Hydrogenation carbonaceous species

Another way of detecting interstellar species is provided by vibrational emission spectra (Figure 3). Let us mention here the so-called Unidentified Infra-Red (UIR) bands, which have not been unambiguously assigned yet. The striking resemblance of the UIR from the Orion bar with the Raman spectrum of an auto soot clearly seems to indicate that the carriers are carbon compounds. Very certainly, it will not be possible to make a one-to-one correspondence between the observed bands and some given species. Very certainly, also, the carriers of these bands are hydrogenated carbonaceous species. Whether these species are Polycylic Aromatic Hydrocarbons [24,25,26] (thereafter PAHs), coals [27,28,29], or amorphous carbon [30] is still a matter of debates and controversies that we shall not discuss further here. The interested reader can refer to a recent series published in the Faraday Discussions (1998). 

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. 

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-... 

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... 

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. 

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 presence of carbon contamination on SrTi03 surfaces raises the question of whether a carbonaceous species, rather than water, is oxidized during hydrogen photogeneration on Pt-free... 

Heterogeneous catalysts for hydrocarbon conversion may require metal sites for hydrogenation-dehydrogenation and acidic sites for isomerisation-cyclisation and these reactions may be more or less susceptible to the effect of carbonaceous overlayers depending on the size of ensembles of surface atoms necessary for the reaction. In reality we must expect species to be transferred and spilled-over between the various types of sites and if this transfer is sufficiently fast then it may affect the overall rate and selectivity observed. If there is spillover of a carbonaceous species [4] then there may be a common coke precursor for the carbonaceous overlayer on the two types of site. Nevertheless, the rate of deactivation of a metal site or an acidic site in isolation may be very different from the situation in which both types of site are present at a microscopic level on the same catalyst surface. The rate at which metal and acid sites deactivate with carbonaceous material may of course not be identical. Indeed metal sites may promote the re-oxidation of a carbonaceous species in TFO at a lower temperature than the acid sites would allow on their own and this may allow differentiation of the carbonaceous species held on the two types of site. 

Catalyst coking may involve carbonaceous species such as partially hydrogenated fragments (QHy) and may be initiated on metal or than acidic-oxide sites [1]. Three types of carbonaceous deposits may be formed on say Pt [2], which may be differentiated by temperature-programmed oxidation. SnO,-promoted Pt catalysts are important in reforming of alkanes [3] and low temperature CO oxidation [4]. Of course Sn02 is an n-type semiconductor and certainly in photoelectrolysis one expects metal-oxide electron transfers across the junction [51, but the nature of the Pt-SnOt interaction in catalytic systems remains unclear. 

Methanol adsorption and decomposition on noble metals have been the subject of many surface-analytical investigations (e.g., References 94,171,320,350,378, 478 94). CH3OH dehydrogenation on palladium catalysts could be a valuable source of synthesis gas or hydrogen, but unfortunately catalyst deactivation by carbon deposits (coking) seriously limits this process (495-498). In this respect, the probability of O H vs. C O bond scission is important, the first path resulting in CO and H2, and the second in carbon or carbonaceous species (CH x = 0-3), CH4, and H2O (see scheme in Fig. 49 details are discussed below). 

Some aspects of methanation were discussed in Section 11, and some transient results were discussed in connection with Figs. 1 and 10 for Ni/Al20,3. We now discuss some data for a 10% Fe/ALOs catalyst. After a certain time on stream in a 9/1 H2/CO mixture at 285°C, the feed is changed to He and the reactor cooled to the desired constant hydrogenation temperature. The He is then replaced by H2, and the methane concentration arising from reaction with the surface carbonaceous species is measured as... 

From the above results, we conclude that the first peak in the IPO profiles corresponds to carbonaceous species rich in hydrogen that deposit mainly on the metal surface, while the second peak corresponds to a more graphitic ike carbon that deposits on the suppon. 

Consistent with this model It has been shown on platinum (223) that the reaction occurs on the surface covered with a near monolayer of carbonaceous species In an apparently structure-insensitive loanner but that cyclohexene hydrogenation and dehydrogenation reactions proceed on the clean metal surface In a structure sensitive manner and in addition there Is then a striking variation In catalytic behaviour between various crystal surfaces ... 

Figure 9 shows rapid KDN deactivation of Co-Mo catalysts on both alumina and on carbon. The expectation that the carbon catalyst would deactivate quickly because it has a larger median pore diameter was observed. However, deactivation of the Co-Mo on alumina catalyst in Figure 9 was much faster than the Ni-Mo on alumina catalyst in Figure 8. An explanation for these differences may involve both the chemical coanposition of the catalyst surface as well as the diffusion path length. However, the deactivation of the 3.2 mm Co-Mo on alumina catalyst in Figure 9 was much faster than the 3.2 mm Ni-Mo on almnina catalyst in Figure 8. Since Ni-Mo is often considered to be a better hydrogenation catalyst more hydrogenation of adsorbed carbonaceous species, less coke formation, and less deactivation might be expected.

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