Unsaturated hydrocarbons surface sites

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

One may inquire whether the evidence that 77-allyl complexes yield desorbed olefins when formed from dienes and hydrogen, or from alkenes, is pertinent to the question concerning the course of the exchange of such complexes formed by the adsorption of saturated hydrocarbons. The composition of the surface must be different under the two circumstances in one there must be few sites not occupied by olefin or half-hydrogenated intermediates, while in the other (the exchange of saturated hydrocarbons) many sites must be vacant. Consequently, in the absence of an excess of any unsaturated hydrocarbon, there is no driving force for the desorption (or displacement) of the unsaturated intermediates which are formed on the surface and intermediates of any degree of unsaturation remain bonded to the surface and leave it only as saturated hydrocarbon. Yet the evidence obtained from the reactions of the unsaturated hydrocarbons must indicate the paths which may be traversed under either circumstance. 

Although the surface models for anatase and rutile, as proposed by different authors, are idealized and differ from each other in details, it can certainly be concluded that coordinatively unsaturated Ti4+cations, O2- ions, and OH groups in widely varying configurations should be exposed on partially hydrated and/or hydroxylated surfaces. Depending on the local environments of these sites, a wide spectrum of possible intermolecular interactions should be the consequence which may render specific adsorption processes possible. Finally, the ease of the surface reduction of titanium dioxides due to hydrocarbon contamination (19) leads to the formation of new types of surface sites and to drastic changes of the surface properties. 

The principles described above may serve as a useful guide for unsaturated hydrocarbons, but substituents other than alkyl groups may involve attractive interactions with a catalytic site. A compound in which a phenyl group is attached to a double bond in a five- or six-membered cycloalkene and is also vicinal to an alkyl substituent yields a larger fraction of the cis stereoisomer than is formed from the related dialkylcycloalkene. Cis isomers are formed preferentially on Pd and Rh, and are believed to involve structures in which the phenyl group is bound to the surface through its ir-electrons. ... 

Just above, we have discussed the catalytic activity changes in terms of the possible modifications of the active sites with respect to the unsaturated hydrocarbon, which is often supposed to be of the main importance (see 2.), but we have neglected the possible influence of hydrogen, the second partner of the reaction. In a recent theoretical paper, Sousa et al. [51] argue that the coordination of the surface Pd atoms to other Pd atoms in the second layer are necessary for hydrogen to dissociate and to be trapped with a low energy cost. This points out the importance of the atomic composition and arrangement not only in the outer layer but also in the sublayers to understand the chemical reactivity of alloy surfaces. Anyhow, Cu has a noticeable electronic influence on... 

When satnrated and unsaturated hydrocarbons are used as the probe molecules, the value of the difference in the AGch2 of alkane and alkene is used to study the effect of the it-bond interactions with electron acceptor sites on the surface [203,288-290]. AGch2 is defined as... 

For writing the mechanism of HDS (of thiophene) and HYD over the created sites, we took into account data from literature (5,18). It was assumed that the adsorption of hydrogen is rapid, so that the hydrogen atoms ne ed in HYD and HDS reactions are already on the metal sites where thiophene or unsaturated hydrocarbons are sorbed or, alternatively, on the sulfur sites direcdy surrounding these metal sites. In addition, we took into consideration the experimental observations that active sites exist even in the absence of a remote control. This is due to the defect structure of the catalyst surface. These are "original" sites, for which no creation process is needed. 

ITQ-2, a novel zeolitic structure prepared by swelling and delaminating a MWW precursor, has been studied by IR spectroscopy. The same precursor yields, when calcined, the zeolite MCM-22. Bronsted acidity has been measured as the propensity either to engage in H-bonds or to transfer the proton to unsaturated hydrocarbons. Comparison with MCM-22 shows that dealumination accompanies the process of delamination, but no appreciable change in residual Bronsted acidity takes place. Reaction of propene with Bronsted sites to branched oligomers occurs mainly on the external surface. Oligomers show no tendency to evolve to allylic cationic species, in contrast with MCM-22. 

Because sites that are active for adsorption also tend to be active catalytic sites, compounds in the streams that would not normally cause deactivation may react to form nondesorbables. Olefins, diolefins, and other unsaturated hydrocarbons are especially difficult since they easily polymerize to long-chain species in the presence of high-surface-atea solids. Hydrocarbons in the presence of oxygen can form oxygenated species such as aldehydes and ketones, which can further react by aldol condensation to finrn heavier components. The presence of oxygen with sulfur compounds can create elemental sulfur. 

Although the SERS of benzene has been reported at an Ag electrode, it has been difficult to duplicate, and absorbed hydrocarbons at Ag electrodes appear to give very weak SERS. Nonpolar unsaturated hydrocarbon molecules can chemisorb to metal surfaces via 7r-bonding interactions, and these interactions seem to be stronger on Au than on Ag. In addition, the SERS-active sites seem to be more stable at lower adsorbate coverage on Au than on Ag. Alkenes and alkynes adsorb on pretreated Au electrodes from saturated aqueous solutions primarily by donation of electron density from TT-orbitals to vacant Au orbitals. This process leads to lower-frequency bands for the C=C stretching modes. 

When fuel contains heavier hydrocarbons than methane, or it is biofuel, or contains alcohols, the feedstock often contains additional compounds such as sulphur and phosphorus, that is, fertiliser impurities. In the petrochemical industry, gas-borne reactive spedes (i.e., sulphur, arsenic, chlorine, mercury, zinc, etc.) or unsaturated hydrocarbons (i.e., acetylene, ethylene, propylene and butylene) may act as contaminating agents (Deshmukh et al, 2007). These impurities cause catalyst deactivation by poisoning. The effect of a poison on an active surface is seen as site blockage or atomic surface structure transformation (Babita et a/., 2011). Therefore, it is important to choose poisoning-resistant catalyst materials. For example, nickel is not the most effective MSR catalyst although it is widely used in industry due to its low market price compared to ruthenium and rhodium. Both Ru and Rh are more effective in MSR and less carbon is formed in these systems, than in the case of Ni. However, due to the cost and availability of precious metals, these are not widely used in industrial applications. 

The above is formally equivalent to the picture of a coordinatively unsaturated surface (CUS) put forward by Burwell et al. (8) in their discussion of chromia. The acid-base formalism does have the advantage of drawing attention to the analogy of acid and base catalyzed reactions. If a hydrocarbon undergoes reaction at these sites via loss of a proton to the oxide site, the reaction should be analogous to a base catalyzed reaction if it undergoes reaction via the loss of a hydride to the zinc site or addition of a proton from the oxide site, the reaction should be analogous to an acid catalyzed reaction. This view, which we find useful, is implicit in the discussion that follows. 

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