Selectivity among Dehydrogenation Steps for Alkyl Groups on Metal Surfaces:  Comparison between Nickel and Platinum

Abstract

The thermal chemistry of ethyl and neopentyl iodides on Pt(111) surfaces was investigated by temperature-programmed desorption and reflection−absorption infrared (RAIRS) spectroscopies. The analysis using RAIRS of the isotopic composition of the ethylidyne formed from adsorption of CH3CD2I at different temperatures provided a reasonable estimate for the difference in activation energies between α- and β-hydride elimination steps from alkyl groups adsorbed on Pt(111). A study of the reactivity of neopentyl groups on the same surface using selective deuterium labeling yielded additional information on the relative rates of α- versus γ-hydride eliminations. It was determined that C−H bond-scission steps at the α and γ positions display comparable rates and that both are several orders of magnitude slower than dehydrogenation at the β carbon. A comparison is also presented here to similar chemistry on nickel substrates, where dehydrogenation reactions are usually much faster and where α-elimination dominates over the γ counterpart. The implications of these results to catalysis are discussed in the text.

An Investigation of the Reaction Pathway for Ethylene Hydrogenation on Pd(111)

“…43 The onset of ethylene desorption is at ∼250 K, while the infrared data of Figure 4 show that the formation of surface ethyl species is complete by 250 K indicating that -hydride elimination is the rate-limiting step in the formation of ethylene.…”

Regiospecificity in deuterium labeling determined by mass spectrometry

“…On the other hand, it is also clear that a sizable amount of asymmetrically exchanged prod- ucts are made in these reactions as well. This indicates that, under the pressures and temperatures used for hydrocarbon catalytic conversion, ␣-hydride elimination may compete with dehydrogenation at the ␤ position [22]. Such competition explains the ability for platinum to promote hydrocarbon reforming processes.…”

“…Such competition explains the ability for platinum to promote hydrocarbon reforming processes. Further experiments with longer-chain reactants suggest that elimination from the ␥ position is also possible, but it would be nice to obtain direct confirmation of this via H™D catalytic exchange studies of appropriate molecules such as neopentane [22][23][24].…”

Mechanisms of hydrocarbon conversion reactions on heterogeneous catalysts: analogies with organometallic chemistry

“…Only a few examples are available for c-H eliminations from alkyl groups adsorbed on surfaces. In one study, a close competition between a-and chydride elimination from neopentyl moieties was observed on Pt(1 1 1) [69][70][71], in contrast with the exclusive elimination at the a-carbon reported on Ni(1 0 0) [62,65]. It is quite possible that c-hydide elimination and metallacycle formation are central to the mechanism of isomerizations and other skeletal rearrangement catalytic reactions [72].…”

“…On the other hand, hydrogen removal at the alpha position also displays a higher activation energy, so the difference in rates between the two hydride elimination steps is reduced at the higher temperatures typical of catalytic reforming processes [23]. A couple of experiments in our laboratory have indicated that a-and b-H eliminations may in fact display similar rates about 800-900 K [53,62]. In particular, NMR and mass spectrometry analysis of a reaction mixture produced by catalytic isotope exchange of normal ethane with deuterium gas promoted by a platinum foil pointed to the formation of comparable amounts of 1,1-and 1,2-didueriated ethanes, implying close competition between eliminations of hydrogen atoms from adsorbed ethyl groups at the alpha versus beta positions (respectively) [53].…”