乙醇在二氧化鈰金屬催化系統上之吸附及轉化

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(1)國立台灣師範大學化學系 Department of Chemistry, National Taiwan Normal University. 碩. 士. 論. 文. Master’s Thesis. 乙醇在二氧化鈰金屬催化系統上之吸附及轉化. Ethanol Adsorption and Conversion over Ceria Catalyst System. 研究生：廖恆毅指導教授：張一知博士徐新光博士. 中華民國. 九十八. 年. 六. 月二十五日.

(2) For Mom, Dad, Heng-Tzeng and all my dear family members. i.

(3) Acknowledgements First, I would like to thank Prof. Shyu and Prof. Chang for being my research advisers and giving me the opportunity to work and study in this wonderful place with these lovely people. The experiences during this period are very helpful for me and shall become impressive and pleasant memories of my life. I am grateful to Prof. Lin for attending my defense committee and attentive reading of this thesis. It’s fortunate for me to meet so much gratifying companions in Prof. Shyu’s group. About the sundry works of lab, Chia-Kai is a reliable consultant who has helped me to understand and get myself into this group when I was a newcomer; Chia-Min is a patient instructor and aided me the most in my first year in this group; Chih-Min always decently answers all my inquiries and is also a great listener; Regardless of dirtiness and mess, Yu-Chuan has undoubted professional knowledge and enthusiasm; Dr. Hossain is a wise elder who frequently share his unique life philosophy and experience with me; Chih-Hung is a guy with gentle personality. He conscientiously explained for all of my questions until his last minute in this lab; Chi-Cheng and Yang-Chieh are faithful assistants. Our team work make the experiments proceed more efficiently; Zih-Min, Guo-Ping, and Yu-Chu are mischievous in ordinary time. However, they won’t disappoint me when I really need some help; Hsuan-Yin is a silent but funny guy, usually working hard without other’s attention; When I need some advices to dealing with my graduation, Sih-Ting always cheerfully assist me with constant smile; About the arrangement for my oral, Hui-Tie has also do me great help on contacting job. In addition, I would like to thank the members of Prof. Chang’s group for their help during this period. Finally, I would like to thank all my dear family members. Thank them for offering sweet home, affluent living, good upbringing, and outstanding model in my growing. Encourage me when I am frustrated, console me when I fell sad, support me when I face plight, and share joy when I am happy. I am proud of my family and hope someday I could bring them glory too. This is for my dear family.. ii.

(4) 謝. 誌. 首先感謝徐新光老師和張一知老師擔任我的指導教授，並且給我這個機會能夠在如此美好環境下和這些可愛的夥伴們一起工作和學習。這段期間的經歷和體驗帶給我很大的幫助，同時也將成為我人生中難忘的美好回憶。感謝林秀美老師擔任我的口試委員，並且用心地閱讀我的論文後給我許多建議。. 很慶幸能夠在徐老師的實驗室遇到了許多令人愉快的同伴。對於實驗室的各種大小事務，家凱是可靠的諮詢對象，在我剛加入這個團隊時幫助我了解並融入環境；加明是相當有耐心的指導者，實驗室生活的第一年，他是給我最多幫助的人；志銘總是親切和善地回答我各種問題，同時也是很好的聆聽者；裕川雖然有點髒亂，但他的專業能力和拼勁是不容質疑的；Dr. Hossain 是個有智慧的長者，常會跟我分享他獨特的人生哲學和經驗；個性溫和的智鴻直到離開實驗室的前一刻仍然認真地回答著我每個問題；麒承和揚傑是忠實而可靠的助手，我們的團隊合作使實驗進行地更快更有效率；姿旻、國蘋和郁筑平時雖然調皮，但在真正需要幫忙時都不會讓我失望；低調又帶點悶騷的宣尹，總是在沒人注意的地方默默地努力；當需要畢業相關經驗分享的時候，活潑開朗的思婷總是面帶笑容地給我協助；在最後準備安排口試期間，惠澤在聯絡訊息方面也幫了我很大的忙。此外，我也要感謝張一知老師實驗室的同學們在這段期間給我的幫助。. 最後我要感謝我所有親愛的家族成員，感謝他們在我成長的過程中提供了溫暖的家庭、優渥的環境、良好的教養以及優秀的模範。在挫折時給我鼓勵、難過時給我安慰、困難時給我支持、快樂時分享喜悅。我以我的家族為榮，希望有朝一日我也能成為他們的驕傲。本文僅獻給我親愛的家人。. iii.

(5) Abstract Ethanol adsorption and conversion on a series of CeO2- and SiO2- supported catalysts was investigated in order to further understand about ethanol reforming process. These CeO2- and SiO2- supported catalysts were prepared and characterized through N2 physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), ethanol pulse chemisorption, temperature programmed desorption (TPD), and ethanol pulse reaction. The result indicated that oxidation pretreatment sample can not only adsorb ethanol, but also shows greater ethanol adsorption than that of reduction pretreatment sample. And the mechanisms of ethanol adsorption for reduced and oxidized surface seems to be different. The ethanol reforming process can be accomplished by bare CeO2. For Pt/CeO2 catalyst, participation of Pt merely accelerates some steps in the process by promoting the decomposition of adsorbed ethanol and making it feasible at lower temperature. For the long time, Pt causes deactivation of catalyst. Both of Pt/SiO2 and Pt/Ce/SiO2 shows unexpected good ethanol conversion capability and deactivation of these catalysts were not observed. Pt/SiO2 reveals considerable ethanol adsorption and conversion capability while both of bare SiO2 and diluted Pt (PtO2 and SiO2 mixed) showed no sign of ethanol adsorption and conversion. That may imply that ethanol adsorption on Pt/SiO2 associates with the interaction between Pt and SiO2.. iv.

(6) 摘. 要. 針對乙醇在一系列以 CeO2 及 SiO2 為載體的觸媒上之吸附和轉化現象進行研究以進一步了解乙醇的重組反應。首先製備出這一系列的觸媒，並對其進行氮氣表面吸脫附、Ｘ光粉末繞射、溫度程序控制還原、乙醇脈衝吸附、溫度程序控制脫附、乙醇脈衝反應等鑑定及測試。實驗結果顯示，經氧化處理的觸媒表面表現出較還原處理的觸媒更高的乙醇吸附量，且其吸附機制似乎亦有所不同。乙醇重組反應在單純的 CeO2 表面上即可能完成。在 Pt/SiO2 中 Pt 的參與作用僅在於促進表面吸附物種的分解，藉此加速反應進行的流程。但就長時間作用而言，Pt 的存在會造成觸媒反應性衰減。Pt/SiO2 和 Pt/Ce/SiO2 均表現出較預期良好的乙醇轉化能力，且沒有出現觸媒反應性衰減的情形。在 Pt/SiO2 表現出可觀乙醇吸附及轉化能力的同時，單純 SiO2 和 diluted Pt (混合 PtO2 和 SiO2)的表面上似乎完全沒有乙醇吸附。因此在 Pt/SiO2 上觀察到的吸附及轉化能力可能與 Pt 及 SiO2 之間的連結作用有關。. v.

(7) Table of Contents Acknowledgements........................................................................................................ii Abstract .........................................................................................................................iv 摘. 要........................................................................................................................v. Table of Contents ..........................................................................................................vi List of Figures ..............................................................................................................vii List of Tables.................................................................................................................ix. Chapter 1 ~ Introduction .......................................................................................1 Chapter 2 ~ Experimental .....................................................................................7 2.1 Catalysts preparation............................................................................................7 2.2 X-ray diffraction (XRD) ......................................................................................9 2.3 N2 physisorption...................................................................................................9 2.4 Temperature programmed reduction (TPR).......................................................10 2.5 Ethanol pulse chemisorption..............................................................................10 2.6 Temperature-programmed desorption (TPD) of ethanol ...................................10 2.7 Ethanol pulse reaction........................................................................................11. Chapter 3 ~ Results and Discussion .................................................................12 3.1 Catalyst preparation ...........................................................................................12 3.2 X-ray diffraction (XRD) ....................................................................................12 3.3 N2 physisorption.................................................................................................18 3.4 Temperature programmed reduction (TPR).......................................................18 3.5 Ethanol pulse chemisorption..............................................................................29 3.6 Temperature-programmed desorption (TPD) of ethanol ...................................33 3.7 Ethanol pulse reaction........................................................................................54 3.8 Discussion ..........................................................................................................67. Chapter 4 ~ Conclusion........................................................................................69 References....................................................................................................................70. vi.

