Adsorbed Hydrocarbon Species

Higher alkynes may form structures analogous to numbers 11 to 16 in Table 4.2 but analogues of 16A to 21 are only formed by terminal alkynes. Molecules containing more than one multiple bond, e.g. 1,2-propadiene, 1,2- and 1,3-butadiene, etc. can have their adsorbed states formulated in many different ways, but in general both C=C bonds are connected to the surface in the same way, i.e. both are either TT or di-a. It will be evident that species containing an odd number of hydrogen 

Ethyne 2ECo4(CO)i0 Ethyne Ethylylidyne Ethenylidene Ethenylidene Ethenylidene 

TABLE 4.5. Selected Structures Formed by Dissociative Chemisorption of Propene 

1-absorbed 2-absorbed 1,2-diadsorbed 1,3-diadsorbed 2,3-diadsorbed 1,4-diadsorbed 

Simple hydrocarbons form a bewildering variety of adsorbed species those containing an even number of hydrogen atoms correspond to a parent molecule, although they may have been formed by the loss of a hydrogen molecule or have undergone severe structural reorganisation (see for example species 5 and 15-19 in Table 4.2). 

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In discussing the reaction pathways, we believe that the general evidence leads to the conclusion that hydrogenolysis proceeds via adsorbed hydrocarbon species formed by the loss of more than one hydrogen atom from from the parent molecule, and that in these adsorbed species more than one carbon atom is, in some way, involved in bonding to the catalyst surface. In the case of ethane, this adsorption criterion is met via a 1-2 mode or a v-olefin mode. Mechanistically it is difficult to see how the latter could be involved in C—C bond rupture in ethane. With molecules larger than ethane, other reaction paths are possible One is via adsorption into the 1-3 mode, and another involves adsorption as a ir-allylic species. 

We have already noted (p. 16) that the interaction of an olefin with hydrogen or deuterium may lead to the occurrence of any of a number of processes. There is much evidence to suggest that each of these processes may be accounted for by considering a number of elementary steps in which a hydrogen atom, from a meantime unspecified source, is added to or removed from an adsorbed hydrocarbon species. 

Results obtained from the reaction of ethylene with deuterium have been used to obtain information regarding the probabilities of the various changes which the adsorbed hydrocarbon species may undergo. The procedure, due originally to Kemball [102] and subsequently used by Bond et al. [103—105] and Wells and co-workers [106], is based upon a steady state analysis of the following general mechanism. 

The dipolar MSSR applies strictly to a flat metal surface. However, the consideration by Pearce and Sheppard (93) that adsorbed layers are typically a few angstroms (tenths of nanometers) thick in relation to the diameters of larger metal particles in catalysts (up to tens of nanometers) led to the consideration that the MSSR could have substantial effects on the intensities of infrared absorptions from adsorbed species on metal catalysts with large particles. It has been estimated that parallel modes of vibration will have their infrared absorption bands substantially attenuated at metal particle diameters of greater than 2 nm (94). This is proving to be a very important consideration in the interpretation of the infrared spectra from adsorbed hydrocarbon species on metal catalysts (20, 95, 96) and has recently become widely accepted as valid (52, 54, 57, 62, 97). 

The arguments outlined previously lead to 88 steps for oligomerization and /3-scission processes involving 74 adsorbed hydrocarbon species. Although the choice of these steps is arbitrary, we later show that relatively few of them are kinetically significant. 

The 74 adsorbed hydrocarbon species can be interconverted via isomerization steps. We have included 71 representative isomerization steps to allow interconversion between the various C isomers. We categorize each of these steps as a nonbranching rearrangement (involving hydrogen and methyl shifts) or a branching rearrangement ... 

