1. 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. 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. 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. 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 2-3. 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).

5. 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. 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. 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. 8 RESULTS Three kinds of physisorption: CH 4 -Pt C∙∙∙PtH∙∙∙PtH∙∙∙Pt∙∙∙H 1.12 2.15 3.69 2.12 1.181.17 E ads = 0.74 eV 0.27 eV -0.36 eV

9. 9 CH 4 -Pt 2 E ads = 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 C∙∙∙Pt

10. 10 Structures CH 4 H∙∙∙Pt∙∙∙HC∙∙∙PtH∙∙∙PtH∙∙∙Pt 3 E ads (eV) 0.580.360.220.028 CH 4 -Pt 3 The most preferred adsorption occurs along the C n axis. 1.141.16 2.25 1.94 1.13 2.33 1.11 3.23 3.25 CnCn

11. 11 CH 4 -Pt 4 The most preferred adsorption occurs at the C n direction of a surface of the three-dimensional cluster. CnCn 1.14 2.32 1.14 1.12 2.37 C∞C∞ E ads = 0.46 eV 0.41 eV H∙∙∙Pt∙∙∙H

12. 12 CH 4 -Pt 5 H∙∙∙Pt∙∙∙H E ads (eV) 0.18-0.047 CH 4 2.33 1.14 1.12 CnCn 1.13 2.40 CnCn The most preferred isomer tends to adsorb methane stably.

13. 13 Adsorption energy versus cluster size

14. 14 Structures CH 4 H∙∙∙Pt∙∙∙HC∙∙∙PtH∙∙∙PtH∙∙∙Pt 3 E ads (eV) 0.580.360.220.028 Frequency (cm -1 ) 3093.793102.293026.373124.853085.93 3090.683024.952932.233122.443083.58 3089.452593.372907.593016.363078.17 2973.582432.712809.862647.642961.83 1.141.16 2.25 CH 4 -Pt 3 1.94 1.13 2.33 1.11 The larger the E ads, the larger the red-shift. 3.23 3.25 CnCn

15. 15 CH 4 -Pt 5 H∙∙∙Pt∙∙∙H E ads (eV) 0.18-0.047 Frequency (cm -1 ) 3089.452715.032661.16 2973.582629.552562.40 2.33 1.14 1.12 CnCn 1.13 2.40 CnCn CH 4 However …

16. 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. 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. 18 FUTURE WORK To explore the electronic properties of Pt n -CH 4 systems. To study chemisorption of methane on Pt n clusters.

19. 19 AKNOWLEDGEMENT The American Chemical Society Petroleum Research Fund