High specific-area materials

By definition, the turnover frequency is expressed per number of active sites. So, catalytic samples that differ only in the amount active sites must exhibit the same values of turnover frequency. If not, heat and mass transfer phenomena are present. Specifically, the correct measurement of intrinsic kinetic data in heterogeneous catalysis is difficult due to the effect of heat and mass transfer, especially inside the pores of high specific-area materials. The turnover frequency reveals these phenomena. In other words, in the case of supported... 

Although this is a sensitive technique 10 H atoms were necessary for the analysis hence the use of a high specific area material, 70—80m g , and 30—40 g Ni. Interpretation of the results was done as follows although computation of dynamics of adlayers is possible on the basis of assumed structures and force constants, there is little fundamental information available for Ni—H. Thus the authors chose current models for HyN molecules as a starting point where other spectroscopic information was available. This research technique is promising but still in its early stages as a surface technique. 

Hence, for double-layers established at high specific-area materials, such as powdered or fibrous carbons, or metals, or their oxides, very high specific capacitance per gram can be attained. This is the principle that has underlain the development of electrochemical capacitors, now often referred to as supercapacitors (Conway [1999]) or ultracapacitors . 

Desiccants. A soHd desiccant is simply an adsorbent which has a high affinity and capacity for adsorption of moisture so that it can be used for selective adsorption of moisture from a gas (or Hquid) stream. The main requkements for an efficient desiccant are therefore a highly polar surface and a high specific area (small pores). The most widely used desiccants (qv) are siHca gel, activated alumina, and the aluminum rich zeoHtes (4A or 13X). The equiHbrium adsorption isotherms for moisture on these materials have characteristically different shapes (Fig. 3), making them suitable for different appHcations. 

We have also tried the trapping reactor system, in which ammonia is trapped on the catalyst/adsorbent and microwave is irradiated intermittently. However, due to the small specific surface area and the small ammonia adsorption capacity on the employed CuO, the trapping system was not effective compared to the continuous irradiation. Further study should be made to develop a material having high ammonia adsorption capacity and high efficiency for microwave absorption. Supported CuO on high surface area material or preparation of high surface area CuO can be effective. 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

Oxide materials which are attractive because of their catalytic activity are often employed in the form of finely divided powders of considerable surface area. The history of the material and the preparation technique employed are important aspects to be considered when electrokinetic data are compared. Oxide powders can be hot pressed into sintered pellets, supported, or impregnated, on to carbons of high specific area, and bonded with Teflon or other inert material into composite electrodes. Microporosity of the system may produce an ill-defined surface zone flooded by electrolyte with imprecise ratio of real surface area to geometric cross-section. Sput-... 

A great general interest in the sorption technologies with silica adsorbents, as well as their widespread use, have motivated numerous experimental and theoretical studies of the structure and chemistry of these materials. In particular, it is true for the investigations of surfaces of amorphous silica, both its bulk and high specific area varieties. Several books and monographs are fully or partially concerned with this topic [33-36]. Below, we shall selectively cite some more recent journal publications they contain references to more sources. 

The use of porous materials attracts a significant part of research efforts because of their ability to provide electrodes with high specific area and their capacity for storing reaction products (gaseous products, in particular). 

In the case of carbon, the EDL capacitance is directly connected with its nature, the conditions of preparation and its surface features. For instance, carbons having large specific areas and in suitable electrolyte solution, show large accessible capacitances (higher than 150 faradays). FTowever, the ideally attainable capacitance of 250 Fg" for a high surface area material is usually reduced to the more realistic value of a few tens of F g due to the limited accessibility of the carbon surfece to electrolyte [6]. 

Niu, J., W. G. Pelf and B. E. Conway. 2006. Requirements for performance characterization of C double-layer supercapacitors Applications to a high specific-area C-cloth material. Journal of Power Sources 156 725-740. 

FIG U RE 2.3 Ragone plots for the fully charged carbon-cloth electrode for various constant currents as marked on the figure for four electrolyte eoncentrations 0.01 M ( ), 0.05 M ( ), 0.5 M (A), and 5 M (t). (Reprinted from Journal of Power Sources, 156, Niu, J., W. G. Pell, and B. E. Conway, Requirements for performance characterization of C double-layer supercapacitors Applications to a high specific-area C-cloth material, 725-740, Copyright 2006, with permission from Elsevier.)... 

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