Porous carbons description

Porous carbons were prepared using MCM-48 and SBA-15 mesoporous templates and various carbon precursors (see Chapter 3 for preparation description). Figure 8.11 displays the nitrogen adsorption isotherms at 77 K of SBA-15 and of the corresponding templated carbon obtained by carbonization of sucrose in the template. Both isotherms show a bimodal porosity in the templated carbon, mesopores are generated by the removal of the silica walls, and micropores are present in... 

Except for the fullerenes, carbon nanotubes, nanohoms, and schwarzites, porous carbons are usually disordered materials, and cannot at present be completely characterized experimentally. Methods such as X-ray and neutron scattering and high-resolution transmission electron microscopy (HRTEM) give partial structural information, but are not yet able to provide a complete description of the atomic structure. Nevertheless, atomistic models of carbons are needed in order to interpret experimental characterization data (adsorption isotherms, heats of adsorption, etc.). They are also a necessary ingredient of any theory or molecular simulation for the prediction of the behavior of adsorbed phases within carbons - including diffusion, adsorption, heat effects, phase transitions, and chemical reactivity. 

More descriptively, tar sand is an unconsolidated-to-consolidated sandstone or a porous carbonate rock, impregnated with bitumen. In simple terms, an unconsolidated rock approximates the consistency of dry or moist sand, and a consolidated rock may approximate the consistency of set concrete. Alternative names, such as bituminous sand or (in Canada) oil sand, are gradually finding usage, with the former name more technically correct. The term oil sand is also used in the same way as the term tar sand, and the terms are used interchangeably. The term oil sand is analogous to the term oil shale. Neither material contains oil, but oil is produced therefrom by application of thermal decomposition methods. It is important to understand that tar sand and the bitumen contained therein are different components of the deposit. The recovery of the bitumen, a hydrocarbonaceous material that can be converted into synthetic crude oil (Speight, 1990,... 

To describe the behavior and the performance of porous carbon electrodes in a CDI cell, one approach is based on a general description of EC processes, and points out the importance that each electrode s potential must be positioned appropriately relative to a reference potential, or within a voltage window, required to have optimized ion adsorption and minimal faradaic, parasitic electrode reac-tions. ° Instead, if the potentials are not chosen correctly, then ion adsorption is not optimized. 

Tortuosity is one of the most important parameters to characterize a porous medium, and reflects the reduction in transport within the electrode due to the complex porous structure comprised of active particles, binder, and conductive carbon [59-61]. Complex, tortuous nanostructures can lead to decreased effective electrolyte conductivity and diffusivity for porous electrodes by limiting transport in the electrolyte phase. The concept of electrode tortuosity (t) is used along with electrode porosity (e) as a measure for the decrease in effective electrolyte conductivity and diffusivity due to the structure of the electrode within the confines of the porous electrode description the tortuosity of a material should decrease as the porosity increases, and Bruggeman suggested a quantitative relationship where tortuosity is inversely proportional to the square root of porosity [62, 63] note that... 

The purpose of this paper Is 1) to describe the electrochemistry of ferrl-/ferro-cyanlde and the oxidation of ascorbic at an activated glassy carbon electrode which Is prepared by polishing the surface with alumina and followed only by thorough sonlcatlon 2) to describe experimental criteria used to bench-mark the presence of an activated electrode surface and 3) to present a preliminary description of the mechanism of the activation. The latter results from a synergistic Interpretation of the chemical, electrochemical and surface spectroscopic probes of the activated surface. Although the porous layer may be Important, Its role will be considered elsewhere. 

Fig. 1 shows the cyclic voltammetry of an FePc/XC-72 dispersion, heated at 280°C in an inert atmosphere, in the form a thin porous Teflon bonded coating electrode in a 1 M NaOH solution. A description of the methodology involved in the preparation of this type of electrode may be found in Ref. 3. As can be clearly seen, the voltammetry of this specimen exhibits two sharply defined peaks separated by about 330 mV. The potentials associated with these features are essentially identical to those found by other workers for the reduction and oxidation of films of iron oxy-hydroxide formed on a number of host surfaces, including iron and carbon.(5)... 

After the description of chemical structure and control of meso-architecture and surface area, selected applications of such carbon materials as battery electrodes, supercapacitors, and in the design of controlled hybrid heterojunctions were presented. In the Li battery, coating or hybridization with hydrothermal carbon brought excellent capacities at simultaneous excellent stabilities and rate performances. This was exemplified by hybridization with Si, Sn02 (both anode materials) as well as LiFeP04 (a cathode material). In the design of supercapacitors, porous HTC carbons could easily reach the benchmark of optimized activated traditional carbons, with better stability and rate performance. 