(8) List of Figures Figure 1. Possible reaction pathways in steam reforming of ethanol over metal catalysts..........................................................................................................2 Figure 2. Summary of the proposed ethanol steam reforming mechanism. .................5 Figure 3. XRD profile of CeO2. ..................................................................................13 Figure 4. XRD profile of Pt/CeO2...............................................................................13 Figure 5. XRD profile of High surface area CeO2. .....................................................14 Figure 6. XRD profile of Pt/High surface area CeO2. ................................................14 Figure 7. XRD profile of SiO2. ...................................................................................15 Figure 8. XRD profile of Ce/SiO2...............................................................................15 Figure 9. XRD profile of Pt/SiO2................................................................................16 Figure 10. XRD profile of Pt/Ce/SiO2. .......................................................................16 Figure 11. XRD profile of diluted Pt...........................................................................17 Figure 12. XRD profile of PtO2. .................................................................................17 Figure 13. TPR profile of CeO2 obtained by TCD......................................................20 Figure 14. TPR profile of CeO2 obtained by MS........................................................20 Figure 15. TPR profile of Pt/CeO2 obtained by TCD. ................................................21 Figure 16. TPR profile of Pt/CeO2 obtained by MS. ..................................................21 Figure 17. TPR profile of High surface area CeO2 obtained by TCD. .......................22 Figure 18. TPR profile of High surface area CeO2 obtained by MS...........................22 Figure 19. TPR profile of Pt/High surface area CeO2 obtained by TCD. ...................23 Figure 20. TPR profile of Pt/High surface area CeO2 obtained by MS. .....................23 Figure 21. TPR profile of SiO2 obtained by TCD.......................................................24 Figure 22. TPR profile of SiO2 obtained by MS.........................................................24 Figure 23. TPR profile of Ce/SiO2 obtained by TCD. ................................................25 Figure 24. TPR profile of Ce/SiO2 obtained by MS. ..................................................25 Figure 25. TPR profile of Pt/SiO2 obtained by TCD. .................................................26 Figure 26. TPR profile of Pt/SiO2 obtained by MS. ...................................................26 Figure 27. TPR profile of Pt/Ce/SiO2 obtained by TCD.............................................27 Figure 28. TPR profile of Pt/Ce/SiO2 obtained by MS...............................................27 Figure 29. TPR profile of diluted Pt obtained by TCD...............................................28 Figure 30. TPR profile of diluted Pt obtained by MS. ................................................28 Figure 31. Ethanol pulse chemisorption profile of CeO2 ............................................30 Figure 32. Ethanol pulse chemisorption profile of Pt/CeO2. ......................................30 Figure 33. Ethanol pulse chemisorption profile of High surface area CeO2...............31 Figure 34. Ethanol pulse chemisorption profile of Pt/High surface area CeO2. .........31 vii.

(9) Figure 35. Ethanol pulse chemisorption profile of SiO2. ............................................31 Figure 36. Ethanol pulse chemisorption profile of Ce/SiO2. ......................................31 Figure 37. Ethanol pulse chemisorption profile of Pt/SiO2. .......................................32 Figure 38. Ethanol pulse chemisorption profile of Pt/Ce/SiO2. ..................................32 Figure 39. Ethanol pulse chemisorption profile of diluted Pt. ....................................32 Figure 40. TPD profiles of adsorbed ethanol over CeO2 (R)......................................36 Figure 41. TPD profiles of adsorbed ethanol over CeO2 (O)......................................37 Figure 42. TPD profiles of adsorbed ethanol over Pt/CeO2 (R). ................................38 Figure 43. TPD profiles of adsorbed ethanol over Pt/CeO2 (O) .................................39 Figure 44. TPD profiles of adsorbed ethanol over High surface area CeO2 (R).........40 Figure 45. TPD profiles of adsorbed ethanol over High surface area CeO2 (O) ........41 Figure 46. TPD profiles of adsorbed ethanol over Pt/High surface area CeO2 (R). ...42 Figure 47. TPD profiles of adsorbed ethanol over Pt/High surface area CeO2 (O) ....43 Figure 48. TPD profiles of adsorbed ethanol over SiO2 (R). ......................................44 Figure 49. TPD profiles of adsorbed ethanol over SiO2 (O).......................................45 Figure 50. TPD profiles of adsorbed ethanol over Ce/SiO2 (R). ................................46 Figure 51. TPD profiles of adsorbed ethanol over Ce/SiO2 (O) .................................47 Figure 52. TPD profiles of adsorbed ethanol over Pt/SiO2 (R)...................................48 Figure 53. TPD profiles of adsorbed ethanol over Pt/SiO2 (O) ..................................49 Figure 54. TPD profiles of adsorbed ethanol over Pt/Ce/SiO2 (R) .............................50 Figure 55. TPD profiles of adsorbed ethanol over Pt/Ce/SiO2 (O).............................51 Figure 56. TPD profiles of adsorbed ethanol over diluted Pt (R). ..............................52 Figure 57. TPD profiles of adsorbed ethanol over diluted Pt (O)...............................53 Figure 58. Ethanol pulse reaction profiles of blank. ...................................................57 Figure 59. Ethanol pulse reaction profiles of CeO2. ...................................................58 Figure 60. Ethanol pulse reaction profiles of Pt/CeO2................................................59 Figure 61. Ethanol pulse reaction profiles of High surface area CeO2. ......................60 Figure 62. Ethanol pulse reaction profiles of Pt/High surface area CeO2...................61 Figure 63. Ethanol pulse reaction profiles of SiO2. ....................................................62 Figure 64. Ethanol pulse reaction profiles of Ce/SiO2................................................63 Figure 65. Ethanol pulse reaction profiles of Pt/SiO2. ................................................64 Figure 66. Ethanol pulse reaction profiles of Pt/Ce/SiO2. ..........................................65 Figure 67. Ethanol pulse reaction profiles of diluted Pt..............................................66. viii.

(10) List of Tables Table 1. BET surface area, pore volume, and pore size of catalysts.………....…..…..18 Table 2. Results of catalyst ethanol pulse chemisorption measurements.………...…..29. ix.

(11) Chapter 1 Introduction Up to date, most of our daily use energy comes directly or indirectly from fossil fuels ~ a non-renewable energy source. This long-term dependence on fossil fuel is already about to exhaust its limited reserve. Furthermore, massive usage of fossil fuels also causes serious environmental problems, such as air pollutants, acid rain and green house effect. The need of renewable, dependable, and environment-friendly alternatives is becoming more and more important. Compare to other potential energy sources, such as solar, wind, tide, and terrestrial heat, hydrogen seems to be one of the best choice because of its clean combustion (the only product is water), high energy content (120.7 kJ/g), and easy availability. It has claimed that hydrogen could be an ideal energy carrier to support sustainable energy development [1-4]. The most abundant hydrogen storage in nature is in bound form, such as water, natural gas, gasoline, diesel oil, liquefied petroleum gas, and other hydrocarbon or biomass. Hydrogen can be produced from these sources by different techniques. These techniques are generally grouped into four categories [5]: (1) Thermochemical: reforming of hydrocarbons by heat, usually with catalyst. (2) Electrochemical: electrolysis of water. (3) Photobiological: photosynthesis of bacteria or algae to produce hydrogen. (4) Photoelectrochemical: water splitting by photoelectric semiconductor. Among these sources and techniques, steam reforming of ethanol is a good candidate for several reasons: (i) ethanol is renewable and easily available; (ii) ethanol is low in toxicity and biodegradable; (iii) ethanol is a stable liquid and easy to transport and store; (iv) steam reforming of ethanol can generate hydrogen with high efficiency; (v) steam reforming of ethanol is thermodynamically feasible [6,7]. These advantages make it a expectant successor of the current hydrogen production processes. Stoichiometrically, the overall reaction of steam reforming of ethanol could be represented as follows [8].. C2H5OH + 3H2O → 2CO2 + 6H2 (∆Ho298 = +347.4 kJ/mol) However, there are several possible pathways included in the whole process. The 1.

(12) tendency of going each pathway depends on the catalyst and reaction condition.. Figure 1. Possible reaction pathways in steam reforming of ethanol over metal catalysts [5]. A breakdown of the reactions is given below: (1) Ethanol (C2H5OH) dehydration to ethylene (C2H4) and water, follow by polymerization of ethylene to form coke [9-13]:. dehydration: polymerization:. C2H5OH → C2H4 + H2O C2H4 → coke. (2) Ethanol (C2H5OH) decomposition to methane (CH4), followed by steam reforming of methane [12]: 2.

(13) decomposition: steam reforming:. C2H5OH → CH4 + CO + H2 CH4 + 2H2O → 4H2 + CO2. (3) Ethanol (C2H5OH) dehydrogenation to acetaldehyde (C2H4O), followed by decrabonylation or steam reforming of acetaldehyde [9-20]:. dehydrogenation:. C2H5OH → C2H4O + H2. decarbonylation:. C2H4O → CH4 + CO. steam reforming:. C2H4O + H2O → 3H2 + 2CO. (4) Ethanol (C2H5OH) decomposition into acetone (CH3COCH3), followed by steam reforming [14-15] [20-21]:. decomposition: steam reforming:. 2C2H5OH → CH3COCH3 + CO + 3H2 CH3COCH3 + 2H2O → 5H2 + 3CO. (5) Steam reforming (C2H5OH) of ethanol to syngas (CO + H2) [9]:. C2H5OH + H2O → 2CO + 4H2 (6) Water gas shift:. CO + H2O → CO2 + H2 (7) Methanation:. CO + 3H2 → CH4 + H2O CO2 + 4H2 → CH4 + 2H2O (8) Coking from the decomposition of methane (CH4):. CH4 → 2H2 + C 3.

