The Building of UHV System with the Molecular Beam, IRAS, and TPD

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姓名

簡渾淇(Huen-Chi Jian) 查詢紙本館藏

畢業系所

物理學系

論文名稱

(The Building of UHV System with the Molecular Beam, IRAS, and TPD)

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摘要(中)

我們建立一套超高真空系統，其中包含三項實驗技術: 分子束、反射式紅外光譜吸收儀、熱脫附質譜術，這些實驗技術在表面科學上是有用的工具可以研究催化反應過程。我們的設計包括：主腔體、分子束的反應腔體、紅外光路徑腔體。
除了系統架設外，我們利用此系統研究甲醇在鉑單晶上實際時間上的分解反應，在溫度500 K及甲醇曝量在高氣壓狀態(2×10-7- 4×10-7 托耳)，利用我們的熱脫附質譜圖與之前Stuve等人所做的文獻比較之下，我們推測因甲醇分解所產生的一氧化碳比較容易累積在鉑表面上而造成一氧化碳的脫附量增加，而存留在表面的一氧化碳也會使甲醇分解效率降低。另外我們利用氧分子束與甲醇分解反應在溫度480 K下作用，發現氧分子不會與鉑反應分解形成氧原子。

摘要(英)

We built an ultrahigh vacuum system with molecular beam, infrared reflection absorption spectroscopy (IRAS), and temperature programmed desorption (TPD). These techniques are useful tools to study reaction dynamics in surface science. Our designs include a main chamber, a QMS chamber for molecular beam, and IR chambers. We investigated the real-time catalyzed methanol decomposition on single crystal Pt(100) surface. At 500 K and at high pressure (2×10-7 - 4×10-7 Torr), we compare our TPD results to the previous study by Stuve et al. which suggests that the produced CO accumulated on Pt(100) surface in the reactions. The accumulated CO continued to desorb during the exposure to methanol, leading to increase of desorbing CO. The remained CO decreases the probability of methanol decomposition. The O2 molecules refuse to dissociate into O atoms on Pt (100) at 480 K according to the experiments of methanol decomposition with O2 molecular beams.

關鍵字(中)

★ 分子束
★ 反射式吸收紅外光譜儀
★ 熱脫附質譜術
★ 甲醇分解

關鍵字(英)

★ molecular beam
★ IRAS
★ TPD
★ methanol decomposition

論文目次

Contents
摘要.............................................i
Abstract .......................................ii
致謝............................................iii
Contents.........................................iv
List of figures.................................vii
List of Tables...................................ix
Chapter 1: Introduction...........................1
References of Chapter 1..........................3
Chapter 2: Literature survey......................4
2.1: Introduction to Catalysis and Model Catalysts.........................................4
2.2: Molecular Beam, Infrared Reflection Adsorption Spectroscopy (IRAS), and Temperature Programmed Desorption (TPD) Approach.........................8
2.2.1: Methane dissociative adsorption on oxygen-precovered Pt(110) (1×2)........9
2.2.2: The adsorbate-induced lifting of the Pt(100) surface reconstruction...........................12
2.2.3: Reactions of Methanol on Pt(100)......14
2.2.4: Adsorption and decomposition of methanol
on supported Pd modal catalysis..................16
References of Chapter 2........................20
Chapter 3: Experiment Apparatus and Methods......21
3.1: Ultrahigh Vacuum Chambers and Vacuum Pumps............................................21
3.1.1: Introduction to Vacuum................21
3.1.2: An UHV System.........................22
3.1.3: Vacuum Chambers and Pumps of Our System...........................................22
3.2: Experiment Methods........................23
3.2.1: Sample Cleaning.......................23
3.2.2: Methanol Adsorption and Reaction......24
3.3: The Principle of Molecular Beam...........25
3.4: The Pulsed Molecular Beam and Pulsed Valve Control Instrument.......................................28
3.5 Temperature Programmed Desorption..........30
3.6: The principle of IRAS.....................33
3.7: Fourier Transform Interferometers.........36
References of Chapter 3.........................39
Chapter 4: Results and Conclusion................40
4.1: The Designs in Our Systems................40
4.1.1: The Design of the Main Chamber........40
4.1.2: The Design of QMS Chamber of Molecular Beam.............................................45
4.1.3: The Design of IRAS System.............48
4.2: The Alignment of IR Light.................53
4.3: The Real-time Catalyzed Methanol Decomposition on Pt(100)..........................................55
Chapter 5 Conclusions............................63

