Transcript Pt n and propane, the 244th ACS national meeting (Physical chemistry), Philadelphia, PA, 8/22/2012

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
100
E (kJ/mol)
50
0
-50
• 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−propene
3. (H)2−Ptn−propene + ½ O2 → Ptn−propene + H2O + heat
4. Ptn−propene + heat → Ptn+ propene
• We studied the Pt cluster size and charge effects regarding step 1.
separated
reactants
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
BE PtO2
reactant
complex
transition
state
product
Ptn + C3H8 → Ptn---C3H8 → H−Ptn−CH(CH3)2
50
Neutral Ptn
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.
E (kJ/mol)
Method comparison against exp. data
Pt10(+, M=2)
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
•
•
•
•
-100
Pt10(M=3)
0
BE PtO
-50
BE PtC
Global minima of Pt2-6
BE Pt2
B3LYP
IE PtO
B3PW91
IE PtC
PBE
IE Pt2
Pt3 (M=1)
Pt6 (M=5)
Pt4 (M=3)
reactant
complex
transition state
Conclusions
-100
separated
reactants
product
50
+1 charged Ptn
PW91
E (kJ/mol)
IE PtO2
Pt2 (M=3)
Pt5 (M=5)
MP2
IE Pt
Global minima of +1 charged Pt2-6
EA Pt2
0
Pt2 (M=4)
Pt5 (M=4)
Pt3 (M=4)
Pt6 (M=6)
Pt4 (M=4)
-50
EA Pt
-75% -50% -25%
0%
25%
50%
75%
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.
-100
Acknowledgements
-150
separated
reactants
reactant
complex
References
1.
2.
3.
4.
• 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, electronpushing metal oxide surfaces may hinder the electron
transfer from propane to Ptn and thereby lower the catalytic
ability of the surface-supported Ptn clusters.
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
transition state
product
• CWU SEED Grant
• CWU College of the Sciences Faculty Development Fund
• CWU Department of Chemistry