Ethene, hydrogenation kinetics

Soma s excellent infrared and kinetic study of ethene hydrogenation catalyzed by Pt/Al203 (423) showed clearly the dominant role played by the 77-adsorbed ethene species and by the reversibly adsorbed hydrogen that occurs at higher pressures in the form of on-top PtH. It also pointed to a Langmuir-Hinshelwood mechanism as the 77-adsorbed ethene was shown to compete with adsorbed H atoms for surface sites. 

In the UHV the successful results of the TPR experiments towards ethene hydrogenation on different cluster sizes are further investigated. Vibrational information (IRRAS) in combination with the electronic stfucture (EES) of cluster adsorbates is believed to elucidate the size dependent behavior. In this respect, first isotope experiments with deuterium look promising to elucidate the role of the hydrogen activation in the hydrogenation reaction. In addition, the application of isothermal pMBRS will help to gain kinetic data which will make the results more widely applicable [1 ]. 

The kinetics of the pyrolysis of Pb(C2H5)4 have been investigated in a static system over the temperature range 233 to 267 by means of gas chromatography. In the Initial stages, only ethane, ethene, hydrogen, and n-butane are formed. Over the pressure range... 

Kinetic Aspects of the Interconversion and Hydrogenation of Ethene and Other C2H Species on Platinum Surfaces... 

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. 

Kinetic Data for Ethane Desorption from Ethene Self-Hydrogenation on Pt(lll) and for Ethane Desorption from a... 

In the absence of reactant hydrogen, these two steps describe self-hydro-genation. To the best of our knowledge, only one kinetic parameter has been reported in the literature for the overall rate expression of ethene self-hydrogenation (381). This is the activation energy, which has been estimated from TPD data to be 18 kcal mol-1. 

The Kinetic Hydrogenation or Deuteration of Ethene on Oxide-Supported Pt Catalysts... 

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. 

At the remaining surface, where reactive species are adsorbed, a competition takes place between adsorbed hydrogen and ethene. The formal kinetic equation in the form of a power law is then (approx.) rate = k pgr Ph2 where a is near to zero or slightly negative. Since adsorption of hydrogen is dissociative, one would expect p 2, if H atoms are added one by one. The reasons why a can be different are discussed elsewhere (see Section 5.2 on syngas reactions). The reactive form... 

Thus for rhodium, catalyst cost is offset by selectivity. It should be noted that alkene hydrogenation is thermodynamically more favourable than hydroformylation (e.g. by 34 kJ mol-1 for ethene) and so a requirement of the catalyst is that it kinetically diverts the reaction away from simple hydrogenation. 

Reprinted from Chemical Engineering and Processing, 32, A.N.R. Bos et. al., A kinetic study of the hydrogenation of ethyne and ethene on a commercial Pd/AljO, catalyst, 53-63,1993, with kind permission from Elsevier Science S.A., P.O. Box 564,1001 Lausanne, Switzerland. 

Bos ANR, Westerterp K (1978) Mechanism and kinetics of the selective hydrogenation of ethyne and ethene. Chem Eng Process 32 1... 

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... 

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