(14) (9) Coking from the Boudouard reaction:. CO2 → O2 + C (10) Dissociative adsorption of water to from acetic acid (CH3COOH):. C2H5OH + H2O → CH3COOH + 2H2 Basically, the intent of the whole process is to make as much hydrogen with carbon dioxide and to reduce other byproducts as possible. In order to achieve this objective, it’s necessary for us to further understand the reaction mechanism ~ how the reactions really work on the surface of the catalysts. The nature of the metal and support in a catalyst strongly affects its stability and product distribution of the reaction [22,23]. Recently, metal-loaded ceria catalyst system had received considerable attention. Ceria and ceria-containing mixed oxides have been proposed as a catalytically active component for ethanol conversion reactions due to their high oxygen storage capacity which improves the stability of catalyst [24-26], and their ability to promote the dissociation of the ROH type molecules [27]. In addition, the strong metal-support interaction prevents metal particle sintering, which contributes to catalyst deactivation [28,29]. Many studies on this kind of catalyst have been reported, including some investigations about the reaction mechanism on its surface. Romero-Sarria et al. [29] reported that the use of a catalytic system based on Ce-Zr mixed oxides with Co, Ni, Rh, Rh-Co, or Rh-Ni metal-loading reduced the formation of carbonaceous deposits during steam reforming. Aupretre et al. [30] and Diagne et al. [31] have observed that ceria/zirconia supported metal catalysts exhibit higher H2 yield than other supports during steam reforming. The performance of ceria/zirconia supported Pt catalysts was investigated for partial oxidation by Mattos et al. [32] and for steam reforming by de Lima et al. [33]. The Pt/CeZrO2 catalyst exhibited good activity and stability for partial oxidation. It also showed good selectivity to H2 for steam reforming. Jacobs et al. [26-27] [34-35] have carried out a series of researches to gain insight into the mechanism operating for the steam reforming of ethanol reaction over Pt/ceria catalyst system. The mechanism was explored using a combination of reaction test, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study, and comparing with their previous investigations of the reactions of other ROH 4.

(15) type model compounds such as methanol steam reforming for methanol (MeOH) and water-gas shift for water (HOH).. Methanol steam reforming: CH3OH + H2O ↔ 3H2 + CO2 (∆Ho298 = +131 kJ/mol) Water-gas shift: CO + H2O ↔ 3H2 + CO2 (∆Ho298 = −41.2 kJ/mol) The results of their researches can be summarized as Figure 2.. Figure 2. Summary of the proposed ethanol steam reforming mechanism. 5.

(16) Although the basic skeleton of the ethanol steam reforming reaction seems to be exposed gradually, some details of the mechanism are still ambiguous, such as the roles of metal and ceria support, the effect of the nature of the metal, and how does deactivation happen. Jacobs et al. [35] have also made some efforts on these topics in their latest report. According to their report, the Pt metal appears to associate with the decomposition of surface species, while the dehydrogenation of surface species attributes to the ceria support. Moreover, the function of the Pt metal likely causes the deactivation of the catalyst. The objective of this work is trying to get more information and insight about the reaction, which may be helpful to further clarify the details of the mechanism on metal-loaded ceria catalyst system.. 6.

(17) Chapter 2 Experimental 2.1 Catalysts preparation Pt/CeO2 (1 wt% of Pt) The platinum metal was loaded on 3.0 g of the purchased CeO2 (Aldrich, 99.9%) by incipient wetness impregnation method using tetraammineplatinum(Ⅲ) nitrate (STREM, 99%) aqueous solution (0.06 g dissolved in 1.14 ml of deionized water ). After impregnation, the paste was dried at room temperature overnight, subsequently ground into powder and calcined in air at 500 oC for 4 hour. The resulting sample was pressed to tablet, following by crushed and sieved to 30-40 mesh size particles. CeO2 For contrast, 3.0 g of the purchased CeO2 (Aldrich, 99.9%) was processed with the similar treatment mentioned above, but using deionized water instead of aqueous solution of metal precursor. After treatment, the paste was dried at room temperature overnight, subsequently ground into powder and calcined in air at 500 oC for 4 hour. The resulting sample was pressed to tablet, following by crushed and sieved to 30-40 mesh size particles. High surface area CeO2 High surface area ceria was prepared via homogeneous precipitation method similar to Li et la. [36]. The idea is utilizing the decomposition of urea to slowly release the precipitation agent, OH anion, resulting in a slower, more homogeneous precipitation. On a basis of 30 g CeO2, 76 g of cerium(Ⅲ) nitrate hexahydrate (ACROS, 99.5%) and 240 g of urea (ACROS, 99.5%) were dissolved in 900 ml of deionized water. The mixture was heated to about 90-100 oC with constant stirring for 24 hour. The precipitate was filtered, washed with deionized water, and dried at 100 o C overnight. The dry precipitate was crushed and calcined at 400 oC for 4 hour. The resulting sample was pressed to tablet, following by crushed and sieved to 30-40 mesh size particles.. 7.

(18) Pt/ high surface area CeO2 (1 wt% of Pt)I The platinum metal was loaded on 2.0 g of the high surface area ceria by incipient wetness impregnation method using tetraammineplatinum(Ⅲ) nitrate (STREM, 99%) aqueous solution (0.04 g dissolved in 0.96 ml of deionized water). After impregnation, the paste was dried at room temperature overnight, subsequently ground into powder and calcined in air at 500 oC for 4 hour. The resulting sample was pressed to tablet, following by crushed and sieved to 30-40 mesh size particles. Ce/SiO2 (10 wt% of Ce) The cerium metal was loaded on 10 g of the purchased SiO2 fume (ACROS, CAB-O-SIL® M-5, scintillation grade) by incipient wetness impregnation method using cerium(Ⅲ) nitrate hexahydrate (ACROS, 99.5%) aqueous solution (3.1 g dissolved in 29 ml of deionized water). After impregnation, the paste was dried at room temperature overnight, subsequently ground into powder and calcined in air at 500 oC for 4 hour. The resulting sample was pressed to tablet, following by crushed and sieved to 30-40 mesh size particles. Pt/SiO2 (1 wt% of Pt) The platinum metal was loaded on 10 g of the purchased SiO2 fume (ACROS, CAB-O-SIL® M-5, scintillation grade) by incipient wetness impregnation method using tetraammineplatinum(Ⅲ) nitrate (STREM, 99%) aqueous solution (0.2 g dissolved in 29 ml of deionized water). After impregnation, the paste was dried at room temperature overnight, subsequently ground into powder and calcined in air at 500 oC for 4 hour. The resulting sample was pressed to tablet, following by crushed and sieved to 30-40 mesh size particles. Pt/Ce/SiO2 (1 wt% of Pt, 10 wt% of Ce) The cerium metal was loaded on 5.0 g of the purchased SiO2 fume (ACROS, CAB-O-SIL® M-5, scintillation grade) by incipient wetness impregnation method using cerium(Ⅲ) nitrate hexahydrate (ACROS, 99.5%) aqueous solution (1.55 g dissolved in 14.5 ml of deionized water). After impregnation, the paste was dried at room temperature overnight, subsequently ground into powder and calcined in air at 500 oC for 4 hour. The resulting Ce/SiO2 sample was again treated with incipient wetness impregnation method using tetraammineplatinum(Ⅲ) nitrate (STREM, 99%) 8.

(19) aqueous solution (0.1 g dissolved in 14.5 ml of deionized water) to load on platinum metal. After impregnation, the paste was dried at room temperature overnight, ground into powder, then calcined in air at 500 oC for 4 hour. The resulting sample was pressed to tablet, following by crushed and sieved to 30-40 mesh size particles. SiO2 For contrast, 10 g of the purchased SiO2 fume (ACROS, CAB-O-SIL® M-5, scintillation grade) was processed with the similar treatment mentioned above, but using deionized water instead of aqueous solution of metal precursor. After impregnation, the paste was dried at room temperature overnight, subsequently ground into powder and calcined in air at 500 oC for 4 hour. The resulting sample was pressed to tablet, following by crushed and sieved to 30-40 mesh size particles. Diluted Pt (mixture of 1 wt% of PtO2 and SiO2) 0.02 g of the purchased PtO2 (ACROS, 83% Pt) was adequately mixed with 2.0 g of the processed SiO2 (mentioned above) power, subsequently pressed to tablet, then crushed and sieved to 30-40 mesh size particles. 2.2 X-ray diffraction (XRD) X-ray diffraction measurements of samples were carried out using a Philips X’Pert PRO MPD (Model PW3040/60) instrument, which was equipped with Cu Kα radiation source (λ = 1.5406 Å). The operating voltage was 45 KV and the current was 40mA. The conditions of measurements were as follow: (1) scan rate of 0.02 o per step, scan time of 10 s per step over a 2θrange of 5-80 degree; (2) scan rate of 0.02 o per step, scan time of 20 s per step over a 2θrange of 20-90 degree. 2.3 N2 physisorption The N2 physisorption analyses of samples were carried out using a Micromeritics ASAP 2010 analyzer. The adsorptive gas was nitrogen (N2) and the adsorption was performed at the boiling temperature of liquid nitrogen. A full 64-point isotherm was obtained in the measurement. Before each trial, the sample was degassed at 150 oC in high vacuum (lower than 0.002 mmHg). The surface area was calculated by the BET method, while the pore size distribution curve was obtained by the BJH method [37].. 9.

