Carbon surface properties

Quantifying their relative importance as a function of carbon surface properties and of solution electrochemical conditions will undoubtedly keep carbon electrochemists occupied for the next quarter of a century. 

The basic problem with activated carbon is that, intrinsically, it is a poor electrical conductor. Moreover, the use of small particles instead of a bulk crystal adds a contribution to the contact resistance. A binder must be mixed with the powder to stick the carbon particles together. The choice of binder material type and amount is influenced by the carbon surface properties. 

In this review we concentrate on the studies that attempt to elucidate the importance of carbon surface properties in controlling the equilibrium uptakes of aromatic and aliphatic adsorbates. Rather than comparing model parameters, such as Langmuir or Freundlich constants, we examine the uptakes at comparable equilibrium concentrations and attempt to rationalize the differences observed under different conditions and on different adsorbents. 

In the following sections I attempt to offer a consistent explanation of the importance of the carbon surface properties that influence the adsorption processes, which is valid for different organic solutes, from nonelectrolytes to polyelectrolytes and bacteria. 

Sulfide collectors ia geaeral show Htfle affinity for nonsulfide minerals, thus separation of one sulfide from another becomes the main issue. The nonsulfide collectors are in general less selective and this is accentuated by the large similarities in surface properties between the various nonsulfide minerals (42). Some examples of sulfide flotation are copper sulfides flotation from siUceous gangue sequential flotation of sulfides of copper, lead, and zinc from complex and massive sulfide ores and flotation recovery of extremely small (a few ppm) amounts of precious metals. Examples of nonsulfide flotation include separation of sylvite, KCl, from haUte, NaCl, which are two soluble minerals having similar properties selective flocculation—flotation separation of iron oxides from siUca separation of feldspar from siUca, siUcates, and oxides phosphate rock separation from siUca and carbonates and coal flotation. 

Catalysis. Catalytic properties of the activated carbon surface are useful in both inorganic and organic synthesis. For example, the fumigant sulfuryl fluoride is made by reaction of sulfur dioxide with hydrogen fluoride and fluorine over activated carbon (114). Activated carbon also catalyzes the addition of halogens across a carbon—carbon double bond in the production of a variety of organic haUdes (85) and is used in the production of phosgene... 

The surface oxide groups on carbon play a major role in its surface properties for example, the wettability in aqueous electrolytes, work function, and pH in water are strongly affected by the presence of surface groups on the carbonaceous material. Typically, the wettability of carbon... 

A comprehensive review which discusses the surface properties and their role in the electrochemistry of carbon surfaces was written by Leon and Radovic [26]. This review provides a useful complement to the following discussion on the role of carbon in aqueous batteries. Four key parameters that are important for carbonaceous materials in batteries, which were identified by Fischer and Wissler [24], are ... 

The physicochemical properties of carbon are highly dependent on its surface structure and chemical composition [66—68], The type and content of surface species, particle shape and size, pore-size distribution, BET surface area and pore-opening are of critical importance in the use of carbons as anode material. These properties have a major influence on (9IR, reversible capacity <2R, and the rate capability and safety of the battery. The surface chemical composition depends on the raw materials (carbon precursors), the production process, and the history of the carbon. Surface groups containing H, O, S, N, P, halogens, and other elements have been identified on carbon blacks [66, 67]. There is also ash on the surface of carbon and this typically contains Ca, Si, Fe, Al, and V. Ash and acidic oxides enhance the adsorption of the more polar compounds and electrolytes [66]. 

At the electrode surface there is competition among many reduction reactions, the rates of which depend on iQ and overpotential q for each process. Both /0 and q depend on the concentration of the electroactive materials (and on the catalytic properties of the carbon surface). However, the chemical composition of the SEI is also influenced by the solubility of the reduction products. As a result, the voltage at... 

