Hydrocarbon Hydrogenation-Dehydrogenation

One of our long-term projects is to study the reaction mechanism for the catalytic hydrogenation and dehydrogenation of olefins and other unsaturated hydrocarbons.  Our initial studies, based mainly on the thermal chemistry of ethylene on Pt(111) single-crystal surfaces [F. Zaera, Langmuir 12 (1996) 88; F. Zaera, T. V. W. Janssens and H. Öfner, Surf. Sci. 368 (1996) 371; F. Zaera, Isr. J. Chem. 38 (1998) 293], clearly showed that olefin hydrogenation proceeds via the stepwise and reversible incorporation of hydrogen atoms.  However, it became apparent that while in vacuum these reactions occur on the clean metal [F. Zaera, J. Phys. Chem. 94 (1990) 5090], under the atmospheric pressures typical in catalysis they take place in the presence of a carbonaceous layer that grows on the surface soon after its exposure to the reactant gases [F. Zaera and G. A. Somorjai, J. Am. Chem. Soc. 106 (1984) 2288].  These strongly bonded hydrocarbon fragments have been shown to consist of alkylidyne moieties, the structure of which is exemplified for ethylidyne in the figure below.

Infrared and low-energy electron diffraction evidence for the formation of ethylidyne from room-temperature ethylene adsorbed on a Pt(111) surface.  The vibrational modes seen for the C-C stretching at 1118 cm-1, the symmetric methyl deformation (umbrella mode) at 1339 cm-1, and the symmetric C-H stretching at 2883 cm-1 are characteristic of ethylidyne moieties.  The structure of the carbon backbone of ethylidyne determined by LEED experiments is shown in the inset.

A careful study into the conversion of ethylene to ethylidyne revealed the complex mechanism illustrated in the figure below.  Vinyl intermediates were ruled out because they themselves undergo a series of side reactions to form a number of intermediates –including ethylene– before ultimately transforming into ethylidyne [F. Zaera and N. Bernstein, J. Am. Chem. Soc. 116 (1994) 4881].  The participation of ethyl groups was also discarded on the grounds that those decompose readily via beta-hydride elimination into ethylene [F. Zaera, Surf. Sci. 219 (1989) 453; H. Hoffmann, P. R. Griffiths and F. Zaera, Surf. Sci. 262 (1992) 141].  Lastly, the involvement of ethylidene in a simple two-step mechanism was initially found to be inconsistent with results from kinetic studies using trideuteroethylene [F. Zaera, J. Am. Chem. Soc. 111 (1989) 4240], but later shown to be possible if a fast ethylene-ethylidene pre-equilibrium were to precede its slow decomposition to ethylidyne [T. V. W. Janssens and F. Zaera, J. Phys. Chem. 100 (1996) 14118; F. Zaera and C. R. French, J. Am. Chem. Soc. 121 (1999) 2239].  The involvement of alkylidenes in this reaction has also been corroborated by the lack of dependence of the rate of alkylidyne formation on the coverage of hydrogen on the surface.  Finally, the mechanism of the overall reaction was found to change with surface coverage. 

The conversion of adsorbed olefins to alkylidynes competes kinetically with other reactions.  Of particular importance for catalysis is the occurrence of H/D exchange and hydrogenation steps [T. V. W. Janssens and F. Zaera, Surf. Sci. 344 (1995) 77].  H/D exchange in particular was shown to take place via the interchange between di-sigma adsorbed alkenes and alkyl intermediates, and to require temperatures well below those needed for other steps [T. V. W. Janssens, D. Stone, J. C. Hemminger and F. Zaera, J. Catal. 177 (1998) 284].  RAIRS kinetic experiments have pointed to the fact that the extent of deuterium substitution during the formation of alkylidynes is largely determined by the previous H/D exchange between ethylene and surface hydrogen.  It has also been observed that the dosing order of the reactants influences the kinetics of the reaction, indicating that specific surface ensembles are required: both the rate and the degree of H/D exchange reaction can be increased by dosing hydrogen prior to the olefin.  All these data can be explained on the basis of a mechanism in which the H/D exchange reaction and formation of ethylidyne are parallel processes involving different surface species (see above).

Typical mass spectra from laser-induced thermal desorption/Fourier-transform infrared spectroscopy experiments designed to characterize the kinetics of H/D exchange reactions between C2D4 and surface H on Pt(111).  The exchange is manifested here both by a decrease of the 32 amu signal (C2D4) and by the simultaneous growth of the 31 (C2D3H) and 29 and 27 (multiply exchanged C2X4) amu signals during the course of the reaction.

