B3LYP study of the dehydrogenation of propane catalyzed by Pt clusters: Size and charge effects

1. B3LYP study of the dehydrogenation of propane catalyzed by Pt clusters: Size and charge effects T. Cameron Shore, Drake Mith, Staci McNall, and Yingbin Ge* Department of Chemistry, Central Washington University, Ellensburg, WA 98926 Introduction Global optimization of Pt clusters (e.g. Pt5) Potential energy surface (Pt5 + C3H8) Pt10 and Pt10+ local minima + C3H8 • Vajda et al. find Pt8-10 clusters are much more active than traditional catalysts towards propane in 4 steps1: • Ptn + C3H8 → H−Ptn−CH(CH3)2 • H−Ptn−CH(CH3)2 → (H)2−Ptn−propene • (H)2−Ptn−propene + ½ O2 → Ptn−propene + H2O + heat • Ptn−propene + heat → Ptn+ propene • We studied the Ptcluster size and charge effects regarding step 1. Removal of a 2nd H produces propene Relative energies are in kJ/mol. M stands for multiplicity. The quintet PES is the lowest energy reaction path for Pt5. Global minimum • Computational method • B3LYP density functional theory • 6-31G(d) on C and H atoms • LanL2DZ (f) basis set and LanL2 effective core potential (ECP) on Pt • Transition states are verified by minimum energy path calculations • Ptn + C3H8 → Ptn---C3H8 → H−Ptn−CH(CH3)2 Neutral Ptn Method comparison against exp. data Each label consists of point group, relative energy in kJ/mol, and # of imaginary frequencies if applicable. Energy includes electronic energy and zero-point vibrational energy. Global minima of Pt2-6 • Conclusions • The energy barrier for the Ptn + C3H8 → H−Ptn−CH(CH3)2 reaction decreases as the size of the neutral Ptn cluster increases from 2 to 6, and then it starts to level off. • +1 charged Pt clusters are significantly more active than their neutral counterparts. • Pt4+ is the least active among all studied +1 charged Ptn clusters; this finding agrees with Adlhart et al. experiments.3 • We conjecture that, in heterogeneous catalysis, electron-pushing metal oxide surfaces may hinder the electron transfer from propane to Ptn and thereby lower the catalytic ability of the surface-supported Ptn clusters. +1 charged Ptn Global minima of +1 charged Pt2-6 Percent errors of the calculated bond energy (BE), ionization energy (IE), and electron affinity (EA) using various computational methods with the LANL2DZ (f) basis set and ECP on Pt and 6-31G(d) basis set on C & O. • Acknowledgements • CWU SEED Grant • CWU College of the Sciences Faculty Development Fund • CWU Department of Chemistry References Vajda S, Pellin MJ, Greeley JP, Marshall CL, Curtiss LA, Ballentine GA, Elam JW, Catillon-Mucherie S, Redfern PC, Mehmood F, Zapol P (2009) Subnanometre platinum clusters as highly active and selective catalysts for the oxidative dehydrogenation of propane. Nat Mater 8:213-216 Xiao L, Wang LC (2004) Structures of platinum clusters: Planar or spherical? J Phys Chem A 108:8605-8614; Xiao L, Wang LC (2007) Methane activation on Pt and Pt4: A density functional theory study. J Phys Chem B 111:1657-1663 Adlhart C, Uggerud E (2007) Mechanisms for the dehydrogenation of alkanes on platinum: Insights gained from the reactivity of gaseous cluster cations, Ptn+, n=1-21. Chemistry-a European Journal 13:6883-6890 Ge YB, Shore TC, Mith D, McNall SA (2012) Activation of a central C−H bond in propane by neutral and +1 charged platinum clusters: A B3LYP study, submitted to Journal of Theoretical and Computational Chemistry