Active surface area

Fig. 5.12 (a) Water adsorption isotherms at 20°C on Graphon activated to 24-9 % burn-off, where its active surface was covered to varying extents by oxygen complex. (b) The results of (a) plotted as amount adsorbed per of active surface area (left-hand scale) and also as number of molecules of water per atom of chemisorbed oxygen (right-hand scale). (After Walker.)... 

Because of demands for improved fuel consumption through reduced rolling resistance, a new series of carbon blacks referred to as LH, ie, N300 with this innovation would be N300 LH. Basically this series of blacks has a wider size range in both the primary particles and primary aggregates in addition to a more chemically active surface area. 

The effectiveness of a porous electrode over a plane surface electrode is given by the product of the active surface area S in cm /mL and the penetration depth Tp of the reaction process into the porous electrode. 

Since catalyst activity is dependent on how much catalytically active surface is available, it is usually desirable to maximi2e both the total surface area of the catalyst and the active fraction of the catalytic material. It is often easier to enlarge the total surface area of the catalyst than to increase the active component s surface area. With proper catalyst design, however, it is possible to obtain a much larger total active surface area for a given amount of metal or other active material in a supported catalyst than can be achieved in the absence of a support. 

Surface Area. Overall catalyst surface area can be determined by the BET method mentioned eadier, but mote specific techniques are requited to determine a catalyst s active surface area. X-ray diffraction techniques can give data from which the average particle si2e and hence the active surface area may be calculated. Or, it may be necessary to find an appropriate gas or Hquid that will adsorb only on the active surface and to measure the extent of adsorption under controUed conditions. In some cases, it maybe possible to measure the products of reaction between a reactive adsorbent and the active site. Radioactively tagged materials are frequentiy usehil in this appHcation. Once a correlation has been estabHshed between either total or active surface area and catalyst performance (particulady activity), it may be possible to use the less costiy method for quaHty assurance purposes. 

Design and Construction The capacity and efficiency of biofilter operation are a direc t function of active surface area, filter void... 

Kinetics The capacity and efficiency of biofilter operation is a function of active surface area, filter void space, target removal efficiency, gas species, gas concentration, and gas flow rate. A simphfied theoretical model described by S.P.P. Ottengraf et al. is schematically represented by in Fig. 25-18. The mass balance made around the hq-uid-phase biolayer can be described as follows ... 

Decrease in E-cat activity, surface area, fresh catalyst activity, and rare earth content... 

Trend changes in catalyst activity, surface area, rare earth, and metals content. Consider adding/increasing metals inhibitor. 

As discussed below, the porosity and surface area of the catalyst film is controllable to a large extent by the sintering temperature during catalyst preparation. This, however, affects not only the catalytically active surface area AG but also the length, t, of the three-phase-boundaries between the solid electrolyte, the catalyst film and the gas phase (Fig. 4.7). 

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

Chemical deactivation. In chemical deactivation the active surface area changes by strong chemisorption of impurities in the feed, by blocking of active sites by heavy products formed in parallel or sequential reactions, etc. The most important chemical causes of deactivation are poisoning by impurities in the feed and deposition of carbonaceous material, usually referred to as coke . 

In this section, we demonstrate the real ORR activities (apparent rate constant per real active surface area, fe pp) and P(H202) at bulk Pt and nanosized Pt catalysts dispersed on carbon black (Pt/CB) with dp,= 1.6 + 0.4, 2.6 + 0.7, and 4.8 1.0 nm in the practical temperature range 30-110 °C [Yano et al., 2006b]. The use of a channel flow double-electrode (CFDE) cell allowed us to evaluate fe pp and P(H202) precisely. 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

The so-called indicator electrodes must be considered as microelectrodes, which means that the active surface area is very small compared with the volume of the analyte solution as a consequence, the electrode processes cannot perceptibly alter the analyte concentration during analysis in either non-faradaic potentiometry or faradaic voltammetry. 

Assume that current is passed either through the total nucleus surface area or through part thereof, such as the edge of a two-dimensional nucleus of monoatomic thickness. The transition of the ion Mz+ to the metallic state obeys the equation for an irreversible electrode reaction, i.e. Eqs (5.2.12), (5.2.23) and (5.2.37). The effect of transport processes is neglected. The current density at time t thus depends on the number of nuclei and their active surface area. If there is a large number of nuclei, then the dependence of their number on time can be considered to be a continuous function. For the overall current density at time t we have... 

The hydrogenation processes were performed at a relatively low temperature and pressure in the presence of promoted Raney Ni 2400 and Raney Co 2724 catalysts (13) in this study but any common nitrile hydrogenation catalysts (e.g. Fe, Ru, Rh, bulk or supported catalysts) could be used. The advantage of using a low temperature and pressure process is that it lowers the investment cost of an industrial process. Raney Ni 2400 is promoted with Cr and Raney Co 2724 is promoted with Ni and Cr. The particle sizes for both catalysts were in the range 25 - 55 pm. The BET surface area of Raney Ni 2400 and Raney Co 2724 are 140 m2/g and 76 m2/g, respectively, and the active surface area of the Ni and Co catalysts are 52 and 18 m2/g, respectively, based on CO chemisorption (Grace Davison Raney Technical Manual, 4th Edition, 1996). 

Adsorption versus Polymerization. It is instructive to examine further the time dependence of the corrosion inhibition. In acid corrosion inhibition tests, steady state is customarily assumed to be reached within 10 to 20 min after initial exposure of the metal specimen. Since the inhibitors function by reducing the available active surface area, we expect an increase in and a corresponding decrease in P. The degree of corrosion protection the inhibitor provides is given by... 

This facilitates the flow of degradable species through these tunnels onto the surface of the TiC>2 where electron could be donated to the holes of the anatase phase and the photocatalytic action in combination with the cavitational effect of the ultrasound can accelerate the fragmentations of pollutants. The details of this mechanism are however discussed at the end of chapter. Ultrasound also breaks TiC>2 particles to still smaller size and increases the active surface area manifold. 

This phenomenon, however, is not difficult to understand in view of the mechanism of dissolution under such conditions. Since the number of active sites increases linearly with current density and these sites are characterized by a film structure (or thickness or both) different from that at the OCP, one could expect corresponding increases in the corrosion rate. However, as was mentioned earlier, the active surface area in the pits increases with time, and hence one should expect the corrosion rate to increase correspondingly. Therefore, since the effect is not time dependent, one... 

Uses of adsorption studies Determination of catalytically active surface area and elucidation of reaction kinetics Determination of specific surface areas and pore size distributions... 

In this paper, new highlights are proposed to interpret the reversible and irreversible capacities of nanostructured carbons in lithium batteries. A proportional dependence between the irreversible capacity and the active surface area (ASA) of carbon materials will be demonstrated, showing the ASA concept more universal than any of the other parameters which were previously considered. In-situ 7Li NMR will be also presented as a means to... 

RELATION BETWEEN THE IRREVERSIBLE CAPACITY AND THE ACTIVE SURFACE AREA... 

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