Ethene on platinum

Fig. 7.17 INS spectrum of ethene on platinum black (BT4, NIST), (a) Adsorbed at 120 K. (b) Annealed at 300 K (without hydrogen). Reproduced from [64] with permission from the American Institute of Physics.

TABLE 7.2. Kinetic Parameters for the Hydrogenation of Ethene on Platinum Catalysts ... 

With the advent of sophisticated experimental techniques for studying surfaces, it is becoming apparent that the structure of chemisorbed species may be very different from our intuitive expectations.10 For example, ethylene (ethene, H2C=CH-2) chemisorbs on platinum, palladium, or rhodium as the ethylidyne radical, CH3—C= (Fig. 6.2). The carbon with no hydrogens is bound symmetrically to a triangle of three metal atoms of a close-packed layer [known as the (111) plane of the metal crystal] the three carbon-metal bonds form angles close to the tetrahedral value that is typical of aliphatic hydrocarbons. The missing H atom is chemisorbed separately. Further H atoms can be provided by chemisorption of H2, and facile reaction of the metal-bound C atom with three chemisorbed H atoms dif-... 

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. 

It is seen that vibrational spectroscopy, when the results are considered in conjunction with the results of other physical techniques, has already made useful contributions to the study of the kinetics of ethene-related reactions on platinum surfaces. 

Ethene adsorption, in particular on platinum and palladium surfaces, has received much attention. The atomic structure of the various adsorbed ethene... 

Table 7.9 INS spectra ( a/cm )of ethene and ethylidyne on platinum black [64].

Initial heats of adsorption of ethene reported many years ago " for condensed metal films were remarkably similar to those given in Tables 4.8 and 4.10 (Fe, 272 Ni, 232 Rh, 210 kJ mol ). Plus ga change, plusc est la meme chose. More recently values have been obtained for a number of hydrocarbons on platinum films (see Table 4.10 for ethene), but adsorbed states could not be identified, and in many cases (e.g. methane, ethane) substantial decomposition must have occurred. 

Ethylidyne (8) has been recognised on Pt/SiOa at 300 K using the SEDOR NMR technique applied to heavily C-labelled ethene the C—C bond length was 149 pm. This seemed to occur on large platinum particles, where areas of (111) face are most likely it was also seen by SIMS on platinum black, but on small particles vinylidene (17) predominated. Similar SEDOR experiments with ethyne showed 75% vinylidene and 25% ethyne as 12A or 15. " Adsorbed benzene was shown to rotate freely at 300 K, and cyclopropane was adsorbed, but not strongly, i.e. without loss of hydrogen. ... 

When alkene is in excess, the reaction in a constant volume system stops when the deuterium is used up, and the deuterium alkene ratio decreases continuously as the reaction proceeds. Unreacted but partially exchanged alkene remains at the end, and the deuterium number of the alkanes M is less than two, the alkane-d<, becoming a major product (see Figure 7.9 for the propene-deuterium reaction on Pt/pumice ). Contrarily when deuterium is in excess, the final alkane deuterium number is greater than two, the alkene-rfo falls to near zero, and the alkane-d2 becomes the major product (see also Figure 7.9). Similar but less complete results were seen with propene ° and ethene on a variety of supported platinum catalysts. 

Gootzen JFE, Wonders AH, Visscher W, vanVeen JAR (1997) Adsorption of C-3 alcohols, 1-butanol, and ethene on platinized platinum as studied with FTIRS and DBMS. Langmuir 13 1659-1667... 

Ethene can add on to certain metal salts it is believed that the extra electrons of the double bond can be donated to some extent an example is the compound PtCl2-C2H4 formed with platinum(II) chloride which has the structure... 

Kolbe noted also the formation of traces of methyl acetate and butyl valerate from electrolysis of acetate and valerate respectively. Careful analysis of reaction products by Petersen (1900) identified compounds which are today formulated as being derived from carbocations formed by loss of one electron from the alkyl radical [50]. Propanoic acid gives mostly ethene while butanoic acid and 2-methyl-propanoic acid give mostly propene. Acetate and long chain alkylcarboxylates give mostly the Kolbe type dimer hydrocarbon on electrolysis of their potassium salts in concentrated solution at a platinum electrode, using high current density and low temperatures [51]. 

The first organometallic compound of the transition metals to be characterized (1827) was Zeise s salt, K[(C2H4)PtCl3]-H20 (Fig. 18.1). It forms when K2[PtCl4] in aqueous ethanol is exposed to ethylene (ethene) a dimeric Pt—C2H4 complex with Cl bridges is also formed. In both species, the ethylene is bonded sideways to the platinum(II) center so that the two carbon atoms are equidistant from the metal. This is called the dihapto-or T]2 mode. A ligand such as an allyl radical with three adjacent carbons directly bonded to a metal atom would be trihapto- or t 3, and so on. 

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