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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Presentation transcript:

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

2 INTRODUCTION Methane (CH 4 ) 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

3 Pt is extensively used for heterogeneous catalysis. Why Pt clusters: They are dominated by surface species which is critical for the activity of catalysts.

4 Methane activation on Pt clusters: –Pt 2-5 are the most reactive. 1 –Theoretical study has been done on reaction mechanism of CH 4 activation on Pt D. J. Trevor et al, J. Am. Chem. Soc. 112, (1990). 2.Q. Cui et al, J. Chem. Phys. 108, (1998). 3.Q. Cui et al, J. Phys. Chem. A 102, (1998).

5 OBJECTIVES To study the physisorption of CH 4 on Pt 1-7 clusters. To explore the structure effect of Pt clusters on CH 4 adsorption. To investigate the vibrational spectra of CH 4 adsorbed on Pt n.

6 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)

7 The adsorption energy (E ads ) of methane on a Pt cluster: E ads = (E Pt n + E CH 4 ) – E CH 4 Pt n CH 4 + Pt n  CH 4 Pt n + E ads E ads > 0, CH 4 Pt n is more stable than CH 4 +Pt n E ads < 0, CH 4 Pt n is less stable than CH 4 +Pt n

8 RESULTS Three kinds of physisorption: CH 4 -Pt C∙∙∙PtH∙∙∙PtH∙∙∙Pt∙∙∙H E ads = 0.74 eV 0.27 eV eV

9 CH 4 -Pt 2 E ads = 0.12 eV 0.12 eV eV The most preferred adsorption occurs along the C ∞ axis H∙∙∙Pt∙∙∙H C∙∙∙Pt

10 Structures CH 4 H∙∙∙Pt∙∙∙HC∙∙∙PtH∙∙∙PtH∙∙∙Pt 3 E ads (eV) CH 4 -Pt 3 The most preferred adsorption occurs along the C n axis CnCn

11 CH 4 -Pt 4 The most preferred adsorption occurs at the C n direction of a surface of the three-dimensional cluster. CnCn C∞C∞ E ads = 0.46 eV 0.41 eV H∙∙∙Pt∙∙∙H

12 CH 4 -Pt 5 H∙∙∙Pt∙∙∙H E ads (eV) CH CnCn CnCn The most preferred isomer tends to adsorb methane stably.

13 Adsorption energy versus cluster size

14 Structures CH 4 H∙∙∙Pt∙∙∙HC∙∙∙PtH∙∙∙PtH∙∙∙Pt 3 E ads (eV) Frequency (cm -1 ) CH 4 -Pt The larger the E ads, the larger the red-shift CnCn

15 CH 4 -Pt 5 H∙∙∙Pt∙∙∙H E ads (eV) Frequency (cm -1 ) CnCn CnCn CH 4 However …

16 CONCLUSION The adsorption of methane molecules on Pt 1-7 was investigated using Density Functional Theory calculations. The interaction between the CH 4 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 C n axis of the planar geometries, and 3) at the C n direction of a surface of the three-dimensional clusters.

17 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 CH 4 depend strongly on the adsorption site and orientation of CH 4 as well as the structure of Pt isomers. The larger the adsorption energy, the larger the red- shift. Interestingly, there are exceptions …

18 FUTURE WORK To explore the electronic properties of Pt n -CH 4 systems. To study chemisorption of methane on Pt n clusters.

19 AKNOWLEDGEMENT The American Chemical Society Petroleum Research Fund