Carbon surface relative inertness

The existence of surface hydride groups of the types known in classic organic chemistry is very probable in most carbons. Direct chemical evidence is very difficult to obtain due to the relative inertness of the carbon-hydrogen bond. However, the fact that hydrogen is strongly chemisorbed on carbons and released at high temperatures only in the form of hydrocarbons is sufficient proof of the existence of true carbon-hydrogen bonds. 

The most common metal indicator electrode is platinum, which is relatively inert—it does not participate in many chemical reactions. Its purpose is simply to transmit electrons to or from species in solution. Gold electrodes are even more inert than Pt. Various types of carbon are used as indicator electrodes because the rates of many redox reactions on the carbon surface are fast. A metal electrode works best when its surface is large and clean. To clean the electrode, dip it briefly in hot 8 M HN03 and rinse with distilled water. 

The details of the sample preparation and studies of the nature of the supported-metal samples have been described in a paper dealing with the effect of surface coverage on the spectra of carbon monoxide chemisorbed on platinum, nickel, and palladium (1). The samples consist of small particles of metal dispersed on a nonporous silica which is produced commercially under the names Cabosil or Aerosil.f This type of silica is suitable as a support because it is relatively inert and has a small particle size (150-200 A.). The small particle size is important because it reduces the amount of radiation which is lost by scattering. A nonporous small particle form of gamma-alumina, known as Alon-C, is also available. This material is not so inert as the silica and will react with gases such as CO and CO2 at elevated temperatures. 

Table 1(c) on the formation or removal in vacua of carbon dioxide by reaction of the surface oxides with carbon in the metal shows the results of these calculations. The reactions are feasible for tungsten and iron but not for zirconium and magnesium. Chromium presents an intermediate case with an equilibrium pressure of 10-12-46 at 800°C., 10-9,88 at 1000°C., and 10 768 at 1200°C. The reverse reaction is feasible for zirconium and magnesium and for chromium at low temperatures. From a kinetic viewpoint the probability that this reaction will occur is small compared to the reaction to form carbon monoxide gas. In this case zirconium will act as a getter for carbon dioxide, while tungsten, iron, and chromium will be relatively inert to carbon dioxide molecules. 

Because carbon black is the preferred support material for electrocatalysts, the methods of preparation of (bi)metallic nanoparticles are somewhat more restricted than with the oxide supports widely used in gas-phase heterogeneous catalysis. A further requirement imposed by the reduced mass-transport rates of the reactant molecules in the liquid phase versus the gas phase is that the metal loadings on the carbon support must be very high, e.g., at least lOwt.% versus 0.1-1 wt.% typically used in gas-phase catalysts. The relatively inert character of the carbon black surface plus the high metal loading means that widely practiced methods such as ion exchange [9] are not effective. The preferred methods are based on preparation of colloidal precursors, which are adsorbed onto the carbon black surface and then thermally decomposed or hydrogen-reduced to the (bi)metallic state. This method was pioneered by Petrow and Allen [10], and in the period from about 1970-1995 various colloidal methods are described essentially only in the patent literature. A useful survey of methods described in this literature can be found in the review by Stonehart [11]. Since about 1995, there has been more disclosure of colloidal methods in research journals, such as the papers by Boennemann and co-workers [12]. 

It was replaced later by the sextet-doublet mechanism (106), in which physically adsorbed cyclopentane lies parallel to the surface (Fig. lb) (107, 108). In the transition state, the five carbon atoms of the ring are located over the interstices of the (111) plane of platinum. The distance between two contiguous interstices allows them to accommodate two consecutive carbon atoms, but one C-C bond in cyclopentane is necessarily stretched, and this favors the hydrogenolysis of the ring. In contrast, all the carbon atoms of cyclohexane or paraffins may be brought in contact with the metal surface, which would explain the selectivity of platinum for Cj-ring hydrogenolysis. In the first presentation of the model, the relative inertness of the tertiary-secondary C-C bonds in substituted cyclopentanes was explained by some kind of steric hindrance. 

The relatively low reactivity or inertness of the carbon surface is also very useful in the preparation of bimetallic catalysts, since the low interaction between the carbon surface and the two metals or metal precursors facilitates their mutual interaction. This is especially interesting when the objective is the formation of bimetallic particles. One clear example is the preparation of bimetallic Pt-Sn catalysts for selective hydrogenations. The catalytic behavior of this system is determined by at least three aspects [29,30] that determine the catalytic activity and the selectivity toward the desired product (1) the oxidation state of tin in... 

Another interesting example of the exploitation of the relative inertness of the carbon surface is provided by the use of Fe/C catalysts in the hydrogenation of carbon monoxide. In this case, the inertness of the carbon surface facilitates the presence of zero-valent iron in the catalyst [32,33], which is more difficult for other supports, such as alumina, on which the reduction of the oxidized iron species is hindered. Vannice et al. carried out an extensive study of carbon-supported iron catalysts using different carbons and preparation methods and concluded that highly dispersed Fe/C catalysts could be prepared on high-surface-area carbons, due to the weak chemical interactions between oxidized iron precursors and the carbon surface [32-34]. 

The relative inertness of the carbon surface is of paramount importance when carbon materials are going to be used as supports for hydrogenation catalysts. These systems usually consist of more than one metallic phase (bimetallic systems) and even by metals promoted by metal oxides. The carbon inertness facilitates interaction between the metals and/or between the metals and the promoters, yielding more active and selective catalysts than those supported on other common supports. These aspects will be illustrated by examples of the application of carbon-supported catalysts to the hydrogenation of carbon oxides. 

As mentioned above, the addition of promoters, and even the formation of bimetallic particles, can provide carbon-supported iron catalysts with better performances in CO hydrogenation. The method of preparation of these systems is going to determine the final effect, always taking advantage of the relative inertness of the carbon surface. The interaction between the different components of the active phase can be maximized by using mixed-metal carbonyl complexes. Furthermore, use of these precursors allows for the preparation of catalysts with... 

Porous carbons constitnte a fascinating kind of material. Different types with distinctive physical forms and properties (i.e., activated carbons, high-surface-area graphites, carbon blacks, activated carbon cloths and fibers, nanofibers, nanotubes, etc.) find a wide range of indnstrial applications in adsorption and catalysis processes. The main properties of these materials that make them very useful as catalyst supports, as well as some of their applications, have been described. The use of carbon as a catalyst support relies primarily on the relative inertness of its surface, which facilitates the interaction between active phases or between active phases and promoters, thus enhancing the catalytic behavior. This makes porous carbons an excellent choice as catalyst support in a great number of reactions. 

Ceramics used in fabricating implants can be classified as nonabsorbable (relatively inert), bioactive or surface reactive (semi-inert) [Hench, 1991,1993] and biodegradable or resorbable (non-inert) [Hentrich et al., 1971 Graves et al., 1972]. Alumina, zirconia, silicone nitrides, and carbons are inert bioceramics. Certain glass ceramics and dense hydroxyapatites are semi-inert (bioreactive) and calcium phosphates and calcium aluminates are resorbable ceramics [Park and Lakes, 1992]. 

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