B3LYP study of the dehydrogenation of propane catalyzed by Pt clusters: Size and charge effectsT. Cameron Shore, Drake Mith, Staci McNall, and Yingbin Ge*

Department of Chemistry, Central Washington University, Ellensburg, WA 98926

Method comparison against exp. data

Global optimization of Pt clusters (e.g. Pt5)

Ptn + C3H8 → Ptn---C3H8 → H−Ptn−CH(CH3)2

References1. 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 8:213-2162. 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 111:1657-16633. 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 13:6883-68904. 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

Potential energy surface (Pt5 + C3H8)

EA Pt

EA Pt2

IE Pt

IE Pt2

IE PtC

IE PtO

IE PtO2

BE Pt2

BE PtC

BE PtO

BE PtO2

-75% -50% -25% 0% 25% 50% 75%

B3LYPB3PW91PBEPW91MP2

Global minima of Pt2-6

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.

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

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.

Acknowledgements• CWU SEED Grant• CWU College of the Sciences Faculty Development Fund• CWU Department of Chemistry

Removal of a 2nd H produces propene

separated reactants

reactant complex

transition state

product-100

-50

0

50

Pt2 (M=3) Pt3 (M=1) Pt4 (M=3)Pt5 (M=5) Pt6 (M=5)

E (k

J/m

ol)

Pt10 and Pt10+ local minima + C3H8

separated reactants

reactant complex

transition state

product-100

-50

0

50

100

Pt10(M=3) Pt10(+, M=2)

E (k

J/m

ol)

Introduction

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.

Relative energies are in kJ/mol.M stands for multiplicity. The quintet PES is the lowest energy reaction path for Pt5.

• Vajda et al. find Pt8-10 clusters are much more active than traditional catalysts towards propane in 4 steps1:

1. Ptn + C3H8 → H−Ptn−CH(CH3)2

2. H−Ptn−CH(CH3)2 → (H)2−Ptn−propene3. (H)2−Ptn−propene + ½ O2 → Ptn−propene + H2O + heat4. Ptn−propene + heat → Ptn+ propene

• We studied the Pt cluster size and charge effects regarding step 1.

separated reactants

reactant complex

transition state

product-150

-100

-50

0

50

Pt2 (M=4) Pt3 (M=4) Pt4 (M=4)Pt5 (M=4) Pt6 (M=6)

E (k

J/m

ol)

Neutral Ptn

+1 charged Ptn

Global minimum