(20) 2.4 Temperature programmed reduction (TPR) Temperature programmed reduction (TPR) analysis was conducted on samples in a Micromeritics AutoChem Ⅱ 2920 instrument, which was equipped with a thermal conductivity detector (TCD). Argon was used as the reference gas, and 10% H2 (balance Ar) was flowed at a rate of 20 ml/min as the temperature was increased from room temperature to 900 oC at a ramp rate of 10 oC/min. A liquid nitrogen cold trap was used to capture any water that evolved from the sample prior to entering the TCD. In each trial, a weight of approximately 0.1 g of sample was used. A Thermo SCIENTFIC ProLab mass spectrometer was attached to the system as a secondary detector. 2.5 Ethanol pulse chemisorption Ethanol pulse chemisorption experiments were carried out using a Micromeritics AutoChem Ⅱ 2920 instrument. A Thermo SCIENTFIC ProLab mass spectrometer was attached and used as the detector. Two types of process had been practiced in this experiment: (1) A weight of approximately 0.1 g of sample was first reduced under 20 ml/min of 10% H2 in Ar flow at 600 oC for 20 min (the temperature was increased to 600 oC at a ramp rate of 30 oC/min, then hold for 20 min). After reduction, the system was purged with 50 ml/min of He flow to remove residual hydrogen and cooled down to room temperature, subsequently sent pulse of an ethanol/helium mixture, which was obtained by flowing He through a saturator containing ethanol at 60 oC. The number of moles of ethanol adsorbed was determined. (2) A weight of approximately 0.1 g of sample was first oxidized under 20 ml/min of 10% O2 in He flow at 600 oC for 20 min (the temperature was increased to 600 oC at a ramp rate of 30 oC/min, then hold for 20 min). After oxidation, the system was purged with 50 ml/min of He flow to remove residual oxygen and cooled down to room temperature, subsequently sent pulse of an ethanol/helium mixture, which was obtained by flowing He through a saturator containing ethanol at 60 oC. The number of moles of ethanol adsorbed was determined. 2.6 Temperature-programmed desorption (TPD) of ethanol Temperature programmed desorption (TPD) experiments of adsorbed ethanol 10.

(21) were carried out using a Micromeritics AutoChem Ⅱ 2920 instrument. A Thermo SCIENTFIC ProLab mass spectrometer was attached and used as the detector. The fragments monitored for each compound listed as follow: hydrogen (m/z = 2), methane (m/z = 16), ethane (m/z = 27), CO (m/z = 28), acetaldehyde (m/z = 29), ethanol (m/z = 31), CO2 (m/z = 44). Two types of process had been practiced in this experiment: (1) A weight of approximately 0.1 g of sample was first reduced under 20 ml/min of 10% H2 in Ar flow at 600 oC for 20 min (the temperature was increased to 600 oC at a ramp rate of 30 oC/min, then hold for 20 min). After reduction, the system was purged with 50 ml/min of He flow to remove residual hydrogen and cooled down to room temperature. The adsorption of ethanol was carried out at this temperature with pulse of an ethanol/helium mixture, which was obtained by flowing He through a saturator containing ethanol at 60 oC. After absorption, the sample was heated at a rate of 20 oC/min to 600 oC under 50 ml/min of He flow. (2) A weight of approximately 0.1 g of sample was first oxidized under 20 ml/min of 10% O2 in He flow at 600 oC for 20 min (the temperature was increased to 600 oC at a ramp rate of 30 oC/min, then hold for 20 min). After oxidation, the system was purged with 50 ml/min of He flow to remove residual oxygen and cooled down to room temperature. The adsorption of ethanol was carried out at this temperature with pulse of an ethanol/helium mixture, which was obtained by flowing He through a saturator containing ethanol at 60 oC. After absorption, the sample was heated at a rate of 20 oC/min to 600 oC under 50 ml/min of He flow. 2.7 Ethanol pulse reaction Ethanol pulse reaction experiments were carried out using a Micromeritics AutoChem Ⅱ 2920 instrument. A Thermo SCIENTFIC ProLab mass spectrometer was attached and used as the detector. Prior to the experiment, a weight of approximately 0.1 g of sample was first reduced under 20 ml/min of 10% H2 in Ar flow at 600 oC for 20 min (the temperature was increased to 600 oC at a ramp rate of 30 oC/min, then hold for 20 min). After reduction, the system was purged with 50 ml/min of He flow to remove residual hydrogen and cooled down to room temperature, subsequently ramped to the reaction temperature and sent pulse of an ethanol/helium mixture, which was obtained by flowing He through a saturator containing ethanol at 70 oC. The pulse reaction was carried out at 200 oC, 300 oC, 400 o C, 500 oC, and 600 oC respectively. 11.

(22) Chapter 3 Results and Discussion 3.1 Catalyst preparation A series of CeO2- and SiO2- supported catalysts was prepared and investigated in order to further understand about ethanol reforming process. For CeO2- supported catalysts (CeO2, Pt/CeO2, high surface area CeO2, and Pt/high surface area CeO2), two kinds of CeO2 supports were used in this work. One is purchased CeO2, and the other is high surface area CeO2. Pt was loaded to these supports by incipient wetness impregnation method. High surface area CeO2 provided a contrast with more available active site. For SiO2- supported catalysts (SiO2, Ce/SiO2, Pt/SiO2, and Pt/Ce/SiO2), the support was commercial fume SiO2. Ce and Pt were loaded to the support by incipient wetness impregnation method.Ce/SiO2 and Pt/Ce/SiO2 provided another alternative to investigate the effect of Pt addition on ceria while Pt/SiO2 was taken as a ceria-free contrast. The diluted Pt (mixture of PtO2 and SiO2) was also prepared for comparing with Pt/SiO2 catalyst. 3.2 X-ray diffraction (XRD) X-ray diffraction measurements provide the information about crystallization and crystal size of samples. The XRD patterns of the catalysts are shown in Figure 3 ~ 12. For CeO2 (Figure 3) and high surface area CeO2 (Figure 5), the patterns exhibited the characteristic peaks of ceria at 2θ= 28.6 o, 33.2 o, 47.5 o, 56.3 o, 59.1 o, 69.3 o, 76.7 o, 79.0 o, and 88.3 o. CeO2 exhibits very sharp and strong peaks while high surface area CeO2 shows broader peaks because of its smaller crystal size. The pure SiO2 (Figure 7) itself exhibited no peaks (amorphous structure), while Ce/SiO2 (Figure 8) revealed the peaks corresponding to ceria (2θ= 28.6 o, 33.2 o, 47.5 o, 56.3 o), indicating crystallized ceria was formed on the surface of SiO2. The peaks observed for Ce/SiO2 are much weaker and broader than which observed for CeO2 because of its smaller crystal size and less amount of ceria. For all the samples with Pt loading (Pt/CeO2, Pt/high surface area CeO2, Pt/SiO2, and Pt/Ce/SiO2), no Pt characteristic peaks was observed. This may due to the low loading of Pt. Similarly, no characteristic peaks of PtO2 (Figure 12) was observed on the diluted Pt (Figure 11). 12.

(23) CeO2 10. 20. 30. 40. 50. 60. 70. 80. 90. 100. 80. 90. 100. 2θ. Figure 3. XRD profile of CeO2.. Pt/CeO2 10. 20. 30. 40. 50. 60. 70. 2θ. Figure 4. XRD profile of Pt/CeO2. 13.

(24) High surface area CeO2 10. 20. 30. 40. 50. 60. 70. 80. 90. 100. 90. 100. 2θ. Figure 5. XRD profile of high surface area CeO2.. Pt / High surface area CeO2 10. 20. 30. 40. 50. 60. 70. 80. 2θ. Figure 6. XRD profile of Pt/high surface area CeO2. 14.

(25) SiO2 10. 20. 30. 40. 50. 60. 70. 80. 90. 100. 80. 90. 100. 2θ. Figure 7. XRD profile of SiO2.. Ce / SiO2 10. 20. 30. 40. 50. 60. 70. 2θ. Figure 8. XRD profile of Ce/SiO2. 15.

(26) Pt / SiO2 10. 20. 30. 40. 50. 60. 70. 80. 90. 100. 80. 90. 100. 2θ. Figure 9. XRD profile of Pt/SiO2.. Pt / Ce / SiO2 10. 20. 30. 40. 50. 60. 70. 2θ. Figure 10. XRD profile of Pt/Ce/SiO2. 16.

(27) PtO2 SiO2 10. 20. 30. 40. 50. 60. 70. 80. 90. 100. 2θ. Figure 11. XRD profile of diluted Pt.. PtO2. 0. 10. 20. 30. 40. 50. 60. 2θ. Figure 12. XRD profile of PtO2. 17. 70. 80. 90.

