Hydrogenation, adsorbed hydrocarbons ethene

Naito et al. studied hydrogenation with use of adsorption measurements, mass spectrometry, and microwave spectroscopy for product analysis. In the room temperature deuteriation of propene, butene, and 1,3-butadiene, the main products were [ H2]-propane, [ H2]-butane, and l,2-[ H2]-but-l-ene, respectively. They showed, using mixtures of H2 and D2, that deuterium was added in the molecular form and at a rate proportional to the partial pressure of D2, as opposed to D surface coverage the reaction rates were zero order in hydrocarbon. They proposed, therefore, in contrast to the model of Dent and Kokes for ethene (but note in this case that reaction rate was 0.5 order in hydrogen pressure and proportional to ethene surface coverage), that hydrogenation proceeded by interaction of adsorbed hydrocarbon with gas-phase D2, that is by an Eley-Rideal mechanism. 

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

In 1954 R. P. Eischens, W. A. Pliskin, and S. A. Francis (5) of the Texaco Research Center in New York published the first infrared spectra of chemisorbed species, namely of carbon monoxide adsorbed on the silica-supported finely divided metal catalysts of Ni, Pd, Pt, and Cu. Also, in 1956, Pliskin and Eischens (5) were the first to obtain spectra of the hydrocarbons ethylene (ethene), acetylene (ethyne), and propene adsorbed on an oxide-supported metal catalyst, Ni/Si02. Eischens and his colleagues followed this up with further studies of chemisorbed zj-alkenes and their surface-hydrogenation products on Ni/Si02 (7). 

In this section we shall consider the results recorded in the literature that pertain to the structures of the adsorbed species. Kinetic or catalytic aspects, as could be relevant to hydrogenation, hydrogenolysis, or metathesis processes, will be treated in Part 11. Spectra of the much-investigated alkenes are discussed in detail in Part I. The spectra of the other principal types of hydrocarbon adsorbates, viz. alkynes, alkanes, cycloalkanes, and aromatics, will be analyzed in Part II. Most results are available for the type-molecules ethene, ethyne, ethane, and benzene as well as for the metals, Pt, Pd, Ni, Rh, and Ru. 

Coking, widely experienced in the catalysis of hydrocarbon conversion (7), can deactivate both metallic and acid catalytic sites for hydrocarbon reactions (2). Accumulation of such carbonaceous deposits affects selectivity in hydrocarbon conversion (5). Adsorbed ethene even inhibits facile o-p-Hj conversion over Ni or Pt (4 ), the surface of which it appears is very nearly covered at lower temperatures in such deposits. H spillover may enhance hydrocarbonaceous residue formation (6). Accumulated carbonaceous residues can be removed by temperature programmed oxidation, reduction and hydrogenation TPO, TPR, TPH, etc (7) as part of catalyst regeneration. 

Taking the hydrogenation of ethyne as a case study, it is well established now that the interaction of ethyne and/or ethene with a metal surface leads to a wide variety of adsorbed species, the proportion of which depends on the hydrocarbon pressure, the temperature, and the nature of the metal [76]. Earlier identihcations of such species—in particular, the ethylidyne =C-CH3—have been reported hrst on Pt(l 11) single crystals [77,78], then on Pd/Al203 [79]. With all available spectroscopic and kinetic data at various conditions, a nearly complete reaction scheme for the hydrogenation of ethyne on Pd(lll) has recently been proposed [80] and illustrated in Figure 6. The choice between the two routes through ethylidyne or Ti-bonded adsorbed species obviously depends on the nature of the metal, but also on some... 

ICI catalyst showed that ethene adsorbed competitively with the ethyne, but the results required two types of site (see Table 9.6) (or two modes of chemisorption of the ethyne) to explain them. Their properties did not however match any of those proposed by Webb. Type X, in the majority, adsorbed both hydrocarbons, but ethene was favoured by a factor of 2200 Type Y adsorbed ethene only, perhaps because of its high concentration. The main source of the ethane was confirmed as ethene, since in the reaction with deuterium the main product was ethane-d2- When the pressure of ethyne was varied in the presence of excess ethene, its rate of removal (and that of formation of dimers) passed through a maximum, while that of ethane formation feU to zero at an ethyne pressure of 2 kPa (see Figure 9.7). The ethane rate was almost independent of the ethene pressure. Extensive work by Borodzinski and colleagues led " to detailed proposals for the identity of two types of site, designated A and E, that were thought to be created as the carbonaceous overlayer developed, and a third type (E ) that may play a role on certain supports. Type A sites, in the majority, were small, so that only ethyne and hydrogen could adsorb on them, the former perhaps as vinylidene (>C=CH2),... 

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