List of figures
Figure 2-1: Energy diagram for the oxidation of CO on Pt catalysts. All energies are given in kJ mol-1. For comparison, the heavy dashed lines show a noncatalytic route.............................................5
Figure 2-2 : Structural and chemical complexity of a catalyst surface illustrated through the example of an oxide-supported metal catalyst (left side) and kinetic phenomena (right side), which are expected arise on this type of system....................................6
Figure 2-3: An example of the oxygen transient reaction after CH4 exposure at Ts= 650 K. At first, the molecular beam of O2 did not hit the sample. This showed the interaction between chamber and the oxygen beam. At time = 150 sec, the oxygen beam hit the sample. The CO and CO2 was yield by O2 beam and detected by quadrupole mass spectrometer.....................................10
Figure 2-4: (a) The true initial sticking probability s0 (solid line), and the apparent value, sc (dash line), versus Oa precoverage at Et= 188 meV, Tn=800 K, Ts=400 K. (b) The variation of s0 with oxygen adatom precoverage at Et = 90 and 500 meV..............................11
Figure 2-5: (a) IR spectra of CO adsorbed on Pt(100) (1×1) (b) on Pt(100) (hex) at 300 K shown as a function of exposure. Resolution 4 cm-1...................12
Figure 2-6: High resolution IR spectra of CO adsorbed on Pt(100)-hex at 300 K shown as a function of exposure. Resolution 1 cm-1................................13
Figure 2-7: TPD spectra for CH3OH, CO, H2O, and H2 following adsorption of methanol on the Pt(100)-hex at 100 K............................................14
Figure 2-8: The comparison of CO and H2 TPD spectra following methanol adsorption on the (1×1) hex surface of Pt(100) at 130 K…………………...15
Figure 2-9: (a) RAIR spectra of the CO stretching frequency region recorded for the methanol exposure at 100 K. (b) Comparison of TPD spectra for methanol as a function of exposure at 100 K adsorbed on Pd/Al2O3/NiAl (stabilized) (solid line) and on the pristine support (dash line)......................................17
Figure 2-10: RAIR spectra of the CH, CD, and CO stretching frequency regions displayed at different temperatures with a beam of CH3OH or CD3OD, respectively.....................................18
Figure 3-1: Schematic drawing of several pumps with their working ranges...................................23
Figure 3-2: Flow charts of our experiment process..........................................25
Figure 3-3: The schematic structure of continuum free-jet expansion........................................26
Figure 3-4: Schematic view of typical molecular beam source setup.....................................27
Figure 3-5: Diagram if the connection of each instrument and direction of signal flow.....................29
Figure 3-6: Screen shot of the pulsed valve control program..........................................29
Figure 3-7: Schematic diagram of the experimental setup used for TPD.....................................30
Figure 3-8: The diagram of a typical (a) first order and (b) second order desorption......................31
Figure 3-9: The diagram of the connection of Analog-Digital/Digital-Analog interface.................32
Figure 3-10: Electric vectors of the p- and s- components of IR radiation. Primed and umprimed vectors refer to reflected and incident beams respectively........34
Figure 3-11: The surface electric field, E⁄E_0 , and the quantity ((E⁄(E_0 )^2 ))⁄cos⁡θ for platinum at (a) ν ̃=500 〖cm〗^(-1) (ϵ=-3500-4200i) and (b) ν ̃=2100 〖cm〗^(-1) (ϵ=-375-200i) as a function of the angle incidence, θ................................................35
Figure 3-1: (a) The structure of Michelson interferometer and (b) the interferogram........................37
Figure 4-1: The top view of the main chamber. It shows each port, the corresponding technique and the distance between the center of the main chamber and the flange...........................................42
Figure 4-2: The engineering drawing with dimensions of the main chamber. Top left and right are show the angles between the port used for the molecular source chamber and the other ports. Bottom left shows the length 400 mm between the top and the bottom flanges. Bottom right is the cross section view showing the horizontal inclination of nipple........................................43
Figure 4-3: A top-view photograph for all instruments on the main chamber.................................44
Figure 4-4: The photograph of the molecular beam QMS system...........................................46
Figure 4-5: The schematic drawing of the entire molecular beam system......................................47
Figure 4-6: The setup of IRAS on an UHV system...48
Figure 4-7: The photograph of fixed mirrors at the (a) IR chamber 1 (b) IR chamber 2.......................50
Figure 4-8: The schematic drawing of IR chamber 1 with the dimensions...................................50
Figure 4-9: The schematic drawing of IR chamber 2 with the dimensions...................................51
Figure 4-10: (a) the schematic drawing and (b) the photograph for the sealed flange.................52
Figure 4-11: The schematic diagram illustrating how to seal liquid nitrogen tube........................52