(28) 3.3 N2 physisorption In order to understand the difference in surface area between the catalysts, N2 physisorption analyses were performed and the results are presented in Table 1. The surface area of the purchased CeO2 is very low (9.9 m2/g), while the high surface area CeO2 prepared via homogeneous precipitation method revealed much higher surface area (129.2 m2/g). Loading of Pt (Pt/CeO2 and Pt/high surface area CeO2) didn’t cause considerable change in surface area of these supports. The surface area of SiO2 is 179.2 m2/g and the Ce and Pt loaded samples (Ce/SiO2, Pt/SiO2, and Pt/Ce/SiO2) also didn’t reveal considerable change in surface area. Table 1. BET surface area, pore volume, and pore size of catalysts. Catalyst. Surface area. Pore volume (cm /g). (Ǻ). CeO2. 9.9. 0.048. 196.0. Pt/CeO2. 9.1. 0.050. 186.5. High surface area CeO2. 129.2. 0.105. 46.6. Pt/high surface area CeO2. 127.9. 0.095. 48.2. SiO2. 179.2. 0.891. 186.9. Ce/SiO2. 172.6. 0.802. 179.9. Pt/SiO2. 179.0. 0.900. 190.0. Pt/Ce/SiO2. 167.3. 0.887. 205.3. (m /g). 3. Pore size. 2. 3.4 Temperature programmed reduction (TPR) The TPR profiles of catalysts are shown in Figure 13 ~ Figure 30. As proposed by Jacobs et al. [38], the TPR profiles of CeO2 (Figure 13) presented peaks around 500 oC. This peak can be assigned to the reduction of the ceria layers on and close to the surface. Signal around 800 oC are bulk ceria reduction. After loaded with Pt (Figure 15), the reduction peaks of the surface ceria layers was shifted to lower temperature, leaving the bulk ceria reduction peak remained unchanged. This observation indicated that the reduction of surface ceria layers was catalyzed by the Pt loading. For high surface area CeO2 and Pt/high surface area CeO2 (Figure 17 and Figure 19), similar results were observed, except the much greater reduction peaks of the surface ceria layers due to their high surface area. The shifting of surface ceria layers 18.

(29) reduction peaks to lower temperature by Pt addition was observed, and the reduction peak of bulk ceria remained unchanged. For SiO2 and metal-loaded SiO2 samples (Ce/SiO2, Pt/SiO2, and Pt/Ce/SiO2), bare SiO2 (Figure 21) exhibited no reduction peak at all. After loaded with Ce (Ce/SiO2), the reduction peaks similar to CeO2 but with smaller scale was observed (Figure 23). Further Pt loading shifted the reduction peaks of surface ceria layers to lower temperature (Figure 27). Because the loading of the Pt metal was very low (1 wt%), Pt/SiO2 (Figure 23) exhibited very small and ambiguous reduction peaks around 100 oC and 400 oC. No peak can be identified on diluted Pt (Figure 29). The signal of hydrogen (m/z = 2) obtained by mass spectrometer provides a way to make sure that if the response of TCD signal actually attributed to the change of hydrogen in the flow. The interference from the instability of TCD or other component in the flow could be easily recognized by comparing the signal of mass spectrometer with TCD signal. The results of TPR analysis shows that the surface shell ceria of all samples could be completely reduced below 600 oC. This temperature was used for reduction pretreatment in the following experiments.. 19.

(30) H2 comsumption. 483. 525. CeO2 0. 200. 400. 600. 800. o. Temperature ( C ). Figure 13. TPR profile of CeO2 obtained by TCD.. Figure 14. TPR profile of CeO2 obtained by mass spectrometer. 20. 1000.

(31) H2 comsumption. 188. 385. 517. Pt / CeO2 0. 200. 400. 600. 800. o. Temperature ( C ). Figure 15. TPR profile of Pt/CeO2 obtained by TCD.. Figure 16. TPR profile of Pt/CeO2 obtained by mass spectrometer.. 21. 1000.

(32) H2 comsumption. 475. High surface area CeO2. 0. 200. 400. 600. 800. 1000. o. Temperature ( C ). Figure 17. TPR profile of high surface area CeO2 obtained by TCD.. Figure 18. TPR profile of high surface area CeO2 obtained by mass spectrometer.. 22.

(33) H2 comsumption. 260. 217 422. Pt / High surface area CeO2 0. 200. 400. 600. 800. 1000. o. Temperature ( C ). Figure 19. TPR profile of Pt/high surface area CeO2 obtained by TCD.. Figure 20. TPR profile of Pt/high surface area CeO2 obtained by mass spectrometer. 23.

(34) H2 comsumption. SiO2. 0. 200. 400. 600. 800. o. Temperature ( C ). Figure 21. TPR profile of SiO2 obtained by TCD.. Figure 22. TPR profile of SiO2 obtained by mass spectrometer.. 24. 1000.

(35) H2 comsumption. 557. Ce / SiO2 0. 200. 400. 600. 800. o. Temperature ( C ). Figure 23. TPR profile of Ce/SiO2 obtained by TCD.. Figure 24. TPR profile of Ce/SiO2 obtained by mass spectrometer.. 25. 1000.

(36) H2 comsumption. 100. 409. Pt / SiO2. 0. 200. 400. 600. 800. o. Temperature ( C ). Figure 25. TPR profile of Pt/SiO2 obtained by TCD.. Figure 26. TPR profile of Pt/SiO2 obtained by mass spectrometer.. 26. 1000.

(37) H2 comsumption. 142. 353. Pt / Ce / SiO2 0. 200. 400. 600. 800. 1000. o. Temperature ( C ). Figure 27. TPR profile of Pt/Ce/SiO2 obtained by TCD.. Figure 28. TPR profile of Pt/Ce/SiO2 obtained by mass spectrometer.. 27.

(38) H2 comsumption. PtO2 SiO2 TPR. 0. 200. 400. 600. 800. 1000. o. Temperature ( C ). Figure 29. TPR profile of diluted Pt obtained by TCD.. Figure 30. TPR profile of diluted Pt obtained by mass spectrometer.. 28.

(39) 3.5 Ethanol pulse chemisorption The ethanol pulse chemisorption experiments were performed in order to investigate the ethanol adsorption ability of samples. The results of samples pretreated (reduction and oxidation) at 600 oC are shown in Table 2. For CeO2, the oxidation pretreated sample revealed greater ethanol adsorption than that of reduction pretreated sample. For Pt/CeO2, addition of Pt to CeO2 increased the ethanol adsorption of reduction pretreated sample but has no effect (even slightly decreased its ethanol adsorption) on oxidation pretreated sample. For high surface area CeO2, the oxidation pretreated sample showed greater ethanol adsorption than that of reduction pretreated one. Addition of Pt (Pt/high surface area CeO2) still increased the ethanol adsorption of reduction pretreated sample. However, the effect of Pt loading for oxidation pretreated sample can not be identified in this experiment, since all of the ten ethanol pulses were exhausted in both cases (Figure 33 and Figure 34). Table 2. Results of catalyst ethanol pulse chemisorption measurements. (R) stands for the reduction pretreatment sample and (O) stands for the oxidation pretreatment sample. Catalyst. Surface area 2. (m /g). Ethanol uptake (R). Ethanol uptake (O). (µmol/gcat). (µmol/gcat). CeO2. 9.9. 12.0. 108.0. Pt/CeO2. 9.1. 82.0. 96.0. High surface area CeO2. 129.2. 311.0. >600.0. Pt/high surface area CeO2. 127.2. 341.0. >600.0. SiO2. 179.2. --------. --------. Ce/SiO2. 172.6. --------. --------. Pt/SiO2. 179.0. --------. --------. Pt/Ce/SiO2. 167.3. --------. --------. In cases of SiO2, metal-loaded SiO2 (Ce/SiO2, Pt/SiO2, and Pt/Ce/SiO2), and diluted Pt, signals of pulse ethanol did not exhibit distinct peaks appropriate for quantitative determination (Figure 35 ~ Figure 39). However, we can still obtain some clues from the results of TPD experiments (see in 3.6). The TPD results of SiO2 (Figure 48 and Figure 49) revealed almost no signal of desorption for ethanol or any other compound, implying that ethanol can barely adsorb on the surface of SiO2. By contrast, TPD results of Ce/SiO2 (Figure 50 and Figure 51) and Pt/SiO2 (Figure 52 and Figure 53) revealed considerable amount of ethanol adsorption, indicating that the adsorption of ethanol was attributed to the existence of Ce and Pt. In addition, the 29.

(40) TPD results of Diluted Pt (Figure 56 and Figure 57) showed no signal of ethanol or any other compound. From the observation mentioned above, ceria itself showed ability of ethanol adsorption. Considerable quantity of ethanol adsorbed on Pt/SiO2 while both of SiO2 and Diluted Pt showed no sign of adsorption. These results imply that ethanol adsorption on Pt/SiO2 associates with the interaction between Pt and SiO2. However, the lacking of ethanol adsorption on Diluted Pt may due to the poor dispersion of Pt. The possibility that ethanol may adsorbs on Pt still can’t be totally excluded. In order to obtain more information (ethanol adsorption quantities of all samples) and reduce measurement error, better experimental design should be carried out for the future work (e.g. lower vapor concentration with more pulse).. Figure 31. Ethanol pulse chemisorption profile of CeO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment). Figure 32. Ethanol pulse chemisorption profile of Pt/CeO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment). 30.

(41) Figure 33. Ethanol pulse chemisorption profile of high surface area CeO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment). Figure 34. Ethanol pulse chemisorption profile of Pt/high surface area CeO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment). Figure 35. Ethanol pulse chemisorption profile of SiO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment). Figure 36. Ethanol pulse chemisorption profile of Ce/SiO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment). 31.

(42) Figure 37. Ethanol pulse chemisorption profile of Pt/SiO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment). Figure 38. Ethanol pulse chemisorption profile of Pt/Ce/SiO2. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment). Figure 39. Ethanol pulse chemisorption profile of diluted Pt. (Left: sample with reduction pretreatment; Right: sample with oxidation pretreatment). 32.

