Hydrogen-Carbon Species

Hydrogen is present on a carbon surface as chemisorbed water, as surface functionalities (e.g., carboxylic acids, phenolic groups, amines), or is bonded directly to carbon atoms as a part of aromatic or aliphatic structures. The carbon-hydrogen bond is very stable but breaks on heating at about 1273 K. Nevertheless, the complete desorption of hydrogen does not happen at temperatures below 1473 K. 

Hydrogen influences the carbon properties for example, the electrical resistance of carbon black is related to its hydrogen content [112]. Moreover, the presence of hydrogen at the edge sites on carbon blacks inhibits graphitization 

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The term unidentified infrared emission is used to refer to the long-known emission features of interstellar dusts in the spectral region from just over 3,000 cm-1 to below 800 cm-1 (Gillett et al. 1973). These features comprise sharp IR bands at 2,920,1,610, and 880 cm-1, as well as a broader envelope near 1,300 cm-1. In addition, a recurrent mode at 3,050 cm-1, a weak mode near 1,450 cm-1, and a shoulder near 1,150 cm-1 are observed. These spectral features can all be attributed to vibrational modes of hydrogenated carbon species, as summarized in Table 2.1. The chemical structure of these species remains the subject of debate. Furthermore, a number of carbon-rich astronomical objects reveal an emission feature in the far-IR at 490 cm-1, of unclear attribution (Kwok et al. 1989). 

Isobutane Dehydrogenation. - The coke formed on a heavily sulfided nickel catalyst, used in the dehydrogenation of isobutane, was characterized by XPS . The XPS spectra showed two different carbon states on the catalysts with low amounts of coke. One state can be ascribed to carbidic carbon (282 eV) and partially hydrogenated carbon species, CHx (285.3 eV). Essentially one dominant feature was observed when the amount of carbon deposited became large. This was ascribed to graphitic carbon (283 eV). These results are in agreement with those found with TPO and DSC as above described. 

Several other transition metal-hydroxo systems have been investigated. The insertion reaction affords in general a hydrogen carbonate species (4.14) which can be stable or can react further. 

Xs are surface fractions (or active centers), free and covered by chemisorbed species of hydrogen, carbon monoxide, and methanol. H, C, and M are activities of hydrogen, carbon monoxide, and methanol. Primes indicate equilibrium values. 

The Bronsted definition also includes the possibility that an ion is an acid (an option not allowed by the Arrhenius definition). For instance, a hydrogen carbonate ion, HC03, one of the species present in natural waters, can act as an acid and lose a proton, and the resulting carbonate ion is removed by precipitation if suitable cations are present (Fig. 10.2) ... 

The deposition is a complex mechanism which is not fully understood at this time. Two conditions seem necessary (a) activation of the carbon species and (b) the action of atomic hydrogen. These factors are reviewed in the following sections. 

By increasing the electrical energy in a fixed amount of gas, the temperature is raised and may reach 5000°C or higher.P i Such high temperatures produce an almost complete dissociation of the hydrogen molecules, the CH radicals, and other active carbon species. From this standpoint, arc-plasma deposition has an advantage over microwave-plasma or thermal CVD since these produce much less atomic hydrogen. 

The sudden expansion of the gases, as they are heated in the arc plasma, causes the formation of a high-speed arc jet so that the atomic hydrogen and the reactive carbon species are transported almost instantly to the deposition surface and the chances of hydrogen recombination and of vapor-phase reactions are minimized. 

C03-0108. What species are present in solution when the following compounds dissolve in water (a) sodium dichromate (b) copper(II) chloride (c) barium hydroxide (d) methanol (e) sodium hydrogen carbonate and (f) iron(III) nitrate. 

C16-0033. What are the major species present in each of the following solutions (a) 1.00 M perchloric acid (b) 0.25 M ammonia (c) 0.50 M potassium hydrogen carbonate and (d) 0.010 M hypochlorous acid, HCIO... 

C16-0088. When the foiiowing substances dissoive in water, what major species are present (a) sodium hydrogen carbonate (b) methanoi (c) hydrogen bromide and (d) benzoic acid... 

Because 1 is several orders of magnitude larger than 2 or, we identify 1 as dominant. Notice, however, that the water equilibrium generates some hydroxide ions in the solution, so this equilibrium must be used to find the concentration of hydroxide ions. The third reaction involves a minor species, HCO3, as a reactant, so it cannot be the dominant equilibrium. However, just as the water equilibrium generates some hydroxide ions, the hydrogen carbonate equilibrium generates some carbonate anions, whose concentration must be determined. 

These reactions do not occur at lower temperatures because of activation energy barriers and because H2 becomes the dominant form of hydrogen. Aromatic species are produced initially from acetylene via Diels-Alder type processes, in which a two-carbon and a four-carbon hydrocarbon condense into an aromatic species. Once PAHs are synthesized, they may continue to grow to form carbonaceous small grains. 

These different types of carbon tend to have different reactivities toward gases such as hydrogen, oxygen, or steam. Hence, a relatively simple technique such as temperature-programmed hydrogenation or oxidation can be used to classify them. Table 4.2 summarizes different reactivities of carbon species toward hydrogen. 

Examples of Various Carbon Species on Cobalt FTS Catalysts along with Their Hydrogenation Temperatures... 

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