For description of textural properties of carbonaceous adsorbents, adsorption/desorption isotherms of vapours and gases in static conditions as well as mercury porosimetry are used. The latter method often leads to destruction of porous structure of investigated materials while the usage of the former one is affected by the specific properties of molecular sieves described above. Taking into account these limitations, in this work the authors have made an attempt of determination of porous structure of carbon molecular sieves with the used of the pycnometric technique. 

Abiotic and biotic reactions may occur that can produce or consume chemical species in porous media, and transport equations must therefore include descriptions of such processes for prediction accuracy. For example, microorganisms may use ethanol as a carbon source, and consume ethanol as it is transported through the porous system. Examples of other reactive processes include radioactive decay and abiotic degradation. Reactions of chemical species in porous media are often expressed using ... 

Riccardo and coworkers [50, 51] reported the results of a statistical thermodynamic approach to study linear adsorbates on heterogeneous surfaces based on Eqns (3.33)—(3.35). In the first paper, they dealt with low dimensional systems (e.g., carbon nanotubes, pores of molecular dimensions, comers in steps found on flat surfaces). In the second paper, they presented an improved solution for multilayer adsorption they compared their results with the standard BET formalism and found that monolayer capacities could be up to 1.5 times larger than the one from the BET model. They argued that their model is simple and easy to apply in practice and leads to new values of surface area and adsorption heats. These advantages are a consequence of correctly assessing the configurational entropy of the adsorbed phase. Rzysko et al. [52] presented a theoretical description of adsorption in a templated porous material. Their method of solution uses expansions of size-dependent correlation functions into Fourier series. They tested... 

The series of 10 chapters that constitute Part 3 of the book deals mainly with the use of adsorption as a means of characterizing carbons. Thus, the first three chapters in this section complement each other in the use of gas-solid or liquid-solid adsorption to characterize the porous texture and/or the surface chemistry of carbons. Porous texture characterization based on gas adsorption is addressed in Chapter 11 in a very comprehensive manner and includes a description of a number of classical and advanced tools (e.g., density functional theory and Monte Carlo simulations) for the characterization of porosity in carbons. Chapter 12 illustrates the use of adsorption at the liquid-solid interface as a means to characterize both pore texture and surface chemistry. The authon propose these methods (calorimetry, adsorption from solution) to characterize carbons for use in such processes as liquid purification or liquid-solid heterogeneous catalysis, for example. Next, the surface chemical characterization of carbons is comprehensively treated in Chapter 13, which discusses topics such as hydrophilicity and functional groups in carbon as well as the amphoteric characteristics and electrokinetic phenomena on carbon surfaces. 

R. Kaliszan, K. Osmialowski, B. J. Bassler and R. A. Hartwick, Mechanism of retention in high-performance liquid chromatography on porous graphitic carbon as revealed by principal component analysis of structural descriptions of solutes,/. Chromatogr., 1990, 499, 333-344. 

The pore size distribution in the carbon support is an important factor for a well performance of the catalyst. Pores in the nanometric scale are classified by lUPAC in three groups the micropores are those with diameters lower than 2 nm, the mesopores with diameters between 2 and 50 nm and the macropores with diameters larger than 50 run. Each pores size offers different benefits, the micropores produce materials with high surfaces area but could be inaccessible to liquid solutions or have slow mass transport. A material with mesopores has a lower surface area but better accessibility than those with micropores. FinaUy, materials with macropores show the lowest surface area, but they are easily accessible to liquid fuel. For this reason, the structured carbons, principally mesoporous carbon, have attracted considerable attention due to their potential application in the catalyst area, where the challenging is to favour the dispersion of catalyst and allow the accessibility of liquids that feed the anode side of a DMFC. In the following sections a description of different carbons support wdl be discussed stressing on the effect of the porous structure. 

The geometrical structure of pores is of great concern, but the three-dimensional description of pores is not established in less-crystalline porous solids. Only intrinsic crystalline intra-particle pores offer a good description of the structure. The hysteresis analysis of molecular adsorption isotherms and electron microscopic observation estimate the pore geometry such as cylinder (cylinder closed at one end or cylinder open at both ends), cone shape, slit shape, interstice between closed-packing spheres and inkbottle. However, these models concern with only the unit structures. The higher order structure of these unit pores such as the network structure should be taken into accoimt. The simplest classification of the higher order structures is one-, two- and three-dimensional pores. Some zeolites and aluminophosphates have one-dimensional pores and activated carbons have basically two-dimensional slit-shaped pores with complicated network structures [95]. 

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