Chapter 6 Conclusions
A. 3 Results and discussion
Figure A. 1 shows the C1s, O1s and Pt4f spectra of the Pt(110) surface under different CO and O2 pressures and at different temperatures. We analyze each core level using different photon energies to ensure the same surface sensitivity. When 200 mTorr CO was added to the high pressure cell, a broad peak that can be de-convoluted into two peaks1 appeared in the C1s spectrum (Spectrum A) with binding energies of 286.0±0.1 and 286.7±0.1 eV. And two distinct peaks1 appeared in the O1s spectrum (Spectrum A), with binding energies of 531.0±0.1 and 532.7±0.1 eV. We assigned these peaks to CO molecules adsorbed on bridge sites and on-top sites, respectively. These binding energies are in good agreement with CO adsorption experiments under UHV conditions [52]. The Pt4f peak of the CO covered surface is broader compared to the clean Pt surface due to CO adsorption.
Upon addition of 200 mTorr O2, the population of CO on bridge sites decreased (Figure A.
1, Spectra B). The area ratio between on-top and bridge sites changed from 2.5:1 to 3.1:1 without the appearance of additional peaks. This is consistent with asymmetric inhibition of CO and O2 on Pt(110) [53]. The sample was subsequently heated to 150 ℃ under the same CO and O2pressure. Most bridge site CO molecules were removed above 100 ℃ and only ontop CO remained (Spectra c). Upon further heating, CO2 was first detected at 120℃. At 150 ℃, the surface was still CO covered. However, a new peak at 287.9 eV appeared in the C1s spectrum at the high binding energy side of the on-top CO peak. This peak was only observed during CO oxidation (i.e. when CO2 formation was observed). The origin of such a feature is not completely known, though it may belong to a reaction intermediate [48].
Although it is similar to the CO vibrational fine structure, the intensity is much larger than what has been reported.
The sample temperature was maintained at 150 ℃ during the entire CO oxidation experiment. Figure A. 2 shows the reaction data, i.e., the partial pressures of CO, O2, and CO2 gases during CO oxidation. By controlling the CO pressure, we can switch the reaction conditions from a CO rich to an O2 rich environment and vice versa. In each region, XPS data (Figure A. 3) was collected after the pressure stabilized. First, we introduced 180 mTorr O2 into the high pressure cell (Region A), creating a Pt surface that is covered with chemisorbed atomic oxygen [54]. Only one O1s peak at 529.7 eV, belonging to chemisorbed oxygen, is observed [55]. A corresponding high BE shoulder also appears in
the Pt4f spectrum [51, 52].
At t = 4200 s (start of Region B), CO was introduced at 230 mTorr. At the same time, the O2 partial pressure dropped to 140 mTorr. This drop is partly due to the consumption of O2by the reaction, but the major reason is the gas delivery system, i.e., a decrease of the pressure difference between the gas manifold and the high pressure cell. As soon as CO is introduced, a sharp increase in CO2production is observed in Region B of Figure A. 2, which is then followed by a decrease to a steady state value of ~14 mTorr. The sudden increase in CO2 production results from reaction between CO gas and chemisorbed oxygen on the Pt surface. However, this chemisorbed oxygen is not stable under these reaction conditions and disappears immediately. Due to the limited flow rate of the chamber, the CO2 decreases slowly to the steady state value, 14 mTorr. O1s spectra taken after the pressure stabilized clearly show that the chemisorbed oxygen on the surface is completely removed. The main peak at 532.7 eV in the O1s spectrum results from on-top site CO molecules on the Pt surface. This is also supported by the corresponding C1s peak and Pt4f spectrum (Figure A. 3a, c Spectrum B). Furthermore, the new peak observed in Figure A. 1a (Spectrum D) at 287.8±0.1 eV is present in the C1s spectrum. The rapid increase and decay of the CO2signal in the early stage of Region B demonstrates that the oxygen covered Pt(110) surface has a higher reaction rate than that of the CO covered surface, but this surface is not stable under CO rich conditions.
In Region C, we reduce the CO pressure to achieve an O2 rich condition. The CO2
production starts to increase as the CO pressure decreases. The CO2 production reaches a maximum when the CO pressure reaches a minimum at 14 mTorr. The O1s, C1s and Pt4f core level spectra taken in this region are almost identical to those of Region B, except for a slight decrease of the high BE shoulder of the Pt4f peaks. This shows that the Pt surface is still CO covered, but with lower CO coverage. There is no chemisorbed oxygen or oxide formation on the Pt surface. Furthermore, the CO2pressure decreases from 50 to 36 mTorr as the CO pressure increases from 14 to 28 mTorr in Region C, a negative order in CO.
This result is consistent with the classical L-H mechanism. Because of the asymmetric adsorption of CO and O2on the Pt surface, a state with low (high) CO coverage exhibits a high (low) rate of CO2 production [53]. This high reaction rate region is different from the high reaction rate region observed in Ref. [47], where the Mars-Van Krevelen mechanism
is proposed. In their experiment (performed under higher pressures and higher O2/CO ratio), the CO2pressure increases as the pressure of CO increases in the high reaction rate region.