The kinetics of the hydrogenation of ethylene to ethane on Pt(111) was also studied isothermally under vacuum by using a variation of the dynamic molecular beam method originally devised by King and Wells [H. Öfner and F. Zaera, J. Phys. Chem. B 101 (1997) 396; H. Öfner and F. Zaera, J. Am. Chem. Soc., 124 (2002) 10982].  The uptake of ethylene on platinum (111) single-crystal surfaces was shown to involve two adsorption states, a di-sigma species irreversibly bonded on the clean surface, and a second pi reversibly-adsorbed state which develops at high coverages.  The two states display significantly different kinetic behavior, but can slowly interchange, suggesting an initial adsorption on empty metal atoms in an imperfect monolayer followed by a collective rearrangement of the neighboring molecules into a new compressed layer.  Direct infrared spectroscopic evidence was obtained with propylene for the formation of the second pi-bonded olefin [F. Zaera and D. Chrysostomou, Surf. Sci. 457 (2000) 71], and a reduction of the total ethylene uptake but an increase the amount of the weakly-adsorbed ethylene with hydrogen coadsorption was also proven.  The first-order dependence measured for the rate of ethylene hydrogenation on the coverage of this weakly bonded species provided additional support for the idea of it being the direct intermediate in the hydrogenation of ethylene.  Finally, the presence of ethylidyne on the surface was found irrelevant for the hydrogenation reaction except for their blocking of surface sites. 

Typical isothermal kinetic data obtained for the uptake of ethylene on Pt(111) at 270 K using a molecular beam technique.  The initial irreversible adsorption seen within the first 20 s (blue points) is followed by an additional small reversible adsorption of weakly (pi) bonded olefin (red points).  A comparison between the coverages of the strongly (di-sigma) and weakly (pi) bonded ethylene and the initial rate of ethane formation indicate that it is the latter weakly adsorbed ethylene the one involved in the hydrogenation reaction.  In fact, the formation of an analogous pi species has been directly determined by infrared spectroscopy for the case of propylene.  The catalytic conversion of olefins on supported metal catalysts is believed to involve a similar pi adsorption on top of strongly adsorbed carbonaceous deposits.

More complex reactions resulting from the sequential hydrogenation-dehydrogenation steps between alkanes and alkenes can be probed with heavier olefins.  For instance, the participation of allylic intermediates during hydrogenation or H/D exchange reactions has been clearly ruled out using propylene on Pt(111) [F. Zaera and D. Chrysostomou, Surf. Sci. 457 (2000) 89; 144. D. Chrysostomou, C. French and F. Zaera, Catal. Lett. 69 (2000) 117;D. Chrysostomou and F. Zaera, J. Phys. Chem. B 105 (2001) 1003].  Double-bond migration and cis-trans isomerization has also been probed with butylene [I. Lee and F. Zaera, J. Phys. Chem. B 109 (2005) 2745].  Particularly curious is the fact that the isomerization of trans-2-butene to its cis conformer was found to be easier on Pt(111) surfaces than the opposite cis to trans conversion.  This kinetic trend, which is opposite to what would be expected on thermodynamic grounds, is explained by an increased stability of the cis isomer upon adsorption.  A model where adsorption energies are affected by steric interactions between the side moieties of the olefin and the surface suggests that selectivity toward cis vs. trans formation may be manipulated by controlling the structure of the surface of the catalyst.

Energy diagram for the interconversion of butenes on metal surfaces.  Shown are the elementary steps responsible for double-bond migration between 1-butene and 2-butene as well as for cis-trans isomerization within 2-butene.  Of particular importance here is the role that the subtle relative differences among the energy barriers of the different beta-hydride elimination steps from the common 2-butyl surface intermediate play in defining the selectivity of these reactions.

Our H/D exchange studies have been extended to other compounds [A. Loaiza, M. Xu and F. Zaera, J. Catal. 159 (1996) 127].  The isotope exchange in ethane in particular has been studied over platinum surfaces under catalytic conditions.  A detailed analysis of the resulting products always indicates a U-shaped distribution, with maxima at the singly- and fully-deuterated ethane molecules [F. Zaera and G. A. Somorjai, J. Phys. Chem., 89 (1985) 3211].  Additional 13C-NMR [A. Loaiza, D. Borchardt and F. Zaera, Spectrochim. Acta A, 53 (1997) 2481] and high-resolution and tandem mass spectrometry [A. Loaiza and F. Zaera, J. Am. Soc. Mass Spectrom. 15 (2004), 1366] analysis has also pointed to the preferential formation of CH2D-CH2D over that of CH3CHD2, suggesting that adsorbed ethylene is one of the main intermediates in the mechanism for the complete exchange.  The chemistry observed has been explained on the basis of a sequence of steps including the formation of ethyl, ethylene, and ethylidyne surface intermediates.

13C NMR analysis of a mixture obtained after the catalytic H-D exchange between normal ethane and D2 over a platinum foil.  The use of this technique allows for the identification of all possible isotopic substitutions in the ethane molecule, and provides specific information about the regioselectivity for the exchange reaction.  The data not only show that there are two competing mechanisms responsible for mono and perdeutero exchange, respectively, but also prove that the latter involves the formation of a chemisorbed ethylene intermediate.

Last modified on June 27, 2005