B3LYP study on the lowest energy Pt clusters and their reactivity towards small alkanes T. Cameron Shore, Drake Mith, an

1. B3LYP study on the lowest energy Pt clusters and their reactivity towards small alkanes T. Cameron Shore, Drake Mith, and Yingbin Ge* Department of Chemistry, Central Washington University, Ellensburg, WA 98926 Introduction Global minima of Pt2-6 and Pt2-6+ Potential energy surfaces of Ptn + RH → H-Ptn-R & Ptn+ + RH → H-Ptn-R+ Ptn + C3H8 • Vajda et al. found Pt8-10 clusters are much more active than tradition catalysts towards propane in a 4-step mechanism.1 • 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 focus on neutral and +1 charged Ptn + H−R → H−Ptn−R, where R = -CH3, -C2H5, and -CH(CH3)2,to study the size and charge effects. Relative energy of the reactant complex vs. the Mulliken charge on the Pt atoms in the Ptn---C3H8 reactant complex. Apparent energy barrier vs. relative energy of reactant complex for the Ptn + C3H8 → Ptn---C3H8 → H-Ptn-C3H7 reaction. • Computational method • B3LYP density functional theory • 6-31G(d) on C and H atoms • LANL2DZ(f) basis set and effective core potential on Pt • Possible low-energy structures were studied exhaustively. • Neutral clusters and corresponding potential energy surfaces were calculated with a multiplicity of 1,3,5,7; the +1 charged ones with a multiplicity of 2,4,6,8. Ptn + RH → H-Ptn-R Method comparison Potential energy surfaces of Ptn + RH → H-Ptn-R & Ptn+ + RH → H-Ptn-R+ Ptn+ + RH → H-Ptn-R+ endothermic • Conclusions • The Ptn + R-H → H-Ptn-R reaction involves electron density transfer from alkane to Ptn. • Reactivity: C3H8 > C2H6 > CH4. This is because -CH3 pushes electrons. E.g., the Mulliken charge on Pt2 is -0.183, -0.208, and -0.224 in the Pt2---RH reactant complex, where R=CH3, C2H5, and C3H7, respectively. • Larger Ptn clusters better disperse the negative charge and hence have lower energy barrier and more negative reaction energy. • Positively charged Ptn+ clusters are generally more active than their neutral counterparts. Pt4+ is the least active among all Ptn+ clusters; this finding agrees with experiments.4 • Acknowledgements • CWU SEED Grant • CWU College of the Sciences Faculty Development Fund • CWU Department of Chemistry Percent errors of the calculated bond energy (BE), ionization energy (IE), and electron affinity (EA) using the various computational methods with the LANL2DZ (f) basis set on Pt and 6-31G(d) on main group elements. The studied species include Pt, Pt2, PtC, PtO, PtO2. 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 (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