic active phase. Different coating techniques are developed and tested over the last years as there is a steady increase of the number of catalytic appUcations in pharmaceutical and fine chemical industries driven by strict environmental regulations and policies introduced during the last decade [1]. 

The sol-gel coating technique may be well suitable for the coating of highly porous substrates such as metal foams, because it generates uniform coatings, which are difficult to achieve by wash-coating using foams. 

The porous coat system (Nordson) allows the application of discrete, random, and open patterns of hot-melt adhesive to substrates such as films, papers, fabrics, and nonwovens. With the control coat system, hot melt adhesives are applied continuously or intermittently by air-controlled nozzles without contact to the substrate. This technique is used as well for reactive hot-melt adhesives such as moisture-curing polyurethane hot-melt adhesives. 

They also formed the condensed polynuclear aromatic (COPNA) resin film on a porous a-alumina support tube. Next, a pinhole-free CMSM was produced by carbonization at 400-1,000°C [29], The mesopores of the COPNA-based caibon membranes did not penetrate through the total thickness of each membrane and served as channels which increased permeances by linking the micropores. CMSMs produced using COPNA and BPDA-pp ODA polyimdes showed similar permeation properties even though they had different pore stractures. This suggests that the micropores are responsible for the permselectivities of the carbonized membrane. Besides that, Fuertes [30] used phenohc resin in conjunction with the dip coating technique to prepare adsorption-selective carbon membrane supported on ceramic tubular membranes. 

Fuertes and co-woikers [12, 46 8, 78] investigated intensively the preparation of caibon membranes from PR precursors. According to Centeno and Fuertes [12], they coated a small quantity of liquid phenohc resin (Novolak type) on the finely polished surface of a porous carbon disk by means of a spin coating technique. The supported phenolic resin film was cured in air at 150°C for 2 h, and then carbonization was carried out in a vertical tubular furnace (Carbolite) at different temperatures (between 500 and 1,000°C) under vacuum. Figure 4.6 shows the SEM micrograph of the membranes. The polymeric film (Fig. 4.6a) coated on top of the porous substrate is dense and has a thickness of around 2 pm. The thickness of the carbon membrane shown in Fig. 4.6b is also about 2 pm. Figure 4.6c shows the top view of the fractured membrane. The top smface is veiy smooth. Helium gas permeance of membranes carbonized at different temperatures is shown in Fig. 4.7. 

The dip-coating technique is widely used for the preparation of porous ceramic membranes [1]. Figure 2.7 illustrates the flow sheet of a dip-coating process. The prepared coatings may be adjustable between lOOnm and 100pm in thickness and the pore size covers from micropore to mesopore and part of the macropore range. 

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