The SEI is formed by parallel and competing reduction reactions and its composition thus depends on i0, t], and the concentrations of each of the electroactive materials. For carbon anodes, (0 also depends on the surface properties of the electrode (ash content, surface chemistry, and surface morphology). Thus, SEI composition on the basal plane is different from that on the cross—section planes. 

Donnet, J. B., Vidal A. Carbon Black-Surface Properties and Interactions with Elastomers. Vol. 76, pp. 103-128. 

The simplest example of oxygen spillover is found in the adsorption of oxygen on carbon. The spillover oxygen migrates from the basal carbon (donor) to carbon atoms exposed at steps between layers of the graphite surface, where it reacts with the edge carbons (acceptor).71 In this case the donor and acceptor phase consist of the same material with different surface properties. 

Dole, M. Calorimetric Studies of States and Transitions in Solid High Polymers. Vol. 2, pp. 221-274. Donnet, J. B., Vidal, A. Carbon Black-Surface Properties and Interactions with Elastomers. Vol. 76, pp. 103-128. 

TEM observation and elemental analysis of the catalysts were performed by means of a transmission electron microscope (JEOL, JEM-201 OF) with energy dispersion spectrometer (EDS). The surface property of catalysts was analyzed by an X-ray photoelectron spectrometer (JEOL, JPS-90SX) using an A1 Ka radiation (1486.6 eV, 120 W). Carbon Is peak at binding energy of 284.6 eV due to adventitious carbon was used as an internal reference. Temperature programmed oxidation (TPO) with 5 vol.% 02/He was also performed on the catalyst after reaction, and the consumption of O2 was detected by thermal conductivity detector. The temperature was ramped at 10 K min to 1273 K. 

The surface properties of these nano-objects match those of metal nano crystals prepared in ultrahigh vacuum, for example the C - O stretch of adsorbed carbon monoxide or the magnetic properties of cobalt particles embedded in PVP. This demonstrates the clean character of the surface of these particles and its availabihty for reactivity studies. 

It is unknown at this point why the gold particles on the decolorizing carbon had the largest average particle size. The decolorizing carbon had a surface area comparable to the X40S but the surface chemistry has not been studied. Previous studies have demonstrated the critical role carbon support properties have on the properties of... 

The absorption property exhibited by active carbon certainly depends on the large specific surface area of the material, though an interpretation that it is based solely on this is incomplete. This is borne out by the fact that equal amounts of two activated carbon specimens, prepared from different raw materials or by different processes and having the same total surface area, may behave differently with regard to adsorption. Such differences can be partly explained in terms of the respective surface properties of the carbon samples and partly in terms of their relative pore structure and pore distribution. Every activated carbon particle is associated with at least two types of pores of distinctly different sizes. They are the macropores and the micropores. The macropores completely permeate each particle and act as wide pathways for the diffusion of material in and out of carbon, but they contribute very little to the total surface area. The micropores are more important since they... 

Polyvinyl chloride has been modified by photochemical reactions in order to either produce a conductive polymer or to improve its light-stability. In the first case, the PVC plate was extensively photochlorinated and then degraded by UV exposure in N2. Total dehydrochlorination was achieved by a short Ar+ laser irradiation at 488 nm that leads to a purely carbon polymer which was shown to exhibit an electrical conductivity. In the second case, an epoxy-acrylate resin was coated onto a transparent PVC sheet and crosslinked by UV irradiation in the presence of both a photoinitiator and a UV absorber. This superficial treatment was found to greatly improve the photostability of PVC as well as its surface properties. 

The object of the present study was to use in the above mentioned hydrogenations improved carbon supported catalysts, which could compete with the Pd black catalyst. Carbon materials are common supports, their surface properties can be modified easily and it is possible to prepare carbons with different proportion of micro-, meso- and macropores, which can be key factors influencing their performances. A highly mesoporous carbon was synthesised and used as support of Pd catalysts in the enantioselective hydrogenations. To our knowledge this is the first report on the use of highly mesoporous carbon for the preparation of Pd catalysts for liquid-phase hydrogenation. 

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