1 Li Xiao and Lichang Wang Department of Chemistry & Biochemistry Southern Illinois University Carbondale The Structure Effect of Pt Clusters on the Vibrational.

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Li Xiao and Lichang WangDepartment of Chemistry & BiochemistrySouthern Illinois University Carbondale

The Structure Effect of Pt Clusters on the Vibrational

Spectra of Adsorbed Methane Molecules

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INTRODUCTION

Methane (CH4) is the principle component of

natural gas (80-90%)– Energy production – Chemical synthesis – Hydrogen production for fuel cell applications

Problem: Large cost of transport Solution: Conversion of methane to liquid chemicals

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Pt is extensively used for heterogeneous catalysis.

Why Pt clusters: They are dominated by surface species which

is critical for the activity of catalysts.

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Methane activation on Pt clusters:

– Pt2-5 are the most reactive.1

– Theoretical study has been done on reaction mechanism of CH4 activation on Pt2-3. 2-3

1. D. J. Trevor et al, J. Am. Chem. Soc. 112, 3742-3749 (1990).2. Q. Cui et al, J. Chem. Phys. 108, 8418-8428 (1998).3. Q. Cui et al, J. Phys. Chem. A 102, 6373-6384 (1998).

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OBJECTIVES

• To study the physisorption of CH4 on Pt1-7 clusters.

• To explore the structure effect of Pt clusters on CH4 adsorption.

• To investigate the vibrational spectra of CH4 adsorbed on Ptn.

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METHODOLOGY

• Density functional theory (DFT)

• The electron-ion interactions: Ultrasoft pseudopotentials

• The exchange and correlation energies: PW91

• Plane wave basis set

• Implementation: Vienna Ab-initio Simulation Package (VASP)

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The adsorption energy (Eads) of methane on a Pt cluster:

Eads = (EPtn + ECH4) – ECH4Ptn

CH4 + Ptn CH4Ptn + Eads

Eads > 0, CH4Ptn is more stable than CH4+Ptn

Eads < 0, CH4Ptn is less stable than CH4+Ptn

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RESULTSThree kinds of physisorption:

CH4-PtC∙∙∙Pt H∙∙∙PtH∙∙∙Pt∙∙∙H

1.12

1.121.12

2.15 3.692.12

1.18 1.17

Eads= 0.74 eV 0.27 eV -0.36 eV

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CH4-Pt2

Eads= 0.12 eV 0.12 eV -0.36 eV

The most preferred adsorption occurs along the C∞ axis.

2.37

1.14 1.12

2.39

1.17

3.85

H∙∙∙Pt∙∙∙H H∙∙∙Pt∙∙∙H C∙∙∙Pt

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Structures

CH4 H∙∙∙Pt∙∙∙H

C∙∙∙Pt H∙∙∙Pt H∙∙∙Pt3

Eads (eV) 0.58 0.36 0.22 0.028

CH4-Pt3

The most preferred adsorption occurs along the Cn axis .

1.14 1.16

2.251.94

1.13

2.33

1.111.111.11

3.233.23

3.25

Cn

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CH4-Pt4

The most preferred adsorption occurs at the Cn direction of a surface of the three-dimensional cluster.

Cn

1.141.14

2.32

1.141.12

2.37

C∞

Eads= 0.46 eV 0.41 eV

H∙∙∙Pt∙∙∙H

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CH4-Pt5

H∙∙∙Pt∙∙∙H

Eads (eV) 0.18 -0.047

CH4

2.331.141.12

Cn

1.131.13

2.40

Cn

The most preferred isomer tends to adsorb methane stably.

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Adsorption energy versus cluster size

Cluster Size

1 3 5 70 2 4 6 8

Ead

s (e

V)

-0.2

0.0

0.2

0.4

0.6

0.8

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Structures

CH4 H∙∙∙Pt∙∙∙H

C∙∙∙Pt H∙∙∙Pt H∙∙∙Pt3

Eads (eV) 0.58 0.36 0.22 0.028

Frequency

(cm-1)

3093.79 3102.29 3026.37 3124.85 3085.93

3090.68 3024.95 2932.23 3122.44 3083.58

3089.45 2593.37 2907.59 3016.36 3078.17

2973.58 2432.71 2809.86 2647.64 2961.83

1.14 1.16

2.25

CH4-Pt3

1.94

1.13

2.33

1.111.111.11

The larger the Eads, the larger the red-shift.

3.233.23

3.25

Cn

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CH4-Pt5

H∙∙∙Pt∙∙∙H

Eads (eV) 0.18 -0.047

Frequency(cm-1)

3089.45 2715.03 2661.16

2973.58 2629.55 2562.40

2.331.141.12

Cn

1.131.13

2.40

CnCH4

However …

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CONCLUSION

• The adsorption of methane molecules on Pt1-7 was investigated using Density Functional Theory calculations.

• The interaction between the CH4 and Pt clusters depends strongly on the adsorption site and orientation.

• The most preferred adsorption occurs 1) along the C∞ axis of the linear structures, 2) along the main Cn axis of the planar geometries, and 3) at the Cn direction of a surface of the three-dimensional clusters.

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•The H∙∙∙Pt∙∙∙H interaction is strong compared with the other possible interactions, such as C∙∙∙Pt and H∙∙∙Pt.

•The adsorption energy decreases with cluster size.

•The vibrational spectra of CH4 depend strongly on the adsorption site and orientation of CH4 as well as the structure of Pt isomers.

•The larger the adsorption energy, the larger the red-shift. Interestingly, there are exceptions …

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FUTURE WORK

• To explore the electronic properties of Ptn-CH4 systems.

• To study chemisorption of methane on Ptn clusters.

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AKNOWLEDGEMENT

The American Chemical Society Petroleum Research Fund