Transcript osu.ppt

The Structure Effect of Pt Clusters
on the Vibrational Spectra of
Adsorbed Methane Molecules
Li Xiao and Lichang Wang
Department of Chemistry & Biochemistry
Southern Illinois University Carbondale
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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:
1.
2.
3.
–
Pt2-5 are the most reactive.1
–
Theoretical study has been done on reaction
mechanism of CH4 activation on Pt2-3. 2-3
D. J. Trevor et al, J. Am. Chem. Soc. 112, 3742-3749 (1990).
Q. Cui et al, J. Chem. Phys. 108, 8418-8428 (1998).
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:
CH4 + Ptn  CH4Ptn + Eads
Eads = (EPtn + ECH4) – ECH4Ptn
Eads > 0, CH4Ptn is more stable than CH4+Ptn
Eads < 0, CH4Ptn is less stable than CH4+Ptn
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RESULTS
Three kinds of physisorption:
CH4-Pt
H∙∙∙Pt∙∙∙H
1.18
Eads=
1.17
C∙∙∙Pt
1.12
H∙∙∙Pt
1.12
1.12
2.12
2.15
3.69
0.74 eV
0.27 eV
-0.36 eV
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CH4-Pt2
The most preferred adsorption occurs along the C∞ axis.
H∙∙∙Pt∙∙∙H
1.14
1.12
2.37
H∙∙∙Pt∙∙∙H
C∙∙∙Pt
1.17
2.39
3.85
Eads=
0.12 eV
0.12 eV
-0.36 eV
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CH4-Pt3
The most preferred adsorption occurs along the Cn axis .
CH4
H∙∙∙Pt∙∙∙H
Cn
1.14
1.16
2.25
1.11
1.11
1.11
2.33
Structures
Eads (eV)
C∙∙∙Pt
H∙∙∙Pt
H∙∙∙Pt3
1.13
1.94
3.25
3.23
3.23
0.58
0.36
0.22
0.028
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CH4-Pt4
The most preferred adsorption occurs at the Cn direction
of a surface of the three-dimensional cluster.
Cn
H∙∙∙Pt∙∙∙H
1.14
1.14
2.32
Eads=
0.46 eV
C∞
1.12
2.37
1.14
0.41 eV
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CH4-Pt5
The most preferred isomer tends to adsorb methane stably.
CH4
Cn
1.12 1.14
2.33
1.13
1.13
2.40
H∙∙∙Pt∙∙∙H
Eads (eV)
Cn
0.18
-0.047
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Adsorption energy versus cluster size
0.8
Eads (eV)
0.6
0.4
0.2
0.0
-0.2
0
1
2
3
4
5
6
7
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Cluster Size
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CH4-Pt3
The larger the Eads, the larger the red-shift.
CH4
H∙∙∙Pt∙∙∙H
Cn
1.14
1.16
2.25
1.11
1.11
1.11
2.33
Structures
H∙∙∙Pt
H∙∙∙Pt3
1.13
1.94
3.25
3.23
3.23
0.58
0.36
0.22
0.028
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
Eads (eV)
Frequency
(cm-1)
C∙∙∙Pt
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CH4-Pt5
However …
CH4
Cn
1.12 1.14
2.33
1.13
1.13
2.40
H∙∙∙Pt∙∙∙H
0.18
-0.047
3089.45
2715.03
2661.16
2973.58
2629.55
2562.40
Eads (eV)
Frequency
(cm-1)
Cn
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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 redshift. 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
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