Ethene, hydrogenation

The decomposition of ethene under vacuum has been proven to occur over a family of single crystal surfaces, Pt, Rh, Pd, Pm as well as supported Pt, Prf and M particles. For Pt(l 11), but also for other rf-metals, in the absence of hydrogen, ethane is formed and can be detected when studying thermal desorption of ethene, thus the formation of ethane occurs via a self-hydrogenation [34]. 

During the thermal decomposition, hydrogen is formed on the surface and is able to hydrogenate ethene to ethane. The rate determining step (RDS) is the C —. W bond breaking [35] and the overall reaction is described by the following steps (Eqs.2.6-2.10). 

Based on isotope labeling experiments (TPD, IRRAS) [36-39] it was concluded that the catalytic ethene hydrogenation reaction on surfaces proceeds as a step wise process of hydrogen incorporation. This step wise general mechanism is called the Horiuti-Polanyi mechanism [40] and is shown in Eqs.2.11-2.15 [41].  

Despite the numerous publications available, two major questions remain (part wise) unanswered [36]. First, the exact mechanism for the formation of ethylidyne and its actual role in the catalytic formation of ethane and second, a detailed picture of the ethane formation including all relevant C2 moieties. 

For the ethylidyne species the question arises as to whether it is a simple spectator [52] or does it have a more active role in the hydrogenation of ethene [36]. The current opinion is [53], that it does not actively participate in the reaction, however blocks the sites available for ethene adsorption [54] or reacts with hydrogen and thus indirectly affects the reaction kinetics of hydrogenation [51, 55]. 

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Data showing how the catalytic activity for ethene hydrogenation of La203-supported Rhe clusters increased as hydride ligands built up on the clusters are presented in Fig. 8 [37]. These results suggest that hydride ligands are intermediates in the catalytic reaction. 

Fig. 7 Dependence of IR band intensities on H2 partial pressure during ethene hydrogenation catalyzed by Ir4/y-Al203 at 288 K and 760 Torr (40 Torr C2H4, 50-300 Torr H2, and the balance He). The bands at 2990 (diamonds) and 2981 cnr (squares) were chosen to represent di-cr-bonded ethene and that at 1635 cnr (circles) to represent water on the y-AbOs support. These IR bands were chosen as the best ones to minimize error caused by overlap with other bands. The triangles represent the reaction rate expressed as a turnover frequency (TOF), the rate of reaction in units of molecules of ethene converted per Ir atom per second. The data indicate a correlation of the band intensities with the TOF, consistent with the suggestion that the ligands represented by the bands are reaction intermediates (but the data are not sufficient to identify the reaction intermediates) [39]...

Fig. 8 Dependence of catalytic activity measured by TOP (rate of reaction per Rh atom) (squares) and IR intensity of hydride (2020-cm mode) (diamonds) during the induction period for ethene hydrogenation catalyzed by Rhg supported on La203 at 298 K and atmospheric pressure in a flow reactor (partial pressures in feed H2, 348 Torr C2H4, 75Torr He, 337 Torr) [37]...

Fig. 9 Dependence of catalytic activity of MgO-supported catalysts containing cationic gold and (except in the most active catalyst) gold clusters for ethene hydrogenation at 760 Torr and 353 K (reactive mixture of He, ethene, and H2—ethene partial pressure, r ethene. 40 Torr Phydrogen. 160 Torr the balance He). Note the nonlinearity of the scale at the top [53]...

Beeck at Shell Laboratories in Emeryville, USA, had in 1940 studied chemisorption and catalysis at polycrystalline and gas-induced (110) oriented porous nickel films with ethene hydrogenation found to be 10 times more active than at polycrystalline surfaces. It was one of the first experiments to establish the existence of structural specificity of metal surfaces in catalysis. Eley suggested that good agreement with experiment could be obtained for heats of chemisorption on metals by assuming that the bonds are covalent and that Pauling s equation is applicable to the process 2M + H2 -> 2M-H. 

Carbon monoxide oxidation, ethane dehydrogenation, ethane hydrogenolysis, ethene hydrogenation. Pt, Mg, Zn catalysts placed either in the pores of the membrane or at the entrance of the membrane pores. 