To complete this study, we then increase the CO pressure back to 200 mTorr to reproduce a CO rich condition similar to Region B. Both reaction data in Figure A. 2 (Region D) and XPS data in Figure A. 3 shows identical results as those obtained in Region B. The production of CO2decreases again when the CO pressure increases, which is consistent with the L-H mechanism.
Figure A. 1 The C1s, O1s and Pt4f core level spectra of the Pt(110) surface under different conditions. A: 200 mTorr CO at room temperature; B: 200 mTorr CO+200 mTorr O2 at room temperature; C: 200 mTorr CO+200 mTorr O2 at 100 ℃; D: 200 mTorr CO+200 mTorr O2 at 150 ℃.
Figure A. 2 Partial pressures of CO, O2, and CO2 at a constant temperature of 150 ℃.
We controlled the CO pressure to switch the reaction from a CO rich to oxygen rich environment and vise versa. Region A: 180 mTorr O2; Region B: 140 mTorr O2+230 mTorr CO; Region C: 150 mTorr O2+14 mTorr -28 mTorr CO; Region D: 140 mTorr O2+200 mTorr CO. These regions correspond to XPS spectra in Figure A. 3.
Figure A. 3 XPS spectra at each region in Figure A. 2 (A-D): (a) C1s spectra taken at 540 eV; (b) O1s spectra taken at 800 eV; (c) Pt4f taken at 340 eV.
A. 4 Conclusion
To summarize, we investigated the CO and O2co-adsorption and catalytic oxidation of CO on a Pt(110) surface using APXPS at various temperatures and pressures. We found that the CO molecules occupy both bridge and on-top sites at room temperature. As the temperature increases, the number of the bridge site CO molecules is reduced and completely removed above 100 ℃ and only CO molecules absorbed at on-top site remain.
We also observe a reaction peak in the C1s spectra that is associated with CO oxidation.
During CO oxidation, we monitor the surface chemical composition and surface reactivity under both O2 rich and CO rich environments. No surface oxides were detected under either condition; the chemisorbed oxygen surface formed under 200 mTorr O2can lead to a high reaction rate, but it is not stable under the reaction conditions we investigated. The high reaction rate found in Region C (O2rich) can be explained by the Langmuir-Hinshelwood mechanism. We also want to emphasize that no direct comparison should be made between this study and that of Ref. [47] due to the difference in CO/O2ratio and total pressure. The surface chemical compositions may be different under those conditions.
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Curriculum vitae 鐘 仁 陽
Jen-Yang Chung
terry06190619@yahoo.com.tw NATIONALITY ______________________________ __________________
- Taiwan (R.O.C.)
PERSONAL DETAILS____________________________________________
- Date of Birth: 19 June 1982.
SKILLS ______________________________________________________
- Ultra-High-Vacuum Scanning Tunneling Microscopy (UHV- STM) - Ultra-High-Vacuum Atomic Force Microscopy (AFM)
- X-ray Photoelectron Spectroscopy (XPS)
EXPERIENCE _________________________________________________
- Research in surface science and interested in adsorbates/semiconductor system.
- Study synchrotron radiation research at National Synchrotron Radiation Research Center ( SRRC ).
- Semiconductor processing technology at National Nano Device laboratories (NDL).
- Operate UHV-AFM and UHV-STM
- Exchange student at Advanced Light Source of Lawrence Berkeley National Laboratory, USA (2007/09~2008/08).
EDUCATION __________________________________________________
Tamkang University (TKU) Taipei City
- undergraduate degree in department of physics - 2000~2004
National Chiao Tung University (NCTU) Hsinchu City - Ph. D. degree in institute of physics - 2004~2010
PUBLICATIONS LIST____________________________________________
[1] J. Y. Chung, F. Aksoy, M. E. Grass, H. Kondoh, P. Ross, Z. Liu, and B. S. Mun, Surf.
Sci. 603, L35 (2009).
[2] J. Y. Chung, W. H. Wang, H. D. Li, C. T. Lou, and D. S. Lin, submitted (2009).
[3] S. F. Tsay, J. Y. Chung, M. F. Hsieh, S. S. Ferng, C. T. Lou, and D. S. Lin, Surf. Sci.
603, 419 (2009).
[4] C. T. Lou, H. D. Li, J. Y. Chung, D. S. Lin, and T. C. Chiang, Phys. Rev. B 80, 5 (2009).
[5] F. Tao, M. E. Grass, Y. W. Zhang, D. R. Butcher, J. R. Renzas, Z. Liu, J. Y. Chung, B.
S. Mun, M. Salmeron, and G. A. Somorjai, Science 322, 932 (2008).
[6] M. F. Hsieh, J. Y. Chung, D. S. Lin, and S. F. Tsay, J. Chem. Phys. 127, 6 (2007).
[7] K. M. Yang, J. Y. Chung, M. F. Hsieh, and D. S. Lin, Jpn. J. Appl. Phys. Part 1 - Regul.
Pap. Brief Commun. Rev. Pap. 46, 4395 (2007).
[8] K. M. Yang, J. Y. Chung, M. F. Hsieh, S. S. Ferng, D. S. Lin, and T. C. Chiang, Phys.
Rev. B 74, 4 (2006).