銅基觸媒對氨氣選擇性氧化反應之研究

Selective Catalytic Oxidation of Ammonia to Nitrogen Over Copper-based Catalysts

陳鈺沛(Yu-Pei Chen)

指導教授 ： 林錕松

元智大學/工程學院/化學工程與材料科學學系/碩士(2012年)

https://doi.org/10.6838/YZU.2012.00185

摘要

本研究利用臨溼含浸法製備5 ~ 15 wt.% CuO/Al2O3觸媒，有系統地以NH3選擇性氧化反應(SCO)、探討製備變因對銅基觸媒活性的影響，藉以篩選出最佳製備條件，製備變因包括銅負載量、觸媒煅燒溫度、添加Pt、Ce及Ag等促進劑。並以三段式控溫固定床反應器，進行NH3選擇性氧化反應，探討銅負載量、觸媒煅燒溫度、促進劑、NH3/O2進料流率比及不同流質比對觸媒反應活性的影響，並探討觸媒穩定性。SCO於進料氣體比(NH3 : O2 = 1 ~ 5)及流質比(F/W = 42,000 ~ 63,000 ml h-1 gcat-1)下進行，另以BET、TPR、XRD、XPS、SEM-EDS、XANES、EXAFS與In situ DRIFTS等分析，瞭解其物理特性與表面性質。
於NH3選擇性氧化反應，x% CuO/Al2O3觸媒最適製備條件是銅負載量10 ~ 12%及10% CuO/Al2O3觸媒經550°C煅燒。在製備變因的探討中，本研究發現除了銅負載量外，Cu1+/Cu2+比值扮演重要角色，隨著10% CuO/Al2O3觸媒表面Cu1+/Cu2+比值遞減，觸媒於NH3選擇性氧化反應活性也隨之遞減。經過200小時反應後觸媒仍能保持接近初始的轉化率(100%)與選擇率(96%)，且有不錯的穩定性，顯示CuO/Al2O3觸媒於NH3選擇性氧化反應是一理想的觸媒。10% CuO/Al2O3觸媒添加鈰、白金及銀均能促進反應活性，其中又以銀的促進效果最佳，T90下降至94°C，S90都可維持98%的選擇率。
Al2O3擔體與5 ~ 15% CuO/Al2O3觸媒經550°C煅燒後，分別在無氧(2% NH3/Ar)及有氧(NH3/O2/Ar = 2%/10%/88%)的氣氛下進行In situ DRIFTS光譜分析，間接證明Al2O3擔體表面有較活潑的OH基參與-NH2基與-NH基反應產生NO與NO2；於5 ~ 15% CuO/Al2O3觸媒，觀察到N2H2 (Hydrazine)吸收帶、NH2 (Amide)吸收帶、NH (Imide)吸收帶、HNO (Nitroxyl)吸收帶、 NH3吸附在Al2O3擔體的Bronst acid sites 吸收帶及N2O吸收帶，隨溫度增加，吸收帶減弱，而3733 cm–1吸收帶是屬擔體上的OH基增強，間接證明NH3選擇性氧化與銅負載量有關。此外，將5 ~ 15% CuO/Al2O3觸媒於程溫選擇性氧化反應下進行In situ DRIFTS動態分析，觀察到OH基吸收帶同時增強，間接證明反應過程產生的OH基或H2O與NH3選擇性氧化反應有關。
本研究藉NH3選擇性氧化反應，與XPS、XANES、EXAFS及DRIFTS分析提出5 ~ 15% CuO/Al2O3觸媒觸媒於NH3選擇性氧化反應機制。氧引入時可與-NH基作用而生成中間產物-HNO基，接著中間產物-HNO基再與-NH基作用而產生N2與H2O，或中間產物-HNO基自偶合生成N2O與H2O。同時CuO有促進NH3的氧化作用，而本身CuO可以被還原成Cu2O或Cu0，再經氧的氧化作用回復至原來的CuO狀態。