(43) 3.6 Temperature-programmed desorption (TPD) of ethanol Temperature-programmed desorption of adsorbed ethanol was performed in order to investigate the desorption composition at different temperature. So that conversion of adsorptive surface species can be studied. Experimental results of 600 o C pretreated (reduction and oxidation) samples are mentioned as following. CeO2 The TPD profiles of adsorbed ethanol over CeO2 are shown in Figure 40 (reduction pretreated) and Figure 41 (oxidation pretreated). All signals were weak due to the small surface area of CeO2 and low adsorption on its surface. For reduction pretreated sample, broad signals for ethanol and a acetaldehyde between 80 and 300 o C were observed. A small peak corresponding to hydrogen exhibited around 300 oC. Peaks corresponding to CO2 between 200 and 600 oC were witnessed. For oxidation pretreated sample, the results are almost the same, except the slight increasing in CO2 signal at higher temperature. Pt/CeO2 The TPD profiles of adsorbed ethanol over Pt/CeO2 are shown in Figure 42 (reduction pretreated) and Figure 43 (oxidation pretreated). All signals were weak due to the small surface area and low adsorption. For reduction pretreated sample, peak corresponding to ethanol exhibited between about 80 and 200 oC. Signal corresponding to acetaldehyde exhibited between 80 and 200 oC, and seems weaker than that observed in CeO2. Peaks between 300 and 500 oC were observed for both hydrogen and CH4 formation. Peaks of CO2 exhibited between 300 and 600 oC. For oxidation pretreated sample, the positions of peaks basically remain unchanged. Signals corresponding to CH4, acetaldehyde, and CO2 were stronger, while peak corresponding to hydrogen at lower temperature seems weaker than those observed in Figure 42. High surface area CeO2 The TPD profiles of adsorbed ethanol over high surface area CeO2 are shown in Figure 44 (reduction pretreated) and Figure 45 (oxidation pretreated). For reduction pretreated sample, peak corresponding to ethanol exhibited between 80 and 400 oC. Peaks corresponding to hydrogen exhibited around 100 oC, 300 oC, and 500 oC. 33.

(44) Signals corresponding to ethene and acetaldehyde exhibited between 80 and 400 oC, with a higher peak at 330 oC. CO also exhibited a maxima peak at 330 oC, with a minor signal detected around 500 oC. Signal corresponding to CH4 was observed between 300 and 500 oC, and CO2 exhibited two broad signals around 200 oC and 500 o C respectively. For oxidation pretreated sample, signals for most of compounds became stronger without much change in position. Pt/ high surface area CeO2 The TPD profiles of adsorbed ethanol over Pt/high surface area CeO2 are shown in Figure 46 (reduction pretreated) and Figure 47 (oxidation pretreated). For reduction pretreated sample, peak corresponding to ethanol exhibited between 80 and 200 oC, the signal around higher temperature was remarkably reduced (comparing to high surface area CeO2). The peaks corresponding to hydrogen could be detected all the way from 100 to 600 oC, with several maxima peaks in this range. Signals corresponding to ethene and acetaldehyde exhibited mainly between 80 and 400 oC, but much weaker than those observed in high surface area CeO2. Signal corresponding to CH4 was observed between 100 and 450 oC, while CO exhibited between 400 and 600 oC. The signals corresponding to CO2 exhibited from 100 to 600 o C, with the maxima at about 400 oC. For oxidation pretreated sample, the signals for most of compounds became stronger without much change in position. SiO2 The TPD profiles of adsorbed ethanol over SiO2 are shown in Figure 48 (reduction pretreated) and Figure 49 (oxidation pretreated). For both reduction and oxidation pretreated sample, it shows very small and ambiguous ethanol signal without signal of any other compound, implying that there should be almost no adsorption of ethanol on the surface of SiO2. Ce/SiO2 The TPD profiles of adsorbed ethanol over Ce/SiO2 are shown in Figure 50 (reduction pretreated) and Figure 51 (oxidation pretreated). For reduction pretreated sample, peak corresponding to ethanol exhibited between 80 and 300 oC. Peaks corresponding to hydrogen were observed around 100 oC and 300 oC. Signals corresponding to ethene and acetaldehyde exhibited between 80 and 300 oC. CO2 signal could be detected all the way from 100 to 600 oC, with maxima at 100 oC and 34.

(45) 500 oC. For oxidation pretreated sample, signals corresponding to hydrogen, ethene, and acetaldehyde at higher temperature were reduced. CO2 signal around 500 oC decreases and exhibited a signal around 600 oC. Pt/SiO2 The TPD profiles of adsorbed ethanol over Pt/SiO2 are shown in Figure 52 (reduction pretreatment) and Figure 53 (oxidation pretreatment). For reduction pretreated sample, peak corresponding to ethanol exhibited between 80 and 200 oC. Peaks corresponding to hydrogen exhibited two minor peaks around 100 oC and 200 o C with a major peak at 550 oC. Signals corresponding to ethene and acetaldehyde were observed between 80 and 200 oC, while signal corresponding to CH4 exhibited between 100 and 300 oC. CO2 signal could be detected all the way from 100 to 600 oC, with maxima at 100 oC and 400 oC. For oxidation pretreated sample, signals corresponding to CH4, ethene, and CO2 were weaker than those observed in Figure 52. Pt/Ce/SiO2 The TPD profiles of adsorbed ethanol over Pt/Ce/SiO2 are shown in Figure 54 (reduction pretreatment) and Figure 55 (oxidation pretreatment). For reduction pretreated sample, peak corresponding to ethanol exhibited between 80 and 200 oC, signal around higher temperature remarkably reduced (comparing to Ce/SiO2). The peaks corresponding to hydrogen could be detected all the way from 100 to 600 oC, with several maxima peaks in this range. Signals corresponding to ethene and acetaldehyde exhibited between 80 and 200 oC, while signal corresponding to CH4 was observed between 100 and 400 oC. Signals corresponding to CO2 exhibited from 100 to 600 oC, with the maxima at about 300 oC. For oxidation pretreated sample, only small variations were observed in signals corresponding to hydrogen and CO2. Diluted Pt The TPD profiles of adsorbed ethanol over Diluted Pt are shown in Figure 56 (reduction pretreatment) and Figure 57 (oxidation pretreatment). For both reduction and oxidation pretreated sample, it shows very small and ambiguous ethanol signal without signal of any other compound. Similar to SiO2, it shows no adsorption of ethanol.. 35.

(46) Figure 40. TPD profiles of adsorbed ethanol over CeO2 with reduction pretreatment.. 36.

(47) Figure 41. TPD profiles of adsorbed ethanol over CeO2 with oxidation pretreatment.. 37.

(48) Figure 42. TPD profiles of adsorbed ethanol over Pt/CeO2 with reduction pretreatment. 38.

(49) Figure 43. TPD profiles of adsorbed ethanol over Pt/CeO2 with oxidation pretreatment. 39.

(50) Figure 44. TPD profiles of adsorbed ethanol over high surface area CeO2 with reduction pretreatment. 40.

(51) Figure 45. TPD profiles of adsorbed ethanol over high surface area CeO2 with oxidation pretreatment. 41.

(52) Figure 46. TPD profiles of adsorbed ethanol over Pt/high surface area CeO2 with reduction pretreatment. 42.

(53) Figure 47. TPD profiles of adsorbed ethanol over Pt/high surface area CeO2 with oxidation pretreatment. 43.

(54) Figure 48. TPD profiles of adsorbed ethanol over SiO2 with reduction pretreatment.. 44.

(55) Figure 49. TPD profiles of adsorbed ethanol over SiO2 with oxidation pretreatment.. 45.

(56) Figure 50. TPD profiles of adsorbed ethanol over Ce/SiO2 with reduction pretreatment. 46.

(57) Figure 51. TPD profiles of adsorbed ethanol over Ce/SiO2 with oxidation pretreatment. 47.

(58) Figure 52. TPD profiles of adsorbed ethanol over Pt/SiO2 with reduction pretreatment. 48.

(59) Figure 53. TPD profiles of adsorbed ethanol over Pt/SiO2 with oxidation pretreatment. 49.

(60) Figure 54. TPD profiles of adsorbed ethanol over Pt/Ce/SiO2 with reduction pretreatment. 50.

(61) Figure 55. TPD profiles of adsorbed ethanol over Pt/Ce/SiO2 with oxidation pretreatment. 51.

(62) Figure 56. TPD profiles of adsorbed ethanol over diluted Pt with reduction pretreatment. 52.

(63) Figure 57. TPD profiles of adsorbed ethanol over diluted Pt with oxidation pretreatment. 53.

