銅鋅基質觸媒於甲醇部分氧化反應之微動力學分析與可調式氧空缺修飾之研究

The Study of Microkinetic Analysis and Tunable Oxygen Vacancies Modification over CuZn-based Catalysts for Partial Oxidation of Methanol Reaction

李冠誼(Kuan-Yi Lee)

指導教授 ： 黃鈺軫

國立清華大學/原子科學院/生醫工程與環境科學系/碩士(2013年)

摘要

本研究利用一系列即時監測實驗來探討Cu30Mn20Zn50觸媒於甲醇部分氧化反應之反應機制及反應微動力學。與Cu30Zn70基質觸媒相比，由於CuMnZn觸媒會形成CuMn2O4的尖晶石結構，此結構可增進反應的活性位置及整體反應速率，進而提升甲醇轉化率及氫氣選擇率；在POM反應過程中，觸媒未啟動前 ( < 180℃)，觸媒上主要以甲氧基、單芽甲酸基吸附物種為主，當反應溫度達啟動溫度時 ( > 180℃)，銅及錳會還原成具活性的Cu0、Cu+、Mn2+價態，而表面只剩下穩定的甲酸物種，且氫氣會快速的產生。進一步的利用動力學去模擬實際數據發現CuMnZn觸媒能明顯的將活化能降低到16.5 kcal/mole，此數值遠低於以往的CuZn觸媒；由此可推論在CuMnZn觸媒中，錳金屬會與銅發生協同效應，進而導致反應活性提升。
另一方面，本研究利用簡單及便宜的方法來調控CuZn觸媒氧空缺的含量，並利用螢光光譜儀的強度對於氧空缺作定量。一系列具氧空缺的vZ (具有氧空缺的ZnO) 載體在氮氣環境下利用不同的鍛燒溫度 (450℃、500℃及550℃) 先製成，接著利用沉積沉澱的方法負載銅金屬製成DP-CvZ (具有氧空缺的CuZn觸媒)，另外也利用共沉澱方法製備出CP-CvZ (具有氧空缺的CuZn觸媒)，來與Cu30Zn70、Cu30Zr70 基質觸媒相比較。從反應結果得知，CP-CvZ-450及DP-CvZ-450兩種觸媒在250℃時，甲醇轉化率接近100%而氫氣選擇率接近95%，且在低溫150℃時，甲醇轉化率還能維持70%而氫氣選擇率也能維持75%；重要的是DP-CvZ-450觸媒CO選擇率只有0~4%。此外，本研究也發現當觸媒的氧空缺含量增加時 (DP-CvZ-500及DP-CvZ-550)，更可以大幅度的降低CO產生 (SCO = 0%)。故推測這些氧空缺不僅在低溫就可以幫助POM反應氣體的吸附，並且提升中間產物的分解及轉移，同時也能催化CO氧化反應之進行。

關鍵字

機制 ； 微動力學 ； 活化能 ； 氧空缺

並列摘要

A series of in-situ experiments were carried out on mechanisms and microkinetic modeling analysis was conducted of a CuMnZn (ca. 28.0 wt.% Cu, 23.3 wt.% Mn, and 48.7 wt.% Zn) catalyst for the partial oxidation of methanol reaction. In comparison with CuZn (ca. 29.2 wt.% Cu, and 70.8 wt.% Zn), CuMnZn catalyst with the structure of copper-manganese spinel CuMn2O4 performed with higher methanol conversion and hydrogen selectivity. During POM reaction, copper and manganese might reduce to more active species, such as Cu0, Cu+, and Mn2+, and enhance the adsorption at lower temperature. Furthermore, ethoxy and monodentate formate must be the dominators below ignition temperature (≦180 ℃). In contrast, when reaching ignition temperature (>180 ℃), the consumption of formate species were, correspondingly, to generate hydrogen. For more in-depth understanding, kinetic modeling analysis of these data was conducted with rate equations which have rendered it possible to derive six steps comprising methanol adsorption, oxygen adsorption, surface reaction, hydrogen desorption, water desorption, and carbon dioxide desorption. The apparent activation energy of 16.5 kcal/mole of CuMnZn catalyst, in contrast, is much lower than a general CuZn-based catalyst and can be initiated at lower temperature. Thus, loading manganese with copper seems to be a synergistic phenomenon which would lead to effective catalytic activity in the partial oxidation of methanol.
A simple and very inexpensive method was created to modulate oxygen vacancies on the non-precious metallic CuZn-based catalyst surface. The identification and quantification of these oxygen vacancies on vZ (ZnO containing oxygen vacancies) and the catalytic activities of CvZ (Cu on ZnO containing oxygen vacancies, ca. 30 wt.% Cu and 70 wt.% Zn) during partial oxidation of methanol (POM) reaction are discussed. The vZ was calcined in a nitrogen atmosphere at various temperatures (450 ℃, 500 ℃ and 550 ℃), and catalytic activities of CvZ catalysts prepared in deposition precipitation (DP) and co-precipitation (CP) and CZr (Cu on ZrO2, ca. 30 wt.% Cu and 70 wt.% Zr) catalysts were performed. Both CP-CvZ-450 and DP-CvZ-450 catalysts present excellent catalytic performance with 100% of CMeOH and 95% of SH2 at 250℃. They also maintained 70% of CMeOH and 75% SH2 at 150℃; especially, Sco was kept at 0~4% at T < 250 ℃, with outstanding stability for DP-CvZ-450 catalyst, as well. Moreover, a CO-free situation could be achieved for both DP-CvZ-500 and DP-CvZ-550 which contain more oxygen vacancies. These oxygen vacancies on the surface: enhanced an affinity for adsorbing reactant oxygen atoms; induced decomposition of intermediates species, and; can even catalyze CO oxidation at a lower temperature.

並列關鍵字

mechanisms ； microkinetic ； activation energy ； oxygen vacancies ； CO-free

參考文獻

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