Iridium carbonyl clusters of several nuclearities (2, 4 and 6) have been prepared by a controlled carbonylation of [lr(CO)2(acac)] complex adsorbed in the cages of a NaY zeolite. Then, decarbonylation of the clusters gave rise to lr2, lr4 and Ir frames. Studies of the dependence of the catalytic activity on the size of the iridium frames in NaY zeolites show that there is no simple explanation for the variation in catalytic performance in ethene hydrogenation with cluster size [208]. 

Similarly, j -elimination from a propyl chain is easier than from an ethyl chain, and propyl-to-ethene hydrogen transfer is easier than ethyl-to-ethene transfer (but propyl-to-propene transfer is not). This explains why higher olefins are only dimerized at aluminium insertion becomes more difficult, while elimination (from a -branched alkyl, leading to a strongly stabilized olefin) becomes much easier. 

The lack of any important effect of ethylidyne on the ethene hydrogenation has presented something of a dilemma because (CCH3)Pt(lll) models imply that there is little room left for the adsorption and reaction of ethene (400). Surface hydrogen (or deuterium) atoms which could be present despite the high coverage of ethylidyne would somehow have to be transferred to ethene weakly adsorbed on top of the ethylidyne adlayer (399), or, alternatively, the ethylidyne species would need to move apart from each other under reaction conditions to allow ethene to reach the surface for reaction (400). 

In a hydrogen atmosphere at 195 K, (what we now know to be) the ethylidyne species was shown by Soma 406) to be more stable than the CT-C2H4 species because the band at 1337 cm 1 (<5CH3 5) scarcely changed its intensity. However, the intensity of this band decreased in the presence of hydrogen when the cell temperature was raised to 243 K, and ethane was produced in the gas phase. Ethylidyne is therefore not strictly a spectator species, but its contribution (when present) to the rate of ethene hydrogenation is very low. 

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. 

It should also be recalled that a particularly weakly perturbed 77-type species, designated 77, has been identified on Pt(110) and Pt(210) surfaces when preadsorbed hydrogen was present (424). On the basis of TPD work, Bowker etal. (425) recently provided evidence that such a species is primarily involved in ethene hydrogenation catalyzed by Rh(lll). It remains to be seen whether this is more generally the case. A distinction between 77 and 77 required the detection of the vC=CI8—CH2 absorption in the... 

In these case studies, in addition to a brief discussion of the catalytic applications, representative reactions are discussed with the aim of illustrating in detail the relationships between surface structures (as inferred from investigations with probe molecules) and catalytic activity. The following topics are discussed in detail (i) MgO as a model catalyst for base-catalyzed reactions (ii) the mechanism of ethene hydrogenation on ZnO (iii) Cu20 as an oxidation catalyst for the conversion of methanol to formaldehyde, with... 

It is also shown that carbonaceous deposits built upon oxides via spill-over from metal sites is active in ethene hydrogenation in its own right. Thus during catalysis of hydrocarbon reactions some sites (metallic and acidic) are lost or made unavailable while simultaneously others are generated. This complex yet intriguing situation emphasises the need for in-situ characterisation of catalytic surfaces. 

A little work on structure-insensitive reactions has been reported [18]. Both catalysts were very active for ethene hydrogenation, and rapid deactivation occurred even at 176 K. Ethyne and 1,3-butadiene react in a more controlled manner study of ethyne hydrogenation using both l4C-labeled ethyne and ethene showed that ethane formation took place directly from adsorbed ethyne, without the intervention of gas-phase ethene. 

TABLE 4 EXAFS best-fit parameters characterizing the lr4/7-Al203 catalyst during ethene hydrogenation at 1 bar and 298 K (Argo et al., 2003). 

FIGURE 40 Dependence of the Ir-lr distance of lr4 (upper) and lr6 (lower) on the partial pressure of H2 during ethene hydrogenation catalysis at 298 K (Argo et al., 2003). Reprinted with permission from (Argo et al., 2003). Copyright 2003 American Chemical Society. 

Interesting information exists about the crystal-face specificity/reactivity [75]. In ethene hydrogenation the (001) face of Ni is inactive, because this face, under the standard reaction conditions applied, is completely covered by carbonaceous deposits. The (111) and (Oil) faces are both active and differ by less than a factor of 2 in their activity. The data just mentioned are just another example of a frequently encountered phenomenon the crystal face specificity or the particle size sensitivity of a reaction is induced by a side reaction and is not caused by the reaction in question [76]. 

The order of activities of various metals in benzene hydrogenation recalls that for ethene hydrogenation. According to ref. 85 ... 

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