Mechanisms for the dehydrogenation of alkanes on platinum: Insights gained from the reactivity of gaseous cluster cations, Pt-n(+) n=1-21

作者：Adlhart Christian; Uggerud Einar^*

来源：Chemistry - A European Journal, 2007, 13(24): 6883-6890.

DOI：10.1002/chem.200700501

• cluster compounds
• gas-phase reactions
• heterogeneous catalysis
• isotope scrambling
• platinum

Rates for the dihydrogen elimination of methane, ethane, and propane with cationic platinum clusters, Pt-n(+) (1 <= n <= 21), were measured under binary collision conditions in a Fourier transform ion cyclotron resonance mass spectrometer (FTICR). The reaction rate for a given cluster, Pt-n(+), follows the trend k(CH4)<k (C2H6)<k(C3H8). Methane is particular in the sense that reactivity is highly variable; some clusters (n=1-3, 5-9, 11, 12, 15) are very reactive towards methane, while all other clusters react with low efficiency or not at all. For propane, all clusters react efficiently, while the reactivity of ethane lies in-between that of methane and propane. By necessity, dihydrogen elimination of methane occurs according to a 1,1-elimination mechanism. Ethane dehydrogenation takes place according to both a 1,1-and a 1,2-mechanism. The difference between the 1,1-and 1,2-mechanisms is well displayed in specifically increased rates for those clusters that were inefficient in the reaction with methane, as well as in the observed selectivity for H-2, HD, and D-2 elimination in the reaction with [D-3]-1,1,1-ethane. Some twofold dihydrogen elimination is observed as well. The outcome of reactions with C2H6 in the presence of D2 demonstrates exchange of all hydrogen atoms in [PtnC2H4](+) with deuterium atoms. A potential energy diagram with a high barrier for the second H2 elimination summarizes these observations. For propane twofold dihydrogen elimination is dominating, and for these reactions a far less regiospecific and more random loss of the hydrogens can be inferred, as was demonstrated by the reactions with [D-6]-1,1,1,3,3,3-propane.

• 出版日期2007