Higher hydrocarbons do not exist at equilibrium and any risk of whisker formation from these compounds can be disregarded at these conditions. Nevertheless whiskers may still form from higher hydrocarbons because at nonequilibrium conditions a potential for the irreversible carbon formation [e.g.. Reaction (11) in Table 3] may exist. The formation of whisker carbon at these conditions depends on a kinetic balance between the rate of the carbon forming and steam-reforming reactions. A simplified reaction sequence outlining the kinetic balance is shown in Fig. 8. The key step is whether the adsorbed hydrocarbon species will react to form adsorbed carbon and whiskers or react with oxygen species to produce gas. ... 

There are two possible avenues to this information. A number of very useful direct physical methods are now being employed to establish the nature of adsorbed hydrocarbon species the principal ones are infrared spectroscopy (8, 9) and the magnetic method (2). These and other methods may suggest but cannot prove what species may exist during a reaction unless measurements are made under the appropriate conditions (see Section II, A). For gaining information on what reactions occur on the surface there is no substitute at the moment for the rational analysis of kinetic measurements, and it is with this indirect approach that the remainder of this article will be chiefly concerned. 

Although the first object of mechanistic studies is the identification of adsorbed hydrocarbon species, the nature and relative importance... 

Evidence from the kinetics of both olefin 27, 31) and acetylene 71) hydrogenation and from the exchange of cycloalkanes with deuterium 12) (in which 7r-allylic intermediates are formed) has shown almost unequivocally that molecular hydrogen can interact with adsorbed hydrocarbon species. When this is the case, addition takes place from above the axis of unsaturation. The importance of molecular hydrogen interaction, as a general phenomenon, has yet to be determined. However, the evidence is sufficiently strong to compel the consideration of steps such as... 

Table XXIII has shown that, for each of the seven metals studied, the hydrogen available for reaction is composed of about 80% D and 20% H when acetylene reacts with deuterium. Therefore, if an adsorption/de-sorption equilibrium is set up for hydrogen, and is fast compared to the rate of hydrogen atom addition to adsorbed hydrocarbon species, then HD should be observed in the gas phase. The concentrations of HD observed in the gas phase at about 50% reaction are shown in Table XXVI. Most exchange was observed over ruthenium and osmium, less...

A simple comparison between the reference spectra of Pt foil, hulk Pt02 and Pt-Zn/AI2O3 spectrum (Figure 1) allows the conclusion that after the pre-treatment, the platinum is fully oxidised. It is less than likely that it could be reduced during the course of the reaction since this one occurs in a very oxidising media (6% O2). This result is very important in the sense that NO does not dissociate on platinum oxide and therefore implying that one of the intermediate is likely to be an adsorbed hydrocarbon species reacting with NO. 

They have recently begun to be the object of some study (227,228). In a combined experimental and theoretical study, Williams et al. (229) demonstrated the direct pickup of relatively large adsorbed hydrocarbon species by polycyclic hydrocarbon ions. Chang et al. (230) have studied the abstraction of H atoms from diamond by H(g) H(g) + H(ad) - H2(g). 

Further research and development work it is necessary to proof this hypothesis. Especially the identification of the adsorbed hydrocarbon species is a major task for this work. 

C—C bond rupture in the adsorbed hydrocarbon species, the decomposition occurring either spontaneously ... 

For all perovskites at room temperature after calcination in air, XPS analysis revealed substantial accumulation of carbonate species in the surface layer. In addition, the surface of the samples contains considerable amount of adsorbed hydrocarbon species trapped from the air. Typical Cls spectrum (sample Lao sSro 2Feo.6Nio.4O3- 700°C) presented in Fig 81 consists of two components. The more intense component with Eb(Cls) = 284.8 eV is assigned to the elementary carbon or hydrocarbons on the surface of samples. The component with Eb(Cls) 288.5 eV is imambiguously related to carbonate species, and its share increases with the calcination temperature. Since samples after calcination were cooled under air, and XPS spectra were recorded without any pretreatment of samples, these carbonates are clearly formed due to capture of CO2 from air by the surface layer of perovskites forming carbonate species. 

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