Figure 4-12: The photograph of supported mirrors on the (a) IR chamber 1 (b) IR chamber 2................53
Figure 4-13: The photograph of IR light on the IR card.............................................54
Figure 4-14: The signal of IR in the computer....54
Figure 4-15: TPD spectra for (a) CO (m/z=28), H2 (m/z=2), and CH4 (m/z=16) from Pt (100) single crystal exposed to 21 L CH3OH at 300 K; (b) TPD spectra for CO (m/z=28), D2 (m/z=4), and CD4 or D2O (m/z=20) from Pt (100) single crystal exposed to 50 L CD3OD at 300 K................................................55
Figure 4-16: The spectra of real-time catalyzed methanol decomposition on Pt (100): (a) D2 (m/z=4), (b) CO (m/z=28), (c) D2O or CD4 (m/z=20). The sample was exposed continuously to CD3OD (p=7.8×10-8 Torr) at 300 K for 250 sec, then heated to 900 K with 3 K/sec...........56
Figure 4-17: The (a) D2 (m/z=4) and (b) CO (m/z=28) spectra of real-time catalyzed methanol decomposition on Pt (100) exposed continuously to CD3OD (p=4×10-7 Torr) at 300 K for 120 sec, then heat to 500 K with 3 K/sec. After that, decrease pressure from 4×10-7 to 9×10-8 Torr.............................................58
Figure 4-18: The background experiment: heating surface temperature from 300 to 500 K (3 K/sec) without any exposure.........................................58
Figure 4-19: The spectra of real-time catalyze reaction with methanol decomposition on Pt (100) for (a) D2 (m/z=4) (b) CO (m/z=28), exposed continuous CD3OD (p=4×10-7 Torr) at 300 K for 120 sec, then heat to 500 K with 3 K/sec. After that control the surface temperature between 400 K and 500 K..........................59
Figure 4-20: The difference in the integrated D2 signals at 500 and 400 K per second with the times of controlled temperature cycle................................60
Figure 4-21: The spectra of the real-time methanol reaction with O2 molecular beam; (a) the stagnation pressure of O2=55 psi, pulse width=170 μs and (b) the stagnation pressure of O2=40 psi, pulse width=1 ms. In both cases, the same repetition rate (8 Hz) and voltage (200 V) are used. The spectra include D2O or CD4 (m/z=20), CO (m/z=28), O2 (m/z=32), and CO2 (m/z=44) from Pt(100) exposed continuously to CD3OD at surface temperature= 480 K...............................60
Figure 4-22: The temporal profile of the optimized pulsed helium beam scattered specularly from the Pt (100)............................................62

List of Tables
Table 3-1: The quality of vacuum and corresponding gas flow mode........................................21
Table 3-2: The vacuum pumps used in our molecular beam system...........................................23

參考文獻

References of Chapter 1
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[5] Sandra M. Lang, Thorsten M. Bernhardt, Phys. Chem. Chem. Phys., 2012, 14, 9255–9269

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References of Chapter 3
[1] Ellaine M. McCash, Surface Chemistry
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[7] Hans Lu ̈th, Surfaces and Interfaces of Solid (2nd), Springer-Verlag (1993)
[8] John B. Hudson, Surface Science: an introduction, J. Wiley & Sons (1998)
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[16] ABB FT-IR reference manual
[17]李冠卿, 近代光學, 聯經出版社 (1988)

指導教授

羅夢凡(Meng-Fan Luo)

審核日期

2015-7-8

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