Hydrocarbons surface intermediates

Detailed studies by Patzlaff et al.918 have shown that addition of ethene causes an increased fraction I, of the distribution characterized by a and a small increase of ctj. This indicates that ethene mainly acts as a chain initiator of hydrocarbons formed according to distribution 1, and to a very small extent as a surface intermediate for insertion into a growing chain. Concurrent experimental results were obtained by Schulz and Claeys.19 Distribution 2 and also a2 are not affected by co-feeding of ethene. Figure 11.4 shows that ethene changes the ASF plot only in the range of low carbon numbers. 

Apart from the above mentioned redox type reactions, we like to consider (in connection with work to be published by us elsewhere) another type of relaxations, due to the possible reorganisations of sorption intermediates on the catalyst surface, as suggested by some investigations in our laboratory. This structuring on the catalyst surface is equivalent to a change in the entropy of the system catalyst surface / adsorbed intermediates and seems to be responsible e.g. for the selectivity change under transient conditions in the oxidation of hydrocarbons. Actually this structural organization of the surface intermediates is also a rate process which can be observed under transient conditions. 

The proposed surface intermediates are summarized in Table II. The intermediates in each reaction of alkanes includes alkox-ide ions, regardless of the type of active oxygen species. Although carboxylate ions are believed to be the intermediate in the reactions of C2 and C3 alkenes, the type of carboxylate formed with 0 as a reactant is different from the type of carboxylate ion formed when 07 or 07 was a reactant. With 0 ions the carbon number of the carboxylate ions is the same as that of the hydrocarbon reactant, but with 07 or 07 the carboxylate ions have carbon numbers smaller than the parent hydrocarbon. The reaction schemes of Ci alkenes are somewhat complicated, yet it appears that they react in a manner more similar to C2-C1 alkanes than to C2 or C3 alkenes. 

The existence of several adsorbed states of an olefin on metal surfaces is shown by infrared spectroscopic studies [68]. This technique has the advantage that it yields direct information regarding the chemical identity of the various adsorbed species, although there are limitations to its use. One of the main limitations is that the presence of surface intermediates may not be revealed if the appropriate band intensities are too weak [69]. In this context, it has been suggested [70] that the C—H bands associated with carbon atoms which are multiply bonded to the surface are too weak to be observed. Pearce and Sheppard [71] have also proposed the operation of an optical selection rule, similar to that found with bulk metals [72], in determining the bands observed with adsorbed species on supported metal catalysts. In spite of these limitations, however, the infrared approach has contributed significantly to the understanding of the nature and reactivity of adsorbed hydrocarbons. 

Aliphatic hydrocarbon solutes are primarily solubilized within the hydrocarbon core region of the surfactant micelles. Solubilization isotherms (activity coefLcient versus mole fraction, X) for these hydrophobic solutes exhibit curves that decrease from relatively large values at inLnite dilution to lower values as X increases toward unity (Figure 12.6). The aromatic hydrocarbons are intermediate in behavior between highly polar solutes, which are anchored in the micelle surface region, and aliphatic hydrocarbons, which preferentially solubilize in the hydrocarbon core region (Kondo et al., 1993). 

The ELS spectra for these two hydrocarbon fragments are shown in Figure 8. Assignment of the observed vibrational frequencies is discussed in detail by Demuth and Ibach for the decomposition of acetylene on Ni(lll) (104). It is possible that species such as these are important surface intermediates under high pressure catalytic conditions (105, 106). Further studies in this area are in progress. 

The formation of such multiply coordinated surface intermediates would be expected to be enhanced by adsorption of multi-functional reagents, e.g., oxygenates with hydrocarbon chains more reactive than saturated alkyl ligands. To test this hypothesis, we have also examined the adsorption and reaction of allyl alcohol (CH2=CH-CH20H) and acrolein (CH2=CH-CHO) on the Rh(lll) surface. While these molecules do exhibit evidence for interaction with the surface via both their oxygen and vinyl functions, and while they appear to preserve the divergence of decarbonylation pathways observed for their aliphatic counterparts, their reactivity patterns add yet another layer of complexity to the puzzle of oxygenate decarbonylation. 

An economical process for the low-temperature non-oxidative coupling of methane to give higher hydrocarbons would be commercially attractive. Direct observation of surface intermediates would be valuable in improving the efficiency of the process. The reaction has been characterized on ruthenium single-crystal surfaces by surface science techniques including HREELS 32) and references cited therein). 

