Reaction mechanism on the activation of ethane C-H and C-C bonds by a diplatinum cluster

摘要

The reaction mechanism of Pt2 cluster with C2H6 has been theoretically investigated on the singlet and triplet potential energy surfaces at BPW91/aug-cc-pvtz, Lanl2tz level. The deethylenation channel (Pt2H2 + C2H4) is kinetically favorable with the rate constant (in dm~3 mol~(-1) s~(-1)) of k = 1.8 x 10~6exp(-18,277/RT), while the dehydrogenation channel (Pt2C2H4 + H2) is fhermodynamically preferable. The twofold dehydrogenation of ethane to acetylene by Pt2 cluster is both thermo-dynamically and kinetically almost equivalent to the single dehydrogenation to ethylene. In addition, both the C-H cleavage intermediate (H)2Pt2C2H4 and the C-C cleavage intermediate CH3HPtPtCH2 are thermodynamicaily favored in the Pt2 + C2H6 reaction. Furthermore, (H)2Pt2C2H4 is kinetically preferred, while CH3HPtPtCH2 is kinetically hindered. According to analysis of activation strain model, the stabilizing interaction ΔE_(int)~≠ prefers the C-H bond cleavage, while the activation strain ΔE_(strain))~≠ favors the C-C bond cleavage. From C-C to C-H bond cleavage, the lowering of activation barrier is mainly caused by the transition states interaction ΔE_(int)~≠ becoming more stabilizing. Compared with mononuclear Pt atom, Pt2 cluster exhibits a more promising catalyst, because the synchronous effect of the atoms of Pt2 cluster makes two C-H bonds of C2H6 break simultaneously and then kinetically releases C2H4 readily from C2H6. This result is in good agreement with the experimental observation. Moreover, Pt2 cluster exhibits more efficient twofold dehydrogenation from ethane.