Porous carbon surfaces

The Competitive Role of Water in Sorption Processes on Porous Carbon Surfaces... 

Keywords Sorption Activated carbon Porous carbon surfaces... 

Having presented briefly the woiking procedures of the various methods, we now would like to illustrate their applications to adsorption of super and sub-critical fluids on non-porous carbon surface and in porous carbonaceous solids having slit pores. But first it is worthwhile to compare the time scales of computation of these methods ... 

The process of SO2 removal on activated coke followed by a simultaneous reduction of NO with ammonia has been successfully applied in industry [176]. Mitsui Mining Process [177] (Table 12) and the Sumitomo Heavy Industry Process [178] are examples of the simultaneous desulphurization and NO removal with the application of moving beds of carbon adsorbents. Apart from the two above mentioned target gases, these processes also exhibit high removal efficiency for heavy metals and dioxins. Removal of NO on activated carbons can also be carried out using two separate processes. As NO2 can be easily removed from gas streams by water, low-temperature oxidation of NO to NO2 on the porous carbon surface is considered as feasible for the removal of NO without the ammonia addition [179]. 

Bone Char. Bone charcoal is a carbonaceous substance derived from the carbonization of selected grades of animal bones by heating dry bones in an airtight iron retort at 500-700°C for about four to six hours. Comparing the capacity of metal ions removal with aetivated carbon, bone charcoal provides not only a porous carbon surface for physical adsorption, but also a hydroxyapatite lattice—Ca,o(P04)g(OH)2 for ion exchange of metal ions. Based on these properties, this sorbent should have excellent adsorption capacities for metal ions. The charaeteristies of typieal bone char are shown in Table 15.9. 

Inorganic membranes (29,36) are generaUy more stable than their polymeric counterparts. Mechanical property data have not been definitive for good comparisons. IndustriaUy, tube bundle and honeycomb constmctions predominate with surface areas 20 to 200 m. Cross-flow is generaUy the preferred mode of operation. Packing densities are greater than 1000 /m. Porous ceramics, sintered metal, and metal oxides on porous carbon support... 

Ceramic Membranes Alumina-based microfiltration membranes and porous carbon substrates are tightened for use as UF membranes usually by depositing a layer of zirconium oxide on the surface. 

From the above data, it would appear that methane densities in pores with carbon surfaces are higher than those of other materials. In the previous section it was pointed out that to maximize natural gas or methane storage, it is necessary to maximize micropore volume, not per unit mass of adsorbent, but per unit volume of storage vessel. Moreover, a porous carbon filled vessel will store and deliver more methane than a vessel filled wnth a siliea based or polymer adsorbent which has an equivalent micropore volume fraction of the storage vessel. 

Several mechanisms have been proposed to explain the activation of carbon surfaces. These have Included the removal of surface contaminants that hinder electron transfer, an Increase In surface area due to ralcro-roughenlng or bulld-up of a thin porous layer, and an Increase In the concentrations of surface functional groups that mediate electron transfer. Electrode deactivation has been correlated with an unintentional Introduction of surface contaminants (15). Improved electrode responses have been observed to follow treatments which Increase the concentration of carbon-oxygen functional groups on the surface (7-8,16). In some cases, the latter were correlated with the presence of electrochemical surface waves (16-17). However, none of the above reports discuss other possible mechanisms of activation which could be responsible for the effects observed. 

If we assume a quasi-cylinder pore structure of the electrode material as in Fig. 1, an average effective pore radius r can be evaluated from the relationship r = 2V/A, where V is the total pore volume, and A is the total pore surface that can also be determined using the DFT method (see also [5]). Then the migration coefficients determined as shown in Fig. 5 can be plotted vs. the pr2 product - see, e.g., Fig. 7 for five electrodes, which were made of various porous carbons produced by Skeleton Technologies. 

Porous carbons are among the most attractive electrode materials for electric double layer capacitors (EDLC), where the charge accumulation occurs mainly by electrostatic attraction forces at the clcctrode/electrolyte interface [1-3]. Advantages of this class of materials include high surface... 

The activation with KOH of selected parent materials under appropriate process conditions (temperature, time, reagent ratio) can provide highly porous carbons of controlled pore size distribution and surface chemistry, also suitable for use as electrode materials in supercapacitors. 

In practical application, it was reported that the platinum particles dispersed in highly porous carbonized polyacrylonitrile (PAN) microcellular foam used as fuel-cell electrocatalyst160 have the partially active property. The fractal dimension of the platinum particles was determined to be smaller than 2.0 by using the potentiostatic current transient technique in oxygen-saturated solutions, and it was considered to be a reaction dimension, indicating that not all of the platinum particle surface sites are accessible to the incoming oxygen molecules. 

Caillard, A., Brault, R, Mathias, J., Charles, C., Boswell, R. W., and Sauvage, T. Deposition and diffusion of platinum nanoparticles in porous carbon assisted by plasma sputtering. Surface and Coatings Technology 2005 200 391-394. 

Concurrent stream of the development of nanomaterials for solid-state hydrogen storage comes from century-old studies of porous materials for absorption of gasses, among them porous carbon phases, better known as activated carbon. Absorption of gases in those materials follows different principles from just discussed absorption in metals. Instead of chemisorption of gas into the crystalline structure of metals, it undergoes physisorption on crystalline surfaces and in the porous structure formed by crystals. The gases have also been known to be phy-sisorbed on fine carbon fibers. 

There is also another topological limit on C H ratio. Physisorption proceeds only in the monolayer above the boiling point of hydrogen, and the adsorption must follow the Langmuir isotherm. Hence, the storage capacity depends on the pressme of Hj gas above the carbon surface at a fixed temperatme. It is also greatly limited by available SSA of carbon phase as provided by porous stmctme. 

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