Carbonaceous species

Odor. Except for highly carbonaceous species, most limestones are odorless. Quick and hydrated limes possess a mild odor that is characteristic but difficult to describe except that it is faintly musty or earthy, not offensive. 

Allen GC, Tucker PM, Capon A, Parsons R. 1974. X-ray photoelectron-spectroscopy of adsorbed oxygen and carbonaceous species on platinum-electrodes. J Electroanal Chem 50 335-343. 

At least for ethylene hydrogenation, catalysis appears to be simpler over oxides than over metals. Even if we were to assume that Eqs. (1) and (2) told the whole story, this would be true. In these terms over oxides the hydrocarbon surface species in the addition of deuterium to ethylene would be limited to C2H4 and C2H4D, whereas over metals a multiplicity of species of the form CzH D and CsHs-jD, would be expected. Adsorption (18) and IR studies (19) reveal that even with ethylene alone, metals are complex. When a metal surface is exposed to ethylene, selfhydrogenation and dimerization occur. These are surface reactions, not catalysis in other words, the extent of these reactions is determined by the amount of surface available as a reactant. The over-all result is that a metal surface exposed to an olefin forms a variety of carbonaceous species of variable stoichiometry. The presence of this variety of relatively inert species confounds attempts to use physical techniques such as IR to char-... 

The first and second weight losses are due to the surfactant decomposition whereas the third loss can be attributed to the oxidative removal of carbonaceous species and the condensation of the remaining silanol groups [2,3,5]. 

Carbonaceous species on metal surfaces can be formed as a result of interaction of metals with carbon monoxide or hydrocarbons. In the FTS, where CO and H2 are converted to various hydrocarbons, it is generally accepted that an elementary step in the reaction is the dissociation of CO to form surface carbidic carbon and oxygen.1 The latter is removed from the surface through the formation of gaseous H20 and C02 (mostly in the case of Fe catalysts). The surface carbon, if it remains in its carbidic form, is an intermediate in the FTS and can be hydrogenated to form hydrocarbons. However, the surface carbidic carbon may also be converted to other less reactive forms of carbon, which may build up over time and influence the activity of the catalyst.15... 

From the work reported in literature it can be thus concluded that there will be various forms of carbonaceous species, which vary in reactivity, that exist on the catalyst or support during FTS. Some forms of this carbon are active (atomic surface carbide and CHX species) and even considered as intermediate species in FTS. However, it is also clear that especially during extended runs there may be a build up/transformation to less reactive forms of carbon (e.g., polymeric carbon). The amounts of these species may be small, but depending on their location, they may be responsible for a part of deactivation observed on cobalt-based FTS catalysts. The electronic interaction of carbon with the catalyst surface may also result in decreased activity. 

Microprobe laser desorption laser ionisation mass spectrometry (/xL2MS) is used to provide spatial resolution and identification of organic molecules across a meteorite sample. Tracking the chemical composition across the surface of the meteorite requires a full mass spectrum to be measured every 10 p,m across the surface. The molecules must be desorbed from the surface with minimal disruption to their chemical structure to prevent fragmentation so that the mass spectrum consists principally of parent ions. Ideally, the conventional electron bombardment ionisation technique can be replaced with an ionisation that is selective to the carbonaceous species of interest to simplify the mass spectrum. Most information will be obtained if small samples are used so that sensitivity levels should be lower than attomolar (10—18 M) fewer than 1000 molecules can be detected and above all it must be certain that the molecules came from the sample and are not introduced by the instrument itself. 

The necessity of forcing activation conditions has been attributed to the production of various carbonaceous species during activation. Under oxidizing atmospheres, the appearance of stretching frequencies consistent with the formation of surface carboxy-lates has been reported. Similarly, several persistent surface bands consistent with coke-type species were identified when activating supported Ru DENs under H2. °... 

A working model for dendrimer thermolysis during calcination involves the PA-MAM dendrimer backbone initially reacting with oxygen (which may or may not be activated by a nanoparticle) in a relatively facile process to generate carboxylates and other surface species. Removal of carbonaceous species closely associated with the nanoparticle is required for complete activation of the catalyst. For Pt DENs, the surface carboxylates may be strongly adsorbed to the nanoparticle surface and extended O2 treatments are required for deep oxidation of the hydrocarbon to reach reasonably volatile species. Once formed, however, it appears that they can be removed more readily with a hydrogen treatment than with further oxidation. 

Catalyst stability with time on stream is an important characteristic. Acidic catalysts can be deactivated by basic poisons such as nitrogen. Carbonaceous species can build up on both metal and acid sites. These are the two prevalent mechanisms for catalyst deactivation. Other ways that a catalyst can be damaged, such as a temperature excursion, may be more likely to occur during the initial start up or during coke burning regenerations. Regeneration is discussed in the next section. 

The CsHe desorption was essentially inhibited in the presence of SO2 because sulfur species can react with Fe O radical to form a relatively stable Fe SOs Fe (see Eq. 23), resulting in a significant decline in the density of available adsorption sites for CsH . Simultaneously, the scarcity of a-02 surface species (Fe 02") due to a competitive SO2 adsorption (Eq. 22) leads to a decrease in both rates of propene oxidation and carbonaceous species (CO and CO2) formation. 

Lawson, D. R., H. W. Biermann, E. C. Tuazon, A. M. Winer, G. I. Mackay, H. I. Schiff, G. L. Kok, P. K. Dasgupta, and K. Fung, Formaldehyde Measurement Methods Evaluation and Ambient Concentrations during the Carbonaceous Species Methods Comparison Study, Aerosol Sci. Technol., 12, 6A-16 (1990). 

What is common to all these results is that the adsorption of ethyne on Pd (as on Ni) leads to gradual self-hydrogenation to give first alkenyl and then to give alkyl-type surface species. For chemical balance these must coexist with hydrogen-deficient carbonaceous species. 

Vibrational spectroscopy has also shown that ethylidyne is the dominant carbonaceous species up to temperatures >300 K and that this is hydrogenated only very slowly. This finding does not support the suggestion of Thomson and Webb (428) that the carbonaceous species is active in this reaction as a carrier of hydrogen. 

The composition of the carbonaceous species retained by sorption on sampling media has rarely been evaluated. Fung (59) reported that the carbonaceous materials retained by sorption on quartz-fiber filters were relatively polar organic compounds, including phthalate esters and nitrogen-containing heterocyclic compounds. Additional studies of this type would be very helpful. 

The problems associated with sampling particulate organic C in the atmosphere crudely parallel those with sampling nitrates. However, the very large number of carbonaceous species, the range of vapor pressures involved, and the ubiquitous nature of carbon severely complicate development of improved sampling techniques based on total (or organic) carbon measurements. 

These factors combine to make impactors less precise and accurate than filters. Very few comparisons have been made between sizing impactors and those that have provided mixed results. The 1977 Environmental Protection Agency-Department of Energy Sampler Intercomparison included the Multi-Day Sampler, which performed well ( 15%) for fine aerosols such as sulfur, lead, and zinc (15). The 1986 Carbonaceous Species tests at Glendora, California, included the DRUM sampler. It performed well for sulfur ( 18%), as compared to the fine filter sampler (PM-2.5), but no other sizing impactor was available for comparison and no element other than sulfur was reported. DRUM versus filter comparisons were reported as part of the Southern California Air Quality Study of 1987 (2). Again, no other impactor was available for comparison, and the comparisons with filters were only fair (r2 0.7 r, linear correlation coefficient). 

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