Another interesting feature of the AMO photocatalysts is the effect of diluent substrates such as MgO or activated C. Addition of substrates causes an increase in the rate of photoassisted catalytic oxidation of isopropanol. A synergistic effect is clear specific amounts of diluent lead to an increase. Too much or too little diluent leads to a decrease in rate. The exact explanation of this synergistic effect is not known, however, it may related to the ability of species such as OH or adsorbed hydrocarbons and intermediates to travel back and forth across the AMO/substrate interface. There does not seem to be a correlation of rate with the surface area, acid base character, particle size or other physical/chemical properties of the substrate. 

Metal alkylidyne fragments are frequently invoked as intermediates in the transformation of hydrocarbons on metal surfaces. These species are usually formulated as triply bridging alkylidynes however, terminal surface alkylidynes may be considered as reactive surface intermediates (30). Evidence for metal carbyne intermediates on Pt—Co bimetallic surfaces was found in a study of the isomerization and hydrogenolysis of alkanes (3]). 

In order to elucidate intermediate species of electrochemical reduction of CO and CO to hydrocarbons, surface species at Cu, Ni, and Fe electrodes were investigated by infrared spectroscopy. The results show that adsorbed CO is intermediate species to hydrocarbons. The reduction activities of adsorbed CO on metal electrodes were discussed. 

We first present some general information on the structure of ZnO, and then continue to discuss various types of catalytic process, principally for hydrocarbons. It should become clear to the reader that ZnO provides one of the best characterized examples among oxide catalysts, at least as far as the identity of surface intermediates and the mechanisms of reaction are concerned. 

The nature and structure of surface intermediates in hydrocarbon adsorption has been investigated using galvanostatic (constant current) and potential sweep techniques (7, 10, 172-174 or radiotracer methods (175. Niedrach s (172, 173 galvanostatic results with C1-C4 alkanes and with ethylene indicate the existence of common, partially oxidized surface species, despite differences in the initially adsorbed hydrocarbons. Methane adsorption is very slow, but higher saturated hydrocarbons adsorb faster and at similar rates. Potentiostatic adsorption followed by an anodic potential sweep gives two peaks [Fig. 14 (174 corresponding to different adsorbed species. The intermediate responsible for the peak at low potentials (0.7-... 

From the above discussion it becomes apparent that some conflicting experimental evidence exists on hydrocarbon adsorption and on surface intermediates. This arises primarily from the use of electrocatalysts of varying histories and pretreatments. It should be stressed that many adsorption studies were performed on anodically pretreated platinum. The removal of surfaces oxides from such electrodes may have not been always accomplished when the surface was cathodically reduced in some experiments, as outlined in Section IV,D. Obviously, different surface species could exist on bare or on oxygen-covered electrocatalysts. Characterization of surface structure and activity and of adsorbed species using modern spectroscopic techniques would provide useful information for fuel cell and selective electrocatalytic oxidations and reductions. 

We previously undertook a study of hydrocarbon activation over different transition metal based oxide catalysts mainly in relation to their total oxidation [7-9]. We proposed that hydrocarbons are activated at their weakest C-H bond on high-oxidation-state transition metal cationic centers with the formation of alkoxy species [7,8]. In the case of propene and 1-butene we suggested that the primary surface intermediates are allyl-oxy species [7,8]. 

The direct reaction of hydrocarbon radical intermediates with Oj would yield peroxo species, ROO or ROOM, which could eventually form alcohols and ketones. Although these species should form irrespective of whether the surface is modified or not, there could still be an influence of the surface on how they evolve. For instance, a possible route on TiOg is... 

Other Oxygenated Hydrocarbons Reductants. Other oxygenated hydrocarbons— 2-propanol, ethanol, methanol, iso-butanol, ethyl ether—were also tested. The inlet carbon atom concentration of these hydrocarbons was calculated to be equal to that of acetone in the tests reported above, e.g., equivalent to 1,300 ppm acetone. The NO conversion at steady state for each reductant is shown in Table II. All reductants showed 100% selectivity to N2. Since the carbon concentration of all reductants was the same, the activity of the reductants can be compared by comparing the NO conversion. The only reductant with activity close to acetone is 2-propanol with 31% conversion. Others showed much less activity than acetone. Specifically, methanol showed negligible NO reduction activity. It is speculated that the NO selective reduction activity is closely related to the ability of hydrocarbons to form oxygenated surface intermediates at these reaction conditions. This is being investigated further. 

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