(64) 3.7 Ethanol pulse reaction Ethanol pulse reaction experiments were performed in order to estimate the ethanol conversion ability of samples. The experimental results of samples reduced at 600 oC are displayed as follow. Blank The ethanol pulse reaction profiles of blank at different temperature are shown in Figure 58. No significant change for ethanol signals was observed from 200 to 600 oC, which means the conversion ethanol was very low (or no conversion at all) with absence of catalyst. The small hydrogen signal appeared at low temperature was the noise coming with ethanol pulse, which shall be ignored in the following discussion. The increasing in hydrogen signal revealed at 500 oC and 600 oC may due to the slightly decomposition of ethanol at high temperature. CeO2 The ethanol pulse reaction profiles of CeO2 at different temperature are shown in Figure 59. Considerable decreasing in ethanol signals began to reveal at 500 oC with increasing in signals of hydrogen observed at 400 oC. At higher temperature, the reaction process was accelerated, which caused the further improvement in ethanol conversion. Pt/CeO2 The ethanol pulse reaction profiles of Pt/CeO2 at different temperature are shown in Figure 60. Both of slight decreasing in ethanol signals and slight increasing in hydrogen signals began to reveal at 300 oC. At higher temperature, the overall conversion of ethanol was further increased. For all temperatures, Pt/CeO2 shows better conversion than bare CeO2 in the beginning (first few peaks). However, the conversion of ethanol was gradually weakened. These observations implying that addition of Pt promoted the conversion of ethanol but also cause deactivation of catalyst, which are in agreement with the results proposed by Jacobs et al. [35]. High surface area CeO2 The ethanol pulse reaction profiles of high surface area CeO2 at different 54.

(65) temperature are shown in Figure 61. At 300 oC, slight decreasing in ethanol signals and slight increasing in hydrogen signals for the first few peaks were observed. Referring to the reaction mechanism proposed by Jacobs et al. [26-27] [34-35], the small amount of ethanol consumption observed at 300 oC may attribute to ethanol adsorption and evolution of adsorbed ethoxy to other species (produce hydrogen). Without sufficient further decomposition of surface species, ethanol can’t be effectively converted at this temperature. At 400 oC, it showed stable ethanol conversion and hydrogen production. The overall ethanol conversion capability was greater than observed on CeO2 due to the higher surface area. Pt/ high surface area CeO2 The ethanol pulse reaction profiles of Pt/high surface area CeO2 at different temperature are shown in Figure 62. Both of slight decreasing in ethanol signals and slight increasing in hydrogen signals began to reveal at 200 oC. At higher temperature, the overall conversion of ethanol was further increased. At 200 oC and 300 oC, Pt/high surface area CeO2 shows better conversion than high surface area CeO2 in the beginning (first few peaks) and the conversion of ethanol was gradually weakened. Similar to Pt/CeO2, the promotion of ethanol conversion and deactivation of catalyst cause by Pt addition were also observed in this case. SiO2 The ethanol pulse reaction profiles of SiO2 at different temperature are shown in Figure 63. No significant change for ethanol signals was observed from 200 to 500 oC, which means the conversion ethanol was very low (or no conversion at all) on the surface of SiO2. At 600 oC, considerable decreasing in ethanol signals and increasing in hydrogen signals were observed, which may due to the thermal decomposition of ethanol at high temperature. With the existence of SiO2 sample in the reaction tube, thermal decomposition shall be promoted since ethanol molecules in the flow have to pass through those hot SiO2 particles. Ce/SiO2 The ethanol pulse reaction profiles of Ce/SiO2 at different temperature are shown in Figure 64. Considerable decreasing in ethanol signals began to reveal at 500 oC with increasing in signals of hydrogen observed at 400 oC. Although with the greater surface area, the ethanol conversion capability of Ce/SiO2 was much lower than high 55.

(66) surface area CeO2. Pt/SiO2 The ethanol pulse reaction profiles of Pt/SiO2 at different temperature are shown in Figure 65. Increasing in signals of hydrogen was observed at 200 oC and ethanol signal was no more detected at 300 oC. Surprisingly, Pt/SiO2 showed even greater ethanol conversion capability than Pt/high surface area CeO2 and the deactivation of catalyst was not observed. However, the TPD profiles show that product distribution of Pt/SiO2 (Figure 52) is different from Pt/high surface area CeO2 (Figure 54), indicating that absence of ceria causes effect to reaction selectivity. Pt/Ce/SiO2 The ethanol pulse reaction profiles of Pt/Ce/SiO2 at different temperature are shown in Figure 66. Increasing in signals of hydrogen was observed at 200 oC and ethanol signal was no more detected at 300 oC. The performance of ethanol conversion for Pt/Ce/SiO2 seems identical to Pt/SiO2 and the deactivation of catalyst was also not observed. The TPD profiles show similar product distribution for Pt/Ce/SiO2 and Pt/high surface area CeO2. Diluted Pt The ethanol pulse reaction profiles of diluted Pt at different temperature are shown in Figure 67. As observed on SiO2, no significant change for ethanol signals was observed from 200 to 500 oC while considerable decreasing in ethanol signals and increasing in hydrogen signals were observed at 600 oC, which attributes to the thermal decomposition of ethanol at high temperature. The existence of PtO2 showed totally no effect on ethanol conversion performance.. 56.

(67) Figure 58. Ethanol pulse reaction profiles of blank at different temperature (200, 300, 400, 500, 600 oC from top to bottom). The hydrogen signals are in left column and the ethanol signals are in right column.. 57.

(68) Figure 59. Ethanol pulse reaction profiles of CeO2 at different temperature (200, 300, 400, 500, 600 oC from top to bottom). The hydrogen signals are in left column and the ethanol signals are in right column.. 58.

(69) Figure 60. Ethanol pulse reaction profiles of Pt/CeO2 at different temperature (200, 300, 400, 500, 600 oC from top to bottom). The hydrogen signals are in left column and the ethanol signals are in right column.. 59.

(70) Figure 61. Ethanol pulse reaction profiles of high surface area CeO2 at different temperature (200, 300, 400, 500, 600 oC from top to bottom). The hydrogen signals are in left column and the ethanol signals are in right column.. 60.

(71) Figure 62. Ethanol pulse reaction profiles of Pt/high surface area CeO2 at different temperature (200, 300, 400, 500, 600 oC from top to bottom). The hydrogen signals are in left column and the ethanol signals are in right column.. 61.

(72) Figure 63. Ethanol pulse reaction profiles of SiO2 at different temperature (200, 300, 400, 500, 600 oC from top to bottom). The hydrogen signals are in left column and the ethanol signals are in right column.. 62.

(73) Figure 64. Ethanol pulse reaction profiles of Ce/SiO2 at different temperature (200, 300, 400, 500, 600 oC from top to bottom). The hydrogen signals are in left column and the ethanol signals are in right column.. 63.

(74) Figure 65. Ethanol pulse reaction profiles of Pt/SiO2 at different temperature (200, 300, 400, 500, 600 oC from top to bottom). The hydrogen signals are in left column and the ethanol signals are in right column.. 64.

(75) Figure 66. Ethanol pulse reaction profiles of Pt/Ce/SiO2 at different temperature (200, 300, 400, 500, 600 oC from top to bottom). The hydrogen signals are in left column and the ethanol signals are in right column.. 65.

(76) Figure 67. Ethanol pulse reaction profiles of diluted Pt at different temperature (200, 300, 400, 500, 600 oC from top to bottom). The hydrogen signals are in left column and the ethanol signals are in right column.. 66.

(77) 3.8 Discussion The adsorption of ethanol had been associated with the reduced surface shell of ceria in previous researches [26, 27, 34, 38-42]. Recently, Jacobs et al. [35] suggested that adsorption of ethanol through reduced ceria surface probably is not the only route. In this work, the results of ethanol pulse chemisorption measurements shown in Table 2 reveal that oxidation pretreated sample can not only adsorb ethanol, but also show greater ethanol adsorption than reduction pretreated sample. This result indicates that oxidation pretreatment can produce more active site than reduction pretreatment. In TPD profiles of high surface area CeO2 (Figure 44, 45) and Pt/high surface area CeO2 (Figure 46, 47), the stronger signals observed for oxidation pretreated sample, which may also suggests the greater ethanol adsorption on oxidized surface. The difference in effect of Pt addition indicates that ethanol adsorbs to reduced and oxidized surface with different mechanisms, which consists with the mechanisms proposed by Jacobs et al. [35]. A comparison between TPD profiles of CeO2, high surface area CeO2, Ce/SiO2 and Pt/CeO2, Pt/high surface area CeO2, Pt/Ce/SiO2 reveals that addition of Pt makes increasing in signals of hydrogen and CH4. Signals corresponding to molecular ethanol desorption were also found to be limited in lower temperature. According to these observations, addition of Pt appears to promote the decomposition of adsorbed ethanol and makes it feasible at lower temperature. In the mass spectrometer analysis of the TPD experiment, the mass/charge ratio of fragments we chose to monitor for ethene and acetaldehyde were 27 and 29 respectively, these are also minor fragments of ethanol. This means if a signal of ethane (mass/charge ratio = 27) or acetaldehyde (mass/charge ratio = 29) appears simultaneously with ethanol signal (in the same temperature area and with the same shape), it might be the interference from ethanol. Moreover, the baseline of CO in mass spectrometer was somehow much higher than other compound, which makes the small amount of CO formation unable to be detected. The TPD profiles of high surface area CeO2 (Figure 44) and Pt/high surface area CeO2 (Figure 46) showed similar result as the previous report [35]. In Figure 44, the signal of ethanol represents the molecular ethanol desorption cause by temperature rising. Hydrogen signal at lower temperature might come from evolution of adsorbing ethanol to other surface species. Simultaneous appearance of hydrogen, CH4, CO, ethene, and acetaldehyde signals around 400 oC could be assigned to desorption and 67.

