Mechanisms for the dehydrogenation of alkanes on platinum: insights gained from the reactivity of gaseous cluster cations, Ptn + n=1-21.

Rates for the dihydrogen elimination of methane, ethane, and propane with cationic platinum clusters, Pt(n) (+) (1<or=n<or=21), were measured under binary collision conditions in a Fourier transform ion cyclotron resonance mass spectrometer (FTICR). The reaction rate for a given cluster, Ptn +, 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 H2, HD, and D2 elimination in the reaction with [D3]-1,1,1-ethane. Some twofold dihydrogen elimination is observed as well. The outcome of reactions with C2H6 in the presence of D(2) 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 [D6]-1,1,1,3,3,3-propane.