關鍵字

臨溼含浸法 ； NH3選擇性氧化 ； 銅負載量 ； 煅 ； 燒溫度 ； 穩定性

並列摘要

Copper-based catalysts with 5 ~ 15wt.% CuO were prepared by incipient wetness impregnation method. The effect of various parameters on the preparation of catalysts, including the copper loading amount, the Cu/Ce molar ratio and the calcination temperatures of the CuO/Al2O3 catalysts for the selective catalytic oxidation of ammonia were systematically discussed.. The reactor was placed in a three-sectional temperature-controlled oven. The catalysts were placed in a quartz tube and secured between two quartz wool plugs. The composition of reactant gas was typically 2% NH3, 2% O2 with the balance He. This mixture of gases was diverted to the reactor at a flow rate of 1,050 ml/min (F/W = 42,000 ~ 63,000 ml h-1 gcat-1) by mass flow controllers. The catalysts were characterized by the specific surface areas analysis (SBET), inductively coupled plasma spectroscopy (ICP), temperature programmed reduction(TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM-EDX), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and in situ diffuse reflection infrared Fourier transform spectroscopy (In situ DRIFTS).
The optimal catalyst, 10 ~ 12% CuO/Al2O3 was obtained using the following preparation parameters: loading CuO amount 10 ~ 15wt% and x% CuO/Al2O3 catalyst calcined at 550°C for NH3 selective oxidation. This investigation confirms that not only copper loading of the catalyst is important, the ratio of copper states (Cu1+/Cu2+) is also critical in determining the activity of the catalyst, and the activity declined markedly as the Cu1+/Cu2+ ratio decreased for NH3 selective oxidation. As the x% CuO/Al2O3 catalyst had the largest Cu1+/Cu2+ ratio, the activity of the catalyst and the selectivity of nitrogen are also highest. First, the catalyst optimum calcinations temperature was 550°C. Then loading CuO amount 10 ~ 15wt% exhibited the best activity. The x% CuO/Al2O3 catalysts exhibited good activity and stability toward ammonia, and could eliminate a wide range of ammonia gas. Even after 200 h of complete oxidation reaction of ammonia, it also showed 100% conversion. It is a very practical and ideal catalyst for complete oxidation reaction of ammonia. The effect of addition of Ce、Pt and Ag on the activity and selectivity is also discussed. The results show that copper and siliver are is very active and selective towared N2.
In situ DRIFTS were performed to investigate Al2O3 and 5 ~ 15% CuO/Al2O3 catalyst calcined at 550°C, respectively, in the presence (NH3/O2/Ar = 2%/10%/88%) or absence (2% NH3/Ar) of O2.. Undoubtedly, the formation of NO and NO2 in an oxygen-free environment resulted from the reaction between -NH2 and -NH and the active OH groups on Al2O3 supports calcined at 550°C. These results demonstrate that the NH3 selective oxidation reaction may be related to the Cu1+/Cu2+ ratio. Otherwise, for 5 ~ 15% CuO/Al2O3 catalyst during the programmed temperature reaction of selective oxidation of NH3 could be observed that the absorption intensity of the OH group N2H2 (Hydrazine), NH2 (Amide), NH (Imide), HNO (Nitroxyl), NH3 molecules coordinated Al2O3 supports on Bronst acid sites and N2O adsorption band decreased with the temperature increased. These results demonstrate that the generation of NH4+ may be related to the formation of the OH groups and H2O groups during the reaction.
Based on XPS, XANES, EXAFS and in situ DRIFTS analyses, a mechanism for NH3 selective oxidation reaction on 5 ~ 15% CuO/Al2O3 catalyst was proposed. In the first step ammonia is oxidized with the formation of N2 and H2O. N2O is formed by the reaction of two nitrosyl species, while the reaction between NH and O2 leads to NO. Simultaneously, NH3 oxidation promoted over CuO, and CuO reducted to Cu2O or Cu0, and subsequently Cu2O or Cu0 oxidized by atomic oxygen to CuO.

並列關鍵字

Stability ； Incipient wetness impregnation method ； Selective catalytic oxidation of ammonia ； Copper loading ； Calcination temperature

參考文獻

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