(78) decomposition of surface species with temperature high enough. In Figure 46, the effects of Pt addition similar to the previous report were also witnessed. However, the signals of CO2 were observed for every catalyst shows ethanol conversion in this work (different from the previous report [35]), including CeO2 and high surface area CeO2, the addition of Pt just increases the amount of CO2. From these results, we might infer that ethanol reforming process could be somewhat accomplished with bare CeO2. Participation of Pt merely promotes some steps in the whole process. In ethanol pulse reaction experiments, comparison between CeO2 and Pt/CeO2 results reveals that addition of Pt promoted the conversion of ethanol. However, the deactivation of catalyst was also observed, which is in agreement with the result proposed by Jacobs et al. [35]. Comparison between high surface area CeO2 and Pt/high surface area CeO2 exhibited the same effects of Pt addition. The ethanol pulse reaction profiles of SiO2 exhibited no ethanol conversion capability while its TPD result indicates that ethanol can barely adsorb on the surface of SiO2, which means SiO2 can be taken as inert support. Although with the greater surface area, Ce/SiO2 showed much lower ethanol conversion capability than high surface area CeO2. Surprisingly, both of Pt/SiO2 and Pt/Ce/SiO2 showed even greater ethanol conversion capability than Pt/high surface area CeO2 and the deactivation of catalyst was not observed. These results might suggest that the existence of SiO2 somehow prevented the deactivation effect caused by Pt. From the comparison between ethanol pulse reaction profiles and TPD results, Pt/SiO2 reveals considerable ethanol adsorption and conversion capability while both of SiO2 and Diluted Pt showed no sign of ethanol adsorption and conversion, which may imply that ethanol adsorption on Pt/SiO2 associates with the interaction between Pt and SiO2. However, the lacking of ethanol adsorption on diluted Pt might also due to the poor dispersion of Pt. The possibility that ethanol may adsorbs on Pt still can’t be totally excluded.. 68.

(79) Chapter 4 Conclusion The results of ethanol pulse chemisorption measurements and TPD experiments reveal that oxidation pretreated sample can not only adsorb ethanol, it even shows greater ethanol adsorption than reduction pretreated sample. And the mechanisms of ethanol adsorption for reduced and oxidized surface are different. A comparison between TPD profiles and ethanol pulse reaction experiment results suggests that ethanol reforming process could be somewhat accomplished with bare CeO2. The participation of Pt merely accelerates some steps in the process by promoting the decomposition of adsorbed ethanol and makes it feasible at lower temperature. The addition of Pt also causes deactivation of catalyst. Both of Pt/SiO2 and Pt/Ce/SiO2 shows unexpected good ethanol conversion capability and the deactivation on these catalysts were not observed. These results might suggest that the existence of SiO2 somehow prevented the deactivation effect caused by Pt. Pt/SiO2 reveals considerable ethanol adsorption and conversion capability while both of SiO2 and Diluted Pt showed no sign of ethanol adsorption and conversion, which may imply that ethanol adsorption on Pt/SiO2 associates with the interaction between Pt and SiO2. However, the lacking of ethanol adsorption on Diluted Pt might also due to the poor dispersion of Pt. The possibility that ethanol may adsorbs on Pt still can’t be totally excluded.. 69.

(80) References. 1. 2.. Goltsov, V. A.; Veziroglu, T. N.; Goltsova, L. F. Int. J. Hydrogen Energy. 2006, 31, 153. Ni, M.; Leung, D. Y. C.; Leung, M. K. H.; Sumathy, K. Fuel Process Technol.. 3.. 2006, 87, 461. Ni, M.; Leung, M. K. H.; Sumathy, K.; Leung, D. Y. C. Int. J. Hydrogen Energy.. 4.. 2006, 31, 1401. Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K. Renewable Sustainable. 5.. Energy Rev. 2007, 11, 401. Haryanto, A.; Fernando, S.; Murali, N.; Adhikari, S. Energy & Fuels. 2005, 19, 2098.. 6. 7.. Garcı´a, E. Y.; Laborde, M. A. Int. J. Hydrogen Energy. 1991, 16, 307. Vasudeva, K.; Mitra, N.; Umasankar, P.; Dhingra, S. C. Int. J. Hydrogen Energy.. 8. 9.. 1996, 21, 13. Velu, S.; Satoh, N.; Gopinath, C. S.; Suzuki, K. Catal. Lett. 2002, 82, 145. Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E. Science. 2004, 303, 993.. 10. Cavallaro, S. Energy Fuels. 2000, 14, 1195. 11. Llorca, J.; Homs, N.; Sales, J.; Fierro, J.-L. G.; de la Piscina, P. R. J. Catal. 2004, 222, 470. 12. Fatsikostas, A. N.; Verykios, X. E. J. Catal. 2004, 225, 439. 13. Therdthianwong, A.; Sakulkoakiet, T.; Therdthianwong, S. ScienceAsia. 2001, 27, 193. 14. Llorca, J.; de la Piscina, P. R.; Sales, J.; Homs, N. Chem. Commun. 2001, 641. 15. Vargas, J. C.; Sternenberg, F.; Roger, A. C.; Kiennemann, A. Presented in the Technical Program, Pisa, Italy, May. 16-19, 2004. 16. Frusteri, F.; Freni, S.; Chiodo, V.; Bonura, G.; Donato, S.; Cavallaro, S. Presented in the Technical Program, Pisa, Italy, May. 16-19, 2004. 17. Goula, M. A.; Kontou, S. K.; Tsiakaras, P. E. Appl. Catal. B. 2004, 49, 135. 18. To´th, M.; Do¨mo¨k, M.; Rasko´x, J.; Hancz, A.; Erdohelyi, A. Presented in the Technical Program, Pisa, Italy, May 16-19, 2004. 19. Fatsikostas, A. N.; Kondarides, D. I.; Verykios, X. E. Chem. Commun. 2001, 851. 20. Sheng, P. Y.; Idriss, H. J. Vac. Sci. Technol. A. 2004, 22, 1652. 21. Zhao, S.; Luo, T.; Gorte, R. J. J. Catal. 2004, 221, 413. 70.

(81) 22. Vaidya, P. D.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2006, 45, 6614. 23. Haryanto, A.; Fernando, S.; Murali, N.; Adhikari, S. Energy Fuels. 2005, 19, 2098. 24. Cai, W.; Wanga, F.; Zhan, E.; Van Veen, A.C.; Mirodatos, C.; Shen, W. J. Catal. 2008, 257, 96. 25. Kugai, J.; Subramani, V.; Song, C.; Engelhard, M. H.; Chin, Y. H. J. Catal. 2006, 238, 430. 26. de Lima, S. M.; da Cruz, I. O.; Jacobs, G.; Davis, B. H.; Mattos, L.V.; Noronha, F. B. J. Catal. 2008, 257, 356. 27. Jacobs, G.; Keogh, R. A.; Davis, B. H. J. Catal. 2007, 245, 326. 28. Roh, H.; Wang, Y.; King, D. L.; Platon, A.; Chin, Y. Catal. Lett. 2006, 108, 15. 29. Romero-Sarria, F.; Vargas, J. C.; Roger, A.; Kiennemann, A. Catal. Today. 2008, 133, 149. 30. 31. 32. 33.. Aupretre, F.; Descorne, C.; Duprez, D. Catal. Commun. 2002, 3, 263. Diagne, C.; Idriss, H.; Kiennemann, A. Catal. Commun. 2002, 3, 565. Mattos, L. V.; Noronha, F. B. J. Power Sources. 2005, 145, 10. de Lima, S. M.; Silva, A. M.; da Cruz, I. O.; Mattos, L.V.; Noronha, F. B. in:. Proc. EUROPACAT VIII, CD ROM, paper P11-16, 2007. 34. de Lima, S. M.; Silva, A. M.; da Cruz, I. O.; Jacobs, G.; Davis, B. H.; Mattos, L.V.; Noronha, F. B. Catal. Today. 2008, 138, 162. 35. de Lima, S. M.; Silva, A. M.; da Cruz, I. O.; Jacobs, G.; Davis, B. H.; Mattos, L.V.; Noronha, F. B. Appl. Catal. A. 2009, 352, 95. 36. Li, Y.; Fu, Q. Appl. Cata.l B. 2000, 27, 179. 37. Barret, E. P.; Joyner L. G.; Halenda, P. H. J. Am. Chem. Soc. 1951, 73, 373. 38. Jacobs, G.; Williams, L.; Graham, U.; Thomas, G. A.; Sparks, D. E.; Davis, B. H. Appl. Catal. A. 2003, 252, 107. 39. Jacobs, G.; Williams L.; Graham, U.; Sparks, D. E.; Davis, B. H. J. Phys. Chem. B. 2003, 107, 10398. 40. Jacobs, G.; Crawford, A.; Williams, L.; Patterson, P. M.; Davis, B. H. Appl. Catal. A. 2004, 267, 27. 41. Jacobs, G.; Patterson, P. M.; Graham, U.; Sparks, D. E.; Davis, B. H. Appl. Catal. A. 2004, 269, 63. 42. Jacobs, G.; Patterson, P. M.; Williams, L.; Sparks, D.; Davis, B. H. Catal. Lett. 2004, 96, 97.. 71.

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